A new experimental drug may change how doctors treat a stubborn blood disorder that leaves patients at risk of dangerous bleeding. A Phase 2 clinical trial published in the New England Journal of Medicine reports that mezagitamab, a novel antibody therapy, achieved a striking 91% response rate in patients with immune thrombocytopenia (ITP), with many seeing their platelet counts return to normal within just 48 hours.

The study, led by researchers at Mass General Brigham's Cancer Institute, offers fresh hope for patients who do not respond to existing treatments. ITP is an autoimmune condition in which the body’s defence system mistakenly attacks its own platelets—the tiny blood cells that help stop bleeding. When platelet levels drop too low, even minor injuries can become serious, and daily life can feel uncertain.

“This is a novel therapy that works fast and attacks the underlying mechanism of the disease,” said lead author David J. Kuter, MD, DPhil, a haematologist with Mass General Brigham Cancer Institute. “We saw that it could have a rapid effect, normalising platelet counts in 48 hours and improving quality of life.”

What makes mezagitamab different is how it works. Originally developed for cancer, it targets a protein called CD38 found on certain immune cells. These cells, including plasma cells, are believed to play a key role in driving the body’s mistaken attack on platelets. By blocking CD38, the drug effectively calms this harmful immune response. In simple terms, it tells the body to stop destroying its blood components.

The trial enrolled 41 adults across multiple countries, including Bulgaria, China, Greece, Italy, Japan and Spain, all of whom had been living with ITP for at least three months. Patients were divided into different groups receiving varying doses of the drug or a placebo. Early safety checks allowed researchers to move forward with a higher 600 mg dose, which ultimately showed the most promise in improving platelet counts and reducing symptoms of ITP among the participants.

Inside the trial setting, patients were closely monitored, with regular blood tests tracking platelet counts. For many, the change was swift. Within days, numbers that had remained dangerously low began to climb. By week 16, 10 out of 11 patients receiving the 600 mg dose showed a strong platelet response, compared to just 3 out of 13 in the placebo group. The safety profile, importantly, was similar to placebo, suggesting the drug was not only effective but also well tolerated.

This matters because nearly 20% of ITP patients fail to respond to currently available therapies. Existing options often focus on managing symptoms or boosting platelet production temporarily, rather than addressing the root cause. For clinicians, this new approach may feel less like a patch and more like fixing the source of the problem.

The business implications are also difficult to ignore. With a clear unmet need and strong early data, mezagitamab could emerge as a valuable addition to the autoimmune treatment market. Developed with support from Takeda Development Center Americas, the therapy reflects a growing trend in drug development—repurposing cancer-targeting antibodies to treat immune disorders.

“This study was designed as a dose-ranging, proof-of-concept trial. We are now conducting a phase 3 clinical trial of a 600-mg dose of mezagitamab with Mass General Brigham Cancer Institute serving as the leading site in North America,” said Kuter.

For now, the findings mark an important step, but not the final word. Larger Phase 3 trials will be critical to confirm the results and assess long-term safety. If successful, mezagitamab could reshape treatment strategies for ITP, offering faster, more targeted relief for patients who have long run out of options.