Investigation of the selectivity of oxymorphone- and naltrexone-derived ligands via site-directed mutagenesis of opioid receptors: exploring the "address" recognition locus

Biased Activation Mechanism Induced by GPCR Heterodimerization: Observations from μOR/δOR Dimers

GPCRs regulate multiple intracellular signaling cascades. Biasedly activating one signaling pathway over the others provides additional clinical utility to optimize GPCR-based therapies. GPCR heterodimers possess different functions from their monomeric states, including their selectivity to different transducers. However, the biased signaling mechanism induced by the heterodimerization remains unclear. Motivated by the issue, we select an important GPCR heterodimer (μOR/δOR heterodimer) as a case and use microsecond Gaussian accelerated molecular dynamics simulation coupled with potential of mean force and protein structure network (PSN) to probe mechanisms regarding the heterodimerization-induced constitutive β-arrestin activity and efficacy change of the agonist DAMGO. The results show that only the lowest energy state of the μOR/δOR heterodimer, which adopts a slightly outward shift of TM6 and an ICL2 conformation close to the receptor core, can selectively accommodate β-arrestins. PSN further reveals important roles of H8, ICL1, and ICL2 in regulating the constitutive β-arrestin-biased activity for the apo μOR/δOR heterodimer. In addition, the heterodimerization can allosterically alter the binding mode of DAMGO mainly by means of W7.35. Consequently, DAMGO transmits the structural signal mainly through TM6 and TM7 in the dimer, rather than TM3 similar to the μOR monomer, thus changing the efficacy of DAMGO from a balanced agonist to the β-arrestin-biased one. On the other side, the binding of DAMGO to the heterodimer can stabilize μOR/δOR heterodimers through a stronger interaction of TM1/TM1 and H8/H8, accordingly enhancing the interaction of μOR with δOR and the binding affinity of the dimer to the β-arrestin. The agonist DAMGO does not change main compositions of the regulation network from the dimer interface to the transducer binding pocket of the μOR protomer, but induces an increase in the structural communication of the network, which should contribute to the enhanced β-arrestin coupling. Our observations, for the first time, reveal the molecular mechanism of the biased signaling induced by the heterodimerization for GPCRs, which should be beneficial to more comprehensively understand the GPCR bias signaling.

Tyrosine 7.43 is important for mu-opioid receptor downstream signaling pathways activated by fentanyl

G protein–coupled receptors can signal through both G proteins and ß-arrestin2. For the µ-opioid receptor (MOR), early experimental evidence from a single study suggested that G protein signaling mediates analgesia and sedation, whereas ß-arrestin signaling mediates respiratory depression and constipation. Then, receptor mutations were used to clarify which residues interact with ligands to selectively regulate signals in a ligand-specific manner. However, there is no systematic study on how to determine these residues and clarify the molecular mechanism of their influence on signal pathways. We have therefore used molecular docking to predict the amino acid sites that affect the binding of ligands and MOR. Then, the corresponding sites were mutated to determine the effect of the structural determinant of MOR on G_i/o protein and ß-arrestin pathways. The pharmacological and animal behavioral experiments in combination with molecular dynamics simulations were used to elucidate the molecular mechanism of key residues governing the signaling. Without affecting ligand binding to MOR, MOR^Y7.43A attenuated the activation of both G_i/o protein and ß-arrestin signaling pathways stimulated by fentanyl, whereas it did not change these two pathways stimulated by morphine. Likewise, the activation peak time of extracellular regulated protein kinases was significantly prolonged at MOR^Y7.43A compared with that at MOR^wildtype stimulated by fentanyl, but there was no difference stimulated by morphine. In addition, MOR^Y7.43A significantly enhanced analgesia by fentanyl but not by morphine in the mice behavioral experiment. Furthermore, the molecular dynamics simulations showed that H6 moves toward the cellular membrane. H6 of the fentanyl–Y7.43A system moved outward more than that in the morphine–Y7.43A system. Y7.43 mutation disrupted hydrophobic interactions between W6.48 and Y7.43 in the fentanyl–Y7.43A system but not in the morphine–Y7.43A system. Our results have disclosed novel mechanisms of Y7.43 mutation affecting MOR signaling pathways. Y7.43 mutation reduced the activation of the G_i/o protein pathway and blocked the ß-arrestin2 recruitment, increased the H6 outward movement of MOR, and disrupted hydrophobic interactions. This may be responsible for the enhanced fentanyl analgesia. These findings are conducive to designing new drugs from the perspective of ligand and receptor binding, and Y7.43 is also expected to be a key site to structure optimization of synthesized compounds.

Discovery of an M-Substituted N-Cyclopropylmethyl-7α-phenyl-6,14-endoethanotetrahydronorthebaine as a Selective, Potent, and Orally Active κ-Opioid Receptor Agonist with an Improved Central Nervous System Safety Profile

The search for selective kappa opioid receptor (κOR) agonists with an improved safety profile is an area of interest in opioid research. In this work, a series of m-substituted analogs were designed, synthesized, and assayed, resulting in the identification of compound 6c (SLL-1206) as a κOR agonist with single-digit nanomolar activities. The subtype selectivity of compound 6c appeared to be a consequence of an enormous decrease in the affinity for μOR and δOR, rather than a significant increase in the affinity for κOR, which was not the case for SLL-039, another selective and potent κOR agonist identified in our previous work. Besides reduced central nervous system effects, SLL-1206 exhibited substantially improved physicochemical and pharmacokinetic properties compared with SLL-039, with increases of over 20-fold in aqueous solubility and approximately 40-fold in oral bioavailability in rats.

Conformational Constraint between Aromatic Residue Side Chains in the "Message" Sequence of the Peptide Arodyn Using Ring Closing Metathesis Results in a Potent and Selective Kappa Opioid Receptor Antagonist

Kappa opioid receptor (KOR) antagonists have recently shown potential for treating drug addiction and mood disorders. The linear acetylated dynorphin A analog arodyn (Ac[Phe^1,2,3,Arg⁴,d-Ala⁸]dynorphin A-(1-11)NH₂), synthesized in our laboratory, demonstrated potent and selective KOR antagonism. Cyclization of arodyn could potentially stabilize the bioactive conformation and enhance its metabolic stability. The cyclization strategy employed involved ring closing metathesis between adjacent meta- or para-substituted Tyr(allyl) residues in the "message" sequence that were predicted in a docking study to yield analogs that would bind to the KOR with binding poses similar to arodyn. Consistent with the modeling, the resulting analogs retained KOR affinity similar to arodyn; the peptides involving cyclization between para O-allyl groups also retained high KOR selectivity, with one analog exhibiting KOR antagonist potency (K_B = 15 nM) similar to arodyn. These promising cyclized analogs with constrained aromatic residues represent novel leads for further exploration of KOR pharmacology.

Computational insights into the subtype selectivity and "message-address-efficacy" mechanisms of opioid receptors through JDTic binding and unbinding

In drug design and discovery, binding affinity and selectivity are two basic properties of a drug candidate. Opioid receptors (ORs) are the main targets of strong analgesics. Like some other class A members of G-protein-coupled receptors (GPCRs), ORs exhibit complex selectivity on their ligands. The diversity of binding activity and selectivity among opioids has deeply attracted researchers for a long time. To investigate the subtype selectivity of μ, δ and κ ORs in detail, using the κ-selective antagonist JDTic as a probe, we performed a series of computational simulations, including molecular dynamics and metadynamics, on JDTic-μ/δ/κ-OR complexes. From the simulations, we found that the decisive factor of JDTic selectivity on the μ-subtype was the 2.63 position, which affected the efficacy of JDTic through changing the dynamics of the Q2.60 residue. In addition to the 2.63-position residue, the 7.35 position was the other crucial aspect of JDTic selectivity for the δ-subtype. Based on the results, we suggest a new concept, the "message-address-efficacy" hypothesis, to explain the relationships among the affinity, selectivity and function between ORs and opioids. Thus, all the detailed dynamics of JDTic-bound ORs might be helpful to deeply understand the subtype selectivity and binding mechanisms of other GPCRs.

C-Homomorphinan Derivatives as Lead Compounds to Obtain Safer and More Clinically Useful Analgesics

Buprenorphine shows strong analgesic effects on moderate to severe pain. Although buprenorphine can be used more safely than other opioid analgesics, it has room for improvement in clinical utility. Investigation of compounds structurally related to buprenorphine should be an approach to obtain novel analgesics with safer and improved profiles compared to buprenorphine. In the course of our previous studies, we observed that derivatives obtained by cyclizing C-homomorphinans were structurally related to buprenorphine. Hence, we synthesized cyclized C-homomorphinan derivatives with various oxygen functionalities on the side chains and evaluated their in vitro pharmacological profiles for the opioid receptors. Among the tested compounds, methyl ketone 2a with an N-methyl group showed full agonistic activities for the μ and the δ receptors and partial agonistic activity for the κ receptor. These properties were similar to those of norbuprenorphine, a major metabolite of buprenorphine, which reportedly contributes to the antinociceptive effect of buprenorphine. From these results, we concluded that cyclized C-homomorphinan would be a possible lead compound to obtain novel analgesics with buprenorphine-like properties.

Probing ligand recognition of the opioid pan antagonist AT-076 at nociceptin, kappa, mu, and delta opioid receptors through structure-activity relationships

Few opioid ligands binding to the three classic opioid receptor subtypes, mu, kappa and delta, have high affinity at the fourth opioid receptor, the nociceptin/orphanin FQ receptor (NOP). We recently reported the discovery of AT-076 (1), (R)-7-hydroxy-N-((S)-1-(4-(3-hydroxyphenyl)piperidin-1-yl)-3-methylbutan-2-yl)-1,2,3,4-tetrahydroisoquinoline-3-carboxamide, a pan antagonist with nanomolar affinity for all four subtypes. Since AT-076 binds with high affinity at all four subtypes, we conducted a structure-activity relationship (SAR) study to probe ligand recognition features important for pan opioid receptor activity, using chemical modifications of key pharmacophoric groups. SAR analysis of the resulting analogs suggests that for the NOP receptor, the entire AT-076 scaffold is crucial for high binding affinity, but the binding mode is likely different from that of NOP antagonists C-24 and SB-612111 bound in the NOP crystal structure. On the other hand, modifications of the 3-hydroxyphenyl pharmacophore, but not the 7-hydroxy Tic pharmacophore, are better tolerated at kappa and mu receptors and yield very high affinity multifunctional (e.g. 12) or highly selective (e.g. 16) kappa ligands. With the availability of the opioid receptor crystal structures, our SAR analysis of the common chemotype of AT-076 suggests rational approaches to modulate binding selectivity, enabling the design of multifunctional or selective opioid ligands from such scaffolds.

Residues W320 and Y328 within the binding site of the μ-opioid receptor influence opiate ligand bias

The development of G protein-biased agonists for the μ-opioid receptor (MOR) offers a clear drug discovery rationale for improved analgesia and reduced side-effects of opiate pharmacotherapy. However, our understanding of the molecular mechanisms governing ligand bias is limited, which hinders our ability to rationally design biased compounds. We have investigated the role of MOR binding site residues W320 and Y328 in controlling bias, by receptor mutagenesis. The pharmacology of a panel of ligands in a cAMP and a β-arrestin2 assay were compared between the wildtype and mutated receptors, with bias factors calculated by operational analysis using ΔΔlog(τ/K_A) values. [³H]diprenorphine competition binding was used to estimate affinity changes. Introducing the mutations W320A and Y328F caused changes in pathway bias, with different patterns of change between ligands. For example, DAMGO increased relative β-arrestin2 activity at the W320A mutant, whilst its β-arrestin2 response was completely lost at Y328F. In contrast, endomorphin-1 gained activity with Y328F but lost activity at W320A, in both pathways. For endomorphin-2 there was a directional shift from cAMP bias at the wildtype towards more β-arrestin2 bias at W320A. We also observe clear uncoupling between mutation-driven changes in function and binding affinity. These findings suggest that the mutations influenced the balance of pathway activation in a ligand-specific manner, thus identifying residues in the MOR binding pocket that govern ligand bias. This increases our understanding of how ligand/receptor binding interactions can be translated into agonist-specific pathway activation.

Pharmacodynamic effects of oral oxymorphone: abuse liability, analgesic profile and direct physiologic effects in humans

Oxymorphone is a semisynthetic μ-opioid agonist, marketed as a prescription analgesic purported to be twice as potent as oxycodone for pain relief. Oral formulations of oxymorphone were reintroduced in the United States in 2006 and reports of abuse ensued; however, there are limited data available on its pharmacodynamic effects. The current study aimed to examine the direct physiologic effects, relative abuse liability, analgesic profile and overall pharmacodynamic potency of oxymorphone in comparison with identical doses of oxycodone. Healthy, non-dependent opioid abusers (n = 9) were enrolled in this within-subject, double-blind, placebo-controlled, 3-week inpatient study. Seven experimental sessions (6.5 hours) were conducted, during which an oral dose of immediate-release formulations of oxymorphone (10, 20 and 40 mg), oxycodone (10, 20 and 40 mg) or placebo was administered. An array of physiologic, abuse liability and experimental pain measures was collected. At identical doses, oxymorphone produced approximately twofold less potent effects on miosis, compared with oxycodone. Oxymorphone also produced lesser magnitude effects on measures of respiratory depression, two experimental pain models and observer-rated agonist effects. However, 40 mg of oxymorphone was similar to 40 mg of oxycodone on several abuse-related subjective ratings. Formal relative potency analyses were largely invalid because of the substantially greater effects of oxycodone. Overall, oxymorphone is less potent on most pharmacodynamic measures, although at higher doses, its abuse liability is similar to oxycodone. These data suggest that the published clinical equianalgesic estimates may not be consistent with the observed direct physiologic effects of opioids, results of experimental pain models or abuse liability measures, as assessed in the human laboratory.

The effects of morphine, naloxone, and κ opioid manipulation on endocrine functioning and social behavior in monogamous titi monkeys (Callicebus cupreus)

The μ opioid receptor (MOR) and κ opioid receptor (KOR) have been implicated in pair-bond formation and maintenance in socially monogamous species. Utilizing monogamous titi monkeys (Callicebus cupreus), the present study examined the potential role opioids play in modulating the response to separation, a potent challenge to the pair-bond. In Experiment 1, paired male titi monkeys were separated from their pair-mate for 30-min and then received saline, naloxone (1.0mg/kg), morphine (0.25mg/kg), or the KOR agonist, U50,488 (0.01, 0.03, or 0.1mg/kg) in a counter-balanced fashion, immediately prior to a 30-min reunion with their mate. Blood samples were collected immediately prior to and after the reunion. Males receiving morphine approached females less, initiated contact less, and females broke contact with the males less. The increase in cortisol in response to naloxone was greater compared to vehicle, and the increase in cortisol in response to the high dose of U50,488 compared to vehicle approached significance. In Experiment 2, paired males were treated with the KOR antagonist, GNTI (0.1, 0.3, or 1.0mg/kg), or saline 24h prior to a 60-min separation from their mate. Blood samples were collected at the time of injection and immediately before and after separation. Administration of the low dose of GNTI decreased the locomotor component of the separation response compared to vehicle. The present study found that the opioid system is involved in both the affiliative and separation distress components of a pair-bond, and these components are regulated by different opioid receptors.

Chemotype-selective modes of action of κ-opioid receptor agonists

The crystal structures of opioid receptors provide a novel platform for inquiry into opioid receptor function. The molecular determinants for activation of the κ-opioid receptor (KOR) were studied using a combination of agonist docking, functional assays, and site-directed mutagenesis. Eighteen positions in the putative agonist binding site of KOR were selected and evaluated for their effects on receptor binding and activation by ligands representing four distinct chemotypes: the peptide dynorphin A(1-17), the arylacetamide U-69593, and the non-charged ligands salvinorin A and the octahydroisoquinolinone carboxamide 1xx. Minimally biased docking of the tested ligands into the antagonist-bound KOR structure generated distinct binding modes, which were then evaluated biochemically and pharmacologically. Our analysis identified two types of mutations: those that affect receptor function primarily via ligand binding and those that primarily affect function. The shared and differential mechanisms of agonist binding and activation in KOR are further discussed. Usually, mutations affecting function more than binding were located at the periphery of the binding site and did not interact strongly with the various ligands. Analysis of the crystal structure along with the present results provide fundamental insights into the activation mechanism of the KOR and suggest that "functional" residues, along with water molecules detected in the crystal structure, may be directly involved in transduction of the agonist binding event into structural changes at the conserved rotamer switches, thus leading to receptor activation.

Bivalent ligands that target μ opioid (MOP) and cannabinoid1 (CB1) receptors are potent analgesics devoid of tolerance

Given that μ opioid (MOP) and canabinoid (CB1) receptors are colocalized in various regions of the central nervous system and have been reported to associate as heteromer (MOP-CB1) in cultured cells, the possibility of functional, endogenous MOP-CB1 in nociception and other pharmacologic effects has been raised. As a first step in investigating this possibility, we have synthesized a series of bivalent ligands 1-5 that contain both μ agonist and CB1 antagonist pharmacophores for use as tools to study the functional interaction between MOP and CB1 receptors in vivo. Immunofluorescent studies on HEK293 cells coexpressing both receptors suggested 5 (20-atom spacer) to be the only member of the series that bridges the protomers of the heteromer. Antinociceptive testing in mice revealed 5 to be the most potent member of the series. As neither a mixture of monovalent ligands 9 + 10 nor bivalents 2-5 produced tolerance in mice, MOR-CB1 apparently is not an important target for reducing tolerance.

Structure of the δ-opioid receptor bound to naltrindole

The opioid receptor family comprises three members, the µ-, δ- and κ-opioid receptors, which respond to classical opioid alkaloids such as morphine and heroin as well as to endogenous peptide ligands like endorphins. They belong to the G-protein-coupled receptor (GPCR) superfamily, and are excellent therapeutic targets for pain control. The δ-opioid receptor (δ-OR) has a role in analgesia, as well as in other neurological functions that remain poorly understood. The structures of the µ-OR and κ-OR have recently been solved. Here we report the crystal structure of the mouse δ-OR, bound to the subtype-selective antagonist naltrindole. Together with the structures of the µ-OR and κ-OR, the δ-OR structure provides insights into conserved elements of opioid ligand recognition while also revealing structural features associated with ligand-subtype selectivity. The binding pocket of opioid receptors can be divided into two distinct regions. Whereas the lower part of this pocket is highly conserved among opioid receptors, the upper part contains divergent residues that confer subtype selectivity. This provides a structural explanation and validation for the 'message-address' model of opioid receptor pharmacology, in which distinct 'message' (efficacy) and 'address' (selectivity) determinants are contained within a single ligand. Comparison of the address region of the δ-OR with other GPCRs reveals that this structural organization may be a more general phenomenon, extending to other GPCR families as well.

Neuropharmacology of the naturally occurring kappa-opioid hallucinogen salvinorin A

Salvia divinorum is a perennial sage native to Oaxaca, Mexico, that has been used traditionally in divination rituals and as a treatment for the "semimagical" disease panzón de borrego. Because of the intense "out-of-body" experiences reported after inhalation of the pyrolized smoke, S. divinorum has been gaining popularity as a recreational hallucinogen, and the United States and several other countries have regulated its use. Early studies isolated the neoclerodane diterpene salvinorin A as the principal psychoactive constituent responsible for these hallucinogenic effects. Since the finding that salvinorin A exerts its potent psychotropic actions through the activation of KOP receptors, there has been much interest in elucidating the underlying mechanisms behind its effects. These effects are particularly remarkable, because 1) salvinorin A is the first reported non-nitrogenous opioid receptor agonist, and 2) its effects are not mediated by the 5-HT(2A) receptor, the classic target of hallucinogens such as lysergic acid diethylamide and mescaline. Rigorous investigation into the structural features of salvinorin A responsible for opioid receptor affinity and selectivity has produced numerous receptor probes, affinity labels, and tools for evaluating the biological processes responsible for its observed psychological effects. Salvinorin A has therapeutic potential as a treatment for pain, mood and personality disorders, substance abuse, and gastrointestinal disturbances, and suggests that nonalkaloids are potential scaffolds for drug development for aminergic G-protein coupled receptors.

Analogues of (3R)-7-hydroxy-N-[(1S)-1-{[(3R,4R)-4-(3-hydroxyphenyl)-3,4-dimethyl-1-piperidinyl]methyl}-2-methylpropyl)-1,2,3,4-tetrahydro-3-isoquinolinecarboxamide (JDTic). Synthesis and in vitro and in vivo opioid receptor antagonist activity

The synthesis of compounds 6, 7a,b, 8a,b, 9a,b, and 10a,b where the amino -NH- group of JDTic (3) was replaced with an aromatic horizontal lineCH-, CH(2), O, S, or SO group was accomplished and used to further characterize the SAR of the compound 3 class of kappa opioid receptor antagonists. All of the compounds showed subnanomolar to low nanomolar K(e) values at the kappa opioid receptor. The most potent compound was 7a, where the amino -NH- group of 3 was replaced by a methylene (-CH(2)-) group. This compound had a K(e) = 0.18 nM and was 37- and 248-fold selective for the kappa relative to the mu and delta opioid receptors, respectively. Similar to compound 3, compound 7a antagonized selective kappa agonist U50,488-induced diuresis after sc administration in rats. In contrast to 3, where kappa antagonist activity lasted for three weeks, compound 7a did not show any kappa antagonist activity after one week.

INTERACTION OF BRIDGED PIPERAZINE DERIVATIVES WITH THE μ-OPIOID RECEPTOR — A THEORETICAL MODEL

The flexible molecular docking was used to study interactions between a series of 3,6-diazabicyclo[3.1.1]heptanes, 9,10-diazatricyclo[4.2.1.1]decanes, and 2,7-diazatricyclo [4.4.0.0]decanes N-substituted by propanoyl and by arylalkenyl groups, and a model of the μ-opioid receptor. It has been found that the optimal position and orientation of the compounds in the ligand–receptor complex resemble that of fentanyl analogs described earlier.1 This model explains stereochemical effects on binding of the two series of 3,6-diazabicyclo[3.1.1]heptanes, suggesting that the steric interaction of the bridge methylenic group plays the major role in modulating μ-receptor affinity of those molecules. Ab initio B3LYP method was used to determine electrostatic potentials of different bridged piperazine derivatives, and to estimate electrostatic contribution to the ligand–receptor complex stability.

Design, synthesis, and biological evaluation of 6alpha- and 6beta-N-heterocyclic substituted naltrexamine derivatives as mu opioid receptor selective antagonists

Opioid receptor selective antagonists are important pharmacological probes in opioid receptor structural characterization and opioid agonist functional study. Thus far, a nonpeptidyl, highly selective and reversible mu opioid receptor (MOR) antagonist is unavailable. On the basis of our modeling studies, a series of novel naltrexamine derivatives have been designed and synthesized. Among them, two compounds were identified as leads based on the results of in vitro and in vivo assays. Both of them displayed high binding affinity for the MOR (K(i) = 0.37 and 0.55 nM). Compound 6 (NAP) showed over 700-fold selectivity for the MOR over the delta receptor (DOR) and more than 150-fold selectivity over the kappa receptor (KOR). Compound 9 (NAQ) showed over 200-fold selectivity for the MOR over the DOR and approximately 50-fold selectivity over the KOR. Thus these two novel ligands will serve as leads to further develop more potent and selective antagonists for the MOR.

Receptor reserve reflects differential intrinsic efficacy associated with opioid diastereomers

Structure-activity relationships built around receptor binding or cell-based assays are designed to reveal physiochemical differences between ligands. We hypothesized that agonist receptor reserve may provide a unique approach to distinguish structurally-related agonists exhibiting similar functional characteristics. An intracellular calcium activation assay in Chinese Hamster Ovary (CHO) cells expressing cloned human mu-opioid receptors was developed. We examined two isomers exhibiting indistinguishable receptor binding and in vitro potency profiles. Oxymorphone, a clinically-available congener of codeine has at least two active diastereomeric metabolites (6alpha- and 6beta-oxymorphols) found to be similar for mu-opioid receptor binding affinity (K(d) = 15 versus 14 nM) and calcium activation (EC(50) = 22 versus 14 nM). Calcium activation was then inhibited in CHO cells in a concentration-dependent manner using the irreversible mu-opioid receptor antagonist, beta-funaltrexamine (beta-FNA). Under these conditions, approximately 10-fold greater receptor reserve was found for 6alpha-oxymorphol compared to 6beta-oxymorphol. This difference between the oxymorphols corresponded to a rank order of intrinsic efficacy (Emax): DAMGO > oxymorphone = 6alpha-oxymorphol = oxycodone > 6beta-oxymorphol. In addition, 6alpha-oxymorphol exhibited greater relative potency than the 6beta-oxymorphol in mouse tail-flick, hot-plate and phenylquinone writhing antinociceptive assays, regardless of route of administration. Thus the beta-FNA/calcium model provides a novel, cell-based approach to distinguish structurally related mu-opioid agonists, and in the specific case of the oxymorphols, receptor reserve differences provided a means to bridge functional in vitro and in vivo models.

Docking studies suggest ligand-specific delta-opioid receptor conformations

An automated docking procedure was used to study binding of a series of delta-selective ligands to three models of the delta-opioid receptor. These models are thought to represent the three ligand-specific receptor conformations. Docking results are in agreement with point mutation studies and suggest that different ligands--agonists and antagonists--may bind to the same binding site under different receptor conformations. Docking to different receptor models (conformations) also suggests that by changing to a receptor-specific conformation, the receptor may open or close different binding sites to other ligands.

Review of oral oxymorphone in the management of pain

Chronic cancer and nonmalignant pain (CNMP) is a common and major health problem afflicting approximately 40 million persons in the US. Most cancer patients, and many patients with CNMP, require opioid analgesics to obtain adequate pain relief. Oral oxymorphone is a new formulation of an existing parenteral opioid that has become available for the treatment of significant pain: acute postoperative, chronic arthritis, chronic low back, and chronic cancer pain. Oxymorphone is a typical mu-opioid agonist that is effective in both immediate- and extended-release (IR and ER) formulations. Oxymorphone is more lipid soluble than morphine, resulting in a rapid onset of action when given in tablet formulation, with a duration of action of approximately 4-6 hours in IR and 12 hours in ER preparations. Oxymorphone provides excellent pain relief for significant pain, with typical opioid side effects that are usually mild or moderate in intensity. Multiple double-blind, prospective, placebo-controlled clinical trials have demonstrated the clinical efficacy and safety of this new oral opioid preparation. Oral oxymorphone is an effective opioid that provides a new therapeutic option for the physician.

Oral extended-release oxymorphone: a new choice for chronic pain relief

Opana ER (oxymorphone extended release [ER]) is a new oral long-acting formulation indicated for the treatment of moderate to severe chronic pain. Because the ER matrix slowly releases oxymorphone over 12 h, consistent plasma levels are produced with low peak-to-trough fluctuations. Oxymorphone ER is the only long-acting opioid that contains oxymorphone, which exhibits some distinct pharmacologic properties compared with most other opioids, including a longer half-life, higher affinity for the micro-opioid receptor, and lack of interaction with the CYPP450 drug-metabolizing system. With a safety and tolerability profile similar to other opioids and documented efficacy in several models of chronic pain (low back, cancer, and osteoarthritis), oxymorphone ER provides a new option for clinicians and patients in the treatment of chronic pain.

Indium-Labeled Macrocyclic Conjugates of Naltrindole: High-Affinity Radioligands for In Vivo Studies of Peripheral δ Opioid Receptors

We have identified a series of hydrophilic indium-labeled DOTA and DO3A conjugates of naltrindole (NTI) that are suited to in vivo studies of peripheral delta opioid receptors. Indium(III) complexes, linked to the indole nitrogen of NTI by six- to nine-atom spacers, display high affinities (0.1-0.2 nM) and excellent selectivities for binding to delta sites in vitro. The [111In]-labeled complexes can be prepared in good isolated yields ( approximately 65%) with high specific radioactivities (>3300 mCi/mumol). The spacers serve as pharmacokinetic modifiers, and log D7.4 values range from -2.74 to -1.79. These radioligands exhibit a high level of specific binding (75-94%) to delta opioid receptors in mouse gut, heart, spleen, and pancreas in vivo. Uptakes of radioactivity are saturable by the non-radioactive complexes, inhibited by naltrexone, and blocked by NTI. Thus, these radiometal-labeled NTI analogues warrant further study by single-photon emission computed tomography.

A 12-week, randomized, placebo-controlled trial assessing the safety and efficacy of oxymorphone extended release for opioid-naive patients with chronic low back pain

OBJECTIVE

Determine the efficacy and tolerability of oxymorphone extended release (OPANA ER) in opioid-naive patients with moderate to severe chronic low back pain (CLBP).

DESIGN AND METHODS

Patients > or = 18 years of age were titrated with oxymorphone ER (5- to 10-mg increments every 12 h, every 3-7 days) to a well-tolerated, stabilized dose. Patients were then randomized to continue their oxymorphone ER dose or receive placebo every 12 h for 12 weeks. Oxymorphone immediate release was available every 4-6 h, as needed, for the first 4 days and twice daily thereafter.

RESULTS

Sixty-three percent of patients (205/325) were titrated to a stabilized dose of oxymorphone ER, most (203/205) within 1 month. During titration, 18% discontinued from adverse events (AEs) and 1% from lack of efficacy. For patients completing titration, average pain intensity decreased from 69.4 mm at screening to 22.7 mm (p < 0.0001). After randomization, 68% of oxymorphone ER and 47% of placebo patients completed 12 weeks of double-blind treatment. Approximately 8% of patients in each group discontinued because of AEs. Placebo patients discontinued significantly sooner from lack of efficacy than those receiving oxymorphone ER (p < 0.0001). Pain intensity increased significantly more in the placebo group (least squares [LS] mean change 26.9 +/- 2.4 [median 28.0]) than in the oxymorphone ER group (LS mean change 10.0 +/- 2.4 [median 2.0]; p < 0.0001). Oxymorphone ER was generally well tolerated without unexpected AEs. Although limitations of a randomized withdrawal study include the potential for unblinding and opioid withdrawal in placebo patients, opioid withdrawal was limited to two patients in the placebo group and one in the oxymorphone ER group.

CONCLUSIONS

Stabilized doses of oxymorphone ER were generally safe and effective over a 12-week double-blind treatment period in opioid-naive patients with CLBP.

Efficacy and safety of OPANA ER (oxymorphone extended release) for relief of moderate to severe chronic low back pain in opioid-experienced patients: a 12-week, randomized, double-blind, placebo-controlled study

Opioid-experienced (N = 250) patients with chronic, moderate to severe low back pain (LBP) were converted from their prestudy opioid(s) to an approximately equianalgesic dose of OPANA ER (oxymorphone extended release). Patients continued slow titration, with 56% stabilized within 1 month to a dose of OPANA ER that reduced average pain to <40 mm on a visual analog scale with good tolerability. Stabilized patients (n = 143) were randomized to placebo or their stabilized dose of OPANA ER every 12 hours for a 12-week double-blind period. Pain intensity increased significantly more for patients randomized to placebo than for patients who continued their stabilized dose of OPANA ER; the increase from baseline (at randomization) to final visit was 31.6 mm for placebo versus 8.7 mm with OPANA ER (P < .0001). During double-blind treatment, placebo patients were approximately 8-fold more likely than OPANA ER patients to discontinue because of lack of efficacy (P < .001). Discontinuations as a result of adverse events were similar between groups, 10% with placebo and 11% with OPANA ER. Opioid-related adverse events included constipation (6%), somnolence (3%), and nausea (3%). Fifty-seven percent of opioid-experienced patients with chronic, moderate to severe LBP achieved a stable dose of OPANA ER that was efficacious and generally well-tolerated for up to 12 weeks.

PERSPECTIVE

In a 12-week, double-blind, randomized, placebo-controlled trial in opioid-experienced patients with chronic, moderate to severe LBP, OPANA ER provided efficacious, long-term analgesia and was generally well-tolerated. OPANA ER may provide clinicians with a new treatment option for patients experiencing suboptimal analgesic responses or poor tolerability with other opioids.

A unique binding epitope for salvinorin A, a non-nitrogenous kappa opioid receptor agonist

Salvinorin A is a potent kappa opioid receptor (KOP) agonist with unique structural and pharmacological properties. This non-nitrogenous ligand lacks nearly all the structural features commonly associated with opioid ligand binding and selectivity. This study explores the structural basis to salvinorin A binding and selectivity using a combination of chimeric and single-point mutant opioid receptors. The experiments were designed based on previous models of salvinorin A that locate the ligand within a pocket formed by transmembrane (TM) II, VI, and VII. More traditional sites of opioid recognition were also explored, including the highly conserved aspartate in TM III (D138) and the KOP selectivity site E297, to determine the role, if any, that these residues play in binding and selectivity. The results indicate that salvinorin A recognizes a cluster of residues in TM II and VII, including Q115, Y119, Y312, Y313, and Y320. Based on the position of these residues within the receptor, and prior study on salvinorin A, a model is proposed that aligns the ligand vertically, between TM II and VII. In this orientation, the ligand spans residues that are spaced one to two turns down the face of the helices within the receptor cavity. The ligand is also in close proximity to EL-2 which, based on chimeric data, is proposed to play an indirect role in salvinorin A binding and selectivity.

Steric interactions and the activity of fentanyl analogs at the μ-opioid receptor

Fentanyl is a highly potent and clinically widely used narcotic analgesic. The synthesis of its analogs remains a challenge in the attempt to develop highly selective mu-opioid receptor agonists with specific pharmacological properties. In this paper, the use of flexible molecular docking in a study of the formation of complexes between a series of active fentanyl analogs and the mu-opioid receptor is described. The optimal position and orientation of fourteen fentanyl analogs in the binding pocket of the mu-receptor were determined. The major receptor amino acids and the ligand functional groups participating in the complex formation were identified. Stereochemical effects on the potency and binding are explained. The proposed model of ligand-receptor binding is in agreement with point mutation experiments explaining the role of the amino acids: Asp147, Tyr148, Asn230, His297, Trp318, His319, Cys321, and Tyr326 in the complex formation. In addition, the following amino acids were identified as being important for ligand binding or receptor activation: Ile322, Gly325, Val300, Met203, Leu200, Val143, and Ile144.

Molecular recognition of opioid receptor ligands

The cloning of the opioid receptors and subsequent use of recombinant DNA technology have led to many new insights into ligand binding. Instead of focusing on the structural features that lead to increased affinity and selectivity, researchers are now able to focus on why these features are important. Site-directed mutagenesis and chimeric data have often been at the forefront in answering these questions. Herein, we survey pharmacophores of several opioid ligands in an effort to understand the structural requirements for ligand binding and selectivity. Models are presented and compared to illustrate key sites of recognition for both opiate and nonopiate ligands. The results indicate that different ligand classes may recognize different sites within the receptor, suggesting that multiple epitopes may exist for ligand binding and selectivity.

A 2-week, multicenter, randomized, double-blind, placebo-controlled, dose-ranging, phase III trial comparing the efficacy of oxymorphone extended release and placebo in adults with pain associated with osteoarthritis of the hip or knee

BACKGROUND

Oxymorphone extended release (ER) is a tablet formulation of the mu-opioid agonist oxymorphone designed to achieve a low peak-to-trough fluctuation in plasma concentrations over a 12-hour dosing period.

OBJECTIVE

This study compared the analgesic efficacy, dose response, and tolerability of 3 doses of oxymorphone ER given every 12 hours with those of placebo in patients with pain related to osteoarthritis (OA) of the hip or knee.

METHODS

This was a 2-week, multicenter, randomized, double-blind, placebo-controlled, dose-ranging, Phase III trial. Patients with OA of the hip or knee who were receiving an opioid medication for chronic, moderate to severe pain or who were judged by the investigator to have received suboptimal analgesia with nonopioid analgesics entered a 2- to 7-day washout of analgesic medication. When pain in the index joint was >40 mm on a 100-mm visual analog scale (VAS), patients were randomized to receive 1 of 4 regimens: oxymorphone ER 10 mg q12h during weeks 1 and 2; oxymorphone ER 20 mg q12h in week 1 and 40 mg q12h in week 2; oxymorphone ER 20 mg q12h in week 1 and 50 mg q12h in week 2; or placebo q12h during weeks 1 and 2. The primary end point was the change in VAS score for arthritis pain intensity. Other assessments included the Western Ontario and McMaster Universities (WOMAC) OA Index subscales for pain, stiffness, and physical function and the composite index; the Medical Outcomes Study 36-Item Short-Form Health Survey (SF-36) physical health component summary (PCS) score; the Chronic Pain Sleep Inventory (CPSI) score; vital signs; clinical laboratory parameters; and adverse events (AEs). AEs were recorded at each clinic visit.

RESULTS

Three hundred seventy patients were randomized to treatment (95 oxymorphone ER 10 mg, 93 oxymorphone ER 40 mg, 91 oxymorphone ER 50 mg, and 91 placebo), and 198 completed the study. Least squares mean changes from baseline in the VAS arthritis pain intensity score were -21, -28, -29, and -17 mm in the oxymorphone ER 10, 40, and 50 mg and placebo groups, respectively (P = 0.002, modified Tukey linear trend test). Oxymorphone ER 40 and 50 mg produced significant improvements from baseline compared with placebo in the WOMAC subscale scores for pain (least squares mean change: -85.1, -108.0, and -42.5, respectively; P < or = 0.025 for 40 mg, P < or = 0.001 for 50 mg), stiffness (-40.5, -48.1, and -17.0; both, P < or = 0.001), and physical function (-256.8, -310.8, and -116.5; P < or = 0.01 and P < or = 0.001, respectively); the SF-36 PCS score (4.6, 3.6, and -0.1; P < 0.001); and the CPSI score (-21.2, -22.2, and -10.7; P < 0.05). The 10-mg dose also was associated with significant improvements compared with placebo in the WOMAC pain (-83.6; P < or = 0.025) and physical function subscales (-232.9; P < or = 0.025) and the SF-36 PCS score (3.9; P < 0.001). The most frequently reported AEs (> or =5% of patients) in the oxymorphone ER groups were nausea (39.4%), vomiting (23.7%), dizziness (22.6%), constipation (22.2%), somnolence (17.6%), pruritus (16.5%), and headache (15.0%). The majority of AEs with oxymorphone ER were mild or moderate in intensity. Three serious AEs (urinary retention, central nervous system depression, and pancreatitis) were considered possibly or probably related to study medication.

CONCLUSION

In these patients with chronic, moderate to severe pain related to OA of the hip or knee, oxymorphone ER administered twice daily for 2 weeks produced dose-related reductions in arthritis pain intensity and improvements in physical function.

Molecular modeling and 3D-QSAR studies of indolomorphinan derivatives as kappa opioid antagonists

Molecular modeling and 3D-QSAR studies were performed on 31 indolomorphinan derivatives to evaluate their antagonistic behaviors on kappa opioid receptor and provide information for further modification of this kind of compounds. Best predictions were obtained with CoMFA standard model (q2 = 0.693, N = 4, r2 = 0.900) and CoMSIA combined model (q2 = 0.617, N = 4, r2 = 0.904). Both models were further validated by an external test set of eight compounds with satisfactory predictions: r2 = 0.607 for CoMFA and r2 = 0.701 for CoMSIA. In addition, the 3D structure of human kappa opioid receptor was constructed based on the crystal structure of bovine rhodopsin, and the CoMSIA contour plots were then mapped into the structural model of kappa opioid receptor-GNTI complex to identify key residues, which might account for kappa antagonist potency and selectivity. The roles of nonconserved Glu297 and conserved Lys227 of human kappa opioid receptor were then discussed.

Kappa opioid antagonists: past successes and future prospects

Antagonists of the kappa opioid receptor were initially investigated as pharmacological tools that would reverse the effects of kappa opioid receptor agonists. In the years following the discovery of the first selective kappa opioid antagonists, much information about their chemistry and pharmacology has been elicited and their potential therapeutic uses have been investigated. The review presents the current chemistry, ligand-based structure activity relationships, and pharmacology of the known nonpeptidic selective kappa opioid receptor antagonists. This manuscript endeavors to provide the reader with a useful reference of the investigations made to define the structure-activity relationships and pharmacology of selective kappa opioid receptor antagonists and their potential uses as pharmacological tools and as therapeutic agents in the treatment of disease states.

Kappa opioid antagonists: past successes and future prospects

Antagonists of the kappa opioid receptor were initially investigated as pharmacological tools that would reverse the effects of kappa opioid receptor agonists. In the years following the discovery of the first selective kappa opioid antagonists, much information about their chemistry and pharmacology has been elicited and their potential therapeutic uses have been investigated. The review presents the current chemistry, ligand-based structure activity relationships, and pharmacology of the known nonpeptidic selective kappa opioid receptor antagonists. This manuscript endeavors to provide the reader with a useful reference of the investigations made to define the structure-activity relationships and pharmacology of selective kappa opioid receptor antagonists and their potential uses as pharmacological tools and as therapeutic agents in the treatment of disease states.

Kappa opioid antagonists: past successes and future prospects

Antagonists of the kappa opioid receptor were initially investigated as pharmacological tools that would reverse the effects of kappa opioid receptor agonists. In the years following the discovery of the first selective kappa opioid antagonists, much information about their chemistry and pharmacology has been elicited and their potential therapeutic uses have been investigated. The review presents the current chemistry, ligand-based structure activity relationships, and pharmacology of the known nonpeptidic selective kappa opioid receptor antagonists. This manuscript endeavors to provide the reader with a useful reference of the investigations made to define the structure-activity relationships and pharmacology of selective kappa opioid receptor antagonists and their potential uses as pharmacological tools and as therapeutic agents in the treatment of disease states.

Characterisation of the in vitro modulation of splenocyte proliferation by non-4,5-epoxymorphinan opioids

Opioids, such as morphine, can directly alter immune function via receptors expressed on immunocompetent cells. However, several studies have questioned the classical opioid nature of this change in immune response. Therefore, it is unclear how opioids that are not from the same structural class as morphine (4,5-epoxymorphinan), will modulate the immune system, if they do not behave in a classical opioid manner. Therefore, the aim of this study was to investigate the in vitro modulatory effects of a range of non-4,5-epoxymorphinan opioids on splenocyte proliferation and compare the response characteristics to their central opioid characteristics. The modulation of concanavalin A stimulated mouse splenocyte proliferation by non-4,5-epoxymorphinan opioids resulted in three types of responses: an inhibitory concentration-response curve (e.g. methadone, inhibitory EC(50)=79.4 microM), an inverted bell shaped curve (e.g. fentanyl, inhibitory EC(50)=0.06 microM) and an induction concentration response curve (e.g. nor-binaltorphimine, induction EC(50)=0.16 microM). Non-stereoselectivity, naloxone-insensitivity, naloxone-sensitivity and non-classical opioid rank order of effect were all observed. These data support the non-classical opioid nature of direct opioid modulation of splenocyte proliferation.

Oxymorphone Extended-Release Tablets Relieve Moderate to Severe Pain and Improve Physical Function in Osteoarthritis: Results of a Randomized, Double-Blind, Placebo- and Active-Controlled Phase III Trial

OBJECTIVE

To compare oxymorphone extended release (ER) and placebo on indices of pain, function, and safety in patients with chronic osteoarthritis (OA) pain.

DESIGN

In this multicenter, double-blind, placebo- and active-controlled, parallel-group, dose-ranging study, patients were randomized to oxymorphone ER 20 mg (N = 121), oxymorphone ER 40 mg (N = 121), oxycodone controlled release 20 mg (N = 125), or placebo (N = 124) every 12 hours. The primary efficacy end point was change in arthritis pain intensity (visual analog scale, 0-100) from baseline to week 3 for the oxymorphone ER 40 mg group versus placebo.

RESULTS

The primary end point was achieved: the week 3 oxymorphone ER least squares mean difference (LSMD) from placebo was -9.0 (95% confidence interval [CI]: -16.2 to -1.8; P = 0.015). Secondary efficacy analysis showed similar improvements at week 4 (LSMD from placebo, -10.3 [95% CI: -17.7 to -2.8]; P = 0.007) and with oxymorphone ER 20 mg at week 3 (LSMD from placebo, -7.7 [95% CI: -15.0 to -0.4]; P = 0.039) and week 4 (LSMD from placebo, -7.5 [95% CI: -15.0 to 0.0]; P = 0.050). Weeks 3 and 4 pain intensity decreased by approximately 30-40%. Oxymorphone ER 20 and 40 mg improved from baseline on the Western Ontario and McMaster Universities Osteoarthritis Composite Index and pain and physical function subscales at week 4. Adverse events in all opioid groups included mild to moderate nausea, constipation, and somnolence.

CONCLUSIONS

In this short-term study, oxymorphone ER was superior to placebo for relieving pain and improving function in patients with moderate to severe chronic OA pain, and is an alternative to other sustained-release opioids.

Efficacy and safety of oxymorphone extended release in chronic low back pain: results of a randomized, double-blind, placebo- and active-controlled phase III study

This multicenter, randomized, double-blind, placebo- and active-controlled trial was conducted to compare the analgesic efficacy and safety of oxymorphone extended release (ER) with placebo and oxycodone controlled release (CR) in ambulatory patients with moderate to severe chronic low back pain requiring opioid therapy. Patients (N = 213) aged 18 to 75 years were randomized to receive oxymorphone ER (10 to 110 mg) or oxycodone CR (20 to 220 mg) every 12 hours during a 7- to 14-day dose-titration phase. Patients achieving effective analgesia at a stable opioid dose entered an 18-day double-blind treatment phase and either continued opioid therapy or received placebo. With stable dosing throughout the treatment phase, oxymorphone ER (79.4 mg/day) and oxycodone CR (155 mg/day) were superior to placebo for change from baseline in pain intensity as measured on a visual analog scale; the LS mean differences were -18.21 and 18.55 (95% CI, -25.83 to -10.58 and -26.12 to -10.98, respectively; P = .0001). Use of rescue medication was 20 mg per day. Adverse events for the active drugs were similar; the most frequent were constipation and sedation. Oxymorphone ER and oxycodone CR were generally safe and effective for controlling low back pain. Oxymorphone ER was equianalgesic to oxycodone CR at half the milligram daily dosage, with comparable safety.

PERSPECTIVE

Definitive studies of long-acting opioids in patients with chronic low back pain are lacking. We report the results of a multicenter, randomized, placebo-controlled, double-blind study evaluating the analgesic efficacy and safety of oxymorphone ER and oxycodone CR in opioid-experienced patients with chronic low back pain.

Specific cross-linking of Lys233 and Cys235 in the mu opioid receptor by a reporter affinity label

The first example of the use of a reporter affinity label (NNA) that contains a fluorogenic naphthalene dialdehyde moiety to identify neighboring lysine and cysteine residues at a recognition site is described. The opioid receptors have served as the proof-of-concept because they contain multiple lysine and cysteine residues. The kinetics of isoindole formation resulting from covalent binding of NNA to wild-type and mutant opioid receptors were followed in cultured cells using flow cytometry. The finding that NNA bound to mutant mu opioid receptors (K233R and C235S) without producing specific fluorescence enhancement suggested that covalent bonding occurred at these positions to produce an isoindole fluorophore in the wild-type mu receptor. The similar kinetics of fluorophore formation for wild-type mu, delta, and kappa opioid receptors suggest that these conserved residues are the cross-linking sites in all three types of opioid receptors. The combined utilization of a reporter affinity label and site-directed mutagenesis offers a more expeditious method of identifying cross-linking at a recognition site when compared to classical procedures.

Homology Modeling and Molecular Dynamics Simulations of the Mu Opioid Receptor in a Membrane-Aqueous System

Three types of opioid receptors-mu, delta, and kappa-belong to the rhodopsin subfamily in the G protein-coupled receptor superfamily. With the recent characterization of the high-resolution X-ray crystal structure of bovine rhodopsin, considerable attention has been focused on molecular modeling of these transmembrane proteins. In this study, a homology model of the mu opioid receptor was constructed based on the X-ray crystal structure of bovine rhodopsin. A phospholipid bilayer was built around the receptor, and two water layers were placed on both surfaces of the lipid bilayer. Molecular-dynamics simulations were carried out by using CHARMM for the entire system, which consisted of 316 amino acid residues, 92 phospholipid molecules, 8327 water molecules, and 11 chloride counter ions-40 931 atoms altogether. The whole system was equilibrated for 250 ps followed by another 2 ns dynamic simulation. The opioid ligand naltrexone was docked into the optimized model, and the critical amino acid residues for binding were identified. The mu opioid receptor homology model optimized in a complete membrane-aqueous system should provide a good starting point for further characterization of the binding modes for opioid ligands. Furthermore, the method developed herein will be applicable to molecular model building to other opioid receptors as well as other GPCRs.

Relationship between 4,5-epoxymorphinan structure and in vitro modulation of cell proliferation

Morphine belongs to the class of compounds known as 4,5-epoxymorphinans, which can alter immune function directly via receptors expressed by immune cells. However, the opioid characteristics of these receptors are not clear. Therefore, the aim of this study was to investigate the in vitro immunomodulatory effects of 24 structurally related 4,5-epoxymorphinans to allow further characterisation of the receptor that mediates the immunomodulation and to ascertain if there is any structure-effect relationship. The immunomodulation of 4,5-epoxymorphinans using isolated mouse splenocytes stimulated with concanavalin A resulted in five types of responses: an inverted bell shaped curve (oxycodone, inhibitory EC(50)=1.6 nM), an inhibitory concentration response curve (buprenorphine, inhibitory EC(50)=12.6 microM), an inverted bell-shaped curve with induction (morphine, induction EC(50)=1.7 microM), an induction concentration response curve (oxymorphone, induction EC(50)=20 nM), and the lack of any response (e.g. noroxycodone). Non-stereoselectivity, naloxone-insensitivity, naloxone-sensitivity and non-classical opioid rank order of effect were all observed. A structure-effect relationship was developed and significant evidence for non-classical opioid receptor function on immune cells was concluded.

3D-QSAR Comparative Molecular Field Analysis on Opioid Receptor Antagonists: Pooling Data from Different Studies

Three-dimensional quantitative structure-activity relationship (3D-QSAR) models were constructed using comparative molecular field analysis (CoMFA) on a series of opioid receptor antagonists. To obtain statistically significant and robust CoMFA models, a sizable data set of naltrindole and naltrexone analogues was assembled by pooling biological and structural data from independent studies. A process of "leave one data set out", similar to the traditional "leave one out" cross-validation procedure employed in partial least squares (PLS) analysis, was utilized to study the feasibility of pooling data in the present case. These studies indicate that our approach yields statistically significant and highly predictive CoMFA models from the pooled data set of delta, mu, and kappa opioid receptor antagonists. All models showed excellent internal predictability and self-consistency: q(2) = 0.69/r(2) = 0.91 (delta), q(2) = 0.67/r(2) = 0.92 (mu), and q(2) = 0.60/r(2) = 0.96 (kappa). The CoMFA models were further validated using two separate test sets: one test set was selected randomly from the pooled data set, while the other test set was retrieved from other published sources. The overall excellent agreement between CoMFA-predicted and experimental binding affinities for a structurally diverse array of ligands across all three opioid receptor subtypes gives testimony to the superb predictive power of these models. CoMFA field analysis demonstrated that the variations in binding affinity of opioid antagonists are dominated by steric rather than electrostatic interactions with the three opioid receptor binding sites. The CoMFA steric-electrostatic contour maps corresponding to the delta, mu, and kappa opioid receptor subtypes reflected the characteristic similarities and differences in the familiar "message-address" concept of opioid receptor ligands. Structural modifications to increase selectivity for the delta over mu and kappa opioid receptors have been predicted on the basis of the CoMFA contour maps. The structure-activity relationships (SARs) together with the CoMFA models should find utility for the rational design of subtype-selective opioid receptor antagonists.

The efficacy and safety of oral immediate-release oxymorphone for postsurgical pain

In this double-blind, parallel-group study, we compared 3 oxymorphone immediate-release (IR) doses with placebo for efficacy and with oxycodone IR and placebo for safety in patients with acute moderate-to-severe postsurgical pain. During the single-dose phase (n = 300), patients received oxymorphone IR 10, 20, or 30 mg; oxycodone IR 10 mg; or placebo. All oxymorphone IR doses were superior for providing pain relief for 8 h (P < 0.05), with a significant analgesic dose response (P < 0.001). Significant pain intensity differences occurred by 45 min (20- and 30-mg doses; P < 0.05). Discontinuations for lack of efficacy totaled 42% among placebo-treated patients and 27% among those treated with oxymorphone IR. Patients requiring rescue medication after 3 h were allowed to receive additional study drug every 4 to 6 h as needed for the multiple-dose phase (n = 164). All oxymorphone groups maintained analgesia for 48 h. The median dosing interval was >9.5 h for oxymorphone IR 30 mg and > or =7 h for the other groups. Opioid-related adverse events, similar among groups, were generally mild or moderate. Oxymorphone IR 10, 20, or 30 mg provided significant dose-related pain relief compared with placebo, and this relief was maintained over several days with a safety profile comparable to that of oxycodone IR.

Effectiveness and safety of oral extended-release oxymorphone for the treatment of cancer pain: a pilot study

GOALS OF WORK

Inadequate analgesia and/or unmanageable adverse events frequently result in the need to rotate patients with cancer pain to a different opioid. The availability of a novel oral extended-release (ER) formulation of oxymorphone provides clinicians with another treatment option. In this study, we assessed the analgesic effectiveness and safety of the new oral ER formulation of oxymorphone following treatment with controlled-release (CR) morphine sulfate or oxycodone.

PATIENTS AND METHODS

Adults with moderate to severe cancer pain were stabilized for > or =3 days on morphine CR or oxycodone CR, and then treated for 7 days at their stabilized dose. Drug selection was based upon patients' previous use or investigator preference. Patients were then crossed over for 7 days of treatment at an estimated equianalgesic dosage of oxymorphone ER. Pain was assessed using a visual analog scale, and adverse events were recorded.

MAIN RESULTS

A total of 86 patients entered open-label treatment. Of 34 patients assigned to morphine CR and 52 assigned to oxycodone CR, 21 (61.8%) and 42 (80.8%) completed stabilization and began treatment with oxymorphone ER, respectively; 59 of 63 (93.7%) completed treatment with oxymorphone. There were no significant differences in daily pain intensity scores between oxymorphone ER and comparators (paired t -test). Rescue medication use, expressed as the percent of the daily dose of scheduled opioid, was greater during morphine CR treatment than after crossover to oxymorphone ER (25.2% vs 13.3%; P <0.05, Wilcoxon's test). The tolerability/safety profiles (e.g., nausea, drowsiness, somnolence) were similar for all opioids.

CONCLUSIONS

Cancer patients stabilized on morphine CR or oxycodone CR were safely and rapidly converted to a lower milligram dose of oxymorphone ER that provided adequate pain relief with comparable tolerability. These results justify additional trials with oxymorphone ER.