Setting: The present study assesses bioavailability indices for rifampicin, isoniazid and pyrazinamide when administered to healthy volunteers separately or in a fixed triple-drug formulation, Rifater 125 SCT.
Objective: To compare the pharmacokinetics of rifampicin, isoniazid and pyrazinamide based on their blood concentrations up to 12 hours with the proportions of the doses of the drugs and their metabolites excreted in urine up to 12 hours, and to assess the bioavailability indices for the free and fixed triple drug formulations.
Design: An open cross-over study was conducted in 18 healthy volunteers with normal hepatic and renal functions to whom the drug combinations were administered in free and fixed dose formulations a week apart, to the same subject.
Results: Concentrations of the three drugs/metabolites were assessed in blood and urine. The results indicated the absence of negative pharmacokinetic interactions between the drugs when administered in both the free and the new fixed triple drug formulation.
Conclusion: Human bioavailability studies provide direct straightforward information, particularly when studying compounds such as rifampicin and other major anti-tuberculosis drugs. The results of the present study indicate that the pharmacokinetic properties of rifampicin, isoniazid and pyrazinamide as assessed after individual and combined administration do not change when combined in a single pharmaceutical preparation. The bioavailability indices calculated based on plasma concentrations and urinary levels for all three drugs compared well.