Induction of thymidine kinase 1 after 5-fluorouracil as a mechanism for 3′-deoxy-3′-[18F]fluorothymidine flare

Imaging the pharmacodynamics of anti-cancer drugs may allow early assessment of anti-cancer effects. Increases in 3′-deoxy-3′-[18F]fluorothymidine ([18F]FLT) uptake early after thymidylate synthase inhibition (TS) inhibition, the so-called flare response, is considered to be largely due to an increase in binding sites for type-1 equilibrative nucleoside transporter. We investigated the induction of thymidine kinase 1 (TK1) after 5-fluorouracil (5-FU) treatment as one of mechanisms for [18F]FLT flare. Exposure of nine cancer cell lines to 5-FU for 24 h induced a 2.5- to 3.5-fold increase in [18F]FLT uptake, significantly higher than the 1.5-fold increase observed 2 h after treatment. The increase of [18F]FLT uptake 24 h after 5-FU exposure accompanied TK1 induction in most cell lines. In representative cell lines (A431 and HT29), 5-FU time-dependently increased [18F]FLT uptake, kinase activity and the levels of protein and mRNA for TK1, sequential cyclin E and A induction, and G1-S phase transition. Cycloheximide treatment and knockdown of TK1 completely inhibited 5-FU-induced [18F]FLT flare. On the other hand, HCT8 cells showed a biphasic [18F]FLT flare with lacked TK1 induction in response to the dosage of 5-FU. Cycloheximide did not inhibit 5-FU-induced [18F]FLT flare in this cells. In vivo dynamic [18F]FLT-PET and ex vivo analysis in HT29 tumor-bearing mice showed significantly increased [18F]FLT flux and TK1 activity of tumor tissue 24 h after 5-FU administration (P < 0.05). Conclusively, 5-FU induced TK1 and TK1-mediated high [18F]FLT flare in most of cell lines. [18F]FLT-PET may be used to assess pharmacodynamics of TS inhibitor by a mechanism involving TK1 induction.