Early Pseudoprogression After Tarlatamab in Small-Cell Lung Cancer: A Case Report.

Pseudoprogression, a transient radiographic flare caused by immune infiltration, is common after immune-checkpoint inhibitors but has not been reported with tarlatamab, a bispecific T-cell engager approved for third-line small-cell lung cancer (SCLC). A 57-year-old woman with extensive-stage SCLC and syndrome of inappropriate antidiuretic hormone secretion (SIADH) received tarlatamab. Within hours, she developed bone pain; Day 7 imaging showed marked tumour swelling and pleural effusion despite negative cytology and rising serum sodium. Therapy continued. By Day 13, computed tomography demonstrated regression of thoracic and hepatic lesions and falling pro-gastrin-releasing peptide (pro-GRP). Early pseudoprogression and paraneoplastic biomarker improvement may predict efficacy.