The overall goals of this project are to further define the safety and toxicity of helper-dependent adenoviral (HD-Ad) vectors, to delineate the pathogenesis of the thrombocytopenia observed with the HD-Ad vectors, to attempt to identify mechanisms to circumvent the thrombocytopenia, to develop experience with these vectors in primates, and to move incrementally towards clinical trials with these very promising HD-Ad vectors. The pathogenesis of the thrombocytopenia will be studied in mice, focusing on direct interactions between platelets and vector and between endothelial cells and vector. One very important aim will be to evaluate safety, toxicity, and short-term expression using high doses of HD-Ad vectors in juvenile baboons. Because of the desire to initiate clinical trials with maximum safety, we are comparing the expression of factor IX and apolipoprotein A-I (apo A-I) with IM administration utilizing a muscle-specific promoter and IV administration aimed at expression in hepatocytes. Another aim will be to put in place all of the necessary reagents and commitments to prepare GMP quality HD-Ad vector suitable for use in pre-clinical and clinical studies. Toxicity studies with GMP quality vector will be conducted in mice and baboons. In longer-term experiments in baboons, we will test whether over-expression of apo A-I will protect against atherosclerosis in baboons. Finally, we propose to develop a clinical trial to introduce the HD-Ad vectors into the clinic using either IM or IV administration. No IRB-approved protocol is available at present, but the major possibilities under consideration include expression of factor IX in patients with hemophilia B or expression of apo A-I in patients with coronary artery disease and low production of apo A-I. The long-term significance of this project is to attempt to develop HD-Ad vectors designed to increase expression of the LDL receptor and/or apo A-I in humans. If successful, this approach could have a major impact on prevention or reduction of atherosclerosis in the human population.