Outcomes of Third-Line Trastuzumab Deruxtecan in a Patient with De Novo Stage 4 HER2-Positive Gastric Adenocarcinoma with Enteroblastic Differentiation: A Case Report

This case report describes the treatment of a patient diagnosed with de novo stage 4 human epidermal growth factor 2 (HER2)-positive gastric adenocarcinoma with enteroblastic differentiation (GAED), a rare and aggressive form of gastric cancer characterized by a tubulopapillary growth pattern and enteroblastic cell lineage markers such as GPC3, SALL4, and alpha fetoprotein. Given the patient’s symptomatic, advanced-stage cancer, treatment objectives were focused on effectively deterring disease progression and ameliorating symptoms throughout the anticipated multiple lines of therapy. Subsequent to standard first- and second-line therapies for HER2-positive metastatic GC, third-line treatment using the antibody-drug conjugate trastuzumab deruxtecan (T-DXd) for seven cycles resulted in satisfactory tumor control and well-preserved physical performance and quality of life, with minimal hematologic and pulmonary toxicities. The patient retained acceptable physical performance to receive subsequent lines of therapies, and still showed a tumor marker response to 5L trastuzumab-based chemotherapy. As the tumor was positive for both HER2 and programmed death-ligand 1 (PD-L1) expressions, the selection and sequencing of anti-HER2 and anti-PD-L1 therapies were discussed in relation to the latest U.S. Food and Drug Administration approvals and trial results.