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Purpose: Common resistance mechanisms to endocrine therapy (ET) in estrogen receptor (ER)-positive metastatic breast cancers include, among others, ER loss and acquired activating mutations in the ligand-binding domain of the ER gene ( ESR1 LBD m ). ESR1 mutational mediated resistance may be overcome by selective ER degraders (SERD). During the first-in-human study of oral SERD AZD9496, early changes in circulating tumor cells (CTCs) and circulating tumor DNA (ctDNA) were explored as potential noninvasive tools, alongside paired tumor biopsies, to assess pharmacodynamics and early efficacy., Experimental Design: CTC were enumerated/phenotyped for ER and Ki67 using CellSearch in serial blood draws. ctDNA was assessed for the most common ESR1 LBD m by droplet digital PCR (BioRad)., Results: Before starting AZD9496, 11 of 43 (25%) patients had ≥5 CTC/7.5 mL whole blood (WB), none of whom underwent reduction to <5 CTC/7.5 mL WB on C1D15. Five of 11 patients had baseline CTC-ER + , two of whom had CTC-ER + reduction. CTC-Ki67 status did not change appreciably. Patients with ≥5 CTC/7.5 mL WB before treatment had worse progression-free survival (PFS) than patients with <5 CTC ( P = 0.0003). Fourteen of 45 (31%) patients had ESR1 LBD m + ctDNA at baseline, five of whom had ≥2 unique mutations. Baseline ESR1 LBD m status was not prognostic. Patients with persistently elevated CTC and/or ESR1 LBD m + ctDNA at C1D15 had worse PFS than patients who did not ( P = 0.0007)., Conclusions: Elevated CTC at baseline was a strong prognostic factor in this cohort. Early on-treatment changes were observed in CTC-ER + and ESR1 LBD m + ctDNA, but not in overall CTC number. Integrating multiple biomarkers in prospective trials may improve outcome prediction and ET resistance mechanisms' identification over a single biomarker., (©2018 American Association for Cancer Research.)