Studies on Analgesic Oligopeptides. II. Structure-Activity Relationship among Thirty Analogs of a Cyclic Dipeptide, Cyclo (-Tyr-Arg-)

Abstract

Thirty diketopiperazines were synthesized as analogs of cyclo (-Tyr-Arg-). The analgesic activities of these analogs were evaluated after intracerebral administration in mice. In the cyclo (-X-Arg-) series of analogs, cyclo [-Tyr (Et)-Arg-] showed the most potent activity. In the cyclo (-Tyr-Y-) series of analogs, the activity decreased in the order Y=homoarginine, p-guanidinophenylalanine, 2-amino-4-guanidinobutyric acid, Lys, Orn, His, α, γ-diaminobutyric acid and Pro. Among the analogs synthesized, cyclo-[-Tyr (Et)-Har-], which was designed on the basis of the above results, exhibited remarkably potent analgesic activity, being 17 times more potent than cyclo (-Tyr-Arg-) and nearly as potent as morphine on a molar basis. The structure-activity relation of cyclo(-Tyr-Arg-) is discussed in the light of these results.