Analgesic properties of N-terminal substituted phenylalanyl-alanine

Phenylalanylalanine, an in vitro inhibitor of enkephalinase, and some of its N alpha-derivatives are shown to possess an analgesic action when injected i.p. and i.c.v. into mice in the presence or absence of Leu5-enkephalin. In the second case a synergistic response is observed. The intensity of the analgesic response depends markedly on the nature of the N-terminal substituent which affects the hydrophobic character of the resulting dipeptide, its subsequent transport and probably its rate of biotransformation by cleaving enzymes.