Pembrolizumab is used in advanced gastric and gastroesophageal cancers when tumors are PD-L1 positive.

• A. PD-L1
• B. HER2/neu
• C. CK20
• D. FGR2B

Answer: (A)

The main idea is how pembrolizumab works. It is a PD-1 inhibitor, meaning it blocks the PD-1 receptor on T cells. Tumors that express PD-L1 use this PD-1/PD-L1 interaction to dampen the immune response and avoid attack. By blocking PD-1, pembrolizumab prevents this inhibitory signal, reactivating T cells to recognize and kill tumor cells. In advanced gastric and gastroesophageal cancers, testing for PD-L1 helps identify tumors likely to respond to pembrolizumab, so treatment is given when PD-L1 is present. Other markers like HER2/neu point to different targeted therapies, and they don’t serve the same predictive role for pembrolizumab in this setting.