1015 Background: Selection of resistance mutations may play a major role in the development of endocrine resistance. ESR1 mutations are rare in primary breast cancer but have a high prevalence in patients treated with aromatase inhibitors (AI) for advanced breast cancer. We investigated the evolution of genetic resistance to first line AI therapy using sequential ctDNA sampling in patients with advanced breast cancer. Methods: Seventy-one patients on first line AI therapy for metastatic breast cancer were enrolled in a prospective study to collect plasma samples for ctDNA analysis every three months on therapy, and at disease progression. All plasma samples were analysed with ESR1 multiplex digital PCR assays, and samples at disease progression were analysed by InVision (enhanced tagged-amplicon sequencing). Mutations were tracked back through samples prior to disease progression to study the evolution of mutations on therapy. Results: Of the 34 patients who progressed on first line AI, 53% (18/34) had ESR1 mutations detectable at progression. Sequencing of progression plasma ctDNA identified polyclonal RAS mutations in 10.7% (3/28) progressing patients (2 polyclonal KRAS, 1 monoclonal HRAS), all of whom also had ESR1 mutations, and a patient with an activating p.R248C FGFR3 mutation. ESR1 mutations were subclonal in 78.6% (11/14) patients, with all RAS mutations being rare subclones. In serial tracking prior to progression, ESR1 mutations were detectable in plasma with a median of 5.3 months (95% CI 2.9-NA) prior to clinical progression. Conclusions: ESR1 mutations are found at high frequency in patients progressing on AI, but are frequently sub-clonal and may not be the sole driver of AI resistance in these patients. Poly-clonal KRAS mutations are identified as a novel mechanism of resistance to AI, associated with detection of ESR1 mutations.
The Impact of ESR1 Mutations on the Treatment of Metastatic Breast Cancer
