Dual-tracer receptor concentration imaging using tracers with different tissue delivery kinetics

Simultaneous dynamic fluorescent imaging of a suitable untargeted tracer in conjunction with any molecular targeted fluorescent agent has been shown to be a powerful approach for quantifying cancer-specific cell surface receptors in vivo in the presence of non-specific uptake and tracer delivery variability. The identification of a “suitable” untargeted tracer (i.e., one having equivalent plasma and tissue delivery pharmacokinetics to the targeted tracer) for every targeted tracer, however, may not always be feasible or could require extensive testing. This work presents a “deconvolution” approach capable of correcting for plasma and tissue-delivery pharmacokinetic differences between tracers by quantifying dynamic differences in targeted and untargeted tracer uptake in a receptor-free tissue (one devoid of targeted molecular species) and correcting uptake in all other tissues accordingly. This deconvolution correction approach is evaluated in theoretical models and explored in an in vivo mouse xenograft model of human glioma. In the animal experiments, epidermal growth factor receptor (EGFR: a receptor known to be overexpressed in the investigated glioma cell line) was targeted using a fluorescent tracer with very different plasma pharmacokinetics than a second untargeted fluorescent tracer. Without correcting for these differences, the dual-tracer approach yielded substantially higher estimations of EGFR concentration in all tissues than expected; however, deconvolution correction was able to produce estimates that matched ex vivo validation.