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Efficacy and Safety of DNTH103 in Adults with Generalized Myasthenia Gravis: A Phase 3 Randomized, Double‑Blind, Placebo‑Controlled Study

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What is this trial about?

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

generalized Myasthenia gravis is a rare condition in which the immune system interferes with the signals that tell muscles to move, leading to fluctuating weakness that can affect the eyes, face, swallowing, and breathing. The study evaluates a new injectable medicine called Claseprubart (code name DNTH103) that is designed to improve muscle strength by targeting the underlying immune problem.

The aim of the trial is to confirm that the medicine works better than no active treatment in people with this disease. The study is a phase 3 trial, meaning it is a large test conducted after earlier safety checks. Participants are assigned by chance (randomized) to receive either the medicine or a dummy injection (placebo) and neither the participants nor the study staff know which one is given (double‑blind), which helps keep the results unbiased.

During the study, participants receive an injection of the assigned product at regular intervals for about 17 weeks and attend scheduled visits where they answer simple questionnaires that track daily activities and muscle strength. After the treatment period, they are monitored for a short follow‑up to check for any side effects and to see how long any benefit lasts.

The research process

The trial runs in 9 steps – from screening to follow-up. Each step says what happens and what the team monitors.

  1. Step 1

    Baseline visit

    You will attend an initial clinic visit after joining the study. during this visit, study staff will record your medical history, current medications, and perform baseline assessments using the mg-adl scale, quantitative myasthenia gravis (qmg) scale, and myasthenia gravis composite (mgc) scale. these scores establish your condition before treatment begins.

  2. Step 2

    Randomization and first dose

    Based on a computer‑generated random assignment, you will receive either dnth103 (the investigational drug) or a matching placebo. the product is supplied as a solution for injection. the exact dose is determined by the study protocol and will be administered by a qualified professional. the study is double‑blind, so neither you nor the staff will know which product you receive.

  3. Step 3

    Regular dosing visits

    You will return to the clinic at scheduled intervals for additional injections. the frequency and total number of injections follow the study schedule, which continues for a total of 17 weeks. each visit includes administration of the assigned injection and review of any symptoms you have experienced since the previous visit.

  4. Step 4

    Efficacy assessments during treatment

    Throughout the 17‑week treatment period, you will complete the mg‑adl questionnaire, the qmg examination, and the mgc rating at predetermined visits. these assessments allow the study team to measure changes in muscle weakness and daily functioning.

  5. Step 5

    Safety monitoring

    At each visit, you will be asked about any new or worsening symptoms, which are recorded as treatment‑emergent adverse events (teae). laboratory tests may be performed to monitor your health. any serious adverse event will be reported promptly.

  6. Step 6

    Final week‑17 evaluation

    At the end of week 17, you will undergo a comprehensive evaluation that repeats the mg‑adl, qmg, and mgc assessments. the results determine whether you achieved a reduction of at least five points on the mg‑adl scale, reached minimal symptom expression (mg‑adl score of 0 or 1), or showed improvement on the qmg scale.

  7. Step 7

    Blinded extension period (bep)

    If you choose to continue, you may enter a blinded extension period during which you will keep receiving the same injection (active drug or placebo) without knowing which you are taking. the same safety and efficacy assessments will continue.

  8. Step 8

    Open‑label extension (ole)

    After the blinded extension, an open‑label extension may be offered. in this phase, all participants receive the active investigational drug (dnth103) and continue regular visits, dosing, and monitoring as described earlier.

  9. Step 9

    Safety follow‑up

    After the final study visit, you will be asked to return for a safety follow‑up appointment. this visit assesses any late‑appearing side effects and may include a blood sample to measure serum concentrations of dnth103 and antibodies against the drug.

Who can join the trial?

10 criteria

  • Provide written informed consent before any study activities start, meaning you sign a form that shows you understand and agree to take part.
  • Be an adult male or female aged between 18 and 80 years at the time of screening.
  • Weigh between 40 kg and 130 kg at the time of screening.
  • Have a confirmed diagnosis of generalized Myasthenia gravis (gMG). This must be shown by a positive acetylcholine receptor antibody test (a blood test that looks for specific antibodies) and at least one of the following: an abnormal test that measures how nerves talk to muscles, a positive anticholinesterase test (a medication that temporarily improves muscle strength), or a clear improvement when taking anticholinesterase medicines.
  • Fit into Myasthenia Gravis Foundation of America (MGFA) Class II‑IVa, which describes a moderate to more severe level of disease symptoms.
  • Score 6 or higher on the MG‑ADL scale, a questionnaire that measures how Myasthenia Gravis affects everyday activities.
  • Score 10 or higher on the QMG scale, a test that evaluates muscle strength in different parts of the body.
  • Have documented vaccinations against encapsulated bacteria (such as certain types of pneumonia or meningitis) according to local health guidelines.
  • If you are male, you must either agree not to donate sperm and use an acceptable method of contraception (birth control) if you have sexual contact with a woman who could become pregnant, or you must be permanently surgically sterile (for example, after a vasectomy) for at least 90 days before screening.
  • If you are female, you must either be unable to become pregnant (non‑childbearing potential) or, if you could become pregnant, agree not to donate eggs, not try to become pregnant, and use a highly effective method of contraception if you have sexual contact with a male partner.

Who cannot join the trial?

12 criteria

  • Having a serious medical problem, recent major surgery, or a serious mental health condition that could affect safety or study results.
  • Taking certain immune‑system medicines such as Rituximab or other B‑cell targeting drugs (e.g., inebilizumab) within the past 6 months, or receiving IVIg (intravenous immunoglobulin) or PLEX (plasma exchange) within the past 4 weeks.
  • Being in another clinical trial of a new drug within the last 90 days (or before the drug has cleared from your body, which is measured by its “half‑life”).
  • Having been diagnosed with systemic lupus erythematosus (SLE) or having a close family member (parent, sibling, or child) with SLE.
  • Having a known deficiency of the “complement” system, which is a part of the immune system that helps fight infections.
  • Having ever had an infection with N. meningitidis, a bacteria that can cause meningitis.
  • Testing positive for an active infection with HIV‑1 or HIV‑2, hepatitis B (HBV) (unless the test is positive only because of vaccination), or hepatitis C (HCV) during the screening.
  • Having previously taken the study drug claseprubart (DNTH103) or having been in a trial that used this drug.
  • Having had any surgery or biopsy of the thymus gland within the past year.
  • Having a known thymoma (a tumor in the thymus) that has not been treated.
  • Having a history of thymic carcinoma or any other thymus‑related cancer.
  • Having an active cancer diagnosed within the past 5 years, except for certain low‑risk skin cancers, treated cervical pre‑cancer, or low‑grade prostate cancer that is only being watched.
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Investigated drugs

DNTH103 is an experimental drug being studied to see if it can help people with generalized myasthenia gravis, a condition that causes muscle weakness. In this trial, the medication is given by injection directly into a vein. Researchers are looking at how well it works to improve muscle strength, how safe it is for patients, and whether it is tolerated without causing serious side effects. This drug is the main treatment being tested, and its effects will be compared to a placebo that looks the same but does not contain the active medicine.

What is already known about the treatment

DNTH103 - DNTH103 is given as a sterile solution for injection that is administered intravenously or subcutaneously by a healthcare professional. It is an investigational drug currently in Phase 3 clinical trials and not yet approved for general medical use. The medication is being studied for the treatment of generalized Myasthenia Gravis, an autoimmune disease that causes muscle weakness. It works by selectively blocking the complement protein C5, which reduces immune‑mediated damage at the neuromuscular junction, and it belongs to the class of complement inhibitors.

Investigated diseases

generalized myasthenia gravis - an autoimmune condition where the body’s immune system attacks the connection between nerves and muscles, causing muscle weakness. The weakness typically starts in the eyes, leading to drooping eyelids or double vision, and then spreads to facial, neck, and limb muscles. As the disease progresses, everyday activities such as chewing, swallowing, and walking become increasingly difficult. Symptoms tend to worsen with continued use of the muscles and improve after rest. The pattern of weakness can vary from day to day, and the condition may evolve over months or years.
Trial detailsLast updated 7 Oct 2026
Age18+ yearsPhasePhase IIITrial ID2026-525298-38-00Protocol codeDNTH103-MG-301Estimated enrolment195 patientsSponsorDianthus Therapeutics Inc.

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