Cancer risk for multiple sclerosis patients treated with azathioprine and disease-modifying therapies: an Italian observational study.

Background: The risk of malignancy associated with sequential disease-modifying therapies (DMTs) for patients with multiple sclerosis (MS) is uncertain. The aim of this study was to analyze the risk of cancer in patients with MS treated with azathioprine (AZA) and the influence of sequential DMTs on the risk.

Method: We retrospectively enrolled a cohort of AZA-treated MS patients followed in two Italian centers from 1987 to 2019. The ratio between observed and expected cancers in the Italian general population was calculated as standardized incidence ratio (SIR). Associations between AZA and DMTs and cancer were estimated by Cox proportional hazards model.

Results: We identified 500 AZA-treated MS patients, followed for a median time of 9.7 (0.1-45.7) years: 61.8% of them were treated with DMTs. We found 22 cases of cancer (4.4%). The SIR was 1.14 (95% CI 0.98-1.29), not significantly increased in comparison with the general population. However, the risk was significantly higher in the quintiles of age 32-45, SIR 1.21 (95% CI 1.21-1.42), and 46-51, SIR 1.11 (95% CI 1.11-1.32) than in older cases. Age at AZA treatment onset was the only covariate significantly related to cancer incidence (HR = 1.049, 95% CI 1.007-1.093). The exposure to other DMTs did not modify the risk.

Conclusion: The risk of malignancy in MS patients after AZA was similar to that of the general population and did not change with other DMTs sequential treatments. The increased risk in the younger ages should be considered in treatment assessment.