Investigation of peripheral cholecystokinin receptor heterogeneity by cyclic and related linear analogs of CCK26-33. Synthesis and biological properties

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Abstract
A possible heterogeneity of peripheral receptors for CCK26-33 [Asp-Tyr(SO3H)-Met-Gly-Trp-Met-Asp-Phe-NH2] (CCK8) was investigated by replacement of the flexible Gly29 residue, reported to be crucially involved in the CCK8 folding, by a D-Lys residue in Boc[Nle28,31]CCK27-33, a derivative as active as CCK8. The linear peptide Boc-Asp-Tyr(SO3H)-Nle-D-Lys-Trp-Nle-Asp-Phe-NH2 (1) was cyclized through amide bond formation between the side chains of Asp26 and D-Lys29 to give the peptide BocAsp26-Tyr(SO3H)-Nle-D-Lys29-Trp-Nle-Asp-Phe-NH2(2). Analogues 1 and 2 were shown to stimulate secretion of amylase from rat pancreas with a potency that was respectively 40 and 80 times lower than that of CCK8. In contrast, both peptides acted as weak antagonists (EC50 .apprx. 10-5 M) of the CCK8-induced contractions of guinea pig ileum. Peptides 3 and 4 obtained by removal of the phenylalanine from 1 and 2 were inactive in all bioassays despite amidification of their C-terminal Asp32 residue, a modification known to induce antagonist properties in CCK7. Cyclization between residues 28 and 31 in Boc[Asp28,Lys31]CCK27-33 gave compound Boc-Tyr(SO3H)-Asp28-Gly-Trp-Lys31-Asp-Phe-NH2(5), which was inactive in all bioassays. The pharmacological properties of these first described cyclic analogues of CCK8 were in agreement with their binding affinity to brain and pancreas receptors, suggesting the existence of a heterogeneity of peripheral receptors and emphasizing the usefulness of cyclic peptides in structure-activity studies.

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