Metformin (dimethylbiguanide) is an established oral anti-diabetic drug that has been used in humans for >25 years. Epidemiological studies have revealed that diabetics treated with metformin have reduced risk of developing various types of cancer. Studies completed on human cancer cell lines have demonstrated inhibition of cell growth including cancer stem cells or tumor initiating cells when exposed to metformin. Tumor initiating cells (TIC) are self-sustaining cancer cells with the ability to self-renew and are believed to cause regrowth or metastasis of a tumor despite aggressive therapy. Based on this theory, combinatorial chemotherapy, in which one agent targets cancer stem cells (TIC) and the other kills non-TIC, would be more effective than single agent therapy. To this end, work completed in human cell lines demonstrated that metformin has activity in human non-TIC and TIC. When human carcinoma cell lines are exposed to metformin and a conventional chemotherapy drug, doxorubicin as combinatorial therapy, a synergistic effect is noted with more profound effect on cell proliferation than either drug given alone. Through the work completed on this study funded through the AKC Canine Health Foundation, we successfully demonstrated a significant decrease in cell growth after exposure to metformin or metformin and doxorubicin in canine mammary and bladder carcinoma cells lines. Combinatorial therapy was more effective than either drug alone in decreasing cell viability. Based on these results we can conclude that the oral anti-diabetic drug, metformin has anti-cancer activity and anti-TIC activity in canine mammary and bladder carcinoma cell lines. Importantly, these results support the next phase of study in which this agent is used in companion animals with spontaneous cancers. We are actively seeking funding for a phase I followed by a phase II clinical trial in which we aim to establish a biologically active dose of metformin, and then use this dose to assess for a response in tumor bearing dogs when given in combination with doxorubicin. Success in this final endeavor would inform the use of an affordable, well tolerated, oral medication either as an alternative to less tolerated conventional chemotherapy drugs or as an agent that can act synergistically with conventional anti-cancer agents.