Thus the extremely high levels of hGAA expression afforded by use of an Ad mediated gene therapy approach find more overcame many of the issues noted with ERT approaches when tested in similar, animal models. High dose intravenous injection of Ad vectors is not without untoward side effects. For example, at extremely high doses, Ad injection ultimately results in generation of high-titer, neutralizing anti-Ad capsid antibodies, that prohibit re-infection of many tissues with same serotype Ad vectors (28, 29). Furthermore, after high dose injection, the Ad capsid proteins themselves appear to immediately elicit “innate” immune responses, such as increased plasma cytokine and

chemokine levels and activation of the complement Inhibitors,research,lifescience,medical system (30–32). Many

of these same responses Inhibitors,research,lifescience,medical have been noted after [E1-]Ad injections into non-human primates, and humans (33). Initial studies of AAV vectors demonstrated that the efficacy of AAV-hGAA vectors was hampered by choice of AAV serotype and promoter used, low-level hGAA expression, and production issues that still limit large-scale AAV production (34–36). Furthermore, it was noted that upon intramuscular injection of certain AAV serotypes, the virus could be found in uninjected sites, suggesting that the virus could cross normal tissue barriers (18, 37, 38). Recently, intravenous administration of newer serotypes of AAV, such as AAV serotype 8 expressing hGAA from a Inhibitors,research,lifescience,medical liver-specific promoter, resulted in high level GAA enzyme production, glycogen reduction in some muscle groups, and minimization of the anti-hGAA humoral immune responses normally noted in GAA-knockout (KO) mice treated with vectors expressing hGAA from non-viral promoter elements (39). Also AAV-9 has shown an improved ability Inhibitors,research,lifescience,medical to infect and allow for expression of hGAA genes from cardiac tissues in vivo (40). Inhibitors,research,lifescience,medical AAV vectors can feasibly be used for GSD-II treatment, but these vectors face similar issues as those noted with Ad based vectors,

issues that have become more detectable as the titer (and thus therapeutic potential) Cell Research of AAV preparations have increased. These include cytokine responses, and elicitation of anti-AAV specific antibody responses (41). Some further considerations include the oncogenic potential (due to integration) and possible germline transmission of AAV vectors, and a limited genetic carrying capacity (< 5 kb) (42–46). Finally, high dose intravenous administration of alternative serotype AAV based vectors allows for dissemination of the vector beyond endothelial barriers into not only muscle tissues, but also gonadal sites (43), (47). Whether this property is a benefit or limitation (i.e. what is the mechanism for this capability?) awaits further research. In conclusion, gene therapy strategies for GSD-II have demonstrated a number of potential benefits when tested in several animals models of GSD-II.