CCK-B agonist or antagonist activities of structurally hindered and peptidase-resistant Boc-CCK4 derivatives

Article abstract of DOI:10.1021/jm00053a022

Replacement of Met³¹ by (N-Me)Nle in CCK₈ or CCK₄ has been shown to improve the affinity and selectivity for CCK-B receptors. In order to obtain molecules with enhanced bioavailability, two novel series of protected tetrapeptides of the general formula Boc-Trp³⁰-X-Asp-Y³³ have been developed. Introduction of (N-Me)Nle and the bulky, aromatic naphthylalaninamide (Nal-NH₂) in positions X and Y, respectively, does not greatly modify the affinity for guinea pig brain CCK-B receptors. In contrast, incorporation of hindering N-methyl amino acids such as (N-Me)Phe, (N-Me)Phg, or (N-Me)Chg, but not their non-methylated counterparts, in position X induced a large decrease in affinity for the CCK-B binding sites. Among the various peptides synthesized, Boc-[(N-Me)Nle³¹,1Nal- NH₂³³]CCK₄ (2) (K(I) = 2.8 nM), Boc-[Phg³¹,1Nal-NH₂³³]CCK₄ (15) (K(I) = 14 nM), and Boc-[Phg³¹,1Nal-N(CH₃)₂³³]CCK₄ (17) (K(I) = 39 nM) displayed good affinities for brain CCK-B receptors and had good selectivity ratios. These pseudopeptides, in which the presence of unnatural and hydrophobic residues is expected to improve their penetration of the central nervous system, were shown to be very resistant to brain peptidases. Interestingly, whereas compounds 2 and 15 proved to be full agonists for rat hippocampal CCK-B receptors when measured in an electrophysiological assay, compound 17 behaved as a potent antagonist in the same test and displayed a good affinity in rat brain K(I)(CCK-B) = 51 nM as compared to the Merck antagonist L365,260, K(I)(CCK-B) = 12 nM. This illustrates a simple means to obtain CCK-B antagonists and suggests that the free, CONH₂ group plays a critical role in the recognition of the agonist state of brain CCK-B receptors.