In Vitro Metabolic Stability of Moisture-Sensitive Rabeprazole in Human Liver Microsomes and Its Modulation by Pharmaceutical Excipients

• Published in: Archives of pharmacal research Vol. 31; no. 3; pp. 406 - 413
• Main Authors: Ren, Shan, Park, Mi-Jin, Kim, Ae-Ra, Lee, Beom-Jin
• Format: Journal Article
• Language: Korean
• Published: 2008
• Subjects: Metabolic stability; Thioether form; Rabeprazole; Human liver Microsomes; Pharmaceutical excipients
• ISSN: 0253-6269

A reliable method to assess in vitro metabolic stability of rabeprazole and its modulation by Generally Recognized As Safe (GRAS)-listed pharmaceutical excipients was established in human liver microsomes. The metabolic stability of rabeprazole decreased as a function of incubation time, resulting in the formation of thioether rabeprazole via nonenzymatic degradation and enzymatic metabolism, Buffer type was also a determining factor for the degree of both nonenzymatic degradation and enzymatic and enzymatic metabolism. The net extent of enzymatic drug metabolism, obtained by calculating the difference in drug degradation between a microsome-present reaction System and a microsome-free solution, was about $9.20\;{\pm}\; 0.67$% in phosphate buffer and $2.27{\pm}1.76$% in Tris buffer, respectively. Rabeprazole exhibited first-order kinetics in Microsome-free solution but showed non-linear kinetics in the microsome-present reaction system. The maximal velocity, $V_{max}$ in phosphate buffer was 5.07 ${\mu}g\;mL^{-1}\;h^{-1}$ and the michaelis-Menten constant, $K_m$, was 10.39 ${\mu}g\;mL^{-1}$ by computer-fitting to the classical Michaelis-Menten equation for pattern of time-dependent change in the substrate concentration. The intact drug and its thioether form were well resolved and successfully identified by HPLC chromatography and liquid chromatography mass spectroscopy (LC/MS). The metabolic stability of rabeprazole was also modulated by the presence of pharmaceutical excipients. Among the five pharmaceutical excipients tested, poloxamer 188 and Gelucire 44/14 had potentially inhibitory effects on rabeprazole metabolism in human liver microsomes (p < 0.05). A greater understanding of metabolic stability and its modulation by pharmaceutical excipients would be useful for optimizing the bioavailability of rabeprazole at the early formulation stages.