Amino acids and peptides. XXIV. Preparation and antinociceptive effect of [D-Ala2,(N-Me)Phe4]enkephalin analog-poly(ethylene glycol) hybrids.

Hybrids of amino-poly(ethylene glycol) (aPEG) and [D-Ala2,(N-Me)Phe4] enkephalin analogs, H-Tyr-D-Ala-Gly-(Me)Phe-aPEG, H-Tyr-D-Ala-Gly-(Me)Phe-Leu-aPEG and H-Tyr-D-Ala-Gly-(Me)Phe-D-Leu-aPEG, were prepared by the solution method and their antinociceptive properties were examined in comparison with those of the peptides. H-Tyr-D-Ala-Gly-(Me)Phe-OH and H-Tyr-D-Ala-Gly-(Me)Phe-Leu-OH themselves at intracerebroventricular (i.c.v.) doses of 10-30 nmol/animal produced an antinociceptive effect which was less potent than that of i.c.v. morphine, 3 micrograms/animal, and H-Tyr-D-Ala-Gly-(Me)Phe-D-Leu-OH did not have any marked effect. However, the antinociceptive effects of H-Tyr-D-Ala-Gly-(Me)Phe-Leu-OH and H-Tyr-D-Ala-Gly-(Me)Phe-D-Leu-OH were remarkably potentiated by hybrid formation with aPEG to levels higher than that of 3 micrograms/mouse of morphine, and the effect lasted at least 120 min. In contrast, the effect of H-Tyr-D-Ala-Gly-(Me)Phe-OH was rather diminished by hybrid formation. In view of the low toxicity and weak immunogenic properties of aPEG, the hybrids could be useful in therapy of patients for relieving chronic and severe pain.