Abstract. Hereditary angioedema (HAE) is a potentially life-threatening autosomal dominant disorder characterized by recurrent swelling of cutaneous tissues, gastrointestinal and respiratory tracts. Almost all cases are caused by deficiency in plasma C1-esterase inhibitor (C1EI, a serine protease inhibitor coded by SERPING1 which functions to regulate the contact, complement and fibrinolytic systems. The deficiency of C1EI leads to uncontrolled, spontaneous activation of C1 and consumption of C2 and C4, resulting in increased levels of bradykinin which promotes vascular leak. Our focus is to restore functional levels of C1 esterase inhibitor by providing a copy of the human C1EI cDNA by gene therapy. ENYX Therapeutics, an early-stage biotechnology company, is collaborating with the R. Crystal laboratory at Weill Cornell to develop adeno-associated (AAV)-mediated gene therapy for HAE. In a murine model of C1EI deficiency, one-time administration of an AAVrh.10 gene transfer vector expressing human C1EI (AAVrh.10hC1EI, ENX05) provides sustained C1EI activity levels in plasma, sufficient to prevent angioedema episodes. AAVrh10.hC1EI-treated mice displayed a marked decrease in dye extravasation in paws and organs, compared with untreated littermates, i.e., a single treatment with ENX05 has the potential to provide long-term protection from angioedema attacks. The goal of this phase 1 STTR is the next step in translation of this therapy to humans: demonstration of the safety of the therapy. As detailed in the proposal, C1EI deficient mice were corrected with an ENX05 dose of 4x1012 gc/kg, a dose for which prior studies have shown that the AAVrh.10 capsid should not pose a safety risk. However, there is the theoretical risk that overexpression of C1EI could evoke dysfunction of the complement system. We will test this in nonhuman primates. Specific aim 1. To assess the hypothesis that overexpression of ENYX05 (AAVrh.10hC1EI) will not cause dysfunction of the complement system. We will assess the complement system (serum CH50, AH50, C3, C4) and other safety parameters in male and female nonhuman primates (NHP) following high dose (1013 gc/kg) intravenous administration of ENX05 (to assess ENX05 administration without preexisting anti-AAVrh.10 immunity) and then in the same NHP over the next 6 wk following a 2nd high dose (1013 gc/kg) intravenous administration of ENX05 (to assess ENX05 administration in the context of existing anti-AAVrh.10 immunity). Controls will include nonhuman primates treated with AAVrh.10Null, a vector identical to ENX05 but without the human C1EI coding sequence. At the end of the study, all major organs will be assessed at the histologic level. With this study, assuming there is no observed dysfunction in the complement system evoked by ENX05, we will have eliminated an important theoretical safety issue in the translation of ENX05 to the clinic.