Rifampicin (RIF) hydrolyzes in acidic medium to form insoluble and poorly a
bsorbed 3-Formyl rifamycin SV (3-FRSV). This study describes development of
two principally different methods, Dual Wavelength UV-Vis. spectrophotomet
ry (DW spectrophotometry) and HPTLC, to determine 3-FRSV in presence of RIF
. Using DW spectrophotometry, RIF was estimated by using wavelengths 475.0
and 507.0 nm and 3-FRSV was estimated using 457.0 and 492.0 nm. In HPTLC me
thod, a mixture of chloroform:methanol:water (80:20:2.5 v/v) was used as th
e mobile phase to resolve 3-FRSV from RIF and 3-FRSV was quantified at 333
nm. The linearity range for 3-FRSV was 2-10 mu g/ml and 50-250 ng/spot for
DW spectrophotometric method and HPTLC method, respectively, and 5-50 mu g/
ml for RIF using DW spectrophotometric method. Both the methods were found
to be specific, accurate and reproducible. The proposed methods were succes
sfully applied to determine the rate of degradation of RIF to 3-FRSV in dis
solution medium (0.1 N HCl) and also in presence of isoniazid (INH). The ra
te of degradation of RIF in presence of INH was almost two times more than
that of RIF alone. These methods were utilized to study the stability of RI
F in market formulations of RIF and RIF with INH in dissolution medium. It
has been observed that RIF degrades by 12.4% to form 3-FRSV (RIF formulatio
ns) while in presence of INH the degradation is catalyzed to about 21.5% (R
IF + INH formulations), in 45 min. Thus, lower concentration of RIF may be
available for absorption leading to poor bioavailability of RIF from combin
ation dosage forms (RIF + INH) as compared to formulations containing only
RIF. It is proposed that specific analytical method should be used to measu
re RIF in presence of 3-FRSV in a dissolution study. (C) 1999 Elsevier Scie
nce B.V. All rights reserved.