Association of

Objective Variants in the SLCO1B1 gene, encoding a hepatic methotrexate (MTX) transporter, affect clearance of high‐dose MTX. We tested whether in the *14 and *15 alleles of SLCO1B1 influenced the response to low‐dose MTX in juvenile idiopathic arthritis (JIA) patients. Methods The study included 310 JIA patients genotyped for three single nucleotide polymorphisms (SNPs) in SLCO1B1 (rs4149056, rs2306283, and rs11045819). A patient's SLCO1B1 diplotype was determined by combining the SNPs into the *1a, *1b, *4, *5, *14, and *15 alleles. Number of active joints at follow‐up (visit closest to 6 months of treatment and prior to starting a tumor necrosis factor inhibitor) was used as the dependent variable in a negative binomial regression model that included active joint count at baseline as a covariate. Results The SLCO1B1*14 allele was associated with less response to MTX (P = 0.024) and the *15 allele was not associated with response to MTX (P = 0.392). Conclusion SLCO1B1 alleles may be associated with poor response to MTX in JIA patients. The *14 allele has been associated with fast clearance (low exposure) after high‐dose MTX in patients with leukemia. Thus, the SLCO1B1 gene may be informative for precision dosing of MTX in JIA patients. Patients carrying the *14 allele may require a higher dose than noncarriers to achieve a similar response to MTX.