PET imaging of HSV1-tk mutants with acquired specificity toward pyrimidine- and acycloguanosine-based radiotracers

• DOI: 10.1007/s00259-009-1089-x
• Publication Date: 2009-03-03T06:19:52Z
• Authors: Yury Likar; Konstantin Dobrenkov; Malgorzata Olszewska; Larissa Shenker; Shangde Cai; Hedvig Hricak; Vladimir Ponomarev
• Keywords: Acyclovir; Herpesvirus 1, Human; Thymidine Kinase; Substrate Specificity; 3. Good health; Gene Expression Regulation, Neoplastic; Mice; 03 medical and health sciences; Pyrimidines; 0302 clinical medicine; Amino Acid Substitution; Genes, Reporter; Cell Line, Tumor; Neoplasms; Positron-Emission Tomography; Drug Resistance, Viral; Mutation; Animals; Humans; Phosphorylation; Radioactive Tracers

The aim of this study was to create an alternative mutant of the herpes simplex virus type 1 thymidine kinase (HSV1-tk) reporter gene with reduced phosphorylation capacity for acycloguanosine derivatives, but not pyrimidine-based compounds that will allow for successful PET imaging.A new mutant of HSV1-tk reporter gene, suitable for PET imaging using pyrimidine-based radiotracers, was developed. The HSV1-tk mutant contains an arginine-to-glutamine substitution at position 176 (HSV1-R176Qtk) of the nucleoside binding region of the enzyme.The mutant-gene product showed favorable enzymatic characteristics toward pyrimidine-based nucleosides, while exhibiting reduced activity with acycloguanosine derivatives. In order to enhance HSV1-R176Qtk reporter activity with pyrimidine-based radiotracers, we introduced the R176Q substitution into the more active HSV1-sr39tk mutant. U87 human glioma cells transduced with the HSV1-R176Qsr39tk double mutant reporter gene showed high (3)H-FEAU pyrimidine nucleoside and low (3)H-penciclovir acycloguanosine analog uptake in vitro. PET imaging also demonstrated high (18)F-FEAU and low (18)F-FHBG accumulation in HSV1-R176Qsr39tk+ xenografts. The feasibility of imaging two independent nucleoside-specific HSV1-tk mutants in the same animal with PET was demonstrated. Two opposite xenografts expressing the HSV1-R176Qsr39tk reporter gene and the previously described acycloguanosine-specific mutant of HSV1-tk, HSV1-A167Ysr39tk reporter gene, were imaged using a short-lived pyrimidine-based (18)F-FEAU and an acycloguanosine-based (18)F-FHBG radiotracer, respectively, administered on 2 consecutive days.We conclude that in combination with acycloguanosine-specific HSV1-A167Ysr39tk reporter gene, a HSV1-tk mutant containing the R176Q substitution could be used for PET imaging of two different cell populations or concurrent molecular biological processes in the same living subject.