Structure activity studies of tryptophan30 modified analogs of Ac-CCK-7

Structural studies on radiopharmaceutical DOTA-minigastrin analogue (CP04) complexes and their interaction with CCK2 receptor

Background

The cholecystokinin receptor subtype 2 (CCK-2R) is an important target for diagnostic imaging and targeted radionuclide therapy (TRNT) due to its overexpression in certain cancers (e.g., medullary thyroid carcinoma (MTC)), thus matching with a theranostic principle. Several peptide conjugates suitable for the TRNT of MTC have been synthesized, including a very promising minigastrin analogue DOTA-(DGlu)₆-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH₂ (CP04). In this contribution, we wanted to see whether CP04 binding affinity for CCK-2R is sensitive to the type of the complexed radiometal, as well as to get insights into the structure of CP04-CCK2R complex by molecular modeling.

Results

In vitro studies demonstrated that there is no significant difference in CCK-2R binding affinity and specific cellular uptake between the CP04 conjugates complexed with [⁶⁸Ga]Ga³⁺ or [¹⁷⁷Lu]Lu³⁺. In order to investigate the background of this observation, we proposed a binding model of CP04 with CCK-2R based on homology modeling and molecular docking. In this model, the C-terminal part of the molecule enters the cavity formed between the receptor helices, while the N-terminus (including DOTA and the metal) is located at the binding site outlet, exposed in large extent to the solvent. The radiometals do not influence the conformation of the molecule except for the direct neighborhood of the chelating moiety.

Conclusions

The model seems to be in agreement with much of structure-activity relationship (SAR) studies reported for cholecystokinin and for CCK-2R-targeting radiopharmaceuticals. It also explains relative insensitivity of CCK-2R affinity for the change of the metal. The proposed model partially fits the reported site-directed mutagenesis data.

Therapeutic potential for novel drugs targeting the type 1 cholecystokinin receptor

Cholecystokinin (CCK) is a physiologically important gastrointestinal and neuronal peptide hormone, with roles in stimulating gallbladder contraction, pancreatic secretion, gastrointestinal motility and satiety. CCK exerts its effects via interactions with two structurally related class I guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs), the CCK(1) receptor and the CCK(2) receptor. Here, we focus on the CCK(1) receptor, with particular relevance to the broad spectrum of signalling initiated by activation with the natural full agonist peptide ligand, CCK. Distinct ligand-binding pockets have been defined for the natural peptide ligand and for some non-peptidyl small molecule ligands. While many CCK(1) receptor ligands have been developed and have had their pharmacology well described, their clinical potential has not yet been fully explored. The case is built for the potential importance of developing more selective partial agonists and allosteric modulators of this receptor that could have important roles in the treatment of common clinical syndromes.

Comparative conformational analysis of CCK-B agonist Boc-Trp-Phg-Asp-(1-Nal)-NH2 and CCK1-B antagonist Boc-Trp-Phg-Asp-(1-Nal)-N(Me)2 using 1HNMR spectroscopy and restrained molecular dynamics

The tetrapeptide Boc-Trp-Phg-Asp-(1-Nal)-NH2 is a potent CCK-B agonist. Interestingly, bis-methylation of the C-terminal carboxamide group of this compound leads to Boc-Trp-Phg-Asp-(1-Nal)-N(Me)2 which behaves as a CCK-B antagonist in electrophysiological studies on hippocampal neurones (Corringer et al., 1993). In order to ascertain whether bismethylation of the terminal carboxamide group has an influence on the conformational preferences of the peptide, we have undertaken a comparative conformational analysis of the two tetrapeptides by the combined use of 2D NMR spectroscopy and restrained molecular dynamics. The solution conformation of the two peptides were examined by 1H NMR in a d6-DMSO/H2O (80:20) mixture. 1H-1H distance constraints, derived from 2D NOESY and ROESY experiments, were used as inputs for subsequent restrained molecular dynamics simulations. Comparison of the NMR and molecular modeling data indicates different conformational preferences for these two peptides. Interestingly, the aromatic side chains of the CCK-B antagonist Boc-Trp-Phg-Asp-(1-Nal)-N(Me)2 in its preferential conformation, overlap their corresponding moieties in the two non peptide CCK-B antagonists L-362,260 and LY-288,513. The differences in conformational behaviour of the studied tetrapeptides could, at least in part, account for their opposite agonist/antagonist profile, a findings which could serve for the design of new conformationally restricted CCK-B analogs.

Evidence for cholecystokinin receptor subtype in endocrine pancreas

Cholecystokinin (CCK) is a gut hormone that regulates pancreatic endocrine functions via CCKA receptors. CCK4 (Trp-Met-Asp-Phe-NH2) has an insulinotropic effect, but is 1000-fold less potent than CCK8. The in vitro potencies and selectivity of newly synthesized CCK4 analogs were investigated. Exchanging various a amino acids, for example Met by Nle and modifying Phe and/or Trp, led to compounds that were much more effective than CCK4 itself and show insulinotropic effects comparable with those of CCK8. Compounds that possess electron withdrawing groups on the C-terminal phenylalanine were especially effective; compounds with electron-donating groups had no effect. In contrast to CCK8 the synthetic CCK4 compounds were selective for the endocrine pancreas: they had no agonistic or antagonistic effect on the contraction of the guinea pig ileum, amylase release from isolated acini, and no major effect on the feeding behavior of mice being supplied with either compound by an implantable AlzetR pump for 8 days. The data indicate that some of the synthetic tetrapeptides exhibit a high affinity for the CCK receptor of the endocrine pancreas and that they are highly selective for this (peripheral) CCKA receptor subtype. The beta-cell CCKA receptors are different from those in exocrine pancreas, smooth muscle, and those for regulating appetite; these peripheral receptor subtypes can be discriminated for the first time.

Structure activity of C-terminal modified analogs of Ac-CCK-7

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.