Effect of reduced folate carrier gene polymorphism (G80A) on the methotrexate level in patients with rheumatoid arthritis and it’s relation to the disease activity

Methotrexate (MTX) is the primary conventional synthetic disease-modifying anti-rheumatic drug (csDMARD) used to treat rheumatoid arthritis (RA). Interindividual variability in its efficacy and toxicity has been partially attributed to genetic polymorphisms affecting folate metabolism and transport. The reduced folate carrier-1 (RFC1), encoded by the SLC19A1 gene, is a key transporter responsible for MTX cellular uptake. The RFC1 G80A (rs1051266) polymorphism may alter transporter function, influencing MTX plasma levels and therapeutic outcomes. This study attempted to evaluate the impact of RFC1 G80A polymorphism on MTX plasma concentration and its association with the activity of RA disease. This cross-sectional study was carried out at the Rheumatology Center in Duhok, Iraq, from September 2024 to June 2025. Ninety RA patients receiving oral MTX therapy were recruited. MTX levels were quantified by ELISA, and genotyping of the RFC1 G80A polymorphism was performed by PCR-RFLP. Statistical analyses were conducted to evaluate genotype associations with MTX levels, DAS-28 activity score, and laboratory parameters. The study cohort (mean age: 50.1 ± 11.9 years) was predominantly female (91.1%). Genotype frequencies were GG (53.3%), GA (35.6%), and AA (11.1%). Although MTX levels were numerically highest in AA individuals (330.1 ± 70.5 ng/mL), the difference was not significant in statistical terms (p=0.82). However, AA carriers had significantly higher DAS-28 scores (4.3 ± 1.5) than GG carriers (3.8 ± 1.2; p=0.009), suggesting poorer disease control. No substantial variation has been noticed in inflammatory or hematological parameters between genotype groups. The RFC1 G80A polymorphism may influence MTX pharmacokinetics and RA disease activity. The AA genotype appears to be associated with non-significantly higher MTX levels and worse disease control. Pharmacogenetic profiling could help optimize MTX therapy in RA.