Breast cancer is a multi-stage process which leads to the accumulation of abnormal cells arising from excessive proliferation, lack of apoptosis or a combination of both. Natural compounds such as g-tocotrienol have been shown to selectively inhibit cancer cell growth without harming normal cell viability with little or no adverse side effects. The antiproliferative and apoptotic effects of the tocotrienol isoform, γ-tocotrienol, have been firmly established in various cancer types. However, in vivo studies have provided mixed results, attributed to γ-tocotrienol rapid clearance and low bioavailability. In order to improve anticancer potency and bioavailability in vivo, γ- and Δ-tocotrienol were chemically modified with electrophilic substitution reactions on their chromane ring using Mannich and Lederer-Manasse reactions, resulting in the synthesis of various oxazine derivatives. Several of these oxazine derivatives (compounds 26, 31, 39, 40 and 44) were found to display potent anticancer activity as compared to their parent compounds when tested on +SA mammary cancer cells grown in culture. These in vitro studies were followed up with in vivo studies to determine the anticancer effects of oxazine derivatives on the growth of mammary tumors in mice. Female syngeneic BALB/c mice, 4-6 week old were inoculated with 1×106 +SA mammary tumor cells in the left mammary pad. Once tumor size reached 5mm in diameter, animals were divided into different treatment groups and received an intra-tumoral injection injected of 0-120μg/20μl tocotrienol or its derivative every other day for 11 days. Afterwards, mice were sacrificed, tumors removed and placed in -80°C until further analysis. Results from these studies showed that tumor growth rate was significantly reduced in the oxazine derivative treated animals as compared to the vehicle-treated controls. Western blot analysis of tumor samples showed that the growth inhibitory effects of tocotrienol derivatives was also associated with a significant reduction in phosphorylated (activated) Akt and reductions in cell cycle regulatory proteins cyclin D1 and cyclin dependent kinases (CDK2, CDK4 and CDK6). In addition, oxazine derivative treatment was also associated with a large increase in CDK inhibitors p21 and p27, as compared to tumors obtained from the vehicle-treated control mice. Western blot analysis also showed that tumor from oxazine-derivative treated mice displayed a large reduction in NFκB levels and its downstream gene product COX-2. In summary, oxazine derivatives of tocotrienols display more potent anticancer activity both in vitro and in vivo, as compared to their parent compounds and suggest that these tocotrienol derivatives may provide some benefit as novel anticancer therapeutic agents. This work was supported, in part, by First Tec International Ltd. (Hong Kong), Malaysian Palm oil Council and the Louisiana Cancer Foundation. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P3-03-11.