Stability Indicating HPTLC Method for Analysis of Rifaximin in Pharmaceutical Formulations and an Application to Acidic Degradation Kinetic Study

A specific stability indicating high-performance thin-layer chromatographic method for analysis of rifaximin both as a bulk drug and in formulations was developed and validated. The method employed HPTLC aluminium plates precoated with silica gel 60 F254 as the stationary phase. The optimized mobile phase system consisted of n-hexane？:？2-propanol？:？acetone？:？ammonia (5？:？4.1？:？1, v/v/v/v), which gave compact spots for rifaximin at of 0.59 ± 0.03. Rifaximin was subjected to forced degradation studies in order to check the specificity of the method. Densitometric analysis of rifaximin was carried out in the absorbance reflectance mode at 443？nm. The calibration plots showed linear relationship in the concentration range of 400–3200？ng per band. Moreover, linearity was also confirmed by verification of homoscedasticity of variance. According to validation studies, the developed method was repeatable and specific as revealed by % RSD less than 2 and hence can be used for routine analysis of pharmaceutical formulation. Moreover, the method could effectively separate the drug from its degradation products; hence it can be employed as a stability indicating one. The kinetics of acid degradation process at various temperatures was also investigated and first-order rate constant, half-life, shelf life, and activation energy were computed. 1. Introduction Rifaximin, a benzimidazole derivative, is a structural analogue of rifampicin. Chemically, it is a 2S,16Z,18E,20S,21S,22R,23R,24R,25S,26S,27S,28E-5,6,21,23,25-pentahydroxy-27-methoxy-2,4,11,16,20,22,24,26-octamethyl-2,7-epoxypentadeca-[1,11,13] trienimino) benzofuro [4,5-e] pyrido [1,2 benzimidazole 1,15(2H)-dione,25-acetate (Figure 1) [1]. Figure 1: Chemical structure of rifaximin. Rifaximin is a newer antibiotic, used for the treatment of patients having more than 12 years of age with traveller’s diarrhoea caused by noninvasive strains of Escherichia coli [4]. Rifaximin binds to the beta-subunit of bacterial DNA-dependent RNA polymerase and prevents catalysis of polymerization of deoxyribonucleotides into a DNA strand, thereby inhibiting bacterial RNA synthesis. In vitro studies of rifaximin have demonstrated broad-spectrum coverage including gram-positive, gram-negative, and anaerobic bacteria as well as a limited risk of bacterial resistance [5]. Literature reports various analytical methods like spectrophotometric [6, 7], RP-HPLC [8, 9], and stability indicating HPLC [10] for the determination of rifaximin in pharmaceutical formulations. Moreover, bioanalytical methods, that is, HPLC-TMS [11, 12], LC-ESI-MS