Factors influencing comparative bioavailability of spironolactone tablets

The bioavailability of spironolactone from 10 tablet formulations, selected to provide a wide range of specifications and in vitro dissolution rates, was assessed from the plasma and urinary levels of its major unconjugated metabolite, canrenone, in a study of balanced incomplete block design using 11 healthy subjects. Significant but weak correlations existed between the amount of spironolactone in solution at 40 min in vitro and the area under the plasma concentration-time curve for canrenone and urinary canrenone excretion. The correlations between in vitro dissolution and bioavailability parameters appeared to be weakened by two tablet formulations, one with dibasic calcium phosphate as the principal excipient and the other formulated from micronized spironolactone bulk drug. Measurement of in vitro dissolution of spironolactone tablets is of value for quality control purposes, provided no major alteration is made in the formulation.