Gene therapy proves effective for hemophilia B

A single treatment with gene therapy, an experimental technique for fixing faulty genes, has been shown to boost output of a vital blood clotting factor, possibly offering a long-term solution for people with hemophilia B.

Researchers said the same technology was also being studied as a treatment for hemophilia A, the far more common type of the inherited bleeding disorder.

"It is a technique for potentially permanently curing patients," said Dr. Charles Abrams, American Society of Hematology secretary and associate chief of hematology/oncology at the University of Pennsylvania in Philadelphia.

Both safety and efficacy have held back the field of gene therapy. One experiment cured two French boys with a rare immune disorder but gave them leukemia in 2002, and an Arizona teenager died in a 1999 gene therapy experiment.

The approach used by researchers at the University College London Cancer Institute and St. Jude Children's Research Hospital in Memphis, Tennessee, involved the use of a novel viral "vector," designed to target the liver specifically.

The strategy involves replacing the defective gene that causes the bleeding disorder with a correct version delivered via the virus to the patient's liver cells - the only cells in the body capable of producing certain clotting factors missing or deficient in people with hemophilia.

The factors are numbered using Roman numerals. The two main forms of the disease are hemophilia A, caused by a lack of clotting factor VIII, and hemophilia B, caused by a lack of clotting factor IX.

Researchers have so far treated six men with severe hemophilia B who were producing clotting factor IX at less than 1 percent of normal levels. The general goal of current treatment with recombinant factor IX is to achieve factor levels greater than 1 percent of normal.

Four of the six trial participants have stopped routine treatment and remain free of spontaneous bleeding. The other two have increased the interval between factor infusions to once every 10 days to two weeks from two to three times a week, said Dr. Andrew Davidoff, chairman of the department of surgery at St. Jude's and co-author of the study.

HIGH COST FOR CURRENT TREATMENT

Frequent treatments with manufactured factor IX, known as recombinant factor concentrates, can cost hundreds of thousands of dollars a year, making hemophilia a tempting target for gene therapy.

The trial "is truly a landmark study," Dr. Katherine Ponder, hematology and oncology professor at Washington University in St. Louis, said in a New England Journal of Medicine editorial.

"If further studies determine that this approach is safe, it may replace the cumbersome and expensive protein therapy currently used for patients with hemophilia B," she wrote.