Under the multinational, double-blind placebo-controlled Phase III HERCULES study, the company will enroll 92 patients at clinical sites across 17 countries.
The primary endpoint of the study is time to platelet count normalization, which avoids further microvascular thrombosis.
Other endpoints include the avoidance of recurrence of the presenting TTP episode following the halt of daily PE, the effect on biomarkers of organ damage, severe morbidity related with ischemia, and the mortality rate.
Caplacizumab secured orphan drug designation in the US and EU in 2009. It inhibits the interaction between Von Willebrand factor (vWF) and platelets by targeting the A1 domain of vWF.
Acquired TTP results in the formation of microvascular thrombosis and organ damage throughout the body, including the brain and the heart. It affects about 11 per million people globally.
Ablynx CEO Edwin Moses said: "The ability of caplacizumab to rapidly inhibit the formation of small blood clots, resulting in the more rapid restoration of normal platelet counts and an important reduction in exacerbations, was well demonstrated in the Phase II TITAN study.
"Based on the clinical effect seen in this TITAN study, we are planning to submit caplacizumab for conditional approval to the European Medicines Agency (EMA) in 2017."
Acquired TTP accounts for more than 90% of the patients. Its characteristics include severe thrombocytopenia, haemolytic anaemia and indications and symptoms of tissue ischemia including stroke or myocardial infarctions.