In 10 patients with severe haemophilia B, the infusion of a single dose of gene therapy with an AAV8 vector resulted in long-term therapeutic factor IX expression associated with clinical improvement. With a follow-up period of up to 3 years, no late toxic effects from the therapy were reported.
A recently published study updates an earlier one from 2011, in which six volunteers were successfully treated with various doses of the treatment that uses a virus to insert genetic material into the liver. The four additional patients received the highest dose. Dr. Andrew Davidoff of St. Jude Children's Research Hospital in Memphis, Tennessee and his colleagues found that the higher the dose of the engineered virus, the more factor IX the body produced. The researchers used an adeno-associated virus or AAV, which infects the liver without causing apparent illness.
The therapy seemed to have no serious side effects and is designed to eliminate the need to give patients injections of factor IX.
The treatment produced factor IX levels in a range of 1 percent to 6 percent of normal. But even a few percentage points can be enough to prevent dangerous bleeding. A single infusion was sufficient. For the six high-dose recipients, the factor IX level averaged 5.1 percent, which resulted in a reduction of more than 90 percent in both bleeding episodes and the use of prophylactic factor IX concentrate.
The researchers have since noted that a problem with the treatment is how difficult the genetically engineered virus is to make. Better methods to produce the treatment are desperately needed as to engineer enough virus to treat the 10 patients required six months of work.