Conformationally constrained CCK8 analogues obtained from a rationally designed peptide library as ligands for cholecystokinin type B receptor

A library of 14 cyclic peptide analogues derived from the octapeptide C-terminal sequence of the human cholecystokinin hormone (CCK(26-33), or CCK8) was designed, synthesized, and characterized. The 14 peptide analogues were rationally designed to specifically interact with the CCK type B receptor (CCK(B)-R) on the basis of the structure of the bimolecular complex between CCK8 and the third extracellular loop of CCK(B)-R, namely CCK(B)-R(352-379). The rational design of new ligands for CCK(B)-R has relied on stabilization by cyclic constraints of the structural motifs that bring the key residues of the ligand (especially Trp 30, Met 31, and Phe 33) in the proper spatial orientation for optimal interaction with the receptor. The binding affinity of the new ligands for CCK(B)-R was assessed by displacement experiments of (111)In-radiolabeled CCK8 in cells that overexpress the CCK(B) receptor. The new ligands generally showed binding affinities lower than that of parent CCK8, with the best compounds having IC50 values around 10 microM. Structure-activity relationship data show that preservation of the Trp 30-Met 31 motif is essential and that the Phe 33 side chain must be present. NMR conformational studies of the compound with maximal binding affinity (cyclo-B11, IC50=11 microM) in DPC micelles shows that this compound presents a turn-like conformation centered at the Trp 30-Met 31 segment, as planned by rational design. Such a conformation is stabilized by its interaction with the micelle rather than by the cyclic constraint.

Synthesis and biological characterisation of [3H]BBL454, a new CCK2 selective radiolabelled agonist displaying original pharmacological properties

[(3)H]BBL454, a new CCK(2) selective tritiated agonist was prepared via the reductive tritiation of a 5-aminopentyn-1-yl moiety introduced on the N-terminal end of a pentapeptide derivative of cholecystokinin. The binding properties of this labelled compound were determined on CHO cells transfected with the rat CCK(2) receptor. [(3)H]BBL454 is able to discriminate two affinity states of the CCK(2) receptor a supplementary indication of its validity for further exploring the heterogeneity of this receptor.

Differential bile-pancreatic secretory effects of CCK-58 and CCK-8

In this work, we 1) synthesized rat CCK-58, 2) determined the amounts and forms of rat CCK in whole blood after stimulation of its release by casein, 3) determined the potency of CCK-8 and CCK-58 peptides to displace labeled CCK-8 from CCK(A) and CCK(B) receptors transfected into Chinese hamster ovary (CHO) cells, and 4) examined the biological actions of CCK-8 and rat CCK-58 in an anesthetized rat model. CCK-58 was the only detected endocrine form of CCK in rat blood. Synthetic rat CCK-58 was less potent than CCK-8 for displacing the label from CCK(A) and CCK(B) receptors in transfected CHO cells. However, rat CCK-58 was more potent than CCK-8 for stimulation of pancreatic protein secretion in the anesthetized rat. In addition, CCK-58 but not CCK-8 stimulated fluid secretion in this anesthetized rat model. These data suggest that regions outside the COOH terminus of rat CCK-58 influence the expression of CCK biological activity. The presence of only CCK-58 in the circulation and the fact that its biological activity differs from CCK-8 suggests that CCK-58 deserves scrutiny in other physiological models of CCK activity.

Synthesis of biologically active canine CCK-58

The carboxyl terminal octapeptide of cholecystokinin (CCK-8) has been hypothesized to account for the bioactivity of all the molecular forms of cholecystokinin. However, the physiological relevance of CCK-58 has not been rigorously examined because of the lack of sufficient amounts of the peptide and concerns about inactivation of natural peptides during their purification. Therefore, canine-sulfated CCK-58 was synthesized and conditions determined for its unblocking and purification that preserved the sulfated tyrosine. Synthetic CCK-58 was indistinguishable from natural CCK-58 by amino acid analysis and by mass spectrometry. Synthetic CCK-58 and CCK-8 have different patterns of pancreatic stimulation: both caused a dose-related increase in amylase release, while only CCK-58 stimulated bile-pancreatic output volume. Thus, CCK-58 and CCK-8 are biased agonists at the CCK-A receptor (they have distinct patterns of action mediated by the same receptor). Previous work has demonstrated that the identical carboxyl termini of CCK-8 and CCK-58 have different solution conformations. Taken together, the physiological and structural results support the hypothesis that different carboxyl terminal conformations of CCK-58 and CCK-8 alter the expression of their biological activity.

CCK8 peptide derivatized with diphenylphosphine for rhenium labelling: synthesis and molecular mechanics calculations

A novel CCK8 derivative bearing a chelating agent at its N- end and its oxo-rhenium(V) complex have been synthesized and characterized. The chelating agent N-[N-13-(diphenylphosphino)propionyl]glycyl]cysteine (PN2S) ligand, the coordination set of which is made by the phosphorus atom of phosphine, the nitrogen atoms of the two amido groups and the sulphur atom of cysteine, has been used due to its high affinity towards the oxo-rhenium(V) moiety. Molecular modelling studies indicate that the CCK8 peptide adopts the right conformation for cholecystokinin receptor binding, and that modifications on the N-terminal side of CCK8 obtained by introducing chelating agents and its metal complexes should not affect the interaction with CCK(A) receptor.

Facile solid-phase synthesis of sulfated tyrosine-containing peptides: total synthesis of human big gastrin-II and cholecystokinin (CCK)-39

Chemical synthesis of tyrosine O-sulfated peptides is still a laborious task for peptide chemists because of the intrinsic acid-lability of the sulfate moiety. An efficient cleavage/deprotection procedure without loss of the sulfate is the critical difficulty remaining to be solved for fluoren-9-ylmethoxycarbonyl (Fmoc)-based solid-phase synthesis of sulfated peptides. To overcome the difficulty, TFA-mediated solvolysis rates of a tyrosine O-sulfate [Tyr(SO3H)] residue and two protecting groups, tBu for the hydroxyl group of Ser and 2,2,4,6,7-pentamethyldihydrobenzofuran-5-sulfonyl (Pbf) for the guanidino group of Arg, were examined in detail. The desulfation obeyed first-order kinetics with a large entropy (59.6 J.K-1.mol-1) and enthalpy (110.5 kJ.mol-1) of activation. These values substantiated that the desulfation rate of the rigidly solvated Tyr(SO3H) residue was strongly temperature-dependent. By contrast, the SN1-type deprotections were less temperature-dependent and proceeded smoothly in TFA of a high ionizing power. Based on the large rate difference between the desulfation and the SN1-type deprotections in cold TFA, an efficient deprotection protocol for the sulfated peptides was developed. Our synthetic strategy for Tyr(SO3H)-containing peptides with this effective deprotection protocol is as follows: (i) a sulfated peptide chain is directly constructed on 2-chlorotrityl resin with Fmoc-based solid-phase chemistry using Fmoc-Tyr(SO3Na)-OH as a building block; (ii) the protected peptide-resin is treated with 90% aqueous TFA at 0 degree C for an appropriate period of time for the cleavage and deprotection. Human cholecystokinin (CCK)-12, mini gastrin-II (14 residues), and little gastrin-II (17 residues) were synthesized with this method in 26-38% yields without any difficulties. This method was further applied to the stepwise synthesis of human big gastrin-II (34 residues), CCK-33 and -39. Despite the prolonged acid treatment (15-18 h at 0 degree C), the ratios of the desulfated peptides were less than 15%, and the pure sulfated peptides were obtained in around 10% yields.

A convenient synthesis of novel bifunctional prochelators for coupling to bioactive peptides for radiometal labelling

New DOTA-based bifunctional prochelators, e.g., 1-(1-carboxy-3-carbotertbutoxypropyl)-4,7,10-(carbotertbutoxyme thyl)-1,4,7,10-tetraazacyclodode-cane (DOTAGA(tBu)4), (6d) for a broad application in the modification of biomolecules with metal ions were prepared. The five-step synthesis of 6d has an overall yield of about 20%. The coupling of 6d to a bioactive peptide on solid-phase was exemplified with use of a CCK-B (cholecystokinin) analogue.

Solid-phase peptide synthesis at elevated temperatures: a search for and optimized synthesis condition of unsulfated cholecystokinin-12

A systematic investigation of solid-phase peptide synthesis at elevated temperatures using the well-known aggregating peptide acyl carrier protein (65-74) and the unsulfated cholecystokinin-8 as models is presented. The main goal of the investigation was the determination of an optimized experimental condition for the synthesis of unsulfated cholecystokinin-12. Of the elevated temperatures used, 60 degrees C was the most appropriate. The efficiency of N,N'-diisopropylcarbodiimide/1-hydroxybenzotriazole (DIC/HOBt) in 25% dimethyl sulfoxide (DMSO)/toluene at this temperature was similar to that of 2-(1-H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate (TBTU). Interestingly, this coupling reagent was more efficient than TBTU, benzotriazol-1-yl oxy-tris(dimethylamino)phosphonium and O-(7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate in N-methylpyrrolidone. 25% DMSO/toluene proved to be suitable for the swelling of the resins phenylacetamidomethyl, methylbenzhydrylamine, hydroxymethylphenoxy, 4-(benzyloxy)-2',4'-dimethoxybenzhydrylamine, 4-(2',4'-dimethoxyphenyl-Fmoc-aminomethyl)phenoxy and (4-succinylamido-2',2',4'-trimethoxy)benzhydrylamine. Those polymeric supports were fully compatible with the approach. Under the optimized synthesis condition found in these studies (temperature of 60 degrees C, DIC/HOBt as coupling reagent and 25% DMSO/toluene as solvent), no difficulties related to the aggregation phenomenon were encountered. These data confirm the usefulness of solid-phase peptide synthesis at elevated temperatures and extend its applicability.

[Synthesis of peptide and pseudo-peptide analogs of cholecystokinin]

We describe selective CCKA receptor antagonists, based on the 1-oxo-1,2,3,4-tetrahydro-beta-carboline-3-carboxylic acid core. Selectivity A vs B is discussed on the basis of molecular modelling. Chemical preparation uses electrophilic cyclization of isocyanates derivating from unnatural tryptophan esters. A stereoselective version of the reaction is given. A few peptides incorporating unnatural tryptophans are prepared, with a view of SAR.

Conformationally readdressed CCK-B/delta-opioid peptide ligands

The sequence of a cholecystokinin (CCK) related peptide was modified to obtain analogues, which interact selectively either with CCK-B, or with delta-opioid receptors. Two kinds of peptides were designed, namely, the cyclic peptides of the H-Tyr-cyclo (D-Pen-Gly-Trp-L/D-3-transmercaptoproline)-Asp-Phe-NH2 sequence (compounds 1a and 1b, respectively), and the linear peptides of the H-Tyr-D-Val-Gly-Trp-L/D-3-trans-methylmercaptoproline-Asp-Phe- NH2 sequence (compounds 2a and 2b, respectively). The only difference between the chemical structures of the linear analogues compared to the cyclic ones is that one covalent bond has been eliminated and a sulfur atom is replaced by a methyl group. Molecular modeling showed that, among low-energy conformers of cyclic compounds 1, there are three-dimensional structures compatible to the model for delta-receptor-bound conformer, suggested earlier [G. V. Nikiforovich, V.J. Hruby, O. Prakash, and C.A. Gehrig (1991) Biopolymers, vol. 31, pp. 941-955]. Results of binding assays fully supported the rationale for the design of compounds 1 and 2. The cyclic analogue 1a has Ki values of 4.5 and > 5000 nM at delta- and mu-opioid receptors, respectively; and IC50 values of 1.6 and > 10,000 nM for CCK-A and CCK-B receptors, respectively. The results of this study demonstrate a possibility to redirect a peptide sequence that interacts with one type of receptors (CCK-B receptors) toward interaction with another type (delta-opioid receptors) belonging to a different physiological system. This redirection could be performed by changing the conformational properties of the peptide with very minimal changes in its chemical structure.

The 'peptoid' approach to the design of non-peptide, small molecule agonists and antagonists of neuropeptides

This article describes a rationale and strategy for the design of low molecular weight, non-peptide ligands (peptoids), using the chemical structure of mammalian neuropeptides as a starting point. These peptoids may act as either agonists or antagonists at neuropeptide receptors. As they are non-peptides, they can serve as robust tools to help establish the role of the peptides in models of physiological and pathophysiological processes and indeed they may emerge as therapeutic agents in their own right. The strategy is exemplified by the first rational design of 'peptoid' ligands for cholecystokinin (CCK) and tachykinin receptors.

Photocleavable biotinylated ligands for affinity chromatography

Two derivatives of the peptide hormone cholecystokinin (CCK-8s) containing a photocleavable o-nitrobenzylester group have been synthesized. One analog, [3H]4-(biotin-epsilon-Ahx-oxymethyl)-3-nitrobenzoyl-Gly-Orn (propionyl)-epsilon-Ahx-CCK-8s ([3H]BANA-CCK-8s), was biotinylated, while the other, 4-alanyloxymethyl-3-nitrobenzoyl-epsilon-Ahx-CCK-8s (ANA-CCK-8s) had a free amino group for coupling to amino-reactive affinity matrices. The analogs retained high affinity to both CCK receptors and anti-CCK antibodies. [3H]BANA-CCK-8s was bound to a streptavidin-agarose affinity matrix and was subsequently released by irradiation with uv light of wavelengths > 320 nm. The ability to recover a biotinylated peptide ligand from a streptavidin column under very mild conditions should be useful in the purification of specifically binding proteins. As a demonstration of this approach, [3H]BANA-CCK-8s was incubated with anti-CCK-antiserum and was subsequently passed over a streptavidin-agarose affinity matrix. After washing, the bound antibodies were eluted by photocleavage of the affinity ligand. Forty-six percent of the bound antibodies were recovered after 40 min of irradiation. The eluted antibodies showed essentially unchanged binding characteristics. The suitability of ANA-CCK-8s for the purification of anti-CCK antibodies was also demonstrated. The approach may prove to be generally useful in the isolation of labile proteins in an intact form.

[Synthesis of protected active peptide analogues of cholecystokinin and study of their biological activities]

A number of protected active peptide analogues of Cholecystokinin were prepared in solution by N-end stretching method with active esters. In the synthesis t-butoxycarbonyl (Boc-) and benzyloxycarbonyl (Z-) were applied as protecting groups. Six oligopeptides which contained protecting groups have not yet been reported in literature. Their structures were identified by amino acid analysis, opticity, IR and FAB-MS. The results of preliminary pharmacological tests show all the samples have biological activities in various degrees.

Total synthesis, purification, and characterization of human [Phe(p-CH2SO 3Na)52, Nle32,53,56, Nal55]-CCK20-58, [Tyr52, Nle32,53,56, Nal55]-CCK-58, and [Phe(p-CH2SO3Na)52, Nle32,53,56, Nal55]-CCK-58

The synthesis of [Phe(p-CH2SO3Na)52, Nle32,53,56 Nal55]-CCK20-58, [Tyr52, Nle32,53,56, Nal55]-CCK-58 and of [Phe(p-CH2SO3Na)52, Nle32,53,56, Nal55]-CCK-58 using the (9-fluorenylmethyloxy)-carbonyl (Fmoc) strategy on a 2,4-DMBHA resin is described. The crude peptide preparations were extremely complex when analyzed by RP-HPLC, capillary zone electrophoresis (CZE), and ion-exchange chromatography (IE-FPLC). We found that the most effective strategy for purification included cation-exchange chromatography followed by a RP-HPLC desalting step. The highly purified peptides (purity greater than 90%) were characterized by RP-HPLC, size exclusion HPLC (SEC), IE-FPLC, CZE, mass spectrometry, amino acid analysis, and Edman sequence analysis (for [Tyr52, Nle32,53,56, Nal55]-CCK-58). The results demonstrate the applicability of the 2,4-DMBHA resin for Fmoc solid-phase synthesis of long peptides amides (58 residues in length in this case) as well as the efficacy of an FPLC/RP-HPLC approach for the purification of very long, heterogeneous crude peptides, allowing a true assessment of the biological properties of these analogs to be carried out. [Phe(p-CH2SO3Na)52, Nle32,53,56, Nal55]-CCK20-58 was less than 1% as potent as CCK-8 while [Tyr52, Nle32,53,56, Nal55]-CCK-58 and [Phe(p-CH2SO3Na)52, Nle32,53,56, Nal55]-CCK-58 were inactive at the doses tested (< 0.01%).

Double bond isosteres of the peptide bond: synthesis and biological activity of cholecystokinin (CCK) C-terminal hexapeptide analogs

New and existing methodologies were used to prepare a series of modified CCK analogs in which each amide bond was replaced by a trans-alkene unit. The data indicate that every amide linkage at C-terminal tetrapeptide (CCK-4) region is crucial for biological activity. While the amide bond beyond the Trp residue in the N-terminal direction can be replaced by a trans-alkene and still retain most of the binding potency and functional activity.

Evaluation of the final deprotection system for the solid-phase synthesis of Tyr(SO3H)-containing peptides with 9-fluorenylmethyloxycarbonyl (Fmoc)-strategy and its application to the synthesis of cholecystokinin (CCK)-12

Acidolytic deprotection and cleavage conditions for an acid-labile Tyr(SO3H)-containing peptide were systematically examined with respect to acid, temperature, and scavenger. The 90% aqueous trifluoroacetic acid (TFA)-based reagent systems (90% aqueous TFA/m-cresol and 90% aqueous TFA/m-cresol/2-methylindole) at 4 degrees C were found to minimize the deterioration of Tyr(SO3Na) in the peptide. The latter deprotection/cleavage system was applied to the 9-fluorenylmethyloxycarbonyl (Fmoc)-based solid-phase synthesis of cholecystokinin (CCK)-12 on an acid-labile PAL-linked support (PAL = peptide amide linker), with Fmoc-Tyr(SO3Na)-OH as a building block.

Synthesis and biological activity of CCK heptapeptide analogues. Effects of conformational constraints and standard modifications on receptor subtype selectivity, functional activity in vitro, and appetite suppression in vivo

A series of modifications of the CCK7 analogue (des-NH2)Tyr(SO3-)-Nle-Gly-Trp-Nle-Asp-Phe-NH2 was prepared and tested for binding to guinea pig CCK-A and CCK-B receptors and in CCK-A-mediated functional assays. Selected analogues also were tested for appetite suppressant activity in rats. Several conformationally restricted residues in the C-terminal tetrapeptide region, including delta Z-Phe33, (N-Me)Phe33, (N-Me)Asp32, (N-Me)Leu31, and 3PP31 (3PP = trans-3-n-propyl-L- proline) were found to be acceptable modifications at one or both receptor subtypes. The (N-Me)Asp32 and (N-Me)Leu31 modifications afforded potent and selective CCK-A and CCK-B ligands, respectively. SAR studies in the N-terminal acyldipeptide region examined structural requirements for the side chain at position 28, where Gly and Pro replacements were found to possess high affinity at both receptor subtypes. Other conformationally restrictive modifications were less active. All of the analogues that showed high affinity (less than 10 nM) for the CCK-A receptor also were full agonists in amylase release and most were full or nearly full agonists in the phosphoinositide (PI) turnover assay, the most notable exception being the delta Z-Phe33 analogue, which showed 69% of the maximal response in the PI assay. Potent activity in suppression of food intake in rats was found for selected analogues.

Rationally designed "dipeptoid" analogues of CCK. A Free-Wilson/Fujita-Ban analysis of some alpha-methyltryptophan derivatives as CCK-B antagonists

A Free-Wilson/Fujita-Ban (FW/FB) analysis is reported on 36 "dipeptoid" antagonists of the CCK-B receptor. This series of compounds includes [R-(R*,R*)]-4-[[2-[[3-(1H-indol-3-yl)-2-methyl-1-oxo-2- [[(tricyclo[3.3.1.1] dec-2-yloxy)carbonyl]amino]propyl]amino]-1-phenylethyl]amino]- 4-oxobutanoic acid (CI-988, 1, Figure 1), the first rationally designed non-peptide antagonist of a neuropeptide receptor. The analysis treats the compounds in three parts: the N-terminus, variants on the tryptophan moiety, and the C-terminus. A highly significant correlation was found (n = 36, r2 = 0.97, s = 0.22, F = 57, p = 2 x 10(-8)), suggesting that these three domains of these compounds contribute to binding affinity independently of each other, and are therefore additive in their effects on receptor affinity. The relative free-energies of binding of the individual substituents are calculated from the coefficients of the regression equation. The substitution of D-alpha-methyltryptophan for L-tryptophan increases the free-energy of binding by 3.5 kcal mol-1. This increase in binding energy is explained by a 300-fold difference in conformational entropy between the methylated and desmethyl analogues.

Synthesis and biological activity of novel, potent and long-acting analogs of AC-CCK-7 with high affinity for peripheral (type A) receptors

Analogs of cholecystokinin (CCK-7) in which the N-terminal Tyr(SO3H) was acetylated, Asp6 replaced with Thr(SO3H) and Phe7 replaced with N-methyl-Phe were prepared by solid-phase peptide synthesis and evaluated for their receptor-binding activity and ability to suppress appetite. The receptor binding activities of these synthetic analogs of CCK-7 and their selectivity for the CCK-A and CCK-B receptor subtypes were determined using solubilized membrane preparations from rat pancreatic tissue and bovine striatum. The synthetic peptide Ac-Tyr(SO3H)-Met-Gly-Trp-Met-Thr(SO3H)-N-methyl-Phe-NH2 (referred to as Ro 23-7014) demonstrated superior satiating potency (ED50 = 0.3 micrograms/kg, i.p.), increased selectivity for CCK-A receptors (400-fold), increased resistance to peptidergic degradation and a longer duration of action (4 to 5 hours). This analog also effectively suppressed food intake following intranasal administration (ED50 = 100 micrograms/kg). These studies demonstrate the feasibility of designing analogs of CCK-8 with greater selectivity, potency and duration of action, which may be useful as nonsystemically administered appetite suppressants.

Rationally designed "dipeptoid" analogues of CCK. alpha-Methyltryptophan derivatives as highly selective and orally active gastrin and CCK-B antagonists with potent anxiolytic properties

This paper describes the synthesis and structure-activity relationships (SAR) leading to the first rational design of "dipeptoid" analogues of the neuropeptide cholecystokinin (CCK). Compounds [R-(R*,S*)]-4-[2-[3-(1H-indol-3-yl)-2-methyl-1-oxo-2-[(tricyclo [3.3.1.1(3,7)]dec-2-yloxy)carbonyl]amino]propyl]amino]-3- phenylpropyl]-amino]-4-oxo-2-butenoic acid, [R-(R*,R*)]-4-[2-[3-(1H-indol-3-yl)-2-methyl-1-oxo-2-[(tricyclo [3.3.1.1(3,7)]dec-2-oxy)carbonyl]amino]propyl]amino]-1- phenylethyl]amino]-4-oxo-2-butenoic acid, and [R-(R*,R*)]-4-[2-[3-(1H-indol-3-yl)-2-methyl-1-oxo-2-[(tricyclo [3.3.1.1(3,7)]dec-2-yloxy)carbonyl]amino]propyl]amino]-1- phenylethyl]amino]-4-oxobutanoic acid (29d) have CCK-B binding affinities of IC50 = 0.8, 0.7, and 1.7 nM with a CCK-A/CCK-B ratio of 550, 1100, and 2500, respectively. Compound 27 is well-absorbed and is equiactive by the subcutaneous (sc) and intravenous (iv) routes of administration in the Ghosh and Schild test in rats in inhibiting pentagastrin stimulated gastric acid secretion with ED50 = 0.07 (0.01-0.34) mumol/kg. Compound 29d is anxiolytic in mice in the black-white test box over the range 0.0001-30 mg/kg sc, comparable in activity to diazepam over the range 0.125-1 mg/kg ip), and also active in this test when dosed orally over a wide range from 0.0001 to 10 mg/kg.

[Structure-activity relationship of cystokinins: analogs exhibiting selective activity]

Structure-activity relationship on cholecystokinin is presented. C-terminal heptapeptide analogues of cholecystokinin exhibiting selective agonist or antagonist activities were synthesized and their biological and pharmacological properties studied. We showed that: 1) Suppression of the C-terminal phenylalanine residue leads to peripheral as well as central cholecystokinin receptor antagonists; 2) Suppression of the C-terminal amide function produces "partial agonists" exhibiting interesting biological and pharmacological activities; 3) Replacement of L-tryptophan residue by D-tryptophan in such "partial agonist analogues" resulted in potent CCK receptor antagonists; 4) The peptide bond between methionine28 and glycine29, as well as the glycine residue are quite significant for the central biological activity; 5) It is possible to obtain highly potent and selective CCK analogues for the central receptor (CCK-B) by cyclization including the C-terminal tetrapeptide. Synthesis and pharmacological studies with these analogues have allowed to precise the significance of some amino acid residues as well as of some peptide bonds. They are significant pharmacological tools for the study of CCK-A (peripheral) and CCK-B (central) receptors, their biological actions and their associated intracellular messengers.

Synthesis of cyclic analogues of cholecystokinin highly selective for central receptors

Cyclic CCK analogues in which positions 28 and 31 have been replaced by lysine residues and whose side chains are bridged by a succinic moiety, were synthesized. They were tested for their ability to inhibit the binding of 125I-BH-CCK-8 to isolated rat pancreatic acini and to guinea pig brain membranes. These cyclic CCK-analogues were compared to the potent CCK analogue Boc-[Nle28,31]-CCK-7 and to Boc-Trp-Leu-Asp-Phe-NH2, analogue of CCK-4. These cyclic compounds appeared to be highly selective for central CCK receptors.

A new probe for affinity labelling pancreatic cholecystokinin receptor with minor modification of its structure

Biochemical studies on receptors for peptides are most often carried out on affinity-labelled (peptide-receptor) complexes. Necessarily, the assumption is made that a covalent (peptide-receptor) complex behaves as the native receptor. The validity of this assumption is dependent on both the affinity-labelling technique and the resolution of the analytical method used for biochemical characterization. We designed a new affinity-labelling probe in order to minimize structural modifications occurring within the affinity-labelled cholecystokinin (CCK) receptor protein. The probe was 125I-labelled 2-(p-azidosalicylamido)-1,3-dithiopropionate-[Thr28,Ahx31 ]CCK-25-33, (125I-ASD-[Thr28,Ahx31]CCK-25-33), the peptide moiety of which was released from its binding site by reduction. It was obtained by coupling a photoactivable chemical to [Thr28,Ahx31]CCK-25-33 via its N-terminus. The resulting peptide was HPLC purified and radioiodinated in the presence of chloramine T. Binding of 125I-ASD-[Thr28,Ahx31]CCK-25-33 was time- and temperature-dependent and reversible. At 25 degrees C, a steady-state level was reached after 60 min and half-maximal dissociation after 38 min. Binding was inhibited by [Thr28,Ahx31]CCK-25-33 and L-364-718 antagonist with IC50 0.4 nM and 0.9 nM, respectively. Photoaffinity labelling of pancreatic plasma membranes by 125I-ASD-[Thr28,Ahx31]CCK-25-33 identified a glycoprotein of Mr 85,000-100,000 which was retained on immobilized wheat germ agglutinin. Enzyme cleavage by endoproteinase Glu-C generated a main fragment of Mr 30,000-34,000. The same glycoprotein was photoaffinity labelled with 125I-DTyr-Gly-[Ahx28,31,pNO2Phe33]CCK-26-33 (Ahx, 2-aminohexanoic acid; pNO2Phe,p-nitrophenylalanine) an intrinsic probe having its photolabile group sited in the binding domain of cholecystokinin. 125I-ASD-[Thr28,Ahx31]CCK-25-33 is a potentially powerful tool for biologically and biochemically studying cholecystokinin receptors.

Synthesis and biological activities of some cholecystokinin analogues substituted in position 29 by a beta-alanine

Syntheses of analogues of the C-terminal heptapeptide of cholecystokinin are described. These analogues were obtained by replacing glycine 29 by a beta-alanine. The C-terminal phenylalanine amide was in some cases substituted by 2-phenylethyl alcohol and/or residues of the C-terminal tetrapeptide by their D-enantiomers. These compounds were tested for their action on stimulation of amylase release from rat pancreatic acini and for their ability to inhibit binding of labeled CCK to rat pancreatic acini and guinea pig brain membranes. Some of these derivatives behaved as CCK receptor antagonists.

Synthesis and biological activity of some partially modified retro-inverso analogues of cholecystokinin

Syntheses of some partially modified retro-inverso analogues of the C-terminal octa- or heptapeptide of cholecystokinin are described. These analogues (in which the C-terminal carboxamide was deleted or not) were obtained by reverting one or several peptide bonds in the parent molecule. All these compounds were able to inhibit binding of labeled CCK-8 to rat pancreatic acini and guinea pig brain membranes and to stimulate amylase release from rat pancreatic acini with various potencies. Some of these derivatives reproduce only part of the biological response of CCK on amylase release.

Conformational behavior of cyclic CCK-related peptides determined by 400-MHz 1H-NMR: relationships with affinity and selectivity for brain receptors

The conformational study of a homogenous series of cyclic analogues of CCK8, selective for central receptors, such as Boc-X-Tyr(SO3H)-Nle-D-Lys-Trp-Nle-Asp-Phe-NH2, where X = L-Glu, D-Glu, or gamma-D-Glu, was performed by 400-MHz 1H-nmr. The regular increase in affinity for central receptors when going from [L-Glu] to [gamma-D-Glu] is correlated to (a) an enhancement in internal flexibility of the cyclic moiety, (b) an external orientation of the tyrosine side chain, and (c) a restructuring of the C-terminal part of the peptide. All these results could permit a modeling of biologically active conformation of CCK8 for both receptors types to be performed.

Enzyme-resistant CCK analogs with high affinities for central receptors

Based on the results of the in vitro metabolism of CCK8 by various peptidases, we have synthesized three CCK analogs: Boc-Tyr(SO3H)-Nle-Gly-Trp-(N- Me)Nle-Asp-Phe-NH2 (compound I), Boc-Tyr(SO3H)-gNle-mGly-Trp-Nle-Asp-Phe-Nh2 (compound II), Boc-Tyr(SO3H)-gNle-mGly-Trp-(N-Me)Nle-Asp-Phe-NH2 (compound III). In in vitro enzymatic degradation studies, these compounds showed a high stability toward either enkephalinase or the enzymes present in crude rat brain membranes preparations. Moreover, in binding studies on guinea pig tissues, these CCK-related peptides were characterized by high apparent affinities for brain CCK receptors and by a broader range of affinities for pancreatic CCK receptors. This broad range of affinities was reflected by their pharmacological potencies in the guinea pig pancreatic amylase release and ileum contraction assays. These enzyme-resistant CCK analogs provide therefore valuable tools to investigate the pharmacology of CCK.

Cholecystokinic activity of N alpha-hydroxysulfonyl-[Nle28,31]CCK26-33 analogues modified at the C-terminal residue

Three new analogues of N alpha-hydroxysulfonyl-[Nle28,31]CCK26-33 are reported in which the C-terminal L-Phe33 residue has been replaced by L-Leu, D-Phe or N-methyl-L-Phe. Biological evaluation in a series of binding and bioassays demonstrates that both L-stereochemistry and an aromatic side chain at position-33 are essential for full agonist activity. While the L-Leu33 and D-Phe33 analogues had reduced potencies in stimulating contraction of the guinea pig ileum or gall bladder, the D-Phe33 analogue was fourfold selective for the ileum. This latter analogue also exhibited apparent partial agonism in the rat pancreatic amylase release assay. The N-methyl-L-Phe33 analogue was almost equipotent to the parent analogue in all bioassays, suggesting that this modification might be useful for introducing enzymatic stability in CCK analogues.

Use of N,O-bis-Fmoc-D-Tyr-ONSu for introduction of an oxidative iodination site into cholecystokinin family peptides

We report the synthesis of a new reagent for the introduction of an oxidative iodination site into the amino terminus of acid-labile peptides, and the use of this reagent to synthesize a novel affinity-labeling probe for the cholecystokinin (CCK) receptor. The acylation reagent, N,O-bis-fluorenylmethyloxycarbonyl-D-tyrosine hydroxysuccinimide ester, utilizes base-labile protection of both the alpha amino group and the aromatic ring hydroxyl. This can be safely removed to expose a cross-linkable free amino group on the aminopeptidase-resistant D-enantiomer of tyrosine. The synthetic probe, D-Tyr-Gly-Asp-Tyr(OSO3H)-Nle-Gly-Trp-Nle-Asp-Phe-NH2, was fully biologically active, could be radioiodinated to high-specific radioactivity (2000 Ci/mmol), bound with high affinity to the pancreatic CCK receptor, and covalently labeled the hormone-binding site. This reagent should be useful for the synthesis of a wide variety of analogues of CCK and other acid-labile peptides.

Studies on immunoassays of peptide factors. V. Synthesis of cholecystokinin-58-[1-11]/iso-1-cytochrome c conjugate

In previous studies on model compounds we have found that the maleimide function is sufficiently stable under the conditions of peptide synthesis to allow its incorporation at preselected positions of a peptide chain in earlier steps of the synthetic route. Taking advantage of this observation the N-terminal undecapeptide of canine cholecystokinin-58 containing at its N-terminus the maleimide group became accessible in high yields as chromatographically homogenous and analytically well characterized compound. Via the incorporated anchor group the undecapeptide was linked selectively at its N-terminus to the cysteine residue 107 of iso-1-cytochrome c to yield a well characterized conjugate of 1:1 stoichiometry for immunization experiments.

Preparation and characterization of a new cholecystokinin receptor probe that can be oxidatively radioiodinated

Oxidative iodination is the simplest, most efficient, and least expensive method for radiolabeling peptide hormones. It has not been applied to cholecystokinin (CCK), however, because this peptide lacks an unsubstituted tyrosine residue and oxidation abolishes its biologic activity. We report the synthesis, purification, and characterization of the first derivative of CCK that can be radioiodinated with an oxidative method while maintaining full biologic activity and receptor binding affinity. Boc-Tyr-[(Thr28, Nle31)CCK-25-33] was synthesized, iodinated using a solid-phase oxidant, and purified on reverse-phase high-pressure liquid chromatography to a specific activity of 2125 Ci/mmol. Both native and iodinated Boc-Tyr-[(Thr28, Nle31)CCK-25-33] exhibited efficacy and potency for stimulation of in vitro pancreatic enzyme secretion that were identical to CCK-8. Binding of Boc-125I-Tyr-[(Thr28, Nle31)CCK-25-33] to rat pancreatic plasma membranes was rapid, reversible, temperature-dependent, saturable, and specific. The abilities of various molecular forms of CCK to compete for this binding paralleled their potencies for stimulation of pancreatic secretion (Kd: CCK-8, 0.8 nM; CCK-33, 3 nM; CCK-8DS, 1 microM; CCK-4, 50 microM), whereas structurally unrelated pancreatic ligands (1 microM) demonstrated no significant competition. These findings suggest that Boc-125I-Tyr-[(Thr28, Nle31)CCK-25-33] interacts with the same high-affinity pancreatic receptor as the CCK receptor probes previously reported, i.e., the Bolton-Hunter-labeled CCK-33 and CCK-8. This probe also possesses the major advantages of simplicity, efficiency, and cost of the labeling procedure, and the stability and relative oxidation-resistance of the product. Boc-125I-Tyr-[(Thr28, Nle31)CCK-25-33] will be useful for the characterization of binding interactions between CCK and its target tissues.

Synthesis of Z-CCK-27-32-NH2, Z-Tyr(SO-3)-Met-Gly-Trp-Met-Asp-NH2, a cholecystokinin receptor antagonist and an inhibitor of gastrin-induced acid secretion

The synthesis of the hexapeptide Z-Tyr(SO-3)-Met-Gly-Trp-Met-Asp-NH2, representing the C-terminal sequence of cholecystokinin minus the C-terminal phenylalanyl residue is described. This peptide was shown to be the most potent cholecystokinin receptor antagonist in vitro described to date. It is also able to inhibit gastrin-induced acid secretion in vivo, in the rat and was proved to antagonize the action of the C-terminal octapeptide of cholecystokinin in the central nervous system.

One-step isocratic high-performance liquid chromatographic purification of radioiodinated and radioiodinated-photoactivable derivatives of cholecystokinin

N-Hydroxysuccinimidyl-3-(4-hydroxy-3-[125I]iodophenyl)propionate (the Bolton-Hunter reagent) was conjugated with (Thr 34, NLeu 37) cholecystokinin (CCK) 31-39 in anhydrous dimethylformamide-pyridine in 20% yield. The radiolabelled peptide was purified from the reaction mixture in one step, by isocratic elution from a C18 high-performance liquid chromatographic (HPLC) column with 35% aqueous acetonitrile-0.13% heptafluorobutyric acid as eluent. The concentration of the radiolabelled peptide was estimated by UV monitoring. The acylating photoactivable radioiodinated reagent N-hydroxysuccinimidyl-N-(4-azido-2-nitrophenyl)-3-[125I]iodotyrosi ne was synthesized, purified on a C18 HPLC column by isocratic elution with 65% acetonitrile-1 mM hydrochloric acid, then conjugated with (Thr 34, NLeu 37) CCK31-39. The resulting photoactivable radioiodinated CCK analogue was purified by isocratic elution on a C18 HPLC column with 39% aqueous acetonitrile-0.1 M triethylamine phosphate (pH 3.5). The binding ability of both tracers and their non-radioactive analogues to CCK receptors was tested on rat pancreatic plasma membranes. As compared to a KD of 4.5 nM for unmodified (Thr 34, NLeu 37) CCK31-39, the KD of the radioiodinated Bolton-Hunter derivative was 3 nM, and that of the photoactivable radioiodinated derivative was 19 nM.

Protease-catalyzed peptide bond formation: application to synthesis of the COOH-terminal octapeptide of cholecystokinin

This study of protease-catalyzed peptide synthesis reports the preparation of the COOH-terminal octapeptide amide of cholecystokinin. The octapeptide was assembled by chemical condensation of two tetrapeptide segments that had been synthesized through the concerted catalytic reactions of several proteases of different specificities. The resulting octapeptide derivative was subjected to catalytic transfer hydrogenation, followed by sulfation of its tyrosine residue and removal of the N alpha-protecting group. The homogeneous target peptide was obtained after purification via partition chromatography, gel filtration, and ion-exchange chromatography. The synthetic octapeptide stimulated amylase release from pancreatic acinar cells.

Synthesis and some pharmacological properties of Z-Tyr(SO3H)-Met-Gly-Trp-Met-Asp(Phe-NH2)-OH, a 32-beta-aspartyl analogue of cholecystokinin (pancreozymin) 27-33

The heptapeptide carbobenzoxy-L-tyrosyl(O-sulfate)-L-methionylglycyl-L-tryptophyl-L-methionyl-L- aspartyl-beta-L-phenylalanine amine (Z-32-beta-Asp-CCK-27-33) was synthesized and tested for its ability to stimulate secretion from dispersed pancreatic acini in vitro, to increase protein secretion from cat pancreas in vivo, and to cause contraction of guinea pig gallbladder in situ. In increasing amylase secretion in vitro, the Z-32-beta-Asp-CCK-27-33 was equal in efficacy with but approximatively one-third as potent as the Boc-CCK-27-33, and when tested in vivo its activity is approximately 10 Ivy dog units (Idu)/microgram. In stimulation of the contraction of the gallbladder, it showed an activity lower than 1 Idu/microgram. This analogue has more pancreozyminic activity than cholecystokin-like activity. This seems to indicate different affinities for the two receptors.

[Cholecystokinin-pancreozymin synthesis. Synthesis of [28-threonine,31-norleucine]- and [28-threonine,31-leucine]cholecystokinin-pancreozymin-(25-33)-nonapeptide]

The syntheses of two analogues related to the C-terminal nonapeptide amide sequence 25-33 of cholecystokinin-pankreozymin are described. Based on the primary structure of the CCK-PZ-active caerulein and the experiences gained from the methionine replacement with leucine or norleucine in human little gastrin I, the analogues were designed by substituting methionine 28 with threonine, and methionine 31 with leucine and norleucine, respectively. Using a new method for the synthesis of tyrosine-O-sulfate-containing peptides, developed in our laboratory, and applying acid-labile side-chain protection in combination with the benzyloxycarbonyl group, the fully protected nonapeptide amide derivatives Z-Arg(Z2)-Asp(OBut)-Tyr-(SO3Ba1/2)-Thr(But)-Gly-Trp-Leu-Asp(OBut)-Phe-NH2 and Z-Arg(Z2)-Asp(OBut)-Tyr(SO3-Ba1/2)-Thr(But)-Gly-Trp-Nle-Asp(OBut)-Phe-NH2, were obtained. Upon hydrogenolytic and subsequent acidolytic removal of the protecting groups, followed by purification via chromatographic procedures the nonapeptide amides were isolated in satisfactory yields at a high degree of purity. In vivo and in vitro assays showed that a substitution of methionine 31 by norleucine with concomitant replacement of methionine 28 by threonine produced a fully active analogue, whereas for the threonine 28, leucine 31 analogue the pankreozymin-activity was lowered by a factor 10.

[Studies on the total synthesis of cholecystokinin-pancreozymin. Synthesis of the suitably protected fragment corresponding to the sequence 24--33 (author's transl)]

The synthesis is described of a suitably protected decapeptide derivative corresponding to the C-terminal tyrosin-O-sulfate-containing sequence 24--33 of cholecystokinin-pancreozymin as key fragment for the total synthesis of this gastrointestinal hormone or its 39-variant. This peptide derivative has been prepared according to our newly developed procedure based on the direct use of N-acyltyrosine-O-sulfate barium salt. Additionally, removal of the protecting groups followed by purification via chromatographic methods produced the decapeptide amide in good yields and with a high degree of purity as a fully active cholecystokinin-pancreozymin related peptide.

Cholecystokinin (pancreozymin). Synthesis and properties of the N alpha-acetyl-derivative of cholecystokinin 27-33

The acetyl-derivative of the biologically active C-terminal 7-peptide portion of cholecystokinin (CCK), N-acetyl-O-sulfate-L-tyrosyl-L-methionyl-glycyl-L-tryptophyl-L-methionyl-L-aspartyl-L-phenylalanine amide was prepared by the condensation of 2-peptide segments with 1-isobutyloxycarbonyl-2-isobutyloxy-1,2-dihydroquinoline as coupling reagent. The N-terminal residue, tyrosine was incorporated by the active ester method. The same 7-peptide was prepared also by stepwise chain-lengthening, starting with the C-terminal residue. The 9-fluorenylmethyloxycarbonyl group was applied for the protection of alpha-amino functions. In the release of amylase from acinar cells of the pancreas of guinea pigs, the acetyl-7-peptide amide was about 3 times more potent than CCK 27-33 and equal in potency to CCK 26-33. The new derivative strongly stimulated the contraction of the in situ guinea pig gall bladder.

Tryptophan replacement in the C-terminal octapeptide of CCK-PZ. Syntheses of six peptide analogues bearing tryptophan congeners

Six sulphated octapeptide amide analogues, representing the C-terminal sequence of pancreozymin have been prepared in addition to the natural sequence. The tryptophyl residue which is essential for biological activity was replaced in this series by structurally, closely related analogues each of which resemble the properties of the original moiety to varying extents. The syntheses were so conducted that the C-terminal tetrapeptide amides (modified tetragastrins) could be secured. The results of the measurement of one of the biological parameters are indicated. The activity was determined to elucidate which of the physicochemical properties of the tryptophyl residue is most essential for exertion of biological activity in the described test.

Syntheses and biological activities of methoxinine-analogues of the C-terminal octapeptide of cholecystokinin-pancreozymin

The C-terminal octapeptide of cholecystokinin-pancreozymin and its analogues in which the methionine residues were successively replaced by methoxinine have been synthesized. When these analogues were tested for their gallbladder-contraction activity in an in vitro system the following relative potencies were observed: [Mox3]-CCK-PZ 8 (95%), [Mox6]-CCK-PZ 8 (5%) and [Mox3, Mox6]-CCK-PZ 8 (2%).