Comparative pharmacokinetics of metronidazole and tinidazole as influenced by administration route

Serum kinetics of metronidazole and tinidazole were compared in four separate randomized crossover studies. Single doses of each drug were given to healthy volunteers through intravenous infusion (500 mg over 20 min, six persons), by mouth (500 mg, nine persons), by rectum (1,000 mg, six persons), or intravaginally (500 mg, six persons). Concentrations of the unchanged drugs in serum, measured by high-pressure liquid chromatography, were similar after oral and intravenous administration, with mean peaks of 9.0 and 9.4 micrograms/ml for metronidazole and 7.5 and 10.1 micrograms/ml for tinidazole. Concentrations of tinidazole were significantly higher than those of metronidazole from 4 h onwards after intravenous infusion, and from 3 h onwards after administration by mouth. After rectal administrations, a significant difference was seen only at 48 h. After vaginal dosing, however, concentrations of metronidazole were significantly higher than those of tinidazole between 1.5 and 12 h. Bioavailability of either drug, calculated according to the formula (area under the curve for oral administration)/(area under the curve for infusion), was practically complete after oral administration and was poorer after rectal and especially vaginal administration. Whenever the parameters were calculable, the elimination half-life of tinidazole (range of means, 14.0 to 14.7 h) was significantly longer and total clearance (40.3 to 47.6 ml/min) was lower than the corresponding values of metronidazole (7.9 to 8.8 h and 71.8 to 80.1 ml/min, respectively).