Iron in neoplastic disease

Estimation of iron overload with T2*MRI in children treated for hematological malignancies

Iron overload may contribute to long-term complications in childhood cancer survivors. There are limited reports of assessment of tissue iron overload in childhood leukemia by magnetic resonance imaging (MRI). A cross-sectional, observational study in children treated for hematological malignancy was undertaken. Patients ≥6 months from the end of therapy who had received ≥5 red-cell transfusions were included. Iron overload was estimated by serum ferritin (SF) and T2*MRI. Forty-five survivors were enrolled among 431 treated for hematological malignancies. The median age at diagnosis was 7-years. A median of 8 red-cell units was transfused. The median duration from the end of treatment was 15 months. An elevated SF (>1,000 ng/ml), elevated liver iron concentration (LIC) and myocardial iron concentration (MIC) were observed in 5 (11.1%), 20 (45.4%), and 2 (4.5%) patients, respectively. All survivors with SF >1,000 ng/ml had elevated LIC. The LIC correlated with SF (p < 0.001). MIC lacked correlation with SF or LIC. Factors including the number of red-cell units transfused and duration from the last transfusion were associated with elevated SF (p = 0.001, 0.002) and elevated LIC (p = 0.012, 0.005) in multiple linear regression. SF >595 ng/ml predicted elevated LIC with a sensitivity of 85% and specificity of 91.6% (AUC 91.2%). A cutoff >9 units of red cell transfusions had poor sensitivity and specificity of 70% and 75% (AUC 76.6%) to predict abnormal LIC. SF >600 ng/ml is a robust tool to predict iron overload, and T2*MRI should be considered in childhood cancer survivors with SF exceeding 600 ng/ml.

Iron: Key player in cancer and cell cycle?

BACKGROUND

Iron is an essential element for growth and metabolic activities of all living organisms but remains in its oxyhydroxide ferric ion form in the surrounding. Unavailability of iron in soluble ferrous form led to development of specific pathways and machinery in different organisms to make it available for use and maintain its homeostasis. Iron homeostasis is essential as under different circumstances iron in excess as well as deprivation leads to different pathological conditions in human.

OBJECTIVE

This review highlights the current findings related to iron excess as well as deprivation with regards to cellular proliferation.

CONCLUSIONS

Iron excess is extensively associated with different types of cancers viz. colorectal cancer, breast cancer etc. by producing an oxidative stressed condition and alteration of immune system. Ironically its deprivation also results in anaemic conditions and leads to cell cycle arrest at different phases with mechanism yet to be explored. Iron deprivation arrests cell cycle at G1/S and in some cases at G2/M checkpoints resulting in growth arrest. However, in some cases iron overload arrests cell cycle at G1 phase by blocking certain signalling pathways. Certain natural and synthetic iron chelators are being explored from few decades to combat diseases caused by alteration in iron homeostasis.

Iron, microbiota and colorectal cancer

Iron deficiency and anaemia are common in colorectal cancer. Replacement with oral or intravenous iron effectively treats this deficiency. However, mechanistic and population studies suggest that excess iron promotes colorectal carcinogenesis. Growing research into gut microbiota and dysbiosis suggests one explanation for this association. Iron is growth limiting for many pathogenic bacteria and may promote a shift in the ratio of pathogenic to protective bacteria. This may increase the toxic bacterial metabolites, promoting inflammation and carcinogenesis. This has important implications as we seek to correct anaemia in our patients.

Effects of cellular iron deficiency on the formation of vascular endothelial growth factor and angiogenesis. Iron deficiency and angiogenesis

Background

Young women diagnosed with breast cancer are known to have a higher mortality rate from the disease than older patients. Specific risk factors leading to this poorer outcome have not been identified. In the present study, we hypothesized that iron deficiency, a common ailment in young women, contributes to the poor outcome by promoting the hypoxia inducible factor-1α (HIF-1α and vascular endothelial growth factor (VEGF) formation. This hypothesis was tested in an in vitro cell culture model system.

Results

Human breast cancer MDA-MB-231 cells were transfected with transferrin receptor-1 (TfR1) shRNA to constitutively impair iron uptake. Cellular iron status was determined by a set of iron proteins and angiogenesis was evaluated by levels of VEGF in cells as well as by a mouse xenograft model. Significant decreases in ferritin with concomitant increases in VEGF were observed in TfR1 knockdown MDA-MB-231 cells when compared to the parental cells. TfR1 shRNA transfectants also evoked a stronger angiogenic response after the cells were injected subcutaneously into nude mice. The molecular mechanism appears that cellular iron deficiency elevates VEGF formation by stabilizing HIF-1α. This mechanism is also true in human breast cancer MCF-7 and liver cancer HepG2 cells.

Conclusions

Cellular iron deficiency increased HIF-1α, VEGF, and angiogenesis, suggesting that systemic iron deficiency might play an important part in the tumor angiogenesis and recurrence in this young age group of breast cancer patients.

Interactions between microenvironment and cancer cells in two animal models of bone metastasis

The preferential proliferation of cancer cells in the bone microenvironment is poorly characterised. Expression pattern of bone marrow and other organ microenvironment in contact with osteolytic (Walker W256) and osteoblastic (MatLyLu MLL) metastases were investigated. Fisher and Copenhagen rats received, respectively, W256 and MLL cells injection. Bone and soft tissues were analysed by immunochemistry for DKK1, cathepsin K, RANKL, MCSF or IL6 expression. Tartrate-resistant acid phosphatase (TRAcP)-positive cells were detected by a histoenzymatic technique. In bone, expressions of MCSF and DKK1 were shown in stromal cells of the bone marrow, in contact with metastatic foci of both tumours. Many stromal cells were found RANKL positive in the vicinity of the tumours. Cells expressing cathepsin K and multinucleated TRAcP+ cells were found in direct contact with trabeculae but also in bone marrow spaces near metastatic cells. In extraosseous tumours, cells in contact with malignant cells did not expressed DKK1, MCSF, cathepsin K and IL6. Some RANKL+ cells were found in the periphery of subcutaneous tumours but may represent Langerhans cells. Abnormal presence of TRAcP+ cells was never observed in the vicinity of malignant cells. Interaction between stromal and cancer cells induces the expression on the formers of characteristics leading to osteoclastogenesis only in the bone microenvironment.

Venous thromboembolism prevention in cancer patients: the search for common antecedents

Venous thromboembolism (VTE) is a well-recognized concomitant of cancer. Although treatment with warfarin is often difficult and tedious, the heparins, and particularly the low molecular weight heparins, have afforded improved care of the patient with cancer-associated VTE, but with increased cost and the need for self-injection. Development by the pharmaceutical industry of inhibitors of specific activated coagulation factors and P-selectin holds promise for improved control of thrombosis with reduced toxicity. Increasing understanding of the interplay between the coagulation mechanism and neoplasia has yielded clues to the upstream origins of both, which may lead to experimental intervention potentially capable of preventing both.

Hazard classification of chemicals inducing haemolytic anaemia: An EU regulatory perspective

Haemolytic anaemia is often induced following prolonged exposure to chemical substances. Currently, under EU Council Directive 67/548/EEC, substances which induce such effects are classified as dangerous and assigned the risk phrase R48 'Danger of serious damage to health by prolonged exposure.' Whilst the general classification criteria for this endpoint are outlined in Annex VI of this Directive, they do not provide specific information to assess haemolytic anaemia. This review produced by the EU Working Group on Haemolytic Anaemia provides a toxicological assessment of haemolytic anaemia and proposes criteria that can be used in the assessment for classification of substances which induce such effects. An overview of the primary and secondary effects of haemolytic anaemia which can occur in rodent repeated dose toxicity studies is given. A detailed analysis of the toxicological significance of such effects is then performed and correlated with the general classification criteria used for this endpoint. This review intends to give guidance when carrying out an assessment for classification for this endpoint and to allow for better transparency in the decision-making process on when to classify based on the presence of haemolytic anaemia in repeated dose toxicity studies. The extended classification criteria for haemolytic anaemia outlined in this review were accepted by the EU Commission Working Group on the Classification and Labelling of Dangerous Substances in September 2004.

Phototherapy and malignancy: Possible enhancement by iron administration and hyperbaric oxygen

Photodynamic therapy (PDT) is a new therapeutic approach for the treatment of malignant tumors. Hyperbaric oxygen (HBO(2)) shows beneficial effects in various modalities of cancer interventions. Tumor cells tend to accumulate large amount of iron. There is interaction between tissue content of oxygen, iron, free radical production and tissue damage. Accumulation of intracellular iron is necessary for the production of oxygen radicals. HBO(2) increases tissue oxygen and hydrogen peroxide production in the cells. Malignant cells require iron, and exhibit more transferrin receptors. The photodynamic sensitization of human leukemic cells is achieved with accumulation of porphyrins stimulated by 5-aminolaevulanic acid (ALA) plus hemin. Further, a significant improvement in tumor response is obtained when PDT is delivered during hyperoxygenation. When PDT is combined with hyperoxygenation, the hypoxic condition is improved and the cell killing rate at various time points after PDT is significantly enhanced. Photosensitization with use of porphyrins is used with HBO(2) and PDT for treatment of certain tumors. PDT with ALA is used for treatment of actinic keratosis (AK). The combination of iron administration (by injection or oral rout), hemin, or transferrin, as a source for iron, HBO(2) as a source of oxygen under pressure and PDT as a source of generating free-radical tissue damage may be useful in the treatment of tumors. The possibility of combining HBO(2), iron, light and local photosensitizers to overcome skin tumors deserve extensive laboratory and clinical research work. Conclusively, iron, HBO(2), and PDT may have synergistic effect to hamper tumor cells.

Expression of E-cadherin and other paracellular junction genes is decreased in iron-loaded hepatocytes

Iron overload in the liver may occur in the clinical conditions hemochromatosis and transfusion-dependent thalassemia or by long-term consumption of large amounts of dietary iron. As iron concentrations increase in the liver, cirrhosis develops, and subsequently the normal architecture of the liver deteriorates. The underlying mechanisms whereby iron loading of hepatocytes leads to the pathology of the liver are not understood. Similarly, a direct relationship between the expression levels of paracellular junction genes and altered hepatocellular physiology has been reported; however, no relationship has been identified between iron loading and the expression of paracellular junction genes. Here, we report that the expression of numerous paracellular junction genes was decreased in iron-loaded hepatocytes, leading to increased cellular permeability, increased baculovirus-mediated gene transfer, and decreased gap junction communication. Iron loading of hepatocytes resulted in decreased E-cadherin promoter activity and subsequently decreased E-cadherin mRNA and protein expression. The data presented in this study describe a clear relationship between iron overload and decreased expression of paracellular junction genes in hepatic cells of rat and human origin.

Suppression of colonic cancer by dietary phytic acid

Large differences exist between human populations in the frequency of colonic cancer. Epidemiological evidence indicates that these differences are strongly influenced by country of residence, and a negative correlation has been found between the fiber content of the diet and frequency of colonic cancer. This has prompted the hypothesis that high-fiber diets are in some way protective. However, reanalysis of the dietary data provides equally strong support for the hypothesis that the protective element may be phytic acid (inositol hexaphosphate). This heat- and acid-stable substance is present in high concentration in many food items, including cereal grains, nuts, and seeds. Phytic acid forms chelates with various metals and suppresses damaging iron-catalyzed redox reactions. Inasmuch as colonic bacteria have been shown to produce oxygen radicals in appreciable amounts, dietary phytic acid might suppress oxidant damage to intestinal epithelium and neighboring cells. Indeed, rapidly accumulating data from animal models indicate that dietary supplementation with phytic acid may provide substantial protection against experimentally induced colonic cancer. Should further investigations yield additional support for this hypothesis, purposeful amplification of dietary phytic acid content would represent a simple method for reducing the risk of colonic carcinogenesis.

Influence of dietary iron overload on cell proliferation and intestinal tumorigenesis in mice

Iron-enriched diets caused an increase of tumor rate in two models of dimethylhydrazine(DMH)-induced colon tumorigenesis in mice. The effect was independent of the time the iron-diet was fed, i.e. during DMH-treatment or following the DMH-treatment period. The increase of tumor rate depended on the iron concentration in the diet (0.5-3.5%). The concentration-dependent iron accumulation in the colonic mucosa of mice was paralleled by increments of malonaldehyde contents indicating lipid peroxidation, another factor known to be involved in tumor development. It is suggested that iron exerts cocarcinogenic activity in the DMH-model by stimulating cell proliferation and inducing oxidative stress in the colonic mucosa. This effect of iron is independent of the time of tumor-initiation by DMH, as it is also observed in the period of tumor-promotion/progression after DMH-treatment.

Does the lens serve as a 'sink' for iron during ocular inflammation?

Twenty-four hours after induction of ocular inflammation by intravitreal injection of endotoxin (10 ng), the intraocular fluid (IOF, aqueous and vitreous humors) concentration of iron (Fe) increased. This was presumably due to entry of the plasma Fe-binding protein, transferrin, into the IOFs through disrupted blood ocular barriers. After 1 day of inflammation the Fe concentration in lenses from the inflamed eye was 60% greater than that measured in contralateral control lenses. By day 15, lens Fe concentration had returned to levels of the contralateral control lenses. There was a distinct relationship between the dose of endotoxin used and the amount of Fe accumulated by the lens. The Fe concentration in lenses from eyes injected with 100 ng endotoxin was 0.376 +/- 0.027 micrograms g-1 wet weight compared to 0.214 +/- 0.014 micrograms g-1 in lenses from eyes injected with 0.25 ng endotoxin. In a previous study, copper (Cu) concentration in the IOFs was elevated to the same extent as Fe in response to intravitreal injection of endotoxin. However, in the present study, lenticular Cu concentration was unaltered at the highest (100 ng) dose of endotoxin. Since the increase in lens uptake was selective for Fe, there may be a specific Fe uptake mechanism in this ocular tissue. The physiological reasons for and possible pathological consequences of such a process are discussed.

The release of iron and transferrin from the human melanoma cell

The role of the transferrin homologue, melanotransferrin (p97), in iron metabolism has been studied using the human melanoma cell line, SK-MEL-28, which expresses this antigen in high concentrations. The release of iron and transferrin were studied after prelabelling cells with human transferrin doubly labelled with iron-59 and iodine-125. Approx. 45% of internalised iron was in ferritin with little redistribution during reincubation. Iron release was linear with time, while transferrin release was biphasic, suggesting that iron was leaving the cell independently of transferrin. Unlabelled diferric transferrin increased transferrin release, implying a degree of coupling between cell surface binding, internalisation and release of transferrin. Increasing the preincubation time increased the amount of transferrin which remained internalised within the cell. A membrane-bound, iron-binding component with properties consistent with melanotransferrin was observed. Desferrioxamine or pyridoxal isonicotinoyl hydrazone could not remove iron from this compartment, suggesting a high affinity for iron. The number of membrane iron-binding molecules per cell was estimated to be 387,000 +/- 7000 . The non-transferrin-bound membrane Fe did not decrease during reincubation periods up to 5 h, suggesting that the cell was not utilising it. Hence, melanotransferrin may not have a role in internalising iron in melanoma cells.

Iron, radiation, and cancer

Increased iron content of cells and tissue may increase the risk of cancer. In particular, high available iron status may increase the risk of a radiation-induced cancer. There are two possible mechanisms for this effect: iron can catalyze the production of oxygen radicals, and it may be a limiting nutrient to the growth and development of a transformed cell in vivo. Given the high available iron content of the western diet and the fact that the world is changing to the western model, it is important to determine if high iron increases the risk of cancer.

T-cell mitogenesis stimulates the synthesis of a mRNA species coding for a 43-kDa peptide reactive with CM-H-9, a monoclonal antibody specific for placental isoferritin

In studying the changes that occur in concanavalin A-activated T lymphocytes, an mRNA species was discovered by hybridization of poly(A)+ mRNA with a human ferritin heavy chain cDNA probe. This ferritin mRNA, termed superheavy chain mRNA, differed from the known human ferritin heavy chain mRNAs by its larger size and degree of homology. The superheavy chain mRNA was isolated by sucrose-gradient centrifugation and translated in vitro in a cell-free system. The products obtained included two peptides (superheavy) of 43kDa that reacted with CM-H-9, a monoclonal antibody specific for placental isoferritin. De novo synthesis in intact transformed T cells revealed the synthesis of the superheavy chain peptides that were immunoprecipitated by anti-ferritin monoclonal antibody CM-G-8 and by placental isoferritin specific monoclonal antibody CM-H-9. The above results indicated that blast transformation of human T cells stimulated the appearance of a unique mRNA species that coded for a superheavy chain peptide associated with placental isoferritin, which was not detected in resting T cells.

Neuroblastomas contain iron-rich ferritin

The ultrastructure of neuroblastoma was examined using unstained sections so that ferritin particles could be identified by the electron density of their iron cores. Ferritin and hemosiderin were found in ten of 11 neuroblastomas that were examined when the patients first presented. The study was therefore expanded to an additional group of children, including some diagnosed by noninvasive procedures and given chemotherapy before the excision of their tumors. In this second group 12 of 20 specimens contained ferritin and hemosiderin in variable amounts. In both groups there was a tendency for patients with advanced disease to have increased amounts of iron compounds in the tumor tissue (Stage III and particularly Stage IV). Most Stage IV patients also had elevated serum ferritin levels. However, based on the available heterogenous material, no absolute relationship could be established between age, disease stage, tumoral storage iron, and the level of serum ferritin. The presence of ferritin in neuroblastoma may be linked to the elevated serum ferritin levels and may be implicated in tumorigenesis.

In vitro antiproliferative activity of 2'-(2-hydroxyphenyl)-2'-thiazoline-4'-carboxylic acid and its methyl ester on L1210 and P388 murine neoplasms

The activity of three iron chelators, methyl [2'-(2-hydroxyphenyl)-2'-thiazoline-4'-carboxylate] (MTL); 2'-(2-hydroxyphenyl)-2'-thiazoline-4'-carboxylic acid (TFAL); and 2-hydroxyphenyl-imido-ethyl-ether (Imidate), regarding antiproliferative, cytocidal, and cell-cycle effects are reported and compared with hydroxyurea (HU). In vitro, against L1210 and P388 murine neoplasms, MTL and TFAL displayed substantially greater antiproliferative activity than HU, although Imidate displayed no appreciable activity. MTL also induced a statistically more complete G1/S cell-boundary block than did HU at equimolar concentrations (100 microM). The IC50 values produced by MTL and TFAL were low enough (less than or equal to 20 microM) to warrant further testing of these chelators as potential antineoplastic agents.

Iron, ferritin, hepatitis B surface and core antigens in the livers of Chinese patients with hepatocellular carcinoma

Surgically resected specimens, consisting of tumor and adjacent non-neoplastic liver tissue, were obtained from 40 patients with primary liver cancer at Zhong Shan Hospital, Shanghai Medical University, the People's Republic of China, between March 1983 and July 1984. All were hepatocellular carcinomas (HCC), one being admixed with cholangiocarcinoma. The relationship of hepatitis B virus (HBV) markers with iron and ferritin was evaluated in liver tissues from patients with primary liver cancers. The serum HBsAg (Hepatitis B surface antigen) positive rate was 80.0% (32/40). Cirrhosis was observed in 97.5% (39/40). HBsAg was identified in 82.5% (33/40) of uninvolved liver, and 35.0% (14/40) of HCC tissues (P less than 0.001). HBcAg (hepatitis B core antigen) was detected in 25.0% (10/40) of liver, and 7.5% (3/40) of HCC tissues (P less than 0.05). Stainable iron was found in 65.0% (26/40) of unaffected livers, and 10.0% (4/40) of HCC tissues (P less than 0.001). Ferritin was demonstrated in 75% (30/40) of non-neoplastic liver, and 40% (16/40) of HCC tissues (P less than 0.001). Twenty-two of 33 HCC patients (66.7%) with HBsAg positive cells in their livers also showed stainable iron. Of 16 patients positive for ferritin in HCC cells, iron was found in only two. Iron was found in nine of ten patients with HBcAg in non-neoplastic hepatocytes (P = 0.056); a finding compatible with the hypothesis that iron accumulates in cells replicating HBV. The other results indicate that: immunohistologic ferritin in HCC is not due to increased stainable iron; tumor cells may produce ferritin; polyclonal antibodies to human liver ferritin react better with non-neoplastic hepatocytes than with HCC cells; the high prevalence of HBsAg and cirrhosis in HCC suggests that HBV plays a major etiologic role in hepatocarcinogenesis in China; and one case of HCC is attributed to Schistosoma japonicum infestation via cirrhosis.

Structure and expression of ferritin genes in a human promyelocytic cell line that differentiates in vitro

HL-60 is a human promyelocytic cell line with the capability of differentiating in vitro to give neutrophils, macrophages, or eosinophils. We screened libraries of HL-60 cDNA clones representing different time points during these differentiation processes to isolate clones corresponding to mRNAs whose expression is regulated during terminal differentiation. Upon sequencing this group of regulated clones, one clone encoding the heavy subunit and two clones encoding the light subunit of human ferritin were identified by reference to published amino acid sequences. Southern blot analyses showed that these clones are encoded by distinct multigene families. These clones identify two mRNAs whose ratios vary in a complex manner during both neutrophil and macrophage differentiation.

Structure and expression of ferritin genes in a human promyelocytic cell line that differentiates in vitro

HL-60 is a human promyelocytic cell line with the capability of differentiating in vitro to give neutrophils, macrophages, or eosinophils. We screened libraries of HL-60 cDNA clones representing different time points during these differentiation processes to isolate clones corresponding to mRNAs whose expression is regulated during terminal differentiation. Upon sequencing this group of regulated clones, one clone encoding the heavy subunit and two clones encoding the light subunit of human ferritin were identified by reference to published amino acid sequences. Southern blot analyses showed that these clones are encoded by distinct multigene families. These clones identify two mRNAs whose ratios vary in a complex manner during both neutrophil and macrophage differentiation.

Potentiation of hydroxyurea cytotoxicity in human chronic myeloid leukemia cells by iron-chelating agent

The effect of hydroxyurea (HU) was studied in 15 cases of human chronic myeloid leukemia cells (CML) in combination with iron-chelating agent, 2,2-bipyridine (bipyridine). The extent of (3H)thymidine incorporation in CML cells in vitro was taken as an index for the measurement of cytotoxicity. In the present study, we observed a potentiation in HU cytotoxicity by hydrophobic iron-chelator bipyridine (p less than 0.001) resulting in complete DNA biosynthesis inhibition. Both HU and bipyridine were used at a relatively non-toxic concentrations of 10(-4) M and 10 micrograms/ml, respectively. This inhibition was found to be partially reversible and a partial protective action by iron is also demonstrated. The various other metal chelators failed to sensitise CML cells to HU cytotoxicity. Implications with respect to the efficacy in cancer treatment is discussed.

Inflammation triggers hypoferremia and de novo synthesis of serum transferrin and ceruloplasmin in mice

Oil of turpentine was used to induce an artificial inflammation so that we could study its effect on iron metabolism and on synthesis of serum transferrin and ceruloplasmin in mice. It was found that turpentine-induced inflammation triggered the establishment of a hypoferremic state characterized by low levels of serum iron, followed by recovery and a gradual return to normal plasma iron levels. This turpentine-induced hypoferremia and its subsequent recovery paralleled the hypoferremia obtained during meningococcal infection. Moreover, serum transferrin and ceruloplasmin activity levels increased drastically during the recovery from hypoferremia. [14C]leucine incorporation studies revealed a de novo synthesis of both transferrin and ceruloplasmin. Turpentine-induced hypoferremia was also found to provide a protective effect against meningococcal infection which could be partially reversed by exogenous iron. The results of this study suggest that transferrin and ceruloplasmin may be synthesized partly in response to the altered iron metabolism observed during hypoferremia.

Binding of transferrin and uptake of iron by K-562 cells

Transferrin-mediated uptake of iron has been studied in K-562 human erythroleukaemia cells. K-562 cells accumulate iron from transferrin by a temperature-dependent mechanism linearly with time for at least 150 min. At 37 degrees C the uptake reaches a saturation level of approximately 25 pmol iron/10(6) cells/h at a concentration of iron (as iron-transferrin) of 5 mumol/l. The binding of transferrin reveals saturation kinetics, at 0.2 mumol/l of transferrin specific binding amounts to 1.8 pmol/10(6) cells. Relative to the binding of transferrin the uptake of iron increases with time. The ability of K-562 cells to accumulate iron declines with days in culture by approximately 50% from day 1 to day 5. The results are compatible with iron uptake by receptor-mediated endocytosis of transferrin, dissociation of iron within the cell and exocytosis of apotransferrin.

Antineoplastic and antiherpetic activity of spermidine catecholamide iron chelators

A series of iron chelating agents including the bacterial siderophores, parabactin and bis-N1,N8(2,3 dihydroxybenzoyl )spermidine, and four related compounds were synthesized and tested biologically. They were found: (a) to inhibit growth of cultured L1210 leukemia cells at IC50 values of 2-14 microM, (b) to inhibit replication of the DNA virus, herpes simplex type I, in monkey kidney cells at IC50 values of 0.4 microM ( parabactin ) to 55 microM, and (c) to be inactive against the RNA virus, vesicular stomatitis, at concentrations up to 1 mM. All effects were fully preventable by exogenous Fe (III). The activities correlated generally with the iron formation constants (10(36) to 10(48) moles/1) and more specifically with the lipophilicity of the compounds. The data suggest inhibition of DNA (but not RNA) synthesis by interference with the iron-containing enzyme, ribonucleotide reductase.