Analysis of the variation in the action of L-365,260 at CCKB/gastrin receptors in rat, guinea-pig and mouse isolated gastric tissue assays

Thermodynamic analysis of ligands at cholecystokinin CCK2 receptors in rat cerebral cortex

BACKGROUND AND PURPOSE

Several studies using radioligand binding assays, have shown that measurement of thermodynamic parameters can allow discrimination of agonists and antagonists (Weiland et al., 1979; Borea et al., 1996a). Here we investigate whether agonists and antagonists can be thermodynamically discriminated at CCK(2) receptors in rat cerebral cortex.

EXPERIMENTAL APPROACH

The pK(L) of [(3)H]-JB93182 in rat cerebral cortex membranes was determined at 4, 12, 21 and 37 degrees C in 50 mM Tris-HCl buffer (buffer B pH 6.96; containing 0.089 mM bacitracin). pK(I) values of ligands of diverse chemical structure and with differing intrinsic activity (alpha), as defined by the lumen-perfused rat and mouse stomach bioassays, were determined in buffer B at 4, 12, 21 and 37 degrees C.

KEY RESULTS

[(3)H]-JB93182 labelled a homogeneous population of receptors in rat cerebral cortex at 4, 12, 21 and 37 degrees C and the pK(L) and B(max) were not altered by incubation temperature. [(3)H]-JB93182 binding reached equilibrium after 10, 50, 90 and 220 min at 37, 21, 12 and 4 degrees C, respectively. pK(I) values for R-L-365,260, R-L-740,093, YM220, PD134,308 and JB95008 were higher at 4 degrees C than at 37 degrees C. There was no effect of temperature on pK(I) values for pentagastrin, CCK-8S, S-L-365,260, YM022, PD140,376 and JB93242.

CONCLUSIONS AND IMPLICATIONS

CCK(2) receptor agonists and antagonists at rat CCK(2) receptors cannot be discriminated by thermodynamic analysis using [(3)H]-JB93182 as the radioligand.

Optimization of 1,3,4-benzotriazepine-based CCK(2) antagonists to obtain potent, orally active inhibitors of gastrin-mediated gastric acid secretion

Starting from a novel, achiral 1,3,4-benzotriazepine-based CCK2 receptor antagonist, a process of optimization has afforded further compounds of this type that maintain the nanomolar affinity for recombinant, human CCK2 receptors and high selectivity over CCK1 receptors observed in the initial lead but display more potent inhibition of pentagastrin-stimulated gastric acid secretion in vivo. Moreover, this has largely been achieved without altering their potency at wild-type canine and rat receptors, as judged by their displacement of [125I]-BH-CCK-8S in a radioligand binding assay and by their activity in an isolated, perfused rat stomach bioassay, respectively. 2-(5-Cyclohexyl-1-(2-cyclopentyl-2-oxo-ethyl)-2-oxo-1,2-dihydro-3H-1,3,4-benzotriazepin-3-yl)-N-(3-(5-oxo-2,5-dihydro- [1,2,4]oxadiazol-3-yl)-phenyl)-acetamide (47) was identified as the most effective compound stemming from this approach, proving to be a potent inhibitor of pentagastrin-stimulated gastric acid secretion in rats and dogs by intravenous bolus as well as by enteral administration.

Optimization of the in Vitro and in Vivo Properties of a Novel Series of 2,4,5-Trisubstituted Imidazoles as Potent Cholecystokinin-2 (CCK2) Antagonists

The systematic optimization of the structure of a novel 2,4,5-trisubstituted imidazole-based cholecystokinin-2 (CCK(2)) receptor antagonist afforded analogues with nanomolar receptor affinity. These compounds were now comparable in their potency to the bicyclic heteroaromatic-based compounds 5 (JB93182) and 6 (JB95008), from which the initial examples were designed using a field-point based molecular modeling approach. They were also orally active as judged by their inhibition of pentagastrin stimulated acid secretion in conscious dogs, in contrast to the bicyclic heteroaromatic-based compounds, which were ineffective because of biliary elimination. Increasing the hydrophilicity through replacement of a particular methylene group with an ether oxygen, as in 3-{[5-(adamantan-1-yloxymethyl)-2-cyclohexyl-1H-imidazole-4-carbonyl]amino}benzoic acid (53), had little effect on the receptor affinity but significantly increased the oral potency. Comparison of the plasma pharmacokinetics and the inhibition of pentagastrin-stimulated acid output following bolus intraduodenal administration of both 53 and 6 indicated that 53 was well absorbed, had a longer half-life, and was not subject to the elimination pathways of the earlier series.

Pharmacological comparison of the alternatively spliced short and long CCK2 receptors

(1) The alternatively spliced, short and long cholecystokinin receptors (CCK2S and CCK2L) were expressed in NIH3T3 cells, and compared using radioligand-binding assays with identical buffer and incubation conditions. (2) As judged by a saturation analysis, the selective CCK2-receptor antagonist radioligand [3H]-JB93182 did not discriminate between the CCK2S or CCK2L receptors. (3) A global analysis of competition studies, using a range of structurally diverse, CCK-receptor selective ligands, provided further evidence that these receptor subtypes were pharmacologically indistinguishable. However, when analysed individually a number of small, yet significant differences were observed with some of the compounds. (4) These data are consistent with previous study that suggested a possible pharmacological difference between these isoforms, at least in terms of the CCK2-receptor antagonist, L-365,260. However, it would appear that the pharmacological profile of these compounds is not consistent with their affinity at the putative G1/G2 receptors previously described by Harper et al.

CCK(2) receptor antagonists containing the conformationally constrained phenylalanine derivatives, including the new amino acid Xic

The conformationally constrained analogues of phenylalanine, tetrahydroisoquinoline-3-carboxylic acid (Tic), Sic, Hic and Nic, and the new amino acid Xic have been incorporated into a potent and highly selective cholecystokinin-2 (CCK(2)) receptor antagonist (2) in place of the phenylalanine residue, producing compounds 15a-e. High selectivities for CCK(2) over CCK(1) were observed for compounds 15a-e. The in vitro profile of the analogue containing the Nic residue (15d) was identical to that of compound 2, whereas the alternative conformational constraints resulted in a significant loss of affinity. The apparent advantage of Nic in the context of these CCK(2) ligands was subsequently demonstrated to be statistically significant.

Nonpeptide cholecystokinin-2 receptor agonists

In the course of structural explorations around a series of potent CCK2 receptor antagonists, it was noted that simple N-methylation of the indolic N-H in the parent molecule gave rise to behavior in vivo that was consistent with the compound acting as an agonist. Exploration in vitro confirmed this property, and it was shown that the agonist action could be blocked by the reference CCK2 receptor antagonist, L-365,260. Further examples of this type of modification were explored, and a common theme with regard to agonist behavior was uncovered. Some molecular modeling is also presented in an attempt to throw light on the nature of the ligand receptor interactions that may be giving rise to the differing properties of these, apparently, structurally similar molecules.

Phytohaemagglutinin inhibits gastric acid but not pepsin secretion in conscious rats

Phytohaemagglutinin (PHA), a kidney bean lectin, is known for its binding capability to the small intestinal surface. There has been no data available, however, on the biological activity of PHA in the stomach. Recent observations indicate that PHA is able to attach to gastric mucosal and parietal cells. Therefore, we examined whether PHA affects gastric acid and pepsin secretion in rats. Rats were surgically prepared with chronic stainless steel gastric cannula and with indwelling polyethylene jugular vein catheter. During experiments, animals were slightly restrained. Gastric acid secretion was collected in 30 min periods. Acid secretion was determined by titration of the collected gastric juice with 0.02 N NaOH to pH 7.0. Pepsin activity was estimated by measuring enzymatic activity. Saline, pentagastrin and histamine were infused intravenously. PHA or bovine serum albumin (BSA) were dissolved in saline and given intragastrically through the gastric cannula. PHA significantly inhibited basal acid secretion. Inhibition of acid output reached 72% during the first collection period following PHA administration when compared, then gradually disappeared. Pentagastrin-stimulated acid secretion was repressed dose-dependently by PHA as well. Maximal inhibition was observed during the first 30 min following application of PHA. Histamine-stimulated acid secretion was inhibited by PHA in a similar manner. Pepsin secretion was not affected by PHA under either basal or stimulated conditions. These results provide evidence that PHA is a potent inhibitor of gastric acid secretion in conscious rats, but it does not affect pepsin output from the stomach.

2,7-Dioxo-2,3,4,5,6,7-hexahydro-1H-benzo[h][1,4]diazonine as a new template for the design of CCK(2) receptor antagonists

A novel series of nonpeptide CCK(2) receptor antagonists has been prepared, in which 2,7-dioxo-2,3,4,5,6,7-hexahydro-1H-benzo[h][1, 4]diazonine (5) was used as a chemical template. This uncommon ring system was obtained in a highly substituted form and in high yield by ozonolysis of the enamine bond of 1,2,3,4-tetrahydro-9H-pyrido[3, 4-b]indole derivatives (4), in which the configuration of the substituents was established stereoselectively via the Pictet-Spengler reaction. Further structural manipulation was guided by molecular modeling through comparison of fieldpoint-based structures of candidate compounds with a selected low-energy conformation of the representative CCK(2) receptor antagonist 5-[[[(1S)-[[(3, 5-dicarboxyphenyl)amino]carbonyl]-2-phenylethyl]amino]carbonyl]-6- [[( 1-adamantylmethyl)amino]carbonyl]indole (JB93182 (3)). By this approach compounds such as (3R, 5S)-4-acetyl-3-(1-adamantyl)methyl-1-(2-chlorobenzyl)-5-carboxymet hyl aminocarbonyl-2,7-dioxo-2,3,4,5,6,7-hexahydro-1H-benzo[h][1, 4]diazonine (32) were prepared. Compound 32 behaved as a competitive CCK(2) receptor antagonist in vitro as judged by its inhibition of pentagastrin-stimulated acid secretion in an isolated, lumen-perfused, immature rat stomach assay (pK(B) = 6.74 +/- 0.27) and by its displacement of [(125)I]CCK-8S from CCK(2) sites in mouse cortical homogenates (pK(i) = 6.99 +/- 0.05). Compound 32 was 100-fold selective for CCK(2) over CCK(1) receptors based on the affinity estimate obtained in a guinea pig pancreas radioligand binding assay (pK(i) = 5.0).

Effect of CCK pretreatment on the CCK sensitivity of rat polymodal gastric vagal afferent in vitro

To prevent the blood-borne interference and reflex actions via neighboring organs and the central nervous system, the study was conducted in an in vitro isolated stomach-gastric vagus nerve preparation obtained from overnight-fasted, urethan-anesthetized rats. Afferent unit action potentials were recorded from the gastric branch of the vagus nerve. The left gastric artery was catheterized for intra-arterial injection. In vitro we found that 1) 55/70 gastric vagal afferents (GVAs) were polymodal, responding to CCK-8 and mechanical stimuli, 13 were mechanoreceptive, and 2 were CCK-responsive; 2) sequential or randomized intra-arterial injections of CCK-8 (0.1-200 pmol) dose-dependently increased firing rate and reached the peak rate at 100 pmol; 3) the action was suppressed by CCK-A (Devazepide) but not by CCK-B (L-365,260) receptor antagonist; 4) neither antagonist blocked the mechanosensitivity of GVA fibers. These results are consistent with corresponding in vivo well-documented findings. Histological data indicate that the layered structure of the stomach wall was preserved in vitro for 6-8 h. Based on these results, it seems reasonable to use the in vitro preparation for conducting a study that is usually difficult to be performed in vivo. For instance, because there was no blood supply in vitro, the composition of the interstitial fluid, i.e., the ambient nerve terminals, can be better controlled and influenced by intra-arterial injection of a defined solution. Here we report that acutely changing the ambient CCK level by a conditioning stimulus (a preceding intra-arterial injection of increasing doses of CCK-8) reduced the CCK sensitivity of GVA terminals to a subsequent test stimulus (a constant dose of CCK-8 intra-arterial injection).

Analysis of the behaviour of selected CCKB/gastrin receptor antagonists in radioligand binding assays performed in mouse and rat cerebral cortex

1. The previously described complex behaviour of the CCKB/gastrin receptor antagonist, L-365,260, in radioligand binding assays could be explained by a variable population of two binding sites. We have investigated whether other CCKB/gastrin receptor ligands (PD134,308, PD140,376, YM022 and JB93182) can distinguish between these sites. 2. In the mouse cortex assay, Hill slopes were not different from unity and the ligand pKI values did not differ when either [125I]-BH-CCK-8S or [3H]-PD140,376 was used as label as expected for a single site (G2). 3. In the rat cortex, where previous analysis of replicate (n=48) L-365,260 data indicated the presence of two CCKB/gastrin sites (G1 and G2), the competition data for PD134,308, PD140,376, YM022 and JB93182 could be explained by a homogeneous population of CCKB/gastrin sites because the Hill slope estimates were not significantly different from unity. However, the estimated affinity values for JB93182 and YM022 were significantly higher and that for PD134,308 was significantly lower than those obtained in the mouse cortex when the same radioligand was used. In view of our previous data obtained with L-365,260, the rat cortex data were also interpreted using a two-site model. In this analysis, SR27897 expressed approximately 9 fold, PD134,308 approximately 13 fold and PD140,376 approximately 11 fold selectivity for the G2 site. In contrast, JB93182 expressed approximately 23 fold and YM022 approximately 4 fold selectivity for the G1 site. If the two-site interpretation of the data is valid then, because of its reverse selectivity to L-365,260, JB93182 has been identified as a compound which if radiolabelled could provide a test of this receptor subdivision.

Characterization of the binding of a novel radioligand to CCKB/gastrin receptors in membranes from rat cerebral cortex

1. We have investigated the binding of a novel radiolabelled CCKB/gastrin receptor ligand, [3H]-JB93182 (5[[[(1S)-[[(3,5-dicarboxyphenyl)amino]carbonyl]-2-phenylethyla mino]-carbonyl]-6-[[(1-adamantylmethyl) amino]carbonyl]-indole), to sites in rat cortex membranes. 2. The [3H]-JB93182 was 97% radiochemically pure as assessed by reverse-phase HPLC (RP-HPLC) and was not degraded by incubation (150 min) with rat cortex membranes. 3. Saturation analysis indicated that [3H]-JB93182 labelled a homogeneous population of receptors in rat cortex membranes (pKD=9.48+/-0.08, Bmax=3.61+/-0.65 pmol g(-1) tissue, nH=0.97+/-0.02, n=5). The pKD was not significantly different when estimated by association-dissociation analysis (pKD=9.73+/-0.11; n=10). 4. In competition studies, the low affinity of the CCKA receptor antagonists, L-364,718; SR27897 and 2-NAP, suggest that, under the assay conditions employed, [3H]-JB93182 (0.3 nM) does not label CCKA receptors in the rat cortex. 5. The affinity estimates obtained for reference CCKB/gastrin receptor antagonists were indistinguishable from one of the affinity values obtained when a two site model was used to interpret [125I]-BH-CCK8S competition curves obtained in the same tissue (Harper et al., 1999). 6. This study provides further evidence for the existence of two CCKB/gastrin sites in rat cortex. [3H]-JB93182 appears to label selectively sites previously designated as gastrin-G1 and therefore it may be a useful compound for the further discrimination and characterization of these putative receptor subtypes.

Characterization of CCK receptors in stomach smooth muscle: evidence for two subtypes

Cholecystokinin (CCK) and related peptides such as gastrin are important regulators of gastric smooth muscle contraction. Several studies have shown that these effects of CCK and gastrin are mediated by CCK(B) receptors. However, recent studies suggest the expression of an additional CCK receptor subtype distinct from CCK(B) receptors in this tissue. This study was designed to distinguish between CCK(A) and CCK(B) receptors on guinea-pig stomach smooth muscle cells and to evaluate these cells for additional receptor subtypes. We cloned these receptors by hybridization screening of a guinea-pig smooth muscle cDNA library using 32P random primed labeled cDNA probes from the recently cloned rat CCK(A) and CCK(B) receptor coding regions. In addition to clones representing the CCK(B) subtype, clones of CCK(A) receptor subtype, but no additional CCK receptor subtypes, could be identified. All isolated clones displayed highly homologous nucleotide sequences in comparison to previously characterized CCK(A) and CCK(B) receptors from different species. The results of cDNA hybridization at different levels of stringency and Southern blot analysis using guinea-pig genomic DNA suggest that it is unlikely that additional CCK receptors despite CCK(A) and CCK(B) receptors exist in stomach smooth muscle.

Antagonist inhibition curves and the measurement of dissociation constants

1. Experiments carried out on guinea-pig isolated ileum with carbachol as agonist and diphenyl-acetoxyethyl-dimethyl-ethyl-ammonium (DADMEA) bromide as antagonist gave results which fit the theoretical relation between fractional inhibition (Q) of the effects of an agonist ([A]) and the concentration of a competitive antagonist ([B]): this also involves the Hill coefficient (logistic slope factor, P) for the agonist concentration-response curve and the degree of agonist stimulation, [A]/[A]50, where [A]50 produces a half-maximum response. 2. Values of IC50 and an exponent, P', can be obtained by fitting Q to [B] using a logistic approximation to the relation. Both P' and IC50 should be greater with higher agonist stimulation but the increase in P' may be masked by errors in extreme values of Q. Estimates of IC50, however, invariably increased with higher agonist stimulation but with a steep concentration-response curve (P > 1) and low agonist stimulation ([A]/[A]50 < 1, IC50 can be less than KD. 3. KD was calculated from the results in three ways; (i) by a least-squares fit of Q to [B] using the values of P and [A]/[A]50 calculated from the control concentration-response curve; (ii) from the value of IC50 for each line and the values of P and [A]/[A]50 and (iii) by using the agonist concentration-response curve to calculate the dose-ratio and estimate of KD for each in the presence of the antagonist. The methods gave similar results (nM: 11 experiments), 12.4 +/- 1.1 (i), 11.7 +/- 0.9 (ii), 14.8 +/- 1.6 (iii) but there are advantages in using methods (i) or (ii) rather than (iii). 4. The method by which KD is calculated is less important than the experimental design: the plan used in this work, with alternative small and large responses from the tissue, is very suitable for estimating KD with low concentrations of antagonists and small dose-ratios. Although it is not a sensitive test for competitive behaviour because only a small range of concentrations of antagonist is tested, the estimate of affinity should be free from complications involved in the use of higher concentrations of antagonist (and agonist) and the nature of the antagonism can always be checked by doing further experiments in the presence of a known competitive antagonist.

Analysis of variation in L-365,260 competition curves in radioligand binding assays

1. For several years, we have used the cholecystokinin (CCK)B/gastrin receptor selective antagonist, L-365,260, as a reference compound in a variety of studies in CCKB/gastrin receptor radioligand binding assays. Here, we have analysed the competition curve data sets obtained between L-365,260 and [125I]-BH-CCK8S in guinea-pig gastric gland and mouse and rat cerebral cortex preparations. 2. Competition curves obtained for L-365,260 in the mouse cortex assay were not different from rectangular hyperbolae (slope = 1.01 +/- 0.02) implying the presence of a single population of binding sites (pKI = 8.41 +/- 0.01; data from 47 experiments, slope constrained to unity). However, in the rat cortex and guinea-pig gastric gland assays, the mean slope of the competition curves was significantly less than one and the mean apparent pKI significantly lower than that obtained in the mouse cortex (slope = 0.85 +/- 0.03, 0.90 +/- 0.03; apparent pKI = 7.98 +/- 0.05, 8.07 +/- 0.05; 48 and 45 experiments, in rat and guinea-pig, respectively). The distribution of the individual pKI and slope estimates of the competition curves in these two assays was consistent with expectations for the variable expression (in terms of absolute number and proportion) of two binding sites. The two sites were characterized by pKI values for L-365,260 of 8.50 +/- 0.04 and 8.48 +/- 0.04 for the high affinity site and 7.32 +/- 0.04 and 7.22 +/- 0.06 for the low affinity site in guinea-pig and rat, respectively. 3. The affinity estimates for L-365,260, although obtained on different tissues, are consistent with data obtained from the analysis of L-365,260 antagonism of pentagastrin-stimulated responses in mouse and rat stomach (acid secretion) and guinea-pig gastric muscle (isotonic contraction) assays. To this extent, these data suggest the existence of two CCKB/gastrin receptor subtypes.