A CD33 Antigen-Targeted AAV6 Vector Expressing an Inducible Caspase-9 Suicide Gene Is Therapeutic in a Xenotransplantation Model of Acute Myeloid Leukemia

Current chemotherapeutic regimens for acute myeloid leukemia (AML) have been modestly effective in patients and are associated with poor long-term survival (<30% at 5 years). Viral vector-based suicide gene therapy is an attractive option, if these vectors can target the AML cells with high specificity and efficiency. In this study, we have developed a receptor-specific adeno-associated virus (AAV) based vector to target the CD33 antigen which is overexpressed in leukemic cells. A targeting peptide was rationally designed from the antigen-binding regions of a CD33 monoclonal antibody. This peptide was further expressed on the capsid of the AAV6 vector, since this serotype was most efficient among AAV1-rh10 vectors to infect the pro-monocytic, human myeloid leukemia cells (U937). AAV6-CD33 vectors expressing a suicide gene, the inducible caspase 9 (iCasp9), and its prodrug AP20187 significantly reduced (∼59%) the viability of U937 cells. To further test its efficacy and specificity in vivo, AAV6-CD33 vectors were administered into a xenotransplantation model of AML in zebrafish through systemic delivery. We observed a significant antileukemic effect with AAV6-CD33 vectors, with a markedly higher survival (100% for AAV6-CD33 vectors vs 15% for mock-treated) and a higher number of TUNEL positive apoptotic cells after systemic vector delivery. Taken together, our work demonstrates the efficacy and translational potential of CD33-targeted AAV6 vectors for cytotoxic gene therapy in AML.