July 18th, 2023
About myasthenia gravis
They are two main types of myasthenia gravis (MG), generalised and ocular. Generalised MG is the most common type, accounting for 85 per cent of people with MG. It is a rare condition with a global prevalence of 100–350 cases per every one million people.
Muscle weakness is the hallmark of myasthenia gravis. With generalised MG, this weakness tends to begin in the eye and gradually spread to other muscle groups. People living with generalised MG can experience a variety of symptoms, including severe muscular weakness that can result in life-threatening weakness of the muscles of respiration, as well as double vision, drooping eyelids, and difficulty swallowing, chewing and talking.
The voluntary muscles of the entire body are controlled by nerve impulses that arise in the brain. These impulses travel down the nerves to the place where the nerves meet the muscle fibres. Nerve fibres do not connect with muscle fibres; there is a space between them called the neuromuscular junction.
When the nerve impulse originating in the brain arrives at the nerve ending, it releases a chemical called acetylcholine. Acetylcholine travels across the space to the muscle-fibre side of the neuromuscular junction where it attaches to many receptor sites. The muscle contracts when enough of the receptor sites have been activated by the acetylcholine.
For someone with MG, there can be as much as an 80 per cent reduction in the number of these receptor sites. The reduction in the number of receptor sites is caused by an antibody that destroys or blocks the receptor site.
Antibodies and generalised MG
Antibodies are proteins that attack foreign bodies (bacteria and viruses), to protect the body. However, with an autoimmune condition like MG, the body makes antibodies that attack healthy tissue by mistake. MG makes antibodies against the receptor sites of the neuromuscular junction, which disrupts the ability of the nerves to stimulate the muscle and results in a weaker contraction. This leads to muscle weakness.
Eight out of 10 people with generalised MG test positive for acetylcholine receptor (AChR) antibodies. Less common MG antibodies are muscle-specific kinase (MUSK) and lipoprotein-related protein 4 (LRP4). Only 5 out of every 100 people with MG will be seronegative, meaning doctors cannot find any MG antibodies in their blood.
Latest research news
In June, two new treatments were approved by US Food and Drug Administration (FDA) for generalised MG.
VYVGART® Hytrulo (efgartigimod alfa and hyaluronidase-qvfc), has been approved for administration via a subcutaneous injection for treatment of generalised MG in adults who are anti-acetylcholine receptor (AChR) antibody positive. It was developed by an immunology company called Argenx SE. The clinical trials of VYVGART show significant benefit to people with a favourable safety profile and clear improvements in disease scores.
The FDA equivalent in European is the European Medicines Agency (EMA). The EMA authorises medicines in Europe and overseas safety, effectiveness, and quality of medicines in the European market. An application for VYVGART is under review by the EMA with a decision expected by the end of 2023.
RYSTIGGO (rozanolixizumab-noli) has been approved for adults with generalised MG who are anti-acetylcholine receptor (AChR) or anti-muscle-specific tyrosine kinase (MuSK) antibody positive. It is the only FDA-approved treatment in adults for both anti-AChR AND anti-MuSK antibody-positive generalised MG.
Developed by a pharmaceutical company called UCB, this drug is administered by subcutaneous infusion injection. Results from their Phase 3 clinical trial showed improvements in generalised MG specific outcomes, including everyday activities such as breathing, talking, swallowing and being able to rise from a chair.
Rozanolixizumab is currently under review by the EMA for the treatment of adults with generalised MG. Orphan designation was granted by the European Commission in April 2020 to rozanolixizumab for the treatment of generalised MG. Designated orphan medicinal products are still under investigation. As a consequence, their quality, safety and efficacy need to be demonstrated before a product can be granted a marketing authorisation. A decision from the EMA on approving this drug is expected in first half of 2024.