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Phase 2 Study of Daxdilimab with Prednisone in Adults with Inadequately Controlled Dermatomyositis or Anti‑Synthetase Inflammatory Myositis

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What is this trial about?

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

The study focuses on two rare muscle‑inflammatory conditions, Dermatomyositis and anti‑synthetase inflammatory myositis. Both diseases cause muscle weakness and skin rashes because the immune system mistakenly attacks muscle and skin tissue. Participants will receive a subcutaneous injection, which means the medicine is given just under the skin, of the experimental drug daxdilimab. In addition, they will continue their usual oral steroid medication, prednisone, which helps control inflammation.

The purpose of the trial is to determine whether the experimental drug can lower disease activity compared with a placebo. Adults will be randomly assigned to receive either the study drug or the placebo, and neither the participants nor the study staff will know which treatment is given. Injections will be given at regular intervals over about six months, with clinic visits to assess muscle strength, skin involvement, and any side effects. Researchers will track changes using simple scores that measure overall improvement and skin disease activity, and they will also monitor whether participants can reduce their steroid dose safely.

The research process

The trial runs in 4 steps – from screening to follow-up. Each step says what happens and what the team monitors.

  1. Step 1

    Baseline visit (day 1)

    You attend the clinic for the first study visit. baseline measurements of disease activity and safety tests are performed.

    You receive a 300 mg subcutaneous injection of daxdilimab (or a matching placebo) according to the randomization. the injection is given in the skin under the surface.

    If you are already taking prednisone 20 mg oral tablets, you continue this dose daily throughout the trial.

  2. Step 2

    Regular follow‑up visits

    You return to the clinic at scheduled intervals up to week 24. at each visit safety labs and disease‑activity assessments are repeated.

    At each visit you receive the next dose of the study medication, which is the same 300 mg subcutaneous injection of daxdilimab (or placebo) as given at baseline.

    You continue taking the daily prednisone 20 mg oral tablet unless your doctor adjusts the dose based on trial criteria.

  3. Step 3

    Week 24 primary assessment

    At the visit that marks week 24, the main efficacy measurements are taken, including the total improvement score and the CDASI activity score.

    Blood samples are collected to measure the concentration of daxdilimab and to check for antibodies against the drug.

    The amount of prednisone you are using is reviewed to determine whether a reduction of at least 25 % or a dose of 7.5 mg/day has been achieved.

  4. Step 4

    Study completion

    After the week 24 assessments, the study medication is stopped. you may be monitored for a short period to observe any delayed safety events.

    Final safety evaluations are performed, and you receive information about the overall study results.

Who can join the trial?

13 criteria

  • Age requirement: You must be an adult aged 18 to 75 years when you sign the consent form.
  • Diagnosis of myositis: You need a confirmed diagnosis of definite or probable myositis (muscle inflammation) using the 2017 ACR/EULAR criteria. This can be met either by a score of 5.5 or higher without a muscle biopsy (but you must have a characteristic skin rash) or by a score of 6.7 or higher with a muscle biopsy (the biopsy report or tissue sample must be provided).
  • Skin rash requirement (if no biopsy): The rash must be one of the typical signs of dermatomyositis, such as a violet‑colored eyelid rash (heliotrope rash) or raised bumps on the knuckles (Gottron’s papules or Gottron’s sign).
  • Specific disease group: You must belong to one of two groups:
    • Group 1 – Dermatomyositis (DM) with a current or past DM rash, or
    • Group 2 – Anti‑synthetase inflammatory myositis (ASIM) with positive antibodies: either the anti‑Jo‑1 antibody tested centrally, or a historic positive test for any of anti‑PL‑12, anti‑PL‑7, anti‑KS, anti‑EJ, anti‑OJ, anti‑ZO, or anti‑YRS(HA).
    • Active disease signs: At the screening visit you must have all of the following:
      • MMT8 score (a test of muscle strength) less than 142,
      • At least two of these symptoms each scoring 2 cm or more on a 10‑cm visual analog scale (a simple line where you mark severity): muscle pain, fatigue, or other muscle‑related problems,
      • One muscle‑related blood test (muscle enzyme) that is at least 1.5 times higher than the normal upper limit,
      • HAQ‑DI (a questionnaire measuring daily activity difficulty) score of 0.5 or higher, and
      • Global muscle damage rating of 5 or higher on a 10‑cm visual analog scale.
      • Stable background treatment: If you are able to tolerate standard medicines for myositis, you should be on a stable dose. If you cannot tolerate them or they did not work, those medicines must be stopped (washed out) before joining the trial.
      • Corticosteroid and immunosuppressant rules: If you are taking corticosteroids (such as prednisone), the dose must be 20 mg per day or less, and you may be on up to two other non‑excluded immunosuppressant drugs. All these medicines must have been at the same dose for at least 4 weeks before randomization. Alternatively, if you previously stopped these drugs because they did not work or caused side effects, a washout period must be completed.
      • Willingness to reduce steroids: You must agree to lower your corticosteroid dose according to the study plan if your condition becomes stable or improves.

Who cannot join the trial?

7 criteria

  • Any health condition that the study doctor believes could make it difficult to judge how the study drug works or to keep you safe.
  • Weight greater than 160 kg (352 pounds) at the screening visit.
  • History of an allergy, severe allergic reaction (hypersensitivity), or life‑threatening reaction (anaphylaxis) to any part of the study drug, to other monoclonal antibodies (lab‑made proteins that target specific cells), or to human immunoglobulin therapy (a type of immune‑system medicine).
  • Had major surgery within the 8 weeks before screening, or plans to have scheduled (elective) surgery from screening until the study ends.
  • History of serious heart disease, such as unstable angina (chest pain that comes and goes), myocardial infarction (heart attack), or congestive heart failure (the heart cannot pump blood well) within the 6 months before randomization.
  • Given more than 50 mL of blood or plasma in the 30 days before screening, or more than 499 mL in the 56 days before screening, or plans to give blood or plasma while in the study or up to 6 months after the last dose of the study drug.
  • Received a blood transfusion (red blood cells or platelets) or a procedure that removes blood components (plasmapheresis or plasma exchange) within 8 weeks before randomization and throughout the study.
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Investigated drugs

  • Daxdilimab

    is an experimental drug given as a subcutaneous injection. In this study it is the main treatment being tested to see if it can lower the activity of dermatomyositis or anti‑synthetase inflammatory myositis. Participants receive the injection under the skin, and the researchers compare its effect to a placebo to determine if it works better at reducing disease symptoms.

  • Prednisone

    is a well‑known oral steroid that is taken as a tablet. In this trial it is used as background therapy, meaning all participants continue their regular prednisone treatment while the new drug is tested. The steroid helps control inflammation and is part of the standard care for these muscle diseases, allowing the study to see how the experimental drug works on top of usual treatment.

What is already known about the treatment

  • Daxdilimab

    Daxdilimab is given as a 300 mg subcutaneous injection, meaning it is injected under the skin. It is an investigational monoclonal antibody currently being studied in Phase 2 trials for adults whose dermatomyositis or anti‑synthetase inflammatory myositis is not well controlled. The drug works by attaching to a specific immune protein and blocking its activity, which helps lower the abnormal inflammation that damages muscle and skin. It is classified as an immunomodulating monoclonal antibody.

  • Prednisone

    Prednisone is taken as a 20 mg oral tablet that is swallowed with water. It is an approved glucocorticoid that has been used for many decades to treat a wide range of inflammatory and autoimmune conditions, including dermatomyositis. The medication mimics the body’s natural cortisol hormone, reducing immune‑system activity and swelling by turning off inflammatory signals. It belongs to the corticosteroid class of drugs.

Investigated diseases

  • Dermatomyositis

    Dermatomyositis is an inflammatory disease that affects the skin and muscles, causing a distinctive rash and muscle weakness. The muscle inflammation gradually reduces strength, often starting in the hips, thighs, shoulders, and upper arms. Over time the weakness can spread to other muscle groups, making everyday activities more difficult. The skin rash may appear on the face, neck, chest, and back and can change in appearance as the disease progresses. Symptoms may fluctuate, with periods of worsening followed by relative stability.

  • Anti‑synthetase syndrome

    Anti‑synthetase syndrome is an autoimmune condition characterized by muscle inflammation, joint pain, and lung involvement. Muscle weakness develops slowly and can affect the ability to climb stairs or lift objects. The lungs may develop inflammation that leads to shortness of breath and reduced exercise capacity. Joint symptoms such as swelling and stiffness often accompany the muscle problems. The condition typically progresses in a stepwise fashion, with new organ involvement appearing over months to years.

Trial detailsLast updated 7 Oct 2026
Age18+ yearsPhasePhase IITrial ID2022-502810-10-00Protocol codeHZNP-DAX-205Estimated enrolment96 patientsSponsorHorizon Therapeutics Ireland Designated Activity Company

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