HR+/HER2- Advanced or Metastatic Breast Cancer Patients Treated With Sacituzumab Govitecan

This study will include patients with HR+/HER2- advanced/metastatic breast cancer who have progressed on prior endocrine therapy and CDK4/6i and who have received up to 1 prior regimen of chemotherapy or ADC for metastatic breast canncer. The primary objective is to evaluate the change in the CelTIL score, a combined biomarker based on stromal tumor-infiltrating lymphocytes and tumor cellularity, as surrogate of treatment response after one dose of sacituzumab govitecan (SG). Patients who fulfil all eligibility criteria will start SG at 10 mg/kg as an IV infusion on Days 1 and 8 of a 21-day cycle. SG will be administered continuously until progression of the disease, unacceptable toxicity, investigator's decision, withdrawal of consent, or other reasons described in the protocol. Tumor tissue (newly obtained) will be sent to a central laboratory. After 2 weeks(14-21 days) of treatment a new biopsy of the same lesion will be performed. Tumor biopsy will be also performed at disease progression / EoT. In addition, blood samples (for ctDNA) will be collected at C1D1, C2D1 and at progression / EoT for exploratory objectives. Imaging will be performed prior to day 1 of treatment and target and non-target lesions will be identified as per RECIST 1.1. Tumor assessments will be performed every 9 weeks until the start of a new anti-cancer therapy, withdrawal of consent, progression of disease, death, or the end of the study, whichever occurs first. Tumor assessments will be performed on the specified schedule regardless of treatment delays. Tumor response will be assessed as per RECIST v.1.1. Safety assessments will include the incidence, nature, and severity of AEs and laboratory abnormalities graded per the NCI CTCAE v.5.0. Laboratory safety assessments will include the regular monitoring of hematology, blood chemistry and pregnancy test.