title: Incidence and clinical significance of ESR1 mutations in heavily pretreated metastatic breast cancer patients.
abstract: BACKGROUND
  ESR1 mutation has recently emerged as one of the important mechanisms involved in endocrine resistance. The incidence and clinical implication of ESR1 mutation has not been well evaluated in heavily pretreated breast cancer patients.
  
  
  METHODS
  We conducted a retrospective review of advanced breast cancer patients with tumors who underwent next-generation sequencing genomic profiling using Foundation One test at Cancer Treatment Centers of America(®) regional hospitals between November 2012 and November 2014.
  
  
  RESULTS
  We identified a total of 341 patients including 217 (59%) estrogen receptor (ER)+, 177 (48%) progesterone receptor (PR)+, 30 (8%) hormone receptor+/HER2 positive, and 119 (32%) triple negative patients. ESR1 mutation was noted in 27/222 (12.1%) ER+ or PR+ breast cancer patients. All ER+ patients received at least one line of an aromatase inhibitor. All 28 patients were found to harbor ESR1 mutations affecting ligand-binding domain with the most common mutations affecting Y537 (17/28, 60.7%) and D538 (9/28, 32.1%). In this cohort, 19 (67.9%) patients carried three or more, seven (25%) patients had one or two additional genomic alterations and one (3.6%) patient had an ESR1 mutation only. Of 28 patients, three patients were treated with fulvestrant immediately before and two patients were treated after next-generation sequencing testing; only one patient achieved stable disease for 8 months and the other four patients had progression of disease. In all, 3/3 (100%) patients before testing and 2/4 (50%) after testing treated with exemestane and everolimus achieved stable disease for at least 6 months.
  
  
  CONCLUSIONS
  ESR1 mutation was found in 12.1% of a large cohort of advanced breast cancer patients. Exemestane in combination with everolimus might be a reasonable option. Prospective studies are warranted to validate these findings.
authors.affiliation: Department of Medical Oncology, Western Regional Medical Center at Cancer Treatment Centers of America (CTCA), Goodyear, AZ, USA.
authors.forename: Jiaxin
authors.initials: J
authors.lastname: Niu
authors.affiliation: Department of Medical Oncology, Western Regional Medical Center at Cancer Treatment Centers of America (CTCA), Goodyear, AZ, USA.
authors.forename: Grant
authors.initials: G
authors.lastname: Andres
authors.affiliation: CTCA Medicine and Science, Zion, IL, USA.
authors.forename: Kim
authors.initials: K
authors.lastname: Kramer
authors.affiliation: Department of Medical Oncology, Banner MD Anderson Cancer Center, Gilbert, AZ, USA.
authors.forename: Madappa N
authors.initials: MN
authors.lastname: Kundranda
authors.affiliation: Department of Medical Oncology, Southeastern Regional Medical Center at CTCA, Newnan, GA, USA.
authors.forename: Ricardo H
authors.initials: RH
authors.lastname: Alvarez
authors.affiliation: Department of Medical Oncology, Eastern Regional Medical Center at CTCA, Philadelphia, PA, USA.
authors.forename: Eiko
authors.initials: E
authors.lastname: Klimant
authors.affiliation: Department of Medical Oncology, Eastern Regional Medical Center at CTCA, Philadelphia, PA, USA.
authors.forename: Ankur R
authors.initials: AR
authors.lastname: Parikh
authors.affiliation: Department of Pathology, Midwestern Regional Medical Center at CTCA, Zion, IL, USA.
authors.forename: Bradford
authors.initials: B
authors.lastname: Tan
authors.affiliation: Advanced Individual Medicine, Phoenix, AZ, USA.
authors.forename: Edgar D
authors.initials: ED
authors.lastname: Staren
authors.affiliation: CTCA Medicine and Science, Philadelphia, PA, USA.
authors.forename: Maurie
authors.initials: M
authors.lastname: Markman
keywords: ESR1 mutation; breast cancer; endocrine therapy; genomic alteration; next-generation sequencing; resistance