a) Cytotoxicity of HD NPs, free DOX, and free HCPT toward MCF-7R cells evaluated by CCK-8 assays after 48 h of treatment. (b) Combination index (CI) of HD NPs against MCF-7R cells. CI values <1 indicate synergism, CI values equal to 1 indicate an additive effect, and CI values >1 indicate antagonism. (c) Long-term colony formation assays of MCF-7R cells. Cells were grown in the presence of drugs (4 μM DOX and 1 μM HCPT) for 7 days. For each cell line, all dishes were fixed at the same time, stained, and photographed.