The aim of this study was to investigate the potential of calcineurin inhibitors [cyclosporine A (CsA) and tacrolimus (Tac)] to inhibit cellular uptake of atorvastatin mediated by the liver-specific organic anion-transporting polypeptide 1B1 (OATP1B1) in vitro. Patients with solid organ transplants are frequently treated with HMG-CoA reductase inhibitors (statins). CsA increases atorvastatin systemic exposure severalfold, an effect not observed with Tac. The effect of CsA and Tac on atorvastatin transport via OATP1B1 was investigated in transfected human embryonic kidney 293 cells. An in vitro-in vivo extrapolation (IVIVE) was performed to estimate the clinical potential for CsA and Tac to inhibit OATP1B1-mediated transport. CsA inhibited OATP1B1-mediated uptake of atorvastatin approximately 90-fold more efficiently than Tac, with half-maximal inhibitory concentration (IC(50)) values of 0.021 +/- 0.004 and 1.99 +/- 0.42 muM, respectively. Coincubation compared with preincubation with CsA showed a 20-fold lower inhibitory capacity, with an IC(50) value of 0.47 +/- 0.34 muM. The IVIVE showed that clinically obtainable concentrations of CsA, but not Tac, inhibit OATP1B1 transport of atorvastatin. CsA inhibition ranged from 28 to 77% within a dosing interval, whereas it was less than 1% for Tac, considering free concentrations and assuming competitive inhibition. This does not fully explain the clinically observed interaction with CsA, suggesting that a more complex inhibitory mechanism may be present. This is also supported by the decreased IC(50) value of CsA after preincubation. This study provides evidence that OATP1B1 inhibition is a relevant mechanism for the interaction observed between CsA and atorvastatin.