A JAK inhibitor for myelofibrosis with low platelets
Pacritinib
A daily medicine used to treat myelofibrosis, especially in people who have low platelets.
What is Pacritinib?
Pacritinib is a specialist medicine used to treat myelofibrosis, a bone-marrow condition that can cause an enlarged spleen and troublesome symptoms. It belongs to a group called JAK inhibitors and is particularly suited to people whose platelet counts are low, where some similar medicines are harder to use. The most important safety issues are bleeding, including serious haemorrhage, and diarrhoea, which can be troublesome. It can also affect the heart, including its electrical rhythm (the QT interval), and increase the risk of infections. It is taken by mouth and monitored closely by a blood specialist team.
Education and reference only. This is a plain-language guide to Pacritinib — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Pacritinib is a targeted treatment for myelofibrosis, a condition in which the bone marrow becomes scarred and cannot make blood cells properly, often causing an enlarged spleen and symptoms such as tiredness, night sweats and itching. It is one of a group of medicines called JAK inhibitors, which calm the overactive signalling behind the condition. Its particular place is in people who have a low platelet count, a situation where some other JAK inhibitors are difficult to use safely. It is taken by mouth, usually twice a day, under the supervision of a blood (haematology) specialist team.
How it works
In myelofibrosis, signalling through pathways known as JAK pathways becomes overactive, driving the abnormal bone-marrow activity, the enlarged spleen and the symptoms. Pacritinib blocks these signals, which can shrink the spleen and ease symptoms. It is designed so that it can be used even when platelet counts are low, which is why it is useful for people in whom other similar medicines are harder to give. It is taken regularly to keep the signalling dampened, with close monitoring of blood counts, bleeding, bowel symptoms and the heart, which are the main safety concerns.
Company & origin
Originated / developed by: Specialist manufacturer.
A specialist medicine used to treat myelofibrosis, particularly in people whose platelet counts are low.
Practical use
How to take Pacritinib
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it by mouth as prescribed, usually twice a day, swallowing the capsules whole with water.
- Report any unusual bruising or bleeding, blood in the urine or stool, or bleeding that does not stop, straight away.
- Manage diarrhoea early as advised and keep well hydrated, telling your team if it is troublesome.
- Attend your blood tests and any heart tracings, as it can affect blood counts and the heart's rhythm.
- Report signs of infection such as fever promptly, and do not stop the medicine suddenly without advice.
Weighing it up
Advantages & disadvantages of Pacritinib
Advantages
- Eases the enlarged spleen and symptoms of myelofibrosis even when platelet counts are low.
- Offers an option for people in whom some other JAK inhibitors are difficult to use.
- Taken by mouth under specialist care.
Disadvantages
- Can cause bleeding, including serious haemorrhage, which needs careful monitoring.
- Commonly causes diarrhoea, which can be troublesome.
- Can affect the heart's rhythm and increase the risk of infections.
Practical use
Good to know
Pacritinib is particularly useful for people with myelofibrosis whose platelets are low, but bleeding is still its most important safety concern: serious bleeding can happen, so any unusual bruising, blood in the urine or stool, or bleeding that does not stop must be reported urgently, and the team manages other medicines that thin the blood carefully. Diarrhoea is common and can be troublesome, so early management and good hydration help. It can affect the heart, including its electrical rhythm (the QT interval), so heart tracings (ECGs) and blood-salt checks may be used and medicines that also affect the rhythm are reviewed. Because it dampens the immune system, infections are more likely, so signs such as fever need prompt attention. It is taken under specialist supervision and should not be stopped suddenly without advice.
Who should not take it / use with caution
- People who have had a serious allergic reaction to pacritinib should not take it.
- It is used with particular care in people who are bleeding, on blood-thinning medicines, or with heart-rhythm problems.
- It is generally avoided in pregnancy and breastfeeding unless a specialist judges it necessary.
Monitoring
- Regular blood counts and watching closely for bleeding or unusual bruising.
- Heart tracings (ECGs) and blood-salt checks where appropriate, and attention to diarrhoea and hydration.
- Watching for signs of infection and reviewing spleen size and symptoms over time.
Side effects
- Bleeding, including, rarely, serious haemorrhage, and bruising.
- Diarrhoea, nausea, vomiting and swelling of the legs.
- Low blood counts, infections and tiredness.
- Changes in the heart's rhythm (the QT interval), which can rarely lead to dangerous rhythms.
Key interactions
- Blood-thinning and antiplatelet medicines can add to the risk of bleeding and are reviewed carefully.
- Medicines that affect the heart's QT interval should be reviewed, as the effect can add up.
- Some medicines can change pacritinib levels, so tell your team about everything you take.
Available as: Capsules taken by mouth.
Answers
Pacritinib: frequently asked questions
What is pacritinib used for?
It is used to treat myelofibrosis, easing the enlarged spleen and symptoms, and is particularly suited to people whose platelet counts are low.
Why is bleeding a concern?
Pacritinib can cause bleeding, including serious haemorrhage, so any unusual bruising, blood in the urine or stool, or bleeding that will not stop must be reported urgently.
Why might I have heart tracings?
It can affect the heart's electrical rhythm (the QT interval), so heart tracings (ECGs) and blood-salt checks may be used and other medicines that affect the rhythm are reviewed.
What should I do about diarrhoea?
Diarrhoea is common; manage it early as advised, keep well hydrated, and tell your team if it is troublesome, as the treatment may need adjusting.
Why is it useful when platelets are low?
It is designed to be usable even when platelet counts are low, where some other JAK inhibitors are harder to give safely.
The wider class
About JAK inhibitor (myelofibrosis)
Pacritinib belongs to the jak inhibitor (myelofibrosis) class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
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Authoritative sources
- BNF
- NICE CKS
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