Synthesis and Pharmacology of Halogenated δ-Opioid-Selective [ d -Ala²]Deltorphin II Peptide Analogues

Deltorphins are naturally occurring peptides produced by the skin of the giant monkey frog (Phyllomedusa bicolor). They are δ-opioid receptor-selective agonists. Herein, we report the design and synthesis of a peptide, Tyr-d-Ala-(pI)Phe-Glu-Ile-Ile-Gly-NH₂ 3 (GATE3-8), based on the [d-Ala²]deltorphin II template, which is δ-selective in in vitro radioligand binding assays over the μ- and -opioid receptors. It is a full agonist in [³⁵S]GTPγS functional assays and analgesic when administered supraspinally to mice. Analgesia of 3 (GATE3-8) is blocked by the selective δ receptor antagonist naltrindole, indicating that the analgesic action of 3 is mediated by the δ-opioid receptor. We have established a radioligand in which ¹²⁵I is incorporated into 3 (GATE3-8). The radioligand has a K_D of 0.1 nM in Chinese hamster ovary (CHO) cells expressing the δ receptor. Additionally, a series of peptides based on 3 (GATE3-8) was synthesized by incorporating various halogens in the para position on the aromatic ring of Phe³. The peptides were characterized for binding affinity at the μ-, δ-, and -opioid receptors, which showed a linear correlation between binding affinity and the size of the halogen substituent. These peptides may be interesting tools for probing δ-opioid receptor pharmacology.