Hydroxyurea for Sickle Cell: Dosing, Transfusion, New Drugs

Hydroxyurea for sickle cell disease is the foundation of disease-modifying treatment. In Dr. Doval's protocol, the dose starts at 10 to 15 mg/kg a day, is increased by 5 mg/kg every two months as tolerated, and can be given up to 35 mg/kg. It reduces pain crises, chest crises, transfusions and hospital admissions, and it has been used safely long term.

Dr. Divya Doval, Principal Consultant - Haemato Oncology & BMT, BLK-Max Super Speciality Hospital, New Delhi, India, calls hydroxyurea "the wonder drug". This article sets out how she uses it, where transfusion fits, what the newer drugs offer and where their evidence stands, how sickle cell disease is managed in pregnancy, and when the conversation should turn to cure. The wider framework is in the complete guide to diagnosis and management of sickle cell disease.

The sickle cell treatment ladder

Dr. Doval placed treatment on a clear ladder:

1. General medical care: vaccination, eye screening, nutrition, blood pressure control.
2. Pain management and complication screening, acute and chronic.
3. Disease-modifying therapy: transfusion, hydroxyurea, L-glutamine, crizanlizumab and voxelotor.
4. Curative therapy: haematopoietic stem cell transplant, with gene therapy as the newer option.

The general care component is specific. Patients need pneumococcal vaccination every 5 years, influenza vaccination annually and hepatitis A and B vaccination. An annual eye examination screens for retinopathy, and blood pressure control matters because conventional risk factors add to stroke risk.

How hydroxyurea works in sickle cell disease

Hydroxyurea acts on several parts of the disease at once:

• it increases fetal haemoglobin (HbF), which does not sickle and dilutes HbS inside the cell;
• it reduces the neutrophil and reticulocyte counts and makes these cells less adhesive;
• it improves red cell hydration, which reduces sickling;
• it releases nitric oxide, which dilates blood vessels and relieves pain.

The clinical result is fewer pain crises and chest crises, fewer transfusions and fewer hospital admissions. Dr. Doval described it as quite a safe drug that can be given long term, and her view is that every patient should receive at least 10 mg/kg to reduce these complications. Some literature has raised a concern about malignancy, but in her experience it has been used safely for years.

When to start hydroxyurea

Her indications to start are:

• more than three pain crises a year;
• more than two chest crises.

For children with HbSS or HbS beta-zero thalassaemia, current guidance goes further: hydroxyurea is offered from 9 months of age regardless of clinical severity.

It can also be considered for patients with:

• chronic leg ulcers;
• symptomatic anaemia with alloimmunisation (antibodies against donor red cells) that prevents regular transfusion;
• persistent priapism not responding to standard therapy;
• increased haemolysis;
• pulmonary hypertension.

In stroke prevention, hydroxyurea at the maximum tolerated dose is the option for children who stop regular transfusion after at least a year, as the TWiTCH trial showed. The stroke evidence is set out in the article on sickle cell stroke prevention and chronic organ damage.

Hydroxyurea dose and monitoring

Dr. Doval's hydroxyurea dosing protocol for sickle cell disease:

The drug is generally well tolerated. The toxicities to watch for are gastrointestinal effects (nausea and vomiting), skin hyperpigmentation and pigmentation of the nails, alongside liver, kidney and blood count changes. If toxicity develops, the approach is to stop hydroxyurea, monitor, wait a couple of weeks and resume at a lower dose.

Fertility and pregnancy

Hydroxyurea should be stopped at least three months before a planned conception. In men it can reduce sperm count and sperm motility. It is teratogenic in animal studies (it can harm a developing fetus), so it is avoided throughout pregnancy and breastfeeding.

Newer sickle cell drugs: L-glutamine, crizanlizumab, voxelotor

Three newer drugs have entered sickle cell care. Dr. Doval was measured about each, and the evidence on two of them has shifted.

L-glutamine

L-glutamine is an antioxidant that may increase the availability of reduced glutathione, reducing the oxidative stress that contributes to the disease. It was approved by the US Food and Drug Administration (FDA) in 2017. Dr. Doval noted that she has no personal experience of using it.

Crizanlizumab

Crizanlizumab is a monoclonal antibody against P-selectin, an adhesion molecule that helps sickled cells stick to the vessel wall. The SUSTAIN trial, published in the New England Journal of Medicine in 2017, showed a reduction in pain crises with high-dose crizanlizumab against placebo. It is given as a 30-minute infusion every 4 weeks. Dr. Doval cautioned that patients in the trial may also have been taking hydroxyurea; in SUSTAIN, a majority of participants were.

The later STAND trial did not confirm a reduction in pain crises, and the European Medicines Agency revoked crizanlizumab's EU authorisation in 2023. Its place in treatment is therefore uncertain and varies by country.

Voxelotor

Voxelotor is a haemoglobin oxygen-affinity modulator: it holds haemoglobin in its oxygenated form, which raises haemoglobin and reduces haemolysis. It was taken orally at 1,500 mg a day and approved by the FDA in 2019. In September 2024 its manufacturer voluntarily withdrew it worldwide because of safety concerns, so it is no longer an option.

Blood transfusion in sickle cell disease: simple and exchange

Transfusion is the oldest disease-modifying therapy, and it comes in two forms. A simple transfusion adds donor red cells. A red cell exchange removes the patient's sickle cells and replaces them with donor cells, lowering HbS without raising total haemoglobin or blood volume.

Dr. Doval's centre is a tertiary hospital that performs many red cell exchanges, particularly before surgery. For complicated, high-risk surgery, pre-operative transfusion or exchange brings HbS below 30% so that the operation is safe.

Keep haemoglobin at 10 g/dL or less

Her most important caution: every time a patient is transfused, keep the haemoglobin at 10 g/dL or less. Above this, blood viscosity rises, which increases sickling and painful episodes. The same principle explains why transfusion for splenic sequestration must be slow.

Complications of transfusion

Three complications matter most:

• Iron overload, which needs structured monitoring with ferritin and MRI; the thresholds for chelation are set out in the article on chronic organ damage.
• Alloimmunisation, the development of antibodies against donor red cells, which Dr. Doval noted is a much bigger problem in sickle cell patients than in the thalassaemia patients her team also treats.
• Delayed haemolytic transfusion reactions, in which transfused cells are destroyed days after the transfusion.

Transfusion has defined indications: stroke prevention, acute chest syndrome that is progressing, splenic sequestration, aplastic crisis, retinal artery occlusion and preparation for surgery. It is not indicated in a simple pain crisis. The acute indications are covered in the guide to managing sickle cell crises and acute emergencies.

Sickle cell disease in pregnancy

Pregnancy is high-risk for both mother and baby. Dr. Doval's approach:

• First trimester. Hydroxyurea is not given during pregnancy, which is challenging. The priority is to keep the patient well hydrated and warm and to prevent crises.
• Second and third trimesters. Transfusions can be given if needed; hydroxyurea remains avoided.
• Labour. A crisis during labour or childbirth can be very difficult to manage.

Morbidity and mortality are high. Every patient should be counselled before starting a family that the risks to both mother and baby are significant. A participant in the session added that pregnancy should be managed by a multidisciplinary team under close obstetric supervision.

When to move from management to cure

Dr. Doval's case for transplant is built on what medical management cannot do. Managing sickle cell disease is ongoing, one complication after another, and even with excellent care silent ischaemic events gradually impair cognition. A stem cell transplant is a one-time treatment, and the cure is for life.

Indications for transplant

The published indications she presented are:

• irreversible end-organ damage;
• stroke;
• elevated tricuspid regurgitant jet velocity (TRV) on echocardiography, a marker of raised pulmonary pressure;
• sickle nephropathy;
• sickle hepatopathy;
• two or more vaso-occlusive crises requiring hospitalisation every year;
• any episode of acute chest syndrome while on hydroxyurea.

Her own centre goes further for children. Because half-matched (haploidentical) transplant has widened donor availability and outcomes have improved so much, more families are willing to accept the risk. Her indications now include any child with a single stroke, a single episode of acute chest syndrome, or multiple pain crises requiring admission.

Outcomes

Data from three transplant registries covering about 1,000 matched sibling transplants performed between 1986 and 2013 showed 5-year overall survival of 95% in children under 16 and around 81% in older patients. Reduced-intensity conditioning achieved overall survival of 93%. With haploidentical transplants, Dr. Doval reported overall survival of 70% to 80%.

There is no upper age limit: her centre has transplanted adults. But risk increases with age, and so does organ damage from stroke and liver and kidney disease, which reduces eligibility. Transplant at a younger age gives the best outcome.

Cost

Dr. Doval put the cost of a matched sibling transplant at her centre at around US$20,000 to 24,000. A half-matched transplant costs more: she gave a figure of around US$35,000 in one answer and US$40,000 to 45,000 in another. Jivo Healthcare's cost pages for matched sibling donor BMT and haploidentical half-match BMT set out what is included.

For the transplant pathway in depth, see Dr. Dharma Choudhary's guide to sickle cell transplant outcomes and Dr. Vikas Dua's analysis of sickle cell transplant cure rates by donor, age and regimen.

Gene therapy for sickle cell disease

Gene therapy is the other approved transformative therapy. It works like an autologous stem cell transplant, using the patient's own cells:

1. Haematopoietic stem cells are collected from the patient.
2. The cells are modified in the laboratory, either by gene addition (inserting a working globin gene) or by gene editing.
3. The patient receives myeloablative conditioning chemotherapy (high-dose treatment that clears the bone marrow), usually busulfan.
4. The modified cells are infused back, and the team waits for engraftment.

Both approaches received FDA approval in December 2023 for patients aged 12 and older. The barriers are significant: the cost is prohibitive, and the conditioning chemotherapy carries a risk of infertility and secondary malignancy.

Questions from the session on transplant cost, age limits and success rates, with Dr. Doval's full answers, are on the Live Discussion page.