Engineering the AAV Vector for Enhanced Tumor Targetin

Adding tumor-specific recognition and binding domains to enhance the interaction with cancer cells is the traditional concept used in tumor targeting for adeno-associated virus vectors (AAVs) and other drug delivery platforms. However, the theory that enhanced specificity for tumor antigens contributes to improved gene delivery to tumors remains unconfirmed. We used a complex AAV library, which contains AAV variants with combinatorial mutations introduced in the variable regions (VRs) of the viral capsid. We hypothesized that, with this library, we could use in vivo selection to identify capsid variants with enhanced tumor targeting and to understand the pressures involved in AAV-mediated gene transfer in tumor. A patient-derived osteosarcoma murine xenograft model was used for the in vivo selection. Uniquely, we compared the selective pressures of intratumoral (i.t.) library injection with these of intravenous (i.v.) library injection for systemic circulating and infecting the tumor cell. Intratumoral screening positively selected for mutated motifs in VR I and V, but not VR VIII, which is known as the AAV primary receptor binding region, and determines the AAV native tropism. Intravenous screening negatively selected against the wild type VR VIII and only enriched for AAV variants with mutated VR VIII. Further characterization showed that the AAVs with i.t. motifs demonstrated increased AAV transduction in vitro, but not in vivo. The utilization of a motif in VR VIII derived from i.v. screening achieved hundreds of folds higher transgene expression in vivo. The combination of both systemic and intra-tumoral motifs further increased the selectivity, but not the efficiency, of transgene expression in the tumor. Profiling the genome biodistribution of the above vectors showed that diminishing the native AAV tropism and thereby enabling AAV accessibility to the tumor were critical for effective tumor targeting. By enabling the AAV to freely circulate, the biology of tumor and tumor cells out-competes normal cells and tissues for AAV transduction. Furthermore, the AAV vectors armed with novel motifs demonstrated high accessibility and transgene expression in two different patient cells derived xenografts in vivo. Therefore, only accounting for vector-cell interaction is not enough for AAV-mediated systemic tumor targeting. It appears that tumors are uniquely suited for high AAV uptake and transduction. Future tumor-directed AAV vector design should consider systemically reaching the tumor cells before vector tumor cell interaction.