Abstract P4-07-12: Development of Triple-negative breast cancer (TNBC) syngeneic models and TROP2-directed antibody-drug conjugate (ADC) surrogate to model therapeutic combinations

Abstract Background: Sacituzumab govitecan (SG, Trodelvy®) is a human trophoblast cell surface antigen 2 (TROP-2) directed antibody drug conjugate (ADC) coupled to an active form of irinotecan (SN-38) via our novel hydrolyzable linker (CL2A). SG is the only FDA-approved ADC treatment for TNBC patients in the second-line setting. TROP-2 is a transmembrane protein encoded by the tumor-associated calcium signal transducer 2 (TACSTD2) gene and highly expressed in TNBC, an aggressive type of cancer accounting for approximately 15% of all breast cancers. TROP-2 overexpression is associated with poor survival and relapse, but its biological function in TNBC remains poorly understood. Hypothesis/rationale: To better understand TROP-2 and TROP-2-directed ADC biology, we developed and characterized TROP2high vs TROP2low TNBC syngeneic tumors and an SG surrogate directed to murine TROP-2. Experimental design: We established 2 syngeneic TNBC models with differential TROP-2 expression: 4T1 cells were flow sorted into high (>95%) vs low (< 7%) TROP-2 expressors and EMT6 cells were transduced with a murine TACSTD2-encoding lentivirus. Balb/c mice were subcutaneously implanted with 0.5 × 10 E6 TROP-2high, TROP-2low, or parental tumor cells (4T1 or EMT6). Tumor immunophenotyping and transcriptomic analyses were performed 15 and 24 days after implantation. An SG mouse surrogate was engineered to mimic SG, using an anti-TROP-2 antibody (Rab64) that cross-reacts with human and murine TROP-2 covalently attached to SN-38 by the CL2A linker. SG surrogate activity was characterized in vitro and in 4T1 syngeneic models. Results: SG surrogate demonstrated high affinity for human and mouse TROP-2 (KD=1.1 and 1.4 nM, respectively) with SN-38 release rates and PK similar to that of SG. Flow cytometry analysis after bulk cell sorting of 4T1 or lentivirus transduction of EMT6 confirmed high TROP2 expression after at least 3 in vitro passages. Fifteen days after subcutaneous implantation, flow cytometry analysis of tumor single-cell suspensions revealed significant differences in immune infiltrates between 4T1-derived tumor groups (n=5/group; mean percentages in TROP2high vs TROP2low 4T1-derived tumors of cells expressing CD45: 65% vs 10%, P < 0.0001; CD8: 5.5% vs 1%, P = 0.0033; CD4: 22% vs 4%, P = 0.0055; macrophages: 12.5% vs 2.5%, P = 0.0002; myeloid cells: 52% vs 75%, P = 0.0066). In addition, TROP2high 4T1-derived tumors were smaller and had significantly less necrosis than TROP2low and unsorted 4T1-derived tumors 25 days after implantation. Finally, transcriptomics analyses of TROP2high vs TROP2low 4T1-derived tumors demonstrated the association of TACSTD2 expression levels with regulation of distinct molecular pathways. Conclusion: Syngeneic tumors derived from 4T1 cells with differential TROP2 expression levels are associated with differential cellular states and tumor microenvironment composition. In contrast, no significant phenotypic changes were observed in tumors derived from TACSTD2-transduced compared with mock-transduced EMT6 cells. Taken together, these results suggest that expression of the TACSTD2 gene is associated with, but not causative of, different tumor phenotypic states. Additional studies to investigate TROP-2 expression as a correlative marker of patient prognosis and the antitumor immune response are warranted. The effects of in vivo treatment with an SG surrogate on 4T1 tumor growth and immune phenotype will be discussed at the time of the presentation. Citation Format: Chih-Chien Chou, Jordan Kardos, Becky Yang, Jessica Orf, Rutwij Dave, Yurong Lai, Chingwei V. Lee, Giuseppe A. Papalia, Kelli Boyd, Lauri Diehl, Nathalie Scholler. Development of Triple-negative breast cancer (TNBC) syngeneic models and TROP2-directed antibody-drug conjugate (ADC) surrogate to model therapeutic combinations [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-07-12.