To determine the inhibition effects of drugs on the glucuronidation of
estradiol (E2), 29 drugs that have been reported to induce
gynecomastia were examined in the presence of
UDP-glucuronic acid using human hepatic microsomes (pooled) as the
enzyme source. The percentage inhibition of the E2 glucuronidation was determined at
drug concentrations of 1 microM (approximate therapeutic concentration) and 100 microM (non-clinical overdose concentration) based on the rate constants for the 3- and 17-glucuronidation of E2 (11.2 and 2.52 pmol/min/mg
protein, respectively). The only
drug that exhibited 50% or higher inhibition of the 3-glucuronidation at a concentration of 1 microM was
manidipine (54.4%). When the concentration was 100 microM,
manidipine exhibited 100% inhibition of the 3-glucuronidation, and other drugs that exhibited 50% or higher inhibition of the 3-glucuronidation were
nicardipine (92%),
nisoldipine (90%),
nifedipine (84%),
domperidone (81%),
tacrolimus (80%),
nitrendipine (77%) and
ketoconazole (69%). Conversely,
ipriflavone accelerated the formation of
estradiol 3-glucuronide in the activity of 165% at the concentration of 100 microM. On the 17-glucuronidation, all of the drugs showed less than 50% inhibition at the concentration of 1 microM, but at the concentration of 100 microM, drugs that exhibited 50% or higher inhibition consisted of
manidipine (79%),
chlormadinone acetate (74%),
nisoldipine (66%),
nitrendipine (60%) and
ketoconazole (55%). Although IC(50) values of these drugs were all lower than the K(m) value (285 microM) for the 3-glucuronidation of E2, they were higher than the K(m) value for the 17-glucuronidation (18.8 microM). Thus, the effect of the drugs on the E2 glucuronidation should be greater for hydroxy group at the C-3 than that at the C-17 of E2 molecule. On the other hand, metabolic clearances (V(max)/K(m)) of the 3- and 17-glucuronidation were about 1/14th and 1/18th of that of the 2-hydroxylation of E2, respectively. The result implies that, when the contribution of the glucuronidation to enterohepatic circulation is taken into consideration, the effect of this metabolic inhibition in the
estrogen pool cannot be ignored.