Original Article
Augmentation by Erythropoietin of the Fetal-Hemoglobin Response to Hydroxyurea in Sickle Cell Disease
N Engl J Med 1993; 328:73-80January 14, 1993
- Abstract
Background
Hydroxyurea increases the production of fetal hemoglobin in patients with sickle cell anemia, inhibiting the polymerization of hemoglobin S and potentially improving vaso-occlusive manifestations and hemolysis. Recombinant erythropoietin increases the number of reticulocytes containing fetal hemoglobin in laboratory animals and in humans. We studied whether hydroxyurea and erythropoietin might have a potentiating effect on the production of fetal hemoglobin in patients with sickle cell disease.
Methods
We treated four patients who were receiving hydroxyurea for sickle cell disease (three who were homozygous for sickle cell anemia and one with sickle β0-thalassemia) with escalating doses of intravenous erythropoietin for seven weeks, along with oral iron sulfate. Doses of hydroxyurea on four consecutive days were alternated with doses of erythropoietin on three consecutive days.
Results
There was a 28 percent increase in the number of reticulocytes containing fetal hemoglobin and a 48 percent increase in the percentage of fetal hemoglobin, as compared with the maximal values obtained with hydroxyurea alone. The percentage of erythrocytes containing fetal hemoglobin (F cells) increased from 64 to 78 percent. As compared with hydroxyurea alone, treatment with hydroxyurea and erythropoietin decreased the mean (±SD) serum indirect bilirubin level from 0.8 ±0.2 to 0.5 ±0.1 mg per deciliter (13.3 ±2.9 to 8.9 ±2.2 μmol per liter) (P = 0.02), suggesting a further decrease in hemolysis. Red-cell filterability improved.
Conclusions
Intravenous recombinant erythropoietin with iron supplementation alternating with hydroxyurea elevates fetal-hemoglobin and F-cell levels more than hydroxyurea alone. Such increases decrease intracellular polymerization of hemoglobin S and improve the overall rheologic characteristics of erythrocytes. A reduced dosage of hydroxyurea alternating with erythropoietin may prove less myelotoxic than hydroxyurea given daily or in pulsed-dose regimens. It may also increase levels of fetal hemoglobin in patients with sickle cell disease who have not been helped by hydroxyurea alone.
Media in This Article
Figure 1
Administration of Hydroxyurea and Recombinant Human Erythropoietin to Patients with Sickle Cell Disease and Sickle β0-Thalassemia.Figure 2
Levels of F Reticulocytes (Open Symbols) and Fetal Hemoglobin (Solid Symbols) in Three Patients Treated with Hydroxyurea (HU) and Then Concomitant Iron Sulfate and Recombinant Human Erythropoietin (Epo).Article Activity
- Article
Stimulating fetal hemoglobin by increasing γ-globin synthesis in patients with sickle cell disease would, if the production of βS-globin decreased concomitantly, have a large “sparing” effect on the formation of intracellular hemoglobin S polymer1-3 and would be expected to improve the acute and chronic hemolytic and vaso-occlusive complications of the disease. Azacytidine and hydroxyurea have been shown to increase fetal-hemoglobin levels in some patients with sickle cell disease4-9. When hydroxyurea is used to induce the production of fetal hemoglobin in sickle cell anemia, the responses of individual patients are variable, and prolonged treatment is required for a maximal effect9. A recent multicenter clinical trial involving more than 50 patients10 described increases in mean fetal-hemoglobin levels ranging from 4 to 15 percent and the restriction of fetal hemoglobin to a subgroup of reticulocytes containing fetal hemoglobin (F reticulocytes) and erythrocytes (F cells). There is evidence that higher levels of fetal hemoglobin, in more pancellular distributions, would be necessary to reduce substantially the polymerization of hemoglobin S at physiologic oxygen tensions3. In addition, because of the long-term nature of the treatment, concern has been raised about the myelosuppressive effects and potential carcinogenicity of prolonged hydroxyurea therapy at high doses.
Accordingly, recent efforts have focused on the identification of noncytotoxic agents to stimulate the synthesis of fetal hemoglobin. Hematopoietic growth factors augment the production of F reticulocytes in laboratory animals11-13. Al-Khatti et al.14,15 and McDonagh et al.13 have shown that erythropoietin enhances fetal-hemoglobin production by hydroxyurea in animals. Goldberg and colleagues16 detected no augmentation of fetal hemoglobin by erythropoietin and hydroxyurea given daily in five patients, but studies in animals13 suggest that this dose schedule was suboptimal. We treated patients with sickle cell disease with hydroxyurea, iron sulfate, and intermittent recombinant erythropoietin to see whether there would be enhanced fetal erythropoiesis, as compared with that obtained with hydroxyurea alone.
Methods
Patients
Four patients with sickle cell disease were enrolled in this study. Patients 1, 2, and 3 had homozygous sickle cell anemia, and Patient 4 had sickle β0-thalassemia. Each had had longstanding complications of the disease, including recurrent severe crises and chronic bone pain. All the patients were male; they were 31, 34, 36, and 37 years old. Patients 1, 2, and 3 had been treated with escalating and fixed doses of hydroxyurea9; the data reported herein have not been previously published. These patients had been taking hydroxyurea for 5 to 15 months before entering the erythropoietin study.
During the erythropoietin trial, the patients were hospitalized for approximately two months at the Warren Grant Magnuson Clinical Center of the National Institutes of Health. An exemption for treatment with an investigational new drug was obtained from the Food and Drug Administration. The treatment protocol was approved by the Clinical Research Subpanel of the National Heart, Lung, and Blood Institute.
Study Protocol
After informed consent was obtained from the patients, base-line and serial hematologic tests were performed, including phthalate profiles of red-cell density17 and determinations of the percentage of F reticulocytes and F cells by immunologic methods18. Blood tests of hepatic and renal function were performed three times a week. Base-line serum erythropoietin levels were determined by radioimmunoassay (SmithKline BioScience Laboratories, Philadelphia). Bone marrow aspirates and biopsy samples were obtained on days 0, 14, and 50 of erythropoietin treatment to monitor bone marrow morphologic characteristics, including the estimation of the ratio of myeloids to erythroids, the quantitation of dyserythropoiesis, and the iron and reticulum content of bone marrow.
The patients were monitored in the hospital for two weeks to ensure a steady state with respect to hematologic values before therapy with hydroxyurea and erythropoietin was begun. Data obtained from studies in primates13 provided the basis of the treatment protocol. Cyclic treatment in which hydroxyurea given for four days alternated with erythropoietin given for three days was found to be more effective than therapy with intermittent erythropoietin and daily hydroxyurea. Preliminary observations suggested that the desired F-reticulocyte response to erythropoietin required simultaneous iron therapy19. Therefore, the treatment regimen comprised two phases, both incorporating alternating administration of hydroxyurea (on days 1 through 4) and erythropoietin (on days 5 through 7) together with oral iron sulfate (Figure 1Figure 1
Administration of Hydroxyurea and Recombinant Human Erythropoietin to Patients with Sickle Cell Disease and Sickle β0-Thalassemia.). Phase 1 involved a schedule of erythropoietin treatment in which the timing increased from 1000 units per kilogram of body weight once a week to 1000 units per kilogram three times a week. Phase 2 involved a schedule in which the dosage increased from 1000 units per kilogram three times a week to 3000 units per kilogram three times a week, as indicated. The erythropoietin was diluted in 5 percent albumin and infused intravenously over a 30-minute period.The patients continued to receive hydroxyurea at the dose that had been previously determined to be the maximally tolerable dose for each patient; this dose was defined on the basis of at least a doubling in the levels of fetal hemoglobin and F reticulocytes, without a decline in the white-cell count to less than 5000 per cubic millimeter, in the platelet count to less than 150,000 per cubic millimeter, or in the absolute reticulocyte count to less than 40,000 per cubic millimeter9. Patients 1, 2, and 4 received 20 mg of hydroxyurea per kilogram, whereas Patient 3 received 25 mg per kilogram. Patient 1 was subsequently retreated with a protocol that combined hydroxyurea and erythropoietin after his dose of hydroxyurea was increased by 25 percent (to 25 mg per kilogram); he again received hydroxyurea on four consecutive days each week. The hydroxyurea was administered at this dose for approximately two months in order to achieve a plateau in the fetal-hemoglobin and F-reticulocyte counts. In Patient 1, supplemental treatment with oral iron sulfate was begun approximately two weeks before the institution of erythropoietin to assess the effects of oral iron given before erythropoietin.
Red-Cell Measurements
The number of dense cells was defined as the percentage of cells with an intracellular hemoglobin concentration of more than 37 g per deciliter (23.0 mmol per liter), as measured by phthalate-ester gradient centrifugation17. The median corpuscular hemoglobin concentration (MCHC) was determined from these gradients and used to calculate the tendency toward intracellular polymerization of hemoglobin S for cells with a density less than 23.0 mmol per liter. The majority of circulating cells (>80 percent) had a normal density. Measurements of mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH) were obtained with use of a Coulter electronic cell analyzer. The tendency of F cells and non-F cells toward intracellular polymerization was calculated as described elsewhere3,20 on the basis of measurements of the total intracellular hemoglobin concentration and the percentages of hemoglobin S, fetal hemoglobin, and hemoglobin A2. Although values were calculated for the full range of oxygen saturation, only values obtained at 40 percent and 70 percent oxygen saturation are presented here, as representative of physiologically relevant values on the venous and arteriolar sides of the microcirculation, respectively.
For the measurements of filtration, blood samples were centrifuged at 22 °C at 800 × g for 10 minutes, after which the plasma and buffy coat were carefully removed and replaced with HEPES-buffered sodium chloride solution (141 mM sodium chloride and 10 mM HEPES-sodium buffer, pH 7.4). The red-cell preparations were washed three times by repeated resuspension and then centrifuged at 800 × g, 600 × g, and 500 × g. Filtration studies of the red-cell suspension were then performed by the vertical-tube method, with a nickel-mesh network of uniform 3-micrometer pores, as described elsewhere21,22. The time course of filtration through this network was monitored and compared with the filtration characteristics of normal blood homozygous for hemoglobin A, as well as those of a HEPES-buffered sodium chloride solution. All the studies were performed at 24 °C with red-cell suspensions equilibrated with room air, and all were run within four hours after the collection of venous blood.
Results
Fetal-Hemoglobin Responses
Hydroxyurea was administered for 5 to 15 months to the four patients enrolled in this study. The characteristic increase in the F-reticulocyte count, followed by a more gradual increase in fetal hemoglobin, with the administration of hydroxyurea is clearly seen in the data on the three patients with sickle cell disease (Figure 2Figure 2
Levels of F Reticulocytes (Open Symbols) and Fetal Hemoglobin (Solid Symbols) in Three Patients Treated with Hydroxyurea (HU) and Then Concomitant Iron Sulfate and Recombinant Human Erythropoietin (Epo).). Furthermore, a plateau in fetal-hemoglobin levels had occurred in all four patients receiving a constant dose of hydroxyurea, indicating a new balance between the increased production and the probably decreased destruction of fetal hemoglobin-containing sickle erythrocytes (Figure 2 and Figure 3AFigure 3
Fetal-Hemoglobin Response to Hydroxyurea (HU) and Hydroxyurea Combined with Erythropoietin (Epo) in a Patient with Sickle β0-Thalassemia and in Patient 1 with Sickle Cell Disease during a Second Course of Therapy.). Among the patients with sickle cell disease, fetal-hemoglobin levels had increased 5 to 18 times above the base line during hydroxyurea treatment, with comparable increases in the levels of F reticulocytes. Maximal increases of both F-reticulocyte and fetal-hemoglobin levels in Patients 1, 2, and 3 with a constant dose of hydroxyurea were seen on the average by day 77 (range, days 50 to 100) and day 96 (range, days 70 to 125), respectively.The time course of the F-reticulocyte and fetal-hemoglobin responses to treatment with the combination of hydroxyurea and erythropoietin in the three patients with homozygous sickle cell disease is also shown in Figure 2. The addition of iron and erythropoietin to the hydroxyurea regimen resulted in an immediate further increase in F-reticulocyte numbers and a subsequent increase in fetal-hemoglobin levels. Data on the patient with sickle β0-thalassemia and the second course of treatment of Patient 1 are shown in Figure 3A and Figure 3B, respectively. Treatment of the patient with sickle β0-thalassemia and retreatment of Patient 1 also had an augmented effect of iron and erythropoietin.
Table 1Table 1
Average Peak Fetal Erythropoietic Responses in Four Patients Treated with Hydroxyurea (HU) or Hydroxyurea plus Erythropoietin (EPO). summarizes the base-line and average F-reticulocyte, fetal-hemoglobin, and F-cell responses of the three patients with sickle cell disease during both the administration of hydroxyurea alone and that of hydroxyurea and erythropoietin. (Data on the patient with sickle β0-thalassemia are not included in the averages because of the patient's need for continued transfusion.) Fetal-hemoglobin levels increased by an average of 48 percent during treatment with hydroxyurea and erythropoietin as compared with hydroxyurea therapy alone. The F-reticulocyte count increased by 28 percent with combination therapy, whereas the percentage of F cells increased from an average of 64.0 percent with hydroxyurea alone to 78.4 percent with hydroxyurea and erythropoietin. The peak response to erythropoietin occurred early, on day 17 for F reticulocytes (range, days 12 to 25) and on day 44 for fetal hemoglobin (range, days 40 to 50). There was also a small increase, from 16.5 percent to 20.1 percent, in the amount of fetal hemoglobin per F cell, as calculated from fetal-hemoglobin levels and numbers of F cells. In these three patients, the changes in the percentage of fetal hemoglobin and the amount of fetal hemoglobin per F cell during therapy with hydroxyurea and erythropoietin were statistically significant.Patient 4, who had sickle β0-thalassemia and required periodic blood transfusions because of symptomatic bone pain when his hemoglobin level dropped below 7.0 g per deciliter (4.3 mmol per liter), had an approximate doubling of his fetal-hemoglobin levels during the four months of initial therapy with hydroxyurea (Figure 3A). After the administration of erythropoietin, the maximal fetal-hemoglobin level doubled again, peaking on day 35. Isolated values for F reticulocytes, F cells, and the amount of fetal hemoglobin per F cell suggest comparable increases in these measures (Table 1) but cannot be interpreted in detail. In all the patients, when erythropoietin was stopped, the number of F reticulocytes and the fetal-hemoglobin level returned to the pre-erythropoietin values (Figure 2 and Figure 3). After the cessation of erythropoietin treatment, the calculated half-life for the return of fetal-hemoglobin levels to the levels in patients receiving hydroxyurea alone was greater than 60 days (Figure 2), presumably because of the long survival of the newly created F cells.
In Patient 1, the dose of hydroxyurea was increased by 25 percent (to 25 mg per kilogram, given on four consecutive days per week) (Figure 3B). Within 40 to 50 days, this patient had a large increase in the number of F reticulocytes and a small increase in fetal hemoglobin. The administration of iron sulfate with hydroxyurea, in the absence of erythropoietin, was associated with a transient, though marked, drop in the F-reticulocyte count and the fetal-hemoglobin level. With the concomitant administration of erythropoietin, however, there was a small additional increase in the level of F reticulocytes and fetal hemoglobin. After the erythropoietin had been discontinued and the dose of hydroxyurea returned to the previous level of 20 mg per kilogram, the levels of F reticulocytes and fetal hemoglobin gradually returned to the previous levels over the ensuing several months.
Hematologic Responses
Table 2Table 2
Maximal Laboratory Values in the Four Patients during Treatment with Hydroxyurea (HU) and Erythropoietin (EPO). shows the mean hematologic responses at the time of the patients' maximal response to hydroxyurea or hydroxyurea and erythropoietin. Hydroxyurea produced a small increase in hemoglobin values; the addition of erythropoietin increased hemoglobin levels further. Hydroxyurea treatment alone increased MCV and MCH but caused no change in MCHC. In Patients 1, 2, and 3 the addition of erythropoietin to the hydroxyurea regimen further increased MCV (from 124.5 to 128.5 micrometer3) and MCH (from 41.8 to 43.2 pg) but produced no change in MCHC, as determined by the Coulter electronic counter or directly by the phthalate-ester gradient technique17.There was also improvement in the hemolytic rate with the regimen combining hydroxyurea and erythropoietin, as compared with hydroxyurea therapy alone. For example, the average serum indirect bilirubin level dropped from 0.8 ±0.2 mg per deciliter (13.3 ±2.9 μmol per liter) to 0.5 ±0.1 mg per deciliter (8.9 ±2.2 μmol per liter) (P = 0.02), and the reticulocyte level fell from 6.5 ±3.6 to 3.8 ±2.2 percent (P = 0.09) during combination therapy.
Neutropenia may be a dose-limiting toxic effect when erythropoietin is used to treat the anemia of prematurity23 or in patients with oncologic conditions24. We found little change in white-cell and platelet levels when patients received the combination of hydroxyurea and erythropoietin, as compared with hydroxyurea alone (Table 2).
Bone marrow aspirates and biopsy specimens were obtained on days 0, 14, and 50 during therapy with hydroxyurea and erythropoietin. Before erythropoietin was added to the regimen, all the patients had erythroid hyperplasia, with myeloid:erythroid ratios ranging from 1:2 to 1:4. As previously described in the case of hydroxyurea therapy, the erythroid elements showed mild-to-moderate megaloblastic changes with no evidence of other dyserythropoietic features. During therapy with erythropoietin there was a minimal increase in the erythroid numbers (data not shown); the myeloid, megakaryocytic, and lymphoid elements in the bone marrow changed little; and there were no changes in cellularity, iron deposition, or reticulin staining. All the patients had the expected elevated serum iron and ferritin values, suggesting that none were iron deficient (Table 2). In the three patients in whom serum erythropoietin levels were measured, the levels were only moderately elevated, a finding that has previously been interpreted to indicate a blunted “erythropoietin response” in sickle cell disease25.
Red-Cell Rheologic Properties
Hydroxyurea therapy causes a significant decrease in the percentage of dense cells and a slight decrease in phthalate-density-gradient measurements of red-cell heterogeneity, expressed as R60 values (the values in the middle 60 percent of the density range)9,10,16. Hydroxyurea therapy caused the same effects in these four patients. The addition of erythropoietin further reduced the numbers of dense cells in the one patient with detectable dense cells and caused a further small decrease in R60 values in three patients (Table 2).
Filtration studies were performed before and during erythropoietin therapy with a 3-micrometer nickel-mesh, gravity-filtration system21,22. Red-cell filterability, expressed as the percent drop in pressure at 2.5 minutes as compared with that when a buffer solution was used, improved in the three patients for whom measurements were made (Table 2). Changes in filterability closely paralleled changes in the fetal-hemoglobin level induced by hydroxyurea and erythropoietin and normalization in the red-cell density profiles; the correlation of filterability with fetal-hemoglobin levels was linear (r2 = 0.95, P<0.01).
The polymerization tendency was calculated as described elsewhere, from the values of the phthalate-ester-determined MCHC and the percentage of non-S hemoglobins during the course of treatment with either hydroxyurea or hydroxyurea and erythropoietin. These values are expressed as the polymerization tendencies at 40 percent and 70 percent oxygen saturation, for purposes of comparison (Table 2). During treatment with hydroxyurea and erythropoietin, as compared with hydroxyurea, at 40 percent oxygen saturation there was a further decrease in the polymerization tendency in the F cells of about 16 percent (for Patients 1, 2, and 3). As expected, the values for non-F cells did not change (0.32 at 40 percent saturation). The effect of combination therapy on the polymerization tendency would be expected to be further magnified by an increase in the percentage of F cells in all patients and by the reduction in the percentage of dense cells in Patient 3.
Side Effects
The first three patients (including Patient 1, who had a second course of therapy) tolerated the high doses of erythropoietin well, with no acute vaso-occlusive crisis and only minimal-to-moderate changes in the total hemoglobin level (Table 2). Patient 3, who had an increase in total hemoglobin of about 2 g per deciliter (1.2 mmol per liter) over the seven-week trial of erythropoietin, showed no signs or symptoms of enhanced vaso-occlusion. Patient 4 had a slight increase in chronic bone pain when his dosage of erythropoietin reached 2000 units per kilogram three times a week; therefore, the drug dose was not increased further. At these dosages of erythropoietin, none of the patients had changes in blood pressure or renal or hepatic function, changes that have been reported in patients undergoing dialysis and also receiving erythropoietin26.
Discussion
Hydroxyurea elevates fetal-hemoglobin levels in some patients with β-thalassemia27 and in most patients with sickle cell disease, but the magnitude of the elevation varies. Although hydroxyurea has been used safely for extended periods in patients with myeloproliferative disorders, it can cause generalized myelosuppression and may induce chromosomal changes,28 a special concern in children. For these reasons, a search continues for other agents that might enhance the effect of hydroxyurea on fetal-hemoglobin levels and numbers of F reticulocytes and F cells.
Erythropoietin promotes the differentiation and proliferation of erythroid cells, and it can induce the production of fetal hemoglobin in cultured erythroid progenitors29,30. The administration of erythropoietin to baboons can lead to dramatic elevations of F cells,15 and erythropoietin can stimulate F-cell production in the rhesus monkey13. The use of recombinant erythropoietin alone to treat sickle cell anemia has been disappointing,16,31 but studies of erythropoietin in primates have shown that, combined with hydroxyurea, it led to levels of F reticulocytes that were higher than those with either agent alone13,14.
Goldberg et al.16 recently reported that in five patients with sickle cell disease treated with the maximally tolerated doses of hydroxyurea, there was no additive effect of erythropoietin (given in doses reaching 1500 units per kilogram) on the fetal-hemoglobin or F-reticulocyte response. Our protocol differed from theirs in four important ways (Figure 1). First, we gave hydroxyurea four days out of seven, whereas they gave hydroxyurea daily. Second, our patients received oral iron therapy as a supplement. Third, the doses of erythropoietin in the present study were higher than those in their study. Finally, the patients treated by Goldberg et al. received hydroxyurea at maximal tolerated doses, whereas our patients were treated with lesser degrees of myelosuppression.
Hydroxyurea may be toxic to cycling erythroblasts, but noncycling erythroid precursors may survive and, in the presence of erythropoietin, may undergo accelerated differentiation and maturation32. The alternating-dose schedule may allow this mechanism to operate, whereas continuous doses may not. With respect to iron supplementation, serum ferritin levels may be insufficient to assess the adequacy of “physiologically available” iron stores33,34. Patients in both studies had elevated serum ferritin levels, which implied that iron stores were adequate to generate elevated reticulocyte counts, but the acute erythropoietic stimulus caused by the administration of pharmacologic doses of erythropoietin could impose a “relative” iron deficiency due to kinetic factors in the mobilization of iron. Our doses of erythropoietin were much higher than those used in the previous study,16 but we do not think this explains the difference in outcomes, since the peak F-reticulocyte response occurred at a relatively low dose of erythropoietin (1000 units per kilogram in all patients).
Tolerable levels of hydroxyurea limit the number of erythroid cells that are subsequently sensitive to erythropoietin. Treating Patient 1 again with higher doses of hydroxyurea (Table 1 and Figure 3B) resulted in additional increments in the levels of F reticulocytes and fetal hemoglobin (average increases, 28 percent and 36 percent, respectively) at the expense of a decline in the absolute neutrophil count (to 2100 per cubic millimeter) and the absolute reticulocyte count (to 33,000 per cubic millimeter). Erythropoietin increased F reticulocytes and fetal hemoglobin, but less than before (Figure 3B).
The higher levels of fetal hemoglobin achieved with combination therapy than with hydroxyurea alone may be important for the development of an effective therapy. It was anticipated that sustained fetal-hemoglobin levels of 20 percent or more would probably be needed3,35. Since fetal hemoglobin only inhibits polymerization in cells that contain it, the increases in F-cell levels and in the amount of calculated fetal hemoglobin per F cell caused by erythropoietin may be as important as the increases in fetal-hemoglobin levels themselves. The ability to modulate the expression of fetal hemoglobin in patients with an average erythropoietin dose of 1000 units per kilogram, coupled with the relatively prolonged survival of the F cells (half-life, >60 days in our patients) (Figure 2), raises the possibility that low-dose hydroxyurea together with intermittent erythropoietin therapy could be a promising alternative treatment.
The decrease in red-cell heterogeneity and dense cells also helps by reducing the number of dense cells that contribute disproportionately to intracellular polymerization, despite the lack of an overall change in MCHC. The improvement in measured red-cell filterability and the calculated values of intracellular polymerization are both consistent with these processes. In patients treated with hydroxyurea alone, there is a marked increase in filterability accompanying both the immediate phase of decreased numbers of dense cells and the more gradual phase of increase in fetal-hemoglobin levels36. In the present study, it is presumably the second process that accounts for the continued improvement in measured filterability. These laboratory measurements are reflected in the patient's clinical state, as shown by the evidence of reduced hemolysis.
If sustained, such an elevation in fetal hemoglobin, F cells, and the amount of fetal hemoglobin per F cell, with the subsequent reduction in hemoglobin S polymerization tendency, would simulate the milder sickle cell syndromes, such as those seen in Saudi Arabia and India3. If combination therapy results in higher fetal-hemoglobin levels than does daily hydroxyurea at the maximal dose tolerated, several advantages could be anticipated. A lower total dose of hydroxyurea may suffice, thereby reducing myelotoxicity. This is important, as pediatric hematologists ponder the administration of hydroxyurea to adolescents and children suffering from the severe effects of sickle cell disease and in whom standard therapy has not been beneficial. In patients who respond poorly to hydroxyurea, combination treatment may increase their fetal-hemoglobin and F-cell levels, a particularly important development now that butyrate compounds and bone marrow transplantation are emerging as alternative treatments. The disadvantages of combination treatment with hydroxyurea and erythropoietin are the need for parenteral medication and the extremely high cost of erythropoietin, especially in view of the high doses used. On the other hand, since all four patients enrolled in this study responded to the lowest dose of erythropoietin, the minimal effective dose of erythropoietin alternating with hydroxyurea has yet to be defined. Controlled clinical trials will be required to judge the relative benefits of long-term combination therapy.
We are indebted to Ortho Biotech Pharmaceutical Corporation for its gift of recombinant human erythropoietin; to Bristol-Myers Squibb for the hydroxyurea used in this study; to Dr. Geraldine P. Schechter for her critical review of the manuscript; and to Mrs. La Netta Dietrichson for expert assistance in the preparation of the manuscript.
Source Information
From the Laboratory of Chemical Biology, National Institute of Diabetes and Digestive and Kidney Diseases (G.P.R., C.T.N., A.N.S.), and the Clinical Hematology Branch, National Heart, Lung, and Blood Institute (A.W.N.), National Institutes of Health, Bethesda, Md.; the Department of Pediatrics, Johns Hopkins Medical School, Baltimore (G.J.D.); and the Department of Physiology, Nippon Medical School, Tokyo, Japan (N.U.).
Address reprint requests to Dr. Rodgers at the Laboratory of Chemical Biology, National Institute of Diabetes and Digestive and Kidney Diseases, Bldg. 10, Rm. 9N-307, Bethesda, MD 20892.
- References
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