Opioids are a mainstay for pain management in the perioperative period. That they are effective in a range of types of nociceptive pain is clear where they modulate nociceptive information flow. Their use/efficacy more widely in chronic pain is controversial, particularly in neuropathic pain, but there is utility in the palliative care setting. Alongside the beneficial analgesic actions (antinociception in animals) opioids produce a troublesome set of adverse effects; these include ventilatory depression, constipation, immune suppression, tolerance and opioid use disorder. Opioid-induced hyperalgesia is also a significant clinical problem. Tolerance is often viewed at the centre of an adverse effect circle where increased dosing is required, but this produces more tolerance (and the other adverse effects). The focus of this review is to explore improved analgesia, but it is important to remember that tolerance can manifest secondary to disease progression (pseudo-tolerance) and can develop to many adverse effects; this includes ventilatory depression.

Opiates are natural products from the poppy but also encompass natural endogenous peptides (endorphins for example) whereas opioids are synthetic and not found in nature. Moreover, it is important to remember that not all opioids used in the clinic are the same. At therapeutic concentrations the vast majority display μ (mu, MOP) receptor selectivity. However, there is an approved synthetic peptide κ (kappa, KOP) receptor agonist difelikefalin, but this is used as a treatment for pruritis associated with chronic kidney disease. There are marked differences in clinical MOP ligands with respect to receptor potency (e.g. fentanyl is ∼80× more potent than morphine), efficacy (e.g. buprenorphine is a partial agonist) and in pharmacokinetics (e.g. rapid metabolism for remifentanil). If we consider immune suppression, morphine is strong, oxycodone is weaker and buprenorphine has almost no activity; clearly with respect to immune suppression opioids are not all the same. As covered below in in vitro studies, opioids at ‘equipotent’ concentrations can activate different signalling pathways. Considering the above arguments regarding marked differences in opioid pharmacological behaviour, Emery and Eitan state “… different opioids cannot be made equivalent by merely dose adjustment” other approaches are required.

In this focused review I will cover opioid receptors and mechanisms in some controversial areas of enquiry in a digestible format and use this to frame attempts to design safer opioid medications.

Opioid receptors

Opioid receptors are class A G protein-coupled receptors (GPCRs) and are part of a family; these are the classical naloxone-sensitive MOP, δ (delta, DOP) and KOP along with the non-classical receptor for nociceptin/orphanin FQ (N/OFQ) or NOP (Fig 1). The consequences of receptor interactions will be discussed later. There is historic pharmacological evidence to suggest subtypes of opioid receptors and we have reviewed this in the past. The observation that knock-out of a single gene for each receptor results in full loss of function argues against subtypes but there is evidence for splice variants; with extensive data for MOP. These splice variants can explain some differences in function but not the pre-cloning pharmacological suggestion of subtypes. Moreover, there are now knock-in animals expressing trackable genetic variants to address basic pharmacological-neurobehavioural responses. These animals have natural opioid gene sequence replaced with a modified sequence typically also encoding a fluorescent probe so that expression can be tracked or containing a single nucleotide polymorphism.

Figure 1.

All opioid receptors couple via the inhibitory heterotrimeric G protein (composed of α and β/γ subunits), G_i. Binding of the opioid ligand to the orthosteric site, facilitates G protein interaction and guanine nucleotide (guanosine diphosphate [GDP] for guanosine triphosphate [GTP]) exchange on the α subunit which dissociates from the β/γ dimer. The α_i-GTP and variably β/γ dimer go on to inhibit adenylate cyclase to reduce cyclic adenosine monophosphate (cAMP), open inwardly rectifying K⁺ channels to hyperpolarise, close voltage gated Ca²⁺ channels and activate mitogen-activated protein kinases (MAPKs) (Fig 1). The opioid signal is terminated by GTP metabolism back to GDP (the α subunit is also a GTPase enzyme) and after G protein-coupled receptor kinase (GRK) phosphorylation of the receptor, arrestin recruitment and eventual endocytosis. Arrestin recruitment is important to consider further as there is (disputed) evidence that biased signalling towards G protein and away from arrestin has the potential to produce good quality analgesics with reduced adverse effect profiles.

Opioids modulate both the afferent and efferent parts of the pain pathway. By reducing neurotransmitter release they inhibit pain transmission from first order primary afferent to the second order ascending neurones. These actions are predominantly at K⁺ and Ca²⁺ channels where activation of the former enhances K⁺ efflux leading to hyperpolarisation, while inhibition of the latter reduces Ca²⁺ influx; both resulting in reduced transmitter release. They also affect second to third order transmission and enhance descending inhibitory control activity; the latter being through reduction in GABAergic inhibitory transmission. With respect to the NOP receptor and pain processing there is significant plasticity.

In a seminal series of papers from the laboratory of Laura Bohn, the involvement of β-arrestin-2 in opioid antinociception and adverse effects was explored. To accomplish this, animals deficient in the gene for β-arrestin-2 production (knock-out or KO animals) were generated. KO animals showed greater antinociception and reduced tolerance. In a further series of studies, KO animals showed reduced ventilatory depression and inhibition of gastrointestinal (GI) motility. The overall proposal was that G protein action was beneficial and β-arrestin-2 action was not; this is the basis of functional selectivity or biased agonism covered below. Ligand bias is when a particular ligand can drive one transduction pathway over another. For the MOP receptor this has been questioned in a study by Kliewer and colleagues where fentanyl and morphine did not display reduced adverse effect profiles in KO animals along with some biased opioid receptor knock-in studies. β-Arrestin-2 bias has also been questioned by Gillis and colleagues with a more simple explanation based on efficacy; putative MOP biased agonists being partial agonists. In a reanalysis of the data from the Gillis paper, Stahl and Bohn conclude ‘The data in the Science Signaling paper provide strong corroborating evidence that G protein signaling bias may be a means of improving opioid analgesia while avoiding certain undesirable side effects’. Whilst of great interest pharmacologically and as a potential driver for early phase drug discovery, we have argued that from a therapeutic (drug to market) perspective it does not matter provided any new ligands provide therapeutic efficacy with low adverse effect profiles.

Opioid pharmaco-therapeutic strategies

Opioid receptors should be viewed as dynamic and existing in multiple conformations (e.g. active and inactive). Rather than being thought of as traditional keys to receptor locks, ligands (opioids) stabilise a particular conformation or the equilibrium between different conformations. Full agonists shift the equilibrium and stabilise the active form whereas partial agonists less so. Neutral antagonists simply block but do not activate receptors. For a receptor to exist in the active form in the absence of agonists it must possess constitutive activity. This can occur via a number of mechanisms and of interest here is a series of inherited diseases associated with variants in genes encoding GPCRs. Ligands that interact with this conformation and reduce activity are inverse agonists; they produce the opposite effect to the standard agonist. In experimental systems there are examples of inverse agonists at MOP, DOP, KOP and NOP; less so for NOP receptors, more so for DOP receptors.

Opioids bind to opioid receptors at the orthosteric ligand binding site and some details of this ligand binding site can be obtained from the crystal structures; details of all four opioid receptors have been resolved. Ligand interaction can modulate receptor function by targeting monomer receptors (the traditional view), dimer receptors (homo and hetero), using allosteric modulators and potentially driving one transduction pathway over another (Fig 2).

Figure 2.

To explore pharmacological classification a little more, in a whole organism context, efficacy is the size or strength of a given response in a particular tissue; for opioids this could be analgesia or ventilatory depression. Agonists that return a lower maximum response than a full (typically endogenous) agonist are partial agonists and have reduced efficacy. Can reduced efficacy be used to therapeutic effect (e.g. buprenorphine) and can differences in efficacy be used to explain the pharmacology of some of the newly produced opioid ligands?

Multi-target strategy

Opioid receptors are unlikely to function alone. There is marked interaction between subtypes and this can be as a result of signalling interaction or the formation of dimers. The existence of opioid homodimers and heterodimers has been known for many years and there are some elegant studies demonstrating this using fluorescent tagged receptors and others using receptor probes. For example, activating MOP whilst inhibiting DOP produces antinociception with a reduced adverse effect profile. This can be accomplished with a MOP agonist (morphine) and a DOP antagonist (naltrindole) or by using a bivalent ligand that interacts with both MOP and DOP simultaneously such as MOP agonist/DOP antagonist UFP-505. Whilst offering much there are no clinically available MOP–DOP bivalents. Moreover, there are data that show that the trivalent opioid agonist DPI-125 with marginal selectivity for DOP over MOP/KOP produces antinociception but with reduced ventilatory depression and abuse liability.

There are other combinations that show potential and MOP–NOP is an example with substantial preclinical and clinical development. Cebranopadol is a mixed NOP-opioid ligand and we have reviewed this molecule in the past. The ligand is a high efficacy partial agonist at NOP and other opioid receptors; MOP being of particular interest here. This mixed (non-selective) opioid is antinociceptive in animal models of nociceptive (tail withdrawal), inflammatory (complete Freund's adjuvant) and neuropathic (nerve constriction) pain. Importantly this ligand was more potent (lower doses) in neuropathic pain; representing an area of significant therapeutic need. In animal models there was no ventilatory depression and tolerance developed very slowly. In humans cebranopadol showed efficacy in chronic low back pain, has low abuse potential and respiratory advantage. The CORAL phase III trial in cancer-related chronic pain compared cebranopadol with morphine; cebranopadol was non-inferior. In a longer term safety and efficacy trial, CORAL-XT reported cebranopadol to be safe and well tolerated in prolonged treatment.

From this description of multi-targeting offering adverse effect advantage, non-selectivity is clearly being advocated. This goes against a large part of pharmacological dogma that drives selectivity to reduce adverse effect profile. With respect to opioids a rethink is needed; current development has already moved on (Fig 2).

Biased agonism

The fundamental principle governing biased agonism is that a particular ligand drives one signalling pathway over another and this produces therapeutic advantage. In the case of opioids, β-arrestin-2 KO animals display good quality antinociception but with reduced tolerance and other adverse effects. This has been questioned. The Trevena pharmaceutical company described a MOP receptor biased agonist; TRV130 or oliceridine that drove the G_i pathway over β-arrestin-2 recruitment, behaving as a G protein biased agonist. Based on these data it was presumed that it would have a reduced adverse effect profile. This was the case in preclinical studies and there was evidence of a similar effect in larger phase III clinical trials. These trials; APOLLO-1 (hard tissue, bunionectomy), APOLLO-2 (soft tissue, abdominoplasty) and Athena (safety and efficacy) facilitated Food and Drug Administration (FDA) approval of TRV130 as Olinvyk. There is documented respiratory advantage in humans. However, in a comprehensive series of experiments across several laboratories this biased agonist consistently returns as a partial agonist. G protein-coupled receptors support signal amplification. In the context of TRV130 if the G protein pathway is amplified and the arrestin pathway is not then a partial agonist could return a response at the former and not the latter; showing ‘apparent’ bias and the potential to misclassify. There is much debate in the literature covered above. From a therapeutic perspective this does not really matter as Olinvyk has some advantage over more conventional ‘non-biased’ ligands. Alongside TRV130 there are other G protein biased agonists such as PZM21. This is not in clinical development but is also returning as a partial agonist (Fig 2). In a very recent study Zhuang and colleagues used pharmacological-structural analysis to probe opioid interaction with MOP receptors. The MOP receptor is a GPCR and so composed of seven transmembrane (TM) domains; two areas were investigated, TM-3 face and TM-6/7 face of the ligand binding pocket. Importantly, TM-6/7 is involved in arrestin recruitment. Unbiased ligands (such as morphine and fentanyl) interact at both sites whereas putative biased agonists such as oliceridine and PZM21 preferentially interacted with TM-3.

Allosteric modulators

As noted, opioids bind to the orthosteric site on the receptor to engage G protein and produce an output. There are additional binding sites on the opioid receptor to which other molecules can bind and these can modify the activity of drugs binding to the traditional orthosteric site; these are allosteric modulators. Allosteric modulators can be positive, negative or neutral (silent) but I will focus on positive allosteric modulators (PAMs) here. Consider two situations; (i) endogenous opioid action and (ii) therapeutic intravenous opioid administration. The former is highly selective in site of action and temporal profile but the latter is not, with widespread systemic distribution. A PAM (alone not effective) has the potential to enhance selective endogenous opioid action thereby either reducing the need for a systemic approach or reducing the systemic dose; the net effect is retention of analgesia but with reduced adverse effect profile. The PAM enhances the natural temporal profile produced, in this example by the endogenous opioid ligand. Interestingly, allosteric modulator effects depend on the orthosteric ligand being used; this is called probe dependence. Allosterism concepts are covered in detail in excellent reviews. There are molecules in various stages of development that target opioid receptors. There are a significant amount of data on the BMS (Bristol-Myers Squib) series of allosteric modulators where positive variants (BMS-986122) produce good quality antinociception (mice and rats) against noxious heat and inflammatory stimuli with reduced ventilatory depression, constipation and reward. The orthosteric site was engaged by endogenous (produced or enhanced with an enkephalinase inhibitor) opioids, methadone or morphine. A similar set of data has been produced (also in mice) using MS1, a known MOP–PAM and a molecule with similar chemistry identified from a commercial database (Fig 2).

Opioid receptors, pain and epigenetics

In this review I have covered ligand regulation of receptor function and this makes sense from a pharmacological-therapeutic perspective. Opioid receptor expression and function can also be regulated by epigenetic mechanisms and if these changes can then be manipulated pharmacologically an extra layer to receptor regulation exists. According to the US National Institutes of Health National Human Genome Research Institute, epigenetics is ‘a field of study focused on changes in DNA that do not involve alterations to the underlying sequence’. Epigenetic regulation involves; histone modification, DNA methylation (adding a methyl group to cytosine in DNA) and the activity of non-coding RNAs (ncRNAs are RNA molecules that are not translated into protein). Epigenetic regulation is driven by a family of writers (adding modifications), readers (recognising) and erasers (removing modifications). Epigenetic mechanisms are implicated in a range of diseases including cancer (covered in a journal special issue), asthma and multiple sclerosis and epigenetic changes can be driven by environmental exposure, diet and age. Of relevance to anaesthesia, epigenetics has a role to play in the perioperative period: there are data showing epigenetic links to several types of pain and also opioid addiction-misuse which may have implications for the opioid misuse crisis.

There is thorough recent coverage of epigenetic control of opioid receptors by Reid and colleagues and others, much of this is based on in vitro experiments. Some of the issues relevant to MOP as a mainstay therapeutic opioid target are covered below.

It is generally accepted that in opioid use disorder (addiction) there is hypermethylation of MOP receptor promoters (where transcription is initiated). What are the effects of shorter-term therapeutic use? This has been addressed by Sandoval-Sierra and colleagues who examined genome-wide DNA methylation in the MOP promoter. Thirty-three opioid-naive dental surgery patients were recruited and provided saliva samples pre-surgery then 2.7 [1.5] and 39 [10] days post-surgery. There was a demonstrable hypermethylation of the MOP promoter confirming epigenetic regulation with short-term therapeutic opioid use; this was a small study. If opioid misuse starts with therapeutic use then DNA hypermethylation can be thought of as a continuum; starting in the clinic then continuing with inappropriate or illicit use in the community. DNA methylation of MOP promoters results in reduced expression in the brain.

Histones can be modified by acetyltransferases which add acetyl groups and Histone deacetylase (HDAC) remove them. Enhanced histone acetylation is reported in heroin users and there was a positive correlation with use history. HDACs have a role in several neuropathic pain syndromes where they are generally upregulated resulting in reduced histone acetylation. A study in rat bone cancer pain showed that the associated hyperalgesia is attenuated by the HDAC inhibitor trichostatin A potentially restoring acetylation. In the spinal cord of these animals, bone cancer reduced the expression of MOP and this was restored by the HDAC inhibitor. The authors also showed in vitro in PC12 cells that trichostatin A increased MOP messenger RNA (mRNA) and receptor protein. In a rat model of pancreatitis pain HDAC2 expression was also increased and MOP activity in the dorsal horn of the spinal cord was reduced; the HDAC inhibitor AR-42 attenuated this effect on MOP receptor immunoreactivity. Ricolinostat (an HDAC inhibitor) is currently in phase II evaluation for painful diabetic neuropathy where the investigators are examining pain intensity; there are no results posted (NCT03176472).

An additional mechanism by which opioid receptor expression can be regulated is via the action of non-coding RNAs; these generally target specific mRNAs to effectively silence expression, the best known being microRNA (miRNA). In tolerance (in vivo and in vitro paradigms) the miRNA let-7 inhibits MOP translation; others are variously involved. In a recent systematic review Polli and colleagues explored miRNA in human pains. They report that a wide range of types of pain; complex regional pain syndrome, fibromyalgia, migraine, irritable bowel syndrome, musculoskeletal pain, osteoarthritis and neuropathic pain, all have miRNA associations. We have considered HDACs above and there are data from other neurological diseases showing miRNA regulation of HDAC expression underscoring that epigenetic mechanisms should not be considered in isolation.

Long non-coding RNAs (lncRNAs) are involved in RNA stabilisation (including miRNAs) and the function of translated proteins involved in setting pain syndromes and in substance misuse. For example, an association with peripheral neuropathic pain, diabetic neuropathic pain, trigeminal neuralgia, central pain, inflammatory and cancer pain have been reviewed. Moreover, Michelhaugh and colleagues used Affymetrix microarrays to track five lncRNAs in the nucleus accumbens and these were upregulated in heroin users.

Is there a causal link between epigenetic modification and opioid receptor expression in the brains of patients with opioid use disorder (and as part of the continuum in early therapeutic use) and can this explain opioid misuse? In 1994 Gabilondo and colleagues measured opioid radioligand binding to post-mortem brain tissue. Density in the frontal cortex, thalamus and caudate nucleus in heroin users was similar to that in controls. Ferrer-Alcon examined post-mortem opioid receptor density (immuno technique) in the frontal cortex of patients with opioid use disorder and controls. They showed a reduction of MOP receptor (∼25%) in brains of users. Using positron emission tomography (PET) and [¹¹C]diprenorphine, Williams and colleagues explored opioid receptor binding in living brains during early abstinence; they reported increased binding. If during addiction receptor numbers decrease, then in early abstinence it is not unreasonable to suggest a compensatory upregulation and an increase in [¹¹C]diprenorphine binding. To address the epigenetic issue more directly Knothe and colleagues measured MOP expression by mRNA and protein along with DNA methylation in 27 post-mortem brains. They showed a good correlation between receptor mRNA and protein but not with DNA methylation. In a series of in vitro experiments that they ran concurrently, DNA methylation was an order of magnitude greater—highlighting differences between systems. The authors suggest that epigenetic mechanisms (addiction-related hypermethylation) are unlikely to control expression in the human brain. Epigenetic modulation of neurobiological circuitry is an attractive area of enquiry. Pharmacological manipulation of these epigenetic processes (pharmacoepigenetics) has the potential to affect responses.

Opioids—opioid receptors and immunomodulation

The fact that opioids modulate the immune response has been known for many years and we and others have reviewed this in the past. It is worth re-emphasising that immune modulation is not the same for all opioids. Moreover, there are recent data suggesting an interaction between opioids, COVID infection and outcome. The precise target site for immunomodulation is controversial with three areas of interest; none fully explains immune modulation. These are (i) the immune cell itself, (ii) modulation of the hypothalamic–pituitary–adrenal (HPA) axis and (iii) central actions. Taking these in reverse order, reactive gliosis in central pain is documented as are opioid receptors on glia and minocycline (microglial inhibitor) is effective in neuropathic pain. HPA axis modulation (increased glucocorticoids) appears to show marked species variation along with variation relating to acute or chronic administration. Direct modulation of the immune cell is the most controversial where there is evidence for effects of opioids on immune cell function. In a series of polymerase chain reaction (PCR) experiments we have failed to detect gene expression (mRNA) for any of the classical opioid receptors in mixed or separated human immune cell populations. Without mRNA then there can be no protein. In contrast we have detected mRNA for NOP and, in some cell types, its endogenous ligand N/OFQ. Moreover, we have recently used a novel fluorescent probe for NOP to detect active receptor protein and linked this to cellular function.

Toll-like receptors 4 (TLR4s) respond to the products of Gram-negative bacteria and are important in immune signalling. These receptors are widely distributed and can be found on the vascular endothelium and in a wide range of tumour cells. This receptor seems an unlikely target to be discussing when considering opioids and immune modulation but there is substantial evidence showing a range of opioids interact with this receptor and in a naloxone-sensitive manner. In the absence of definitive evidence for opioid receptor protein on immune cells, TLR4 is a plausible surrogate that can explain immune modulation. In the search for opioid receptor-mediated immunomodulation have we been looking in the wrong place?

Recent work from our laboratory has focussed on examining N/OFQ release from human polymorphonuclear (PMN) cells. We have developed a novel bioassay to measure the interaction of released N/OFQ with chimeric NOP receptors expressed in a biosensor layer of Chinese hamster ovary (CHO) cells. These receptors are forced to couple to Gα_i/q proteins which, when activated, lead to measurable calcium responses. When polymorphs are overlaid onto sensor CHO cells and stimulated to degranulate with N-formyl-L-methionyl-L-leucyl-phenylalanine (fMLP) we can detect real time N/OFQ release from single cells. In unpublished observations we have reported similar responses in isolated human B and T cells.

Collectively, it is clear that NOP activation can modulate immune function and immune cells can produce and release N/OFQ. For classical opioids we should look elsewhere; TLR4 is a compelling target. Immune cells are also capable of the production and release of a range of opioid peptides for classical opioid receptors and these can then interact with neuronal opioid receptors producing a neuro-immune axis, and as discussed below for NOP–N/OFQ, an immune-vascular axis.

Sepsis and an immune-vascular axis; involvement of the N/OFQ opioid receptor

Sepsis is ‘a life-threatening organ dysfunction caused by a dysregulated host immune response to infection’. According to the UK sepsis trust there are 245,000 cases of sepsis each year, while 40% of sepsis survivors suffer permanent, life-changing after-effects and five people die with sepsis every hour. The dysregulated host response in sepsis and septic shock has profound effects on the cardiovascular system resulting in hypotension, organ hypoperfusion and resultant dysfunction/failure. This suggests cross-talk between immune and cardiovascular systems; an immune–vascular axis.

In a retrospective study we measured N/OFQ concentrations in critically ill patients in the ICU and compared these with their own recovery data and a matched control group of volunteers. We reported increased plasma N/OFQ concentrations over the first two days of admission to the ICU. Animal models have explored survival outcomes. In rats with caecal ligation and puncture peritoneal sepsis, mortality increased if N/OFQ was administered. Conversely, mortality was improved if the NOP antagonist UFP-101 was administered. The implications of these data are that N/OFQ is increased in sepsis (agrees with human ICU data) and that NOP antagonism might be beneficial as an adjunct to treat sepsis-induced hypotension. Indeed, in the rat microcirculation in vivo, we have shown that N/OFQ produced hypotension, vasodilation and macromolecular leak which was reversed by the NOP antagonist UFP-101.

In a recent study we examined the expression of NOP on human vascular endothelium cells (HUVECs) and vascular smooth muscle. We showed that unstimulated endothelium expressed mRNA for the NOP receptor but expression of functional protein required treatment with lipopolysaccharide and peptidoglycan G (LPS/PepG) as an in vitro sepsis mimic. These upregulated NOP receptors are functionally active. In sepsis we hypothesise that immune cells release N/OFQ and that this then activates upregulated NOP receptors on the endothelium to support vasodilation and the sequelae of reduced blood pressure; this is an immune-vascular axis (Fig 3).

Figure 3.

Opioid-free analgesia

One school of thought to avoid opioid adverse effects (therapeutic and societal) is to eliminate them with the use of opioid-free analgesia. This seems a rather drastic course of action for a drug class that apparently has good efficacy when used for the right indication, at the right time and for the correct duration. There is literature exploring personalisation of opioid-free anaesthesia; or at least posing the question. There is a wide literature base exploring opioid-free techniques per se and a critical review recently compared opioid and opioid-free approaches concluding that ‘The data indicate that opioid-free strategies, however noble in their cause, do not fully acknowledge the limitations and gaps within the existing evidence and clinical practice considerations.’

Opioid free is worthy of some further consideration here; no opioids at all, no intraoperative opioids, no postoperative opioids or combinations? Olausson and colleagues performed a systematic review and meta-analysis of RCTs published between 2000 and 2021 looking at opioid-free general anaesthesia. The data were derived from 26 trials of 1,934 patients and concluded that opioid-free anaesthesia reduced postoperative adverse effects. Opioids were used in the postoperative period but their use was significantly lower in the opioid-free anaesthesia group. Fiore and colleagues also performed a systematic review and meta-analysis of opioid and opioid-free analgesia after surgical discharge. Their data were derived from 47 trials published after 1 January 1990 and which enrolled 6,607 patients. Opioid prescribing (compared with opioid free) did not reduce postoperative pain although the authors noted that ‘Data were largely derived from low-quality trials’. So what is the optimum? Reduced opioid multimodal intraoperative analgesia followed by limited postoperative (multimodal) use and transition to opioid free? Is this optimal for all procedures? There is guidance.

If this is all considered in the context of poor opioid stewardship (clinical use/prescribing) and the resulting well documented ‘opioid crisis’ then the scene is set for a potential withdrawal of opioids from routine use. This will deny a large patient population access to effective acute pain medications and in the case of cheap generics, effective analgesia in countries with developing health provision. In their report ‘Alleviating the access abyss in palliative care and pain relief—an imperative of universal health coverage: the Lancet Commission report’ Knaul and colleagues state of the 298.5 metric tonnes of morphine-equivalent opioids, 0.03% are distributed to low income countries. Moreover, using Haiti as an index country, the same report concludes 99% of (opioid) need goes unmet.

Conclusions

As pharmacologists and perioperative clinicians we have much to offer in the design of safer opioids (earlier part of this review), but the global community of researchers and perioperative practitioners need to tread carefully. Opioid medicine withdrawal might fix one important and perceived problem but then create another of monstrous proportions. What is clear is that the opioid epidemic has created a ‘hostile’ opioid environment; this hostility is not just from regulators but also from wider society and lawmakers. Whatever mechanisms underlie the actions of opioids and on the backdrop of the opioid crisis, it is clear that the route to market for any new opioid-based analgesics will not be straightforward; we must not give up.

Opioids are widely used for pain management before, during, and after surgery. Their ability to manage various types of pain by changing how pain signals are processed is well-established. However, the broader use of opioids for long-lasting (chronic) pain, especially nerve-related pain, is debated. They do, however, have a clear role in palliative care. While opioids effectively relieve pain, they also cause significant unwanted side effects, such as slowed breathing, constipation, weakened immune function, developing tolerance, and opioid use disorder. Opioid-induced heightened pain sensitivity is another major clinical issue. Tolerance is often seen as central to these problems, where increasing doses are needed, which in turn can worsen tolerance and other side effects. This discussion focuses on improving pain relief, but it is important to remember that tolerance can also occur due to disease progression (pseudo-tolerance) and can develop for many side effects, including slowed breathing.

Opiates are natural compounds derived from the poppy plant, and the term also includes natural substances in the body like endorphins. Opioids, in contrast, are synthetic and not found in nature. It is crucial to recognize that not all opioids used in medical settings are the same. At treatment doses, most opioids primarily act on the mu (μ, MOP) receptor. However, there is a synthetic peptide, difelikefalin, that targets the kappa (κ, KOP) receptor, but it is used to treat itching associated with chronic kidney disease. Opioids that act on the MOP receptor vary greatly in their receptor strength (e.g., fentanyl is about 80 times stronger than morphine), effectiveness (e.g., buprenorphine is a partial agonist, meaning it only partially activates the receptor), and how they are processed by the body (e.g., remifentanil is quickly broken down). For instance, morphine strongly suppresses the immune system, oxycodone is weaker, and buprenorphine has almost no effect; this clearly shows that opioids differ in their immune system impact. Studies in lab settings suggest that even at doses that produce similar effects, different opioids can activate different signaling pathways within cells. Because of these distinct pharmacological behaviors, it has been noted that different opioids cannot simply be made equivalent by adjusting the dose; other approaches are necessary.

The following sections will discuss opioid receptors and their mechanisms in some areas of ongoing debate, presented in an easy-to-understand format, to explore ways of designing safer opioid medications.

Opioid receptors

Opioid receptors are a family of G protein-coupled receptors (GPCRs). These include the classic naloxone-sensitive MOP, delta (δ, DOP), and KOP receptors, as well as the non-classic nociceptin/orphanin FQ (N/OFQ) or NOP receptor. The effects of receptor interactions will be discussed later. While there has been historical evidence for opioid receptor subtypes, the fact that removing a single gene for each receptor leads to a complete loss of function suggests against true subtypes. However, there is evidence for splice variants, particularly for MOP, which can explain some differences in function but not the earlier pharmacological suggestions of subtypes. New animal models with specific genetic modifications are now used to study basic pharmacological and neurobehavioral responses.

All opioid receptors connect through an inhibitory G protein called Gi. When an opioid binds to its receptor, it helps the G protein interact and exchange guanine nucleotides. This causes the G protein's alpha subunit to separate from its beta/gamma dimer. The activated alpha subunit and, to varying degrees, the beta/gamma dimer then work to inhibit adenylate cyclase (reducing cAMP), open potassium channels (making the cell less excitable), close calcium channels, and activate specific protein kinases. The opioid signal ends when the G protein's alpha subunit converts GTP back to GDP, and after the receptor is modified by an enzyme (GRK), leading to the recruitment of arrestin and eventually the receptor being taken inside the cell. Arrestin recruitment is important because there is evidence, though debated, that signaling pathways favoring G protein activity over arrestin recruitment could lead to effective pain relief with fewer side effects.

Opioids affect both the incoming and outgoing parts of the pain pathway. They reduce the release of neurotransmitters, thereby inhibiting pain transmission from the initial sensory neurons to the secondary ascending neurons. These actions primarily occur at potassium and calcium channels: activation of potassium channels increases potassium outflow, making cells less excitable, while inhibition of calcium channels reduces calcium inflow. Both effects lead to reduced release of pain-signaling molecules. Opioids also influence the transmission between secondary and tertiary neurons and enhance the brain's natural pain-inhibiting pathways by reducing the activity of inhibitory GABAergic transmission. The NOP receptor, in particular, shows significant flexibility in pain processing.

Early research explored the role of a protein called β-arrestin-2 in opioid pain relief and side effects. Studies using animals engineered to lack the gene for β-arrestin-2 (knock-out animals) showed that these animals experienced greater pain relief and developed less tolerance. Further studies found that these animals also had reduced respiratory depression and inhibited gastrointestinal movement. The general idea proposed was that G protein activity was beneficial, while β-arrestin-2 activity was not, forming the basis of "functional selectivity" or "biased agonism." Ligand bias occurs when a specific drug activates one signaling pathway more than another. For the MOP receptor, this concept has been questioned by some studies, where fentanyl and morphine did not show reduced side effects in animals lacking β-arrestin-2, and also by some studies on biased opioid receptor modifications. Other research has suggested that apparent β-arrestin-2 bias might simply be explained by the drug's efficacy, with supposed biased agonists actually being partial agonists. However, further analysis of the data still supports the idea that G protein signaling bias could improve opioid pain relief while avoiding certain unwanted side effects. From a treatment perspective, the precise mechanism may not matter as long as new drugs provide effective pain relief with minimal side effects.

Opioid pharmaco-therapeutic strategies

Opioid receptors should be seen as dynamic structures existing in various states, both active and inactive. Instead of acting like traditional keys fitting into locks, opioids stabilize specific receptor states or the balance between different states. Full agonists shift this balance and stabilize the active form, while partial agonists do so to a lesser extent. Neutral antagonists simply block receptors without activating them. For a receptor to be active without any agonist, it must have "constitutive activity," which can happen through several ways, including inherited diseases linked to variations in GPCR genes. Drugs that interact with this active state and reduce its activity are called inverse agonists, producing the opposite effect of standard agonists. Inverse agonists have been observed for MOP, DOP, KOP, and NOP receptors in experimental settings.

Opioids bind to a specific "orthosteric" site on the receptor, and detailed information about this site is available from crystal structures of all four opioid receptors. Drug interaction can change receptor function by targeting individual receptors (the traditional view), receptor pairs (homo- and hetero-dimers), by using other molecules called allosteric modulators, or by potentially favoring one signaling pathway over another.

In the context of a whole organism, "efficacy" refers to the strength of a specific response in a particular tissue; for opioids, this could be pain relief or slowed breathing. Agonists that produce a weaker maximum response than a full agonist (typically an endogenous one) are called partial agonists and have reduced efficacy. The question is whether reduced efficacy can be therapeutically useful (e.g., buprenorphine) and if efficacy differences can explain the pharmacology of newer opioid drugs.

Multi-target strategy

Opioid receptors likely do not function in isolation. There is significant interaction between different opioid receptor types, which can occur through signaling pathways or by forming dimers (pairs of receptors). The existence of opioid homodimers and heterodimers has been known for many years. For example, activating MOP receptors while inhibiting DOP receptors can provide pain relief with fewer side effects. This can be achieved by combining a MOP agonist (like morphine) with a DOP antagonist (like naltrindole), or by using a "bivalent" drug that interacts with both MOP and DOP simultaneously, such as UFP-505. Despite their potential, no MOP–DOP bivalent drugs are currently available clinically. Furthermore, data suggest that the "trivalent" opioid agonist DPI-125, which shows slight preference for DOP over MOP/KOP, produces pain relief with reduced respiratory depression and lower potential for abuse.

Other combinations also show promise, with MOP–NOP being an example that has seen considerable preclinical and clinical development. Cebranopadol is a mixed NOP-opioid drug, which has been previously reviewed. This drug is a highly effective partial agonist at NOP and other opioid receptors, particularly MOP. This mixed opioid has shown pain-relieving effects in animal models of various pain types, including acute, inflammatory, and neuropathic pain. Importantly, this drug was more potent (effective at lower doses) in neuropathic pain, an area with significant unmet medical need. In animal models, cebranopadol did not cause respiratory depression, and tolerance developed very slowly. In humans, cebranopadol was effective for chronic low back pain, had low potential for abuse, and showed respiratory benefits. A phase III trial comparing cebranopadol with morphine for cancer-related chronic pain found cebranopadol to be equally effective. A longer-term safety and efficacy trial reported cebranopadol to be safe and well-tolerated with prolonged treatment.

This description of multi-targeting suggests that non-selectivity can offer advantages in terms of side effects. This contradicts much of traditional pharmacological thinking, which typically promotes selectivity to reduce side effects. With respect to opioids, a re-evaluation is needed, and current drug development has already progressed in this direction.

Biased agonism

The core idea behind biased agonism is that a specific drug can activate one signaling pathway more than another, leading to therapeutic benefits. In the case of opioids, animals lacking β-arrestin-2 showed effective pain relief with reduced tolerance and other side effects. However, this has been questioned. The pharmaceutical company Trevena developed a MOP receptor "biased agonist" called TRV130 (oliceridine) that favored the Gi pathway over β-arrestin-2 recruitment, acting as a G protein-biased agonist. Based on these findings, it was presumed to have fewer side effects. This was observed in preclinical studies and there was similar evidence in larger phase III clinical trials. These trials led to FDA approval of TRV130 as Olinvyk, which has shown respiratory advantages in humans. However, numerous comprehensive experiments across various laboratories consistently classify this biased agonist as a partial agonist. G protein-coupled receptors allow for signal amplification. If, for TRV130, the G protein pathway is amplified while the arrestin pathway is not, a partial agonist could produce a response in the former but not the latter, appearing to show "bias" and potentially leading to misclassification. While this is a topic of much debate in the scientific literature, from a treatment perspective, it may not truly matter as Olinvyk offers some advantages over conventional opioids. Besides TRV130, other G protein-biased agonists exist, such as PZM21. Although not in clinical development, it also appears to act as a partial agonist. Recent pharmacological and structural analyses investigating opioid interaction with MOP receptors found that unbiased drugs (like morphine and fentanyl) interact at two specific areas within the binding pocket, while proposed biased agonists (like oliceridine and PZM21) primarily interact with one of these areas, which is less involved in arrestin recruitment.

Allosteric modulators

As previously mentioned, opioids bind to the "orthosteric site" on the receptor to activate G proteins and produce a response. However, opioid receptors also have additional binding sites where other molecules, called "allosteric modulators," can bind and modify the activity of drugs at the orthosteric site. Allosteric modulators can be positive, negative, or neutral. This discussion will focus on positive allosteric modulators (PAMs). Consider two situations: the action of the body's natural opioids and the therapeutic administration of opioids intravenously. Natural opioid action is highly specific in its location and timing, but intravenous administration leads to widespread distribution throughout the body. A PAM, which is not effective on its own, has the potential to enhance the specific action of the body's natural opioids, thereby reducing the need for systemic opioid use or lowering the systemic dose. The overall effect is retained pain relief with fewer side effects. The PAM enhances the natural timing of the effect produced by the endogenous opioid. Interestingly, the effects of allosteric modulators depend on the specific orthosteric drug being used, a phenomenon known as "probe dependence." Various molecules targeting opioid receptors are currently in different stages of development. There is significant data on a series of allosteric modulators from Bristol-Myers Squibb, where positive variants (BMS-986122) effectively relieve pain in animal models against heat and inflammatory stimuli, with reduced respiratory depression, constipation, and reward. These effects were observed when the orthosteric site was activated by the body's natural opioids (either naturally present or enhanced by an enzyme inhibitor), methadone, or morphine. Similar results have been shown using MS1, a known MOP-PAM, and a chemically similar molecule identified from a commercial database.

Opioid receptors, pain and epigenetics

This review has covered how drugs regulate receptor function, which is relevant from a pharmacological and therapeutic standpoint. However, the expression and function of opioid receptors can also be controlled by epigenetic mechanisms. If these changes can be manipulated by drugs, an additional layer of receptor regulation exists. Epigenetics refers to changes in DNA that do not alter the underlying genetic sequence. Epigenetic regulation involves modifying histones (proteins around which DNA is wrapped), adding methyl groups to DNA (DNA methylation), and the activity of non-coding RNAs (RNA molecules that are not translated into proteins). These processes are managed by "writers" (adding modifications), "readers" (recognizing modifications), and "erasers" (removing modifications). Epigenetic mechanisms are linked to various diseases, including cancer, asthma, and multiple sclerosis, and can be influenced by environmental factors, diet, and age. In the context of anesthesia, epigenetics plays a role in the perioperative period, with data showing connections to several types of pain and also opioid addiction and misuse, which may be relevant to the current opioid crisis.

Recent thorough discussions of epigenetic control of opioid receptors, much of which is based on lab experiments, highlight issues relevant to the MOP receptor as a key opioid target.

It is generally accepted that in opioid use disorder (addiction), there is increased DNA methylation in the promoters of MOP receptors, which reduces the initiation of gene transcription. The effects of shorter-term therapeutic opioid use have also been examined. One study investigated genome-wide DNA methylation in the MOP promoter in 33 opioid-naïve dental surgery patients. They collected saliva samples before surgery, and then at 2.7 and 39 days post-surgery. This small study showed a clear increase in DNA methylation of the MOP promoter after short-term therapeutic opioid use, confirming epigenetic regulation. If opioid misuse begins with therapeutic use, then DNA hypermethylation could be seen as a continuous process, starting in clinical settings and progressing with inappropriate or illegal use in the community. DNA methylation of MOP promoters leads to reduced MOP receptor expression in the brain.

Histones can be modified by enzymes called acetyltransferases, which add acetyl groups, and Histone deacetylases (HDACs), which remove them. Increased histone acetylation has been reported in heroin users, with a positive correlation to their history of use. HDACs play a role in several types of neuropathic pain, where they are typically overactive, leading to reduced histone acetylation. A study on rat bone cancer pain showed that the associated heightened pain was reduced by an HDAC inhibitor, trichostatin A, which potentially restored acetylation. In the spinal cords of these animals, bone cancer reduced MOP expression, and this was restored by the HDAC inhibitor. The authors also demonstrated in lab cell cultures that trichostatin A increased MOP messenger RNA (mRNA) and receptor protein. In a rat model of pancreatitis pain, HDAC2 expression was also increased, and MOP activity in the spinal cord was reduced; the HDAC inhibitor AR-42 lessened this effect on MOP receptor levels. Ricolinostat, another HDAC inhibitor, is currently in phase II trials for painful diabetic neuropathy, where investigators are evaluating pain intensity.

Another way opioid receptor expression can be regulated is through the action of non-coding RNAs, particularly microRNA (miRNA), which generally target specific mRNAs to effectively silence gene expression. In studies of opioid tolerance (both in living organisms and in lab settings), miRNA let-7 inhibits MOP protein production, and other miRNAs are also involved. A recent review explored miRNA in various human pain conditions, reporting that a wide range of pain types, including complex regional pain syndrome, fibromyalgia, migraine, irritable bowel syndrome, musculoskeletal pain, osteoarthritis, and neuropathic pain, are associated with miRNA changes. Considering HDACs, there is also data from other neurological diseases showing miRNA regulation of HDAC expression, highlighting that epigenetic mechanisms should not be viewed in isolation.

Long non-coding RNAs (lncRNAs) are involved in stabilizing RNA molecules (including miRNAs) and influencing the function of proteins that contribute to pain syndromes and substance misuse. For example, lncRNAs have been linked to peripheral neuropathic pain, diabetic neuropathic pain, trigeminal neuralgia, central pain, inflammatory pain, and cancer pain. Furthermore, studies have shown that five specific lncRNAs were increased in the nucleus accumbens of heroin users.

A key question is whether there is a direct link between epigenetic changes and opioid receptor expression in the brains of individuals with opioid use disorder (and in early therapeutic use) that could explain opioid misuse. Early research in 1994 found similar opioid receptor binding in post-mortem brain tissue of heroin users compared to controls. However, a later study found a reduction of MOP receptors (approximately 25%) in the frontal cortex of individuals with opioid use disorder. Using advanced imaging techniques (PET scans) in living brains during early abstinence, an increase in opioid receptor binding was reported. If receptor numbers decrease during addiction, it is plausible that a compensatory increase occurs in early abstinence. To directly address the epigenetic link, one study measured MOP expression (mRNA and protein) and DNA methylation in 27 post-mortem brains. They found a good correlation between receptor mRNA and protein but not with DNA methylation. Concurrent lab experiments showed DNA methylation to be significantly higher, highlighting differences between systems. The authors suggested that epigenetic mechanisms, such as addiction-related hypermethylation, may not be the primary control of MOP expression in the human brain. Nonetheless, epigenetic modulation of brain circuits remains an interesting area of study, and pharmacological manipulation of these epigenetic processes (pharmacoepigenetics) has the potential to influence responses.

Opioids—opioid receptors and immunomodulation

The fact that opioids alter the immune response has been known for many years. It is important to reiterate that immune modulation is not uniform across all opioids. Furthermore, recent data suggest an interaction between opioids, COVID infection, and its outcomes. The exact target site for immunomodulation is debated, with three main areas of interest: the immune cell itself, modulation of the hypothalamic–pituitary–adrenal (HPA) axis, and central nervous system actions. Starting with central actions, reactive gliosis (a response of glial cells in the brain) is documented in central pain, and opioid receptors are found on glial cells. Minocycline, which inhibits microglia (a type of glial cell), is effective in neuropathic pain. HPA axis modulation (leading to increased stress hormones) appears to vary significantly between species and depending on whether opioid administration is acute or chronic. Direct modulation of the immune cell is the most controversial area, despite evidence for opioid effects on immune cell function. However, in laboratory experiments using human immune cells, we have not detected gene expression (mRNA) for any of the classical opioid receptors. Without mRNA, there can be no protein. In contrast, we have detected mRNA for NOP and, in some cell types, its natural signaling molecule N/OFQ. More recently, a novel fluorescent probe has been used to detect active NOP receptor protein and link it to cellular function.

Toll-like receptors 4 (TLR4s) respond to products from Gram-negative bacteria and are crucial for immune signaling. These receptors are widely distributed, found on blood vessel lining cells and various tumor cells. While TLR4 might seem an unlikely target in discussions of opioids and immune modulation, substantial evidence shows that various opioids interact with this receptor in a way that can be blocked by naloxone. In the absence of definitive evidence for classical opioid receptor proteins on immune cells, TLR4 presents a plausible alternative target that could explain immune modulation. This raises the question of whether we have been looking in the wrong place for opioid receptor-mediated immunomodulation.

Recent work from our laboratory has focused on studying N/OFQ release from human polymorphonuclear (PMN) cells. A new bioassay has been developed to measure the interaction of released N/OFQ with modified NOP receptors expressed in a biosensor layer of Chinese hamster ovary (CHO) cells. These receptors are engineered to couple to specific G proteins (Gαi/q), which, when activated, lead to measurable calcium responses. When PMN cells are placed over sensor CHO cells and stimulated to degranulate, real-time N/OFQ release from single cells can be detected. Similar responses have been observed in isolated human B and T cells.

Collectively, it is clear that NOP activation can modulate immune function, and immune cells can produce and release N/OFQ. For classical opioids, alternative mechanisms should be considered; TLR4 is a compelling target. Immune cells are also capable of producing and releasing various opioid peptides for classical opioid receptors, which can then interact with neuronal opioid receptors, creating a neuro-immune axis. As discussed below for NOP–N/OFQ, this also extends to an immune-vascular axis.

Sepsis and an immune-vascular axis; involvement of the N/OFQ opioid receptor

Sepsis is a life-threatening organ dysfunction caused by a dysregulated immune response to infection. In the UK, there are 245,000 cases of sepsis each year, with 40% of survivors experiencing permanent, life-altering after-effects, and five people dying from sepsis every hour. The body's dysregulated response in sepsis and septic shock profoundly affects the cardiovascular system, leading to low blood pressure, reduced blood flow to organs, and subsequent organ dysfunction or failure. This suggests a communication pathway between the immune and cardiovascular systems, an "immune–vascular axis."

In a retrospective study, N/OFQ concentrations were measured in critically ill patients in the ICU and compared with their recovery data and a matched control group. Increased plasma N/OFQ concentrations were observed over the first two days of ICU admission. Animal models have explored survival outcomes. In rats with peritonitis (inflammation of the abdominal lining) caused by infection, mortality increased if N/OFQ was administered. Conversely, mortality improved if a NOP antagonist (UFP-101) was given. These data imply that N/OFQ levels rise in sepsis (consistent with human ICU data) and that blocking NOP might be beneficial as an additional treatment for sepsis-induced low blood pressure. Indeed, in live rat microcirculation studies, N/OFQ caused low blood pressure, blood vessel widening (vasodilation), and leakage of large molecules, which was reversed by the NOP antagonist UFP-101.

A recent study examined NOP expression on human vascular endothelial cells (HUVECs) and vascular smooth muscle cells. It showed that unstimulated endothelium expressed mRNA for the NOP receptor, but functional protein expression required treatment with specific bacterial components (LPS/PepG) as a lab model of sepsis. These upregulated NOP receptors were functionally active. In sepsis, it is hypothesized that immune cells release N/OFQ, which then activates the increased NOP receptors on the endothelium, leading to vasodilation and the consequences of reduced blood pressure; this mechanism describes an immune-vascular axis.

Opioid-free analgesia

One approach to avoid opioid side effects (both medical and societal) is to eliminate their use entirely, opting for "opioid-free analgesia." This seems a rather drastic measure for a class of drugs known to be effective when used appropriately for the correct indication, duration, and timing. There is literature exploring personalized opioid-free anesthesia, or at least questioning its universal applicability. A wide body of literature examines opioid-free techniques specifically. A critical review recently compared opioid and opioid-free approaches, concluding that opioid-free strategies, however well-intentioned, do not fully acknowledge the limitations and gaps in existing evidence and clinical practice considerations.

Opioid-free practices warrant further consideration: does it mean no opioids at all, no opioids during surgery, no opioids after surgery, or combinations of these? A systematic review and meta-analysis of randomized controlled trials published between 2000 and 2021, focusing on opioid-free general anesthesia, included data from 26 trials with 1,934 patients. It concluded that opioid-free anesthesia reduced postoperative side effects. Although opioids were used in the postoperative period, their use was significantly lower in the opioid-free anesthesia group. Another systematic review and meta-analysis examined opioid and opioid-free pain relief after surgical discharge. This review included data from 47 trials published after January 1, 1990, enrolling 6,607 patients. Opioid prescribing, compared to opioid-free methods, did not reduce postoperative pain, though the authors noted that the data largely came from low-quality trials. So, what is the optimal approach? Perhaps reduced opioid multimodal intraoperative pain relief, followed by limited postoperative (multimodal) use, transitioning to opioid-free strategies. Is this optimal for all procedures? Guidance exists.

Considering this alongside poor opioid management (clinical use and prescribing) and the resulting well-documented "opioid crisis," the stage is set for a potential withdrawal of opioids from routine use. This would deny a large patient population access to effective acute pain medications, and in the case of inexpensive generic drugs, effective pain relief in countries with developing healthcare systems. A report on global palliative care and pain relief highlights that of the 298.5 metric tonnes of morphine-equivalent opioids produced, only 0.03% are distributed to low-income countries. Furthermore, using Haiti as an example, the same report concludes that 99% of the opioid need goes unmet.

Conclusions

As pharmacologists and perioperative clinicians, much can be contributed to the design of safer opioids. However, the global community of researchers and healthcare providers must proceed cautiously. Withdrawing opioid medicine entirely might solve one significant and perceived problem but could create another of immense scale. It is evident that the opioid epidemic has fostered a "hostile" environment towards opioids, not only from regulators but also from broader society and lawmakers. Regardless of the mechanisms underlying opioid actions and against the backdrop of the opioid crisis, the path to market for any new opioid-based pain relievers will undoubtedly be challenging; nevertheless, this effort must not be abandoned.

Opioids for Pain Management

Opioid medications are widely used to manage pain during and after surgery. Their effectiveness in reducing many types of pain is clear because they influence how pain signals are processed in the body. However, using opioids for chronic pain, especially nerve pain, is debated, though they are useful in palliative care. While opioids are excellent pain relievers, they cause many problematic side effects. These include reduced breathing, constipation, weakened immune function, tolerance, and opioid use disorder. Opioid-induced hyperalgesia, where pain sensitivity increases, is also a significant concern. Tolerance often leads to higher doses, which can worsen tolerance and other side effects.

It is important to distinguish between opiates, which are natural compounds from the poppy plant or natural peptides in the body (like endorphins), and opioids, which are synthetic and not found in nature. Not all opioids used in medicine are the same. Most opioids used therapeutically primarily target the mu (μ) receptor. However, there is a synthetic peptide called difelikefalin that targets the kappa (κ) receptor, approved for treating itching related to chronic kidney disease. Different opioids that target the mu receptor vary significantly in strength (potency), how well they work (efficacy), and how the body processes them (pharmacokinetics). For example, fentanyl is much stronger than morphine, and buprenorphine is a partial agonist, meaning it produces a weaker maximum response. Even for side effects like immune suppression, opioids differ: morphine is strong, oxycodone is weaker, and buprenorphine has almost no effect. This highlights that different opioids cannot simply be made equivalent by adjusting the dose; other approaches are necessary.

Opioid Receptors

Opioid receptors are a group of specialized proteins on cell surfaces, including the classical mu, delta (δ), and kappa receptors, along with the non-classical nociceptin/orphanin FQ (NOP) receptor. While there was historic evidence for different subtypes of these receptors, genetic studies show that losing a single gene for each receptor leads to a complete loss of its function, suggesting against subtypes. However, there is evidence for variations in how the receptor genes are put together (splice variants), especially for the mu receptor, which can explain some functional differences.

All opioid receptors work by coupling with an inhibitory G protein called Gi. When an opioid binds to the receptor, it triggers a chain of events inside the cell. This ultimately leads to a reduction in certain chemical messengers (like cAMP), the opening of specific potassium channels (which calms the cell), the closing of calcium channels (which reduces neurotransmitter release), and the activation of other signaling pathways. The opioid signal is eventually turned off through a process involving the breakdown of a G protein component and the recruitment of other proteins, such as arrestin, which helps in ending the receptor's activity and removing it from the cell surface. The role of arrestin is important because some research suggests that designing opioids that favor G protein signaling over arrestin recruitment might lead to effective pain relief with fewer side effects. Opioids influence both incoming and outgoing pain signals by reducing the release of pain-transmitting chemicals and enhancing the body's natural pain-inhibiting systems.

Studies have explored the role of β-arrestin-2 in opioid effects. Animals engineered to lack β-arrestin-2 showed stronger pain relief and less tolerance to opioids. These animals also experienced fewer side effects like reduced breathing and slower gut movement. This led to the idea that G protein actions are beneficial, while β-arrestin-2 actions are not, forming the basis of "biased agonism" or "functional selectivity"—where a drug preferentially activates one signaling pathway over another. However, this concept has been debated, with some studies suggesting that what appears to be biased signaling might simply be a drug acting as a partial agonist, meaning it produces a weaker response overall. Regardless of the exact mechanism, the goal is to develop new drugs that provide good pain relief with minimal adverse effects.

Opioid Therapeutic Strategies

Opioid receptors should be understood as dynamic, existing in various forms. Opioid drugs act by stabilizing a specific form of the receptor. For example, full agonists fully activate the receptor, while partial agonists do so to a lesser extent. Neutral antagonists block the receptor without activating it. Some receptors can be active even without a drug binding, a state called constitutive activity. Drugs that reduce this activity are called inverse agonists. Scientists have mapped the binding sites where opioids interact with their receptors. This interaction can modify receptor function in several ways, including by affecting individual receptors, pairs of receptors (dimers), or by using additional molecules called allosteric modulators, or by favoring certain signaling pathways over others.

The effectiveness of an opioid, or its efficacy, refers to the strength of its response in a given tissue, such as its ability to relieve pain or suppress breathing. Partial agonists, which produce a lower maximum response than full agonists, have reduced efficacy. This reduced efficacy can be used therapeutically, as seen with buprenorphine. Researchers are exploring ways to design safer opioids by targeting multiple opioid receptors simultaneously. This "multi-target strategy" aims to combine the benefits of different receptor interactions while minimizing side effects. For instance, drugs that activate the mu receptor while inhibiting the delta receptor have shown promise in animal studies, providing pain relief with fewer side effects. An example is cebranopadol, a drug that acts on both NOP and other opioid receptors, including mu. In animal models, cebranopadol effectively relieved various types of pain, including neuropathic pain, without causing significant breathing problems or rapid tolerance. Clinical trials in humans have shown it to be effective for chronic low back pain and cancer-related pain, with low abuse potential and respiratory advantages. This suggests that non-selective targeting, rather than high selectivity, might be beneficial for opioids, challenging older pharmacological ideas.

Another strategy is "biased agonism," where a drug activates one signaling pathway more than another, ideally leading to therapeutic benefits with fewer side effects. Oliceridine (Olinvyk) is an example of a mu receptor biased agonist that was designed to favor the G protein pathway over the β-arrestin-2 pathway. Preclinical and clinical trials suggested it had a better side effect profile, especially concerning breathing, leading to its FDA approval. However, there is ongoing debate about whether its effects are truly due to bias or simply because it acts as a partial agonist. Regardless, Olinvyk offers some advantages over traditional opioids.

"Allosteric modulators" are molecules that bind to a different site on the opioid receptor than the opioid itself, thereby altering the opioid's activity. Positive allosteric modulators (PAMs) can enhance the effects of natural opioids produced by the body or reduce the amount of an administered opioid needed. This approach could lead to effective pain relief with lower doses of opioids, thereby reducing side effects. Such modulators are being investigated and have shown promise in animal studies, producing good pain relief with fewer adverse effects like reduced breathing, constipation, and reward-seeking behavior.

Opioid Receptors, Pain, and Epigenetics

Opioid receptor activity and expression can also be influenced by "epigenetic mechanisms," which involve changes to DNA that do not alter the underlying genetic code. These changes can be driven by factors like environment, diet, and age. Key epigenetic mechanisms include modifying histones (proteins around which DNA is wrapped), adding methyl groups to DNA (DNA methylation), and the actions of non-coding RNAs. These mechanisms are linked to various diseases, including different types of pain and opioid addiction.

In opioid use disorder, there is often an increase in DNA methylation in the regions that control the mu opioid receptor, which typically leads to reduced receptor expression in the brain. Even short-term therapeutic opioid use has been shown to cause similar hypermethylation of the mu opioid receptor promoter. If opioid misuse begins with therapeutic use, this methylation could be part of a continuous process. Histone modifications also play a role; increased histone acetylation has been observed in heroin users. Conversely, enzymes called histone deacetylases (HDACs), which remove acetyl groups from histones, are often upregulated in certain types of nerve pain, leading to reduced histone acetylation. Studies in animal models of pain have shown that drugs inhibiting HDACs can alleviate pain and restore mu opioid receptor expression. Clinical trials are currently evaluating HDAC inhibitors for conditions like painful diabetic neuropathy.

Non-coding RNAs, particularly microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), can also regulate opioid receptor expression and are associated with various pain conditions and substance misuse. For example, some miRNAs can inhibit mu opioid receptor function during the development of tolerance. While these epigenetic changes are clearly linked to pain and opioid use, the direct causal link between epigenetic modification and opioid receptor expression in the human brain, especially in the context of opioid use disorder, is still under investigation and debated by researchers. However, manipulating these epigenetic processes through drugs (pharmacoepigenetics) holds potential for influencing responses to opioids.

Opioids and Immune Modulation

It has long been known that opioids affect the immune system, and these effects differ among various opioids. There is also recent research suggesting an interaction between opioids, COVID-19 infection, and its outcomes. The exact cellular targets for opioid-induced immune changes are debated. Three main areas of interest include direct effects on immune cells, modulation of the body's stress response system (the HPA axis), and actions within the central nervous system, such as on glial cells. However, direct modulation of immune cells is the most controversial. Research using specific gene detection methods has failed to find classical opioid receptor genes (mu, delta, kappa) in human immune cells, suggesting these receptors may not be present on them. In contrast, genes for the NOP receptor and its natural ligand (N/OFQ) have been found in some immune cell types, and NOP receptors have been shown to be functionally active.

Given the uncertainty about classical opioid receptors on immune cells, another target, Toll-like receptor 4 (TLR4), is considered a plausible alternative. TLR4s are important in immune signaling and respond to bacterial products; they are widely distributed, including on blood vessel linings and tumor cells. Evidence suggests that various opioids can interact with TLR4 in a way that can be blocked by naloxone, similar to classical opioid receptors, making it a compelling target to explain opioid-related immune effects. Additionally, immune cells can produce and release various opioid peptides that can then interact with neuronal opioid receptors, creating a "neuro-immune axis." For NOP-N/OFQ, there's evidence for an "immune-vascular axis."

Sepsis and an Immune-Vascular Axis

Sepsis is a life-threatening condition where the body's overwhelming response to infection causes organ dysfunction. This dysregulated immune response profoundly affects the cardiovascular system, leading to low blood pressure, reduced blood flow to organs, and organ failure. This suggests a crucial interaction between the immune and cardiovascular systems, forming an "immune-vascular axis."

Studies have shown increased levels of N/OFQ in the plasma of critically ill patients with sepsis. In animal models of sepsis, administering N/OFQ increased mortality, while blocking the NOP receptor with an antagonist improved survival. This implies that N/OFQ levels rise during sepsis and that NOP receptor blockade might help treat sepsis-induced low blood pressure. Further research has shown that N/OFQ can cause low blood pressure, vasodilation (widening of blood vessels), and leakage from blood vessels in animal models, effects that were reversed by an NOP antagonist.

Research on human vascular endothelial cells (cells lining blood vessels) and vascular smooth muscle has shown that while unstimulated endothelium expresses NOP receptor genes, functional NOP protein only appears after exposure to substances that mimic sepsis. These activated NOP receptors become functional. It is hypothesized that during sepsis, immune cells release N/OFQ, which then activates these upregulated NOP receptors on the endothelium, leading to vasodilation and the subsequent drop in blood pressure. This proposed mechanism describes an "immune-vascular axis" at play in sepsis.

Opioid-Free Analgesia

One approach to avoid the adverse effects of opioids is to eliminate their use entirely, opting for "opioid-free analgesia." However, this seems like a drastic measure for a class of drugs that can be highly effective when used correctly for the appropriate condition and duration. The literature exploring opioid-free techniques is extensive, but a critical review concluded that these strategies, while noble, do not always fully acknowledge the limitations and gaps in existing evidence and clinical practice.

The concept of "opioid-free" can mean different things: no opioids at all, no opioids during surgery, or no opioids after surgery. A systematic review and meta-analysis of studies on opioid-free general anesthesia found that it reduced postoperative side effects and led to less opioid use after surgery. However, another meta-analysis looking at opioid and opioid-free pain relief after hospital discharge found that opioid prescribing did not necessarily reduce postoperative pain, though these studies were often of lower quality. An optimal approach might involve using reduced opioids during surgery, followed by limited multimodal pain relief after surgery, and then transitioning to opioid-free methods.

Considering the global opioid crisis and issues of poor opioid management, there is a push to potentially reduce or withdraw opioids from routine use. However, a widespread withdrawal could deny a large population access to effective acute pain medications, especially cheap generic opioids in countries with developing healthcare systems. Reports highlight that a tiny fraction of morphine-equivalent opioids is distributed to low-income countries, leaving a vast unmet need for pain relief worldwide.

Conclusions

As medical professionals involved in pain management, there is a significant role in designing safer opioid medications. However, the global medical community must proceed cautiously. While withdrawing opioids might address the current opioid crisis, it risks creating a new, enormous problem of untreated pain. The opioid epidemic has fostered a challenging environment for opioids, not just from regulators but also from society and lawmakers. Despite the mechanisms underlying opioid actions and the backdrop of the opioid crisis, developing and bringing new opioid-based pain relievers to market will be difficult, but researchers and clinicians must not give up on this important endeavor.

Opioid Pain Management and Its Challenges

Opioids are commonly used to manage pain during and after surgery. They are known to effectively reduce various types of pain by changing how pain signals are processed. However, their broader use for long-term pain, especially nerve pain, is debated. Opioids do have a clear role in easing suffering for those receiving palliative care.

While opioids offer pain relief, they also cause many difficult side effects. These include slowed breathing, constipation, weakened immune system, tolerance (needing higher doses for the same effect), and opioid use disorder. Another significant problem is opioid-induced hyperalgesia, where the body becomes more sensitive to pain. Tolerance often leads to a cycle where higher doses are needed, which then increases tolerance and other side effects. This review focuses on improving pain relief, but it is important to remember that tolerance can also be mistaken for worsening disease, and it can develop to many side effects, including slowed breathing.

Not all opioids used in medical settings are the same. Opiates are natural substances from the poppy plant, including natural body chemicals like endorphins. Opioids are synthetic, meaning they are man-made. Most opioids work on specific receptors in the body, primarily the mu (MOP) receptor, at common doses. However, differences exist among MOP-targeting drugs in terms of how strongly they work (e.g., fentanyl is much stronger than morphine), how fully they activate receptors (e.g., buprenorphine is a partial activator), and how the body processes them. For instance, morphine strongly affects the immune system, oxycodone less so, and buprenorphine has almost no effect. Different opioids cannot simply be made equal by adjusting the dose; other approaches are needed due to their varied effects.

Opioid Receptors

Opioid receptors are a group of specialized proteins on cell surfaces, including the classical mu (MOP), delta (DOP), and kappa (KOP) receptors, and the non-classical nociceptin/orphanin FQ (NOP) receptor. While there was historical belief in receptor subtypes, modern genetic studies suggest each receptor comes from a single gene. However, there is evidence for different versions of these receptors (splice variants), especially for MOP, which can explain some functional differences. Scientists now also use special animals with modified genes to track how these receptors work.

When an opioid drug binds to its receptor, it triggers a series of actions inside the cell. This involves a G protein that helps change chemical signals, leading to reduced cell activity, altered ion channels (potassium and calcium), and activation of certain enzymes. The opioid signal eventually stops as the G protein breaks down the activating chemical, and other proteins modify the receptor. A key area of research involves "biased signaling," where some opioids might activate beneficial G protein pathways more than others that cause side effects, potentially leading to better pain relief with fewer problems.

Opioids affect how pain signals travel through the body. They reduce the release of chemicals that transmit pain from the first nerve cells to the next, mainly by influencing potassium and calcium channels. This reduces the ability of nerve cells to send pain messages. Opioids also influence later stages of pain transmission and boost the body's natural pain-inhibiting systems.

Research has explored the role of a protein called β-arrestin-2 in opioid pain relief and side effects. Studies in animals that lacked the gene for β-arrestin-2 showed more effective pain relief and less tolerance to opioids. These animals also had less slowed breathing and reduced digestive issues. This led to the idea that G protein activation by opioids is beneficial, while β-arrestin-2 activity is not, a concept known as "biased agonism." However, other studies have questioned this idea, suggesting that some so-called biased opioids might just be partial activators of the receptor. Regardless of the exact mechanism, the goal is to develop new opioid medications that provide strong pain relief with fewer adverse effects.

Opioid Treatment Strategies

Opioid receptors are not static; they can exist in different active or inactive states. Opioid drugs act like keys, stabilizing the receptor in a particular state. "Full agonists" strongly activate receptors, while "partial agonists" activate them less. "Neutral antagonists" block receptors without activating them. Some receptors have natural activity even without an opioid present; drugs that reduce this activity are called "inverse agonists." Examples of inverse agonists have been found for MOP, DOP, KOP, and NOP receptors.

Opioids bind to a specific spot on the receptor called the orthosteric site. Detailed structures of all four opioid receptors have been identified, providing insights into this binding. Ligands can affect receptor function in various ways, including by interacting with single receptors, linked pairs of receptors, or by using other molecules called allosteric modulators, and potentially by activating specific signaling pathways more than others.

"Efficacy" refers to the strength of a drug's effect in the body, such as pain relief or slowed breathing. Partial agonists, like buprenorphine, produce a weaker maximum response compared to full agonists. Scientists are exploring whether reduced efficacy can be therapeutically useful and help explain how newer opioid drugs work.

Multi-Target Strategies

Opioid receptors likely do not work in isolation but interact with each other. These interactions can involve signaling pathways or the formation of receptor pairs. Activating the MOP receptor while blocking the DOP receptor, for example, can produce pain relief with fewer side effects. This can be achieved with a combination of a MOP activator and a DOP blocker, or with special "bivalent" drugs that interact with both receptors at once. Currently, no MOP-DOP bivalent drugs are available in clinics. However, other multi-target drugs are being explored, such as a trivalent opioid that shows promise in reducing slowed breathing and abuse potential.

Another promising combination involves MOP and NOP receptors, with drugs like cebranopadol in development. Cebranopadol is a "mixed" opioid that partially activates both NOP and other opioid receptors, including MOP. In animal studies, it effectively relieved various types of pain, showing particular strength against nerve pain. Importantly, it did not cause slowed breathing, and tolerance developed very slowly. In humans, cebranopadol has shown effectiveness for chronic lower back pain, has a low risk of abuse, and offers breathing advantages. Clinical trials comparing it to morphine for cancer pain found it to be equally effective, and long-term studies confirmed its safety.

This multi-target approach suggests that targeting multiple receptors, even in a "non-selective" way, can offer advantages in reducing side effects. This idea challenges the traditional pharmacological belief that highly selective drugs are always better for avoiding side effects, and current drug development reflects this shift in thinking.

Biased Agonism

The core idea behind "biased agonism" is that a drug can favor one signaling pathway over another, leading to better therapeutic effects with fewer side effects. For opioids, animal studies suggested that activating the G protein pathway while avoiding the β-arrestin-2 pathway could improve pain relief and reduce tolerance.

Trevena Pharmaceutical developed oliceridine (TRV130), a MOP receptor drug designed to be a G protein-biased agonist. This meant it was intended to activate the G protein pathway more than the β-arrestin-2 pathway, predicting fewer side effects. Preclinical studies and later large-scale clinical trials supported this, showing reduced side effects, and oliceridine was approved by the FDA as Olinvyk, with documented breathing advantages in humans. However, some research suggests that oliceridine might simply be a partial agonist, meaning it doesn't fully activate the receptor. Even with this debate, Olinvyk offers some benefits over older opioids. Other G protein-biased agonists, like PZM21, are also being studied and often show partial agonist activity. Recent structural studies indicate that traditional opioids bind to two main areas on the receptor, while these newer biased agonists tend to bind to only one, which may explain their unique effects.

Allosteric Modulators

Opioids typically bind to a primary site on the receptor, called the orthosteric site, to produce their effects. However, other molecules can bind to different spots on the receptor, known as allosteric sites. These "allosteric modulators" can change how effectively drugs bind to the orthosteric site. "Positive allosteric modulators" (PAMs) enhance this activity.

PAMs, which are not effective on their own, could boost the effects of the body's natural opioids, or enhance the effects of administered opioids. This could allow for lower doses of traditional opioids, leading to fewer side effects while maintaining pain relief. The effects of PAMs can vary depending on the specific opioid drug being used. Several PAMs targeting opioid receptors are currently under development. For example, some PAMs have shown effective pain relief in animals with reduced slowed breathing, constipation, and addictive potential. These PAMs worked by enhancing the effects of endogenous opioids, methadone, or morphine.

Opioid Receptors, Pain, and Epigenetics

Beyond how drugs directly interact with receptors, the activity and number of opioid receptors can also be influenced by "epigenetics." Epigenetics refers to changes in gene activity that do not involve altering the underlying DNA sequence. These changes include chemical tags added to DNA (DNA methylation), modifications to proteins called histones that DNA wraps around, and the activity of small RNA molecules (non-coding RNAs). These epigenetic processes are involved in various diseases, including pain and opioid addiction, and can be influenced by lifestyle and age.

In opioid use disorder (addiction), there is often an increase in DNA methylation in the areas that control MOP receptor production. This reduces the number of MOP receptors in the brain. Even short-term therapeutic opioid use has shown similar epigenetic changes, suggesting a continuous process that might begin in medical settings and continue with inappropriate use.

Histone modifications also play a role. Increased histone acetylation has been observed in heroin users. In nerve pain syndromes, enzymes called HDACs (Histone deacetylases) are often overactive, reducing histone acetylation. Studies in animal models of bone cancer pain found that an HDAC inhibitor drug reduced pain and restored MOP receptor levels. Another HDAC inhibitor is currently in clinical trials for painful diabetic nerve damage.

Non-coding RNAs, particularly microRNAs (miRNAs), can also regulate opioid receptor levels by silencing gene expression. In opioid tolerance, certain miRNAs have been found to reduce MOP receptor production. These miRNAs are also linked to various types of human pain. Similarly, long non-coding RNAs (lncRNAs) are involved in maintaining pain conditions and substance misuse.

While these epigenetic changes are clearly linked to opioid use and pain, the exact causal link to opioid receptor levels in the human brain, particularly in addiction, is still being studied. Some research shows reduced MOP receptors in the brains of people with opioid use disorder, while others have found no significant difference or even an increase in early abstinence. However, direct studies on human brain tissue have found a good correlation between MOP receptor mRNA and protein, but less so with DNA methylation, suggesting epigenetic mechanisms might not be the sole controlling factor in the human brain compared to lab experiments. Despite these complexities, influencing these epigenetic processes through medication (pharmacoepigenetics) remains a promising area for future treatments.

Opioids, Opioid Receptors, and Immune System

It has been known for a long time that opioids affect the immune system, and these effects vary depending on the specific opioid. Recent data also suggest a link between opioids, COVID-19 infection, and patient outcomes. The exact way opioids modulate the immune system is debated, with three main areas of interest: direct effects on immune cells, effects on stress hormone pathways, and central nervous system actions.

While some central nervous system effects are clear, and effects on stress hormones show variation, direct modulation of immune cells is the most controversial. Our own research, using PCR experiments, has not found evidence of classical opioid receptor genes in human immune cells. However, we have detected the NOP receptor and its natural activating chemical in these cells, and we have observed that active NOP receptors are present and influence immune cell function.

An alternative explanation involves Toll-like receptors 4 (TLR4s), which are important for immune signaling and found on various cells. There is strong evidence that many opioids interact with TLR4 in a way that can be blocked by naloxone. If classical opioid receptors are not present on immune cells, then TLR4 could be a key target that explains how opioids affect the immune system.

Our laboratory has also focused on how N/OFQ (the natural activator for the NOP receptor) is released from human immune cells. We developed a special test to measure N/OFQ release from single immune cells in real-time, and we observed similar responses in B and T cells.

Overall, it's clear that NOP activation can influence immune function, and immune cells can produce N/OFQ. For how traditional opioids affect the immune system, TLR4 is a strong candidate. Immune cells can also produce and release their own opioid-like chemicals, creating communication pathways between the nervous system and the immune system, and between the immune system and blood vessels.

Sepsis and an Immune-Vascular Axis: N/OFQ Opioid Receptor Involvement

Sepsis is a severe, life-threatening condition where the body's immune response to an infection harms its own organs. In the UK, there are hundreds of thousands of sepsis cases each year, leading to permanent disabilities for many survivors and a high death rate. The uncontrolled immune response in sepsis and septic shock profoundly affects the cardiovascular system, causing low blood pressure, poor blood flow to organs, and organ failure. This highlights a crucial interaction between the immune system and the blood vessel system, known as an immune-vascular axis.

In a study of critically ill patients, researchers measured N/OFQ levels and found them to be elevated during the first two days in the intensive care unit. Animal studies have also explored survival outcomes in sepsis models. Administering N/OFQ to septic rats increased their mortality, while blocking the NOP receptor with a drug called UFP-101 improved survival. These findings suggest that N/OFQ levels rise during sepsis and that blocking NOP receptors could help treat the dangerously low blood pressure associated with sepsis. Indeed, in live animal models, N/OFQ was shown to cause low blood pressure, widen blood vessels, and increase fluid leakage, all of which were reversed by the NOP blocker.

More recent research examined NOP receptor levels on human blood vessel lining cells (HUVECs) and smooth muscle. They found that these cells naturally expressed NOP receptor genes, but the functional protein only appeared when the cells were treated with a substance that mimicked sepsis. This suggests that in sepsis, immune cells might release N/OFQ, which then activates these upregulated NOP receptors on blood vessel cells, leading to vessel widening and reduced blood pressure, thereby forming an immune-vascular axis.

Opioid-Free Pain Management

One approach to avoid the side effects and societal problems associated with opioids is to eliminate their use entirely, known as opioid-free pain management. However, this seems like an extreme measure for a class of drugs that can be very effective when used appropriately for the right patients and durations. There is growing interest in tailoring opioid-free anesthesia, but a critical review found that current opioid-free strategies often overlook limitations in existing evidence and clinical realities.

The term "opioid-free" itself needs clearer definition: does it mean no opioids at all, or just no opioids during surgery, or no opioids after surgery? A review of studies on opioid-free general anesthesia found that it reduced postoperative side effects and led to lower overall opioid use after surgery, even though some opioids were still given afterward. Another large review comparing opioid and opioid-free pain relief after hospital discharge found that opioid use did not necessarily reduce postoperative pain, though the quality of evidence was often low. The optimal approach likely involves a strategy of reduced opioid use, multimodal pain management during surgery, followed by limited postoperative (multimodal) use, eventually transitioning to opioid-free methods. Specific guidelines exist for different procedures.

When considering widespread poor opioid prescribing practices and the resulting "opioid crisis," there is a strong push to reduce or withdraw opioids from routine use. However, such a move could deny a vast number of patients access to effective acute pain medications, particularly in developing countries where affordable generic opioids are crucial. Reports indicate that only a tiny fraction of the world's morphine-equivalent opioids reach low-income countries, leaving a significant portion of the global need for pain relief unmet.

Conclusions

As medical experts involved in pharmacology and surgical care, there is much to contribute to developing safer opioid medications, as discussed earlier in this review. However, the global community of researchers and healthcare providers must proceed with caution. Completely withdrawing opioid medications, while addressing the current opioid crisis, could inadvertently create another problem of immense scale by depriving many patients of essential pain relief. The opioid epidemic has led to a challenging environment for these drugs, influenced by regulators, society, and lawmakers. Regardless of the scientific mechanisms behind opioid actions, it is clear that bringing any new opioid-based pain medications to market will be difficult. Despite these challenges, the effort to create better and safer pain relief options must continue.

Pain Relief Medicines Called Opioids

Opioids are often used to help with pain during and after surgery. They work well for many kinds of pain by changing how pain signals travel in the body. However, using opioids for long-lasting pain is not always agreed upon, especially for nerve pain. They are useful for comfort care at the end of life. Opioids do help with pain, but they also cause many problems. These problems include slow breathing, constipation, weakened immune system, needing more of the drug to feel the same effect (tolerance), and opioid use disorder. Opioids can also sometimes make people feel more pain. Tolerance often leads to needing higher doses, which then causes more tolerance and other side effects. This paper looks at ways to get better pain relief, but it is important to remember that tolerance can also happen if an illness gets worse, and it can affect how the body reacts to side effects like slow breathing.

Natural products from the poppy plant are called opiates, which also includes natural chemicals in the body like endorphins. Opioids, on the other hand, are man-made and not found in nature. It is important to know that not all opioids used in hospitals are the same. Most of them work on a specific part of the body called the mu (μ) receptor. There is one approved man-made drug that works on the kappa (κ) receptor, but it is used for itching caused by long-term kidney disease. Different opioids used for the mu receptor can be very different. For example, fentanyl is much stronger than morphine, buprenorphine only partly works, and remifentanil leaves the body very quickly. If we look at how they affect the immune system, morphine has a strong effect, oxycodone has a weaker effect, and buprenorphine has almost no effect. This shows that opioids are clearly not all the same. Research has also shown that even at similar strengths, opioids can work in different ways inside cells. Because of these big differences, experts say that "different opioids cannot be made equal by just changing the dose"; other ways are needed.

This paper will explain how opioid receptors work in the body, especially in areas that are still being debated. The goal is to understand how to make safer opioid medicines.

Opioid Receptors

Opioid receptors are a group of special proteins on cells. They include the well-known mu, delta (δ), and kappa (κ) receptors, along with another type called NOP. How these receptors work together will be discussed later. For a long time, scientists thought there might be different kinds of each opioid receptor. However, studies show that if the gene for just one receptor is removed, the receptor stops working completely, which means there are likely no different kinds. But there can be slight changes in the genes that cause small differences in how the receptors work. Scientists now use special animals with changed genes to study how these receptors affect the body.

All opioid receptors work by connecting with a protein called Gi. When an opioid drug attaches to its receptor, it helps the Gi protein to work. This starts a chain of events inside the cell. It lowers a chemical called cAMP, opens channels for potassium to leave the cell (making the cell less active), closes channels for calcium to enter the cell (which reduces how many signals the cell sends), and turns on other cell processes. The opioid signal stops when the Gi protein returns to its inactive state. Another important step is when a protein called arrestin attaches to the receptor. Some experts think that if a drug makes the Gi protein work more and the arrestin protein attach less, it could lead to good pain relief with fewer side effects. This idea is debated.

Opioids change how pain signals travel through the body. They reduce the release of chemicals that carry pain messages from the first nerve cells to the next. They do this by making potassium channels open more and calcium channels close more. Both actions lead to fewer pain signals being sent. Opioids also affect how signals travel between other nerve cells and can boost the body's natural pain-blocking system. How the NOP receptor affects pain signals can also change over time.

A lot of important research has looked at a protein called beta-arrestin-2 and its role in how opioids relieve pain and cause problems. In studies with animals that lacked the gene for beta-arrestin-2, these animals had more pain relief and developed tolerance more slowly. Other studies showed that these animals also had less slow breathing and less slowed gut movement. The main idea was that the Gi protein's actions were good, while beta-arrestin-2's actions were not. This is the basis of "biased signaling," where a drug favors one pathway over another. However, this idea has been questioned by other studies. Some research suggests that drugs thought to be "biased" might just be less powerful overall. Despite the debate, experts agree that if new drugs can provide good pain relief with fewer side effects, the exact way they work might not matter as much.

Opioid Treatment Strategies

Opioid receptors should be seen as changing and existing in different states, either active or inactive. Instead of just thinking of drugs as keys fitting into locks, it's better to think of them as helping the receptor stay in a certain state. Strong drugs (full agonists) push the receptor to stay in an active state, while weaker drugs (partial agonists) do so less. Drugs that block receptors but don't activate them are called neutral antagonists. If a receptor is active even without a drug, it has "natural activity." There are cases of diseases where changes in genes cause receptors to be naturally active. Drugs that reduce this natural activity are called inverse agonists; they do the opposite of standard drugs. These inverse agonists have been found for different opioid receptors in lab studies.

Opioids attach to a specific spot on the receptor called the orthosteric site. Scientists have been able to get detailed pictures of what these sites look like for all four opioid receptors. When drugs attach to receptors, they can change how the receptor works in several ways. They can act on single receptors, on two receptors joined together (called dimers), use other helper molecules (allosteric modulators), or make the receptor favor one internal cell pathway over another.

In a living body, "efficacy" means how strong a drug's effect is in a certain part of the body. For opioids, this could be pain relief or slow breathing. Drugs that cause a weaker maximum effect than a natural, strong drug are called partial agonists. They have reduced efficacy. Can this reduced efficacy be helpful, as seen with buprenorphine? Can differences in how well drugs work explain how some newer opioid drugs act?

Multi-Target Strategy

Opioid receptors likely do not work alone. They often interact with each other, either through shared signals or by forming pairs. Scientists have known for many years that opioid receptors can form pairs of the same type or different types. For example, activating the mu receptor while blocking the delta receptor can lead to pain relief with fewer side effects. This can be done by using a mu receptor drug (like morphine) and a delta receptor blocker, or by using a single drug that acts on both at the same time. While promising, no such dual-acting drugs are currently available for patients. Also, some studies show that a drug that acts on three types of opioid receptors can relieve pain with less slow breathing and less risk of misuse.

Other drug combinations also look promising, such as drugs that act on both mu and NOP receptors. One such drug is cebranopadol. This drug works partly on NOP and other opioid receptors, especially mu. It relieves pain in animals with different types of pain, including nerve pain, which is very hard to treat. Importantly, this drug worked better at lower doses for nerve pain. In animals, it did not cause slow breathing, and tolerance developed very slowly. In people, cebranopadol helped with chronic low back pain, had a low risk of misuse, and was better for breathing. A study comparing cebranopadol with morphine for cancer pain found cebranopadol worked just as well. A long-term study also showed cebranopadol was safe for long-term use.

From this, it seems that drugs that act on multiple targets can have fewer side effects. This goes against the common idea in drug development that drugs should be very specific to avoid side effects. For opioids, we might need to rethink this, and drug development has already started to move in this direction.

Biased Agonism

The main idea behind biased agonism is that a specific drug can make one internal cell pathway work more than another, leading to better treatment results. For opioids, animals without the beta-arrestin-2 protein showed good pain relief but less tolerance and other side effects. This idea, however, has been questioned. A drug company developed an opioid called TRV130 (oliceridine) that was designed to favor the Gi protein pathway over beta-arrestin-2. It was thought this drug would have fewer side effects. This was true in early lab studies, and there was some evidence of similar benefits in larger human studies. These studies led to TRV130 being approved as Olinvyk. It has been shown to have advantages for breathing in humans. However, many studies have since shown that this "biased" drug often acts more like a partial agonist (a weaker drug). Cells can increase signals from one pathway but not another. So, if the Gi protein pathway is boosted and the arrestin pathway is not, a partial agonist might seem "biased." This is a big debate among scientists. From a patient's view, it might not really matter, as Olinvyk does offer some benefits over older opioids. Other drugs designed to be Gi protein-biased also often act as partial agonists. Recent studies suggest that unbiased drugs (like morphine) attach to two spots on the receptor, while so-called biased drugs (like oliceridine) mostly attach to one spot.

Allosteric Modulators

As mentioned, opioids attach to the main spot on the receptor to start their effects. But there are other spots on the opioid receptor where different molecules can attach. These molecules, called allosteric modulators, can change how well the main opioid drug works. Allosteric modulators can either boost, reduce, or have no effect on the main drug. This discussion will focus on positive allosteric modulators (PAMs) which boost the drug's activity. Think of two situations: first, when the body's own natural opioids work, and second, when an opioid drug is given through a vein. The body's natural opioids work very specifically in certain places and for a short time. But a drug given through a vein spreads everywhere. A PAM, which does nothing on its own, can make the body's own natural opioids work better. This could mean less need for a strong drug given to the whole body, or a lower dose of it. The result would be good pain relief with fewer side effects. How an allosteric modulator works can depend on which main opioid drug is being used. Many drugs are being developed that target these other spots on opioid receptors. For example, some PAMs have shown good pain relief in animals for different types of pain, with fewer problems like slow breathing, constipation, and addiction. These PAMs worked by boosting the body's own opioids or drugs like methadone or morphine. Similar results have been found with other PAMs.

Opioid Receptors, Pain, and Epigenetics

This paper has discussed how drugs control receptor function. But how many opioid receptors are made and how they work can also be changed by something called epigenetics. If we can control these epigenetic changes with drugs, it adds another layer to how receptors are regulated. Epigenetics refers to changes in DNA that do not change the basic DNA sequence itself. These changes involve how DNA is packaged (histone modification), adding small chemical groups to DNA (DNA methylation), and the activity of small RNA molecules that don't make proteins. These epigenetic changes are controlled by special proteins that add, read, or remove the changes. Epigenetic changes are linked to many diseases like cancer, asthma, and multiple sclerosis, and can be caused by things in the environment, diet, and age. For surgery, epigenetics plays a role during and after the operation. There is evidence that epigenetic changes are linked to several types of pain and also to opioid addiction and misuse, which might be important for the opioid crisis.

Much recent research, mainly from lab experiments, has looked at how epigenetics controls opioid receptors. Here are some key points about the mu opioid receptor, which is a main target for pain relief.

It is generally thought that in opioid use disorder (addiction), there is too much DNA methylation in the parts of the gene that make the mu opioid receptor. This typically leads to fewer receptors being made. What happens with short-term medical use? One study looked at DNA methylation in dental surgery patients who had never used opioids. They collected saliva before surgery and then a few days and weeks after. They found increased DNA methylation in the mu opioid receptor gene after short-term opioid use. This was a small study. If opioid misuse starts with medical use, then DNA methylation could be seen as a continuous process, starting in the clinic and continuing with inappropriate use. DNA methylation in these gene areas leads to fewer mu opioid receptors in the brain.

Histone proteins, which DNA wraps around, can be changed by adding or removing certain chemical groups. Increased changes to histones have been seen in heroin users and were linked to how long they had been using. Certain proteins (HDACs) play a role in several types of nerve pain, where they are usually increased. This leads to fewer changes to histones. A study in rats with bone cancer pain showed that a drug that blocks HDACs helped reduce the increased pain, possibly by restoring the histone changes. In the spinal cord of these animals, bone cancer reduced the number of mu opioid receptors, and the HDAC blocker brought them back to normal. Lab studies also showed that this HDAC blocker increased the amount of mu opioid receptor parts in cells. In a rat model of pancreas pain, HDAC2 protein was also increased, and mu opioid receptor activity in the spinal cord was reduced. An HDAC blocker helped lessen this effect on the mu opioid receptor. Another HDAC blocker (Ricolinostat) is currently being tested in human studies for painful nerve damage from diabetes.

Another way opioid receptor production can be controlled is by small RNA molecules called non-coding RNAs. These often stop specific messenger RNAs from making proteins. For example, in tolerance to opioids, one type of non-coding RNA (miRNA let-7) stops the mu opioid receptor from being made. Other similar RNAs are also involved. A recent review looked at these small RNAs in different human pains, finding they are linked to many conditions like complex regional pain syndrome, migraine, and nerve pain. HDACs, which were discussed earlier, can also be controlled by these small RNAs, showing that these epigenetic processes are all connected.

Long non-coding RNAs (lncRNAs) help stabilize RNA molecules and affect how proteins work, playing a role in pain conditions and substance misuse. They have been linked to various nerve pains, central pain, and cancer pain. Studies have also found increased levels of certain lncRNAs in the brains of heroin users.

Is there a direct link between epigenetic changes and the number of opioid receptors in the brains of people with opioid use disorder, and can this explain misuse? Older studies found similar amounts of opioid receptors in the brains of heroin users and non-users after death. However, another study found a reduction (about 25%) in mu opioid receptors in the brains of users. Studies using special brain scans in living people during early recovery from addiction found an increase in opioid receptor binding. If receptor numbers go down during addiction, it makes sense that they might go up again during early recovery. To directly look at epigenetics, one study measured mu opioid receptor levels and DNA methylation in brains after death. They found a good link between receptor levels and the protein made, but not with DNA methylation. Lab experiments they did at the same time showed much higher DNA methylation, pointing out differences between lab models and real human brains. The authors suggest that epigenetic changes from addiction might not directly control receptor levels in the human brain. Still, studying how epigenetics affects brain circuits is an interesting area. Using drugs to control these epigenetic processes could change how the body responds to opioids.

Opioids, Opioid Receptors, and Immune System Changes

It has been known for many years that opioids change how the immune system works. It's important to remember that not all opioids affect the immune system in the same way. Recent studies also suggest a link between opioids, COVID infection, and how sick people get. The exact place where opioids affect the immune system is still debated, with three main ideas: (1) acting directly on immune cells, (2) changing a hormone system called the HPA axis, and (3) acting in the brain. For the brain actions, inflammation in the brain in chronic pain is known, and opioid receptors are found on brain support cells. For the HPA axis, how much this system is affected by opioids can differ between animals and whether the opioid is used for a short or long time. The most debated idea is whether opioids directly affect immune cells. In many lab tests, scientists have not found genes for the main opioid receptors in human immune cells. Without these genes, the cells cannot make the receptor proteins. However, genes for the NOP receptor and its natural chemical have been found in some immune cells. Also, new tools have been used to find active NOP receptor protein and link it to how cells work.

Receptors called Toll-like receptors 4 (TLR4s) respond to harmful substances from certain bacteria and are important for immune signaling. These receptors are found in many places, including blood vessel linings and tumor cells. While it might seem strange to discuss this receptor with opioids, there is strong evidence that many opioids interact with TLR4s, and this interaction can be blocked by naloxone. If there is no clear evidence that opioid receptors are on immune cells, then TLR4 could be a good explanation for how opioids change the immune system. Perhaps we have been looking in the wrong place for how opioids affect immunity.

Recent work has looked at how immune cells release the N/OFQ chemical. Scientists have developed a way to measure how this released N/OFQ interacts with NOP receptors. When certain immune cells are stimulated, they release N/OFQ, and this can be measured in real-time. Similar results have been seen in other human immune cells.

Overall, it's clear that activating the NOP receptor can change immune function, and immune cells can make and release N/OFQ. For other common opioids, we should look elsewhere for their immune effects; TLR4 is a strong candidate. Immune cells can also make and release other opioid chemicals that can then affect nerve opioid receptors, creating a link between the immune system and the nervous system, and as discussed below for NOP–N/OFQ, a link between the immune system and blood vessels.

Sepsis and the Immune-Vascular Link; Role of the N/OFQ Opioid Receptor

Sepsis is a serious condition where the body's immune system overreacts to an infection, causing organ damage that can be deadly. In the UK, there are many cases of sepsis each year, and many survivors face lasting health problems. The uncontrolled immune response in sepsis has a severe effect on the heart and blood vessels, leading to low blood pressure, poor blood flow to organs, and organ failure. This suggests a connection between the immune system and blood vessels.

One study measured N/OFQ levels in very sick patients in the intensive care unit (ICU) and compared them to when they recovered and to healthy people. The study found higher N/OFQ levels in patients during the first two days in the ICU. Animal studies have also looked at survival rates. In rats with a type of sepsis, giving N/OFQ increased deaths. On the other hand, giving a drug that blocks NOP receptors improved survival. These findings suggest that N/OFQ levels go up in sepsis (matching human data), and blocking NOP receptors might help treat the low blood pressure caused by sepsis. Indeed, in live animal studies, N/OFQ caused low blood pressure, widening of blood vessels, and leaky blood vessels, all of which were reversed by the NOP blocker.

A recent study looked at NOP receptors on human blood vessel lining cells and blood vessel muscle cells. It found that resting blood vessel lining cells had the gene for the NOP receptor, but they only made active NOP receptor protein after being treated with substances that mimic sepsis in the lab. These activated NOP receptors then become active. In sepsis, it is thought that immune cells release N/OFQ, which then activates these increased NOP receptors on blood vessel lining cells. This causes blood vessels to widen and leads to low blood pressure. This is an example of an immune-vascular link.

Opioid-Free Pain Relief

One idea to avoid the problems with opioids is to stop using them completely. This seems like a strong step for a group of drugs that work well when used correctly for the right type of pain and for the right amount of time. There is research on making pain relief without opioids personal for each patient. Many studies have looked at "opioid-free" methods. A recent review compared opioid and opioid-free methods and concluded that "opioid-free" strategies, while well-intended, do not fully consider all the limits and gaps in current evidence and practice.

"Opioid-free" needs more thought. Does it mean no opioids at all, no opioids during surgery, no opioids after surgery, or a mix? One review looked at studies between 2000 and 2021 on general anesthesia without opioids. Data from 26 studies with many patients showed that opioid-free anesthesia reduced problems after surgery. Patients still used opioids after surgery, but much less than those who had opioids during anesthesia. Another review looked at opioid and opioid-free pain relief after leaving the hospital. Data from 47 studies with many patients showed that prescribing opioids did not reduce pain more than opioid-free methods, though the authors noted the studies were often not high quality. So, what is the best way? Perhaps using fewer opioids with a mix of other pain relievers during surgery, followed by limited opioid use after surgery, and then switching to completely opioid-free methods? Is this best for all types of operations? There are guidelines available.

If all this is seen in the context of poor opioid management and the well-known "opioid crisis," then the stage is set for opioids to be removed from common use. This would mean many patients would not have access to effective pain medications, and in poorer countries, they would lose access to cheap and effective pain relief. A report stated that only a tiny fraction of opioid pain relief goes to low-income countries. For example, in Haiti, 99% of the need for opioid pain relief is not met.

Conclusions

As drug experts and doctors who work during and after surgery, we have much to offer in designing safer opioids, as discussed earlier in this paper. But the global research and medical community needs to be careful. Taking away opioid medicines completely might solve one major problem but create another huge one. What is clear is that the opioid crisis has made a "hostile" environment for opioids. This negative view comes not only from regulators but also from society and lawmakers. No matter how opioids work, and against the background of the opioid crisis, it is clear that getting any new opioid-based pain relievers to patients will not be easy. Still, we must not give up.