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Exploration of the fifth position of leu‐enkephalin and its role in binding and activating delta (DOP) and mu (MOP) opioid receptors

Enkephalins are pentapeptidic endogenous ligands that regulate nociception by binding to mu (MOP) and delta (DOP) opioid receptors. To further explore the role of the leucine residue of Leu‐enkephalin, 12 peptidomimetic analogs were synthesized by systematically replacing this residue with non‐natural amino acids. The analogs were tested for their ability to bind DOP and MOP. We also investigated the potency of these analogs to inhibit cAMP production and to recruit β‐arrestin 2 via both receptors. We found that replacement of the leucine residue by substituted non‐natural amino acid derivatives of alanine, cycloleucine, or isoleucine was generally well tolerated. By contrast, substituting leucine with homoproline greatly reduced the affinity for DOP and, to a lesser extent, for MOP. Interestingly, when compared to Leu‐enkephalin, analogs containing either aza‐β‐homoleucine or cycloleucine showed a bias toward inhibition of cAMP production through the activation of DOP but not MOP. By contrast, derivatives containing 4,5‐dehydroleucine or d‐allo‐isoleucine conferred a bias toward β‐arrestin 2 at MOP, but not DOP. Our results suggest that position 5 in Leu‐enkephalin analogs can be further exploited to develop compounds with the potential to produce bias toward G protein or β‐arrestin 2.