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Study of pelabresib (DAK539) plus ruxolitinib versus placebo plus ruxolitinib in adult patients with myelofibrosis who are JAK‑inhibitor‑naive

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What is this trial about?

A plain-language summary of the goals, design and what participants do

Myelofibrosis is a rare blood condition in which scar tissue forms in the bone marrow, the area that makes blood cells. This scar tissue can cause the spleen, an organ that helps filter blood, to become enlarged and can lead to symptoms such as tiredness, night sweats, and weight loss.

The study is testing whether adding the experimental oral drug pelabresib (code name DAK539) to the approved oral medication ruxolitinib improves reduction of spleen size and relief of symptoms compared with taking ruxolitinib together with a placebo. The purpose of the study is to determine if the combination therapy provides a greater benefit than the standard treatment alone.

Participants will take the study tablets each day for several months. Throughout the trial they will undergo imaging tests, such as MRI or CT scan, to measure the size of the spleen, and they will complete simple questionnaires about how they feel. Regular health checks will be performed to monitor safety, and the study will continue for about a year to observe how the treatment works over time.

The research process

The trial runs in 7 steps – from screening to follow-up. Each step says what happens and what the team monitors.

  1. Step 1

    Randomization

    After joining, you are assigned by a computer to receive either pelabresib plus ruxolitinib or placebo plus ruxolitinib. the assignment is double‑blind, meaning neither you nor the study staff know which study medication you receive.

  2. Step 2

    Baseline assessments

    Before starting medication, you undergo imaging of the spleen using mri or ct scan to measure spleen size.

    You complete a questionnaire that records the total symptom score (tss) to evaluate disease‑related symptoms.

    Blood samples are taken to check hemoglobin levels and other laboratory values.

  3. Step 3

    Start of background medication

    You begin taking ruxolitinib tablets orally. the prescribed dose is 25 mg per day, taken as a single oral tablet each day.

    The medication is continued for the duration of the trial unless a dose adjustment is required for safety.

  4. Step 4

    Start of study medication

    On the same day as the background medication, you start the study medication.

    If assigned to the active arm, you take pelabresib 175 mg orally once daily.

    If assigned to the control arm, you take a matching placebo tablet once daily.

    The study medication is taken at the same time each day and continues for the full study period.

  5. Step 5

    Regular clinic visits and assessments

    You attend scheduled visits at week 12, week 24, week 36 and week 48.

    At each visit, a new mri or ct scan is performed to evaluate spleen volume reduction.

    The total symptom score (tss) questionnaire is repeated to track symptom improvement.

    Blood tests are repeated to monitor hemoglobin and other safety parameters.

    Any side effects or adverse events are recorded and managed by the study team.

  6. Step 6

    Continuation of medication

    You continue taking both ruxolitinib and the assigned study medication (pelabresib or placebo) daily throughout the trial.

    The medication course lasts at least 48 weeks, or longer if the study protocol allows continued treatment after the final assessment.

  7. Step 7

    Final assessment and study end

    At the end of the study period, a final imaging scan and symptom questionnaire are performed.

    Overall results, including spleen volume change, symptom score change, and safety data, are collected.

    After the final visit, study medication is stopped according to the protocol.

Who can join the trial?

8 criteria

  • You must have been diagnosed with primary myelofibrosis or with myelofibrosis that developed after another blood disorder called polycythemia vera or essential thrombocythemia, according to the latest international classification system for blood cancers.
  • Your disease must be classified as intermediate‑1, intermediate‑2, or high‑risk using the DIPSS scoring system, which measures how advanced the condition is.
  • Your spleen (the organ that helps filter blood) must be enlarged to a size of at least 450 cubic centimeters as shown on a CT (computed tomography) or MRI (magnetic resonance imaging) scan.
  • You need an average total symptom score (TSS) of 15 or higher, measured within the week before you join the study, using the Myelofibrosis Symptom Assessment Form (MFSAF) version 4.0. At least four out of seven daily symptom assessments are required to calculate this average.
  • Your overall health and ability to perform daily activities must be rated as ECOG performance status 0, 1, or 2. This scale ranges from fully active (0) to able to care for yourself but unable to work (2).
  • The number of immature blood cells called blasts in your circulating blood must be less than 5%. This test is done during the screening visit.
  • Your platelet count (cells that help blood clot) must be at least 100 × 10⁹ per liter, and you must not have received medicines that boost blood cell production (growth factors) or blood transfusions in the four weeks before screening.
  • You must be an adult, typically 18 years of age or older, and you can be either male or female.

Who cannot join the trial?

6 criteria

  • Anyone who has had a splenectomy (surgical removal of the spleen) at any time, or who received splenic irradiation (radiation treatment to the spleen) in the last 6 months cannot join the study.
  • People who have already had a hematopoietic cell transplant (stem‑cell transplant to replace blood‑forming cells) or who are expected to have one within 24 weeks after randomization are not eligible.
  • If a bone‑marrow test shows blasts (immature blood cells) making up 5% or more, or if the patient has a history of an accelerated phase (rapid disease progression) or leukemic transformation (the disease turning into leukemia), they cannot participate.
  • Anyone who has had another malignancy (cancer) – other than myelofibrosis variants – that required systemic treatment (medicine that works throughout the whole body) within the past 3 years is excluded.
  • Patients who took any approved or investigational drug for myelofibrosis, other than hydroxyurea or anagrelide, within 14 days before the first study dose—or within five times the drug’s half‑life (the time it takes for half of the drug to leave the body)—cannot join.
  • Anyone who has previously been treated with a JAK inhibitor (a medicine that blocks a specific signaling pathway) or a Bromodomain and extraterminal domain (BET) inhibitor (a newer class of drugs targeting certain proteins) is not allowed in the trial.
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Investigated drugs

  • Ruxolitinib

    is a medicine taken by mouth that blocks a protein called JAK, which is involved in the growth of certain blood cells. In this study, it is used as the standard treatment that all participants receive. The drug helps to control the disease by reducing inflammation and abnormal cell activity, which can lessen the size of an enlarged spleen and improve symptoms such as fatigue and night sweats.

  • Pelabresib

    (also called DAK539) is an experimental oral medication being tested in this trial. It works in a different way from ruxolitinib, aiming to further reduce the abnormal growth of blood cells and the scar tissue in the bone marrow. When added to ruxolitinib, pelabresib is being studied to see if it can shrink the spleen more than ruxolitinib alone and improve the overall symptoms experienced by patients with myelofibrosis.

What is already known about the treatment

  • Ruxolitinib

    This medication is taken by mouth as a tablet and is already approved and widely used for treating myelofibrosis, a rare blood disease. It works by blocking the activity of JAK1 and JAK2 proteins, which helps reduce abnormal cell growth and inflammation. Ruxolitinib belongs to the class of drugs called Janus kinase (JAK) inhibitors. It is given to patients who need relief from an enlarged spleen and disease‑related symptoms.

  • Pelabresib

    Pelabresib is an oral tablet that is still under investigation and not yet approved for general medical use. It is being studied for myelofibrosis and works by inhibiting BET proteins, which changes the expression of genes that drive disease growth. Pelabresib is classified as a BET bromodomain inhibitor, a type of epigenetic modulator. The drug aims to improve spleen size and symptom scores when used together with ruxolitinib.

Investigated diseases

Myelofibrosis - Myelofibrosis is a blood disorder in which the bone marrow becomes scarred and produces fewer healthy blood cells. As the scar tissue expands, the marrow shrinks and blood production shifts to other organs such as the spleen and liver. This shift often causes an enlarged spleen and a gradual decline in red cells, white cells, and platelets. Symptoms may develop slowly, with increasing fatigue, weight loss, and changes in blood counts. The disease can progress from mild early changes to extensive marrow fibrosis and organ enlargement.
Trial detailsLast updated 2 Oct 2026
Age18+ yearsPhasePhase IIITrial ID2025-523555-66-00Protocol codeCDAK539A12303Estimated enrolment461 patientsSponsorNovartis Pharma AG

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