Rifaximin (L/105), a new topical intestinal antibiotic: pharmacokinetic study after single oral administration of 3H-rifaximin to rats.

Tritiated rifaximin was administered in a single oral dose of 10 mg/kg (specific activity 8.998 microCi/mg) and 100 mg/kg (specific activity 0.786 microCi/mg) to rats. After treatment, at fixed times, the animals were sacrificed and the radioactivity in plasma, urine, feces, and in the principal organs and tissues was measured. The radioactivity present in the feces of the two groups of rats was more than 95% of the administered dose, while the amounts found in the urine ranged between 1.15% and 1.5% of the dose. In the plasma and tissues the radioactivity levels were very low. This evidence confirmed the scanty gastroenteric absorption of L/105. Furthermore, as the radioactivity levels maintained almost the same value during the trial, there was the possibility that the amounts found were in part due to isotopic exchange between tritium of 3H-L/105 and hydrogen of water.