Assessment of stereoselectivity of trimethylphenylalanine analogues of δ-opioid [D-Pen²,D-Pen⁵]-enkephalin

[D-Pen²,D-Pen⁵]-Enkephalin (DPDPE) is an enzymatically stable δ- opioid receptor-selective peptide, which was modified by the trimethylation of the Phe⁴ residue to give β-methyl-2',6'-dimethylphenylalanine (TMP), resulting in four conformations: (2S,3S)-β-Phe-DPDPE, (2R,3R)-β-Phe-DPDPE, (2R,3S)-β-Phe-DPDPE, and (2S,3R)-β-Phe-DPDPE. Synthesis was by solid-phase techniques using enantiomerically pure amino acids to give the four optically pure diastereoisomer peptides. The potency and selectivity (δ- versus μ- opioid receptor) were evaluated by radioreceptor binding in rat brain, with a μ/δ ratio decrease for all TMP conformations, compared with the parent compound (DPDPE). Octanol/buffer distribution analysis showed enhanced lipophilicity of all TMP forms, with a sixfold enhancement associated with (2S,3S)-TMP. In situ vascular perfusion in anesthetized rats showed a 1.6- fold (p < 0.01) increase in the ratio of brain uptake for (2S,3S)-TMP and a 1.5-fold (p < 0.01) decrease in uptake for (2R,3R)-TMP. Saturability of (2S,3S)-TMP was shown (p < 0.01) against 100 μM unlabeled DPDPE, showing a shared nondiffusionary transport system. P-glycoprotein affinity was shown in situ for the parent and (2S,3S)-TMP (p < 0.01). Protein binding capacity of the TMP compounds in rat plasma and in situ mammalian bovine serum albumin- Ringer showed (2R,3S)-TMP and (2S,3R)-TMP with the lowest degree of protein binding (p < 0.01), and (2S,3S)-TMP and (2R,3R)-TMP with comparable affinities to DPDPE. Analgesia, via intravenous administration, showed significantly reduced (p < 0.01) end effect and time course for (2R,3R)-TMP, (2R,3S)-TMP, and (2S,3R)-TMP as compared with DPDPE. These results demonstrate that topographical modification in a conformationally restricted peptide can significantly modulate potency and receptor selectivity, binding capacity, enzymatic stability, lipophilicity, P-glycoprotein affinity, and blood-brain barrier permeability, resulting in a change of bioavailability, and thereby provides insight for future peptide drug design.