Dynamic Tracking of <i>In Vivo</i> Receptor Availability in Tumor Using Paired-Agent Imaging

Quantitative assessment of receptor availability (RA) provides valuable insight into therapeutic outcomes in drug development and clinical practice. Here, paired-agent imaging (PAI) is used to dynamically track the availability of the epidermal growth factor receptor (EGFR) in response to <i>in vivo</i> ligand or inhibitor binding in individual mice with head and neck cancer (HNC). Naïve (<i>n</i> = 3) or xenograft HNC tumor-bearing (<i>n</i> = 21) mice were coadministered 0.15, 0.3, or 0.9 nmol ABY-029, and 2.5 nmol of IRDye 700DX. Fluorescence images were acquired for 300 min and then for an additional 60 min after administration of Z03115 (test group), human EGF (positive control), or PBS (vehicle control). Kinetic fluorescence and PAI curves were evaluated to determine the effects of the ABY-029 dose and EGFR blocking on tumor RA estimation. Nonquantifiable increases in ABY-029 fluorescence in tumor and muscle were observed after <i>in vivo</i> blocking, while PAI produced the expected decrease in RA. No statistically significant difference in preblocking RA was observed with different doses of ABY-029. RA decreased in response to blocking in positive control and test group animals, while the vehicle group exhibited no significant change in RA. This study demonstrated that RA can be monitored dynamically in individual animals using PAI regardless of imaging agent dose, while fluorescence from the receptor-targeted imaging agent alone could not. These results demonstrate PAI as a simple imaging strategy that could allow dose optimization in pharmaceutical development and patient-specific dosing for molecular therapeutics.