Circulating Tumor Cells Predict Response to the DLL3-Targeting Bispecific Antibody Tarlatamab

The bispecific antibody tarlatamab recruits T cells to cancers expressing the neuroendocrine epitope DLL3. Tarlatamab is effective in small cell lung cancer (SCLC), but clinical outcomes vary, and no biomarkers enable patient selection. Single-cell RNA sequencing of SCLC biopsies identifies heterogeneity in DLL3 expression, and analysis of circulating tumor cells (CTCs) distinguishes individual patients as predominantly DLL3Pos or DLL3Low. In a prospective cohort of 20 patients, pretreatment DLL3 expression on CTCs predicts tarlatamab clinical benefit (85% sensitivity, 100% specificity). Necrotic CTC clusters in blood accompany treatment-induced tumor lysis. Acquired resistance to tarlatamab is associated in some cases with loss of DLL3 expression, but persistence of other targetable neuro-endocrine epitopes; in other patients, DLL3 is retained on CTCs, but accompanied by systemic markers of T cell dysfunction. Quantitation of DLL3-positive CTCs identifies patients likely to benefit from tarlatamab, and longitudinal monitoring may guide therapeutic decision-making at the time of acquired resistance.