Clinical outcomes of MSI-high (MSI-H) versus stable (MSS) endometrial carcinoma (EC) after front-line platinum chemotherapy and subsequent matched therapy.

Abstract

6086 Background: Precision oncology approaches in EC patients (pts) are evolving with emerging targeted therapy. We reviewed the effect of genomic findings on treatment choice and outcome in an Asian EC cohort. Methods: Recurrent or metastatic ECs were prospectively profiled with next-generation sequencing (NGS) and relevant immunohistochemistry. Clinical data were collected to assess outcomes. Results: Between 12/2014 to 12/2019, 51 Asian EC pts of endometrioid (26/51), serous (7/51), carcinosarcoma (4/51), clear cell (4/51) and mixed (10/51) histology were enrolled. 35/51(69%) of tumors were high grade. The median age at diagnosis was 56 (range 37-77), and the median lines of treatment received was 3 (range 1-8). 45/51(88%) of pts had successful NGS profiling, 31/45(69%) using FoundationOne CDx, and 14/45 (31%) on an in-house platform. Frequent mutations (>20%) occurred in PTEN (60%), PIK3CA (49%), TP53 (46%), ARID1A (27%), CTNNB1 (24%) and KRAS (22%). There were 12/51(24%) MSI-H, 25/51(49%) MSS, and 14/51(27%) MSI-unknown tumors. The 6 mth progression free survival (PFS) rate for MSS versus MSI-H pts treated with front-line carboplatin+paclitaxel (CP) was 83% versus 50% (RR 1.67, fisher’s exact 2-sided p=0.09), with a shorter median PFS after 1st line CP for MSI-H versus MSS pts (median 5.2 mth vs. 8.3 mth, not sig). Upon progression, 29/51(57%) of pts were matched to therapy based on tumor profiles. Of these, 7/29(24%), 13/29(45%) and 9/29(31%) matched to anti-PD1/PD-L1, endocrine therapy and other targeted therapy, respectively. Among 7 MSI-H pts matched to anti-PD1/PD-L1 therapy, median PFS was 14.6 mth (95% CI 0.4-29), and objective response rate was 57%(4/7). In subsequent-line, matching to endocrine therapy (HR 4.3 95% CI 0.95-19.0, p=0.06) or other targeted therapy (HR 5.1, 95% CI 1.1-24.5, p=0.04) was associated with worse PFS compared to anti-PD1/PD-L1 therapy. Despite a short median PFS after front-line CP, median overall survival (OS) was not reached for MSI-H pts, compared to 38 mth (95% CI 30.7-46.0) for MSS and MSI-unknown pts. Conclusions: MSI-H EC pts appear to have shorter PFS to front-line CP chemotherapy compared with MSS pts, but may derive durable responses from immunotherapy in subsequent-line therapy. Early use of immunotherapy in advanced MSI-H EC pts should be considered. Further optimisation of therapy is urgently needed in advanced MSS EC.