McArdle Disease Clinical Trial
Official title:
A Phase II Pilot Study to Explore Treatment With Sodium Valproate in Adults With McArdle Disease (Glycogen Storage Disorder Type V, GSDV)
McArdle disease is a metabolic myopathy characterised by the absence of glycogen
phosphorylase in skeletal muscle. Sodium Valproate is part of a group of drugs known as
histone deacetylase inhibitors, which have a direct effect on chromatin. Recently a drug
trial in an animal model of McArdle disease showed that sodium valproate stimulated the
expression of a different isoform of the missing enzyme in skeletal muscle.
A safety and feasibility study of sodium valproate in people with McArdle disease has been
carried out in London (UK) and Copenhagen (DK) since January 2015. Participants will receive
20mg/Kg/day of sodium valproate for 6 months. The primary outcome measure is exercise
performance assessed by cycle ergometry. Pre and post-treatment skeletal muscle biopsies will
be performed to assess for glycogen phosphorylase. Together with blood analyses for safety.
Additional functional exercise tests will be performed.
McArdle disease (Glycogen storage disease type V, GSDV) is an inherited metabolic disorder of
skeletal muscle. Affected patients are unable to perform strenuous exercise due to a
congenital absence of the enzyme muscle glycogen phosphorylase, essential for glycogen
metabolism. This enzyme deficiency results in the inability to mobilise muscle glycogen
stores from muscle, required for energy during strenuous exercise. In affected people
symptoms of fatigue and cramps occur within minutes of initiating any activity and during
strenuous activity such as lifting heavy weights or walking uphill, if activity is continued
despite severe cramping, a contracture occurs which leads to muscle damage (rhabdomyolysis),
myoglobinuria (dark brown/black discolouration of urine) and, when severe, acute renal
failure.
Currently no satisfactory treatment can be recommended other than aerobic exercise. Although
most people with McArdle disease have complete absence of skeletal muscle phosphorylase,
there are a small minority of patients who possess splice site mutations that enable
production of very small amounts (1-2%) of functional enzyme. These people have a milder
phenotype with less severe symptoms, and functional exercise assessments have shown better
exercise capacity than typical patients with the condition. Findings from these atypical
individuals suggest potential therapeutic agents might only need to produce very small
amounts of enzyme for significant functional improvement. Furthermore, finding a therapeutic
agent to 'switch on' expression of the foetal isoenzyme may be a potential therapeutic
strategy. There is some evidence from animal studies to suggest that sodium valproate can
'switch on' the foetal phosphorylase isoenzyme. The current proposes to undertake a
feasibility study of 8 participants recruited in the UK and 7 in Denmark.
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