Synthesis and Characterization of 9-(4-[ 18 F]Fluoro-3-(hydroxymethyl)butyl)-2-(phenylthio)-6-oxopurine as a Novel PET Agent for Mutant Herpes Simplex Virus Type 1 Thymidine Kinase Reporter Gene Imaging

Purpose [ 18 F]FHBG has been used as a positron emission tomography (PET) imaging tracer for the monitoring of herpes simplex virus type 1 thymidine kinase (HSV1-tk), a reporter gene for cell and gene therapy in humans. However, this tracer shows inadequate blood-brain barrier (BBB) penetration and, therefore, would be limited for accurate quantification of reporter gene expression in the brain. Here, we report the synthesis and evaluation of 9-(4-[ 18 F]fluoro-3-(hydroxymethyl)butyl)-2(phenylthio)-6-oxopurine ([ 18 F]FHBT) as a new PET tracer for imaging reporter gene expression of HSV1-tk and its mutant HSV1-sr39tk, with the aim of improved BBB penetration. Procedures [ 18 F]FHBT was prepared by using a tosylate precursor and [ 18 F]KF. The cellular uptake of [ 18 F]FHBT was performed in HSV1-sr39tk-positive (+) or HSV1-sr39tk-negative (−) MDA-MB-231 breast cancer cells. The specificity of [ 18 F]FHBT to assess HSV1-sr39tk expression was evaluated by in vitro blocking studies using 1 mM of ganciclovir (GCV). Penetration of [ 18 F]FHBT and [ 18 F]FHBG across the BBB was assessed by dynamic PET imaging studies in normal mice. Results The tosylate precursor reacted with [ 18 F]KF using Kryptofix2.2.2 followed by deprotection to give [ 18 F]FHBT in 10 % radiochemical yield (decay-corrected). The uptake of [ 18 F]FHBT in HSV1-sr39tk (+) cells was significantly higher than that of HSV1-sr39tk (−) cells. In the presence of GCV (1 mM), the uptake of [ 18 F]FHBT was significantly decreased, indicating that [ 18 F]FHBT serves as a selective substrate of HSV1-sr39TK. PET images and time-activity curves of [ 18 F]FHBT in the brain regions showed similar initial brain uptakes (~ 12.75 min) as [ 18 F]FHBG ( P > 0.855). Slower washout of [ 18 F]FHBT was observed at the later time points (17.75 − 57.75 min, P > 0.207). Conclusions Although [ 18 F]FHBT showed no statistically significant improvement of BBB permeability compared with [ 18 F]FHBG, we have demonstrated that the 2-(phenylthio)-6-oxopurine backbone can serve as a novel scaffold for developing HSV1-tk/HSV1-sr39tk reporter gene imaging agents for additional research in the future.