Flow Cytometric Analysis of Hydroxyurea Effects on Fetal Hemoglobin Production in Cultures of β‐Thalassemia Erythroid Precursors

An increase in fetal hemoglobin (Hb F) ameliorates the clinical symptoms of the underlying disease in the β hemoglobinopathies—sickle cell anemia and β‐thalassemia (thal). Hydroxyurea (HU) can elevate Hb F production in erythroid cells and is the agent currently in clinical use for patients with sickle cell anemia; it is presently being tested in clinical trials for thalassemia. We have developed a two‐phase liquid culture system that mimics the in vivo hematological changes that are observed in patients treated with HU. Adding HU during the second phase of the culture increases the proportion of Hb F, increases the levels of total hemoglobin (Hb) content per cell and increases cell size, but it decreases the numbers of cells and the total amount of Hb produced. In the present study we developed and utilized a double labeling procedure for flow cytometric analysis of the cellular distribution of Hb F. Cells exposed to various concentrations of HU on day 6 of the second phase of the culture were harvested on day 12, and stained simultaneously with fluorochrome‐conjugated monoclonal antibodies specific for human glycophorin A, an erythroid specific marker, and human Hb F. Both the percentage of the Hb F‐containing cells and their intensity of fluorescence were recorded. The latter value gives a semi‐quantitative estimation of the mean cellular Hb F content. The results indicated that cultures derived from different β‐thalassemic patients differ in their response to HU. In most patients, low doses of HU decreased the percentage of Hb F‐cells as well as their Hb F content. At high doses, some patients showed an increase in both parameters, while others showed an increase in the percentage of Hb F‐cells with minimal increase in their mean Hb F content, while still other patients showed little effect at all. In all patients, high doses of HU caused a decrease in cell numbers. These results suggest that HU has mixed effects on erythroid precursors. Both the two‐phase liquid culture and the flow cytometric analysis procedures described herein provide the experimental tools for screening of Hb F‐inducing drugs and for evaluating patients' cell response prior to treatment.     

Hydroxyurea increases fetal hemoglobin in cultured erythroid cells derived from normal individuals and patients with sickle cell anemia or beta-thalassemia

Hydroxyurea (HU), an inhibitor of DNA synthesis, has been shown to increase fetal hemoglobin (HbF) levels in patients with sickle cell anemia and in some patients with beta-thalassemia. However, until now there have not been good in vitro model systems that simulate this effect for study of the molecular and cellular mechanism(s) involved in perturbing the normal ontogeny of the globin genes. We analyzed the cellular effects of HU using a two-phase liquid culture procedure (Fibach et al: Blood 73:100, 1989) in which human peripheral blood- derived progenitor cells undergo proliferation and differentiation. HU was found to have multiple effects on these cultured cells: (1) an increase in the proportion of HbF produced; (2) a decrease in cell number due to inhibition of cell proliferation; (3) an increase in hemoglobin content per cell (mean corpuscular hemoglobin [MCH]); and (4) an increase in cell size (mean corpuscular volume). The extent of these effects was related to the HU dose and time of addition. When added to cell cultures from normal individuals, 4 days following their exposure to erythropoietin (EPO), 100 mumol/L HU caused a 1.3- to 3.5- fold increase in the proportion of HbF, from 0.4% to 5.2% (mean 1.6) in untreated to 1.5% to 8.2% (mean 3.1) in HU-treated cultures and a 45% +/- 10% increase in MCH but only a 25% +/- 7% decrease in cell number on day 13. Cultures of cells derived from five patients with sickle cell anemia have shown a twofold to fivefold increase in the percentage of Hb F following addition of HU while four patients with beta- thalassemia showed a 1.3- to 6.2-fold increase. We believe that this primary cell culture procedure should prove useful in studying the cellular and molecular mechanisms of pharmacologic induction of HbF and might provide a valuable predictive assay system for evaluation of the response of individual patients with hemoglobinopathies to HU and similar agents.     

Quantitation of hemoglobin in single erythrocytes with and without fetal hemoglobin

The hemoglobin content was determined by microspectrophotometry in single erythrocytes with and without fetal hemoglobin (Hb F) from 16 normal subjects, 30 patients with anemia of different etiology and severity and 20 individuals with thalassemic disorders. Hb F-containing cells were identified by an indirect immunofluorescent method. The relative single cell value: total extinction (TE) at 415 nm, cell size (A) and the ratio TE/A, were used to indicate single cell values of MCH, MCV and MCHC respectively. The TE (MCH) and/or TE/A (MCHC) did not differ significantly between Hb F-containing (F-cells) and non-F cells in normal subjects and in cases with various forms of acquired anemia. On the contrary, the TE and/or TE/A of F-cells was found significantly higher in F compared to non-F cells in 11 of 20 (55%) of the cases with thalassemia. The results suggest that, in some cases of thalassemia, hemoglobin F is an important substitute for hemoglobin A and may improve the level of hemoglobinization.     

Zileuton induces hemoglobin F synthesis in erythroid progenitors: role of the L-arginine-nitric oxide signaling pathway

Induction of fetal hemoglobin (Hb F) is an important therapeutic tool in ameliorating complications of sickle cell disease. Nitric oxide has been implicated in the mechanism of Hb F synthesis induced by hydroxyurea (HU). This study examined whether zileuton (ZL), a structural analog of hydroxyurea, possessed Hb F-inducing properties and the potential role nitric oxide plays. ZL caused a dose-dependent increase in gamma-globin expression in K562 cells. This effect was confirmed by a dose-dependent increase in Hb F synthesis in erythroid progenitors from individuals with sickle cell anemia and normal hemoglobin genotypes. l-arginine had no effect on Hb F production; however, it dose-dependently inhibited ZL's ability to induce Hb F. The nitric oxide synthase inhibitor N(G)-monomethyl-l-arginine (l-NMMA) inhibited l-arginine's effect and restored ZL-mediated increase in Hb F synthesis. In addition, 8-PCPT-cGMP (8-(4-chlorophenylthio)guanosine 3',5'-cyclic monophosphate) inhibited ZL-mediated induction of Hb F synthesis. When comparing l-NMMA effects alone on ZL and HU, a partial reversal of increased Hb F synthesis was seen only with HU. Neither l-arginine alone nor l-arginine in combination with l-NMMA effected hydroxyurea-mediated induction of Hb F synthesis. This study demonstrates that ZL induces Hb F through a mechanism that involves l-arginine/nitric oxide/cGMP in a manner distinctly different from HU.     

Enhancement of hemoglobin and F-cell production by targeting growth inhibition and differentiation of K562 cells with ribonucleotide reductase inhibitors (didox and trimidox) in combination with streptozotocin

Upon appropriate drug treatment, the human erythroleukemic K562 cells have been shown to produce hemoglobin and F-cells. Fetal hemoglobin (Hb F) inhibits the polymerization events of sickle hemoglobin (Hb S), thereby ameliorating the clinical symptoms of sickle cell disease. Ribonucleotide reductase inhibitors (RRIs) have been shown to inhibit the growth of myeloid leukemia cells leading to the production of Hb F upon differentiation. Of the RRIs currently in use, hydroxyurea is the most effective agent for Hb F induction. We have examined the capacity of two novel RRIs, didox (DI) and trimidox (TRI), in combination with streptozotocin (STZ), to induce hemoglobin and F-cell production. The K562 cells were cultured with different concentrations of didox-STZ or trimidox-STZ at a fixed molar ratio of 3:1 and 1:5 for 96 hr, respectively. At pre-determined time intervals, aliquots of cells were obtained and total hemoglobin (benzidine positive) levels, number of F-cells, and Hb F were determined by the differential staining technique, fetal hemoglobin assay kit, and fluorescence cytometry respectively. The effect of combined drug treatment on the growth of K562 cells was examined by isobologram analysis. Our results indicate that a synergistic growth-inhibitory differentiation effect occurred when didox or trimidox was used in combination with STZ on K562 cells. There was an increase in the number of both benzidine-positive normoblasts and F-cells, accompanied by morphologic appearances typical of erythroid maturation. On day 4, the number of benzidine-positive cells showed a 6-9-fold increase and the number of F-cells was between 2.5- and 5.7-fold higher than the respective controls. Based upon these results, treatment with a ribonucleotide reductase inhibitor, such as didox or trimidox, in combination with STZ, might offer an additional promising option in sickle cell disease therapy.     

Fetal Hemoglobin in the Maternal Circulation – Contribution of Fetal Red Blood Cells

The major hemoglobin (Hb) during fetal life is fetal Hb (Hb F). It is mostly replaced by adult Hbs before birth and during the first year of life. In adults, where Hb F comprises <2.0% of the total Hb, it is not homogenously distributed among the red blood cells (RBCs) but is concentrated in a few RBCs, termed F-cells. Interestingly, for reasons that are unclear, Hb F increases in the maternal circulation during pregnancy. This increased Hb F could have two potential origins that are not mutually exclusive: A) maternal origin, due to inducing environment of Hb F in the maternal erythroid precursors; B) fetal origin, due to fetal cells crossing the placenta and entering the maternal circulation. The question we present herein is whether the observed increased Hb F in the maternal circulation during pregnancy is, at least partially, derived from the fetal origin. Peripheral blood was obtained from normal neonates (1–3 days old), adult men and pregnant and non pregnant women. The RBCs were stained for Hb F and carbonic anhydrase (CA) using a fetal cell count kit and analyzed by flow cytometry. Fetal and adult F-cells were distinguished by their expression of Hb F and CA. Fetal F-cells were Hb F⁺⁺/CA^?, while adult F-cells were Hb F⁺/CA⁺. Comparing pregnant and non pregnant women samples (n?=?10), we found six samples of pregnant women with 0.2–1.7% fetal cells, but none in the non pregnant group. These results support the possibility that at least part of the increase in Hb F during pregnancy is due to fetal cells entering the maternal circulation.     

Low fixed-dose hydroxyurea in severely affected Indian children with sickle cell disease

There is limited data on the efficacy of hydroxyurea (HU) in Indian sickle cell anemia patients who have severe manifestations despite high fetal hemoglobin (Hb F). Sixty sickle cell anemia children (5-18 years) with more than three episodes of vasoocclusive crises or blood transfusions per year were randomized to receive HU (n = 30) or placebo (n = 30) therapy. Fixed dose (10 mg/kg/day) of HU was administered for 18 months and the patients were followed-up monthly with clinical assessment and laboratory monitoring. In the HU group, hemoglobin (Hb) and Hb F levels increased significantly along with a significant decrease in the number of painful crises, blood transfusion requirements and hospitalizations compared to the placebo group. No major adverse events were observed in this study. In conclusion, low-fixed dose HU therapy was effective for the treatment of Indian sickle cell anemia children. However, there is a need for long-term studies to evaluate the efficacy and toxicity in a larger number of Indian sickle cell anemia patients.     

Hydroxyurea and sodium phenylbutyrate therapy in thalassemia intermedia

Hydroxyurea (HU) and sodium phenylbutyrate (SPB) have been shown to increase fetal hemoglobin (Hb F) levels in patients with thalassemia intermedia. The reported effects of these agents in increasing total Hb, however, have been inconsistent and there have been no studies on the combination of these medications. We describe the clinical response, as determined by increases in total Hb and decreased transfusion needs, in five patients with thalassemia intermedia treated with HU alone or in combination with SPB. All of the patients responded with increased levels of Hb F, but the responses in total Hb varied. Of the five patients, two had a marked response in total Hb in excess of 3 g/dl, two responded modestly with an increase in total Hb of 1-2 g/dl, and one did not respond. Prolonged responses were achieved with low doses of HU (3-10 mg/kg/day) and higher doses were associated with mild reversible hematologic or hepatic toxicity and no further increases in Hb. Sodium phenylbutyrate was added to treatment with HU in two patients, but failed to produce an increase in total Hb despite increasing Hb F levels. Of the four patients who responded to HU with an increase in total Hb, all reported symptomatic improvement and three have not required further transfusions. We conclude that low-dose HU therapy in patients with thalassemia intermedia may increase total Hb levels sufficiently to eliminate the need for transfusions. We, therefore, recommend a trial of HU for thalassemia intermedia patients in whom chronic transfusion therapy is being contemplated.     

Influence of hydroxyurea on fetal hemoglobin production in vitro

Cytotoxic drugs increase circulating fetal hemoglobin levels. We examined the mechanism by measuring the fetal hemoglobin produced per BFU-E-derived erythroblast following hydroxyurea treatment in vivo and in vitro. Treatment of four sickle cell patients increased the percentage of circulating F reticulocytes. The frequencies of bone marrow or peripheral blood BFU-E or CFU-E-derived colonies and their fetal hemoglobin content were unaffected. In all cases, the number of erythroid cells/progenitor-derived colony increased. To explore further the effect of hydroxyurea on fetal hemoglobin production, we added 50 mumol/L hydroxyurea to cultures of peripheral blood BFU-E-derived erythroblasts on 1 of 9 days (day 5 through 13) to nine samples. These BFU-E were derived from the peripheral blood of normal donors, sickle trait donors, and sickle cell anemia patients and from the bone marrows of monkeys. This concentration of hydroxyurea was selected so that the frequency of BFU-E and their size was moderately decreased. Addition of hydroxyurea to these progenitor-derived erythroid cells had no effect on fetal hemoglobin content per cell. Neither did transient exposure of progenitors to hydroxyurea prior to culture in nontoxic concentrations (0 to 500 mumol/L) result in a significant increase in fetal hemoglobin content in progenitor-derived erythroblasts. These data suggest that hydroxyurea does not directly alter the HbF program expressed by progenitor-derived erythroid cells. Instead, it enhances hemoglobin F content secondarily, possibly by inducing alterations in erythropoiesis.     

Enhanced fetal hemoglobin production by phenylacetate and 4- phenylbutyrate in erythroid precursors derived from normal donors and patients with sickle cell anemia and beta-thalassemia

In both sickle cell (SS) anemia and beta-thalassemia (beta-thal), an increase in fetal hemoglobin (HbF) ameliorates the clinical symptoms of the underlying disease. Several pharmacologic agents have been used to elevate HbF levels in adults; however, concerns regarding adverse effects of the prevailing drugs raise an urgent need for other agents capable of stimulating HbF production. We show here that sodium phenylacetate (NaPA) and its precursor, sodium 4-phenylbutyrate (NaPB), can enhance HbF production in cultured erythroid progenitor derived from normal donors and patients with SS anemia or beta-thal, when used at pharmacologic concentrations. Treatment resulted in (1) reduced cell proliferation, (2) elevated hemoglobin (Hb) content per cell (mean cellular Hb [MCH]), and (3) an increased proportion of HbF produced, associated with elevated levels of gamma-globin mRNA. Moreover, the active phenyl-fatty acids, with NaPA as a prototype, potentiated HbF induction by other drugs of clinical interest, including hydroxyurea (HU), sodium butyrate, and 5-azacytidine (5AzaC). Efficacy could be further enhanced by introducing chlorine substituents at the phenyl ring to increase drug lipophilicity. Our findings indicate that NaPA and NaPB, both already proven safe and effective in treatment of children with urea cycle disorders, might benefit also patients with severe hemoglobinopathies. The two-phase liquid culture procedure used in this study should prove valuable in further studies exploring the mechanisms of HbF induction by these agents, and might provide an assay to predict patient response in the clinical setting.     

Hydroxyurea exerts bi-modal dose-dependent effects on erythropoiesis in human cultured erythroid cells via distinct pathways

Hydroxyurea (HU) has been shown to increase the proportion of fetal haemoglobin (HbF) in most sickle cell patients. A low-dosage regimen increased total haemoglobin (Hb) levels in some thalassaemia intermedia patients by preferentially increasing beta-globin biosynthesis. To further characterize these apparent dose-dependent effects of HU, we examined erythroid cells exposed to HU (5-100 micro mol/l) in two-phase liquid culture. Low doses (from 5 to 25 micro mol/l) increased Hb levels by up to 2.7-fold, and a high dose (100 micro mol/l) increased Hb levels when added at d 3-6 of phase II, with no significant changes in response to HU during the late stage of phase II culture (> or = 9 d). HU exposure during d 0-3 of phase II culture increased the number of erythroid colonies to a maximum of fivefold at 5 micro mol/l HU. GATA-1 mRNA was downregulated at a high dose and GATA-2 was dose dependently upregulated over a lower dosage range. Treatment with 100 micro mol/l HU dramatically upregulated the death receptor DR-5, caspase 3, as determined by cDNA microarray analysis. In contrast, 10 micro mol/l HU modestly upregulated mRNA levels of the early growth response gene. Our results suggest that HU exerts concentration-dependent effects on HbF production and erythropoiesis and that these two effects are mediated by distinct molecular mechanisms.     

On the mechanism of Hb F elevations in the baboon by erythropoietic stress and pharmacologic manipulation

Maximal fetal hemoglobin (Hb F) elevations in the baboon subsequent to phenyl hydrazine-induced hemolytic anemia, bleeding, bleeding plus hydroxyurea (HU), or cytosine arabinoside were two to three times lower than those achieved with bleeding plus 5-azacytidine (azaC). Because, in the baboon, maximal elevations in F cell numbers occurred with bleeding alone, changes in the levels of Hb F in hemolysates and in Hb F per F cell could be considered to be due to the administered drugs. Erythropoietic toxicity of azaC was minimal, making it unlikely that the marked elevations of Hb F were due to shifts in the population of erythroid progenitors and precursors and more likely that they were related to a biochemical effect of the drug on DNA. The data indicate marked DNA hypomethylation. This was also found to be associated, but to a much lesser extent, with the modest Hb F elevations after bleeding, hemolysis, and treatment with HU. This drug had greater erythroid toxicity than azaC, and it appeared that the Hb F elevations occurred mainly on the rebound from the early cytotoxicity. The explanation of the molecular DNA changes with this drug and in erythropoietic stress alone remains unknown.     

Hydroxyurea increases hemoglobin F levels and improves the effectiveness of erythropoiesis in beta-thalassemia/hemoglobin E disease

Hydroxyurea (HU) is one of several agents that have been shown to enhance hemoglobin (Hb) F levels in patients with sickle cell disease and may be useful as a therapy for beta-globinopathies. However, limited information exists on the effects of HU in patients with thalassemia. Accordingly, we examined the hematologic effects of orally administered HU in 13 patients with beta-thalassemia/Hb E, including four patients who had been splenectomized. These patients were treated with escalating doses (final range, 10 to 20 mg/kg/d) for 5 months and were observed in the outpatient hematology clinic every 2 to 4 weeks. Complete blood counts including reticulocyte counts, amounts of Hb E and Hb F, G gamma:A gamma and alpha:non-alpha globin biosynthetic ratios were evaluated before and during treatment. Almost all patients responded with an average increase of 33% in Hb F levels, from a mean (+/- SD) of 42% +/- 11% to 56% +/- 8% (P < .0001), and a reciprocal decline in the percentage of Hb E from 59% +/- 9% to 49% +/- 8% (P < .001). Reticulocytosis was decreased from a mean (+/- SD) of 18.0% +/- 15.6% to 11.7% +/- 9.1% (P < .05); there was also a slight (10%) but statistically significant increase in hemoglobin levels and an improved balance in alpha:non-alpha globin chains ratios. The side effects were minimal in most patients, although these patients tended to tolerate a lower dose of HU before significant myelosuppression than has been our previous experience in sickle cell disease. One splenectomized patient died of sepsis during the trial. We conclude that increased Hb F production in beta-thalassemia/Hb E patients, with an improvement in the alpha:non-alpha globin ratios and, probably, the effectiveness of erythropoiesis, can be achieved using HU. Longer trials of HU in this population, including at other doses and in combination with other agents, appear warranted.     

Monoclonal antibody-based methods for quantitation of hemoglobins: application to evaluating patients with sickle cell anemia treated with hydroxyurea

Abstract: High-titer monoclonal antibodies (mAb) were raised against chromatographically purified human hemoglobin (Hb) species. These mAb were specific for either Hb A, Hb F, Hb S or Hb C. Based on these antibodies, which were directly conjugated with either fluorochromes or an enzyme (horseradish peroxidase), we developed immunoassays for determining the Hb profile in the peripheral blood; an enzyme-linked immunosorbent assay (ELISA) for determining the absolute and relative quantities of various Hb species and one-step immunolabeling for fluorescence microscopic and flow cytometric analyses of the distribution of RBC with respect to their Hb types. We utilized these methods for monitoring the Hb F level and the percentage of Hb F-containing cells in patients with sickle cell anemia undergoing treatment with hydroxyurea.     

Hydroxyurea affects cell morphology, cation transport, and red blood cell adhesion in cultured vascular endothelial cells

Hydroxyurea (HU) significantly increases fetal hemoglobin (Hb) production and concomitantly affects passive erythrocyte K transport and cell volume in patients homozygous for Hb S, thus decreasing disease severity. Red blood cells (RBCs) with Hb S display a greater adherence to vascular endothelial cells (VECs) than do Hb A cells, thus increasing the probability of vaso-occlusive crisis. The effect of HU on the structure and function of VECs is still unknown. In the present study, HU significantly changed, in a dose-dependent manner, the morphology and monovalent cation composition of cultured VECs after incubation in normal culture medium for up to 10 days in the absence and presence of 0.3 (therapeutic dose) and 3.0 (toxic dose) mmol/L HU. Treated cells showed significant morphologic changes such as an increase in apparent cell size and the formation of multinucleated giant cells. The protein content per dish decreased by 50% and 80% at 0.3 and 3.0 mmol/L HU, respectively, accompanied by an increase in cell Na (maximum, approximately 200%) and cell K (maximum, approximately 50%) contents at about days 4 to 6 and 8 to 10, respectively. In addition, HU decreased RBC adherence to VECs in experiments with 51Cr- loaded Hb A or Hb S RBCs. The HU-induced changes in VEC morphology, cation composition, and RBC adherence may be caused or accompanied by alterations in cell membrane permeability, transformation of endothelial cells, or decreased number/density of VEC adhesion molecules. Precise mechanisms of the HU effects warrant further investigation in light of the reported beneficial effects of HU in the treatment of sickle cell anemia.     

Fetal hemoglobin synthesis in sickle cell anemia: Some molecular considerations

An ability to maintain high levels of fetal hemoglobin (Hb F) has been associated with the amelioration of the clinical severity of the sickle cell disease (SS). Clinical efforts to increase the Hb F level of the patients have, however, yielded variable therapeutic response. In an attempt to further elucidate the underlying molecular basis, in vitro Hb F synthesis was studied in erythroid progenitor (BFU-E) cells obtained from SS patients and their heterozygous (AS) relatives with varying genetic backgrounds. This allows us to study the Hb F biosynthesis pattern uncomplicated by the influence of the preferential survival of “F cells” In vivo. The Hb F levels and the relative concentrations of its constituent gamma globin chains, G_γ and A_γ, were assayed using the reversed phase HPLC method. A percentage increase in the fetal hemoglobin content was observed in the lysates of the erythroid progenitor cells relative to the circulating peripheral blood erythrocyte values in SS patients and their AS relatives with different β_s haplotypes reflecting the intrinsic capacity of fetal hemoglobin synthesis in these subjects. No such increase was observed in the patient with the Mor haplotype. Furthermore, the Hb F synthesized in the BFU-E colonies was more of the adult type, as evidenced by the decrease in the percent G_γlevel relative to the corresponding peripheral blood values of the subjects in all the haplotype groups studied. The Mor haplotype was again an exception, synthesizing fetal hemoglobin more of the fetal type. © 1994 Wiley-Liss, Inc.     

Effects of hydroxyurea on hemoglobin F and water content in the red blood cells of dogs and of patients with sickle cell anemia.

A rationale for clinical trials of hydroxyurea (HU) treatment in sickle cell disease is that the agent increases red blood cell (RBC) fetal hemoglobin content. However, an additional effect of HU is to raise the mean corpuscular volume (MCV). To investigate the action of HU in a species that makes no electrophoretically distinguishable fetal hemoglobin, we treated dogs with the drug and compared their response to that of five patients with sickle cell anemia. Both dogs and patients had an increase in MCV, but the effect of HU treatment on the mean corpuscular hemoglobin concentration (MCHC), density, and water content of the RBCs differed in the two species. The dog RBCs became low in MCHC, high in ion and water content, and low in mean density. Thus, HU can raise MCV and lower MCHC without influencing fetal hemoglobin synthesis. A different pattern was seen in the sickle cell patients during HU treatment. Although the MCV of their RBCs increased, there was no change in MCHC, ion content, or mean density. A notable change in the sickle cell patients' blood was that two subpopulations of cells were nearly eliminated during HU treatment; the hypodense reticulocyte fraction and the hyperdense fraction that contains irreversibly sickled cells. These findings lead us to suggest that trials of HU in sickle cell disease must recognize the possibility that any beneficial effect of this agent might be due not only to an increase in hemoglobin F alone, but perhaps also to the associated increase in MCV or the altered RBC density profile.     

Early Detection of Response to Hydroxyurea Therapy in Patients with Sickle Cell Anemia

Red blood cells (RBC) and reticulocyte parameters were determined on peripheral blood from a subset of patients enrolled in the multicenter study of hydroxyuea (HU) in sickle cell anemia. Multiple blood samples were obtained every 2 weeks. Cellular indices were measured by flow cytometry. Generalized linear models were used to determine the relationship between the longitudinal trajectories of RBC and reticulocyte indices and HU usage. There was a significant relationship between HU usage and most of the RBC and reticulocyte indices. Hydroxyurea produced higher value trajectories than those generated by placebo usage for the hemoglobin (Hb) content of both the RBCs and reticulocytes and for the mean corpuscular volume (MCV) of reticulocytes. These changes were first detected 10 weeks after starting HU and before the increase in Hb F levels. The data suggest that subtle and early markers of response to HU reside in the hemogram.     

Stimulation of fetal hemoglobin synthesis in bone marrow cultures from adult individuals.

The regulation of fetal hemoglobin in adult erythroid cells was investigated with bone marrow cultures. Fetal hemoglobin (Hb F) was identified in individual erythroid colonies with fluorescent antibodies against Hb F and synthesis of gamma chains was determined with analyses of radioactive globins. The appearance of fetal hemoglobin in erythroid colonies was clonal. All the cells of the Hb F synthesizing colonies contained fetal hemoglobin. The frequency of erythroid colonies showing Hb F was higher than expected compared to the frequency of Hb F containing cells in the blood. Production of Hb F in culture, as shown by analysis of the radioactive globins, was 5 to 14 times higher than baseline Hb F synthesis. These results suggest that the ability for gamma chain synthesis in erythroid cells is determined at or above the level of the precursor cell from which the erythroid colonies, in vitro, derive (probably an erythropoietin responsive stem cell), and that stimulation of fetal hemoglobin synthesis in adult erythroid cells is possible.     

Effects of hydroxurea, stem cell factor, and erythropoietin in combination on fetal hemoglobin in the baboon

OBJECTIVE: Augmentation of the level of fetal hemoglobin (HbF) is considered therapeutic for patients with sickle cell disease. The objective of this study was to determine the effect of treatment with a combination of erythropoietin (Epo), stem cell factor (SCF), and hydroxyurea (HU) on HbF levels. MATERIALS AND METHODS: The effect of treatment with a combination of Epo, SCF, and HU on HbF, F-cell numbers, and globin chain synthesis was evaluated in a baboon model. RESULTS: Treatment with a combination of SCF+Epo resulted in a two-fold increase in HbF, F-cells, and F-reticulocytes compared to Epo alone. The combination of SCF+Epo+HU resulted in an additional two-fold increase in HbF, whereas F-cells and F-reticulocytes increased only 25% compared to the SCF+Epo regimen. Measurement of differential globin chain synthesis indicated that the SCF+Epo+HU treatment also increased the I gamma/V gamma (homologous to human G gamma and A gamma) synthetic ratio toward the fetal ratio. CONCLUSIONS: HU can effectively augment growth factor-induced HbF synthesis in vivo. Because I gamma/V gamma ratios are unaffected by erythropoietic stress and similar increases in this ratio have only been observed following administration of 5-azacytidine, we suggest that these two agents may share a common mechanism of action involving the recruitment of a similar target cell population to terminal erythroid differentiation.