[18F]fluorothymidine PET Informs the Synergistic Efficacy of Capecitabine and Trifluridine/Tipiracil in Colon Cancer

In cancer therapy, enhanced thymidine uptake by the salvage pathway can bypass dTMP depletion, thereby conferring resistance to thymidylate synthase inhibition. We investigated whether sequential combination therapy of capecitabine and trifluridine/tipiracil (TAS-102) could synergistically enhance antitumor efficacy in colon cancer xenograft models. We also examined 3'-deoxy-3′-[18F]fluorothymidine ([18F]FLT) PET as a means to predict therapeutic response to a sequential combination of capecitabine and trifluridine/tipiracil. [3H]FLT uptake after 5-fluorouracil treatment in vitro and [18F]FLT uptake after capecitabine (360 mg/kg/day) in athymic nude mice (Balb/c-nu) with xenografts (n = 10–12 per group) were measured using eight human colon cancer cell lines. We determined the synergistic effects of sequential combinations of 5-fluorouracil and trifluridine in vitro as well as the sequential combination of oral capecitabine (30–360 mg/kg) and trifluridine/tipiracil (trifluridine 75 or 150 mg/kg with tipiracil) in six xenograft models (n = 6–10 per group). We observed significant increases in [3H]FLT uptake in all cell lines and [18F]FLT uptake in five xenograft models after 5-fluorouracil and capecitabine treatment, respectively. Increased [18F]FLT uptake after capecitabine followed by extinction of uptake correlated strongly with tumor growth inhibition (ρ = −0.81, P = 0.02). The effects of these combinations were synergistic in vitro. A synergy for sequential capecitabine and trifluridine/tipiracil was found only in mouse xenograft models showing increased [18F]FLT uptake after capecitabine. Our results suggest that the sequential combination of capecitabine and trifluridine/tipiracil is synergistic in tumors with an activated salvage pathway after capecitabine treatment in mice, and [18F]FLT PET imaging may predict the response to capecitabine and the synergistic antitumor efficacy of a sequential combination of capecitabine and trifluridine/tipiracil. Cancer Res; 77(24); 7120–30. ©2017 AACR.

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