Siklos (Hydroxyurea) for Sickle Cell Anemia

Siklos (hydroxyurea) is a prescription medication approved by the U.S. Food and Drug Administration (FDA) for adults and children ages 2 and older with sickle cell anemia who have repeated moderate to severe pain crises. It helps reduce the frequency of pain crises and the need for blood transfusions.

How Siklos Works and How It’s Taken

Siklos is a type of medicine called an antimetabolite. It blocks an enzyme involved in making DNA, but the exact way Siklos helps people with sickle cell anemia is not fully known. It may increase fetal hemoglobin, lower certain white blood cells called neutrophils, increase the amount of water in red blood cells, and help sickled red blood cells stay more flexible.

Doctors prescribe Siklos when adults and children ages 2 and older with sickle cell anemia have repeated moderate to severe pain crises. It helps reduce how often these pain crises happen and reduces the need for blood transfusions.

Siklos is taken by mouth once a day at the same time each day. Swallow the prescribed dose with a glass of water. If you cannot swallow the tablets, you can dissolve your prescribed dose in a small amount of water in a teaspoon and swallow it right away.

Siklos is a cytotoxic medicine and must be handled carefully. You or your caregiver should wear disposable gloves when handling the tablets or bottle and wash your hands with soap and water before and after handling them. Avoid touching crushed tablets.

If crushed medicine touches your skin, wash the area right away with soap and water. If it gets in your eyes, rinse them with water or isotonic eyewash for at least 15 minutes. Wipe up spilled powder with a damp disposable towel, throw the towel away in a closed container, and clean the area with detergent and water.

Typical Dosing for Sickle Cell Anemia

The recommended starting dose is 15 milligrams per kilogram once daily for adults and 20 milligrams per kilogram once daily for children. The dose is based on the person’s actual or ideal body weight, whichever is lower.

Blood counts should be checked every two weeks. If blood counts stay within an acceptable range, the dose may be increased by 5 milligrams per kilogram per day every eight weeks, or sooner if a painful crisis occurs. The dose may be increased up to the highest dose the person can tolerate or 35 milligrams per kilogram per day.

If blood counts become too low, Siklos should be stopped until the blood counts recover. Treatment may then be restarted at a dose that is 5 milligrams per kilogram per day lower than the dose linked to the blood-count problem.

For people with a creatinine clearance below 60 milliliters per minute or with end-stage kidney disease, the starting dose of Siklos should be reduced by 50 percent. Creatinine clearance is a measure of how well the kidneys are working.

This information is based on prescribing information, but your healthcare provider may tailor your treatment plan. Always follow their guidance.

Siklos Side Effects

In clinical studies of Siklos, the most common side effects occurred in more than 10 percent of people.

Side effects in children include:

Infections
Low white blood cell counts (neutropenia)

Side effects in adults include:

Infections
Headache
Dry skin

Serious Side Effects and Warnings

Siklos can cause serious side effects that may require immediate medical attention. These include:

Myelosuppression — Dangerously low blood cell counts can raise the risk of infections, bleeding, and severe anemia.
Cancer — This includes leukemia and skin cancer.
Embryo-fetal toxicity — Siklos can harm a developing baby.
Cutaneous vasculitic toxicities — These can cause serious blood vessel and skin problems, including leg ulcers and, in severe cases, skin tissue death.
Hemolytic anemia — This is the fast breakdown of red blood cells, which can worsen anemia and may cause yellowing of the skin or blood in the urine.

Get medical help right away if you think you are having a serious reaction.

How To Save on Siklos

Medunik USA, the manufacturer of Siklos, offers the Siklos Savings Programs. Eligible people with commercial insurance may pay as little as $0. Savings programs may also be available if you do not have commercial insurance or your insurance does not cover Siklos.

The Siklos Savings Programs include a Siklos Copay Card and a Patient Assistance Program.

To learn more, visit the Siklos Savings Programs page or call 844-633-8645.

What To Know Before Taking Siklos

Before starting Siklos, your doctor will check your blood cell counts. If you can become pregnant, your healthcare provider will also check your pregnancy status before treatment.

Tell your doctor if you plan to get any vaccines. You should not receive live vaccines while taking Siklos.

Do not take Siklos if you have had an allergic reaction to hydroxyurea or any ingredient in Siklos.

Tell your doctor about all medicines you take, including prescription drugs, over-the-counter medicines, vitamins, and herbal supplements.

Before taking Siklos, tell your doctor if you:

Have kidney problems or receive hemodialysis
Have liver problems
Have HIV or take HIV medicines
Have high uric acid levels in your blood
Have received interferon treatment in the past or are currently receiving it
Have leg wounds or ulcers
Use a continuous glucose monitor (CGM) to check your blood sugar

Siklos may affect some sensor glucose readings and could lead to low blood sugar if those readings are used to decide how much insulin to take.

Siklos can harm a developing baby. If you can become pregnant, use effective birth control while taking Siklos and for at least six months after treatment ends. Avoid becoming pregnant during treatment.

Tell your healthcare provider right away if you become pregnant or think you may be pregnant.

If you are male and have a female partner who can become pregnant, use effective birth control while taking Siklos and for at least six months after treatment ends. Siklos can affect sperm and may cause fertility problems in males.

Do not breastfeed while taking Siklos.

Community FAQs

These answers are fact-checked by our editorial staff.

How effective is Siklos for sickle cell anemia?

In a study of children with sickle cell disease who had not previously taken hydroxyurea, fewer children had sickle cell-related problems after 12 months of Siklos treatment. The percentage who had at least one vaso-occlusive episode, or pain crisis caused by blocked blood flow, was 69 percent in the 12 months before treatment and 42.5 percent after 12 months of treatment. The percentage who needed at least one blood transfusion was 46 percent before treatment and 23 percent after treatment.

Adults who had not previously taken hydroxyurea also had fewer sickle cell-related problems after 12 months of Siklos treatment. The percentage who had at least one vaso-occlusive episode was 63.8 percent before treatment and 38.4 percent after treatment. The percentage who needed at least one blood transfusion was 43.3 percent before treatment and 18.9 percent after treatment.

The study also found decreases in acute chest syndrome and hospitalizations related to sickle cell disease.

How long does Siklos take to work for sickle cell anemia?

The clinical study does not say exactly how long Siklos takes to start working. In the study, fetal hemoglobin levels were measured at around six months and were higher than before treatment. Sickle cell-related events were compared after 12 months of treatment.

Does Siklos interact with other medicines?

Yes. Siklos can cause serious problems when taken with certain HIV medicines, including didanosine and stavudine (both now discontinued in the United States). These combinations have been linked to inflammation of the pancreas, liver problems, and nerve damage.

Tell your healthcare provider about all medicines you take. You should also avoid live vaccines while taking Siklos.