Use of azathioprine for non-thymoma myasthenia and risk of cancer: a nationwide case-control study in Denmark.

BACKGROUND AND PURPOSE To evaluate the association between the use of azathioprine and risk of cancer in patients with non-thymoma myasthenia gravis (MG) in a nationwide setting. METHODS Case-control study based on population-based registries. Cases were patients with MG with a first time diagnosis of cancer (except non-melanoma skin cancer) registered during 2000-2009, and controls were patients with MG with no history of cancer. Prior use of azathioprine in cases and controls was assessed through prescription records (1995-2009). We used unconditional logistic regression to calculate odds ratios (ORs) with 95% confidence intervals (CIs) for cancer associated with a high cumulative dose [≥ 1000 defined daily doses (DDD)] or long-term use (≥ 5 years) of azathioprine, compared with never use of the drug and adjusted for potential confounders. RESULTS We identified 89 cases and 873 controls. The prevalence of ever use of azathioprine was similar among cases (39.3%) and controls (39.4%). We observed a slightly elevated OR for cancer overall associated with long-term use of azathioprine (1.22; 95% CI: 0.62-2.40, P = 0.56). The highest ORs were observed for use of 2000 DDD or more of azathioprine; however, these risk estimates were based on small numbers. CONCLUSIONS Use of azathioprine in patients with non-thymoma MG may be associated with a slightly increased risk of cancer overall. Larger studies are necessary to address the risk of site-specific cancers.