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1009 Background: The selective estrogen receptor modulator (SERD) fulvestrant is commonly used to treat patients with hormone receptor positive advanced breast cancer, although potential mechanisms of acquired resistance are poorly understood. plasmaMATCH cohort A (NCT03182634) investigated the activity of fulvestrant in patients with activating ESR1 mutations in circulating tumor DNA (ctDNA). Here we present analysis of baseline and end-of-treatment (EOT) ctDNA to identify potential resistance mutations to fulvestrant. Methods: Paired baseline and EOT plasma samples from patients enrolled into plasmaMATCH underwent ctDNA sequencing (Guardant360, Guardant Health) to identify acquired mutations. For F404 analysis, MCF-7 cells were transiently transfected with estrogen receptor expression constructs containing either wildtype ESR1 (WT), single ESR1 mutations (D538G, E380Q, F404L), or compound mutations (D538G_F404L, E380Q_F404L), alongside an estrogen response element (ERE)-luciferase reporter construct. Transfected cells were treated with or without fulvestrant and ERE-luciferase activity compared. Results: Of 84 patients enrolled in cohort A, 69 had paired baseline and EOT sequencing. Patients with baseline ESR1 Y537S had shorter progression free survival (PFS), and Y537C longer PFS, than those wild-type for each respective mutation (p = 0.03 and p = 0.04). Patients frequently acquired mutations at EOT (n = 35, 51%), including potentially targetable mutations in 25% (including 3 PTEN, 3 BRCA1/2, 2 PIK3CA, 2 HER2, 1 BRAF). Three (4%) patients acquired ESR1 p.F404 mutations (F404L, F404I, F404V), with 7 mutations in total. Of 26 patients with PFS of ≥16 weeks, 3 patients (12%) acquired ESR1 F404. In 800 patients screened for entry to plasmaMATCH, one harbored a F404 mutation (0.13%), with a prior history of fulvestrant. F404 mutations resided in cis with E380Q in 6/7 assessable mutations. In vitro structural modelling revealed that ESR1 p.F404 resides within the ESR1 ligand binding domain, and contributes to estrogen and fulvestrant binding through a pi-stacking bond between the aromatic ring of phenylalanine and estrogen/fulvestrant, with all F404 mutations disrupting this bond. Transient transfection demonstrated that single mutations D538G, E380Q, F404L and wild-type ESR1 were sensitive to fulvestrant (p < 0.0001, p = 0.0006, p = 0.04 and p = 0.0001), whereas compound mutations D538G_F404L and E380Q_F404L were resistant. Further investigation of relative sensitivity of the F404 mutant ESR1 to other anti-estrogens will be presented. Conclusions: We have identified a novel resistance mechanism to fulvestrant, with F404 mutations acquired in patients with pre-existing activating ESR1 mutations. F404 confers fulvestrant resistance through the loss of a pi-stacking bond and likely reduced fulvestrant binding affinity, identifying a new potential target to overcome endocrine therapy resistance.

e13039Background: Palbociclib is approved in the UK in first line for hormone receptor (HR)-positive HER2-negative advanced breast cancer (ABC). A Compassionate Access Program (CAP) allowed patient...