Abstract PD2-02: Longitudinal ctDNA sequencing using an expanded genomic panel in the PALOMA3 trial of palbociclib plus fulvestrant

Background: CDK4/6 inhibition combined with endocrine therapy (ET) is now the standard of care for advanced hormone receptor (HR) positive breast cancer patients (pts). Previous ctDNA analysis of paired pre-treatment (pre-T) and end of treatment (EoT) plasma samples from the PALOMA-3 trial of the CDK4/6 inhibitor P with/without F demonstrated that acquired PIK3CA, ESR1, RB1 and rare growth factor receptor mutations (mut) likely contribute to resistance (Turner et al, ASCO 2018, Abstract 1001). We now report results from an extended ctDNA analysis of paired PALOMA-3 plasma samples using hybridization capture-based, next-generation sequencing (NGS) assay covering 87 breast cancer, HR signaling, and cell cycle-related genes. Methods: Pre-/post-menopausal pts whose disease progressed after prior ET (N=521) were randomized 2:1 to receive F 500 mg + P (125 mg/d, 3 wk on/1 wk off) or placebo. Cell-free DNA extracted from paired plasma samples (n=194) was analyzed with a custom 87-gene NGS assay, to identify single nucleotide variants and copy number amplification (CNA), with molecular barcoding, high NGS coverage (10,000-20,000 reads per nucleotide position), and background error correction. Differences in ctDNA mut frequencies detected in P+F compared to F alone were assessed and their association to clinical outcome estimated. Results: Blinded assay qualification and droplet digital PCR validation suggested a mut detection frequency cutoff at 0.3%. A high coefficient of correlation with previously presented data was observed for ESR1 and PIK3CA variants (r=0.94 and 0.97, respectively), with acquisition of PIK3CA and ESR1 mutations at EoT in P+F and F groups. Gene level mut analysis of EoT plasma revealed no significant difference between P+F vs F alone (Table), although there was an increase in RB1 mutations in P+F consistent with previous data. Other acquired muts at EoT (SMAD4, NOTCH2, and CDKN1B) were observed at low frequencies. Muts in CDK4 and CDK6 were rarely observed ( Conclusions: Our results corroborate the previously reported results demonstrating that genomic profiles of treatment resistant cancer are similar between P + F and F therapy. Expanded analysis of cell-cycle genes identified no new recurrently mutated genes, and confirmed that RB1 mutations are selected at low frequency after P+F treatment. Alterations in cell cycle genes may not be a common mechanism of resistance to CDK4/6 inhibitors in HR+ HER2- advanced breast cancer. Sponsor: Pfizer Citation Format: O9Leary B, Lira ME, Huang S, Deng S, Xie T, Kinong JK, Hardwick J, Rejto P, Liu Y, Huang X, Huang Bartlett C, Andre F, Loibl S, DeMichele A, Cristofanilli M, Turner NC. Longitudinal ctDNA sequencing using an expanded genomic panel in the PALOMA3 trial of palbociclib plus fulvestrant [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr PD2-02.