Small Cell Lung Cancer Transformation as a Mechanism of Resistance to PD-1 Therapy in KRAS-Mutant Lung Adenocarcinoma: A Report of Two Cases

KRAS G12C–mutant metastatic lung adenocarcinoma was diagnosed in a 67-year-old woman with a 50-pack-year smoking history (Fig. 1). The patient responded to first-line carboplatin and paclitaxel but then progressed 19 months after diagnosis. At first progression she declined a biopsy and started taking nivolumab. Two weeks after her 36th cycle of nivolumab she experienced widespread progression, including development of a pericardial effusion with tamponade requiring pericardiocentesis. The pericardial fluid and pleural fluid from a new effusion revealed SCLC.