Nitric oxide production from hydroxyurea

Oxidant-antioxidant status in Egyptian children with sickle cell anemia: a single center based study

OBJECTIVE

the present study was conducted to investigate the oxidant-antioxidant status in Egyptian children with sickle cell anemia.

METHODS

the serum levels of total antioxidant capacity (TAO), paraoxonase (PON), vitamin E, nitrite, and malondialdehyde (MDA) were measured in 40 steady state children with homozygous sickle cell anemia (24 males and 16 females) and 20 apparently healthy age- and gender-matched controls.

RESULTS

mean serum TAO, PON, vitamin E, and nitrite levels were significantly lower in the group with sickle cell anemia, whereas mean serum MDA was significantly higher in these children compared to controls. No significant differences in mean levels of TAO, PON, nitrite, vitamin E, and MDA were found in sickle cell anemia patients receiving hydroxyurea when compared with those not receiving hydroxyurea. A significant negative correlation between serum nitrite and the occurrence of vaso-occlusive crises (VOC) was observed (r=-0.3, p=0.04). PON level was found to be positively correlated with patients' weight and BMI (r=-0.4, p=0.01; r=-0.7, p<0.001, respectively), but not with frequency of VOC. The area under the curve of serum nitrite in predicting occurrence of VOC was 0.782, versus 0.701 for PON, and 0.650 for TAO (p=0.006). Serum MDA was not correlated with nitrite, PON, TAO, or vitamin E levels. No significant correlations were detected between serum nitrite and hemoglobin or antioxidant enzymes.

CONCLUSION

children with sickle cell anemia have chronic oxidative stress that may result in increased VOC, and decreased serum nitrite may be associated with increases in VOC frequency. A novel finding in this study is the decrease in PON level in these patients, which is an interesting subject for further research.

Hydroxycarbamide decreases sickle reticulocyte adhesion to resting endothelium by inhibiting endothelial lutheran/basal cell adhesion molecule (Lu/BCAM) through phosphodiesterase 4A activation

Vaso-occlusive crises are the main acute complication in sickle cell disease. They are initiated by abnormal adhesion of circulating blood cells to vascular endothelium of the microcirculation. Several interactions involving an intricate network of adhesion molecules have been described between sickle red blood cells and the endothelial vascular wall. We have shown previously that young sickle reticulocytes adhere to resting endothelial cells through the interaction of α4β1 integrin with endothelial Lutheran/basal cell adhesion molecule (Lu/BCAM). In the present work, we investigated the functional impact of endothelial exposure to hydroxycarbamide (HC) on this interaction using transformed human bone marrow endothelial cells and primary human pulmonary microvascular endothelial cells. Adhesion of sickle reticulocytes to HC-treated endothelial cells was decreased despite the HC-derived increase of Lu/BCAM expression. This was associated with decreased phosphorylation of Lu/BCAM and up-regulation of the cAMP-specific phosphodiesterase 4A expression. Our study reveals a novel mechanism for HC in endothelial cells where it could modulate the function of membrane proteins through the regulation of phosphodiesterase expression and cAMP-dependent signaling pathways.

Endothelium-derived nitric oxide production is increased by ATP released from red blood cells incubated with hydroxyurea

Red blood cells (RBCs) release adenosine triphosphate (ATP) in response to a variety of stimuli, including flow-induced deformation. Hydroxyurea (HU), a proven therapy for individuals with sickle cell disease (SCD), is known to improve blood flow. However, the exact mechanism leading to the improved blood flow is incomplete. Here, we report that the incubation of human RBCs with HU enhances ATP release from these cells and that this ATP is capable of stimulating nitric oxide (NO) production in an endothelium. RBCs incubated with HU were pumped through micron-size flow channels in a microfluidic device. The release of ATP from the RBCs was measured using the luciferin-luciferase assay in detection wells on the device that were separated from the flow channels by a porous polycarbonate membrane. NO released from a layer of bovine artery endothelial cells (bPAECs) cultured on the polycarbonate membrane was also measured using the extracellular NO probe DAF-FM. ATP release from human RBCs incubated with 100 μM HU was observed to be 2.06±0.37-fold larger than control samples without HU (p<0.05, N ≥ 3). When HU-incubated RBCs were flowed under a layer of bPAECs, NO released from the bPAEC layer was measured to be 1.34±0.10-fold higher than controls. An antagonist of the P2Y receptor established that this extra 30% increase in NO release is ATP mediated. Furthermore, when RBCs were incubated with L-NAME, a significant decrease in endothelium-derived NO production was observed. Control experiments suggest that RBC-generated NO indirectly affects endothelial NO production via its effects on RBC-derived ATP release.

Emerging science of hydroxyurea therapy for pediatric sickle cell disease

Hydroxyurea (HU) is the sole approved pharmacological therapy for sickle cell disease (SCD). Higher levels of fetal hemoglobin (HbF) diminish deoxygenated sickle globin polymerization in vitro and clinically reduce the incidence of disease morbidities. Clinical and laboratory effects of HU largely result from induction of HbF expression, though to a highly variable extent. Baseline and HU-induced HbF expression are both inherited complex traits. In children with SCD, baseline HbF remains the best predictor of drug-induced levels, but this accounts for only a portion of the induction. A limited number of validated genetic loci are strongly associated with higher baseline HbF levels in SCD. For induced HbF levels, genetic approaches using candidate single-nucleotide polymorphisms (SNPs) have identified some of these same loci as being also associated with induction. However, SNP associations with induced HbF are only partially independent of baseline levels. Additional approaches to understanding the impact of HU on HbF and its other therapeutic effects on SCD include pharmacokinetic, gene expression-based, and epigenetic analyses in patients and through studies in existing murine models for SCD. Understanding the genetic and other factors underlying the variability in therapeutic effects of HU for pediatric SCD is critical for prospectively predicting good responders and for designing other effective therapies.

Oxidative stress in sickle cell disease: an overview of erythrocyte redox metabolism and current antioxidant therapeutic strategies

Erythrocytes have an environment of continuous pro-oxidant generation due to the presence of hemoglobin (Hb), which represents an additional and quantitatively significant source of superoxide (O2(-)) generation in biological systems. To counteract oxidative stress, erythrocytes have a self-sustaining antioxidant defense system. Thus, red blood cells uniquely function to protect Hb via a selective barrier allowing gaseous and other ligand transport as well as providing antioxidant protection not only to themselves but also to other tissues and organs in the body. Sickle hemoglobin molecules suffer repeated polymerization/depolymerization generating greater amounts of reactive oxygen species, which can lead to a cyclic cascade characterized by blood cell adhesion, hemolysis, vaso-occlusion, and ischemia-reperfusion injury. In other words, sickle cell disease is intimately linked to a pathophysiologic condition of multiple sources of pro-oxidant processes with consequent chronic and systemic oxidative stress. For this reason, newer therapeutic agents that can target oxidative stress may constitute a valuable means for preventing or delaying the development of organ complications.

The Preparation, Structural Characteristics, and Physical Chemical Properties of Metal-Nitrosyl Complexes

The preparation and characterization of a representative group of novel non-heme metal nitrosyl complexes that have been synthesized over the last decade are discussed here. Their structures are examined and classified based on metal type, the number of metal centers present, and the type of ligand that is coordinated with the metal. The ligands can be phosphorus, nitrogen, or sulfur based (with a few exceptions) and can vary depending on the presence of chelation, intermolecular forces, or the presence of other ligands. Structural and bonding characteristics are summarized and examples of reactivity regarding nitrosyl ligands are given. Some of the relevant physical chemical properties of these complexes, including IR, EPR, NMR, UV-vis, cyclic voltammetry, and X-ray crystallography are examined.

Fetal hemoglobin and hydroxycarbamide moduate both plasma concentration and cellular origin of circulating microparticles in sickle cell anemia children

Microparticles are cell membrane-derived microvesicles released during cell apoptosis and activation processes. They have been described as bio-markers in various vascular diseases, including sickle cell anemia, and associated with an increased risk of thrombosis. We investigated the effects of fetal hemoglobin level, a factor known to modulate the clinical expression of sickle cell anemia, and that of hydroxycarbamide treatment which reduces the frequency of vasoocclusive crises, the canonical clinical manifestation of the disease, on both the plasma concentration and the cellular origin of circulating microparticles. Flow cytometry was used to characterize microparticles in 62 sickle cell anemia children at steady state aged 2 months-16 years; 13 of them were treated with hydroxycarbamide. In untreated children, we observed negative correlations between fetal hemoglobin levels and the absolute plasma concentration of microparticles as well as that of microparticles specifically derived from platelets, erythrocytes, and monocytes. Compared to untreated children, those treated with hydroxyurea showed lower concentrations of total microparticles as a consequence of decreased microparticles shed by platelets and erythrocytes. In conclusion, in our sickle cell patients, neonatal decline of fetal hemoglobin coincided with an increase in circulating microparticles derived from erythrocytes, platelets, and monocytes. Hydroxyurea treatment was associated with a decrease in microparticles derived from erythrocytes and platelets.

Impact of hydroxyurea on clinical events in the BABY HUG trial

The Pediatric Hydroxyurea Phase 3 Clinical Trial (BABY HUG) was a phase 3 multicenter, randomized, double-blind, placebo-controlled clinical trial of hydroxyurea in infants (beginning at 9-18 months of age) with sickle cell anemia. An important secondary objective of this study was to compare clinical events between the hydroxyurea and placebo groups. One hundred and ninety-three subjects were randomized to hydroxyurea (20 mg/kg/d) or placebo; there were 374 patient-years of on-study observation. Hydroxyurea was associated with statistically significantly lower rates of initial and recurrent episodes of pain, dactylitis, acute chest syndrome, and hospitalization; even infants who were asymptomatic at enrollment had less dactylitis as well as fewer hospitalizations and transfusions if treated with hydroxyurea. Despite expected mild myelosuppression, hydroxyurea was not associated with an increased risk of bacteremia or serious infection. These data provide important safety and efficacy information for clinicians considering hydroxyurea therapy for very young children with sickle cell anemia. This clinical trial is registered with the National Institutes of Health (NCT00006400, www.clinicaltrials.gov).

Detailed mechanism of the autoxidation of N-hydroxyurea catalyzed by a superoxide dismutase mimic Mn(III) porphyrin: formation of the nitrosylated Mn(II) porphyrin as an intermediate

The in vitro autoxidation of N-hydroxyurea (HU) is catalyzed by Mn(III)TTEG-2-PyP(5+), a synthetic water soluble Mn(III) porphyrin which is also a potent mimic of the enzyme superoxide dismutase. The detailed mechanism of the reaction is deduced from kinetic studies under basic conditions mostly based on data measured at pH = 11.7 but also including some pH-dependent observations in the pH range 9-13. The major intermediates were identified by UV-vis spectroscopy and electrospray ionization mass spectrometry. The reaction starts with a fast axial coordination of HU to the metal center of Mn(III)TTEG-2-PyP(5+), which is followed by a ligand-to-metal electron transfer to get Mn(II)TTEG-2-PyP(4+) and the free radical derived from HU (HU˙). Nitric oxide (NO) and nitroxyl (HNO) are minor intermediates. The major pathway for the formation of the most significant intermediate, the {MnNO} complex of Mn(II)TTEG-2-PyP(4+), is the reaction of Mn(II)TTEG-2-PyP(4+) with NO. We have confirmed that the autoxidation of the intermediates opens alternative reaction channels, and the process finally yields NO(2)(-) and the initial Mn(III)TTEG-2-PyP(5+). The photochemical release of NO from the {MnNO} intermediate was also studied. Kinetic simulations were performed to validate the deduced rate constants. The investigated reaction has medical implications: the accelerated production of NO and HNO from HU may be utilized for therapeutic purposes.

Correlation of low levels of nitrite and high levels of fetal hemoglobin in patients with sickle cell disease at baseline

Background

Sickle cell disease is a hemoglobinopathy characterized by hemolytic anemia, increased susceptibility to infections and recurrent vaso-occlusive crises that reduces the quality of life of sufferers.

Objective

To evaluate the correlation of the levels of lactate dehydrogenase, malonaldehyde and nitrite to fetal hemoglobin in patients with sickle cell disease not under treatment with hydroxyurea in outpatients at a university hospital in Fortaleza, Ceará, Brazil.

Methods

Forty-four patients diagnosed with sickle cell disease were enrolled at baseline. Diagnosis was confirmed by evaluating the beta globin gene using polymerase chain reaction-restriction fragment length polymorphism. The concentration of fetal hemoglobin was obtained by high-performance liquid chromatography. Serum levels of nitrite, malonaldehyde and lactate dehydrogenase were measured by biochemical methods.

Results

Significantly higher levels of lactate dehydrogenase, nitrite and malonaldehyde were observed in patients with sickle cell disease compared to a control group. The study of the correlation between fetal hemoglobin levels and these variables showed a negative correlation with nitrite levels. No correlation was found between fetal hemoglobin and malonaldehyde or lactate dehydrogenase. When the study population was stratified according to fetal hemoglobin levels, a decrease in the levels of nitrite was observed with higher levels of fetal hemoglobin (p-value = 0.0415).

Conclusion

The results show that, similar to fetal hemoglobin levels, the concentration of nitrite can predict the clinical course of the disease, but should not be used alone as a modulator of prognosis in patients with sickle cell disease.

Hydroxyurea and a cGMP-amplifying agent have immediate benefits on acute vaso-occlusive events in sickle cell disease mice

Inhibition of leukocyte adhesion to the vascular endothelium represents a novel and important approach for decreasing sickle cell disease (SCD) vaso-occlusion. Using a humanized SCD-mouse-model of tumor necrosis factor-α-induced acute vaso-occlusion, we herein present data demonstrating that short-term administration of either hydroxyurea or the phosphodiesterase 9 (PDE9) inhibitor, BAY73-6691, significantly altered leukocyte recruitment to the microvasculature. Notably, the administration of both agents led to marked improvements in leukocyte rolling and adhesion and decreased heterotypic red blood cell-leukocyte interactions, coupled with prolonged animal survival. Mechanistically, these rheologic benefits were associated with decreased endothelial adhesion molecule expression, as well as diminished leukocyte Mac-1-integrin activation and cyclic guanosine monophosphate (cGMP)-signaling, leading to reduced leukocyte recruitment. Our findings indicate that hydroxyurea has immediate beneficial effects on the microvasculature in acute sickle-cell crises that are independent of the drug's fetal hemoglobin-elevating properties and probably involve the formation of intravascular nitric oxide. In addition, inhibition of PDE9, an enzyme highly expressed in hematopoietic cells, amplified the cGMP-elevating effects of hydroxyurea and may represent a promising and more tissue-specific adjuvant therapy for this disease.

Study of correlation of nitrite levels with malonaldehyde and the prognosis of patients with sickle cell disease on hydroxyurea, Ceará-Brazil

Hydroxyurea (HU) is the only drug approved for the induction of fetal hemoglobin. Besides this benefit, there are others such as the reduction of leukocyte and generation of nitric oxide (NO). Sickle cell anemia (SCA) is characterized by chronic hemolytic anemia and vaso-occlusive phenomena. The aim of this study was to evaluate the correlation of parameters MDA and NO2 with the prognosis of patients with SCA as outpatients at Hospital Universitário Walter Cantídeo. In all, 65 patients with SCA--51 without the use of HU (group I) and 14 chronically treated with HU (group II)--were recruited. Nitrite and malonaldehyde were determined by biochemical methods. We found that in group II there was a significant difference of serum MDA with clinical variables: two or more transfusions during the year (P<0.0469), the presence of malleolar ulcers (P<0.0400), and the occurrence of vaso-occlusive episodes (P<0.0031), and Group I with the occurrence of three or more vaso-occlusive episodes (P<0.0051). Correlating the malonaldehyde with clinical variables in groups I and II, we observed a statistically significant relationship with two or more transfusions during the year and the presence of malleolar ulcer. Our results demonstrate that MDA levels can be used as parameter for prognosis in SCA.

Impact of hydroxyurea on perioperative management and outcomes in children with sickle cell anemia

Hydroxyurea has enhanced the treatment for children with sickle cell anemia. The objectives of this study were to compare perioperative transfusions and outcomes for children taking hydroxyurea versus those not taking hydroxyurea. We retrospectively reviewed perioperative management and outcomes for 51 children with sickle cell anemia (HbSS genotype) who underwent surgery in our center between January 2003 and April 2008. Of the 51 patients, 30 (59%) were taking hydroxyurea and 21 (41%) were not taking hydroxyurea. Eight of 30 (27%) in the hydroxyurea group were not transfused preoperatively, 12 of 30 (40%) received a single transfusion and 10 of 30 (33%) received serial transfusions, compared with 1 of 21 (5%) children in the nonhydroxyurea group who was not transfused, 2 of 21 (10%) who received a single transfusion and 18 of 21 (85%) who received serial transfusions or pheresis (P=0.004; for comparison across groups). One patient not taking hydroxyurea developed a delayed hyperhemolytic transfusion reaction, and 2 children taking hydroxyurea developed acute chest syndrome. Overall, children taking hydroxyurea had substantially fewer transfusions compared with children not taking hydroxyurea. Both groups of children had a low complication rate. Further research should be done to optimize perioperative management of children taking hydroxyurea.

Evaluation of the concentration of malondialdehyde and nitrite in patients with sickle cell anemia treated or not with hydroxyurea

ABSTRACT Objective: To determine the serum levels of malondialdehyde and nitrite in patients with sickle cell anemia treated or not with hydroxyurea in outpatient's setting. Methods: Of the 65 patients with sickle cell anemia selected for the study, 51 were not treated with hydroxyurea (Group 1), 14 made chronic use of hydroxyurea (Group 2) and 20 individuals had no hemoglobinopathies (Control Group). Results: The Control Group had a lower and more homogeneous concentration of malondialdehyde levels as compared to the other groups. The results of Groups 1 and 2 showed increased values of malondialdehyde levels when compared to the Control Group. Considering the values of Groups 1 and 2, there were no significant changes in the malondialdehyde levels. There was no significant difference in the serum levels of nitrite between the groups. Group 2 presented a statistically significant correlation between serum malondialdehyde levels and the clinical variables investigated. In turn, Group 1 showed correlation only with occurrence of three or more vaso-occlusive crises. There was no correlation between nitrite levels and the clinical variables. Conclusion: The results revealed that during the pathogenesis of sickle cell anemia, an increase in lipid peroxidation was observed. On the other hand, no changes in oxidative parameters were detected during treatment with hydroxyurea, probably due to the short period of treatment of the patients studied.

Hydroxyurea stimulates the release of ATP from rabbit erythrocytes through an increase in calcium and nitric oxide production

Hydroxyurea, a proven therapy for sickle cell disease, is known to improve blood flow and reduce vaso-occlusive crises, although its exact mechanism of action is not clear. The objective of this study was to determine if hydroxyurea results in an increase of ATP release from the red blood cell (RBC) via the drug's ability to stimulate nitric oxide (NO) production in these cells. A system enabling the flow of RBCs through microbore tubing was used to investigate ATP release from the RBC. Incubation of rabbit RBCs (7% hct) with 50 microM hydroxyurea resulted in a significant increase in the release of ATP from these cells. This level of ATP release was not detected in the absence of flow. Studies also showed that increments in hydroxyurea and NO (from spermine NONOate) resulted in an initial increase in ATP release, followed by a decrease in this release at higher concentrations of hydroxyurea and the NO donor. Incubation with L-NAME abolished the effect of the hydroxyurea, suggesting that NO production by the RBC was involved. Indeed, in the presence of 50 microM hydroxyurea, the amount of total Ca(2+) measured (by atomic absorption spectroscopy) in a 7% solution of RBCs increased from 363+/-47 ng/ml and 530+/-52 ng/ml. Finally, EPR studies suggest that an increase in nitrosylated Hb in the RBC is only measured for those studies involving hydroxyurea and a Ca(2+)-containing buffer.

How I use hydroxyurea to treat young patients with sickle cell anemia

Hydroxyurea has many characteristics of an ideal drug for sickle cell anemia (SCA) and provides therapeutic benefit through multiple mechanisms of action. Over the past 25 years, substantial experience has accumulated regarding its safety and efficacy for patients with SCA. Early proof-of-principle studies were followed by prospective phase 1/2 trials demonstrating efficacy in affected adults, then adolescents and children, and more recently infants and toddlers. The phase 3 National Heart, Lung and Blood Institute-sponsored Multicenter Study of Hydroxyurea trial proved clinical efficacy for preventing acute vaso-occlusive events in severely affected adults. Based on this cumulative experience, hydroxyurea has emerged as an important therapeutic option for children and adolescents with recurrent vaso-occlusive events; recent evidence documents sustained long-term benefits with prevention or reversal of chronic organ damage. Despite abundant evidence for its efficacy, however, hydroxyurea has not yet translated into effective therapy for SCA. Because many healthcare providers have inadequate knowledge about hydroxyurea, patients and families are not offered treatment or decline because of unrealistic fears. Limited support for hydroxyurea by lay organizations and inconsistent medical delivery systems also contribute to underuse. Although questions remain regarding its long-term risks and benefits, current evidence suggests that many young patients with SCA should receive hydroxyurea treatment.

Decreased sickle red blood cell adhesion to laminin by hydroxyurea is associated with inhibition of Lu/BCAM protein phosphorylation

Sickle cell disease is characterized by painful vaso-occlusive crises during which abnormal interactions between erythroid adhesion molecules and vessel-wall proteins are thought to play a critical role. Hydroxyurea, the only drug with proven benefit in sickle cell disease, diminishes these interactions, but its mechanism of action is not fully understood. We report that, under hydroxyurea, expression of the unique erythroid laminin receptor Lu/BCAM was increased, but red blood cell adhesion to laminin decreased. Because Lu/BCAM phosphorylation is known to activate cell adhesion to laminin, it was evaluated and found to be dramatically lower in hydroxyurea-treated patients. Analysis of the protein kinase A pathway showed decreased intracellular levels of the upstream effector cyclic adenosine monophosphate during hydroxyurea treatment. Using a cellular model expressing recombinant Lu/BCAM, we showed that hydroxyurea led to decreased intracellular cyclic adenosine monophosphate levels and diminished Lu/BCAM phosphorylation and cell adhesion. We provide evidence that hydroxyurea could reduce abnormal sickle red blood cell adhesion to the vascular wall by regulating the activation state of adhesion molecules independently of their expression level.

Increased adhesive properties of platelets in sickle cell disease: roles for alphaIIb beta3-mediated ligand binding, diminished cAMP signalling and increased phosphodiesterase 3A activity

Whilst high pro-coagulant activity is reported in sickle cell disease (SCD), the precise role of platelets (PLTs) in SCD inflammatory and vaso-occlusive processes is unclear. Adhesion of PLTs from healthy controls (CON), SCD individuals (SCD) and SCD patients on hydroxycarbamide (SCDHC) to fibrinogen (FB) was compared using static adhesion assays. PLT adhesion molecules and intraplatelet cyclic adenosine monophosphate (icAMP) were observed by flow cytometry and enzyme-linked immunosorbent assay. SCD-PLTs demonstrated significantly greater adhesion than CON-PLTs to FB. Participation of the alpha(IIb)beta(3)-integrin in SCD-PLT adhesion was implicated by increased alpha(IIb)beta(3) activation and data showing that an alpha(IIb)beta(3)-function-inhibiting antibody significantly diminished SCD-PLT adhesion to FB. Platelet activation was potentiated by reductions in icAMP; cAMP levels were decreased in SCD-PLTs, being comparable to those of thrombin-stimulated CON-PLTs. Furthermore, SCD-PLT adhesion to FB was significantly reduced by cilostazol, an inhibitor of cAMP-hydrolyzing phosphodiesterase 3A (PDE3A). Both alpha(IIb)beta(3)-integrin activation and icAMP correlated significantly with fetal haemoglobin in SCD. Accordingly, hydroxycarbamide therapy was associated with lower PLT adhesion and higher icAMP. SCD-PLTs may be capable of adhering to proteins encountered on the inflamed vascular wall and, potentially, participate in vaso-occlusive processes. Hydroxycarbamide and, speculatively, nitric oxide donor or cyclic-nucleotide-targeted therapies may aid in the reversal of PLT adhesive properties in SCD.

The effect of prolonged administration of hydroxyurea on morbidity and mortality in adult patients with sickle cell syndromes: results of a 17-year, single-center trial (LaSHS)

The aim of this prospective study was to evaluate the long-term efficacy and safety of hydroxyurea (HU) in patients with sickle cell disease (SCD). Thirty-four patients with sickle cell anemia (hemoglobin S [HbS]/HbS), 131 with HbS/beta(0)-thal, and 165 with HbS/beta(+)-thal participated in this trial. HU was administered to 131 patients, whereas 199 patients were conventionally treated. The median follow-up period was 8 years for HU patients and 5 years for non-HU patients. HU produced a dramatic reduction in the frequency of severe painful crises, transfusion requirements, hospital admissions, and incidence of acute chest syndrome. The probability of 10-year survival was 86% and 65% for HU and non-HU patients, respectively (P = .001), although HU patients had more severe forms of SCD. The 10-year probability of survival for HbS/HbS, HbS/beta (0)-thal, and HbS/IVSI-110 patients was 100%, 87%, and 82%, respectively, for HU patients and 10%, 54%, and 66%, for non-HU patients. The multivariate analysis showed that fetal hemoglobin values at baseline and percentage change of lactate dehydrogenase between baseline and 6 months were independently predicted for survival in the HU group. These results highlight the beneficial effect of HU, which seems to modify the natural history of SCD and raise the issue of expanding its use in all SCD patients.

Hydroxyurea generates nitric oxide in human erythroid cells: mechanisms for gamma-globin gene activation

Hydroxyurea (HU) induces fetal hemoglobin synthesis through activation of cyclic guanine monophosphate (cGMP) signaling. Studies in sickle cell patients demonstrated increased circulating nitric oxide (NO) levels after oral HU treatment. However, the direct measurement of NO in erythroid cells and its role in fetal hemoglobin induction have not been defined. Therefore, we quantified the level of nitrate and nitrite (NOx) generated by HU in human erythroid progenitors in the presence of three nitric oxide synthase inhibitors (NOS), including N(G)-monomethyl-L-arginine (L-NMMA). In addition, cGMP levels were measured in the presence or absence of the pathway inhibitor 1H-(1,2,4)ox-adiazolo(4,3-a)quinoxalin-1-one, which blocks soluble guanylyl cyclase formation. HU treatment increased NOx levels and gamma-globin transcription in K562 and primary erythroid cells, which was augmented when HU was combined with L-NMMA. Pretreatment with the cGMP pathway inhibitor reversed gamma-gene activation by HU. These data demonstrate the direct stimulation of cellular NO and cGMP signaling in erythroid progenitors by HU as a possible mechanism for gamma-globin gene activation.

Hemoglobin disorders and endothelial cell interactions

Endothelial damage and inflammation make a significant contribution to the pathophysiology of sickle cell disease (SCD) and the beta-thalassemia syndromes. Endothelial dysfunction and ensuing vasculopathy are implicated in pulmonary hypertension in the hemoglobinopathies and endothelial activation and endothelial-blood cell adhesion, accompanied by inflammatory processes and oxidative stress, are imperative to the vaso-occlusive process in SCD. Herein, we discuss the role that the endothelium plays in all of these processes and the effect that genetic modifiers and hydroxyurea therapy may have upon endothelial interactions. Therapies targeting the endothelium and endothelial interactions may represent a promising approach for treating these diseases.

Cyanide, peroxide and nitric oxide formation in solutions of hydroxyurea causes cellular toxicity and may contribute to its therapeutic potency

Hydroxyurea (HU) is a potent remedy against a variety of ailments and an efficient inhibitor of DNA synthesis, yet its pharmacology is unclear. HU acts in Escherichia coli by the same mechanism as it does in eukaryotes, via inhibition of ribonucleotide reductase. When examining a controversy about concentrations of HU that prevent thymineless death in E. coli, we found instability in HU solutions that avoided prior detection due to its peculiar nature. In contrast to freshly dissolved HU, which did not affect respiration and was bacteriostatic, 1-day-old HU solutions inhibited respiration and were immediately bactericidal. Respiration was inhibited by two gases, hydrogen cyanide (HCN) and nitric oxide (NO), whose appearance we detected in "aged" HU stocks by gas chromatography-mass spectrometry; however, neither gas was bactericidal. While determining the cause of toxicity, we found that HU damages DNA directly. We also demonstrated accumulation of peroxides in HU solutions by enzymatic assays, which explains the toxicity, as both NO and HCN are known to kill bacteria when combined with hydrogen peroxide. Remarkably, we found that bactericidal effects of NO+H(2)O(2) and HCN+H(2)O(2) mixtures were further synergistic. Accumulation of decomposition products in solutions of HU may explain the broad therapeutic effects of this drug.

Newer aspects of the pathophysiology of sickle cell disease vaso-occlusion

Sickle cell disease is an inherited disorder of hemoglobin (Hb) synthesis, caused by a single nucleotide substitution (GTG>GAG) at the sixth codon of the beta-globin gene, leading to the production of a defective form of Hb, Hb S. When deoxygenated, Hb S polymerizes, damaging the sickle erythrocyte and it is this polymerization that is the primary indispensable event in the molecular pathogenesis of sickle cell disease. Hb S polymerization results in a series of cellular alterations in red cell morphology and function that shorten the red cell life span and leads to vascular occlusion. Sickle cell disease vaso-occlusion is now known to constitute a complex multifactorial process characterized by recurrent vaso-occlusion, ischemia-reperfusion injury, and oxidative stress with consequent vascular endothelial cell activation that induces a chronic inflammatory state in sickle cell disease individual and is propagated by elevated levels of circulating inflammatory cytokines. Activation of the endothelium results in the induction of endothelial adhesion molecule expression that mediates red and white cell adhesion to the vessel wall and the formation of heterocellular aggregates, followed by secondary red cell trapping, all of which contribute to reduced blood flow and eventually obstruction of the micro-circulation. Reduced nitric oxide bioavailability, caused principally by its consumption by cell-free Hb, liberated during intravascular hemolysis, contributes to this process by facilitating vasoconstriction and adhesion molecule activity.

Synthesis, characterization and crystal structure of a dinuclear iron nitrosyl complex with 2-mercapto-1-[2-(4-pyridyl)-ethyl]-benzimidazolyl

A new dinuclear iron nitrosyl complex [Fe(2)(C(14)H(12)N(3)S)(2)(NO)(4)] (1) (C(14)H(12)N(3)S = 2-mercapto-1-[2-(4-pyridyl)-ethyl]-benzimidazolyl) has been obtained by the reaction of Fe(NO)(2)(CO)(2) with 2-mercapto-1-[2-(4-pyridyl)-ethyl]-benzimidazole in CH(3)OH under moderate condition. Complex 1 was characterized by IR, UV-vis, electrochemistry and single crystal X-ray diffraction. IR spectrum displays two strong characteristic NO stretching frequencies (nu(NO)) in solution and in solid state. Cyclic voltammetry shows one irreversible, two quasi-reversible and two reversible one-electron reductions and irreversible oxidizations. This result is consistent with the fact that complex 1 is very unstable and ready to lose NO in the air. As showing in the single crystal X-ray diffraction, complex 1 forms a "chair-shape" structure by the connections of two iron centers and S-C-N frames of benzimidazole. The dihedral angle of benzimidazole ring and 2Fe-2S plane is 73.6 degrees . The crystal data are the following: 1, monoclinic, space group P2(1)/c, a = 10.43940(10) A, b = 16.0900(2) A, c = 10.13240(10) A, alpha = 90 degrees , beta = 111.0940(10) degrees , gamma = 90 degrees , V = 1587.89(3) A(3), Z = 4.

Neutral and reduced Roussin's red salt ester [Fe(2)(mu-RS)(2)(NO)(4)] (R = n-Pr, t-Bu, 6-methyl-2-pyridyl and 4,6-dimethyl-2-pyrimidyl): synthesis, X-ray crystal structures, spectroscopic, electrochemical and density functional theoretical investigations

A series of Roussin's red salt esters [Fe(2)(micro-RS)(2)(NO)(4)] (R = n-Pr (), t-Bu (), 6-methyl-2-pyridyl () and 4,6-dimethyl-2-pyrimidyl ()) were synthesized by the reaction of Fe(NO)(2)(CO)(2) with thiols or thiolates. Complexes were characterized by IR, UV-vis, (1)H-NMR, electrochemistry, and single-crystal X-ray diffraction analysis. The IR spectra of complexes display one weak and two strong NO stretching frequencies (nu(NO)) in solution, but only two strong nu(NO) in the solid. Density functional theoretical (DFT) calculations using complex as model suggest that two spatial isomers of these complexes bear a 3 kcal energy difference in solution. Frequency calculations of the two isomers provide insight on the origin of the vibrational bands and explain the IR observation of complexes in the solid state and in solution. Cyclic voltammetry shows two quasi-reversible, one-electron reductions for complexes and one quasi-reversible, one-electron reduction for complexes . The paramagnetic complexes [Fe(2)(micro-RS)(2)(NO)(4)](-) ((-)-(-)), which are prepared by the chemical reduction of neutral complexes [Fe(2)(micro-RS)(2)(NO)(4)] (), have also been investigated by EPR spectroscopy. Interestingly, the EPR spectra of complexes [Fe(2)(micro-RS)(2)(NO)(4)](-) ((-)-(-)) exhibit an isotropic signal of g = 1.998-2.004 without hyperfine splitting in the temperature range 180-298 K. The observations are consistent with the results of the calculations, which reveal that the unpaired electron is dominantly delocalized over the two sulfur and two iron atoms. The difference of the g values between the reduced form of Roussin's red ester and the typical dinitrosyl iron complexes is explained, for the first time, by the difference in unpaired electron distributions between the two types of complexes, which provides the theoretical bases for the use of g values as a spectroscopic tool to differentiate these biologically active complexes.

Resolution of chronic hypoxemia in pediatric sickle cell patients after treatment with hydroxyurea

Chronic hypoxemia is a common manifestation among patients with sickle cell anemia (SCA) who develop chronic lung disease. We report the beneficial effect of hydroxyurea on chronic hypoxemia in three pediatric patients with SCA and recurrent episodes of acute chest syndrome (ACS). All three patients improved rapidly after they were treated with hydroxyurea despite having additional risk factors for hypoxemia. A prospective trial is warranted to assess whether there is a therapeutic effect from the use of hydroxyurea in pediatric SCA patients with chronic hypoxemia.

Sickle cell disease vasculopathy: a state of nitric oxide resistance

Sickle cell disease (SCD) is a hereditary hemoglobinopathy characterized by microvascular vaso-occlusion with erythrocytes containing polymerized sickle (S) hemoglobin, erythrocyte hemolysis, vasculopathy, and both acute and chronic multiorgan injury. It is associated with steady state increases in plasma cell-free hemoglobin and overproduction of reactive oxygen species (ROS). Hereditary and acquired hemolytic conditions release into plasma hemoglobin and other erythrocyte components that scavenge endothelium-derived NO and metabolize its precursor arginine, impairing NO homeostasis. Overproduction of ROS, such as superoxide, by enzymatic (xanthine oxidase, NADPH oxidase, uncoupled eNOS) and nonenzymatic pathways (Fenton chemistry), promotes intravascular oxidant stress that can likewise disrupt NO homeostasis. The synergistic bioinactivation of NO by dioxygenation and oxidation reactions with cell-free plasma hemoglobin and ROS, respectively, is discussed as a mechanism for NO resistance in SCD vasculopathy. Human physiological and transgenic animal studies provide experimental evidence of cardiovascular and pulmonary resistance to NO donors and reduced NO bioavailability that is associated with vasoconstriction, decreased blood flow, platelet activation, increased endothelin-1 expression, and end-organ injury. Emerging epidemiological data now suggest that chronic intravascular hemolysis is associated with certain clinical complications: pulmonary hypertension, cutaneous leg ulcerations, priapism, and possibly stroke. New therapeutic strategies to limit intravascular hemolysis and ROS generation and increase NO bioavailability are discussed.

Platelet aggregation profile as a marker of hydroxyurea bioavailability through nitric oxide generation in chronic myelogenous leukemia

Platelet aggregation profiles were studied in chronic myelogenous leukemia patients who were undergoing hydroxyurea therapy. Nitric oxide (NO) generation induced by hydroxyurea was measured from the altered aggregatory response, in which the platelet suspension exhibits a de-aggregatory behaviour. NO caused platelet de-aggregation by generation of cyclic guanidine monophosphate through the activation of soluble guanylate cyclase (SGC). The fact that the observed response is specific to NO was confirmed by the reversal of the de-aggregatory behaviour in the presence of (1)H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), an inhibitor of SGC. Among the subjects studied, one subset showed an hydroxyurea-induced de-aggregatory effect that was inhibited by ODQ, whereas another subset did not show any such effect. The observed inter-individual variability in platelet aggregometric response after the ingestion of drugs may be an indicator for NO generation from hydroxyurea, and this may help to explain the drug efficacy encountered in such cases.

Sickle cell disease: role of reactive oxygen and nitrogen metabolites

1. Sickle cell disease (SCD) is an inherited disorder of haemoglobin synthesis that is associated with significant morbidity and mortality due to sequelae of episodic vaso-occlusive events: pain crises and multiorgan damage. The microvascular responses to the initiation, progression and resolution of vaso-occlusive events are consistent with an inflammatory phenotype as suggested by activation of multiple cell types, an oxidatively stressed environment and endothelial cell dysfunction. 2. Decreased anti-oxidant defences in SCD patients and mice are accompanied by activation of enzymatic (NADPH oxidase, xanthine oxidase) and non-enzymatic (sickle haemoglobin auto-oxidation) sources of reactive oxygen species. The resultant oxidative stress leads to dysfunction/activation of arteriolar and venular endothelial cells, resulting in impaired vasomotor function and blood cell-endothelial cell adhesion. 3. Changes in substrate and cofactor availability for endothelial cell nitric oxide synthase may underlie reactive oxygen- and nitrogen-induced events that contribute to SCD-induced vasculopathy. 4. The emerging role of reactive oxygen and nitrogen species in the pathogenesis of SCD provides a platform for the development of novel agents to treat this painful and lethal disease.

Therapy with hydroxyurea is associated with reduced adhesion molecule gene and protein expression in sickle red cells with a concomitant reduction in adhesive properties

Propagation of the vaso-occlusive process in sickle cell anaemia (SCA) is a complex process involving the adhesion of steady-state SCA patients red cells and reticulocytes to the vascular endothelium. The effect of hydroxyurea therapy (HUT) on the adhesive properties of sickle cells and the expression of adhesion molecule genes by erythroid cells of SCA individuals is not yet fully understood. The expressions of the CD36 gene and the VLA-4-integrin subunit genes, CD49d (alpha-subunit) and CD29 (beta-subunit), were compared in the reticulocytes of steady-state SCA patients and patients on HUT using real-time PCR. Basal adhesion of red cells from these subjects was also compared using static adhesion assays, as was surface protein expression, using flow cytometry. Basal sickle red cell adhesion to fibronectin was significantly greater than that of normal cells (P < 0.01); in contrast, HUT was associated with significantly lower levels (P < 0.01) of red cell adhesion that were similar to those of control cells; this decrease could not be justified solely by altered reticulocyte numbers in this population. Accordingly, flow cytometry demonstrated that reticulocytes from patients on HUT had significantly lower CD36 and CD49d surface expressions (P < 0.01) and, importantly, significantly lower expressions of the CD36, CD49d and CD29 genes (P < 0.05) than reticulocytes of SCA patients not on HUT. Taken together, data support the hypothesis that HUT reduces the adhesive properties of sickle cells and that this decrease appears to be mediated, at least in part, by a decrease in the gene and, consequently, surface protein expression of adhesion molecules such as VLA-4 and CD36.

Effects of hydroxyurea and L-arginine on the production of nitric oxide metabolites in cultures of normal and sickle erythrocytes

Previous in vitro studies suggest that erythrocytes may be a source of nitric oxide (NO) produced by nitric oxide synthase (NOS) or by oxyhemoglobin-mediated oxidation of hydroxyurea (HU). This study was performed to determine the roles of HU and NOS in the production of NO by normal and sickle erythrocytes. Red blood cells (RBCs) from normal adult hemoglobin (HbAA) and homozygous sickle cell subjects (HbSS) were incubated with PBS containing 0.2 mM hydrogen peroxide (control) for 2 h at 37 degrees C in the presence and absence of l-arginine, the substrate for NOS, and with l-arginine plus HU in the presence and absence of l-NMMA, a specific inhibitor of NOS. The nitrate and nitrite metabolites of NO, expressed as [NOx], were measured. [NOx] in the HbAA and HbSS RBC cultures was not significantly different in the presence and absence of 1.0 mM l-arginine (p>0.1). [NOx] in the HbAA and HbSS cultures treated with a clinically relevant dose of HU (1.0 mM) plus 1.0 mM l-arginine was significantly greater than that in controls incubated with PBS and with l-arginine p < 0.01. However, [NOx] in the HbAA and HbSS cultures treated with 50 microg/ml l-NMMA was not significantly different than that in the cultures treated with HU plus l-arginine in the absence of l-NMMA. These findings suggest that NOx production by erythrocytes may be increased by treatment with HU and may not be decreased by inhibiting NOS. Therefore, we conclude that a therapeutic dose of HU may increase the plasma concentration of NO by a mechanism that does not require erythrocytes NOS activity.

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-1beta 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.

Sickle cell disease and nitric oxide: a paradigm shift?

Traditionally the pathophysiology of sickle cell disease is thought to result from the polymerization of hemoglobin S in red cells, under hypoxic conditions, resulting in the occlusion of blood vessels. Adhesion of cells to the venular endothelium also appears to play a role. Recent studies have also suggested that in addition to the polymerization of hemoglobin S in the red blood cell, a deficiency of the endogenous vasodilator, nitric oxide may be involved. Hemoglobin released as a result of hemolysis rapidly consumes nitric oxide resulting in a whole program of events that inhibit blood flow. Therapies directed at decreasing the destruction of nitric oxide, increasing the production of nitric oxide, or amplifying the nitric oxide response may prove beneficial.

Modulation of erythrocyte arginase activity in sickle cell disease patients during hydroxyurea therapy

An elevated erythrocyte arginase activity with a corresponding decrease in nitric oxide (NO) level has been implicated in the pathophysiology of sickle cell disease (SCD). Recent studies have shown that hydroxyurea (HU) increases the production of NO, which increases the soluble guanylate cyclase activity and fetal haemoglobin (HbF) synthesis. To study the effects of HU on the arginase and nitric oxide synthase (NOS) activities in SCD patients, we compared levels of arginase activity and NO metabolites in red blood cells and plasma, respectively, from 23 patients with SCD (HbSS) receiving HU therapy, with those of 12 SCD patients not receiving HU treatment. Patients on HU therapy showed significantly lower arginase activity than that of HbSS patients not on HU therapy (1.36+/-0.2 U/10(8) cells vs. 3.31+/-0.29 U/10(8) cells). NOS activity was higher in patients on HU therapy than in untreated patients (0.72+/-0.4 nmol/ml/min vs. 0.35+/-0.15 nmol/ml/min, P<0.05). Among the HU-treated patients, the decreased level of arginase activity correlated (r=0.71) with HbF level as well as the mean corpuscular haemoglobin content. These data suggest that one of the beneficial effects of HU in vivo may involve the regulation of arginase activity and a concomitant induction of NOS activity, which may lead to an increased production of NO. The outcome of this study may lead to the development of improved NO-based treatments for SCD.

Polymorphisms of chemokine receptors and eNOS in Brazilian patients with sickle cell disease

Sickle cell disease (SCD) is an inherited disorder that presents extremely variable clinical manifestations. For the past few decades, it has been approached as an inflammatory disorder, and several researchers have tried to determine the factors involved in such characteristic. In order to contribute to the identification of the genetic differences underlying this phenotypic diversity in SCD, we proposed to study the distribution of polymorphic variants of the genes encoding the chemokine receptors CCR2 and CCR5, as well as three polymorphisms in the NOS3 gene, in Brazilian SCD patients. These genes are involved in the development of inflammatory immune reactions, a feature believed to be of extreme importance in SCD pathology. Our results indicate that the polymorphisms studied here are not directly associated with severe clinical manifestations in SCD patients. Nevertheless, we observed a tendency for the development of a severe clinical course in carriers of the variant alleles CCR2-64I and CCR5delta32 and in homozygotes for the -786C variant of the NOS3 gene. Further studies should be carried out in order to assess the role of such variants in the clinical picture of SCD.