The company noted that in a Phase I trial, low-dose veltuzumab, as a single agent, produced an objective response rate of 55% in 38 response-assessable patients with relapsed ITP, including 11 patients (29%) who reported a complete response.
In addition, ten responders achieved durable responses for more than one year, including three patients in remission for up to 4.3 years after veltuzumab treatment.
The Phase II expansion trial has completed patient accrual and they are being followed for up to five years.
Immunomedics president and chief executive officer Cynthia Sullivan said: "We are pleased to receive the orphan drug designation for veltuzumab in ITP from the FDA.
"We are currently evaluating various options for further clinical development of veltuzumab in ITP and other autoimmune disease indications, including licensing arrangements and collaborations with outside study groups."
In ITP, the immune system attacks the platelets (or thrombocytes) resulting in their accelerated destruction.
ITP is a bleeding disorder characterized by low blood platelet counts of less than 50,000/uL.