| Author |
Tsujii K, Yajima K, Akiyoshi T, Sakamoto K, Suzuki Y, Oka T, Imaoka A, Yamamura H, Kurokawa J, Ohtani H.
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| Abstract |
Several genetic variants of OATP1B1, a hepatic uptake transporter, increase the blood concentrations of substrate drugs, e.g., ∗15 carriers exhibit higher blood substrate concentrations than ∗1b carriers. It remains unclear whether these differences are due to changes in expression or intrinsic activity (transport activity per OATP1B1 molecule). This study compared the intrinsic activity of four OATP1B1 variants, ∗1a, ∗1b, ∗5, and ∗15, using HEK293 cell lines that co-expressed large-conductance Ca2+-activated K+ (BK) channels and one of the OATP1B1 variants. To estimate the kinetic parameters Km and Vmax, 2', 7'-dichlorofluorescein uptake was evaluated. The number of OATP molecules per cell (QT) was calculated from BK channel-mediated whole-cell conductance and the OATP1B1/BK channel expression ratio (ρ) (determined by LC-MS/MS). Vmax,int (maximum intrinsic transport velocity) was obtained by dividing Vmax by QT, and intrinsic clearance (CLint) was calculated as Vmax,int/Km. The Km values of ∗1a, ∗1b, ∗5, and ∗15 were 12.5, 9.19, 7.53, and 10.4 μM, and their Vmax,int values were 3.0, 7.0, 1.5, and 1.2 × 10-21 mol/OATP molecule/min, respectively. Accordingly, the CLint value for OATP1B1∗15 was 15 % lower than that for OATP1B1∗1b, suggesting that the increased blood substrate concentrations observed in OATP1B1∗15 carriers may be due to the decreased intrinsic activity of OATP1B1∗15.
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