Background/Purpose: Methotrexate (MTX) is the most commonly used drug in rheumatoid arthritis (RA). 30% of patients fail to respond to the drug or suffer from adverse events. Therefore, there is a need for therapeutic drug monitoring (TDM). MTX plasma concentrations decrease in a few hours, whereas MTX polyglutamates (MTX-PG) are accumulated intracellular over months inside cells, and therefore may be a tool for TDM.

Methods: The study included patients from two longitudinal cohorts who were diagnosed with RA according to the 2010 ACR criteria and were treated with MTX therapy: 285 patients from the treatment in Rotterdam Early Arthritis Cohort (tREACH) (1) and 99 from the Methotrexate in Rotterdam (MTX-R) study. We measured MTX with a tail of 1,2,3,4 and 5 glutamates in erythrocytes at 3 months after MTX start with an LC-MS/MS assay. As outcome measure for MTX response we defined disease activity score (DAS) 28 at 0,3,6 and 9 months after MTX start and EULAR response criteria at 3,6 and 9 months. Adverse events were measured as gastro intestinal intolerance and overall complaints at 3,6 and 9 months. DAS28 was analyzed with analysis of covariance (ANCOVA). EULAR response and adverse events were analyzed with logistic regression. All MTX-PGs were analyzed as continuous variables, quintiles and tertiles. All analysis were adjusted for age, gender, DAS28 at baseline, MTX dose, MTX route of administration, other DMARDs, NSAIDs, steroids and study cohort.

Results: Concentrations at 3 months (mean/SD) were: MTX-PG1: 17.99 nmol/l (18.28), MTX-PG2: 9.25 nmol/l (4.61), MTX-PG3: 18.26 nmol/l (7.66), MTX-PG4: 7.78 nmol/l (5.95), MTX-PG5: 2.41 nmol/l (3.02) and total MTX-PG: 58.07 nmol/l (27.47). DAS28 was 4.76 (1.26) at baseline and 3.08 (1.20), 2.91 (1.22), 2.68 (1.16) at 3,6 and 9 months, respectively. At 3 months 52% had gastrointestinal complaints, at 6 months 26% and at 9 months 24%. 46%, 49% and 47% of the patients reported to have one or more adverse events at 3, 6 and 9 months. Lower DAS28 scores at 3,6 and 9 months were associated with higher MTX PG1, MTX-PG2 and total MTX-PG concentrations (table). Analysis in quintiles revealed that MTX-PG1,2 and total MTX-PG concentrations were non-linearly associated with EULAR response. As compared with patients in the lowest quintiles, patients in the highest quintiles had a higher chance to respond. No associations were found between any of the MTX-PG concentrations and adverse events.

Conclusion: In this first longitudinal study, higher MTX-PG1,2 and total MTX-PG concentrations in erythrocytes were associated with MTX response over the first 9 months treatment and are therefore a potential tool for TDM of MTX in RA.

Funds: RDJ: Dutch Arthritis Association (nr. 06-02-402, 09-1-402).

Reference: 1. De Jong PH, et al. Ann Rheum Dis. 2012 Jun 7.