ARTICLE IN BRIEF
Dantrolene has the potential to augment the exon-skipping drugs currently moving through clinical trials for Duchenne muscular dystrophy (DMD), a new study found. The therapy improved exon skipping in human and mouse models of DMD.
Dantrolene, a drug that is approved by the Food and Drug Administration (FDA) for controlling spasticity resulting from upper motor neuron disorders, improves exon-skipping therapy in a mouse model of Duchenne muscular dystrophy (DMD), and in a human cell model, according to a Dec. 12, 2012, paper in the journal Science Translational Medicine.
The results suggest that the modest improvements in dystrophin production seen in clinical trials of exon-skipping drugs in DMD may not be the final word for these therapies, and that increasing their efficacy or reducing their costs and side effects may be possible.
Researchers not involved in the study are cautious about the results for the moment, since so many successes in lab models have not translated into benefits for boys with DMD. But the same researchers also expressed optimism that the quality of the study and the clear effects it produced may signal that dantrolene, or a compound like it, has the potential to augment the exon-skipping drugs currently moving through clinical trials.
[Dantrolene is a ryanodine receptor antagonist; the ryanodine receptor is known to mediate the release of calcium from the sarcoplasmic reticulum.]
“It's an exciting time right now in the treatment of muscular dystrophy,” said Kevin M. Flanigan, MD, professor of neurology at Ohio State University and principal investigator at the Center for Gene Therapy at Nationwide Children's Hospital in Columbus, OH.
Two different exon-skipping compounds are currently in trials for DMD. Each is an antisense oligonucleotide, whose sequence is designed to bind to the dystrophin RNA at a critical exon-intron border, causing the splicing machinery to omit a mutation-containing exon from the final messenger RNA. The result is a shorter, but still functional, dystrophin protein. The two compounds differ in their backbone chemistry, and each has shown the ability to increase the amount of dystrophin at the muscle membrane in boys with DMD, but not sufficiently to yet lead to functional improvement.
COMPOUNDS THAT ENHANCE EXON-SKIPPING
That modest effect, combined with the high cost and potential for side effects of both therapies, led Stanley F. Nelson, MD, and Carrie Miceli, PhD, to look for compounds that could enhance the exon-skipping effect. Dr. Nelson is professor of human genetics and Dr. Miceli is associate professor of microbiology, immunology, and molecular genetics, both at the University of California, Los Angeles, where they are also co-directors (along with Melissa Spencer, PhD) of the Center for Duchenne Muscular Dystrophy.
They conducted a high-throughput screen of FDA-approved compounds that increased exon skipping in the presence of antisense compounds targeted to the dystrophin gene. “One of the reasons we had an idea it might work,” Dr. Miceli said, “was that we know that in many cases, a low level of skipping happens naturally. So we thought that if we combined an antisense oligonucleotide with a drug targeting this machinery, we might be able to optimize the outcome.”
The best results from the screen were for dantrolene, a compound that targets the ryanodine receptor in muscle, and is used to treat malignant hyperthermia and spasticity. In the early 1990s, dantrolene was given to boys with DMD for over two years to test its ability to reduce creatine kinase and improve muscle strength. The results were not impressive, but it proved to be well tolerated.
TESTING IN ANIMAL MODEL, HUMAN DMD CELLS
Drs. Nelson and Miceli tested dantrolene in cells from the mdx mouse, the standard animal model of DMD. They used a suboptimal dose of an antisense molecule with the same backbone chemistry used in one of the clinical trials. The dose chosen was sufficient to induce about 10 percent of the maximal skipping possible. Then they washed out the antisense compound, and treated with dantrolene.
Dantrolene turned a suboptimal dose into an optimal one, inducing the same level of skipping seen in the higher dose without dantrolene. “We are sparing oligonucleotides but getting the same effect,” Dr. Nelson said.
The same pattern was seen in human DMD cells, with dantrolene triggering a ten-fold increase in exon skipping from a suboptimal dose of the other antisense molecule used in clinical trials. There was no measurable effect from dantrolene alone.
In mdx mice, dantrolene combined with antisense therapy produced the same amount of dystrophin as ten times the antisense dose without dantrolene. Combined treatment continued to be effective when administered multiple times over three weeks, and led to reduced creatine kinase levels and improved strength.
Not all muscle groups benefited equally. Significantly, the heart, which has not been successfully treated with antisense alone, did not respond to combination therapy.
FOCUS FOR STUDY
“What we really asked was whether a suboptimal dose could become optimal,” Dr. Miceli said. “An open question we hope to pursue is whether dantrolene could also optimize the highest dose.” That may be critical, she said, if it turns out the highest doses used in the clinical trials are insufficient by themselves to provide a meaningful benefit for DMD patients.
Another open question is the exact mechanism through which dantrolene is acting. The researchers know the beginning of the answer, since they found that other ryanodine receptor antagonists, structurally unrelated to dantrolene, also promoted exon skipping, strongly suggesting the drugs were all acting through their effect on the receptor. How that receptor antagonism at the sarcoplasmic reticulum is translated to a change in RNA processing in the nucleus is unknown. Unraveling that mystery may lead to even better drug candidates, they said.
One of the lessons of the study, Dr. Nelson said, was the potential for “repurposing the rich cornucopia of drugs that are already approved.” Increasingly, researchers in many fields are using high-throughput screening of these drugs to identify potential therapies that can, if effective, jump quickly to the head of the line for clinical trials in new applications. (Another recent exampleis the testing of tadalafil in Becker muscular dystrophy patients to improve muscle ischemia. [See Neurology Today story, “Tadalafil Improves Muscle Oxygenation in Becker Muscular Dystrophy,” on page 12.]
In the past, Dr. Miceli said, the expectation was often for “a big hit that would give you a cure. But the truth is that if you take a number of drugs that each can give an incremental effect, and see if they can synergize, there is the potential for tremendous value.”
“These data, with an FDA approved drug, raise the possibility of using lower doses of drug, or getting more efficacy, either of which would probably be of great benefit in the field,” said Dr. Flanigan of Ohio State University. “We have no idea how it does this, but the data are quite compelling. Being able to dose with less drug, and at less cost, is one of the promising findings.”
He also noted that the results were not dependent on the backbone chemistry of the individual agents, or the sequence of the gene segment targeted. “Those are two critical findings,” he said, since they broaden the likely applicability of the therapy. Both current antisense agents target exon 51, since it stands to benefit the most patients, but there are a large handful of other mutations that could potentially benefit from exon-skipping therapy.
Kathryn Wagner, MD, PhD, associate professor of neurology and neuroscience at Johns Hopkins University and director of the Center for Genetic Muscle Disorders at Kennedy Krieger Institute, concurred that the results are promising. “We may be nearing a meaningful therapy,” she said. She noted that the decision to undertake this study while the exon-skipping clinical trials were still in the planning stages “showed incredible forethought.” The result may be that combination clinical trials can be undertaken rapidly, especially since dantrolene has been used in the past in boys with DMD. “It is an extraordinarily exciting time for the Duchenne muscular dystrophy field,” she said.
The study was funded from grants from the NIH, the Department of Defense, and the Foundation to Eradicate Duchenne, as well as through NIH funding for the UCLA Muscular Dystrophy Core Center.