Noninvasive imaging of transgene expression by use of positron emission tomography in a pig model of myocardial gene transfer

Radionuclide imaging of reporter gene expression may be useful for noninvasive monitoring of clinical cardiac gene therapy. Experience until now, however, has been limited to small animals. To evaluate feasibility in a clinically applicable setting, pigs were studied by conventional positron emission tomography (PET) 2 days after regional intramyocardial injection of control adenovirus or adenovirus carrying herpesviral thymidine kinase reporter gene (HSV1-tk). Myocardial blood flow was quantified by use of [13N]ammonia. Subsequently, kinetics of the reporter substrate [124I]-2'-fluoro-2'-deoxy-5-iodo-1-beta-d-arabino-furanosyluracil (FIAU) were assessed over a period of 2 hours. Areas infected with adenovirus expressing HSV1-tk showed significantly elevated FIAU retention during the first 30 minutes after injection. At later times, washout was observed, and retention was not different from that in areas infected with control virus or remote myocardium. Early in vivo FIAU uptake correlated with ex vivo images, autoradiography, and immunohistochemistry for reporter gene product after euthanasia. After intramyocardial injection of both adenoviruses, myocardial blood flow was mildly elevated compared with that in remote areas, consistent with histological signs of regional inflammation. In vivo quantification of regional myocardial transgene expression is feasible with clinical PET methodology, the radioiodinated reporter probe FIAU, and the HSV1-tk reporter gene. Radioactivity efflux after specific initial uptake was not observed previously in tumor studies, suggesting that tissue-specific differences in nucleoside metabolism influence reporter probe kinetics. By coregistering reporter gene expression with additional biological parameters such as myocardial blood flow, PET allows for noninvasive characterization of the success of cardiac gene transfer along with its functional correlates.