381. Prohypertrophic Agents Enhance the Response to Gene Therapy in Pompe Disease

Abstract

Pompe disease causes a progressive myopathy resulting from acid α-glucosidase (GAA) deficiency in the heart and skeletal muscle. Enzyme replacement therapy (ERT) with recombinant human (rh) GAA has prolonged the survival of patients. However, complete reversal neuromuscular involvement has not been possible in Pompe disease by treating with ERT. The paucity of cation-independent mannose-6-phosphate receptor (CI-MPR) in skeletal muscle, where it is needed to take up rhGAA, correlated with a poor response to ERT by muscle in Pompe disease. Clenbuterol, a selective β2 receptor agonist, enhanced the CI-MPR expression in striated muscle, and increased the efficacy of either ERT or gene therapy in murine Pompe disease. The underlying mechanism of clenbuterol's therapeutic action is Igf-1 mediated muscle hypertrophy, which has correlated with increased CI-MPR (also the Igf-2 receptor) expression. In this study we have evaluated 4 new drugs in GAA knockout (KO) mice in combination with an adeno-associated virus (AAV) vector encoding human GAA. The dosage for each drug was selected to induce muscle hypertrophy with an associated increased expression of CI-MPR, analogous to clenbuterol's effects. Three alternative β2 agonists and dehydroepiandrosterone (DHEA) were evaluated in combination with gene therapy in GAA-KO mice. Mice were transgenic for a liver-specific human GAA transgene to induce immune tolerance to introduced GAA. The 3 new β2 agonists were chosen to be long-acting like clenbuterol. Furthermore, DHEA caused muscle hypertrophy similar to the β2 agonists, is available in the US, and was well-tolerated in rodent experiments. Mice were injected with AAV2/9-CBhGAA (1E+11 vector particles) at a dose previously found to be partially effective at clearing glycogen storage from the the heart. Heart GAA activity was significantly increased by either salmeterol (p 0.001), salmeterol (p<0.05), formoterol (p<0.01), or clenbuterol (p<0.01) in combination with the AAV vector, in comparison with untreated mice. Functional testing was performed subsequently, and the wirehang test at 18 weeks following vector administration revealed that the combination of salmeterol and the AAV vector significantly increased latency in comparison with untreated mice (p<0.01), AAV vector alone (p<0.001). Similarly, salmeterol with the vector increased latency significantly more than either DHEA (p<0.001), formoterol (p<0.05), fenoterol (p<0.05), or clenbuterol (p<0.05) with the vector. An important consideration with regard to adjunctive therapy is whether any effects are due to the adjuvant rather than the combined treatment. The most effective individual drugs were evaluated by themselves, and no significant effect upon GAA activity of heart, glycogen content of heart, or wirehang latency was observed, in comparison with untreated mice. Thus, salmeterol should be further developed as adjunctive therapy in combination with either ERT or gene therapy for Pompe disease.