To increase muscle mass by reducing the levels of the muscle growth inhibitor myostatin and related factors.
There are factors that enhance the formation of muscle and factors that inhibit muscle formation (not all tissues should be muscle and because muscles use a lot of energy, they should not be bigger than necessary). Myostatin is one of the main factors that inhibit muscle growth (it lowers the volume setting of many muscle-related genes), but there are related proteins with similar functions.
Myostatin and related proteins bind to “receptors” on the muscle cells. The binding is a signal for the muscle fiber to stop growing (i.e. the volume of muscle related genes is turned down, so less muscle proteins are made). When the gene for the myostatin protein is mutated and no myostatin is made, this leads to increased muscle formation in animals (Belgium blue cattle, Texel sheep, greyhounds, mice) and humans.
This observation resulted in proposing myostatin inhibition as a potential way to improve muscle mass for Duchenne patients, i.e. if it is possible to prevent myostatin from doing its job, this should enhance muscle formation and compensate for the loss of muscle tissue in Duchenne patients. Myostatin inhibition can be achieved by antibodies for myostatin. These antibodies bind to myostatin and prevent it from reaching the gene switches and turning down the volume. The same can be achieved by making a soluble receptor for myostatin. These will bind to myostatin, but because they are soluble, there is no relaying of the signal. At the same time, the binding to the soluble receptors will prevent the myostatin from binding to the muscle-linked receptors.
Myostatin antibodies have been tested in healthy volunteers and were deemed safe. They were consecutively tested in adult patients with muscle diseases. While treatment was safe, it did not result in an increase in muscle mass in the patients. However, patients were only treated for 28 days, which might not have been long enough.
A new trial to test a myostatin antibody (domagrozumab, PF06252616 from Pfizer) has been completed in healthy volunteers. A phase 2 trial to test this three different doses antibody in Duchenne patients has been discontinued. Patients were be treated for 96 weeks, receiving the antibody for the first or the last 48 weeks or for 96 weeks. However, Pfizer announced in August 2018 that the primary end point (time to climb 4 stairs) was not met after 48 weeks of treatment and that none of the secondary outcome measures were suggestive of a treatment effect. Accordingly, they stopped the development of this antibody for Duchenne.
Bristol-Meyers-Squibb has developed another myostatin antibody-like drug called BMS-986089 (adnectin). Adnectin development has now been taken over by Roche. The compound has been tested in healthy volunteers and was well tolerated. A trial to assess safety is ongoing in ambulatory Duchenne patients in the USA and Canada. A global follow up study to assess efficacy is ongoing and has been fully recruited.
The company Acceleron (now taken over by Shire) has generated a soluble myostatin receptor (ACE-031) that outperformed the myostatin antibodies in Duchenne mouse models, probably because in addition to myostatin it can also bind other factors that reduce muscle size. This soluble receptor has been tested in healthy volunteers. This was well tolerated and led to increased muscle mass in a dose dependent manner, with an increase of ~1 kg for the highest dose in a period of 2 weeks.
A dose escalation safety trial with ACE-031 in Duchenne patients has been terminated because some patients suffered from unexplained nose and gum bleeding. The most likely explanation is that the soluble receptor could bind other signal peptides in addition to myostatin (i.e. it is less specific than the myostatin antibodies described above). Additional tests in animal models have been performed and unfortunately the results did not support further development of this compound.
Follistatin gene delivery
To increase muscle mass by antagonizing the muscle growth inhibitor myostatin.
Follistatin is a protein that inhibits myostatin. As is described above, myostatin is a protein that inhibits muscle growth. Thus, by increasing the levels of follistatin, the inhibitor is inhibited, which will lead to an increase in muscle mass. The follistatin gene has been delivered to mice and monkeys using an AAV viral vector (see gene therapy for more details about the challenges and prospects of gene therapy). The injections resulted in an increase in muscle mass and muscle strength.
A clinical trial where AAV viral vectors with the follistatin gene are injected in the quadriceps of Becker patients is ongoing at Nationwide Children’s Hospital (Columbus Ohio). The aim is to assess whether this is safe and whether it can improve quadriceps muscle mass and strength. In a follow up study this approach has been tested in Duchenne patients.
Other ways to increase muscle strength
Tamoxifen is an approved drug to treat estrogen-dependent breast cancer. Studies by Urs Ruegg and Olivier Dorchies in Geneva in mdx mice have shown that tamoxifen treatment reduces fibrosis formation and improves muscle repair. Based on this finding, a clinical trial in DMD patients is ongoing.
Alternative ways to improve muscle quality: HDAC Inhibition
Our body consists of proteins. Most of these proteins are made by our own cells using genes as a genetic blueprint (or a recipe) for protein production. Each cell contains a copy of all genes and could in theory thus produce all proteins. However, muscle cells will produce only proteins needed in muscle and e.g. liver cells will produce only proteins needed in liver. Humans have 20,000 genes but generally only a subset is used in any given tissue. To make the process easier, a cell will highlight genes it often uses (like using a sticky note in a recipe book for a favourite recipe) and will also mark genes that are not used.
Because the proteins produced in muscle differ from the proteins produced in scar tissue, the genes that are marked as ‘used’ and ‘not-used’ differ between these tissues. This means that once muscle tissue becomes fibrotic, the way genes are marked will change as well, leading to a further tendency of the muscle to become fibrotic (as the cell has more difficulty finding the muscle genes, while the fibrotic genes are highlighted).
HDAC-inhibitors are compounds that can ‘reset’ this system, thus removing the highlights of fibrotic genes and also clearing the “not used” makers for muscle genes. In the mdx mouse model treatment with HDAC-inhibitors improved regeneration and muscle quality and decreased formation of fibrosis.
Givinostat is an HDAC-inhibitor that has been shown safe in children and has been tested in DMD patients in a trial in Italy by Italpharmaco. Results from the first small trial showed that treatment for one year was well tolerated. For a small number of patients, reductions in platelets were observed shortly after treatment initiation. Analysis of muscle biopsies suggested a reduction of fibrosis, necrosis and fat when comparing pre- and post-treatment biopsies. An open label extension trial is currently ongoing and patients have been treated for up to 5 years. Analysis of these patients suggests a delay in loss of ambulation compared to natural history controls. An international phase 3 trial to test for efficacy in ambulant DMD patients is now recruiting.
Normalizing calcium homeostasis
Due to the lack of dystrophin the calcium channel in muscle fibers is leaky. This leads to abnormal calcium levels within the muscle, leading to muscle damage, oxidative stress and fibrosis. A drug that can normalize the calcium levels in muscle fibers is rimeporide (from Esperare). A phase 1 trial in DMD patients has been completed and showed good tolerability of remiporide. Esperare is currently planning a phase 2 trial to further test this drug in DMD patients.