|dc.description.abstract||Controlling metastatic and invasive tumors is the challenging part of treating cancer. The overall aim of this dissertation was to target these aggressive cells, called tumor-initiating cells (TICs). They exploit embryonic stem cell genes that should not otherwise be activated, allowing them their aggressive capabilities. Osteosarcoma is the most common bone cancer of children. Canine osteosarcoma was selected as the type of cancer to study because of its predictable and similar behavior to human osteosarcoma. Human osteosarcoma has an analogous gene signature to canine osteosarcoma, but a slightly prolonged length of remission due to more aggressive therapy. The goals accomplished in this dissertation were: 1) Optimized a method using epirubicin to enrich for TICs; 2) Used molecular techniques to determine the phenotypic differences in canine osteosarcoma TICs compared to typical cancer cells; 3) Evaluated two classes of drugs that specifically target the unique phenotypes in TICs in vitro and in vivo. These drug classes were smoothened-inhibitors and Erb-B2 receptor tyrosine kinase 2 (HER2)-inhibitors.
Epirubicin reliably enriched for phenotypically different TICs. Embryonic stem cell genes were expressed in TICs but not the differentiated tumor cells. A second embryonic stem cell pathway, the hedgehog pathway, is also upregulated in TICs. Two smoothened-inhibiting drugs, cyclopamine and vismodegib, were used to block the hedgehog pathway in canine osteosarcoma cell lines in both in vitro and in vivo studies.
The second target, HER2, was upregulated in both differentiated osteosarcoma cells and TICs. Lapatinib was used to block HER2 of TICs in vitro resulting in a significant cell death, which was demonstrated in viability assays. HER2 was also downregulated in RT-qPCR. Canine osteosarcoma mouse xenografts treated with lapatinib showed inhibition of tumor growth and tumor cell death.
In conclusion, these studies have demonstrated the benefit of using therapies that target specific cancer cell subtypes through TIC-specific targets. Vismodegib performs best at slowing the growth of metastases formed by TICs, whereas lapatinib also causes TIC and differentiated tumor cell death. Future studies are intended to evaluate these drugs in dogs post-amputation. Human clinical trials will follow if successful.||en