Purpose: Fluoropyrimidines like 1-(2'-deoxy-2'-fluoro-beta-D: -arabinofuranosyl)-thymine (FMAU) and 3'-deoxy-3'-fluorothymidine (FLT) accumulate in tumors and are being used as positron emission tomography tumor-imaging tracers. Proliferating tissues with high thymidine kinase 1 (TK1) activity retain FLT; however, the mechanism of selective accumulation of FMAU in tumors and certain other tissues requires further study.
Methods: Retention of [(3)H]FLT and [(3)H]FMAU was measured in prostate cancer cell lines PC3, LNCaP, DU145, and the breast cancer cell line MD-MBA-231, and the tracer metabolites were analyzed by high-performance liquid chromatography (HPLC). FMAU retention, thymidine kinase 2 (TK2) activity, and mitochondrial mass were determined in cells stressed by depleted cell culture medium or by treating with oxidative, reductive, and energy stress, or specific adenosine monophosphate-activated protein kinase activator, or eIF2 inhibitor. TK1 and TK2 activities and mitochondrial mass were determined by FLT phosphorylation, 1-beta-D: -arabinofuranosylthymine (Ara-T) phosphorylation, and flow cytometry, respectively.
Results: FMAU retention in rapidly proliferating cancer cell lines was five to ten times lower than FLT after 10 min incubation. HPLC analysis of the cellular extracts showed that phosphorylated tracers are the main retained metabolites. Nutritional stress decreased TK1 activity and FLT retention but increased retained FMAU. TK2 inhibition decreased FMAU retention and phosphorylation with negligible effects on FLT. Oxidative, reductive, or energy stress increased FMAU retention and correlated with mitochondrial mass (r (2) = 0.88, p = 0.006). FMAU phosphorylation correlated with increased TK2 activity (r (2) = 0.87, p = 0.0002).
Conclusion: FMAU is preferably phosphorylated by TK2 and can track TK2 activity and mitochondrial mass in cellular stress. FMAU may provide an early marker of treatment effects.