Abstract 1867: Combined and sequential chemotherapy efficacy of trifluridine/tipiracil and regorafenib in colorectal cancer cell lines

Background: Trifluridine/tipiracil (FTD/TPI or TFTD, also known as TAS-102) is recommended for treating metastatic colorectal cancers refractory to standard chemotherapies. Regorafenib is an oral multikinase inhibitor that blocks the activity of several protein kinases associated with angiogenesis, oncogenesis, and the tumor microenvironment. We evaluated the antitumor effects of FTD/TPI combined with regorafenib on a colorectal tumor xenograft mouse model. We also investigated whether cytotoxicity was enhanced when FTD was used sequentially with regorafenib in vitro. Method: The colorectal cancer cell lines SW620 and COLO 205 were implanted into nude mice subcutaneously. FTD/TPI (150 mg/kg/day) was orally administered twice daily from days 1 to 14 and regorafenib (10 mg/kg/day) was administered once a day at approximately 2-4 h after FTD/TPI. For the in vitro cytotoxic assay, SW620 cells were treated with FTD and regorafenib as follows: (1) exposure to 0.1-4.0 µM FTD alone, (2) sequential exposure to 0.1-4.0 µM FTD for 24 h followed by 10 µM regorafenib for 24 h, or (3) sequential exposure to 10 µM regorafenib for 24 h followed by 0.1-4.0 µM FTD for 24 h. All treatments were evaluated by a colony formation assay. Molecules involved in mechanism of action of FTD and regorafenib and in apoptosis were evaluated by western blot. Results: Tumor growth inhibition by combination therapy was 86.4% and 92.3% for SW620 and COLO 205 xenografts, respectively; this was significantly superior to the growth inhibition observed after either monotherapy for the two evaluated cancer xenografts (P < 0.01). Thus, FTD/TPI combined with regorafenib is significantly more effective than either monotherapy in colorectal cancer xenografts. This combination therapy was tolerated without 20% body-weight reduction and drug-related death. FTD followed by regorafenib treatment in SW620 cells showed stronger cell killing effect than FTD alone. In contrast, regorafenib followed by FTD treatment attenuated its cell killing effect rather than that to FTD alone. In comparison with FTD alone, FTD followed by regorafenib reduced thymidine synthase (TS) and inhibited phosphorylation of ERK1/2 (pERK1/2) with the induction of proapoptotic marker, cleavage of PARP. In contrast, regorafenib followed by FTD induced TS and pERK1/2. Thus, FTD followed by regorafenib treatment is more effective than regorafenib followed by FTD treatment in vitro. Conclusion: FTD/TPI combined with regorafenib is signif