Noninvasive Evaluation of Metabolic Tumor Volume in Lewis Lung Carcinoma Tumor-Bearing C57BL/6 Mice with Micro-PET and the Radiotracers 18F-Alfatide and 18F-FDG: A Comparative Analysis

Preliminary Clinical Application of RGD-Containing Peptides as PET Radiotracers for Imaging Tumors

Angiogenesis is a common feature of many physiological processes and pathological conditions. RGD-containing peptides can strongly bind to integrin αvβ3 expressed on endothelial cells in neovessels and several tumor cells with high specificity, making them promising molecular agents for imaging angiogenesis. Although studies of RGD-containing peptides combined with radionuclides, namely, ¹⁸F, ⁶⁴Cu, and ⁶⁸Ga for positron emission tomography (PET) imaging have shown high spatial resolution and accurate quantification of tracer uptake, only a few of these radiotracers have been successfully translated into clinical use. This review summarizes the RGD-based tracers in terms of accumulation in tumors and adjacent tissues, and comparison with traditional ¹⁸F-fluorodeoxyglucose (FDG) imaging. The value of RGD-based tracers for diagnosis, differential diagnosis, tumor subvolume delineation, and therapeutic response prediction is mainly discussed. Very low RGD accumulation, in contrast to high FDG metabolism, was found in normal brain tissue, indicating that RGD-based imaging provides an excellent tumor-to-background ratio for improved brain tumor imaging. However, the intensity of the RGD-based tracers is much higher than FDG in normal liver tissue, which could lead to underestimation of primary or metastatic lesions in liver. In multiple studies, RGD-based imaging successfully realized the diagnosis and differential diagnosis of solid tumors and also the prediction of chemoradiotherapy response, providing complementary rather than similar information relative to FDG imaging. Of most interest, baseline RGD uptake values can not only be used to predict the tumor efficacy of antiangiogenic therapy, but also to monitor the occurrence of adverse events in normal organs. This unique dual predictive value in antiangiogenic therapy may be better than that of FDG-based imaging.

18F-RGD PET/CT imaging reveals characteristics of angiogenesis in non-small cell lung cancer

Background

The objective of this study was to explore the benefit of ¹⁸F-AlF-NOTA-PRGD2 positron emission tomography/computed tomography (denoted as ¹⁸F-RGD PET/CT) imaging for determining the clinical pathologic features of non-small cell lung cancer (NSCLC).

Methods

Seventy-two patients with NSCLC (37 cases of adenocarcinoma and 35 cases of squamous carcinoma) were enrolled to receive ¹⁸F-RGD PET/CT scanning pretreatment. The peak standard uptake value (SUV_peak), mean standard uptake value (SUV_mean), angiogenic tumor volume (ATV) and total lesion angiogenesis (TLA) of tumors were determined using an automated contouring program. Cases were classified according to the tumor, lymph node, metastasis (TNM) stage.

Results

Significant differences in ATV and TLA were observed among T1, T2, T3 and T4 cases (ATV, P=0.000; TLA, P=0.000). ATV and TLA also differed significantly among cases of clinical stage I, II, III and IV (ATV, P=0.002; TLA, P=0.011). However, no significant differences in any values were observed between stage III and IV NSCLC cases (SUV_peak, P=0.675; SUV_mean, P=0.668; ATV, P=0.52; TLA, P=0.634). All assessed values were higher in squamous cell carcinoma cases than in adenocarcinoma cases (SUV_peak, P=0.045; SUV_mean, P=0.014; ATV, P=0.003; TLA, P=0.001). For clinical stage III and IV cases specifically, SUV_peak, SUV_mean, and TLA were higher for squamous cell carcinoma than for adenocarcinoma (SUV_peak, P=0.015; SUV_mean, P=0.009; TLA, P=0.036).

Conclusions:¹⁸F-RGD PET/CT imaging revealed the presence of increased angiogenesis in the tumor microenvironment of NSCLC, especially squamous cell carcinoma, and thus may be valuable in planning therapeutic regimens for individual patients.

A Comprehensive Review of Non-Covalent Radiofluorination Approaches Using Aluminum [18F]fluoride: Will [18F]AlF Replace 68Ga for Metal Chelate Labeling?

Due to its ideal physical properties, fluorine-18 turns out to be a key radionuclide for positron emission tomography (PET) imaging, for both preclinical and clinical applications. However, usual biomolecules radiofluorination procedures require the formation of covalent bonds with fluorinated prosthetic groups. This drawback makes radiofluorination impractical for routine radiolabeling, gallium-68 appearing to be much more convenient for the labeling of chelator-bearing PET probes. In response to this limitation, a recent expansion of the ¹⁸F chemical toolbox gave aluminum [¹⁸F]fluoride chemistry a real prominence since the late 2000s. This approach is based on the formation of an [¹⁸F][AlF]²⁺ cation, complexed with a 9-membered cyclic chelator such as NOTA, NODA or their analogs. Allowing a one-step radiofluorination in an aqueous medium, this technique combines fluorine-18 and non-covalent radiolabeling with the advantage of being very easy to implement. Since its first reports, [¹⁸F]AlF radiolabeling approach has been applied to a wide variety of potential PET imaging vectors, whether of peptidic, proteic, or small molecule structure. Most of these [¹⁸F]AlF-labeled tracers showed promising preclinical results and have reached the clinical evaluation stage for some of them. The aim of this report is to provide a comprehensive overview of [¹⁸F]AlF labeling applications through a description of the various [¹⁸F]AlF-labeled conjugates, from their radiosynthesis to their evaluation as PET imaging agents.