Assessment of predictive biomarker prevalence in molecularly defined adult-type ovarian granulosa cell tumors.

5567 Background: The FDA has separately approved pembrolizumab for all advanced solid tumors with either microsatellite instability or high tumor mutational burden (≥10 mutations per megabase), and homologous recombination deficiency is an established biomarker for response to poly ADP ribose polymerase inhibitors in epithelial ovarian cancer. The detection of these predictive biomarkers in adult-type ovarian granulosa cell tumors could identify novel treatment strategies in this rare disease. The primary objective of this study was to determine the prevalence of established predictive biomarkers among molecularly defined adult-type ovarian granulosa cell tumors. Methods: With institutional review board approval, we performed a cross-sectional study examining de-identified FoundationOne companion diagnostic molecular profiles for 423 women with molecularly defined (FOXL2 c.C402G positive) adult type ovarian granulosa cell tumors. The dataset was comprised of coding variants for up to 406 genes as well as genomic signatures including microsatellite instability, tumor mutational burden, and genome wide loss of heterozygosity. PD-L1 expression by immunohistochemistry was also available for a subset of tumors. Descriptive statistics were used for comparison between groups and all statistical tests were two-sided. Results: Women in this cohort had a mean age of 57 years (range 24-87) at the time of sample submission for molecular profiling. The median tumor mutational burden was 1.3 mutations per megabase [mut/Mb] (range 0-8.8 mut/Mb). TP53-mutated aGCT had a higher tumor mutation burden than TP53 non-mutated tumors (median 2.4 mut/Mb, 95% CI 1.7-3.0 mut/Mb vs median 1.3 mut/Mb, 95% CI 1.5-1.9 mut/Mb; P=.02). All 384 tumors with available microsatellite instability testing were microsatellite stable. Sixty-seven tumors had PD-L1 expression measured and of these 94% (63/67) were negative with the remainder “low positive.” No tumors were positive for genome wide loss of heterozygosity. Apart from FOXL2 c.C402G, the most frequent short variants were TERT promoter mutations (-124C>T: 190/423, 45.0%; -146C>T: 39/423, 9.2%). Other frequently observed variants included truncating mutations in KMT2D/MLL2 (71/423, 16.8%), pathogenic TP53 mutations (35/423, 8.3%), CDKN2A/B deletions (43/423, 10.2%), and activating PIK3CA mutations (23/423, 5.4%). Conclusions: No women with molecularly defined adult-type ovarian granulosa cell tumors in this large cross-sectional study would be eligible for FDA-approved pembrolizumab based on either microsatellite instability or high tumor mutational burden. No tumors exhibited evidence of homologous recombination deficiency and molecularly targetable mutations were rare. The development of novel precision treatment options remains a critical unmet need for this rare disease.