Establishment and characterization of a panel of breast XPDX models representing innate or acquired resistance to trastuzumab deruxtecan (T-DXd)

Abstract Background: Trastuzumab deruxtecan (T-DXd) is an ADC consisting of an anti-HER2 antibody linked to a novel topoisomerase I inhibitor payload approved for T-DM1-refractory, HER2+ breast cancer (BC) and metastatic, recurrent HER2-low BC. While this therapy is effective in some patients, resistance often develops. To aid in developing new therapies for T-DXd-resistant BC and better understand potential resistance mechanisms, we established a panel of six breast XPDX models representing innate or acquired resistance to T-DXd. These models, designated ST4565C, ST4565D, STM148B, STM148C, STM148D, and ST4480B/EHR were developed and characterized for receptor expression, genomics, and drug sensitivities toward chemotherapies and targeted agents including T-DXd, T-DM1, and margetuximab. Methods: ST4565C/D was established from a patient with ER+/HER2+, metastatic BC who was T-DXd treatment naïve. ST4565C was collected post chemo- and HER2-targeted therapies; ST4565D was collected following eribulin/margetuximab. STM148B/C/D were established from a patient with ER-/HER2+ metastatic BC following trastuzumab, T-DM1, and eleven months of T-DXd. ST4480B/EHR was established by chronic in vivo T-DXd dosing to the parent ST4480B ER+/HER2+ model until resistance developed. Resulting models were passaged and receptor expression confirmed by IHC and genomic analyses, including WES and RNAseq, were performed to further characterize models. In vivo, models were tested with agents including: trastuzumab, T-DM1, and T-DXd; endpoints included tumor volume (TV) and time from treatment initiation (TTI) 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=<0) versus Day 0 TV was also reported. Results: Models retained receptor expression and similar histology comparable with archival clinical or model samples. In vivo, all ST4565 and STM148 models were refractory to trastuzumab, T-DM1, and T-DXd up to 10 mg/kg weekly with an average %T/C of 75%. ST4480B/EHR showed similar resistance to trastuzumab and T-DM1 at 10 mg/kg, and T-DXd at 3 mg/kg; T-DXd tested at 3 mg/kg in the ST4480B parent model resulted in tumor regressions. Sequencing of ST4565C/D identified a novel fusion, MTAP/CDKN2A/B deletions and elevated expression of FGFR1 and CCND1. In STM148 models an AKAP8L-NOTCH3 fusion and PIK3CA and CCNE1 mutations were reported. ST4480B/EHR reported PIK3CAE545K and an ESR1-CCDC170 fusion identical to the parent model, but differential expression was noted in several genes in the RICTOR/TORC2 pathway. Conclusion: We established and characterized six breast XPDX models representing innate or acquired resistance to T-DXd. These models are valuable tools in understanding resistance mechanisms and in developing novel therapies for T-DXd-resistant patients. Citation Format: Maci DeBoer, Albert Paez, Johnnie Flores, Robyn Baeza, Alyssa Simonson, Jim Lund, Kyriakos Papadopoulos, Thomas Gribbin, Amy Lang, Gladys Rodriguez, Lorena Mozas, Lorena Gonzalez, Manish Sharma, Emiliano Calvo, Tatiana Hernandez, Michael J. Wick. Establishment and characterization of a panel of breast XPDX models representing innate or acquired resistance to trastuzumab deruxtecan (T-DXd). [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3855.