Scientists have created a thimble-sized implant which may solve the problems of current haemophilia treatment.

The device sits inside the body and activates inert blood-clotting factors which are already present in the patient's blood.

The device has not yet been tested in humans, but raised the clotting ability in a rhesus monkey during a month-long trial, as well as in several guinea pigs.

'Viable alternative'

Professor Harvey Pollard, at the Uniformed Services University School of Medicine, Maryland, US, led the research team and said: "The rhesus monkey is about as close as you can get to a human."

He said trials in humans were planned but could not say when they would begin. However, his team believes that the implant "could eventually provide a viable alternative to replacement therapies".

Dr Steven Pipe, at the Department of Pediatrics, University of Michigan, comments on the research in the journal Nature Biotechnology, and told BBC News Online that the approach was "definitely promising".

He said the implant had the potential to overcome several problems which plague existing treatments for haemophilia.

Genetic mutations

Haemophilia is caused by genetic mutations which prevent the formation of clotting factors, usually VIII or IX. Usual treatments involve replacing the missing factors by transfusion.

This often requires several sessions a week and exposes the patients to the risk of other blood-borne diseases, tragically illustrated in the 1980s when many haemophiliacs contracted the HIV virus and hepatitis.

Even if the clotting factors are synthetically produced, avoiding the risk of disease, the immune systems of many patients start to reject the foreign material after a while.

Activated factor

The new implant avoids all these problems by using a factor which is already present in the patient's blood. Factor VII is inactive and unable to help clot blood but when it seeps through the tiny pores of the implant it encounters an enzyme which activates it - factor XIIa.

The active factor VII then seeps out and helps compensate for the missing blood clotting factors.

Professor Pollard said the monkey achieved 20% of the clotting ability of a non-haemophiliac equivalent. By contrast, he said, patients receiving the best current treatment only manage 5% of the clotting ability: "People are chronically under treated at the moment."

The centimetre-sized implant sits in the peritoneal cavity, next to the stomach, and pumps by diffusion, needing no power. Professor Pollard claimed the implant would be a much cheaper treatment than transfusions.

Doubly unique

Comparing the implant to transfusions and to new gene therapy approaches, Dr Pipe said it was unique in two ways: "First, there is continuous generation of the coagulation factors and, second, it will be applicable to patients who have developed immune responses to the replacement factors."

However, he said it had not yet been shown whether the implant raised the levels of factor VIIa to high enough levels to improve blood clotting significantly: "That would generate more excitement for this strategy."

It also remained to be seen how long the factor XIIa in the implant would last before it needed replenishing.