A Phase III, Double-blind, Randomised Study to Assess Switching to AZD9833 (a Next Generation, Oral SERD) + CDK4/6 Inhibitor (Palbociclib or Abemaciclib) vs Continuing Aromatase Inhibitor (Letrozole or Anastrozole)+ CDK4/6 Inhibitor in HR+/HER2-MBC Patients With Detectable ESR1Mutation Without Disease Progression During 1L Treatment With Aromatase Inhibitor+ CDK4/6 Inhibitor- A ctDNA Guided Early Switch Study


The study is intended to show superiority of AZD9833 in combination with CDK4/6 inhibitor (palbociclib or abemaciclib) versus aromatase inhibitors (anastrozole or letrozole) in combination with CDK4/6 inhibitor in patients with hormone receptor-positive (HR-positive), human epidermal growth factor receptor 2-negative (HER2-negative) metastatic breast cancer with detectable ESR1 mutation.

Study Arms

  • Experimental: AZD9833 + palbociclib or abemaciclib Therapy
  • Active Comparator: Anastrozole or letrozole + palbociclib or abemaciclib


  • Proven diagnosis of adenocarcinoma of the breast with evidence of locoregionally recurrent or metastatic disease not amenable to resection or radiation therapy with curative intent.
  • Documentation of histologically confirmed diagnosis of estrogen receptor-positive (ER+) /HER2- breast cancer based on local laboratory results.
  • Currently on AI (letrozole or anastrozole) + CDK4/6 inhibitor (palbociclib or abemaciclib) ± LHRH as the initial endocrine-based treatment for advanced disease
  • Eastern Cooperative Oncology Group performance status of 0 or 1.
  • ESR1m positive detected by central testing of ctDNA
  • Willingness and ability to comply with scheduled visits, treatment plan, laboratory tests, and other study procedures.
  • Adequate organ and marrow function