Model System Using Controlled Receptor Expression for Evaluating Targeted Ultrasound Contrast Agents

This report details a model system for evaluating targeted ultrasound (US) contrast agents using adenoviral (Ad) vectors to regulate target receptor expression. Receptor density in vitro was modulated in breast cancer cells by varying the multiplicity of infection (MOI) from 0 to 100. Target receptors were induced using a green fluorescent protein (GFP) reporter Ad vector for gene transfer and expression of the hemagglutinin (HA) tag. These reporter genes were under the control of the ubiquitous cytomegalovirus (CMV) promoter. Subsequently, receptor expression and anti-HA antibody (Ab) binding was examined with flow cytometry. Targeted US contrast agents, or microbubbles (MB), were created by conjugating either biotinylated anti-HA or isotype control Ab to the surface of biotin coated MBs via a streptavidin bridge. Targeted MBs were incubated with Ad infected 2LMP cells to evaluate in vitro MB binding. Experimental results found GFP expression to be directly correlated with Ad MOI (r ² = 0.96). Increasing the Ad MOI produced a corresponding increase in binding and accumulation of anti-HA Ab on the cell surface (p < 0.01). However, no difference was found between Cy5-labeled anti-HA Ab exposed cell groups at an MOI of 0 (p > 0.29). Additionally, no difference was found between the isotype control Ab group (p > 0.44) indicating minimal nonspecific binding. No difference was found between cell groups incubated with isotype-targeted MBs (p > 0.42) regardless of receptor density. However, cells exposed to HA-targeted MBs showed increased levels of cell binding proportional to induced receptor expression levels (p < 0.02).