The iron chelators desferrioxamine and 1-alkyl-2-methyl-3-hydroxypyrid-4-ones inhibit vascular prostacyclin synthesis in vitro

The iron chelators desferrioxamine (DFO), 1,2-dimethyl(L1)-, 1-ethyl-2-methyl(L1NEt)- and 1-propyl-2-methyl(L1NPr)-3-hydroxypyrid-4-ones inhibited rat aortic prostacyclin (PGI2) synthesis in vitro (rank order of potency: DFO greater than L1 greater than L1NEt greater than L1NPr) when stimulated with adrenaline, arachidonate and the Ca2+ ionophore A23187. The inhibitory action of the chelators was blocked by Fe3+ and Al3+ and reversed by washing and H2O2, but not by ascorbate. These data suggest that iron chelators inhibit prostanoid synthesis in intact tissue through the removal or binding of Fe3+ linked to cyclo-oxygenase. These iron chelators may be of therapeutic value in the treatment of inflammatory and other diseases via two mechanisms: (1) the inhibition of pro-inflammatory prostanoid synthesis and (2) the inhibition of toxic-free-radical generation by cyclo-oxygenase.

Effective chelation of iron in beta thalassaemia with the oral chelator 1,2-dimethyl-3-hydroxypyrid-4-one

The main iron chelator used for transfusional iron overload is desferrioxamine, which is expensive, has toxic side effects, and has to be given subcutaneously. An orally active iron chelator is therefore required. The effects of oral 1,2-dimethyl-3-hydroxypyrid-4-one on urinary iron excretion were studied in eight patients who had received multiple transfusions: four had myelodysplasia and four beta thalassaemia major. Different daily doses of the drug up to 100 mg/kg/day, alone or in combination with ascorbic acid, were used. In three patients with thalassaemia the effect of the drug was compared with that of subcutaneous desferrioxamine at the same daily dose. In all eight patients a single dose of oral 1,2-dimethyl-3-hydroxypyrid-4-one resulted in substantial urinary iron excretion, mainly in the first 12 hours. Urinary iron excretion increased with the dose and with the degree of iron loading of the patient. Giving two or three divided doses over 24 hours resulted in higher urinary iron excretion than a single dose of the same amount over the same time. In most patients coadministration of oral ascorbic acid further increased urinary iron excretion. 1,2-Dimethyl-3-hydroxypyrid-4-one caused similar iron excretion to that achieved with subcutaneous desferrioxamine at a comparable dose. In some cases the iron excretion was sufficiently high (maximum 99 mg/day) to suggest that a negative iron balance could be easily achieved with these protocols in patients receiving regular transfusions. No evidence of toxicity was observed on thorough clinical examination or haematological and biochemical testing in any of the patients. None of the patients had any symptoms that could be ascribed to the drug. These results suggest that the oral chelator 1,2-dimethyl-3-hydroxypyrid-4-one is as effective as subcutaneous desferrioxamine in increasing urinary iron excretion in patients loaded with iron. Its cheap synthesis, oral activity, and lack of obvious toxicity at effective doses suggest that it should be developed quickly and thoroughly tested for the management of transfusional iron overload.

1,2-Dimethyl-3-hydroxypyrid-4-one, an orally active chelator for treatment of iron overload

Subcutaneous desferrioxamine, though effective in preventing or reducing iron overload in transfusion-dependent refractory anaemia, is expensive and inconvenient. One potentially cheaper and orally active alternative is 1,2-dimethyl-3-hydroxypyrid-4-one (L1). This drug has been tested in three multiply transfused patients with myelodysplasia. Gelatin capsules were taken at doses ranging from 0.5 g to 3.0 g. Urinary iron excretion increased substantially in all three patients and in the one tested was equal to that achieved with comparable doses of subcutaneous desferrioxamine. The amounts of iron excreted were related to the dose of L1 administered and the iron load of the patients. The urinary excretion of zinc, magnesium, and calcium did not increase, and the drug was well tolerated.

Iron chelation studies using desferrioxamine and the potential oral chelator, 1,2-dimethyl-3-hydroxypyrid-4-one, in normal and iron loaded rats

A novel iron chelator, 1,2-dimethyl-3-hydroxypyrid-4-one, and desferrioxamine were compared for their ability to remove iron and for their site of action in iron release in rats. Repeated intraperitoneal injections of the chelators in rats with widespread tissue labelling by 59Fe derived from transferrin showed comparable 59Fe mobilisation by each chelator in normal and iron loaded rats. Specific labelling of a chelatable "cold" iron pool in hepatocytes by 59Fe derived from ferritin showed this pool to be equally accessible to parenteral doses of both chelators and also to oral 1,2-dimethyl-3-hydroxypyrid-4-one, which is an effective oral iron chelating agent that removes iron from parenchymal cells. This and other alpha-ketohydroxypyridines need further development as potential therapeutic agents in human iron overload.

Comparative study of iron mobilization from haemosiderin, ferritin and iron(III) precipitates by chelators

The heteroaromatic chelators 1,2-dimethyl-3-hydroxypyrid-4-one, maltol, mimosine and 2,4-dihydroxypyridine-N-oxide, have been shown to mobilize iron from human spleen haemosiderin, ferritin and also from iron(III) precipitates, all containing equal amounts of iron, at physiological pH. In the case of almost every chelator, the least-solubilized polynuclear iron form was ferritin, whereas haemosiderin was more soluble and the iron(III) precipitate the most soluble of all. Most of the chelators were more efficient than desferrioxamine at releasing iron from ferritin, but less efficient in the removal of iron from the other two polynuclear iron forms. It is suggested that the chelator differences in iron mobilization may be related to variations in the chelator molecular structure, the protein structure, iron forms and in the mechanism of iron release.

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.

Orally active alpha-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.

Iron mobilization from ferritin using alpha-oxohydroxy heteroaromatic chelators

Several alpha-oxohydroxy heteroaromatic chelators have been shown to mobilize iron from horse spleen ferritin. Although the reactions were slow, taking up to 3 days to reach completion, the amounts of iron mobilized were higher than those reported for other chelators. These results increase the prospects for the clinical use of alpha-oxohydroxy chelators in the treatment of iron overload.

Site specificity of iron removal from transferrin by alpha-ketohydroxypyridine chelators

The site specificity of the removal of iron from diferric human transferrin, at pH 7.4, by two alpha-ketohydroxypyridine chelators, 1,2-dimethyl-3-hydroxypyrid-4-one (L1) and mimosine, has been investigated using urea-polyacrylamide gel electrophoresis. Chelator L1 removes iron preferentially from the C-terminal site whereas mimosine shows a small preference for iron in the N-terminal site. The removal of iron has also been followed spectrophotometrically and by monitoring the loss of 59Fe from [59Fe]transferrin. Both chelators are able to remove iron completely from diferric transferrin without additional mediators or reducing agents.