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A "miracle" gene therapy could effectively cure people with haemophilia, doctors claimed today.
Researchers say a one-time transfusion could stop people with the rare condition that prevents their blood from clotting from taking weekly injections to manage it.
The phase 1-2 trial of 10 patients with hemophilia B found that nine did not need to take vaccinations more than two years after the treatment, known as FLT180a, as a replacement for clotting drugs.
Lead author Professor Pratima Chowdary, a haematologist at University College London, said curing the condition "will be a reality for the majority of adults within the next one to three years".
Gene therapies have already shown promise in treating hemophilia A patients, who make up 85 percent of patients with the disease.
But this new treatment is one of the first to be tested for people with hemophilia B, which is caused by a missing protein called clotting factor.
None are yet available on the NHS and there have been concerns over price, with a haemophilia A treatment which showed similar results earlier this month estimated to cost between £1.5 million and £2.5 million per dose.
Current clotting factor injections can cost between £150,000 and £200,000 a year.
A new gene therapy has dramatically reduced the risk of bleeding in people with the rare disease haemophilia B, experts say (David Davies/PA).
Around 6,000 people in the UK have haemophilia, while there are 20,000 in the US with the disease.
It is usually inherited and mainly affects men because the missing protein is on the X chromosome.
Normally, when a person cuts themselves, clotting factors mix with blood cells called platelets to stop the bleeding.
However, people with hemophilia lack clotting factors and are at risk of severe bleeding. This can be fatal and also causes severe joint pain in some sufferers.
Professor Chowdary, who also works at the Royal Free Hospital in London, said BBC: “We have many young patients who are suffering excruciating pain and there is nothing we can do to reverse the joint damage.”
The study, published in New England Journal of Medicine was led by experts from UCL, the Royal Free Hospital and biotech company Freeline Therapeutics.
The ten patients selected for the study all had “severe or moderate” hemophilia B – the rarer form of the disease, which accounts for 15 percent of cases.
It is caused by a deficiency of clotting factor IX, while hemophilia A patients lack factor VIII.
They received one of four different doses of therapy at the start of the study, and doctors measured their factor IX levels at the end to see how it affected their long-term condition.
After an average of 27 months, five patients had normal factor IX levels (ranging from 51 to 78 percent), three patients had levels of 23 to 43 percent, and one had a level of 260 percent.
Nine patients no longer needed to take weekly injections. Professor Chowdary said: “We are very excited by the results.
“The fact that hemophilia patients no longer have to regularly inject themselves with the missing protein is an important step towards improving their quality of life.
“The long-term follow-up study will monitor patients for duration of expression and monitoring for late effects.”
FLT180a works by using a small part of a virus to deliver a copy of a gene directly into the patient's tissue to compensate for a missing gene.
This new gene can then create the missing FIX proteins that allow normal blood clotting.
Patients in the study had to take immunosuppressive medications for several weeks to several months to prevent their immune systems from rejecting the therapy.
While the treatment was generally well tolerated, all patients experienced side effects, with an abnormal blood clot in one who received the highest dose of FLT180a and had the highest protein levels.
Freeline co-founder Professor Amit Nathwani, a hematologist at UCL who co-authored the study, said: “Gene therapy is still an emerging field that is pushing the boundaries of science for people with serious genetic diseases.”
He said the new study adds to "the growing evidence that gene therapy has the potential to free patients from the challenges of adhering to lifelong therapy or could provide a treatment where none exists today."