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.