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A series of analogs of Ac-CCK-7 [Ac-Tyr(SO3H)-Met28-Gly29-Trp-Met-Asp- Phe-NH2, (1)] were prepared in which the Met28-Gly29 dipeptide was replaced by omega-aminoalkanoic acids. Compounds were assessed in binding assays using homogenated rat pancreatic membranes and bovine striatum as the source of CCK-A and CCK-B receptors, respectively, and for anorectic activity after intraperitoneal administration to rats. The analog incorporating 4-aminobutanoic acid (5) was only 8 times less potent than 1 in the pancreatic binding assay, was more potent in the striatal binding assay, and was more potent than 1 in reducing food intake in rats. Using a bioactive cyclic analog of Ac-CCK-7 as a template, several rigid spacers were designed and tested as substitutes for the Met28-Gly29 dipeptide. The analogs incorporating 3-aminobenzoic acid (20) and (1S)-trans-2-aminocyclopentanecarboxylic acid (26) proved highly effective in the binding assays and as anorectic agents. We hypothesize that for stimulation of CCK-A receptors, the main function of the N-terminal tripeptide of Ac-CCK-7 is to orient the tyrosine sulfate with respect to Trp30 and that the bioactive arrangement of these elements lies among those which are readily available to both 20 and 26. NOESY and distance-constrained molecular dynamics experiments carried out on 20 and 26 identified conformations in which the relative orientation of the tyrosine hydroxide and the alpha-carbon atom of tryptophan were similar, providing the basis for further drug design efforts.

Cholecystokinin represents a family of gut hormones which among other activities, have been proposed to participate in satiety signaling. Ac-CCK-7[Ac-Tyr(SO3H)-Met-Gly-Trp30-Met-Asp-Phe-NH2 (2)] possesses the full spectrum of activity and potency of the intact hormone; thus analogs of 2 may be useful as anorectic agents. A series of derivatives has been prepared in which the tryptophan indole moiety of 2 has been modified. The new compounds were assayed in CCK binding assays using homogenated rat pancreatic membranes and bovine striatum as a source of CCK-A and CCK-B receptors respectively and in vivo in rats for anorectic activity. Although previous studies have concluded that the indole ring of Trp30 is a critical pharmacophore for the interaction of CCK with both its A and B type receptors, we find 2-Nal30-Ac-CCK-7 (20) to be nearly equipotent to 2 in both CCK binding and as an anorectic agent sensitive to blockade by the Merck CCK-A receptor antagonist MK-329. The extreme structural sensitivity of this anorectic activity is illustrated by the 1-naphthylalanine30 (19) and (benzo[b]thien-2-yl)alanine30 (21) analogs which are 30 and 100 times less potent than 2 respectively. Other mono- and bicyclic Trp30 replacements, including substituted phenylalanines, 3-quinolinylalanine, and 2-(5,6,7,8-tetrahydro)naphthylalanine, gave inactive compounds.

Previous work indicates that both the C-terminal phenylalanine amide and the tryptophan moieties of cholecystokinin (CCK) are critical pharmacophores for interaction with either the A or B receptor subtypes. We have examined a series of analogs of Ac-CCK-7 [Ac-Tyr(SO3H)-Met-Gly-Trp-Met-Asp-Phe33-NH2] (2) in which the phenyl ring of the C-terminal Phe-NH2 has been modified. Compounds were assessed in binding assays using homogenated rat pancreatic membranes and bovine striatum as the source of CCK-A and CCK-B receptors respectively and for anorectic activity after intraperitoneal administration to rats. Substitution of a number of cycloalkyl or bicyclic aryl moieties for the phenyl ring of phenylalanine33 including cyclopentyl (20), cyclohexyl (21), cyclooctyl (23), 2-(5,6,7,8-tetrahydro)naphthyl (26), 2-naphthyl (27), and 1-naphthyl (29) led to analogs with 10-70 times the anorectic potency of 2. The anorectic activity of 21 was blocked by the specific CCK-A receptor antagonist MK-329. Other bulky aliphatic groups in place of the phenylalanine33 aromatic ring such as isopropyl, 2-adamantyl and cyclohexylmethyl gave derivatives similar to 2 in potency. While most of the new compounds were comparable to CCK in binding assays, 23, 26, 27 and 29 were exceptionally potent with IC50s 10(-11)-10(-14) M in the pancreas. Compounds 23 and 29 were further evaluated for their ability to stimulate amylase secretion and found to have potencies similar to that of CCK. The dissociation between potency in the binding and amylase secretion assays suggests that they may interact with a high affinity binding site which is not coupled to amylase secretion. We conclude that CCK receptors possess a generous hydrophobic pocket capable of accommodating large alkyl groups in place of the side chain of phenylalanine33 and that the pharmacological profile of CCK analogs can be tailored by appropriate exploitation of this finding.