Synthesis and biological activity of partially modified retro-inverso pseudopeptide derivatives of the C-terminal tetrapeptide of gastrin

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Abstract
The effects of partial retro-inverso modifications of selected peptide bonds of the N-terminal tetrapeptide of gastrin have been studied. In some of the synthesized compounds, the phenylalanyl residue has been replaced by the (R,S)-2-benzylmalonyl, 3-phenylpropionyl, benzylcarbamoyl, or benzyloxycarbonyl moieties. All pseudopeptides showed affinity for the gastrin receptor, in vitro, with potencies varying from IC50 = 10-7 to IC50 = 10-4 M. These compounds exhibited little or no activity on acid secretion in the anesthetized rat but were able to antagonize the action of gastrin. Among the most potent were Boc-Trp-Leu-gAsp-CO-CH2CH2C6H5 (20) (ED50 = 0.15 .mu.M/kg), Boc-Trp-Leu-gAsp-m(R,S)Phe-NH2 (3) (ED50 = 0.15 .mu./kg), and Boc-Trp-gLeu-D-Asp-m(R,S)Phe-NH2(7) (ED50 = 0.3 .mu.M/kg).

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