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Although iron is a bio-essential metal, dysregulated iron acquisition and metabolism result in production of reactive oxygen species (ROS) due to the Fenton catalytic reaction, which activates ferroptotic cell death pathways. The lipophilic Fe(III)-chelator chlorquinaldol (L; i.e. , 5,7-dichloro-8-hydroxy-2-methylquinoline) strongly favors the formation of a highly stable binuclear Fe(III) complex [(L 2 Fe) 2 (μ-O)] (1) that can mimic the function of the Fe(III)-transferrin complex in terms of the strong binding to Fe(III) and facile release of Fe(II) when the metal center is reduced. It should be noted that the cellular uptake of 1 is not transferrin receptor-mediated but enhanced by the high lipophilicity of chlorquinaldol. Once 1 is transported across the cell membrane, Fe(III) can be reduced by ferric reductase or other cellular antioxidants to be released as Fe(II), which triggers the Fenton catalytic reaction, thus harnessing the anticancer activity of iron. As the result, this transferrin-inspired iron-delivery strategy significantly reduces the cytotoxicity of 1 in normal human embryonic kidney cells (HEK 293) and the hemolytic activity of 1 in human red blood cells (hRBCs), giving rise to the unique tumor-specific anticancer activity of this Fe(III) complex.

The judicious selection of 5,7-dibromo-2-methy-8-quinolinol (BQ) as a Mn(II) ionophore results in the synthesis of Mn(BQ) 2 (DMSO) 2 ·DMSO (1), a potent metalloantibiotic with a dual antimicrobial mode of action against four different strains of Staphylococcus aurous (SA) bacteria (MIC = 0.625 μg mL -1 ). Additionally, 1 can overcome ciprofloxacin-resistance in methicillin-resistant SA bacteria.

Although iron is essential for all forms of life, it is also potentially toxic to cells as the increased and unregulated iron uptake can catalyze the Fenton reaction to produce reactive oxygen species (ROS), leading to lipid peroxidation of membranes, oxidation of proteins, cleavage of DNA and even activation of apoptotic cell death pathways. We demonstrate that Fe(hinok) 3 (hinok = 2-hydroxy-4-isopropyl-2,4,6-cycloheptatrien-1-one), a neutral Fe(III) complex with high lipophilicity is capable of bypassing the regulation of iron trafficking to disrupt cellular iron homeostasis; thus, harnessing remarkable anticancer activity against a panel of five different cell lines, including Pt-sensitive ovarian cancer cells (A2780; IC 50 = 2.05 ± 0.90 μM or 1.20 μg/mL), Pt-resistant ovarian cancer cells (A2780cis; IC 50 = 0.92 ± 0.73 μM or 0.50 μg/mL), ovarian cancer cells (SKOV-3; IC 50 = 1.23 ± 0.01 μM or 0.67 μg/mL), breast cancer cells (MDA-MB-231; IC 50 = 3.83 ± 0.12 μM or 2.0 μg/mL) and lung cancer cells (A549; IC 50 = 1.50 ± 0.32 μM or 0.82 μg/mL). Of great significance is that Fe(hinok) 3 exhibits unusual selectivity toward the normal HEK293 cells and the ability to overcome the Pt resistance in the Pt-resistant mutant ovarian cancer cells of A2780cis.

8-Hydroxyquinoline (8-hq) exhibits potent antimicrobial activity against Staphylococcus aureus (SA) bacteria with MIC = 16.0-32.0 µM owing to its ability to chelate metal ions such as Mn 2+ , Zn 2+, and Cu 2+ to disrupt metal homeostasis in bacterial cells. We demonstrate that Fe(8-hq) 3 , the 1:3 complex formed between Fe(III) and 8-hq, can readily transport Fe(III) across the bacterial cell membrane and deliver iron into the bacterial cell, thus, harnessing a dual antimicrobial mechanism of action that combines the bactericidal activity of iron with the metal chelating effect of 8-hq to kill bacteria. As a result, the antimicrobial potency of Fe(8-hq) 3 is significantly enhanced in comparison with 8-hq. Resistance development by SA toward Fe(8-hq) 3 is considerably delayed as compared with ciprofloxacin and 8-hq. Fe(8-hq) 3 can also overcome the 8-hq and mupirocin resistance developed in the SA mutant and MRSA mutant bacteria, respectively. Fe(8-hq) 3 can stimulate M1-like macrophage polarization of RAW 264.7 cells to kill the SA internalized in such macrophages. Fe(8-hq) 3 exhibits a synergistic effect with both ciprofloxacin and imipenem, showing potential for combination therapies with topical and systemic antibiotics for more serious MRSA infections. The in vivo antimicrobial efficacy of a 2% Fe(8-hq) 3 topical ointment is confirmed by the use of a murine model with skin wound infection by bioluminescent SA with a reduction of the bacterial burden by 99 ± 0.5%, indicating that this non-antibiotic iron complex has therapeutic potential for skin and soft tissue infections (SSTIs).