Use drugs to treat the various aspects of disease pathology.
Due to the loss of dystrophin, patient’s muscle fibers are continuously damaged during exercise. Lost muscle tissue is replaced by scar tissue (‘fibrosis’)and fat tissue (“adiposis”). This process is irreversible and is exacerbated by an immune response that is initiated by the muscle damage. Drugs can help to increase muscle growth, to compensate for the lost muscle tissue. Alternatively, they can suppress the immune system or inhibit scar tissue formation.
The drugs only treat the symptoms of the disease, not the cause. Nevertheless, when successful, this can slow down disease progression.
Generally drugs can be taken orally and act on all muscles in the body (no delivery problems as observed for gene and cell therapy). Sometimes drugs already used to treat other diseases can also be used to treat Duchenne patients. This speeds up the transition to clinical application, since a lot of information is already known for the drug (e.g. toxicity, dose etc).
There is a vast number of drugs that are reputedly beneficial in Duchenne patients and/or dystrophic mouse models. In this section we list the ones that have been tested in patients and the ones that gave very promising results in mouse models.
We have divided the various drugs in to the following areas.
Corticosteroids (prednisone, deflazacort etc)
To suppress the immune system in order to reduce the formation of scar tissue.
Corticosteroids are a group of drugs that can suppress the immune system. When muscle tissue is damaged, this will elicit an immune response (the body does not know what causes the damage – it could well be a virus, bacteria etc). While the immune response has the best intensions (to protect the body from infections), in this case the immune system increases the severity of the disease. Immune cells excrete toxic substances (aimed to kill bacteria etc), which further increase muscle damage and enhance the formation of scar tissue.
By suppressing the immune system with corticosteroids, muscle damage will be less severe and less scar tissue will be formed.
Not that many trials have been done to compare patients treated with corticosteroids and those without, or to compare one corticosteroid to another (e.g. prednisone to deflazacort), but the general consensus is that corticosteroids delay the disease progression and therefore they are part of the standards of care for DMD. Long term analyses have revealed treatment will delay wheelchair dependency by ~1-3 years, will improve muscle strength and function, will delay the loss of respiratory function and improve survival.
It is very likely that corticosteroids work on other levels than only immune suppression (it is thought they can increase expression of utrophin and/or stabilize muscle fibers, so they are less sensitive to damage). This is still under investigation. However, the finding that drugs that only suppress the immune system are less effective than corticosteroids (see e.g. below in the section on cyclosporine),
underlines this idea.
Corticosteroids have to be taken regularly and chronically. This results in side effects in most patients. The most common are weight gain, depression, behavioral problems, growth retardation, delayed puberty and loss of bone mass but many more have been described.
For some patients side effects can be reduced by an “on-off” treatment regime. Steroids are e.g. taken only every other week, or only during weekdays and not weekends or high doses are taken only during the weekends.
Some patients cannot tolerate chronic treatment with corticosteroids. If the side effects outweigh the benefits (e.g. weight gain to such an extent it impairs rather than enhances muscle functionality) it may be best to stop treatment (this should of course only be done after discussion with the treating clinician because abruptly stopping steriod treatment can lead to severe side effects and even death).
Many different corticosteroid treatment regimes are in use by different patients. It is not yet known whether one regime is more optimal than others. Furthermore, for clinical trials testing therapeutic approaches, it would be preferred if a more standardized regime would be used by all patients in the trial.
The FOR-DMD trial compares beneficial effects and side effects of the most used dosing regimes of prednisolone (daily treatment vs. 10 days on – 10 days off) and daily treatment with deflazacort. The study takes place in at least 40 muscle clinics in the USA, Canada, UK, Germany and Italy. This study is finished and data-cleaning and analysis is currently ongoing.
It is unknown when best to start with corticosteroids. Due to the adverse effects (stunting growth, obesity and increased osteoporosis) most clinicians do not start before age 3-4 years.
To test whether starting before age 3 years is beneficial, a clinical trial is ongoing in 1-30 month old DMD patients. Patients take high doses of prednisone only twice per week to lower the chance of side effects.
Vamorolone (previously VBP15) is a non-steroid compound developed by Reveragen Biopharma. The hope is that this compound induces the beneficial effects of corticosteroids, but not the side effects. In the mdx mouse model this was indeed observed. A phase 1 trial in healthy volunteers has been completed. A phase 2a trial in patients has been completed in the USA. Dose dependent functional improvements have been observed in patients in the open label phase after 18 months of treatment compared to untreated patients from natural history datasets. At the same time, treated patients had normal growth rates and less weight gain and Cushingoid features were reported than for published reports for deflazacort or prednisolone. Phase 2b trials are currently ongoing in the USA and Europe where vamorolone treatment is compared to placebo and prednisone treatment.
CAT1004 (edasalonexent) is an anti-inflammatory drug developed by Catabasis. It has been tested in healthy adults. A two stage phase 1/2 study in DMD patients has recently been completed. Edasalonexent was safe and well tolerated. No anti-inflammatory effect could be picked up by MRI when comparing placebo and treatment groups after 12 weeks of treatment. All boys were then switched to the high dose group. After 48-60 weeks of treatment, muscle function seems to be stabilized compared to the progression before treatment initiation. Patients still receive treatment in an open label phase. A phase 3 confirmatory trial is currently ongoing. Results are expected in the second half of 2020.
MNK1411 (cosyntropin) is a manmade hormone that has similar anti-inflammatory effects as corticosteroids. Results in mdx mice suggest that MNK1411 treatment results in less inflammation. The company Mallinckrodt Pharmaceuticals was conducting a placebo-controlled clinical trial in DMD patients to test whether injections with MNK1411 have therapeutic effects. However, because enrollment was very slow, this trial has been terminated.
Marathon Pharmaceuticals has conducted an open label study for deflazacort in the US in DMD patients which was at the time not marketed. They have obtained approval for treatment of DMD in the US by the FDA. PTC has obtained the rights to deflazacort in the US and is currently marketing the drug.
Tested but found ineffective:
Cyclosporin is a drug that suppresses the immune system. When muscle tissue is damaged, this will elicit an immune response (the body does not know what causes the damage – it could well be a virus, a bacteria etc). While the immune response has the best intensions (to protect the body from infections), in this case the immune system increase the pathology. Immune cells excrete toxic substances (aimed to kill bacteria etc), which further increases muscle damage and enhances the formation of scar tissue.
By suppressing the immune system with cyclosporin, muscle damage will thus be less severe and less scar tissue will be formed. It is thought to induce less side effects than corticosteroids.
A clinical trial was performed in Germany (Rudolf Korinthenberg in Freiburg) to assess whether cyclosporine treatment indeed is beneficial for patients. Unfortunately, no difference was observed between patients treated with and without cyclosporine.
To reduce scar formation.
Due to the loss of dystrophin, skeletal and heart muscle of Duchenne patients are under continuous stress (oxidative stress), which is yet another process that leads to the formation of scar tissue and also impairs the energy production of muscle fibers by the mitochondria (power generators). In muscle this leads to loss of muscle function. In heart, it results in a reduced pump function (the heart becomes “stiffer”). Idebenone helps the mitochondria (power generators of the cells) to generate energy. Idebenone also reduces oxidative stress (it is an antioxidant), in heart and skeletal muscle, to prevent scar tissue formation. Thus, the heart pathology that is seen in many adolescent patients could be delayed or even prevented and skeletal muscle quality should be maintained longer.
Santhera has tested idebenone in Duchenne patients and has shown it is safe. A Phase III study has also been performed to assess the effectiveness of Idebenone on pulmonary function, motor function, muscle strength and quality of life in patients not using corticosteroids. Results show that idebenone was well tolerated and a slower decline in respiratory function was observed for treated patients compared to the placebo group. A placebo-controlled phase 3 study in patients using corticosteroids is ongoing. Santhera has filed for marketing authorization with both the European Medicine Agencies (EMA) and Food and Drug Administration (FDA) in 2016. July 2016 the FDA indicated that they need trial results from steroid-treated patients at the time of filing and therefore cannot consider the current application. In 2017 and 2018 the Committee for Human Medicinal Products gave a negative opinion on the use of Idebenone for DMD. In May 2019 Santhera filed for conditional marketing authorization with EMA for the treatment of respiratory dysfunction. It is anticipated that the CHMP will have an opinion in Q3 2020.
To reduce inflammation and fibrosis and improve regeneration.
Halofuginone is a compound that helps muscle regeneration, reduces inflammation and reduces fibrosis.
Halofuginone is not very well tolerated (gastro-intestinal problems).
A different formulation of this compound has been generated (HT-100) by Akashi Therapeutics that is better tolerated.
A first clinical trial testing the safety of HT-100 was completed for DMD patients in the USA. This trial was temporarily put on hold because toxicity was observed in dogs at very high doses. However, after further testing and re-evaluation of the data FDA allowed the trial to continue. Preliminary results suggested that HT-100 is well tolerated. An improvement in muscle strength compared to baseline was observed for treated patients. A confirmatory study was initiated but has been suspended after a patient participating in the trial sadly passed away. It was revealed that this patient received a much higher dose of HT-100 than expected. The development of HT-100 for treatment of DMD has been terminated.
Other compounds to reduce fibrosis
FibroGen’s FG-3019 (pamrevlumab) is an antibody against CTGF, a growth factor that plays a key role in the production and maintenance of fibrotic tissue. Pamrevlumab is anticipated to prevent CTGF from functioning and in this way reduce the amount of fibrosis formation. A clinical trial to evaluate Pamrevlumab in non-ambulatory DMD patients is ongoing. This is a 3 year trial with an interim analysis planned after a year.
Epicatechin is a molecule that is similar to a hormone produced by the energy producing organelles of the cells (mitochondria) upon exercise. Epicatechin results in the production of mitochondria and improved tissue regeneration in animal models and resulted in reduced fibrosis in muscular dystrophy models. A small pilot study in Becker muscular dystrophy patients was encouraging and showed treatment with epicatechin was well tolerated. A trial in non-ambulant Duchenne patients coordinated by Cardero Therapeutics and UC-Davis is currently completed. Analysis of the results is ongoing. A trial to assess the safety of Coenzyme Q10 (another antioxidant) with and without co-treatment of lisinopril is completed in DMD, Becker and Limb-girdle muscular dystrophy patients. No results have been communicated yet.
Additional compounds to reduce fibrosis are evaluated in preclinical studies, e.g. MTB-1 from Astellas which aims to improve the function and number of mitochondria.
Pentoxifylline has been tested in a clinical trial and was found not to slow down disease progression and was poorly tolerated by patients. Flavocoxid is another antioxidant that has been tested in a phase 1 trial in Duchenne patients to assess safety.
To improve heart and/or muscle function
When muscles contract blood is pushed out of the blood vessels, while contracting muscles actually need more blood (oxygen and nutrients). To compensate normally blood vessels in the muscle will dilate upon contraction. This is in part regulated by production of nitric oxide (NO) by the nNOS synthase enzyme. This enzyme is scaffolded to the membrane of muscle in blood vessels by dystrophin. Without dystrophin the nNOS synthase enzyme is not located properly and thus the ability to dilate blood vessels in heart and muscle is reduced. This can lead to insufficient oxygen supply to muscle and heart, which leads to damaged muscle and heart cells. There are many approved drugs that can improve the dilation of blood vessels. The drugs listed below are or have been tested in Duchenne patients in clinical trials.
Lisinopril is an ACE inhibitor (ACE is an abbreviation of angiotensin converting enzyme). The molecule angiotensin 2 leads to constriction of blood vessels. This molecule is converted from angiotensin 1 by ACE. Thus, by inhibiting this conversion, less angiotensin 2 is produced, reducing constriction of blood vessels. The effect of lisinopril on heart function has been tested in Duchenne patients in sites in the USA, Japan and Canada. This trial also tests whether combined use of Coenzyme Q10 (an antioxidant) and lisinopril has further benefits on heart muscle function. Results have not yet been reported.
Other compounds for dilation of blood vessels
Eplerenone has been tested in Duchenne patients in a clinical trial in a placebo controlled trial and an extension study. This revealed a reduction in cardiac pathology in eplerenone treated patients in young patients while in older patients cardiac pathology stabilized.
A clinical study comparing spironolactone and eplerenone treatment was performed in DMD patients. Results have been published showing that short term (1 year) treatment with spironolactone and eplerenone are comparable with regards to tolerability and efficacy. However, since cardiomyopathy develops slowly in DMD patients over the course of many years, continued follow up analysis is required to assess whether effects are also comparable after long term use.
Revatio (Sildenafil) and Tadalafil
Revatio® (also known as Sildenafil or Viagra) and Tadalafil are PDE5 inhibitors. When NO is produced, this leads to a cascade of reactions that lead to dilation of blood vessels. PDE enzymes target compounds produced during this cascade, counteracting the dilatation of the blood vessel. Inhibition of PDE enzymes leads to a prolonged effect of dilatation.
Revatio has been tested in a clinical trial in DMD and BMD patients in Baltimore (MD, USA). However, this trial has been suspended, since results showed that Revatio is unlikely to provide benefit to adult DMD heart and muscle function, while there was a potential risk for cardiac events.
Based on the encouraging results of a pilot trial with tadalafil performed in a small cohort of DMD patients by Ron Victor, Eli Lilly as started a double-blind placebo-controlled trial in 330 DMD patients. Unfortunately, the results did not reveal any evidence for efficacy of tadalafil in slowing down disease progression in the whole group or in subgroups. As such, extension trials have been stopped. Eli Lilly has indicated that they are willing to share data obtained in this trial with the DMD community to help the clinical development of other therapeutic products.
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 in ambulatory Duchenne patients in the USA and Canada and a global follow up study to assess efficacy have been discontinued after interim analysis revealed no clinical benefit.
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 have been injected in the lower limbs of 3 Duchenne patients has been completed at Nationwide Children’s Hospital (Columbus Ohio). The aim was to assess whether this is safe and whether it can improve lower limb muscle mass and strength. Initial results have been posted. Treatment appears well tolerated. Effects on muscle function measured by North Star Ambulatory Assessment over the course of 2 years are being collected.
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.
To increase the levels of the dystrophin homologue utrophin in muscle.
Utrophin is a protein that is very similar to dystrophin and forms the same link between cell skeleton and connective tissue dystrophin does, but primarily in non muscle tissues. During muscle development or regeneration, utrophin is located at the membrane of muscle fibers. However, when dystrophin production is initiated, dystrophin will replace utrophin. In adult muscles utrophin is expressed at very low levels and mainly located at the transition of nerve to muscle (neuromuscular junction). In Duchenne patients and animal models, however, utrophin is expressed at the muscle fiber membrane as well. In patients these increased levels are still too low to be beneficial. Mouse studies have revealed that 300-500% increased levels of utrophin can functionally compensate for lack of dystrophin and delay disease progression.
Genes have a volume switch, which is regulated by special proteins that can turn a gene off, or set it to low or high in different tissues (resulting in low or high levels of protein, respectively). The utrophin gene switch is set to a very low volume in muscle. Thousands of drugs are screened in order to find those that can increase the volume of the utrophin gene.
Drugs to enhance utrophin expression in cultured cells and animal models have been identified by Summit PLC (John Tinsley and Kay Davies, UK). Ezutromid has been tested in clinical trials in DMD patients by Summit in 3 different doses and uptake resulted in sufficient levels in blood for two of the 12 patients. A follow up trial has been performed where the formulation was tested with a high-fat meal. This improved uptake with 6/12 Duchenne patients having drug levels expected to increase utrophin expression by 30-50% based on mouse data. A phase 2 trial to assess efficiency in ambulant Duchenne patients is currently ongoing (and fully recruited) in the UK and the USA. For an interim analysis, a biopsy was taken after 24 weeks of treatment showing a 7% increase in utrophin levels. Analysis after 48 weeks revealed that the primary endpoint and secondary outcomes were not met (i.e. the drug had no therapeutic effect) and Summit announced that the development of ezutromid will be discontinued for Duchenne.
Other ways to enhance utrophin levels
Additional approaches to enhance utrophin levels are in the preclinical phase. Biglycan from Tivorsan aims to increase both utrophin and nNOS. Laminin-111 from Prothelia aims to increase utrophin and integrin (another protein that links muscle fibers to connective tissue and thus provides stability).