Development of a potent bombesin receptor antagonist with prolonged in vivo inhibitory activity on bombesin-stimulated amylase and protein release in the rat

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Of the various types of potent bombesin(Bn)/gastrin releasing peptide receptor antagonists that have been discovered, the desMet 14-methyl ester peptides are devoid of residual agonist activity and are among the most potent in terms of in vitro receptor blockade and also in terms of their prolonged inhibition of bombesin-stimulated amylase and protein release in the rat. We have now examined the in vitro and in vivo properties of a new series of methyl ester analogues, [ d-Phe 6]Bn(6–13)OMe, [ d-Phe 6, d-Ala 11]Bn(6–13)OMe, N α- propionyl-[ d- Ala 24] GRP(20–26) OMe , and [ d-pentafluoro-Phe 6, d-Ala 11]Bn(6–13)OMe, which have an additional d-amino acid substituent and some highly lipophilic moieties at the N-terminus. All analogues were able to potently antagonize the ability of Bn to stimulate amylase release from rat acinar cells, with IC 50 values of 2.4, 2.5, 0.6, and 1.3 n M, respectively. The four peptides were found to have binding affinities for these cells comparable to Bn itself, with K i s of 10.3, 2.8, 5.5, and 3.6 n M, respectively, but all had little or no affinity for neur B receptors on murine C6 cells. Single bolus IV injections of these peptides were found to potently inhibit amylase and protein release caused by IV infusion of bombesin into the rat. Generally the peptides containing the d-Ala substituent were longer acting than [ d-Phe 6]Bn(6–13)OMe, so that [ d-Phe 6, d-Ala 11]Bn(6–13)OMe and N α- propionyl-[ d-Ala 24] GRP(20–26) OMe displayed significant inhibitory effects for up to 1.5 h after administration. The most lipopholic analogue, [ d-pentafluoro-Phe 6, d-Ala 11]Bn(6–13)OMe, however, was active for up to 4.5 h after injection and, at the same dose level as the other peptides, it had a fifteenfold longer duration of action. This analog appears to be a good candidate for clinical studies where prolonged blockade of endogenous bombesin-like peptides is desirable.