GNE myopathy is a rare, autosomal recessive, inborn error of sialic acid metabolism, caused by mutations in GNE, the gene encoding UDP-N-acetyl-glucosamine-2-epimerase/N-acetylmannosamine kinase. The disease manifests as an adult-onset myopathy characterized by progressive skeletal muscle weakness and atrophy. There is no medical therapy available for this debilitating disease. Hyposialylation of muscle glycoproteins likely contributes to the pathophysiology of this disease. N-acetyl-D-mannosamine (ManNAc), an uncharged monosaccharide and the first committed precursor in the sialic acid biosynthetic pathway, is a therapeutic candidate that prevents muscle weakness in the mouse model of GNE myopathy. We conducted a first-in-human, randomized, placebo-controlled, double-blind, single-ascending dose study to evaluate safety and pharmacokinetics of ManNAc in GNE myopathy subjects. Single doses of 3 and 6 g of oral ManNAc were safe and well tolerated; 10 g was associated with diarrhea likely due to unabsorbed ManNAc. Oral ManNAc was absorbed rapidly and exhibited a short half-life (~2.4 h). Following administration of a single dose of ManNAc, there was a significant and sustained increase in plasma free sialic acid (Neu5Ac) (Tmax of 8-11 h). Neu5Ac levels remained above baseline 48 h post-dose in subjects who received a dose of 6 or 10 g. Given that Neu5Ac is known to have a short half-life, the prolonged elevation of Neu5Ac after a single dose of ManNAc suggests that intracellular biosynthesis of sialic acid was restored in subjects with GNE myopathy, including those homozygous for mutations in the kinase domain. Simulated plasma concentration-time profiles support a dosing regimen of 6 g twice daily for future clinical trials.

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