The full realization of conditionally replicative adenoviruses (CRAds) for cancer therapy has been hampered by the limited knowledge of CRAd function in vivo and particularly in an immunocompetent host. To address this issue, we previously proposed a canine adenovirus type 2 (CAV2)-based CRAd for clinical evaluation in canine patients with osteosarcoma (OS). In this study, we evaluated infectivity-enhancement strategies to establish the foundation for designing a potent CAV2 CRAd with effective transduction capacity in dog osteosarcoma cells. The results indicate that the native CAV2 fiber-knob can mediate increased binding, and consequently gene transfer, in both canine osteosarcoma immortalized and primary cell lines relative to previously reported Ad5 infectivity-enhancement strategies. Gene delivery was further enhanced by incorporating a polylysine polypeptide onto the carboxy terminus of the CAV2 knob. This vector demonstrated improved gene delivery in osteosarcoma xenograft tumors. These data provide the rationale for generation of infectivity-enhanced syngeneic CAV2 CRAds for clinical evaluation in a dog osteosarcoma model. © 2006 Nature Publishing Group All rights reserved.