AbstractObjectives2-Methoxyestradiol (2ME2), a metabolite of estradiol, has antitumour activity in vitro. However, potential clinical applicability has been limited by low oral bioavailability. Probenecid was evaluated in vitro as an inhibitor of 2ME2 glucuronidation for purposes of enhancing 2ME2 oral bioavailability.MethodsHuman liver microsomes were used to determine kinetic parameters for transformation of 2ME2 to its glucuronide metabolites (M1, M2) and inhibition of the reactions by probenecid.Key findingsM1 and M2 formation from 2ME2 proceeded with features of substrate inhibition. Probenecid inhibited metabolite formation, with mean inhibition constant (Ki) values of 0.9 and 2.6 mM, respectively. Inhibition was reversible, with mixed competitive–non-competitive characteristics.ConclusionThe Ki values for probenecid inhibition of 2ME2 glucuronide formation, when compared to maximum probenecid plasma concentrations anticipated clinically, indicate that probenecid co-administration has the potential to augment systemic plasma levels of 2ME2 after oral dosage in humans.