Assessment Of Esr1 And Erbb2 Mutations In Estrogen Receptor Positive (Er Plus ) Metastatic Breast Cancers (Mbc).

1040Background: Mutations (mut) in ESR1 have been reported in ER+ breast cancers (BC) as an acquired resistance mut to aromatase inhibitor (AI) therapy. Acquired ERBB2 mut have also been reported in MBC patients (pts) that cause activated ERBB2 signaling. The emergence of acquired secondary mut presents challenges in effective treatment approaches. Methods: Comprehensive genomic profiling was performed on 83 BC samples with 48 metastatic; 34 primary samples. Targeted next-generation sequencing was performed on 562 cancer associated genes in paired tumor and blood DNA (germline) samples. Results: ESR1-mut were found in 23%(11/48) of ER+ MBC tissues with no mut detected in primary ER+ BCs. Mutations-D538G, Y537S and E380Q in the ligand binding domain of ER were the most common alterations, found in 54.5%, 18% and 18% of ESR1 mut samples, respectively. An ER+ HER2- liver biopsy obtained after 20 mos on AI + everolimus had ESR1-D538G and TSC2 structural event. Protein array showed high expression of androgen ...