49. Minimum Effective Dose of Liver-Targeted Gene Therapy for Pompe Disease Reduced 10-Fold from Prior Estimates

Abstract

Pompe disease (glycogen storage disease type II; acid maltase deficiency) is a devastating myopathy resulting from acid alpha-glucosidase (GAA) deficiency in striated and smooth muscle. Despite the availability of enzyme replacement therapy (ERT) with recombinant human GAA, many patients have poor outcomes including mortality. ERT has increased survival rates, improved hypotonia, and resolved heart involvement in patients with Pompe disease. However ERT is limited by the short half-life of GAA and the formation of antibody responses that reduced its clinical benefits. The present study compared the efficacy of ERT against an adeno-associated viral (AAV) vector containing a liver-specific promoter, AAV2/8-LSPhGAApA. We hypothesized that liver-specific expression of GAA with AAV2/8-LSPhGAA would suppress the antibody response, continually express GAA, and improve efficacy in comparison with ERT. Preclinical experiments with Pompe disease therapies demonstrated that a very low amount of AAV2/8-LSPhGAApA, 2×1010 vector particles (vp), equivalent to 8×1011 vp/kg body weight, was as effective as long-term ERT. Therefore, we evaluated the biochemical efficacy of 3 lower dosages of AAV2/8-LSPhGAA in GAA knockout (KO) mice, reduced to as low as 2×1010 vp/kg, either alone or in combination with ERT. Biochemical correction in GAA-KO mice was evaluated 8 weeks following vector administration. The minimum effective dose (MED) was at least 10-fold lower than previously estimated, because 8×1010 vp/kg significantly reduced glycogen content in the striated muscle of GAA-KO mice. This MED of AAV2/8-LSPhGAA significantly increased GAA activity in liver (p<0.01), in comparison with untreated GAA-KO mice. Importantly, the MED significantly reduced the glycogen content of heart (p<0.01; Fig. A), and diaphragm (p<0.01; Fig. B), which demonstrated that the glycogen storage in muscle associated with Pompe disease was substantially cross-corrected by GAA secretion from liver accompanied by receptor-mediated uptake in striated muscle. Glycogen clearance, or reduction in comparison with no treatment, was 36% in heart, and 35% in diaphragm. Furthermore, administering ERT by itself had no significant effect on the glycogen content of quadriceps, but ERT following administration of the MED of AAV2/8-LSPhGAA significantly reduced glycogen content of quadriceps (p<0.05; Fig. C) by 38%, indicating that gene therapy with AAV2/8-LSPhGAA made ERT more effective. Vector dose correlated inversely with anti-GAA antibody formation, confirming that immune tolerance to GAA was critical to the efficacy from AAV2/8-LSPhGAApA. In conclusion, the MED for AAV2/8-LSPhGAA has been estimated at least 10-fold lower than previous data suggested, and this dosage would be acceptable as a starting dose for a Phase I clinical trial of gene therapy in Pompe disease.View Large Image | Download PowerPoint Slide