PIK3CA mutation detection by circulating tumor cell sequencing guides the effective PI3K inhibitor treatment of a patient with hormone receptor-positive breast cancer transformed into triple-negative breast cancer: A case report.

Most patients with hormone receptor-positive breast cancer (BC) prefer endocrine therapy and chemotherapy. However, some HR+ BC patients often exhibit resistance to CDK4/6 inhibitors and even undergo molecular subtyping changes during disease progression. Therefore, precise detection and treatment of these patients after disease progression are crucial. In October 2021, a 51-year-old Chinese female (Han ethnicity) with a palpable mass in her left breast was diagnosed with HR+ BC. After endocrine therapy and CDK4/6 inhibitor treatment, the patient's disease still progressed and transformed into triple-negative BC. Peripheral blood circulating tumor cells were isolated from the patient and performed targeted sequencing by using next-generation sequencing, revealing the presence of a PIK3CA mutation in the patient. The patient's condition was effectively controlled after treatment with PI3K inhibitors. The patient's progression free survival was prolonged to 7 months after treatment with the PI3K inhibitor. All adverse reactions were tolerated after symptomatic treatment. Her adverse reactions remained tolerable until further progression. Our case fully demonstrates the importance of the early detection of PIK3CA mutations in treatment strategies. In addition, circulating tumor cells-targeted sequencing technology could be used for the concomitant diagnosis of advanced BC and evaluate gene mutations under treatment pressure.