Myositis – A common but underreported adverse effect of osimertinib: Case series and review of the literature

Lung cancer is the second most common cancer in both men and women and the leading cause of cancer death worldwide. The development of novel tyrosine kinase inhibitors (TKIs) represented a paradigm shift in the management of lung cancer and has resulted in markedly prolonged survival. Osimertinib is a TKI that was fast-tracked by the United States Food and Drug Administration in 2015 and subsequently approved for the treatment of metastatic epidermal growth factor receptor T790M mutation-positive non-small cell lung cancer. However, despite the generally favorable outcomes associated with osimertinib, rapid development and deployment of any new drug increases the risk of unforeseen adverse effects. Post-marketing surveillance studies therefore play an important role in further elucidating the risks and benefits of novel anti-neoplastic agents. We describe four patients with non-small cell lung cancer who developed myositis after beginning treatment with osimertinib. In addition, we review the literature on osimertinib-associated myositis. Using PubMed, the following terms were searched and relevant citations assessed: creatine phosphokinase, myositis, osimertinib, rhabdomyolysis, osimertinib, and Tagrisso. Thirty-eight patients were treated with osimertinib in our community clinic. Four with non-small cell lung cancer developed myositis after beginning treatment. The onset of symptoms and/or elevation of creatine phosphokinase occurred between two weeks and eleven months after osimertinib was initiated. Alternative causes for myositis were not identified. In two patients, myositis resolved within one month of withdrawing treatment. Two other patients continued osimertinib treatment with close monitoring. Myositis is a serious and potentially underreported adverse effect of osimertinib. Previous studies suggest that osimertinib-associated myositis is rare, occurring in less than 1% of patients. However, myositis occurred in over 10% of patients treated with osimertinib in our clinic population. We suggest regular monitoring for myositis among patients being treated with osimertinib and dose-reduction or cessation of treatment if clinically indicated.