The planning and individualization of gene therapy with suicide genes such as herpes simplex virus thymidine kinase (HSV-tk) necessitates the assessment of the enzyme activity expressed in the tumor. This can be done by uptake measurements of specific substrates for HSV-tk. Due to the molecular structure of 5-fluoro-1-(2'-deoxy-fluoro-beta-D-ribofuranosyl)uracil (FFUdR), it may be a substrate for both the mammalian thymidine kinase and HSV-tk. METHODS Using a HSV-tk-expressing rat hepatoma cell line and a control cell line (bearing the empty vector) the uptake of 3H-FFUdR was determined with increasing incubation periods. Furthermore, measurements with graded mixtures of HSV-tk-expressing cells and control cells were made. To elucidate the mechanism of FFUdR transport into cells, a series of inhibition/competition experiments was performed with challenge inhibitors of the nucleoside and the nucleobase transport systems. RESULTS The uptake studies with tritiated FFUdR revealed a 14- to 19-fold higher accumulation in the HSV-tk-expressing cell line compared to the control cell line. While the 3H-FFUdR uptake was 3- to 4-fold higher than the 3H-ganciclovir uptake in the HSV-tk-expressing cells, it was also higher in control cells (5-fold). Furthermore, FFUdR accumulation was linearly correlated with the amount of HSV-tk-expressing cells. FFUdR uptake and growth inhibition by therapeutic doses of ganciclovir were highly correlated, with r = 0.96. Inhibition/competition experiments showed that FFUdR is transported mainly by the equilibrative and the concentrative nucleoside transporter but not by the nucleobase transport systems. CONCLUSIONS The FFUdR uptake is an indicator of the HSV-tk activity in tumor cells and can be used as a prognostic marker during gene therapy with HSV-tk. The relative merits of ganciclovir and FFUdR as specific substrates for HSV-tk will need to be further explored in vivo.