A single treatment with a gene therapy drug has shown promise in curing patients with haemophilia — a disorder in which blood does not clot normally, results of a clinical trial in Britain show.

Over one year on from a single treatment with a gene therapy drug, participants with haemophilia A (the most common type) are showing normal levels of the previously missing protein, and effectively curing them, showed the study published in the New England Journal of Medicine.

“We really now have the potential to transform care for people with haemophilia using a single treatment for people who at the moment must inject themselves as often as every other day. It is so exciting,” said John Pasi, Professor at Queen Mary University of London.

A hereditary genetic condition dominantly affecting men, people with severe haemophilia A have virtually none of the protein factor VIII which is essential for blood to clot.

It puts those affected at risk of excessive bleeding even from the slightest injury as well as causing spontaneous internal bleeding, which can be life-threatening.

Recurring bleeding into joints can also lead to progressive joint damage and arthritis.

The only current treatment involves multiple weekly injections to control and prevent bleeding, but there is no cure.

In the trial, a single infusion of the gene therapy drug showed improved levels of the essential blood clotting protein Factor VIII, with 85 per cent of patients achieving normal or near-normal Factor VIII levels even many months after treatment.

The trial saw patients across England injected with a copy of the missing gene, which allows their cells to produce the missing clotting factor.

Following patients for up to nineteen months, tests show that eleven out of thirteen patients in the trial now have normal or near normal levels of the previously missing factor and all thirteen patients have been able to stop their previously regular treatment.

“We have seen mind-blowing results which have far exceeded our expectations. When we started out we thought it would be a huge achievement to show a five per cent improvement, so to actually be seeing normal or near normal factor levels with dramatic reduction in bleeding is quite simply amazing,” Pasi said.