High-dose methotrexate in pediatric acute lymphoblastic leukemia: impact of ABCC2 polymorphisms on plasma concentrations.

The adenosine triphosphate-binding cassette (ABC) class transporter ABCC2 (MRP2 [multidrug resistance related protein 2] or cMOAT [canalicular multispecific organic anion transporter]) is involved in the cellular outward transport and elimination of methotrexate. We hypothesized that common genetic variations may contribute to the variability of high-dose methotrexate pharmacokinetics. Polymorphisms in all 32 exons of the ABCC2 gene were analyzed in a reference group of 59 healthy white subjects by polymerase chain reaction, single-strand conformation polymorphism, and sequencing. Subsequently, we assessed the association of polymorphisms with the methotrexate plasma concentrations in 44 pediatric patients with acute lymphoblastic leukemia (ALL) (29 male and 15 female patients; mean age, 6.8+/-4.8 years). Patients received 4 cycles of 5000 mg/m2 body surface area according to the ALL-Berlin-Frankfurt-Muenster (BFM) 95 or ALL-BFM 2000 protocol. In the reference group we detected 8 frequent single-nucleotide polymorphisms. Five of these were in complete linkage disequilibrium. Overall, 5 new polymorphisms are described. The genotype distribution of the patient cohort was not significantly different from the reference collective. The mean plasma methotrexate area under the curve from 36 to 48 hours after the start of the infusion was significantly 2-fold higher in female patients carrying at least 1 -24T allele as compared with all other patients (14.2+/-12.8 h.micromol/L versus 6.9+/-4.2 h.micromol/L, P<.001). The risk to have 2 or more cycles necessitating an intensification of folinate rescue was 9-fold (95% confidence interval, 1.8- to 44-fold) in female patients carrying at least 1 T allele (P=.0067). The data suggest a hitherto unknown gender-specific impact of the -24C>T ABCC2 gene polymorphism on high-dose methotrexate pharmacokinetics. Whereas a nonfunctional MRP2 variant has been described in a patient with severe impairment of methotrexate excretion, our study is the first to suggest that a frequent ABCC2 polymorphism contributes to variability of methotrexate kinetics.