Desmethionine Alkylamide Bombesin Analogues: A New Class of Bombesin Receptor Antagonists with Potent Antisecretory Activity in Pancreatic Acini and Antimitotic Activity in Swiss 3T3 Cells

Bombesin-related peptides have a large number of physiological functions as well as having an autocrine growth mechanism for the regulation of small cell lung cancer cells. In the present study we have synthesized 21 des-Met amide or alkylamide analogues of bombesin and compared their abilities to function as bombesin receptor antagonists in guinea pig pancreatic acini and Swiss 3T3 cells with those of the previously most potent antagonist described, [Leu¹³ψCH₂NH)Leu¹⁴]bombesin (analogue I). All des-Met analogues functioned as antagonists. Bn(l-13)NH₂ was approximately equipotent to I (K_i = 60−80 nM) whereas Bn(6–13)NH₂ was 30-fold less potent (K_i = 1800 nM). Formation of an ethylamide, Bn(6–13)ethylamide, increased the potency 30-fold such that this octapeptide was equipotent to I. The addition of a D-Phe⁶ moiety to I did not change potency but caused a 30-fold increase in potency of Bn(6–13)NH₂ and a 8-fold increase in the potency of Bn(6–13)ethylamide (K_i= 16 nM). Additional studies of both NH₂- and COOH-terminal alterations in Bn(6–13)NH₂ demonstrated that the most potent antagonist was [D-Phe⁶]Bn(6–13)propylamide (PA), having IC₅₀'s of 1.6 nM and 0.8 nM for bombesin-stimulated amylase release and Swiss 3T3 cell growth, respectively. Detailed studies of the most potent amide analogue, [D-Phe⁶]Bn(6–13)NH₂, and alkylamide analogue, [D-Phe⁶]Bn(6–13)PA, demonstrated that these analogues functioned as competitive antagonists and that their action was selective for the bombesin receptor. These results demonstrate that, as with CCK- and gastrin-related peptides, the C-terminal amino acid is important for initiating a biologic response but not essential for determining receptor affinity. Furthermore, the most potent des-Met analogue, [D-Phe⁶]Bn(6–13)PA, is 30-fold more potent than any previously described bombesin receptor antagonist. This member of this new class of antagonists can be easily synthesized, offers fewer proteolytic degradation sites, and should be useful for in vivo studies.