Noninvasive quantification of target availability during therapy using paired-agent fluorescence tomography

Abstract

Immuno-oncological treatment strategies that target abnormal receptor profiles of tumors are an increasingly important feature of cancer therapy. Yet, assessing receptor availability (RA) and drug-target engagement, important determinants of therapeutic efficacy, is challenging with current imaging strategies, largely due to the complex nonspecific uptake behavior of imaging agents in tumors. Herein, we evaluate whether a quantitative noninvasive imaging approach designed to compensate for nonspecific uptake, MRI-coupled paired-agent fluorescence tomography (MRI-PAFT), is capable of rapidly assessing the availability of epidermal growth factor receptor (EGFR) in response to one dose of anti-EGFR antibody therapy in orthotopic brain tumor models. Methods: Mice bearing orthotopic brain tumor xenografts with relatively high EGFR expression (U251) (N=10) or undetectable human EGFR (9L) (N=9) were considered in this study. For each tumor type, mice were either treated with one dose of cetuximab, or remained untreated. All animals were scanned using MRI-PAFT, which commenced immediately after paired-agent administration, and values of RA were recovered using a model-based approach, which uses the entire dynamic sequence of agent uptake, as well as a simplified “snapshot” approach which requires uptake measurements at only two time points. Recovered values of RA were evaluated between groups and techniques. Hematoxylin & eosin (H&E) and immunohistochemical (IHC) staining was performed on tumor specimens from every animal to confirm tumor presence and EGFR status. Results: In animals bearing EGFR(+) tumors, a significant difference in RA values between treated and untreated animals was observed (RA = 0.24 ± 0.15 and 0.61 ± 0.18, respectively, p=0.027), with an area under the curve - receiver operating characteristic (AUC-ROC) value of 0.92. We did not observe a statistically significant difference in RA values between treated and untreated animals bearing EGFR(-) tumors (RA = 0.18 ± 0.19 and 0.27 ± 0.21, respectively; p = 0.89; AUC-ROC = 0.55), nor did we observe a difference between treated EGFR(+) tumors compared to treated and untreated EGFR(-) tumors. Notably, the snapshot paired-agent strategy quantified drug-receptor engagement within just 30 minutes of agent administration. Examination of the targeted agent alone showed no capacity to distinguish tumors either by treatment or receptor status, even 24h after agent administration. Conclusions: This study demonstrated that a noninvasive imaging strategy enables rapid quantification of receptor availability in response to therapy, a capability that could be leveraged in preclinical drug development, patient stratification, and treatment monitoring.

Examining the Feasibility of Quantifying Receptor Availability Using Cross-Modality Paired-Agent Imaging

“…Even at these long time points, a significant proportion of the retained agent in the tumor can be due to nonspecific retention and thus is generally not considered quantitative. By contrast, paired-agent techniques can produce high-contrast, quantitative images of specific binding within minutes of administration [ 11 ]. This feature could be particularly impactful for radionuclides which lose signal strength through decay.…”

“…This study represents an extended analysis of an earlier study that evaluated single-modality PAI using the same animals [ 11 ]. As such, agent and animal preparation and fluorescence tomographic image reconstruction procedures are only briefly summarized here and referenced accordingly.…”

“…The MRI-coupled fluorescence tomography (FMT) system has been described in previous publications [10,11,[25][26][27][28]. Briefly, the instrument consists of a clinical Philips Achieva 3.0 T scanner and a spectrometer-based FMT system that includes eight optical fibers extending into the bore of the MRI scanner and positioned around the head of the animal using a custom rodent optical/RF coil.…”

“…The general PAI framework involves imaging the dynamic behavior of a pair of contrast agents (often co-administered): one targeted to the receptor of interest and the other a non-targeted isotype [8,9]. RA can be estimated by either fitting a kinetic model to time-activity curves or applying ratio-based mathematical operations, distinguishing the specific agent-receptor signal from the confounding non-specific accumulation and delivery effects exhibited by the targeted agent [8][9][10][11].…”

“…To date, PAI has been developed and validated primarily using optical imaging of two spectrally distinct fluorescently labeled agents, largely due to the modality's capacity to separate multiple colocalized agents with different fluorescent labels [10]. Optical imaging is exceptionally well-suited to image superficial tissues, such as exposed tumors in preclinical models or fluorescence-guided surgery applications in humans, and we have previously reported on using fluorescence tomography-based PAI to estimate RA and drug-receptor engagement in subsurface tumor xenografts in mice [10][11][12][13]. Yet, even with sophisticated tomographic techniques, the high scattering behavior of visible and nearvisible photons in tissue limits optical PAI applicability in human tissue volumes greater than a few centimeters in diameter.…”