Divergent effects of different enzyme-inducing agents on endogenous and exogenous testosterone

The effects of three different enzyme-inducing drugs (antipyrine 1200 mg, phenobarbital 100 mg, rifampicin 600 mg per day for 7 days) on plasma and urinary testosterone concentrations, plasma gonadotropin levels, antipyrine kinetics, and urinary 6 beta-hydroxycortisol excretion were studied in 18 healthy volunteers. Changes in plasma and urinary testosterone concentrations following exogenous testosterone undecanoate (TU) were also investigated. Although both antipyrine and rifampicin increased antipyrine clearance by about 60%, they produced contrary effects on testosterone: antipyrine lowered the total morning plasma testosterone and plasma testosterone AUC following TU, while rifampicin led to increases of about 20% and 78%, respectively. By contrast, phenobarbital did not significantly alter the endogenous and exogenous plasma testosterone concentrations, but it increased the urinary excretion of testosterone by more than 60%. The other two enzyme inducers did not alter this parameter. Gonadotropin levels remained unchanged. The results indicate that different enzyme-inducing agents exert divergent effects on endogenous and exogenous testosterone concentrations and suggest that the effect of enzyme induction on endogenous testosterone depends on the type of microsomal enzyme-inducing drug used rather than on the extent of the induction achieved.