Abstract
Tarlatamab is a bispecific T-cell engager immunotherapy that targets delta-like ligand 3 and CD3. We report a case of pseudoprogression (PsPD) in a patient with small-cell lung cancer treated with tarlatamab. Due to its mechanism, tarlatamab is known to induce T-cell infiltration into tumors. In PsPD, the tumor transiently enlarges because of massive T-cell accumulation within the tumor. In this case, the peripheral blood lymphocyte count decreased, and the interleukin-6 level was markedly elevated, which was thought to reflect T-cell accumulation in the tumor. These trends in the blood tests may be useful in easily differentiating PsPD from true progressive disease during tarlatamab treatment.