Assessment of stereoselectivity of trimethylphenylalanine analogues of delta-opioid [D-Pen(2),D-Pen(5)]-enkephalin

[D-Pen(2), D-Pen(5)]-Enkephalin (DPDPE) is an enzymatically stable delta-op ioid receptor-selective peptide, which was modified by the trimethylation o f the Phe(4) residue to give beta-methyl-2',6'-dimethylphenylalanine (TMP), resulting in four conformations: (2R,3S)-beta-Phe-DPDPE, (2R,3R)-beta-Phe- DPDPE, (2R,3S)-beta-Phe-DPDPE, and (2S,3R)-beta-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 (delta- versus mu-opioid receptor) were evaluated by radioreceptor binding in rat brain, with a mu/delta ratio decrease for ail TMP conformations, com pared with the parent compound (DPDPE). Octanol/buffer distribution analysi s showed enhanced lipophilicity of all TMP forms, with a sixfold enhancemen t associated with (2S,3S)-TMP. In situ vascular perfusion in anesthetized r ats 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. S aturability of (2S,3S)-TMP was shown (p < 0.01) against 100 mu M unlabeled DPDPE, showing a shared nondiffusionary transport system. P-glycoprotein af finity 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 low est degree of protein binding (p < 0.01), and (2S,3S)-TMP and (2R,3R)-TMP w ith comparable affinities to DPDPE. Analgesia, via intravenous administrati on, 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 res ults demonstrate that topographical modification in a conformationally rest ricted peptide can significantly modulate potency and receptor selectivity, binding capacity, enzymatic stability, lipophilicity, P-glycoprotein affin ity, and blood-brain barrier permeability, resulting in a change of bioavai lability, and thereby provides insight for future peptide drug design.