Evaluation of Pharmaceutical Excipients as Cosolvents in 4-Methyl Umbelliferone Glucuronidation in Human Liver Microsomes: Applications for Compounds with Low Solubility

Summary:Standard incubation procedures for carrying out microsomal assays involve the use of less than 1% w/v organic solvents to minimize the potential inhibitory effects of organic solvents on metabolic activity. 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. 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. We observed that HPBCD (Hydroxypropyl-β-cyclodextrin) may be employed in in vitro glucuronidation incubations up to 0.5% w/v without affecting the Clint of 4-MU. Although NMP (N-methyl-2pyrrolidone) and DMA (N,N-dimethylacetamide); showed low IC75 values approximately 0.1% w/v each, neither excipients altered the Clint of 4-MUG (4-methylumbelliferyl-β-D-glucuronide) formation. Our studies point toward possible applications 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.