Search

This study evaluates the effects of agonists and antagonists of cholecystokinin (CCK) on contractile and myoelectrical activity in isolated longitudinal muscle strips from colon or ileum of guinea pigs or beagle dogs. Caerulein and CCK-8 caused a dose-dependent increase of contractile and myoelectrical spike activity in both species with maximal effects seen between 10−8 and 3 × 10−8 M. The dose responses were identical for both CCK agonists and species. The dose-related effects of CCK compounds on colonic muscle were slightly shifted to the right when compared to ileum in both species. All antagonists, the proglumide-derivatives CR1409, CR1392, and CR1505, as well as the nonpeptide substances asperlicin and L-364,718, caused a parallel rightward shift of CCK's dose-dependent motor activity response, indicating the competitive nature of inhibition. The antagonists displayed a rank order of potency in antagonizing CCK's action on intestinal motility similar to their ability to antagonize CCK's action on pancreas and gallbladder. L-364,718 was the most potent antagonist, followed by CR1409, CR1505, CR1392, asperlicin, and proglumide. The antagonists did not affect contractile or myoelectrical responses to acetylcholine, histamine, motilin, or substance P. Thus compounds that have been described as CCK antagonists for pancreas and gallbladder also act as specific and competitive antagonists of CCK's action on contractile and myoelectrical activity of Heal and colonie muscle.

A series of C-terminal anthranoyl-anthranilic acid derivatives arising from a strict bond disconnection approach of asperlicin were synthesized and examined for their CCK receptor affinities. These compounds represent the second step of our investigation directed toward the search for alternative substructures of asperlicin as a starting point for the development of a new class of CCK ligands. The obtained micromolar affinities for CCK-A rather than CCK-B receptor confirm that the anthranilic acid dimer represents a useful template for the development of selective CCK-A receptor ligands.

A series of 2-aralkyl-4H-pyridothiadiazine 1,1-dioxides and 3-aralkylamino-2-aryl-2H-pyrido[4,3-e]-1,2,4-thiadiazine 1,1-dioxides structurally related to quinazolinone CCK receptor antagonists were synthesized and evaluated as CCK-A and CCK-B receptor ligands. The compounds were effective as cholecystokinin-ligands in the micromolar range of concentration, c.f. the cholecystokinin receptor antagonists asperlicin, lorglumide or benzotript, and were thus less potent than the best quinazolinones previously reported. Although the compounds were unsuitable for drug use, the work contributed to our understanding of the chemistry of unusual 2,3-disubstituted pyridothiadiazinedioxides.

A novel hybrid series of nonpeptide CCK-B receptor antagonists has been designed from two known series derived from asperlicin. An efficient synthesis of 2-aminoquinazolinone, an intermediate for the synthesis of a targeted analog, has been developed.

The Val353 residue of the rat cholecystokinin-B (CCK-B) receptor was mutated to Leu to test whether this residue is part of a binding site for antagonists having different chemical structures. The agonist radioligand [3H]SNF 8702 showed similar affinity for both wild-type and mutant receptors. Mutation of the CCK-B receptor reversed the order of affinities for the asperlicin derivatives from L-365,260 > devazepide (wild-type) to devazepide > L-365,260 (mutant) but had no effect on the affinity of the peptoid CCK-B receptor antagonist Cam-1028. The results show that Val353 is not part of a general CCK-B receptor antagonist recognition site and that Cam-1028 is recognized at a receptor site distinct from that binding asperlicin derivatives.

Members of the asperlicin family of fungal metabolites produced by Aspergillus alliaceus are known potent CCK(A) antagonists. Herein, we report the identification of the gene cluster responsible for directing their biosynthesis. We validate and probe the pathway by genetic manipulation, and provide the first biochemical characterization of the oxidative cyclization en route to the heptacyclic asperlicin E by reconstituting the activity of the FAD depend monooxygenase AspB. This report provides the first genetic characterization of a NRPS assembly line that efficiently activates two anthranilate building blocks and illustrates the remarkably efficient biosynthesis of the complex heptacyclic asperlicin E.

New derivatives of 1,3,4-benzotriazepin-5-one were designed and synthesized as structural analogues to the antitumor agents devazepide and asperlicin. An efficient and novel approach to the synthesis of 2-amino-1,3,4-benzotriazepin-5-one 2 was developed and its structure was confirmed. The newly synthesized derivatives were evaluated for their in vitro antitumor activity on 60 different cell lines. Compounds 8 and 9 displayed the most potent antitumor activity against several cell lines specifically ovarian cancer, renal cancer and prostate cancer, while compounds 5, 10 and 12 showed significant activities against UO-31 renal cancer cell line.

Isolated gastric glands from rabbit were used to characterize the functional cholecystokinin (CCK)-like peptide receptors that mediate pepsinogen secretion. Pepsinogen secretion was stimulated by both CCK octapeptide sulfate (CCK-8) and A-71378, a selective CCK-A-type receptor agonist, with similar mean effective doses (1.0 and 0.8 nM, respectively). Compared with CCK-8, gastrin-17 (G-17-I) showed reduced potency and only partial efficacy for stimulation of pepsinogen secretion while inhibiting the maximal CCK-8-stimulated response. The nonpeptide inhibitors, asperlicin and L-364,718, inhibited pepsinogen secretion with identical pA2 values for antagonism of both CCK and gastrin, indicating that both peptides interact with the same functional receptor. Specific binding of [3H]CCK-8 to isolated chief cell membranes was displaced fully by both CCK and gastrin, indicating full receptor occupancy by both peptides. A novel synthetic peptide analogue, pseudogastrin [(Glu)5-Ala-Tyr-Nle-Gly-Trp-Nle-Asp-Phe-NH2], was used to investigate the structural basis for the lower potency and efficacy of G-17-I. The potency of CCK and gastrin analogues for pepsinogen secretion was found to be dependent on both sulfation of a tyrosine residue and the position of the tyrosine residue relative to the COOH-terminal phenylalanine amide. The efficacy appears to be determined partially by the extended NH2-terminal sequence of G-17-I. The results of the present study are interpreted to show that pepsinogen secretion is mediated by a CCK-A-type receptor and gastrin acts at the same receptor as a partial agonist.

Peptide receptors represent important targets for a variety of potentially important disease modifying drugs. Over the past 20 years, considerable effort has been spent on modifying the natural ligands for peptide receptors, the peptides themselves, with limited success. Using a targeted screen, namely cholecystokinin (CCK)-A receptor binding, the Merck group in 1984 discovered asperlicin, a potent non-peptide cholecystokinin antagonist selective for peripheral tissues, that was isolated from Aspergillus alliaceaus. This compound was used as a lead to develop MK-329, a CCK-A receptor antagonist, the evolution of which is discussed within the context of the drug discovery process and receptor modeling strategies. © 1993 Wiley-Liss, Inc.

A novel hybrid series of nonpeptide CCK-B receptor antagonists has been designed from two known series derived from asperlicin. An efficient synthesis of 2-aminoquinazolinone, an intermediate for the synthesis of a targeted analog, has been developed.

We have described previously an innovative bond disconnection strategy of asperlicin, a naturally occurring CCK receptor antagonist, leading to anthranilic acid dimer and tryptophan synthons. We have also demonstrated that when the tryptophan residue is connected to the C- or N-terminal sides of the anthranilic acid dimer, compounds with similar micromolar CCK-A receptor affinities are obtained. In order to investigate the binding effects of different N-terminal substitution, in this paper we describe a new series of anthranilic acid dimer derivatives, characterized by the presence of the tryptophan residue in the C-terminus of the dimer. Among the compounds synthesized, the N-1H-indol-3-propionyl derivative exhibited an improved, at the micromolar range, affinity for the CCK-A receptor in comparison to that of either, the N-unsubstituted derivative and asperlicin. The lead compound emerging from this key step of our investigation represents the new starting point for the development of a new class of CCK-A receptor ligands.

A series of new N-substituted anthranilic acid dimer derivatives having a C-terminal Phe residue was synthesized and evaluated for their affinity for CCK receptors. These compounds resulted from a blended approach based firstly on the use of an alternative substructure embedded within asperlicin and secondly on the derivatization of this template with substituents chosen considering the C-terminal primary structure of the endogenous ligand. Although these compounds exhibited a regnylogical-type organization similar to that of CCK-4, they are characterized by about 1000-fold greater affinity for CCK-A receptor than the C-terminal tetrapeptide.

The role of endogenous opiates and cholecystokinin (CCK) in the control of postprandial pyloric myoelectric activity was investigated in conscious dogs with chronically implanted intraparietal electrodes at the gastroduodenal junction. Meals consisted of either 20 g/kg of canned food (standard meal) or the same food supplemented with 0.5 mL/kg of arachis oil (fat meal). During the 6 hours after standard and fat meals, the number of pyloric spike bursts, 2-4 seconds in duration, was 61.8 +/- 15.8 and 49.9 +/- 12.7/15 minutes, respectively. Administered 15 minutes before a fat meal, naloxone (50 micrograms/kg IV) decreased the number of spike bursts by 31.4%, whereas methyl-levallorphan, a peripheral opiate antagonist, increased postprandial spike activity by 22.2% when administered IV (0.5 mg/kg) and decreased it when administered intracerobroventricularly at a dose of 10 micrograms/kg. These two antagonists administered in the same conditions before a standard meal had no effect on the postprandial spike activity. A 1-hour infusion of cholecystokinin octapeptide (CCK-8), 500 ng.kg-1.h-1 IV and 50 ng.kg-1.h-1 intracerebroventricularly, performed 1 hour after a standard meal induced a 19.6% and 15.8% decrease in the number of pyloric spike bursts, respectively. Both naloxone IV (50 micrograms/kg) and methyl-levallorphan intracerebroventricularly (10 micrograms/kg) administered before the infusion of CCK-8 reinforced this pyloric inhibition, which was antagonized by methyl-levallorphan IV (0.5 mg/kg). The CCK antagonist asperlicin, 200 micrograms/kg IV and 20 micrograms/kg intracerebroventricularly, administered before a fat meal increased pyloric spike bursts by 22.0% and 31.5%, respectively. These results indicate that after a fat meal, endogenous opiates exert a peripheral inhibitory and central stimulatory control of pyloric motility; they suggest the involvement of both peripheral and central release of CCK.

[reaction: see text] Imidazoindolones are present as the key structural motif in the family of antifungals, fumiquinazolines, and the antagonist asperlicin. The first example of a CuI-catalyzed tandem intramolecular amidation forming substituted imidazoindolones from readily accessible ortho gem-dibromovinylanilines is described.

Cholecystokinin (CCK) is a regulatory peptide hormone, predominantly found in the gastrointestinal tract, and a neurotransmitter present throughout the nervous system. In the gastrointestinal system CCK regulates motility, pancreatic enzyme secretion, gastric emptying, and gastric acid secretion. In the nervous system CCK is involved in anxiogenesis, satiety, nociception, and memory and learning processes. Moreover, CCK interacts with other neurotransmitters in some areas of the CNS. The biological effects of CCK are mediated by two specific G protein coupled receptor subtypes, termed CCK1 and CCK2. Over the past fifteen years the search of CCK receptor ligands has evolved from the initial CCK structure derived peptides towards peptidomimetic or non-peptide agonists and antagonists with improved pharmacokinetic profile. This research has provided a broad assortment of potent and selective CCK1 and CCK2 antagonists of diverse chemical structure. These antagonists have been discovered through optimization programs of lead compounds which were designed based on the structures of the C-terminal tetrapeptide, CCK-4, or the non-peptide natural compound, asperlicin, or derived from random screening programs. This review covers the main pharmacological and therapeutic aspects of these CCK1 and CCK2 antagonist. CCK1 antagonists might have therapeutic potential for the treatment of pancreatic disorders and as prokinetics for the treatment of gastroesophageal reflux disease, bowel disorders, and gastroparesis. On the other hand, CCK2 antagonists might have application for the treatment of gastric acid secretion and anxiety disorders. © 2003 Wiley Periodicals, Inc. Med Res Rev, 23, No. 5, 559–605, 2003

A series of new N-substituted anthranilic acid dimer derivatives having a C-terminal Phe residue was synthesized and evaluated for their affinity for CCK receptors. These compounds resulted from a blended approach based firstly on the use of an alternative substructure embedded within asperlicin and secondly on the derivatization of this template with substituents chosen considering the C-terminal primary structure of the endogenous ligand. Although these compounds exhibited a regnylogical-type organization similar to that of CCK-4, they are characterized by about 1000-fold greater affinity for CCK-A receptor than the C-terminal tetrapeptide.

A series of C-terminal anthranoyl-anthranilic acid derivatives arising from a strict bond disconnection approach of asperlicin were synthesized and examined for their CCK receptor affinities. These compounds represent the second step of our investigation directed toward the search for alternative substructures of asperlicin as a starting point for the development of a new class of CCK ligands. The obtained micromolar affinities for CCK-A rather than CCK-B receptor confirm that the anthranilic acid dimer represents a useful template for the development of selective CCK-A receptor ligands.

We have described previously an innovative bond disconnection strategy of asperlicin, a naturally occurring CCK receptor antagonist, leading to anthranilic acid dimer and tryptophan synthons. We have also demonstrated that when the tryptophan residue is connected to the C- or N-terminal sides of the anthranilic acid dimer, compounds with similar micromolar CCK-A receptor affinities are obtained. In order to investigate the binding effects of different N-terminal substitution, in this paper we describe a new series of anthranilic acid dimer derivatives, characterized by the presence of the tryptophan residue in the C-terminus of the dimer. Among the compounds synthesized, the N-1H-indol-3-propionyl derivative exhibited an improved, at the micromolar range, affinity for the CCK-A receptor in comparison to that of either, the N-unsubstituted derivative and asperlicin. The lead compound emerging from this key step of our investigation represents the new starting point for the development of a new class of CCK-A receptor ligands.

Publisher Summary This chapter presents an overview of privileged structures. The term “privileged structure” was introduced by B. Evans in describing development of benzodiazepine-based CCK-A antagonists from the natural product lead asperlicin. A privileged structure such as a benzodiazepine was defined as “a single molecular framework able to provide ligands for diverse receptors…” Benzodiazepines are found in several types of central nervous system (CNS) agents and are in ligands of both ion channel and G-protein coupled receptors (GPCRs). Many of the privileged structures in GPCR ligands are found in enzyme inhibitors. Examples include benzodiazepine-containing K-secretase inhibitors and farnesyl transferase inhibitors including the tricycle-based SCH-66336 and the tetrahydrobenzodiazepine. Some examples of benzodiazepines in ion channel ligands include the delayed rectifier K+ current modulator. The chapter focuses on privileged structure based ligands of GPCRs. An overview of privileged structure based antagonists is presented and peptidomimetic GPCR agonists are discussed. An overview of agonists via privileged structure modification is presented and examples of peptidyl privileged structure agonists are presented.

The synthesis of 3-aralkyl-4-aryl-4H-, 3-aralkylamino-4-aryl-4H- and 3-aralkylsulfanyl-4-aryl-4H-pyrido[4,3-e]-1,2,4-thiadiazine 1, 1-dioxides is described. Moreover, the affinity of the different compounds towards the cholecystokinin CCK-A and CCK-B receptors was evaluated. For selected compounds, affinity on the two receptor subtypes was expressed in the micromolar range. This was comparable to the affinity observed with the naturally occurring CCK receptor antagonist asperlicin.

A large interest is devoted today to endogenous peptides that possess specific binding sites associated with some physiological properties. The first challenge for medicinal chemists is to design and synthesize potent and specific ligands of these receptors. Most of the successes are generally achieved through systematic screening of synthetic collections of chemicals originated from natural sources 1 (asperlicin as CCK antagonist), or synthetic compounds from combinatorial chemistry. However rational design starting from the natural peptidic ligand may help to design a first lead compound that presents at least a μM range IC50 value.

A family of novel compounds has been detected and isolated following an assay for the attenuation of multiple drug resistance in tumor cells from the fermentation broth and mycelia of a strain of Aspergillus fischeri which we have designated var. brasiliensis. The structures of three components were determined employing 1-D and 2-D homonuclear and heteronuclear NMR spectroscopy and mass spectrometry. The structure of 5-N-acetylardeemin was confirmed by single crystal X-ray diffraction. These compounds are most closely structurally related to asperlicin E1).

The C 3 -substituted benzodiazepines derived from asperlicin, e.g. devazepide (L-364,718, MK-329), constitute the most potent class of cholecystokinin A-type (CCK A ) receptor antagonists. In order to gain insight into the prerequisites for binding, we examined the conformational properties of both potent and weak representati3es of this class with computer assisted molecular modelling (CAMM) techniques. The CAMM results indicate that the binding site for C 3 -substituents is a planar slot on the CCK A receptor surface and, in addition, allow the proposal of a model which describes the relative binding mode of the less potent R isomers versus that of the S isomers. The latter model illustrates the unique spatial properties of the benzodiazepine moiety, which we suggest functions primarily as an invertible core which assures an optimal arrangement of attached substituents.

MK-329 is a nonpeptidal, highly specific cholecystokinin (CCK) receptor antagonist, with affinity for pancreatic and gallbladder CCK receptors similar to CCK itself. MK-329 and its progenitor, asperlicin, can inhibit the growth of CCK receptor-positive human pancreatic cancer in athymic mice. Based on these activities and the ability of MK-329 to transiently increase food intake and enhance morphine analgesia in murine models, we conducted an open trial of MK-329 in 18 patients with advanced pancreatic cancer in whom the CCK receptor status of the tumors was unknown. Tumor response, pain control, and nutritional parameters (hunger rating, caloric intake, body weight, and anthropometrics) were serially assessed. The results of the study failed to demonstrate any impact of MK-329 on tumor progression, pain, or nutrition. Toxicity was mild and limited to nausea, vomiting, diarrhea, and abdominal cramps, with 17 of 18 patients able to tolerate treatment. While a role for MK-329 in the management of patients with advanced pancreatic cancer cannot be supported by the results of this trial, additional studies of this agent in patients with known CCK receptor-positive tumors, at escalated doses, and possibly in conjunction with other growth antagonists, appear warranted.

Publisher Summary The search for novel pharmacophores from natural sources has continued apace. The goal has shifted from random screening to a more direct approach. Advances in molecular biology have allowed the development of mechanism-based receptor screens to probe interactions among large molecules and find small natural product molecules as potential agonists and antagonists. The choice of organisms to be examined has also been refined in response to the recognition that evolutionary pressures have exerted a marked effect on secondary metabolite production. This is particularly true for those organisms living in close association with each other. Special emphasis has been placed on the selection of those types of organisms as the raw material for screening programs. This chapter provides a selective, illustrative account of the literature, with special emphasis on the data published in 1990. The chapter highlights the chemical diversity and the origin of fungi, Actinomycetales, marine organisms (including algae), and plants, as well as discusses the molecular structures of the compounds. Natural materials are fertile sources of novel chemical structures, possessing unique pharmacological activities. Many natural products that are routinely discovered become biochemical tools for the mechanism of action studies, particularly as enzyme inhibitors or receptor ligands. In those cases, where novel structures possess truly valuable biological activities without toxicity, the path to drug development is clear as in the cases of the immunomodulatory agent FK-506 or the cholecystokinin (CCK) antagonist asperlicin. The marketplace success of natural products, such as cyclosporin, avermectin, and mevinolin, has provided renewed impetus to the screening of plants, marine sources, and microbial broths as the sources of novel structures with new activities.

Cholecystokinin belongs together with gastrin and secretin to the classical gut hormones [1]. Cholecystokinin is produced by the I cells in the upper small intestinal mucosa and is released into the circulation in response to ingestion of nutrients or infusion of the neurotransmitter bombesin/gastrin releasing peptide. The polypeptide hormone is named cholecystokinin because of its property to stimulate gallbladder contraction. In addition to its gallbladder-contracting property, cholecystokinin has been shown to be a potent stimulus of pancreatic growth and pancreatic enzyme and hormone secretion, to affect motility of the gastrointestinal tract, and to play a role in the regulation of satiety. Because of its pancreatic growth promoting action, cholecystokinin has been studied in pancreatic carcinogenesis [2–4]. The recent development of highly effective and specific cholecystokinin-receptor antagonists has enabled to delineate the various actions of cholecystokinin in great detail [5,6]. Two types of cholecystokinin-receptor antagonists can be used for in vivo studies, glutaramic derivates (proglumide, lorglumide, loxiglumide) and non-peptide compounds (asperlicin, L-364718). Most in vivo studies have been performed using the glutaramic derivates lorglumide (CR-1409) and loxiglumide (CR-1505) and the non-peptide antagonist L-364718 (MK-329). Studies on the role of cholecystokinin on gastrointestinal tumor growth have been concentrated on the role of cholecystokinin in pancreatic cancer.

Benzodiazepine derivative CCK antagonists originated from the discovery at Merck, by receptor-based screening, of a novel benzodiazepine-like fermentation product from Aspergillus alliaceus [1], asperlicin. Its lack of oral activity and the need for greater potency stimulated the design of improved antagonists. On the basis of structural relationships between asperlicin and the molecules diazepam and D-tryptophan, an indolylmethyl benzodiazepine was designed and synthesized [2]. This simpler compound proved to be comparable in potency and selectivity to asperlicin as a CCK antagonist. Structure-activity studies based on this lead produced a series of compounds with marked increases in potency and selectivity [3,4]. One optimized antagonist, devazepide (MK-329 or L-364718, Fig. 1) [5], binds to the CCK-A (peripheral) receptor with an affinity comparable to CCK-8 (IC50 0.08 nM) and shows greater than 1000-fold selectivity over the CCK-B receptor [6].

In 1981 proglumide and benzotript were shown to act as specific and competitive CCK-receptor antagonists in vitro [32]. However, these compounds had low potencies in antagonizing the action of CCK in the intact organ [62]. More recently, several new CCK antagonists have been described which are more potent in vitro compared to proglumide. First, C-terminal CCK fragments or analogs such as CCK-(27–32)-amide were shown to act as specific CCK antagonists up to 75 times more potent than proglumide [77]. CCK fragments, however, were shown to be rapidly degraded in physiological fluids [45]. Subsequently, peptide molecules with a proglumide-like structure were synthesized which were up to 500–5000 times more potent than proglumide in vitro and in vivo [53, 64–66] (Fig. 1). More recently, asperlicin, a nonpeptide substance isolated from Aspergillus aliaceus (Fig. 1) was shown to act as a specific CCK-antagonist which was 200 times more potent than proglumide in inhibiting CCK-stimulated secretion in vitro [11] (Fig. 2). Asperlicin, however, was poorly water soluble and, thus, difficult to use for in vivo experiments. In 1986, several nonpeptide substances, structurally similar to asperlicin, were described which were more water soluble than asperlicin. The most potent of these asperlicin derivatives, L-364,718, was shown to be 10000 to 3000000 times more potent than proglumide in inhibiting CCK’s action and binding in vitro and in vivo [10, 12, 18, 66] (Figs. 1, 2). Both the new potent peptide and nonpeptide antagonist only inhibit the actions of those agonists which bind to the CCK receptor [66].

To compare receptors for cholecystokinin (CCK) in pancreas and gallbladder, we measured binding of 125I-Bolton-Hunter-labeled CCK-8 (125I-BH-CCK-8) to tissue sections from guinea pig gallbladder and pancreas under identical conditions. In both tissues, binding had similar time-, temperature-, and pH dependence, was reversible, saturable and inhibited only by CCK related peptides or CCK receptor antagonists. Autoradiography localized 125I-BH-CCK-8 binding to the smooth muscle layer in the gallbladder. Binding of 125I-BH-CCK-8 to gallbladder sections was inhibited by various agonists with the following potencies (IC50):CCK-8 (0.4 nM) greater than des(SO3)CCK-8 (0.07 microM) greater than gastrin-17-I (1.7 +/- 0.3 microM) and by various receptor antagonists with the following potencies: L364,718 (1.5 nM) greater than CR 1409 (0.19 microM) greater than asperlicin = CBZ-CCK-(27-32)-NH2 (1 microM) greater than Bt2cGMP (120 microM). Similar potencies were found for the agonists and antagonists for pancreas sections. Inhibition of binding of 125I-BH-CCK-8 by 11 different analogues of proglumide gave similar potencies for both pancreas and gallbladder. The potencies of agonists in stimulating and antagonists in inhibiting CCK-stimulated contraction or amylase release correlated closely with their abilities to inhibit 125I-BH-CCK-8 binding to gallbladder or pancreas sections or acini, respectively. The present results demonstrate and characterize a method that can be used to compare the CCK receptors in guinea pig gallbladder and pancreas under identical conditions. Moreover, this study demonstrates that gallbladder and pancreatic CCK receptors have similar affinities for the various agonists and antagonists tested and, therefore, provides no evidence that they represent different subtypes of CCK receptors that can be distinguished pharmacologically.

Asperlicin (1, C31H29N5O4) is a novel cholecystokinin antagonist produced by Aspergillus alliaceus. The structure of asperlicin has been determined by NMR and mass spectral analysis, and X-ray crystallography.

The natural product asperlicin is the first nonpeptide antagonist of cholecystokinin isolated from a microbial source. At discovery, production of asperlicin by the original soil isolate ofAspergillus alliaceus was between 15 and 30 mg/l. Selection of natural variants ofA. alliaceus, use of Plackett & Burman and Simplex experimental designs; formulation of synthetic media; amino acid supplementation of production media; analysis of complex nitrogen sources for their amino acid content; evaluation of promising media in fermentors; substitution of glycerol for glucose as a carbon source and rational mutant selection all contributed to titer increases to >900 mg/l.

Recent studies have demonstrated gastrin receptors in some pancreatic tumors and that gastrin is a potent stimulant of pancreatic Na+-H+ exchange. In the present study we used 125I-labeled gastrin (125I-gastrin) to characterize gastrin receptors on guinea pig pancreatic acini. Binding of 125I-gastrin was temperature dependent, saturable, and specific for gastrin-related peptides. Analysis demonstrated a single class of receptors with high affinity for gastrin (Kd = 1.5 nM) and a binding capacity of 1 fmol/mg protein. Binding of 125I-gastrin was inhibited with the following relative potencies (Kd): cholecystokinin octapeptide (CCK-8) (0.35 nM) greater than gastrin-17-I = gastrin-34-I (1.5 nM) greater than pentagastrin (7 nM) greater than desulfated [des(SO3)]CCK-8 (28 nM) greater than CCK-4 (508 nM) and by the receptor antagonists CBZ-CCK-27-32-NH2 (3.5 microM) greater than proglumide analogue 10 (30 microM) greater than asperlicin (265 microM) greater than Bt2-guanosine 3',5'-cyclic monophosphate (828 micron). In contrast, for both stimulation of enzyme secretion and inhibition of binding of 125I-CCK-8 the relative potencies were CCK-8 much greater than des(SO3)CCK-8 greater than gastrin-17-I = gastrin-34-I greater than pentagastrin greater than CCK-4. For each receptor antagonist the dose-inhibition curve for gastrin-stimulated amylase release was superimpossible with that for CCK-8-stimulated amylase release. Gastrin-17-I at concentrations less than 0.1 microM did not potentiate carbachol or vasoactive intestinal peptide-stimulated amylase secretion and did not affect basal or stimulated adenosine 3',5'-cyclic monophosphate or 45Ca outflux.(ABSTRACT TRUNCATED AT 250 WORDS)

SummaryThe bimodular 276 kDa nonribosomal peptide synthetase AspA from Aspergillus alliaceus, heterologously expressed in Saccharomyces cerevisiae, converts tryptophan and two molecules of the aromatic β-amino acid anthranilate (Ant) into a pair of tetracyclic peptidyl alkaloids asperlicin C and D in a ratio of 10:1. The first module of AspA activates and processes two molecules of Ant iteratively to generate a tethered Ant-Ant-Trp-S-enzyme intermediate on module two. Release is postulated to involve tandem cyclizations, in which the first step is the macrocyclization of the linear tripeptidyl-S-enzyme, by the terminal condensation (CT) domain to generate the regioisomeric tetracyclic asperlicin scaffolds. Computational analysis of the transannular cyclization of the 11-membered macrocyclic intermediate shows that asperlicin C is the kinetically favored product due to the high stability of a conformation resembling the transition state for cyclization, while asperlicin D is thermodynamically more stable.

The discovery and biological properties of four novel cholecystokinin antagonists produced by Aspergillus alliaceus is described. One of these was seven times more potent than the previously reported asperlicin.

Seventeen analogues of the selective, competitive cholecystokinin (CCK) antagonist asperlicin 1 were prepared. These compounds were tested as inhibitors of the binding of [125I]CCK to rat pancreas and guinea pig brain receptors. Compounds 4, 7, and 8 were more potent than asperlicin on the pancreatic CCK receptor. One analogue, 17, displayed potency equivalent to asperlicin on the pancreas CCK receptor and showed a marked improvement in aqueous solubility, thereby facilitating the use of this class of CCK antagonists in physiological and pharmacological studies.

We examined the effect of L364718, a new cholecystokinin (CCK) receptor antagonist, on amylase release stimulated by CCK or different secretagogues in isolated rat pancreatic acini. L364718 caused a parallel rightward shift of the dose-response curve of CCK8. Schild plots showed a slope of 1.05 +/- 0.15 and a pA2 value of 10.01 +/- 0.31. L364718 inhibited maximally stimulated amylase release by CCK in a dose-dependent manner, with half maximal inhibition (ID50) at 1.7 nM and complete inhibition at 30 nM. Asperlicin, a prototype compound of L364718, also caused dose-dependent inhibition, but L364718 was approximately 400 times more potent than asperlicin (ID50 = 761 nM). L364718 significantly inhibited amylase release in response to CCK33 and CCK8 but had no effect on amylase release stimulated by other receptor secretagogues or agents by passing receptors. The results indicate that L364718 acts as an extremely potent, competitive, and specific antagonist of CCK's action on pancreatic acini.

Recent studies have demonstrated that cholecystokinin (CCK) receptors are heterologous in peripheral tissues and in the central nervous system and that CCK-gastrin (CCK-G) peptides are potent trophic factors for the gastrointestinal tract. In the present study we used 125I-labeled gastrin and 125I-labeled CCK to demonstrate the heterogeneity of CCK receptors on a rat pancreatic acinar cell line (AR4-2J) and analyze the role of these receptors in increasing the activity of ornithine decarboxylase. Pharmacological analysis of radioligand binding fit well with the presence of two different receptors: 1) a CCK-selective receptor having the characteristics of the CCK receptor present on normal pancreatic cells and 2) a high-affinity, low-selectivity CCK-G binding site that interacts with all CCK-G peptides sulfated and nonsulfated. CCK-G peptides stimulate ornithine decarboxylase activity with the following order of potencies (EC50): G-(2-17)-ds (0.1 nM) greater than or equal to CCK-9 (0.25 nM) greater than or equal to pentagastrin (0.4 nM) greater than CCK-4 (6 nM). This stimulation was not inhibited by CCK antagonist (asperlicin) at a concentration range that blocks the CCK receptor but does not compete with 125I-labeled gastrin binding to the CCK-G receptor. These results, obtained with CCK-G agonists and antagonists, demonstrate that ornithine decarboxylase stimulation in these cells is mediated via the CCK-G receptor.

The role of endogenous opioids and cholecystokinin (CCK) in gastric emptying was investigated in mice killed 30 min after gavage with 51Cr-radiolabeled liquid meals. The meals consisted of 0.5 ml of milk or one of five synthetic meals containing arabic gum, glucose and/or arachis oil and/or casein. Naloxone (0.1 mg/kg sc) significantly (P less than 0.01) accelerated gastric emptying of milk and meals containing fat but did not modify gastric emptying of nonfat meals. The CCK antagonist asperlicin (0.1 mg/kg ip) increased by 25% gastric emptying of milk. The gastric emptying of meals containing glucose and casein but not fat was reduced after administration of the COOH-terminal octapeptide of cholecystokinin (CCK-8, 4 micrograms/kg ip). This decrease was antagonized by both asperlicin (10 mg/kg ip) and naloxone (0.1 mg/kg sc). Intracerebroventricular (icv) administration of an opiate antagonist that poorly crosses the blood-brain barrier, methyl levallorphan (10 micrograms/kg), did not modify gastric emptying of milk but accelerated it when peripherally administered (0.1 mg/kg sc). Similarly, asperlicin (icv) administered at a dose of 1 mg/kg did not affect milk emptying. These results indicate that endogenous opiates are involved at peripheral levels in the regulation of gastric emptying of fat meals only and that such regulation involves release of CCK.

The effect of cholecystokinin (CCK)-gastrin family peptides (caerulein, unsulfated gastrin-17, and pentagastrin) and secretin in activating amiloride-sensitive 22Na uptake were investigated in guinea pig pancreatic acini. Secretin had no effect, but CCK-gastrin peptides stimulated the amiloride-sensitive 22Na uptake. The effect of caerulein was inhibited by dibutyryl guanosine 3',5'-cyclic monophosphate (cGMP) and asperlicin, indicating that activation of the Na+-H+ antiport caused by caerulein is mediated by CCK receptors. The effect of gastrin was dibutyryl cGMP and asperlicin insensitive, whereas the effect of pentagastrin was inhibited by the CCK antagonists but with a low affinity, indicating that the effect of gastrin and that of pentagastrin was CCK receptor independent. The calcium ionophore A23187 caused an increase in amiloride-sensitive 22Na uptake. However, the effect of caerulein, which increased internal calcium concentration, was not modified after depletion of intracellular calcium, and that of CCK-gastrin family peptides was not dependent on external calcium concentration. Activation of amiloride-sensitive 22Na uptake was also induced by 12-O-tetradecanoylphorbol 13-acetate and 1-oleoyl-2-acetyl-glycerol. Activation of protein kinase c may be involved in the mechanism of caerulein or gastrin in activating the Na+-H+ exchange.

A new simple mouse assay for the in vivo evaluation of CCK antagonists which is based upon visual determination of the gastric emptying of a charcoal meal is described. CCK-8 (24 micrograms/kg s.c.) but not various other peptide and nonpeptide agents effectively inhibited gastric emptying in this test system. The effect of CCK-8 was antagonized by established peripheral CCK antagonists but not representative agents of various other pharmacological classes. The rank order of potency of the CCK antagonists were: L-364,718 (ED50 = 0.01 mg/kg, i.v.; 0.04 mg/kg, p.o.) greater than Compound 16 (ED50 = 1.5 mg/kg, i.v.; 2.0 mg/kg p.o.) greater than asperlicin (ED50 = 14.8 mg/kg i.v.) greater than proglumide (ED50 = 184 mg/kg i.v.; 890 mg/kg, p.o.). Duration of action studies based upon ED50 values determined at various time intervals after oral administration showed that L-364,718 and proglumide are considerably longer acting than Compound 16. Asperlicin (ED50 greater than 300 mg/kg, p.o.) was ineffective as a CCK antagonist when administered orally. These data provide the first direct comparisons of the in vivo potencies of current CCK antagonists and demonstrate the utility of a new simple mouse assay for the in vivo characterization of peripheral CCK antagonists.

Asperlicin, a product derived from the fungus Aspergillus alliaceus, antagonized the multiple stimulatory effects of cholecystokinin (CCK-8S) on isolated islets. At a level of 10 microM, asperlicin completely inhibited insulin release in response to 25 nM CCK-8S. Increasing the level of CCK-8S to 100 nM partially restored a secretory response, while an even greater insulin stimulatory effect was noted with 500 nM CCK-8S. The inhibitory effect of asperlicin on CCK-8S-induced release was reversible. Asperlicin exposure had no effect on glucose or glyceraldehyde-induced secretion. Asperlicin reduced, in parallel with secretion, the increase in 3H efflux from [3H]inositol prelabeled islets usually noted with CCK-8S addition. Asperlicin did not influence the small glucose-stimulated increase in 3H efflux. The results support the notion that asperlicin is a specific and potent antagonist of the multiple stimulatory effects of CCK-8S on islet tissue.

Feeding of 14C-labeled amino acids to resting cells of Aspergillus alliaceus strongly supported the intuitive hypothesis that asperlicin is biosynthesized from tryptophan, anthranilate and leucine. The resting cell system was used also to prepare 25 asperlicin analogs via directed biosynthesis in presence of analogs of tryptophan and leucine.

Seventeen analogues of the selective, competitive cholecystokinin (CCK) antagonist asperlicin 1 were prepared. These compounds were tested as inhibitors of the binding of [125I]CCK to rat pancreas and guinea pig brain receptors. Compounds 4, 7, and 8 were more potent than asperlicin on the pancreatic CCK receptor. One analogue, 17, displayed potency equivalent to asperlicin on the pancreas CCK receptor and showed a marked improvement in aqueous solubility, thereby facilitating the use of this class of CCK antagonists in physiological and pharmacological studies.

Cholecystokinin octapeptide (CCK-8) (EC50 = 5 nM) was considerably more potent than pentagastrin (EC50 = 161 nM) in stimulating acid secretion in the isolated perfused mouse stomach suspended in a medium containing a phosphodiesterase inhibitor. The maximum acid response to CCK-8 was not significantly different from that produced by pentagastrin. The nonselective CCK/gastrin antagonist, proglumide, but not the selective CCK antagonist, asperlicin, antagonized the acid response to both pentagastrin and CCK-8. The data suggest that CCK-8 acts as a potent, full agonist on gastrin receptors for acid secretion in the isolated mouse stomach.

The present experiments evaluate in vivo effects of recently described cholecystokinin (CCK) receptor antagonists on rat pancreatic secretion. Pancreaticobiliary secretion was studied after bile duct cannulation in anesthetized rats. After two basal 10-min fractions were selected, secretion was stimulated by intravenous caerulein (0.1-30.0 micrograms/kg) or secretin, and collected for seven further 10-min fractions. Peptide antagonists (CR 1409, CR 1392, and CR 1505) and nonpeptide antagonists (asperlicin and L364,718) were given intravenously 10 min before agonists. Increasing doses of antagonists gradually reduced secretion of protein and enzymes stimulated by submaximal and maximal doses of caerulein. The antagonists did not alter nonstimulated or secretin-stimulated secretion, indicating their specificity for the CCK receptor. Except for proglumide and asperlicin, all antagonists were able to abolish caerulein-stimulated pancreatic secretion, as evaluated by the mean integrated 1-h response to a near-maximal dose of caerulein. The caerulein dose-response curve was gradually shifted to the right by increasing doses of CR 1409, indicating competitive-like kinetics. Inhibition of secretion due to supramaximal doses of caerulein, however, could be reversed by doses of CR 1409 smaller than expected from extrapolating truely competitive kinetics from an in vitro situation to the in vivo situation. The rank order of potency of the compounds to antagonize caerulein-stimulated secretion in vivo agreed with their relative potencies to antagonize caerulein-stimulated amylase secretion from pancreatic acini in vitro as well as with their affinity to bind to peripheral CCK receptors in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Publisher Summary This chapter discusses the recent advances in various strategies available for the discovery of non-peptide ligands, such as peptide mimetics, for peptide receptors and peptidases. Empirical approaches to the discovery of peptide ligands have involved the screening of pure compounds and complex mixtures in biological assays. The screening of pure entities has generally afforded little reported success, with the exception of ligands, for the promiscuous μ-opioid receptor. The discovery of receptor ligands has proved to be more elusive. Apart from the report of a structurally uncharacterized bradykinin antagonist from Mandevilla relutina, the discovery of the CCK antagonist asperlicin remains as the single significant success. Several new applications of conformationally restricted amino acids have been described in this chapter. Analogues containing amide bond surrogates have frequently been utilized to investigate the aspects of peptide structure and function. Recently, several improved syntheses of (E)-alkene peptide mimetics have appeared, in which good stereochemical control of appropriate peptidic side chains was achieved. The design, synthesis, and pharmacology of novel dipeptide mimetics have recently been described. In all the preceding, dipeptide mimics seek to duplicate the configuration of the trans-amide bond. An intriguing challenge of peptide mimetic design is the discovery of structures that can model the 3-dimensional orientation of amino acid residues into the known secondary conformations of proteins: turns, helices, and sheets. The design of β-sheet and α-helix mimetics has also been discussed in the chapter. The goal of peptide mimicry is the discovery of structural leads or the development of strategies for molecular design that result in bioavailable peptide analogues. Although it may not be necessary to remove all peptide-like character to ach eve this objective, reduction of the molecular weight and resistance to peptidase action is required.