New endomorphin analogs with μ-agonist and δ-antagonist properties

Endomorphins (endomorphin-1, H-Tyr-Pro-Trp-Phe-NH₂, endomorphin-2, Tyr-Pro-Phe-Phe-NH₂) are potent and selective μ-opioid receptor agonists. In order to improve the affinity and chemical stability of endomorphins, we have designed, synthesized, and characterized novel analogs with unnatural (2′, 6′-dimethyltyrosine, Dmt) and/or β-alicyclic amino acids (ACPC and ACHC). Radioligand binding assay indicated that several of the novel analogs exhibit high affinity for both μ- and δ-opioid receptors in rat- or mouse-brain membrane preparations. The most promising derivatives-such as Dmt-Pro-Trp/Phe-Phe-NH₂, Dmt-(1S,2R)-ACPC-Phe-Phe-NH₂, and Dmt-(1S,2R)-ACHC-Phe-Phe-NH ₂)-were characterized in recombinant cell lines expressing human μ- or δ-opioid receptors. Interestingly, while these novel peptides were potent opioid agonists in the functional [³⁵S] GTPγS binding assays in Chinese hamster ovary cells expressing the μ-opioid receptors, some behaved as antagonist or inverse agonist in the human δ-opioid receptor-expressing CHO cells. Since it has previously been demonstrated that the coadministration of δ-antagonists with μ-analgesics attenuates the development of analgesic tolerance, introduction of high-affinity δ-antagonist properties into the μ-agonist endomorphins is expected to lead to potent analgesics that produce limited tolerance.