Between 2% and 6% of epithelial ovarian cancer (EOC) patients develop brain metastases (brain mets), which are incurable and invariably result in death. This poor outcome is associated with a lack of established guidelines for the detection and treatment of brain mets in EOC patients. In this study, we characterize an unusual case of low-grade serous ovarian carcinoma (LGSOC) that metastasized to the brain. Using a spatially oriented single-cell proteomics platform, we compared sequential biopsies of a primary tumor with a peritoneal recurrence and brain mets. We identified several targetable oncogenic pathways and immunosuppressive mechanisms that are amplified in the brain mets and could be involved in the progression of LGSOC to the brain. Furthermore, we were able to identify cell populations that are shared between the primary tumor and the brain mets, suggesting that cells that have a propensity for metastasis to the brain could be identified early during the course of disease. Taken together, our findings further a path for personalized therapeutic decisions in LGSOC.

Background: Clear cell ovarian carcinoma (CCOC) is characterized by a distinct histologic and molecular profile, and associated with very poor responses to standard treatment consisting of surgery and carboplatin:taxol chemotherapy. CCOC is chemo-resistant at the time of diagnosis and response to chemotherapy in the recurrent setting is less than 10%. Literature suggests a potential role for immune checkpoint inhibitors (ICI). However, only a subgroup of patients (20%) responds to ICI and little is known about the mechanisms of response and resistance to therapy. We postulate that a better understanding of CCOC tumor microenvironment (TME) could help predict patients’ response to ICI. Objective: The aim of this study is to investigate the relationship between TME immune infiltrate and clinical/outcome in 22 CCOC cases and identify subsets of CCOC who may benefit from immunotherapy. Material & Methods: We characterized the immune landscape of 11 early and 11 advanced CCOC through a multiplex IHC Discovery Platform. Spatial single-cell proteomics analyses (cyclic-IF) and spatially-resolved RNAseq in 10 CCOC cases using an OC tissue microarray (TMA) were performed. Results were corelated with clinical and treatment outcome. Results: The percent of CD8, CD4, CD20 B cells Tregs, PD1, PDL1, monocytes and M2 monocytes and myeloid cells was significantly higher in advanced than early-stage cancers. Recurrent cancers were more immunosuppressive than cases with no recurrence. Tumor infiltrate was dense in 4 cases. Four patients with early-stage disease had a high number of CD8 naïve cells and experienced no recurrence. TME analysis of single case of advanced CCOC (before and after chemotherapy), revealed that hot tumor changed to cold tumor, suggesting the resistance to treatment. Cyclic IF analyses identified 3 tumor phenotype groups in a subset of 10 CCOC present on the OC TMA. The majority of CCOC had high expression of CCNE and high PI3K-AKT-mTor activity, low expression of hormone receptors (AR, ERa, PRg) and low cell cycle activity. Some tumors were also high for HER2. The stromal compartment was enriched in collagen VI, aSMA and PDGFR. The immune monitoring of several of those samples also revealed an immunosuppressive microenvironment, including the presence of M2 macrophages and expression of immune checkpoint proteins PD-L1 and B7-H4. Conclusion: Our study shows that various phenotypes of CCOC are defined by the cancer cell profiles and TME content. Importantly, a strong immunosuppressive microenvironment was detected in many samples, suggesting a potential response to ICI. Furthermore, we detected the expression of several therapeutic targets (MAPK, CCNE, PI3K-AKT-mTOR, HSP90, HER2), including oncogenic signaling pathways (RTK, MAPK). Different tumor phenotypes identified across our CCOC samples suggest that clear cell carcinoma could be subclassified into subtypes that should be treated differently. Citation Format: Tanja Pejovic, Sonali Joshi, Shawn Campbell, Dhanir Tailor, Joanna Pucilowska, Benjamin Tate, Pierre-Valérien Abate, Korina Mouzakitis, Marilyne Labrie, Elizabeth Munro, Jenna Emerson, Sanjay V. Malhotra. Study of tumor microenvironment of ovarian clear cell carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 79.