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Improving the delivery of therapeutics to disease-affected tissues can increase their efficacy and safety. Here, we show that chemical conjugation of a synthetic oligosaccharide harboring mannose 6-phosphate (M6P) residues onto recombinant human acid alpha-glucosidase (rhGAA) via oxime chemistry significantly improved its affinity for the cation-independent mannose 6-phosphate receptor (CI-MPR) and subsequent uptake by muscle cells. Administration of the carbohydrate-remodeled enzyme (oxime-neo-rhGAA) into Pompe mice resulted in an approximately fivefold higher clearance of lysosomal glycogen in muscles when compared to the unmodified counterpart. Importantly, treatment of immunotolerized Pompe mice with oxime-neo-rhGAA translated to greater improvements in muscle function and strength. Treating older, symptomatic Pompe mice also reduced tissue glycogen levels but provided only modest improvements in motor function. Examination of the muscle pathology suggested that the poor response in the older animals might have been due to a reduced regenerative capacity of the skeletal muscles. These findings lend support to early therapeutic intervention with a targeted enzyme as important considerations in the management of Pompe disease.

Enzyme replacement therapy (ERT) became a reality for patients with Pompe disease, a fatal cardiomyopathy and skeletal muscle myopathy caused by a deficiency of glycogen-degrading lysosomal enzyme acid alpha-glucosidase (GAA). The therapy, which relies on receptor-mediated endocytosis of recombinant human GAA (rhGAA), appears to be effective in cardiac muscle, but less so in skeletal muscle. We have previously shown a profound disturbance of the lysosomal degradative pathway (autophagy) in therapy-resistant muscle of GAA knockout mice (KO). Our findings here demonstrate a progressive age-dependent autophagic buildup in addition to enlargement of glycogen-filled lysosomes in multiple muscle groups in the KO. Trafficking and processing of the therapeutic enzyme along the endocytic pathway appear to be affected by the autophagy. Confocal microscopy of live single muscle fibers exposed to fluorescently labeled rhGAA indicates that a significant portion of the endocytosed enzyme in the KO was trapped as a partially processed form in the autophagic areas instead of reaching its target--the lysosomes. A fluid-phase endocytic marker was similarly mistargeted and accumulated in vesicular structures within the autophagic areas. These findings may explain why ERT often falls short of reversing the disease process and point toward new avenues for the development of pharmacological intervention.

Pompe disease (type II glycogen storage disease) is an autosomal recessive disorder caused by a deficiency of lysosomal acid alpha-glucosidase (GAA) leading to the accumulation of glycogen in the lysosomes primarily in cardiac and skeletal muscle. The recombinant human GAA (rhGAA) is currently in clinical trials for enzyme replacement therapy of Pompe disease. Both clinical data and the results of preclinical studies in our knockout model of this disease show that rhGAA is much more effective in resolving the cardiomyopathy than the skeletal muscle myopathy. By contrast, another form of human GAA--transgenic enzyme constitutively produced in liver and secreted into the bloodstream of knockout mice (Gaa-/-)--completely prevented both cardiac and skeletal muscle glycogen accumulation. In the experiments reported here, the transgenic enzyme was much less efficient when delivered to skeletal muscle after significant amounts of glycogen had already accumulated. Furthermore, the transgenic enzyme and the rhGAA have similar therapeutic effects, and both efficiently clear glycogen from cardiac muscle and type I muscle fibers, but not type II fibers. Low abundance of proteins involved in endocytosis and trafficking of lysosomal enzymes combined with increased autophagy in type II fibers may explain the resistance to therapy.