Trifluridine-tipiracil hydrochloride mixture (TAS-102), a novel antimetabolite combination chemotherapy agent, consists of a rediscovered antimetabolite agent, trifluorothymidine (trifluridine, FTD) combined with the metabolic inhibitor of thymidine phosphorylase, tipiracil (TPI), in a 1:0.5 molar ratio. FTD is the active antitumor component of Trifluridine-tipiracil hydrochloride mixture (TAS-102); its monophosphate form inhibits thymidylate synthase, and its triphosphate form is incorporated into DNA in tumor cells. The incorporation into DNA is known to have antitumor effects, since the inhibition of thymidylate synthase caused by oral FTD rapidly disappears after the drug's elimination. When FTD is administered orally, it is rapidly degraded to its inactive form by thymidine phosphorylase in the intestines and liver (first-pass effect). Consequently, TPI is synthesized to maintain adequate plasma concentrations of orally-administered FTD and to potentiate the antitumor activity of FTD.