One of biotechnology’s greatest unfulfilled promises is that of gene therapy. The concept of delivering therapeutic genes directly to cells using viral vectors has proved far more difficult than researchers predicted two decades ago. But such therapies are slowly making their way towards the clinic, and one of the first major applications of the technique could be the treatment of chronic pain.
Currently the “gold standard” for chronic pain treatment is morphine, a drug that binds tightly to so-called opioid receptors on pain-sensitive neurons of the brain and spinal cord, effectively switching off pain signals. But when delivered into the bloodstream, morphine also binds to opioid receptors on cells elsewhere in the body, causing unwanted side-effects that include grogginess and constipation. And like other treatments for pain, morphine also must be administered every few hours to maintain its effectiveness.
Gene therapy techniques have the potential to overcome such drawbacks by targeting pain-sensing neurons directly, producing long-term effects with single treatments. “As long as the gene is being expressed,” says Joseph Glorioso, a prominent gene-therapy researcher at the University of Pittsburgh, “you get pain control.”
One such technique was described in a paper published Jan. 22, 2008, in the Proceedings of the National Academy of Sciences. Andreas Beutler and colleagues at the Mount Sinai School of Medicine in New York used a gene-delivery vehicle, or “vector,” known as adeno-associated virus (AAV) to implant the gene for a pain-killing protein known as beta-endorphin into pain-sensing neurons.
Two to four weeks after a solution of the gene-carrying vector was injected into the spinal fluid of rats, there was clear evidence that the virus had implanted the gene within neurons of the dorsal root ganglia, a cluster of way-stations for pain signals in the rats’ spinal cords. The beta-endorphin produced after this single injection was sufficient to reverse standard indicators of pain in the rats for three months or more.
Beutler, who devised the first experimental gene therapy for chronic pain when he was a visiting graduate student at UC-San Diego and the Salk Institute in 1995, now wants to test his advanced, AAV-based therapy in larger mammals or in a phase 1 trial of people with chronic pain (phase 1 trials test whether a treatment is safe and has few side-effects in humans). “I’m in the process of talking to colleagues in order to form a consensus about how to proceed,” he says.
As Beutler notes, AAV is increasingly popular among gene therapy researchers, among other reasons because it does not replicate within cells as normal disease-causing viruses do, and at least in rats and mice, provokes relatively little immune response.
But in humans, AAV has had its problems. In 2006, researchers led by Katherine High of the Children’s Hospital of Philadelphia and Mark Kay at Stanford ran a clinical trial of an AAV-based therapy targeting liver cells to treat hemophilia. They found that the therapeutic gene stopped producing within eight weeks. As Beutler puts it, “there was evidence that the gene expression was shut down by a cellular immune response.”
Beutler thinks that within the central nervous system, AAV-based therapies might be more protected from an immune response, but given the immunological differences among different species, he says, “this is an issue that can probably only be answered by tests in humans.”
Meanwhile, Joseph Glorioso and his colleagues have devised a chronic pain gene therapy based on a much larger gene-carrying vector, herpes simplex virus (HSV). The virus, which in its “wild-type” form has evolved to infect the human neuron, enters nerve endings in the skin and follows them “upstream” to the spinal cord.
Glorioso doesn’t think his HSV-based vector would have the same immune problems as AAV. “Delivery to the neurons happens very quickly,” he says. “And once it gets into the nerve cell it’s protected.”
HSV’s size also makes it easier to add complex genetic payloads. “You can put a lot of different gene cassettes in it,” says Glorioso, “whereas AAV is a very small virus and you can do only one.” A gene cassette is a manipulable fragment of DNA that carries and can express genes.
Glorioso’s group reported success last year in using an HSV vector to deliver the gene for an endorphin protein known as proenkephalin to pain-sensing neurons in mice. The effect in mice, according to Glorioso, is “as good as morphine. It’s not like a trivial dampening of the response. It’s dramatic.” He says he expects the HSV-based therapy, which is now backed by a Swedish drug company, Diamyd, to begin phase 1 clinical trials within a few months.
His rival, Beutler, praises the elegance of Glorioso’s approach, saying that “it very effectively takes advantage of the biology of HSV.” But Beutler observes that Glorioso’s HSV technique requires multiple injections to cover the patch of skin, or dermatome, covered by each nerve root: “So something that’s very easy and practical in a small rodent becomes fairly tricky in a large human.” He suggests that the current approach might be most applicable to pain conditions in which only one or a few nerve roots are affected, so that a relatively small number of injections are needed.
“There are probably better delivery systems we’ll develop in the future,” Glorioso notes, “for example, to cover a whole dermatome with microinjection needles.”
Seeking other avenues
NIH’s Michael Iadarola, who is co-inventor on a patent covering the AAV technique, believes that both Beutler’s and Glorioso’s approaches need more development before they can be used in the clinic. During the late 1990s and early 2000s, he says, his laboratory looked at AAV and HSV vectors, as well as others, but “we couldn’t make the pain-killing effect strong enough and long enough, despite the fact that we evaluated a fair number of vectors and tried to modify them in various ways.”
Iadarola says he hopes one or both approaches eventually work, but for the past few years he has been developing his own more radical technique, for use in patients with the most severe chronic pain. The technique involves injections of a protein called resiniferatoxin, derived from a cactus-like plant native to Morocco. In high enough doses, says Iadarola, resiniferatoxin “lesions the fiber that goes from the [pain-sensing] neuron cell body to the spinal cord, basically cutting the wire—but not on all pain cells, only on the ones critical for chronic pain.” Having tested resiniferatoxin successfully in dogs with severe bone-cancer pain—"blocking the pain for the rest of their life”—Iadarola says he expects soon to file an application with the FDA to test the drug in people.