Abstract 5533: Cell-free DNA sequencing in ERBB2-mutant breast cancer patients treated with neratinib and fulvestrant: Exploratory analysis from the Phase 2 SUMMIT ‘basket' trial
Authors: Schram Alison M. , Selcuklu S. Duygu , Nagy Rebecca J. , Smyth Lillian M. , Eli Lisa D. , Cutler Richard E. , Lalani Alshad S. , Hyman David M. , Lanman Richard B. 
Published: August 2018

Abstract
Background: Mutations in ERBB2, encoding the HER2 protein, occur in up to 4% of breast cancers. A subset of ERBB2 mutations have been shown to have oncogenic potential through constitutive activation of the HER2 kinase. SUMMIT (NCT01953926) is a phase II ‘basket' study investigating the use of the irreversible pan-HER tyrosine kinase inhibitor neratinib, in combination with the estrogen degrader fulvestrant, for the treatment of patients with advanced hormone-positive breast cancers harboring ERBB2 mutations. We collected and sequenced serial samples of circulating cell-free DNA (cfDNA) from participating patients in order to identify biomarkers of response and resistance.
Methods: Patients were treated with neratinib 240 mg daily and fulvestrant at the labelled 500 mg dose. Plasma was collected and cfDNA was extracted from patients at baseline and on Day 1 of each cycle (occurring every 4 weeks). Next generation sequencing (NGS) was performed on cfDNA samples collected from 12 patients using the Guardant360 test (Guardant Health, CA), which identifies select point mutations, small insertions and deletions, copy number changes, and structural rearrangements in 73 genes. Paired samples were sequenced from 9 of the 12 patients, including baseline and progression samples from 7 patients. cfDNA NGS results were compared to pretreatment tissue NGS results obtained 0-6 years prior (median: 6 months).
Results: 90% (9/10) baseline cfDNA NGS detected the locally reported ERBB2 mutation identified by tissue NGS. In one patient with only a sample obtained at progression, the ERBB2 mutation was identified. No ERBB2 mutation was identified in a singular on-treatment sample obtained from a patient with continued response to therapy. Eight ERBB2 mutations were identified in the baseline cfDNA sample of one patient with a hypermutated tumor, 6 of which were not identified by tissue NGS. Emergence of at least one additional ERBB2 mutation was seen in 3/7 patients with paired baseline and progression samples, including patients with 2 and 5 acquired ERBB2 mutations. The ERBB2 mutant allele fraction (MAF) increased compared to baseline in 4/7 of patients at the time of progression and declined in 2/2 patients with continued response to therapy. cfDNA NGS detected mutations, unidentified by tissue NGS in 9/10 baseline samples and in 11/12 patients, including 5 ESR1 mutations in 3 patients.
Conclusion: cfDNA NGS was sensitive for the detection of ERBB2 mutations. MAF dynamics typically tracked response to, or progression on therapy, at the time of cfDNA collection. The emergence of additional ERBB2 mutations was common at the time of progression; however, no T798 gatekeeper mutation was observed. Many subclonal mutations were identified on cfDNA NGS that were not observed on tissue NGS.
Citation Format: Alison M. Schram, S. Duygu Selcuklu, Rebecca J. Nagy, Lillian M. Smyth, Lisa D. Eli, Richard E. Cutler, Alshad S. Lalani, David M. Hyman, Richard B. Lanman. Cell-free DNA sequencing in ERBB2-mutant breast cancer patients treated with neratinib and fulvestrant: Exploratory analysis from the Phase 2 SUMMIT ‘basket' trial [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5533.