Investigation of the Importance of Multidrug Resistance-Associated Protein 4 (Mrp4/Abcc4) in the Active Efflux of Anionic Drugs Across the Blood-Brain Barrier.

The importance of multidrug resistance-associated protein 4 (Mrp4/Abcc4) in limiting the penetration of Mrp4 substrate compounds into the central nervous system across the blood-brain barrier was investigated using Mrp4-/- mice. Significant adenosine triphosphate-dependent uptake by MRP4 was observed for ochratoxin A, pitavastatin, raltitrexed (Km = 43.7 μM), pravastatin, cyclic guanosine monophosphate, 2,4-dichlorophenoxyacetate, and urate. The defect in the Mrp4 gene did not affect the brain-to-plasma ratio (Kp,brain) of quinidine and dantrolene. Following intravenous infusion in wild-type and Mrp4-/- mice, the plasma concentrations of the tested compounds (cefazolin, cefmetazole, ciprofloxacin, cyclophosphamide, furosemide, hydrochlorothiazide, methotrexate, pitavastatin, pravastatin, and raltitrexed) were identical; however, Mrp4-/- mice showed a significantly higher (1.9- to 2.5-fold) Kp,brain than wild-type mice for methotrexate, raltitrexed, and cyclophosphamide. GF120918, a dual inhibitor of P-gp and Bcrp, significantly decreased Kp,cortex and Kp,cerebellum only in Mrp4-/- mice. Methotrexate and raltitrexed are also substrates of multispecific organic anion transporters such as Oatp1a4 and Oat3. GF120918 showed an inhibition potency against Oatp1a4, but not against Oat3. These results suggest that Mrp4 limits the penetration of methotrexate and raltitrexed into the brain across the blood-brain barrier, which is likely to be facilitated by some uptake transporters.