Intraoperative imaging of positron emission tomographic radiotracers using Cerenkov luminescence emissions

Abstract

Imaging the location and extent of cancer provides invaluable information before, during, and after surgery. The majority of "image-guided" methods that use, for example, positron emission tomography (PET) involve preoperative imaging and do not provide real-time information during surgery. It is now well established that the inherent optical emissions (Cerenkov radiation) from various β-emitting radionuclides can be visualized by Cerenkov luminescence imaging (CLI). Here we report the full characterization of CLI using the positron-emitting radiotracer 89Zr-DFO-trastuzumab for target-specific, quantitative imaging of HER2/neu-positive tumors in vivo. We also provide the first demonstration of the feasibility of using CLI for true image-guided, intraoperative surgical resection of tumors. Analysis of optical CLIs provided accurate, quantitative information on radiotracer biodistribution and tissue uptake that correlated well with the concordant PET images. CLI, PET, and biodistribution studies revealed target-specific uptake of 89Zr-DFO-trastuzumab in BT-474 (HER2/neu positive) versus MDA-MB-468 (HER2/neu negative) xenografts in the same mice. Competitive inhibition (blocking) studies followed by CLI also confirmed the in vivo immunoreactivity and specificity of 89Zr-DFO-trastuzumab for HER2/neu. Overall, these results strongly support the continued development of CLI as a preclinical and possible clinical tool for use in molecular imaging and surgical procedures for accurately defining tumor margins.

Figure 1

Temporal images of ⁸⁹Zr-DFO-trastuzumab uptake (4.07–4.26 MBq [110–115 μCi], 23–24 μg mAb; 200 μL sterile saline) recorded in dual BT-474 (HER2/neu positive; lower right flank) and MDA-MB-468 (HER2/neu negative; lower left flank) tumor-bearing mice between 3 and 144 hours postadministration. A, Optical CLI signal observed for ⁸⁹Zr-DFO-trastuzumab tumor uptake in four normal mice (left panel) and four mice treated with a blocking dose (125 μg/mouse) of nonradiolabeled trastuzumab (see Figure S1 for control optical images). Corresponding temporal maximum intensity projection (MIP) PET images of two representative mice from the (B) normal (mouse 1) and (C) blocking (mouse 7) groups. The upper and lower thresholds of the CLIs and PET images have been adjusted for visual clarity, as indicated by the scale bars. For the MIP images, the higher and lower intensity thresholds were set at 100% and 15%, respectively.

Figure 2

Time-activity curves (TACs) showing change in ⁸⁹Zr-DFO-trastuzumab accumulation in BT-474 and MDA-MB-468 tumors in both normal and trastuzumab-treated mice. TACs are derived from region-of-interest (ROI) and volume-of-interest (VOI) analysis of the CLIs and PET images, respectively. (A) Average and (B) maximum radiance (p/s/cm²/sr) versus time for the optical CLI data. (C) Mean and (D) maximum %ID/g tumor uptake of ⁸⁹Zr-DFO-trastuzumab as measured from the PET image data.

Figure 3

Scatterplots showing the correlation between the observed background and decay-corrected CLI optical signal intensity and the quantitative decay-corrected PET image data for (A) the average radiance and mean %ID/g and (B) the maximum radiance and maximum %ID/g data. Each scatterplot contains image data recorded at 3, 24, 48, 96, and 144 hours postinjection of the radiotracer for all normal and trastuzumab-treated (block) groups of mice (total n = 8 mice; 8 BT-474 and 8 MDA-MB-468 tumors; 5 time points).

Figure 4

Bar chart showing selected tissue biodistribution data (%ID/g) for the uptake of ⁸⁹Zr-DFO-trastuzumab in female, athymic nu/nu mice at the end of the optical CLI and PET imaging experiments (144 hours postinjection). *p < .05.

Figure 5

Intraoperative optical CLI of mouse 4 during surgical resection of the BT-474 (HER2/neu positive) tumor at 144 hours postadministration of ⁸⁹Zr-DFO-trastuzumab. A, Background CLI of the scanner bed recorded immediately prior to commencing surgery. B, Preoperative optical CLI of mouse 4 prior to surgical incision. C, Intraoperative optical CLI of the exposed tumor immediately prior to resection. Note the increased intensity of the CLI signal owing to reduced attenuation and scattering from removal of the skin. D, Resected tumor (upper left corner) and the exposed incision site showing the complete loss of CLI signal in the exposed region of the mouse. E, CLI of the excised BT-474 tumor alone. F, Postoperative CLI of the mouse after closing the incision site with sutures. All CLIs are shown at the same radiance scale for direct quantitative comparison.