Effect of pefloxacin on the urinary excretion of rifampicin.

The effect of pefloxacin on the urinary excretion of rifampicin was investigated in 5 healthy volunteers between the ages of 20 and 35 years. The investigation was carried out in 2 different phases, with a 1-week drug washout separating the phases. Each subject received 600 mg rifampicin with 350 mL of water. After 1 week, the subjects were given 600 mg rifampicin plus 500 mg pefloxacin with 350 mL of water. Urinary levels of rifampicin were measured spectrophotometrically for the 2 phases from 0 to 72 hours. Coadministration of rifampicin with pefloxacin led to 20.1% urinary recovery of rifampicin. The increased rifampicin excretion rate following pefloxacin coadministration is supported by the competitive liver clearance between rifampicin and pefloxacin, which favors pefloxacin and causes rifampicin secretion, thus increasing its elimination through the kidney. Pefloxacin increases the absorption and urinary excretion of rifampicin by decreasing the gastrointestinal motility through chelation mechanisms.