1. ¹Crump Institute for Molecular Imaging, David Geffen School of Medicine at UCLA, Los Angeles, CA, USA
2. ²Department of Molecular and Medical Pharmacology, David Geffen School of Medicine at UCLA, Los Angeles, CA, USA
3. ³Department of Pharmacology, Nagasaki University School of Medicine, Nagasaki, Japan
4. ⁴Jonsson Comprehensive Cancer Center, David Geffen School of Medicine at UCLA, Los Angeles, CA, USA
5. ⁵Molecular Imaging Program at Stanford, Department of Radiology and Bio-X Program, Stanford, CA, USA

Correspondence: SS Gambhir, Molecular Imaging Program at Stanford, Department of Radiology and Bio-X Program, The James H Clark Center, 318 Campus Drive, East Wing, 1st Floor, Stanford, CA, 94305-5427, USA. E-mail: sgambhir@stanford.edu

Received 9 February 2003; Accepted 9 September 2003; Published online 15 January 2004.

Abstract

Site-specific recombination tools such as the Cre–loxP system are used to create animal models where conditional gene deletion/activation studies are required. In the current proof of principle study, we have demonstrated that a PET reporter gene (PRG), the herpes simplex virus type 1 thymidine kinase (HSV1-tk), can be made to remain silent and can be activated by Cre–loxP-mediated recombination in cell culture and in living mice. An adenovirus carrying a silent HSV1-tk was tail-vein injected (1x10⁹ PFU) in six transgenic mice that express Cre recombinase in their liver (Cre+) and in four control mice (Cre-). The liver-specific expression of the PRG in Cre+ mice was detected in the microPET following injection of the reporter probe, 9-[4-fluoro-3-(hydroxymethyl)butyl]guanine ([¹⁸F]-FHBG). The [¹⁸F]-FHBG accumulation in the liver in terms of percent-injected dose per gram of tissue was 7.721.13 for the Cre+ mice and 0.100.02 for the Cre- mice (P<0.05) 48 h after adenoviral injection. These results were further validated by quantitative RT-PCR, western blotting and by in vitro assays for herpes simplex virus type 1 thymidine kinase enzyme activity. Thus by using the Cre–loxP system it is possible to modulate a PRG and noninvasively monitor the extent of Cre–loxP-mediated gene activation by imaging in a microPET scanner.