The kinetics of a sustained-release formulation of pentoxifylline were compared with those of a capsule and an intravenous infusion. Ten healthy subjects received each of the oral pentoxifylline formulations (400 mg) three times a day for 9 days in a random crossover fashion. Pentoxifylline (200 mg) was also given intravenously on a separate day. After intravenous pentoxifylline, plasma levels declined in a biphasic manner, with a terminal t1/2 of 1.63 +/- 0.8 hr. Plasma clearance was 1333 +/- 481 ml/min and the volume of distribution was 168 +/- 82.3 l. Cumulation of pentoxifylline in plasma after repeated dosing was minimal. Plasma levels of the active 5-hydroxylated metabolite were generally higher than those of the parent drug after both routes of administration. Urinary excretion of two acid metabolites after oral and intravenous dosing indicated almost complete absorption of drug-related substances from both of the oral formulations, although bioavailability averaged 20% to 30%.

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1. Incubation conditions were established in experiments with human-prostate homogenates for almost complete conversion of [4-(14)C]testosterone into at least ten transformation products. 2. Whole homogenates of tissue with benign hypertrophy were shown to contain 3alpha-, 3beta- and 17beta-hydroxy steroid dehydrogenases, Delta(4)-3-oxo steroid 5alpha- and 5beta- reductases and unidentified hydroxylases. 3. Most of the 17beta-hydroxy steroid-dehydrogenase activity was located in the mitochondria, which showed little other activity. 4. The 3alpha- and 3beta-hydroxy steroid dehydrogenases and the 5beta-reductase were located in the high-speed supernatant and required supplementation with NADPH for activity. 5. The 5alpha-reductase was located in both microsomal and high-speed supernatant fractions and also required supplementation with NADPH.

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