Low‐efficacy mu‐opioid receptor (MOR) agonists represent promising therapeutics, but existing compounds (e.g., buprenorphine, nalbuphine) span a limited range of low MOR efficacies and have poor MOR selectivity. Accordingly, new and selective low‐efficacy MOR agonists are of interest. A novel set of chiral C9‐substituted phenylmorphans has been reported to display improved MOR selectivity and a range of high‐to‐low MOR efficacies under other conditions; however, a full opioid receptor binding profile for these drugs has not been described. Additionally, studies in mice will be useful for preclinical characterization of these novel compounds, but the pharmacology of these drugs in mice has also not been examined. Accordingly, the present study characterized the binding selectivity and in vitro efficacy of these compounds using assays of opioid receptor binding and ligand‐stimulated [35S]GTPɣS binding. Additionally, locomotor effects were evaluated as a first step for in vivo behavioral assessment in mice. The high‐efficacy MOR agonist and clinically effective antidepressant tianeptine was included as a comparator. In binding studies, all phenylmorphans showed improved MOR selectivity relative to existing lower‐efficacy MOR agonists. In the ligand‐stimulated [35S]GTPɣS binding assay, seven phenylmorphans had graded levels of sub‐buprenorphine MOR efficacy. In locomotor studies, the compounds again showed graded efficacy with a rapid onset and ≥1 h duration of effects, evidence for MOR mediation, and minor sex differences. Tianeptine functioned as a high‐efficacy MOR agonist. Overall, these in vitro and in vivo studies support the characterization of these compounds as MOR‐selective ligands with graded MOR efficacy and utility for further behavioral studies in mice.

The acid-mediated condensation of acetamide with butanal dimethylacetal and EtSCHCH(OMe), followed by dehydration, was investigated by electronic structure calculations that supported the prediction that the Z-geometry would be favored in the product. The reaction was investigated experimentally using suitably functionalized cysteine building blocks. Some side reactions and optimization of reaction conditions are reported, en route to identifying a mild, inexpensive Lewis acid that achieves a reasonable yield of (Z)-thioenamide 21 with high stereoselectivity.

N-Phthalimido-d-cysteine allyl ester was S-alkylated with 2-iodoethanol. The derived β-thioaldehyde was condensed with Nα-tetrachlorophthalimidovalinamide to afford a Z-thioenamide. Removal of the tetrachlorophthalimido protecting group and homologation with N-Boc-l-leucine afforded the linear tripeptide. Removal of the Boc and allyl protecting groups, followed by carbodiimide-mediated cyclization, led to the 13-membered ring with the aminovinylcysteine moiety embedded. This constitutes the C-terminal macrocycle of all known members of the linardin family of peptides, including the antileukemia agent, cypemycin.

All possible diastereomeric C9-hydroxymethyl-, hydroxyethyl-, and hydroxypropyl-substituted 5-phenylmorphans were synthesized to explore the three-dimensional space around the C9 substituent in our search for potent MOR partial agonists. These compounds were designed to lessen the lipophilicity observed with their C9-alkenyl substituted relatives. Many of the 12 diastereomers that were obtained were found to have nanomolar or subnanomolar potency in the forskolin-induced cAMP accumulation assay. Almost all these potent compounds were fully efficacious, and three of those chosen for in vivo evaluation, 15, 21, and 36, were all extremely G-protein biased; none of the three compounds recruited beta-arrestin2. Only one of the 12 diastereomers, 21 (3-((1S,5R,9R)-9-(2-hydroxyethyl)-2-phenethyl-2-azabicyclo[3.3.1]nonan-5-yl)phenol), was a MOR partial agonist with good, but not full, efficacy (Emax = 85%) and subnanomolar potency (EC50 = 0.91 nM) in the cAMP assay. It did not have any KOR agonist activity. This compound was unlike morphine in that it had a limited ventilatory effect in vivo. The activity of 21 could be related to one or more of three well-known theories that attempt to predict a dissociation of the desired analgesia from the undesirable opioid-like side-effects associated with clinically used opioids. In accordance with the theories, 21 was a potent MOR partial agonist, it was highly G-protein biased and did not attract beta-arrestin2, and it was found to have both MOR and DOR agonist activity. All the other diastereomers that were synthesized were either much less potent than 21 or had either too little or too much efficacy for our purposes. It was also noted that a C9-methoxymethyl compound with 1R,5S,9R stereochemistry (41) was more potent than the comparable C9-hydroxymethyl compound 11 (EC50 = 0.65 nM for 41 vs. 2.05 nM for 11). Both 41 and 11 were fully efficacious.