The progress of several experimental therapies currently in development for neuromuscular diseases was discussed at MDA's 2012 Clinical Conference, held in Las Vegas March 4-7.
Most of these emerging therapies are unlikely to be curative, though they may be disease-modifying, meaning they may slow or stop progression or perhaps even return someone to an earlier and better functional level. However, many questions remain to be answered before these therapies can become available to patients.
The main experimental strategies discussed at the conference were:
Annemieke Aartsma-Rus from Leiden University Medical Center in the Netherlands gave the audience a status report on exon skipping for DMD.
Aartsma-Rus has been working with the Dutch biotechnology company Prosensa on developing the DMD exon-skipping drug PRO051/GSK2402968. This experimental drug, now in clinical trials, targets a section of the dystrophin gene known as exon 51 and would, Aartsma-Rus said, potentially help about 13 percent of DMD patients.
She reported that, in a trial of this drug in which 12 DMD patients were given subcutaneous injections of PRO051/GSK2402968 for five weeks, 10 of them produced dystrophin protein, the main goal of exon skipping in DMD. Most muscle fibers sampled produced dystrophin protein in these trial participants, she said.
The 12 participants are now in an open-label, extension study, in which there is no control group (no group taking a placebo). Aartsma-Rus said they now have nearly two years of data available.
"The patients are doing well, and some are even improving," she said. However, without a control group for comparison, it’s impossible to say how much of the apparent maintenance or improvement is due to the drug and how much to other factors, such as normal variation in the disease course.
Aartsma-Rus added that there have been some adverse events seen in the trial, such as local reactions to the injections; protein excreted in the urine; changes in liver function; and a decrease in one type of blood cell. None of these have been considered serious, she noted, and all were reversible when the treatment was stopped.
Of interest is that there were no antibodies (immune system proteins) detected against the dystrophin protein, an indication that the immune system appears to be tolerating the newly synthesized dystrophin.
Aartsma-Rus also discussed another experimental exon-skipping drug, AVI-4658, also known as eteplirsen, in development by biotechnology company AVI BioPharma. This drug also targets exon 51 of the dystrophin gene but has a different chemical formulation.
This drug was given by intravenous infusion in a recent clinical trial, in which there were no serious adverse events, she said.
There was clear dystrophin protein production in seven of the 19 trial participants, of which three responded particularly well. There were indications of decreased inflammation in the muscles, but the investigators did not see an improvement in function or a reduction in blood levels of creatine kinase, an enzyme that leaks out of damaged muscle cells. (A lower creatine kinase level in the blood suggests less muscle damage is occurring.)
Aartsma-Rus said that regulatory agencies, such as the U.S. Food and Drug Administration (FDA) and similar bodies in Europe, see each exon-skipping drug as a separate entity, potentially posing barriers to moving these drugs through clinical trials and into the clinic.
Lee Sweeney from the University of Pennsylvania explored the puzzling finding that an experimental drug recently developed by PTC Therapeutics for people with DMD and BMD with a specific type of dystrophin mutation was more effective at a low dose than at a high dose.
The experimental drug, ataluren (formerly PTC124), is designed to help muscle cells "read through" (ignore) a type of gene flaw known as a premature stop codon, or nonsense mutation, which prematurely terminates the synthesis of a protein. In DMD and BMD, that protein is dystrophin.
Reading through a premature stop codon can allow a cell to produce full-length dystrophin protein even though the mutation is still there. However, the "read-through" process works with variable efficiency.
In 2010, PTC announced that, in a phase 2b trial, those who took the lower dose of ataluren showed a slower decline in walking ability over the course of nearly a year than those in the high-dose or placebo groups. (MDA did not fund the phase 2b trial but did fund earlier development of ataluren.)
Sweeney said that once the mechanism behind this effect is better understood, it may be possible to "select a subset of patients likely to respond" to ataluren.
Utrophin is a naturally occurring protein that is similar in structure to dystrophin and can probably substitute for dystrophin's absence in muscle fibers to some extent.
Absence of dystrophin protein characterizes DMD, while a partially functional, shorter dystrophin protein characterizes BMD.
With this in mind, said Justin Fallon of Brown University, several strategies aimed at raising utrophin levels in DMD and perhaps in BMD are now in development.
Fallon noted that the benefits of utrophin-based therapy appear more likely to occur in DMD; in BMD, he said, the effects are unclear and need further investigation.
Fallon said there are now four utrophin-increasing strategies being investigated:
Fallon's laboratory did the initial research on a protein called biglycan, on which TVN-102 is based. In dystrophin-deficient mice, Fallon said, biglycan increased utrophin levels in muscle tissue and decreased signs of muscle damage without apparent toxicity.
Utrophin levels are naturally increased in DMD-affected muscles, Fallon said, which may be the body's attempt to compensate for dystrophin's absence.
This "compensation" hypothesis is supported by the fact that mice missing both dystrophin and utrophin have a more severe muscle disease than mice missing dystrophin alone; and that a dystrophin-deficient patient in whom utrophin was not properly positioned had a particularly severe disease course.
If increasing utrophin is a helpful response to the loss of dystrophin, then utrophin-based strategies are employing what Fallon termed a "natural compensatory mechanism" to treat DMD and possibly BMD.
Gene therapy— the insertion of new genes for therapeutic purposes — as a treatment strategy for DMD was the subject addressed by Dongsheng Duan from the University of Missouri. Duan's research, much of which has been MDA-supported, has focused on delivering miniaturized versions of the dystrophin gene inside adeno-associated viral (AAV) vectors.
Dystrophin is the protein that's missing in DMD, and adeno-associated viral (AAV) vectors are viral "shells" used to transport genes to tissues.
Duan's recommended "to-do" list for DMD gene therapy includes:
Duan also noted the importance of including the part of the dystrophin gene that sticks to a molecule called nNOS when designing miniaturized dystrophin genes. This molecule helps regulate blood flow to exercising muscle fibers.
Charles Thornton, who directs the MDA/ALS Center and co-directs the MDA Clinic at the University of Rochester Medical Center, gave an update on the development of antisense-based therapies for ALS and MMD.
All these effects can be used for drug development in different diseases.
Thornton has been working with Isis Pharmaceuticals on development of an antisense-based drug for the type of ALS that’s caused by any of several mutations in the SOD1 gene. Toxic SOD1 mutations, he noted, are the second most common cause of familial ALS.
In addition, Thornton said, the consequences of having a lower-than-normal level of SOD1 protein appear to be “not too dire,” making targeted destruction of SOD1 RNA with antisense a viable option for therapeutic development.
A phase 1 trial of the experimental antisense drug ISIS-SOD1-Rx in 32 people with SOD1-related ALS has now been completed, and the drug is likely to undergo further testing in a phase 2 trial, Thornton said. ISIS-SOD1-Rx is given intrathecally, meaning into the spinal fluid. Its development has been supported by MDA.
Thornton also discussed antisense-based drug development for type 1 MMD (MMD1), noting that Isis also is involved in this effort (as is MDA).
In MMD1, an expansion in the DNA in the gene for the DMPK protein causes a similar expansion in the RNA for this protein, with many harmful effects on nerve and muscle cells.
The goal of antisense for MMD1 is destruction of DMPK RNA. There have been several types of experimental antisense strategies in laboratory development for type 1 MMD, Thornton said, and until recently, they have failed to live up to expectations in rodents. Now, however, at least one of the compounds is getting to muscle and having beneficial effects, including suppression of myotonia, the prolonged muscle contraction seen in mice and humans with the disease.
Thornton noted that the effects of one antisense compound have lasted a year after administration in rodents.
“There is hope for reversibility of the disease,” he said. “We may see substantial recovery in some patients.”
Thornton said the compound is now being further refined to target human DMPK RNA (instead of rodent DMPK RNA), and the plan is for it to enter into phase 1 trials in MMD1 patients.
Scott Harper from Nationwide Children’s Hospital in Columbus, Ohio, spoke about RNA interference, a therapeutic strategy being investigated for FSHD that may also have application in other diseases where the root of the problem is toxic RNA or toxic protein made from the RNA.
RNA interference, or RNAi, is similar to antisense in that it targets a specific RNA message and reduces the synthesis of protein from it. However, it works via a different mechanism from antisense, utilizing natural pathways that cells employ to reduce or stop production of a protein
In FSHD, Harper explained, inappropriate "switching on" of production of a protein called DUX4 appears likely to be the molecular root of the disease.
Inappropriate production of another protein, known as FRG1, may also be involved, he said, and both could be good targets at which to aim RNA-reducing therapies.
Until recently, there was no mouse model of FSHD that inappropriately produced DUX4, Harper noted, because DUX4 is highly toxic to the developing mouse
Now, Harper and his research group have, with MDA support, developed a mouse model of FSHD that produces DUX4 only temporarily, allowing the mouse to develop and survive and allowing investigators to experiment with RNAi against DUX4 genetic instructions.
When DUX4 production was reduced with RNAi, Harper said, the strength of the DUX4-overproducing mice increased.
Brian Kaspar at Nationwide Children's Hospital in Columbus, Ohio, discussed the potential for gene therapy in SMA, a disease that requires targeting cells in the spinal cord.
In SMA, a deficiency of full-length SMN ("survival of motor neurons") protein, caused by mutations in the SMN1 gene, causes loss of spinal cord motor neurons, the nerve cells that control muscle movement.
Kaspar and his colleagues have used type 9 AAV (AAV9) vectors to successfully transfer SMN1 genes into the motor neurons of mice with an SMA-like disease. Kaspar said they targeted 60 to 80 percent of these cells in the mice with a single intravenous injection the day after birth
This type of SMA mouse dies very early in life if untreated, but Kaspar said the treated mice are surviving "well past 150 days with normal function." (He also noted that, although the mice had normal function, they were only about half the size of non-SMA mice.)
A caveat is that the gene therapy treatment had to be given very early — within the first few days of life — for it to be effective. It isn't clear how that translates into a required time frame for treatment of human babies with SMA.
Kaspar noted that the "window of opportunity" for targeting motor neurons in pigs and monkeys does not seem to be as narrow as it is for mice, but evidence suggests that "it's potentially important to do early treatment.
Kaspar and his colleagues are now testing delivering genes directly into the spinal fluid in pigs, hoping this delivery method will decrease the required dose and therefore increase the safety of this type of therapy.
The investigators are designing a clinical trial for infants with SMA who are 1 to 6 months old. "We would like to go into newly diagnosed SMA patients," Kaspar said, adding that they're working with the Food and Drug Administration to identify the best ways to measure the effects of gene therapy in this disease.
Arthur Burghes, also at Nationwide Children's Hospital, continued the discussion of experimental therapies for SMA.
Burghes is working with Brian Kaspar on gene therapy for SMA and is also working on other strategies, particularly antisense for this disease.
In ALS, MMD and FSHD, the goal of antisense and similar strategies is to block production of a toxic protein. However, in SMA, the goal of using antisense is to change the way genetic instructions for a protein are interpreted by cells. Specifically, in SMA, the goal is to change the way cells "read" the SMN2 genetic instructions (RNA) so that more of the needed, full-length SMN protein can be made from them.
"You can get significant correction of SMA [in mouse models of the disease] with antisense," Burghes said. He favors a type of antisense formulation called morpholino antisense, but he noted that others favor different formulations.
Burghes said there are now three main strategies in the pipeline for treating SMA:
"Three different methods are better than one," Burghes said, noting that we don't yet know what will work best in humans.
Like Kaspar, Burghes believes early treatment of SMA-affected babies may be needed. He said newborn screening for the disease might be a good way to identify these babies.
The March 2012 MDA Clinical Conference had three major themes, all related to neuromuscular disease: new developments in genetics and immunology; targeted therapies; and best practices.
The first two themes have been summarized in this article and in the Genetics and Immunology Upate published online March 29, 2012.
Coming soon: A Quest News Online article about the best practices sessions from this conference.