Liver background uptake of [¹⁸F]FLT in PET imaging

High liver uptake presents a problem for 3'-deoxy-3'-[¹⁸F]fluorothymidine ([¹⁸F]FLT) as a radiotracer for imaging cellular proliferation in the liver with positron emission tomography (PET). This investigation re-visited some issues related to the high liver background uptake of [¹⁸F]FLT with an animal model of woodchucks. Several enzymes involved in the hepatic catabolism of FLT, thymidine phosphorylase (TP, TYMP), uridine 5'-diphospho-glucuronosyl-transferases (UDP-GTs, short for UGTs), and β-glucuronidase (GUSB), their homology as well as hepatic expression between the human and the woodchuck was examined. Inhibitors of these enzymes, TP inhibitor (TPI) tipiracil hydrochloride, UGT inhibitor probenecid, β-glucuronidase inhibitor L-aspartate, were administered to the animals at human equivalent doses either intravenously (i.v.) and orally before the injection of tracer-dose [¹⁸F]FLT for PET imaging to examine any changes in liver uptake. Liver tissue samples were harvested from the animals after PET imaging and used to perform polymerase chain reaction (PCR) for TP expression or assays for enzymatic activities of TP and β-glucuronidase. Non-radiolabeled (cold) FLT was also applied for enzyme saturation. Animals administered with TPI displayed lower radioactivity in the liver in comparison with the baseline scan. The application of probenecid did not change [¹⁸F]FLT liver uptake even though it reduced renal uptake. L-aspartate reduced the liver background uptake of [¹⁸F]FLT slightly. The application of cold FLT reduced overall uptake of [¹⁸F]FLT including the liver background. Therefore, the combined application of cold FLT and [¹⁸F]FLT merits further clinical investigation for reducing liver background uptake of [¹⁸F]FLT.