Effect of the 3- and 4-methyl groups on the opioid receptor properties of N-substituted trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidines

Searching for Synthetic Opioid Rescue Agents: Identification of a Potent Opioid Agonist with Reduced Respiratory Depression

While in the process of designing more effective synthetic opioid rescue agents, we serendipitously identified a new chemotype of potent synthetic opioid. Here, we report that conformational constraint of a piperazine ring converts a mu opioid receptor (MOR) antagonist into a potent MOR agonist. The prototype of the series, which we have termed atoxifent (2), possesses potent in vitro agonist activity. In mice, atoxifent displayed long-lasting antinociception that was reversible with naltrexone. Repeated dosing of atoxifent produced antinociceptive tolerance and a level of withdrawal like that of fentanyl. In rats, while atoxifent produced complete loss of locomotor activity like fentanyl, it failed to produce deep respiratory depression associated with fentanyl-induced lethality. Assessment of brain biodistribution demonstrated ample distribution of atoxifent into the brain with a Tmax of approximately 0.25 h. These results indicate enhanced safety for atoxifent-like molecules compared to fentanyl.

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.

Design, Synthesis, and Biological Evaluation of Structurally Rigid Analogues of 4-(3-Hydroxyphenyl)piperidine Opioid Receptor Antagonists

In order to gain additional information concerning the active conformation of the N-substituted trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidine (1) class of opioid receptor antagonists, procedures were developed for the synthesis of structurally rigid N-substituted-6-(3-hydroxyphenyl)3-azabicyclo[3.1.0]hexane and 3-methyl-4-(3-hydroxyphenyl)-4-azabicyclo[4.1.0]heptanes. Evaluation of the conformationally constrained series in a [³⁵S]GTPγS assay showed that structural rigid compounds having the 3-hydroxyphenyl group locked in the piperidine equatorial orientation had potencies equal to or better than similar compounds having more flexible structures similar to 1. The studies of the rigid compounds also suggested that the 3-methyl group present in compound 1 type antagonists may not be necessary for their pure opioid antagonist properties.

Design, synthesis, and pharmacological evaluation of JDTic analogs to examine the significance of the 3- and 4-methyl substituents

The design and discovery of JDTic as a potent and selective kappa opioid receptor antagonist used the N-substituted trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidine pharmacophore as the lead structure. In order to determine if the 3-methyl or 4-methyl groups were necessary in JDTic and JDTic analogs for antagonistic activity, compounds 4a-c, and 4d-f which have either the 3-methyl or both the 3- and 4-methyl groups removed, respectively, from JDTic and analogs were synthesized and evaluated for their in vitro opioid receptor antagonist activities using a [(35)S]GTPγS binding assay. Other ADME properties were also assessed for selected compounds. These studies demonstrated that neither the 3-methyl or 3,4-dimethyl groups present in JDTic and analogs are required to produce potent and selective κ opioid receptor antagonists.

Discovery of the first small-molecule opioid pan antagonist with nanomolar affinity at mu, delta, kappa, and nociceptin opioid receptors

The trans-(3R,4R)-dimethyl-4-(3-hydroxyphenyl)piperidine scaffold is a known pharmacophore for mu opioid (MOP), kappa opioid (KOP), and delta opioid (DOP) receptor antagonists; however, it has not been explored in nociceptin opioid (NOP/ORL-1) receptor ligands. We recently found that the selective KOP antagonist JDTic, (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, containing this opioid antagonist pharmacophore, has significant binding affinity at the NOP receptor (Ki 16.67 ± 0.76 nM), with no intrinsic activity in the [(35)S]GTPγS functional assay. Since this is the first ligand containing the trans-(3R,4R)-dimethyl-4-(3-hydroxyphenyl)piperidine opioid antagonist pharmacophore to have affinity for the NOP receptor, we explored the structural determinants of its NOP binding affinity. When rational chemical modifications of JDTic were carried out, based on our previously established NOP pharmacophoric structure-activity relationship (SAR) model, most modifications led to a significant decrease in NOP and opioid binding affinity compared to JDTic. Interestingly, however, removal of the 3,4-dimethyl groups of the trans-(3R,4R)-dimethyl-4-(3-hydroxyphenyl)piperidine antagonist scaffold of JDTic increased the binding affinity at NOP by 10-fold (Ki 1.75 ± 0.74 nM) while maintaining comparable affinity for KOP, MOP, and DOP receptors (Ki 1.14 ± 0.63, 1.67 ± 0.6, and 19.6 ± 1.3 nM, respectively). In vitro functional efficacy studies using the [(35)S]GTPγS assay showed that this compound AT-076 functions as an antagonist at all four opioid receptors. Detailed characterization of the antagonist activity of AT-076 shows that it has a noncompetitive antagonist profile at the NOP and KOP receptors (insurmountable antagonism), but is a potent competitive antagonist at the MOP and DOP receptors, with Ke values 3-6-fold more potent than those of JDTic. AT-076 is the first opioid pan antagonist with high affinity at all four opioid receptor subtypes. Our SAR studies show that the 3,4-dimethyl groups of the well-known trans-(3R,4R)-dimethyl-4-(3-hydroxyphenyl)piperidine opioid antagonist scaffold may be removed without significant loss in binding affinity or antagonist potency to obtain an opioid pan antagonist such as AT-076.