Role of damage-sensing/processing genes in the radiation response of haemopoietic in vitro colony-forming cells.

PURPOSE: To characterize the role of various cellular damagesensing, processing and survival genes in the in-vitro radiosensitivity of haemopoietic colony-forming cells. MATERIALS AND METHODS: Bone marrow cells from a range of different gene-knockout mice were irradiated in vitro with graded radiation doses and assayed for colony-forming efficiency. RESULTS: Colony-forming efficiency in the nulls was often lower by up to threefold compared with the wild-types. This was noticeable in particular for the atm, bax and p21 nulls. Radiosensitivity was markedly increased in the scid mouse (about 2.3-fold), more than in the atm null mouse (about 1.7-fold). There was resistance in the p53 nulls compared with the wild-types, using two different background strains, that gave similar results. There was slight sensitization in the p21 nulls. In the bcl-2 nulls, there was sensitization at low dose, but not at high dose. In contrast, in the bax nulls, there was protection at low dose, but again not at high dose. The heterozygotes for p53, bcl-2 and bax responded similarly to the wild types, so that no gene dosage effects were identified. CONCLUSIONS: These studies are the first to elucidate the role of as many as six relevant genes in the radiosensitivity of a single cell type. They show the greater importance of 'survival' genes at lower cytotoxic doses of radiation compared with the greater importance of 'damage-sensing' genes at higher doses.

Transgenerational effects of preconception paternal contamination with (55)Fe.

The conjecture that germline mutations induced by radiation exposure before conception may predispose subsequent offspring to cancer remains contentious. Previous experimental studies have shown that preconception paternal irradiation with (239)Pu induces perturbations in the hemopoietic systems of offspring and influences sensitivity to a secondary carcinogen. In the present study, male DBA2 mice were injected intravenously with the Auger electron emitter (55)Fe (4 kBq g(-1)) 18 or 84 days before mating with normal females. Comet analysis showed an increased incidence of DNA strand breaks in sperm from contaminated animals after 84 days, but not after 18 days, indicating spermatogonial rather than spermatid damage. Offspring were either assayed for changes in bone marrow stem cells and committed progenitors or challenged with the chemical carcinogen methyl nitrosourea (MNU, 50 mg/kg) at 10 weeks of age and monitored for the onset of malignancy. Offspring from irradiated fathers had normal peripheral blood profiles, although the stem cell population was amplified in offspring arising from those exposed to (55)Fe at 84 days before conception. Exposure to MNU significantly increased the incidence of lympho-hemopoietic malignancies in offspring from the 84-day group, but not in those from the 18-day group. These findings support the hypothesis that aberrations that are potentially leukemogenic may be transmitted to offspring after radiation damage to the paternal germline.

Bone marrow toxicity in mice treated with indium-114m-labelled blood cells.

Clinical trials with autologous indium-114m-labelled lymphocytes have revealed significant anti-tumour effects in chronic lymphocytic leukaemia patients with highly resistant disease. Substitution of the lymphocyte vector with heat-damaged red blood cells (HDRBC) may make this treatment more universally applicable and reduce the dose-limiting myelosuppression encountered with labelled lymphocytes. Therefore, the bone marrow localization and toxicities of indium-labelled lymphocytes or HDRBC have been investigated in BDFI mice. At 24 hours approximately 4% and 1.2% of 114In(m) administered as labelled lymphocytes or HDRBC respectively was localized within the bone marrow and remained constant for 57 days thereafter. Toxicity towards bone marrow stem cells, measured as CFU-S, was equivalent for both cellular vectors. However, at clinically relevant activities, 114In(m) HDRBC were less toxic than labelled lymphocytes towards committed progenitors, assayed as in vitro-CFC and CFU-Meg. These data suggest that substitution of HDRBC for lymphocytes as the 114In(m) vector may be beneficial in reducing the myelosuppression associated with this technique.

Effect of bcl-2 deficiency on the radiation response of clonogenic cells in small and large intestine, bone marrow and testis.

PURPOSE: Overexpression of bcl-2 protects against radiation induced apoptosis in lymphohaematopoietic cell types in vivo, whilst bcl-2 deficiency radiosensitizes murine T-lymphocytes in vitro. However, there are few data regarding the influence of bcl-2 deficiency on the radiosensitivity of non-lymphoid cell types. The purpose of this study was to investigate the role of bcl-2 in the clonogenic radiation response of intestinal crypts, bone marrow progenitor cells and testicular stem cells. METHOD: Survival curves were obtained for each cell type from bcl-2 null (-/-), heterozygote (+/-) and wild type (+/+) mice. Crypt survival in the small and large intestine was assessed using the crypt microcolony assay. Committed haemopoietic progenitors were assayed using in vitro colony-forming cell (CFC) assays and survival of clonogenic spermatogonia was assessed by scoring regenerative tubules at 35 days post-irradiation. RESULTS: There was no difference in small intestine crypt survival between the three genotypes. In the colon, there was a tendency towards lower clonogen survival in the +/- and -/- animals. Haemopoietic in vitro CFC from -/- animals showed lower survival in comparison to +/+ mice, but spermatogonial stem cells were comparatively more radioresistant. CONCLUSIONS: Deficiencies in bcl-2 affect the radiation response of different cell populations in small but different ways. This may be due to variations between cells in their innate capacity for apoptosis, their dependence on different members of the bcl-2 family gene and their cell-cycle status and p53 expression.

Modification of murine adult haemopoiesis and response to methyl nitrosourea following exposure to radiation at different developmental stages.

PURPOSE: To investigate the hypothesis that the developmental phase at which an individual encounters radiation damage affects its long-term sensitivity to a subsequent tumourigenic insult. MATERIALS AND METHODS: Either the pregnant C57B16 mouse was exposed to 137Cs gamma-rays at 4 or 15 days post-conception (embryonic and foetal stages respectively) or BDF1 offspring were irradiated at 4 or 21 days of age (neonatal and juvenile stages). Offspring were either assayed for changes in bone marrow stem cells and committed progenitors at 6, 12 and 18 weeks of age, or injected with the chemical carcinogen methyl nitrosourea (MNU) at 10 weeks of age and monitored for onset of neoplasia. RESULTS: Gamma-irradiation induced a persistent long-term deficit in stem cells in all irradiated animals, with the foetal stage appearing most radiosensitive. However, femoral cellularity, committed progenitor cell numbers and peripheral blood counts were unaffected. When offspring were exposed to MNU, the incidence of malignancy was significantly enhanced in animals irradiated at the foetal, neonatal and juvenile stages. CONCLUSIONS: This study has shown that exposure to ionizing radiation at the foetal, neonate or juvenile stages of development induces residual haemopoietic damage and increases oncogenic susceptibility to a subsequent exposure to MNU.

Hemopoietic damage and induction of leukemia in offspring due to preconception paternal irradiation from incorporated plutonium-239.

Preconception paternal irradiation has been implicated in a localized excess of childhood leukemia and non-Hodgkin's lymphoma close to a nuclear reprocessing plant. Other epidemiological studies, however, threw doubt on the validity of this hypothesis. Experimental evidence implicating preconception paternal X rays in the development of lung and skin cancers has also been questioned. In this study, (239)Pu (0, 128 or 256 kBq kg(-1)) was injected into male CBA-H and DBA-2 mice 12 weeks before they were mated with CBA-H and C57BL/6 females. Their offspring were assessed for hematological status at 6 to 18 weeks of age or were treated with either 3.3 Gy whole-body gamma rays or methylnitrosourea (MNU, 50 mg kg(-1)) and monitored for onset of malignancies of the lymphoid and hemopoietic system. As a group, offspring were normal hematologically, but up to 35% of individual mice had femoral cellularities and numbers of spleen and fibroblastoid colony-forming cells outside the normal range. Exposure of the offspring to radiation or MNU significantly increased the rate of incidence of lymphoid and myeloid leukemias. Simulation of the experiments with preconception gamma irradiation indicated that damage to the spermatogenic stem cell was an important factor. It is concluded that preconception paternal irradiation can influence susceptibility of offspring to a subsequent exposure to a carcinogen.

Mutation studies in lacI transgenic mice after exposure to radiation or cyclophosphamide.

We have used the Big Blue lacI transgenic mouse reporter system to investigate mutation induction in the testes, spleen and liver after exposure to an internally incorporated radionuclide, 114mIn, whole body irradiation with 60Co gamma-rays and systemically administered cyclophosphamide. Spontaneous mutation frequencies were 6-17x10(-6). No statistically significant mutation induction was observed in testes or spleen at 35 days after exposure to any test agent, although mutation frequencies tended to be increased (by approximately 1.5-fold) after exposure to 1 Gy gamma-rays. However, liver mutation frequencies were doubled after treatment with 100 mg/kg cyclophosphamide and were elevated by approximately 2.5-fold after systemic administration of 114mIn and 4.5-fold after 1 Gy 60Co gamma-rays. When data from all organs were pooled, mutation frequency was doubled after exposure to 1 Gy gamma-rays, but no other significant increases were observed. These findings support the hypothesis that the lacI transgenic mouse may be relatively inefficient at detecting mutations induced by exposure to ionizing radiation or other agents which produce a spectrum of deletion sizes, including those which are larger than the lacI transgene.

Loss of atm radiosensitizes multiple p53 null tissues.

An unusual clinical finding in ataxia-telangiectasia, a human disorder caused by mutations in atm, is exquisite sensitivity to gamma irradiation. By contrast, homozygous deletion of p53 is marked by radiation resistance in certain tissue compartments. Previous studies (A. J. Levine, Cell, 88: 323-331, 1997) have shown that, in vitro, p53-deficient bone marrow cells are resistant to gamma irradiation. Furthermore, the gastrointestinal radiosensitization engendered by the loss of atm has recently been shown (C. H. Westphal et al., Nat. Genet., 16: 397-401, 1997) to be independent of p53. Expanding on previous work, we have looked at in vivo bone marrow resistance in p53-deficient mice. Our results indicate that inbred FVB strain p53 null mice survive lethal irradiation doses because of bone marrow resistance. Moreover, the deletion of atm radiosensitizes even p53 null bone marrow and mouse embryonic fibroblast cells. The results presented here argue that the loss of atm radiosensitizes multiple tissues in a p53-independent manner. Hence, functional inhibition of atm in p53 null and p53 wild-type human tumors may be a useful adjunct to gamma irradiation-based antitumor therapy.

Transferrin-dependent uptake and dosimetry of Auger-emitting diagnostic radionuclides in human spermatozoa.

UNLABELLED: Localization of Auger-emitting radionuclides within spermatozoa could lead to the induction of transmissible genetic damage. We have quantified in vitro uptake of the widely used diagnostic Auger-emitters, (111)In and 99mTc, by ejaculated human spermatozoa and investigated the role of transferrin in their cellular localization. The resultant dose to sperm heads, including cellular dosimetry for Auger emissions, has been calculated for each radionuclide and compared with that achieved using conventional macrodosimetry. METHODS: Freshly isolated human spermatozoa were incubated in a physiological salt solution containing (111)In-chloride, 99mTc-pertechnetate or the transferrin-binding isotope 59Fe-citrate as a positive control. Cellular uptake mechanisms were investigated with transferrin competition and temperature dependence studies. The percentage uptake of each radionuclide was determined, and the dose to individual sperm heads was calculated using both conventional macrodosimetric methods and by consideration of radionuclide localization and energy deposition at the cellular level, including Auger electron emissions from (111)In and 99mTc. RESULTS: On in vitro incubation, human spermatozoa were found to accumulate (111)In and 59Fe but not 99mTc. Cell uptake of (111)In and 59Fe was transferrin-mediated; however, an alternative transferrin-independent uptake pathway was also present for (111)In. The dose to sperm heads from (111)In, calculated using measured uptake and cellular dosimetry, was found to be larger than that calculated using conventional dosimetry by a factor of more than 100. In contrast, conventional dosimetry was adequate for 99mTc and 59Fe. CONCLUSION: Isolated human spermatozoa appear to accumulate transferrin-binding isotopes, such as the Auger-emitter (111)In. If this uptake mechanism operates in the male reproductive tract, the resultant high dose to the sperm head could indicate that contraception may be advisable after large diagnostic doses of (111)In and, possibly, other transferrin-binding radionuclides. Such precautions could prevent transmission of any genetic damage from irradiated spermatozoa.

Transferrin-mediated uptake of plutonium by spermatogenic tubules.

Using isolated rat seminiferous tubules as an in vitro model, we have found that 238Pu can cross the blood-tubule barrier and accumulate within tubules in a time dependent manner. Furthermore, similar to 59Fe, tubule 238Pu uptake was inhibited by the addition of excess transferrin, suggesting that plutonium may utilize the physiological iron-transferrin pathway to cross the blood-tubule barrier. However unlike 59Fe, 238Pu was only transiently associated with the tubules, suggesting differences in the intracellular processing of these radionuclides. The assumptions made in the estimation of doses to the human testis from incorporated plutonium are considered.

The relationship of intracellular iron chelation to the inhibition and regeneration of human ribonucleotide reductase.

The depletion of cellular iron can lead to the inhibition of ribonucleotide reductase, preventing new DNA synthesis and hence inhibiting cell proliferation. Electron paramagnetic resonance (EPR) spectroscopy has been used to examine simultaneously for the first time the relationship between chelation of intracellular iron and the rate of removal and regeneration of the tyrosyl radical of ribonucleotide reductase within intact human leukemia K562 cells. The different physiochemical characteristics of relatively hydrophobic low molecular weight bidentate hydroxypyridinone chelators and the higher molecular weight hexadentate ferrioxamine have been exploited to elucidate these interactions further. The base-line concentration of EPR-detectable mononuclear nonheme iron complexes was 3.15 =/- 1.05 microM, rising on incubation with chelators more rapidly with hydroxypyridinones than with desferrioxamine. Hydroxypyridinones also removed the tyrosyl radical more rapidly, apparently as a consequence of depletion of the intracellular iron pools necessary to regenerate the active enzyme and compatible with their reportedly greater cell toxicity. The radical decay rate is consistent with previous models, suggesting that iron is spontaneously removed from mammalian ribonucleotide reductase. Upon removal of extracellular chelator the regeneration of the tyrosyl radical was significantly faster for hydroxypyridinones than for desferrioxamine, consistent with their differential effects on cell cycle synchronization.

Transferrin-mediated uptake of radionuclides by the testis.

UNLABELLED: In an attempt to explain the deleterious effects of gonadal radionuclide localization, we examined the role of transferrin in testicular radionuclide uptake. METHODS: In vivo testicular uptake and retention of the transferrin binding radionuclides 114mIn-citrate and 59Fe-citrate were compared with that of the nontransferrin binding isotopes 137Cs-citrate and Na125I for 63 days postinjection. Isotope uptake mechanisms were investigated in vitro using isolated seminiferous tubules and Sertoli cell monolayers grown in bicameral culture chambers. RESULTS: Indium-114m, 59Fe and 137Cs were localized in the testis by 24 hr postinjection, but accumulation of 125I was minimal. Although testicular 114mIn remained constant, 59Fe declined slowly over the following 63 days and 137Cs fell very rapidly. When 114mIn- or 59Fe-loaded testes were fractionated, and markedly more 114mIn was associated with the seminiferous tubules than 59Fe, suggesting that 114mIn may be retained. In vitro uptake of 59Fe, 67Ga and 114mIn by isolated seminiferous tubules was inhibited by transferrin, but uptake of 137Cs and 125I was unaffected. Iron-59, 67Ga and 114mIn were retained by isolated tubules in contrast to 137Cs and 125I. Whereas 137Cs, 59Fe and 114mIn all crossed Sertoli cell monolayers, the rate of transcellular transport of 137Cs was faster than that of 59Fe or 114mIn, suggesting differences in the intracellular processing of transferrin binding and nontransferrin binding radionuclides. CONCLUSION: These data suggest that some radionuclides may access the seminiferous epithelium through receptor-mediated endocytosis of transferrin. Such radionuclide localization could lead to continuous irradiation of the testes, resulting in mutagenic damage to spermatogenic cells.

Testicular toxicity of the transferrin binding radionuclide 114mIn in adult and neonatal rats.

Adult (70 d) and neonatal (7 d) male rats were dosed (i.p.) with 37 MBq/kg (1 mCi/kg; approximately 1 microgram elemental indium/kg) 114mIn, a transferrin-binding radionuclide. In adults, approximately 0.25% of the injected activity localised within the testis by 48 h postinjection and remained constant for up to 63 d. In neonates, 0.06% of the activity was in the testis by 48 h, and this declined such that by 63 d only 0.03% remained. At 63 d, treated rats had reduced sperm head counts and abnormal testicular histology that was more marked in animals dosed as adults than as neonates. In vitro, uptake of 114mIn into seminiferous tubules isolated from 7-, 20-, or 70-d-old rats was compared with that of 125I. Both radionuclides were readily accumulated by the tubules. Whilst 114In uptake into 20- and 70-d tubules was inhibited by excess transferrin, uptake into 7-d tubules was unchanged. 125I uptake was not affected by excess transferrin. These data support the contention that some radionuclides may cross the blood-testis barrier by utilisation of the physiologic iron-transferrin pathway, which may lead to greater testicular damage in adult compared to neonatal animals.

Spermatogenic and mutagenic damage after paternal exposure to systemic indium-114m.

The cytotoxic and mutagenic consequences of systemic administration of 114mIn have been examined. Adult male rats were dosed intraperitoneally with 14.8 or 3.7 MBq/kg 114mIn. Approximately 0.25% of the injected radioactivity was localized within the testis by 24 h and was retained with an effective half-life of 49.5 days. Breeding studies were started 3 days after injection, males being housed with two females for seven consecutive mating trials of 19 days, separated by 2 days. Indium-114m caused a reduction in litter size and an increase in the incidence of pre- and postimplantation losses and dominant lethal mutations. These effects became evident from 24 days but were most marked between 87-126 days after treatment and persisted up to 147 days. When animals were mated 200 days after treatment, no significant changes were observed. In a parallel study, administration of 14.8 MBq/kg 114mIn resulted in decreased testis and epididymal weight and sperm reserves. Maximal reduction occurred between 87-108 days after injection followed by recovery toward control values, but neither organ had reached normal levels at 200 days. A single dose of 3.7 MBq/kg, however, had no effect on reproductive organ weight or sperm content. Male F1 progeny from the 14.8 MBq/kg group of the second mating period (commencing at 24 days) displayed decreased testis weights and sperm content and provoked a higher incidence of dominant lethal mutations. This effect was not observed in male progeny from any other time or the alternative dose level.

In vivo induction of O6-alkylguanine-DNA-alkyltransferase in response to indium-114m.

The effect of systemic administration of the radionuclide 114mIn on O6-alkylguanine-DNA-alkyltransferase (ATase) activity has been examined in rats. In response to 14.8 MBq/kg 114mIn injected intraperitoneally, hepatic ATase was induced maximally approximately fivefold at 7 days after injection, at which time the cumulative radiation dose to the liver was approximately 2 Gy. At 63 days after injection ATase activity was still approximately twofold elevated and remained so at 126 days after injection. By 200 days after injection ATase activity had returned to control values. The 114mIn content of the liver increased to a maximum of 28.7 kBq/g 48-72 h after injection, after which it began to decrease such that at 126 days only 0.3 kBq/g remained and at 200 days 0.03 kBq/g. In response to 4.44 MBq/kg 114mIn, hepatic ATase was induced twofold by 7 days after injection, when the liver had received a radiation dose of 0.6 Gy, and was still slightly elevated at 63 days. There was no ATase induction after 0.44 MBq/kg 114mIn up to 7 days after injection; however, at 42 days after injection activity was approximately twofold higher. These results suggest that induction of hepatic ATase activity by 114mIn is dependent upon cumulative radiation dose and dose rate; both must be above minimum threshold values for induction to occur. The induction of a DNA repair enzyme by radiation exposure from an internal radionuclide may have important consequences for risk assessments of occupational, medical and environmental exposures.

Subcellular distribution of desferrioxamine and hydroxypyridin-4-one chelators in K562 cells affects chelation of intracellular iron pools.

The interactions of iron chelators with intracellular iron pools have been examined by measuring the subcellular distribution of radiolabelled desferrioxamine (DFO) and the orally active hydroxypyridinone (HPO) chelator 1,2-diethyl-3-hydroxypyridin-4-one (CP94), as well as the ability of these chelators to modify the subcellular distribution of 59Fe delivered by the receptor mediated endocytosis of transferrin. K562 cells were pulsed with 59Fe transferrin and challenged with DFO or CP94 (100 microM IBE) for 20 or 240 min and then subjected to subcellular fractionation. At 20 min there was a significant decrease (P < 0.05) in both lysosomal/particulate 59Fe (75% of control) and cytosolic 59Fe ferritin (50% of control) in cells incubated with CP94, unlike cells treated with DFO where no decrease was observed. By 240 min, in addition to the above, 59Fe accumulation was significantly decreased in the nuclear, mitochondrial, and low molecular weight cytosolic fractions with CP94 (P < 0.05). With DFO a significant decrease in 59Fe in only the lysosomal/particulate and cytosolic ferritin compartments was observed at 240 min (P < 0.05). At this time, however, there was a significant accumulation of both cytosolic low molecular weight 59Fe and cytosolic DFO. The relatively rapid decrease of 59Fe within intracellular compartments seen with CP94 compared to DFO was paralleled by a significantly higher accumulation of CP94 than DFO in nuclear, lysosomal/particulate and low molecular weight cytosolic compartments at 20 min (P < 0.05). These results suggest that transferrin derived endosomal iron may be chelated by HPOs, unlike DFO, due to their faster uptake into these organelles. The more rapid access of HPOs than DFO to certain intracellular iron pools may explain the greater possibility of HPOs to inhibit proliferation of cells in vivo.

Contrasting interspecies efficacy and toxicology of 1,2-diethyl-3-hydroxypyridin-4-one, CP94, relates to differing metabolism of the iron chelating site.

In order to define a predictive animal model for the effects of hydroxypyridinone (HPO) iron chelators in humans, we have compared the 28 d oral efficacy and toxicology of the HPO, 1,2-diethyl-3-hydroxypyridin-4-one (CP94) in rats and guinea-pigs and related the results to the contrasting metabolism of this compound in the two species. CP94 was highly effective at mobilizing liver iron in rats but showed toxicity at higher doses, whereas in the guinea-pig the compound lacked toxicity but was ineffective at mobilizing liver iron. These differences can be explained by the contrasting metabolism of the drug between the two species. In rats, at the top dose of 300 mg/kg intragastrically, all animals died before the end of the study, with no deaths or weight loss at lower doses. At 100 mg/kg, rat liver non-haem iron concentrations were reduced by 53% and 44% in females and males respectively (P < 0.001). At this dose, adrenal medullary cell vacuolation, increased mammary secretory activity, vacuolation of corpora luteal cells and single cell hepatocyte necrosis were seen. There were no reductions in the white cell count. At 50 mg/kg rat liver non-haem iron concentrations were decreased by 50% and 34% in females and males respectively (P < 0.02). In female rats this was associated with increased mammary secretory activity. In iron-overloaded rats given 100 mg/kg by gavage for 28 d, liver non-haem iron concentration was reduced by 39% (P < 0.01) and serum ferritin by 71% (P < 0.001). Ovarian and mammary changes were not influenced by iron loading. In guinea-pigs, CP94 was evaluated at 50 mg/kg, 100 mg/kg or 200 mg/kg by oral insufflation for 28 d. No reduction in liver iron was seen and no systematic dose related histological, biochemical or haematological effects were observed. Whereas in guinea-pigs 99% of urinary recovery following an oral dose of CP94 (100 mg/kg) was as the inactive glucuronide metabolite, in the rat only 23% of the dose was excreted in the urine as the glucuronide with remainder as the free drug or an iron binding metabolite. The lack of both efficacy and toxicity in the guinea-pig may therefore be explained by the rapid inactivation of CP94 by glucuronidation. This metabolism of CP94 in the guinea-pig is closer to humans than the rat, suggesting that both the efficacy and toxicity of this compound in humans may also be limited by glucuronidation.

In vivo and in vitro effects of 3-hydroxypyridin-4-one chelators on murine hemopoiesis.

The effects of 3-hydroxypyridin-4-one (HPO) iron chelators and desferrioxamine (DFO) on murine hemopoiesis in vivo and in vitro have been compared in order to investigate the mechanism by which leucopenia in mice and granulocytopenia in man occurs with 1,2-,dimethyl-HPO (CP20). Administration of 60 doses of 200 mg/kg CP20 to Balb/c mice resulted in significant anemia, lymphopenia and granulocytopenia accompanied by bone marrow hypocellularity. DFO and CP94 (1,2,diethyl-HPO) at the same dose also caused lymphopenia but marrow cellularity was unaffected. When marrow from untreated mice was incubated with HPOs and DFO, erythroid burst-forming cells (BFU-E) and granulocyte/macrophage colony forming units (CFU-G+Mac), colony growth was inhibited in a dose-dependent manner at micromolar concentrations. The addition of iron to saturate the chelators abrogated the effects of DFO, but not those of the HPOs. With the HPO-iron complexes, addition of sufficient iron to saturate the transferrin in the medium reversed the inhibitory effects of the relatively hydrophilic CP20-iron complex but not those of the more lipophilic CP94-iron complex. Addition of further iron-saturated transferrin also corrected inhibition by the CP94-iron complex. These results show that HPO-iron complexes potentially have antiproliferative effects unlike DFO-iron complex (FO). The difference in the relative effects of CP20 to CP94 on hemopoiesis in vivo and in vitro suggests that additional factors to those inhibiting hemopoiesis in marrow cultures may operate with the long-term administration of iron chelators in vivo.