N-Methylated Analogs of Ac[Nle^28,31]^CCK(26-33): Synthesis, Activity, and Receptor Selectivity

A series of singularly N-methylated analogs of Ac[Nle^28,31]CCK(26-33) were synthesized by the solid-phase methodology, and their biological activity was tested in three different in vitro bioassays. The bioassays employed were the guinea pig gallbladder (GPGB), stomach (GPS), and ileum (GPI). All N-methyl analogs were agonists in all three bioassays. N-Methylation at either N-or C-terminals did not affect potency and selectivity, whereas N-methylation of internal residues [Nle²⁸, (N-Me)Nle³¹]- and [Nle^28,31, (N-Me)Trp³⁰]CCK(26-33) in the sequence resulted in analogs which were 10-fold less potent than Ac[Nle^28,31]CCK(26-33) in all three preparations. Different rank orders of potencies observed for [Nle^28,31,Sar²⁹]- and [Nle^28,31, (N-Me) Asp³²] CCK(26-33) analogs correspond to increased selectivity to either GPGB or GPS, respectively. We propose that systematic N-methylation of single amide bonds in a bioactive peptide should be conducted as an additional routine to probe structure-activity relationships.