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Segal JB, Strouse JJ, Beach MC, et al. Hydroxyurea for the Treatment of Sickle Cell Disease. Rockville (MD): Agency for Healthcare Research and Quality (US); 2008 Feb. (Evidence Reports/Technology Assessments, No. 165.)

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Hydroxyurea for the Treatment of Sickle Cell Disease.

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Executive Summary


In February 1998, hydroxyurea was approved by the Food and Drug Administration (FDA) for use in adults with sickle cell disease. In 2002, The National Heart Lung and Blood Institute issued a recommendation that practitioners should consider using hydroxyurea daily in select patients with sickle cell disease. However, physicians are often non-adherent to practice guidelines and slow to change their practices in response to new data. To clarify the role of hydroxyurea in the treatment of patients with sickle cell disease and to improve physician adherence to guidelines regarding its use, the National Institutes of Health Office of Medical Applications of Research (OMAR) and the Agency for Healthcare Research and Quality (AHRQ) requested that the Evidence-based Practice Center (EPC) of the Bloomberg School of Public Health of the Johns Hopkins University prepare an evidence report. We were asked to address the following Key Questions:


What is the efficacy (results from controlled clinical studies) of hydroxyurea treatment for patients who have sickle cell disease?


What is the effectiveness (in everyday practice) of hydroxyurea treatment for patients who have sickle cell disease?


What are the short- and long-term harms of hydroxyurea treatment?


What are the barriers to the use of hydroxyurea treatment (and other therapies) for patients who have sickle cell disease and what are the potential solutions?


What are the future research needs?a

Sickle cell disease is a genetic disorder that decreases life expectancy by 25 to 30 years and affects approximately 80,000 Americans. Individuals are diagnosed with sickle cell disease if they have one of several genotypes that result in at least half of their hemoglobin being hemoglobin S (Hb S). Sickle cell anemia refers specifically to the condition associated with homozygosity for the Hb S mutation (Hb SS). Several other hemoglobin mutations, when occurring with an Hb S mutation, cause a similar but often milder disease than sickle cell anemia. In addition to reduced life expectancy, patients with sickle cell disease experience chronic pain and reduced quality of life. Painful crises, also known as vaso-occlusive crises, are the most common reason for emergency department use and hospitalization, and acute chest syndrome is the most common cause of death.

Prior to the approval of hydroxyurea for use in sickle cell disease, patients with this condition were treated only with supportive therapies. These measures included penicillin in children to prevent pneumococcal disease, routine immunizations, and hydration and narcotic therapy to treat painful events. Red blood cell transfusions increase the blood's oxygen carrying capacity and decrease the concentration of cells with abnormal hemoglobin, but chronic transfusion therapy predictably leads to iron overload and alloimmunization. Therapies such as hydroxyurea that raise fetal hemoglobin (Hb F, α2γ2) levels are promising because they effectively lower the concentration of Hb S within a cell, resulting in less polymerization of the abnormal hemoglobin.

Hydroxyurea's efficacy in sickle cell disease is generally attributed to its ability to raise the levels of Hb F in the blood; however, the mechanisms by which it does so are unclear. Early studies suggested that hydroxyurea is cytotoxic to the more rapidly dividing late erythroid precursors, resulting in the recruitment of early erythroid precursors with an increased capacity to produce Hb F. One recent study supports a nitric oxide-derived mechanism for the induction of Hb F by hydroxyurea, and another study suggests that ribonucleotide reductase inhibition is responsible for this increase in Hb F. Alternatively, hydroxyurea may be of benefit in sickle cell disease for reasons unrelated to Hb F production, including its ability to increase the water content of red blood cells, decrease the neutrophil count, and alter the adhesion of red blood cells to the endothelium.

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This review was conducted by a team from Johns Hopkins University with expertise in the management of sickle cell disease, clinical trial methodology (including clinical trials of hematological agents), systematic literature review, epidemiological studies, and ethics and adherence research. External technical experts, including academic and clinical experts and representatives of patients and public interest groups, provided input regarding the selection and refinement of the questions to be examined and the relevant literature to be considered. The core team worked with the technical experts, the OMAR Consensus Panel chairman, and the AHRQ to develop the Key Questions (see page 1). Literature inclusion criteria were tailored to each question, based on the availability and applicability of trial evidence and the relevance of other study designs.

In Key Questions 1 and 2, we addressed the efficacy and effectiveness of hydroxyurea in children and adults separately. Given the limited amount of evidence available from randomized controlled trials (RCTs), we also included non-randomized trials, cohort studies with a control population, and pre/post studies.

For Key Question 3, which addresses the toxicity of hydroxyurea, we reviewed studies (randomized and non-randomized, as well as observational studies) that addressed toxicities associated with this drug in patients with sickle cell disease. We also incorporated the findings of the experts at the Center for the Evaluation of Risks to Human Reproduction (CERHR); their detailed report, issued in 2007, reviewed toxicities in children and developing fetuses. We updated this information by including data from papers published since their report. In order to examine rare and long-term adverse effects, we also included observational studies, including case reports, together with indirect evidence from randomized trials, observational studies, case reports, and large cohorts of patients without sickle cell disease who had been treated with hydroxyurea.

For Key Question 4, we included information on barriers to the use of hydroxyurea, as well as those related to other therapies for the treatment of sickle cell disease. We included three types of studies encompassing a broad range of study designs: 1) studies that tested an intervention aimed at overcoming barriers to accessing scheduled care, receiving medication prescriptions, or adhering to medications; 2) studies in which patients or providers or family members described what they perceived to be barriers to accessing scheduled care, receiving medication prescriptions, or adhering to medications; and 3) studies that tested whether supposed barriers were actually associated with accessing scheduled care, receiving medication prescriptions, or adhering to medications.

Literature Sources

We searched for articles using both electronic and hand searching. In March 2007, we searched the MEDLINE® and EMBASE databases. We repeated the search in May 2007, adding a supplemental search targeting thrombocythemia. On June 30, 2007, the MEDLINE® and EMBASE® searches were updated and additional searches were executed using TOXLine and CINAHL. All searches were limited to English-language articles involving treatment of humans. Review articles were excluded from the searches. Searches were not limited by date of publication or subject age.

Eligibility Criteria

An article was included if it addressed one of the key questions. An article was excluded if it was (1) not written in English, (2) contained no original data, (3) involved animals only, (4) was solely a report of an in vitro experiment, or (5) was a case series. We excluded studies with fewer than 20 patients unless the article was primarily reporting on toxicities in sickle cell disease. We excluded trials involving other diseases if fewer than 20 patients received hydroxyurea. We allowed cohort studies of diseases other than sickle cell disease only if they described more than 100 patients treated with hydroxyurea. Although we excluded case series because they do not provide sufficient data about the effectiveness of a medication we included case reports if they had information regarding the dose of hydroxyurea and the duration of treatment that could be use to assess a causal relationship with potential toxic effects.

Quality Assessment

We graded the included studies on the basis of their quality with regard to reporting relevant data. For the RCTs, we used the scoring system developed by Jadad et al.b For the observational studies (both cohort studies and controlled clinical trials), we created a quality form, based on those previously used by our EPC, that was aimed at capturing data elements most relevant to study design. We designed questions to evaluate the potential for selection bias (three items) and to assess the potential for confounding (five items). For our assessment of the quality of the qualitative studies we reviewed, we developed a nine-item form to identify key elements that should be reported when describing results from qualitative research, including a description of the population and subjects and transparency of the data collection procedures. Similarly, to assess the quality of the surveys we included, we created an eight-item form assessing information about the survey methods, population, and validity and reliability of the instruments used. A pair of reviewers each performed the quality assessment independently. In the case of the RCTs, a third reviewer reconciled the results of the first two reviewers; for the other study designs, the results of the two reviewers were averaged. The overall score was the percentage of the maximum possible score, ranging from 0 to 100 percent. The results for RCTs were reported as 0 to 5 points. We considered high-quality studies to be those with a Jadad score of 4 or 5, or those receiving 80 percent or more of available quality points. However, no study was excluded from review on the basis of its quality score.

Data Extraction

We used a sequential review process in which the primary reviewer abstracted all the relevant data into abstraction forms, and a second reviewer checked the first reviewer's forms for completeness and accuracy. Reviewer pairs were formed to include personnel with both clinical and methodological expertise. Differences were resolved by discussion. We then created detailed evidence tables containing information extracted from the eligible studies.

Grading of the Evidence

At the completion of our review, we graded the quantity, quality, and consistency of the best available evidence addressing Key Questions 1, 2, and 3 by adapting an evidence grading scheme recommended by the GRADE Working Group and the EPC guide that is was under development at the time of the review. We applied evidence grades to the bodies of evidence about the efficacy and/or effectiveness of hydroxyurea for the treatment of sickle cell disease in one assessment. In terms of the strength of the study designs, we considered RCTs best, followed by non-randomized controlled trials and observational studies. We assessed the quality and consistency of the best available evidence, including an assessment of limitations to individual study quality (using individual quality scores), certainty regarding the directness of the observed effects in studies, precision and strength of findings, and availability (or not) of data to answer the Key Question. We classified evidence bodies pertaining to each Key Question as shown in Table 1. The evidence from case reports was graded according to the criteria of the World Health Organization (WHO) Collaborating Center for Drug Monitoring.

Table 1Summary of Evidence Relating to the Efficacy of Hydroxyurea in Sickle Cell Disease*

OutcomesEvidence GradeBasis for Grade
Key Question 1 and 2—Children
Increase in fetal hemoglobinHighOne good RCT, plus consistent observational studies
Reduction in pain crisesModerateOne good RCT; inconsistent observational studies
Reduction in hospitalizationsHighOne good RCT, plus consistent observational studies
Reduction in neurological eventsLowObservational studies
Reduction in transfusion frequencyInsufficientFew observational studies
Key Question 1 and 2—Adults
Increase in fetal hemoglobinHighOne good RCT, plus consistent observational studies
Reduction in pain crisesHighOne good RCT, plus consistent observational studies
Reduction in hospitalizationsHighOne good RCT, plus consistent observational studies
Reduction in neurological eventsInsufficientNo studies
Reduction in transfusion frequencyHighOne good RCT, plus consistent observational studies
MortalityLowInconsistent observational studies

Evidence grades: “high” (high confidence that the evidence reflects the true effect; further research is very unlikely to change our confidence in the estimate of effect); “moderate” (moderate confidence that the evidence reflects the true effect; further research may change our confidence in the estimate of effect and may change the estimate); “low” (low confidence that the evidence reflects the true effect; further research is likely to change the confidence in the estimate of effect and is likely to change the estimate); and “insufficient” (evidence either is unavailable or does not permit estimation of an effect); RCT=randomized controlled trial


Efficacy and Effectiveness of Hydroxyurea in Children

A single, small, placebo-controlled randomized trial of hydroxyurea for 6 months in Belgian children with sickle cell disease reported that the rate of hospitalization and number of days hospitalized per year were significantly lower in the hydroxyurea group (1.1 admissions, p=0.0016 and 7.1 days, p=0.0027) than in the placebo group (2.8 admissions and 23.4 days). Hb F% increased by an absolute 10.7 percent from baseline in the treated group (p<0.001).

Among the cohort studies, Hb F% was reported as an outcome in 17 studies. The mean pre-treatment Hb F% ranged from 5 to 10 percent, and the on-treatment values were in the range of 15 to 20 percent. The percentage of F cells was less frequently reported, but it increased from baseline in three of the four pediatric studies in which it was reported. Three of these cohort studies were retrospective; two reported increases in Hb F% comparable to those in the prospective studies. Hemoglobin concentrations increased modestly (roughly 1 gm/dL) but significantly across these studies.

The frequency of pain crises was reported as an outcome in five pediatric studies, with a reduction in frequency reported in three. In one retrospective cohort study in a resource-poor environment, the frequency of pain crises declined from a median of 3 per year to a median of 0.8 per year during treatment, with a median followup time of 24 months. Of note is the fact that these results were obtained using a fixed dose of hydroxyurea (15 mg/kg/day). A small, high-quality prospective study found a decrease in pain events, from 3.1 per year in the year prior to hydroxyurea therapy to 1.2 per year during the 18 months of therapy. Hospitalization rates decreased in all four studies describing this outcome. In the retrospective study described above, the hospitalization rates decreased to 0.5 per year during treatment, from a baseline rate of 4 per year. Within the Belgian Registry, hospitalization rates declined to 1.1 per patient-year during the third year of treatment, from 3.2 per patient-year.

One study assessed the impact of hydroxyurea on secondary stroke prevention by enrolling 35 children who needed to discontinue their chronic transfusion protocol. The average hydroxyurea dose was 27 mg/kg/day, and the children were treated for a mean of 42 months. The rate of recurrent ischemic events was 5.7 per 100 patient-years, which is better than was seen in another study in which children discontinued transfusions without starting hydroxyurea. One other study reported that brain images by magnetic resonance imaging (MRI) were stable during the course of treatment in 24 of 25 children. In the Belgian Registry, during 426 patient-years of hydroxyurea treatment, the rate of central nervous system events (stroke or transient ischemic attacks) was 1.3 per 100 patient-years, but no comparison rate was provided.

Based on one randomized trial in children and many observational studies, some of which were high-quality and most of which were consistent in their findings, we graded the evidence as shown in Table 1.

Efficacy and Effectiveness of Hydroxyurea in Adults

Only one randomized trial, the Multicenter Study of Hydroxyurea for Sickle Cell Anemia (MSH Study), tested the efficacy of hydroxyurea in adults with sickle cell anemia, with six additional analyses either based on this trial or on followup studies. The significant hematological effects of hydroxyurea after 2 years (as compared to the placebo arm) included a small mean increase of 0.6 g/dl in total hemoglobin and a moderate absolute increase in fetal hemoglobin of 3.2 percent. The median number of painful crises was 44 percent lower, and the time to the first painful crisis was 3 months, as compared to 1.5 months in the placebo arm. There were fewer episodes of acute chest syndrome and transfusions, but no significant differences in deaths, strokes, chronic transfusion, or hepatic sequestration. Use of hydroxyurea had no significant effect on annualized costs. It improved the quality of life, but only in those patients who experienced a substantial increase in Hb F%.

In all six prospective cohort studies in adults that reported hematological outcomes, Hb F% increased significantly. The mean baseline Hb F% ranged from 4 percent to 12 percent, and during hydroxyurea treatment, it ranged from 10 percent to 23 percent. As in the pediatric studies, there was a small increase in hemoglobin in most studies. The single retrospective study reported hematological outcomes comparable to those seen in the prospective studies. The number of pain crises was described in three studies. In a study of Sicilians with Hb Sβ thalassemia, the frequency of crises decreased significantly, from a mean of 7 (median of 9) per year to a mean of 1.1 (median 1.8) per year. In the non-randomized study comparing patients receiving hydroxyurea to those receiving cognitive behavioral therapy, those receiving hydroxyurea had fewer pain crises (1.4 per year compared to 4.3 per year, p≤0.05) but this was not a strong study design for assessing such an outcome. Similarly, hospitalization rates decreased consistently in adults treated with hydroxyurea. In the study of Sicilians, the number of hospitalized days per year declined from 22.4 days to 1.2 days (SD =2.3) (p<0.0001). In a retrospective effectiveness study, the rates of hospitalization declined from baseline in the group treated for longer than 24 months (2.1 per year from 3.1 per year, p=0.04). However, in the group treated for fewer than 24 months, the hospitalization rates were not significantly different from baseline values.

Based on one high-quality randomized trial in adults and many consistent observational studies, we graded the evidence as shown in Table 1.

Toxicities of Hydroxyurea in Children and Adults

Our assessment of the strength of the evidence regarding the toxicity of hydroxyurea, when used in children, was generally derived from our review of the report by the panel of experts that had been assembled by the National Toxicology Program (NTP)'s Center for the Evaluation of Risks to Human Reproduction (CERHR). The panel reviewed articles, published through January 2007, that pertained to the evaluation of adverse effects of hydroxyurea on development and reproduction in both humans and animals. Their review was not restricted to the use of hydroxyurea for sickle cell disease. The dosing of hydroxyurea for sickle cell disease is comparable to that in other diseases, although in the case of malignant disease, more drug is often given less frequently (such as 80 mg/kg every 3 days rather than 15–20 mg/kg daily).

The panel concluded that treatment of children aged 5 to15 years with hydroxyurea does not cause a growth delay. They felt there were insufficient data to allow them to evaluate the effects of hydroxyurea on pubertal development. The panel found no data regarding the effects on subsequent generations after exposure of germ cells to hydroxyurea, including exposure during fetal life, infancy, childhood, and adolescence. The CERHR report did not describe any studies on the long-term health effects, including carcinogenicity, of childhood exposure to hydroxyurea; we also found no such studies. The expert panel had concerns about the adverse effect of hydroxyurea on spermatogenesis in men receiving hydroxyurea at therapeutic doses; we also identified case reports of impaired spermatogenesis after hydroxyurea treatment in patients with sickle cell disease, as well as in patients with other illnesses. The CERHR report concluded that the use of hydroxyurea in pregnancy was not associated with adverse perinatal outcomes, but that there were no data on long-term outcomes in children who were exposed in utero. However, the panel expressed concern, based on minimal data from experimental studies, that hydroxyurea might increase the risk of congenital anomalies or abnormalities of fetal growth after exposure of pregnant women to the drug.

We found three cases of leukemia, described in observational studies, in patients with sickle cell disease who had been treated with hydroxyurea. We identified another three case reports of hydroxyurea-treated patients with sickle cell disease who developed leukemia, and one report of a child who developed Hodgkin's lymphoma. Toxicities in patients with sickle cell disease that are probably causally related to hydroxyurea are neutropenia, skin rashes, and nail changes.

We reviewed toxicity reports from hydroxyurea-treated patients with other illnesses and found many reports of leg ulcers and skin cancers. Among the randomized trials enrolling patients with other diseases, no trial demonstrated a greater number of cases of leukemia in the group treated with hydroxyurea. This parameter could not be assessed in the trials enrolling patients with chronic myelogenous leukemia (CML), as progression to acute leukemia was considered a poor response to the intervention and could not be considered a toxicity of treatment. We reviewed a case series of 26 patients with acute myelogenous leukemia (AML) with a unique t (3;21) chromosomal translocation. Among these 26 patients were 15 people with CML who had been treated with hydroxyurea. We found no other reports describing an association between this translocation and hydroxyurea.

We concluded that low-grade evidence suggested that hydroxyurea treatment in adults with sickle cell disease is not associated with an increased risk of leukemia. (Table 2)

Table 2Summary of Evidence About the Toxicity of Hydroxyurea in Sickle Cell Disease*

OutcomesEvidence GradeBasis for Grade
Key Question 3—Children
Leukemia (MDS/AML/Cytogenetic abnormalities)InsufficientCERHR report
Developmental toxicities (in utero)Evidence of harm in animalsCERHR report
Leg ulcersInsufficientCERHR report
Growth delaysEvidence of no growth delayCERHR report
Developmental toxicities in next generationInsufficientCERHR report
Key Question 3—Adults
Leukemia (MDS/AML/Cytogenetic abnormalitiesLowIndirect evidence and inconsistent results
Leg ulcersHigh (absence of effect)One good RCT, plus consistent observational studies
Skin neoplasmsInsufficientNo studies in sickle cell; high-grade evidence in other populations
Secondary malignanciesInsufficientNo studies in sickle cell; low-grade evidence in other populations
Adverse pregnancy outcomesInsufficientCEHER report
Spermatogenesis defectsLowCase reports with evidence of causality

Evidence grades as on Table 1;

MDS = myelodysplastic syndromes; AML = acute myelogenous leukemia; CEHER = Center for the Evaluation of Risks to Human Reproduction.

High-grade evidence supported the assertion that hydroxyurea is not associated with leg ulcer development in patients with sickle cell disease, although high-grade evidence indicated that it is associated with leg ulcers in patients with other conditions. The evidence was insufficient in sickle cell disease to indicate whether hydroxyurea contributes to skin neoplasms, although high-grade evidence supported its involvement in patients with other illnesses. Similarly, there was insufficient evidence to establish whether hydroxyurea is associated with secondary malignancies in adults with sickle cell disease; the evidence in other diseases was only low-grade.

Barriers to the Use of Hydroxyurea and Other Treatments for Managing Sickle Cell Disease

Only two studies (one in patients and one in providers) investigated barriers to use of hydroxyurea; both used survey data. The study involving patients used a cross-sectional design and showed that the perceived efficacy and safety of hydroxyurea had the strongest association with patients' (or parents') choice of hydroxyurea therapy over other therapies. In the study of clinicians, the reported barriers to use of hydroxyurea for sickle cell disease included patient concerns about side effects and a variety of clinician concerns: the appropriateness of using hydroxyurea in older patients, patient compliance, a lack of contraception in premenopausal women, side effects and carcinogenic potential, doubts about effectiveness, and costs to patients.

We reviewed an additional 47 studies addressing barriers to the treatment of patients with sickle cell disease and interventions to overcome these barriers. In our review of barriers to adequate pain management, we found two factors that were identified as a barrier in more than two studies: negative provider attitudes and poor provider knowledge. Because of the quantity and consistency of these findings, we concluded that the evidence was high-grade that negative provider attitudes are barriers and moderate-grade that poor provider knowledge is a barrier to the use of pain medications in patients with sickle cell disease. The evidence for the remaining barriers to pain management was insufficient to allow us to draw any conclusions.

In our review of the barriers to other therapies for chronic sickle cell disease management, we concluded that the evidence was of a moderate grade that patient sex is not a barrier to use of therapies. Largely because of the paucity and inconsistency of the studies, we concluded that there was only low-grade evidence that patient/family knowledge, the number of hospital visits, and patient age are barriers to the use of therapies.

We identified three studies that tested interventions to improve patient adherence to established therapies for chronic disease management, but none of these three showed any effect on patient adherence. However, given the small sample sizes and the studies' diverse outcome measures, we concluded that there was only low-grade evidence that interventions did not improve patient adherence. In contrast, we identified nine studies that examined the impact of interventions to improve pain management during vaso-occlusive crises, and we concluded that there was moderate evidence that interventions can overcome barriers to the use of pain medications. We also identified one study that investigated the impact of an intervention to improve receipt of routine healthcare and, partly because of the strength of the effect found in the study, we concluded that there is moderate evidence to indicate that interventions can overcome barriers to the receipt of routine, scheduled healthcare for patients with sickle cell disease.

We found it informative that when researchers chose the barriers to investigate, they most often studied patient-related barriers. When patients were asked to identify barriers to the use of therapies, they most often cited provider-related barriers. The barrier to pain management that was most often identified by patients and providers was negative provider attitudes. However, only one of the nine pain management intervention studies addressed this issue directly through provider sensitivity training.

Limitations of the Evidence

The evidence base described here had significant limitations. Most notably, only two randomized trials addressed hydroxyurea efficacy and safety in patients with sickle cell disease. While the trial enrolling adults was a high-quality trial, it was not long, with only 2 years elapsing since randomization. Two years may be adequate for assessing short-term efficacy, but we had no trial data that made it possible to comment on the long-term efficacy of the drug. We also found no trial data to allow us to assess the effectiveness of this drug in a population who may be asked to take the medication for many years with less intense supervision and encouragement than is received in a trial. The trial conducted in children was a moderate-quality trial, but it was even shorter than the trial in adults, with only 6 months of treatment. Thus, this evidence base is limited by a lack of long-term effectiveness trials, even though the MSH trial may be considered a definitive trial of the short-term efficacy of the drug in adults. In addition, these trial results cannot be generalized to all patients with sickle cell disease, since the trials included only patients with Hb SS; clinical response and toxicities are known to differ to some extent according to genotype.

The most frequently reported outcomes in the observational studies were hematological. The data convincingly demonstrated an increase in Hb F% with the use of this drug; however, there was far less evidence regarding the clinically relevant outcomes of hospitalization, stroke, pain crises, acute chest syndrome, and mortality. Furthermore, observational data may be plagued with issues of regression to the mean. If patients were started on hydroxyurea after a period of increased frequency of disease symptoms, it is expected that they would, in time, return to their usual disease severity, even without a change in therapy. This is a major concern in interpreting the pre/post data from many of these observational studies reporting clinical outcomes.

The evidence was scant regarding benefits for patients with genotypes other than Hb SS. Similarly, there was limited evidence about the use of doses other than the maximally tolerated dose (MTD). Also, there was little evidence to guide dosing based on clinical outcomes.

The evidence regarding toxicities had limitations as well. The relatively short clinical trials we found could not provide strong evidence for toxicities that may require many years of exposure to develop. The follow-up studies from these trials are important contributors to the literature, but they became observational studies after the period of randomization ended, and are thus subject to the limitations of any observational study. The losses to followup were substantial in the majority of the observational studies. Very few studies required active surveillance for toxicities, such as periodic skin examination or cytogenetic studies, with notable exceptions. The studies of toxicities suffered from a lack of control groups; for example, studies that describe impaired spermatogenesis would require a control of group of comparably ill men with sickle cell disease in order to determine whether this symptom was disease- or treatment-related.

In reviewing the evidence, we opted to include toxicity data from patients treated with hydroxyurea for conditions other than sickle cell disease. This approach provided only indirect evidence of toxicity, in that the patient populations were markedly different than patients with sickle cell disease.

Our investigation of barriers to the use of hydroxyurea was limited by the paucity of data regarding this question. Since there were only two studies specifically addressing barriers to the use of hydroxyurea, we needed to bring in supporting evidence from interventions that might have exhibited barriers comparable to those associated with hydroxyurea treatment. The majority of the potential barriers considered in the cross-sectional studies (i.e., those chosen by the researcher) were patient-related factors, which suggested a lack of attention to provider and societal-level contributions. Very few of these studies included adult patients. Only half of the cross-sectional studies used multivariate techniques to adjust for the effects of potential confounders, an omission that limited the value of these studies. Another concern was that many of the intervention studies used indirect outcomes, such as length of stay or total hospital costs, to assess improvement in pain management; these are not the best outcome measures for this question.

Future Research Needs

Several placebo-controlled trials in progress are expected to address some of the research gaps that remain: BABY-HUG is examining the safety and effectiveness of hydroxyurea in infants (results expected in late 2009), and the Stroke With Transfusion Changing to Hydroxyurea (SWiTCH) trial is examining hydroxyurea use for secondary prevention of stroke in patients with sickle cell disease. However, there is still a substantial need for research on the use of this drug.

The paucity of randomized trials suggests that additional randomized trials with other clinical outcomes may be appropriate, including trials that are aimed at preventing or treating other complications of sickle cell disease, including kidney disease, pulmonary hypertension, neurological events in adults, and psychiatric complications. Also, effectiveness trials are needed to assess the use of hydroxyurea in a regular care setting. These could be (1) clustered randomized trials in which some providers are randomized to use hydroxyurea in all patients and others are randomized to usual care, including the use of hydroxyurea when clinically indicated; or (2) effectiveness studies, in which one group of providers is actively encouraged to consider hydroxyurea when appropriate and another clinic is not targeted for education.

Longer studies are needed to assess the potential toxicities of this drug, particularly given its uncertain mechanisms of action. This would include studies in which patients are treated for longer periods of time, as well as studies in which patients are followed for longer periods of time after treatment is discontinued. This need is most relevant to outcomes with a long latency period, such as leukemia and secondary malignancies, including skin cancers. Randomized trials are not feasible for long periods, so a well-designed prospective study may be the optimal design. A registry of users of hydroxyurea could also be considered if the data collection and followup can be sufficiently rigorous and ongoing. Other toxicities requiring further study are the developmental toxicities and risk to subsequent generations that are described in detail in the CERHR report.

Many subgroups require further study, particularly patients with genotypes other than Hb SS. While there have been observational studies of patients with other genotypes, the randomized trials enrolled only patients with Hb SS. Patients with Hb SC are particularly understudied. Additional studies of hydroxyurea at doses other than the MTD are appropriate, particularly since the use of the MTD in resource-poor populations may be impractical. Effectiveness studies of hydroxyurea in resource-poor populations would be particularly beneficial. Other subgroups of interest are patients with comorbid illnesses, specifically HIV/AIDS and/or hepatitis C. More information is needed about the interactions between hydroxyurea and these underlying diseases, and between hydroxyurea and therapies for these diseases. Further research on the place of hydroxyurea in therapy and its comparative effectiveness is also indicated, since the existing studies have not defined the optimal time for initiation of hydroxyurea or identified the indicators that a patient has “failed” therapy with the drug. Other questions remain: Is there a role for rechallenge with the drug if there was no previous efficacy? Is there a role for hydroxyurea as an adjunctive therapy with other drugs? What are the best intermediate outcomes that will predict clinical response to the drug? Given the strong evidence that hydroxyurea reduces the frequency of pain and hospitalization in children and adults with sickle cell disease, some have questioned whether additional placebo-controlled trials of hydroxyurea are ethical. We suggest that additional trials are ethical in understudied subgroups (e.g., patients with genotypes other than Hb SS), and in the evaluation of hydroxyurea for other indications (e.g., treatment of mild pulmonary hypertension or secondary prevention of stroke in adults).

Given that we have concluded that evidence supports the short-term efficacy of hydroxyurea in sickle cell disease, there is clearly a need for further research on the barriers to the use of this drug. These studies should aim to identify barriers at the level of the patient, at the level of the provider, and at a societal level, perhaps with special attention to adult patients. After these barriers are better characterized, interventions to overcome these barriers should be tested, including replication of the one promising study that demonstrated improved receipt of routine care in patients with sickle cell disease. The barriers and interventions that we identified as influencing the use of other treatments in sickle cell disease may provide an appropriate starting point for further study. Comparative effectiveness studies may be appropriate as well, in particular for testing established interventions for improving pain control.

The JHU EPC was not charged with conducting a separate review for Key Question 5 in the original task order; this question is addressed in the “Discussion” section of the report.

Jadad AR, Moore RA, Carroll D, et al. Assessing the quality of reports of randomized clinical trials: is blinding necessary? Control Clin Trials 1996;17(1 ):1–12.



The JHU EPC was not charged with conducting a separate review for Key Question 5 in the original task order; this question is addressed in the “Discussion” section of the report.


Jadad AR, Moore RA, Carroll D, et al. Assessing the quality of reports of randomized clinical trials: is blinding necessary? Control Clin Trials 1996;17(1 ):1–12.

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