Breast cancer is currently classified by immunohistochemistry. However, technological advances in the detection of circulating tumor DNA (ctDNA) have made new options available for diagnosis, classification, biological knowledge, and treatment selection. Breast cancer is a heterogeneous disease and ctDNA can accurately reflect this heterogeneity, allowing us to detect, monitor, and understand the evolution of the disease. Breast cancer patients have higher levels of circulating DNA than healthy subjects, and ctDNA can be used for different objectives at different timepoints of the disease, ranging from screening and early detection to monitoring for resistance mutations in advanced disease. In early breast cancer, ctDNA clearance has been associated with higher rates of complete pathological response after neoadjuvant treatment and with fewer recurrences after radical treatments. In metastatic disease, ctDNA can help select the optimal sequencing of treatments. In the future, thanks to new bioinformatics tools, the use of ctDNA in breast cancer will become more frequent, enhancing our knowledge of the biology of tumors. Moreover, deep learning algorithms may also be able to predict breast cancer evolution or treatment sensitivity. In the coming years, continued research and the improvement of liquid biopsy techniques will be key to the implementation of ctDNA analysis in routine clinical practice.

Purpose: In hormone receptor–positive (HR+)/HER2− metastatic breast cancer (MBC), it is imperative to identify patients who respond poorly to cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) and to discover therapeutic targets to reverse this resistance. Non-luminal breast cancer subtype and high levels of CCNE1 are candidate biomarkers in this setting, but further validation is needed. Experimental Design: We performed mRNA gene expression profiling and correlation with progression-free survival (PFS) on 455 tumor samples included in the phase III PEARL study, which assigned patients with HR+/HER2− MBC to receive palbociclib+endocrine therapy (ET) versus capecitabine. Estrogen receptor–positive (ER+)/HER2− breast cancer cell lines were used to generate and characterize resistance to palbociclib+ET. Results: Non-luminal subtype was more prevalent in metastatic (14%) than in primary tumor samples (4%). Patients with non-luminal tumors had median PFS of 2.4 months with palbociclib+ET and 9.3 months with capecitabine; HR 4.16, adjusted P value < 0.0001. Tumors with high CCNE1 expression (above median) also had worse median PFS with palbociclib+ET (6.2 months) than with capecitabine (9.3 months); HR 1.55, adjusted P value = 0.0036. In patients refractory to palbociclib+ET (PFS in the lower quartile), we found higher levels of Polo-like kinase 1 (PLK1). In an independent data set (PALOMA3), tumors with high PLK1 show worse median PFS than those with low PLK1 expression under palbociclib+ET treatment. In ER+/HER2− cell line models, we show that PLK1 inhibition reverses resistance to palbociclib+ET. Conclusions: We confirm the association of non-luminal subtype and CCNE1 with resistance to CDK4/6i+ET in HR+ MBC. High levels of PLK1 mRNA identify patients with poor response to palbociclib, suggesting PLK1 could also play a role in the setting of resistance to CDK4/6i.

Breast cancer is the most common cancer in women in our country and it is usually diagnosed in the early and potentially curable stages. Nevertheless, around 20–30% of patients will relapse despite appropriate locoregional and systemic therapies. A better knowledge of this disease is improving our ability to select the most appropriate therapy for each patient with a recent diagnosis of an early stage breast cancer, minimizing unnecessary toxicities and improving long-term efficacy.

Despite the improvement achieved by the introduction of HER2-targeted therapy, up to 25% of early human epidermal growth factor receptor 2-positive (HER2+) breast cancer (BC) patients will relapse. Beyond trastuzumab, other agents approved for early HER2+ BC include the monoclonal antibody pertuzumab, the antibody-drug conjugate (ADC) trastuzumab-emtansine (T-DM1) and the reversible HER2 inhibitor lapatinib. New agents, such as trastuzumab-deruxtecan or tucatinib in combination with capecitabine and trastuzumab, have also shown a significant improvement in the metastatic setting. Other therapeutic strategies to overcome treatment resistance have been explored in HER2+ BC, mainly in HER2+ that also overexpress estrogen receptors (ER+). In ER+ HER2+ patients, target therapies such as phosphoinositide-3-kinase (PI3K) pathway inhibition or cyclin-dependent kinases 4/6 blocking may be effective in controlling downstream of HER2 and many of the cellular pathways associated with resistance to HER2-targeted therapies. Multiple trials have explored these strategies with some promising results, and probably, in the next years conclusive results will succeed. In addition, HER2+ BC is known to be more immunogenic than other BC subgroups, with high variability between tumors. Different immunotherapeutic agents such as HER-2 therapy plus checkpoint inhibitors, or new vaccines approaches have been investigated in this setting, with promising but controversial results obtained to date.

Patritumab deruxtecan (HER3-DXd) is a human epidermal growth factor receptor 3 (HER3)-directed antibody-drug conjugate composed of a fully human anti-HER3 monoclonal antibody (patritumab) covalently linked to a topoisomerase I inhibitor payload via a stable, tumor-selective, tetrapeptide-based cleavable linker. TOT-HER3 is a window-of-opportunity study designed to assess the biological activity, measured by CelTIL score [= -0.8 × tumor cellularity (in %) + 1.3 × tumor-infiltrating lymphocytes (TILs) (in %)], and clinical activity of HER3-DXd during short-term (21 days) pre-operative treatment in patients with primary operable HER2-negative early breast cancer. Patients with previously untreated hormone receptor-positive/HER2-negative tumors were allocated to one of four cohorts according to baseline ERBB3 messenger RNA expression. All patients received one dose of HER3-DXd 6.4 mg/kg. The primary objective was to evaluate change from baseline in CelTIL score. Seventy-seven patients were evaluated for efficacy. A significant change in CelTIL score was observed, with a median increase from baseline of 3.5 (interquartile range, -3.8 to 12.7; P = 0.003). Among patients assessable for clinical response (n = 62), an overall response rate of 45% was observed (tumor measurement by caliper), with a trend toward an increase in CelTIL score among responders compared with non-responders (mean difference, +11.9 versus +1.9). Change in CelTIL score was independent of baseline ERBB3 messenger RNA and HER3 protein levels. Genomic changes occurred, including switching toward a less proliferative tumor phenotype based on PAM50 subtypes, suppression of cell proliferation genes, and induction of genes associated with immunity. Treatment-emergent adverse events were observed in 96% of patients (14% grade ≥3); most common were nausea, fatigue, alopecia, diarrhea, vomiting, abdominal pain, and neutrophil count decrease. A single dose of HER3-DXd was associated with clinical response, increased immune infiltration, suppression of proliferation in hormone receptor-positive/HER2-negative early breast cancer, and a tolerable safety profile consistent with previously reported results. These findings support further study of HER3-DXd in early breast cancer.