Grade 4 thrombocytopenia with PD-1 inhibitors following progression of PD-L1 inhibitor therapy in small cell lung cancer: a case report and literature review.

Immune checkpoint inhibitors (ICIs) are now a standard treatment for small cell lung cancer (SCLC). In certain cases, cross-line ICI therapy may still provide clinical benefit after disease progression; however, reports detailing the associated adverse events—particularly hematologic toxicities—remain scarce. We present a case of grade 4 immune thrombocytopenia that occurred in a patient with extensive-stage SCLC shortly after switching from durvalumab (anti-PD-L1) to camrelizumab (anti-PD-1) upon progression. The diagnosis was confirmed by strongly positive anti-platelet antibodies (4+) after excluding other causes. Combination therapy with high-dose methylprednisolone and intravenous immunoglobulin, supplemented with recombinant human thrombopoietin (rhTPO) in accordance with Chinese clinical guidelines, led to rapid platelet recovery within one week. No recurrence occurred during subsequent non-ICI chemotherapy, strongly supporting a probable association with camrelizumab. To our knowledge, this represents the first documented case of severe thrombocytopenia occurring in the context of and likely triggered by camrelizumab following durvalumab-to-camrelizumab sequence in SCLC, which highlights the potential for severe irAEs upon switching ICI classes.