Iron Chelators for Thalassaemia (1998)

The iron-chelating compound desferrioxamine (DFO) was discovered accidentally, as a byproduct of antibiotic research by scientists in the Swiss Federal Institute of Technology in Zurich and Ciba in Basel. Its development into a useful clinical drug was not by design, but thanks to natural curiosity and ingenuity, as described by Keberle (1992). Although DFO has been available for treating transfusional iron overload from the early 1960s, the era of modern and effective iron-chelating therapy started only 20 years ago with the introduction of subcutaneous DFO infusions by portable pumps. Today, long-term DFO therapy is an integral part of the management of thalassaemia and other transfusion-dependent anaemias, with a major impact on well-being and survival. In this Review we will discuss the abnormalities in iron metabolism associated with thalassaemia major and the chelatable iron pools representing the targets of iron-chelating therapy, examine the results of long-term DFO therapy in thalassaemia, and, finally, review the development of new, orally effective, iron chelators which may be considered for present or future use. Iron balance in normal subjects is characterized by an efficient recycling of wastage iron from senescent erythrocytes. Only a fraction of the daily iron requirement is obtained by intestinal absorption ( Cook et al, 1970 ). The primary abnormality in thalassaemia major is a wasteful ineffective erythropoiesis resulting in a 10–15-fold expansion of the erythroid bone marrow ( Hershko & Rachmilewitz, 1979) and a drastic increase in haemoglobin catabolism. Iron accumulation is the consequence of blood transfusions as well as of increased iron absorption caused by erythropoietic activity. The role of increased iron absorption is illustrated by the severe iron overload encountered in patients with thalassaemia intermedia who have never received blood transfusions. The combination of iron overload and increased outpouring of catabolic iron from the reticuloendothelial system overwhelms the iron-carrying capacity of transferrin, resulting in the emergence of toxic non-transferrin bound plasma iron (NTPI). Although our original description of a chelatable, low molecular weight, plasma iron fraction in thalassaemic patients with severe iron overload ( Hershko et al, 1978 ) was greeted with initial scepticism, its existence has been confirmed by subsequent studies using a variety of methods. NTPI promotes the formation of free hydroxyl radicals and accelerates the peroxidation of membrane lipids in vitro ( Gutteridge et al, 1985 ). Long-term treatment with DFO or deferiprone (L1) in thalassaemic patients results in a marked decrease in their NTPI measurements ( Al-Refaie et al, 1992 ). More recently, Porter et al (1996 ) have shown that plasma NTPI is removed by intravenous DFO therapy in a biphasic manner and that upon cessation of DFO infusion it reappears rapidly, lending support to the continuous, rather than intermittent, use of DFO in high-risk patients. The rate of low molecular weight iron uptake by cultured rat heart cells is over 300-times that of transferrin iron ( Link et al, 1985 ). Moreover, transferrin-iron uptake is inhibited at high tissue iron concentrations due to the down-regulation of transferrin receptor production, whereas non-transferrin iron uptake is actually increased at high tissue iron concentrations ( Randell et al, 1994 ). Such uptake results in increased myocardial lipid peroxidation and abnormal contractility, and these effects are reversed by in vitro treatment with DFO ( Link et al, 1989 ). Recognition of NTPI as a potentially toxic component of plasma iron in thalassaemic siderosis has important practical implications for designing better strategies for the effective administration of DFO and other iron chelating drugs. Without iron-chelating therapy, the accumulation of iron progresses relentlessly, and when about 20 g of iron have been retained in the body, significant clinical manifestations of iron toxicity may be anticipated ( Gabutti & Borgna-Pignatti 1994). The most important complications of transfusional siderosis are cardiac, hepatic and endocrine disease. Pathologic findings in the heart include dilated, thickened ventricular walls with particularly heavy iron deposits in the ventricles, epicardium and papillary muscles. These cellular deposits induce increased membrane lipid peroxidation in the sarcolemma resulting in impaired Na,K,ATPase activity ( Link et al, 1994 ), increased lysosomal fragility ( Link et al, 1993 ) and, in particular, impaired mitochondrial inner-membrane respiratory chain activity ( Link et al, 1996 ). It is possible to demonstrate early myocardial dysfunction in asymptomatic patients using a multi-gated imaging (MUGA) scan ( Aldouri et al, 1990 ) or dobutamine stress echocardiography ( Mariotti et al, 1993 ). Progressive cardiac siderosis leads to the development of symptomatic heart failure and life-threatening arrhythmias. Myocardial siderosis is the single most important cause of mortality in inadequately-treated thalassaemics. Hepatic fibrosis is a common complication of thalassaemia and, similar to cardiac problems, its incidence is age-related. It has been proposed that patients with liver iron concentrations < 15 mg/g dry weight may have a prolonged survival free of the clinical complications of iron overload ( Brittenham et al, 1994 ). Iron overload per se is responsible for the development of cirrhosis in many cases. However, the coexistence of chronic hepatitis B or C with an incidence ranging from 8.7% to 70% of thalassaemic patients in various geographic areas ( Gabutti & Borgna-Pignatti, 1994) underlines the complexity of this problem. Endocrine problems caused by direct accumulation of iron in endocrine glands or indirectly through the hypothalamic–pituitary axis are common. Stunted growth, delayed puberty, hypothyroidism, hypoparathyroidism and diabetes mellitus are all well-established complications of transfusional siderosis ( De Sanctis et al, 1992 ). Because diabetes and hypothyroidism appear when most endocrine cells are destroyed and replaced by fibrosis, these complications are rarely reversible. Current models of iron acquisition, sequestration and storage by mammalian cells are based on a regulated adjustment of membrane transferrin receptor and cytosolic ferritin levels. Iron in transit between these two iron-binding proteins is believed to exist in a weakly bound low-molecular-weight complex which is also available for interaction with iron chelating drugs ( Rothman et al, 1992 ). This chelatable labile iron pool (LIP) is assumed to be sensed by a cytosolic iron-responsive protein (IRP) which coordinately represses ferritin mRNA translation and increases transferrin receptor mRNA stability ( Klausner et al, 1993 ). Efficient regulatory mechanisms prevent fluctuations in the size of the labile iron pool under conditions of moderate iron deprivation and iron loading. However, massive iron loading results in uncontrollable expansion of the chelatable pool, which fails to be matched by the sequestrating capacity of cellular ferritin ( Breuer et al, 1997 ). This expanded LIP is an obvious target of intracellular iron chelation by drugs that are able to cross the barrier of the cytoplasmic membrane. Although excess iron may be deposited in almost all tissues, most of it is found in association with two cell types: reticuloendothelial (RE) cells in the spleen, liver and bone marrow, and parenchymal tissues represented by hepatocytes, endocrine cells and the myocardium. In contrast to RE cells in which iron accumulation is relatively harmless, parenchymal siderosis may result in significant organ damage. The source of iron and the proportion retained in ferritin stores or recycled into the circulation from the two cell types is quite different. RE cells have a limited ability to assimilate transferrin iron and they derive iron from the catabolism of haemoglobin in non-viable erythrocytes ( Hershko & Weatherall, 1988). Most of this catabolic iron is recycled to plasma transferrin or NTPI within a few hours. In contrast, hepatic parenchymal cells maintain a dynamic equilibrium with plasma transferrin, with iron uptake predominating when transferrin saturation is high, and release when serum iron and transferrin saturation are low. Unlike RE cells, the turnover of parenchymal iron stores is extremely low. In general, iron overload associated with increased intestinal absorption such as hereditary haemochromatosis results in predominant parenchymal siderosis, whereas in iron overload caused by multiple blood transfusions the primary site of siderosis is the RE cells. Considerable redistribution of iron may take place subsequently. Experimental and clinical observations indicate that the urinary excretion of chelated iron is derived mainly from RE cells. Studies in hypertransfused rats have shown that, in contrast to hepatocellular radioiron excretion which is confined entirely to the bile, most of the radioiron excretion derived from RE cells is recovered in the urine. Moreover, when DTPA or IRC11, water-soluble synthetic chelators which do not enter cells easily, are employed in the same experimental model, there is no enhancement at all of hepatocellular iron excretion, but the enhancement of urinary RE radioiron excretion is similar or higher than that observed previously with DF ( Rivkin et al, 1997 ). Hence, DF obtains iron for chelation by one of two alternative mechanisms: (a) in situ interaction with hepatocellular iron and subsequent biliary excretion, and (b) chelation of iron derived from red blood cell catabolism in the RE system directly or following its release into the plasma in the form of NTPI with subsequent urinary excretion. The introduction of desferrioxamine for the treatment of transfusional iron overload had a major impact on the survival and well-being of thalassaemic patients. Although this has never been proven by prospective, randomized clinical studies, and such randomized studies are no longer justified ethically, the beneficial effects of long-term desferrioxamine treatment are clearly demonstrated by comparison of treatment outcome with historic controls. Experience with long-term DFO therapy in thalassaemic patients has been the subject of a number of excellent recent reviews ( Gabutti & Borgna-Pignatti, 1994; Gabutti & Piga, 1996; Olivieri & Brittenham, 1997). The effect of treatment, measured by urinary iron excretion, is directly proportional to the severity of iron overload. Hence, treatment in subjects without iron overload will result in limited iron excretion. However, treatment should not be introduced too late if the objective is the prevention of iron toxicity. It should be started when serum ferritin levels reach about 1000 μg/l which usually occurs after the first 10 or 20 transfusions ( Gabutti & Piga, 1996). DFO is infused via a thin s.c. needle in the arm or abdomen at night, connected to a portable pump over 8–12 h, five to seven times per week, at a daily dose of 20–60 mg/kg. A urinary iron excretion of 0.5 mg/kg/d is usually sufficient to ensure negative iron balance. Although prolonged subcutaneous DFO infusion is universally recognized as the optimal method of treatment, twice daily s.c. injections may yield similar amounts of urinary iron ( Borgna-Pignatti & Cohen, 1997). A new delivery system for continuous DF infusion has been introduced by Baxter enabling continuous 48 h s.c. or continuous 24 h i.v. delivery for 7 d each week ( Araujo et al, 1996 ). This technology enables effective removal of toxic free iron (NTPI) from the plasma, a significant decrease in serum ferritin within 4 weeks, and improves patient to s.c. DFO pumps. a of DFO has been by by a dose over a longer of it efficient and the proportion of which be to of the drug when by ( et al, 1997 ). studies have shown that the of of the is h, and iron excretion is times that of s.c. DFO of the new in clinical studies to be this new technology may or s.c. the for portable and patient to treatment may be by serum ferritin liver or ( Brittenham et al, ). are low in patients with ( et al, ) and high in liver or serum ferritin is the most and for the long-term of chelating and from cardiac complications may be when ferritin levels are ( Olivieri et al, 1994 ). DFO is but a number of complications may from particularly in subjects with limited iron overload. These complications are their is A rate was first by et al ), in in with low serum has also been in and bone most in the and ( Olivieri et al, 1992 De Sanctis et al, 1996 ). are usually upon DFO a has been observed in patients with low ferritin levels ( Olivieri et al, ). DFO was by in patients with but not with severe high DFO and have also been described ( et al, et al, 1989 et al, 1990 ). of these complications chelation are if is to early of toxicity. they are and to the of iron their be by dose such method is the daily weight to a DFO to ferritin of ( Porter et al, ). to s.c. DFO infusion is This has been in patients considered to be to DFO by i.v. therapy ( et al, 1996 ). The of after of i.v. therapy the that the to s.c. DFO is to a direct effect on cells and is not an The from s.c. to i.v. therapy was associated with a in and a decrease in serum within a A but complication of DFO is a chelated iron as a It occurs in of chelated patients. of may the severity of this life-threatening complication ( et al, ). The impact of desferrioxamine treatment on is demonstrated by comparison of survival in chelated patients. In a major of thalassaemic patients at seven it was shown that 70% of patients 1970 and to the modern era of chelation to 20 years of to in patients after 1970 who received effective chelation from an early ( Gabutti & Borgna-Pignatti, 1994). Most of the in survival was to cardiac This of in survival is in a decrease in cardiac the that prevention of cardiac mortality is the most important beneficial effect of DFO survival in thalassaemic patients has been in other major studies from the ( & and ( et al, 1990 Olivieri et al, 1994 Brittenham et al, 1994 ). The direct the beneficial effect of DFO on heart is the of in cases. in hereditary haemochromatosis has shown that the of iron overload is potentially by effective iron through However, in transfusional the of myocardial was and believed to be to iron chelating indicate that such patients may be to chelating et al ) described first the of symptomatic myocardial in of five patients by continuous i.v. DFO therapy at the of severe toxicity. of symptomatic has been by without significant drug toxicity ( et al, 1985 et al, 1990 ). 24 h intravenous infusion of DFO through using portable infusion or the new Baxter delivery is a effective method for the of heart disease. In it is an excellent for patient delivery of DFO at of and the effective of iron In of the proven of not all patients are to with the of the long-term use of portable pumps. In the high of this treatment is a to its use. In of these there is a for the development of orally effective, iron-chelating drugs. recent years than one have been in These have to the of a few of which may be of possible clinical use. The present will be limited to the most of these deferiprone the and the all the new iron-chelating drugs available only deferiprone has been introduced for clinical use as an orally effective for The of by and ( et al, 1990 to iron in a with a stability of about of higher than The most important compound of this is or Although clinical on the use of deferiprone (L1) in thalassaemic and other patients as early as studies on toxicity and on have only available a daily dose of times the dose in for caused bone marrow in and of tissues and These associated with high of mortality ( et al, 1992 et al, 1993 ). studies have shown that plasma concentrations are within h or of administration ( Al-Refaie et al, ). is an important of drug and is efficient in than in a that may the observed in drug and toxicity between clinical and studies ( Porter et al, 1993 et al, 1990 ). The results of long-term iron-chelating therapy with in thalassaemic patients ( Olivieri et al, et al, 1990 et al, 1992 ) have been in recent reviews ( Al-Refaie & 1994) and the of the major and the clinical use of has been described in a of the for Iron ( Al-Refaie et al, The with thalassaemia major or representing a of of was as excellent in in and in of patients. an dose of urinary iron excretion was at 0.5 with no of a with ferritin a decrease with from an of to μg/l after 48 patients complications of which of treatment severe and liver dysfunction The patients treatment of low and conditions to toxicity. complications treatment increase in liver moderate and was no patients one of heart and the other of with This demonstrated the of in long-term use for the treatment of transfusional iron but also the complications associated with such In a recent major the of patients from and subjects for by clinical and ( et al, 1997 Cohen, 1997). patients received and the was all patients on DFO therapy to the there was no in serum or in liver iron concentrations mg/g dry in patients and in Although the of is in it is clearly an to as illustrated by the development of in and and the of ( & 1997). the the clinical results of the major in the have been ( & et al, 1996 ). have failure in a fraction of patients to hepatic iron concentrations a of 15 mg/g weight, in studies as associated with an increased of heart and early than of the patients in one ( had serum ferritin levels the in the for the of cardiac disease. This failure to a decrease in storage iron is by the in between the two drugs on a weight per weight shown by a recent balance urinary and iron excretion in thalassaemic patients DFO or iron excretion in patients was only of on DFO ( et al, 1996 ). However, in patients was as effective or better than The results of therapy in patients such as in are quite similar to with negative iron balance was in of and the of of treatment and ( et al, 1996 ). of iron chelating therapy with or DFO in is a significant decrease in requirement and increased erythroid activity by serum transferrin receptor to ( et al, 1996 ). these that treatment will not ensure sufficient in all patients and that is to patients in chelating treatment with DFO is to the development of hepatic fibrosis have been based on observations on patients on long-term therapy at the for This important will the future of in clinical and by the hepatic of all other patients on long-term The results of hepatic and liver iron measurements in the Swiss of thalassaemic patients who have been on continuous therapy 1989 are ( et al, ). important cause of limited of deferiprone in is its with the iron-binding of the Hence, present is at the development of by with or ( et al, 1996 ). The of may be by on the of the Such a limited interaction with iron such as and are in but their ability to intracellular iron ( Porter et al, 1997 ). The for orally effective, iron-chelating has to the of a over years ago by et al It a with iron by its or and hydroxyl and The for of is and its for other is relatively low. of the of to results in a marked in its intestinal absorption and a increase in iron excretion. and are and their in rats is ( Hershko et al, ). studies in hypertransfused rats have shown that at the dose of and times effective than Iron balance studies in a number of thalassaemic patients with ( et al, 1994 ) have shown increased and iron excretion following iron excretion with mg/kg/d was but no increase in excretion was at a dose of These limited amounts of iron excretion represented of the excretion for a negative iron balance. Because the absorption and in studies, this compound is as a for clinical a new of has been by and, in with two other orally effective by the same is ( et al, 1997 ). The compound was by ( et al, 1996 ). has a number of (a) it is a at a and without the of possible toxic in contrast with which is a of the drug for each (b) is and its stability with is times that of its in is to for and with the of do not and are anticipated to in In contrast to which promotes the excretion of hepatocellular and RE the major source of iron by is iron derived from the catabolism of red cell haemoglobin in the RE system ( Rivkin et al, 1997 ). Because in thalassaemia the of red cell catabolism to the of iron to the plasma is higher than that of all other it is to that catabolism is the predominant source of the toxic and chelatable non-transferrin bound plasma iron with its ability to with catabolic iron may not only the rate of urinary iron excretion in thalassaemia, but also better of tissues from by the formation or by is a water-soluble its membrane is limited but it may be as a useful compound to a of of its synthetic by may result in interaction with hepatocellular iron and also better activity. Such studies are under DFO the drug of for the management of transfusional siderosis in thalassaemic patients. However, its high and the of its administration by portable are major the for alternative orally effective new iron chelating drugs. In the for new and it is useful to of the the and of iron chelators as in an important recent review by et al (1996 ). (a) of the This is not only by the of the drug to but also by the of their chelators iron at a a complex which iron from in hydroxyl In contrast, chelators such as to one iron to form a complex ( Porter et al, ). Hence, at concentrations of when tissue iron concentrations are high or drug concentrations too or may be which may result in of potentially toxic chelators on be to or (b) the in and the ability to cross lipid with of will have efficient will result in efficient drug from the circulation by the also improves the of the and and increases drug toxicity ( Porter et al, 1990 ). weight is a in designing new chelators with chelators with excellent in vitro interaction with chelatable iron in cell or following their may be ineffective if their size with their intestinal the molecular weight should be by of their molecular and are to have higher absorption than are an excellent for a number of in the development of an of orally effective iron Such be and from the system by the into the they may liver iron in situ to be in the bile, or following release into the circulation with non-transferrin plasma iron and be in the urine. Because of their iron excretion may be the effects of is optimal when the is present in the This similar to continuous s.c. or i.v. drug may be by orally effective in therapy has the of and of thalassaemic patients. However, the high and of desferrioxamine therapy, and the significant toxicity of the for the development of new and orally effective iron Such and the of strategies of iron chelating therapy, better of the of iron toxicity and the of of iron chelating drugs. The of Porter are This was by the by the of and