Abstract A65: Spatial characterization of drug resistance in ovarian cancer

As we strive to prolong patient survival, the advent of targeted therapy for the treatment of ovarian cancer has significantly added to our armamentarium. Unfortunately, both chemotherapy and molecularly targeted PARPi approaches share the overarching limitation of the emergence of drug resistance. One key aspect towards realizing the potential of targeted therapies is a better understanding of the intrinsic and acquired resistance mechanisms that limit their efficacy. Through comprehensive genomic analysis of post-treatment patient samples, we recently identified the most common mechanism of acquired drug resistance in high-grade serous ovarian cancer (HGSC) to date, a transcriptional fusion involving ABCB1. ABCB1 encodes P-gp also known as multidrug resistance protein 1 (MDR1), a multi-transmembrane domain protein that is a member of the superfamily of ATP binding cassette (ABC) transporters involved in the cellular efflux of chemotherapeutic drugs. The SLC25A40-ABCB1 fusion was associated with upregulation of ABCB1 expression, whilst leaving the predicted ABCB1 protein unaltered. Interestingly, fusion events were only detected in patients who had been exposed to chemotherapies that are known substrates of P-gp, with the probability of fusion events closely correlated to the number of lines of P-gp substrate chemotherapy. Surprisingly, WGS analysis of patient samples revealed that not all tumor cells in fusion-positive patients carry the fusion. An intriguing possibility is that resistance within tumor sites is spatially ordered rather than random. Identifying such patterning could explain why tumor eradication has not been effective for the majority of HGSC patients to date. To address the subclonal localization and spatial patterning of ABCB1 fusions in HGSC, CASCADE (rapid autopsy program), biopsy specimens, and PDX tissue are being prescreened (qRT-PCR) to identify those with the highest levels of ABCB1 expression and thus most likely to harbor fusions. In situ DNA and RNA detection assays are being employed to identify ABCB1 fusions. To examine whether fusion negative cells also overexpress P-gp, IHC analysis will then be conducted to co-register fusion positivity and protein expression. We have successfully identified fusion events using ACD BaseScope technology and examined the localization of key HGSC genes including CCNE1 and ABCB1 through PCR FISH assays. In summary, this study will decipher the diversity of resistance mechanisms within individual HGSC patients, thereby providing critical information required for next-generation chemotherapy and PARPi clinical trials aimed at reversing or bypassing acquired resistance. Citation Format: Kathleen I. Pishas, Elizabeth L. Christie, Jessica A. Beach, Kathryn Alsop, Alison Freimund, Nidhi Vashistha, Niyati Jhaveri, Emerald Doolittle, Wei Wei, Bingqing Zhang, Xiao-Jun Ma, David D.L. Bowtell. Spatial characterization of drug resistance in ovarian cancer [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research; 2019 Sep 13-16, 2019; Atlanta, GA. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(13_Suppl):Abstract nr A65.