*2.4. 3D multicellular Tumor Spheroid Cytotoxicity Assays*

MCF-7 or MDA-MB-231 cells were seeded into ultra-low attachment 96-well plates (Sigma-Aldrich, MO, USA), at a density ranging between 625 and 5000 cells per well and incubated at 37 ◦C to promote spheroid formation. Liposomes were serially diluted in PBS and incubated with cells for up to 96 h (each concentration tested in triplicate).
