The potential use of urinary excretion data for assessing the relative bioavailability of rifampicin in fixed dose combination anti-tuberculosis formulations.

Setting: The perceived need for simple, non-invasive methods of assessing the relative bioavailability of rifampicin in fixed-dose combination (FDC) anti-tuberculosis formulations.
Objective: To compare the performance of methods based on urinary excretion data with those utilising plasma concentration-time profiles to assess the relative bioavailability of rifampicin in combined and single-drug formulations.
Design: A two-period randomised crossover bioequivalence study in healthy male volunteers with a 1 week washout period between treatments. Plasma rifampicin concentrations were measured at 0, 1, 2, 4, 6 and 8 hours after each drug administration using a high performance liquid chromatography (HPLC) method. The rifampicin and desacetylrifampicin content of complete urinary collections made from 0-4 and 4-8 hours after dosage were determined using both the HPLC and a much simpler colorimetric procedure.
Results: There was good agreement between the relative bioavailability of the formulations using plasma and urinary excretion data, although the precision of the urinary-based estimates was slightly less than those derived from the plasma findings. There was also good agreement between the HPLC and colorimetric estimates of the combined urinary excretion of rifampicin plus desacetylrifampicin.
Conclusions: Urinary excretion data may be used for ongoing quality control to confirm that commercial combined rifampicin-containing formulations that were initially shown to be satisfactory continue to be so.