A mutant herpes simplex virus type 1 thymidine kinase reporter gene shows improved sensitivity for imaging reporter gene expression with positron emission tomography.

We are developing assays for noninvasive, quantitative imaging of reporter genes with positron emission tomography (PET), for application both in animal models and in human gene therapy. We report here a method to improve the detection of lower levels of PET reporter gene expression by utilizing a mutant herpes simplex virus type 1 thymidine kinase (HSV1-sr39tk) as a PET reporter gene. The HSV1-sr39tk mutant was identified from a library of site-directed mutants. Accumulation (net uptake) of the radioactively labeled substrates [8-(3)H]penciclovir ([8-(3)H]PCV), and 8-[(18)F]fluoropenciclovir (FPCV) in C6 rat glioma cells expressing HSV1-sr39tk is increased by a factor of approximately 2.0 when compared with C6 cells expressing wild-type HSV1-tk. The increased imaging sensitivity of HSV1-sr39tk when FPCV is used is also demonstrated in vivo both with tumor cells stably transfected with either HSV1-tk or HSV1-sr39tk, and after hepatic delivery of HSV1-tk or HSV1-sr39tk by using adenoviral vectors. The use of HSV1-sr39tk as a PET reporter gene and FPCV as a PET reporter probe results in significantly enhanced sensitivity for imaging reporter gene expression in vivo.