Guillain-Barré Syndrome Following the Administration of Tarlatamab in Extensive-stage Small Cell Lung Cancer: A Case Report with Pathological Confirmation.

We report a rare case of Guillain-Barré syndrome (GBS) following treatment with tarlatamab, a bispecific T-cell engager targeting DLL3 and CD3, in a patient with extensive-stage small cell lung cancer (SCLC). After the initial dose, the patient developed grade 2 cytokine release syndrome, followed by progressive weakness and areflexia. A cerebrospinal fluid analysis, nerve conduction studies, and a nerve biopsy confirmed demyelinating GBS with axonal involvement in the patient. No preceding infection was clinically identified, although serological testing for common infectious triggers was not performed. The temporal association between tarlatamab administration, cytokine release syndrome, and neurological symptom onset suggests a possible treatment-related etiology. This is the first reported case of pathologically confirmed tarlatamab-associated GBS, highlighting the need for early recognition and prompt immunomodulatory therapy in such cases.