Drug interaction between methotrexate and salazosulfapyridine in Japanese patients with rheumatoid arthritis

BACKGROUND: Methotrexate (MTX) and salazosulfapyridine (SASP) are disease-modifying drugs that are commonly used in the treatment of rheumatoid arthritis (RA), and combination therapy with MTX and SASP is recommended for RA patients who show an inadequate response to monotherapy with either drug. This study was designed to examine the interaction between the two drugs from the viewpoint of serum MTX concentration in Japanese RA patients, who were receiving combination therapy with relatively low doses of MTX and SASP. METHODS: This is a 24-week open-label intervention study of stable RA patients (n = 10) with low disease activity. In these patients, who had received SASP/MTX combination therapy for at least 12 weeks, SASP was discontinued, and the patients received MTX monotherapy for the next 24 weeks. The primary outcome was change of serum MTX concentration at 12 weeks after discontinuation of SASP. Two disease activity markers, simplified disease activity index (SDAI) and disease activity score-C reactive protein (DAS28-CRP), were assessed as secondary outcomes at 24 weeks after discontinuation of SASP. We also monitored levels of matrix metalloproteinase-3 (MMP-3) and inflammatory cytokines. Patients were asked to complete a questionnaire after the study. RESULTS: Serum MTX concentration in RA patients who discontinued SASP increased more than 2-fold within 4 weeks, and the higher level was maintained thereafter. No significant differences were detected in SDAI, DAS28-CRP, MMP-3 or inflammatory cytokines. Most participants reported no change in physical condition after withdrawal of SASP, and most preferred MTX monotherapy for future treatment. CONCLUSIONS: Withdrawal of SASP from patients receiving SASP/MTX caused a rapid, marked increase of serum MTX concentration, without any apparent change in disease parameters or side effects. Our results suggest that SASP can be discontinued without adverse effects in stable RA patients receiving combination therapy, at least among Japanese patients receiving relatively low doses of the two drugs. TRIAL REGISTRATION: UMIN000024507. October 21, 2016 retrospectively registered.