OBJECTIVETo provide guidance for the high-dose methotrexate (HD-MTX) treatment of pediatric acute lymphoblastic leukemia (ALL), and to understand the impact of SLCO1B1c.521T>C (rs4149056) variant on methotrexate (MTX) pharmacokinetics and clinical outcome in children with ALL.

METHODEighty-two children with ALL in Division of Hematology of Wuhan Children's Hospital from January 2008 to February 2013 were enrolled. All patients were genotyped for rs4149056 single nucleotide polymorphism (SNP) into wild-type group (TT genotype) and variant group (TC/CC genotype). According to the ALL-BFM 2000 protocol, all patients received intravenous infusion of MTX every ten days at 3 to 5 g/m(2). Leucovorin rescue was performed after 36 hours of the MTX administration and its dose was adjusted according to the MTX plasma concentration at 48 hours. The concentrations of MTX and its metabolite at 24, 48 and 72 h were determined by high performance liquid chromatography with solid phase extraction. Population pharmacokinetic parameters were estimated by the NLME software. The pharmacokinetics, toxicity and leucovorin rescue was compared. The relapse rate within 5 years and event-free survival were followed up.

RESULTEighty-two pediatric patients were classified into two groups: variant group including 20 TC genotype carriers and one CC genotype carrier, wild-type group included 61 patients with TT genotype. Compared with wild-type group, plasma concentration of MTX at 48 and 72 h increased significantly [48 h: (1.00±1.41) vs.(0.34±0.17) µmol/L, t=2.131, P=0.046; 72 h: (0.31±0.26) vs.(0.08±0.04) µmol/L; t=3.995, P=0.001]. Area under the concentration time curve (AUC48-∝) of MTX significantly increased in variant group [(23.18±19.91) vs.(5.66±2.01) h·µmol/L] (t=4.025, P=0.001). Time above the MTX safety threshold (TC>0.1 µmol/L) increased significantly in variant group [(95.3±22.0) vs.(67.1±7.5) h, t=5.880, P<0.001]. Rescue dosage of leucovorin in variant group was higher than that in wild-type group [(312.7±287.8) vs.(140.6±27.5) mg/m2, t=2.614, P=0.017]. The children carrying rs4149056 C allele suffered from a higher frequency of serious adverse effect [gastrointestinal toxicity: 33% (7/21) vs. 5% (3/61);hepatic toxicity: 24% (5/21) vs. 2% (1/61)]. The difference was statistically significant (χ2=9.275, 8.289, all P<0.05). Hospital stay of variant group was significantly longer than that of wild-type [(4.95±1.43) vs. (4.05±0.22) d, t=2.881, P=0.009]. The relapse rate within 5 years of variant group and wild-type group were 9% (2/21) and 13% (8/61), respectively. There were no significant differences in the event-free survival between the two groups (χ2=0.001, P=0.971).

CONCLUSIONThe SLCO1B1 c.521T>C variant was an important determinant of MTX pharmacokinetics. An appropriate leucovorin dose raise in variant group was beneficial to reducing the serious toxicity and did not affect the long-term clinical outcome.