Trimethoprim and rifampicin: pharmacokinetic studies in man

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Abstract
The absorption and excretion of trimethoprim (tm), 160 mg, and rifampicin (rif), 300 mg, alone and in combination, has been studied in 12 healthy volunteers. In a single dose study, with triple crossover, there was no interference with the handling of one drug by the other. Next, the two drugs were taken in combination by 12 volunteers every 12 h on 13 consecutive occasions, and pharmacokinetic profiles taken after the first and the last doses. The half-life of rif was significantly lower (1.64 vs. 2.08 h) on the seventh day, but the amount excreted in the urine was unchanged. For tm, a build-up in serum levels occurred (peak levels were 1.87 μg/ml on day 7 vs. 1.39 μg/ml on day 1), the half-life was decreased (7.28 vs. 9.22 h) and the 12-h urinary excretion was increased (44.89% vs. 33.44%). No toxicity was observed. When the levels of the two antimicrobial agents, and their ratios, observed in this study are compared with microbiological findings it would be expected that the former would be adequate to treat infections caused by a variety of pathogenic bacteria. We suggest that a clinical trial of the combination rif/tm is now indicated.

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