Many forms of canine cancer are currently incurable, and novel treatments are needed. The phytochemical silibinin, the bioactive constituent in the herb milk thistle, has been shown to induce growth arrest and apoptosis, inhibit angiogenesis, and decrease tumor cell invasion in various human tumor cells, and inhibit tumor growth in a variety of mouse models of cancer. In parallel with phase-I dose escalation and pharmacokinetic studies currently ongoing in tumor-bearing dogs at the Animal Cancer Center at Colorado State University, researchers propose to evaluate silibinin in vitro against a panel of canine tumor cell lines, including hemangiosarcoma, osteosarcoma and transitional cell carcinoma. Silibinin will be evaluated for its ability to: 1) induce growth arrest; 2) induce apoptosis; 3) enhance chemosensitivity; 4) inhibit transwell migration and Matrigel invasion. Successful demonstration of in vitro antitumor activity in these currently incurable cancers will provide important preliminary data justifying further clinical evaluation of silibinin in canine cancer.