Positron Emission Tomography of Cardiac Transgene Expression - Comparison of two Approaches Based on Herpesviral Thymidine Kinase Reporter Gene

Background: PET imaging of reporter gene expression holds promise for noninvasive monitoring of gene therapy. Previously, two approaches based on the herpes simplex virus type 1 thymidine kinase gene(HSV1-tk) have been applied to the heart. Wild-type HSV1-tk was imaged with [¹²⁴I]-fluoro-deoxy-iodo-arabinofuranosyluracil(FIAU) and mutant HSV1-sr39tk with [¹⁸F]-fluoro-hydroxymethylbutyl-guanine(FHBG), but a direct comparison has not yet been performed.
Methods and Results: In H9c2 cardiac cells, in vitro FIAU accumulation was superior to FHBG following infection with adenovirus expressing wild-type HSV1-tk(Ad_tk), while FHBG uptake was superior to FIAU using adenovirus expressing mutant HSV1-sr39tk(Ad_tk). Subsequent in vivo studies employed dynamic PET two days after intramyocardial vector injection. FIAU was used after Ad_tk infection. Highest cardiac uptake compared to negative controls occurred at 10-30min after tracer injection. Specific uptake disappeared at later times due to FIAU washout. FHBG was used after Ad_sr39tk infection. Specific tracer uptake continuously increased over time and was highest vs controls at latest imaging times(105-120min). Global cardiac reporter probe kinetics in rats were confirmed by regional analyses in pigs. Transgene expression was specifically visualized by both approaches. Highest traget/backgrounde ratio of FIAU in Ad_tk infected pig myocardium was 1.50±0.20 vs 2.64±0.49 for FHBG in Ad_sr39tk infected areas(P=0.01). In vivo results were confirmed by ex vivo counting and autoradiography.
Conclusions: Both reporter gene/probe combinations were feasible for noninvasive imaging of cardiac transgene expression in different species. Specific probe kinetics suggest different myocardial handling of pyrimidine(FIAU) and acycloguanosine(FHBG) derivatives. Results are in favour of FHBG and mutant HSV1-sr39tk because of continuous accumulation over time and higher imaging contrast.
Condensed Abstract: Two approaches for PET imaging of cardiac reporter gene expression based on herpesviral thymidine kinase gene (HSV1-tk) were compared in small and large animals. Using wild-type HSV1-tk, dynamically imaged with [¹²⁴I]-fluoro-deoxy-iodo-arabinofuranosyluracil (FIAU), highest myocardial FIAU uptake occurred early after injection, but tracer washout was observed at later times. Using mutant HSV1-sr39tk, imaged with [¹⁸F]-fluoro-hydroxymethylbutyl-guanine (FHBG), specific uptake increased over time and was highest at late imaging. Both reporter gene/probe combinations were feasible for noninvasive imaging of cardiac transgene expression, but results are in favour of FHBG because of continuous accumulation over time and higher imaging contrast.