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Myasthenia gravis (MG) is a neuromuscular disease that results in compromised transmission of electrical signals at the neuromuscular junction (NMJ) from motor neurons to skeletal muscle fibers. As a result, patients with MG have reduced skeletal muscle function and present with symptoms of severe muscle weakness and fatigue. ClC-1 is a skeletal muscle specific chloride (Cl−) ion channel that plays important roles in regulating neuromuscular transmission and muscle fiber excitability during intense exercise. Here, we show that partial inhibition of ClC-1 with an orally bioavailable small molecule (NMD670) can restore muscle function in rat models of MG and in patients with MG. In severely affected MG rats, ClC-1 inhibition enhanced neuromuscular transmission, restored muscle function, and improved mobility after both single and prolonged administrations of NMD670. On this basis, NMD670 was progressed through nonclinical safety pharmacology and toxicology studies, leading to approval for testing in clinical studies. After successfully completing phase 1 single ascending dose in healthy volunteers, NMD670 was tested in patients with MG in a randomized, placebo-controlled, single-dose, three-way crossover clinical trial. The clinical trial evaluated safety, pharmacokinetics, and pharmacodynamics of NMD670 in 12 patients with mild MG. NMD670 had a favorable safety profile and led to clinically relevant improvements in the quantitative myasthenia gravis (QMG) total score. This translational study spanning from single muscle fiber recordings to patients provides proof of mechanism for ClC-1 inhibition as a potential therapeutic approach in MG and supports further development of NMD670. Editor's summary: Myasthenia gravis (MG) is caused by an autoimmune response against postsynaptic components of the neuromuscular junction, leading to impaired communication between motor neurons and muscle fibers. Here, Skov et al. show that inhibition of the ClC-1 chloride channel with an orally bioavailable small molecule (NMD670) augmented muscle excitability in response to motor commands and thereby improved muscle function in rat models of MG. Data from a small randomized clinical trial demonstrate that treatment with NMD670 was well tolerated in patients with MG and improved a quantitative disease severity score. These findings support further clinical development of ClC-1 inhibitors for MG. —Daniela Neuhofer [ABSTRACT FROM AUTHOR]

NMD670 is a first-in-class inhibitor of skeletal muscle-specific chloride channel ClC-1, developed to improve muscle weakness and fatigue in neuromuscular diseases. Preclinical studies show that ClC-1 inhibition enhances muscle excitability, improving muscle contractility and strength. We describe the first-in-human, randomized, double-blind, placebo-controlled study, which evaluated the safety, pharmacokinetics, and pharmacodynamics of single and multiple doses of NMD670 in healthy male and female subjects. Single-ascending doses (50-1,600 mg) were administered in a (partial) cross-over design; multiple-ascending doses (200-600 mg q.d.; 400 mg b.i.d.) were administered in a parallel design. Differences in pharmacokinetics between males/females and fed/fasted states were evaluated. Pharmacodynamic effects were evaluated using muscle velocity recovery cycles (MVRC) and analyzed using mixed-effects modeling. NMD670 was generally safe and well-tolerated in healthy subjects, with the only dose-related adverse event being myotonia occurring at the highest dose levels tested (single dose of 1,200, and 1,600 mg). Moreover, NMD670 significantly increased the following MVRC parameters after a single dose of 1,200 mg compared with placebo: early supernormality (estimated difference (ED) 2.04; 95% confidence interval (CI) 0.379, 3.70; P = 0.0242); early supernormality after 5 conditioning stimuli (ED 2.51; 95%CI 0.599, 4.41; P = 0.0177); supernormality at 20 ms (ED 2.78; 95%CI 1.377, 4.181; P = 0.0021). Importantly, the results of this study indicate pharmacological target engagement at well-tolerated dose levels in healthy subjects; firstly, because myotonia was an expected exaggerated on-target pharmacological effect, and secondly, because the effects on MVRC indicate increased muscle cell excitability. This study in healthy subjects indicates proof-of-mechanism and provides a solid base for translation to patients with neuromuscular diseases., (© 2024 The Author(s). Clinical Pharmacology & Therapeutics published by Wiley Periodicals LLC on behalf of American Society for Clinical Pharmacology and Therapeutics.)