Risks of malignant lymphoma in rheumatoid arthritis patients receiving methotrexate-alone and in combination therapy compared with the general population: A study based on a Japanese medical claims database.

Objective: The risk of malignancy in patients with rheumatoid arthritis (RA) treated with methotrexate (MTX) and biological disease-modifying antirheumatic drug (bDMARD) combination therapy is unknown. This study aimed to clarify the incidence of malignancy and the recommended monitoring period in patients receiving this combination therapy.

Materials and methods: A retrospective, observational study based on a large Japanese medical claims database was conducted between April 2013 and February 2020. Patients with RA were classified into MTX-alone and combination therapy groups, and the standardized incidence rates (SIR) of malignancy were calculated. The time of onset of malignancy in both groups was calculated.

Results: In total, 2,052 patients received MTX-alone and 782 received combination therapy. The incidence of malignant lymphoma was significantly higher with MTX-alone therapy (SIR: 6.09, 95% confidence interval (CI): 1.58 - 10.61) and combination therapy (SIR: 20.86, 95% CI: 8.53 - 33.19) than in the general Japanese population. Furthermore, the combination therapy had a significantly higher risk of malignant lymphoma than the MTX-alone therapy (adjusted odds ratio: 4.27, 95% CI: 1.64 - 11.12). The median time from MTX prescription to the onset of malignant lymphoma was 3.58 years (interquartile range (IQR): 2.00 - 5.34 years) for MTX-alone and 3.42 years (IQR: 1.25 - 4.92 years) for combination therapy.

Conclusion: The incidence of malignant lymphoma in the combination therapy group was extensively higher than that in the general Japanese population. Special attention is required for early symptoms of malignant lymphoma, particularly in the 3^rd - 4^th year after initiating MTX therapy.