Abstract

Organic solvents are extensively used in in vitro drug metabolism studies to overcome the solubility issues of lipophilic substrate/new chemical entities. The effects of ten common water miscible organic solvents on cytochrome P450s and non-cytochrome P450 enzymes, namely, flavin monooxygense, aldehyde oxidase, xanthine oxidase, esterases and glutathione S-transferases were evaluated. Rat liver microsomes and rat liver cytosol were used for cytochrome P450, flavin monooxygenase and glutathione S-transferase assays. Partially purified guinea pig liver aldehyde oxidase and rat liver xanthine oxidase fractions were used for molybdenum hydroxylase activity assays. Human plasma was used for esterase activity. Para-nitro phenol, metoprolol, imipramine, methyl para-tolyl sulfide, para-nitrophenyl acetate, vanillin, xanthine and chloro dinitrobenzene, were used as substrates for evaluating CYP2E1, CYP2D6, CYP1A2, flavin monooxyenase, esterase, aldehyde oxidase, xanthine oxidase and glutathione S-transferase activities, respectively. Ten solvents (acetonitrile, acetone, dioxane, dimethyl formamide, dimethyl sulfoxide, ethanol, methanol, polyethylene glycol 400, n-propanol, and isopropanol) were evaluated at four concentrations (0.1, 0.25, 0.5 and 1 % v/v). The following results were obtained. Dioxane (1 % v/v) was found to a potent inhibitor inhibiting all the tested CYP’s (>90 % inhibition) and flavin monooxygenase (71 % inhibition). Seven of ten organic solvents inhibited esterases in a concentration dependent manner out of which n-propanol showed the maximal inhibition (79 % inhibition). Methanol was the only solvent that inhibited aldehyde oxidase (78 % inhibition) and xanthine oxidase mediated metabolism (64 % inhibition). Minimal inhibition effect was observed on glutathione S-transferase mediated activity by all solvents. Polyethylene glycol 400 showed least inhibition of enzyme activities. Among the 10 solvents studies, acetonitrile overall appears to be the safest organic solvent at concentrations of 1 % v/v or less. The results of this study indicate that many water miscible organic solvents must be used very judiciously in drug metabolism studies.