1043 Background: In estrogen receptor (ER) positive MBC, mutations (e.g. ESR1), identified from tumor biopsies or cfDNA, confer resistance. The concordance between mutations observed in tumor and cfDNA and the implications for response to Z-endoxifen, a potent anti-estrogen, are unknown. Methods: We previously conducted a phase I trial of Z-endoxifen in endocrine refractory, ER positive MBC. Seven dose levels were considered ranging from 20 to 160 mg/day followed by expansion cohorts (EC) of 40, 80, and 100 mg/day. Pretreatment blood samples (all pts) and fresh tumor biopsies (EC) were collected prospectively. Tumor and cfDNA were evaluated by targeted NGS. Results: 41 pts (38 evaluable) were enrolled. Prior endocrine therapy included aromatase inhibitors (37/38, 97%), fulvestrant (22/38, 58%) and tamoxifen (26/38, 68%). Substantial endoxifen exposure without DLTs at doses above 80 mg/day led to a halt in dose escalation and opening of the EC. Overall clinical benefit (stable > 6 months [7 pts.] or partial response by RECIST criteria [3 pts.]) rate was 26.3% (95%CI: 13.4-43.1%). cfDNA was obtained from 36 pts and mutations were identified in 13 (36%) including ESR1 [ Y537N or D538G] (5), PIK3CA [ H1047R or E542K] (8), TP53 [ K132R, R248Q, R267Q, or H179Y] (4), AKT ( Q79K) (1), and KRAS ( G12D) (1). In 5 pts with cf ESR1 mutations, 4 had additional cfDNA mutations including PIK3CA (3), and TP53 (1). PFS was shorter in pts with cfDNA mutations relative to those without (median: 61 vs.132 days; log-rank p = 0.021). Discordance was observed between tumor and cfDNA mutations where 3/7 PIK3CA tumor mutations were detected by cfDNA, 1/2 TP53 tumor mutations were detected by cfDNA, and 0/1 AKT tumor mutations were detected by cfDNA. Conversely, 2 pts had cfDNA mutations (either ESR1, TP53 or AKT) undetected in tumor. Conclusions: The absence of cfDNA mutations in patients with endocrine resistant, MBC treated with Z-Endoxifen was associated with significantly longer PFS. Given the discordance between tumor and cfDNA sequence data, future studies must determine which approach maximizes prognosis and prediction of benefit for estrogen-targeted therapy. Clinical trial information: NCT01327781.