Microsatellite Instability as a Prognostic Marker for Resectable Gastric Cancer and a Potential Biomarker for Perioperative Chemotherapy Benefit

Dr. Osama Rahma, an Assistant Professor of Medicine at Harvard Medical School, Dana-Farber Cancer Institute, discusses the role of MSI in gastric cancer. TRANSCRIPT This JCO Podcast provides observations and commentary on the JCO article "Individual Patient Data Meta-Analysis of the Value of Microsatellite Instability as a Biomarker in Gastric Cancer" by Pietrantonio et al. My name is Osama Rahma, and I am an Assistant Professor of Medicine at Harvard Medical School, Dana-Farber Cancer Institute in Boston, Massachusetts. My oncologic specialty is Gastrointestinal Cancer and Cancer Immunotherapy. Despite the advances in surgical approach and perioperative multimodalities in gastric cancer, the recurrence rate remains over 50%. While many patients benefit from perioperative chemotherapy many others don't derive such benefits and experience toxic side effects. Accordingly, there is an unmet need to identify prognostic markers to guide  chemotherapy decisions in the neoadjuvant and adjuvant settings. Microsatellite instability has been well established as a prognostic marker in colon cancer that correlates with better overall survival and potential lack of benefit from adjuvant chemotherapy in stage II disease. The question is whether microsatellite instable gastric tumors have similar behavior? To answer this question, subgroups analyses of patients with MSI high tumors were performed in both the MAGIC and the CLASSIC trials which established perioperative ECF chemotherapy in non-Asian population and adjuvant capecitabine and oxaliplatin in Asia, respectively as the standard of care in resectable gastric cancer. Although MSI tumors were found to be less likely to benefit from chemotherapy, the low incidence of MSI tumors in both trials of 8-10% made it very challenging to draw  meaningful conclusions.   In the article that accompanies this podcast, Pietrantonio et al performed individual patient level data meta-analysis testing the prognostic significance of microsatellite instability in both the MAGIC and CLASSIC trials. In addition, the analysis included the ARTIST study, which tested adding radiation to adjuvant capecitabine and cisplatin in Asia and the ITACA-S study which tested adjuvant FOLFIRI, docetaxel and cisplatin vs 5-fluorouracil and folinic acid in Europe in radically resected gastric cancer. The meta-analysis included 1556 patients, 63% Asian and 37% Europeans. Patients were stratified using microsatellite instability status and based on whether they received multimodality therapy in all 4 trials or surgery only in the MAGIC and CLASSIC trials. 7.8% of patients had MSI high tumors and  70.8 % received multimodality treatment. Consistent with prior reports of individual trials, patients with MSI high tumors had better outcomes compared to MSI low or MSS tumors. Specifically, the 5-year disease free survival rate was 71.8% vs 52.3% and 5-year overall survival was 77.5% vs 59.3%. MSI remained a significant independent prognostic factor for improved disease-free survival and overall survival in a multivariate analysis in addition to Asian ethnicity, low TNM staging, and receiving multimodality therapy. In addition, MSI was noted to be more prevalent in the Asian population and was a better prognostic factor in Asians compared to Europeans.   Importantly, MSI high tumors did not benefit from perioperative chemotherapy with 5-year disease free and overall survival of 70% and 75%, respectively for those who received chemotherapy compared to 77% and 83% for MSI high patients who did not receive chemotherapy. In contrast patients with MSI low or MSS experienced benefit from perioperative chemotherapy compared to surgery alone. Finally, MSI status did not affect post-recurrence survival. However, this was limited by the small number of MSI high patients who experience disease recurrence.   This study has several limitations including its retrospective nature, the relatively small number of patients with MSI high tumors of 121, and the heterogenicity of administered treatment in the 4 trials including different regimens of neoadjuvant and adjuvant chemotherapy with or without radiation. In addition, D1 surgery was performed in Europe while D2 surgery was performed in Asia. In summary, I think this study supports the adoption of microsatellite testing as a routine biomarker in patients with operable gastric cancer. The result of microsatellite testing should be discussed in a multidisciplinary fashion since perioperative chemotherapy may result in more harm than benefit in MSI high tumors. This study among others highlights the unique behavior of microsatellite instable tumors which is driven by their unique immune profile including increased T-cell infiltration and activation due to higher mutational load and neoantigens compared to microsatellite stable tumors. PD-1 inhibitors are effective in more than 50% of MSI tumors in the advanced setting and in 15-25% of gastric tumors with positive PD-L1 expression.  Accordingly, future clinical trials should stratify gastric cancer patients based on their microsatellite status and PD-L1 expression while incorporating immunotherapy in the early disease setting.   This concludes this JCO Podcast. Thank you for listening.