In Vivo Quantification of Tumor Receptor Binding Potential with Dual-Reporter Molecular Imaging

Abstract

Purpose Receptor availability represents a key component of current cancer management. However, no approaches have been adopted to do this clinically, and the current standard of care is invasive tissue biopsy. A dual-reporter methodology capable of quantifying available receptor binding potential of tumors in vivo within a clinically relevant time scale is presented. Procedures To test the methodology, a fluorescence imaging-based adaptation was validated against ex vivo and in vitro measures of epidermal growth factor receptor (EGFR) binding potential in four tumor lines in mice, each line expected to express a different level of EGFR. Results A strong correlation was observed between in vivo and ex vivo measures of binding potential for all tumor lines (r=0.99, p<0.01, slope=1.80±0.48, and intercept=−0.58±0.84) and between in vivo and in vitro for the three lines expressing the least amount of EGFR (r=0.99, p<0.01, slope=0.64±0.32, and intercept=0.47±0.51). Conclusions By providing a fast and robust measure of receptor density in tumors, the presented methodology has powerful implications for improving choices in cancer intervention, evaluation, and monitoring, and can be scaled to the clinic with an imaging modality like SPECT.

Dynamic dual-tracer MRI-guided fluorescence tomography to quantify receptor density in vivo

“…As discussed elsewhere (21), the model system assumes first-order binding kinetics and therefore loses accuracy when a large percentage of receptors are bound with targeted tracer. Although receptor saturation may obtain if large quantities of tracer are administered, an analysis of plasma and binding kinetics, and receptor density in the U251 tumor line shows that receptor saturation at the injected dose used herein (0.2 nmol) is unlikely (SI Text).…”

“…Derivation of the dual-tracer model system has been described elsewhere (21). Briefly, the behavior of the targeted tracer in this model is defined by a series of rate constants that describe the movement of the tracer between the blood plasma and extravascular space and once the tracer is in the extravascular space, the rates at which it will bind to and be released from the available receptors.…”

“…The required identification of a reference tissue region largely precludes the application of this approach in cancer imaging, because solid tumors have unique vasculature (12,(18)(19)(20) that alters tracer kinetics. A recent study established a more robust method applicable in any tissue region, including solid tumors, by adapting the compartmental model principle to a dualtracer model developed around dynamic imaging of two tracers in the same tissue simultaneously (21). Although optical imaging methods are well-suited for this multitracer approach because of their ability to discriminate multiple compounds based on their spectral signatures, imaging deep-seated tumors is severely limited by photon scattering in tissue.…”

Computed Tomography-guided Time-domain Diffuse Fluorescence Tomography in Small Animals for Localization of Cancer Biomarkers

“…The ability of the system to excite and detect fluorescence at multiple wavelengths allows the simultaneous detection of multiple fluorescent markers. Additional fluorescent markers provide a means of interrogating multiple aspects of a pathology, simultaneously, or could be used, as in this study, to employ more quantitative imaging approaches such as dual-reporter methods of measuring in vivo binding potential, a marker of receptor density 26,27 .…”

“…The tumor was made to fluoresce by injecting a fluorescent tracer, IRDye 800CW-EGF (LI-COR Biosciences, Lincoln, NE) targeted to epidermal growth factor receptor, a cell membrane protein known to be overexpressed in the U251 tumor line and many other cancers 18 . A second, untargeted fluorescent tracer, Alexa Fluor 647 (Life Technologies, Grand Island, NY) was also injected to account for non-receptor mediated effects on the uptake of the targeted tracers to provide a means of quantifying tracer binding and receptor availability/density 27 . A CT-guided, time-domain algorithm was used to reconstruct the location of both fluorescent tracers (i.e., the location of the tumor) in the mouse brain and their ability to localize the tumor was verified by contrast-enhanced magnetic resonance imaging.…”