A multi-year, ongoing study suggests that a new kind of gene therapy for hemophilia B could be safe and effective for human patients. Published in the journal Science Translational Medicine, the research showed that a reprogrammed retrovirus could successfully transfer new factor IX (clotting) genes into animals with hemophilia B to dramatically decrease spontaneous bleeding. Thus far, the new therapy has proven safe.
"The result was stunning," said Timothy Nichols, MD, director of the Francis Owen Blood Research Laboratory at the University of North Carolina School of Medicine and co-senior author of the paper. "Just a small amount of new factor IX necessary for proper clotting produced a major reduction in bleeding events. It was extraordinarily powerful."
The idea behind gene therapy is that doctors could give hemophilia patients a one-time dose of new clotting genes instead of a lifetime of multiple injections of recombinant factor IX that until very recently had to be given several times a week. A new FDA-approved hemophilia treatment lasts longer than a few days but patients still require injections at least once or twice a month indefinitely.
This new gene therapy approach, like other gene therapy approaches, would involve a single injection and could potentially save money while providing a long-term solution to a life-long condition. A major potential advantage of this new gene therapy approach is that it uses lentiviral vectors, to which most people do not have antibodies that would reject the vectors and make the therapy less effective.
In human clinical studies, approximately 40 percent of the potential participants screened for a different kind of viral vector - called adeno-associated viral vectors - have antibodies that preclude them from entering AAV trials for hemophilia gene therapy treatment. This means that more people could potentially benefit from the lentivirus gene therapy approach.
Hemophilia is a bleeding disorder in which people lack a clotting factor, which means they bleed much more easily than people without the disease. Often, people with hemophilia bleed spontaneously into joints, which can be extremely painful and crippling. Spontaneous bleeds into soft tissues are also common and can be fatal if not treated promptly. Hemophilia A affects about one in 5,000 male births. These patients do not produce enough factor VIII in the liver. This leads to an inability to clot. Hemophilia B affects about one in 35,000 births; these patients lack factor IX.
This new method was spearheaded by Luigi Naldini, PhD, director of the San Raffaele Telethon Institute for Gene Therapy and co-senior author on the Science Translational Medicine paper.
For this study, Naldini and Nichols developed a way to use a lentivirus, which is a large retrovirus, to deliver factor IX genes to the livers of three dogs that have naturally occurring hemophilia. The researchers removed the genes involved in viral replication. "Essentially, this molecular engineering rendered the virus inert," Nichols said. "It had the ability to get into the body but not cause disease." This process turned the virus into a vector - simply a vehicle to carry genetic cargo.
Unlike some other viral vectors that have been used for gene therapy experiments, the lentiviral vector is so large that it can carry a lot of payload - namely, the factor IX genes that people with hemophilia B lack. (This approach could also be used for hemophilia A where the FVIII gene is considerably larger.)
These viral vectors were then injected directly into the liver or intravenously. After more than three years, the three dogs in the study experienced zero or one serious bleeding event each year. Before the therapy, the dogs experienced an average of five spontaneous bleeding events that required clinical treatment. Importantly, the researchers detected no harmful effects.
"This safety feature is of paramount importance," Nichols said. "Prior work elsewhere during the early 2000s used retroviruses for gene therapy to treat people with Severe Combined Immunodeficiency, but some patients in clinical trials developed leukemia." Newer retroviral vectors, though, have so far proved safe for SCID patients.
To further demonstrate the safety of this new hemophilia treatment, Nichols and Naldini used three different strains of mice that were highly susceptible to developing complications, such as malignancies, when introduced to lentiviruses. But the researchers found no harmful effects in the mice. The researchers think that turning the lentivirus into a lentiviral vector made it safe.
"Considering the mouse model data and the absence of detectible genotoxicity during long-term expression in the hemophilia B dogs, the lentiviral vectors have a very encouraging safety profile in this case," Nichols said.
More work still needs to be done before this gene therapy approach can be used in human clinical trials. For instance, researchers hope to increase the potency of the therapy to decrease spontaneous bleeding even more while also keeping the therapy safe.
Before the treatment, the hemophilia dogs had no sign of factor IX production. After the treatment, they exhibited between 1 and 3 percent of the production found in normal dogs. This slight increase was enough to substantially decrease bleeding events.
Still, Nichols said it would be best if they could boost factor IX production to between 5 and 10 percent of normal while still remaining safe. This amount of factor IX expression could potentially eliminate spontaneous bleeding events for people with hemophilia B.
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