In order to explain the potent antihypertensive activity of the modestly active (IC50 = 3.2 microM) dihydropyrimidine calcium channel blocker 5, we carried out drug metabolism studies in the rat and found 5 is metabolized to compounds 6-10. Two of the metabolites, 6 (IC50 = 16 nM) and 7 (IC50 = 12 nM), were found to be responsible for the antihypertensive activity of compound 5. Potential metabolism of 6 into 7 in vivo precluded our interest in pursuing compounds related to 6. Structure-activity studies aimed at identifying additional aryl-substituted analogues of 7 led to 17g,j,p with comparable potential in vivo, though these compounds were less potent than 7 in vitro. To investigate the effects of absolute stereochemistry on potency, we resolved 7 via diastereomeric ureas 19a,b, prepared from 18 by treatment with (R)-alpha-methylbenzylamine. Our results demonstrate that the active R-(-)-enantiomer 20a of 7 is both more potent and longer acting than nifedipine (1) as an antihypertensive agent in the SHR. The in vivo potency and duration of 20a is comparable to the long-acting dihydropyridine amlodipine. The superior oral antihypertensive activity of 20a compared to that of previously described carbamates 2 (R2 = COOEt) could be explained by its improved oral bioavailability, possibly resulting from increased stability of the urea functionality.

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