Genetic polymorphisms in candidate genes predict increased toxicity with methotrexate therapy in Lebanese children with acute lymphoblastic leukemia.

BACKGROUND: The aim of this study is to analyze polymorphisms in genes involved in 6-mercaptopurine detoxification (TPMT); methotrexate (MTX) metabolism including ABCB1 (or MDR1), ABCC2, SLC19A1 (or RFC1), and SLCO1B1; and the MTX effect mainly MTHFR and TYMS, and to assess whether these polymorphisms are predictors of treatment toxicity and/or MTX clearance.
MATERIALS AND METHODS: This study included 127 Lebanese acute lymphoblastic leukemia patients, of whom 117 were treated following the St Jude's Children Research Hospital protocol. Genotyping was performed using real-time PCR or restriction fragment length polymorphism. MTX levels were measured using a polarization fluorescence assay from Roche. MTX clearance was estimated on the basis of all available MTX levels measured after high-dose MTX treatment during the consolidation phase.
RESULTS: Five variants in four genes (MTHFR, ABCB1, ABCC2, and TYMS) were shown to be associated with toxicity, but neither was associated with MTX pharmacokinetic parameters. For instance, during the consolidation phase, a statistically significant association was found between MTHFR rs1801133 variant allele carriers and a decrease in hemoglobin levels [odds ratio (OR)=3.057; 95% confidence interval (CI): 1.217; 7.680]. In addition, a statistically significant association was found among neutropenia (absolute neutrophil count<500) and variant allele carriers of ABCB1 rs1045642 (OR=5.174; 95% CI: 1.674; 15.989) and ABCB1 rs1128503 (OR=3.364; 95% CI: 1.257; 9.004), respectively. ABCC2 rs717620 variant allele carriers needed significantly more time to reach a MTX level below 0.1 µmol/l (β=5.122; 95% CI: 1.412; 8.831). During the continuation phase, a statistically significant association was found between ABCC2 rs717620 and TYMS 28-bp tandem repeats carriers with the need to decrease weekly MTX doses (β=-4.905; 95% CI: -9; -0.809 and β=-5.770; 95% CI: -10.138; -1.403), respectively.
CONCLUSION: Genotyping for MTHFR, ABCB1, ABCC2, and TYMS polymorphisms may be useful in identifying patients at risk of increased MTX toxicity and the need for dose optimization before treatment initiation.