Prognostic and Predictive Value of Microsatellite Instability, Inflammatory Reaction and PD-L1 in Gastric Cancer Patients Treated with Either Adjuvant 5-FU/LV or Sequential FOLFIRI Followed by Cisplatin and Docetaxel: A Translational Analysis from the ITACA-S Trial

Background: Patients with high microsatellite instability (MSI) gastric cancer (GC) show improved survival and no benefit or harm from adjuvant and/or perioperative chemotherapy. The role of immune microenvironment in GC is largely unknown.

Materials and methods: In the present study, 256 tumor tissue blocks were centrally collected from patients enrolled in ITACA-S, a randomized adjuvant trial of 5-FU/LV versus sequential FOLFIRI and cisplatin-docetaxel. MSI status was assessed by multiplex PCR, inflammatory reaction by H&E morphological assessment, and programmed death-ligand 1 (PD-L1) expression by immunohistochemistry.

Results: Overall, 9% patients had MSI-high tumors, 23% had high inflammatory reaction, 11% had tumor PD-L1 ≥ 1%, and 11% had stromal PD-L1 ≥ 1%. A significant association with disease-free survival (DFS) and overall survival (OS) was found for MSI-high (hazard ratio [HR], 0.43; p = .02; HR, 0.40; p = .02) and high inflammatory reaction (HR, 0.55; p = .010; HR, 0.53; p = .008) but not for PD-L1. At multivariable analysis, only MSI showed an independent association with both DFS (p = .02) and OS (p = .01), whereas inflammatory reaction showed an independent association only with OS (p = .04). Patients with tumor PD-L1 ≥ 1% had a significantly longer DFS in sequential chemotherapy than in than 5-FU/LV arm (interaction p = .04) and a trend for OS (interaction p = .12).

Conclusion: Our data suggest that MSI status could be a useful prognostic biomarker in patients with radically resected stage II-III GC and should be used as stratification factor in future trials. Tumor PD-L1 ≥ 1% should be further investigated as a potential predictor of benefit from intensive chemotherapy.

Implications for practice: In this post hoc analysis of patients with radically resected gastric cancer randomized to an intensive sequential chemotherapy regimen versus 5-FU/LV monotherapy as adjuvant treatment in the ITACA-S trial, MSI-high status was independently associated with better disease-free survival and overall survival (OS) and inflammatory reaction was independently associated with better OS. Moreover, tumor PD-L1 expression ≥1% was associated with greater benefit from intensive sequential chemotherapy compared with 5-fluorouracil plus leucovorin (5-FU/LV), whereas PD-L1 expression <1% was not, conditioning a statistically significant interaction between such biomarker and treatment arms. The meta-analysis of individual patients' data from available studies could yield data on the role of MSI status that could inform clinical decisions.