PT - JOURNAL ARTICLE AU - Pauline Breedveld AU - Dick Pluim AU - Greta Cipriani AU - Femke Dahlhaus AU - Maria A. J. van Eijndhoven AU - Cornelia J. F. de Wolf AU - Annemieke Kuil AU - Jos H. Beijnen AU - George L. Scheffer AU - Gerrit Jansen AU - Piet Borst AU - Jan H. M. Schellens TI - The Effect of Low pH on Breast Cancer Resistance Protein (ABCG2)-Mediated Transport of Methotrexate, 7-Hydroxymethotrexate, Methotrexate Diglutamate, Folic Acid, Mitoxantrone, Topotecan, and Resveratrol in In Vitro Drug Transport Models AID - 10.1124/mol.106.028167 DP - 2007 Jan 01 TA - Molecular Pharmacology PG - 240--249 VI - 71 IP - 1 4099 - http://molpharm.aspetjournals.org/content/71/1/240.short 4100 - http://molpharm.aspetjournals.org/content/71/1/240.full SO - Mol Pharmacol2007 Jan 01; 71 AB - Some cellular uptake systems for (anti)folates function optimally at acidic pH. We have tested whether this also applies to efflux from cells by breast cancer resistance protein (BCRP; ABCG2), which has been reported to transport folic acid, methotrexate, and methotrexate di- and triglutamate at physiological pH. Using Spodoptera frugiperda-BCRP membrane vesicles, we showed that the ATP-dependent vesicular transport of 1 μM methotrexate by BCRP is 5-fold higher at pH 5.5 than at physiological pH. The transport of methotrexate was saturable at pH 5.5, with apparent Km and Vmax values of 1.3 ± 0.2 mM and 44 ± 2.5 nmol/mg of protein/min, respectively, but was linear with drug concentration at pH 7.3 up to 6 mM methotrexate. In contrast to recent reports, we did not detect transport of methotrexate diglutamate at physiological pH, but we did find transport at pH 5.5. We also found that 7-hydroxy-methotrexate, the major metabolite of methotrexate, is transported by BCRP both at physiological pH and (more efficiently) at low pH. The pH effect was also observed in intact BCRP-overexpressing cells: we found a 3-fold higher level of resistance to both methotrexate and the prototypical BCRP substrate mitoxantrone at pH 6.5 as at physiological pH. Furthermore, with MDCKII-BCRP monolayers, we found that resveratrol, which is a neutral compound at pH ≤ 7.4, is efficiently transported by BCRP at pH 6.0, whereas we did not detect active transport at pH 7.4. We conclude that BCRP transports substrate drugs more efficiently at low pH, independent of the dissociation status of the substrate. The American Society for Pharmacology and Experimental Therapeutics