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Abstract
The present study describes comparative bioavailability of rifampicin (RIF) after
administration of a single component RIF (450 mg) capsule and rifampicin-isoniazid
(RIF-INH) (450+300 mg) fixed dose combination (FDC) capsule formulations. Six healthy
male volunteers participated in a single dose, two treatment, two period, cross-over
study. A sensitive, specific and accurate HPTLC method was developed, validated and
employed for estimation of RIF and its major active metabolite, 25-Desacetylrifampicin
(25-DAR) levels, in urine. Using the urinary excretion data various pharmacokinetic
parameters: AUC(0-24), AUC(0-infinity), cumulative amount excreted in 24 h, peak excretion
rate, etc. for both RIF and 25-DAR were calculated and compared statistically (ANOVA,
90% confidence interval for ratio). Significant decrease in the bioavailability (
approximately 32% as RIF and approximately 28% as 25-DAR) of RIF from FDC capsules
was observed. The present bioavailability study confirms our serious doubts about
the stability of RIF in presence of INH in acidic environment of stomach, which probably
is the main factor responsible for the reduced bioavailability of RIF from RIF-INH
combination formulations. This study underlines the fact that there is an urgent need
to reconsider the formulation of the FDC product in order to minimize or avoid the
decomposition of RIF in gastrointestinal tract.