Hyperprogressive Disease in Lung Cancer with Transformation of Adenocarcinoma to Small-cell Carcinoma during Pembrolizumab Therapy

Hyperprogressive disease (HPD) is a paradoxical phenomenon involving the acceleration of tumor progression after treatment with immune checkpoint inhibitors (ICIs). A 66-year-old male smoker with advanced lung adenocarcinoma started pembrolizumab for progressive disease following first-line chemotherapy. He developed HPD after two cycles, and a re-biopsy revealed transformation to small-cell carcinoma. He subsequently underwent two lines of chemotherapy for small-cell carcinoma until progression and ultimately died. Transformation to small-cell carcinoma may be a cause of HPD during ICI therapy. The possibility of pathological transformation should be considered in cases of HPD with resistance to ICI therapy.