Optical Imaging of Cardiac Reporter Gene Expression in Living Rats

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Abstract
Background — Studies of cardiac gene transfer rely on postmortem analysis using histologic staining or enzyme assays. Noninvasive imaging of the temporal and spatial characteristics of cardiac gene expression in the same subject offers significant advantages. Methods and Results — Rats underwent direct myocardial injection via left thoracotomy with adenovirus-expressing firefly luciferase (Ad-CMV-Fluc; n=30). The reporter substrate d -luciferin was injected intraperitoneally. Serial images were acquired by use of a cooled charged couple detector (CCD) camera. Results are expressed as relative light unit per minute (RLU/min). Rats transduced with 1×10 9 plaque-forming units show decremental cardiac luciferase activity over time: 152 070±21 170 (day 2), 195 806±62 630 (day 5), 7250±2941 (day 8), and 2040±971 RLU/min (day 14). To assess the detection sensitivity, serially diluted titers of Ad-CMV-Fluc were injected: 1×10 9 (195 393±14 896), 1×10 8 (33 777±18 179), 1×10 7 (417±91), 1×10 6 (185±64), 1×10 5 (53±1), and control (54±1) ( P 9 , 1×10 8 , and 1×10 7 plaque-forming units versus control adenovirus-expressing mutant thymidine kinase [Ad-CMV-HSV1-sr39tk]; n=3). Finally, rats were euthanized, and in vitro luciferase activity correlated with in vivo CCD signals ( r 2 =0.92). Conclusions — This study demonstrates for the first time the feasibility of imaging the location, magnitude, and time course of cardiac reporter gene expression in living rats. Cardiac gene therapy studies could be aided with wider application of this approach.