A novel class of 2-(4-heterocyclylphenyl)-1,4-dihydropyridines (2-38) possessing antagonist activity against platelet activating factor (PAF) was prepared by the Hantzsch synthesis from a variety of ethyl 4'-heterocyclic-substituted benzoylacetates, aryl or heteroaryl aldehydes, and substituted 3-aminocrotonamides or 3-aminocrotonate esters. Structure-activity relationships were evaluated where PAF antagonist activity was measured in vitro by determining the concentration of compound (IC50) required to inhibit the PAF-induced aggregation of rabbit washed platelets, and in vivo by determining the oral dose (ED50) which protected mice from a lethal injection of PAF. The nature of the substituent at the dihydropyridine 2-position was found to be important for both in vitro and in vivo activity, whereas there was greater flexibility for structural variation at the 4- and 5-positions. The most potent compound was 4-(2-chlorophenyl)-1,4-dihydro-3-(ethoxycarbonyl)-6-methyl-2-[4-(2- methylimidazo[4,5-c]pyrid-1-yl)phenyl]-5-[N-(2- pyridyl)carbamoyl]pyridine (17, UK-74,505), IC50 = 4.3 nM, ED50 = 0.26 mg/kg po, which was found to be approximately 33 times more potent in vitro (rabbit platelet aggregation) and about 8 times more potent in vivo (murine lethality) than WEB2086. Compound 17 also exhibited a long duration of action in the dog (inhibition of PAF-induced whole blood aggregation ex vivo was maintained for greater than 24 h following a single oral dose of 75 micrograms/kg) and was highly selective as a PAF antagonist, showing only weak affinity (IC50 = 6600 nM) for the [3H]nitrendipine binding site. As a result of its high oral potency, selectivity, and duration of action, UK-74,505 has been selected for clinical evaluation.

1-(2-Phenoxyethyl)-1H-imidazole was found to be an inhibitor of thromboxane (TxA2) synthetase, but it also inhibited the adrenal cytochrome P-450 enzyme steroid 11 beta-hydroxylase. The preparation of a series of analogues is described, and activity against TxA2 synthetase, PGI2 synthetase, cyclooxygenase, and steroid 11 beta-hydroxylase is discussed. Potency against TxA2 synthetase was increased by introduction of a carboxyl group at a suitable distance from the imidazole ring. A distance of 8.1-8.8 A between N-1 of the imidazole and the carboxyl carbon was found to be optimal. Introduction of a carboxyl group also had the effect of reducing activity against steroid 11 beta-hydroxylase. The most potent and selective compound was found to be 4-[2-(1H-imidazol-1-yl) ethoxy]benzoic acid (14).

Following the discovery of moderately potent antagonist activity platelet-activating factor (PAF) in 2-methyl-1-phenylimidazo[4,5-c]pyridine (2) (IC50 = 840 nM), 19 derivatives (3-21) were prepared which incorporated various lipophilic groups attached to the phenyl 4-position. Structure-activity relationships were evaluated where PAF antagonist activity was measured in vitro by determining the concentration of compound (IC50) required to inhibit the PAF-induced aggregation of rabbit washed platelets and in vivo by determining the oral dose (ED50) which protected mice from a lethal injection of PAF. [1,5]Benzodiazepines, e.g., 14 (2,3-dihydro-1-methyl-4-[4-(2-methylimidazo[4,5-c] pyrid-1-yl)phenyl]-1H-[1,5]benzodiazepin-2-one) (IC50 = 4.9 nM, Ed50 = 0.03 mg/kg po), were found to possess equivalent or superior potency to the 1,4-dihydropyridine PAF antagonist UK-74,505 (1,4-(2-chlorophenyl)-1,4-dihydro-3-(ethoxycarbonyl)-6-methyl-2- [4-(2-methylimidazo[4,5-c]pyrid-1-yl)phenyl]-5-[N-(2-pyridyl) carbamoyl]pyridine) in vitro and in vivo. Furthermore, a potent benzazepine, 21 (7,8-dichloro-1-methyl-4-[4-(methylimidazo[4,5-c]pyrid-1-yl) phenyl]-2,3,4,5-tetrahydro-1H-1-benzazepin-2-one) (IC50 = 0.5 nM, ED50 = 0.03 mg/kg po), was discovered. These investigations prompted the synthesis and evaluation of additional diazepine derivatives, which are described in the following paper. The relationship between the key PAF antagonist pharmacophores of 2-methyl-1-phenylimidazo[4,5-c]pyridine, a triazolothienodiazepine (WEB2170), and a pyrrolothiazolidine (RP-52,770) is discussed.

The optimization of in vitro activity and oral potency and duration of action in vivo is described for three novel structural types of platelet-activating factor (PAF) antagonist: [1,5]benzodiazepines 5-12 onto which a variety of other heterocyclic rings were fused, pyrido[2,3-b][1,4]-diazepinones 13-26, and pyrazolo[3,4-b][1,4]diazepinones 27-46. Compounds 5-12 were prepared by elaboration of the [1,5]benzodiazepine-2-thiones 47 and 48, and 13-46 were prepared by cyclocondensation reactions of a variety of 2,3-diaminopyridine and 4,5-diaminopyrazole derivatives with ethyl 4'-(2-methylimidazo[4,5-c] pyrid-1-yl)benzoylacetate (53). The presence of imine-enamine tautomerism was observed in certain diazepine derivatives and is discussed. Structure-activity relationships were evaluated where PAF antagonist activity was measured in vitro by determining the concentration of compound (IC50) required to inhibit PAF-induced aggregation of rabbit washed platelets and in vivo by determining the oral dose (ED50) which protected mice from a lethal injection of PAF. In addition, the duration of action in conscious dogs as measured by determining the oral dose of selected compounds required to inhibit completely PAF-induced whole blood aggregation ex vivo. The most potent compound was 1,6,7,8-tetrahydro-1,8-dimethyl-5-[4-(2-methylimidazo [4,5-c]pyrid-1-yl)phenyl]-7-oxo-3-(3-pyridyl) pyrazolo[3,4-b][1,4]diazepine (43, UK-91,473) (IC50 = 2.4 nM, ED50 = 0.01 mg/kg po), which was found to be significantly more potent in vivo (murine lethality) than the dihydropyridine PAF antagonist 4-(2-chlorophenyl)-1,4-dihydro-3-(ethoxycarbonyl)-6-methyl- 4-[(2-methylimidazo[4,5-c]pyrid-1-yl)phenyl]-5- [N-(2-pyridyl)carbamoyl]pyridine (4, UK-74,505) (ED50 = 0.26 mg/kg po). Compound 43 also possessed a longer duration of action than compound 4 in the conscious dog at one-fourth of the dose. The crystal structure of compound 43, established by X-ray diffraction, is reported.

The preparation of a series of 1H-imidazol-1-yl-substituted benzo[b]furan-, benzo[b]thiophene-, and indolecarboxylic acids is described. Most of the compounds were potent inhibitors of TxA2 synthetase in vitro, and the distance between the imidazole and carboxylic acid groups was found to be important for optimal potency. The most potent compound in vivo was 6-(1H-imidazol-1-ylmethyl)-3-methylbenzo[b]thiophene-2-carboxylic acid (71), which, in conscious dogs, showed a similar profile of activity to that of dazoxiben (1).