Published Online
on July 26, 2004

A more recent version of this article appeared on August 10, 2004

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Submitted on April 9, 2004
Revised on May 20, 2004
Accepted on June 11, 2004

Molecular Imaging of the Kinetics of Vascular Endothelial Growth Factor Gene Expression in Ischemic Myocardium

Joseph C. Wu MD, PhD, Ian Y. Chen MSE, Yanling Wang PhD, Jeffrey R. Tseng MD, Ankush Chhabra MD, Mahdi Salek BS, Jung-Joon Min MD, Michael C. Fishbein MD, Ronald Crystal MD, and Sanjiv S. Gambhir MD, PhD^*

From the Department of Molecular and Medical Pharmacology, the Crump Institute of Molecular Imaging (J.C.W., Y.W., M.S., J.J.M., S.S.G.), and Division of Nuclear Medicine (J.R.T.); the Department of Pathology (M.C.F.); and the Department of Medicine, Division of Cardiology (J.C.W., A.C.), UCLA School of Medicine, Los Angeles, Calif; the Department of Medicine (R.C.), Weill Medical College of Cornell University, Ithaca, NY; and the Department of Radiology and Bio-X Program (J.C.W., I.Y.C., S.S.G.), Stanford University, Palo Alto, Calif.

^* To whom correspondence should be addressed. E-mail: sgambhir{at}stanford.edu.

Background--Angiogenic gene therapy is a promising treatment paradigm for patients with ischemic heart disease. In this study, we used micro-positron emission tomography (microPET) to monitor the transgene expression, function, and effects in a whole-body system.

Methods and Results--Adenovirus with cytomegalovirus promoter driving an angiogenic gene (vascular endothelial growth factor [VEGF]) linked to a PET reporter gene (herpes simplex virus type 1 mutant thymidine kinase; Ad-CMV-VEGF₁₂₁-CMV-HSV1-sr39tk) was used to transfect rat embryonic cardiomyoblasts in vitro. Expression of both genes correlated strongly (r=0.98; P<0.001). Afterward, rats underwent ligation of the left anterior descending artery followed by injection of 1x10¹⁰ pfu of Ad-CMV-VEGF₁₂₁-CMV-HSV1-sr39tk (study; n=35) or Ad-null (control; n=15) at the peri-infarct region. Noninvasive microPET imaging was used to assess the uptake of 9-(4-[¹⁸F]-fluoro-hydroxymethylbutyl)guanine ([¹⁸F]-FHBG) PET reporter probe by cells expressing the HSV1-sr39tk PET reporter gene. Cardiac transgene expression peaked at day 1 and declined over the next 2 weeks. Repeat adenoviral injections at day 60 yielded no detectable signal. The in vivo reporter gene expression (% injected dose/g of [¹⁸F]-FHBG) correlated well with ex vivo gamma counting (r=0.92), myocardial tissue HSV1-sr39TK enzyme activity (r=0.95), and myocardial tissue VEGF level (r=0.94; P<0.001 for all). The VEGF₁₂₁ isoform induced significant increases in capillaries and small blood vessels. However, the level of neovasculature did not translate into significant improvements in functional parameters such as myocardial contractility by echocardiography, perfusion by nitrogen-13 ammonia imaging, and metabolism by [¹⁸F]-fluorodeoxyglucose imaging.

Conclusions--Taken together, these findings establish the feasibility of molecular imaging for monitoring angiogenic gene expression with a PET reporter gene and probe noninvasively, quantitatively, and repetitively. The principles demonstrated here can be used to evaluate other therapeutic genes of interest in animal models before future clinical trials are initiated.

Key words: gene therapy • angiogenesis • imaging • myocardium • nuclear medicine

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