ARTICLE IN BRIEF
Investigators reported that intrathecal delivery of an antisense oligonucleotide suppressed mutant huntingtin in a Huntington's mouse model, and the benefi ts lasted months after the treatment was stopped.
Antisense oligonucleotide treatment in a mouse model of Huntington's disease (HD) leads to widespread suppression of mutant protein synthesis in the mouse brain, and continues to provide benefit for months after treatment has stopped. The results have researchers looking forward to a trial in humans within two years, and rethinking how the mutant protein causes the problems it does.
Previous attempts to silence the mutant HD gene have employed virally-delivered interfering RNA. Because of an abundance of virus receptors in brain tissue, most of the injected virus has been absorbed by a very small volume of brain tissue immediately surrounding the injection site.
In contrast, in a June 21 paper in Neuron, investigators reported that the antisense oligonucleotide (“oligo”), a modified form of DNA that caused the destruction of the messenger RNA for both mutant and normal huntingtin, was injected directly into the CSF. Because there are no known cellular receptors for such molecules, they are much more widely circulated before being taken into cells, said the study's senior investigator Don Cleveland, PhD, professor of medicine, neurosciences, and cellular and molecular medicine at the Ludwig Institute for Cancer Research at the University of California, San Diego. Holly Kordasiewicz, PhD, who was formerly in Dr. Cleveland's lab and is now at Isis Pharmaceuticals in Carlsbad, CA, led the study.
“We show that intrathecal delivery can achieve broad distribution, not just in the spinal cord but in almost all brain regions, and reaches the major brain regions that you would want to get to treat Huntington's disease,” Dr. Cleveland said.
Mice bearing the mutant human HD gene were continuously infused with the antisense oligo for two weeks, and then the process was stopped. The oligo was not detected in regions of the brain composed primarily of white matter.
The level of oligo in the brain decreased steadily over the next 16 weeks. Human huntingtin mRNA was reduced to 38 percent of control levels immediately after the end of infusion, and continued to remain suppressed for the next 12 weeks, rising back to untreated levels over the next four weeks. The level of mutant protein followed a similar pattern.
To determine the ability of treatment to reverse disease symptoms after they had developed, and to determine the duration of short-term treatment, the investigators treated 6-month old HD mice for two weeks, and then followed them. Treatment partially reversed the motor deficit mice had developed by the start of treatment, although not to the level of wild-type animals. The improvement was sustained for up to nine months. There were also sustained improvements in hypoactivity and anxiety, as measured by their willingness to explore a lit arena. These improvements developed slowly, and persisted long after mutant huntingtin had been restored to its pretreatment levels.
“This was perhaps the most surprising feature,” Dr. Cleveland said. “Our inference from that is that the reduction of huntingtin synthesis has allowed the clearance of some toxic intermediate species that then take time to rebuild up, and that the anxiety [improvements] are measures of neuronal rewiring of the remaining neurons, and that takes time.”
“There remains a controversy about how safe it will be to lower normal huntingtin” in humans, Dr. Cleveland said, so safety as well as efficacy were critical questions in the mouse. That transient suppression of the protein led to prolonged symptomatic improvement “mitigates the risk of lowering the normal protein because we don't have to suppress it continuously to have a long-term benefit.”
The researchers also infused the oligo into the CSF of Rhesus monkeys for 21 days, using technology similar to that used in a recent clinical trial of antisense oligonucleotides for amyotrophic lateral sclerosis. They found that the oligo distributed widely, accumulating in both cortex and the caudate nucleus of the striatum, and reduced levels of huntingtin mRNA by 25 percent to 63 percent, depending on the brain region. These levels remained reduced for four weeks after stopping treatment, and returned to normal after an additional four weeks.
Based on these results, Dr. Cleveland said, “it would take a true pessimist to think that we can't deliver the drug in an effective way to parts of the [human] brain that are centrally involved in the disease. And that's pretty good. It's not perfect but it's pretty good.” Based on these results, Isis, the company that produces the oligo, is moving ahead with plans for a clinical trial within two years.
“One of the big surprises is that this method seems remarkably effective” at delivery, said Steven Finkbeiner, MD, PhD, director of the Taube-Koret Center for Huntington's Disease Research and professor of neurology and physiology at the University of California, San Francisco, who was not involved with the study. “There does seem to be reasonable confidence that you can actually deliver the drug widely throughout the nervous system. The other thing that's really new is that you can get behavioral benefits that last quite a bit longer than the measurable suppression in the Huntington's gene.”
“Our conception of neurodegenerative disease has been hugely influenced by pathologists,” he noted, leading to a primary emphasis on cell loss. The results of this study combine with others suggest that, instead, “the deficits you see are the consequence of ongoing dysfunction triggered by mutant huntingtin, and if you lower it, you have the possibility of recovering function that apparently had been lost. To the extent that holds in humans, that should affect the way we think about neurodegenerative disease. It suggests that at least some aspects of it may be reversible. Almost certainly the deficits we see are the interplay between helpful and maladaptive changes the brain undergoes” to cope with the accumulation of mutant protein.
The exact pathogenic cascade through which mutant huntingtin causes disease remains unclear, said Robert Pacifici, PhD, chief scientific officer of CHDI, a nonprofit research organization dedicated to Huntington's disease. “But if there is one therapeutic approach that really skirts these arguments, it is huntingtin lowering.”
Both Dr. Finkbeiner and Dr. Pacifici were encouraged and excited about the prospects for a clinical trial. The key question in any clinical trial, Dr. Finkbeiner said, “is are you working on a validated target?” Whatever the downstream mechanisms, it is clear that in HD, the mutant protein is the ultimate validated target, and lowering it should be the initial efficacy measure in any trial of antisense therapy. “I think if you show that, I would be very committed to keep doing clinical trials,” even if symptomatic improvement is more difficult to demonstrate initially.
It must also be shown that the treatment is safe in humans. “I think all of us feel there is going to be a minimum amount of protein lowering that is going to be necessary to have a therapeutic effect,” Dr. Pacifici said, “but also a maximum beyond which you might end up getting deleterious effects,” due to a reduction in normal huntingtin. The protein is required during development, but its role in the adult brain is unknown, with some researchers suggesting it may be unnecessary.
But the bottom line, he said, is that “we're very excited about the prospects” for antisense treatment in HD. “It's going to be high risk in terms of its chances for success, but we think it's well worth trying.”