Duchenne muscular dystrophy causes progressive muscle degeneration (and eventually death), with average life expectancies for those with DMD at only 25 years. DMD is rarely seen in females, even though females can carry the genetic mutation responsible for DMD. Most DMD patients are dependent on a wheelchair by age 12, and progressive deterioration of muscle leads to loss of movement and eventually paralysis.
Becker muscular dystrophy is a less severe type of muscular dystrophy that causes slow-progressing weakness of the pelvis and legs. In this condition, there is insufficient dystrophin produced by muscle cells, causing unstable muscle cell structure. In DMD, there is no dystrophin produced, making it much more severe.
Thankfully, recent studies have shown promise in the treatment of DMD with tadalafil, the generic name for the erectile dysfunction (ED) drug Cialis. New clinical studies are planned testing Cialis as a treatment for Becker muscular dystrophy as well.
Current Study Results: Why Cialis (Tadalafil)?
In patients with DMD, there is insufficient widening of blood vessels due to lack of neuronal nitric oxide synthase, or nNOS. Drugs known as PDE5 inhibitors, like Cialis and Viagra, are designed to lead to prolonged widening or lessened constriction of blood vessels. Cialis is believed to improve blood flow and, hence, muscle integrity. It may also improve cardiac function and reduce fibrosis of the respiratory tract.
Studies have been done in dogs demonstrating that Cialis slows down muscle loss. It is not a cure for DMD, and its use in humans may be as a palliative treatment, but there are very few treatment options for those with DMD. Studies in mice show a pronounced effect on the diaphragm, and this has also encouraged researchers, even though DMD in mice is not a perfect correlate with DMD in humans. However, by keeping the diaphragm functioning, doctors may be able to improve quality of life and possibly extend life in those with DMD. Furthermore, Cialis could be easily combined with current drugs used by those with DMD.
Why Cialis Instead of Similar Drugs?
Earlier in 2013, a study assessing Viagra in patients with DMD aged 18 and older was suspended when patients demonstrated worse cardiac function. In addition to inhibiting PDE5, Viagra also inhibits a substance called PDE1c, whereas Cialis does not affect PDE1c. Cialis is not expected to cause the same cardiac effects, though patients will be closely monitored in future trials.
Future Investigations
A study involving 306 patients will be starting in August, and will measure the ability of Cialis to slow the decline in walking ability in boys age 7 to 14 with DMD who are ambulatory. Additionally, Cedars Sinai Medical Center in Los Angeles is conducting a two-phase trial involving Cialis in males age 18 to 50 who have been diagnosed with Becker muscular dystrophy, as well as healthy controls in the same age group. In phase 1, baseline measurements of blood flow before and after exercise will be studied in Becker patients and healthy controls; in phase 2, patients with Becker muscular dystrophy will take Cialis for two days, after which measurements will be repeated.
Potential Implications
Ronald G. Victor, MD, leader of the Cedars Sinai research team told Medscape Medical News, “A key question is will the findings translate into a clinically meaningful outcome? Will the drug slow the course of the disease? That’s what we’re working on next.” It is possible that the use of the drugs for treatment of another type of muscular dystrophy, called limb-girdle muscular dystrophy, and early cases of amyotrophic lateral sclerosis may be investigated in the future.
Dr. Sanjay Bidichandani, Vice President for Research for the Muscular Dystrophy Association, is also encouraged by the early trials, telling Medscape Medical News, “These results are encouraging, and demonstrate a clear need for larger trials to evaluate clinically meaningful effects on muscle strength and disease progression in BMD.”
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