Peripheral antinociceptive effects of a bifunctional µ and δ opioid receptor ligand in rat model of inflammatory bladder pain.

There is a need to develop a novel analgesic for pain associated with interstitial cystitis/painful bladder syndrome (IC/PBS). The use of the conventional µ-opioid receptor agonists to manage IC/PBS pain is controversial due to adverse CNS effects. These effects are attenuated in benzylideneoxymorphone (BOM), a low-efficacy µ-opioid receptor agonist/δ-opioid receptor antagonist that attenuates thermal pain and is devoid of reinforcing effects. We hypothesize that BOM will inhibit bladder pain by attenuating responses of urinary bladder distension (UBD)-sensitive afferent fibers. Therefore, the effect of BOM was tested on responses of UBD-sensitive afferent fibers in L6 dorsal root from inflamed and non-inflamed bladder of rats. Immunohistochemical (IHC) examination reveals that following the induction of inflammation there were significant high expressions of µ, δ, and µ-δ heteromer receptors in DRG. BOM dose-dependently (1-10 mg/kg, i.v) attenuated mechanotransduction properties of these afferent fibers from inflamed but not from non-inflamed rats. In behavioral model of bladder pain, BOM significantly attenuated visceromotor responses (VMRs) to UBD only in inflamed group of rats when injected either systemically (10 mg/kg, i.v.) or locally into the bladder (0.1 ml of 10 mg/ml). Furthermore, oxymorphone (OXM), a high-efficacy µ-opioid receptor agonist, attenuated responses of mechanosensitive bladder afferent fibers and VMRs to UBD. Naloxone (10 mg/kg, i.v.) significantly reversed the inhibitory effects of BOM and OXM on responses of bladder afferent fibers and VMRs suggesting µ-opioid receptor-related analgesic effects of these compounds. The results reveal that a low-efficacy, bifunctional opioid-based compound can produce analgesia by attenuating mechanotransduction functions of afferent fibers innervating the urinary bladder.

Behavioral effects of benzylideneoxymorphone (BOM), a low efficacy µ opioid receptor agonist and a δ opioid receptor antagonist.

RATIONALE: Opioids remain the drugs of choice for treating moderate to severe pain, although adverse effects often limit use. Drugs acting concomitantly as agonists at µ opioid receptors and antagonists at δ opioid receptors produce antinociceptive effects with a reduced profile of adverse effects; one such drug, benzylideneoxymorphone (BOM), might further limit adverse effects because it appears to have lower pharmacological efficacy than other µ opioid receptor agonists. OBJECTIVES: The current study compared the acute behavioral effects of BOM with the effects of other µ opioid receptor agonists. METHODS: Discriminative stimulus and rate-decreasing effects were studied in 1 group of 7 rats discriminating 3.2 mg/kg morphine while responding under a fixed-ratio 10 schedule of food presentation. Antinociceptive effects were determined in a second group of 8 rats using a warm water tail withdrawal procedure. Reinforcing effects were evaluated in a third group of 12 rats with a history of remifentanil self-administration. RESULTS: BOM produced morphine-lever responding and both discriminative stimulus and rate-decreasing effects were antagonized by naltrexone. BOM did not markedly increase tail-withdrawal latencies from water maintained at 50 °C and did not substantially attenuate the antinociceptive effects of morphine. BOM was not self-administered and did not change remifentanil self-administration. CONCLUSIONS: Some effects of BOM (e.g., discriminative stimulus effects) appear to be mediated by µ opioid receptors; however, BOM is not self-administered by rats, suggesting that it might have limited abuse liability and a reduced profile of adverse effects compared with currently prescribed opioids.

Synthesis, Biological, and Structural Explorations of New Zwitterionic Derivatives of 14- O-Methyloxymorphone, as Potent µ/δ Opioid Agonists and Peripherally Selective Antinociceptives.

Herein, the synthesis and pharmacological characterization of an extended library of differently substituted N-methyl-14- O-methylmorphinans with natural and unnatural amino acids and three dipeptides at position 6 that emerged as potent µ/δ opioid receptor (MOR/DOR) agonists with peripheral antinociceptive efficacy is reported. The current study adds significant value to our initial structure-activity relationships on a series of zwitterionic analogues of 1 (14- O-methyloxymorphone) by targeting additional amino acid residues. The new derivatives showed high binding and potent agonism at MOR and DOR in vitro. In vivo, the new 6-amino acid- and 6-dipeptide-substituted derivatives of 1 were highly effective in inducing antinociception in the writhing test in mice after subcutaneous administration, which was antagonized by naloxone methiodide demonstrating activation of peripheral opioid receptors. Such peripheral opioid analgesics may represent alternatives to presently available drugs for a safer pain therapy.

Benzylideneoxymorphone: A new lead for development of bifunctional mu/delta opioid receptor ligands.

Opioid analgesic tolerance remains a considerable drawback to chronic pain management. The finding that concomitant administration of delta opioid receptor (DOR) antagonists attenuates the development of tolerance to mu opioid receptor (MOR) agonists has led to interest in producing bifunctional MOR agonist/DOR antagonist ligands. Herein, we present 7-benzylideneoxymorphone (6, UMB 246) displaying MOR partial agonist/DOR antagonist activity, representing a new lead for designing bifunctional MOR/DOR ligands.

An open-label pharmacokinetic study of oxymorphone extended release in the presence of naltrexone in the older adult.

OBJECTIVE: The aging population generally has greater need for analgesics and is best served by having as many good therapeutic options as possible. Geriatric analgesia requires special consideration of age-associated physiologic changes that can affect drug dosing. The study of extended-release (ER) oxymorphone in older (≥ 65 years of age) versus younger (18-40 years of age) male and female volunteers was described. METHODS: In this multiple-dose, parallel-group, open-label trial, healthy volunteers received a single oral dose of 20 mg oxymorphone ER on day 1, followed by a 48-hour washout period, then two oral doses of 20 mg oxymorphone ER tablets every 12 hours from day 3 to day 8, and a single oral dose of 20 mg oxymorphone ER on day 9. Naltrexone was administered each day to the subjects. RESULTS: The elderly had significantly higher plasma levels of oxymorphone, 6-OH-oxymorphone, and oxymorphone-3-glucuronide than the younger group (1.36-fold higher area under the concentration versus time curve [AUC] and 1.45-fold higher C(max)) when they were treated with a single dose (20 mg) of oxymorphone. Steady-state AUC and C(max) also were higher in the older group. Following adjustment for body weight, AUC values for oxymorphone and its metabolites were about 40 percent higher and the mean C(max) values were 30-35 percent higher in the older group compared to the younger group. CONCLUSION: The results of the current study of an ER formulation revealed no pharmacokinetic features that would preclude dosing in the elderly. As with any drug and any age group (but particularly the elderly), oxymorphone ER should be initiated at lower doses in elderly compared to younger patients and titrated to optimal level.

Simultaneous determination of oxymorphone and its active metabolite 6-OH-oxymorphone in human plasma by high performance liquid chromatography-tandem mass spectrometry.

A selective high performance liquid chromatography-tandem mass spectrometric (LC-MS/MS) method for the simultaneous determination of oxymorphone and its active metabolite 6-OH-oxymorphone in human plasma was developed and validated using oxymorphone-d(3) as the internal standard. Chromatographic conditions were optimized to separate oxymorphone from the other metabolite, oxymorphone-3-glucuronide, which may convert to oxymorphone in MS ion source, resulting in inaccurate quantitation of oxymorphone. Solid phase extraction (SPE) was used to extract oxymorphone and 6-OH-oxymorphone from plasma. SPE offered the advantage of being able to remove the unwanted metabolite, oxymorphone-3-glucuronide, through the wash step during the extraction. The developed method was precise and reproducible as shown by good linearity of calibration curves (correlation coefficients ≥0.9968 for oxymorphone and ≥0.9967 for 6-OH-oxymorphone) with high intraday assay and interday assay precision (CV% ≤11.0% for oxymorphone and ≤12.6% for 6-OH-oxymorphone) over a range of 35/25 - 5000/5000 pg/mL for oxymorphone/6-OH-oxymorphone. The method has been successfully applied to analyze oxymorphone and 6-OH-oxymorphone in plasma from 19 healthy volunteers in a bioequivalence study. A total of 1026 samples were analyzed. Good linearity (average correlation coefficient 0.9988 for oxymorphone and 0.9966 for 6-OH-oxymorphone) was achieved with calibration curves and high precision (CV% ≤5.9% for oxymorphone and ≤10.9% for 6-OH-oxymorphone) was obtained with QCs.

Recent advances in the development of 14-alkoxy substituted morphinans as potent and safer opioid analgesics.

Morphine and other opioid morphinans produce analgesia primarily through µ opioid receptors (MORs), which mediate beneficial but also non-beneficial actions. There is a continued search for efficacious opioid analgesics with reduced complications. The cornerstone in the development of 14-alkoxymorphinans as novel analgesic drugs was the synthesis of the highly potent MOR agonist 14-O-methyloxymorphone. This opioid showed high antinociceptive potency but also the adverse effects associated with morphine type compounds. Further developments represent the introduction of a methyl and benzyl group at position 5 of 14-O-methyloxymorphone leading to the strong opioid analgesics 14-methoxymetopon and its 5-benzyl analogue, which exhibited less pronounced side effects than morphine although interacting selectively with MORs. Introduction of arylalkyl substituents such as phenylpropoxy in position 14 led to a series of extremely potent antinociceptive agents with enhanced affinities at all three opioid receptor types. During the past years, medicinal chemistry and opioid research focused increasingly on exploring the therapeutic potential of peripheral opioid receptors by peripheralization of opioids in order to minimize the occurrence of centrally-mediated side effects. Strategies to reduce penetration to the central nervous system (CNS) include chemical modifications that increase hydrophilicity. Zwitterionic 6-amino acid conjugates of 14-Oalkyloxymorphones were developed in an effort to obtain opioid agonists that have limited access to the CNS. These compounds show high antinociceptive potency by interacting with peripheral MORs. Opioid drugs with peripheral site of action represent an important target for the treatment of severe and chronic pain without the adverse actions of centrally acting opioids.

Generation of novel radiolabeled opiates through site-selective iodination.

Tritiated opioid radioligands have proven valuable in exploring opioid binding sites. However, tritium has many limitations. Its low specific activity and limited counting efficiency makes it difficult to examine low abundant, high affinity sites and its disposal is problematic due to the need to use organic scintillants and its relatively long half-life. To overcome these issues, we have synthesized both unlabeled and carrier-free radioiodinated iodobenzoyl derivatives of 6ß-naltrexamine ((125)I-BNtxA, 18), 6ß-naloxamine ((125)I-BNalA, 19) and 6ß-oxymorphamine ((125)I-BOxyA, 20) with specific activities of 2100Ci/mmol. To optimize the utility of the radioligand, we designed a synthesis in which the radiolabel is incorporated in the last synthetic step, which required the selective iodination of the benzoyl moiety without incorporation into the phenolic A ring. Competition studies demonstrated high affinity of the unlabelled compounds for opioid receptors in transfected cell lines, as did the direct binding of the (125)I-ligands to the opioid receptors. The radioligand displayed very high sensitivity, enabling a marked reduction in tissue, as well as excellent signal/noise characteristics. These new (125)I-radioligands should prove valuable in future studies of opioid binding sites.

Novel approach to demonstrate high efficacy of mu opioids in the rat vas deferens: a simple model of predictive value.

14-O-Methyloxymorphone and 14-methoxymetopon were reported as highly selective and potent micro opioid receptor agonists. The aim of this study was to demonstrate the opioid activity of these compounds in vitro and in vivo in comparison to oxymorphone, morphine and DAMGO. The micro opioid receptor efficacy, full or partial agonist nature of opioids was analyzed in the rat vas deferens (RVD) bioassay. Compared to oxymorphone, 14-O-methyloxymorphone and 14-methoxymetopon showed greater affinities to the rodent brain micro opioid receptors in receptor binding assays. In isolated organs 14-O-methyloxymorphone and 14-methoxymetopon were 3-10-fold more potent than the micro agonist opioid peptide, DAMGO. All tested compounds reached at least 70% maximum inhibition in mouse vas deferens (MVD) except morphine and oxymorphone. In the RVD, morphine could not exceed 50% inhibition of the twitches while 14-O-methyloxymorphone and 14-methoxymetopon showed inhibitory effects more than 70%. Oxymorphone reached only 4% maximal agonist effect and antagonized the inhibitory effect of DAMGO. The investigated morphinans produced dose-dependent antinociceptive activities in mice and rats. Both, 14-O-methyloxymorphone and 14-methoxymetopon are highly efficacious micro opioid receptor agonists in the RVD exhibiting full micro agonist properties. The RVD tissue contains mu receptors indicated by the comparable K(e) values of the micro antagonist naltrexone against DAMGO in the MVD. RVD may be a good alternative to assess the mu receptor efficacy of opioid agonists providing a more physiological environment for the ligand-receptor interaction than other efficacy measuring methods such as the [(35)S]GTPgammaS binding assay.

Comparison of the developmental effects of 5-methoxy-N,N-diisopropyltryptamine (Foxy) to (+/-)-3,4-methylenedioxymethamphetamine (ecstasy) in rats.

RATIONALE: We have previously shown that (+/-)-3,4-methylenedioxymethamphetamine (MDMA) treatment from postnatal days (P)11 to P20 leads to learning and memory deficits when the animals are tested as adults. Recently, the club drug 5-methoxy-N,N-diisopropyltryptamine (5-MeO-DIPT) has gained popularity. OBJECTIVE: Due to the similarities between MDMA and 5-MeO-DIPT and the substitution of 5-MeO-DIPT for MDMA, the purpose of this study was to compare the developmental effects of these drugs. METHODS: Within a litter, animals were treated from P11 to P20 with either MDMA, 5-MeO-DIPT, or saline. RESULTS: MDMA-treated animals showed increased anxiety in a measure of defensive marble burying, as well as deficits in spatial and path integration learning. 5-MeO-DIPT-treated animals showed spatial learning deficits; however, there were no deficits observed in spatial memory or path integration learning. 5-MeO-DIPT-treated animals also showed hyperactivity in response to a challenge dose of methamphetamine. CONCLUSIONS: The results show that treatment with either 5-MeO-DIPT or MDMA during development results in cognitive deficits and other behavioral changes but the pattern of effects is distinct for each drug.

Effectiveness of opioids in the treatment of chronic non-cancer pain.

For thousands of years, opioids have been used to treat pain, and they continue to be one of the most commonly prescribed medications for pain. It is estimated that 90% of patients presenting to pain centers and receiving treatment in such facilities are on opioids. Opioids can be considered broad-spectrum analgesics that act at multiple points along the pain pathway. Unfortunately, opioids also have the potential for great harm, with multiple side effects and potential complications, some of which are lethal. They are also uniquely addictive, which can lead to misuse and diversion. We reviewed the relevant English literature and did thorough manual searches of the bibliographies of known primary and review articles. We utilized pain relief as the primary outcome measure. Other outcome measures were functional improvement, improvement of psychological status, improvement in work status, and evidence of addiction. Short-term use and improvement was defined as less than 6 months and long-term relief was defined as 6 months or longer. The 3 systematic reviews evaluating long-term effectiveness of opioids for chronic non-cancer pain provided unclear and weak evidence. The results of this review showed that many patients in the included studies were dissatisfied with adverse events or insufficient pain relief from opioids and withdrew from the studies. For patients able to continue on opioids, evidence was weak suggesting that their pain scores were lower than before therapy and that this relief could be maintained long-term (> 6 months). There was also weak evidence that long-term opioid therapy with morphine and transdermal fentanyl not only decreases pain but also improves functioning. Limited evidence was available for the most commonly used opioids, oxycodone and hydrocodone. Evidence for the ability to drive on chronic opioid therapy was moderate without major side effects or complications. It is concluded that, for long-term opioid therapy of 6 months or longer in managing chronic non-cancer pain, with improvement in function and reduction in pain, there is weak evidence for morphine and transdermal fentanyl. However, there is limited or lack of evidence for all other controlled substances, including the most commonly used drugs, oxycodone and hydrocodone.

Rapid quantification of urinary oxycodone and oxymorphone using fast gas chromatography-mass spectrometry.

Human urine specimens that were determined to be presumptively positive for oxycodone and its metabolite, oxymorphone, by immunoassay screening were assayed using fast gas chromatography-mass spectrometry to positively identify and quantify the oxycodone and oxymorphone present. Urine specimens were first spiked with deuterated internal standards, oxycodone-d(3) and oxymorphone-d(3), subjected to acid hydrolysis, and then extracted using a positive-pressure manifold and mixed-bed solid-phase cartridge extraction methodology. Extracts were derivatized using methoxylamine and acetic anhydride. The acetylated-oxime derivatives of oxycodone and oxymorphone were identified and quantified using a selective ion monitoring (SIM). The method was found to be linear for both analytes to 1600 ng/mL, and limits of detection for oxycodone and oxymorphone were found to be 40 ng/mL and 20 ng/mL, respectively. Interlaboratory data comparisons (n = 40) showed correlation coefficients of 0.9999 and 0.9997 for oxycodone and oxymorphone, respectively. Twelve semisynthetic, structurally similar compounds at concentrations of 5000 ng/mL were assayed in the presence of oxycodone and oxymorphone and found not to interfere with identification and quantitation by this method. Finally, exact mass and tandem mass spectrometry techniques were employed to elucidate the structures of the SIM ions.

Absence of conditioned place preference or reinstatement with bivalent ligands containing mu-opioid receptor agonist and delta-opioid receptor antagonist pharmacophores.

Treatment of pain with opioids is limited by their potential abuse liability. In an effort to develop analgesics without this side effect, a series of bivalent ligands containing a mu-opioid receptor agonist pharmacophore connected to a delta-opioid receptor antagonist pharmacophore through variable-length spacers (16-21 atoms) was synthesized. Members of this series [mu-opioid receptor (M)-delta-opioid receptor (D)-agonist (A)-antagonists (N): MDANs] are antinociceptive in the tail flick assay, but antinociceptive tolerance and physical dependence do not develop to ligands having spacers with 19-21 atoms. The current studies compared the rewarding properties of three bivalent ligands (MDAN-16, -19 and -21) and a mu-opioid receptor agonist (MA-19) to those of morphine in the conditioned place preference assay in mice after i.v. administration. Place preference developed to morphine and to MA-19, but not to the MDANs. The responses to MDAN-16 were highly variable, although place preference of borderline significance appeared to develop. Reinstatement was also evaluated after extinguishing morphine conditioned place preference; morphine and MA-19, but not the MDANs, reinstated morphine conditioned place preference. Taken together, these results suggest that the bivalents are less rewarding compared to morphine in opioid-naïve mice and do not induce reinstatement in previously morphine-preferring mice. The lack of a conditioned place preference response for MDAN-19 and -21, compared to the equivocal results with MDAN-16, suggests a minimum distance requirement between mu-opioid receptor and delta-opioid receptor recognition sites. This requirement may reflect the binding of MDAN-19 and -21 to mu-opioid receptor-delta-opioid receptor heterodimeric receptors that block reward but not antinociception.

Local peripheral antinociceptive effects of 14-O-methyloxymorphone derivatives in inflammatory and neuropathic pain in the rat.

Antinociception achieved after peripheral administration of opioids has opened a new approach to the treatment of severe and chronic pain. Additionally, opioid analgesics with restricted access to the central nervous system could improve safety of opioid drugs used in clinical practice. In the present study, peripheral components of antinociceptive actions of 6-amino acid-substituted derivatives of 14-O-methyloxymorphone were investigated after local intraplantar (i.pl.) administration in rat models of inflammatory and neuropathic pain. Their antinociceptive activities were compared with those of morphine, the classical mu-opioid receptor agonist. Intraplantar administration of morphine and the 6-amino acid derivatives produced dose-dependent reduction of formalin-induced flinching of the inflamed paw, without significant effect on the paw edema. Local i.pl. administration of the new derivatives in rats with neuropathic pain induced by sciatic nerve ligation produced antiallodynic and antihyperalgesic effects; however, the antinociceptive activity was lower than that observed in inflammatory pain. In both models, the 6-amino acid derivatives and morphine at doses that produced analgesia after i.pl. administration were systemically (s.c.) much less active indicating that the antinociceptive action is due to a local effect. Moreover, the local opioid antinociceptive effects were significantly attenuated by naloxone methiodide, a peripherally acting opioid receptor antagonist, demonstrating that the effect was mediated by peripheral opioid receptors. The present data indicate that the peripherally restricted 6-amino acid conjugates of 14-O-methyloxymorphone elicit antinociception after local administration, being more potent in inflammatory than in neuropathic pain. Opioid drugs with peripheral site of action can be an important target for the treatment of long lasting pain.

Synthesis and biological evaluation of 14-alkoxymorphinans. 22.(1) Influence of the 14-alkoxy group and the substitution in position 5 in 14-alkoxymorphinan-6-ones on in vitro and in vivo activities.

Novel 14-alkoxy-substituted (e.g. allyloxy, benzyloxy, naphthylmethoxy) morphinan-6-one derivatives were synthesized and biologically evaluated. Compounds 6-9 and 11 displayed affinities in the subnanomolar range to mu opioid receptors which were comparable to 14-O-methyloxymorphone (1) and 14-methoxymetopon (3), and higher than oxymorphone (2). Opioid binding affinity was sensitive to the character and length of the substituent in position 14. In smooth muscle preparations they behaved as potent agonists. Antinociceptive potencies of compounds 6-11 in the hot-plate test after sc administration in mice were considerably greater than the potency of morphine. In the colonic propulsion test, the most potent analgesic compound 7 showed negligible constipating activity at the analgesic dose. These findings provide further evidence that the nature of the substituent at position 14 has a major impact on the abilities of morphinans to interact with opioid receptors. Introduction of a 5-methyl group has no significant effect on in vitro biological activities, but resulted in decreased antinociceptive potency.

Pharmacokinetics and dose-proportionality of oxymorphone extended release and its metabolites: results of a randomized crossover study.

STUDY OBJECTIVE: To evaluate the pharmacokinetics and dose-proportionality of four dose strengths (5, 10, 20, and 40 mg) of oxymorphone extended release (ER) under both single-dose and steady-state conditions. DESIGN: Randomized, three-period, four-sequence, crossover study. SETTING: Bioavailability clinic. SUBJECTS: Twenty-four healthy adult volunteers. INTERVENTION: Each subject received three of the four possible doses. The three 8-day administration periods were separated by a 7-day washout. Plasma was collected for up to 48 hours after a single dose on day 1 and during a 12-hour dosage interval at steady state. Naltrexone was administered to reduce opioid-related adverse effects. MEASUREMENTS AND MAIN RESULTS: Twenty-three subjects completed at least one study period. Dose-proportionality and linearity were confirmed after single doses (mean oxymorphone ER area under the concentration versus time curve [AUC] 4.54, 8.94, 17.80, and 37.90 ng x hr/ml for 5-, 10-, 20-, and 40-mg doses, respectively) and at steady state (mean oxymorphone ER AUC 5.60, 9.77, 19.3, and 37.0 ng x hr/ml for 5-, 10-, 20-, and 40-mg doses every 12 hrs, respectively). Similar results were found for maximum plasma concentration. Metabolite (6-hydroxyoxymorphone and oxymorphone-3-glucuronide) plasma levels also increased in a linear fashion after single-dose administration and at steady state. CONCLUSION: The pharmacokinetic profile of oxymorphone ER demonstrates linearity and dose-proportionality under single-dose and steady-state conditions for the parent compound and its metabolites for doses of 5-40 mg.

Antagonism in opioid peptides: the role of conformation.

The availability of new, highly selective antagonists, in the field of opioid peptides and of other pain peptides, is important both for a better understanding of the interaction of the receptors with their ligands and for their practical relevance. The design of antagonists is not obvious even when the essential features of agonists are well known. In this review we have examined the main aspects of the problem using, as leading criteria two theoretical models of antagonism and the subdivision of opioid peptides into two functional domains. The main causes of antagonism have been integrated in two very general models: one, referred to as the participation model, attributes antagonism to the lack, with respect to the parent agonist, of an essential group, whereas another model, attributes antagonism to the misfit of the molecule inside the receptor. The second criterion is the division of the structure of peptide hormones, originally put forward by Robert Schwyzer, in two functional domains, the message domain, which is responsible of the larger part of the binding affinity of opioid agonists, and an address domain, which dictates most of the peptide specificity. The most significant achievements in the design of opioid antagonists are classified according to the relative importance of chemical constitution, conformation and chirality.

Derivatives of 17-(2-methylallyl)-substituted noroxymorphone: variation of the delta address and its effects on affinity and selectivity for the delta opioid receptor.

In an effort to establish the importance of the N-(2-methylallyl) substituent in the noroxymorphone series, several derivatives have been synthesized, retaining that N-substituent and modifying the delta address moiety. A few compounds showed moderate binding affinity and selectivity for the delta receptor; none displayed a pharmacological profile as exceptional as N-(2-methylallyl)noroxymorphindole. A second study showed that 3-O-methylation of all derivatives decreases binding affinity. The present results indicate that only a combination of the N-(2-methylallyl) group and an indole delta address provided high selectivity for the delta receptor.

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

The delta-selective opioid antagonist naltrindole (NTI), as well as the kappa-selective opioid antagonists norbinaltorphimine (norBNI) and 5'-guanidinonaltrindole (GNTI), are derived from naltrexone, a universal opioid antagonist. Previous studies have indicated that extracellular loop III is the key region for discrimination by naltrexone-derived selective ligands between the delta, mu, and kappa opioid receptor types. It has been proposed that selective ligands could bind to all three receptor types if the appropriate portions of the extracellular loops were eliminated. To investigate this possibility, several single-point mutant opioid receptors have been generated with the aim of conferring enhanced affinity of selective ligands for their nonpreferred receptor types. Mutations were made in all three types of opioid receptors with the focus on two positions at the extracellular end of transmembrane regions (TM) VI and VII. It was found that the delta-selective NTI could bind both mu and kappa receptors with significantly enhanced affinity when an aromatic residue in TM VII was replaced with alanine (mu[W318A] and kappa[Y312A]). Similarly, kappa-selective antagonists, norBNI and GNTI, showed enhanced affinity for the mu[W318A] mutant and for both mu and delta receptors when a glutamate residue was incorporated into the extracellular end of TM VI (mu[K303E] and delta[W284E]). These results demonstrate that naltrexone-derived selective ligands achieve their selectivity via a combination of enhanced affinity of the address for a particular subsite along with loss of affinity due to steric interference at nonpreferred types. The results reveal key residues in the "address" recognition locus that contribute to the selectivity of opioid ligands and support the hypothesis that recognition of the naltrexone moiety is essentially the same for all three receptor types.

A three-dimensional model of the delta-opioid pharmacophore: comparative molecular modeling of peptide and nonpeptide ligands.

A comparative molecular modeling study of delta-opioid ligands was performed under the assumption that potent peptide and nonpeptide agonists may have common three-dimensional (3D) arrangement of pharmacophore groups upon binding to the delta-receptor. Low-energy conformations of the agonists 7-spiroindanyloxymorphone (SIOM) and 2-methyl-4a-alpha-(3-hydroxyphenyl)-1,2,3,4,4a,5,12, 12a-alpha-octahydro-quinolino[2,3,3-g]isoquinoline (TAN-67), and a partial agonist oxomorphindole (OMI) were determined by high-temperature molecular dynamics (MD). A good spatial overlap was found for the pharmacophore groups of SIOM, TAN-67, and OMI, including the basic nitrogen, phenol hydroxyl, and two aromatic ring. Based on this overlap we proposed a 3D pharmacophore model for nonpeptide delta-opioid agonists with a distance of 7.0 +/- 1.3 A between the two aromatic rings and of 8.2 +/- 1.0 A between the nitrogen and phenyl ring. The potent and highly delta-opioid receptor selective agonist [(2S,3R)-TMT(1)]DPDPE, which shares global backbone constraints of the 14-membered disulfide cycle and a strong preference for the trans rotamer of the TMT(1) side chain, was chosen as a peptide template of the delta-opioid pharmacophore. Extensive MD simulations at 300 K with the AMBER force field were performed for [(2S,3R)-TMT(1)]DPDPE and the less potent [(2S, 3S)-TMT(1)]DPDPE analogue. Multiple MD trajectories were collected for each peptide starting from the x-ray structures of DPDPE and [L-Ala(3)]DPDPE and from models proposed in the literature. Low-energy MD conformations were filtered by the nonpeptide pharmacophore query and then directly superimposed with SIOM, OMI, and TAN-67. Two conformers of [(2S,3R)-TMT(1)]DPDPE that showed the best overlap with the nonpeptide pharmacophore (rms deviation