Lower motor neurons of the spinal cord are the main cell type affected by SMA. When the lower motor neurons deteriorate, they no longer efficiently contact their target muscles, ultimately leading to loss of voluntary muscle contraction, which causes muscle wasting, or atrophy, due to inactivity.
The goal of muscle protection is to protect the muscles from atrophy, increase muscle mass, and perhaps even restore some muscle function. Similar to neuroprotection, this strategy does not address the underlying genetics of SMA, rather, the aim is to slow or stop the progression of the disease. It could be used in combination with approaches designed to increase SMN protein levels.
It is thought that these approaches may be more appropriate for milder types of SMA. They may include:
- Small molecules that enhance the ability of muscles to contract
Tirasemtiv is being studied in clinical trials for people with Motor Neurone Disease (MND) by Cytokinetiks. The effects of Tirasemtiv on leg and respiratory muscle function are being examined and whether it may reduce fatigue and improve muscle strength during exercise. As motor neurons are affected in both MND and SMA, the possibilities are also being investigated for SMA.
- Muscle mass regulators that may improve muscle strength
A number of compounds that are able to increase muscle mass have been identified in the laboratory. Some of these drugs are currently being tested in SMA model mice. Updates on these drugs will be provided when studies are published.