SPECT imaging of cardiac reporter gene expression in living rabbits

This work is to demonstrate feasibility of imaging the expression of herpes simplex virus 1-thymidine kinase （HSVI-tk） reporter gene in rabbits myocardium by using the reporter probe ^131I-2＇-fluoro-2＇-deoxy-l-β-o- arabinofuranosyl-5-iodouracil （^131I-FIAU） and SPECT. Rabbits of the study group received intramyocardial injection of Ad5-tk and control group received aseptic saline injection. Two sets of experiments were performed on the study group. Rabbits of the 1st set were injected with ^131I-FIAU 600 uCi at Day 2 after intramyocardial transfection of Ad5-tk in 1×10^9, 5×10^8, 1×10^8, 5×10^7 and 1×10^7pfu, and heart SPECT imaging was done at different hours. Rabbits of the 2nd were transferred various titers of Ad5-tk （1×10^9, 5×10^8, 1×10^8, 5×10^7, 1×10^7 pfu） to determine the threshold and optimal viral titer needed for detection of gene expression. Two days later, ^131I-FIAU was injected and heart SPECT imaging was performed at 6, 24 and 48 h, before killing them for gamma counting of the hearts. Reverse transcription-polymerase chain reaction （RT-PCR） was used to verify the transferred HSVI-tk gene expression. Semi-quantitative analysis derived of region of interest （ROI） of SPECT images and RT-PCR images was performed and the relationship of SPECT images with ex vivo gamma counting and mRNA level were evaluated. SPECT images conformed ^131I-FIAU accumulation in rabbits injected with Ad5-tk in the anterolateral wall. The optimal images quality was obtained at 24-48 h for different viral titers. The highest radioactivity in the focal myocardium was seen at 6 h, and then declined with time. The threshold was 5×10^7 pfu of virus titer. The result could be set better in 1-5×10^8 pfu by SPECT analysis and gamma counting. ROI-derived semi-quantitative study on SPECT images correlated well with ex vivo gamma counting and mRNA levels from RT-PCR analysis. The HSVI-tk/^131I-FIAU reporter gene/reporter probe system is feasible for cardiac SPECT reporter gene imaging. The optimal Ad5-tk titer is 1-5×10^8pfu and optimal imaging time is 24-48 h after transferred Ad5-tk in rabbit. The imaging of transgene expression in heart might be used for noninvasive imaging of gene therapy in cardiac diseases in human.