CHELATION THERAPY FOR IRON OVERLOAD

The pathogenesis of iron overload is reviewed and recent developments in chelation therapy aimed at reducing iron overload are discussed.

Key Words: iron chelation, iron overload, thalas- semia major

Iron differs from many other body constituents in that balance is achieved not by changing excre- tion rates but by altering the amount of absorp- tion from the gut. In fact, there is no physiological mechanism for the excretion of excess iron and this lack is partly responsible for the pathologi- cally large amounts of iron which accumulate in the body in certain situations. The congenital cause of iron overload is idiopathic hemo- chromatosis due to genetic enhancement of iron absorption from the gut. The magnitude of iron deposition in certain organs also is dependent on factors such as alcohol intake and menstrual loss. Acquired forms of iron overload include the oft-quoted Bantu siderosis where a combination of iron cooking pots and home-brewed beer re- sults in pathological siderosis. Less avoidable acquired causes of iron overload are a side- effect of the regular blood transfusions neces- sary in certain chronic anemias. The majority of individuals requiring such a regimen suffer from aplastic anemia or thalassemia major. In the latter condition, iron overload is contributed to by high levels of iron absorption related to the high rate of erythropoiesis. A further contributory factor is the hemolytic component of the disorder resulting in higher turnover of iron from red cells with a shortened lifespan.

Normal iron stores have been estimated to be on average 980 mg in men and 450 mg in women.’ These figures have been calculated from estimations of serum ferritin levels which bear a close relation to storage levels. In children with thalassemia major, symptoms begin when there are on average 28 g of iron in the body.* Initial manifestations are endocrine, resulting in slow growth and retarded puberty. Later, hepatic and cardiac dysfunction and other endocrine

disorders such as diabetes mellitus and hypoad- renalism ensue.

The clinical symptomatology points to the or- gans which are functionally most affected by the abnormal deposition of iron both in the congeni- tal and in the acquired forms of iron overload. The liver appears to be the major site of excess iron deposition and iron is found in both the hepatocytes and the Kupffer cells. The organ enlarges and in time portal cirrhosis develops. Myocardial deposits of iron also are marked usually within the muscle fibers in the peri- nuclear region. Excess iron also is found in the pan- creatic tissue, both exocrine and endocrine, and fibrosis eventually develops. Most other tissues show excess iron deposition in idiopathic hemosiderosis and transfusion siderosis. In the later stages of the transfusion siderosis, how- ever, there is a much more marked accumula- tion of iron in the spleen and reticuloendothelial system generally. The actual mechanism of cell damage remains a matter of conjecture. The excess iron is detected ultrastructurally from the characteristic appearance of ferritin under the electron microscope. The iron in the liver is found in the cytoplasm particularly associated with the lysosomes which become markedly iron loaded and show morphological abnormal- ities. Lysosomal enzymes are increased in liver biopsy specimens from both congenital or ac- quired siderosis.3 Whether this causes cell dam- age is unknown. Excess ionized, divalent iron is certainly toxic to cells, but ferric acid exists in a complexed form, mostly as ferritin.

Cells dying in experimental iron overload show no specific changes at the ultrastructural level and give no clue as to the pathogenesis of their demise. One suggestion is that the basic insult is to cell and organelle membranes due to lipid peroxidation and destruction of membrane sulphydryl gr0ups.l Another aspect of the his-

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tological abnormality in iron overload is the ex- cess fibrosis found in the liver and myocardium. One suggestion is that excess iron stimulates fibroblasts to excrete excessive amounts of tropocollagen rather than that this is a response to cell death.4 Much remains to be learned therefore as to what is happening at the cellular level in iron overload and why some tissues, more than others, bear the brunt.

Iron chelation therapy has been a practical possibility since early reports that an iron chelator, desferrioxamine, when given by in- tramuscular injection cwld reduce the amount of iron in the liver in thalassemia major and prevent further fibrosis5 Regular chelation therapy re- sults in a fall in serum ferritin6 levels and in- creased growth in ~hi ldren,~ but intramuscular desferrioxamine cannot reduce pathological amounts of body iron to within the normal range nor, if given concurrently with a regular transfu- sion regimen, prevent the accumulation of ab- normal amounts of body iron.

The major reason for the relative inefficacy of desferrioxamine is probably its short biological lifespan. Its half-life in the plasma is less than ten minutes,7 although some is probably passing in- tracellularly and thus may continue to be effec- tive therein. The next obvious development in therapy therefore was to administer desfer- rioxamine continuously. The most effective way of doing this is by slow subcutaneous infusion by small portable pumps which are suitable for use even by children. It is now apparent that the same amount of desferrioxamine when given subcutaneously or intravenously induces excre tion of more iron than when given intramuscu- larly. For example, in a group of iron-overloaded individuals, intramuscular injection of 750 rng of desferrioxamine removed a mean of 14.5 mg iron, whereas the same dose, given by continu- ous intravenous infusion, resulted in a mean urinary excretion of 44.9 mg.e Dose for dose continuous subcutaneous infusion is about 90 percent as effective as continuous intravenous infusion? By slow subcutaneous injection even early states of iron loading can be reversed. It is possible that overload could be completely pre- vented by the early initiation of this type of therapy.

Although subcutaneous desferrioxamine may well produce an increase in lifespan for thalas-

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semic individuals, the method of administration is cumbersome and costly. Therefore, the search for alternative methods for administration and for better chelating agents continues. One possibility is the development of depot prepara- tions of known chelators such as desfer- rioxamine. Another possibility is the develop- ment of more potent chelating agents. One compound from the same chemical family as desfemoxamine, cholylhydroxamic acid, seems to be worth further study,l0 although it has not yet been tested in human beings. A distinct com- pound, rhodotorulic acid, a derivative of benzoic acid, is also more potent than desferrioxaminel and requires further investigation.

A significant advance would be the develop- ment of an oral chelating agent. Cholylhy- droxamic acid comes into this category as does a complex between isoniazide and pyridoxal. Both of these latter compounds are individually capa- ble of chelating metals and appear very effective when complexed together. The activity of this complex has been studied by Hoy and cowork- ers'* using two experimental protocols. The first used in vivo methods of studying iron excre- tion in iron-loaded rats. Oral administration of isoniazid-pyridoxal resulted in an eight-fold in- crease in fecal iron excretion and probably some increase in urinary iron excretion although this is difficult to quantitate due to fecal contamination.

Having studied its effect on the whole animal, these workers then studied the effect of this compound at the cellular level. For this they employed the Chang cell line derived from liver cells and preincubated with isotopically labeled iron. On incubation of the complex, 22 percent of cell iron and 26 percent of ferritin iron were r e moved. Neither compound when added alone had an effect on iron removal. This new com- pound justified further trials. It is hoped that it will be effective without the toxicity that has bedevil- led so many attempts to find effective iron chelators.

The outlook for the iron-overloaded subject is thus much brighter than it was a decade ago. High transfusion regimens aimed at keeping the hemoglobin permanently above 10 g have im- proved the quality of life and, for the price of wearing a portable pump at regular intervals, it seems likely that death from the consequences of iron overload will be postponed or perhaps

delayed indefinitely. We are, however, almost certainly at an intermediate stage in this im- provement. It seems likely that new oral chelat- ing agents will become the treatment of choice in the not too distant future. 0

1 . A. Jacobs: Iron Overload-Clinical and Pathologic Aspects. Sernin. Haernatol. 14: 89- 113, 1977

2. B. Modell: Management of Thalassemia Major. Br. Med. Bull. 32: 270-276. 1976

3. T.J. Peters and C.A. Seymour: Acid Hydrolase Activities and Lysosomal Integrity in Liver Biop- sies from Patients with Iron Overload. Clin. Sci. Mol. Med. 50: 75-78, 1976

4. G.W. Richter: The Iron-Loaded Cell-The Cytopathology of Iron St0rage.A Review. Am. J. Pathd. 91: 363-396, 1978

5. M. Barry, D.M. Flynn, E.A. Letsky and R.A. Ris- don: Long-Term Chelation Therapy in Thalas- semia Major: Effect on Liver Iron Concentration, Liver Histology and Clinical Progress. Br. Med. J.

6. E.A. Letsky, F. Miller, M. Worwood and D.M. Flynn: Serum Ferritin in Children with Thalas- saemia Regularly Transfused. J. Clin. Pathol. 27:

2: 16-20, 1974

652-655, 1974

7. M.R. Summers, A. Jacobs, D. Tudway, P. Perera and C. Ricketts: Studies in Desferrioxamine and Ferrioxamine Metabolism in Normal and Iron- Loaded Subjects. Br. J. Haernatol. 42: 547-555, 1979

8. R.D. Propper, J.B. Shurin and D.G. Nathan: Reassessment of the Use of Desferrioxamine B in Iron Overload. New fngl. J. Med. 294: 1421- 1423, 1976

9. R.D. Propper, B. Cooper, R.R. Rufo, A.W. Nienhuis, W.F. Anderson, H.F. Bunn, A Rosen- thal and D.G. Nathan: Continuous Subcutaneous Administration of Desferrioxamine in Patients with Iron Overload. New Engl. J. Med. 297: 418- 423, 1977

10. R.W. Grady, J.H. Graziano, G.P. White, A. Jacobs and A. Cerami: The Development of New Iron Chelating Drugs. II. J. Pharrnacol. €xp. Ther.

1 1 . R.W. Grady, J.H. Graziano, H.A. Akers and A. Cerami: The Development of New Iron Chelating Drugs. J. Pharmacol. f x p . Ther. 196: 478-485, 1976

12. T. Hoy, J. Humphrys, A. Jacobs, A. Williams and P. Ponka: Effective Iron Chelation following Oral Administration of an Isoniazid-Pyridoxal Hyd- razone. Br. J. Haernatol. 43: 443-449, 1979

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