In vivo testing of oral iron chelators intended for clinical use.

The effectiveness of 1,2-dimethyl-3-hydroxypyrid-4-one (Ll) in increasing iron excretion in iron-loaded patients improved the propsects for replacing desferrioxamine (DF) with an oral chelating drug.’,’ L1 belongs to a new group of iron chelators, the a-ketohydroxypyridines (KHP); which are cheap to p ~ e p a r e ~ . ~ and orally active. Recently, a detailed article by Porter et al.’ on in vivo studies in mice with KHP iron chelators concluded that several of these compounds appear to be more effective and less toxic than L1 and DF. However, the results of that study contradict earlier findings in the same and other while insufficient references and the use of a single, experimental animal model question the validity of their conclusions. For example, the synthesis and purification of almost all the KHP described’ are not found in references 11 and 12 but can be found in other publication^.^-^ Reference 18 was not submitted or published, nor does reference i 7 describe the distribution of 59Fe from ”Fe la~toferrin.~ The drawbacks of the mouse iron excretion model sed^,',^ are the inability to measure the total iron that is 59Fe and carrier iron (from iron dextran), both of which are distributed in different compartments and which may offer variable accessability to different chelators in animals.”” In mice, the distribution of ”Fe 2 weeks after the intravenous (IV) administration of 59Fe lactoferrin is mainly in hemoglobin8 and not in the liver, as suggested.’ This finding may be relevant to the observations that doses of 50 mgkg are not effective in this model’but are effective in humans, and also that the site of iron excretion varies with the animal species and iron-loading procedures used.” The assumption that highly hydrophilic compounds may be orally inactive? is invalid because pharmacokinetic studies have shown almost 100% recovery of oral L1 in the urine of humans.13 The oral efficacy of L1 in increasing iron excretion in the mouse model is equivalent or higher to the other more lipophilic homologous chelators with the exception of l-allyl-2-methyl-3-hydroxypyrid4-one (LlNAll),” which is the most effective of the series but, like the other lipophilic chelators, is more toxic than L1. Another major discrepancy in the paper of Porter et al’ is the acute toxicity study where the methodology of repeated administrations every 48 hours in unspecified number of mice and doses as well as the unspecified number of deaths is incorrectly related to LD50. In that study, mice may have died with one or more single doses if they were left for over 48 hours. The incorrect “LD50” reported’ should, therefore, be regarded as an overestimation. This may explain the lower intraperitoneal lethal dose (LD50) observed by all these chelators in rats’’ where L1 was the least toxic of the series. It can also explain the lack of correlation between lipophilicity and acute toxicity in their mice study,’ which was clearly demonstrated elsewhere.’’ Based on this discrepancy it is likely that the estimation of the therapeutic safety margin of all the KHP they have tested was incorrect. The higher toxicity margin of lipophilic chelators such as the 1,2,-diethyl-3-hydroxypyrid-4-one (ELlNEt or C94) and l-(2’methoxyethyl)-3-hydroxypyrid-4-one (LlNMeOEt or C52) by comparison with hydrophilic chelators such as L1 and DF has also been shown in the long-term oral administration of 200 mgkg doses, 5 days a week in rats.” All the rats treated with lipophilic derivatives died within 3 (ELlNEt) and 5 (LlNMeOEt) months, but none with L1 and less than 20% with DF. The high toxicity margin of ELlNEt has now been confirmed by the same authors14 who suggested that oral administration of doses only lower than 50 mgkg for a maximum 28 days may have acceptable level of toxicity in rats. However, in our more extended studies (unpublished), oral ELlNEt at 50 mg/kg, 5 days a week causes 50% mortality in rats within 3 months. In addition, the leukopenia observed by oral L1 at 200 mgkg” in long-term studies in rats is a side effect observed by most l-substituted-2-alkyl-3-hydroxypyrid-4-ones, including EL1NEt. However, this latter chelator, unlike L1, causes convulsion in rats, indicating central nervous system involvement. The chronic treatment of transfusional iron overload by an oral KHP chelator will require the daily administration of doses higher than 50 mgkg to bring patients to negative iron balance. The success of DF with regards to low toxicity at high doses in transfusional iron-loaded patients appears to be related to its and its iron complex hydrophilicity, which is also apparent with L1 in animals and humans. The use of lipophilic chelators in humans may be desirable in short-term studies, but these will have to be administered at much lower doses and their safety during longterm administration is questionable.