Hydroxyurea (hydroxycarbamide) for sickle cell disease

Sarah J Nevitt; Ashley P Jones; Jo Howard

doi:10.1002/14651858.CD002202.pub2

. 2017 Apr 20;2017(4):CD002202. doi: 10.1002/14651858.CD002202.pub2

Hydroxyurea (hydroxycarbamide) for sickle cell disease

Sarah J Nevitt ^1,^✉, Ashley P Jones ¹, Jo Howard ²

Editor: Cochrane Cystic Fibrosis and Genetic Disorders Group

PMCID: PMC6478259 PMID: 28426137

Abstract

Background

Sickle cell disease (SCD) is one of the most common inherited diseases worldwide. It is associated with lifelong morbidity and a reduced life expectancy. Hydroxyurea (hydroxycarbamide), an oral chemotherapeutic drug, ameliorates some of the clinical problems of SCD, in particular that of pain, by raising fetal haemoglobin. This is an update of a previously published Cochrane Review.

Objectives

To assess the effects of hydroxyurea therapy in people with SCD (all genotypes), of any age, regardless of setting.

Search methods

We searched the Cochrane Cystic Fibrosis and Genetic Disorders Group Haemoglobinopathies Register, comprising of references identified from comprehensive electronic database searches and handsearches of relevant journals and abstract books of conference proceedings. We also searched online trial registries.

Date of the most recent search: 16 January 2017.

Selection criteria

Randomised and quasi‐randomised controlled trials, of one month or longer, comparing hydroxyurea with placebo, standard therapy or other interventions for people with SCD.

Data collection and analysis

Authors independently assessed studies for inclusion, carried out data extraction and assessed the risk of bias.

Main results

Seventeen studies were identified in the searches; eight randomised controlled trials were included, recruiting 899 adults and children with SCD (haemoglobin SS (HbSS), haemoglobin SC (HbSC) or haemoglobin Sβºthalassaemia (HbSβºthal) genotypes). Studies lasted from six to 30 months.

Four studies (577 adults and children with HbSS or HbSβºthal) compared hydroxyurea to placebo; three recruited individuals with only severe disease and one recruited individuals with all disease severities. There were statistically significant improvements in terms of pain alteration (using measures such as pain crisis frequency, duration, intensity, hospital admissions and opoid use), measures of fetal haemoglobin and neutrophil counts and fewer occurrences of acute chest syndrome and blood transfusions in the hydroxyurea groups. There were no consistent statistically significant differences in terms of quality of life and adverse events (including serious or life‐threatening events). Seven deaths occurred during the studies, but the rates by treatment group were not statistically significantly different.

Two studies (254 children with HbSS or HbSβºthal also with risk of primary or secondary stroke) compared hydroxyurea and phlebotomy to transfusion and chelation; there were statistically significant improvements in terms of measures of fetal haemoglobin and neutrophil counts, but more occurrences of acute chest syndrome and infections in the hydroxyurea and phlebotomy group. There were no consistent statistically significant differences in terms of pain alteration and adverse events (including serious or life‐threatening events). Two deaths occurred during the studies (one in a the hydroxyurea treatment arm and one in the control arm), but the rates by treatment group were not statistically significantly different. In the primary prevention study, no strokes occurred in either treatment group but in the secondary prevention study, seven strokes occurred in the hydroxyurea and phlebotomy group (none in the transfusion and chelation group) and the study was terminated early.

The quality of the evidence for the above two comparisons was judged as moderate to low as the studies contributing to these comparisons were mostly large and well designed (and at low risk of bias); however evidence was limited and imprecise for some outcomes such as quality of life, deaths during the studies and adverse events and results are applicable only to individuals with HbSS and HbSβºthal genotypes.

Of the remaining two studies, one (22 children with HbSS or HbSβºthal also at risk of stoke) compared hydroxyurea to observation; there were statistically significant improvements in terms of measures of fetal haemoglobin and neutrophil counts but no statistically significant differences in terms of adverse events (including serious or life‐threatening events).

The final study (44 adults and children with HbSC) compared treatment regimens with and without hydroxyurea – there was statistically significant improvement in terms of measures of fetal haemoglobin, but no statistically significant differences in terms of adverse events (including serious or life‐threatening events). No participants died in either of these studies and other outcomes relevant to the review were not reported.

The quality of the evidence for the above two comparisons was judged to be very low due to the limited number of participants, the lack of statistical power (as both studies were terminated early with approximately only 20% of their target sample size recruited) and the lack of applicability to all age groups and genotypes.

Authors' conclusions

There is evidence to suggest that hydroxyurea is effective in decreasing the frequency of pain episodes and other acute complications in adults and children with sickle cell anaemia of HbSS or HbSβºthal genotypes and in preventing life‐threatening neurological events in those with sickle cell anaemia at risk of primary stroke by maintaining transcranial doppler velocities. However, there is still insufficient evidence on the long‐term benefits of hydroxyurea, particularly in preventing chronic complications of SCD, recommending a standard dose or dose escalation to maximum tolerated dose. There is also insufficient evidence about the long‐term risks of hydroxyurea, including its effects on fertility and reproduction. Evidence is also limited on the effects of hydroxyurea on individuals with HbSC genotype. Future studies should be designed to address such uncertainties.

Plain language summary

Hydroxyurea (also known as hydroxycarbamide) for people with sickle cell disease

Review question

What is the effect of hydroxyurea on clinical outcomes (changes in pain crises, life‐threatening illnesses, survival, haemoglobin levels, quality of life and side effects) in people with sickle cell disease (SCD) of any genotype?

Background

SCD is an inherited genetic disorder that creates problems with haemoglobin (the substance in red blood cells that carries oxygen around the body). The disease can be inherited in different ways; people can inherit two sickle genes (HbSS genotype) or they can inherit the sickle gene from one parent and a different haemoglobin gene (such as haemoglobin C (HbSC genotype) or a beta thalassaemia gene (HbSβ+ or HbSβºthal genotype)) from the second parent.

In people with SCD the abnormal sickle haemoglobin forms long polymers (chains) within the red blood cells when they become de‐oxygenated. This damages the red blood cells and makes them stickier, leading to blockages and reduced blood flow, causing pain and organ damage. Fetal haemoglobin stops these polymers forming in the sickle haemoglobin within the red blood cell. The drug hydroxyurea is used to raise fetal haemoglobin and can reduce the effects of the disease. This is an update of a previously published Cochrane Review.

Search date

The evidence is current to 16 January 2017.

Study characteristics

We included eight studies (899 adults and children with SCD (HbSS, HbSC or HbSβºthal genotypes)). Studies lasted from six to 30 months.

Key results and quality of the evidence

In four studies, 577 adults and children with SCD were randomly selected to receive hydroxyurea or placebo. In two studies, 254 children with SCD, who were also at an increased risk of having a first or second stroke, were randomly selected to receive hydroxyurea and phlebotomy (collection of blood) or blood transfusion and chelation (administration of agents to remove excess iron from the body). These six studies only recruited people with HbSS or HbSβºthal genotypes so results do not apply to people with the HbSC genotype.

There was moderate quality evidence from these six studies that those receiving hydroxyurea experienced significant reductions in the frequency of pain crises, increases in fetal haemoglobin and decreases in neutrophil (white blood cell) counts compared to the comparator treatment. There was no difference between people receiving hydroxyurea or other treatments in terms of quality of life, deaths during the studies and side effects (including serious and life‐threatening side effects); however, there is less information about these outcomes in the studies, so the quality of this evidence is low.

Two further studies were included in the review. In one study, 22 children with SCD, who were also at an increased risk of having a stroke, were randomly selected to receive hydroxyurea or no treatment (observation only) and in one study 44 adults and children were randomly selected to receive treatments with or without adding hydroxyurea. Both studies showed an increase in fetal haemoglobin for people receiving hydroxyurea compared to the comparator treatment and there were no deaths during the studies. There was no difference between people receiving hydroxyurea or other treatments in terms of pain crises and side effects (including serious or life‐threatening side effects) and these studies did not measure quality of life. The quality of the evidence from these studies is very low, given the studies were very small and only recruited around 20% of the intended number of people and results do not apply to all people with SCD (different genotypes).

Conclusions

The evidence shows that hydroxyurea is likely to be effective in the short term at decreasing the frequency of painful episodes and raising fetal haemoglobin levels in the blood in people with SCD. Hydroxyurea is also likely to be effective in preventing first strokes for those at an increased risk of stroke and does not seem to be associated with an increase in any side effects (including serious and life‐threatening side effects).

There is currently not much evidence on whether hydroxyurea is beneficial over a long period of time, what the best dose to take is, or whether treatment causes any long‐term or serious side effects. More studies are needed to answer these questions.

Summary of findings

Background

Description of the condition

Haemoglobinopathies, including sickle cell disease (SCD), are among the most common inherited disorders in the world. SCD affects people originating from sub‐Saharan Africa, Arab countries, the Mediterranean, the Indian subcontinent, the Caribbean and South America, as well as African‐Americans and descendants from immigrants from the above countries in other parts of the world. The genetic mutation gives carriers some advantage against malaria and for this reason has persisted. There is an estimated global birth rate of around 300,000 affected individuals per year, the majority of which are in Africa (Piel 2013). There are an estimated 90,000 to 100,000 individuals with SCD in the USA and approximately 12,000 to 15,000 in the UK (Brousseau 2010; Hassell 2010; NICE 2012).

Haemoglobin is responsible for transporting oxygen around the body packaged in red blood cells. SCD is caused by the recessive inheritance of abnormal haemoglobins. People who inherit only one sickle gene and the normal gene for adult haemoglobin (HbA) are sickle cell carriers (sickle cell trait or AS) and are healthy. When people are homozygous, because they have inherited two sickle genes (SS), they have sickle cell anaemia, which is a variable clinical condition, with the vast majority of individuals suffering some pain attacks and a reduced life expectancy. Clinically significant SCD also arises when people inherit the sickle gene from one parent and another variant haemoglobin gene from the second parent such as haemoglobin C (SC) or a beta thalassaemia gene (Sβ+ or Sβ0). Sickle haemoglobin, when not carrying oxygen, polymerises distorting the red blood cell into the classic 'sickle' shape. The clinical problems arise predominantly as a result of chronic anaemia and the blockage of small blood vessels, which stops oxygen delivery to the tissues causing pain or organ damage or both.

The most frequent clinical problem is pain which causes over 90% of acute hospital admissions (Brozovic 1987) and significant morbidity in the community (Fuggle 1996). Other acute complications include acute chest syndrome, stroke, splenic sequestration, priapism and an increased risk of infection. SCD is a multi‐organ disease with a high chronic disease burden which increases with increasing age. The majority of individuals are anaemic with haemoglobin levels of 60 to 90g/L in HbSS. Common chronic complications include chronic sickle lung damage, pulmonary hypertension, renal dysfunction, chronic bony damage (avascular necrosis), retinopathy and leg ulceration (Howard 2015).

Whilst the majority of those affected used to die in childhood, and still do within parts of the developing world (Chakravorty 2015; Grosse 2011), death rates in children in higher‐income countries have fallen over the past decades and the majority of children (over 95%) now survive to adulthood (Quinn 2010; Telfer 2007; van der Plas 2011). Data from the USA reviewing national mortality data from 1979 to 2005 has confirmed this decrease in mortality in children, but showed an increased mortality rate in adults with a median age of death of 42 years for women and 38 years for men (Lanzkron 2013). Earlier data from the USA showed a median survival for adults with HbSS in the USA of 42 years in men and 46 years in women and in HbSC of 60 years and 68 years, respectively (Platt 1994). This contrasts with Jamaican figures which show median survival for HbSS of 53 years for men and 58.5 years for women (Wierenga 2001) and recent single‐centre data from the UK has shown median survival of 67 years in people with HbSS and higher in those with HbSC (Gardner 2016). Data from the USA has also shown an increase in deaths following transition from paediatric to adult services (Quinn 2010). Death is generally SCD‐related and is caused either by chronic organ failure consequent to the sickling process (e.g. renal failure, pulmonary hypertension, hepatic failure) (Manci 2003) or as a result of an acute catastrophic event, such as stroke (Powars 1983), acute sickle chest syndrome, splenic sequestration (Rogers 1978), sepsis (Manci 2003; van der Plas 2011) or other complications (Gray 1991; Lanzkron 2013; Perronne 2002). The UK National Confidential Enquiry into Patient Outcome and Death (NCEPOD) report reviewed UK mortality data over 48 months and found six paediatric and 40 adult deaths. One paediatric death was due to pneumococcal sepsis and two were due to sub‐arachnoid haemorrhage. In adults the main causes of death were stroke, multi‐organ failure, acute chest syndrome, renal failure and non‐sickle causes (NCEPOD 2008).

Improvements in paediatric outcomes are related to the introduction of neonatal screening, early enrolment in comprehensive paediatric care, penicillin prophylaxis, vaccination to decrease life‐threatening infection and primary stroke prevention with transcranial Doppler screening. In adults, treatment has relied on the avoidance of factors that precipitate crisis (including dehydration, infection and cold), and symptomatic treatment of the acute painful episodes (Davies 1997a; Davies 1997b; Steinberg 1999; STOP 2006). Hydroxyurea is currently the only licensed treatment for SCD. Blood transfusion is often required and may be used to treat acute complications or in the long term to treat or prevent disease complications. Haemopoeitic stem cell transplantation is the only currently available curative treatment option and is offered to children with severe disease phenotype and an human leukocyte antigen (HLA)‐matched sibling donor. Other donor transplant options and adult transplant are currently only available in the context of clinical trials. Gene therapy offers another potential curative treatment and is currently being investigated in clinical trials.

Description of the intervention

Hydroxyurea (also known as hydroxycarbamide) is an anti‐neoplastic oral drug and an inhibitor of ribonucleotide reductase (BABY HUG 2011). The drug has been shown to have many beneficial effects for treating SCD; including increasing fetal haemoglobin (HbF) concentration in red blood cells, improving nitric oxide metabolism, reducing red cell‐endothelial interaction and erythrocyte density (Ware 2010). Such disease‐modifying effects have been shown to decrease episodes of pain, acute chest syndrome, hospital admissions and the need for transfusions among people with SCD (MSH 1995; Strouse 2008); however, frequent monitoring of the person's blood count is required, in addition to monitoring of toxicity. There are recognised, but mostly reversible, side effects of hydroxyurea, such as low neutrophil count, low platelet count, anaemia, rash, headache and occasionally nausea. Furthermore, it may have teratogenic effects and may have an effect on male fertility (Strouse 2008;Zimmerman 2004). Long‐term or serious adverse effects (or both) of hydroxyurea are rare (Steinberg 2010; Strouse 2008) and observational data suggest a survival advantage for those treated with hydroxyurea (Steinberg 2010; Voskaridou 2010).

How the intervention might work

It has long been recognised that raised HbF levels can ameliorate the clinical effects of SCD (Perrine 1978; Platt 1994). HbF levels are high at birth and decrease over the first year of life and and hence clinical manifestations are often delayed until the HbF levels decrease. In addition, individuals who inherit high levels of HbF display a milder disease phenotype. This is because the HbF interferes with the polymer formation of the sickle haemoglobin within the red blood cell. This polymerisation is the underlying pathology in SCD. The more HbF there is, the greater the inhibition. Hydroxyurea was first shown to raise HbF levels in SCD in the 1980s (Platt 1984; Veith 1985). Its intermittent toxicity on the bone marrow leads to a stress response and enhanced erythropoiesis and levels of HbF. In addition to its effects on SCD via HbF enhancement, hydroxyurea also improves blood flow and reduces vaso‐occlusion via other mechanisms including decrease of adhesion molecules and stimulation of nitric oxide production (Green 2014).

Why it is important to do this review

It is important to examine as a whole, the body of work relating to the use of hydroxyurea in SCD, to evaluate the drug's effectiveness and tolerability in adults and children, and in the different types of SCD, the dosage regimens and whether the setting appears to influence the outcome, i.e. high‐income versus low‐ and middle‐income countries. We aim to review any evidence relating to the impact of hydroxyurea on the natural history of SCD and life expectancy.

Objectives

The aims of this review are to determine whether the use of hydroxyurea in people with SCD:

alters the pattern of acute events, including pain;
prevents, delays or reverses organ dysfunction;
alters mortality and quality of life;
is associated with adverse effects.

In addition we hoped to assess whether the response to hydroxyurea in SCD varies with type of SCD, age of the individual, duration and dose of treatment and healthcare setting.

Methods

Criteria for considering studies for this review

Types of studies

All randomised or quasi‐randomised studies, irrespective of language. Studies with quasi‐randomised methods, such as alternation, were included if there was sufficient evidence that the treatment and control groups were similar at baseline.

Types of participants

This review is limited to studies of hydroxyurea in SCD only. In order to fully quantify the potential harm and toxicity of this drug, a further review may need to be undertaken in all patient groups treated with hydroxyurea.

People of any age with SCD (SS, Sβ₀, SC, Sβ+) proven by electrophoresis and sickle solubility test, with family studies or DNA tests as appropriate.

Types of interventions

Hydroxyurea in any formulation at all doses, compared to either placebo or standard treatment (no placebo) for periods of one month or longer.

Types of outcome measures

Primary outcomes

Pain alteration
1. frequency, duration, severity measured on self‐reported patient scales
2. health service utilisation (e.g. inpatient days, outpatient or accident and emergency department visits)
3. opioid use
Life‐threatening illness (e.g. acute chest syndrome, stroke and acute splenic sequestration)*
Death during the study

* In the 2017 update of the review, serious adverse events reported in included studies (whether treatment‐related or not) were included under the definition of 'Life‐threatening illness'.

Secondary outcomes

Measures of fetal haemoglobin (HbF or F cells) and neutrophil counts
Other surrogate markers of response (e.g. haemoglobin, mean cell volume, platelet count and growth)
Quality of life, time loss to school or employment, integration into society, scales recording feeling of well‐being and global function (e.g. Karnofsky)
Measures of organ damage (e.g. spleen (pitted red cells), chronic sickle lung disease (transfer factor), liver, chronic renal failure (creatinine), priapism, leg ulcer, neurological damage (e.g. intelligence quotient (IQ)))
Any reported adverse effects or toxicity

Search methods for identification of studies

We searched for all relevant published and unpublished trials without restrictions on language, year or publication status.

Electronic searches

Relevant studies were identified from the Cochrane Cystic Fibrosis and Genetic Disorders Group's Haemoglobinopathies Trials Register using the terms: sickle cell AND hydroxyurea.

The Haemoglobinopathies Trials Register is compiled from electronic searches of the Cochrane Central Register of Controlled Trials (Clinical Trials) (updated each new issue of the Cochrane Library) and weekly searches of MEDLINE. Unpublished work is identified by searching the abstract books of five major conferences: the European Haematology Association conference; the American Society of Hematology conference; the British Society for Haematology Annual Scientific Meeting; the Caribbean Public Health Agency Annual Scientific Meeting (formerly the Caribbean Health Research Council Meeting); and the National Sickle Cell Disease Program Annual Meeting. For full details of all searching activities for the register, please see the relevant section of the Cochrane Cystic Fibrosis and Genetic Disorders Group's website.

Date of the most recent search of the Group's Haemoglobinopathies Trials Register: 16 January 2017.

We also searched the following trials registries on 20 March 2017.

ClinicalTrials.gov using the following search terms: hydroxyurea AND sickle.

WHO International Clinical Trials Registry Platform (ICTRP) using the following search terms: hydroxyurea AND sickle.

Searching other resources

The bibliographic references of all retrieved studies and reviews were assessed for additional reports of studies.

Data collection and analysis

Selection of studies

For the initial review, two authors (SCD and AO) independently applied inclusion criteria. For the updates to this review, two authors (AJ and SJN) performed this task. There were no discrepancies between the authors' assessments.

Data extraction and management

For the initial review, two authors (SCD and AO) independently extracted the data. For the updates of the review, two authors (AJ and SJN) performed this task. There were no discrepancies between the authors' assessments.

We intended to group outcome data into those measured at three, six, twelve months and annually thereafter. When outcome data were recorded at other time periods, then consideration was given to examining these as well.

Assessment of risk of bias in included studies

For the initial review, two authors (SCD and AO) independently assessed the methodological quality of each trial using the Cochrane Risk of Bias tool (Higgins 2011). For the updates of the review, two authors (AJ and SJN) performed these tasks. The authors assessed methodological quality on the methods of concealment and generation of randomisation sequence, blinding, whether data were available to analyse on intention‐to‐treat basis and whether all randomised participants were included in the analysis. There were no discrepancies between the authors' assessments.

Measures of treatment effect

For binary outcomes, we aimed to calculate a pooled estimate of the treatment effect for each outcome across studies, (the risk of an outcome among treatment allocated participants to the corresponding risk among controls). For each study, we calculated risk ratios (RR) with 95% confidence intervals (CI) for all important, dichotomous outcomes. We present RR in preference to odds ratios (OR), as OR give an inflated impression of the size of effect where event rates are high, as is the case of these studies. For the continuous outcomes, we aimed to calculate a pooled estimate of treatment effect, using the mean difference (MD).

Unit of analysis issues

One study was cross‐over in design (Belgian Study 1996). We planned to analyse data from this study using the approach recommended by Elbourne (Elbourne 2002); extracting and analysing data from paired analyses if possible.

Outcomes were measured at different time points throughout the course of the MSH study. Methods to analyse aggregate longitudinal data if individual patient data are not available are discussed by Jones (Jones 2009); however, data presented did not allow the use of these methods therefore we have carried out analysis at each individual time point reported.

Dealing with missing data

In order to allow an intention‐to‐treat analysis, we collected data by allocated treatment groups, irrespective of compliance, later exclusion (regardless of cause) or loss to follow‐up.

Assessment of heterogeneity

We assessed clinical heterogeneity by reviewing the differences across trials in the characteristics of recruited participants and treatment protocols. We assessed statistical heterogeneity using a Chi² test for heterogeneity. We assessed heterogeneity using the Q test (P < 0.10 for significance) and the I² statistic (greater than 50% indicating considerable heterogeneity (Higgins 2003)) and visually by inspecting forest plots.

Data synthesis

We analysed data using the fixed‐effect model, if we had found considerable heterogeneity (I² statistic > 50%) then we would have examined it using a random‐effects model and subgroup analyses.

Subgroup analysis and investigation of heterogeneity

We intended to perform subgroup analyses according to age (infant, child, adult etc), type of SCD (SS, Sβ₀, SC, Sβ+), dosage regimen (study specific) and setting (community, hospital, outpatient, etc), however, available data did not allow for these analyses.

Sensitivity analysis

We planned a sensitivity analysis based on the methodological quality of the studies, including and excluding quasi‐randomised studies. However, no quasi‐randomised studies were included in the review, therefore, no sensitivity analyses were performed.

Summary of findings and quality of the evidence (GRADE)

In a post hoc change from the protocol, we have presented four summary of findings tables, one for each comparison of the review (Table 1; Table 2; Table 3; Table 4).

Summary of findings for the main comparison. Summary of findings ‐ Hydroxyurea compared with placebo for sickle cell disease.

Hydroxyurea compared with placebo for sickle cell disease
Patient or population: adults and children with sickle cell disease Settings: outpatients Intervention: hydroxyurea Comparison: placebo
Outcomes	*Illustrative comparative risks (95% CI)**		Relative effect  (95% CI)	No of Participants  (studies)	Quality of the evidence  (GRADE)	Comments
	Assumed risk	Corresponding risk
	Placebo	Hydroxyurea
Pain alteration¹ Follow‐up: 6 ‐ 24 months	See comment	See comment	NA	577 (4 studies)²	⊕⊕⊕⊝  moderate⁵	All studies showed a significant advantage to hydroxyurea compared to placebo (different measures of pain alteration presented)¹.
Life‐threatening illness Follow‐up: 6 ‐ 24 months	See comment	See comment	NA	552 (3 studies)	⊕⊕⊕⊝  moderate⁵	Significantly fewer occurrences of ACS (2 studies) and transfusions (3 studies) on hydroxyurea compared to placebo. No significant differences in terms of stroke, hepatic or splenic sequestration (two studies).
Death during the study (all deaths) Follow‐up: 6 ‐ 24 months	26 per 1000	10 per 1000 (0 to 51 per 1000)	RR 0.39 (0.08 to 1.96)	577 (4 studies)²	⊕⊕⊕⊝  moderate⁵	There was also no significant difference between groups in terms of deaths related to SCD.
Measures of HbF (%) Follow‐up: 6 ‐ 24 months	See comment	See comment	NA	577 (4 studies)²	⊕⊕⊕⊝  moderate⁵	There was a significant increase in HbF(%) in the hydroxyurea group compared to the placebo group in all studies (different measures presented)³.
Measures of ANC Follow‐up: 6 ‐ 24 months	See comment	See comment	NA	517 (3 studies)²	⊕⊕⊕⊝  moderate⁵	There was a significant decrease in ANC in the hydroxyurea group compared to the placebo group in all studies (different measures presented)³.
Quality of life: 'Health Status Survey' the 'Profile of Mood States' and the 'Ladder of Life' Follow‐up: 24 months	See comment	See comment	NA	up to 277 (1 study)	⊕⊕⊝⊝ low^5,6	No significant difference in terms of any domain of any scale except for pain recall at 18 months (MD 0.70, 95% CI 0.11 to 1.29, P = 0.02)⁴.
Adverse events or toxicity: differences in rates of specific adverse events Follow‐up: 6 ‐ 24 months	See comment	See comment	NA	577 (4 studies)²	⊕⊕⊝⊝ low^5,7	Significantly fewer events of dactylitis and gastroenteritis on hydroxyurea compared to placebo. No significant differences between groups in terms of all other events.
The basis for the assumed risk is the event rate in the control group unless otherwise stated in the comments and footnotes. The corresponding risk (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI).  ACS: acute chest syndrome; ANC: absolute neutrophil counts; CI: confidence interval; HbF: fetal haemoglobin; MD: mean difference;NA: not applicable; RR: risk ratio; SCD: sickle cell disease.
GRADE Working Group grades of evidence  High quality: further research is very unlikely to change our confidence in the estimate of effect.  Moderate quality: further research is likely to have an important impact on our confidence in the estimate of effect and may change the estimate.  Low quality: further research is very likely to have an important impact on our confidence in the estimate of effect and is likely to change the estimate.  Very low quality: we are very uncertain about the estimate.

Hydroxyurea and phlebotomy compared to transfusion and chelation for people with sickle cell disease and an increased risk of stroke
Patient or population: adults and children with sickle cell disease and an increased risk of stroke Settings: outpatients Intervention: hydroxyurea and phlebotomy Comparison: transfusion and chelation
Outcomes	*Illustrative comparative risks (95% CI)**		Relative effect  (95% CI)	No of Participants  (studies)	Quality of the evidence  (GRADE)	Comments
	Assumed risk	Corresponding risk
	Transfusion and chelation	Hydroxyurea and phlebotomy
Pain alteration: proportion experiencing serious VOC or sickle‐related pain events Follow‐up: 24 to 30 months	213 per 1000	718 per 1000 (339 to 1000 per 1000)	RR 3.37 (95% CI 1.59 to 7.11)	254 (2 studies)	⊕⊕⊝⊝ low^4,5	No significant difference between treatment groups in terms of all pain events (serious and non‐serious) in 1 study (RR 1.03, 95% CI 0.81 to 1.30).
Life‐threatening illness Follow‐up: 24 ‐ 30 months	See comment	See comment	NA	254 (2 studies)	⊕⊕⊕⊝  moderate⁴	No significant difference between groups in life‐threatening neurological events, hepatobiliary disease and splenic sequestration; significantly more ACS and infections and infestations in the hydroxyurea and phlebotomy compared to the transfusion and chelation group.
Death during the study (all deaths) Follow‐up: 24 ‐ 30 months	1 death occurred in the transfusion and chelation group of 1 study¹.	1 death occurred in the hydroxyurea and phlebotomy group of 1 study¹.	RR 0.99 (95% CI 0.06 to 15.42)	254 (2 studies)	⊕⊕⊝⊝ low^4,5
Measures of HbF (%) Follow‐up: 24 to 30 months	See comment	See comment	NA	254 (2 studies)	⊕⊕⊕⊝  moderate⁴	There was a significant increase in HbF(%) in the hydroxyurea and phlebotomy group compared to the transfusion and chelation group for both studies (different measures presented)².
Measures of ANC Follow‐up: 24 to 30 months	See comment	See comment	NA	254 (2 studies)	⊕⊕⊕⊝  moderate⁴	There was a significant decrease in ANC in the hydroxyurea and phlebotomy group compared to the transfusion and chelation group for both studies (different measures presented)².
Quality of life	Outcome not reported³				NA
Adverse events or toxicity: differences in rates of specific adverse events	See comment	See comment	NA	254 (2 studies)	⊕⊕⊝⊝ low^4,6	There was a statistically significant difference in terms of immune disorders (more in transfusion and chelation group), reticulocytopenia, neutropenia and anaemia (more in hydroxyurea and phlebotomy group) in 1 study and the rate of adverse events was balanced across groups in the other study.
The basis for the assumed risk is the event rate in the control group unless otherwise stated in the comments and footnotes. The corresponding risk (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI).  ACS: acute chest syndrome; ANC: absolute neutrophil counts; CI: confidence interval; HbF: fetal haemoglobin; NA: not applicable; RR: risk ratio; VOC: vaso‐occlusive crisis.
GRADE Working Group grades of evidence  High quality: further research is very unlikely to change our confidence in the estimate of effect.  Moderate quality: further research is likely to have an important impact on our confidence in the estimate of effect and may change the estimate.  Low quality: further research is very likely to have an important impact on our confidence in the estimate of effect and is likely to change the estimate.  Very low quality: we are very uncertain about the estimate.

Hydroxyurea compared with observation for people with sickle cell disease and an increased risk of stroke
Patient or population: adults and children with sickle cell disease and an increased risk of stroke Settings: outpatients Intervention: hydroxyurea Comparison: observation
Outcomes	*Illustrative comparative risks (95% CI)**		Relative effect  (95% CI)	No of Participants  (studies)	Quality of the evidence  (GRADE)	Comments
	Assumed risk	Corresponding risk
	Observation	Hydroxyurea
Pain alteration Follow‐up: NA	Outcome not reported				NA
Life‐threatening illness Follow‐up: 15 months	See comment	See comment	NA	22 (1 study)	⊕⊝⊝⊝  very low^3,4	No significant differences between groups in terms of ACS, blood transfusions required or acute splenic sequestration.
Death during the study Follow‐up: 15 months	No deaths occurred	No deaths occurred	NA	22 (1 study)	⊕⊝⊝⊝  very low^3,4
Measures of HbF Follow‐up: 15 months	See comment	See comment	NA	22 (1 study)	⊕⊝⊝⊝  very low^3,4	There was a significant increase in HbF in the hydroxyurea group compared to the observation group¹.
Measures of ANC Follow‐up: 15 months	See comment	See comment	NA	22 (1 study)	⊕⊝⊝⊝  very low^3,4	There was a significant decrease in ANC in the hydroxyurea group compared to the observation group¹.
Quality of Life Follow‐up: NA	Outcome not reported²				NA
Adverse events or toxicity: differences in rates of specific adverse events Follow‐up: 15 months	See comment	See comment	NA	22 (1 study)	⊕⊝⊝⊝  very low^3,4	No significant differences between groups in terms of transient neutropenia, reticulocytopenia, parasite infestation, headache and dizziness.
The basis for the assumed risk is the event rate in the control group unless otherwise stated in the comments and footnotes. The corresponding risk (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI).  ACS: acute chest syndrome; ANC: absolute neutrophil counts; CI: confidence interval; HbF: fetal haemoglobin; NA: not applicable; RR: risk ratio.
GRADE Working Group grades of evidence  High quality: further research is very unlikely to change our confidence in the estimate of effect.  Moderate quality: further research is likely to have an important impact on our confidence in the estimate of effect and may change the estimate.  Low quality: further research is very likely to have an important impact on our confidence in the estimate of effect and is likely to change the estimate.  Very low quality: we are very uncertain about the estimate.

Hydroxyurea compared with no hydroxyurea for sickle cell disease
Patient or population: adults and children with sickle cell disease Settings: outpatients Intervention: hydroxyurea Comparison: no hydroxyurea
Outcomes	*Illustrative comparative risks (95% CI)**		Relative effect  (95% CI)	No of Participants  (studies)	Quality of the evidence  (GRADE)	Comments
	Assumed risk	Corresponding risk
	No hydroxyurea	Hydroxyurea
Pain alteration Follow‐up: NA	Outcome not reported				NA
Life‐threatening illness Follow‐up: NA	Outcome not reported				NA
Death during the study Follow‐up: 11 months	No deaths occurred	No deaths occurred	NA	up to 44 (1 study)¹	⊕⊝⊝⊝  very low^2,3,4
Measures of HbF Follow‐up: 24 weeks	See comment	See comment	NA	up to 44 (1 study)¹	⊕⊝⊝⊝  very low^2,3,4	There was a significant increase in HbF in the hydroxyurea group compared to the no hydroxyurea group¹.
Measures of ANC Follow‐up: 24 weeks	See comment	See comment	NA	up to 44 (1 study)¹	⊕⊝⊝⊝  very low^2,3,4	There was no significant difference in ANC between treatment groups¹.
Quality of life Follow‐up: NA	Outcome not reported				NA
Adverse events or toxicity Follow‐up: 11 months	See comment	See comment	NA	up to 44 (1 study)¹	⊕⊝⊝⊝  very low^2,3,4	Vaso‐occlusive pain crises, headache / migraine, upper respiratory infection, skin rash diarrhoea and abdominal pain were the most common adverse events during the trial and these events were evenly distributed across treatment groups (not separated by group).
The basis for the assumed risk is the event rate in the control group unless otherwise stated in the comments and footnotes. The corresponding risk (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI).  ANC: absolute neutrophil counts; CI: confidence interval; HbF: fetal haemoglobin; NA: not applicable; RR: risk ratio.
GRADE Working Group grades of evidence.  High quality: further research is very unlikely to change our confidence in the estimate of effect.  Moderate quality: further research is likely to have an important impact on our confidence in the estimate of effect and may change the estimate.  Low quality: further research is very likely to have an important impact on our confidence in the estimate of effect and is likely to change the estimate.  Very low quality: we are very uncertain about the estimate.

	Hydroxyurea		Placebo		P value
	Baseline	18 months	Baseline	18 months	P value
Clinical events (number of events per participant per year)
VOC	12.13 (8.56)	0.60 (1.37)	11.46 (3.01)	10.2 (3.24)	< 0.001
Blood transfusions	2.43 (0.69)	0.13 (0.43)	2.13 (0.98)	1.98 (0.82)	< 0.001
Hospitalisations	10.13 (6.56)	0.70 (1.28)	9.56 (2.91)	9.59 (2.94)	< 0.001
Haematological parameters
Hb (g/dL)	8.1 (0.68)	9.29 (0.55)	8.21 (0.68)	7.90 (0.58)	< 0.001
HbF(%)	19.8 (0.9)	24 (5.9)	19.21 (6.37)	18.92 (5.77)	< 0.001
Reticulocytes (x10⁵ /mm³)	1.83 (0.96)	1.15 (0.1)	1.73 (0.49)	1.81 (0.67)	< 0.001
Leucocytes (x10³ /mm³)	7.36 (6.03)	6.54 (5.54)	7.26 (4.91)	7.38 (2.85)	< 0.001
Platelets (x10³ /mm³)	1.78 (0.26)	2.01 (0.18)	1.91 (0.21)	2.06 (0.26)	0.28
RBC (x10⁶ /mm³)	2.89 (0.57)	1.98 (0.22)	1.84 (0.47)	3.11 (0.20)	0.05
Total bilirubin (mg/dL)	2.32 (1.42)	1.10 (0.42)	2.27 (1.28)	2.71 (0.93)	< 0.001

Baseline	Hyrdoxyurea		Placebo		P value
Baseline	2 years	Baseline	2 years	2 years	P value
WBC (10⁹/L)	12.6 (3.4)	9.9 (3.1)	12.3 (3.2)	12.2 (2.8)	0.0001
Neutrophils (10⁹/L)	6.9 (2.4)	4.9 (2.0)	6.7 (2.3)	6.4 (2.0)	0.0001
Platelets (10⁹/L)	468 (147)	399 (124)	457 (130)	423 (122)	0.12
Hb (g/dL)	8.5 (1.4)	9.1 (1.5)	8.5 (1.2)	8.5 (1.3)	0.0009
PCV (%)	24.9 (4.4)	27 (5)	25.2 (4.0)	25.1 (4.2)	0.0007
MCV (fl)	94 (9)	103 (14)	93 (9)	93 (9)	0.0001
Reticulocytes (10⁹/L)	327 (98)	231 (100)	325 (94)	300 (99)	0.0001
HbF (%)	5 (3.5)	8.6 (6.8)	5.2 (3.4)	4.7 (3.3)	0.0001
F cells (%)	33 (17)	48 (23)	33 (17)	35 (18)	0.0001
F reticulocytes	15 (8)	17 (9)	15 (8)	15 (7)	0.0036
Dense cells (%)	14 (6)	11 (6)	14 (7)	13 (7)	0.004
Creatinine (mg/dL)	0.9 (0.3)	1.0 (0.5)	0.9 (0.2)	1.0 (0.5)	0.64
Total bilirubin (mg/dL)	3.7 (2.4)	2.9 (2.5)	3.7 (2.5)	4.2 (4.6)	0.004
Direct bilirubin (mg/dL)	0.5 (0.3)	0.4 (0.3)	0.5 (0.4)	0.7 (2.2)	0.08
Aspartate aminotransferase	44 (23)	39 (20)	41 (21)	43 (27)	0.16
Alkaline phosphatase	120 (59)	117 (48)	119 (67)	119 (71)	0.71

Outcome	Hydroxyurea and phlebotomy group (n = 67)	Transfusions and chelation group (n = 66)	P value
HbF (%)	17.9 (92 to 22.9)	‐0.2 (‐0.8 to 0.4)	< 0.001
ANC (x10⁹/L)	‐3.3 (‐5.1 to ‐1.4)	0.8 (‐1.3 to 2.4)	< 0.001
Hb (g /dL)	0.0 (‐0.7 to 0.7)	0.0 (‐0.5 to 0.6)	0.898
HbA (%)	‐50.9 (‐66.8 to ‐33.7)	0.0 (‐12.7 to 6.7)	< 0.001
HbS (%)	35.0 (21.7 to 46.2)	0.3 (‐7.5 to 12.3)	< 0.001
MCV (fL)	19.5 (7.5 to 28.5)	0.1 (‐2.0 to 2.5)	< 0.001
WBC (x10⁹/L)	‐5.4 (‐8.1 to ‐2.2)	0.2 (‐2.0 to 2.3)	< 0.001
ARC (x10⁹/L)	‐149.1 (‐231.0 to ‐19.0)	‐11.8 (‐88.2 to 93.2)	< 0.001
Platelets (x10⁹/L)	‐83.0 (‐171.0 to ‐8.0)	‐28.0 (‐70.0 to 18.0)	0.0022
Total bilirubin (mg/dL)	‐1.1 (‐1.9 to ‐0.6)	0.4 (‐0.3 to 1.2)	< 0.001
LIC (mg Fe/g)	‐1.2 (‐2.8 to 7.2)	‐2.2 (‐5.5 to 4.9)	0.48888
Serum ferritin (ng/mL)	‐966.0 (‐1629.0 to 49.0)	1159.5 (‐662.0 to 2724.0)	< 0.001
LDH (U/L)	‐67.0 (‐143.0 to 7.0)	‐8.5 (‐74.0 to 74.0)	0.0015

Outcome	Hydroxyurea (n = 11)	Observation (n = 11)	P value
Hb (g/dL)	1.6	‐0.5	< 0.0001
MCV (fL)	8.7	1	0.0001
ARC ( x10⁹/L)	22.7	‐33.2	0.76
WBC ( x10⁹/L)	‐4.6	1.3	0.07
ANC ( x10⁹/L)	‐2.2	1.4	0.05
Platelets ( x10⁹/L)	‐76	‐35	0.56
HbF (%)	8.9	0.3	0.002
Weight (kg)	2.5	1.8	0.51
Height (cm)	6.8	3.8	0.22

Pregnancy		Hydroxyurea	Placebo
Patients	Normal full‐term delivery	1	2
Patients	Elective termination	2	1
Partners of patients	Normal full‐term delivery	2	0
	Spontaneous abortion	1	0
	Still pregnant	0	1
TOTAL		6	4

Date	Event	Description
10 April 2017	New search has been performed	Six new studies have been added to the review (BABY HUG 2011; Jain 2012; CHAMPS 2011; SCATE 2015; SWiTCH 2012; TWiTCH 2016). Summary of findings tables are now included in the review.
10 April 2017	New citation required and conclusions have changed	Six new trials have been included in the review (BABY HUG 2011; Jain 2012; CHAMPS 2011; SCATE 2015; SWiTCH 2012; TWiTCH 2016). These include participants with different genotypes, different disease severities and participants at risk of primary and secondary stroke. The inclusion of these new trials has impacted upon the conclusions of the review.

Date	Event	Description
10 November 2010	New search has been performed	A search of the Group's Haemoglobinopathies Trials Register identified 25 new references potentially eligible for inclusion in this review. Fifteen of these were additional references to the BABY HUG study which is listed as ongoing (Baby Hug 2007a). Eight references were to the already included MSH trial and provide no further data or information not already included within the review (MSH 1995). The remaining two references (each published in abstract form) have been added to 'Studies awaiting classification', and we await publication of the full papers in order to accurately assess eligibility (Jain 2010; CHAMPS 2011a).
5 November 2008	New search has been performed	No new trials were identified by the search of the Group's Haemoglobinopathies trials Register.
1 July 2008	Amended	Converted to new review format.
1 February 2008	New search has been performed	The search identified four new references. One was an additional reference to the already included MSH 1995 study (Ballas 2006), however, this did not include any new data that can be added to the review. A further reference was to the already excluded De Montalembert 2006 study (De Montalembert 2006). The third reference was an additional reference to the Baby Hug 2007a study now listed in Ongoing studies (Wynn 2007); and the final reference has been added to Excluded studies (Silva‐Pinto 2007).    In addition, the Plain Language Summary has been updated in line with latest guidance from The Cochrane Collaboration.
1 February 2007	New search has been performed	The search identified 12 new references to four studies. Two of the new references were additional references to the already included MSH 1995 study; however, no further data have been included in the review. Seven of the references were to one study, which is listed in 'Studies awaiting assessment' (Baby Hug 2005). The remaining three new references have been excluded (De Montalembert 2005; Voskaridou 2005; Ware 2006).
1 February 2006	New search has been performed	The search identified one new reference (Charache 1993) which is an additional reference to the already included MSH 1995 trial. This reference provided no additional data. Ashley Jones became lead author in September 2005 and acts as guarantor of the review.
1 November 2004	New search has been performed	The search identified one new reference to the already included MSH 1995 trial: Orringer EP, Jones S, Strayhorn D, Hoffman E, Parker J, Greenberg CS. The effect of hydroxyurea (HU) administration on circulating d‐dimer levels in patients with sickle cell anemia [abstract]. Blood 1996;88(10 Suppl 1):496a. Following on from the inclusion of this reference, a further outcome has been added to the review: cost effectiveness of hydroxyurea.
1 November 2003	New search has been performed	Two additional references to the already included MSH 1995 study have been incorporated into the review.
1 February 2002	New search has been performed	Quality of life data, presented in an abstract by Terrin (Terrin 1999), which reports on the MSH study (MSH 1995), has now been incorporated into the review.

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
1 Pain crises	2		Mean Difference (IV, Fixed, 95% CI)	Totals not selected
1.1 Mean annual crisis rate at 2 years (all crises)	1		Mean Difference (IV, Fixed, 95% CI)	‐2.80 [‐4.74, ‐0.86]
1.2 Mean annual crisis rate at 2 years (all crises requiring hospitalisation)	1		Mean Difference (IV, Fixed, 95% CI)	‐1.50 [‐2.58, ‐0.42]
1.3 Number of vaso‐occlusive crises after 18 months of treatment	1		Mean Difference (IV, Fixed, 95% CI)	‐9.6 [‐10.86, ‐8.34]
2 Proportion experiencing pain	1		Risk Ratio (M‐H, Fixed, 95% CI)	Totals not selected
3 Proportion experiencing life threatening events during study	2		Risk Ratio (M‐H, Fixed, 95% CI)	Subtotals only
3.1 Acute chest syndrome	2	492	Risk Ratio (M‐H, Fixed, 95% CI)	0.43 [0.29, 0.63]
3.2 Hepatic sequestration	1	299	Risk Ratio (M‐H, Fixed, 95% CI)	0.32 [0.03, 3.06]
3.3 Stroke	2	492	Risk Ratio (M‐H, Fixed, 95% CI)	0.54 [0.12, 2.53]
3.4 Patients transfused	2	492	Risk Ratio (M‐H, Fixed, 95% CI)	0.66 [0.52, 0.82]
3.5 Splenic sequestration	1	193	Risk Ratio (M‐H, Fixed, 95% CI)	0.90 [0.36, 2.23]
4 Number of life‐threatening events during study	1		Mean Difference (IV, Fixed, 95% CI)	Totals not selected
4.1 Blood transfusions	1		Mean Difference (IV, Fixed, 95% CI)	‐1.85 [‐2.18, ‐1.52]
4.2 Hospitalisations	1		Mean Difference (IV, Fixed, 95% CI)	‐8.89 [‐10.04, ‐7.74]
4.3 Duration of hospitalisation (days)	1		Mean Difference (IV, Fixed, 95% CI)	‐2.00 [‐4.87, ‐3.13]
5 Deaths during the study	3		Risk Ratio (M‐H, Fixed, 95% CI)	Subtotals only
5.1 All deaths	3	552	Risk Ratio (M‐H, Fixed, 95% CI)	0.39 [0.08, 1.96]
5.2 Deaths related to SCD	3	552	Risk Ratio (M‐H, Fixed, 95% CI)	0.48 [0.09, 2.60]
6 Change from baseline in fetal haemoglobin (HbF %)	1		Mean Difference (Fixed, 95% CI)	Totals not selected
7 Fetal haemoglobin (HbF %) after treatment	2	359	Mean Difference (IV, Fixed, 95% CI)	4.07 [2.95, 5.18]
8 Change from baseline in absolute neutrophil count (x10³ per μL)	1		Mean Difference (Fixed, 95% CI)	Totals not selected
9 Neutrophil response (10⁹/L) after treatment	1		Mean Difference (IV, Fixed, 95% CI)	Totals not selected
9.1 Neutrophils (x10 9/l) at 10 weeks	1		Mean Difference (IV, Fixed, 95% CI)	‐1.90 [‐2.51, ‐1.29]
9.2 Neutrophils (x10 9/l) at 2 years	1		Mean Difference (IV, Fixed, 95% CI)	‐1.50 [‐2.01, ‐0.99]
10 Change from baseline in haemoglobin (g/L)	1		Mean Difference (Fixed, 95% CI)	Totals not selected
11 Change from baseline in m corpuscular volume (fL)	1		Mean Difference (Fixed, 95% CI)	Totals not selected
12 Change from baseline in white blood cells (x10³ per μL)	1		Mean Difference (Fixed, 95% CI)	Totals not selected
13 Change from baseline in absolute reticulocyte count (x10³ per μL)	1		Mean Difference (Fixed, 95% CI)	Totals not selected
14 Change from baseline in reticulocytes (%)	1		Mean Difference (Fixed, 95% CI)	Totals not selected
15 Change from baseline in total bilirubin (mg/L)	1		Mean Difference (Fixed, 95% CI)	Totals not selected
16 Change from baseline in platelet count (x10³ per μL)	1		Mean Difference (Fixed, 95% CI)	Totals not selected
17 Haemoglobin (g/dL)	2		Mean Difference (IV, Fixed, 95% CI)	Subtotals only
17.1 At 10 weeks	1	299	Mean Difference (IV, Fixed, 95% CI)	0.5 [0.19, 0.81]
17.2 At the end of the study	2	359	Mean Difference (IV, Fixed, 95% CI)	1.04 [0.82, 1.25]
18 Mean corpuscular volume (fL)	1		Mean Difference (IV, Fixed, 95% CI)	Totals not selected
18.1 At 10 weeks	1		Mean Difference (IV, Fixed, 95% CI)	12.30 [9.69, 14.91]
18.2 At 2 years	1		Mean Difference (IV, Fixed, 95% CI)	10.0 [7.34, 12.66]
19 Total bilirubin (mg/L)	2		Mean Difference (IV, Fixed, 95% CI)	Subtotals only
19.1 At the end of the study	2	359	Mean Difference (IV, Fixed, 95% CI)	‐1.56 [‐1.90, ‐1.23]
20 Reticulocytes	2		Mean Difference (IV, Fixed, 95% CI)	Totals not selected

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
1 Proportion experiencing pain	2		Risk Ratio (M‐H, Fixed, 95% CI)	Subtotals only
1.1 Vaso‐occlusive or sickle cell‐related pain (all)	1	133	Risk Ratio (M‐H, Fixed, 95% CI)	1.03 [0.81, 1.30]
1.2 Vaso‐occlusive or sickle cell‐related pain (serious)	2	254	Risk Ratio (M‐H, Fixed, 95% CI)	3.37 [1.59, 7.11]
2 Proportion experiencing life‐threatening events during study	2		Risk Ratio (M‐H, Fixed, 95% CI)	Subtotals only
2.1 Stroke (secondary)	1	133	Risk Ratio (M‐H, Fixed, 95% CI)	14.78 [0.86, 253.66]
2.2 Transient ischaemic attack (primary)	1	121	Risk Ratio (M‐H, Fixed, 95% CI)	1.02 [0.21, 4.84]
2.3 Transient ischaemic attack (secondary)	1	133	Risk Ratio (M‐H, Fixed, 95% CI)	0.66 [0.25, 1.74]
2.4 Other neurological event (primary)	1	121	Risk Ratio (M‐H, Fixed, 95% CI)	1.02 [0.07, 15.88]
2.5 Acute chest syndrome	2	254	Risk Ratio (M‐H, Fixed, 95% CI)	2.84 [1.25, 6.42]
2.6 Infections and Infestations	2	254	Risk Ratio (M‐H, Fixed, 95% CI)	3.65 [1.05, 12.76]
2.7 Splenic sequestration or splenectomy	2	254	Risk Ratio (M‐H, Fixed, 95% CI)	0.34 [0.01, 8.16]
2.8 Hepatobiliary disease	1	121	Risk Ratio (M‐H, Fixed, 95% CI)	1.02 [0.07, 15.88]
2.9 Total with serious adverse events	2	254	Risk Ratio (M‐H, Fixed, 95% CI)	1.93 [1.17, 3.20]
2.10 Total with sickle cell related, non‐neurological adverse events	1	133	Risk Ratio (M‐H, Fixed, 95% CI)	3.10 [1.42, 6.75]
3 Deaths during the study	2	254	Risk Ratio (M‐H, Fixed, 95% CI)	0.99 [0.06, 15.42]
4 Change from baseline in fetal haemoglobin (HbF %)	1		Mean Difference (IV, Fixed, 95% CI)	Totals not selected
5 Change from baseline in absolute neutrophil count (x10⁹/L)	1		Mean Difference (IV, Fixed, 95% CI)	Totals not selected
6 Change from baseline in mean corpuscular volume (fL)	1		Mean Difference (IV, Fixed, 95% CI)	Totals not selected
7 Change from baseline in sickle haemoglobin (%)	1		Mean Difference (IV, Fixed, 95% CI)	Totals not selected
8 Change from baseline in haemoglobin (g/L)	1		Mean Difference (IV, Fixed, 95% CI)	Totals not selected
9 Change from baseline in absolute reticulocyte count (10⁹ / L)	1		Mean Difference (IV, Fixed, 95% CI)	Totals not selected
10 Change from baseline in white blood count (10⁹ / L)	1		Mean Difference (IV, Fixed, 95% CI)	Totals not selected
11 Change from baseline in platelets (10⁹ / L)	1		Mean Difference (IV, Fixed, 95% CI)	Totals not selected
12 Change from baseline in total bilirubin (mg/L)	1		Mean Difference (IV, Fixed, 95% CI)	Totals not selected
13 Change from baseline in liver iron concentration	1		Mean Difference (IV, Fixed, 95% CI)	Totals not selected
14 Change from baseline in serum ferritin (ng/mL)	1		Mean Difference (IV, Fixed, 95% CI)	Totals not selected
15 Change from baseline in lactate dehydrogenase (U/L)	1		Mean Difference (IV, Fixed, 95% CI)	Totals not selected
16 Signs of organ damage ‐ CNS measures at the end of the study	1		Mean Difference (IV, Fixed, 95% CI)	Totals not selected
16.1 Final TCD ultrasound velocity	1		Mean Difference (IV, Fixed, 95% CI)	‐5.0 [‐9.16, ‐0.84]
17 Proportion of participants experiencing non‐neurological adverse events and toxicity	1		Risk Ratio (M‐H, Fixed, 95% CI)	Totals not selected
17.1 Infections and infestations	1		Risk Ratio (M‐H, Fixed, 95% CI)	1.24 [0.89, 1.72]
17.2 Gastrointestinal disorders	1		Risk Ratio (M‐H, Fixed, 95% CI)	1.16 [0.74, 1.80]
17.3 Fever	1		Risk Ratio (M‐H, Fixed, 95% CI)	1.08 [0.66, 1.75]
17.4 Musculoskelatal disorders	1		Risk Ratio (M‐H, Fixed, 95% CI)	1.14 [0.59, 2.20]
17.5 Immune system disorders	1		Risk Ratio (M‐H, Fixed, 95% CI)	0.12 [0.02, 0.96]
17.6 Cholelithiasis	1		Risk Ratio (M‐H, Fixed, 95% CI)	0.74 [0.17, 3.17]
17.7 Cholecystitis	1		Risk Ratio (M‐H, Fixed, 95% CI)	1.97 [0.18, 21.21]
17.8 Asthma	1		Risk Ratio (M‐H, Fixed, 95% CI)	0.74 [0.17, 3.17]
17.9 Acute chest syndrome	1		Risk Ratio (M‐H, Fixed, 95% CI)	1.72 [0.53, 5.61]
17.10 Renal or urinary disorder	1		Risk Ratio (M‐H, Fixed, 95% CI)	0.66 [0.11, 3.80]

Outcome or subgroup title	No. of studies	Statistical method	Effect size
1 Proportion experiencing life‐threatening events during the study	1	Risk Ratio (M‐H, Fixed, 95% CI)	Totals not selected
1.1 Vaso‐occlusive events	1	Risk Ratio (M‐H, Fixed, 95% CI)	0.4 [0.10, 1.64]
1.2 Acute splenic sequestration	1	Risk Ratio (M‐H, Fixed, 95% CI)	1.0 [0.07, 14.05]
1.3 Blood transfusions required	1	Risk Ratio (M‐H, Fixed, 95% CI)	0.33 [0.04, 2.73]
2 Signs of organ damage ‐ proportion of participants with a change in CNS measures	1	Risk Ratio (M‐H, Fixed, 95% CI)	Totals not selected
2.1 Participants converting from conditional to abnormal TCD ultrasound velocity	1	Risk Ratio (M‐H, Fixed, 95% CI)	0.2 [0.03, 1.45]
3 Signs of organ damage ‐ change from baseline in CNS measures	1	Mean Difference (IV, Fixed, 95% CI)	Totals not selected
3.1 TCD ultrasound velocity (time‐averaged mean maximum velocity)	1	Mean Difference (IV, Fixed, 95% CI)	‐25.7 [‐45.38, ‐6.02]
4 Adverse events and toxicity	1	Risk Ratio (M‐H, Fixed, 95% CI)	Totals not selected
4.1 Dizziness	1	Risk Ratio (M‐H, Fixed, 95% CI)	3.0 [0.14, 66.53]
4.2 Headaches	1	Risk Ratio (M‐H, Fixed, 95% CI)	1.0 [0.07, 14.05]
4.3 Transient neutropenia	1	Risk Ratio (M‐H, Fixed, 95% CI)	3.0 [0.14, 66.53]
4.4 Reticulocytopenia	1	Risk Ratio (M‐H, Fixed, 95% CI)	1.0 [0.07, 14.05]
4.5 Parasite infection	1	Risk Ratio (M‐H, Fixed, 95% CI)	1.0 [0.07, 14.05]

Methods	Randomised, double blind (participant, investigator), placebo control, parallel assignment, efficacy study conducted at 13 centres in the USA.
Participants	Children aged 9 months to 18 months with SCD (HbSS or HbSßº‐thalassemia) irrespective of clinical severity. Exclusion criteria were transfusion within 2 months, height, weight or head circumference below the 5th percentile, mental development index less than 70 or abnormal TCD ultrasound velocity. 193 randomised, 96 participants to hydroxyurea and 97 to placebo. Mean (SD) age: hydroxyurea group : 13.6 (2.7) months, placebo group: 13.5 (2.8) months 187 (97%) with HbSS genotype, 109 (56%) females
Interventions	Hydroxyurea (20 mg/kg/day) versus placebo for 24 months.
Outcomes	Primary outcomes: spleen function (decline in splenic uptake); and glomerular filtration rate with ^99mTc‐diethyl‐enetriaminepentaacetic acid plasma clearance. Secondary outcomes: ratio of nuclear decay counts in the spleen and liver; proportion of red blood cells containing pits or Howell‐Jolly bodies; renal function; growth; and development (including neuro‐developmental assessment).
Notes
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	The telephone randomisation schedule was developed by the medical co‐ordinating centre.
Allocation concealment (selection bias)	Low risk	Centralised telephone randomisation was used.
Incomplete outcome data (attrition bias)   All outcomes	Low risk	Some secondary outcomes reported for only the individuals who completed the study but ITT approach taken for primary outcomes (via multiple imputation) and safety outcomes so risk of bias judged to be low.
Selective reporting (reporting bias)	Low risk	All outcomes well defined in the methods and reported well in the results.
Other bias	Low risk	None identified.
Blinding of participants and personnel (performance bias)   All outcomes	Low risk	Participants, caregivers and medical coordinating centre staff were masked to treatment allocation via packaging and treatment of the same appearance.
Blinding of outcome assessment (detection bias)   All outcomes	Low risk	An unmasked "primary endpoint person" monitored laboratory values and assisted clinical management. Other outcome assessors (e.g. those reading the liver‐spleen scans).

Methods	Randomised placebo‐controlled cross‐over study conducted in a single centre in Belgium.
Participants	25 children with HbSS genotype, age 2 ‐ 22 years (median 9 years) with > 3 vaso‐occlusive events reported in preceding 12 months and/or history of CVA off transfusion (severe alloimmunisation or compliance), ACS, ASS.
Interventions	Hydroxyurea 20 mg/kg/day rising to 25 (unless toxic cytopenia) versus placebo. Treatment period was for 6 months.
Outcomes	Number of hospitalisations.  Number of inpatient days.  FBC.  HbF%.
Notes	Data are not presented in a way that results can be included in the review so results are presented narratively.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Described as "drawing sealed envelopes, patients were randomly allocated to one of the following treatment sequences", therefore the generation of the treatment sequence are not clear from this statement.
Allocation concealment (selection bias)	Unclear risk	Described as "drawing sealed envelopes, patients were randomly allocated to one of the following treatment sequences", therefore the allocation of the treatment sequence are not clear from this statement.
Incomplete outcome data (attrition bias)   All outcomes	Unclear risk	3 participants who were excluded from the analysis due to their failure to attend the monthly evaluation at 4 ‐ 5 months. There was no discussion of whether or not an ITT analysis was used so the risk is unclear.
Selective reporting (reporting bias)	High risk	A planned outcome (number of days in pain) was dropped from analysis due to difficulty in obtaining information to inform this analysis from participants
Other bias	Low risk	Unclear if a washout period was used in this cross‐over study but tests for period effects and carry‐over effect were not significant so the risk of bias in the cross‐over design is low.
Blinding of participants and personnel (performance bias)   All outcomes	Unclear risk	The hospital pharmacy provided the treatment and placebo for each participant and both were described as "indistinguishable," however the physician was aware of the treatment schedule because of the difficulty of blinding the attending physician to the treatment received. Unclear if this could have influenced results.
Blinding of outcome assessment (detection bias)   All outcomes	Unclear risk	No information provided.

Methods	Randomised multicentre, phase II, double blind placebo‐controlled study.
Participants	Eligible participants over the age of 5 years with HbSC and at least 1 vaso‐occlusive event in the previous 12 months (but none in the 4 weeks prior to study entry). 44 participants randomised; 11 to each treatment group (see 'Interventions'). Mean age: 13.6 years (range 5 ‐ 53 years), 43% females.
Interventions	Participants randomised to 1 of 4 arms in a factorial design: 1. hydroxyurea (20 mg/kg/day) and magnesium (0.6 mmol/kg/day in 2 doses); 2. hydroxyurea (20 mg/kg/day) and placebo; 3. placebo and magnesium (0.6 mmol/kg/day in 2 doses); 4. placebo and placebo.
Outcomes	Primary outcome: proportion of hyperdense red blood cells at 8 weeks. Secondary outcomes: central laboratory evaluations (including measurements of red cell density, HbF, red cell cation content, KCl co‐transport and Gardos channel activity, cell adhesion to endothelial cells and laminin, and erythrocyte membrane phosphatidyl serine (PS exposure)) at baseline (twice) and weeks 8, 16, 24, and 44. Participants were evaluated at 2 or 4 week intervals for 11 months (15 visits).
Notes	The study was not designed to measure efficacy and all analyses were considered exploratory. The study was terminated early due to low enrolment after 44 participants had been randomised (target 188). Due to factorial design, no data can be entered into analysis and results for hydroxyurea groups (groups 1 and 2) compared to no hydroxyurea groups are summarised narratively.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	A sequential allocation algorithm (i.e. minimisation) was used due to small numbers within each strata (site and age group).
Allocation concealment (selection bias)	Unclear risk	No information provided.
Incomplete outcome data (attrition bias)   All outcomes	High risk	36 out of 44 participants completed 8 weeks and 22 out of 44 completed 44 weeks. Only those who completed the follow‐up time were included in analysis (8 weeks for primary endpoint and 44 weeks for secondary endpoints). This is not an ITT approach.
Selective reporting (reporting bias)	Low risk	Outcome defined in the methods section well described in the results section. Study was not designed to measure efficacy.
Other bias	Low risk	Study terminated early after only 44 of planned sample size of 188 were recruited. However, the study was not designed to measure efficacy and performed only exploratory analyses therefore the early termination is unlikely to have introduced bias.
Blinding of participants and personnel (performance bias)   All outcomes	Low risk	All tablets were 'over‐capsulated' to disguise appearance.
Blinding of outcome assessment (detection bias)   All outcomes	Unclear risk	No information provided.

Methods	Double‐blind randomised controlled study conducted in a tertiary hospital in Nagpur City, India.
Participants	Indian children between the ages of 5 and 18 years with severe manifestations (defined as 3 or more blood transfusions or VOC requiring hospitalisation per year) despite high HbF. Exclusion criteria included seropositivity for HIV or chronic illness. 60 participants randomised; 30 to each treatment group. 53% females (16 females per group). Mean (SD) age ‐ hydroxyurea group: 12.73 (4.4) years, placebo group: 11.73 (4.08) years.
Interventions	Hydroxyurea (fixed dose 10 mg/mg/day) compared to placebo for 18 months.
Outcomes	Primary outcome: frequency of VOC per participant per year. Secondary outcomes: frequency of blood transfusions, hospitalisations and HbF levels.
Notes
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Participants were randomised using randomisation tables.
Allocation concealment (selection bias)	Unclear risk	No information provided.
Incomplete outcome data (attrition bias)   All outcomes	Low risk	All participants completed 18‐months follow‐up and were included in analysis.
Selective reporting (reporting bias)	Low risk	No protocol available, outcomes defined in the methods reported well in the results.
Other bias	Low risk	None identified.
Blinding of participants and personnel (performance bias)   All outcomes	Low risk	Participants were blinded with placebo tablets of identical appearance. The clinician who assessed participants were not aware of treatment arm.
Blinding of outcome assessment (detection bias)   All outcomes	Low risk	The laboratory technician and the clinician who assessed participants were not aware of treatment arm.

Methods	Randomised multicentre parallel, double‐blind placebo‐controlled study. Conducted in the USA and Canada across 21 sites.
Participants	Adults over 18 years of age with HbSS genotype who had reported more than 3 'crises' to treating physician in the preceding 12 months and who had < 15% HbA. Exclusions included: HbA > 15%, pregnancy, opiate addiction, other potent anti‐sickling agents, cytopenia, CVA in the preceding 6 years, HIV antibody +, prior hydroxyurea therapy 152 randomised to hydroxyurea and 147 given placebo.
Interventions	Hydroxyurea starting at 15 mg/kg/day rising 12‐weekly by 5 mg/kg/day unless marrow depression (then cessation of drug until recovery and restarted at 2.5 mg/kg/day lower, i.e. MTD) compared to placebo for 2 years.
Outcomes	Primary outcome: pain events ‐ attending hospital > 4 hours & parenteral opiate treatment.  Secondary outcomes: 'crises' including ACS, CVA, priapism etc, daily pain charts, time to first and second crisis, FBC, F cells & Hb F%, dense cells.
Notes	93% follow‐up at 2 years, treatment stopped in 14 hydroxyurea and 6 placebo participants. Due to beneficial treatment effects, the study was stopped at a mean follow‐up of 21 months, before the planned 24 months of treatment had been completed for all participants.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Treatment assignments were made centrally using a computer programme to generate separate, randomised block assignment schedules for each clinic.
Allocation concealment (selection bias)	Unclear risk	There was no clear discussion on allocation concealment.
Incomplete outcome data (attrition bias)   All outcomes	Low risk	Withdrawals from the study were well documented, all participants included in analysis in an ITT approach in the primary publications but later reported outcomes (such as quality of life) reported only for those who contributed data.
Selective reporting (reporting bias)	Low risk	All outcomes well defined in the methods and reported well in the results.
Other bias	Low risk	None identified.
Blinding of participants and personnel (performance bias)   All outcomes	Low risk	Stated as double‐blind (physician and participant), where treatment was assigned in combinations of identically appearing capsules.
Blinding of outcome assessment (detection bias)   All outcomes	Unclear risk	There was no clear discussion on blinding of outcome assessors.

Methods	Phase III randomised, partially masked (outcome assessors) multicentre study conducted in 3 centres in the USA, Jamaica and Brazil between May 2012 and August 2013.
Participants	Children with SCA and conditional TCD ultrasound velocities (170 cm ‐ 199 cm per second). Exclusion criteria were prior abnormal TCD velocities or clinical stroke, red blood cell transfusion within 2 months of enrolment, concurrent use of another anti‐sickling medication or contraindication to hydroxyurea therapy (allergy, pregnancy, renal insufficiency). 22 participants randomised, 11 to each treatment group. Mean age (SD): hydroxyurea group: 6.2 (2.4) years, observation group: 6.6 (1.5) years. 64% females, 7 per group. 21 participants with HbSS and 1 with HbSβº‐thalassemia.
Interventions	Hydroxyurea (starting at 20 mg/kg/day escalated to a maximum of 35 mg/kg/day) compared to standard treatment (observation).
Outcomes	Primary outcome: conversion to abnormal maximum abnormal TAMV. Secondary outcomes: changes in serial TCD velocities. Indicence of acute events including stroke. Health‐related quality of life (planned but not recorded, see 'Selective reporting' below).
Notes	The planned length of follow‐up was 30 months but the study was terminated early after 15 months of follow‐up due to slow participant accrual and the unlikelihood of meeting the trial recruitment target (100) and the primary endpoint.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Randomisation was performed using an adaptive blocked algorithm.
Allocation concealment (selection bias)	Low risk	Central pharmacy distribution of allocations.
Incomplete outcome data (attrition bias)   All outcomes	Low risk	Numbers screened, randomised and number receiving randomised treatment reported. Analysis conducted using an ITT approach.
Selective reporting (reporting bias)	High risk	A planned outcome (health‐related quality of life) was not analysed or presented as the outcome was not sufficiently collected due to early study termination.
Other bias	High risk	Study terminated early after only 22 of planned sample size of 100 were recruited therefore study is likely to be statistically underpowered.
Blinding of participants and personnel (performance bias)   All outcomes	Low risk	Participants and personnel could not be masked to treatment allocation by design (hydroxyurea compared to observation only). The primary outcome (conversion to abnormal TAMV) was objective and determined by masked outcome assessors so lack of blinding of participants and personnel is unlikely to have affected results.
Blinding of outcome assessment (detection bias)   All outcomes	Low risk	All TCD site examiners, central reviewers were masked and site clinicians were masked to participants TCD results.

Methods	Phase III, multicentre, single masked (outcome assessors), non‐inferiority study conducted across 26 paediatric sites in the USA.
Participants	Participants with previous clinical stroke, aged 7 to 17 years, PRBC transfusions for at least 18 months and transfusional iron overload. No specific exclusion criteria stated. 133 participants were randomised; 67 randomised to hydroxyurea treatment and 66 to standard treatment. Mean (SD) age at study enrolment in years: hydroxyurea group 13.0 (4.0) years, standard treatment group 13.3 (3.8) years. 132 out of 133 participants with HbSS genotype, 61 females (46%).
Interventions	Hydroxyurea starting at 20 mg/kg/day escalated to MTD and phlebotomy compared to standard treatment (transfusions and chelation) for 30 months.
Outcomes	The primary outcome of the trial was a composite outcome. This involved occurrence of a secondary stroke and quantitative liver iron level change from baseline. Secondary outcomes included quality of life, non‐stroke neurological events, other SCD‐related events, growth and development, functional evaluations, neurocognitive evaluations, transfusion related complications, chelation related complications, hydroxyurea‐related complications, phlebotomy related complications, liver biopsy related complications and adverse and serious adverse events.
Notes	Numerous secondary outcomes were not reported in the main paper or any subsequent papers.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Study described as randomised, no further details given.
Allocation concealment (selection bias)	Unclear risk	No details were given on allocation concealment.
Incomplete outcome data (attrition bias)   All outcomes	Low risk	Withdrawals from treatment reported, all individuals included in analysis in an ITT approach.
Selective reporting (reporting bias)	High risk	Many of the secondary outcomes (such as growth and development, functional evaluations, neurocognitive evaluations) have not yet been reported, If these results can be included at a later date then this judgement will be reconsidered.
Other bias	Low risk	None identified.
Blinding of participants and personnel (performance bias)   All outcomes	Low risk	Participants and personnel could not be masked to treatment allocation by design (hydroxyurea compared to transfusion). The primary outcome (secondary stroke and quantitative liver iron level change from baseline) was objective and determined by masked outcome assessors so lack of blinding of participants and personnel is unlikely to have affected results.
Blinding of outcome assessment (detection bias)   All outcomes	Low risk	The primary outcome (secondary stroke and quantitative liver iron level change from baseline) were determined by a group of treatment masked neurologists and neuroradiologists.

Methods	Multicentre phase III randomised open‐label (partially masked) non‐inferiority study conducted at 26 paediatric hospitals and health centres in the USA and Canada.
Participants	Children aged 4 ‐ 16 years with SCA and abnormal TCD ultrasound velocities (> 200 cm per second) if they had received 12 months of chronic transfusions. Exclusion criteria were documented clinical stroke, TIA or severe vasculopathy. 121 participants were randomised; 60 randomised to hydroxyurea treatment and 61 to standard treatment. Mean (SD) age at study enrolment in years: hydroxyurea group 9.5 (2.6) years, standard treatment group 9.7 (3.2) years. 119 out of 121 participants with HbSS genotype, 73 females (60%).
Interventions	Hydroxyurea starting at 20 mg/kg/day escalated to MTD and phlebotomy compared to standard treatment (transfusions and chelation) for 24 months.
Outcomes	Primary outcome: maximum TCD time averaged mean velocity on the index side (i.e. the cerebral hemisphere with the higher mean arterial velocity at baseline assessment). Secondary outcomes: TCD velocity on the non‐index side, neurological events, new brain lesions, hepatic iron overload, SCD‐related events, treatment‐related complication (reported in this publication) neuropsychological status, quality of life and growth and (to be reported in future publications).
Notes	Study was terminated early at the first interim analysis when non‐inferiority was demonstrated, target sample size was met at early termination.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Centralised block randomisation with block size four, with stratification by site and balanced by baseline age and TCD velocity.
Allocation concealment (selection bias)	Low risk	Central randomisation and treatment allocation.
Incomplete outcome data (attrition bias)   All outcomes	Low risk	Numbers discontinuing the interventions stated, all participants included in analysis in an ITT analysis.
Selective reporting (reporting bias)	High risk	Outcomes of neuropsychological status, quality of life and growth were measured but results are not yet published. If these results can be included at a later date then this judgement will be reconsidered.
Other bias	Low risk	Study was terminated early at the first interim analysis when non‐inferiority was demonstrated, target sample size was met at early termination so the study is adequately powered to detect differences.
Blinding of participants and personnel (performance bias)   All outcomes	Low risk	Participants and personnel could not be masked to treatment allocation by design (hydroxyurea compared to transfusion). The primary outcome (maximum TCD time averaged mean velocity) was objective and determined by masked outcome assessors so lack of blinding of participants and personnel is unlikely to have affected results.
Blinding of outcome assessment (detection bias)   All outcomes	Low risk	All TCD examinations were read centrally by observers blinded to treatment allocation and previous TCD results.

Study	Reason for exclusion
Al‐Nood 2011	This is not a randomised study.
De Montalembert 2006	This study does not fulfil the inclusion criteria. The length of treatment was 8 days, the inclusion criteria state that treatment should be at least 1 month.
George 2013	This does not make a randomised comparison of hydroxyurea and placebo or standard treatment (the randomised comparison is dosing schedules of hydroxyurea).
NCT00004492	This does not make a randomised comparison of hydroxyurea and placebo or standard treatment.
NCT01848925	This study does not fulfil the inclusion criteria. The length of treatment was 7 days, the inclusion criteria state that treatment should be at least 1 month.
NCT01960413	This does not make a randomised comparison of hydroxyurea and placebo or standard treatment.
NCT02149537	This is not an appropriate design to measure the effectiveness of hydroxyurea (cross‐over design of low‐dose hydroxyurea compared to no treatment to monitor those at increased risk of infection).
Pushi 2000	This is not a randomised study.
Silva‐Pinto 2007	This is not a randomised study.
Silva‐Pinto 2014	This is not a randomised study.
Vichinsky 2013	This does not make a randomised comparison of hydroxyurea and placebo or standard treatment.
Voskaridou 2005	This is not a randomised study.

Methods	Prospective, randomised, placebo‐controlled, double‐blinded phase III trial (NOHARM trial)
Participants	Study participants will be recruited from the Mulago Hospital Sickle Cell Clinic (MHSCC) in Kampala, Uganda. Children aged 1 to 4 years with documented HbSS living in an area of meso‐endemic malaria transmission.
Interventions	Hydroxyurea 20 ± 2.5 mg/kg/day compared to placebo
Outcomes	Primary outcome: malaria incidence, defined as episodes of clinical malaria occurring over the 1‐year randomised study treatment period. Secondary outcomes: frequency of haematologic toxicities and AEs, relationships between hydroxyurea treatment and fetal haemoglobin, soluble intracellular adhesion molecule‐1, and nitric oxide levels, and between levels of these factors and risk of subsequent malaria.
Notes	Currently, only a protocol is available for the NOHARM study. We are unsure if this study meets the inclusion criteria of the review of 'Type of Participants' due to the study objectives around determining the incidence of malaria in SCA individuals. We will make an assessment of the eligibility of the population when study results are available.

Methods	Randomised, double‐blind, parallel group, dose‐controlled study (SPRING)
Participants	Participants with HbSS or HbSβº‐thalassemia, S variant with baseline haemoglobin less than 10 g/dL or other sickle cell syndromes apart from HbSC between the ages of 5 and 12 years, living in sub‐Saharan Africa, without prior overt stroke. Inclusion criteria for a 'non‐elevated TCD group' (participants who are not eligible to receive hydroxyurea therapy but are willing to be following for a minimum of 3 years).
Interventions	Hydroxyurea (moderate dose): 20 mg/kg/day (range 17.5 ‐ 26 mg/kg/day) for 24 months. Hydroxyurea (low dose): 10 mg/kg/day (range 7 ‐ 15 mg/kg/day) for 24 months.
Outcomes	Primary outcome: efficacy of moderate vs low‐dose hydroxyurea therapy for primary stroke prevention. Secondary outcome: incidence of all‐cause hospitalizations. Secondary outcome: long‐term safety of hydroxyurea therapy.
Notes	Unclear if this is an appropriate design (dose‐control) and population for the review ('non‐elevated TCD group'). Trial is ongoing (estimated completion date ‐ December 2021), we will make an assessment of the eligibility of the design and population when trial results are available.

PERMALINK

Hydroxyurea (hydroxycarbamide) for sickle cell disease

Sarah J Nevitt

Ashley P Jones

Jo Howard

Abstract

Background

Objectives

Search methods

Selection criteria

Data collection and analysis

Main results

Authors' conclusions

Plain language summary

Summary of findings

Background

Description of the condition

Description of the intervention

How the intervention might work

Why it is important to do this review

Objectives

Methods

Criteria for considering studies for this review

Types of studies

Types of participants

Types of interventions

Types of outcome measures

Primary outcomes

Secondary outcomes

Search methods for identification of studies

Electronic searches

Searching other resources

Data collection and analysis

Selection of studies

Data extraction and management

Assessment of risk of bias in included studies

Measures of treatment effect

Unit of analysis issues

Dealing with missing data

Assessment of heterogeneity

Data synthesis

Subgroup analysis and investigation of heterogeneity

Sensitivity analysis

Summary of findings and quality of the evidence (GRADE)

Summary of findings for the main comparison. Summary of findings ‐ Hydroxyurea compared with placebo for sickle cell disease.

Summary of findings 2. Summary of findings ‐ Hydroxyurea and phlebotomy compared to transfusion and chelation for people with sickle cell disease and an increased risk of stroke.

Summary of findings 3. Summary of findings ‐ Hydroxyurea compared with observation for people with sickle cell disease and an increased risk of stroke.

Summary of findings 4. Summary of findings ‐ Hydroxyurea compared with no hydroxyurea for people with sickle cell disease.

Results

Description of studies

Results of the search

1.

Included studies

Excluded studies

Risk of bias in included studies

2.

Allocation

Blinding

Incomplete outcome data

Selective reporting

Other potential sources of bias

Effects of interventions

1. Hydroxyurea compared to placebo for participants with SCD

2. Hydroxyurea and phlebotomy compared to transfusion and chelation for participants with SCD and an increased risk of stroke

3. Hydroxyurea compared to observation for participants with SCD and an increased risk of stroke

4. Hydroxyurea compared to no hydroxyurea for participants with SCD

1. Clinical events and markers of response in Jain 2012.

2. Laboratory measurements from MSH 1995.

3. Laboratory evaluations from the SWiTCH trial.

4. Laboratory evaluations from the SCATE trial.

Primary outcomes

1. Pain alteration

Hydroxyurea compared to placebo for participants with SCD

1.1. Analysis.

1.2. Analysis.

Hydroxyurea and phlebotomy compared to transfusion and chelation for participants with SCD and an increased risk of stroke

2.1. Analysis.

Hydroxyurea compared to observation for participants with SCD and an increased risk of stroke

Hydroxyurea compared to no hydroxyurea for participants with SCD

2. Life‐threatening illness

Trial name or title	NCT01389024: Hydroxyurea to Prevent Brain Injury in Sickle Cell Disease (HUPrevent)
Methods	Randomised, double‐blind, parallel design, phase 2 study.
Participants	Participants with HbSS or HbSβº‐thalassemia aged between 9 and 48 months of age, with or without central nervous system complications.
Interventions	Hydroxyurea 20 mg/kg/day increased by 5 mg/kg every 8 weeks to maximum of 35 mg/kg/day; placebo (sucrose) 0.2 mL/kg/day increased to max of 0.35 mL/kg/day.
Outcomes	Primary outcome: central nervous system complications (a composite of abnormally elevated cerebral blood flow velocity as measured by TCD ultrasound, SCI, or stroke) Secondary outcome: proportion of participants with severe adverse events attributed to study procedures.
Starting date	October 2011
Contact information	Johns Hopkins University Diane Weiss, BA (dweiss14@jhmi.edu) James F. Casella, MD (jcasella@jhmi.edu)
Notes	Estimated completion date October 2017 (final data collection date for primary outcome measure)