Trastuzumab Deruxtecan (T-DXd): Tailoring Treatment and Companion Diagnostics (CDx) by Liquid Biopsy

This is a prospective single-center pilot study phase II non randomized designed to explore the role of liquid biopsy in metastatic HER2-positive breast cancer patients treated with Trastuzumab deruxtecan (T-Dxd) as second line treatment according to international guidelines. All eligible patients to T-Dxd as second line treatment will receive T-Dxd intravenous at dose of 5,4 mg/kg every three weeks until progression disease or unacceptable toxicities. Subjects eligible to T-Dxd, who agree to participate in the study, will undergo serial blood samples for liquid biopsy (LB) until progression disease. The timing of blood drawing will be scheduled as follows: At each T-DXd administration for the first four cycles of T-Dxd (every three weeks). The next blood drawings will be done every three cycles of T-Dxd (every 9 weeks) until the thirteenth cycle. The next blood drawings will be done every six cycles of T-Dxd (every 18 weeks) until progression disease (documented by medical imaging) For all enrolled patients with available tumour tissue from primary diagnosis or last biopsy from recurrence, tissue samples will be requested for exploratory analysis.