Hemophilia is an ideal model for gene therapy, speaker after speaker repeated during the American Society for Gene Therapy's (ASGT) June meeting in Washington, D.C. Successfully providing a single protein a few times should, in theory, cure the disease, they stated. And once introduced via vector, a therapeutic clotting protein need not be continuously produced or regulated, they added. What they did not mention--at least not as openly--was the double-edged nature of that "ideal" status. In other words, if gene therapy doesn't work in hemophilia models, in what disease model will it work?
"The stakes are incredibly high," acknowledged James Wilson, ASGT president and director of the Institute for Human Gene Therapy at the University of Pennsylvania in Philadelphia, at the end of a session focusing on treating the disease. Within a year, scientists may find out if the stakes are too high. Phase I clinical trials using a retroviral vector to treat hemophilia A have begun in Pittsburgh. And a Philadelphia-based Phase I trial to treat hemophilia B using adeno-associated vectors (AAV) is set to start soon.
Because of those high stakes, Inder M. Verma, American Cancer Society professor of molecular biology at the Salk Institute for Biological Studies, La Jolla, Calif., worries that rushing prematurely to the clinic could deal the field an untimely failure. Neither hemophilia trial is designed to deliver a therapeutic amount of the protein necessary to cure hemophilia, he notes. Both also face the unwelcome possibility of triggering an immune response against the foreign protein. "If the patient has never seen this protein before, you can imagine the body saying, 'This is foreign. Let's build an immune response.'" On the other hand, the body might let the protein remain. And as for the third- and fourth-generation vectors presented at the hemophilia seminar? Most of the data presented came from only a few animals, he notes.
Although the Pittsburgh-based trial is not designed for a complete cure, researchers involved hope to increase the level of Factor VIII (FVIII), which hemophilia A patients lack, by 2 to 7 percent. The first patient has already received one course of treatment (see related story, this page). But Margaret V. Ragni, professor of medicine at the University of Pittsburgh Medical Center, admits she doesn't know how difficult that level will be to deliver with a retroviral vector. "I don't think I have that answer," she comments in a telephone interview. "That's why we're doing the study." Verma, who calls the study, which is sponsored by Chiron Corp. of Emeryville, Calif., "interesting," questions whether the trial harnesses the best vector for the job. "I just don't know if retroviruses will infect nondividing cells."
That's not an issue for the other trial, since AAV transfects both dividing and nondividing cells. But the hemophilia B trial, led by Katherine High, of Children's Hospital of Philadelphia and the University of Pennsylvania, has other weaknesses. High aims for a 1 percent increase in Factor IX (FIX) via injections into muscle. "I would have waited to make a little bit more protein, to make sure that there [are] therapeutic amounts," Verma comments. "They think that will be sufficient to improve the quality of the patient. I think [both trials] are not ideal ...."
Verma notes that his own favored vector for hemophilia, the attenuated AIDS virus, or lentiviral vector, has its shortcomings too. Although the lentivirus, unlike the shorter AAV vector, can accommodate both the smaller FIX and the larger FVIII, it has seen mixed results in animal models. In null dog models, the lentivirus carrying the clotting factors triggered immune responses. Before Verma rules out using the vector for hemophilia, he will try a set of experiments in dogs that are not null.
Unwanted immune responses are plaguing even the newest third- and fourth-generation adenovirus vectors, even though researchers have removed most of the vector's viral backbone, scientists at the AGST hemophilia seminar reported. Wei-Wei Zhang, of GenStar Therapeutics Inc., San Diego, delivered eight to 10 times the therapeutic amount of clotting protein in mice using one of the "minimal adenoviral vectors." But one subgroup of mice in an already-small experiment experienced immune responses. Sheila Connelly, Genetic Therapy Inc./SyStemix Inc., Palo Alto, Calif., reported mixed results with another highly engineered adenovirus. The liver-specific vector phenotypically corrected mice long term, but caused unexpected toxicity in dogs. She tried another less toxic version of the vector, but expression diminished rapidly after a few months. And C. Thomas Caskey, senior vice president at Merck Research Laboratories, West Point, Pa., remained coy about the specifics of his early findings, although he reported that his vector--altered to include a "helper" component--achieved long-term expression in mouse liver and muscle. "The data look extremely favorable," Caskey reported.
Verma replies that it is perhaps not surprising that companies are trying their newest vectors on hemophilia, since their level of success will likely be higher--especially in animal models. "This is a good model system. Everybody wants to validate their vector," he comments. "So the adenoviral people say, 'Look. We have a new vector. You guys are worried about the immune response, but now we have removed all the [viral] protein.'" However, he will remain unconvinced of the new vectors until they are tested in more animals. He also remains cautious about findings reported in mice, because mouse models tend to be highly strain-specific.
At the end of the seminar, Wilson commented that he was glad to hear so many gene therapy approaches for hemophilia. "Every talk had a different strategy. That's good. We simply don't know what's best." Verma suspects that the best has not happened. "We should have gone an extra step to make the very best clinical trials."
| By Ricki Lewis and Paul Smaglik |
In some respects Don Miller, the first person to receive in vivo gene therapy for hemophilia, considers himself fortunate. The retired University of Pittsburgh librarian lived with hemophilia A for 50 years without contracting HIV. That viral status made him eligible for the trial. "If you are HIV positive, you can't be taking treatment because [the virus] would attack the vector." Few people with hemophilia over 25--the target age for the trial--can make that claim. Many contracted the virus through frequent transfusions. Miller reduced his risks by minimizing the amount of blood products he received--even before the specter of AIDS appeared. "I lucked out," he admits.
Any lessening of symptoms would be a relief to Miller. He nearly bled to death when he was circumcised and recalls countless other harrowing incidents. "One time I fell at my grandmother's house and had a one-inch-long cut on the back of my leg. It took five weeks to stop bleeding--it ... leaked real slowly, but I didn't need whole blood replacement. ... [I]f I moved a little the wrong way, it would open and bleed again."
As he grew up, treatment progressed from whole blood infusions to plasma replacement, gamma globulin, cryoprecipitate, thrice-monthly Factor VIII injections, and now, to a total of three gene therapy infusions.