Use of Azathioprine for Non-Thymoma Myasthenia and Risk of Cancer: A Nationwide Case-Control Study in Denmark (P02.191)

OBJECTIVE: To evaluate the association between use of azathioprine and risk of cancer in non-thymoma myasthenia gravis (MG) patients in a nationwide setting. BACKGROUND: Acquired MG is an autoimmune disorder characterized by muscle weakness and fatigability. Treatment often includes long-term exposure to immunosuppressants, including azathioprine. An increased cancer risk associated with use of azathioprine has been reported among patients with inflammatory bowel disease and organ recipients, but only few studies have addressed this pertinent question in MG patients. Furthermore, these studies are hampered by methodological issues reducing comparability. DESIGN/METHODS: Case-control study based on Danish population-based registries. Cases were MG patients with a first time diagnosis of cancer (except non-melanoma skin cancer) during 2000-2009 and controls were MG patients with no history of cancer, selected by incidence density sampling. Prior use of azathioprine in cases and controls was assessed through prescription records (1995-2009). We used unconditional logistic regression to calculate odds ratios (OR) with 95% confidence intervals (CI) for cancer associated with a high cumulative dose (≥150 g) or long-term use (≥5 years) of azathioprine, 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 overall cancer associated with long-term use of azathioprine (1.22; 95% CI: 0.62-2.40) compared with never use of the drug. In addition, use of ≥300g azathioprine was associated with an increased risk of cancer, although the numbers were small. CONCLUSIONS: Use of azathioprine in non-thymoma MG patients may be associated with a slightly increased risk of overall cancer. Larger studies are warranted to address the risk of site specific cancers. Disclosure: Dr. Pedersen has nothing to disclose. Dr. Pottegaard has nothing to disclose. Dr. Hallas has received personal compensation for activities with the Association of Danish Pharmaceutical Industry. Dr. Friis has nothing to disclose. Dr. Hansen has received royalty payments from ThermoFisher Scientific. Dr. Jensen has nothing to disclose. Dr. Gaist has nothing to disclose.