Metabolism and pharmacokinetics of the dihydropyridine calcium antagonist, ryosidine, in man.
The metabolic fate of [14C]ryosidine (ryodipine) has been investigated after oral administration to human subjects (by capsule), and to rats and dogs (in solution). The excretion patterns of 14C were similar for all three species: about 50% dose was excreted in urine, mainly in 24 h, but a proportion was excreted slowly, particularly by humans. Absorption in man appeared to be less than in the animal species, probably as a result of the capsule dosage form used. Mean concentrations of total 14C in human plasma reached a peak value of 0.41 microgram equiv./ml at four hours and declined biphasically thereafter (mean terminal t1/2 = 28 h). Unchanged ryosidine was only detected in plasma from two to six hours (mean t1/2 = 80 min), and never accounted for more than 5% of the plasma 14C. The extent of binding of ryosidine to the plasma proteins (in vitro) was similar (greater than 90%) to that of total 14C (in vivo; mainly metabolites). Less than 0.5% of the dose to human subjects was excreted via the kidneys as unchanged ryosidine, whereas the bulk of the extractable faecal 14C was in the form of unchanged drug and presumably represented unabsorbed material. The principal routes of biotransformation of ryosidine in all three species involved oxidative aromatization of the 1,4-dihydropyridine ring, followed by ester hydrolysis, O-dealkylation, hydroxylation of an alpha-methyl group (and lactonization) and some glucuronidation, although quantitative interspecies differences were apparent.