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Fellowship Research Project Funded in 2005

Approaches to investigate the cause and management of acute toxicities associated with adenovirus-mediated gene therapy in Hemophilia A

Dr. Maha Ahmed Othman

Queen’s University, Kingston, Ontario

Gene therapy is an attractive potential treatment for hemophilia A. The condition is due to mutations in a single, identified gene, the clinical picture is dramatically improved with a small increment of plasma FVIII levels and excellent animal models are available for preclinical testing. Replication deficient adenovirus is an efficient vehicle for liver-directed gene delivery. However, a major obstacle to the successful application of these vectors in humans has been the activation of the host immune and inflammatory response. These responses limit the efficiency of transduction, prevent readministration of the vector and cause adverse effects to the host such as acute liver injury and thrombocytopenia (reduction of the platelet count).

We have evaluated three approaches to reduce the early host immune responses to adenovirus-mediated gene delivery. These three strategies, the infusion of chilled (and thus structurally altered) platelets, intravenous immunoglobulin (IVIG) and the macrophage depleting drug Clodronate have been evaluated in a mouse model of hemophilia A. We have shown that while the transfusion of chilled platelets did not benefit the outcome of the adenoviral gene therapy protocol, there is a potential for using clodronate since it not only enhances the subsequent expression of FVIII but also significantly reduces the development of an antibody response to FVIII. Two other major advantages that we observed were that the acute thrombocytopenia that normally follows adenovirus administration was not encountered in each of the clodronate and IVIG treatment groups and acute liver injury was minimal.

In continuing with this project, we are beginning to investigate the mechanisms responsible for the acute fall in the platelet count following adenovirus-mediated gene therapy. Preliminary studies have shown that adenovirus is capable of “activating” platelets and that this event may subsequently trigger other responses in the hemostatic system. The experimental plan will be focused on studying adenovirus platelet interactions, which will involve in vitro as well as studies in laboratory mice. These studies have the potential to significantly improve our understanding of the mechanism responsible for adenovirus-induced thrombocytopenia and may provide opportunities to prevent this effect.