Probes for narcotic receptor mediated phenomena. 41. Unusual inverse μ-agonists and potent μ-opioid antagonists by modification of the N-substituent in enantiomeric 5-(3-hydroxyphenyl)morphans

3-[3-(Phenalkylamino)cyclohexyl]phenols: Synthesis, biological activity, and in silico investigation of a naltrexone-derived novel class of MOR-antagonists

The development of novel μ-opioid receptor (MOR) antagonists is one of the main objectives of drug discovery and development. Based on a simplified version of the morphinan scaffold, 3-[3-(phenalkylamino)cyclohexyl]phenol analogs were designed, synthesized, and evaluated for their MOR antagonist activity in vitro and in silico. At the highest concentrations, the compounds decreased by 52% to 75% DAMGO-induced GTPγS stimulation, suggesting that they acted as antagonists. Moreover, Extra-Precision Glide and Generalized-Born Surface Area experiments provided useful information on the nature of the ligand-receptor interactions, indicating a peculiar combination of C-1 stereochemistry and N-substitutions as feasibly essential for MOR-ligand complex stability. Interestingly, compound 9 showed the best experimental binding affinity, the highest antagonist activity, and the finest MOR-ligand complex stability. In silico experiments also revealed that the most promising stereoisomer (1R, 3R, 5S) 9 retained 1,3-cis configuration with phenol ring equatorial oriented. Further studies are needed to better characterize the pharmacodynamics and pharmacokinetic properties of these compounds.

Synthesis and Pharmacological Evaluation of Enantiopure N-Substituted Ortho-c Oxide-Bridged 5-Phenylmorphans

The design of enantiopure stereoisomers of N-2-phenylcyclopropylmethyl-substituted ortho-c oxide-bridged phenylmorphans, the E and Z isomers of an N-cinnamyl moiety, and N-propyl enantiomers were based on combining the most potent oxide-bridged phenylmorphan (the ortho-c isomer) with the most potent N-substituent that we previously found with a 5-(3-hydroxy)phenylmorphan (i.e., N-2-phenylcyclopropyl methyl moieties, N-cinnamyl, and N-propyl substituents). The synthesis of the eight enantiopure N-2-phenylcyclopropylmethyl ortho-c oxide-bridged phenylmorphans and six additional enantiomers of the N-substituted ortho-c oxide-bridged phenylmorphans (N-E and Z-cinnamyl compounds, and N-propyl compounds) was accomplished. The synthesis started from common intermediates (3R,6aS,11aS)-10-methoxy-1,3,4,5,6,11a-hexahydro-2H-3,6a-methano-benzofuro[2,3-c]azocine (+)-6 and its enantiomer, (3S, 6aR, 11aR)-(-)-6, respectively. The enantiomers of ±-6 were obtained through salt formation with (S)-(+)- and (R)-(-)-p-methylmandelic acid, and the absolute configuration of the (R)-(-)-p-methylmandelate salt of (3S, 6aR, 11aR)-(-)-6 was determined by single-crystal X-ray analysis. The enantiomeric secondary amines were reacted with N-(2-phenylcyclopropyl)methyl derivatives, 2-(E)-cinnamyl bromide, and (Z)-3-phenylacrylic acid. These products led to all of the desired N-derivatives of the ortho-c oxide-bridged phenylmorphans. Their opioid receptor binding affinity was measured. The compounds with MOR affinity < 50 nM were examined for their functional activity in the forskolin-induced cAMP accumulation assay. Only the enantiomer of the N-phenethyl ortho-c oxide-bridged phenylmorphan ((-)-1), and only the (3S,6aR,11aR)-2-(((1S,2S)-2-phenylcyclopropyl)methyl)-1,3,4,5,6,11a-hexahydro-2H-3,6a-methanobenzofuro[2,3-c]azocin-10-ol isomer ((+)-17), and the N-phenylpropyl derivative ((-)-25) had opioid binding affinity < 50 nM. Both (-)-1 and (-)-25 were partial agonists in the cAMP assay, with the former showing high potency and low efficacy, and the latter with lower potency and less efficacy. Most interesting was the N-2-phenylcyclopropylmethyl (3S,6aR,11aR)-2-(1S,2S)-enantiomer ((+)-17). That compound had good MOR binding affinity (Ki = 11.9 nM) and was found to have naltrexone-like potency as a MOR antagonist (IC50 = 6.92 nM).

Modulation of opioid receptor affinity and efficacy via N-substitution of 9β-hydroxy-5-(3-hydroxyphenyl)morphan: Synthesis and computer simulation study

The enantiomers of a variety of N-alkyl-, N-aralkyl-, and N-cyclopropylalkyl-9β-hydroxy-5-(3-hydroxyphenyl)morphans were synthesized employing cyanogen bromide and K2CO3 to improve the original N-demethylation procedure. Their binding affinity to the μ-, δ-, and κ-opioid receptors (ORs) was determined and functional (GTPγ35S) assays were carried out on those with reasonable affinity. The 1R,5R,9S-enantiomers (1R,5R,9S)-(-)-5-(3-hydroxyphenyl)-2-(4-nitrophenethyl)-2-azabicyclo[3.3.1]nonan-9-ol (1R,5R,9S-16), (1R,5R,9S)-(-) 2-cinnamyl-5-(3-hydroxyphenyl)-2-azabicyclo[3.3.1]nonan-9-ol (1R,5R,9S-20), and (1R,5R,9S)-(-)-5-(3-hydroxyphenyl)-2-(4-(trifluoromethyl)phenethyl)-2-azabicyclo[3.3.1]nonan-9-ol (1R,5R,9S-15), had high affinity for the μ-opioid receptor (e.g., 1R,5R,9S-16: Ki=0.073, 0.74, and 1.99nM, respectively). The 1R,5R,9S-16 and 1R,5R,9S-15 were full, high efficacy μ-agonists (EC50=0.74 and 18.5nM, respectively) and the former was found to be a partial agonist at δ-OR and an antagonist at κ-OR, while the latter was a partial agonist at δ-OR and κ-OR in the GTPγ35S assay. The enantiomer of 1R,5R,9S-16, (+)-1S,5S,9R-16 was unusual, it had good affinity for the μ-OR (Ki=26.5nM) and was an efficacious μ-antagonist (Ke=29.1nM). Molecular dynamics simulations of the μ-OR were carried out with the 1R,5R,9S-16 μ-agonist and the previously synthesized (1R,5R,9S)-(-)-5-(9-hydroxy-5-(3-hydroxyphenyl-2-phenylethyl)-2-azabicyclo[3.3.1]nonane (1R,5R,9S-(-)-NIH 11289) to provide a structural basis for the observed high affinities and efficacies. The critical roles of both the 9β-OH and the p-nitro group are elucidated, with the latter forming direct, persistent hydrogen bonds with residues deep in the binding cavity, and the former interacting with specific residues via highly structured water bridges.

Synthesis of Branched Tryptamines via the Domino Cloke-Stevens/Grandberg Rearrangement

The rearrangement of cyclopropylketone arylhydrazones generated in situ from arylhydrazine hydrochlorides and ketones leads to formation of tryptamine derivatives. The use of (2-arylcyclopropyl)ethanones in the reactions with model 4-bromophenylhydrazine hydrochloride gives branched tryptamines with aryl groups in the α-position to the amino group, while (2-methylcyclopropyl)ethanone gives a mixture of α- and β-substituted products in a ratio of 1:3. The method was found effective in the synthesis of enantiomerically pure tryptamine. Thus, (R,R)-(2-phenylcyclopropyl)ethanone gives the (S)-α-phenyltryptamine derivative with an enantiomeric excess over 99%.

Asymmetric C-H functionalization of cyclopropanes using an isoleucine-NH2 bidentate directing group

The systematic investigation of substrate-bound α-amino acid auxiliaries has resulted in catalytic asymmetric C-H functionalization of cyclopropanes enabled by amino acid amides as chiral bidentate directing groups. The use of an Ile-NH2 auxiliary embedded in the substrate provided excellent levels of asymmetric induction (diastereomeric ratio of up to 72 : 1) in the Pd(ii)-catalyzed β-methylene C(sp3)-H bond activation of cyclopropanes and cross-coupling with aryl iodides.

Synthesis, pharmacological characterization, and structure-activity relationship studies of small molecular agonists for the orphan GPR88 receptor

GPR88 is an orphan G-protein-coupled receptor (GPCR) enriched in the striatum. Genetic deletion and gene expression studies have suggested that GPR88 plays an important role in the regulation of striatal functions and is implicated in psychiatric disorders. The signal transduction pathway and receptor functions of GPR88, however, are still largely unknown due to the lack of endogenous and synthetic ligands. In this paper, we report the synthesis of a GPR88 agonist 2-PCCA and its pure diastereomers, which were functionally characterized in both transiently and stably expressing GPR88 HEK293 cells. 2-PCCA inhibited isoproterenol-stimulated cAMP accumulation in a concentration-dependent manner in cells expressing GPR88 but not in the control cells, suggesting that the observed cAMP inhibition is mediated through GPR88 and that GPR88 is coupled to Gαi. 2-PCCA did not induce calcium mobilization in GPR88 cells, indicating no Gαq-mediated response. A structure-activity relationship (SAR) study of 2-PCCA was also conducted to explore the key structural features for GPR88 agonist activity.

Cocaine dysregulates opioid gating of GABA neurotransmission in the ventral pallidum

The ventral pallidum (VP) is a target of dense nucleus accumbens projections. Many of these projections coexpress GABA and the neuropeptide enkephalin, a δ and μ opioid receptor (MOR) ligand. Of these two, the MOR in the VP is known to be involved in reward-related behaviors, such as hedonic responses to palatable food, alcohol intake, and reinstatement of cocaine seeking. Stimulating MORs in the VP decreases extracellular GABA, indicating that the effects of MORs in the VP on cocaine seeking are via modulating GABA neurotransmission. Here, we use whole-cell patch-clamp on a rat model of withdrawal from cocaine self-administration to test the hypothesis that MORs presynaptically regulate GABA transmission in the VP and that cocaine withdrawal changes the interaction between MORs and GABA. We found that in cocaine-extinguished rats pharmacological activation of MORs no longer presynaptically inhibited GABA release, whereas blocking the MORs disinhibited GABA release. Moreover, MOR-dependent long-term depression of GABA neurotransmission in the VP was lost in cocaine-extinguished rats. Last, GABA neurotransmission was found to be tonically suppressed in cocaine-extinguished rats. These substantial synaptic changes indicated that cocaine was increasing tone on MOR receptors. Accordingly, increasing endogenous tone by blocking the enzymatic degradation of enkephalin inhibited GABA neurotransmission in yoked saline rats but not in cocaine-extinguished rats. In conclusion, our results indicate that following withdrawal from cocaine self-administration enkephalin levels in the VP are elevated and the opioid modulation of GABA neurotransmission is impaired. This may contribute to the difficulties withdrawn addicts experience when trying to resist relapse.

Effect of Iboga alkaloids on µ-opioid receptor-coupled G protein activation

OBJECTIVE

The iboga alkaloids are a class of small molecules defined structurally on the basis of a common ibogamine skeleton, some of which modify opioid withdrawal and drug self-administration in humans and preclinical models. These compounds may represent an innovative approach to neurobiological investigation and development of addiction pharmacotherapy. In particular, the use of the prototypic iboga alkaloid ibogaine for opioid detoxification in humans raises the question of whether its effect is mediated by an opioid agonist action, or if it represents alternative and possibly novel mechanism of action. The aim of this study was to independently replicate and extend evidence regarding the activation of μ-opioid receptor (MOR)-related G proteins by iboga alkaloids.

METHODS

Ibogaine, its major metabolite noribogaine, and 18-methoxycoronaridine (18-MC), a synthetic congener, were evaluated by agonist-stimulated guanosine-5´-O-(γ-thio)-triphosphate ([(35)S]GTPγS) binding in cells overexpressing the recombinant MOR, in rat thalamic membranes, and autoradiography in rat brain slices.

RESULTS AND SIGNIFICANCE

In rat thalamic membranes ibogaine, noribogaine and 18-MC were MOR antagonists with functional Ke values ranging from 3 uM (ibogaine) to 13 uM (noribogaine and 18MC). Noribogaine and 18-MC did not stimulate [(35)S]GTPγS binding in Chinese hamster ovary cells expressing human or rat MORs, and had only limited partial agonist effects in human embryonic kidney cells expressing mouse MORs. Ibogaine did not did not stimulate [(35)S]GTPγS binding in any MOR expressing cells. Noribogaine did not stimulate [(35)S]GTPγS binding in brain slices using autoradiography. An MOR agonist action does not appear to account for the effect of these iboga alkaloids on opioid withdrawal. Taken together with existing evidence that their mechanism of action also differs from that of other non-opioids with clinical effects on opioid tolerance and withdrawal, these findings suggest a novel mechanism of action, and further justify the search for alternative targets of iboga alkaloids.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).