[Life-threatening flare of an underlying «paraneoplastic» dermatomyositis in a patient with lung adenocarcinoma treated with anti-PD-1 pembrolizuma].

Dermatomyositis is an autoimmune disease mainly characterized by muscle and skin involvement. Its association with cancer is known but the term «paraneoplastic» remains debated. We report here the case of a 71-year-old woman with a new diagnosis of dermatomyositis with, at the same time, the discovery of a lung adenocarcinoma. Lung cancer was treated with pembrolizumab, an immune checkpoint inhibitor directed against the "Programmed cell Death protein 1" (PD-1) receptor. Three weeks later, the patient presented a severe flare of dermatomyositis. Administration of intravenous corticosteroids and infliximab were ineffective. Intravenous immunoglobulins were then administered, followed by subcutaneous methotrexate, with a progressive positive evolution. Flares of pre-existing autoimmune diseases are observed under immune check point inhibitors, even when the evolution of the cancer is favourable. These immune-related adverse events are often «mild to moderate» and severe immune related side effects are not more frequent when the patient has a pre-existing autoimmune disease. Treatment can be maintained in the majority of cases. However, as demonstrated in this clinical case, although immune checkpoint inhibitors are not contraindicated in autoimmune diseases, the presence of myositis requires special attention given the potential severity of flares.