Bioequivalence trials of rifampicin containing formulations: extrinsic and intrinsic factors in the absorption of rifampicin.

Rifampicin shows variable bioavailability from solid oral dosage forms and the reasons for this variable absorption reported in literature varies from extrinsic formulations factors to intrinsic variability in rifampicin absorption. Hence, we have undertaken a systematic and comprehensive evaluation of all the factors to study contribution of all the factors on rifampicin absorption. As a first step, data from eight bioequivalence studies conducted at National Institute of Pharmaceutical Education and Research (NIPER) bioavailability center was compared across the trials to understand the effect of extrinsic/intrinsic factors on the bioavailability of rifampicin, isoniazid and pyrazinamide. Out of eight fixed dose combination (FDC) formulations, six formulations were bioequivalent for rifampicin to separate formulations whereas one formulation was below and one was above the limits of bioequivalence. It was observed that more variability in rifampicin blood levels is associated with FDC formulations when compared to rifampicin-only formulations and was attributed to complexity involved in the manufacturing of FDCs. Further, one of the rifampicin-only capsule showed unexpectedly lower plasma levels indicating role of physical characteristics of rifampicin bulk material. It was also seen that rifampicin shows dose-dependent pharmacokinetics even at the modest increase in dose due to saturation of efflux system at absorption site and metabolizing enzymes for elimination. Other components of FDC formulations such as isoniazid and pyrazinamide due to high solubility and permeability have shown very less variability and were bioequivalent to separate formulations even from formulations those were failed for rifampicin. The comparison of data across the trials suggested that rifampicin bioavailability problem is more attributable to the extrinsic factors such as formulation or rifampicin bulk material rather than intrinsic variability of rifampicin absorption.