and Other Imaging Probes

characteristics. These probes were selected for further dynamic PET studies in nude rats bearing subcutaneous RG2TK1 and RG2 tumors. The 2-h postinjection (18F)FEAU uptake levels were 3.3% 6 1.0% and 0.28% 6 0.07% dose/cm3 in subcutane- ous RG2TK1 and RG2 tumors, respectively, and 2.3% 6 0.2% and 0.19% 6 0.01% dose/cm3, respectively, for (18F)FFEAU. The corresponding RG2TK1/RG2 uptake ratios were 11.5 6 1.5 and 12.2 6 1.4, respectively. The inherent problem of comparing different radiolabeled pyrimidine nucleoside and guanosine-based probes for imaging HSV1-tk expression using different transduced cell lines and assay systems in the absence of an independent thymidine kinase-enzyme assay is discussed. Conclusion: For HSV1-tk reporter systems that require a 1- to 4-h PET paradigm, HSV1-tk-(18F)FEAU is the current top