Belgian biotech company Ablynx has received a boost with strong results from a late-stage trial into its experimental drug caplacizumab to treat a rare, life-threatening autoimmune blood clotting disorder.

Acquired thrombotic thrombocytopenic purpura (aTTP) is caused by impaired activity of the ADAMTS13 enzyme and results in very low platelet count plus clot formation in small blood vessels throughout the body, leading to ischemia and widespread organ damage.

In the Phase III HERCULES study, treatment with caplacizumab plus standard-of-care showed a statistically significant reduction in time to platelet count response – this was the primary endpoint and a measure of prevention of further microvascular thrombosis.

Patients on caplacizumab were 1.5 times more likely to achieve platelet count response at any given time point, compared to patients treated with placebo.

Two key secondary endpoints were also met. First, caplacizumab resulted in a 74% reduction in the percentage of patients with aTTP-related death, recurrence of aTTP, or at least one major thromboembolic event during study drug treatment. Second, the proportion of patients with a recurrence of aTTP in the overall study period was 67% lower in the caplacizumab arm compared to placebo.

Current standard-of-care is PEX and immunosuppression, but aTTP is still associated with a mortality rate of up to 20%, with most deaths coming within 30 days of diagnosis.

Patients are at risk of acute thromboembolic complications such as stroke or myocardial infarction, and the recurrence of the disease.

The topline data from HERCULES comes from 145 patients – the biggest study yet in this population - and pushes caplacizumab, an anti-von Willebrand factor Nanobody, one step closer towards marketing approval.

In July, Ablynx received Fast Track designation from the Food and Drug Administration and the company expects to file the drug in the US next year. It submitted a Marketing Authorisation Application to the European Medicines Agency in February.

Dr Edwin Moses, CEO of Ablynx, called the HERCULES results “very important milestone” and added: “I am delighted by this outcome as it reinforces all our beliefs in the potential for caplacizumab to change the lives of patients affected by aTTP.”

“We believe these positive Phase III study results confirm the potential for caplacizumab to address the high unmet medical need in the treatment of aTTP and to have a meaningful impact on the lives of affected patients,” said Dr Robert K. Zeldin, Chief Medical Officer at Ablynx. “We continue to analyse the data and look forward to sharing the results with regulatory authorities and the scientific community.”