Thymidine salvage and <i>de novo</i> synthesis pathways.

<p>In thymidine salvage, thymidine is transported across the cell membrane and phosphorylated by TK1 into thymidine monophosphate (TMP). The thymidine is further phosphorylated into thymidine diphosphate (TDP) and thymidine triphosphate (TTP) and then incorporated into DNA. Alternatively, using the <i>de novo</i> synthesis pathway, deoxyuridine monophosphate (dUMP) is converted to TMP by TS which can then be further phosphorylated and incorporated into DNA. Similarly to thymidine, [¹⁸F]-FLT is transported into the cell and phosphorylated into [¹⁸F]-FLT monophosphate ([¹⁸F]-FLTMP) and trapped by TK1. [¹⁸F]-FLTMP can be further phosphorylated into [¹⁸F]-FLT diphosphate ([¹⁸F]-FLTDP) and [¹⁸F]-FLT triphosphate ([¹⁸F]-FLTTP), however, due to the substitution of OH with ¹⁸F in the 5-prime position, [¹⁸F]-FLTTP is not incorporated into the DNA.