Fig 1.

Mechanisms of cellular [¹⁸F]FLT retention. Similar to thymidine in the salvage pathway, [¹⁸F]FLT is taken up from the extracellular milieu by specialized nucleoside transporters or via passive diffusion. Within the cell [¹⁸F]FLT is phosphorylated by thymidine kinase 1 (TK1), the enzyme also responsible for phosphorylation of thymidine. TK1 activity is dependent on adenosine triphosphate (ATP). The phosphorylated form of thymidine (TMP) is further phosphorylated to thymidine diphosphate (TDP) and thymidine triphosphate (TTP), which is subsequently incorporated into the DNA. The phosphorylated form of [¹⁸F]FLT cannot be incorporated into DNA but is trapped within the cell ⁶⁰. Techniques like PET or gamma counter measurements are capable of quantifying the rate of accumulation of [¹⁸F]FLT within cells. An alternative thymidine metabolism pathway is the de novo synthesis. The key enzyme of this pathway is thymidylate synthase (TS), which methylates deoxyuridine monophosphate (dUMP) to TMP. The two pathways merge at the level of TMP. Studies describing the importance of the different factors for [¹⁸F]FLT uptake are described in detail in the Supplementary Results.