Background/Purpose: Despite widespread use, there remain no clear predictors of methotrexate (MTX) response or toxicity in Juvenile Idiopathic Arthritis (JIA).  With the utilization of new assessment tools, the prevalence of MTX intolerance may be higher than previously recognized.  We explore cellular folate and genotype as potential predictors of MTX intolerance.

Methods: This is a single center prospective cohort study of newly treated JIA patients on standardized doses of MTX (15mg/m²) and folic acid (1mg/day). After obtaining informed consent, samples for assessment of intracellular erythrocyte folate ([folate]; comprised of 5-methyl-tetrahydrofolate + 5-10 methenyl-tetrahydrofolate) and MTX polyglutamate (MTXGlu) measurements by UPLC-tandem mass spectrometry are collected prior to and after 3 and 6 months on therapy.  Genotype analysis for 41 SNPs, indels and repeats in 23 folate related genes are assessed by established methods.  Clinical data are collected at each time point and include the methotrexate intolerance severity score (MISS) in a subset of patients (n=26). Statistical significance was tested using ANOVA, simple linear regression, and chi square, utilizing Wilcoxon Rank Sum or Spearman’s correlations in non-normally distributed data

Results:

MISS scores ranged from 0-11, and the median (IQR) score was 2 (0, 5) in the 26 patients with data at 3 months; of these, 4/26 (15%) had a score of 6 or greater.  Higher pretreatment RBC [folate] was associated with lower 3 month MISS scores (β: -0.005, p=0.008, R² 0.28).  Greater decreases in RBC [folate] from baseline were also associated with lower 3 month MISS scores (β: -3.033, p=0.05, R²0.17).  When MISS scores were dichotomized at a cutoff of 6, patients with scores less than 6 had higher mean baseline RBC [folate] compared to patients with scores of 6 or greater (1107.5 (± 306) nmol/L vs. 767.2 (± 184) nmol/L,  p<0.05).  There were no correlations between 3 month MISS scores and MTXGlu.

The presence of at least 1 variant allele in methionine synthase reductase (MTRR) A>G (rs 1801394) appeared protective based on lower mean MISS scores (1.4 vs. 5.3, p=0.005) and decreased probability of a MISS score of ≥6 (0% vs. 50%, p=0.02).  Homozygous variant patients had higher mean baseline RBC [folate] (1152.7 (± 379) nmol/L vs. 897.4 (± 304) nmol/L, p=0.01) and higher 3 month RBC [folate] (807.4 (± 241) nmol/L vs.  656.9 (± 241) nmol/L, p=0.02).

Conclusion: MTX intolerance is associated with lower baseline folate and reduced folate depletion after MTX initiation, suggesting a difference in folate homeostasis in intolerant children.  Genetic variation in MTRR (rs 1801394) appears to be protective and reflects its important regulatory role in the folate pathway, restoring the activity of methionine synthase (MTR), crucial for transformation of homocysteine to methionine and important for one-carbon metabolism.  The ‘Methyl-trap’ has been described with decreased MTR/MTRR activity due to depletion of the B₁₂ cofactor, resulting in a functionall folate depleted state.  Our data suggest that MTRR AA patients may be at risk for a similar ‘methyl trap’ state, further exacerbated with the introduction of MTX, resulting in increased intolerance.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/disruptions-in-folate-homeostasis-may-lead-to-increased-risk-for-methotrexate-intolerance-in-juvenile-idiopathic-arthritis/