Synthesis and evaluation of nucleoside radiotracers for imaging proliferation.
BACKGROUND
Uncontrolled proliferation is a fundamental characteristic of cancer, and consequently, imaging of tumor proliferative status finds interest clinically both as a diagnostic tool and for evaluation of response to treatment. Positron emission tomography (PET) radiotracers based on a nucleoside core, such as 3'-[18F]fluoro-3'-deoxythymidine ([18F]FLT), have been extensively studied for this purpose. However, [18F]FLT suffers from poor DNA incorporation leading to occasional poor correlation of [18F]FLT tumor uptake with other proliferation indicators such as Ki-67 immunostaining.


METHODS
N3-((1-(2-[18F]fluoroethyl)-1H-[1,2,3]-triazol-4-yl)methyl)thymidine ([18F]2) and N3-((1-(2-[18F]fluoroethyl)-1H-[1,2,3]-triazol-4-yl)methyl)-4'-thio-β-thymidine ([18F]3) were synthesized by click chemistry from [18F]fluoroethyl azide and by direct nucleophilic substitution of a tosylate precursor. Metabolic stability and phosphorylation potential of the radiotracers were evaluated in vitro and compared to [18F]FLT. Further, metabolic stability and biodistribution analysis of [18F]2 and [18F]3 were evaluated in vivo.


RESULTS
Stable isotope standards and radiochemistry precursors were synthesized by modification of existing literature procedures. [18F]2 and [18F]3 were synthesized in a radiochemical yield of 8%-12% (end of synthesis, non-decay corrected). Both nucleosides were stable to metabolic degradation by thymidine phosphorylase, and in vivo stability analysis showed only one metabolite for [18F]3. No phosphorylation of [18F]2 could be detected in HCT116 cell homogenates, and in the same assay, only minor (∼8%) phosphorylation of [18F]3 was observed. Biodistribution in Balb/c mice indicated rapid clearance for [18F]2 and [18F]3 to a lesser extent.


CONCLUSIONS
The favorable biodistribution and metabolic profile of [18F]3 warrant further investigation as a next-generation PET proliferation marker.
Comprehensive Cancer Imaging Centre, Faculty of Medicine, Imperial College London, Hammersmith Hospital, W12 0NN London, UK.
Graham
G
Smith
Roberta
R
Sala
Laurence
L
Carroll
Kevin
K
Behan
Matthias
M
Glaser
Edward
E
Robins
Quang-Dé
QD
Nguyen
Eric O
EO
Aboagye
D000818:Animals; D049109:Cell Proliferation; D060326:Chemistry Techniques, Synthetic; D057930:Click Chemistry; D004355:Drug Stability; D005462:Fluorine Radioisotopes; D045325:HCT116 Cells; D006801:Humans; D008297:Male; D051379:Mice; D049268:Positron-Emission Tomography; D011849:Radioactive Tracers; D011854:Radiochemistry; D012015:Reference Standards; D013936:Thymidine / Q000138:chemical synthesis* / Q000378:metabolism / Q000493:pharmacokinetics; D013937:Thymidine Kinase / Q000378:metabolism
D005462:Fluorine Radioisotopes; D011849:Radioactive Tracers; D013937:Thymidine Kinase; D013936:Thymidine