Multiple doses of polyethylene glycol (PEG) decorated pharmaceuticals cause accelerated blood clearance (ABC) due to the generation of antibodies reactive to the PEG moiety. Using molecular imaging to monitor response to therapy could be complicated by the ABC effect due to PEG chains in microbubble lipid shells. Our objective was to measure the half-life of targeted contrast flowing through non-tumor tissue during longitudinal imaging studies, and to determine which targeted agent returned the highest signal intensity within tumors. The molecular imaging signals from contrast agents targeted to three distinct molecular targets, Secreted Frizzled-Related Protein-2 (SFRP2), Vascular Endothelial Growth Factor Receptor-2 (VEGFR2), AlphaVBeta3 Integrin (avb3) were all significantly correlated to contrast half-life. The molecular imaging signal from SFRP2 remained significantly higher than the signal returned by ultrasound contrast targeted to either VEGFR2 or avb3 before and after restricting analyses to imaging exams with similar half-lives. We hypothesize that increasing immune clearance rates during our longitudinal studies limited the amount of targeted contrast able to perfuse tumor vasculature, and that this resulted in a global dose-dependent decrease in molecular imaging signals. Molecular imaging may underestimate biomarker levels as longitudinal studies progress and as contrast half-lives decrease, unless contrast dosing is normalized by the amount of contrast able to reach the tumor and surrounding tissue rather than by the injected dosage.
Keywords: accelerated blood clearance; contrast half-life; ultrasound molecular imaging.