Abstract P1-18-05: Early changes in circulating tumor DNA and its effect on clinical outcomes in patients with advanced breast cancer receiving the CDK4/6 inhibitor palbociclib: Genotyping results from POLARIS

Abstract Background: POLARIS is a prospective, real-world study of palbociclib in patients with hormone receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2–) advanced breast cancer (ABC) in the United States and Canada. These analyses evaluated the difference in tumor mutation profiles of patients receiving palbociclib in the first line versus second line or greater and examined the applicability of circulating tumor DNA (ctDNA) monitoring in a real-world setting. Methods: The clinical database cut-off date was March 16, 2021. Patients in the biomarker analysis group provided consent for serial blood sample collections, received ≥1 dose of initial palbociclib combination treatment, and had ≥1 ctDNA measurement available. The Guardant360 platform with somatic single-nucleotide variants in complete or critical exons of 73 genes was used. The association between early changes in ctDNA mutation allele fractions at Cycle 2 Day 1 (C2D1) and disease progression at Week 24 was evaluated using univariate and multivariable logistic regression analysis adjusting for line of therapy. Results: Among patients with HR+/HER2– ABC (N=347), 93.9% (n=326) had ctDNA measured at baseline, of which 85.9% (n=280) had ≥1 ctDNA alteration detected; 66.6% of patients received palbociclib treatment in the first line and 33.4% as second line or greater ABC setting. The frequencies of gene mutations at baseline were generally higher in patients receiving palbociclib as a second-line or greater therapy compared to those who received it as first-line therapy, particularly for ESR1 mutations (30% vs 15%) and FGFR1 mutations (13% vs 9%). With a median (range) follow-up duration of 18.5 (0.1-46.3) months, patients with total ctDNA increase at C2D1 (log ratio change >0) were 3.74 times more likely to have disease progression at Week 24 (odds ratio, 3.74 [95% CI, 1.71-8.17]; P=0.001) compared with those without change or with a decrease in ctDNA at C2D1 (log ratio change ≤0). The observed significant association remained after adjusting for line of therapy in the multivariable regression analysis (odds ratio, 2.62 [95% CI, 1.14-6.04]; P=0.023). Conclusions: Among patients with HR+/HER2- ABC receiving palbociclib, early changes in ctDNA mutations were significantly associated with disease progression. Further studies are needed to confirm these findings and to evaluate the clinical utility of ctDNA-guided “adaptive” early therapeutic interventions to improve outcomes in patients with metastatic breast cancer. Pfizer; NCT03280303 Citation Format: Debu Tripathy, Zhe Zhang, Joanne L. Blum, Meghan S. Karuturi, Steven L. McCune, Bijoy Telivala, Shailendra Lakhanpal, Kamal Patel, Richard C. Frank, Kit Lu, Chetan Deshpande, Yao Wang, Yuan Liu, Aditya Bardia. Early changes in circulating tumor DNA and its effect on clinical outcomes in patients with advanced breast cancer receiving the CDK4/6 inhibitor palbociclib: Genotyping results from POLARIS [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-18-05.