18F-FEAU as a radiotracer for herpes simplex virus thymidine kinase gene expression:: in-vitro comparison with other PET tracers

Objective The herpes simplex virus thymidine kinase (HSVtk) gene has frequently been applied as a reporter gene for monitoring transgene expression in animal models. In clinical gene therapy protocols, however, extremely low expression levels of the transferred gene are generally observed. Consequently, sensitive and selective radiotracers for imaging are required. This study describes the in-vitro evaluation of 2'-[F-18]fluoro-5-ethyl-1-beta-D-arabi-nofuranosyluracil (F-18-FEAU) as a candidate tracer for HSVtk imaging with positron emission tomography (PET). Methods In cellular accumulation experiments, the potential of F-18-FEAU as a PET tracer for HSVtk was compared to the known acyclic guanosine derivatives 9-[(3-[F-18]fluoro-1-hydroxy-2-propoxy)methyl]guanine (F-18-FHPG) and 9-[4-[F-18]fluoro-3-(hydroxymethyl)butyl]-guanine (F-18-FHBG), and the thymidine derivatives 3'-deoxy-3'-[F-18]fluorothymidine (F-18-FLT), 2-deoxy-2'-[F-18]fluoro-5-methyl-1-beta-D-arabi-nofuranosyluracil (F-18-FMAU) and 2'-deoxy-2'-[F-18]fluoro-5-iodo-1-beta-D-arabi-nofuranosyluracil (F-18-FIAU). For this purpose, C6 control cells and HSVtk-expressing C6tk cells were incubated with the different tracers for various periods of time and cellular uptake and initial uptake rates were analysed. The initial rate of tracer uptake was determined from the slope of the plot of tracer uptake versus incubation time. Results After 2 h of tracer incubation, the C6tk/C6 accumulation ratio was 1.6 for F-18-FLT, 2.4 for F-18-FMAU, 5.5 for F-18-FHPG, 10.3 for F-18-FIAU, 40.8 for F-18-FHBG and 84.5 for F-18-FEAU. The initial tracer uptake rate in C6tk cells was in the order FLT > FMAU > FEAU > FIAU > FHBG > FHPG, whereas the initial tracer uptake rate in C6 control cells was FLT > FMAU > FIAU > FEAU congruent to FHBG congruent to FHPG. The highest HSVtk specific uptake was observed for FEAU. Conclusion This study indicates that the high uptake rate of FEAU together with its high selectivity make this tracer an excellent candidate as a PET tracer for HSVtk gene expression.