Identification of Opioid Ligands Possessing Mixed μ Agonist/δ Antagonist Activity among Pyridomorphinans Derived from Naloxone, Oxymorphone, and Hydropmorphone

A Crosstalk between the Cannabinoid Receptors and Nociceptin Receptors in Colitis-Clinical Implications

Inflammatory bowel diseases (IBD) refer to a group of gastrointestinal (GI) disorders with complex pathogenesis characterized by chronic intestinal inflammation with a variety of symptoms. Cannabinoid and nociceptin opioid receptors (NOPs) and their ligands are widely distributed in the GI tract. The nociceptin opioid receptor is a newly discovered member of the opioid receptor family with unique characteristics. Both cannabinoid and NOP systems exhibit antinociceptive and anti-inflammatory activity and contribute to maintaining proper motility, secretion and absorption in the GI tract. Furthermore, they influence high and low voltage calcium channels, which play a crucial role in the processing of pain, and share at least two kinases mediating their action. Among them there is NF-κB, a key factor in the regulation of inflammatory processes. Therefore, based on functional similarities between cannabinoid and nociceptin receptors and the anti-inflammatory effects exerted by their ligands, there is a high likelihood that there is an interaction between cannabinoid receptors 1 and 2 and the nociceptin receptor in colitis. In this review, we discuss potential overlaps between these two systems on a molecular and functional level in intestinal inflammation to create the basis for novel treatments of IBD.

Discovery of 3-((dimethylamino)methyl)-4-hydroxy-4-(3-methoxyphenyl)-N-phenylpiperidine-1-carboxamide as novel potent analgesic

Management of moderate to severe pain by clinically used opioid analgesics is associated with a plethora of side effects. Despite many efforts have been dedicated to reduce undesirable side effects, moderate progress has been made. In this work, starting from Tramadol, a series of 3-((dimethylamino)methyl)-4-hydroxy-4-(3-methoxyphenyl)piperidine-1-carboxamide derivatives were designed and synthesized, and their in vitro and in vivo activities were evaluated. Our campaign afforded selective μ opioid receptor (MOR) ligand 2a (K_iMOR: 7.3 ± 0.5 nM; K_iDOR: 849.4 ± 96.6 nM; K_iKOR: 49.1 ± 6.9 nM) as potent analgesic with ED₅₀ of 3.1 mg/kg in 55 °C hot plate model. Its antinociception effect was blocked by opioid antagonist naloxone. High binding affinity toward MOR of compound 2a was associated with water bridge, salt bridge, hydrogen bond and hydrophobic interaction with MOR. The high selectivity of compound 2a for MOR over δ opioid receptor (DOR) and κ opioid receptor (KOR) was due to steric hindrance of compound 2a with DOR and KOR. 2a, a compound with novel scaffold, could serve as a lead for the development of novel opioid ligands.

In vivo effects of μ-opioid receptor agonist/δ-opioid receptor antagonist peptidomimetics following acute and repeated administration

BACKGROUND AND PURPOSE

Agonists at μ-opioid receptors (μ-receptors) are used for pain management but produce adverse effects including tolerance, dependence and euphoria. The co-administration of a μ-receptor agonist with a δ-opioid receptor (δ-receptor) antagonist has been shown to produce antinociception with reduced development of some side effects. We characterized the effects of three μ-receptor agonist/δ-receptor antagonist peptidomimetics in vivo after acute and repeated administration to determine if this profile provides a viable alternative to traditional opioid analgesics.

EXPERIMENTAL APPROACH

Three μ-receptor agonist / δ-receptor antagonist peptidomimetics, AAH8, AMB46 and AMB47, and morphine were evaluated for the development of tolerance and dependence after 5 days of twice daily treatment with escalating doses of drug (10-50 mg·kg^-1 ). Antinociceptive effects were measured in the warm water tail withdrawal assay before and after repeated drug treatment. Physical dependence was evaluated by naltrexone-precipitated withdrawal jumping. The rewarding effects of AAH8 were evaluated using a conditioned place preference (CPP) assay with twice daily conditioning sessions performed for 5 days.

KEY RESULTS

Morphine, AAH8, AMB47 and AMB46 all demonstrated acute antinociceptive effects, but repeated administration only produced tolerance in animals treated with morphine and AMB46. Injection of naltrexone precipitated fewer jumps in mice treated repeatedly with AAH8 as compared with morphine, AMB47 or AMB46. Conditioning with morphine, but not AAH8, produced significant CPP.

CONCLUSIONS AND IMPLICATIONS

AAH8 may be a better alternative than traditional opioid analgesics, producing antinociception with less development of tolerance and dependence and may be less rewarding than morphine.

Pain Therapy Guided by Purpose and Perspective in Light of the Opioid Epidemic

Prescription opioid misuse is an ongoing and escalating epidemic. Although these pharmacological agents are highly effective analgesics prescribed for different types of pain, opioids also induce euphoria, leading to increasing diversion and misuse. Opioid use and related mortalities have developed in spite of initial claims that OxyContin, one of the first opioids prescribed in the USA, was not addictive in the presence of pain. These claims allayed the fears of clinicians and contributed to an increase in the number of prescriptions, quantity of drugs manufactured, and the unforeseen diversion of these drugs for non-medical uses. Understanding the history of opioid drug development, the widespread marketing campaign for opioids, the immense financial incentive behind the treatment of pain, and vulnerable socioeconomic and physical demographics for opioid misuse give perspective on the current epidemic as an American-born problem that has expanded to global significance. In light of the current worldwide opioid epidemic, it is imperative that novel opioids are developed to treat pain without inducing the euphoria that fosters physical dependence and addiction. We describe insights from preclinical findings on the properties of opioid drugs that offer insights into improving abuse-deterrent formulations. One finding is that the ability of some agonists to activate one pathway over another, or agonist bias, can predict whether several novel opioid compounds bear promise in treating pain without causing reward among other off-target effects. In addition, we outline how the pharmacokinetic profile of each opioid contributes to their potential for misuse and discuss the emergence of mixed agonists as a promising pipeline of opioid-based analgesics. These insights from preclinical findings can be used to more effectively identify opioids that treat pain without causing physical dependence and subsequent opioid abuse.

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

Derivatives of furanditerpenes from Pterodon genus: Pharmacological studies disclose their potential as chronic pain relief in mice

Pterodon genus fruits are commercially available at the Brazilian medicinal market used in folk medicine due to their anti-inflammatory, analgesic, and anti-rheumatic effects. Previous studies demonstrated that furanditerpenes possessing vouacapan skeleton, isolated from Pterodon genus, possess expressive antinociceptive activities, with promising moiety for the development of new analgesic products. The antinociceptive properties of compounds 6α,7β-6α-hidroxivouacapan-7β-17β-lactone (HVL) and 6α-oxovouacapan-7β-17β-lactone (OVL), semi-synthetic analogues of furanditerpenes previously reported as analgesic agents were evaluated on animal experimental models (Spindola et al., 2010, 2011). The chemical-induced pain methods used in the present work, demonstrated for the first time that both compounds HVL and OVL have potential as important templates for the development of chronic pain control drugs. The main findings of this work were that both compounds were: effective in the writhing test; reduced paw edema in the carrageenan test; effective in the inflammatory phase of the formalin test corroborating their activity against inflammatory pain conditions; effective on reducing pain through the stimulation of vanilloid receptors sensible to capsaicin (an important pathway for chronic pain maintenance); reduced the pain stimulus caused by PGE₂ injection (a pathway involved in chronic pain hypersensitivity); effective on decreasing mechanical allodynia in the CFA-model, demonstrating their potential use against chronic pain disorders.

Oral Oxymorphone for Pain Management

Objective

To describe the pharmacology, safety and efficacy, and rationale for use of oral oxymorphone for the management of acute and chronic moderate-to-severe pain.

Data Sources

A PubMed/MEDLINE search (1966-March 2007) was conducted using the following terms: oral oxymorphone, oxymorphone, EN 3202, EN 3203, Opana, and Opana ER. Manufacturer-provided data (package inserts) and abstracts presented at the American Pain Society meetings (2003–2006) were also reviewed.

Study Selection and Data Extraction

Human studies evaluating the safety and efficacy of oral oxymorphone in pain management were considered; animal and non–English-language data were excluded.

Data Synthesis

Oral oxymorphone is a semisynthetic opioid agonist that is specific for the μ-opioid receptor and approved to treat both acute and chronic pain. Unlike other opioids, such as oxycodone, oxymorphone does not bind to the κ-opioid receptor. Due to extensive liver metabolism, oral oxymorphone is contraindicated in patients with moderate-to-severe hepatic impairment; however, no clinically significant CYP3A4, 2C9, or 2D6 mediated drug-drug interactions have been noted. Elderly patients may experience a 40% increase in plasma concentrations, while renally impaired patients may have a 57–65% increase in bioavailability. Food can increase the rate of absorption by as much as 50%, necessitating dosing either 1 hour before or 2 hours after a meal. Oxymorphone's primary adverse effects are similar to those of other opioids: nausea, vomiting, pruritus, pyrexia, and constipation.

Conclusions

Oxymorphone is an oral therapeutic option approved for the treatment of acute and chronic moderate-to-severe pain. Oxymorphone has a safety and efficacy profile similar to that of other commonly used pure opioids (morphine, oxycodone, hydromorphone). Like oxycodone and morphine, oxymorphone also has immediate-release and extended-re lease formulations. Since cost alone is not yet favorable for oxymorphone over oxycodone or morphine, further studies of comparative efficacy targeting potential advantages of oxymorphone over other opioids are necessary before considering it for addition to a formulary.

The behavioral effects of a mixed efficacy antinociceptive peptide, VRP26, following chronic administration in mice

RATIONALE

VRP26 displays mu opioid receptor agonist and delta opioid receptor antagonist activity in vitro, a pharmacological profile purported to produce reduced tolerance, dependence, and rewarding effects. We hypothesized that VRP26 would display reduced adverse effects after chronic administration as compared with the traditional opioid analgesic fentanyl.

OBJECTIVE

The aim of this study is to explore the development of tolerance, dependence, and conditioned place preference of VRP26 as compared with the traditional opioid analgesic fentanyl.

METHODS

The antinociceptive effects of VRP26 and fentanyl were assessed using the mouse warm water tail withdrawal (WWTW) assay. Measurement of antinociceptive tolerance and physical dependence occurred after 7 days of continuous administration of either fentanyl (0.3 mg/kg/day) or VRP26 (10 mg/kg/day); tolerance was measured by a shift in the antinociceptive dose response curve in the WWTW assay. Physical dependence was determined by observation of withdrawal symptoms after precipitated withdrawal. Rewarding effects were measured by the ability of VRP26 or fentanyl to produce conditioned place preference.

RESULTS

Fentanyl produced significant tolerance and dependence, as well as significant conditioned place preference. VRP26 produced neither tolerance nor physical dependence, nor did it produce significant conditioned place preference.

CONCLUSIONS

These results suggest that chronic treatment with VRP26 may produce less tolerance or physical dependence than chronic treatment with clinically available mu opioid analgesics such as fentanyl. Additionally, VRP26 produces less rewarding effects than fentanyl. This desirable in vivo profile may be due to the mixed efficacy nature of VRP26 and could provide the framework for safer opioid analgesics.

Multitarget opioid ligands in pain relief: New players in an old game

Still nowadays pain is one of the most common disabling conditions and yet it remains too often unsolved. Analgesic opioid drugs, and mainly MOR agonists such as morphine, are broadly employed for pain management. MOR activation, however, has been seen to cause not only analgesia but also undesired side effects. A potential pain treatment option is represented by the simultaneous targeting of different opioid receptors. In fact, ligands possessing multitarget capabilities led to an improved pharmacological fingerprint. This review focuses on the examination of multitarget opioid ligands which have been distinguished in peptide and non-peptide and further listed as bivalent and bifunctional ligands. Moreover, the potential of these compounds, both as analgesic drugs and pharmacological tools to explore heteromer receptors, has been stressed.

Transporter-Mediated Disposition of Opioids: Implications for Clinical Drug Interactions

Opioid-related deaths, abuse, and drug interactions are growing epidemic problems that have medical, social, and economic implications. Drug transporters play a major role in the disposition of many drugs, including opioids; hence they can modulate their pharmacokinetics, pharmacodynamics and their associated drug-drug interactions (DDIs). Our understanding of the interaction of transporters with many therapeutic agents is improving; however, investigating such interactions with opioids is progressing relatively slowly despite the alarming number of opioids-mediated DDIs that may be related to transporters. This review presents a comprehensive report of the current literature relating to opioids and their drug transporter interactions. Additionally, it highlights the emergence of transporters that are yet to be fully identified but may play prominent roles in the disposition of opioids, the growing interest in transporter genomics for opioids, and the potential implications of opioid-drug transporter interactions for cancer treatments. A better understanding of drug transporters interactions with opioids will provide greater insight into potential clinical DDIs and could help improve opioids safety and efficacy.

Pyrrolo- and pyridomorphinans: non-selective opioid antagonists and delta opioid agonists/mu opioid partial agonists

Opioid ligands have found use in a number of therapeutic areas, including for the treatment of pain and opiate addiction (using agonists) and alcohol addiction (using antagonists such as naltrexone and nalmefene). The reaction of imines, derived from the opioid ligands oxymorphone and naltrexone, with Michael acceptors leads to pyridomorphinans with structures similar to known pyrrolo- and indolomorphinans. One of the synthesized compounds, 5e, derived from oxymorphone had substantial agonist activity at delta opioid receptors but not at mu and/or kappa opioid receptors and in that sense profiled as a selective delta opioid receptor agonist. The pyridomorphinans derived from naltrexone and naloxone were all found to be non-selective potent antagonists and as such could have utility as treatments for alcohol abuse.

Translation of structure-activity relationships from cyclic mixed efficacy opioid peptides to linear analogues

Most opioid analgesics used in the treatment of pain are mu opioid receptor (MOR) agonists. While effective, there are significant drawbacks to opioid use, including the development of tolerance and dependence. However, the coadministration of a MOR agonist with a delta opioid receptor (DOR) antagonist slows the development of MOR-related side effects, while maintaining analgesia. We have previously reported a series of cyclic mixed efficacy MOR agonist/DOR antagonist ligands. Here we describe the transfer of key features from these cyclic analogs to linear sequences. Using the linear MOR/DOR agonist, Tyr-DThr-Gly-Phe-Leu-Ser-NH2 (DTLES), as a lead scaffold, we replaced Phe(4) with bulkier and/or constrained aromatic residues shown to confer DOR antagonism in our cyclic ligands. These replacements failed to confer DOR antagonism in the DTLES analogs, presumably because the more flexible linear ligands can adopt binding poses that will fit in the narrow binding pocket of the active conformations of both MOR and DOR. Nonetheless, the pharmacological profile observed in this series, high affinity and efficacy for MOR and DOR with selectivity relative to KOR, has also been shown to reduce the development of unwanted side effects. We further modified our lead MOR/DOR agonist with a C-terminal glucoserine to improve bioavailability. The resulting ligand displayed high efficacy and potency at both MOR and DOR and no efficacy at KOR.

Synthesis, modeling, and pharmacological evaluation of UMB 425, a mixed μ agonist/δ antagonist opioid analgesic with reduced tolerance liabilities

Opioid narcotics are used for the treatment of moderate-to-severe pain and primarily exert their analgesic effects through μ receptors. Although traditional μ agonists can cause undesired side effects, including tolerance, addition of δ antagonists can attenuate said side effects. Herein, we report 4a,9-dihydroxy-7a-(hydroxymethyl)-3-methyl-2,3,4,4a,5,6-hexahydro-1H-4,12-methanobenzofuro[3,2-e]isoquinolin-7(7aH)-one (UMB 425) a 5,14-bridged morphinan-based orvinol precursor synthesized from thebaine. Although UMB 425 lacks δ-specific motifs, conformationally sampled pharmacophore models for μ and δ receptors predict it to have efficacy similar to morphine at μ receptors and similar to naltrexone at δ receptors, due to the compound sampling conformations in which the hydroxyl moiety interacts with the receptors similar to orvinols. As predicted, UMB 425 exhibits a mixed μ agonist/δ antagonist profile as determined in receptor binding and [(35)S]GTPγS functional assays in CHO cells. In vivo studies in mice show that UMB 425 displays potent antinociception in the hot plate and tail-flick assays. The antinociceptive effects of UMB 425 are blocked by naloxone, but not by the κ-selective antagonist norbinaltorphimine. During a 6-day tolerance paradigm, UMB 425 maintains significantly greater antinociception compared to morphine. These studies thus indicate that, even in the absence of δ-specific motifs fused to the C-ring, UMB 425 has mixed μ agonist/δ antagonist properties in vitro that translate to reduced tolerance liabilities in vivo.

Oxymorphone active uptake at the blood-brain barrier and population modeling of its pharmacokinetic-pharmacodynamic relationship

The aim of this study was to characterize the blood-brain barrier (BBB) transport and pharmacokinetics-pharmacodynamics (PKPD) relationship of oxymorphone and to further elucidate its possible contribution to oxycodone analgesia. The BBB transport of oxymorphone was studied using microdialysis in male Sprague-Dawley rats. Samples from microdialysis blood and brain probes, brain tissue, and plasma were analyzed by liquid chromatography with tandem mass spectrometry. The effect was measured as tail-flick latency. The study consisted of a PKPD experiment with combined microdialysis and antinociceptive measurements (n = 8), and another antinociceptive effect experiment (n = 9) using a 10 times lower dose. The combined data were analyzed with an integrated PKPD model in nonlinear mixed effect modeling utilizing a specific method (M3) for handling missing PD observations. The concentration of unbound oxymorphone was higher in brain than in blood, with a ratio of 1.9 (RSE, 9.7%), indicating active uptake at the BBB. The integrated PKPD model described the oxymorphone BBB transport and PKPD relationship successfully, with an EC50 in the brain of 63 ng/mL, and the M3 method was able to address the issue of censored observations. Oxymorphone has active uptake transport at the BBB in rats, with moderate uptake clearance to the brain. Its contribution to analgesia after oxycodone administration is not significant.

14-Alkoxy- and 14-acyloxypyridomorphinans: μ agonist/δ antagonist opioid analgesics with diminished tolerance and dependence side effects

In the search for opioid ligands with mixed functional activity, a series of 5'-(4-chlorophenyl)-4,5α-epoxypyridomorphinans possessing alkoxy or acyloxy groups at C-14 was synthesized and evaluated. In this series, the affinity and functional activity of the ligands were found to be influenced by the nature of the substituent at C-14 as well as by the substituent at N-17. Whereas the incorporation of a 3-phenylpropoxy group at C-14 on N-methylpyridomorhinan gave a dual MOR agonist/DOR agonist 17h, its incorporation on N-cyclopropylmethylpyridomorphinan gave a MOR agonist/DOR antagonist 17d. Interestingly, 17d, in contrast to 17h, did not produce tolerance or dependence effects upon prolonged treatment in cells expressing MOR and DOR. Moreover, 17d displayed greatly diminished analgesic tolerance as compared to morphine upon repeated administration, thus supporting the hypothesis that ligands with MOR agonist/DOR antagonist functional activity could emerge as novel analgesics devoid of tolerance, dependence, and related side effects.

Modulation of opioid receptor ligand affinity and efficacy using active and inactive state receptor models

Mu opioid receptor (MOR) agonists are widely used for the treatment of pain; however, chronic use results in the development of tolerance and dependence. It has been demonstrated that coadministration of a MOR agonist with a delta opioid receptor (DOR) antagonist maintains the analgesia associated with MOR agonists, but with reduced negative side-effects. Using our newly refined opioid receptor models for structure-based ligand design, we have synthesized several pentapeptides with tailored affinity and efficacy profiles. In particular, we have obtained pentapeptides 8, Tyr-c(S-S)[DCys-1Nal-Nle-Cys]NH(2), and 12, Tyr-c(S-S)[DCys-1Nal-Nle-Cys]OH, which demonstrates high affinity and full agonist behavior at MOR, high affinity but very low efficacy for DOR, and minimal affinity for the kappa opioid receptor (KOR). Functional properties of these peptides as MOR agonists/DOR antagonists lacking undesired KOR activity make them promising candidates for future in vivo studies of MOR/DOR interactions. Subtle structural variation of 12, by substituting D-Cys(5) for L-Cys(5), generated analog 13, which maintains low nanomolar MOR and DOR affinity, but which displays no efficacy at either receptor. These results demonstrate the power and utility of accurate receptor models for structure-based ligand design, as well as the profound sensitivity of ligand function on its structure.

Bifunctional μ/δ opioid peptides: variation of the type and length of the linker connecting the two components

On the basis of evidence that opioid compounds with a mixed μ agonist/δ antagonist profile may produce an antinociceptive effect with low propensity to induce side effects, bifunctional opioid peptides containing the μ agonist H-Dmt-d-Arg-Phe-Lys-NH(2) ([Dmt(1) ]DALDA; Dmt = 2',6'-dimethyltyrosine) connected tail-to-tail via various α,ω-diaminoalkyl- or diaminocyclohexane linkers to the δ antagonists H-Tyr-TicΨ[CH(2) -NH]Cha-Phe-OH (TICP[Ψ]; Cha = cyclohexylalanine, Tic = 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid), H-Dmt-Tic-OH or H-Bcp-Tic-OH (Bcp = 4'-[N-((4'-phenyl)phenethyl)carboxamido]phenylalanine) were synthesized and pharmacologically characterized in vitro. Bifunctional [Dmt(1) ]DALDA→NH-(CH(2) )(n) -NH←TICP[Ψ] compounds (n = -12) showed decreasing μ and δ receptor binding affinities with increasing linker length. As expected, several of the bifunctional peptides were μ agonist/δ antagonists with low nanomolar μ and δ receptor binding affinities. However, compounds with unexpected opioid activity profiles, including a μ partial agonist/δ partial agonist, μ antagonist/δ antagonists and μ agonist/δ agonists, were also identified. These results indicate that the binding affinities and intrinsic efficacies of these bifunctional compounds at both receptors depend on the length and type of the linker connecting the μ and δ components. An important recommendation emerging from this study is that the in vitro activity profiles of bifunctional compounds containing an agonist and an antagonist component connected via a linker need to be determined prior to their pharmacological evaluation in vivo.

Regulation of male fertility by the opioid system

Endogenous opioid peptides are substances involved in cell communication. They are present in various organs and tissues of the male and female reproductive tract, suggesting that they may regulate some of the processes involved in reproductive function. In fact, the opioid system that operates as a multi-messenger system can participate in the regulation of reproductive physiology at multiple levels, for example, at the levels of the central nervous system, at the testes level and at sperm level. A better understanding of the implication of the opioid system in reproductive processes may contribute to clarifying the etiology of many cases of infertility and the effect of opiate abuse on fertility. Indeed, a novel biochemical tool for the diagnosis and treatment of male infertility could be based upon components of the opioid system. The presence of the opioid system in sperm cells also represents a novel opportunity for reproductive management, for either enhancing the probability of fertilization or reducing it through the development of novel targeted contraceptives.

Pentapeptides displaying mu opioid receptor agonist and delta opioid receptor partial agonist/antagonist properties

Chronic use of mu-opioid agonists has been shown to cause neurochemical adaptations resulting in tolerance and dependence. While the analgesic effects of these drugs are mediated by mu-opioid receptors (MOR), several studies have shown that antagonism or knockdown of delta-opioid receptors (DOR) can lessen or prevent development of tolerance and dependence. On the basis of computational modeling of putative active and inactive conformations of MOR and DOR, we have synthesized a series of pentapeptides with the goal of developing a MOR agonist/DOR antagonist peptide with similar affinity at both receptors as a tool to probe functional opioid receptor interaction(s). The eight resulting naphthylalanine-substituted cyclic pentapeptides displayed variable mixed-efficacy profiles. The most promising peptide (9; Tyr-c(S-CH(2)-S)[D-Cys-Phe-2-Nal-Cys]NH(2)) displayed a MOR agonist and DOR partial agonist/antagonist profile and bound with equipotent affinity (K(i) approximately 0.5 nM) to both receptors, but also showed kappa opioid receptor (KOR) agonist activity.

[6,7]-heterocycle-fused 14-hydroxymorphinan derivatives: design, synthesis, and opioid receptor activity

A series of new 14-hydroxymorphinan analogues with a thiazole or imidazo[2,1-b]thiazole fragment as the heterocyclic function fused to ring C were designed and synthesized. These compounds can be viewed as the result of a direct modification at ring C of the 14-hydroxymorphinan scaffold. Among these compounds, three were identified as having potent binding affinity (approximately 1 nM) at both kappa and mu receptors, and acting as agonists at kappa and partial agonists or antagonists at mu receptors. In view of the promising results from studies on compounds with mixed kappa and mu receptor activities, these new compounds warrant further investigation.

The biological activity and metabolic stability of peptidic bifunctional compounds that are opioid receptor agonists and neurokinin-1 receptor antagonists with a cystine moiety

In order to improve metabolic stability, a ring structure with a cystine moiety was introduced into TY027 (Tyr-D-Ala-Gly-Phe-Met-Pro-Leu-Trp-NH-[3',5'-(CF(3))(2)Bzl]), which is a lead compound of our developing bifunctional peptide possessing opioid agonist and NK1 antagonist activities. TY038 (Tyr-cyclo[D-Cys-Gly-Phe-Met-Pro-D-Cys]-Trp-NH-[3',5'-(CF(3))(2)Bzl]) was found as a highly selective delta opioid agonist over mu receptor in conventional tissue-based assays, together with an effective NK1 antagonist activity and good metabolic stability with more than 24h half life in rat plasma.

Synthesis and SAR study of opioid receptor ligands: mono- and bis-indolomorphinans

Mono- and bis-indolomorphinans were synthesized through a multi-step synthetic approach from the alkaloid, thebaine, to further explore the C-ring SAR (structure-activity relationship) of morphinan scaffold. Both mono-indoles displayed good binding affinity and selectivity for the delta receptor, with compound 6b possessed the highest K(i) value of 1.45 nm at this receptor. Bisindolomorphinans 7a,b did not have appreciable affinity for both delta and kappa receptors, but moderate binding at the mu receptor was observed. Functional assays indicated that the newly synthesized mono-indole 6b was delta-agonist, opposite to the delta-antagonist profile of naltrindole. Bisindoles 7a,b were mu-agonists. This work further confirms that the phenol component in opioids is essential for higher binding to the opioid receptors. The different binding ability, receptor selectivity, and the functional activity profiles of naltrindole 2, monoindole 6b, and bisindole 7b clearly indicated that they interact with the opioid receptors in different modes.

Bi- or multifunctional opioid peptide drugs

Strategies for the design of bi- or multifunctional drugs are reviewed. A distinction is made between bifunctional drugs interacting in a monovalent fashion with two targets and ligands containing two distinct pharmacophores binding in a bivalent mode to the two binding sites in a receptor heterodimer. Arguments are presented to indicate that some of the so-called "bivalent" ligands reported in the literature are unlikely to simultaneously interact with two binding sites. Aspects related to the development of bi- or multifunctional drugs are illustrated with examples from the field of opioid analgesics. The drug-like properties of the tetrapeptide Dmt(1)[DALDA] with triple action as a micro opioid agonist, norepinephrine uptake inhibitor and releaser of endogenous opioid peptides to produce potent spinal analgesia are reviewed. Rationales for the development of opioid peptides with mixed agonist/antagonist profiles as analgesics with reduced side effects are presented. Progress in the development of mixed micro opioid agonist/delta opioid antagonists with low propensity to produce tolerance and physical dependence is reviewed. Efforts to develop bifunctional peptides containing a micro opioid agonist and a cholecystokinin antagonist or an NK1 receptor antagonist as analgesics expected to produce less tolerance and dependence are also reviewed. A strategy to improve the drug-like properties of bifunctional opioid peptide analgesics is presented.

Cancer pain and analgesia

Pain ranges in prevalence from 14-100% among cancer patients and occurs in 50-70% of those in active treatment. Cancer pain may result from direct invasion of tumor into nerves, bones, soft tissue, ligaments, and fascia, and may induce visceral pain through distension and obstruction. Cancer pain is multifaceted. Clinicians may describe cancer pain as acute, chronic, nociceptive (somatic), visceral, or neuropathic. Despite implementation of the WHO guidelines, reports of undertreatment of cancer pain persist in various clinical settings and in spite of decades of work to reduce unnecessary discomfort. Substantial obstacles to adequate pain relief with opioids include specific concerns of patients themselves, their family members, physicians, nurses, and the healthcare system. The WHO analgesic ladder serves as the mainstay of treatment for the relief of cancer pain in concert with tumoricidal, surgical, interventional, radiotherapeutic, psychological, and rehabilitative modalities. This multidimensional approach offers the greatest potential for maximizing analgesia and minimizing adverse effects. Primary therapies are directed at the source of the cancer pain and may enhance a patient's function, longevity, and comfort. Adjuvant therapies include nonopioids that confer analgesic effects in certain medical conditions but primarily treat conditions that do not involve pain. Nonopioid medications (over-the-counter agents) are useful in the management of mild to moderate pain, and their continuation through step 3 of the WHO ladder is an option after weighing a drug's risks and benefits in individual patients. Symptomatic treatment of severe cancer pain should begin with an opioid, regardless of the mechanism of the pain. They are very effective analgesics, titrate easily, and offer a favorable risk/benefit ratio. Cancer pain remains inadequately controlled despite the diagnostic and therapeutic means of ensuring that patients feel comfortable during their illness. Therefore, all practitioners need to make control of cancer pain a professional duty, even if they can only use the most basic and least expensive analgesic medications, such as morphine, codeine, and acetaminophen, to reduce human suffering.

Use of oral oxymorphone in the elderly

OBJECTIVE

To review the pharmacodynamics, pharmacokinetics, efficacy, tolerability, dosing, and role of oral oxymorphone immediate-release (IR) and extended-release (ER).

DATA SOURCE

A MEDLINE/PUBMED search (1970 to September 2006) of English language studies. Additional references were obtained from their bibliographies.

STUDY SELECTION

All human studies of oxymorphone were reviewed.

DATA SYNTHESIS

Oral oxymorphone IR/ER tablet formulations were approved in June 2006. Oxymorphone, a semi-synthetic -opioid receptor agonist structurally similar to hydromorphone, has an oral bioavailability of approximately 10%. Oxymorphone is extensively metabolized to oxymorphone-3-glucuronide and the active 6-hydroxyoxymorphone. Rapid clearance mandates every four- to six-hour dosing (IR) and every 12-hour dosing (ER). Hepatic impairment, renal impairment, and aging enhance systemic exposure. Oxymorphone IR was superior to placebo and oxycodone IR (acute pain studies). Oxymorphone ER was superior to placebo and equivalent to oxycodone CR and morphine CR (one acute and five chronic pain studies). Oxymorphone exhibits the expected opioid side effects, being comparable to oxycodone and morphine in clinical trials. Coadministration with ethanol causes "dose-dumping" (ER) and increases intersubject variability in drug absorption. Oxymorphone IR is indicated for the relief of moderate-to-severe pain, while oxymorphone ER is indicated for persistent pain. Initial doses (opioid-naïve) are 10 mg to 20 mg every 4 to 6 hours (IR) and 5 mg every 12 hours (ER). Dosage adjustment is recommended in mild hepatic impairment (Child-Pugh class A), renal impairment (creatinine clearance below 50 mL/min), and in the elderly.

CONCLUSION

Oxymorphone is the newest oral opioid to enter a crowded marketplace now totaling 12 Schedule 2 opioids. It does not appear to have any unique assets or liabilities and should be considered as one of many oral opioids for the management of acute and persistent pain of moderate-to-severe intensity.

Opioid ligands with mixed properties from substituted enantiomeric N-phenethyl-5-phenylmorphans. Synthesis of a micro-agonist delta-antagonist and delta-inverse agonists

Enantiomeric N-phenethyl-m-hydroxyphenylmorphans with various substituents in the ortho, meta or para positions of the aromatic ring in the phenethylamine side-chain (chloro, hydroxy, methoxy, nitro, methyl), as well as a pyridylethyl and a indolylethyl moiety on the nitrogen atom, were synthesized and their binding affinity to the mu-, delta-, and kappa-opioid receptors was examined. The higher affinity ligands were further examined in the [(35)S]GTPgammaS assay to study their function and efficacy. 3-((1R,5S)-(-)-2-(4-Nitrophenethyl)-2-aza-bicyclo[3.3.1]nonan-5-yl)phenol ((-)-) was found to be a mu-agonist and delta-antagonist in that functional assay and was about 50 fold more potent than morphine in vivo. 3-((1R,5S)-(-)-2-(4-Chlorophenethyl)-2-aza-bicyclo[3.3.1]nonan-5-yl)phenol ((-)-) and several other ligands displayed inverse agonist activity at the delta-opioid receptor. The absolute configuration of all of the reported compounds was established by chemical conversion of (-)- to 1R,5S-(-)-.HBr.

Effect of lysine at C-terminus of the Dmt-Tic opioid pharmacophore

Substitution of Gly with side-chain-protected or unprotected Lys in lead compounds containing the opioid pharmacophore Dmt-Tic [H-Dmt-Tic-Gly-NH-CH(2)-Ph, mu agonist/delta antagonist; H-Dmt-Tic-Gly-NH-Ph, mu agonist/delta agonist; and H-Dmt-Tic-NH-CH(2)-Bid, delta agonist (Bid = 1H-benzimidazole-2-yl)] yielded a new series of compounds endowed with distinct pharmacological activities. Compounds (1-10) included high delta- (Ki(delta) = 0.068-0.64 nM) and mu-opioid affinities (Ki(mu) = 0.13-5.50 nM), with a bioactivity that ranged from mu-opioid agonism {10, H-Dmt-Tic-NH-CH[(CH2)4-NH2]-Bid (IC50 GPI = 39.7 nM)} to a selective mu-opioid antagonist [3, H-Dmt-Tic-Lys-NH-CH2-Ph (pA2(mu) = 7.96)] and a selective delta-opioid antagonist [5, H-Dmt-Tic-Lys(Ac)-NH-Ph (pA2(delta) = 12.0)]. The presence of a Lys linker provides new lead compounds in the formation of opioid peptidomimetics containing the Dmt-Tic pharmacophore with distinct agonist and/or antagonist properties.

Discovery of novel triazole-based opioid receptor antagonists

We report the computer-aided design, chemical synthesis, and biological evaluation of a novel family of delta opioid receptor (DOR) antagonists containing a 1,2,4-triazole core structure that are structurally distinct from other known opioid receptor active ligands. Among those delta antagonists sharing this core structure, 8 exhibited strong binding affinity (K(i) = 50 nM) for the DOR and appreciable selectivity for delta over mu and kappa opioid receptors (delta/mu = 80; delta/kappa > 200).

New 2',6'-dimethyl-L-tyrosine (Dmt) opioid peptidomimetics based on the Aba-Gly scaffold. Development of unique mu-opioid receptor ligands

The Aba-Gly scaffold, incorporated into Dmt-Tic ligands (H-Dmt-Tic-Gly-NH-CH2-Ph, H-Dmt-Tic-Gly-NH-Ph, H-Dmt-Tic-NH-CH2-Bid), exhibited mixed micro/delta or delta opioid receptor activities with micro agonism. Substitution of Tic by Aba-Gly coupled to -NH-CH2-Ph (1), -NH-Ph (2), or -Bid (Bid=1H-benzimidazole-2-yl) (3) shifted affinity (Ki(micro)=0.46, 1.48, and 19.9 nM, respectively), selectivity, and bioactivity to micro-opioid receptors. These compounds represent templates for a new class of lead opioid agonists that are easily synthesized and suitable for therapeutic pain relief.

Opioid ligands with mixed mu/delta opioid receptor interactions: an emerging approach to novel analgesics

Opioids are widely used in the treatment of severe pain. The clinical use of the opioids is limited by serious side effects such as respiratory depression, constipation, development of tolerance, and physical dependence and addiction liabilities. Most of the currently available opioid analgesics exert their analgesic and adverse effects primarily through the opioid mu receptors. A large number of biochemical and pharmacological studies and studies using genetically modified animals have provided convincing evidence regarding the existence of modulatory interactions between opioid mu and delta receptors. Several studies indicate that delta receptor agonists as well as delta receptor antagonists can provide beneficial modulation to the pharmacological effects of mu agonists. For example, delta agonists can enhance the analgesic potency and efficacy of mu agonists, and delta antagonists can prevent or diminish the development of tolerance and physical dependence by mu agonists. On the basis of these observations, the development of new opioid ligands possessing mixed mu agonist/delta agonist profile and mixed mu agonist/delta antagonist profile has emerged as a promising new approach to analgesic drug development. A brief overview of mu-delta interactions and recent developments in identification of ligands possessing mixed mu agonist/delta agonist and mu agonist/delta antagonist activities is provided in this report.

Divergent enantioselective synthesis of (-)-galanthamine and (-)-morphine

An efficient divergent synthetic strategy for the synthesis of the opiate and amaryllidaceae alkaloids emerges by employing a Pd-catalyzed asymmetric allylic alkylation (AAA) to set the stereochemistry. Three generations of syntheses of galanthamine are discussed in detail with particular focus on the scope of the palladium-catalyzed AAA reactions and intramolecular Heck reactions. The pivotal tricyclic intermediate is available in six steps from 2-bromovanillin and the monoester of methyl 6-hydroxycyclohexene-1-carboxylate. This intermediate requires only two steps to convert to (-)-galanthamine. Using a Heck vinylation, we found that the fourth ring of codeine/morphine could be formed. The final ring formation involves a novel visible light-promoted hydroamination. Thus, six steps are required to convert the pivotal tricyclic intermediate into codeine, which has been demethylated in high yield to morphine.

Competitive displacement binding assay on rat brain sections and using a beta-imager: application to mu-opioid ligands

A new approach of competitive displacement binding assay using brain sections and a beta-imager is presented to estimate binding parameters such as affinity and selectivity of new compounds or to characterize receptor families or subtypes of receptors in small brain regions. This method includes a preliminary saturation assay intended to define the optimal concentration of displaceable radio-labeled ligand followed by the determination of displacement constants (IC(50) and K(i)) in cerebral regions rich in studied receptor. The technique application was demonstrated in seven rat brain structures, using displacement of the selective tritiated mu-opioid ligand [(3)H]-DAMGO by six opioid ligands: a specific agonist (DAMGO), less specific agonists (morphine, remifentanil), a non-specific antagonist with good affinity for mu receptors (naloxone) and ligands specific of other opioid subtypes (naltrindole, U50.488). Radioactivity counts were collected during 48 h. The assay-validation was performed by measuring intra- and inter-assay variation on determinations and by comparing presently obtained K(i) values with data from recognised methodologies. Both prove the accuracy of the proposed method.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Oxymorphone: a review

Oxymorphone (oxymorphone hydrochloride) (14-hydroxy-dihydromorphinone), a semisynthetic mu-opioid agonist, was first approved by the US Food and Drug Administration in 1959. Oxymorphone is considered a more potent opioid than its parent compound, morphine. Recently, an immediate-release and long-acting oral formulation of this drug was developed that makes oxymorphone a new option in treating moderate to severe pain. This article reviews the pharmacodynamics, pharmacology, and clinical efficacy for this new option in treating moderate to severe pain.

Opioid peptide-derived analgesics

Two recent developments of opioid peptide-based analgesics are reviewed. The first part of the review discusses the dermorphin-derived, cationic-aromatic tetrapeptide H-Dmt-D-Arg-Phe-Lys-NH(2) ([Dmt(1)]DALDA, where Dmt indicates 2',6'-dimethyltyrosine), which showed subnanomolar mu receptor binding affinity, extraordinary mu receptor selectivity, and high mu agonist potency in vitro. In vivo, [Dmt(1)]DALDA looked promising as a spinal analgesic because of its extraordinary antinociceptive effect (3000 times more potent than morphine) in the mouse tail-flick assay, long duration of action (4 times longer than morphine), and lack of effect on respiration. Unexpectedly, [Dmt(1)]DALDA also turned out to be a potent and long-acting analgesic in the tail-flick test when given subcutaneously (s.c.), indicating that it is capable of crossing the blood-brain barrier. Furthermore, little or no cross-tolerance was observed with s.c. [Dmt(1)]DALDA in morphine-tolerant mice. The second part of the review concerns the development of mixed mu agonist/delta antagonists that, on the basis of much evidence, are expected to be analgesics with a low propensity to produce tolerance and physical dependence. The prototype pseudopeptide H-Dmt-TicPsi[CH(2)NH]Phe-Phe-NH(2) (DIPP-NH(2)[Psi], where Tic indicates 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) showed subnanomolar mu and delta receptor binding affinities and the desired mu agonist/delta antagonist profile in vitro. DIPP-NH(2)[Psi] produced a potent analgesic effect after intracerebroventricular administration in the rat tail-flick assay, no physical dependence, and less tolerance than morphine. The results obtained with DIPP-NH(2)[Psi] indicate that mixed mu agonist/delta antagonists look promising as analgesic drug candidates, but compounds with this profile that are systemically active still need to be developed.

β-Phenylethylamines and the isoquinoline alkaloids

This review covers beta-phenylethylamines and isoquinoline alkaloids derived from them, including further products of oxidation, condensation with formaldehyde and rearrangement, some of which do not contain as isoquinoline system, together with napthylisoquinoline alkaloids, which have a different biogenetic origin. The occurrence of the alkaloids with the structures of new bases, together with their reactions and syntheses, are reported. The literature from July 2003 to June 2004 is reviewed, with 145 references cited.

The synthesis and pharmacological evaluation of (±)-2, 3- seco-fentanyl analogues

An efficient, five-step synthetic approach to various acyclic 1,3-diamines has been developed and applied to the preparation of a novel class of open-chained fentanyl analogues. The acyclic derivatives 5.1?5.5 (all new compounds) were synthesized with the aim of estimating the significance of the piperidine ring for the opioid analgesic activity of anilido-piperidines. The starting ?-keto-amide 1.1, prepared by the aminolysis of methyl acetoacetate with methylphenethylamine, (93 % yield), was successively reacted with NaH and BuLi, to form the highly reactive ?,?-dienolate anion 1.1a. Regio and chemoselective ?-alkylation of the dienolate with various primary and secondary alkyl halides furnished the ?-keto-amides 1.2?1.5 (76?91 %). Reductive amination of the keto-amides 1.1?1.5 with aniline and Zn powder in acetic acid, via the enamine intermediates 2.1?2.5, afforded the ?-anilino amides 3.1?3.5 (74?85 %). After reductive deoxygenation of the tertiary amide group, using in situ generated diborane, the corresponding 1,3-diamines 4.1?4.5 were obtained (87?97 %). The synthesis of (?)-2,3-seco-fentanyls 5.1?5.5 was completed by N-acylation of the diamines 4.1?4.5 with propionyl chloride, followed by precipitation of the monooxalate salts (86?95 %). The parent compound, 2,3-seco-fentanyl 5.1, was found to be a 40 times less potent narcotic analgesic than fentanyl but still 5?6 times more active than morphine in rats, while i-Pr derivative 5.3 was inactive. Apart from the pharmacological significance, the general procedure described herein may afford various functionalized, 1,3-diamines as potential complexing agents and building blocks for the synthesis of aza-crown ethers.