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A series of novel CCK tetrapeptide analogues of the general formula Boc-Trp-Lys(Tac)-N(R)-(CH2)nCON(R')Phe-NH2 (Tac = o-tolylaminocarbonyl), where R,R' = H or Me and n = 1-5, have been synthesized and tested. These analogues, which lack an acidic residue at the penultimate position, demonstrated surprisingly high CCK-A receptor affinity and selectivity. The effect of N-methylation pattern on CCK-A receptor affinity showed consistent trends for analogues in which n = 1, 2, or 3, with the di-N-methylated analogues having the highest affinity in each case. However, none of these analogues had full agonist activity, as measured by percent maximal PI hydrolysis. Two conformationally constrained analogues also demonstrated high CCK-A receptor affinity and selectivity, as well as nearly maximal agonist activity. In addition, one of these conformationally-constrained analogues demonstrated anorectic activity in rats.

N-Methylation of backbone amide bonds was conducted on a tetrapeptide that had been identified previously (Shiosaki, K.; et al. J. Med. Chem. 1991, 34, 2387-2842) as a potent and selective CCK-A agonist. N alpha-Methylation at the position corresponding to Asp32 (CCK-33 numbering) was consistent with high affinity, efficacy, and selectivity for the CCK-A receptor. Combination of this (N-Me)Asp with the (N-Me)Phe modification also provided a highly active analogue. The observation of parallel structure-binding affinity profiles with respect to sites of N-methylation in the C-terminal regions of tetrapeptide vs heptapeptide CCK analogues suggests that the two series interact similarly with the CCK-A receptor.

We had reported earlier on a novel series of potent and selective tetrapeptide cholecystokinin-A (CCK-A) agonists of the general structure Boc-Trp-Lys[epsilon-Y]-Asp-N(R)PheNH2 [Y = amides, ureas; R = H, Me] that were potent anorectic agents in rats. In an effort to optimize the potency, selectivity, stability, and efficacy of our lead candidate A-71623 [R = Me, Y = o-tolylaminocarbonyl; Tac] toward development of a clinical candidate, we have explored a series of analogues in which the N-terminal Boc functionality was systematically replaced with various amides, ureas, carbamates, and sulfonamides of differing size, hydrophobicity, and stereoelectronic properties. In general, these analogues maintained good potency and selectivity for the CCK-A receptor (guinea pig pancreas), as well as potent anorectic activity in rats. Those analogues exhibiting equal or superior activity compared to A-71623 but differing physicochemical properties may represent superior drug candidates.

A series of Boc-CCK-4 derivatives represented by the general structure Boc-Trp-Lys(N epsilon-COR)-Asp-Phe-NH2, where R is an aromatic, heterocyclic, or aliphatic group, are potent and selective CCK-A receptor agonists. These amide-bearing compounds complement the previously described urea-based tetrapeptides (Shiosaki et al. J. Med. Chem. 1991, 34, 2837-2842); structure-activity studies revealed parallel as well as divergent trends between these two series. A significant correlation was observed between pancreatic binding affinity and the resonance constant R of the phenyl substituent in one particular series of derivatives. Sulfation of phenolic amides appended onto the epsilon-amino group of the lysine did not affect affinity for the CCK-A receptor in contrast to the 500-fold increase in binding potency observed upon sulfation of CCK-8, suggesting that the lysine appendage and the sulfated tyrosine in CCK-8, both key structural elements that impart high affinity for the CCK-A receptor, are interacting differently with the receptor. The amide-bearing tetrapeptides are full agonists relative to CCK-8 in stimulating pancreatic amylase release while being partial agonists in eliciting phosphoinositide (PI) hydrolysis. Both effects were blocked by selective CCK-A receptor antagonists.

The intriguing structural similarities of glutamic acid based cholecystokinin (CCK) antagonists (A-64718 and A-65186) and the benzodiazepine CCK antagonist MK-329 (L-364,718) have been reported. Efforts to include the weak CCK antagonist benzotript into this construct utilizing a similar approach have resulted in a novel series of benzotript-based hybrid antagonists N alpha-(3'-quinolylcarbonyl)-(R)-tryptophan di-n-pentylamide (9, A-67396), N alpha-(4',8'-dihydroxy-2'-quinolylcarbonyl)-(R)-tryptophan di-n-pentylamide (23, A-70276), and N alpha-(3'-quinolylcarbonyl)-(R)-5'-hydroxytryptophan di-n-pentylamide (36, A-71134) which possess respectively binding affinities of 23, 21, and 11 nM for the pancreatic CCK-A receptor and which inhibit CCK8-induced amylase secretion. Compound 9 possesses a selectivity of greater than 500-fold for the pancreatic CCK-A receptor over the CCK-B receptor.

Novel Boc-CCK-4 derivatives were communicated recently as having high potency and selectivity for the CCK-A receptor (Shiosaki et al. J. Med. Chem. 1990, 33, 2950-2952). While Boc-CCK-4 binds selectively to the CCK-B receptor, replacement of the methionine with an N epsilon-substituted lysine dramatically reversed receptor selectivity, leading to the development of this novel series of tetrapeptides. A detailed structure-activity analysis of a series of urea-substituted tetrapeptides, represented by the general structure Boc-Trp-Lys(N epsilon-CO-NHR)-Asp-Phe-NH2, revealed that a number of substituted phenyl, naphthyl, and aliphatic urea residues in the lysine side chain yielded potent and selective CCK-A ligands. These tetrapeptides elicit full agonist responses in stimulating pancreatic amylase release that are effectively blocked by a selective CCK-A receptor antagonist. Conversion of the urea to a thiourea significantly reduced CCK-A binding potency as did replacement of the lysine with the homologous ornithine or homolysine. Tetrapeptides that were partial agonists (less than 80% efficacy) in phosphoinositide (PI) hydrolysis relative to CCK-8 did not exhibit high-dose inhibition of amylase secretion in guinea pig acini.