In Vivo Imaging of Transplanted Islets with ^(64)Cu-DO3A-VS-Cys^(40)-Exendin-4 by Targeting GLP-1 Receptor

Glucagon-like peptide 1 receptor (GLP-1R) is highly expressed in pancreatic islets, especially on β-cells. Therefore, a properly labeled ligand that binds to GLP-1R could be used for in vivo pancreatic islet imaging. Because native GLP-1 is degraded rapidly by dipeptidyl peptidase-IV (DPP-IV), a more stable agonist of GLP-1 such as Exendin-4 is a preferred imaging agent. In this study, DO3A-VS-Cys^(40)-Exendin-4 was prepared through the conjugation of DO3A-VS with Cys^(40)-Exendin-4. The in vitro binding affinity of DO3A-VS-Cys^(40)-Exendin-4 was evaluated in INS-1 cells, which overexpress GLP-1R. After ^(64)Cu labeling, biodistribution studies and microPET imaging of ^(64)Cu-DO3A-VS-Cys^(40)-Exendin-4 were performed on both subcutaneous INS-1 tumors and islet transplantation models. The subcutaneous INS-1 tumor was clearly visualized with microPET imaging after the injection of ^(64)Cu-DO3A-VS-Cys^(40)-Exendin-4. GLP-1R positive organs, such as pancreas and lung, showed high uptake. Tumor uptake was saturable, reduced dramatically by a 20-fold excess of unlabeled Exendin-4. In the intraportal islet transplantation models, ^(64)Cu-DO3A-VS-Cys^(40)-Exendin-4 demonstrated almost two times higher uptake compared with normal mice. ^(64)Cu-DO3A-VS-Cys^(40)-Exendin-4 demonstrated persistent and specific uptake in the mouse pancreas, the subcutaneous insulinoma mouse model, and the intraportal human islet transplantation mouse model. This novel PET probe may be suitable for in vivo pancreatic islets imaging in the human

Quantitative, Simultaneous PET/MRI for Intratumoral Imaging with an MRI-Compatible PET Scanner

Noninvasive methods are needed to explore the heterogeneous tumor microenvironment and its modulation by therapy. Hybrid PET/MRI systems are being developed for small-animal and clinical use. The advantage of these integrated systems depends on their ability to provide MR images that are spatially coincident with simultaneously acquired PET images, allowing combined functional MRI and PET studies of intratissue heterogeneity. Although much effort has been devoted to developing this new technology, the issue of quantitative and spatial fidelity of PET images from hybrid PET/MRI systems to the tissues imaged has received little attention. Here, we evaluated the ability of a first-generation, small-animal MRI-compatible PET scanner to accurately depict heterogeneous patterns of radiotracer uptake in tumors. Methods: Quantitative imaging characteristics of the MRI-compatible PET (PET/MRI) scanner were evaluated with phantoms using calibration coefficients derived from a mouse-sized linearity phantom. PET performance was compared with a commercial small-animal PET system and autoradiography in tumor-bearing mice. Pixel and structure-based similarity metrics were used to evaluate image concordance among modalities. Feasibility of simultaneous PET/MRI functional imaging of tumors was explored by following ^(64)Cu-labeled antibody uptake in relation to diffusion MRI using cooccurrence matrix analysis. Results: The PET/MRI scanner showed stable and linear response. Activity concentration recovery values (measured and true activity concentration) calculated for 4-mm-diameter rods within linearity and uniform activity rod phantoms were near unity (0.97 ± 0.06 and 1.03 ± 0.03, respectively). Intratumoral uptake patterns for both ^(18)F-FDG and a ^(64)Cu-antibody acquired using the PET/MRI scanner and small-animal PET were highly correlated with autoradiography (r > 0.99) and with each other (r = 0.97 ± 0.01). On the basis of these data, we performed a preliminary study comparing diffusion MRI and radiolabeled antibody uptake patterns over time and visualized movement of antibodies from the vascular space into the tumor mass. Conclusion: The MRI-compatible PET scanner provided tumor images that were quantitatively accurate and spatially concordant with autoradiography and the small-animal PET examination. Cooccurrence matrix approaches enabled effective analysis of multimodal image sets. These observations confirm the ability of the current simultaneous PET/MRI system to provide accurate observations of intratumoral function and serve as a benchmark for future evaluations of hybrid instrumentation

Micro-PET Imaging of αβ-Integrin Expression with F-Labeled Dimeric RGD Peptide

The α v integrins, which act as cell adhesion molecules, are closely involved with tumor invasion and angiogenesis. In particular, α v β 3 integrin, which is specifically expressed on proliferating endothelial cells and tumor cells, is a logical target for development of a radiotracer method to assess angiogenesis and anti-angiogenic therapy. In this study, a dimeric cyclic RGD peptide E[c(RGDyK)] 2 was labeled with 18 F ( t 1 /2 = 109.7 min) by using a prosthetic 4-[ 18 F]fluorobenzoyl moiety to the amino group of the glutamate. The resulting [ 18 F]FB-E[c(RGDyK)] 2 , with high specific activity (200–250 GBq/μmol at the end of synthesis), was administered to subcutaneous U87MG glioblastoma xenograft models for micro-PET and autoradiographic imaging as well as direct tissue sampling to assess tumor targeting efficacy and in vivo kinetics of this PET tracer. The dimeric RGD peptide demonstrated significantly higher tumor uptake and prolonged tumor retention in comparison with a monomeric RGD peptide analog [ 18 F]FB-c(RGDyK). The dimeric RGD peptide had predominant renal excretion, whereas the monomeric analog was excreted primarily through the biliary route. Micro-PET imaging 1 hr after injection of the dimeric RGD peptide exhibited tumor to contralateral background ratio of 9.5 ± 0.8. The synergistic effect of polyvalency and improved pharmacokinetics may be responsible for the superior imaging characteristics of [ 18 F]FB-E[c(RGDyK)] 2

Quantitative serial imaging of an I-124 anti-CEA monoclonal antibody in tumor-bearing mice

Objective: The 4.2-day half-life I-124 favors its use for positron emission tomography (PET) of monoclonal antibodies (mAbs). However, high positron energy and beta(+)-associated cascade gamma rays pose image resolution and background noise problems for I-124. This study evaluated quantitative PET of an I-124 mAb in tumor-bearing mice. Methods: An R4 microPET (TM) (Siemens/CTIMI, Knoxville, TN) was used with standard energy and coincidence timing windows (350-750 keV and 6 ns, respectively), delayed random coincidence subtraction, iterative image reconstruction, and no attenuation or scatter correction. Image resolution, contrast, and response linearity were compared for I-124 and F-18, using phantoms. Nude mice bearing human colon tumors (LS-174T) were injected intravenously with a chimeric I-124 anti-CEA mAb (cT84.66) and imaged serially 1 hour to 7 clays postinjection. Venous blood was sampled to validate image-derived blood curves. Mice were sacrificed after the final scan, and the biodistribution of I-124 was measured by direct tissue assay. Images were converted to units of kBq/g for each tissue of interest by comparing the final scans with the direct assays. Results: Measured resolution (FWHM) 0-16 mm from? the scanner axis was. 2.3-2.7 mm for I-124 versus 1.9-2.0 mm for F-18. Due to true coincidence e vents between annihilation photons and cascade gamma rays, background was greater for I-124 than F-18, but the signal-to-background ratio was still more than 20, and I-124 image intensities varied linearly with activity concentration. Tissue-based calibration worked well (i.e., PET blood curves agreed with direct measurements within 12% at all time points), while calibration, based on a cylindrical phantom approximating the mouse body, yielded tumor quantitation that was 46%-66% low, compared with direct assay. Conclusions: Images of quantitative accuracy sufficient for biodistribution. measurements can be obtained from tumor-bearing mice by using I-124 anti-CEA mAbs with standard. microPET acquisition and processing techniques, provided the calibration is based on the direct assay of excised tissue samples

Integrin αβ3-Targeted Imaging of Lung Cancer

A series of radiolabeled cyclic arginine-glycineaspartic acid (RGD) peptide ligands for cell adhesion molecule integrin αβ3-targeted tumor angiogenesis targeting are being developed in our laboratory. In this study, this effort continues by applying a positron emitter 64Cu-labeled PEGylated dimeric RGD peptide radiotracer 64Cu-DOTA-PEG-E[c(RGDyK)]2 for lung cancer imaging. The PEGylated RGD peptide indicated integrin αβ3 avidity, but the PEGylation reduced the receptor binding affinity of this ligand compared to the unmodified RGD dimer. The radiotracer revealed rapid blood clearance and predominant renal clearance route. The minimum nonspecific activity accumulation in normal lung tissue and heart rendered high-quality orthotopic lung cancer tumor images, enabling clear demarcation of both the primary tumor at the upper lobe of the left lung, as well as metastases in the mediastinum, contralateral lung, diaphragm. As a comparison, fluorodeoxyglucose (FDG) scans on the same mice were only able to identify the primary tumor, with the metastatic lesions masked by intense cardiac uptake and high lung background. 64Cu-DOTA-PEGE[c(RGDyK)]2 is an excellent positron emission tomography (PET) tracer for integrin-positive tumor imaging. Further studies to improve the receptor binding affinity of the tracer and subsequently to increase the magnitude of tumor uptake without comprising the favorable in vivo kinetics are currently in progress