To improve the oral bioavailability of a dermorphin tetrapeptide analog, N α-1-iminoethyl-Tyr- D-MetO-Phe-MeβAla-OH (III), 1) which has a potent analgesic activity after oral administration, various derivatives were synthesized to increase lipophilicity by esterification of the C-terminal carboxyl group and/or acylation of the phenolic hydroxyl group on Tyr 1. Antinociceptive activity was evaluated after subcutaneous or oral administration using the ...