Non-invasive gamma camera imaging of gene transfer using an adenoviral vector encoding an epitope-tagged receptor as a reporter

A model epitope-tagged receptor was constructed by fusing the hemagglutinin (HA) sequence on the extracellular N-terminus of the human somatostatin receptor subtype 2 (hSSTr2) gene. This construct was placed in an adenoviral (Ad-HAhSSTr2) vector. This study evaluated Ad-HAhSSTr2 in vitro and in vivo using FACS, fluorescent microscopy, radioactive binding assays, and gamma camera imaging techniques. Infection of A-427 non-small cell lung cancer cells with Ad-HAhSSTr2 or Ad-hSSTr2 resulted in similar expression of hSSTr2 by FACS analysis and binding assays using a Tc-99m-labeled somatostatin analogue (Tc-99m-P2045). HAhSSTr2 expression in A-427 cells was specific for infection with Ad-HAhSSTr2. FITC-labeled anti-HA antibody (FITC-HA) confirmed surface expression in live A-427 cells and the absence of internalization. Gamma camera imaging and gamma counter analysis of normal mice showed significantly greater (P < 0.05) liver uptake of Tc-99m-labeled anti-HA antibody (Tc-99m-anti-HA) in mice injected i.v. 48h earlier with Ad-HAhSSTr2 (53.6 +/- 6.9% ID/g) as compared to mice similarly injected with Ad-hSSTr2 (9.0 +/- 1.3% ID/g). In a mouse tumor model, imaging detected increased tumor localization of Tc-99m-anti-HA due to direct intratumor injection Ad-HAhSSTr2. Gamma counter analysis confirmed significantly greater (P < 0.05) uptake of Tc-99m-anti-HA in tumors injected with Ad-HAhSSTr2 (12.5 +/- 4.1% ID/g) as compared to Ad-hSSTr2-infected tumors (5.1 +/- 1.5% ID/g). These studies demonstrate the feasibility of using an epitope-tagged reporter receptor for non-invasively imaging gene transfer.