Background: Preclinical data support a key role for the human epidermal growth factor receptor 3 (HER3) pathway in hormone receptor (HR)–positive breast cancer. Recently, new HER3 directed antibody drug conjugates have shown activity in breast cancer. Given the need to better understand the molecular biology, tumor microenvironment, and mechanisms of drug resistance in breast cancer, we designed this window-of-opportunity study with the HER3 directed antibody drug conjugate patritumab deruxtecan (HER3-DXd; U3-1402).Trial Design: Based on these data, a prospective, multicenter, single-arm, window-of-opportunity study was designed to evaluate the biological effect of patritumab deruxtecan in the treatment of naïve patients with HR-positive/HER2-negative early breast cancer whose primary tumors are ≥1 cm by ultrasound evaluation. Patients will be enrolled in four cohorts according to the mRNA-based ERBB3 expression by central assessment. The primary endpoint is a CelTIL score after one single dose. A translational research plan is also included to provide biological information and to evaluate secondary and exploratory objectives of the study.Trial Registration Number: EudraCT 2019-004964-23; NCT number: NCT04610528.

Although the metastasic breast cancer is still an incurable disease, recent advances have increased significantly the time to progression and the overall survival. However, too much information has been produced in the last 2 years, so a well-based guideline is a valuable document in treatment decision making. The SEOM guidelines are intended to make evidence-based recommendations on how to manage patients with advanced and recurrent breast cancer to achieve the best patient outcomes based on a rational use of the currently available therapies. To assign a level of certainty and a grade of recommendation the United States Preventive Services Task Force guidelines methodology was selected as reference.

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Patients with metastatic breast cancer have a wide number of treatment options, including medical, surgical, and supportive care measures. Treatment decisions are based in predictive and prognostic factors and the informed choice of the patients. SEOM has elaborated these guidelines with evidence-based recommendations for the diagnostic work-up, treatment (chemotherapy, endocrine therapy and targeted therapies) and supportive care for the management of these patients.