Abstract 1986: Acquisition of carboplatin resistance corresponds with reduced sacituzumab govitecan efficacy in triple negative breast cancer patient derived xenografts

Abstract Sacituzumab Govitecan (SG) is an antibody drug conjugate that targets the epithelial glycoprotein Trop-2. SG has been approved for the treatment of patients with triple negative breast cancer (TNBC) or estrogen receptor positive (ER+) breast cancer after they have received at least two previous systemic treatments. The objectives of this study were to identify biomarkers of SG response using breast cancer patient-derived xenografts (PDXs) and determine if SG efficacy changes when cells become carboplatin resistant (CR). Analysis of transcriptomic and proteomic levels of Trop2 using bulk RNAseq, scRNA-seq, and tandem mass tag spectrometry found that overall ER+ PDXs had significantly lower RNA (p-value less than 0.001) and protein (p-value less than 0.001) expression of Trop-2 than TNBC PDXs. Immunohistochemical assessment of 42 TNBC and 27 ER+ patient tumor samples identified variation in Trop-2 expression; within each cohort, expression varied from highly positive to completely negative. To test the association of Trop-2 with drug responsiveness, cells from 19 different PDXs were cultured in vitro and administered increasing doses of SG. TNBC cells were more responsive to SG treatments compared to ER+ cells (p-value equaling 0.005). Correlation analysis identified a positive relationship of drug response with protein abundance which was stronger in TNBC samples. In vivo studies with 7 TNBC PDXs found that over 70% were highly susceptible to SG treatments, with some tumors being completely eradicated or exhibiting a total inhibition of tumor growth which resulted in a long-term durable response after 10 weeks of treatment. In vivo across all the PDXs, Trop-2 expression alone did not strongly correlate with SG treatment success. Interestingly, CR sublines derived from 2 carboplatin sensitive PDXs were significantly less responsive to SG than their parental PDXs (p-value less than 0.001). These two CR sublines were also significantly less sensitive to the majority of 555 other drugs identified by the NCI compared to their parental PDXs. Ongoing studies are defining mechanisms of reduced SG efficacy in CR models. Overall, these studies find that SG is highly effective in many TNBC PDX models and should be utilized earlier in treatment protocols, especially in metastatic patients. Citation Format: Carson J. Walker, Julia E. Altman, Emily K. Zboril, Rachel K. Myrick, Nicole S. Hairr, David C. Boyd, Bin Hu, Mikhail G. Dozmorov, J. Chuck Harrell. Acquisition of carboplatin resistance corresponds with reduced sacituzumab govitecan efficacy in triple negative breast cancer patient derived xenografts [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1986.