Synthesis and antinociceptive activity of [D-Met2, Pro5] enkephalin...

Tetra-O-acetylgalactopyranosylamine and tetra-O-acetylglucopyranosylamine of D-Met2, Pro5 enkephalin were designed and synthesized to enhance their membrane penetration, biological activity and resistance to proteolytic hydrolysis. Three approaches to the synthesis were attempted, which lead to a new synthetic scheme with a higher yield and enhanced ease of purification. The improved procedure involved attaching the tetra-O-acetylglycopyranosylamine to a t-Boc-Gly-Phe-Pro-OH peptide, removing the t-Boc, and condensing it with t-Boc-Tyr-D-Met-OH. Biological evaluation in vivo showed that these acetylglycopyranosylamine derivatives bind to mu and delta opioid receptors in homogenate binding assays and possess analgesic activity. The analgesic potency was less than that of the parent compound D-Met2, Pro5 enkephalin. These acetylglycopyranosylamine derivatives showed enhanced lipophilicity compared to their parent compound by a partition coefficient study and they also showed greater membrane permeability, using the rabbit cornea as a model system. These derivatives also are resistant to hydrolytic enzymes as compared to the endogenous met-enkephalin when evaluated in homogenized iris-ciliary body and aqueous humor from rabbit eyes.