Evidence for mu1-opioid receptor involvement in fentanyl-mediated respiratory depression

Safety and effectiveness of adding fentanyl or sufentanil to spinal anesthesia: systematic review and meta-analysis of randomized controlled trials

INTRODUCTION

Spinal infusions of either fentanyl or sufentanil have been reported in international reports, articles, and scientific events worldwide. This study aimed to determine whether intrathecal fentanyl or sufentanil offers safety in mortality and perioperative adverse events.

METHODS

MEDLINE (via PubMed), EMBASE, CENTRAL (Cochrane library databases), gray literature, hand-searching, and clinicaltrials.gov were systematically searched. Randomized controlled trials with no language, data, or status restrictions were included, comparing the effectiveness and safety of adding spinal lipophilic opioid to local anesthetics (LAs). Data were pooled using the random-effects models or fixed-effect models based on heterogeneity.

RESULTS

The initial search retrieved 4469 records; 3241 records were eligible, and 3152 articles were excluded after reading titles and abstracts, with a high agreement rate (98.6%). After reading the full texts, 76 articles remained. Spinal fentanyl and sufentanil significantly reduced postoperative pain and opioid consumption, increased analgesia and pruritus. Fentanyl, but not sufentanil, significantly reduced both postoperative nausea and vomiting, and postoperative shivering; compared to LAs alone. The analyzed studies did not report any case of in-hospital mortality related to spinal lipophilic opioids. The rate of respiratory depression was 0.7% and 0.8% when spinal fentanyl or sufentanil was added and when it was not, respectively. Episodes of respiratory depression were rare, uneventful, occurred intraoperatively, and were easily manageable.

CONCLUSION

There is moderate to high quality certainty that there is evidence regarding the safety and effectiveness of adding lipophilic opioids to LAs in spinal anesthesia.

Pharmacokinetics and pharmacodynamics of cyclopropylfentanyl in male rats

BACKGROUND

Illicitly manufactured fentanyl and its analogs are a major driving force behind the ongoing opioid crisis. Cyclopropylfentanyl is a fentanyl analog associated with many overdose deaths, but limited knowledge is available about its pharmacology. In the present study, we developed a bioanalytical method for the determination of cyclopropylfentanyl and its main metabolite cyclopropylnorfentanyl and evaluated pharmacokinetic-pharmacodynamic relationships in rats.

METHOD

An ultra-high performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) method was developed and validated for determination of cyclopropylfentanyl and cyclopropylnorfentanyl in rat plasma. Male Sprague-Dawley rats fitted with jugular catheters and temperature transponders received cyclopropylfentanyl (30, 100, and 300 μg/kg) or saline subcutaneously. Blood specimens were withdrawn over an 8-h time period, along with measurements of pharmacodynamic endpoints.

RESULTS

The analytical method was validated, and both analytes exhibited a low limit of quantification (15 pg/mL). Cyclopropylfentanyl caused dose-related increases in hot plate latency (ED₅₀ = 48 µg/kg) and catalepsy (ED₅₀ = 87 µg/kg) and produced long-lasting hypothermia at the highest dose. Plasma cyclopropylfentanyl rose rapidly in a dose-related fashion, reaching maximal concentration (C_max) after 15-28 min, whereas metabolite Cmax occurred later at 45-90 min. Cyclopropylfentanyl C_max values were similar to concentrations measured in non-fatal intoxications in humans; however, differences in parent drug: metabolite ratio indicated possible interspecies variance in metabolism.

CONCLUSION

Our study shows that cyclopropylfentanyl produces typical opioid-like effects in male rats. Cyclopropylfentanyl displays much greater analgesic potency when compared to morphine, suggesting that cyclopropylfentanyl poses increased overdose risk for unsuspecting users.

The rising crisis of illicit fentanyl use, overdose, and potential therapeutic strategies

Fentanyl is a powerful opioid anesthetic and analgesic, the use of which has caused an increasing public health threat in the United States and elsewhere. Fentanyl was initially approved and used for the treatment of moderate to severe pain, especially cancer pain. However, recent years have seen a growing concern that fentanyl and its analogs are widely synthesized in laboratories and adulterated with illicit supplies of heroin, cocaine, methamphetamine, and counterfeit pills, contributing to the exponential growth in the number of drug-related overdose deaths. This review summarizes the recent epidemic and evolution of illicit fentanyl use, its pharmacological mechanisms and side effects, and the potential clinical management and prevention of fentanyl-related overdoses. Because social, economic, and health problems that are related to the use of fentanyl and its analogs are growing, there is an urgent need to implement large-scale safe and effective harm reduction strategies to prevent fentanyl-related overdoses.

Noradrenergic Mechanisms in Fentanyl-Mediated Rapid Death Explain Failure of Naloxone in the Opioid Crisis

In December 2018, the Centers for Disease Control declared fentanyl the deadliest drug in America. Opioid overdose is the single greatest cause of death in the United States adult population (ages 18-50), and fentanyl and its analogs [fentanyl/fentanyl analogs (F/FAs)] are currently involved in >50% of these deaths. Anesthesiologists in the United States were introduced to fentanyl in the early 1970s when it revolutionized surgical anesthesia by combining profound analgesia with hemodynamic stability. However, they quickly had to master its unique side effect. F/FAs can produce profound rigidity in the diaphragm, chest wall and upper airway within an extremely narrow dosing range. This clinical effect was called wooden chest syndrome (WCS) by anesthesiologists and is not commonly known outside of anesthesiology or to clinicians or researchers in addiction research/medicine. WCS is almost routinely fatal without expert airway management. This review provides relevant clinical human pharmacology and animal data demonstrating that the significant increase in the number of F/FA-induced deaths may involve α-adrenergic and cholinergic receptor-mediated mechanical failure of the respiratory and cardiovascular systems with rapid development of rigidity and airway closure. Although morphine and its prodrug, heroin, can cause mild rigidity in abdominal muscles at high doses, neither presents with the distinct and rapid respiratory failure seen with F/FA-induced WCS, separating F/FA overdose from the slower onset of respiratory depression caused by morphine-derived alkaloids. This distinction has significant consequences for the design and implementation of new pharmacologic strategies to effectively prevent F/FA-induced death. SIGNIFICANCE STATEMENT: Deaths from fentanyl and F/FAs are increasing in spite of availability and awareness of the opioid reversal drug naloxone. This article reviews literature suggesting that naloxone may be ineffective against centrally mediated noradrenergic and cholinergic effects of F/FAs, which clinically manifest as severe muscle rigidity and airway compromise (e.g., wooden chest syndrome) that is rapid and distinct from respiratory depression seen with morphine-derived alkaloids. A physiologic model is proposed and implications for new drug development and treatment are discussed.

Evidence for nonlinear accumulation of the ultrapotent fentanyl analog, carfentanil, after systemic administration to male rats

The current opioid overdose crisis is being exacerbated by illicitly manufactured fentanyl and its analogs. Carfentanil is a fentanyl analog that is 10,000-times more potent than morphine, but limited information is available about its pharmacology. The present study had two aims: 1) to validate a method for quantifying carfentanil and its metabolite norcarfentanil in small-volume samples, and 2) to use the method for examining pharmacodynamic-pharmacokinetic relationships in rats. The analytical method involved liquid-liquid extraction of plasma samples followed by quantitation of carfentanil and norcarfentanil using ultra-high-performance liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS/MS). The method was validated following SWGTOX guidelines, and both analytes displayed limits of detection and quantification at 7.5 and 15 pg/mL, respectively. Male Sprague-Dawley rats fitted with jugular catheters and temperature transponders received subcutaneous carfentanil (1, 3 and 10 μg/kg) or saline. Repeated blood specimens were obtained over 8 h, along with pharmacodynamic measures including core temperature and catalepsy scores. Carfentanil produced dose-related hypothermia and catalepsy that lasted up to 8 h. Carfentanil C_max occurred at 15 min whereas metabolite C_max was at 1-2 h. Concentrations of both analytes increased in a dose-related fashion, but area-under-the-curve values were much greater than predicted after 10 μg/kg. Plasma half-life for carfentanil increased at higher doses. Our findings reveal that carfentanil produces marked hypothermia and catalepsy, which is accompanied by nonlinear accumulation of the drug at high doses. We hypothesize that impaired clearance of carfentanil in humans could contribute to life-threatening effects of this ultrapotent opioid agonist. This article is part of the Special Issue entitled 'New Vistas in Opioid Pharmacology'.

Mechanisms of respiratory insufficiency induced by methadone overdose in rats

Methadone may cause respiratory depression. We aimed to understand methadone-related effects on ventilation as well as each opioid-receptor (OR) role. We studied the respiratory effects of intraperitoneal methadone at 1.5, 5, and 15 mg/kg (corresponding to 80% of the lethal dose-50%) in rats using arterial blood gases and plethysmography. OR antagonists, including intravenous 10 mg/kg-naloxonazine at 5 minutes (mu-OR antagonist), subcutaneous 30 mg/kg-naloxonazine at 24 hours (micro1-OR antagonist), 3 mg/kg-naltrindole at 45 minutes (delta-OR antagonist) and 5 mg/kg-Nor-binaltorphimine at 6 hours (kappa-OR antagonist) were pre-administered. Plasma concentrations of methadone enantiomers were measured using high-performance liquid chromatography coupled to mass-spectrometry. Methadone dose-dependent inspiratory time (T(I)) increase tended to be linear. Respiratory depression was observed only at 15 mg/kg and characterized by an increase in expiratory time (T(E)) resulting in hypoxemia and respiratory acidosis. Intravenous naloxonazine completely reversed all methadone-related effects on ventilation, while subcutaneous naloxonazine reduced its effects on pH (P < 0.05), PaCO(2) (P < 0.01) and T(E) (P < 0.001) but only partially on T(I) (P < 0.001). Naltrindole reduced methadone-related effects on T(E) (P < 0.001). Nor-binaltorphimine increased methadone-related effects on pH and PaO(2) (P < 0.05) Respiratory effects as a function of plasma R-methadone concentrations showed a decrease in PaO(2) (EC(50): 1.14 microg/ml) at lower concentrations than those necessary for PaCO(2) increase (EC(50): 3.35 microg/ml). Similarly, increased T(I) (EC(50): 0.501 microg/ml) was obtained at lower concentrations than those for T(E) (EC(50): 4.83 microg/ml). Methadone-induced hypoxemia is caused by mu-ORs and modulated by kappa-ORs. Additionally, methadone-induced increase in T(E) is caused by mu1- and delta-opioid receptors while increase in T(I) is caused by mu-ORs.

Characteristics and comparative severity of respiratory response to toxic doses of fentanyl, methadone, morphine, and buprenorphine in rats

Opioids are known to induce respiratory depression. We aimed to characterize in rats the effects of four opioids on arterial blood gases and plethysmography after intraperitoneal administration at 80% of their LD(50) in order to identify opioid molecule-specific patterns and classify response severity. Opioid-receptor (OR) antagonists, including intravenous 10 mg kg(-1)-naloxonazine at 5 min [mu-OR antagonist], subcutaneous 30 mg kg(-1)-naloxonazine at 24 h [mu1-OR antagonist], subcutaneous 3 mg kg(-1)-naltrindole at 45 min [delta-OR antagonist], and subcutaneous 5 mg kg(-1)-Nor-binaltorphimine at 6 h [kappa-OR antagonist] were pre-administered to test the role of each OR. Methadone, morphine, and fentanyl significantly decreased PaO(2) (P<0.001) and increased PaCO(2) (P<0.05), while buprenorphine only decreased PaO(2) (P<0.05). While all opioids significantly increased inspiratory time (T(I), P<0.001), methadone and fentanyl also increased expiratory time (T(E), P<0.05). Intravenous 10 mg kg(-1)-naloxonazine at 5 min completely reversed opioid-related effects on PaO(2) (P<0.05), PaCO(2) (P<0.001), T(I) (P<0.05), and T(E) (P<0.01) except in buprenorphine. Subcutaneous 30 mg kg(-1)-naloxonazine at 24 h completely reversed effects on PaCO(2) (P<0.01) and T(E) (P<0.001), partially reversed effects on T(I) (P<0.001), and did not reverse effects on PaO(2). Naltrindole reversed methadone-induced T(E) increases (P<0.01) but worsened fentanyl's effect on PaCO(2) (P<0.05) and T(I) (P<0.05). Nor-binaltorphimine reversed morphine- and buprenorphine-induced T(I) increases (P<0.001) but worsened methadone's effect on PaO(2) (P<0.05) and morphine (P<0.001) and buprenorphine's (P<0.01) effects on pH. In conclusion, opioid-related respiratory patterns are not uniform. Opioid-induced hypoxemia as well as increases in T(I) and T(E) are caused by mu-OR, while delta and kappa-OR roles appear limited, depending on the specific opioid. Regarding severity of opioid-induced respiratory effects at 80% of their LD(50), all drugs increased T(I). Methadone and fentanyl induced hypoxemia, hypercapnia, and T(E) increases, morphine caused both hypoxemia and hypercapnia while buprenorphine caused only hypoxemia.

Intravenous self-administration of etonitazene alone and combined with cocaine in rhesus monkeys: comparison with heroin and antagonism by naltrexone and naloxonazine

RATIONALE

In humans, micro opioid-cocaine combinations (speedballs) have been reported to heighten pleasurable effects and result in greater abuse potential compared to either drug individually. Emerging evidence in animals suggests that the ability of mu opioids to enhance the reinforcing effects of cocaine might be independent of their mu intrinsic efficacy even though mu agonist efficacy appears to be a determinant in the reinforcing effects of micro opioids themselves.

OBJECTIVES

This study examined the relationship between agonist efficacy, self-administration, and the enhancement of cocaine self-administration using the high-efficacy mu agonist etonitazene.

MATERIALS AND METHODS

Rhesus monkeys self-administered cocaine, heroin, etonitazene, and opioid-cocaine combinations under a progressive-ratio schedule of intravenous drug injection.

RESULTS

Unlike cocaine and heroin, etonitazene did not maintain consistent self-administration at any dose tested (0.001-1.0 microg/kg/injection). However, combining etonitazene (0.1-1.0 microg/kg/injection) with cocaine (0.01 and 0.03 mg/kg/injection) enhanced cocaine self-administration, and this enhancement was attenuated by naltrexone. These effects are similar to those obtained by combining non-reinforcing doses of heroin and cocaine. Antagonism of etonitazene-cocaine and heroin-cocaine self-administration by naloxonazine was short lasting and was not maintained after 24 h (when naloxonazine's purported micro(1) subtype antagonist effects are thought to predominate).

CONCLUSIONS

The results suggest that high micro agonist efficacy does not guarantee consistent drug self-administration and that the ability of mu agonists to enhance cocaine self-administration does not depend exclusively on reinforcing efficacy. Moreover, the results do not support a major role for micro(1) receptor mechanisms in either etonitazene- or heroin-induced enhancement of cocaine self-administration.

Fentanyl transdermal matrix patch (Durotep MT patch; Durogesic DTrans; Durogesic SMAT): in adults with cancer-related pain

The fentanyl transdermal matrix patch is approved in Japan for the management of moderate to severe cancer-related pain in adults. Bioequivalence, in terms of exposure and the maximum and minimum serum concentrations, has been established between the fentanyl transdermal matrix patch 16.8 mg (100microg/h) and the fentanyl transdermal reservoir patch 10 mg (100microg/h) after single and multiple applications. The fentanyl transdermal matrix patch 2.1-8.4 mg (12.5-50microg/h) effectively managed chronic cancer-related pain in adults in a noncomparative, multicentre, phase II study; 89.4% of recipients rated their global assessment of pain as 'very satisfied', 'satisfied' or 'neither satisfied nor dissatisfied'. Adults with cancer- or non-cancer-related chronic pain were switched from fentanyl transdermal reservoir patch to fentanyl transdermal matrix patch therapy without compromising efficacy; no differences in pain intensity or sleep interference scores were seen between the two formulations in an nonblind, multicentre, switching pilot study. Given the nature of the therapy, the tolerability profile of the fentanyl transdermal matrix patch was generally acceptable. Topical adverse events included erythema, application-site irritation and pruritus. In general, patients and physicians preferred the fentanyl transdermal matrix patch over the fentanyl transdermal reservoir patch in the pilot study.

The effect of nitric oxide on fentanyl and haloperidol-induced catalepsy in mice *

BACKGROUND AND OBJECTIVES

This study was designed to investigate the role of nitric oxide on catalepsy induced by fentanyl and haloperidol.

METHODS

Male albino mice were treated either with fentanyl (0.1-0.2 mg kg-1, s.c.) or haloperidol (0.5-2 mg kg-1, i.p.). The non-selective nitric oxide synthase inhibitor, NG-nitro-L-arginine (10 mg kg-1, i.p.), selective neuronal nitric oxide synthase inhibitor, 7-nitroindazole (3 mg kg-1, i.p.), and nitric oxide donors, L-arginine (30-300 mg kg-1, i.p.) and D-arginine (30 mg kg-1, i.p.), were applied 20 min prior to fentanyl or haloperidol injection. A mu-opioid receptor antagonist naloxone (1 mg kg-1, i.p.) was also given in some groups. The cataleptic status of mice was assessed by placing animals in a rearing position in the cage. If the mouse maintained cataleptic posture for more than 20 s, it was scored as cataleptic and duration of catalepsy was expressed in terms of minutes.

RESULTS

Both NG-nitro-L-arginine and 7-nitroindazole prolonged fentanyl-induced catalepsy (fentanyl: 3.6+/-0.8 min; fentanyl+NG-nitro-L-arginine: 77.4+/-14.6 min, fentanyl+7-nitroindazole: 56.0+/-10.4 min; n=6; P<0.01). This effect was reversed by L-arginine and naloxone, but not by D-arginine. Nitric oxide synthase inhibitors also prolonged the cataleptic action of haloperidol but to a lesser extent (haloperidol: 72.0+/-6.3 min; haloperidol+NG-nitro-L-arginine: 98.5+/-6.3 min, haloperidol+7-nitroindazole: 89.6+/-2.2 min; n=6; P<0.05). The prolongation of haloperidol-induced catalepsy with nitric oxide synthase inhibitors was not reversed by L-arginine.

CONCLUSION

These results suggest a common mechanism between mu-opioid receptors and the nitric oxide system in the development of fentanyl-induced catalepsy in mice different from haloperidol-induced catalepsy.

Distribution of mu receptors in the ventral respiratory group neurons; immunohistochemical and pharmacological studies in decerebrate cats

Immunoreactivity for mu receptors was investigated in 21 bulbar respiratory neurons, individually identified by intracellular recording and labeling with neurobiotin. In 14 of these neurons, effects of iontophoresed morphine were examined. Morphine hyperpolarized the membrane and decreased spike discharges in 4/6 augmenting inspiratory (aug-I), 4/5 postinspiratory (post-I) and 3/3 augmenting expiratory (aug-E) neurons. It had no effect on two aug-I and one post-I neurons. Strong immunoreactivity for mu receptor was detected in the soma and dendrites of 5/8 aug-I, 5/7 post-I and 6/6 aug-E neurons. In the remaining three aug-I and two post-I neurons that included cells unresponsive to morphine, weak immunoreactivity was detected only in the dendrites. These results demonstrated wide, but uneven, distribution of mu receptors in bulbar respiratory neurons and suggest their contribution to respiratory depression by opioids.

Opioid-induced quantal slowing reveals dual networks for respiratory rhythm generation

Current consensus holds that a single medullary network generates respiratory rhythm in mammals. Pre-Bötzinger Complex inspiratory (I) neurons, isolated in transverse slices, and preinspiratory (pre-I) neurons, found only in more intact en bloc preparations and in vivo, are each proposed as necessary for rhythm generation. Opioids slow I, but not pre-I, neuronal burst periods. In slices, opioids gradually lengthened respiratory periods, whereas in more intact preparations, periods jumped nondeterministically to integer multiples of the control period (quantal slowing). These findings suggest that opioid-induced quantal slowing results from transmission failure of rhythmic drive from pre-I neurons to preBötC I networks, depressed below threshold for spontaneous rhythmic activity. Thus, both I (in the slice), and pre-I neurons are sufficient for respiratory rhythmogenesis.

Lack of involvement of mu(1) opioid receptors in dermorphin-induced inhibition of hypoxic and hypercapnic ventilation in rat pups

The effects of dermorphin, a mu-selective opioid agonist, on respiratory responses to altered O(2) and CO(2) during postnatal development were investigated in conscious, unrestrained Wistar rats aged 2-21 days. Respiration was recorded by barometric plethysmography. Dermorphin (4 mg kg(-1)) was administered subcutaneously, and the ventilatory responses to hypoxia (11% O(2), 89% N(2)) in 2-21-day-old pups and hyperoxia (100% O(2)), and hypercapnia (8% CO(2), 92% O(2)) in 2-13-day-old pups were assessed in the presence and absence of the mu(1) receptor antagonist naloxonazine (10 mg kg(-1) s.c.) administered 1 day before testing. Six minutes of hypoxia increased ventilation in all age groups, largely via an increase in frequency. Dermorphin inhibited the ventilatory response to hypoxia, and this inhibition was insensitive to naloxonazine. After 5 min of hyperoxia, ventilation was the same as with air breathing except in the presence of dermorphin, when hyperoxic ventilation was depressed by a naloxonazine-insensitive decrease in frequency. Following this 5 min 100% O(2) exposure, pups were exposed to hypercapnia, and respiratory parameters were measured 5 min later. The ventilatory response to CO(2) was inhibited by dermorphin in a naloxonazine-insensitive manner. There was no evidence for endogenous mu(1) receptor modulation of the ventilatory responses to altered gases in rat pups of any age. Thus, mu opioid-induced inhibition of the hypoxic and hypercapnic responses in young rats does not occur via activation of mu(1) opioid receptors.

Opioid antagonists: a review of their role in palliative care, focusing on use in opioid-related constipation

Opioid antagonists have well-established indications in the reversal of life-threatening opioid toxicity, but also hold considerable promise for other applications in palliative care practice, particularly management of opioid-related constipation. We briefly review current understanding of opioid receptors, focusing on their complex role in gastrointestinal physiology. We summarize the pharmacology, conventional indications, and clinical usage of three major groups of opioid antagonists, including a promising new peripherally acting agent, methylnaltrexone, which is not commercially available. We suggest an approach to administering opioid antagonists for reduction of life-threatening opioid toxicity in patients with pain. The literature on opioid-induced constipation and its treatment with opioid-antagonists is reviewed in detail. Finally, other potential uses of opioid antagonists in palliative care are described, especially strategies for reducing such opioid side effects as nausea and pruritus and for improving analgesia or reducing tolerance by concomitantly administrating both an opioid agonist and low dosages of an antagonist.

Transdermal fentanyl: an updated review of its pharmacological properties and therapeutic efficacy in chronic cancer pain control

Fentanyl is a synthetic opioid agonist which interacts primarily with the mu-opioid receptor. The low molecular weight, high potency and lipid solubility of fentanyl make it suitable for delivery by the transdermal therapeutic system. These patches are designed to deliver fentanyl at a constant rate (25, 50, 75 and 100 microg/h), and require replacement every 3 days. Data from randomised, nonblind trials suggest that transdermal fentanyl is as effective as sustained-release oral morphine in the treatment of chronic cancer pain, as reported by patients using visual and numerical analogue scales as well as verbal description scales. No obvious differences in health-related quality of life were found in patients with chronic cancer pain when comparing transdermal fentanyl with sustained-release oral morphine. Nevertheless, significantly more patients expressed a preference for transdermal fentanyl than for sustained-release oral morphine after a randomised, nonblind, crossover trial. Because of the formation of a fentanyl depot in the skin tissue, serum fentanyl concentrations increase gradually following initial application, generally levelling off between 12 and 24 hours. Thereafter, they remain relatively constant, with some fluctuation, for the remainder of the 72-hour application period. Once achieved, steady-state plasma fentanyl concentrations can be maintained for as long as the patches are renewed. The most frequently observed adverse events during transdermal fentanyl administration (as with other opioid agonists) included vomiting, nausea and constipation. Data from a nonblind, randomised trials suggest that constipation occurs less frequently in patients receiving transdermal fentanyl than in those given sustained-release oral morphine. The most serious adverse event reported in US premarketing trials was hypoventilation, which occurred with an incidence of approximately 2%. Adverse reactions related to skin and appendages (i.e. rash and application site reactions - erythema, papules, itching and oedema) were reported in 153 patients with cancer at a frequency between 1 and 3%.

CONCLUSION

Transdermal fentanyl is a useful opioid-agonist for the treatment of moderate to severe chronic cancer pain. The advantages of transdermal fentanyl include ease of administration and the 3-day application interval. These factors coupled with a lower incidence of constipation are likely to contribute to the reported patient preference of transdermal fentanyl over sustained-release oral morphine.

Differentiation of delta, mu, and kappa opioid receptor agonists based on pharmacophore development and computed physicochemical properties

Compounds that bind with significant affinity to the opioid receptor types, delta, mu, and kappa, with different combinations of activation and inhibition at these three receptors could be promising behaviorally selective agents. Working on this hypothesis, the chemical moieties common to three different sets of opioid receptor agonists with significant affinity for each of the three receptor types delta, mu, or kappa were identified. Using a distance analysis approach, common geometric arrangements of these chemical moieties were found for selected delta, mu, or kappa opioid agonists. The chemical and geometric commonalities among agonists at each opioid receptor type were then compared with a non-specific opioid recognition pharmacophore recently developed. The comparison provided identification of the additional requirements for activation of delta, mu, and kappa opioid receptors. The distance analysis approach was able to clearly discriminate kappa-agonists, while global molecular properties for all compounds were calculated to identify additional requirements for activation of delta and mu receptors. Comparisons of the combined geometric and physicochemical properties calculated for each of the three sets of agonists allowed the determination of unique requirements for activation of each of the three opioid receptors. These results can be used to improve the activation selectivity of known opioid agonists and as a guide for the identification of novel selective opioid ligands with potential therapeutic usefulness.

Modulation of breathing by mu1 and mu2 opioid receptor stimulation in neonatal and adult rats

Opioid modulation of breathing during postnatal development through to the adult was investigated in the rat. Respiratory frequency, tidal volume and minute volume were recorded in unanesthetized, unrestrained rat pups and adults using barometric plethysmography. Subjects were administered the highly selective mu opioid agonists dermorphin and fentanyl. Fentanyl, which readily crosses the blood-brain barrier, was included to ensure that developmental changes in blood-brain barrier restrictions did not mask some of the dermorphin effects in older neonates. Drugs were administered subcutaneously in neonates and adults, although dermorphin was given by intracerebroventricular route only in adults. In neonates, mu agonist administration caused a gasping-like pattern of breathing, characterized by a marked fall in frequency and a smaller increase in tidal volume. The gasping response was prevented by pre-treatment with the long-acting mu1 antagonist naloxonazine (NALZ). In the presence of NALZ, mu agonists elicited only a small, but significant, reduction in tidal volume. Both dermorphin and fentanyl showed more potent activity in younger pups than in older pups, possibly in the case of dermorphin because of developmental maturation of blood-brain barrier function. In adults, fentanyl and dermorphin both caused a reduction in frequency and minute volume. The response of adults to fentanyl, but not dermorphin, was prevented by NALZ. These results suggest that both mu1 and mu2 receptors contribute to opioid-induced respiratory depression during neonatal and adult life.

Molecular determinants of non-specific recognition of delta, mu, and kappa opioid receptors

Identification of the molecular determinants of recognition common to all three opioid receptors embedded in a single three-dimensional (3D) non-specific recognition pharmacophore has been carried out. The working hypothesis that underlies the computational study reported here is that ligands that bind with significant affinity to all three cloned opioid receptors, delta, mu, and kappa, but with different combinations of activation and inhibition properties at these receptors, could be promising behaviorally selective analgesics with diminished side effects. The study presented here represents the first step towards the rational design of such therapeutic agents. The common 3D pharmacophore developed for recognition of delta, mu, and kappa opioid receptors was based on the receptor affinities determined for 23 different opioid ligands that display no specificity for any of the receptor subtypes. The pharmacophore centers identified are a protonated amine, two hydrophobic groups, and the centroid of an aromatic group in a geometric arrangement common to all 23, non-specific, opioid ligands studied. Using this three-dimensional pharmacophore as a query for searching 3D structural databases, novel compounds potentially involved in non-specific recognition of delta, mu, and kappa opioid receptors were retrieved. These compounds can be valuable candidates for novel behaviorally selective analgesics with diminished or no side effects, and thus with potential therapeutic usefulness.

Morphine inhibits resting respiration, but it attenuates reflexive respiratory suppression in anesthetized cat through kappa-receptor

Noxious stimulation of thin-fiber muscular afferents induces a reflexive respiratory suppression that we call "poststimulus respiratory suppression." In anesthetized, vagotomized, paralyzed, and artificially ventilated cats, morphine depressed the level of resting respiration (inhibitory effect on resting respiration) and attenuated the magnitude of the poststimulus respiratory suppression (excitatory effect on the reflexively modified respiration). These two kinds of morphine effects were antagonized by naloxone, suggesting the participation of opioid receptors. To clarify the opioid receptor subtypes responsible for these effects of morphine, three type-selective opioid antagonists-naltrindole (delta antagonist), gamma-funaltrexamine (mu antagonist), and Mr2266 (kappa antagonist)-were tested. The morphine-induced depression in the resting respiration was antagonized by pretreatment with the kappa antagonist, not with the mu or delta antagonist. Furthermore, the morphine-induced attenuation in the magnitude of the poststimulus suppression was also blocked by the kappa antagonist, but not by the mu or delta antagonist. In conclusion, (1) morphine inhibits resting respiration, but it attenuates the magnitude of the poststimulus respiratory suppression; (2) both these morphine effects are mediated by kappa opioid receptors. The possibility that the kappa(3) receptor, one of the kappa receptors subtypes, mediates the two kinds of morphine effects has been discussed.

Endogenous opiates: 1996

This paper is the nineteenth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1996 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress, tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.