Comparative bioavailability of a lipophilic steroid.

17beta-Acetoxy-2alpha-chloro-3-(p-nitrophenoxy)imino-5alpha-androstane (I) is a lipophilic steroid with postimplantive antifertility activity in laboratory animals. The bioavailability of micronized I from solutions and suspensions was compared in four groups of adult female Wistar rats. Each group received varying concentrations of micronized 3H-I (specific activity of 0.38--8.94 muCi/mg) in sesame oil by oral gavage. Samples of whole blood and urine collected following drug administration were assayed for radioactive content. Calculation of the mean area under the blood radioactivity versus time curve, when corrected for the quantity of drug administered, indicated that a substantially larger fraction of the dose was absorbed in the two instances where I was present only in solution. A linear relationship between the amount of I absorbed based on whole blood radioactivity and urinary excretion and the administered dose was found primarily for groups receiving the drug in solution. Preliminary results in humans indicate that 3H-I was absorbed to a much greater extent following oral administration of the drug in sesame oil than when admixed with lactose.