Pharmacokinetic interaction studies of tanshinones with tolbutamide, a model CYP2C11 probe substrate, using liver microsomes, primary hepatocytes and in vivo in the rat

The effects of Danshen and its active components (tanshinone I, tanshinone IIA, dihydrotanshinone and cryptotanshinone) on tolbutamide 4-hydroxylation was investigated in the rat. Danshen (0.125-2 mg/ml) decreased 4-hydroxy-tolbutamide formation in vitro and in vivo. Enzyme kinetics studies showed that inhibition of tolbutamide 4-hydroxylase activity was competitive and concentration-dependent. The K_i values of the tanshinones were: dihydrotanshinone (8.92 μM), cryptotanshinone (24.5 μM), tanshinone I (80.3 μM) and tanshinone IIA (242.9 μM). In freshly prepared primary rat hepatocytes, tanshinones inhibited tolbutamide 4-hydroxylation in a concentration-dependent manner, with EC₄₀ values in the order: cryptotanshinone (15.8 μM), tanshinone IIA (16.2 μM), dihydrotanshinone (20.1 μM) and tanshinone I (48.2 μM). In whole animal studies, single dose Danshen treatment (50 or 200 mg/kg, i.p.) increased tolbutamide clearance (17-26.9%), decreased AUC (14.4-20.9%) and increased the Vd (7.26%). Three-day Danshen treatment (200 mg/kg/day, i.p.) decreased the C_initial, increased T_1/2 and Vd but did not affect tolbutamide clearance and AUC. Tolbutamide-4-hydroxylation in vivo was decreased by Danshen after acute and after 3-day treatment, with decreases in the AUC of 4-hydroxy-tolbutamide (15-28%) over the time period studied. Despite competitive inhibition of rat CYP2C11 in vitro and in vivo, as shown by the decrease in tolbutamide 4-hydroxylation, only minor changes in tolbutamide pharmacokinetics was observed. This study illustrated that the herb-drug interaction potential should be monitored by both in vitro and in vivo biotransformation/ pharmacokinetic parameters.