214 ILAR Journal

Successful development of novel cancer drugs depends on well-reasoned scientifi c drug discovery, rigorous preclinical development, and carefully conceived clinical trials. Failure in any of these steps contributes to poor rates of approval for new drugs to treat cancer. As technological and scientifi c advances have opened the door to a variety of novel ap-proaches to cancer drug discovery and development, pre-clinical models that can answer questions about the activity and safety of novel therapies are increasingly necessary. The advance of a drug to clinical trials based on information from preclinical models presupposes that the models convey in-formative data for future use in human patients with cancer. The study of novel cancer drugs using in vitro models is highly controllable, reproducible, relatively inexpensive, and linked to high throughput. However, these models fail to reproduce many of the complex features of human can-cer. Mouse models address some of these limitations but have important biological differences from human cancer. The integration of studies using pet dogs with spontaneously occurring tumors as models in the development path can an-swer questions not adequately addressed in conventional models and is therefore gaining attention and interest in drug development communities. The study of novel cancer drugs in dogs with naturally occurring tumors allows drug assess-ment in a cancer that shares many fundamental features with the human cancer condition, and thus provides an oppor-tunity to answer questions that inform the cancer drug de-velopment path in ways not possible in more conventional models. Key Words: cancer; comparative oncology; dog; drug development; preclinical mode