Monitoring tumor cell proliferation by targeting DNA synthetic processes with thymidine and thymidine analogs.
The use of radiolabeled thymidine (TdR) and thymidine analogs as PET-based tracers of tumor growth rate is based on the assumption that measurement of uptake of these nucleosides, a function primarily of thymidine kinase-1 (TK(1)) activity, provides an accurate measure of active cell proliferation in tumors. The goal of this study was to test this hypothesis and determine how well these tracers track changes in proliferation of tumor cells.


METHODS
TK(1) activity; S-phase fraction; and uptake of TdR, 3'-deoxy-3'-fluorothymidine (FLT), and 2'-fluoro-5-methyl-1-(beta-D-2-arabino-furanosyl) uracil (FMAU) were determined in plateau-phase and exponentially growing cultures of 3 human and 3 murine tumor cell lines.


RESULTS
TK(1) activity and S-phase fraction increased in all cell lines as cells moved from plateau-phase conditions to exponential growth. Some cell lines had relatively large TK(1) activities and S-phase fractions under plateau-phase conditions, consistent with a loss of normal cell cycle checkpoint control in these cells. There were also 2 cell lines in which TK(1) activity changed little as cells moved from the plateau phase to exponential growth, suggesting that in these cell lines, de novo nucleotide synthesis pathways predominate over salvage pathways. Both TdR and FLT detected changes in TK(1) activity. The slope of the relationship between TdR uptake and TK(1) activity was nearly twice that for FLT and more than 40-fold that for FMAU.


CONCLUSIONS
Although not all tumors show a strong TK(1) dependence of proliferation, in all cell lines for which proliferation is highly TK(1) dependent, phosphorylation of TdR or FLT accurately reflects changes in TK(1) enzyme activity.
Schwartz
Jeffrey L
JL
Department of Radiation Oncology, University of Washington School of Medicine, Seattle, Washington 98195-6069, USA. jschwart@u.washington.edu
Tamura
Yasuko
Y
Jordan
Robert
R
Grierson
John R
JR
Krohn
Kenneth A
KA
D000818:Animals; D001086:Arabinofuranosyluracil / Q000031:analogs & derivatives* / Q000493:pharmacokinetics; D002455:Cell Division; D045744:Cell Line, Tumor / Q000000981:diagnostic imaging / Q000378:metabolism / Q000473:pathology; D004247:DNA / Q000096:biosynthesis*; D015224:Dideoxynucleosides / Q000493:pharmacokinetics; D016503:Drug Delivery Systems / Q000379:methods; D006801:Humans; D008657:Metabolic Clearance Rate; D051379:Mice; D009367:Neoplasm Staging / Q000379:methods*; D009369:Neoplasms / Q000000981:diagnostic imaging* / Q000378:metabolism* / Q000473:pathology; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics; D013936:Thymidine / Q000031:analogs & derivatives* / Q000493:pharmacokinetics*; D013937:Thymidine Kinase / Q000378:metabolism*; D014055:Tomography, Emission-Computed / Q000379:methods; D014316:Tritium / Q000493:pharmacokinetics
D015224:Dideoxynucleosides; D019275:Radiopharmaceuticals; D014316:Tritium; D001086:Arabinofuranosyluracil; D004247:DNA; D013937:Thymidine Kinase; C092717:thymidine kinase 1; C034935:clevudine; C002854:alovudine; D013936:Thymidine