Redistribution of nucleoside transporters to the cell membrane provides a novel approach for imaging thymidylate synthase inhibition by positron emission tomography

• Published in: Cancer research (Chicago, Ill.) Vol. 66; no. 17; pp. 8558 - 8564
• Main Authors: PERUMAL, Meg, PILLAI, Radhakrishna G, BARTHEL, Henryk, LEYTON, Julius, LATIGO, John R, FORSTER, Martin, MITCHELL, Fraser, JACKMAN, Ann L, ABOAGYE, Eric O
• Format: Journal Article
• Language: English
• Published: Philadelphia, PA American Association for Cancer Research 01.09.2006
• Subjects: Animals; Biological and medical sciences; Cell Line, Tumor; Cell Membrane - diagnostic imaging; Cell Membrane - physiology; Cell physiology; Cell transformation and carcinogenesis. Action of oncogenes and antioncogenes; Fibrosarcoma - diagnostic imaging; Fluorine Radioisotopes; Fundamental and applied biological sciences. Psychology; Mice; Molecular and cellular biology; Neoplasms, Radiation-Induced - diagnostic imaging; Nucleoside Transport Proteins - metabolism; Positron-Emission Tomography; Thymidylate Synthase - antagonists & inhibitors; Radionuclide study; Methyltransferases; Enzyme; Transferases; Plasma membrane; Medical imagery; Nucleoside; Inhibition; Thymidylate synthase; Positron emission tomography; Carrier protein; Emission tomography
• ISSN: 0008-5472; 1538-7445
• DOI: 10.1158/0008-5472.CAN-06-0898

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.