Signatera-Guided CDK4/6 Inhibitor Therapy in Breast Cancer

Participants with intermediate-risk, early-stage HR+/HER2- breast cancer will be enrolled in a prospective, single-arm, multicenter phase II study. Circulating tumor DNA (ctDNA) surveillance will be performed using the Signater Genome assay, which is customized for each participant from archived tumor and matched normal DNA to detect up to 64 tumor-specific variants in plasma. Participants who are ctDNA positive at baseline will start CDK4/6 inhibitor therapy along with hormone therapy. Those who are ctDNA negative will continue hormone therapy with ctDNA testing every three months. If ctDNA becomes positive during surveillance, participants will first undergo staging to rule out distant disease before beginning CDK4/6 inhibitor therapy in addition to hormone therapy. All participants will be followed for up to nine years to assess cancer outcomes. Four-year outcomes will be compared to historical controls from the NataLEE trial to evaluate whether ctDNA-guided timing maintains efficacy while reducing unnecessary treatment.