Mutation analysis using cell-free DNA for endocrine therapy in patients with HR+ metastatic breast cancer

We prospectively evaluated the utility of ESR1 and PIK3CA mutation analysis with cell-free DNA (cfDNA) using droplet digital PCR (ddPCR) for the efficacy of endocrine therapy (ET) in hormone receptive positive (HR+) metastatic breast cancer (MBC) patients. CfDNA was analyzed just before the start of ET for MBC. E380Q, Y537N, Y537S, and D538G were assessed for ESR1 mutations and H1047R, E545K , and E542K were assessed for PIK3CA mutations. A total of 75 patients were enrolled. Of those, 31 (41.3%) received letrozole with palbociclib, and 28 (37.3%) received exemestane and everolimus (EverX). ESR1 mutations were found in 36 (48.0%) patients, of which 16 (21.3%) had more than one variant. Seventeen (23.6%) patients had one PIK3CA mutation and 8 (11.1%) had two. In the total population, time to progression of the first ET after enrollment (TTP1) decreased significantly as the number of ESR1 mutations increased ( p