Redistribution of nucleoside transporters to the cell membrane provides a novel approach for imaging thymidylate synthase inhibition by positron emission tomography.
Thymidylate synthase (EC 2.1.1.45) is a key enzyme for the de novo synthesis of DNA and as such a target for anticancer drug development. There is a need to develop noninvasive methods for assessing thymidylate synthase inhibition in tumors. The aim of this study was to assess the potential of 3'-deoxy-3'-[(18)F]fluorothymidine ([(18)F]FLT) positron emission tomography (PET) for early measurement of thymidylate synthase inhibition and to elucidate the cellular mechanisms involved. Radiation-induced fibrosarcoma-1 tumor-bearing mice were injected with a single i.p. dose of the thymidylate synthase inhibitor 5-fluorouracil (5-FU; 165 mg/kg) and imaged by [(18)F]FLT-PET at 1 to 2 hours after treatment. Deoxyuridine, thymidine kinase 1 (cytoplasmic thymidine kinase; EC2.7.1.21), and ATP levels in excised tumors were measured. Cellular assays for membrane transport were also done. There was a 1.8-fold increase in the 60-minute [(18)F]FLT tumor/heart radioactivity ratio in drug-treated mice compared with vehicle controls (P = 0.0016). Plasma and tumor deoxyuridine levels increased significantly but thymidine kinase and ATP levels were unchanged. Whole-cell assays implicated a (low level) functional role for the type-1 equilibrative nucleoside transporter (ENT). There was an increase in type-1 ENT-binding sites per cell from 49,110 in untreated cells to 73,142 (P = 0.03) in cells treated with 10 microg/mL 5-FU for 2 hours, without a change in transporter affinity (P = 0.41). We conclude that [(18)F]FLT-PET can be used to measure thymidylate synthase inhibition as early as 1 to 2 hours after treatment with 5-FU by a mechanism involving redistribution of nucleoside transporters to the plasma membrane.
Perumal
Meg
M
Molecular Therapy Group, Faculty of Medicine, Imperial College London, Hammersmith Hospital, London, United Kingdom.
Pillai
Radhakrishna G
RG
Barthel
Henryk
H
Leyton
Julius
J
Latigo
John R
JR
Forster
Martin
M
Mitchell
Fraser
F
Jackman
Ann L
AL
Aboagye
Eric O
EO
D000818:Animals; D045744:Cell Line, Tumor; D002462:Cell Membrane / Q000000981:diagnostic imaging / Q000502:physiology*; D005354:Fibrosarcoma / Q000000981:diagnostic imaging; D005462:Fluorine Radioisotopes; D051379:Mice; D009381:Neoplasms, Radiation-Induced / Q000000981:diagnostic imaging; D033703:Nucleoside Transport Proteins / Q000378:metabolism*; D049268:Positron-Emission Tomography; D013940:Thymidylate Synthase / Q000037:antagonists & inhibitors*
D005462:Fluorine Radioisotopes; D033703:Nucleoside Transport Proteins; D013940:Thymidylate Synthase