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-opioid receptor-selective peptide, which was modified by the trimethylation of 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 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 microM 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.
Witt
K A
KA
Department of Pharmacology, University of Arizona, College of Medicine, Tucson, Arizona, USA.
Slate
C A
CA
Egleton
R D
RD
Huber
J D
JD
Yamamura
H I
HI
Hruby
V J
VJ
Davis
T P
TP
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000378:metabolism; D000409:Alanine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism; D000698:Analgesia; D000818:Animals; D001812:Blood-Brain Barrier; D001921:Brain / Q000098:blood supply / Q000378:metabolism; D002196:Capillaries / Q000378:metabolism; D002417:Cattle; D004730:Endothelium, Vascular / Q000378:metabolism; D020881:Enkephalin, D-Penicillamine (2,5)- / Q000031:analogs & derivatives* / Q000737:chemistry; D005260:Female; D008745:Methylation; D008968:Molecular Conformation; D051381:Rats; D017207:Rats, Sprague-Dawley; D017465:Receptors, Opioid, delta / Q000378:metabolism*; D017450:Receptors, Opioid, mu / Q000378:metabolism*; D013237:Stereoisomerism; D013329:Structure-Activity Relationship
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D017465:Receptors, Opioid, delta; D017450:Receptors, Opioid, mu; C410624:methyl-2',6'-dimethylphenylalanine; D020881:Enkephalin, D-Penicillamine (2,5)-; D000409:Alanine