Abstract: In the present study, roxithromycin solutions of different pH values were prepared with water, simulated gastric fluid (SGF) and simulated intestinal fluid (SIF). Stability of roxithromycin in the above solutions were determined separately by colorimetry, TLC and HPLC. All the three methods achieved an identical conclusion that this drug was stable in water and SIF. However, for the stability test of roxithromycin in SGF, the results obtained by colorimetry were contrary to those by TLC and HPLC. According to the results by colorimetry, this drug was stable in SGF for as long as 168 hours. However, when tested by TLC and HPLC, decomposition of this drug was found within 2 h and 10 min separately. According to the determination by HPLC, 8.15 % and 62.02 % of roxithromycin were decomposed in 10 min and 2 h in SGF, which can be explained like this: the decomposition products reacted with H2SO4 to form a solution with the same photo-absorptivity range as that of the original drug. Accordingly, colorimetry is not suitable for the assay and stability test of roxithromycin in acidic solution like SGF. Similarly, we also have reason to predict that microbacterial assay is not suitable for roxithromycin in acidic solution if the decomposition product still has antibacterial activities.

Key words: Roxithromycin, Roxithromycin, Stability, Stability, Colorimetry, Colorimetry, TLC, TLC, HPLC, HPLC