Abstract P3-02-20: CDK4/6 inhibitor resistance in patients with ER+ breast cancer: Identification and characterization of resistance mechanisms in thirty-two XPDX models

Abstract Background: Resistance to CDK4/6 inhibitors (CDK4/6i) has been well studied with several mechanisms identified, including loss of RB, FAT1, and ERa and altered expression of regulatory genes such as CCND1, CCNE1/2, and CDK2. Additionally, alterations responsible for resistance may be linked to duration of CDK4/6i treatment. Previously (AACR2023), we reported on seventeen ER+ breast XPDX models representing patients who responded to CDK4/6i for up to twelve months (RES12), and greater than one year (RES13+). To better understand if duration of clinical treatment correlates with unique resistance mechanisms, we established and characterized an additional fifteen breast XPDX models from patients post-CDK4/6i and compared molecular profiles and drug sensitivities in the panel of thirty-two models. Methods: Seventeen breast models were previously established and characterized which included 8xRES12 and 9xRES13+. Fifteen new models were established from thirteen patients: eleven originated from fluid samples of which seven were designated as ductal (ST4137L, ST4322C, ST4322D, ST6023B, STM185, STM188, ST188B) and four as lobular carcinomas (ST4680D, ST6133, STM229F, ST383F); three were established from core biopsies (ST4534B, lymph node; ST5400, breast; STM354, liver), and one from circulating tissue (STM223B), all designated as ductal carcinomas. Of the fifteen new models, six were classified as RES12 and nine as RES13+. These models were passaged and challenged with palbociclib to confirm resistance. Receptor expression was determined by IHC and genomic analyses, including WES and RNAseq, were performed to identify mechanisms of resistance. For in vivo studies, CDK4/6i were dosed via oral gavage, once daily at 50 mg/kg; 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: Clinical time to progression (TTP) for RES12 (n=6) was four to twelve months and RES13+ (n=9) from thirteen to thirty-six months. All models, except ST4322C and ST4322D, retained ER expression in evaluated passages with similar histology compared with archival clinical samples. Sequencing identified several variants, including RB1 and/or PTEN loss or deletion, and increased gene expression in CCND1, CCNE1, and the PIK3CA/AKT pathway. PIK3CA mutations were reported in 7/15 models; mutations in RAF1, ESR1, FGFR and other genes were also identified across the panel, some more prevalent based on clinical TTP. Most models were resistant to fulvestrant and insensitive to some or all evaluated CDK4/6i treatments. Conclusion: We have expanded our platform of breast XPDX from patients post-CDK4/6i treatment to thirty-two models and characterized each based-on duration of clinical treatment, molecular profiles, and drug sensitivities. We have also identified potential mechanisms of resistance based on CDK4/6i therapy and TTP. This platform is a valuable resource for developing novel therapies for CDK4/6i-resistant patients. Citation Format: Delaney Rushing, Alyssa Simonson, Johnnie Flores, Anna Stackpole, Morgan Lynch, Amy Cook, Lisa Gonzales, Jim Lund, Tahmineh Rouzbahani, Eliza Abdul, Kyriakos Papadopoulos, Gladys Rodriguez, Lorena Mozas, Lorena Gonzalez, Amita Patnaik, Amy Lang, Murali Beeram, Manish Sharma, Emiliano Calvo, Tatiana Hernandez, Michael J. Wick. CDK4/6 inhibitor resistance in patients with ER+ breast cancer: Identification and characterization of resistance mechanisms in thirty-two XPDX models [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P3-02-20.