Microsatellite instability in sarcomas: Which tests are ideal for diagnosis, and a systematic review

e23519 Background: There is a lack of clinically relevant biomarkers to inform immunotherapy for patients with advanced sarcoma. High levels of microsatellite instability (MSI-H) is a histology-agnostic biomarker of response to pembrolizumab. However, the frequency of MSI-H in sarcoma is low - ranging from < 0.5% to 4%. MSI status can be assessed through Next Generation Sequencing (NGS), immunohistochemistry (IHC), or PCR followed by fragment analysis with capillary electrophoresis (PCR). To date, both the College of American Pathologists and the European Society of Medical Oncology have not determined the best method to evaluate MSI in sarcomas. We performed a systematic review to evaluate the methods to diagnose MSI-H in sarcomas and to evaluate the impact of this biomarker in treatment with pembrolizumab. Methods: We performed a literature search using the PubMed database to identify all English full-text available articles between January 2020 and December 2022, with the terms “sarcoma and microsatellite instability” and “sarcoma and pembrolizumab”. One independent reviewer screened the studies, two independent reviewers analyzed the selected studies, and disparities were resolved with a fourth reviewer. Results: Concerning the diagnosis of MSI-H in sarcomas, 187 abstracts were found, and 31 articles were identified for full analysis. MSI status was reported for 7958 patients - 7762 with a single test and 196 with more than one test. NGS was used for 6649, IHC for 1222, and PCR for 285. MSI-H tumors were 25 (0.4%), 38 (3.1%), and 21 (7.33%) for each technique, respectively. Two investigators performed the three available tests to evaluate concordance, with one concordant case and one discordant case. 194 cases were evaluated for both IHC and PCR, with 4 discordant results. No article that described NGS results performed a second test. To analyze the results of pembrolizumab therapy in MSI-H sarcomas, we analyzed 199 abstracts of which we selected 13 prospective and 31 retrospective papers for full analysis. The K cohort of Keynote-158 is the only prospective trial concerning therapies guided by MSI-H in sarcoma (n = 14 patients, ORR = 33%). Regarding retrospective data, we identified 2 case reports of the use of pembrolizumab in patients with advanced sarcoma – both achieved a response to therapy. Conclusions: There is limited data to establish the best diagnostic test for MSI-H in patients with sarcoma. The concordance between NGS, IHC, and PCR seems to be high, but discrepancies have been described and need to be further evaluated. There is only one prospective trial that enrolled patients with sarcomas utilizing MSI-H as a biomarker. Albeit only a small number of patients were enrolled (n = 14), efficacy is consistent with that seen in other histologies, and scarce retrospective data corroborate the potential role of MSI-H as a predictive biomarker of pembrolizumab efficacy in sarcoma.