Effect of guanidino modification and proline substitution on the in vitro stability and blood–brain barrier permeability of endomorphin II

Abstract

Endomorphin II (ENDII), an endogenous ligand for the μ‐opioid receptor, was investigated as a possible analgesic with fewer side effects than morphine. To improve CNS entry of END II, structural modification was also examined to determine whether Pro4 substitution and cationization affected physico‐chemical characteristics, blood–brain barrier (BBB) transport, and analgesic profile. END II and its Pro4‐substituted analog, Morphiceptin (MOR), were cationized by guanidino (GU)‐addition. MOR was seven times less lipophilic than END II, whereas GU‐addition decreased lipophilicity of both peptides. MOR did not affect in vitro BBB permeability; however, GU‐addition increased permeability of MOR by 31%. MOR decreased protein binding by 23% compared to END II, whereas GU‐addition increased protein binding of both peptides by 71 and 113%, respectively. MOR increased brain t1/2 compared to END II. GU‐addition significantly increased t1/2 of MOR and END II in both brain (sixfold and 10‐fold, respectively) and serum (over 10‐fold). Pro4‐substitution and GU‐addition enhanced the in vivo analgesia profiles of i.v. administered END II and MOR, but decreased i.c.v. analgesia profiles. This study demonstrates Pro4‐substitution decreases protein binding and enhances brain stability while cationization enhances both brain and serum stability with variable effects on BBB permeability. The analgesic profiles show that both Pro4‐substitution and cationization enhance i.v. analgesia and thus, are promising structural modifications for the development of successful opioid drugs. © 2002 Wiley‐Liss Inc. and the American Pharmaceutical Association J Pharm Sci 91:2140–2149, 2002