Ac-31>-CCK4 Analogs: Synthesis and Implications for the CCK-B Receptor-Bound Conformation

Article abstract of DOI:10.1021/jm00001a019

It has been reported that substitution of the Met³¹ residue in Boc-CCK₄ (Boc-Trp³⁰-Met³¹-Asp³²-Phe³³-NH2, CCK₃₃ numbering) by trans-3-propyl-L-proline yields a highly potent and selective CCK-B agonist.To further explore the structural requirements of the Met³¹ side chain in the receptor-bound conformation of CCK₄, we have synthesized several Ac-CCK₄ analogs containing substitution of Met³¹ by 3- and 4-(alkylthio)-substituted proline derivatives.To this end we have developed novel synthetic routes to enantiomerically pure N-Boc-4-cis- and -trans-(methylthio)prolines and racemic N-Boc-3-cis- and -trans-<(4-methylbenzyl)thio>prolines.The protected mercaptoprolines were incorporated into Ac-CCK₄ analogs using SPPS and were alkylated using various electrophiles following cleavage from the solid support.Binding assays reveal that 3-(alkylthio)prolines analogs have higher affinities at the CCK-B receptor than the corresponding 4-(alkylthio)proline analogs, and that trans-3-(alkylthio)proline analogs had higher affinities than corresponding cis-3-(alkylthio)proline analogs.Within both the cis- and trans-3-(alkylthio)proline series, the order of potency was found to be Me < Et < n-Pr.The trans-3-(n-propylthio)-L-proline analog demonstrates a higher affinity than that reported for Boc-CCK₄31>.Comparison of the low-energy structures calculated for several high-affinity Ac-CCK₄ analogs reveal a common geometry which we propose to be the CCK-B receptor-bound conformation.This model shows grouping of the hydrophobic side chains of Trp, Met, and Phe at one side of the molecule and the hydrophilic side chain of Asp and the C-terminal carboxamide at the other side.