Spironolactone. II. Bioavailability.

The bioavailability of commercial 25-mg spironolactone tablets and a new tablet preparation containing 100 mg of the drug has been determined in 12 healthy male subjects. After a 200-mg oral dose of the drug given in a solution of polyethylene glycol-400, the peak plasma level of the dethioacetylated metabolite canrenone was 633 +/- 154 ng/ml (mean +/- SD) and was reached at 1.4 +/- 0.43 hr. This peak was higher and was achieved earlier than after either eight 25-mg tablets (480 +/- 155 ng/ml at 2.9 +/- 1.03 hr) or two 100-mg tablets (474 +/- 182 ng/ml at 3.0 +/- 1.37 hr). From the ratio of the 24-hr area under the plasma concentration-time curves, the bioavailabilities of the two tablet preparations relative to the solution were 99.6 +/- 18.2% and 92.1 +/- 22.9%, respectively. The amount of canrenone excreted in the urine by 48 hr was 4.48 +/- 1.26 mg (solution), 6.36 +/- 2.02 mg (eight 25-mg tablets), and 7.81 +/- 1.87 mg (two 100-mg tablets), representing 2% to 4% of the administered dose. It is concluded that urinary excretion of canrenone alone is not a reliable method for determining the bioavailability of spironolactone.