Selection of a new generation of orally active α-ketohydroxypyridine iron chelators intended for use in the treatment of iron overload

The prospect of selecting oral α-ketohydroxypyridine chelators intended for clinical use in iron overload has been examined using several animal models of efficacy and toxicity. Studies using iron dextran-loaded mice labelled with ⁵⁹Fe have shown that only the 1-substituted methyl, ethyl, (n)propyl, allyl, cyclopropyl, 2′-methoxyethyl, 3′-ethoxypropyl, or 2-methyl- or 2-ethyl- 3-hydroxypyrid-4-one chelators were orally effective in increasing iron (⁵⁹Fe) excretion by comparison to intraperitoneally administered desferrioxamine at the same dose (250 mg/kg). In contrast, chelators containing -H, mono- or dihydroxyalkyl and diethoxyethyl 1-substituents caused very little or no increase in iron (⁵⁹Fe) excretion by the oral or intraperitoneal routes. In vitro studies using ferritin and haemosiderin have shown that equivalent iron release took place with both groups of chelators irrespective of their in vivo effects. In most cases there was no correlation between the n-octanol/water partition coefficient (K_par) and iron removal efficacy but positive correlation between the lipophilicity and acute or subacute toxicity of these chelators in rats. The most toxic chelator in the chronic toxicity studies in rats was the lipophilic 1, 2-diethyl-3-hydroxypyrid-4-one (EL1NEt). The most effective chelator in increasing iron excretion in mice and rabbits was 1-allyl-2-methyl-3-hydroxypyrid-4-one (L1NAll), and the chelator with the highest safety margin in mice and rats was 1, 2-dimethyl-3-hydroxypyrid-4-one (L1). Overall the oral effectiveness in increasing iron excretion by these chelators in animals does not appear to be related to their lipophilicity or their ability to mobilise polynuclear iron in vitro but rather to other properties possibly related to their rate of biotransformation and excretion. © 1993 Wiley-Liss, Inc.     

Orally active α-ketohydroxy pyridine iron chelators intended for clinical use: in vivo studies in rabbits

Increased daily iron excretion from iron overloaded, 59Fe lactoferrin labelled rabbits was observed following the intragastric administration of 1,2-dimethyl-3-hydroxy pyrid-4-one (L1) or 1-ethyl-2-methyl-3-hydroxy pyrid-4-one (L1-NEt) at doses of 200 mg/kg. 59Fe excretion induced by these drugs was predominantly faecal and was comparable to that caused by similar doses of subcutaneous or intramuscular desferrioxamine. The effectiveness of the two alpha-ketohydroxy pyridine chelators was confirmed by repeated administration, intragastrically or by subcutaneous or intramuscular injection, to the same or other rabbits. Examination of the urine during the administration of the chelators revealed their high specificity for iron but not for copper, zinc, calcium or magnesium.     

Determinants of fecal and urinary iron excretion in desferrioxamine- treated rats

Radioiron excretion in the urine and feces after continuous subcutaneous infusion of desferrioxamine (DF) was studied in normal and hypertransfused rats in whom reticuloendothelial and parenchymal iron stores had been labeled by selective 59Fe probes. The existence of two alternative pathways for the chelation of iron in vivo by DF was indicated by the results. The first pathway was intracellular chelation in hepatocytes with chelated iron excreted through the bile only. The second pathway, which was extracellular, could only be activated after saturation of the transferrin iron binding capacity, with the chelated iron excreted by the kidneys. The last mechanism was probably identical with the mode of action of diethylenetriaminepentaacetic acid (DTPA). Further studies will be required in order to establish whether the enhancement in iron chelation obtained by the continuous infusion of DF in patients with complete saturation of circulating transferrin may or may not be related to the extracellular mechanism of iron chelation in hypertransfused rats.     

Uptake of Transferrin-Bound Iron by Rat Cells in Tissue Culture

The in vitro uptake of transferrin-bound iron by rat liver cells and rat embryo cells in culture, and by rat reticulocytes, was studied using labelled rat serum. The uptake of iron by rat liver cells was linear with time and showed a curvilinear relationship with the serum iron concentration. Over 40% of the cell radioactivity was found to be ferritin associated. Incubation of rat transferrin doubly labelled with ⁵⁹Fe and ¹²⁵l showed a difference between the uptakes of the two isotopes suggesting reflux of transferrin from the cell after iron uptake. In order to compare iron uptake from half saturated (Tf-Fe 1) and fully saturated (Tf-Fe2) transferrin, the three cell systems were each incubated in sera labelled with ⁵⁵Fe at 10% saturation and with ⁵⁹Fe at varying iron saturations ranging from 10% to 90%. Similar experiments were performed with sera having been randomly labelled with ⁵⁵Fe and labelled with ⁵⁹Fe but preincubated with reticulocytes in order to produce iron bound to non-reticulocyte orientated transferrin binding sites. The data obtained confirms the presence of a functional heterogeneity with respect to the half and fully saturated transferrin. Thus iron uptake from fully saturated transferrin was almost twice that for the half saturated molecule with rat liver cells or reticulocytes, and over three times that of the half saturated molecule with rat embryo cells. In contrast no such differences could be detected when iron uptake from serum randomly labelled with iron and serum with labelled non-reticulocyte orientated binding sites were compared. Finally, the effect of pre-incubating each of the three cell systems in unlabelled sera of varying percentage iron saturations on subsequent radio-iron uptake was studied. An inverse relationship between percentage saturation of the preincubation serum and subsequent uptake was demonstrated indicating that cellular mechanisms also influence cell iron uptake.     

PCTH: a novel orally active chelator for the treatment of iron overload disease

Our laboratories have prepared a novel class of iron (Fe) chelators of the 2-pyridylcarboxaldehyde isonicotinoyl hydrazone (PCIH) class. This article will review the iron chelation efficacy of this series of chelators, both in cell culture and in animal models. Several PCIH analogs were shown to be effective at inducing iron mobilization and preventing iron uptake from the iron-transport protein, transferrin. Moreover, several of these ligands were effective at permeating the mitochondrion and inducing iron release. Studies in mice demonstrated that the PCIH analog, PCTH, was orally active and well tolerated by mice at doses ranging from 50 to 100 mg kg(-1), twice daily (b.d.). A dose-dependent increase in fecal 59Fe excretion was observed in the PCTH-treated group. This level of iron excretion was similar to that found for the orally effective chelators, pyridoxal isonicotinoyl hydrazone (PIH) and deferiprone (L1). The PCIH group of ligands clearly has the potential for the treatment of beta-thalassemia (thal) and Friedreich's Ataxia (FA).     

Dose response studies using desferrioxamine and orally active chelators in a mouse model

59Fe excretion studies in response to different doses (4-9 mg) of three N-substituted 3-hydroxypyrid-4-one chelators, (1,2-dimethyl-3-hydroxypyrid-4-one, 1-ethyl-2-methyl-3-hydroxypyrid-4-one, 1-propyl-2-methyl-3-hydroxypyrid-4-one), and desferrioxamine in iron overloaded 59Fe lactoferrin labelled mice (40 +/- 4 g) have shown that the former chelators, when administered intraperitoneally and intragastrically, caused comparable 59Fe excretions to intraperitoneal desferrioxamine of equivalent doses. No apparent ill effects were observed when doses of 300 mg/kg were administered for 24 d.     

Relative oral efficacy and acute toxicity of hydroxypyridin-4-one iron chelators in mice [see comments]

The relationship between the oral efficacy and the acute toxicity of hydroxypyridin-4-one iron chelators has been investigated to clarify structure-function relationships of these compounds in vivo and to identify compounds with the maximum therapeutic safety margin. By comparing 59Fe excretion following oral or intraperitoneal administration of increasing doses of each chelator to iron-overloaded mice, the most effective compounds have been identified. These have partition coefficients (Kpart) above 0.3 in the iron-free form with a trend of increasing oral efficacy with increasing Kpart values (r = .6). However, this is achieved at a cost of increasing acute toxicity, as shown by a linear correlation between 59Fe excretion increase per unit dose and 1/LD50 (r = .83). A sharp increase in the LD50 values is observed for compounds with Kpart values above 1.0, suggesting that such compounds are unlikely to possess a sufficient therapeutic safety margin. Below a Kpart of 1.0, acute toxicity is relatively independent of lipid solubility. All the compounds are less toxic by the oral route than by the intraperitoneal route, although iron excretion is not significantly different by these two routes. At least five compounds (CP51, CP94, CP93, CP96, and CP21) are more effective orally than the same dose of intraperitoneal desferrioxamine (DFO) (P less than or equal to .02) or orally administered L1(CP20) (P less than or equal to .02).     

Transferrin uptake by bone marrow macrophages is independent of the degree of iron saturation

The uptake of transferrin by macrophages was studied in relation to the degree of iron saturation. Rat bone marrow derived macrophages were incubated with transferrin labelled with 59Fe and 3H. At 37 degrees C the amount of 59Fe incorporated by macrophages was dependent on the time of incubation. 3H labelled transferrin was found degraded in the supernatants of the cell culture (material not precipitated by trichloroacetic acid) in a time dependent fashion. Taking into account the specific activity of 59Fe-3H labelled transferrin, we found that 95% of the transferrin uptake was degraded. This suggests that most of the uptake of transferrin was not mediated by a receptor-dependent mechanism, but by a phase fluid endocytosis. 3H-labelled apotransferrin appears in the supernatant of the cell culture at the same rate as 59Fe-3H labelled diferric transferrin, showing an identical uptake for the two types of transferrin. Uptake of apo- or diferric transferrin by macrophages was identical in relation to time of incubation and the amount of transferrin used. These studies suggest that most of the transferrin uptake by bone marrow macrophages (non-activated or non-elicited cells) is mediated by a non-receptor mechanism that is independent of the degree of transferrin saturation.     

Effects of cadmium on in vitro and in vivo erythropoiesis: erythroid progenitor cells (CFU-E), iron, and erythropoietin in cadmium-induced iron deficiency anemia

Effects of cadmium (Cd) on in vitro and in vivo erythropoiesis in rats were studied by methylcellulose colony assay. Cd suppressed the in vitro growth of late erythroid progenitors (CFU-E) in a dose-dependent fashion and did not lose its inhibitory potency with increasing doses of erythropoietin (EPO). In addition, in marrow suspension cultures, Cd did not significantly influence 59Fe incorporation into both the cells and heme, and the Cd dose-responsive inhibition curve of the number of living cells was similar to that of CFU-E. These results suggest that the suppression of CFU-E colony formation by Cd is not due to the blocking of either EPO action to stimulate the growth of CFU-E or the iron incorporation into the cells ahd heme, but due to its direct cytotoxic effect. The colony suppression by Cd could be prevented by adding metallothionein to the cultures. On the other hand, oral administration of Cd to animals (100 mg/liter in drinking water) induced an iron deficiency anemia characterized by microcytic hypochromic red cells, decreased plasma iron, and increased total iron binding capacity. Marrow CFU-E density steadily increased as plasma iron decreased due to Cd administration and reached a plateau after 50 days. Plasma EPO titers were also found to be elevated in such a Cd-induced anemia. Parenteral iron administration during the Cd drinking period could completely prevent the development of iron deficiency anemia and the increase of both CFU-E and plasma EPO. There was a hyperbolic correlation between CFU-E and plasma iron or transferrin saturation. These results demonstrate that oral CD administration produces bone marrow hyperplasia at the CFU-E level due to iron deficiency.     

Effects of combined chelation treatment with pyridoxal isonicotinoyl hydrazone analogs and deferoxamine in hypertransfused rats and in iron-loaded rat heart cells

Although iron chelation therapy with deferoxamine (DFO) results in improved life expectancy of patients with thalassemia, compliance with parenteral DFO treatment is unsatisfactory, underlining the need for alternative drugs and innovative ways of drug administration. We examined the chelating potential of pyridoxal isonicotinoyl hydrazone (PIH) analogs, alone or in combination with DFO, using hypertransfused rats with labeled hepatocellular iron stores and cultured iron-loaded rat heart cells. Our in vivo studies using 2 representative PIH analogs, 108-o and 109-o, have shown that PIH analogs given orally are 2.6 to 2.8 times more effective in mobilizing hepatocellular iron in rats, on a weight-per-weight basis, than parenteral DFO administered intraperitoneally. The combined effect of DFO and 108-o on hepatocellular iron excretion was additive, and response at a dose range of 25 to 200 mg/kg was linear. In vitro studies in heart cells showed that DFO was more effective in heart cell iron mobilization than all PIH analogs studied. Response to joint chelation with DFO and PIH analogs was similar to an increase in the equivalent molar dose of DFO alone, rather than the sum of the separate effects of the PIH analog and DFO. This finding was most likely the result of iron transfer from PIH analogs to DFO, a conclusion supported directly by iron-shuttle experiments using fluorescent DFO. These findings provide a rationale for the combined, simultaneous use of iron-chelating drugs and may have useful, practical implications for designing novel strategies of iron chelation therapy.     

A rapid assay for evaluation of iron-chelating agents in rats

The animal assay of potential new iron-chelating agents is at present dependent on cumbersome and imprecise iron balance studies in hypertransfused rodents. We report the development of a radioisotope assay in intact rats based on the transient labeling by ferritin 59Fe of the main source of chelatable iron within hepatocytes. The isotope was maximally available to chelators during the first 6 hr after its injection, nearly all the excretion being in the bile. The bile 59Fe/total iron ratio was independent of both the chelator and its dose. However, in iron-loaded rats, the ratio was reduced, and the isotope excretion was a less sensitive measure of intrahepatic chelation. In the proposed assay, test chelators were given to normal rats 2 hr after an intravenous injection of 59Fe-ferritin. Four hours later, the radioiron in the liver and in the gut gave a sensitive measure of the mobilization of hepatic iron to the bile. In addition, chemical iron determinations identified a small alternative source of urinary chelate with agents known to promote urine excretion in man. The assay gave a rapid and precise screen for chelators given by parenteral and oral routes.     

Iron-balance and dose-response studies of the oral iron chelator 1,2- dimethyl-3-hydroxypyrid-4-one (L1) in iron-loaded patients with sickle cell disease

Several life-threatening complications of the common disorder sickle cell disease require management with red blood cell transfusions and, hence, long-term iron-chelating therapy. The efficacy of the oral iron chelator 1,2-dimethyl-3-hydroxypyrid-4-one (L1) has not previously been determined in patients with sickle cell disease. We compared the efficacy of L1 to that of standard-dose subcutaneous deferoxamine in four regularly transfused patients with homozygous sickle cell disease, who had evidence of severe iron overload and a history of poor compliance with deferoxamine. Determination of 24-hour urinary iron excretion conducted over 5 days immediately after transfusion showed that the mean daily urinary iron excretion induced by L1 at 75 mg/kg/d (0.48 +/- 0.23 mg/kg) was equivalent to that induced by deferoxamine at 50 mg/kg/d (0.39 +/- 0.06 mg/kg). In two of three patients studied, a significant (P < .025) increase in mean daily urinary iron excretion was achieved when the dose of L1 was increased to 100 mg/kg/d. Total iron balance studies, which quantitated both urinary and stool iron excretion on L1 and deferoxamine, determined that mean total daily iron excretion induced by deferoxamine (0.88 +/- 0.05 mg/kg) was significantly greater (P < .05) than that induced by L1 (0.53 +/- 0.17 mg/kg), attributable to the significantly greater stool iron excretion during deferoxamine treatment (0.50 +/- 0.16 mg/kg/d) compared with that measured during L1 treatment (0.12 +/- 0.08 mg/kg/d, P < .01). Stool iron excretion accounted for a significantly greater percentage of total iron excretion during deferoxamine treatment (59% +/- 20%) than during L1 treatment (23% +/- 14%, P < .01). These iron balance studies are the first to compare total iron excretion induced by L1 with that achieved by deferoxamine. They demonstrate that the mean total daily iron excretion during L1 treatment (0.53 +/- 0.17 mg/kg) is sufficient to maintain net negative iron balance in most regularly transfused patients with sickle cell disease. Because long-term compliance with L1 has been shown previously to be superior to that with deferoxamine in patients with homozygous beta-thalassemia, the use of L1 should increase the long-term effectiveness of iron chelation in patients with sickle cell disease.     

PCTH: A Novel Orally Active Chelator for the Treatment of Iron Overload Disease

Our laboratories have prepared a novel class of iron (Fe) chelators of the 2-pyridylcarboxaldehyde isonicotinoyl hydrazone (PCIH) class. This article will review the iron chelation efficacy of this series of chelators, both in cell culture and in animal models. Several PCIH analogs were shown to be effective at inducing iron mobilization and preventing iron uptake from the iron-transport protein, transferrin. Moreover, several of these ligands were effective at permeating the mitochondrion and inducing iron release. Studies in mice demonstrated that the PCIH analog, PCTH, was orally active and well tolerated by mice at doses ranging from 50 to 100 mg kg^?1, twice daily (b.d.). A dose-dependent increase in fecal ⁵⁹Fe excretion was observed in the PCTH-treated group. This level of iron excretion was similar to that found for the orally effective chelators, pyridoxal isonicotinoyl hydrazone (PIH) and deferiprone (L1). The PCIH group of ligands clearly has the potential for the treatment of β-thalassemia (thal) and Friedreich's Ataxia (FA).     

Iron mobilization from transferrin and non-transferrin-bound-iron by deferiprone. Implications in the treatment of thalassemia, anemia of chronic disease, cancer and other conditions

Iron mobilization from transferrin is one of the most important screening methods for the selection of chelators intended for clinical use in the treatment of iron overload in thalassemia and other conditions. In vitro and in vivo screening of approved and experimental chelating drugs has shown that only the alpha-ketohydroxypyridines deferiprone (L1) and 1-allyl-2 methyl-3-hydroxypyrid-4-one (L1NAll), are effective in the mobilization of iron from transferrin. Iron mobilization from transferrin and non-transferrin-bound-iron (NTBI) can be used to optimize existing chelation therapy protocols for the treatment of iron loaded patients. New chelation strategies involving L1 and its combination with deferoxamine (DFO) and other chelators can be used to increase iron excretion and reduce or prevent excess iron deposition in the heart and other vital organs of iron loaded patients by comparison to monotherapies. Deferiprone and its combinations may also have potential applications in the treatment of cancer, the anemia of chronic disease and other conditions.     

The fate of intravenously injected tissue ferritin in pregnant guinea-pigs

The organ distribution of intravenously injected hepatic ferritin either labelled with 59Fe or with 59Fe and 125I, was studied in pregnant guinea-pigs. At 5 h 71.2% of injected 59Fe was present in the placenta and fetus. Transfer of 59Fe to the fetus was slow, with 11.2% present at 5 h and 38.6% at 21 h. Analysis of a placental cellular lysate for 59Fe and 125I revealed that the injected iron was present as intact ferritin at 2 h but by 21 h the ferritin had been catabolized, the 125I excreted and the 59Fe incorporated into endogenous ferritin. Most of the fetal 59Fe counts were detected in the liver, with 35.3% of the transferred 59Fe in ferritin, 30.4% in haemoglobin and 10.6% in a low molecular weight pool. The uptake of labelled ferritin by the placenta was inhibited by a 300-fold molar excess of unlabelled ferritin but not by albumin, asialofetuin or by the injection of carbon particles. A nonsignificant reduction in uptake was noted after injection of mannosylated bovine serum albumin. The mannosidase inhibitor swainsonine had no effect. Iron transfer to the fetus was not affected by various microtubular inhibitors. Presaturation of endogenous transferrin with oral carbonyl iron prevented iron release from the feto-placental unit back into the maternal circulation. In consequence, marrow 59Fe uptake by the maternal marrow was reduced. The ferrous chelator 2,2'-bipyridine significantly reduced 59Fe transfer to the fetus and this occurred irrespective of whether the chelator was given prior to or after 59Fe ferritin administration. The ferric chelator desferrioxamine had no such effect. Electron microscopy of placental tissues revealed endocytosis of ferritin molecules. These results indicate that the guinea-pig placenta takes up homologous tissue ferritin and transfers the iron slowly to the fetus after reductive mobilization. The process is compatible with a receptor-mediated pathway.     

Failure of transferrin to enhance iron absorption in achlorhydric human subjects

There is evidence in experimental animals that transferrin, produced either by gastrointestinal cells or derived from bile, mediates the luminal absorption of iron. The applicability of these findings to human subjects was tested by administering diferric transferrin labelled with 3 mg 59Fe to seven patients with pernicious anaemia. Achlorhydric subjects were chosen to ensure that the iron transferrin complex did not dissociate in the stomach. The geometric mean absorption of 1.4% was similar to that of 3 mg iron given as ferric chloride (1.9%) and much less than that of ferrous ascorbate (18.9%). These findings suggest that transferrin does not play a physiological role in the absorption of iron in human subjects.     

The Uptake of Ferric Iron by Rat Liver Ferritin in Vivo and in Vitro

S ummary . The incorporation of ferric iron labelled with ⁵⁹Fe into rat liver ferritin has been studied in whole animals, into liver homogenate and into purified protein. Uptake of Fe³⁺ into purified rat liver ferritin followed a pattern similar to that with horse spleen ferritin given Fe²⁺ and an oxidant. The distributions of iron incorporated as a function of molecular iron content obtained in vivo resembled the in vitro patterns for iron contents above 500 Fe atoms/molecule and times after injection of up to 12 h. At larger intervals a maximum label moved to molecules of highest iron content as the molecules accumulated more iron. The apparently reduced uptake of injected ⁵⁹Fe into molecules of low iron content might be due either to the chase of cold iron through an existing iron pool or the presence of functionally different ferritins at more than one anatomic site within the cell.     

Defective delivery of iron to the developing red cell of the Belgrade laboratory rat

Erythroid cell iron and transferrin uptake and release was studied in the anemia of the Belgrade laboratory rat (gene symbol, b), an autosomal recessive trait characterized by hypochromia and hyperferrinemia. When reticulocyte-rich red cells were incubated in vitro with doubly (59Fe, 125I) labeled transferrin, b/b cells demonstrated a significantly higher uptake of transferrin (164% of control at 60 min), and a significantly lower uptake of iron (21% of control at 60 min) than control cells. These findings with b/b cells were simulated by sodium-fluoride-treated control cells, but not by trypsin-treated control cells. When reticulocytes exposed to doubly labeled transferrin were incubated in normal rat plasma, there was a substantial loss of 125I from both the b/b cells (mean 71%) and control cells (mean 49%), but only a loss of 59Fe from the b/b cells (mean 21%). These findings suggest a defect in the delivery of iron to the b/b reticulocyte, which is distal to the binding of transferrin to its cell surface receptor.     

Translocation of different forms of transferrin from blood to bile in the rat

Five different forms of transferrin (rat apo [iron-free], rat diferric, diferric rat asialo, human diferric, and diferric human asialotransferrin type 3) were used to monitor the passage of this protein and its metal to the bile. Cumulative biliary excretion of the dose over 3 hours was determined. In addition, an excretion profile was constructed from the concentration of tracer in bile samples collected over 10-minute intervals. The profile obtained with apotransferrin was very similar to that found in an earlier study with albumin, the implication being that the apo form is transferred passively (e.g., by diffusion). Behavior of rat diferric transferrin, however, was consistent with the assumption that this form is transferred both passively and actively (i.e., in vesicles). The three other transferrins were investigated with the intent of broadening the spectrum of ligand affinities for the plasmalemma of hepatocyte. The higher this attraction was, the larger fraction of the dose appeared in bile. When transferrin was targeted to lysosomes, the bile contained several intermediate proteolytic fragments. Double-labeled (¹²⁵I, ⁵⁹Fe) transferrin was used to measure recovery of iron (Fe) relative to the protein (P) in bile. With rat diferric transferrin, the Fe/P ratio was 0.72. Lower values were recorded with transferrins (human or asialo) that had higher affinities for the plasmalemma and therefore were expected to be transported to a larger extent in vesicles. Of the biliary ⁵⁹Fe, 85% to 92% was protein bound. The proportion of the protein-bound fraction was essentially independent of the magnitude of Fe/P ratios.     

Iron chelation therapy

Although iron chelation therapy with deferoxamine (DFO) has changed life expectancy in thalassemic patients, compliance with the rigorous requirements of long-term subcutaneous DFO infusions is unsatisfactory. This problem underlines the current efforts for developing alternative, orally effective chelators to improve compliance and treatment results. For the patient with transfusional iron overload in whom results of DFO treatment are unsatisfactory, several orally effective agents are now available. The most important of the new generation of oral chelators are deferiprone and ICL670. Total iron excretion with deferiprone is less than with DFO, but deferiprone has a better ability to penetrate cell membranes and may have a better cardioprotective effect than DFO. Current studies of the clinical efficacy and tolerability of ICL670 indicate that at a single oral dose of 20 mg/kg daily, it may be as effective as parenteral DFO used at the standard dose of 40 mg/kg daily. Combined chelation treatment, employing a weak chelator that penetrates cells better, and a stronger chelator with efficient urinary excretion, may result in improved therapeutic effect through iron shuttling between the two compounds. The efficacy of combined chelation treatment is additive and offers an increased likelihood of success in patients previously failing DFO or deferiprone monotherapy.