Liquid Biopsy: Insights Into Monitoring Tumor Dynamics and Response to Therapy in Patients with Breast Cancer

 The ability to identify the molecular features of metastatic breast cancer (BC) provides a unique insight into a patient's therapeutic options and the opportunity to follow the BC progress over time. A classical tissue biopsy remains the standard procedure to describe tumor biology and guide treatment choices. However, a liquid biopsy, which can provide medical practitioners with the opportunity to detect genomic mutations and monitor therapeutic effects, can play a prominent role in the diagnosis, therapy, and prognosis of patients with different malignancies, including metastatic BC. In fact, the liquid-biopsy-based therapeutic interventions led to the approval of alpelisib (a PI3K inhibitor) in patients with hormone receptor (HR)-positive, human epidermal growth factor receptor2 (HER2)-negative, advanced or metastatic BC, in whom BC had progressed on or after therapy with an aromatase inhibitor (AI). This chapter describes a liquid biopsy in BC. It explores its potential for clinical applications in early diagnosis, monitoring treatment response, detecting minimal residual lesions, predicting risk of progression or recurrence, and estimating prognosis. It compares a liquid biopsy with a tissue biopsy, and outlines the benefits and limitations of each of these procedures, focusing on patients with metastatic BC. Moreover, this chapter analyses the results from recent studies relevant to liquid biopsies in BC (e.g., circulating tumor cells (CTCs) and circulating tumor DNA (ctDNA)). 

Keywords: Alpelisib (A PI3K Inhibitor), Breast cancer (BC), Circulating tumor cells (CTCs), Circulating tumor DNA (ctDNA), Liquid biopsy, Targeted therapy.