QUANTITATIVE STUDIES OF THE ANTAGONISM BY NALORPHINE OF SOME OF THE ACTIONS OF MORPHINE-LIKE ANALGESIC DRUGS

The fentanyl-specific antibody FenAb024 can shield against carfentanil effects

The surge in opioid-related deaths, driven predominantly by fentanyl and its synthetic derivatives, has become a critical public health concern, which is particularly evident in the United States. While the situation is less severe in Europe, the European Monitoring Centre for Drugs and Drug Addiction reports a rise in drug overdose deaths, with emerging concerns about the impact of fentanyl-related molecules. Synthetic opioids, initially designed for medical use, have infiltrated illicit markets due to their low production costs and high potency, with carfentanil posing additional threats, including potential chemical weaponization. Existing overdose mitigation heavily relies on naloxone, requiring timely intervention and caregiver presence, while therapeutic prevention strategies face many access challenges. To provide an additional treatment option, we propose the use of a fentanyl-specific monoclonal antibody (mAb), as a non-opioid method of prophylaxis against fentanyl and carfentanil. This mAb shows protection from opioid effects in a pre-clinical murine model. mAbs could emerge as a versatile countermeasure in civilian and biodefense settings, offering a novel approach to combat opioid-associated mortality.

The opioid receptor: emergence through millennia of pharmaceutical sciences

Throughout history humanity has searched for an optimal approach to the use of opioids that maximizes analgesia while minimizing side effects. This review reflects upon the conceptualization of the opioid receptor and the critical role that the pharmaceutical sciences played in its revelation. Opium-containing formulations have been delivered by various routes of administration for analgesia and other therapeutic indications for millennia. The concept of a distinct site of opium action evolved as practitioners developed innovative delivery methods, such as intravenous administration, to improve therapeutic outcomes. The introduction of morphine and synthetic opioids engendered the prevalent assumption of a common opioid receptor. Through consideration of structure-activity relationships, spatial geometry, and pharmacological differences of known ligands, the idea of multiple opioid receptors emerged. By accessing the high-affinity property of naloxone, the opioid receptor was identified in central and peripheral nervous system tissue. The endogenous opioid neuropeptides were subsequently discovered. Application of mu-, delta-, and kappa- opioid receptor-selective ligands facilitated the pharmacological characterization and distinctions between the three receptors, which were later cloned and sequenced. Opioid receptor signal transduction pathways were described and attributed to specific physiological outcomes. The crystal structures of mu, delta, kappa, and nociceptin/orphanin FQ receptors bound to receptor-selective ligands have been elucidated. Comparison of these structures reveal locations of ligand binding and engagement of signal transduction pathways. Expanding knowledge regarding the structure and actions of the opioid receptor fuels contemporary strategies for driving the activity of opioid receptors toward maximizing therapeutic and minimizing adverse outcomes.

A trypanosome-derived immunotherapeutics platform elicits potent high-affinity antibodies, negating the effects of the synthetic opioid fentanyl

Poorly immunogenic small molecules pose challenges for the production of clinically efficacious vaccines and antibodies. To address this, we generate an immunization platform derived from the immunogenic surface coat of the African trypanosome. Through sortase-based conjugation of the target molecules to the variant surface glycoprotein (VSG) of the trypanosome surface coat, we develop VSG-immunogen array by sortase tagging (VAST). VAST elicits antigen-specific memory B cells and antibodies in a murine model after deploying the poorly immunogenic molecule fentanyl as a proof of concept. We also develop a single-cell RNA sequencing (RNA-seq)-based computational method that synergizes with VAST to specifically identify memory B cell-encoded antibodies. All computationally selected antibodies bind to fentanyl with picomolar affinity. Moreover, these antibodies protect mice from fentanyl effects after passive immunization, demonstrating the ability of these two coupled technologies to elicit therapeutic antibodies to challenging immunogens.

A Concise Review of Concepts in Opioid Pharmacology up to the Discovery of Endogenous Opioids

This brief review covers concepts in opioid pharmacology that were promoted during the period leading up to the establishment of the International Narcotics Research Conference (INRC) in the early 1970s and the discovery of endogenous opioid peptides in 1975. The founders of INRC, meeting together during the International Union of Pharmacology meeting in Basel in 1969, recognized that the time was ripe for the creation of an international society that would provide a venue for the discussion of research across disciplines in this rapidly expanding area of science. The emphasis here is on studies leading to the demonstration that specific receptors for morphine-like analgesics exist, the search for endogenous ligands for these receptors, and early attempts to elucidate the mechanisms underlying opiate drug tolerance, dependence, and addiction. SIGNIFICANCE STATEMENT: Research on opioids in the 20th century was driven by the search for nonaddicting analgesics. This review discusses the development of the "analgesic" receptor concept, the demonstration that such receptors existed, and the search for an endogenous ligand. Conceptual models were proposed to explain tolerance to the actions of opiate drugs and the development of dependence and addiction. This review explains these models and indicates how they foreshadowed more recent discoveries on the acute and chronic actions of opiate drugs.

Differential activation of the μ-opioid receptor by oxycodone and morphine in pain-related brain regions in a bone cancer pain model

BACKGROUND AND PURPOSE

Bone cancer pain is chronic and often difficult to control with opioids. However, recent studies have shown that several opioids have distinct analgesic profiles in chronic pain.

EXPERIMENTAL APPROACH

To clarify the mechanisms underlying these distinct analgesic profiles, functional changes in the μ-opioid receptor were examined using a mouse femur bone cancer (FBC) model.

KEY RESULTS

In the FBC model, the B(max) of [(3) H]-DAMGO binding was reduced by 15-45% in the periaqueductal grey matter (PAG), region ventral to the PAG (vPAG), mediodorsal thalamus (mTH), ventral thalamus and spinal cord. Oxycodone (10(-8) -10(-5)  M) and morphine (10(-8) -10(-5)  M) activated [(35) S]-GTPγS binding, but the activation was significantly attenuated in the PAG, vPAG, mTH and spinal cord in the FBC model. Interestingly, the attenuation of oxycodone-induced [(35) S]-GTPγS binding was quite limited (9-26%) in comparison with that of morphine (46-65%) in the PAG, vPAG and mTH, but not in the spinal cord. Furthermore, i.c.v. oxycodone at doses of 0.02-1.0 μg per mouse clearly inhibited pain-related behaviours, such as guarding, limb-use abnormalities and allodynia-like behaviour in the FBC model mice, while i.c.v. morphine (0.05-2.0 μg per mouse) had only partial or little analgesic effect on limb-use abnormalities and allodynia-like behaviour.

CONCLUSION AND IMPLICATIONS

These results show that μ-opioid receptor functions are attenuated in several pain-related regions in bone cancer in an agonist-dependent manner, and suggest that modification of the μ-opioid receptor is responsible for the distinct analgesic effect of oxycodone and morphine.

Picenadol

Picenadol is a unique opioid mixed agonist-antagonist analgesic currently under clinical evaluation. Structurally, picenadol is a 4-phenylpiperidine derivative and a racemic mixture whose mixed agonist-antagonist properties are a consequence of the d-isomer being a potent opiate agonist, whereas the l-isomer is an opioid antagonist. In the mouse writhing and rat tail heat tests, the analgesic potency of picenadol is estimated to be 1/3 that of morphine. Picenadol itself has weak antagonist activity, whereas the antagonist potency of the l-isomer is approx. 1/10 that of nalorphine. Evaluation of picenadol's affinity for opioid receptors reveals that picenadol, unlike other mixed agonist-antagonists has high affinity for both the mu and delta receptors but a markedly lower affinity for the kappa receptor. Extensive pharmacological investigations show picenadol to have a low potential to produce opiate-like side effects, including a low liability for abuse and physical dependence.

Subcellular distribution of etorphine in rat brain and evidence for in vitro stereospecific binding

1 Control experiments were carried out by homogenizing rat brain at 0 degrees C with sucrose containing various concentrations of [3H]-etorphine. Subcellular fractionation of this homogenate showed that the distribution of the labelled drug amongst the primary fractions was dependent on the concentration of etorphine in the homogenate. 2 Rats were injected intravenously with 0.2 and 20 microgram/kg of [3H]-etorphine. The brains were homogenized and fractionated in sucrose containing 4.2 x 10(-5) M unlabelled etorphine in order to control redistribution artifacts. Different distribution profiles in the subcellular fractions were observed at these two dose levels. 3 Concurrent administration of either cyprenorphine or naloxone with intravenous etorphine, caused a shift of the labelled drug from the P3 fraction to the supernatant fraction. 4 The subcellular distribution of intravenously administered [3H]-etorphine was also studied by homogenizing brains in etorphine-free sucrose, and sucrose containing either levorphanol or dextrorphan. From these experiments it was concluded that the P3 microsomal fraction is a major site to which in vivo etorphine is stereospecifically bound in the rat brain.

The enkephalins and opiates: structure-activity relations

The X-ray structures of 9 "opiate" drugs which exhibit a range of pharmacological activity have been examined in detail leading to the theory that one of the reasons why the enkephalins and related peptides possess morphine-like activity is because they have a tyrosine, and hence a "tyramine", residue at the amino terminal position. This residue or a conformationally similar moiety, can be shown to be present in many opiates and analogues.

The offset of morphine tolerance in rats and mice

1. In rats and mice made tolerant to morphine by pretreatment with the drug, the shift to the right of the log dose/analgesic response line for in naive animals occurs without significant change in slope provided that sufficient time is allowed for elimination of pretreatment drug. 2. Responsiveness to the analgesic effects of morphine, given together with cycloheximide to prevent reinforcement of tolerance, was measured in rats (paw pressure method) and mice (hot plate method) at intervals during 1-23 days following cessation of a variety of regimens of tolerance-inducing drug treatments. 3. A biphasic pattern of recovery of responsiveness was observed, which was independent of the regimen or the drug (morphine, methadone or diamorphine) used to induce tolerance. Estimates of the rates of the first, fast phase are imprecise but the rate of the second phase of offset, from 4th day after cessation of pretreatment had, in rats, a mean half-time of 13.2 plus or minus 0.53 days-for all pretreatments combined, there being no significant differences between the various pretreatment regimens employed. In mice, similarly, a biphasic recovery of analgesic responsiveness was seen after morphine pretreatment, the mean half-time of the slower phase being 17.4 days. 4. Precipitation of an acute withdrawal syndrome in rats by naloxone HCl given 6 h after the final injection of a tolerance-inducing treatment with morphine did not affect the subsequent rate of recovery from tolerance. 5. During the period following a tolerance-inducing pretreatment with morphine in mice, the rate of attenuation of the naloxone-evoked jumping response was faster than the rate of offset of tolerance.

Properties of opiate-receptor binding in rat brain

[(3)H]Naloxone, a potent opiate antagonist, binds stereospecifically to opiate-receptor sites in rat-brain tissue. The binding is time, temperature, and pH dependent and saturable with respect to [(3)H]naloxone and tissue concentration. The [(3)H]naloxone-receptor complex formation is bimolecular with a dissociation constant of 20 nM. 15 Opiate agonists and antagonists compete for the same receptors, whose density is 30 pmol/g. Potencies of opiates and their antagonists in displacing [(3)H]naloxone binding parallel their pharmacological potencies.

Monoamine mediation of the morphine-induced activation of mice

1. The dose-response relationship for hyperactivity in grouped mice following the injection of morphine sulphate has been established.2. The activation response can be modified by drugs which affect either catecholamines or indoleamines.3. The monoamine precursors L-DOPA and 5-hydroxytryptophan potentiate the response.4. The monoamine synthesis inhibitors alpha-methyl-p-tyrosine and p-chlorophenylalanine reduce the response.5. Inhibition of monoamine oxidase activity by pargyline caused a great increase in the response. The simultaneous administration of reserpine resulted in a further potentiation.6. Reserpine blocked the response whenever it was given alone, either before, with or after the injection of morphine.7. Blockade of alpha-adrenoceptors with phentolamine or phenoxybenzamine reduced the response.8. Blockade of tryptaminergic receptors with methysergide or cinanserin also antagonized the response.9. The major tranquillizers haloperidol and chlorpromazine reduced the response. Haloperidol was especially effective in this regard.10. The tricyclic antidepressant drug imipramine potentiated the response.11. The morphine antagonist nalorphine completely prevented the response.12. The anticholinergic agent atropine and the antihistaminic drug mepyramine did not affect the response.13. We conclude that dopamine, noradrenaline and 5-hydroxytryptamine are all involved in the normal activation response of grouped mice to morphine, with dopaminergic mechanisms being of primary importance.

Quantitative studies on the antagonism by naloxone of some narcotic and narcotic-antagonist analgesics

1. Naloxone was used to study the antagonism of the analgesic effects of some narcotics (morphine sulphate, levorphanol tartrate, and methadone hydrochloride) and narcotic antagonists (pentazocine, cyclazocine, and nalorphine hydrochloride). The analgesic assay used was the mouse phenylbenzoquinone stretching test.2. The in vivo equivalent of a pA(2) value (apparent pA(2)) for naloxone was determined with each agonist. These values were found to be significantly larger with the narcotics than with the narcotic antagonists.3. The slopes in the apparent pA(2) plots were also found to be significantly different. It was concluded that this difference in slopes was probably not due to a lack of equilibrium in one of the two groups of analgesics.4. The results suggest that the narcotic and the narcotic-antagonist analgesics may inhibit stretching in this assay by interacting either with two different receptors or with the same receptor in a different manner.

Effects of opiates and opiate antagonists on the Straub tail reaction in mice

1. Subcutaneous injections of opiates produced the Straub tail reaction in mice. The potencies of the opiates in mice were consistent with previous estimates of the analgesic potencies in animals and in man.2. The potencies of sixteen antagonists in counteracting the reaction were consistent with those previously obtained with the rat tail-flick test.3. The (-) isomers of four benzomorphan derivatives were much more potent in counteracting the reaction than their (+) isomers and about twice as potent as their racemates. The activity of the isomers seemed to follow Pfeiffer's rule: the lower the effective dose of a drug, the greater the difference in the pharmacological effects of the optical isomers. One of the trans isomers acted like an opiate, while its cis isomer acted like an antagonist.4. Naloxone and nalorphine fulfilled conventional criteria for competitive antagonism, whereas atropine and the (-) and the (+) isomers of pentazocine and of cyclazocine did not do so.5. The Straub tail test seems to be useful for studying structure-activity relations among opiates and opiate antagonists.