Abstract 3869: Short or long-term treatment with CDK4/6 inhibitors in patients with ER+ breast cancer: characterization and comparative analysis of resistance in seventeen XPDX models

Abstract Background: Mechanisms of resistance to CDK4/6 inhibitors (CDK4/6i) have been well studied and several alterations identified including RB loss and altered expression of related genes including CCNE1, E2F4 and CDK6. However, whether duration of clinical treatment might elicit specific mechanism(s) of CDK4/6i resistance is unclear. To better understand if duration of clinical treatment correlates with unique resistance mechanisms, we established, characterized, and compared a panel of ER+ breast XPDX models from patients who benefitted then progressed on a CDK4/6i. Patients were separated into two groups by time to progression (TTP): those who responded up to twelve months (RES12) and patients with clinical response greater than one year (RES13+). Methods: Seventeen breast cancer XPDX models were analyzed, including eight previously described (7xRES12; 1xRES13+: SABCS2021: T Hernandez et al). Nine new models were established from seven patients: six from fluid samples with three designated as ductal (ST3105B, ST3105C, STM001B) and three lobular carcinoma (STM182, STM229, STM229B); two from lymph node biopsies (ST5676, STM127) and one from a liver core biopsy (ST4887B), all reported as ductal carcinoma. STM182 was classified as RES12 and the remaining eight as RES13+. These models were passaged and challenged with CDK4/6i 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 PO 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 TTP for RES12 (n=8) was four to twelve months and RES13+ (n=9) from thirteen to forty-two months. All models retained ER expression in evaluated passages with similar histology compared with archival clinical samples. Sequencing identified several variants including RB1 truncations or deletions and increased gene expression in CCND1, CCNE1 and the PIK3CA/AKT pathway. Interestingly, 5/8 RES12 models reported ESR1 mutations or fusions versus 1/9 RES13+ and PIK3CA mutations were reported in 1/8 RES12 versus 5/9 RES13+. Several RES13+ models also reported variants and increased amplification in the RICTOR/TORC2 pathway versus RES12. Conclusion: We have established, characterized, and compared a panel of seventeen breast XPDX models from fourteen female patients representing early or late acquired resistance to CDK4/6i therapy and identified potential differences in each set. These models and resulting data are useful in developing novel therapies for CDK4/6i-resistant patients. Citation Format: Alyssa Simonson, Johnnie Flores, Morgan Lynch, Emily Carpenter, Justine Hruzek, Jim Lund, Natalia Baños Herraiz, Kyriakos Papadopoulos, Amy Vander Woude, Gladys Rodriguez, Sreenivasa Chandana, Thomas Gribbin, Nehal Lakhani, Tatiana Hernandez, Maria Jose de Miguel, Amy Lang, Michael J. Wick. Short or long-term treatment with CDK4/6 inhibitors in patients with ER+ breast cancer: characterization and comparative analysis of resistance in seventeen XPDX models. [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 3869.