In photodynamic therapy (PDT), a non-invasive anticancer treatment, visible light is used as a magic bullet selectively destroying cancer cells by a photosensitizer that is non-toxic in the dark. Protoporphyrin IX (PpIX ) is a natural photosensitizer synthesized in the cell which is also a chelating agent that if bonded to Fe2+ forms heme, a central com-ponent of hemoglobin. Therefore, xenobiotic iron chelators can disturb iron homeostasis, increasing the accumulation of PpIX, obstructing the last step of heme biosynthesis, and enhancing PDT efficiency. However, the attempts to use this promising idea have not proved to be hugely successful. Herein, we revisited this issue by analyzing the application of iron chelators highly toxic in the dark which should have higher Fe2+ affinity than the non-toxic chelators used so far. We have designed and prepared thiosemicarbazones (TSC) with the highest dark cellular cytotoxicity among TSCs ever reported. We demonstrate that compound 2 exerts powerful PDT enhancement when used in combination with 5-aminolevulinic acid (ALA), a precursor of PpIX.