The effect of ABCG2 and ABCC4 on the pharmacokinetics of methotrexate in the brain

Methotrexate (MTX) is the cornerstone of chemotherapy for primary central nervous system lymphoma, yet how the blood-brain barrier (BBB) efflux transporters ABCG2 and ABCC4 influence the required high-dose therapy is unknown. To evaluate their role, we used four mouse strains, C57BL/6 (wild-type; WT), Abcg2(-/-), Abcc4(-/-), and Abcg2(-/-);Abcc4(-/-) (double knockout; DKO) to conduct brain microdialysis studies after single intravenous MTX doses of 50 mg/kg. When the area under the concentration-time curve for plasma (AUC(plasma)) was used to assess systemic exposure to MTX, the rank order was Abcc4(-/-) < WT < Abcg2(-/-) < Abcg2(-/-)Abcc4(-/-). Only the DKO exposure was significantly higher than that of the WT group (P < 0.01), a reflection of the role of Abcg2 in biliary excretion and Abcc4 in renal excretion. MTX brain interstitial fluid concentrations obtained by microdialysis were used to calculate the area under the concentration-time curve for the brain (AUC(brain)), which found the rank order of exposure to be WT < Abcc4(-/-) < Abcg2(-/-) < Abcg2(-/-)Abcc4(-/-) with the largest difference being 4-fold: 286.13 ± 130 μg*min/ml (DKO) versus 66.85 ± 26 (WT). Because the transporters affected the systemic disposition of MTX, particularly in the DKO group, the ratio of the AUC(brain)/AUC(plasma) or the brain/plasma partition coefficient Kp was calculated, revealing that the DKO strain had a significantly higher value (0.23 ± 0.09) than the WT strain (0.11 ± 0.05). Both Abcg2 and Abcc4 limited BBB penetration of MTX; however, only when both drug efflux pumps were negated did the brain accumulation of MTX significantly increase. These findings indicate a contributory role of both ABCG2 and ABCC4 to limiting MTX distribution in patients.