Mutational Status in Genes of Folate Metabolism Is Associated with Toxicity of High-Dose Methotrexate in Patients with Immunocompetent Primary Central Nervous System Lymphoma.

Background: Outcome of patients with immunocompetent primary central nervous system lymphoma (PCNSL) has improved since the introduction of high-dose (HD) methotrexate (MTX). However, treatment may be perturbed by considerable toxicity. Acute toxicity comprises hematologic, renal, hepatic, pulmonary events and mucositis while neurologic impairment is essentially restricted to long-term survivors. MTX acts via inhibition of 5,10-methylentetrahydrofolate reductase (MTHFR) as well as thymidylate synthase (TYMS) in target cells what eventually results in decreased DNA synthesis. Intracellular uptake of folates is mediated by the reduced folate carrier (RFC). Two common MTHFR single nucleotide polymorphisms (SNPs) affect enzyme activity: 677 C→T and 1298A→C. A 28 base pair (bp) tandemly repeated sequence in the TYMS enhancer region (5′-UTR) containing either 2 or 3 repeats alters enzyme activity as does a 6 bp deletion (del) polymorphism in the 3′-UTR of TYMS. An RFC SNP (80G→A) is known to influence folate and MTX transport. We hypothesized that these polymorphisms may be directly linked to toxicity in PCNSL patients receiving high-dose MTX.

Patients and methods: Genomic DNA was prospectively collected from patients (pts) with immunocompetent PCNSL who received HD MTX 4 g/m² every 2 weeks after enrolment onto a German multicenter trial. They had to have histological confirmation of their disease as well as adequate organ and bone marrow function. Genotyping of the two MTHFR and the RFC SNPs was performed by melting point analysis using the Light Cycler® technology. Genotyping of the TYMS 28bp polymorphism was performed by conventional PCR followed by agarose gel electrophoresis. The TYMS 6bp del polymorphism was analyzed by PCR, restriction enzyme digest and agarose gel electrophoresis. Hematologic (Hb, WBC, ANC, platelets) and non-hematologic toxicity (renal, hepatic, pulmonary, mucositis) were assessed prospectively and correlated with polymorphisms with respect to presumable functional relevance.

Results: 123 pts with a median age of 62 (range, 27 – 80) years received a total of 506 cycles of high-dose MTX. 39 of 119 evaluable pts (33%) experienced any severe (°3/4) toxicity. Incidence of severe toxicity was significantly different for pts with MTHFR 677 TT (95%) when compared to CC/CT genotype (48%; p=.0024). Not considering hematologic parameters, severe toxicity was even more strongly associated with homozygosity for 677TT (p=.0006). A moderate association with lower ANC was found for MTHFR C677T (p=.049) while °3/4 neutropenia was strongly associated with RFC 80 GG vs. AA/GA (p=.0057). In hand, lower WBC nadirs occurred in pts with RFC 80 GG vs. AA/GA (p=.035).

Conclusion: We demonstrate for the first time that pharmacogenetic studies might identify PCNSL pts who are at risk for severe acute hematologic and non-hematologic toxicity when treated with HD MTX. Correlation of (late) neurotoxicity with polymorphisms of folate metabolizing genes, however, will require longer follow-up.