# Pseudoprogression Associated With T-cell Infiltration During Tarlatamab Therapy for Small-Cell Lung Cancer: A Case Report

## TL;DR

This case report describes pseudoprogression in a small-cell lung cancer patient treated with tarlatamab, showing how T-cell infiltration can temporarily enlarge tumors.

## Contribution

The study highlights blood test trends as potential indicators to distinguish pseudoprogression from true disease progression during tarlatamab therapy.

## Key findings

- The patient experienced pseudoprogression due to T-cell infiltration during tarlatamab treatment.
- Peripheral blood lymphocyte count decreased while interleukin-6 levels increased, suggesting T-cell accumulation in the tumor.
- Blood test trends may help differentiate pseudoprogression from true disease progression.

## 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.