Study Aiming to Compare the Plasma Exposure of the Payload (Free-DXd) in Patients Treated by T-DXd for Locally Advanced or Metastatic Breast Cancer According to Their BMI.

This study is examining whether body weight (BMI) affects how much of the active chemotherapy component (called free-DXd) from the breast cancer drug trastuzumab deruxtecan (T-DXd) is absorbed into the bloodstream, to understand if dosing should be adjusted for people with higher or lower body weight. **You may be eligible if...** - You are 18 or older with a confirmed diagnosis of metastatic or locally advanced breast cancer - Your cancer has HER2 overexpression or amplification, low HER2 expression, or ultra-low HER2 expression - You are already eligible for and will be receiving trastuzumab deruxtecan (T-DXd) as part of your treatment - Female participants of childbearing potential must have a negative pregnancy test within 72 hours before treatment **You may NOT be eligible if...** - You do not have a confirmed breast cancer diagnosis - You are not eligible to receive T-DXd per your doctor's assessment - Your blood tests, organ function, or pregnancy status do not meet the study requirements Talk to your doctor to see if this trial is right for you.