Abstract Osteosarcoma (OS) is the most common bone tumor in both humans and dogs, and has a nearly ten-fold higher incidence in dogs than humans. For the past twenty years, despite advances in treatment of other cancers, the overall survival rates for OS have stagnated. Thus, there is a great need for novel therapeutic strategies. In osteosarcoma, MYC activation is critical for both initiation and maintenance of tumorigenesis and tissue invasion. While targeting c-MYC is a rational strategy to treat OS, c-MYC has evaded therapeutic manipulation to date. In this study, we evaluated using a novel proteasome modulator, TCH-165, which induces c-MYC degradation, and compared it with convention treatments in in vitro studies. We demonstrated dose dependent decrease of c-MYC protein expression with TCH-165 treatment. Utilizing seven canine OS cell lines and three human OS cell line, we determined that the IC50 values for TCH-165 range from 2.2 to 11.6 μM in canine OS, 3.1 to 4.5 μM in human OS cell lines, while IC50 of several primary canine ranged from 22.3 μM to >200μM, suggesting a wide margin of safety. In cell cycle studies, we showed that TCH-165 resulted in G1 arrest after 12 hours of incubation. In addition, we examined the potential of combing TCH-165 with convention therapeutic agents. We selected carboplatin, one of the standard anti-tumor drugs for OS. Combination index calculations using TCH-165 and carboplatin in the canine D17 cell line support synergistic effects of this combination. The data point to potential novel avenues for treatment of OS which can be further examined in proof of concept studies in dogs with OS, serving as a relevant translational model, and accelerate drug development for human OS patients. Citation Format: Ya-Ting Yang, Vilma Yuzbasiyan-Gurkan, Jetze J. Tepe. Targeting c-MYCdegradation as a novel therapeutic strategy for osteosarcoma: Studies in canine and human osteosarcoma cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4729.