Martin Bradley discusses the complexities of mexiletine use following the licensing of NaMuscla® for the treatment of myotonia in adults with non‑dystrophic myotonic disorders
Mexiletine is a sodium channel blocker, and was originally developed as an anticonvulsant drug.1,2 However, it was found to have anti-arrhythmic properties, and has been approved for this indication in several countries since the 1960s.2 There is no licensed preparation of mexiletine in the UK; consequently, it is imported for use in ventricular arrhythmia because there are limited oral sodium channel blocker (class 1b) anti-arrhythmic agents available, and it is a useful agent in patients with long QT syndrome.3
The therapeutic effect of mexiletine on myotonic disorders (rare diseases caused by a malfunction of skeletal ion channels) was investigated in the 1980s.4 Since the 1990s, mexiletine has been the agent of choice for patients with myotonia according to expert opinion,5 and in 2010 and 2012, positive results from two small randomised controlled trials indicated that mexiletine was effective for this condition.6,7 Until recently, no manufacturer has taken mexiletine through regulatory approval for use in myotonia; therefore, an imported product is also used in UK neurology centres for patients with myotonic syndromes.
Since the 1990s, mexiletine has been the agent of choice for patients with myotonia …
In December 2018, a new formulation of mexiletine, NaMuscla®, was licensed for the treatment of myotonia in adult patients with non-dystrophic myotonic disorders in the European Union.8 Because myotonia is a rare disease, NaMuscla® is classed as an orphan medicinal product (OMP) by the European Medicines Agency.8 OMPs are guaranteed a period of at least 10 years’ market exclusivity (no other, similar agents are allowed to be marketed for similar indications),9 and are also generally marketed at a premium price because of the limited volumes used. A commercial in-confidence access price for use in the NHS has been agreed. Owing to its high cost, mexiletine for use in neuromuscular disorders was added to the tariff-excluded drugs list for 2019/20.10 In a communication with trusts, NHS England confirmed that specialist neurosciences centres will be reimbursed for NaMuscla® used for non-dystrophic myotonic disorders, but that NaMuscla® or imported mexiletine capsules must be funded within the tariff by CCGs for other indications.11