Antiulcer agents. 4. Conformational considerations and the antiulcer activity of substituted imidazo[1,2-a]pyridines and related analogues.

Definition of the interrelationship between the conformational characteristics of a series of substituted imidazo[1,2-a]pyridines and their antiulcer activity was investigated by examining the conformational properties of 3-cyano-2-methyl-8-(phenylmethoxy)imidazo[1,2-a]pyridine (1), using a variety of experimental and theoretical methods. The results of these studies was the identification of two distinctly different candidates, designated the "folded" and the "extended" conformation, respectively, to represent the two possible minimum-energy conformations of 1. In order to select the biologically relevant conformer, a group of 3-substituted 2-methylimidazo[1,2-a]pyridines, having either a cis or a trans 2-phenylethenyl substituent at the 8-position were designed as conceptually simple and synthetically accessible semirigid analogues of the respective candidate conformers. Gastric antisecretory activity was found to reside only in the trans isomers (compounds 11, 15, and 17), which mimic the "extended" conformation. This observation led to the construction of 8,9-dihydro-2-methyl-9-phenyl-7H-imidazo[1,2-a]pyrano[2,3-c]pyridi ne-3- acetonitrile (40), a rigid tricyclic analogue that is effectively locked in the "extended" conformation and that exhibited an antiulcer profile comparable to that of prototype 1. These results unequivocally demonstrate that, in accord with expectation for a drug operating at a specific receptor, the conformational characteristics of the molecule have a substantial effect in determining its antiulcer activity. More precisely, it has been demonstrated that it is the "extended" conformation of 1 that represents the "bioactive" form of the drug. These results constitute the basis for a molecular probe that should aid in the investigation of the as yet uncharacterized gastric proton pump enzyme (H+/K+-ATPase), by means of which 1 and its analogues presumably exert their pharmacologic actions.

Antiinflammatory activity of a series of substituted 2,3-dihydro-6-hydroxypyrimido[2,1-f]purine-4,8(1H,9H)-diones.

A series of substituted analogues based on the novel 2,3-dihydro-6-hydroxypyrimido[2,1-f]purine-4,8(1H,9H)-dione ring system have been synthesized and shown to exhibit antiinflammatory activity in the adjuvant-induced arthritis rat model (AAR). The activity exhibited by the pyrimidopurinediones in this model of chronic inflammation is comparable to that of their previously studied 2-oxo congeners, the 6-hydroxypyrimido[2,1-f]purine-2,4,8-(1H,3H,9H)-triones, the best of which show potency levels approximately equal to that of naproxen. On the basis of its potency in the AAR assay, 9-benzyl-2,3-dihydro-1,3-dimethyl-6-hydroxy-7-(3-methyl-2-butenyl) pyrimido-[2,1-f]purine-4,8(1H,9H)-dione was selected for further evaluation and found to exhibit cyclooxygenase inhibitory activity in the in vitro rat neutrophil model. With respect to side-effect liability, this prenylated derivative has been shown to be devoid of gastric ulcer inducing potential, as well as the ocular toxicity observed previously with the 2-oxo series.

Antiulcer agents. 2. Gastric antisecretory, cytoprotective, and metabolic properties of substituted imidazo[1,2-a]pyridines and analogues.

The search for a successor to 3-(cyanomethyl)-2-methyl-8-(phenylmethoxy)imidazo[1,2-a]pyridine, Sch 28080 (27), a compound that exhibits gastric antisecretory and cytoprotective properties and has undergone clinical evaluation as an antiulcer agent, has culminated in the identification of four related compounds that exhibit pharmacologic profiles similar to that of 27. In three of these potential successors an amino group functions as a surrogate for the 3-cyanomethyl substituent of the prototype. The present work concerns, in addition to an evaluation of the structure-activity relationships of a series of analogues of 27, preliminary studies of the pharmacodynamics and metabolism of 27, performed with the aid of cyano carbon labeled versions of the drug (13C labeled; 28; 14C labeled, 29). These studies have shown that 27 is well-absorbed and extensively metabolized and that the major metabolite of 27 is the thiocyanate anion. A similar study performed on 3-amino-2-methyl-8-(phenylmethoxy)imidazo[1,2-a]pyridine, labeled at the 3-position with carbon-13 (41) or carbon-14 (42), revealed that this compound, which has an antisecretory/cytoprotective profile comparable to that of 27, is also metabolized to thiocyanate anion, although this must occur via a different mechanism. The chemistry section includes a discussion of the potential sites of protonation of the pharmacologically similar 3-amino analogue 40 and the structurally related imidazo[1,2-a]pyrazine 67. Predictions based on charge density and protonation product stabilities are presented. That N1 is the site of protonation in these analogues has been definitively demonstrated by X-ray crystal structure analysis, which also unequivocally established the assigned imidazo[1,2-a]pyrazine ring structure.

Antiinflammatory activity of substituted 6-hydroxypyrimido[2,1-f]purine-2,4,8(1H,3H,9H)-triones. Atypical nonsteroidal antiinflammatory agents.

A novel class of antiinflammatory drugs, which are substituted derivatives of the fused tricyclic system 6-hydroxypyrimido[2,1-f]purine-2,4,8(1H,3H,9H)-trione, is described. Synthetic procedures and structure determination with the assistance of X-ray crystallography are discussed. Semiempirical molecular orbital calculations are used to investigate the relative stability of the possible isomers and tautomers of the title compounds. A biological profile of the class, and of several of the more potent analogues, in several antiinflammatory models, including the adjuvant-induced arthritis and the collagen II models, is defined. Several members of the class are shown to possess extremely low ulcerogenic effects in spite of exhibiting cyclooxygenase inhibition. A preliminary bioavailability study of two of the lead structures is presented. The compounds 6-72 appear to constitute a class of drugs that shows interesting potential antiarthritic activity and also exhibits an activity profile different from that of the standard classical NSAI drugs, as determined by a comparison of the profile of this class of drug with that of several standard agents. Certain findings from toxicological studies have precluded the further development of compounds within this group, although related structural types are being investigated.

Antiulcer agents. 1. Gastric antisecretory and cytoprotective properties of substituted imidazo[1,2-a]pyridines.

A novel class of antiulcer agents, the substituted imidazo[1,2-a]pyridines, is described. The present compounds are not histamine (H2) receptor antagonists nor are they prostaglandin analogues, yet they exhibit both gastric antisecretory and cytoprotective properties. The mechanism of gastric antisecretory activity may involve inhibition of the H+/K+-ATPase enzyme. Structure-activity studies led to the identification of 3-(cyanomethyl)-2-methyl-8-(phenylmethoxy)imidazo[1,2-a]pyridine, SCH 28080 (27), which was selected for further development and clinical evaluation.