Abstract PO4-15-06: Genomic landscape characterization after exposure to cyclin dependent kinase 4/6 inhibitors: a retrospective multi-institutional consortium analysis

Abstract Background: Limited data are available to determine the best therapeutic strategy for hormone-receptor positive (HR+) HER2 negative (HER2-) advanced breast cancer (ABC) after progression on cyclin dependent kinase 4/6 inhibitors (CDK4/6i). The aim of this study was to characterize the genomic and prognostic profile of patients (pts) that experienced disease progression after first-line therapy with a CDK4/6i. Methods: The study retrospectively analyzed a multi-institutional cohort of 75 patients (pts) with HR+/HER2- ABC after experiencing progression on first-line endocrine therapy (ET) with CDK4/6i and characterized by circulating tumor DNA (ctDNA) next-generation sequencing (Guardant 360). Oncogenic pathways (i.e., RTK, RAS, RAF, MEK, NRF2, ER, WNT, MYC, p53, cell cycle, notch, and PI3K) were defined based on previous research (Sanchez-Vega et al. Cell, 2018). Associations across single nucleotide variations (SNVs), copy number variations (CNVs), and pathway classification were tested by uni- and multivariable logistic regression with respect to a CDK4/6i naïve group comprising 247 patients with HR+/HER2- ABC (control). Prognosis was analyzed through Cox regression for overall survival (OS). Result: Our study cohort included 75 ABC pts who experienced progression after first line CDK4/6i plus ET: 43 pts (57.3%) were treated with ET while 31 (41.3%) with a non-ET- second line therapy. The most common histology was invasive ductal carcinoma (70.6%), 37.3% of pts were negative for progesterone expression and 26.7% had de novo metastatic disease. The PI3K (33.3%), p53 (28%) and ER (25.3%) pathways were the most mutated in the CDK4/6i-treated cohort. Comparing our population with a CDK4/6i naïve cohort (N=247), the multivariate analysis showed a higher prevalence in the study cohort of SNVs mutations in: i) ESR1 gene Odds ratio (OR) 2.93; p=0.005; ii) cell-cycle pathway (OR 4.24; p=0.033); iii) ER pathway (OR 2.04; p= 0.034). Moreover, in the study cohort a numerical but not significant increase of RB1 SNVs was also observed. Multivariable analyses of the 43 pts that received ET second line therapy showed a negative prognostic impact with TP53 SNVs in both progression-free survival (PFS) Hazard ratio (HR) = 3.34; 95% CI: 1.15-9.66, p=0.026 and OS (HR = 3.85; 95% CI: 1.52-9.77, p=0.005). The prognostic role of p53 pathway mutations was also confirmed for both PFS (HR = 4.71, 95% CI: 1.64-13.50; p=0.004) and OS (HR = 3.25; 95% CI: 1.27-8.3; p=0.014). The potential interaction between the prognostic oncogenic pathways and the outcome was investigated according to the treatment strategy (ET vs non-ET). A consistent impact was observed across the above prognostic pathways both for PFS and OS. A numerical difference was observed in terms of PFS in pts with p53 pathway mutations that underwent non-ET second line. Conclusions: The genomic landscape of progressive disease after CDK4/6i exposure is significantly different from the genomic alterations detectable in treatment-naïve pts, which could potential impact future treatment strategies for patients with disease progression after adjuvant CDK4/6i exposure. Our data suggest that the choice of second-line treatment could potentially be guided by the identification of actionable mutation by ctDNA, although further prospective studies are needed to validate the clinical utility of this approach. Citation Format: Letizia Pontolillo, Carolina Reduzzi, Andrew Davis, Lorenzo Gerratana, Arielle Medford, Katherine Clifton, Whitney L. Hensing, Marko Velimirovic, Ami N. Shah, Jeannine Donahue, Laura Munoz Arcos, Charles S. Dai, Jennifer Keenan, Amir Behdad, William Gradishar, Emilio Bria, Cynthia Ma, Aditya Bardia, Massimo Cristofanilli. Genomic landscape characterization after exposure to cyclin dependent kinase 4/6 inhibitors: a retrospective multi-institutional consortium analysis abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-15-06.