A Window-of-opportunity Study of U3-1402, a HER3-targeting Antibody-drug Conjugate in Operable Breast Cancer According to ERBB3 Expression

This is a prospective, multicenter, single arm, window-of-opportunity study evaluating the biological effect of U3-1402 in treatment naïve patients with early breast cancer, whose primary tumors are ≥1 cm by ultrasound evaluation. The primary objective is to evaluate the biological activity of U3-1402, measured as the CelTIL score increase at post-treatment (C1D21) in HR+/HER2-negative BC included patients. The study will consist of 2 parts enrolling \~115 patients. * Part A will target to treat, with 6.4 mg/kg dose, 80 patients with HR-positive/HER2-negative tumors and * Part B will target to treat with 5.6 mg/kg dose 20 patients with HR-positive/HER2-negative and 15 patients with TNBC tumors Part A will test U3-1402 in patients with HR-positive/HER2-negative early breast cancer with a dose of 6.4 mg/kg. Part B will consist in testing 5.6 mg/kg dose of U3-1402 in patients with HR-positive/HER2-negative early breast cancer and in triple-negative early breast cancer and will be performed sequentially after Part A.