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Objectives: Montoring of gene expression through external imaging may be a useful tool for following gene therapy procedures. In particular, the use of PET may allow quantitative evaluation in these situations. Given the relatively low endogenous expression of the Somatostatin Receptor 2 (SSTR2) and the availability of very efficient positron emitter labeled radiotracers capable of binding this receptor such as 68Ga-DOTA-Tyr(3)-Thr(8)-octreotate (68Ga-DOTATATE), we have evaluated the use of this combination of reporter gene/reporter compound for non invasive monitoring of gene transfer.
Methods: Adeno Associated Virus (AAV) vectors encoding the human SSTR2 gene or green fluorescent protein (GFP) as control, under the cytomegalovirus (CMV) or the thyroxin binding globulin (TBG) promoters were constructed. Different sets of C57/BL6 mice received intravenous (IV) AAV-TBG-SSTR2, intramuscular (IM) or inhaled AAV-CMV-SSTR2. Control animals received both IV AAV-TBG-GFP and IM AAV-CMV-GFP. PET imaging was carried out over a 6 month period following the AAV administration in treated and control animals with 1-5 MBq of 68Ga-DOTATATE using a clinical PET scanner.
Results: IV AAV-TBG-SSTR2 transduced animals showed markedly increased uptake in the liver, that was several-fold above control levels. IM AAV-CMV-SSTR2 transduced animals showed a 3 to 4 fold increase in muscle uptake compared to controls. In both instances, the increase in 68Ga-DOTATATE uptake was dependent on the amount of administered AAV. Liver transduced animals showed a tendency for a decrease in liver uptake over time whereas transduction in muscle showed a steady increased uptake over the 6 month observation period. Inhaled AAV-CMV-SSTR2 did not yield significant increase in 68Ga-DOTATATE uptake in the lungs.
Conclusions: Use of the SSTR2 as reporter gene and repetitive imaging with 68Ga-DOTATATE demonstrates that monitoring is feasible and high uptake levels are maintained for long periods of time. This approach may be useful for quantitative monitoring of gene therapy in animal models.
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