Pharmacokinetics of Antituberculosis Drugs in HIV-Positive and HIV-Negative Adults in Malawi

Limited data address the impact of HIV coinfection on the pharmacokinetics (PK) of antituberculosis drugs in sub-Saharan Africa. A total of 47 Malawian adults underwent rich pharmacokinetic sampling at 0, 0.5, 1, 2, 3, 4, 6, 8, and 24 h postdose. Of the subjects, 51% were male, their mean age was 34 years, and 65% were HIV-positive with a mean CD4 count of 268 cells/μl. Antituberculosis drugs were administered as fixed-dose combinations (150 mg rifampin, 75 mg isoniazid, 400 mg pyrazinamide, and 275 mg ethambutol) according to recommended weight bands. Plasma drug concentrations were determined by high-performance liquid chromatography (rifampin and pyrazinamide) or liquid chromatography-mass spectrometry (isoniazid and ethambutol). Data were analyzed by noncompartmental methods and analysis of variance of log-transformed summary parameters. The pharmacokinetic parameters were as follows (median [interquartile range]): for rifampin, maximum concentration of drug in plasma (Cmax) of 4.129 μg/ml (2.474 to 5.596 μg/ml), area under the curve from 0 to 24 h (AUC0-∞) of 21.32 μg/ml · h (13.57 to 28.60 μg/ml · h), and half-life of 2.45 h (1.86 to 3.08 h); for isoniazid, Cmax of 3.97 μg/ml (2.979 to 4.544 μg/ml), AUC0-24 of 22.5 (14.75 to 34.59 μg/ml · h), and half-life of 3.93 h (3.18 to 4.73 h); for pyrazinamide, Cmax of 34.21 μg/ml (30.00 to 41.60 μg/ml), AUC0-24 of 386.6 μg/ml · h (320.0 to 463.7 μg/ml · h), and half-life of 6.821 h (5.71 to 8.042 h); and for ethambutol, Cmax of 2.278 μg/ml (1.694 to 3.098 μg/ml), AUC0-24 of 20.41 μg/ml · h (16.18 to 26.27 μg/ml · h), and half-life of 7.507 (6.517 to 8.696 h). The isoniazid PK data analysis suggested that around two-thirds of the participants were slow acetylators. Dose, weight, and weight-adjusted dose were not significant predictors of PK exposure, probably due to weight-banded dosing. In this first pharmacokinetic study of antituberculosis drugs in Malawian adults, measures of pharmacokinetic exposure were comparable with those of other studies for all first-line drugs except for rifampin, for which the Cmax and AUC0-24 values were notably lower. Contrary to some earlier observations, HIV status did not significantly affect the AUC of any of the drugs. Increasing the dose of rifampin might be beneficial in African adults, irrespective of HIV status. Current co-trimoxazole prophylaxis was associated with an increase in the half-life of isoniazid of 41% (P = 0.022). Possible competitive interactions between isoniazid and sulfamethoxazole mediated by the N-acetyltransferase pathway should therefore be explored further.