Cholecystokinin type B receptor antagonist PD-136,450 is a partial secretory agonist in the stomach and a full agonist in the pancreas of the rat

Attenuation of stress-induced gastric lesions by lansoprazole, PD-136450 and ranitidine in rats

Combining restraint with cold temperature (4°C) consistently induces gastric ulceration in rats after 3.5 h. The cold restraint-stress (CRS) method provides a suitable model for acute ulcer investigations. This study compares the antiulcer activities of lansoprazole (a proton pump inhibitor), PD-136450 (CCK(2)/gastrin receptor antagonist) and ranitidine (histamine H(2) receptor antagonist) on CRS-induced gastric ulcers in rats. The results have shown that lansoprazole, which is a potent anti-secretory agent, provides complete protection in this model of ulcer formation. The use of indomethacin pretreatment to inhibit the prostaglandin (PG) synthesis and N(G)-nitro L-arginine methyl ester (L-NAME) pretreatment to inhibit nitric oxide synthase did not alter the lansoprazole-induced inhibition of ulcer index obtained in the untreated Wistar rats indicating that these two systems were not involved in the activation of lansoprazole. PD-136450, an effective anti-secretory agent against gastrin- but not dimaprit-induced stimulation, evoked a dose-dependent inhibition of CRS-induced gastric ulcers. The results show that both PG and nitric oxide pathways can influence the inhibitory effect of PD-136450 against CRS-induced gastric ulcer. The antiulcer activities of both lansoprazole and PD-136450 were compared to that of ranitidine. The results showed that ranitidine was more potent than lansoprazole and PD-136450 in inhibiting CRS-induced gastric ulcers and its effect was shown to be influenced by PG as well as nitric oxide synthase. The results of this study have demonstrated that although lansoprazole, PD-136450 and ranitidine were protective against CRS-induced gastric ulcers, the antiulcer activities of PD-136450 and ranitidine involved both PG and nitric oxide pathways, while lansoprazole acted independently of these two systems during CRS.

Rationalizing the activities of diverse cholecystokinin 2 receptor antagonists using molecular field points

Cholecystokinin 2 receptor antagonists encompass a wide range of structures. This makes them unsuitable candidates for existing 3D-QSAR methods and has led us to develop an alternative approach to account for their observed biological activities. A diverse set of 21 antagonists was subjected to a novel molecular field-based similarity analysis. The hypothesis is that compounds with similar field patterns will bind at the same target site regardless of their underlying structure. This initial report demonstrates a linear correlation between ligand similarity and biological activity for this challenging data set. A model generated with three molecules was used to predict the activity of 18 test compounds, with different chemotypes, with a root-mean-square error of 0.68 pKB units. The ability to automatically derive a molecular alignment without knowledge of the protein structure represents an improvement over existing pharmacophore methods and makes the method particularly suitable for scaffold-hopping.

Mechanism of Activation of a G Protein-coupled Receptor, the Human Cholecystokinin-2 Receptor

G protein-coupled receptors (GPCRs) represent a major focus in functional genomics programs and drug development research, but their important potential as drug targets contrasts with the still limited data available concerning their activation mechanism. Here, we investigated the activation mechanism of the cholecystokinin-2 receptor (CCK2R). The three-dimensional structure of inactive CCK2R was homology-modeled on the basis of crystal coordinates of inactive rhodopsin. Starting from the inactive CCK2R modeled structure, active CCK2R (namely cholecystokinin-occupied CCK2R) was modeled by means of steered molecular dynamics in a lipid bilayer and by using available data from other GPCRs, including rhodopsin. By comparing the modeled structures of the inactive and active CCK2R, we identified changes in the relative position of helices and networks of interacting residues, which were expected to stabilize either the active or inactive states of CCK2R. Using targeted molecular dynamics simulations capable of converting CCK2R from the inactive to the active state, we delineated structural changes at the atomic level. The activation mechanism involved significant movements of helices VI and V, a slight movement of helices IV and VII, and changes in the position of critical residues within or near the binding site. The mutation of key amino acids yielded inactive or constitutively active CCK2R mutants, supporting this proposed mechanism. Such progress in the refinement of the CCK2R binding site structure and in knowledge of CCK2R activation mechanisms will enable target-based optimization of nonpeptide ligands.

Cholecystokinin and gastrin receptors

Cholecystokinin and gastrin receptors (CCK1R and CCK2R) are G protein-coupled receptors that have been the subject of intensive research in the last 10 years with corresponding advances in the understanding of their functioning and physiology. In this review, we first describe general properties of the receptors, such as the different signaling pathways used to exert short- and long-term effects and the structural data that explain their binding properties, activation, and regulation. We then focus on peripheral cholecystokinin receptors by describing their tissue distribution and physiological actions. Finally, pathophysiological peripheral actions of cholecystokinin receptors and their relevance in clinical disorders are reviewed.

Partial Agonism, Neutral Antagonism, and Inverse Agonism at the Human Wild-Type and Constitutively Active Cholecystokinin-2 Receptors

Cholecystokinin receptor-2 (CCK2R) is a G protein receptor that regulates a number of physiological functions. Activation of CCK2R and/or expression of a constitutively active CCK2R variant may contribute to human diseases, including digestive cancers. Search for antagonists of the CCK2R has been an important challenge during the last few years, leading to discovery of a set of chemically distinct compounds. However, several early-discovered antagonists turned out to be partial agonists. In this context, we carried out pharmacological characterization of six CCK2R antagonists using COS-7 cells expressing the human CCK2R or a CCK2R mutant having a robust constitutive activity on inositol phosphates production, and we investigated the molecular mechanisms which, at a CCK2R binding site, account for these features. Results indicated that three compounds, 3R(+)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepin-3-yl)-N'-(3-methylphenyl)urea (L365,260), 4-{[2-[[3-(lH-indol-3-yl)-2-methyl-1-oxo-2-[[[1.7.7-trimethyl-bicyclo[2.2.1]hept-2-yl)-oxy]carbonyl]amino]propyl]amino]-1-phenylethyl]amino-4-oxo-[lS-la.2[S*(S*)]4a]}-butanoate N-methyl-D-glucamine (PD135,158), and (R)-1-naphthalenepropanoic acid, b-[2-[[2-(8-azaspiro-[4.5]dec-8-ylcarbonyl)-4,6-dimethylphenyl]amino]-2-oxoethyl] (CR2945), were partial agonists; one molecule, 1-[(R)-2,3-dihydro-1-(2,3-dihydro-1-(2-methylphenacyl)-2-oxo-5-phenyl-1H-1,4-benzodiazepin-3-yl]-3-(3-methylphenyl)urea (YM022), was a neutral antagonist; and two compounds, N-(+)-[1-(adamant-1-ylmethyl)-2,4-dioxo-5-phenyl2,3,4,5-tetrahydro-1H-1,5-benzodiazepin-3-yl]-N'-phenylurea (GV150,013X) and ([(N-[methoxy-3 phenyl] N-[N-methyl N-phenyl carbamoylmethyl], carbomoyl-methyl)-3 ureido]-3-phenyl)2-propionic acid (RPR101,048), were inverse agonists. Furthermore, target- and pharmacophore-based docking of ligands followed by molecular dynamic simulation experiments resulted in consistent motion of aromatic residues belonging to a network presumably important for activation, thus providing the first structural explanations for the different pharmacological profiles of tested compounds. This study confirms that several referenced so-called antagonists are in fact partial agonists, and because of this undesired activity, we suggest that newly generated molecules should be preferred to efficiently block CCK2R-related physiological effects. Furthermore, data on the structural basis for the different pharmacological features of CCK2R ligands will serve to further clarify CCK2R mechanism of activation.

Structure-based design in drug discovery - the application of a peptoid drug design strategy for the development of non-peptide neuropeptide receptor ligands

Over the last decade the increasing availability of metabolically- stable non-peptide antagonists targeted at neuropeptide receptors has led directly to a more thorough understanding of the role of neuropeptides in mammalian physiology. By far the majority of these non-peptide neuropeptide receptor antagonists thus far disclosed have been developed from leads identified from broad screening of company compound files or natural product collections, and may thus bear little obvious structural resemblance to the endogenous peptide ligand. This review will focus on an alternative structure-based approach to non-peptide neuropeptide receptor ligand design, referred to as the 'peptoid' drug design strategy, in which an appreciation of the structure of the neuropeptide is the key to the success of this approach. The development and current clinical progress of peptoid cholecystokinin and tachykinin receptor ligands that have thus far resulted from this process will be highlighted and used to exemplify the importance of this novel approach.

PD-136,450: a CCK2 (gastrin) receptor antagonist with antisecretory, anxiolytic and antiulcer activity

This study investigated the effects of PD-136,450 (PD), a highly selective ligand for the CCK2 receptor, on gastric acid and pancreatic secretions, gastric cytoprotection and anxious behaviour in the rat and rabbit. PD inhibited gastrin (but not dimaprit) stimulated acid secretion in anaesthetized and conscious rats (IC50 of 1 mg kg(-1) sc) and inhibited 14C-aminopyrine uptake in isolated gastric glands from rabbits. In addition, PD decreased dose-dependently gastric haemorrhagic lesions in rats treated orally with acidified ethanol. Both, the antisecretory effects on gastric acid secretion and the gastric cytoprotective effects were less potent compared with the proton pump inhibitor omeprazole. PD strongly increased pancreatic secretion, which was substantially inhibited by the CCK1 antagonist L-364,718 (but not by the CCK2 antagonist L-365,260). PD also showed significant anxiolytic activity as assessed by a black and white box two-compartment activity assay. Both, time spent in the dark compartment and latency for movement from the light to the dark compartment was increased by PD (similarly with 5 mg kg(-1) diazepam). In conclusion, PD inhibited gastrin-stimulated gastric acid secretion, decreased ethanol-induced damage to the gastric mucosa, stimulated pancreatic secretion (via CCK1 receptors) and displayed anxiolytic activity. Thus, PD may have utility as an adjunct therapy in peptic ulcer disease by countering the actions of gastrin and increasing acid neutralization and mucosal protection.

A comparative study on the activity of lansoprazole, omeprazole and PD-136450 on acidified ethanol- and indomethacin-induced gastric lesions in the rat

1. The proton pump inhibitors lansoprazole (LP) and omeprazole (OP) and the cholecystokinin (CCK)-receptor antagonist PD-136450 (PD) provide a broad spectrum of activities in their ability to inhibit gastric acid secretion and protect the stomach against ulcerogens. In the present study, we investigated the protective effects of these compounds against gastric ulcers induced by acidified ethanol (AE) and indomethacin. 2. Both AE (60% ethanol in 150 mmol/L HCl, 1 mL/rat) and indomethacin (30 mg/kg) produced gastric haemorrhagic lesions in the rat 1 and 6 h after oral administration, respectively. 3. The gastric mucosal protective effects of LP (1-20 mg/kg), OP (0.5-10 mg/kg) and PD (1-20 mg/kg), administered either orally or subcutaneously (s.c.) 30 min before the administration of AE or indomethacin, were dose dependent against both models of ulcer induction. 4. To determine whether the cytoprotective effect of LP, OP and PD (each 10 mg/kg) was mediated by endogenous prostaglandins (PG), indomethacin (10 mg/kg, s.c.) was administered 15 min before AE to inhibit prostanoids biosynthesis. Indomethacin reduced the cytoprotective effects of OP, but not LP, administered either orally or s.c. Indomethacin reduced the cytoprotective effect of PD administered orally, although the effect was much less significant than when PD was administered s.c. The results exclude the role of PG in mediating the protective effects of LP, whereas the possibility exists for PG to have a role in mediating the protective effects of OP and PD. 5. To investigate the possible involvement of endogenous nitric oxide (NO) in the cytoprotective action of LP, OP and PD, we treated rats with a selective inhibitor of NO synthesis, namely NG-nitro-L-arginine methyl ester (L-NAME; 25 mg/kg, s.c.). Administration of L-NAME 15 min prior to LP, OP or PD (each 10 mg/kg) orally or s.c. and challenge with AE or indomethacin did not significantly increase the degree of the ulcer index and L-NAME was not able to antagonize the protective effects of LP, OP and PD, thus excluding the role of NO in mediating the protective effects of these drugs. However, the effects of PD in reducing the indomethacin-induced ulcer index were less significant in the presence than the absence of L-NAME (P < 0.05 vs P < 0.001, respectively), suggesting a role for NO. 6. In conclusion, the results of the present study suggest that LP and OP are equally effective against AE- as well as indomethacin-induced gastric ulcers and were more potent than PD in protecting the stomach against ulcer formation. Lansoprazole, OP and PD bring about their cytoprotective action through the reduction of acid secretion and some other unknown mechanisms. However, OP and PD may exert their cytoprotective action through PG and NO pathways.

CCK-B/Gastrin receptor transmembrane domain mutations selectively alter synthetic agonist efficacy without affecting the activity of endogenous peptides

Recent efforts have focused on identifying small nonpeptide molecules that can mimic the activity of endogenous peptide hormones. Understanding the molecular basis of ligand-induced receptor activation by these divergent classes of ligands should expedite the process of drug development. Using the cholecystokinin-B/gastrin receptor (CCK-BR) as a model system, we have recently shown that both affinity and efficacy of nonpeptide ligands are markedly affected by amino acid alterations within a putative transmembrane domain (TMD) ligand pocket. In this report, we examine whether residues projecting into the TMD pocket determine the pharmacologic properties of structurally diverse CCK-BR ligands, including peptides and synthetic peptide-derived partial agonists (peptoids). Nineteen mutant human CCK-BRs, each including a single TMD amino acid substitution, were transiently expressed in COS-7 cells and characterized. Binding affinities as well as ligand-induced inositol phosphate production at the mutant CCK-BRs were assessed for peptides (CCK-8 and CCK-4) and for peptoids (PD-135,158 and PD-136, 450). Distinct as well as overlapping determinants of peptide and peptoid binding affinity were identified, supporting that both classes of ligands, at least in part, interact with the CCK-BR TMD ligand pocket. Eight point mutations resulted in marked increases or decreases in the functional activity of the synthetic peptoid ligands. In contrast, the functional activity of both peptides, CCK-8 and CCK-4, was not affected by any of the CCK-BR mutations. These findings suggest that the mechanisms underlying activation of G-protein-coupled receptors by endogenous peptide hormones versus synthetic ligands may markedly differ.

Altered influence of CCK-B/gastrin receptors on HDC expression in ECL cells after neoplastic transformation

Gastrin is one of the main factors controlling enterochromaffin-like (ECL) cell endocrine function and growth. Long-standing hypergastrinemia may give rise to ECL cell carcinoids in the gastric corpus in man and in experimental models. We have analysed the expression and function of CCK-B/gastrin receptors in normal ECL cells and in ECL cell tumours (gastric carcinoids) of the African rodent Mastomys natalensis. Hypergastrinemia induced by short-term (5 days) histamine2-receptor blockade (loxtidine) resulted in increased histidine decarboxylase (HDC) mRNA expression in the gastric oxyntic mucosa. This increase was significantly and dose-dependently reversed by selective CCK-B/gastrin receptor blockade (YM022). Long-term (12 months) hypergastrinemia, induced by histamine2-receptor blockade, gave rise to ECL cell carcinoids in the gastric oxyntic mucosa. CCK-B/gastrin receptor mRNA was only slightly elevated while HDC mRNA expression was eight-fold elevated in ECL cell carcinoids and was not influenced by CCK-B/gastrin receptor blockade. Thus CCK-B/gastrin receptor blockade of hypergastrinemic animals reduces the HDC mRNA expression in normal mucosa but not in ECL cell carcinoids. These results demonstrate that HDC mRNA expression in neoplastic ECL cells is not controlled by CCK-B/gastrin receptors.

Pharmacological analysis of CCK2 receptor antagonists using isolated rat stomach ECL cells

1. Gastrin stimulates rat stomach ECL cells to secrete histamine and pacreastatin, a chromogranin A (CGA)-derived peptide. The present report describes the effect of nine cholecystokinin2 (CCK2) receptor antagonists and one CCK1 receptor antagonist on the gastrin-evoked secretion of pancreastatin from isolated ECL cells. 2. The CCK2 receptor antagonists comprised three benzodiazepine derivatives L-740,093, YM022 and YF476, one ureidoacetamide compound RP73870, one benzimidazole compound JB 93182, one ureidoindoline compound AG041R and three tryptophan dipeptoids PD 134308 (CI988), PD135158 and PD 136450. The CCK1 receptor antagonist was devazepide. 3. A preparation of well-functioning ECL cells (approximately 80% purity) was prepared from rat oxyntic mucosa using counter-flow elutriation. The cells were cultured for 48 h in the presence of 0.1 nM gastrin; they were then washed and incubated with antagonist alone or with various concentrations of antagonist plus 10 nM gastrin (a maximally effective concentration) for 30 min. Gastrin dose-response curves were constructed in the absence or presence of increasing concentrations of antagonist. The amount of pancreastatin secreted was determined by radioimmunoassay. 4. The gastrin-evoked secretion of pancreastatin was inhibited in a dose-dependent manner. YM022, AG041R and YF476 had IC50 values of 0.5, 2.2 and 2.7 nM respectively. L-740,093, JB93182 and RP73870 had IC50 values of 7.8, 9.3 and 9.8 nM, while PD135158, PD136450 and PD134308 had IC50 values of 76, 135 and 145 nM. The CCK1 receptor antagonist devazepide was a poor CCK2 receptor antagonist with an IC50 of about 800 nM. 5. YM022, YF476 and AG041R were chosen for further analysis. YM022 and YF476 shifted the gastrin dose-response curve to the right in a manner suggesting competitive antagonism, while the effects of AG041R could not be explained by simple competitive antagonism. pK(B) values were 11.3 for YM022, 10.8 for YF476 and the apparent pK(B) for AG041R was 10.4.

CCK2 receptor antagonists: pharmacological tools to study the gastrin-ECL cell-parietal cell axis

Gastrin-recognizing CCK2 receptors are expressed in parietal cells and in so-called ECL cells in the acid-producing part of the stomach. ECL cells are endocrine/paracrine cells that produce and store histamine and chromogranin A (CGA)-derived peptides, such as pancreastatin. The ECL cells are the principal cellular transducer of the gastrin-acid signal. Activation of the CCK2 receptor results in mobilization of histamine (and pancreastatin) from the ECL cells with consequent activation of the parietal cell histamine H2 receptor. Thus, release of ECL-cell histamine is a key event in the process of gastrin-stimulated acid secretion. The oxyntic mucosal histidine decarboxylase (HDC) activity and the serum pancreastatin concentration are useful markers for the activity of the gastrin-ECL cell axis. Powerful and selective CCK2 receptor antagonits have been developed from a series of benzodiazepine compounds. These agents are useful tools to study how gastrin controls the ECL cells. Conversely, the close control of ECL cells by gastrin makes the gastrin-ECL cell axis well suited for evaluating the antagonistic potential of CCK2 receptor antagonists with the ECL-cell HDC activity as a notably sensitive and reliable parameter. The CCK2 receptor antagonists YF476, YM022, RP73870, JB93182 and AG041R were found to cause prompt inhibition of ECL-cell histamine and pancreastatin secretion and synthesis. The circulating pancreastatin concentration is raised, was lowered when the action of gastrin on the ECL cells was blocked by the CCK2 receptor antagonists. These effects were associated with inhibition of gastrin-stimulated acid secretion. In addition, sustained receptor blockade was manifested in permanently decreased oxyntic mucosal HDC activity, histamine concentration and HDC mRNA and CGA mRNA concentrations. CCK2 receptor blockade also induced hypergastrinemia, which probably reflects the impaired gastric acid secretion (no acid feedback inhibition of gastrin release). Upon withdrawal of the CCK2 receptor antagonists, their effects on the ECL cells were readily reversible. In conclusion, gastrin mobilizes histamine from the ECL cells, thereby provoking the parietal cells to secrete acid. While CCK2 receptor blockade prevents gastrin from evoking acid secretion, it is without effect on basal and vagally stimulated acid secretion. We conclude that specific and potent CCK2 receptor antagonists represent powerful tools to explore the functional significance of the ECL cells.

Characterization of antisecretory and antiulcer activity of CR 2945, a new potent and selective gastrin/CCK(B) receptor antagonist

The antigastrinic, antisecretory and antiulcer activities of CR 2945, (R)-1-naphthalenepropanoic acid,beta-[2-[[2-(8-azaspiro[4.5]dec-8-yl-carbonyl)-4,6-dimethylph enyl] amino]-2-oxoethyl], were investigated in vitro and in vivo in rats and cats. Its activities were compared with those of two gastrin/CCK(B) receptor antagonists, L-365,260 (3R(+)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepin -3-yl)-N'-(3-methylphenyl)urea and CAM-1028 (4-[[2-[[3-(1H-indol-3-yl)-2-methyl-1-oxo-2-[[[1,7,7-trimethylbicyclo [2.2.1]hept-2-yl)oxy]carbonyl]amino]propyl]amino]-1-phenylethyl]amino -4-oxo-[1S-1alpha,2beta[S'(S')4alpha]]-butanoate -N-methyl-D-glucamine), of the histamine H2 receptor antagonist, ranitidine, and the proton pump inhibitor, omeprazole. Cytosolic Ca2+ elevation in rabbit parietal cells induced by gastrin (50 nM) was blocked by CR 2945 with an IC50 value of 5.9 nM. CAM-1028 and L-365,260 showed similar activity. CR 2945 antagonized pentagastrin-stimulated gastric acid secretion in rats (ED50 = 1.3 mg kg(-1) i.v. and 2.7 mg kg(-1) i.d.) and cats (1.6 mg kg(-1) i.v.). CR 2945 was slightly less potent than the reference compounds after i.v. administration, whereas after intraduodenal (i.d.) administration, it was more potent than both ranitidine and omeprazole. In the rat, the gastrin antagonism exhibited by CR 2945 was reversible and competitive, with a pA2 value of 7.33. CR 2945 had specific antigastrin activity, as it was unable to antagonize the gastric acid secretion stimulated by histamine or carbachol in rats up to the dose of 30 mg kg(-1). CR 2945 was about as efficacious as ranitidine against the indomethacin- and ethanol-induced gastric ulcers and the cysteamine-induced duodenal ulcer in rats. On the contrary, L-365,260 was only slightly effective. These results suggest that CR 2945 might be a promising compound for the therapy of acid-related disorders, and that its clinical use could help clarify the therapeutic potential of gastrin/CCK(B) receptor antagonists in the gut.

YM022, a highly potent and selective CCKB antagonist inhibiting gastric acid secretion in the rat, the cat and isolated rabbit glands

We investigated the effects of the novel CCKB/gastrin antagonist YM022 on gastric acid secretion in vivo and in vitro, compared to CI-988 and L365,260 as reference antagonists. In the anaesthetized rat, pentagastrin-induced stimulation of gastric acid secretion was dose-dependently and up to 100% inhibited by i.v. administration of YM022 with an ID50 of 0.009 +/- 0.0006 mumol/kg h in comparison to 0.6 +/- 0.03 and 3.40 +/- 0.05 mumol/kg h for CI-988 and L-365,260, respectively. In the gastric fistula cat, i.v. administration of YM022 produced a similar inhibitory effect with an ID50 of 0.02 mumol/kg in comparison to 1.6 and 2.5 mumol/kg for CI-988 and L-365,260, respectively. Furthermore, bolus injection of 0.6 mumol/kg YM022 produced 100% inhibition within 30 min and 85% inhibition was still observed after 3 h. In the isolated rabbit gastric glands, CCK8-stimulated 14C-aminopyrine uptake was inhibited according to the following rank order of potency: YM022 (IC50 = 0.0012 microM) > > CI-988 (IC50 = 0.2 microM) > > L365,260 (IC50 = 2.8 microM). Unlike with L365,260, no influence of CI-988 and YM022 on histamine-stimulated acid output was shown in this study. Thus, YM022 is a highly potent and selective gastric CCKB/gastrin receptor antagonist and has a long-lasting inhibitory effect on gastric acid secretion.

Minor modifications of a cholecystokinin-B/gastrin receptor non-peptide antagonist confer a broad spectrum of functional properties

The development of non-peptide agonists for peptide hormone receptors would markedly expand the treatment options for a large number of diseases. However, difficulty in identifying non-peptide molecules which possess intrinsic activity has been a major obstacle in achieving this goal. At present, most of the known non-peptide ligands for peptide hormone receptors appear in standard functional assays to be antagonists. Here, we report that a constitutively active mutant of the human cholecystokinin-B/gastrin receptor, Leu325 --> Glu, offers the potential to detect even trace agonist activity of ligands which, at the wild type receptor isoform, appear to lack efficacy. The enhanced functional sensitivity of the mutant receptor enabled us to detect intrinsic activity of L-365,260, an established non-peptide antagonist for the cholecystokinin-B/gastrin receptor. Extending from this observation, we were able to demonstrate that minor structural modifications could convert L-365, 260 into either: (i) an agonist or (ii) an inverse agonist (attenuates ligand-independent signaling). The ability to confer functional activity to small non-peptide ligands suggests that the properties of endogenous peptide hormones can be mimicked, and even extended, by considerably less complex molecules.

Small synthetic ligands of the cholecystokinin-B/gastrin receptor can mimic the function of endogenous peptide hormones

The gastric cholecystokinin-B/gastrin receptor (CCK-BR) is a key regulator of enterochromaffin-like cell function and proliferation. Over the last decade, a number of small non-peptide CCK-BR "antagonists" have been discovered. Here, we demonstrate that some of these non-peptide ligands in fact possess significant ability to activate the human CCK-BR, and are, therefore, more properly categorized as partial agonists. When tested in COS-7 cells transiently expressing the recombinant human CCK-BR, saturating concentrations of the small "peptoid" ligands PD 135,158 and PD 136,450 stimulated inositol phosphate formation to 23 and 43 percent, respectively, of the maximum response induced by a considerably larger endogenous peptide agonist, cholecystokinin octapeptide. In contrast, the benzodiazepine-derived CCK-BR ligand, YM022, acted as a "true" high-affinity antagonist of cholecystokinin-induced inositol phosphate formation (pA2 = 9.69). Consistent with recent findings in animal experiments, our data reveal that small synthetic ligands have the potential to function as either CCK-BR agonists or antagonists. These dual properties of synthetic molecules must be considered when evaluating candidate drugs for human disease.

Cholecystokinin (CCK) regulates somatostatin secretion through both the CCK-A and CCK-B/gastrin receptors in sheep

1. Cholecystokinin (CCK) and gastrin both stimulate gastric somatostatin (SOM) secretion in vitro and thus have the potential to modulate their direct effects on the parietal cell. However, the relative potencies and the mechanisms of action of CCK and gastrin on SOM secretion in vivo have not been determined. 2. The objectives of the present study were to compare the in vivo potencies of the sulphated(s) and non-sulphated (ns) forms of gastrin heptadecapeptide (G-17) and CCK octapeptide (CCK-8) on SOM secretion, and to determine the nature of the receptors involved by repeating the studies in the presence of the CCK-A and CCK-B/gastrin receptor antagonists L-364,718 and L-365,260, respectively. All experiments were performed in the chronically cannulated sheep. 3. Dose-response experiments revealed the following potencies for SOM secretion: G-17s = CCK-8s > G-17 ns >> CCK-8ns. However, based on the plasma levels achieved and a higher metabolic clearance rate (MCR) for CCK, CCK-8s was the most potent. 4. Both the CCK-A and CCK-B/gastrin receptor antagonists suppressed CCK-8s-stimulated SOM output. In contrast, G-17s-stimulated SOM output was inhibited by only the CCK-B/gastrin receptor antagonist. 5. Both receptor antagonists increased basal plasma gastrin and CCK levels. 6. The predominant circulating SOM molecular form after both gastrin and CCK stimulation was SOM-14. 7. In conclusion, the sulphated forms of CCK and gastrin are more potent than the non-sulphated forms. Despite sharing a common biologically active carboxy terminus, CCK stimulates SOM secretion by both the CCK-A and CCK-B/gastrin receptors, while gastrin acts via the CCK-B/gastrin receptor alone. These findings explain in part why CCK is a net inhibitor of gastric acid secretion in vivo.

Time-course of deactivation of rat stomach ECL cells following cholecystokinin B/gastrin receptor blockade

1 The so-called enterochromaffin-like (ECL) cells constitute 65-75% of the endocrine cells in the acid-producing part of the rat stomach. They produce and secrete histamine and pancreastatin, a chromogranin A (CGA)-derived peptide, in response to gastrin, Cholecystokinin (CCK)B/gastrin receptor blockade is known to suppress their activity. 2 We have examined the time course of the deactivation of the ECL cells following treatment with the selective CCKB receptor antagonists RP73870 and YM022. The drugs were given by continuous subcutaneous infusion for a time span of 1 h to 3 weeks and the serum gastrin concentration and various ECL cell parameters were measured (oxyntic mucosal histidine decarboxylase (HDC) activity, histamine and pancreastatin concentrations, HDC mRNA and CGA mRNA levels, and circulating pancreastatin concentration). 3 The two antagonists caused a prompt and dramatic decline in the oxyntic mucosal HDC activity and HDC mRNA level. The HDC activity started to decline after 1-2 h, was reduced by 60-70% after 6 h and was maximally suppressed (80-90%) after 24-48 h. The HDC mRNA level was reduced after 12 h and was at about 20% of the pretreatment level after 2-4 days of infusion. The ECL cell histamine concentration was lowered by about 50% after 7-10 days. 4 RP73870 and YM022 lowered the serum pancreastatin concentration and the oxyntic mucosal CGA mRNA level. The serum pancreastatin concentration was reduced by 40% after 6 h and the reduction was maximal after 2-3 days. A decline in the oxyntic mucosal CGA mRNA level was noted after 12 h with a maximal reduction after 2-4 days of infusion. The ECL cell pancreastatin concentration was reduced by 30-40% after 3 weeks. 5 The infusion of RP73870 and YM022 induced hypergastrinaemia. The serum gastrin concentration started to rise after 2-4 h, there was a 2 fold increase after 6 h and maximal increase (3-4 fold) after 2-3 days of treatment. 6 In conclusion, CCKB/gastrin receptor blockade promptly deactivates the ECL cells. Deactivation, manifested in a greatly reduced HDC activity, was apparent after 1-2 h of the infusion. The serum pancreastatin concentration and the oxyntic mucosal HDC mRNA and CGA mRNA levels were greatly reduced after 1-2 days. The ECL cell concentrations of histamine and pancreastatin declined quite slowly by comparison.

Cholecystokinin-B/gastrin receptor blockade suppresses the activity of rat stomach ECL cells

Gastrin controls the histamine- and chromogranin A-producing enterochromaffin-like (ECL) cells, the predominant endocrine cell population in the acid-producing part of the rat stomach. They are responsible for most of the circulating pancreastatin, a chromogranin A-derived peptide. The present study examines the ability of two potent and highly selective cholecystokinin-B/gastrin receptor antagonists, RP73870 and YM022, to incapacitate the ECL cells. The two antagonists were given by continuous subcutaneous infusion to otherwise untreated rats and to hypergastrinaemic rats treated with gastrin-17 (continuous subcutaneous infusion) or omeprazole (orally) for 7 days. Several parameters reflecting ECL cell activity were measured: The oxyntic mucosal histidine decarboxylase activity, the histamine concentration, the histidine decarboxylase mRNA and chromogranin A mRNA concentrations, and the serum pancreastatin concentration. In addition, the serum gastrin concentration was measured. RP73870 and YM022 greatly lowered the oxyntic mucosal histidine decarboxylase activity and the histidine decarboxylase mRNA and chromogranin A mRNA concentrations, and also reduced the oxyntic mucosal histamine concentration and the serum pancreastatin concentration. Moreover, they raised the serum gastrin concentration. With respect to blockade of histidine decarboxylase activity, 1.0 mumol.kg-1.hr-1 was an almost maximally effective dose for both RP73870 and YM022. The corresponding ID50 values were 0.04 and 0.05 mumol.kg-1.hr-1. RP73870 and YM022 inhibited the hypergastrinaemia-evoked rise in all ECL-cell parameters. The results suggest that sustained cholecystokinin-B/gastrin receptor blockade causes lasting deactivation of the ECL cells.

Cloning and functional characterization of the ornithokinin receptor. Recognition of the major kinin receptor antagonist, HOE140, as a full agonist

Kinins are proinflammatory peptides that dilate vessels, increase vascular permeability, contract smooth muscles, and provoke pain. The known mammalian kinin receptors are classified as two subtypes, i.e. the B1 receptor triggered by [des-Arg9]bradykinin and inhibited by [des-Arg9,Leu8]bradykinin, and the B2 receptor stimulated by bradykinin and antagonized by HOE140. Here we report the cloning of a non-mammalian kinin receptor gene amplified from genomic chicken DNA. The protein predicted from the open reading frame shows 31 and 49% sequence identity to the human B1 and B2 receptors, respectively, suggesting that it represents a G protein-coupled receptor of the kinin receptor family. The recombinantly expressed chicken receptor had IC50 values of 4.7 nM for the authentic ligand, ornithokinin ([Thr6,Leu8]bradykinin), 3.8 nM for HOE140, and >/=10 microM for bradykinin, [des-Arg9]bradykinin, and [des-Arg9,Leu8]bradykinin. Ornithokinin and HOE140 at nanomolar concentrations stimulated intracellular inositol phosphate accumulation and induced a significant transient rise in intracelluar free Ca2+, whereas bradykinin was ineffective even at 100 nM. Hence the principal B2 receptor antagonist HOE140 is a potent agonist of the chicken kinin receptor. This unique pharmacological profile classifies the ornithokinin receptor as a novel subtype among kinin receptors and will facilitate further molecular studies on ligand binding and receptor activation.

Use of the chemical structure of peptides as the starting point to design nonpeptide agonists and antagonists at peptide receptors: examples with cholecystokinin and tachykinins

This review summarizes a design strategy to give examples of nonpeptides starting from cholecystokinin (CCK-A and -B) and tachykinins (substance P) (NK-1, -2, -3) as potent functional agonists and antagonists with utility as therapeutic agents.

Characterization of novel peptoid agonists for the CCK-A receptor

The successful design of peptoid CCK-B receptor antagonists using rational approaches suggested that it might be feasible to develop similar non-peptide small molecule agonists with potential therapeutic applications. We now report the characterization of such a compound with full agonist activity at CCK-A receptors on rat exocrine pancreatic acinar cells. The compound, PD149164, stimulated a similar maximal response to CCK8 from the exocrine pancreas in anaesthetized rats in vivo, and from isolated pancreatic acini in vitro it also generated intracellular Ca2+ oscillations similar to those evoked by CCK8. These effects were inhibited by the CCK-A antagonist L-364,718. Interestingly, the enantiomer of PD149164, PD151932, was a CCK-A antagonist and blocked PD149164 stimulated effects on the exocrine pancreas. The data indicate that it is possible to develop both agonist and antagonist activities in enantiomers of small non-peptide molecules.

'Targeted' molecular diversity: design and development of non-peptide antagonists for cholecystokinin and tachykinin receptors

A drug design strategy to non-peptide small molecule antagonists of neuropeptides is described that targets the molecular diversity which exists in the 'privileged' data set of the physico-chemical properties represented by the side-chains of the 20 genetically encoded amino acids. The strategy is exemplified by the design of a selective and high affinity cholecystokinin CCK-A antagonist PD 140548, CCK-B antagonist CI-988 (formerly PD 134308) tachykinin NK-1 antagonist PD 154075 and NK-2 antagonist Cam-2291. The NK-3 antagonists, PD 157672 and the non-peptide PD 161182, were developed from an information-rich dipeptide library constructed from 256 N-protected dipeptides and 64 hydrophobic biased dipeptides.

CCKB/gastrin receptor antagonists: recent advances and potential uses in gastric secretory disorders

Cholecystokinin (CCK) and the structurally related peptide, gastrin, have numerous effects on tissues in the central nervous system and gastrointestinal tract. Recent studies show these effect are mediated by a CCKA and CCKB receptor. Knowledge of the physiological role and role of CCKB receptors in pathologic processes has been particularly limited by the availability of selective, potent receptor antagonists. Recently, new members of five different classes of non-peptide CCKB receptor antagonists are reported and are reviewed briefly. these include compounds isolated from Streptomyces (tetronothiodin, virginiamycin analogues), ureido-acetamide analogues (RP 69758, RP 72540, RP 73870), newer benzodiazepine analogues (L-368,935, L-740,093, YM022), pyrazolidimine analogues (LY 262,691) and glutamic acid analogues (CR2194). Many of these compounds have greater than 1000-fold selectivity for the CCKB over the CCKA receptor and some have greater than 10,000-fold selectivity. The pharmacology and effects of CCKB receptor antagonists on gastric acid secretion is briefly reviewed. Furthermore, the possible clinical usefulness of CCKB receptor antagonists in treating disorders of gastric acid secretion, in inhibiting the trophic effects of gastrin and in other clinical conditions is briefly discussed.

Cholecystokinin-B receptor ligands of the dipeptoid series act as agonists on rat stomach histidine decarboxylase

BACKGROUND & AIMS

The effect of gastrin on the enterochromaffin-like cells in the rat stomach is mediated by cholecystokinin (CCK)-B receptors and manifested as activation of histidine decarboxylase (HDC). The dipeptoids PD 136450, PD 135158, and PD 134308 are thought to be selective CCK-B receptor antagonists. The effects of the dipeptoids and of gastrin-17 and sulfated CCK-8 on rat stomach HDC activity were examined.

METHODS

Drugs were infused intravenously or subcutaneously to fasted rats, and the HDC activity was determined.

RESULTS

The dipeptoids activated HDC with maximal responses (50%-60% of the maximal response to gastrin) at 1 mumol.kg-1.h-1. Rat gastrin-17 activated HDC maximally at 3 nmol.kg-1.h-1, and sulfated CCK-8 produced maximal response at 20 nmol.kg-1.h-1. The CCK-B receptor antagonist L-365,260 inhibited the HDC activation induced by gastrin, sulfated CCK-8, or the dipeptoids. The dipeptoids did not inhibit the gastrin-induced HDC activation.

CONCLUSIONS

Gastrin, sulfated CCK-8, and the dipeptoids activated rat stomach HDC. L-365,260 but not devazepide inhibited the HDC activation. Thus, the dipeptoids, which failed to inhibit the gastrin-induced HDC activation, act as CCK-B receptor agonists and not as antagonists. It is important to recognize this to ensure appropriate interpretation of data obtained with these drugs.