In vivo and in vitro comparison of the short-term hematopoietic toxicity between hydroxyurea and trimidox or didox, novel ribonucleotide reductase inhibitors with potential anti-HIV-1 activity

Targeting the Cell Cycle, RRM2 and NF-κB for the Treatment of Breast Cancers

A hallmark of cancer is the dysregulation of the cell cycle. The CDK4/6 inhibitor palbociclib is approved for treating advanced estrogen-receptor-positive breast cancer, but its success is limited by the development of acquired resistance owing to long-term therapy despite promising clinical outcomes. This situation necessitates the development of potential combination strategies. Here, we report that didox, an inhibitor of ribonucleotide reductase in combination with palbociclib, can overcome palbociclib resistance in ER-positive and ER-negative breast cancers. This study shows didox downregulates an element of the cell cycle checkpoint, cyclin D1, accompanied by a reduction in NF-κB activity in vitro and tumor growth inhibition of palbociclib-resistant ER positive breast cancer tumor growth in vivo. Furthermore, didox induces cell cycle arrest at G1 as well as reduces ROS generated by on-target effects of palbociclib on the cell cycle. Our current study also reports that the CCND1 and RRM2 upregulation associated with palbociclib-resistant breast cancers decreases upon ribonucleotide reductase inhibition. Our data present a novel and promising biomarker-driven combination therapeutic approach for the treatment of ER-positive and ER-negative breast cancers that involves the inhibition of the CDK4/6-cyclinD1/pRb cell cycle axis that merits further clinical investigation in human models.

Nitric Oxide Mediation in Hydroxyurea and Nitric Oxide Metabolites' Inhibition of Erythroid Progenitor Growth

In several systems, hydroxyurea has been shown to trigger nitric oxide (NO) release or activation of NO synthase (NOS). To elucidate this duality in its pharmacological effects, during myelosuppression, we individually examined hydroxyurea's (NO releasing agent) and NO metabolites' (stable NO degradation products) effects on erythroid colony growth and NOS/NO levels in mice using NO scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO). Hydroxyurea and nitrite/nitrate decreased the bone marrow cellularity that was blocked by PTIO only for the NO metabolites. Hydroxyurea inhibition of colony-forming unit-erythroid (CFU-E) formation and reticulocytes was reversed by PTIO. Moreover, hydroxyurea, through a negative feedback mechanism, reduced inducible NOS (iNOS) expressing cells in CFU-E, also prevented by PTIO. Nitrate inhibition of burst-forming units-erythroid (BFU-E) colony growth was blocked by PTIO, but not in mature CFU-E. The presented results reveal that NO release and/or production mediates the hydroxyurea inhibition of mature erythroid colony growth and the frequency of iNOS immunoreactive CFU-E.

Nitric Oxide Synthase Dependency in Hydroxyurea Inhibition of Erythroid Progenitor Growth

Hydroxyurea (HU) causes nitric oxide (NO) bioactivation, acting as both a NO donor and a stimulator of NO synthase (NOS). To examine whether HU effects are NO mediated by chemical degradation or enzymatic induction, we studied human and mouse erythroid cells during proliferation, apoptosis, and differentiation. The HU and NO donor demonstrated persisted versus temporary inhibition of erythroid cell growth during differentiation, as observed by γ- and β-globin gene expression. HU decreased the percentage of erythroleukemic K562 cells in the G2/M phase that was reversed by N-nitro l-arginine methyl ester hydrochloride (L-NAME). Besides activation of endothelial NOS, HU significantly increased apoptosis of K562 cells, again demonstrating NOS dependence. Administration of HU to mice significantly inhibited colony-forming unit-erythroid (CFU-E), mediated by NOS. Moreover, burst-forming-units-erythroid (BFU-E) and CFU-E ex vivo growth was inhibited by the administration of nitrate or nitrite to mice. Chronic in vivo NOS inhibition with L-NAME protected the bone marrow cellularity despite HU treatment of mice. NO metabolites and HU reduced the frequency of NOS-positive cells from CFU-E and BFU-E colonies that was reverted by NOS inhibition. HU regulation of the G2/M phase, apoptosis, differentiation, cellularity, and NOS immunoreactive cells was NOS dependent. Inhalation of NO therapy as well as strategies to increase endogenous NO production could replace or enhance HU activity.

Didox (3,4-dihydroxybenzohydroxamic acid) suppresses IL-33-induced cytokine production in primary mouse mast cells

While IgE is considered the primary mediator of mast cell activation, IL-33 contributes substantially in asthma, allergic rhinitis, and atopic dermatitis. To develop effective treatments for allergic disease, it is important to understand the role of therapeutic agents on IL-33 activation. We examined the effect of Didox (3,4-dihydroxybenzohydroxamic acid), an antioxidant and ribonucleotide reductase (RNR) inhibitor, on IL-33-mediated mast cell activation. Didox suppressed IL-6, IL-13, TNF, and MIP-1α (CCL3) production in bone marrow derived mast cells following IL-33 activation. This suppression was observed in different genetic backgrounds and extended to peritoneal mast cells. The antioxidant N-acetylcysteine mimicked the suppression of Didox, albeit at a much higher dose, while the RNR inhibitor hydroxyurea had no effect. Didox substantially suppressed IL-33-mediated NFκB and AP-1 transcriptional activities. These results suggest that Didox attenuates IL-33-induced mast cell activation and should be further studied as a potential therapeutic agent for inflammatory diseases involving IL-33.

Didox and resveratrol sensitize colorectal cancer cells to doxorubicin via activating apoptosis and ameliorating P-glycoprotein activity

Doxorubicin (DOX) has limited efficacy in colorectal cancer due to multi-drug resistance. Resveratrol (RES) and didox (DID) are polyhydroxyphenols with potential chemosensitizing effects. Herein, we assessed the chemomodulatory effects of RES and DID to DOX in colorectal cancer cells. Equitoxic combination of DOX with RES and DID in HCT 116 reduced the IC₅₀ of DOX from 0.96 ± 0.02 μM to 0.52 ± 0.05 μM and 0.4 ± 0.06 μM, respectively. Similarly, combination of DOX with RES and DID in HT-29 decreased the IC₅₀’s of DOX from 0.88 ± 0.03 μM to 0.47 ± 0.02 μM and 0.29 ± 0.04 μM, respectively. The expressions of p53 and Bax genes were markedly elevated in HCT 116 cells after exposure to DOX/DID. In HT-29 cells, the expression of Bcl-XL gene was significantly decreased after exposure to DOX/DID. In addition, combination of DOX with RES significantly increased the expression of Bax gene in HCT 116 cells. RES treatment induced significant S-phase arrest in DOX-treated HCT 116 cells, while DID induced G₂/M- and S-phase arrest in HCT 116 and HT-29, respectively. Both RES and DID significantly enhanced the intracellular entrapment of DOX due to blocking the efflux activity of p-glycoprotein pump. In conclusion, RES and DID sensitize colorectal cancer cells to DOX via facilitating apoptosis and enhancing intracellular entrapment of DOX.

The efficacy of the ribonucleotide reductase inhibitor Didox in preclinical models of AML

Acute Myeloid Leukemia (AML) is an aggressive malignancy which leads to marrow failure, and ultimately death. There is a desperate need for new therapeutics for these patients. Ribonucleotide reductase (RR) is the rate limiting enzyme in DNA synthesis. Didox (3,4-Dihydroxybenzohydroxamic acid) is a novel RR inhibitor noted to be more potent than hydroxyurea. In this report we detail the activity and toxicity of Didox in preclinical models of AML. RR was present in all AML cell lines and primary patient samples tested. Didox was active against all human and murine AML lines tested with IC₅₀ values in the low micromolar range (mean IC₅₀ 37 µM [range 25.89–52.70 µM]). It was active against primary patient samples at concentrations that did not affect normal hematopoietic stem cells (HSCs). Didox exposure resulted in DNA damage and p53 induction culminating in apoptosis. In syngeneic, therapy-resistant AML models, single agent Didox treatment resulted in a significant reduction in leukemia burden and a survival benefit. Didox was well tolerated, as marrow from treated animals was morphologically indistinguishable from controls. Didox exposure at levels that impaired leukemia growth did not inhibit normal HSC engraftment. In summary, Didox was well tolerated and effective against preclinical models of AML.

Synthesis and anticancer evaluation of benzyloxyurea derivatives

A series of novel benzyloxyurea derivatives was designed, synthesized by substituting different benzyls or phenyls on N,N'-positions of the hydroxyurea (HU). These target compounds were evaluated for their anticancer activity in vitro against human leukemia cell line K562 and murine leukemia cell line L1210 in comparison with HU by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Some of the compounds showed promising anticancer activity against the cells. Molecular docking experiments with Saccharomyces cerevisiae R1 domain indicated that 4a and 4f' have stronger affinity than 4m and 4n. Flow cytometry study showed that compound 4g exerted greater apoptotic activity against K562 cells line than HU.

Evaluating the therapeutic potential of a non-natural nucleotide that inhibits human ribonucleotide reductase

Human ribonucleotide reductase (hRR) is the key enzyme involved in de novo dNTP synthesis and thus represents an important therapeutic target against hyperproliferative diseases, most notably cancer. The purpose of this study was to evaluate the ability of non-natural indolyl-2'-deoxynucleoside triphosphates to inhibit the activity of hRR. The structural similarities of these analogues with dATP predicted that they would inhibit hRR activity by binding to its allosteric sites. In silico analysis and in vitro characterization identified one particular analogue designated as 5-nitro-indolyl-2'-deoxyribose triphosphate (5-NITP) that inhibits hRR. 5-NITP binding to hRR was determined by isothermal titration calorimetry. X-ray crystal structure of 5-NITP bound to RR1 was determined. Cell-based studies showed the anti-cancer effects of the corresponding non-natural nucleoside against leukemia cells. 5-NITP binds to hRR with micromolar affinity. Binding does not induce hexamerization of hRR1 like dATP, the native allosteric inhibitor of hRR that binds with high affinity to the A-site. The X-ray crystal structure of Saccharomyces cerevisiae RR1-5-NITP (ScRR1-5-NITP) complex determined to 2.3 Å resolution shows that 5-NITP does not bind to the A-site but rather at the S-site. Regardless, 5-nitro-indolyl-2'-deoxynucleoside (5-NIdR) produces cytostatic and cytotoxic effects against human leukemia cells by altering cell-cycle progression. Our studies provide useful insights toward developing new inhibitors with improved potency and efficacy against hRR.

The structural basis for peptidomimetic inhibition of eukaryotic ribonucleotide reductase: a conformationally flexible pharmacophore

Eukaryotic ribonucleotide reductase (RR) catalyzes nucleoside diphosphate conversion to deoxynucleoside diphosphate. Crucial for rapidly dividing cells, RR is a target for cancer therapy. RR activity requires formation of a complex between subunits R1 and R2 in which the R2 C-terminal peptide binds to R1. Here we report crystal structures of heterocomplexes containing mammalian R2 C-terminal heptapeptide, P7 (Ac-1FTLDADF7) and its peptidomimetic P6 (1Fmoc(Me)PhgLDChaDF7) bound to Saccharomyces cerevisiae R1 (ScR1). P7 and P6, both of which inhibit ScRR, each bind at two contiguous sites containing residues that are highly conserved among eukaryotes. Such binding is quite distinct from that reported for prokaryotes. The Fmoc group in P6 peptide makes several hydrophobic interactions that contribute to its enhanced potency in binding to ScR1. Combining all of our results, we observe three distinct conformations for peptide binding to ScR1. These structures provide pharmacophores for designing highly potent nonpeptide class I RR inhibitors.

Rapid and quantitative activation of Chlamydia trachomatis ribonucleotide reductase by hydrogen peroxide

We recently showed that the class Ic ribonucleotide reductase (RNR) from the human pathogen Chlamydia trachomatis ( Ct) uses a Mn (IV)/Fe (III) cofactor in its R2 subunit to initiate catalysis [Jiang, W., Yun, D., Saleh, L., Barr, E. W., Xing, G., Hoffart, L. M., Maslak, M.-A., Krebs, C., and Bollinger, J. M., Jr. (2007) Science 316, 1188-1191]. The Mn (IV) site of the novel cofactor functionally replaces the tyrosyl radical used by conventional class I RNRs to initiate substrate radical production. As a first step in evaluating the hypothesis that the use of the alternative cofactor could make the RNR more robust to reactive oxygen and nitrogen species [RO(N)S] produced by the host's immune system [Högbom, M., Stenmark, P., Voevodskaya, N., McClarty, G., Gräslund, A., and Nordlund, P. (2004) Science 305, 245-248], we have examined the reactivities of three stable redox states of the Mn/Fe cluster (Mn (II)/Fe (II), Mn (III)/Fe (III), and Mn (IV)/Fe (III)) toward hydrogen peroxide. Not only is the activity of the Mn (IV)/Fe (III)-R2 intermediate stable to prolonged (>1 h) incubations with as much as 5 mM H 2O 2, but both the fully reduced (Mn (II)/Fe (II)) and one-electron-reduced (Mn (III)/Fe (III)) forms of the protein are also efficiently activated by H 2O 2. The Mn (III)/Fe (III)-R2 species reacts with a second-order rate constant of 8 +/- 1 M (-1) s (-1) to yield the Mn (IV)/Fe (IV)-R2 intermediate previously observed in the reaction of Mn (II)/Fe (II)-R2 with O 2 [Jiang, W., Hoffart, L. M., Krebs, C., and Bollinger, J. M., Jr. (2007) Biochemistry 46, 8709-8716]. As previously observed, the intermediate decays by reduction of the Fe site to the active Mn (IV)/Fe (III)-R2 complex. The reaction of the Mn (II)/Fe (II)-R2 species with H 2O 2 proceeds in three resolved steps: sequential oxidation to Mn (III)/Fe (III)-R2 ( k = 1.7 +/- 0.3 mM (-1) s (-1)) and Mn (IV)/Fe (IV)-R2, followed by decay of the intermediate to the active Mn (IV)/Fe (III)-R2 product. The efficient reaction of both reduced forms with H 2O 2 contrasts with previous observations on the conventional class I RNR from Escherichia coli, which is efficiently converted from the fully reduced (Fe 2 (II/II)) to the "met" (Fe 2 (III/III)) form [Gerez, C., and Fontecave, M. (1992) Biochemistry 31, 780-786] but is then only very inefficiently converted from the met to the active (Fe 2 (III/III)-Y (*)) form [Sahlin, M., Sjöberg, B.-M., Backes, G., Loehr, T., and Sanders-Loehr, J. (1990) Biochem. Biophys. Res. Commun. 167, 813-818].

Toxicity testing using hematopoietic stem cell assays

For over 40 years, in vitro assays have been used to understand the complex system of hematopoiesis. Now, several of these assays are undergoing resurgence as scientists in academia and industry are discovering how these assays can be utilized in drug discovery and development. These assays use primary cells from various hematopoietic tissues in multiple species to provide high content information. While conditions in the human body cannot be completely reproduced in vitro, hematopoietic colony-forming cell assays are proving to be a clinically relevant tool to evaluate potential toxic effects of new compounds. The ability to use these assays as a replacement of, or in conjunction with, high-throughput screening assays and high priced in vivo assays can improve the success of the decision-making process, saving time and costs during drug development.

Quantitative analysis of Howell-Jolly bodies in children with sickle cell disease

OBJECTIVES

Although functional asplenia in sickle cell disease (SCD) begins early in life and has important clinical consequences, quantitative measurement of splenic function is not readily available. A novel high-throughput flow cytometric method for quantitating Howell-Jolly bodies (HJB) has been developed which isolates HJB-containing CD71(+) and CD71(-) erythrocytes. Analysis of these cell populations allows quantitative measurement of splenic filtrative function and possible chromosomal damage.

METHODS

Blood specimens from 147 children with SCD were analyzed using a high-throughput flow cytometric method. Enumeration of the following populations was accomplished: 1) CD71(+) reticulocytes among total erythrocytes, identifying the youngest erythroid cell population; 2) HJB-containing CD71(+) reticulocytes, which isolate young erythrocytes containing micronuclei as an index of cytogenetic damage; and 3) HJB-containing CD71(-) erythrocytes, identifying older erythrocytes containing micronuclei, indirectly measuring splenic function.

RESULTS

Children with HbSC (n = 24) had slightly elevated HJB frequencies, while children with HbSS (n = 125) had highly elevated frequencies within CD71(+) cells (0.44% +/- 0.40%, normal 0.12% +/- 0.06%, p < 0.001) and CD71(-) cells (2493 +/- 2303 per million RBC, normal 20 +/- 11, p < 0.001). Using a multiple regression model, the frequency of HbSS CD71(+) reticulocytes containing HJB was significantly influenced by hydroxyurea use (p < 0.0001), age (p = 0.0288), and splenectomy (p = 0.0498). Similarly, mature CD71(-) erythrocytes containing HJB were positively correlated with hydroxyurea (p = 0.0001), age (p < 0.0001), and splenectomy (p = 0.0104).

CONCLUSIONS

HJB quantitation by flow cytometry is a novel assay for measuring splenic function and may be valuable for investigating the efficacy and safety of therapeutic options for children with SCD.

DNA damage in blood leukocytes of individuals with sickle cell disease treated with hydroxyurea

Hydroxyurea (HU) plays an important role in the treatment of patients with sickle cell disease (SCD). Although HU has been associated with an increased risk of leukemia in some patients with myeloproliferative disorders, the mutagenic and carcinogenic potential of HU has not been established. This study investigated levels of DNA damage using the alkaline (pH>13) comet assay to analyze peripheral blood leukocytes sampled from 28 patients with SCD treated with HU (SCHU) and from 28 normal individuals. The damage index (DI) in the SCHU group was significantly higher than in controls (p<0.05). Gender, smoking or age were not associated with DNA damage in controls or SCHU individuals. In the group of SCHU individuals, mean HU dose and DI were positively correlated, and individuals who received a mean dose of >20 mg/kg HU (DI=24.9+/-5.5) showed significantly more DNA damage than those who received < or =20 mg/kg HU (DI=14.6+/-1.8) (p<0.05). Individuals treated for > or =42 months (DI=23.1+/-4.2) showed significantly greater DNA damage than those treated for <42 months (13.6+/-1.9) (p<0.05). DI was inversely correlated with body mass index in the SCHU group.

Structures of eukaryotic ribonucleotide reductase I define gemcitabine diphosphate binding and subunit assembly

Ribonucleotide reductase (RNR) catalyzes the conversion of nucleoside diphosphates to deoxynucleoside diphosphates. Crucial for rapidly dividing cells, RNR is a target for cancer therapy. In eukaryotes, RNR comprises a heterooligomer of alpha(2) and beta(2) subunits. Rnr1, the alpha subunit, contains regulatory and catalytic sites; Rnr2, the beta subunit (in yeast, a heterodimer of Rnr2 and Rnr4), houses the diferric-tyrosyl radical crucial for catalysis. Here, we present three x-ray structures of eukaryotic Rnr1 from Saccharomyces cerevisiae: one bound to gemcitabine diphosphate (GemdP), the active metabolite of the mechanism-based chemotherapeutic agent gemcitabine; one with an Rnr2-derived peptide, and one with an Rnr4-derived peptide. Our structures reveal that GemdP binds differently from its analogue, cytidine diphosphate; because of unusual interactions of the geminal fluorines, the ribose and base of GemdP shift substantially, and loop 2, which mediates substrate specificity, adopts different conformations when binding to GemdP and cytidine diphosphate. The Rnr2 and Rnr4 peptides, which block RNR assembly, bind differently from each other but have unique modes of binding not seen in prokaryotic RNR. The Rnr2 peptide adopts a conformation similar to that previously reported from an NMR study for a mouse Rnr2-based peptide. In yeast, the Rnr2 peptide binds at subsites consisting of residues that are highly conserved among yeast, mouse, and human Rnr1s, suggesting that the mode of Rnr1-Rnr2 binding is conserved among eukaryotes. These structures provide new insights into subunit assembly and a framework for structure-based drug design targeting RNR.

Inhibition of Ribonucleotide Reductase Reduces Neointimal Formation following Balloon Injury

Percutaneous transluminal coronary angioplasty (PTCA) has greatly benefited patients with occluded coronary arteries, but its benefits have been undermined by a high incidence of restenosis. The introduction of coronary stents has significantly improved the short and long term outcome but restenosis still occurs in approximately 15 to 30% of patients within 6 months. Research efforts are now being directed toward combination stenting and drug delivery. Among the therapeutic targets being pursued are agents that can impede smooth muscle cell migration and proliferation, as these processes are critical components of restenosis injury. We propose that inhibiting the conversion of ribonucleotides to deoxyribonucleotides will impede cell proliferation and, as such, limit the degree of restenosis. Therefore, we tested whether the potent ribonucleotide reductase inhibitors Didox (3,4-dihydroxybenzohydraxamic acid) and Imidate (ethyl-3,4,5-hydroxybenzimidate) can limit the neointimal proliferation associated with restenosis using a rat carotid model of balloon dilatation injury. Results demonstrated that both Didox and Imidate significantly reduced intimal thickening, resulting in a 71 and 62% decrease in the intima/media ratio, respectively. Similar efficacy was seen with the commercially available ribonucleotide reductase inhibitor hydroxyurea, demonstrating the importance of this enzyme in vascular remodeling. Results from cell proliferation studies suggest that the mechanism of protection is inhibition of smooth muscle cell (SMC) proliferation. In addition, Didox and Imidate (100 microM) are potent inhibitors of SMC migration, which may also contribute to their vascular protective effects. These results suggest that inhibition of ribonucleotide reductase may provide a potent strategy to prevent post-PTCA restenosis.

Combination of inhibitors of lymphocyte activation (hydroxyurea, trimidox, and didox) and reverse transcriptase (didanosine) suppresses development of murine retrovirus-induced lymphoproliferative disease

The ribonucleotide reductase inhibitor hydroxyurea (HU) has demonstrated some benefit as a component of drug cocktails for the treatment of HIV-1 infection. However, HU is notoriously myelosuppressive and often administered only as salvage therapy to patients with late-stage disease, potentially exacerbating the bone marrow toxicity of HU. In this report we have compared the antiviral effects of HU and two novel RR inhibitors trimidox (3,4,5-trihydroxybenzamidoxime) and didox (3,4-dihydroxybenzohydroxamic acid) in combination with didanosine (2,3-didoxyinosine; ddI) in the LPBM5 MuLV retrovirus model (murine AIDS). We also evaluated the effects of these drug combinations on the hematopoietic tissues of LPBM5 MuLV-infected animals. The combination of RR inhibitors and ddI was extremely effective (DX>TX>HU) in inhibiting development of retrovirus-induced disease (splenomegaly, hypergammaglobulinemia, activated B-splenocytes and loss of splenic architecture). In addition, relative levels of proviral DNA were significantly lower in combination drug-treated animals compared to infected controls. Evaluation of femur cellularity, numbers of marrow-derived myeloid progenitor cells (CFU-GM and BFU-E) and peripheral blood indices revealed that TX and DX in combination with ddI were well-tolerated. However, treatment with HU and ddI induced moderate myelosuppression. These data demonstrate that RR inhibitors in combination with ddI provide significant protection against retroviral disease in murine AIDS. Moreover, the novel RR inhibitors TX and DX appear to be more effective and less myelosuppressive than HU when administered with ddI in this model.

Mutations in the R2 subunit of ribonucleotide reductase that confer resistance to hydroxyurea

Ribonucleotide reductase is an essential enzyme that catalyzes the reduction of ribonucleotides to deoxyribonucleotides for use in DNA synthesis. Ribonucleotide reductase from Escherichia coli consists of two subunits, R1 and R2. The R2 subunit contains an unusually stable radical at tyrosine 122 that participates in catalysis. Buried deep within a hydrophobic pocket, the radical is inaccessible to solvent although subject to inactivation by radical scavengers. One such scavenger, hydroxyurea, is a highly specific inhibitor of ribonucleotide reductase and therefore of DNA synthesis; thus it is an important anticancer and antiviral agent. The mechanism of radical access remains to be established; however, small molecules may be able to access Tyr-122 directly via channels from the surface of the protein. We used random oligonucleotide mutagenesis to create a library of 200,000 R2 mutants containing random substitutions at five contiguous residues (Ile-74, Ser-75, Asn-76, Leu-77, Lys-78) that partially comprise one side of a channel where Tyr-122 is visible from the protein surface. We subjected this library to increasing concentrations of hydroxyurea and identified mutants that enhance survival more than 1000-fold over wild-type R2 at high drug concentrations. Repetitive selections yielded S75T as the predominant R2 mutant in our library. Purified S75TR2 exhibits a radical half-life that is 50% greater than wild-type R2 in the presence of hydroxyurea. These data represent the first demonstration of R2 protein mutants in E. coli that are highly resistant to hydroxyurea; elucidation of their mechanism of resistance may provide valuable insight into the development of more effective inhibitors.

Suppression of retrovirus-induced immunodeficiency disease (murine AIDS) by trimidox and didox: novel ribonucleotide reductase inhibitors with less bone marrow toxicity than hydroxyurea

Recently, the use of the ribonucleotide reductase (RR) inhibitor hydroxyurea (HU) in combination with nucleoside analogs has gained attention as a potential strategy for anti-HIV-1 therapy. However, appeal for the long-term use of HU in HIV-1 infection may be limited by its propensity to induce hematopoietic toxicity. We report a comparison of the efficacy and bone marrow toxicity of HU (400 and 200 mg/kg/day) with the novel RR inhibitors and free radical-scavenging compounds didox (DX; 3,4-dihydroxybenzohydroxamic acid; 350 mg/kg/day) and trimidox (TX; 3,4,5-trihydroxybenzamidoxime; 175 mg/kg/day) in the murine AIDS (LPBM5 MuLV) model of retrovirus infection. Infected mice received daily drug treatment for 8 weeks. Efficacy was determined by measuring drug effects on retroviral-induced disease progression (i.e. development of splenomegaly and hypergammaglobulinemia) and by evaluating splenic levels of proviral DNA. Bone marrow toxicity was evaluated by measuring peripheral blood indices (WBC, hematocrit and reticulocyte counts), femoral cellularity and by determining the numbers of hematopoietic progenitor cells (CFU-GM, BFU-E) per femur and spleen. Compared to infected controls receiving no drug treatment, disease progression was significantly suppressed by TX, DX and HU. However, HU was associated with mortality and induced significant hematopoietic toxicity in a time- and dose-dependent manner. Conversely, TX and DX effectively inhibited retrovirus-induced disease but did not induce hematopoietic toxicity. These results suggest that due to their reduced hematopoietic toxicity and ability to inhibit disease progression in murine AIDS, TX and DX may offer effective alternatives to HU therapy in HIV-1 infection.

Short-term treatment with novel ribonucleotide reductase inhibitors Trimidox and Didox reverses late-stage murine retrovirus-induced lymphoproliferative disease with less bone marrow toxicity than hydroxyurea

We evaluated the ability of a short course of treatment with the ribonucleotide reductase (RR) inhibitor hydroxyurea (HU) and two novel RR inhibitors Trimidox (TX) and Didox (DX) to influence late-stage murine retrovirus-induced lymphoproliferative disease. LPBM5 murine leukaemia virus retrovirus-infected mice were treated daily with HU, TX or DX for 4 weeks, beginning 9 weeks post-infection, after development of immunodeficiency and lymphoproliferative disease. Drug effects on disease progression were determined by evaluating spleen weight and histology. Effects on haematopoiesis were determined by measuring peripheral blood indices (white blood cells and haematocrit) and assay of femur cellularity and femoral and splenic content of colony-forming units granulocyte-macrophage (CFU-GM) and burst-forming units-erythroid (BFU-E). HU, TX and DX partially reversed late-stage retrovirus-induced disease, resulting in spleen weights significantly below pre-treatment values. Spleen histology was also improved by RR inhibitor treatment (DX>TX>HU). However, as expected, HU was significantly myelosuppressive, inducing a reduction in peripheral indices associated with depletion of femoral CFU-GM and BFU-E. In contrast, although TX and DX were moderately myelosuppressive, both drugs were significantly better tolerated than HU. In summary, short-term treatment in late-stage murine retroviral disease with HU, TX or DX induced dramatic reversal of disease pathophysiology. However, the novel RR inhibitors TX and DX had more effective activity and significantly less bone marrow toxicity than HU.

Hydroxyurea significantly enhances tumor growth delay in vivo with herpes simplex virus thymidine kinase/ganciclovir gene therapy

We have previously demonstrated with several cell lines in vitro that hydroxyurea (HU) synergistically enhances ganciclovir (GCV)-mediated cytotoxicity in bystander cells. In this study, we evaluated the role of DNA synthesis inhibition on enhanced bystander killing and assessed whether addition of HU would improve the efficacy of the HSV-TK/GCV system in vivo. Compared with GCV treatment alone, addition of HU resulted in increased DNA synthesis inhibition and delayed progression through S phase following removal of drug. In a xenograft tumor model, 1:10 and 1:1 mixtures of HSVtk- and LacZ-expressing SW620 cells were injected s.c. in the flanks of nude mice and treated i.p. (100 mg/kg GCV, 1500 mg/kg HU) daily for 5 days. Tumors from mice treated with GCV alone grew rapidly and increased to 10 times their initial size in 15.7 +/- 1.8 and 16.0 +/- 0.9 days for 1:10 and 1:1 mixtures, respectively. However, when both GCV and HU were administered in combination, a single complete tumor regression was observed in both the 1:10 and 1:1 groups. In the remaining mice treated with GCV/HU, it took 23.2 +/- 2.1 (1:10) and 26.4 +/- 3.8 days (1:1) to obtain a similar 10-fold increase in tumor size.

Long-term hydroxyurea in combination with didanosine and stavudine for the treatment of HIV-1 infection. Swiss HIV Cohort Study

OBJECTIVE AND METHODS

In 1998 we reported on a randomized comparison between stavudine plus didanosine plus placebo versus stavudine plus didanosine plus hydroxyurea (HU), in patients with a CD4 count of 200-500 x 10(6) cells/l. After 3 months, the HU group had a higher proportion of patients with viral load < 200 x 10 cells/l. At the end of the 3 months blinded period, patients in the placebo group had the option to add HU if their viral load remained > 200 x 10(6) cells/l. We report results after 24 months.

RESULTS

Seventy-two patients were randomized to the HU arm, and a further 30 elected to add HU after 12 weeks. Twenty-four months after the start of the trial, only 25% of the 72 patients originally randomized to HU, and 20% of the 30 who added HU after week 12, were still taking it. The reasons for stopping HU were: lack of efficacy (45%), adverse events (37%) and patient or physician preference (18%). Side effects were more frequent in the didanosine/stavudine/HU group than in the didanosine/stavudine group: neuropathy (35 versus 15%, P< 0.02), fatigue (22 versus 7%, P< 0.01), and nausea or vomiting (26 versus 9%, P< 0.01). Of those who had discontinued HU, 73% were taking three drugs including a protease inhibitor. Patients who had started HU were compared with similar patients who had started protease inhibitors in the Swiss cohort. The probability of stopping HU was higher than the probability of stopping nelfinavir or indinavir, and similar to the probability of stopping ritonavir.

CONCLUSION

HU increased the antiviral effect of stavudine plus didanosine. However, side effects were more frequent, and after 24 months the majority of patients had switched to protease inhibitor regimens.

Consensus statement: anemia in HIV infection--current trends, treatment options, and practice strategies. Anemia in HIV Working Group

BACKGROUND

Despite important advances in antiretroviral therapy, anemia remains a problem in many HIV-infected patients. Although the incidence of anemia in these patients has decreased, its prevalence appears to have stabilized or decreased only slightly. Anemia has a deleterious effect on both functional capacity and quality of life, and has been associated with shortened survival.

OBJECTIVE

The Anemia in HIV Working Group, an expert panel of physicians and researchers involved in the care of HIV-infected patients, met to determine the impact of anemia in this patient population; to develop practice strategies for the clinician treating HIV-infected patients with anemia; and to identify future research directions.

METHODS

The proposed practice strategies are based on results of the available clinical trials (as identified through a MEDLINE search), a review of the literature, and the clinical experience and expert opinion of the panel. The present report is based on meetings held in February and June of 1998; as further experience with various treatment options accumulates and the impact of highly active antiretroviral therapy becomes clearer, the panel will reconvene to develop evidence-based guidelines.

RESULTS

The working group considers HIV-associated anemia to be an important contributor to the morbidity and mortality of this infection. Recent reports indicate that recovery from anemia is associated with improved quality of life and survival.

CONCLUSIONS

As HIV-infected persons live longer, maintaining quality of life becomes an increasingly important goal of treatment. When planning treatment strategies, clinicians should consider the quality-of-life decrement caused by anemia. Transfusions should be used when rapid recovery is required, and underlying conditions causing anemia should be treated, if possible. Recombinant human erythropoietin (rHuEPO) therapy is appropriate in certain HIV-infected persons and should be considered to maintain hemoglobin concentrations. The target hemoglobin level is 12 g/dL for men and 11 g/dL for women. Weekly rHuEPO dosing is suggested, initiated at 40,000 U, as has been established in patients with cancer.