Evaluation of pharmaceutical excipients as cosolvents in 4-methyl umbelliferone glucuronidation in human liver microsomes: An application for compounds with low solubility

Abstract

Introduction: In order to minimize the potential inhibitory effects of organic solvents on metabolic activity, standard incubation procedures for carrying out microsomal assays involve the use of less than 1% w/v organic solvents. Often, solvents needed to dissolve the substrate add-up nearly to this concentration. This presents a practical limitation for poorly soluble xenobiotics, which cannot be incubated at concentrations high enough to obtain a Vmax, and therefore subsequent values for Km and Clint cannot be calculated. Our goal was to study the application of a variety of pharmaceutical excipients to aid the solubilization of compounds in vitro in glucuronidation incubations, without affecting the reaction kinetics. Methods: In vitro glucuronidation incubations were carried out in human liver microsomes with 4-methylumbelliferone (4-MU) and the kinetics of 4-MU glucuronidation in the presence of excipients were compared to that in control incubations without any excipients. In addition, IC75 values were calculated for each excipient. Results and Summary: We observed that HPBCD may be employed in in vitro glucuronidation incubations up to 0.5% w/v without affecting the Clint of 4-MU. Although NMP and DMA showed low IC75 values approximately 0.1% w/v each, neither excipients altered the Clint of 4-MUG formation. Our studies point toward a possible application of pharmaceutical excipients to carry out in vitro glucuronidation of substrates with poor aqueous solubility, in order to estimate Clint and subsequently scaled organ clearance values