In these two Hot Papers, teams led by Mark Kay, a professor of pediatrics and genetics and director of the Human Gene Therapy program at Stanford University, Stanford, Calif.; and pediatrics professor Katherine High, University of Pennsylvania, chose two similar vectors, but different pathways, and achieved comparable results. Kay and his colleagues chose to deliver vector to the liver, while High and her colleagues, following a long tradition, delivered genes intramuscularly.
|Courtesy of Childrens Hospital of Pennsylvania|
"If you look at the papers, you see that the maximum levels of clotting factor achieved are the same using either muscle or liver as the target. However, you have to use higher doses [of the intramuscularly injected vector]," says High, who is also director of research, hematology division at Children's Hospital of Philadelphia. That difference could be attributable to differences in the preparation of the vector, she adds.
Injecting into the Liver
Kay focused on delivering factor IX to the liver because that is the site of its production in healthy individuals. The transduction need not be overly efficient: a mere 5 percent of liver cells churning out factor IX can do the job because those cells can over-produce the protein, resulting in therapeutic blood concentrations. "We're up to near normal levels [of factor IX], and you probably need only 20-25 percent to essentially correct [the disease]," says Kay.
In part, the work was a big step forward because it involved dogs with hemophilia B. These are good human disease models because a dog is a large animal and, genetically, are far-removed from mice, which do not always accurately predict responses in humans. "If you can treat the disease in two animals that are far apart and one is a large animal, it makes one more confident that the therapy is likely to do the same thing in humans," Kay says. His paper, he says, "goes on to prove the concept that you could treat hemophilia in a clinically relevant manner that was safe; there was no toxicity and we didn't have to do fancy manipulations."
Vector injection into skeletal muscle is also a simple procedure that could be easily done outside of a standard clinical setting. "Delivering it to the liver is more complicated, though I'm convinced that can be done safely, she says. "But it's appealing to have something you can just give as a shot.... It is estimated that over half of the world's hemophilia population doesn't have access to [standard treatments]. The possibility of a procedure that would be relatively simple, that you could do with access to sterile needles and alcohol, is appealing."
The jury is still out on which method works best. The two groups have begun to collaborate since the publication of these papers, and they recently completed initial human trials with intramuscular injection. In eight subjects with three different doses, they found good evidence for gene transfer and expression. "So the animal models were good predictors of what happened in humans," says High.3
Still, they didn't reach the factor IX target levels that they accomplished in dogs, and they didn't witness consistent elevation to levels greater than 1 percent of normal, which is the minimum required to ease the clinical effects of hemophilia. But those patients who did achieve intermittent levels higher than 1 percent have gone on to require less frequent factor IX treatments, which suggests that the gene therapy may have garnered some clinical effect, says High.
With that work completed, the two groups are about to start a Food and Drug Administration-approved human trial using liver delivery, though High anticipates they might return to intramuscular work as they refine their methods. "My feeling is that it would be ideal to develop approaches for both muscle and liver, and that way there will be an option if patients have hepatitis and can't take the liver approach," High says. Kay's group, which is also working on hepatitis, continues to investigate other viral and nonviral vectors, as well as strategies to increase the efficiency of transduction and ways of increasing the AAV's capacity.
1. P. Smaglik, "Gene therapy crossroads," The Scientist, 13:1, July 5, 1999.
2. M.A. Kay, et al., "In vivo gene therapy of hemophilia B: sustained partial correction in factor-IX-deficient dogs" Science, 262:117-9, 1993.
3. M.A. Kay, et al., "Evidence for gene transfer and expression of factor IX in haemophilia B patients treated with an AAV vector," Nature Genetics, 24:257-61, 2000.