Pharmacologic Evidence for a Putative Conserved Allosteric Site on Opioid Receptors.

Allosteric modulators of G protein-coupled receptors, including opioid receptors, have been proposed as possible therapeutic agents with enhanced selectivity. BMS-986122 is a positive allosteric modulator (PAM) of the µ-opioid receptor (µ-OR). BMS-986187 is a structurally distinct PAM for the δ-opioid receptor (δ-OR) that has been reported to exhibit 100-fold selectivity in promoting δ-OR over µ-OR agonism. We used ligand binding and second-messenger assays to show that BMS-986187 is an effective PAM at the µ-OR and at the κ-opioid receptor (κ-OR), but it is ineffective at the nociceptin receptor. The affinity of BMS-986187 for δ-ORs and κ-ORs is approximately 20- to 30-fold higher than for µ-ORs, determined using an allosteric ternary complex model. Moreover, we provide evidence, using a silent allosteric modulator as an allosteric antagonist, that BMS-986187 and BMS-986122 bind to a similar region on all three traditional opioid receptor types (µ-OR, δ-OR, and κ-OR). In contrast to the dogma surrounding allosteric modulators, the results indicate a possible conserved allosteric binding site across the opioid receptor family that can accommodate structurally diverse molecules. These findings have implications for the development of selective allosteric modulators.

The δ-opioid receptor positive allosteric modulator BMS 986187 is a G-protein-biased allosteric agonist.

BACKGROUND AND PURPOSE: The δ-opioid receptor is an emerging target for the management of chronic pain and depression. Biased signalling, the preferential activation of one signalling pathway over another downstream of δ-receptors, may generate better therapeutic profiles. BMS 986187 is a positive allosteric modulator of δ-receptors. Here, we ask if BMS 986187 can directly activate the receptor from an allosteric site, without an orthosteric ligand, and if a signalling bias is generated. EXPERIMENTAL APPROACH: We used several clonal cell lines expressing δ-receptors, to assess effects of BMS 986187 on events downstream of δ-receptors by measuring G-protein activation, ß-arrestin 2 recruitment, receptor phosphorylation, loss of surface receptor expression, ERK1/ERK2 phosphorylation, and receptor desensitization. KEY RESULTS: BMS 986187 is a G protein biased allosteric agonist, relative to ß-arrestin 2 recruitment. Despite showing direct and potent G protein activation, BMS 986187 has a low potency to recruit ß-arrestin 2. This appears to reflect the inability of BMS 986187 to elicit any significant receptor phosphorylation, consistent with low receptor internalization and a slower onset of desensitization, compared with the full agonist SNC80. CONCLUSIONS AND IMPLICATIONS: This is the first evidence of biased agonism mediated through direct binding to an allosteric site on an opioid receptor, without a ligand at the orthosteric site. Our data suggest that agonists targeting δ-receptors, or indeed any GPCR, through allosteric sites may be a novel way to promote signalling bias and thereby potentially produce a more specific pharmacology than can be observed by activation via the orthosteric site.

Biased agonism: the quest for the analgesic holy grail.

Opioids alleviate pain, but adverse effects severely limit their usefulness. To solve this problem, biased ligands favoring 1 signaling pathway downstream of the µ-opioid receptor over another are being developed. In the target article, the authors synthesize compounds that preferentially activate G-protein or ß-arrestin signaling. They find that increased bias towards G-protein signaling produces better antinociception with minimal side effects in mice models. G-protein-biased opioids may provide a safer treatment strategy.