Locally Advanced or Metastatic Breast Cancers

This is a multicenter, prospective, non-randomized, open-label, pharmacokinetic 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. The primary objective is to compare plasma exposition of the payload (free-DXd) between overweight or obese (BMI\>25) and normal weight (BMI≤25) breast cancer patients during the first 3 cycles of Trastuzumab-Deruxtecan (T-DXd). A total of 210 patients will have to be enrolled in this study with the following repartition: N = 105 patients with a BMI ≤ 25 (normal weight patients). N = 105 patients with a BMI \> 25 (overweight or obese patients) with at least 30 obese patients (BMI\>30).