Influence of rifampicin on drug metabolism: differences between hexobarbital and antipyrine

Six healthy volunteers were treated with 1,200 mg of rifampicin daily for 8 days. Before and immediately afterward each received indocyanine green, hexobarbital, galactose, and antipyrine by intravenous infusion on 3 consecutive days. The plasma concentrations of the drugs were determined several times after infusion. The average elimination half-life of hexobarbital had decreased from 407 to 171 min and its metabolic clearance had increased almost threefold. In contrast, the average elimination half-life of antipyrine was virtually the same on both occasions (6.9 and 7.2 hr) and there was no change in metabolic clearance. In a tuberculous patient treated with rifampicin the antipyrine elimination rate was unaffected. Rifampicin did not influence indocyanine green clearance or galactose elimination capacity. Serum gamma glutamyl transferase was not affected but urinary D-glucaric acid excretion was increased during rifampicin treatment. The experiment with hexobarbital was repeated after 2 weeks in all subjects; half-lives and clearance values had returned to near control values. It appears that rifampicin is a selective inducer of oxidative drug metabolism in man.