For quantification of cellular uptake of the different hemin-based compounds, MDA-MB231 cells were seeded in T-75 flasks until a confluency of 80 % was reached. These were then treated with FBS-free RPMI containing 4 or 8 μM of each tested compound with/without 300 μM DETA-NO, and cultured for 12 hours. After that, the media were removed, the cells were detached, centrifuged, counted and washed by PBS. Iron quantification was performed using the iron assay kit. For that, the separated cells were vigorously homogenized in the iron Assay buffer, centrifuged at 13,000 g for 15 min at 4 •C, and ferric iron in the supernatant was reduced by the supplied iron reducer for 30 min. Following this, an iron probe was added to each tested solution, which was incubated for 60 min at room temperature after which the absorbance of solution was measured at 593 nm using the microplate reader. The total cellular iron concentration was normalized to the cell count under each treatment.