Hydroxyurea Toxicity Research Articles

Effect of Hydroxyurea Exposure before Puberty on Sperm Parameters in Males with Sickle Cell Disease

Background: Sperm parameters alteration is documented in untreated men with sickle cell disease (SCD). An aggravating effect of hydroxyurea (HU) on sperm parameters is also well established: HU, at current doses, causes significant, rapid, and unpredictable impairment of spermatogenesis. Reversal of its effect when treatment is stopped has also been documented, albeit in patients given very low doses of treatment (10 mg/kg/d). In France, sperm banking is free of charge and recommended whenever possible before initiation of HU treatment. While recent guidelines have broadened the indication of HU for asymptomatic infantssuch banking is impossible in younger boys before puberty. In addition, little is known on the effect of HU on sperm parameters when given at this specific period. Objectives and Methods: The main objective of this study was to compare sperm parameters after treatment resolution in young males treated by HU prior to puberty with sperm parameters of untreated males. Secondary objective was to analyze longitudinally sperm parameters during HU washout in those treated prior to puberty.Data regarding indication of HU, dosage, date of initiation and stop, clinical profile including vaso occlusive events (VOE) transfusion episodes and date of puberty was collected. Alternative treatment before or during semen analysis was also documented. A period of 3 months of HU wash out was required prior to semen analysis in treated patients. Results: A total of 26 patients (43 semen samples) were studied, with 16 patients treated with HU prior to puberty (HU-PP) and 10 untreated (HU-naive). Characteristics of patients are presented in Table 1. Indication of HU was cerebral vasculopathy (n=3), VOE (n=5), severe anemia (n=1) or combined (n= 7). The median stopping of the HU before CECOS is 4.5 years [0.5-11.0]. An alternative treatment based on a transfusion program was initiated in 14 patients (87.5%) at the time of sperm analysis in the HU-PP group versus 5 patients (50%) in the HU-naive group. Duration of transfusion program was 126 months [3-188] in HU-PP versus 13 [7-100] in the HU naive group. We compared the fraction of abnormal values in semen samples in both groups (25 samples in the HU-PP group and 18 in the HU-naive). No significant difference was observed regarding volume of ejaculate, spermatozoid concentration, total sperm count, spermatozoid motility, morphology and vitality, and sexual abstinence before sampling in both groups (Table 1). In the HU-PP group, there was a trend in improvement of sperm parameters with the duration of transfusion program. In addition, a kinetic analysis of sperm parameters during the period of HU wash out was performed in 3 patients exposed to HU prior to puberty, demonstrating reversibility of HU toxicity in all. All 26 patients were offered semen cryopreservation. For 22 of them, semen parameters after thawing indicated a possible use for assisted reproductive technologies, mostly in vitro fertilization with intracytoplasmic sperm injection. In the remaining 4 (3 in HU-PP group and 1 in HU naïve), there was serious concern about the possible use because of a very low initial sperm concentration and the absence of motile spermatozoids after thawing. Discussion/Conclusion: Toxicity of HU upon spermatogenesis has been well documented in animal studies and in adult males. In many settings, such issues may be a drawback for parents and/or caregivers to treat young boys before semen banking can be performed, given the lack of robust information on reversibility after treatment resolution. Abnormalities of sperm count are also common in non-treated males so that demonstrating the specific effect of HU may be complex. Oseggbe et al. and Berthaud et al. showed that 91% of untreated SCD males have at least one abnormal sperm parameter. Here, we show that after treatment resolution, there are no differences in sperm parameters in patients exposed to HU before puberty compared to untreated males. Because a majority of patients benefitted from transfusion therapy at the time of analysis, we were however unable to demonstrate whether reversal of toxicity occurs spontaneously or requires transfusion. Notwithstanding possible limitations due to small sample size, this study shows that in boys with severe disease requiring HU treatment before puberty, toxicity of HU on sperm parameters should not be a major drawback. Disclosures Bernaudin: AddMedica: Honoraria, Other: Help for travel; GBT: Membership on an entity's Board of Directors or advisory committees; BlueBirdBio: Consultancy. De Montalembert:Novartis: Consultancy, Honoraria; Addmedica: Consultancy, Honoraria; Bluebird Bio: Membership on an entity's Board of Directors or advisory committees. Brousse:bluebird bio: Consultancy; Add medica: Consultancy.

Replication Rapidly Recovers and Continues in the Presence of Hydroxyurea in Escherichia coli.

In both prokaryotes and eukaryotes, hydroxyurea is suggested to inhibit DNA replication by inactivating ribonucleotide reductase and depleting deoxyribonucleoside triphosphate pools. In this study, we show that the inhibition of replication in Escherichia coli is transient even at concentrations of 0.1 M hydroxyurea and that replication rapidly recovers and continues in its presence. The recovery of replication does not require the alternative ribonucleotide reductases NrdEF and NrdDG or the translesion DNA polymerases II (Pol II), Pol IV, and Pol V. Ribonucleotides are incorporated at higher frequencies during replication in the presence of hydroxyurea. However, they do not contribute significantly to the observed synthesis or toxicity. Hydroxyurea toxicity was observed only under conditions where the stability of hydroxyurea was compromised and by-products known to damage DNA directly were allowed to accumulate. The results demonstrate that hydroxyurea is not a direct or specific inhibitor of DNA synthesis in vivo and that the transient inhibition observed is most likely due to a general depletion of iron cofactors from enzymes when 0.1 M hydroxyurea is initially applied. Finally, the results support previous studies suggesting that hydroxyurea toxicity is mediated primarily through direct DNA damage induced by the breakdown products of hydroxyurea, rather than by inhibition of replication or depletion of deoxyribonucleotide levels in the cell.IMPORTANCE Hydroxyurea is commonly suggested to function by inhibiting DNA replication through the inactivation of ribonucleotide reductase and depleting deoxyribonucleoside triphosphate pools. Here, we show that hydroxyurea only transiently inhibits replication in Escherichia coli before replication rapidly recovers and continues in the presence of the drug. The recovery of replication does not depend on alternative ribonucleotide reductases, translesion synthesis, or RecA. Further, we show that hydroxyurea toxicity is observed only in the presence of toxic intermediates that accumulate when hydroxyurea breaks down, damage DNA, and induce lethality. The results demonstrate that hydroxyurea toxicity is mediated indirectly by the formation of DNA damage, rather than by inhibition of replication or depletion of deoxyribonucleotide levels in the cell.

Gap Junctional Intercellular Communication Increases Cytotoxicity and Reduces Resistance to Hydroxyurea

Background: Gap junctions enable small molecules to diffuse between adjacent cells and have been associated with greater cytotoxicity of radiation and anti-cancer drugs. We investigated whether this gap junctional intercellular communication (GJIC) affected the cytotoxicity of the classic ribonucleotide reductase (RR) inhibitor and anti-cancer agent, hydroxyurea (HU). Materials and Methods: We used GJIC-proficient and deficient, connexin 43-expressing WB rat liver epithelial cell lines. We compared HU toxicity by crystal violet assay, effects of the drug on deoxynucleotide pools by HPLC, and ability of GJIC to increase toxicity of HU-resistant cells through a bystander effect in co-culture experiments. Results: GJIC-proficient cells were three- to five-fold more sensitive (IC50 0.1 mM) to HU than GJIC-deficient derivatives (IC50 0.3 - 0.5 mM). This sensitivity depended upon GJIC because treatment of GJIC-proficient cells with the GJIC blocker oleamide decreased HU toxicity by approximately 60% - 80% and restoration of GJIC in GJIC-deficient cells by stable transduction of connexin 32-encoding Gjb1 increased HU toxicity (IC500.1 mM). The effects were not due to connexin expression per se or its localization since all cell lines expressed comparable quantities of connexin 43 that was localized to the plasma membrane. Also HU sensitivity was not related to differential effects on nucleotide metabolism in the cells. Thymidine triphosphate levels increased and deoxyadenosine triphosphate levels decreased similarly (15% - 20%) in GJIC-proficient and deficient cells over 24 h of HU treatment. More importantly, when HU-resistant cells were co-cultured with sensitive cells, the resistant cells were killed only when GJIC was present. Conclusion: The data suggest that GJIC enhances cytotoxicity and decreases resistance to HU. These results may be important clinically if GJIC can be enhanced in drug-resistant cells.

Identification of C18:1-Phytoceramide as the Candidate Lipid Mediator for Hydroxyurea Resistance in Yeast

Recent studies showed that deletion of ISC1, the yeast homologue of the mammalian neutral sphingomyelinase, resulted in an increased sensitivity to hydroxyurea (HU). This raised an intriguing question as to whether sphingolipids are involved in pathways initiated by HU. In this study, we show that HU treatment led to a significant increase in Isc1 activity. Analysis of sphingolipid deletion mutants and pharmacological analysis pointed to a role for ceramide in mediating HU resistance. Lipid analysis revealed that HU induced increases in phytoceramides in WT cells but not in isc1Δ cells. To probe functions of specific ceramides, we developed an approach to supplement the medium with fatty acids. Oleate (C18:1) was the only fatty acid protecting isc1Δ cells from HU toxicity in a ceramide-dependent manner. Because phytoceramide activates protein phosphatases in yeast, we evaluated the role of CDC55, the regulatory subunit of ceramide-activated protein phosphatase PP2A. Overexpression of CDC55 overcame the sensitivity to HU in isc1Δ cells. However, addition of oleate did not protect the isc1Δ,cdc55Δ double mutant from HU toxicity. These results demonstrate that HU launches a lipid pathway mediated by a specific sphingolipid, C18:1-phytoceramide, produced by Isc1, which provides protection from HU by modulating Swe1 levels through the PP2A subunit Cdc55.

The in Vivo Toxicity of Hydroxyurea Depends on Its Direct Target Catalase

Hydroxyurea (HU) is a well tolerated ribonucleotide reductase inhibitor effective in HIV, sickle cell disease, and blood cancer therapy. Despite a positive initial response, however, most treated cancers eventually progress due to development of HU resistance. Although RNR properties influence HU resistance in cell lines, the mechanisms underlying cancer HU resistance in vivo remain unclear. To address this issue, we screened for HU resistance in the plant Arabidopsis thaliana and identified seventeen unique catalase mutants, thereby establishing that HU toxicity depends on catalase in vivo. We further demonstrated that catalase is a direct HU target by showing that HU acts as a competitive inhibitor of catalase-mediated hydrogen peroxide decomposition. Considering also that catalase can accelerate HU decomposition in vitro and that co-treatment with another catalase inhibitor alleviates HU effects in vivo, our findings suggests that HU could act as a catalase-activated pro-drug. Clinically, we found high catalase activity in circulating cells from untreated chronic myeloid leukemia, offering a possible explanation for the efficacy of HU against this malignancy.

Role of Blm and collaborating factors in recombination and survival following replication stress in Ustilago maydis

Inactivation of the structural gene for the RecQ family member, BLM in human, Sgs1 in budding yeast, or Rqh1 in fission yeast leads to inappropriate recombination, chromosome abnormalities, and disturbed replication fork progression. Studies with yeasts have demonstrated that auxiliary gene functions can contribute in overlapping ways with Sgs1 or Rqh1 to circumvent or overcome lesions in DNA caused by certain genotoxic agents. In the combined absence of these functions, recombination-mediated processes lead to severe loss of fitness. Here we performed a genetic study to determine the role of the Ustilago maydis Blm homolog in DNA repair and in alleviating replication stress. We characterized the single mutant as well as double mutants additionally deleted of genes encoding Srs2, Fbh1, Mus81, or Exo1. Unlike yeasts, neither the blm srs2, blm exo1, nor blm mus81 double mutant exhibited extreme loss of fitness. Inactivation of Brh2, the BRCA2 homolog, suppressed toxicity to hydroxyurea caused by loss of Blm function. However, differential suppression by Brh2 derivatives lacking the canonical DNA-binding region suggests that the particular domain structure comprising this DNA-binding region may be instrumental in promoting the observed hydroxyurea toxicity.

Hydroxyurea in the Management of Pediatric Sickle Cell Patients

Low levels of fetal hemoglobin (HbF) in sickle cell disease (SCD) patients (pts) are associated with a variety of vaso-occlusive complications and an increased risk of early death. Raising HbF levels can reduce the effect of the disease. Hydroxyurea (HU) reduces the production of HbS containing red cells and favors the production of HbF containing red cells. HU has bean used successfully in the management of adults with SCD but still there is a limited data on its efficacy and safety in pediatric age group. This study reviews our clinical experience with HU in the treatment of pediatric pts with SCD attending the Pediatric Hematology Clinic of Cairo University. Sixty SCD pts from 2001 to 2007 who received HU over the period of 6 years (yrs) and who continued therapy for at least 6 months were included. Four pts were excluded because of non compliance to treatment. Response to HU was assessed both clinically and by laboratory findings. Pts were considered responders if they showed ≥50% improvement in clinical and laboratory data. These data included number of blood transfusions/yr, vasoocclusive crisis (VOC)/yr requiring admission to the emergency unit, hospital admissions/yr. Laboratory data included Hb (g/dl), MCV (fl), HbF%, total leucocytic count (TLC), (×103/ml) absolute neutrophil count (ANC), platelets (×103/ml) and reticulocytic count (%) and serum ferritin level (ng/ml). Fifty six pts (44 children and 12 adults [≥ 18 yrs]) were included. Their mean age was 14.05 ± 5.3 yrs (range of 6–28 yrs). Thirty seven were females and 19 males. Twenty four (43%) were sickle-β thalassemia while 32 (57%) were homozygous sickle cell anemia (SS). Splenectomy was done for 26 pts (46.5%), 17 before and 9 after start of HU. Forty four (79%) received blood transfusion > 5 times per yr and 12 had sporadic transfusions. Forty pts (71%) were HCV positive. The main indications for starting HU therapy were frequent VOC, transfusion dependency and acute chest syndrome (91%, 86% and 16% respectively). Other indications included hepatic crisis (5%), bone infarction (7%), sequestration crisis (5%) and pulmonary hypertension in one case. HU was started in a dose of 15mg/kg/day with careful monthly monitoring for side effects. There was no attempt to achieve maximum tolerated dose. Dose increase or decrease was done depending on clinical and laboratory response with a maximum dose of 30mg/kg/day. The mean dose of HU was 15.8mg/kg/day (range 10–30 mg/kg/day) and the mean duration of therapy was 3.25 yrs (range of 0.5–6 yrs). Forty four pts (79%) were found to be responders. There was a significant (p<0.05) improvement in all clinical parameters with VOC/yr reduced from a mean of 6 to 1.98, hospitalization/yr from 3 to 0.32, blood transfusion/yr from 9.25 to 1.45. The mean Hb level of responders increased from 7.58g/dl before HU to 8.01g/dl after HU, although not statistically significant, 28/44 pts (63.64%) showed increase in Hb varies between 2-<1g/dl. Patients who did not show increase in Hb level (16/44) were clinically stable and had decreased number of blood transfusion by >50% or stop blood transfusion. Responders showed a significant decrease in TLC, ANC, Retics and serum ferritin (P values 0.002, 0.019, 0.000, 0.001 respectively). Significant increase in HbF and MCV (p= 0.000, 0.001 respectively) was also observed. HU toxicity was defined by > 3 fold increase in ALT, platelet count < 80,000 μl, ANC <1500 or increase in serum creatinine >50% above baseline. Twenty pts (36%) showed signs of HU toxicity: elevated ALT (n=9), neutropenia (n=7), thrombocytopenia (n=1), unexplained jaundice (n=1) and both neutropenia and jaundice (n=2). Thirteen pts continued therapy with reduction of the dose or temporary stopping of HU while 7 stopped HU. It was noticed that all pts who developed hepatotoxicity were HCV positive (p= 0.036). It was also shown that hepatotoxicity was significantly higher among those receiving Deferiprone with HU (n=20) (p=0.001). There was no relation between response to HU and patients' age, sex, spleen status or phenotype. HU provides the best available strategy to achieve clinical and hematological improvement in SCD in pediatrics, but requires periodic monitoring of blood count and ALT levels especially for HCV positive pts and those on Deferiprone therapy

Hydroxyurea induces vasopressin release and cytokine gene expression in the rat hypothalamus

We previously showed that the cytostatic drug hydroxyurea (HU) activates the hypothalamo–pituitary–adrenal (HPA) axis in intact rats, whereas it is lethal in rats with impaired HPA function. In these animals, HU toxicity is mediated by increased circulating levels of proinflammatory cytokines, whose secretion cannot be counteracted by glucocorticoids, suggesting that HPA activation blunts HU toxicity. Here we investigated the mechanisms through which HU activates the HPA axis, looking at the direct effects of the drug on the isolated hypothalamus. We found that HU significantly increases the release of arginine vasopressin but not that of corticotrophin-releasing hormone in short-term incubation experiments. The levels of arginine vasopressin are also increased in the hypothalamus and systemic circulation 2 h after the in vivo administration of the drug. Furthermore, HU increased significantly the expression of interleukin-6 and, to a lesser extent, interleukin-1β in the hypothalamus. Interestingly, experiments with HU on primary cultures of rat microglia and astrocytes suggested that the increase in cytokine gene expression observed in hypothalamic explants is not accounted for by glial cells. Since both vasopressin and cytokines can activate the HPA axis, our present findings provide a reasonable explanation of the HPA activation elicited by HU in vivo in the rat.

Pharmacokinetics of Hydroxyurea in Young Children with Sickle Cell Anemia: A Report from the BABY HUG Trial.

There is a paucity of pharmacokinetic (PK) data about hydroxyurea (HU) in persons with sickle cell anemia (SCA) and none in very young children. HU clearance is predominantly renal. The kidney may be damaged during infancy in SCA, but the first abnormality is usually hyperfiltration which may lead to enhanced HU clearance. Following a 15 mg/kg oral dose of commercially available HU capsules, PKs in 7 adults with SCA and normal renal function have been reported to be a mean±SD half-life (T1/2) of 3.14±0.9 hrs, a maximal concentration (Cmax) of 28.32±11.0 ug/mL, and an area under the curve (AUC) of 81.66±15.5 ug•hr/mL. The Phase II Study of HU in adults concluded that initial 2 hour clearance of HU was not associated with either the maximum tolerated dose or fetal hemoglobin response to treatment. However, the AUC did predict HU toxicity and, in another study, the need for dose adjustment in adults with SCA and renal insufficiency. BABY HUG is an NHLBI-NICHD sponsored Pediatric Phase III Clinical Trial designed to critically assess the efficacy of a novel liquid HU preparation in preventing organ damage in young children with SCA. Forty-five African-American infants, 12 to 18 months (mo) of age (mean 14.7 mo) were recruited without regard to disease severity to the just completed randomized, double-blind, placebo-controlled BABY HUG Feasibility and Safety Pilot Trial. First dose PKs were obtained 0, 1, 2, and 4 hours after oral administration of 20 mg/kg of HU in 22 consecutive patients (mean age 14.5 mo), coincident with measurement of glomerular filtration rate (GFR) by nuclear medicine DTPA clearance. An additional PK sample was obtained between 4 and 8 hours in 15 of the 22 (68%). Samples were frozen at −70°C, shipped, and assayed by high resolution gas chromatography with mass spectrometric detection (limit of detection 0.5 ug/mL). The T1/2 (2.36±0.99 hrs), Cmax (19.81±5.8 ug/mL; 0.26±0.8 uM/L), and AUC (68.82±11.5 ug•hr/mL) were somewhat lower in BABY HUG patients than in the small group of adults with SCA from the literature. There were no apparent relationships between PKs and baseline GFR measured by DTPA or calculated from the Schwartz equation. Younger patients ≤15 mo (n=15) had a trend toward a shorter T1/2 (2.1 vs 2.8 hours; p=0.118) and a lower predicted measurable HU concentration at 8 hours (1.2 vs 2.1 ug/mL; p=0.056) than those 16–18 mo (n=7). There were no age related differences in AUC (67.4 vs 71.8 ug•hr/mL; p=0.421) or Cmax (20.5 vs 18.2 ug/mL; p=0.409). PKs of this novel liquid preparation of HU in young children with SCA may be different than that of adults with the standard capsule formulation. The BABY HUG Trial will continue to refine the PK model by additional evaluations including samples from patients as young as 9 mo of age, the new lower age of eligibility for the open study currently accruing patients.

The effects of inhibitors of cyclo-oxygenase, lipoxygenase and nitric oxide synthase pathways on the toxicity of hydroxyurea in adrenalectomized rats

Our previous observations on the toxic effects of hydroxyurea (HU) in adrenalectomized (ADX) rats prompted us to suggest that these effects might be mediated by an increased synthesis of proinflammatory cytokines, which display similar toxicological profiles (e.g. increased lethality in adrenal-ablated animals). The present study was aimed at investigating whether some of the mediators of the biological effects of pro-inflammatory cytokines (i.e. prostaglandins, leukotrienes and nitric oxide) are involved in the severe HU toxicity seen in ADX rats. The latter were treated over a 5-day period with HU alone or HU plus specific inhibitors of prostaglandin, leukotriene and nitric oxide synthetic pathways. Mortality was recorded throughout the experiments. Results showed that, while corticosterone and dexamethasone afford significant protection against HU toxicity, the latter is not reduced and may even be aggravated by treatments with specific inhibitors of cyclo-oxygenase, lipoxygenase or nitric oxide synthase.

Hydroxyurea: Relationship between Toxicity and Centrally‐Induced Adrenal Activation

The anticancer drug hydroxyurea (HU) at doses of 300-800 mg/kg/day causes a dramatic lethality (up to 100% after a 5-day treatment) in hypophysectomized as well as in adrenalectomized rats drinking physiological saline + 5% glucose. Mortality in controls was less than 10% over a 5-day period. Adrenal stimulatory or replacement therapies protect pituitary- or adrenal-ablated rats against HU toxicity. They also counteract white blood cell changes induced by the drug. HU (30-800 mg/kg) induces a dose-dependent increase of plasma corticosterone in normal rats after single or repeated treatments that is not observed in hypophysectomized animals. HU also increases plasma levels of epinephrine, although this finding cannot account for the rise in plasma corticosterone; indeed, it is secondary to a strong rise in plasma corticosterone. The stimulation of the hypothalamic-hypophyseal-adrenal axis induced by HU is responsible for the drug-induced adrenocortical activation. This activation appears to be a valuable defence mechanism protecting intact rats against HU lethality, and its failure causes the dramatic HU lethality in pituitary- or adrenal-ablated animals.

Protection of L5178Y cells by vitamin E against acute hydroxyurea toxicity does not change the efficiency of ribonucleotide reductase-mediated hydroxyurea-induced cytotoxic events

Exposure of L5178Y cells in culture to 0.2 mM hydroxyurea (HU) for up to 48 h induces inhibition of DNA synthesis, killing of about 20% of cells during the first 24 h exposure, gradual progress of cells into the state of unbalanced growth (measured as progressive increase in cell size, DNA protein ratio and acid phosphatase activity) with final death of the majority of cells. These effects were used to compare effectiveness of rapid and late HU cytotoxicity in cells treated with HU in the presence or absence of vitamin E, employed as a protective agent against non-specific toxicity of HU. It has been found that while the presence of vitamin E prevents cell killing induced by HU treatment during first 24 h, its presence does not change the progress of cells into the state of unbalanced growth and final cell killing. It is suggested that the protection exerted by vitamin E against HU-induced side toxicity does not change the effectiveness of ribonucleotide reductase-mediated DNA synthesis inhibition, leading to cell death through the state of unbalanced growth.

Replacement therapy against increased hydroxyurea toxicity in pituitary or adrenal ablated rats.

The main rat adrenocortical hormone, corticosterone, the mineralocorticoid, 11-deoxycorticosterone (DCA) acetate given alone or together (2:1 ratio) twice daily at doses of 2-4 and 1-8 mg/kg, DCA enanthate given in a single injection of 20 mg/kg 0-3 days before the beginning of the experiments and a highly-concentrated injectable extract of the adrenal cortex (4 mg/kg as hydrocortisone twice a day) given by the intramuscular route, delay and partially protect against the increased toxicity following administration of the anticancer drug, hydroxyurea (800 mg/kg/day for 5 days) in adrenalectomized or hypophysectomized animals (80-100% lethality; in control non ablated rats 0-10% lethality). ACTH1-24 (tetracosactide) also proved effective in pituitary ablated rats. The best protection was afforded with the joint administration of corticosterone and DCA (2-4 and 1-2 mg/kg twice a day) or with corticosterone alone at doses (4 mg/kg twice a day) capable of giving plasma levels, six hours after administration on the third day, similar to those observed in non ablated rats receiving HYD in the morning. The adrenocortical hormones may replace a possible unique defense mechanism against drug toxicity, which is lacking in pituitary or adrenal ablated rats.

Resistance to bromodeoxyuridine mutagenesis and toxicity in mammalian cells selected for resistance to hydroxyurea.

Mutant cell lines resistant to hydroxyurea (HU), an inhibitor of the enzyme ribonucleotide reductase, were selected from a line of Syrian hamster melanoma cells. Mutant lines were selected for resistance to 0.3 mM HU, and from these lines, second-step mutants were selected for resistance to 1.9 mM HU. The HUr lines were tested ffor their responses to 5-bromodeoxyuridine (brdU), in terms of toxicity, mutagenesis, and incorporation of BrdU into DNA. All of the HUr lines showed increased resistance to the toxic effects of BrdU. in addition, the HUr lines all were resistant to BrdU mutagenesis. Overall, there was good correlation among the levels of resistance to HU toxicity, BrdU toxicity, and BrdU mutagenesis in the HUr lines. These tests were carried out under conditions such that the parental and HUr cells incorporated equal amounts of BrdU into nuclear DNA. Therefore, the resistance of the HUr cells to the effects of BrdU cannot be attributed to decreased incorporation of BrdU into DNA. These results suggest that the HUr cells have an lateration in ribonucleotide reductase activity that simultaneously confers resistance to HU and BrdU. The properties of the HUr cells suggest that the perturbation of deoxcytidine metabolism by BrdU. The properties of the HUr cells suggest that the perturbation of deoxcytidine metabolism by BrdU triphosphate inhibition of ribonucleotide reductase activity plays a key role in the toxic and mutagenic effects of BrdU in mammalian cells.