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Most researchers in the field agree that the adenovirus and retrovirus vectors are imperfect, to say the least. In addition to having immunological side effects, both lack the carrying capacity to accommodate the larger, more complex genes that would be useful in therapy. "There are only three problems in gene therapy," says Salk's Verma, "delivery, delivery and delivery. It isn't going to be a problem to make gene therapy work--if we have an appropriate set of tools to deliver the genes."
For his heart-patient trial, St. Elizabeth's Isner found a novel way around the delivery problem. Eschewing virus carriers, he fashioned a construct called "naked DNA." It consists of part of a human gene called VEG-F, which stimulates the growth of blood vessels, and includes its signal segments. These segments, Isner explains, "order the cell, once it has manufactured the gene product, to export it from the cell."
In his Phase I trial, Isner injected a saline solution containing his naked DNA through a small "keyhole" incision in the chest of his heart patients and directly into their heart muscle. A few weeks later, tests on everyone in the trial group showed greatly improved blood flow to the heart muscle though tiny new blood vessels that bypassed clogged arteries.
How does the naked DNA, without viral assistance, penetrate the walls of the heart-muscle cells? "To be perfectly honest," Isner confesses, "no one really understands how it gets there." But unlike most other therapeutic genes, which must find their way into millions of cells to have a therapeutic effect, VEG-F needs to invade only relatively few. Its protein product, issuing from the cell, can act on untold numbers of surrounding, untreated cells. Quips Isner in a parody of the Marine Corps slogan, "All we're looking for are a few good cells."
The fact that the VEG-F gene seems to turn off after three or four weeks makes little difference in this trial because the new blood vessels have already sprouted and remain in place. Still, for this and other reasons, the naked-DNA approach is applicable to only a handful of disorders.
For the vast majority of other trials, scientists are hard at work developing a new generation of viral vectors. One promising candidate, says Pennsylvania's Wilson, is the AAV (adeno-associated virus), a small, benign human virus that does not seem to cause any disease. "It doesn't elicit the same kind of inflammatory response that the other vectors do," Wilson explains. "It's somehow evolved the way to get around that." The AAV also efficiently insinuates itself into nondividing cells and, in tests with monkeys and mice, has enabled the therapeutic gene engineered into it to express itself for more than two years.
Wilson expects Phase I trials using AAV to begin later this year, first for the treatment of hemophilia and later for a form of muscular dystrophy, a liver metabolic disease and retinitis pigmentosa, an eye disorder. "It's kind of a new wave," he says.
The other new vector is being fashioned by Salk's Verma. "What we want," he says, "is a virus that is easy to make, that delivers genes at very high efficiency, that can infect a nondividing cell and that enables its therapeutic gene to become part and parcel of the chromosome."