Effect of pharmaceutical excipients on aqueous stability of rabeprazole sodium

The chemical stability of a proton-pump inhibitor, rabeprazole sodium, was evaluated in simulated intestinal fluid (pH 6.8) containing various 'Generally Recognized As Safe (GRAS)'-listed excipients, including Brij^® 58, Poloxamer 188, Cremophor RH40, Gelucire 44/14 and PEG 6000. After incubation at 37 and 60 °C, the amounts of rabeprazole and its degradation product, thioether-rabeprazole, were quantitated by HPLC analysis. The main degradation product was separated and characterized by LC/MS. The degradation of rabeprazole followed first-order kinetics. In the absence of any excipients, the rate constants (k) obtained at 37 and 60 °C were 0.75 and 2.78 h^-1, respectively. In contrast, the addition of excipients improved its stability. Among several excipients tested in this study, Brij^® 58 displayed the greatest stabilizing effect. For instance, at 37 and 60 °C, Brij^® 58 reduced the k values to 0.22 and 0.53 h^-1, respectively. The stabilizing mechanisms of these hydrophilic polymeric excipients with optimal HLB values could be partially explained in terms of their solubilizing efficiency and micellar formation for thioether-rabeprazole. In conclusion, rabeprazole formulations that contain suitable excipients would improve its stability in the intestinal tract, thereby maximizing bioavailability.