Abstract P1-13-03: Establishment and characterization of two T-DM1-resistant, ER+/HER2+ breast XPDX models developed sequentially from the same patient with differential in vivo sensitivity to trastuzumab deruxtecan (DS-8201a)
Authors: Plasko George , Flores Johnnie , Simonson Alyssa , Forofontov Peter , Varma Ashwin , Lang Amy , Rodriguez Gladys , Papadopoulos Kyriakos P. , Rasco Drew , Patnaik Amita , Conway Bruce , Johnston Joe , Wick Michael 
Published: March 2023

Abstract
Background: Trastuzumab deruxtecan (DS-8201a) is an antibody-drug conjugate (ADC), consisting of a humanized anti-HER2 (human epidermal growth factor receptor 2) monoclonal antibody linked to a topoisomerase I inhibitor payload using a cleavable tetrapeptide-based linker, approved for the treatment of HER2+ metastatic breast cancer patients refractory to anti-HER2 therapy including T-DM1. While some mechanisms for clinical T-DM1 resistance have been identified, less is known about innate or acquired resistance to DS-8201a. We established two XenoSTART Patient-Derived Xenograft (XPDX) models from tissue samples collected two years apart from a patient with ER+/HER2+ breast cancer before and after HER2 directed therapies. These models designated ST4565 and ST4565C were developed and characterized for receptor expression, genomic alterations, and in vivo drug sensitivities toward multiple chemotherapies and targeted agents, including T-DM1 and DS-8201a. Methods: Models ST4565 and ST4565C were established from breast samples collected from a Caucasian female with ER+/HER2+ metastatic breast cancer; ST4565 was collected at age 35 prior to therapy and ST4564C at age 37 following several treatment regimens including 5-FU/doxorubicin/cyclophosphamide, docetaxel/trastuzumab/pertuzumab, and T-DM1/anastrozole. Both were grown subcutaneously in female athymic nude mice supplemented with exogenous estradiol. The resulting models were passaged, and receptor expression confirmed immunohistochemically; genomic analysis, including WES and RNAseq, was performed to further characterize the models. For in vivo studies, both models were evaluated using several chemotherapy and targeted agents alone and in combination including: trastuzumab, pertuzumab, T-DM1, DS-8201a, neratinib, tucatinib, fulvestrant, alpelisib, sacituzumab, and irinotecan. 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. Tumor regression (%T/C=&lt; 0) versus Day 0 tumor volume was also reported. Results: ST4565 and ST4565C retained comparable receptor expression (ER=2+/HER2=3+) over tested passages with similar histology compared to archival clinical samples. DNA/RNA sequencing identified several conserved variants including loss of CDKN2A/B and MTAP and a CNV=14 for CCND1. In vivo, ST4565 and ST4565C were found resistant to T-DM1 at 3 mg/kg weekly with a T/C of 100% in both models. However, DS-8201a treatment at 3 mg/kg weekly resulted in partial tumor regressions in ST4565 (T/C=-51%) while ST4565C was found resistant (T/C=49%). Both models were found resistant to all tested chemotherapies and all other targeted therapies but reported similar sensitivity to fulvestrant (T/C=~40%). Conclusion: We established two XPDX models representing T-DM1-resistant, ER+/HER2+ breast cancer from breast samples collected two years apart from the same patient that were found differentially responsive to DS-8201a. These models can be utilized as a valuable tool in better understanding innate resistance to T-DM1 and acquired resistance to DS-8201a.
Citation Format: George Plasko, Johnnie Flores, Alyssa Simonson, Peter Forofontov, Ashwin Varma, Amy Lang, Gladys Rodriguez, Kyriakos P. Papadopoulos, Drew Rasco, Amita Patnaik, Bruce Conway, Joe Johnston, Michael Wick. Establishment and characterization of two T-DM1-resistant, ER+/HER2+ breast XPDX models developed sequentially from the same patient with differential in vivo sensitivity to trastuzumab deruxtecan (DS-8201a) [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 P1-13-03.