A Journey through Diastereomeric Space: The Design, Synthesis, In Vitro and In Vivo Pharmacological Activity, and Molecular Modeling of Novel Potent Diastereomeric MOR Agonists and Antagonists

Abstract

Four sets of diastereomeric C9-alkenyl 5-phenylmorphans, varying in the length of the C9-alkenyl chain, were designed to examine the effect of these spatially distinct ligands on opioid receptors. Functional activity was obtained by forskolin-induced cAMP accumulation assays and several compounds were examined in the [35S]GTPgS assay and in an assay for respiratory depression. In each of the four sets, similarities and differences were observed dependent on the length of their C9-alkenyl chain and, most importantly, their stereochemistry. Three MOR antagonists were found to be as or more potent than naltrexone and, unlike naltrexone, none had MOR, KOR, or DOR agonist activity. Several potent MOR full agonists were obtained, and, of particular interest partial agonists were found that exhibited less respiratory depression than that caused by morphine. The effect of stereochemistry and the length of the C9-alkenyl chain was also explored using molecular modeling. The MOR antagonists were found to interact with the inactive (4DKL) MOR crystal structures and agonists were found to interact with the active (6DDF) MOR crystal structures. The comparison of their binding modes at the mouse MOR was used to gain insight into the structural basis for their stereochemically induced pharmacological differences.

Role of efficacy as a determinant of locomotor activation by mu‐opioid receptor (MOR) ligands in female and male mice. II. Effects of novel MOR‐selective phenylmorphans with high‐to‐low MOR efficacy

“… a Values at MOR‐mediated stimulation of GTPɣS binding for morphine, DC‐1‐128.1, DC‐1‐76.2, DC‐1‐90.2, and DC‐1‐76.1 were reported in Chambers et al 15 …”

“…Although binding across all three opioid receptor subtypes has not been reported previously for the other phenylmorphans, these compounds were shown to have functional MOR selectivity in an in vitro assay of forskolin‐stimulated cAMP accumulation. Thus, all of these compounds displayed higher potency to inhibit cAMP accumulation in cells expressing MOR than to function as either an agonist or antagonist for inhibition of cAMP accumulation in cells expressing KOR or DOR 15,16,18 . Of particular relevance for the development of these compounds as candidate therapeutics, the lower‐efficacy phenylmorphans all showed improved MOR selectivity relative to nalbuphine as an existing, clinically available, low MOR efficacy opioid.…”

“…Moreover, these data agree with other in vivo data that have been reported for a subset of these compounds. For example, the antinociceptive and respiratory depressant effects of DC‐1‐76.2, JL‐2‐39, and DC‐1‐76.1 have been examined in squirrel monkeys 15,18 . Although results across compounds were not compared statistically, they showed a general trend of declining antinociceptive and respiratory depressant effects in monkeys similar to their declining locomotor stimulant effects in mice.…”

“…Competition binding assays were performed using mMOR‐, mKOR‐, or hDOR‐CHO membrane homogenates containing 20 μg membrane protein as previously described 15,21,25 . Homogenates were incubated with approximate K D concentrations of 1.4 nM [ 3 H]naloxone (for mMOR‐CHO), 0.25 nM [ 3 H]diprenorphine (for mKOR‐CHO), or 1 nM [ 3 H]diprenorphine (for hDOR‐CHO) in the presence 0.2nM EGTA (pH 7.4) for 1.5 h at 30°C.…”

“…In a series of recent publications, the synthesis and initial pharmacological evaluation was described for a series of chiral C9‐substituted phenylmorphans that include new MOR ligands with graded levels of low, sub‐buprenorphine MOR efficacy and improved MOR selectivity 15–19 . However, a full opioid receptor binding profile for these drugs has not been described.…”