Iron uptake from ferrioxamine and from ferrirhizoferrin by germinating spores of Rhizopus microsporus.

Mucormycosis caused by the fungus Rhizopus has been documented in iron overloaded patients and more particularly in dialysis patients, both when treated with desferrioxamine B (DFO). This iron and aluminium chelator is thought to play a role in the pathogenesis of this infection. We therefore investigated in vitro the cellular pharmacology of iron chelated by DFO in the fungus Rhizopus. In a medium, designed for fungal cultivation, Rhizopus microsporus var. rhizopodiformis takes up iron from ferric-DFO complex (55Fe.DFO) and from 55Fe.rhizoferrin, the siderophore synthesized and secreted by Rhizopus [Drechsel et al., Biol. Metals 4: 238-243, 1991]. In both cases, iron accumulation is partially saturable with the duration of exposure and the chelator concentration. Fe.DFO binds to Rhizopus; iron becomes trapped and remains associated with the fungus, whereas the iron-depleted siderophore is released in the extracellular medium. In a medium designed for mammalian cell cultivation and in the absence of human serum, the fungal iron accumulation both from 55Fe.DFO and from 55Fe.rhizoferrin is proportional to the chelator concentration. Human serum at 40% does not influence the iron accumulation from Fe.DFO but it significantly affects that from Fe.rhizoferrin which, in the presence of serum, only occurs at concentration > 5 microM. This difference finds its explanation in the iron transfer observed between Fe.rhizoferrin and seric apotransferrin, the latter making the metal unavailable to Rhizopus. By contrast, no iron transfer takes place between Fe.DFO and apotransferrin, allowing fungal iron utilization from this complex, even at very low concentrations. The iron uptake, being inhibited by NaN3 and KCN, is energy-dependent; being inhibited by bipyridyl, it requires prior reduction of ferric iron; being unaffected by the covalent linkage of Fe.DFO to albumin, it does not require the entry of Fe.DFO within the fungus. These in vitro results strongly suggest that, upon administration of DFO to iron overloaded or dialysis patients, the formed Fe.DFO is efficiently used as an iron source by Rhizopus, even in the presence of serum apotransferrin or rhizoferrin. The consequent promotion of the growth of Rhizopus helps explain the increased risk of mucormycosis in DFO-treated patients.

Deferoxamine augments growth and pathogenicity of Rhizopus, while hydroxypyridinone chelators have no effect.

Deferoxamine (DFO), when used in dialysis patients, is a well recognized risk factor for the development of mucormycosis caused by Rhizopus. This study compares, both in vivo and in vitro, the effects produced on Rhizopus by DFO and by two chelators of the hydroxypyridinone class, L1 and CP94. Experimental systemic mucormycosis was induced in the guinea pig by an i.v. injection of two different strains of Rhizopus: R. microsporus and R. arrhizus. Concomitant i.p. administration of DFO for four days shortened animal survival (P < 0.05), whereas concomitant administration of either L1 or CP94 did not. In vitro radioiron uptake by R. microsporus was 100-fold higher from the 55ferric complex of DFO than of L1 or CP94. In vitro fungal growth was stimulated sevenfold by the ferric complex of DFO (P < 0.0001) but not significantly by the ferric complex of either L1 or CP94. These results indicate that the ferric complex of DFO but not that of L1 or CP94 specifically stimulates both the iron uptake and the growth of Rhizopus. They suggest that the risk of developing mucormycosis should be minimal with L1 or CP94, as opposed to DFO.

Mucormycosis during deferoxamine therapy is a siderophore-mediated infection. In vitro and in vivo animal studies.

This study investigates the pathophysiology of mucormycosis caused by Rhizopus, which has been reported in 46 dialysis patients, while treated with deferoxamine (DFO). This drug aggravates mucormycosis, which we experimentally induced in guinea pigs and which lead to a shortened animal survival (P < or = 0.01). The drug's effect on Rhizopus is not mediated through the polymorphonuclear cells. Fe.DFO, the iron chelate of DFO, abolishes the fungistatic effect of serum on Rhizopus and increases the in vitro growth of the fungus (P < or = 0.0001). This effect is present at Fe.DFO concentrations > or = 0.01 microM, at which fungal uptake of radioiron from 55Fe.DFO is observed. A 1,000-fold higher concentration of iron citrate is required to achieve a similar rate of radioiron uptake and of in vitro growth stimulation as observed with Fe.DFO. These in vitro effects of Fe.DFO (1 microM) in serum on radioiron uptake and on growth stimulation are more striking for Rhizopus than for Aspergillus fumigatus and are practically absent for Candida albicans. For these three fungal species, the rates of radioiron uptake from 55Fe.DFO and of growth stimulation in the presence of Fe.DFO in serum are directly related (r = 0.886). These results underscore the major role of Fe.DFO in the pathogenesis of DFO-related mucormycosis. Pharmacokinetic changes in uremia lead to a prolonged accumulation of Fe.DFO after DFO administration, which helps explain the increased sensitivity of dialysis patients to DFO-related mucormycosis.