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Liquid biopsy has emerged as an alternative source of nucleic acids for the management of Epidermal Growth Factor Receptor (EGFR)-mutant non-Small Cell Lung Cancer (NSCLC). The use of circulating cell-free DNA (cfDNA) has been recently introduced in clinical practice, resulting in the improvement of the identification of druggable EGFR mutations for the diagnosis and monitoring of response to targeted therapy. EGFR-dependent (T790M and C797S mutations) and independent (Mesenchymal Epithelial Transition [MET] gene amplification, Kirsten Rat Sarcoma [KRAS], Phosphatidyl-Inositol 4,5-bisphosphate 3-Kinase Catalytic subunit Alpha isoform [PI3KCA], and RAF murine sarcoma viral oncogene homolog B1 [BRAF] gene mutations) mechanisms of resistance to EGFR tyrosine kinase inhibitors (TKIs) have been evaluated in plasma samples from NSCLC patients using highly sensitive methods (i.e., digital droplet PCR, Next Generation Sequencing), allowing for the switch to other therapies. Therefore, liquid biopsy is a non-invasive method able to detect the molecular dynamic changes that occur under the pressure of treatment, and to capture tumor heterogeneity more efficiently than is allowed by tissue biopsy. This review addresses how liquid biopsy may be used to guide the choice of treatment strategy in EGFR-mutant NSCLC.

Objectives To investigate if full‐length androgen receptor (AR‐FL) is associated with resistance to androgen receptor (AR)‐directed therapy independently and/or combined with AR splice variant 7 (AR‐V7). Patients and Methods Plasma samples were prospectively collected from 73 patients with castrate‐resistant prostate cancer before first‐ or second‐line AR‐directed therapy. mRNA was isolated from exosomes and AR‐FL and AR‐V7 were analysed by droplet digital PCR. Results AR‐FL was detected in all patients and 22% of them were AR‐V7‐positive at baseline. AR‐FL expression was significantly higher in AR‐V7‐positive vs AR‐V7‐negative patients (P < 0.0001). After stratifying patients by tertile for AR‐FL expression, progression‐free survival (PFS) was 22 vs 18 vs 4 months for lower vs intermediate vs higher tertile, respectively (P = 0.0003). The median PFS and overall survival were significantly longer in AR‐V7‐negative vs AR‐V7‐positive patients (20 vs 4 months, P < 0.0001; not reached vs 9 months, P < 0.0001, respectively). Conclusions Resistance to AR‐directed therapy was associated with the presence of AR‐V7; however, AR‐FL expression may help better refine response and survival of patients to AR‐directed therapy. Both biomarkers, if validated in prospective trials, could be used to select the best treatment strategy.

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