Abstract #0675

Transforming the Herpes Simplex Virus Type-1 Thymidine Kinase (HSV1-Tk) Into an MRI Reporter Gene

Amnon Bar-Shir¹,², Guanshu Liu¹,³, Nirbhay N. Yadav¹,³, Yoshinori Kato¹, Galit Pelled¹,³, Piotr Walczak¹,², Michael T. McMahon¹,³, Martin G. Pomper¹, Marc M. Greenberg⁴, Peter C. van Zijl¹,³, Jeff W. Bulte¹,², Assaf A. Gilad¹,²

¹Department of Radiology, Johns Hopkins University, Baltimore, MD, United States; ²Cellular Imaging Section, Johns Hopkins University, Baltimore, MD, United States; ³F.M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, MD, United States; ⁴Department of Chemistry, Johns Hopkins University, Baltimore, MD, United States

By modifying a substrate for herpes simplex virus type-1 thymidine kinase (HSV1-tk), an existing PET reporter gene, we successfully transformed it into a CEST-MRI reporter gene. Thymidine analogues were synthesized with higher pKa values for imino protons, thus reducing their exchange rate to an optimum range for CEST detection. The substrate 5-methyl-5,6-dihydrothymidine provided the highest contrast after saturation at 5 ppm from the water protons. It is efficiently phosphorylated by HSV1-tk but not mammalian thymidine kinase, making it a specific reporter. Transplanted cells expressing HSV1-tk were easily detected in vivo following i.v. administration of 5-methyl-5,6-dihydrothymidine.