The additional ligand N6L on the surface of DOX-functionalized MNPs increases their internalization into BT474 cells and further inhibits tumor cell growth in combination with magnetic hyperthermia in vitro. (A) Intracellular iron content measured in the cells at 24 h after incubation with MNP formulations. Mean ± standard deviation of n ≥ 6 samples/group, p ≤ 0.01 (**) and p ≤ 0.001 (***). (B) The relative dehydrogenase activity was measured 24 h or 48 h after incubation with MNP formulations and the alternating magnetic field (AMF) treatment, (C) MNP application without AMF and (D) treatment with the free ligands. AMF settings: H = 23.9 kA/m, f = 410 kHz, 43 °C for 60 min. Means and standard deviation of three individual experiments with three parallels each, (*) p ≤ 0.05 and (***) p ≤ 0.001.