Functional selectivity of EM-2 analogs at the mu-opioid receptor

Abstract

The mu opioid receptor agonists are the most efficacious pain controlling agents but their use is accompanied by severe side effects. More recent developments indicate that some ligands can differentially activate receptor downstream pathways, possibly allowing for dissociation of analgesia mediated through the G protein from the opioid-related side effects mediated by β-arrestin pathway. In an effort to identify such biased ligands, here we present a series of thirteen endomorphin-2 (EM-2) analogs with modifications in positions 1, 2, and/or 3. All obtained analogs behaved as mu receptor selective agonists in calcium mobilization assay carried out on cells expressing opioid receptors and chimeric G proteins. A Bioluminescence Resonance Energy Transfer (BRET) approach was employed to determine the ability of analogs to promote the interaction of the mu opioid receptor with G protein or β-arrestin 2. Nearly half of the developed analogs showed strong bias towards G protein, in addition four compounds were nearly inactive towards β-arrestin 2 recruitment while blocking the propensity of EM-2 to evoke mu-β-arrestin 2 interaction. The data presented here contribute to our understanding of EM-2 interaction with the mu opioid receptor and of the transductional propagation of the signal. In addition, the generation of potent and selective mu receptor agonists strongly biased towards G protein provides the scientific community with novel tools to investigate the in vivo consequences of biased agonism at this receptor.

Pathology of pain and its implications for therapeutic interventions

“… 602 Biased agonists with reduced β-arrestin 2 recruitment, like 2S-LP2 and EM-2, are being developed to alleviate these effects. 603 , 604 However, there are dissenting opinions that β-arrestin 2 as a scaffolding protein is unlikely to be an ideal pharmacological target. Evidences have shown that severe side effects are not observed in mice with ablation of β-arrestin 2 functions, 605 , 606 implying that unknown mechanisms independent of β-arrestin 2 may contribute to chronic opioid tolerance.…”