ABCSG 61 / TEODOR : Neoadjuvant TrEatment Optimization Driven by Circulating Tumor DNA and endOcrine Responsiveness

This is a prospective, randomized, controlled, open-label phase II performance study for participants with ER+/ HER2- early or locally advanced breast cancer. At the start of the trial, ctDNA is assessed for all participants with the SignateraTM test by Natera, using (archived) tumor tissue and blood samples. Eligible patients start AI therapy per Standard of Care in the Run-in Phase and after 3 weeks of treatment, Ki-67 levels are measured locally, to determine endocrine response. Following the Run-in Phase, participants, whose SignateraTM test result shows no detectable ctDNA and whose Ki-67 value is ≤ 10% are randomized 2:1 to receive either neoadjuvant AI or, if AI is not tolerated, tamoxifen in arm A or neoadjuvant chemotherapy in arm B. Participants with a Ki-67 value of \>10% or detectable ctDNA, according to the SignateraTM test, receive chemotherapy in the third treatment arm C. All study treatment is applied as per standard of care. The planned duration of treatment is 4 weeks in the Run-in phase and 6-8 months in either arm of the Main Treatment Phase. The primary endpoint is the modified PEPI score. Patients will be followed for 5 years from surgery.