Acquired Hemophilia A Secondary to an Immune Checkpoint Inhibitor: A Case Report

We report a case of a patient with extensive-stage SCLC who developed acquired hemophilia A during maintenance atezolizumab therapy. The patient initially presented with asymptomatic anemia, a prolonged acquired prothrombin time, and factor VIII (FVIII) deficiency. Acquired FVIII autoantibodies were detected, confirming the diagnosis of acquired hemophilia. Atezolizumab was ceased and high-dose prednisolone was initiated. He subsequently developed an extensive spontaneous upper limb subcutaneous hematoma and shoulder hemarthrosis despite improving FVIII inhibitor titers on prednisolone. His acute bleeding was successfully treated with recombinant factor VII, and rituximab was added to prednisolone. Given the quiescent malignancy, 16 months of preceding treatment with atezolizumab, and improvement with immunosuppression, a diagnosis of immune checkpoint inhibitor–induced hemophilia A was made. Severe hematologic immune-related adverse events such as this case of acquired hemophilia have rarely been reported in the literature.