Modulating β Arrestin-2 Recruitment at the δ- and μ-Opioid Receptors

Opioid receptors can trigger two distinct pathways (G protein coupling and arrestin recruitment) that differentially regulate a host of desired and undesired pharmacological effects. Increasingly, “biased” opioids that selectively activate one pathway over the other are being developed to treat disorders in which µ- and κ-opioids receptors are involved, though the development of biased δ-opioid receptor agonists has remained rather quiescent. Herein, we identify the C-terminus of Tyr-ψ[(Z)CF=CH]-Gly-Leu-enkephalin as a key site to regulate bias of both δ- and µ-opioid receptor agonists. Using in vitro assays, substitution of the Leu⁵ carboxylate reduced β-arrestin recruitment through both the δ- and µ-opioid receptors in a predictable structure-dependent fashion, while retaining affinity and cAMP potency comparable to the C-terminal carboxylate. These substitutions should enable discovery of a range of tool compounds for exploring δ-opioid receptor pharmacology and toxicology, which will enable reevaluation of this target within the context of biased signaling.