Background: CDK4/6-inhibitors (-i) and endocrine therapy (ET) are first-line treatment for HR+/HER2- metastatic breast cancer (mBC). Clinical evaluation and cross-sectional imaging are used for treatment response assessment. The relationship between clinical/radiographic response and ctDNA dynamics using a high sensitivity tumor-agnostic assay is unknown in HR+/HER2- mBC. ctDNA monitoring in mBC may support clinical decision-making, enable new strategies for response assessment in clinical trials, and facilitate enrolment of patients lacking measurable disease (bone-only). Methods: Patients (pts) with HR+/HER2- mBC receiving ET and CDK4/6i were enrolled in a prospective cohort study. Plasma samples were collected at baseline (BL) and regularly on-treatment. Samples were analyzed using Guardant Infinity, a tumor-agnostic genomic and epigenomic platform. ctDNA-response (-R) was defined as a ≥50% decrease (dec) in methylation tumor fraction (mTF) from BL. ctDNA-progression (-P) was defined as a <50% dec in mTF from BL or any subsequent increase in mTF above an absolute mTF of 0.001% (cohort specific definition). Imaging outcomes and dates of treatment discontinuation (TD) were collected. Results: 57 pts with 350 clinical timepoints were evaluated. Median follow up was 28.2m (range: 1.6-66.0m). 42/57 (74%) pts received palbociclib and 40/57 (70%) received an aromatase inhibitor. ctDNA-R/-P was evaluable in 49/57 patients. ctDNA-P identified within the first 90d on CDK4/6i was not prognostic of TD (p=0.7972); when assessed between 90-180d it was prognostic of early TD (med: 6.3m vs. not reached; HR: 5.17, 95%CI: 1.04-25.7; p=0.0003). 156 matched radiographic and ctDNA assessments (blood collected within 30d of CT) were evaluable for ctDNA-R/-P and imaging concordance. Excluding those with clinician-identified sclerotic bone changes suggestive of treatment effect (n=5 events), adjudicated radiographic progression did not occur in the setting of ctDNA-R (NPV: 100%, sensitivity: 100%); no pts had clinical progression in the presence of ctDNA-R. ctDNA rise in the absence of radiographic progression occurred in 13 pts; 7/13 had TD (med: 10.1m, range: 3.0-21.9m) in the follow up period reflecting a molecular lead time. Blood first monitoring, using these metrics, could have avoided 104/150 (69%) staging CT scans in this cohort. Conclusions: ctDNA dynamics are associated with clinical outcomes in HR+/HER2- mBC. The use of a high sensitivity assay permits assessment of response in nearly all patients and suggests the possibility that radiographic surveillance could be reduced in patients with ctDNA-R. Prospective validation of these findings are required. Additional ctDNA-R/-P definitions and relationship with RECIST 1.1 criteria will be presented at the meeting.