Abstract P5-01-04: Correlation of HER2 receptor expression and in vivo activity of the HER2-targeting therapies trastuzumab deruxtecan (DS-8201a) and T-DM1 activity in a panel of breast XPDX models

Abstract Background: 15-20% of metastatic breast cancers are characterized by overexpression or amplification of human epidermal growth factor receptor 2 (HER2). First-line treatment for HER2-positive metastatic breast cancer includes the anti-HER2 monoclonal antibodies trastuzumab and pertuzumab in combination with chemotherapy. Second-line therapy includes T-DM1, an antibody-drug conjugate (ADC) consisting of trastuzumab conjugated to maytansinoid (DM1). Until recently, patients who progress on T-DM1 were left with few treatment options. Trastuzumab deruxtecan (DS-8201a) is an antibody-drug conjugate (ADC) consisting of an anti-HER2 (human epidermal growth factor receptor 2) antibody linked to a novel topoisomerase I inhibitor payload using a cleavable tetrapeptide-based linker and was recently approved for unresectable or T-DM1-refractory HER2+ breast cancer. In addition, DS-8201a is currently being evaluated as a treatment in breast cancers with low and medium expression of HER2. To better understand the potential for DS-8201a as a treatment in these cancer types, we evaluated this therapy in a panel of ER+/- XPDX models with differential HER2 expression. In addition, we compared DS-8201a activity to T-DM1 and benchmarked efficacy of the topoisomerase I inhibitor irinotecan in all tested models. Methods: One hundred (50ER+/50ER-) breast XPDX models were evaluated in this study. Models were established and characterized for estrogen receptor and HER2 expression by IHC and profiled using WES and RNAseq. For in vivo studies, DS-8201a and T-DM1 were administered IV at 3 mg/kg and irinotecan IP at 100 mg/kg on a weeklyx3 schedule. In vivo study endpoints included tumor volume and time from treatment initiation with %T/C values and tumor regression reported at study completion; a %T/C of ≤ 20 versus control was considered sensitive. Results: Models were grouped by ER positivity (+/-) and model HER2 score and designated as null (0), low (1+), medium (2+) or high (3+); models established from clinically HER2+ patients were also noted. In the ER+ group, HER2 low and medium expressing models accounted for over 80% of the total, while 15% were categorized as high and less than 5% as null. All models with high and 30% with medium HER2 staining were from clinically HER2+ patients, no low or null models were established from breast cancer assigned HER2+ clinically. In the ER- group, HER2 low and medium expressing models accounted for 75% of the total, while 12% were categorized as high and 13% as null. All models with high and 13% with medium HER2 staining were from clinically HER2+ patients; 5% of low or null models were established from breast cancer assigned HER2+ clinically. In vivo, 65% of the ER+ group reported sensitivity to DS-8201a versus 25% to T-DM1; 10% of DS-8201a, and 40% of T-DM1 sensitive models were high expression models and the remaining low and medium with no null models reporting sensitivity to either agent. 35% of tested ER+ models were sensitive to both HER2 therapies and 50% of models sensitive to DS-8201a were also sensitive to irinotecan. In the ER- group, 70% reported sensitivity to DS-8201a versus 25% to T-DM1; 20% of DS-8201a and 25% of T-DM1 sensitive models were high expression models and the remaining low and medium with one null model (ST069) sensitive to both agents. 25% of tested ER- models were sensitive to both HER2 therapies and 40% of models sensitive to DS-8201a were also sensitive to irinotecan. Conclusion: We have compared activity of DS-8201a, T-DM1 and irinotecan in a panel of ER+/- XPDX models and correlated activity based on HER2 expression. This data and panel can be utilized as a valuable tool in better understanding the potential for DS-8201a and other HER2-targeting therapies as a treatment in cancers driven by HER2 expression. Citation Format: Alyssa Simonson, Peter Forofontov, Johnnie R Flores, Kimberly Hernandez, April Cabang, Amy Lang, Gladys Rodriguez, Kyriakos Papadopoulos, Murali Beeram, Arthur Rosenthal, Brittany DeBerry, Lon Smith, Ronald Drengler, Amita Patnaik, Drew Rasco, Luis Rodriguez, Steven Abbate, Scott Ulmer, Michael Wick. Correlation of HER2 receptor expression and in vivo activity of the HER2-targeting therapies trastuzumab deruxtecan (DS-8201a) and T-DM1 activity in a panel of breast XPDX models [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-01-04.