Action of chelators in iron-loaded cardiac cells: Accessibility to intracellular labile iron and functional consequences.

Labile iron in hemosiderotic plasma and tissue are sources of iron toxicity. We compared the iron chelators deferoxamine, deferiprone, and deferasirox as scavengers of labile iron in plasma and cardiomyocytes at therapeutic concentrations. This comprised chelation of labile plasma iron (LPI) in samples from thalassemia patients; extraction of total cellular iron; accessing labile iron accumulated in organelles and preventing formation of reactive-oxidant species; and restoring impaired cardiac contractility. Neonatal rat cardiomyocytes were used for monitoring chelator extraction of LCI (labile cell iron) as 59Fe; assessing in situ cell iron chelation by epifluorescence microscope imaging using novel fluorescent sensors for iron and reactive oxygen species (ROS) selectively targeted to organelles, and monitoring contractility by time-lapse microscopy. At plasma concentrations attained therapeutically, all 3 chelators eliminated LPI but the orally active chelators rapidly gained access to the LCI pools of cardiomyocytes, bound labile iron, attenuated ROS formation, extracted accumulated iron, and restored contractility impaired by iron overload. The effect of deferoxamine at therapeutically relevant concentrations was primarily by elimination of LPI. The rapid accessibility of the oral chelators deferasirox and deferiprone to intracellular labile iron compartments renders them potentially efficacious for protection from and possibly reversal of cardiac damage induced by iron overload.

Intracellular labile iron pools as direct targets of iron chelators: a fluorescence study of chelator action in living cells.

The primary targets of iron chelators used for treating transfusional iron overload are prevention of iron ingress into tissues and its intracellular scavenging. The present study was aimed at elucidating the capacity of clinically important iron chelators such as deferiprone (DFP), desferrioxamine, and ICL670 to (a) gain direct access to intracellular iron pools of key cells of iron accumulation (macrophages, hepatocytes, and cardiomyocyte cell lines); (b) chelate the labile iron present in discrete cell compartments/organelles; and (c) prevent labile iron involvement in the generation of reactive oxidant species. Chelation of cytosolic and organellar cell iron was visualized dynamically and quantitatively in living cells by fluorescence microscopic imaging of fluorescent metallosensors (used as iron-quenched complexes of calceins) targeted to either cytosol, endosome-lysosomes, or mitochondria. The rate and extent of fluorescence recovery provided an in situ measure of the accessibility of chelators to particular cell sites/organelles. Complementary, fluorogenic redox probes associated with cell compartments enabled identification of chelator-sensitive, localized reactive oxidant production. Our studies indicate that chelation by desferrioxamine is slow and is enhanced in cells with relatively high endocytic activities, while ICL670 and DFP readily enter most cells and efficiently reach the major intracellular sites of iron accumulation.