Quantitative analysis and comparison study of [18F]AlF-NOTA-PRGD2, [18F]FPPRGD2 and [68Ga]Ga-NOTA-PRGD2 using a reference tissue model

Noninvasive assessment of liver fibrosis in mini pigs using an 18F-AlF-NOTA-RGD2 PET/CT molecular probe

To evaluate the efficacy of the 18F-AlF-NOTA-RGD2 positron emission tomography (PET)/computed tomography (CT) molecular probe for the noninvasive staging of liver fibrosis in mini pigs, a potential alternative to invasive diagnostic methods was revealed. This study used 18F-AlF-NOTA-RGD2 PET/CT imaging of mini pigs to assess liver fibrosis. The methods included synthesis and quality control of the molecular probe, establishment of an animal model of liver fibrosis, blood serum enzymatic tests, histopathological examination, PET/CT imaging, collagen content and expression, and mitochondrial reserve function assessment. The 18F-AlF-NOTA-RGD2 PET/CT molecular probe effectively differentiated various stages of liver fibrosis in mini pigs. Blood serum enzymatic tests revealed distinct stages of liver fibrosis, revealing significant increases in AST, ALT, TBIL, and DBIL levels as fibrosis advanced. Notably, ALT levels increased markedly in severe fibrosis patients. A gradual increase in collagen deposition and increasing α-SMA RNA expression and protein levels effectively differentiated between mild and severe fibrosis stages. Pathological examinations and Sirius Red staining confirmed these findings, highlighting substantial increases in collagen accumulation. PET/CT imaging results aligned with histopathological findings, showing that increased radiotracer uptake correlated with fibrosis severity. Assessments of mitochondrial function revealed a decrease in total liver glutathione content and mitochondrial reserve capacity, especially in patients with severe fibrosis. The 18F-AlF-NOTA-RGD2 PET/CT molecular probe is a promising tool for the noninvasive assessment of liver fibrosis, offering potential benefits over traditional diagnostic methods in hepatology.

EGFR- and Integrin αVβ3-Targeting Peptides as Potential Radiometal-Labeled Radiopharmaceuticals for Cancer Theranostics

The burgeoning field of cancer theranostics has witnessed advancements through the development of targeted molecular agents, particularly peptides. These agents exploit the overexpression or mutations of specific receptors, such as the Epidermal Growth Factor receptor (EGFR) and αVβ3 integrin, which are pivotal in tumor growth, angiogenesis, and metastasis. Despite the extensive research into and promising outcomes associated with antibody-based therapies, peptides offer a compelling alternative due to their smaller size, ease of modification, and rapid bioavailability, factors which potentially enhance tumor penetration and reduce systemic toxicity. However, the application of peptides in clinical settings has challenges. Their lower binding affinity and rapid clearance from the bloodstream compared to antibodies often limit their therapeutic efficacy and diagnostic accuracy. This overview sets the stage for a comprehensive review of the current research landscape as it relates to EGFR- and integrin αVβ3-targeting peptides. We aim to delve into their synthesis, radiolabeling techniques, and preclinical and clinical evaluations, highlighting their potential and limitations in cancer theranostics. This review not only synthesizes the extant literature to outline the advancements in peptide-based agents targeting EGFR and integrin αVβ3 but also identifies critical gaps that could inform future research directions. By addressing these gaps, we contribute to the broader discourse on enhancing the diagnostic precision and therapeutic outcomes of cancer treatments.

Ultrasensitive detection of uveal melanoma using [18F]AlF-NOTA-PRGD2 PET imaging

BACKGROUND

Uveal melanoma (UM) is the most common primary intraocular tumor in adults, and early detection is critical to improve the clinical outcome of this disease. In this study, the diagnostic effectiveness of [18F]AlF-NOTA-PRGD2 (an investigational medicinal product) positron emission tomography (PET) imaging in UM xenografts and UM patients were evaluated. The cell uptake, cell binding ability and in vitro stability of [18F]AlF-NOTA-PRGD2 were evaluated in 92-1 UM cell line. MicroPET imaging and biodistribution study of [18F]AlF-NOTA-PRGD2 were conducted in 92-1 UM xenografts. Then, UM patients were further recruited for evaluating the diagnostic effectiveness of [18F]AlF-NOTA-PRGD2 PET imaging (approval no. NCT02441972 in clinicaltrials.gov). In addition, comparison of [18F]AlF-NOTA-PRGD2 and 18F-labelled fluorodeoxyglucose ([18F]FDG) PET imaging in UM xenografts and UM patients were conducted.

RESULTS

The in vitro data showed that [18F]AlF-NOTA-PRGD2 had a high cell uptake, cell binding ability and in vitro stability in 92-1 UM cell line. The in vivo data indicated that 92-1 UM tumors were clearly visualized with the [18F]AlF-NOTA-PRGD2 tracer in the subcutaneous and ocular primary UM xenografts model at 60 min post-injection. And the tumor uptake of the tracer was 2.55 ± 0.44%ID/g and 1.73 ± 0.15%ID/g at these two tissue locations respectively, at 7 days after animal model construction. The clinical data showed that tumors in UM patients were clearly visualized with the [18F]AlF-NOTA-PRGD2 tracer at 60 min post-injection. In addition, [18F]AlF-NOTA-PRGD2 tracer showed higher sensitivity and specificity for PET imaging in UM xenografts and UM patients compared to [18F]FDG tracer.

CONCLUSION

[18F]AlF-NOTA-PRGD2 PET imaging may be a more preferred approach in the diagnosis of primary UM compared to [18F]FDG PET imaging. Additionally, due to the high tumor-to-background ratio, [18F]AlF-NOTA-PRGD2 PET imaging seems also to be applicable for the diagnosis of UM patients with liver metastasis.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT02441972, Registered 1 January 2012, https://clinicaltrials.gov/study/NCT02441972 .

Comparison of 18 F-AlF-NOTA-PRGD2 and 18 F-FDG PET/CT Imaging in a Case of Primary Uveal Melanoma

ABSTRACT

Uveal melanoma is the most common intraocular malignancy in adults with a high rate of metastasis and mortality. This study presented the PET/CT imaging of 18 F-AlF-NOTA-PRGD2 and 18 F-FDG in a patient with primary uveal melanoma. In addition to fundus photograph and ophthalmic ultrasonography, both 18 F-AlF-NOTA-PRGD2 and 18 F-FDG PET/CT imaging showed increased radioactive uptake in the lesions within the scan area. The tumoral lesions presented significantly higher uptake of 18 F-AlF-NOTA-PRGD2 compared with that of 18 F-FDG.

Automated Synthesis and Preclinical Evaluation of Optimized Integrin α6-Targeted Positron Emission Tomography Imaging of Pancreatic Cancer

Integrin α6 has been considered a promising biomarker, is overexpressed in many tumors, and plays a vital role in tumor formation, recurrence, and metastasis. In this study, we identified a novel high-affinity integrin α6-targeted peptide named RD2 (Arg-Trp-Tyr-Asp-PEG4)2-Lys-Lys and developed a 18F-radiolabeled peptide tracer ([18F]-AlF-NOTA-RD2) and evaluated its potential application in positron emission tomography (PET) imaging of pancreatic cancer. [18F]-AlF-NOTA-RD2 was produced using GMP (Good Manufacturing Practice of Medical Products)-compliant automatic radiosynthesis on a single GE FASTLab2 cassette-type synthesis module. The stability of [18F]-AlF-NOTA-RD2 was analyzed in phosphate-buffered saline (PBS) and fetal bovine serum (FBS). The cell uptake assay of the tracer was assessed using PANC-1 cells. In addition, small-animal PET imaging and biodistribution studies of [18F]-AlF-NOTA-RD2 were performed in pancreatic cancer subcutaneous tumor-bearing mice. The PET tracer [18F]-AlF-NOTA-RD2 was obtained with a radiochemical yield of 23.7 ± 4.7%, radiochemical purity of >99%, and molar activity of 165.7 ± 59.1 GBq/μmol. [18F]-AlF-NOTA-RD2 exhibited good in vitro stability in PBS and FBS. LogP octanol water value for the tracer was -2.28 ± 0.05 (n = 3). The binding affinity of RD2 to the integrin α6 protein (Kd = 0.13 ± 3.65 μM, n = 3) was significantly higher than that of the RWY (CRWYDENAC) (Kd = 6.97 ± 1.44 μM, n = 3). Small-animal PET imaging and biodistribution also revealed that [18F]-AlF-NOTA-RD2 displayed rapid and good tumor uptake and lower liver background uptake in PANC-1 tumor-bearing mice. [18F]-AlF-NOTA-RD2 showed significant radioactivity accumulation in tumors and was successfully blocked by NOTA-RD2. Compared with [18F]-FDG, [18F]-AlF-NOTA-RD2 PET imaging and biodistribution studies in PANC-1 xenograft tumor-bearing mice confirmed a good tumor-to-muscle ratio (8.69 ± 2.03 vs 1.41 ± 0.23, respectively) at 0.5 h and (2.99 ± 3.02 vs 1.43 ± 0.17, respectively) at 1 h post injection. Autoradiography of human pancreatic cancer tumor tissues further confirmed high accumulation of [18F]-AlF-NOTA-RD2. In summary, we developed an optimized integrin α6-targeted imaging tracer and obtained high radioactivity products with a cassette-type synthesis module; moreover, the tracer exhibited good binding affinity with integrin α6 and good target specificity for PANC-1 cells in xenograft pancreatic tumor-bearing mice, demonstrating its promising application as a noninvasive PET radiotracer of integrin α6 expression in pancreatic cancer.

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.

The Feasibility of Targeted Magnetic Iron Oxide Nanoagent for Noninvasive IgA Nephropathy Diagnosis

IgA nephropathy is the most common glomerular disease in the world and has become a serious threat to human health. Accurate and non-invasive molecular imaging to detect and recognize the IgA nephropathy is critical for the subsequent timely treatment; otherwise, it may progress to end-stage renal disease and lead to glomerular dysfunction. In this study, we have developed a sensitive, specific, and biocompatible integrin αvβ3-targeted superparamagnetic Fe₃O₄ nanoparticles (NPs) for the noninvasive magnetic resonance imaging (MRI) of integrin αvβ3, which is overexpressed in glomerular mesangial region of IgA nephropathy. The rat model of IgA nephropathy was successfully established and verified by biochemical tests and histological staining. Meanwhile, the clinical ¹⁸F-AlF-NOTA-PRGD2 probe molecule was utilized to visualize and further confirmed the IgA nephropathy in vivo via positron emission computed tomography. Subsequently, the Fe₃O₄ NPs were conjugated with arginine–glycine–aspartic acid (RGD) molecules (Fe₃O₄-RGD), and their integrin αvβ3-targeted T2-weighted imaging (T2WI) potential has been carefully evaluated. The Fe₃O₄-RGD demonstrated great relaxation in vivo. The T2WI signal of renal layers in the targeted group at 3 h after intravenous injection of Fe₃O₄-RGD was distinctly lower than baseline, indicating MRI signal decreased in the established IgA nephropathy rat model. Moreover, the TEM characterization and Prussian blue staining confirmed that the Fe₃O₄-RGD was located at the region of glomerulus and tubular interstitium. Moreover, no obvious signal decreased was detected in the untargeted Fe₃O₄ treated and normal groups. Collectively, our results establish the possibility of Fe₃O₄-RGD serving as a feasible MRI agent for the noninvasive diagnosis of IgA nephropathy.

PET Diagnostic Molecules Utilizing Multimeric Cyclic RGD Peptide Analogs for Imaging Integrin αvβ3 Receptors

Multimeric ligands consisting of multiple pharmacophores connected to a single backbone have been widely investigated for diagnostic and therapeutic applications. In this review, we summarize recent developments regarding multimeric radioligands targeting integrin α_vβ₃ receptors on cancer cells for molecular imaging and diagnostic applications using positron emission tomography (PET). Integrin α_vβ₃ receptors are glycoproteins expressed on the cell surface, which have a significant role in tumor angiogenesis. They act as receptors for several extracellular matrix proteins exposing the tripeptide sequence arginine-glycine-aspartic (RGD). Cyclic RDG peptidic ligands c(RGD) have been developed for integrin α_vβ₃ tumor-targeting positron emission tomography (PET) diagnosis. Several c(RGD) pharmacophores, connected with the linker and conjugated to a chelator or precursor for radiolabeling with different PET radionuclides (¹⁸F, ⁶⁴Cu, and ⁶⁸Ga), have resulted in multimeric ligands superior to c(RGD) monomers. The binding avidity, pharmacodynamic, and PET imaging properties of these multimeric c(RGD) radioligands, in relation to their structural characteristics are analyzed and discussed. Furthermore, specific examples from preclinical studies and clinical investigations are included.

Small molecule drug conjugates (SMDCs): an emerging strategy for anticancer drug design and discovery

Mechanisms of how SMDCs work. Small molecule drugs are conjugated with the targeted ligand using pH sensitive linkers which allow the drug molecule to get released at lower lysosomal pH. It helps to accumulate the chemotherapeutic agents to be localized in the tumor environment upon cleaving of the pH-labile bonds.

Review: PET imaging with macro- and middle-sized molecular probes

Recent progress in radiolabeling of macro- and middle-sized molecular probes has been extending possibilities to use PET molecular imaging for dynamic application to drug development and therapeutic evaluation. Theranostics concept also accelerated the use of macro- and middle-sized molecular probes for sharpening the contrast of proper target recognition even the cellular types/subtypes and proper selection of the patients who should be treated by the same molecules recognition. Here, brief summary of the present status of immuno-PET, and then further development of advanced technologies related to immuno-PET, peptidic PET probes, and nucleic acids PET probes are described.

Quantitative evaluation of hepatic integrin αvβ3 expression by positron emission tomography imaging using 18F-FPP-RGD2 in rats with non-alcoholic steatohepatitis

Background

Integrin α_vβ₃, which are expressed by activated hepatic stellate cells in non-alcoholic steatohepatitis (NASH), play an important role in the fibrosis. Recently, we reported that an RGD peptide positron emission tomography (PET) probe is useful as a predictor of hepatic fibrosis. Kinetic analysis of the RGD PET probe has been performed in tumours, but not in hepatic fibrosis. Therefore, we aimed to quantify hepatic integrin α_vβ₃ in a model of NASH by kinetic analysis using ¹⁸F-FPP-RGD₂, an integrin α_vβ₃ PET probe.

Methods

¹⁸F-FPP-RGD₂ PET/CT scans were performed in control and NASH rats. Tissue kinetic analyses were performed using a one-tissue, two-compartment (1T2C) and a two-tissue, three-compartment (2T3C) model using an image-derived input function (IDIF) for the left ventricle. We then conducted correlation analysis between standard uptake values (SUVs) or volume of distribution (V_T), evaluated using compartment kinetic analysis and integrin α_v or β₃ protein expression.

Results

Biochemical and histological evaluation confirmed the development of NASH rats. Integrin α_vβ₃ protein expression and hepatic SUV were higher in NASH- than normal rats. The hepatic activity of ¹⁸F-FPP-RGD₂ peaked rapidly after administration and then gradually decreased, whereas left ventricular activity rapidly disappeared. The 2T3C model was found to be preferable for ¹⁸F-FPP-RGD₂ kinetic analysis in the liver. The V_{T (IDIF)} for ¹⁸F-FPP-RGD₂, calculated using the 2T3C model, was significantly higher in NASH- than normal rats and correlated strongly with hepatic integrin α_v and β₃ protein expression. The strengths of these correlations were similar to those between SUV_60–90 min and hepatic integrin α_v or β₃ protein expression.

Conclusions

We have demonstrated that the V_{T (IDIF)} of ¹⁸F-FPP-RGD₂, calculated using kinetic modelling, positively correlates with integrin α_v and β₃ protein in the liver of NASH rats. These findings suggest that hepatic V_{T (IDIF)} provides a quantitative assessment of integrin α_vβ₃ protein in liver.

[18F]-Alfatide PET imaging of integrin αvβ3 for the non-invasive quantification of liver fibrosis

BACKGROUND & AIMS

The vitronectin receptor integrin αvβ3 drives fibrogenic activation of hepatic stellate cells (HSCs). Molecular imaging targeting the integrin αvβ3 could provide a non-invasive method for evaluating the expression and the function of the integrin αvβ3 on activated HSCs (aHSCs) in the injured liver. In this study, we sought to compare differences in the uptake of [18F]-Alfatide between normal and injured liver to evaluate its utility for assessment of hepatic fibrogenesis.

METHODS

PET with [18F]-Alfatide, non-enhanced CT, histopathology, immunofluorescence staining, immunoblotting and gene analysis were performed to evaluate and quantify hepatic integrin αvβ3 levels and liver fibrosis progression in mouse models of fibrosis (carbon tetrachloride [CCl4] and bile duct ligation [BDL]). The liver AUC divided by the blood AUC over 30 min was used as an integrin αvβ3-PET index to quantify fibrosis progression. Ex vivo analysis of frozen liver tissue from patients with fibrosis and cirrhosis verified the animal findings.

RESULTS

Fibrotic mouse livers showed enhanced [18F]-Alfatide uptake and retention compared to control livers. The radiotracer was demonstrated to bind specifically with integrin αvβ3, which is mainly expressed on aHSCs. Autoradiography and histopathology confirmed the PET imaging results. Further, the mRNA and protein level of integrin αvβ3 and its signaling complex were higher in CCl4 and BDL models than controls. The results obtained from analyses on human fibrotic liver sections supported the animal findings.

CONCLUSIONS

Imaging hepatic integrin αvβ3 with PET and [18F]-Alfatide offers a potential non-invasive method for monitoring the progression of liver fibrosis.

LAY SUMMARY

Integrin αvβ3 expression on activated hepatic stellate cells (aHSCs) is associated with HSC proliferation during hepatic fibrogenesis. Herein, we show that a radioactive tracer, [18F]-Alfatide, binds to integrin αvβ3 with high affinity and specificity. [18F]-Alfatide could thus be used as a non-invasive imaging biomarker to track hepatic fibrosis progression.

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.

Rapid one-step 18F-radiolabeling of biomolecules in aqueous media by organophosphine fluoride acceptors

Currently, only a few ¹⁸F-radiolabeling methods were conducted in aqueous media, with non-macroelement fluoride acceptors and stringent conditions required. Herein, we describe a one-step non-solvent-biased, room-temperature-driven ¹⁸F-radiolabeling methodology based on organophosphine fluoride acceptors. The high water tolerance for this isotope-exchange-based ¹⁸F-labeling method is attributed to the kinetic and thermodynamic preference of F/F over the OH/F substitution based on computational calculations and experimental validation. Compact [^18/19F]di-tert-butyl-organofluorophosphine and its derivatives used as ¹⁸F-labeling synthons exhibit excellent stability in vivo. The synthons are further conjugated to several biomolecular ligands such as c(RGDyk) and human serum albumin. The one-step labeled biomolecular tracers demonstrate intrinsic target imaging ability and negligible defluorination in vivo. The current method thus offers a facile and efficient ¹⁸F-radiolabeling pathway, enabling further widespread application of ¹⁸F.

The synthesis of ¹⁸F-labeled positron emission tomography (PET) tracers is difficult and typically requires anhydrous conditions. Here, the authors developed organophosphine precursors that allow for quick, high-yield synthesis of ¹⁸F-labeled probes in either organic solvents or aqueous media.

18F-AlF Labeled Peptide and Protein Conjugates as Positron Emission Tomography Imaging Pharmaceuticals

The clinical applications of positron emission tomography (PET) imaging pharmaceuticals have increased tremendously over the past several years since the approval of ¹⁸fluorine-fluorodeoxyglucose (¹⁸F-FDG) by the Food and Drug Administration (FDA). Numerous ¹⁸F-labeled target-specific potential imaging pharmaceuticals, based on small and large molecules, have been evaluated in preclinical and clinical settings. ¹⁸F-labeling of organic moieties involves the introduction of the radioisotope by C-¹⁸F bond formation via a nucleophilic or an electrophilic substitution reaction. However, biomolecules, such as peptides, proteins, and oligonucleotides, cannot be radiolabeled via a C-¹⁸F bond formation as these reactions involve harsh conditions, including organic solvents, high temperature, and nonphysiological conditions. Several approaches, including ¹⁸F-labeled prosthetic groups, silicon, boron, and aluminum fluoride acceptor chemistry, and click chemistry have been developed, in the past, for ¹⁸F labeling of biomolecules. Linear and macrocyclic polyaminocarboxylates and their analogs and derivatives form thermodynamically stable and kinetically inert aluminum chelates. Hence, macrocyclic polyaminocarboxylates have been used for conjugation with biomolecules, such as folate, peptides, affibodies, and protein fragments, followed by ¹⁸F-AlF chelation, and evaluation of their targeting abilities in preclinical and clinical environments. The goal of this report is to provide an overview of the ¹⁸F radiochemistry and ¹⁸F-labeling methodologies for small molecules and target-specific biomolecules, a comprehensive review of coordination chemistry of Al³⁺, ¹⁸F-AlF labeling of peptide and protein conjugates, and evaluation of ¹⁸F-labeled biomolecule conjugates as potential imaging pharmaceuticals.

Imaging the expression of glypican-3 in hepatocellular carcinoma by PET

The glypican-3 (GPC3) receptor is overexpressed in hepatocellular carcinoma (HCC) and is a potential diagnostic and therapeutic target. GPC3-targeted molecular imaging will be helpful to differentiate diagnosis and guide therapy. In the present study, we will develop a novel PET probe for imaging the expression of GPC-3. L5 (sequence: RLNVGGTYFLTTRQ), a GPC3 targeting peptide, was labeled with 5-carboxyfluorescein (FAM) and ¹⁸F-fluoride. Cell binding tests were performed to identify the binding specificity of FAM-L5 and ¹⁸F radiolabeled peptide. MicroPET/CT imaging was used to determine the potential of a novel PET tracer for visualizing HCC tumors with a high expression of GPC3. In vitro binding tests showed that the uptake of FAM-L5 in HepG2 cells (high expression of GPC3) was significantly higher than that of HL-7702 cells (negative expression of GPC3) (mean fluorescent intensity: 14,094 ± 797 vs. 2765 ± 314 events, t = 32.363, P = 0.000). Confocal fluorescent imaging identified that FAM-L5 accumulated where the GPC3 receptor was located. A novel PET tracer (¹⁸F-AlF-NODA-MP-6-Aoc-L5) was successfully labeled by chelation chemistry. In vitro cell uptake studies showed that ¹⁸F-AlF-NODA-MP-6-Aoc-L5 can bind to HepG2 tumor cells and was stable in PBS and mouse serum stability tests. MicroPET/CT showed that HepG2 tumors could be clearly visualized with a tumor/muscle ratio of 2.46 ± 0.53. However, the tumor/liver ratio was low (0.93 ± 0.16) due to the high physiological uptake in the liver. This study demonstrates that FAM and the ¹⁸F-labeled L5 peptide can selectively target HCC with a high expression of GPC3 in vitro and in vivo. ¹⁸F-AlF-NODA-MP-C6-L5 has the potential to be a GPC3 target tracer but requires some chemical modifications to achieve a high enough tumor/liver ratio for detection of the tumor in the liver.

Imaging angiogenesis using 68Ga-NOTA-PRGD2 positron emission tomography/computed tomography in patients with severe intracranial atherosclerotic disease

Angiogenesis is a critical compensation route, which has been demonstrated in the brain following ischemic stroke; however, few studies have investigated angiogenesis in chronic intracranial atherosclerosis disease (ICAD). We used ⁶⁸Ga-NOTA-PRGD2 positron emission tomography/computed tomography based imaging to detect angiogenesis in chronic ICAD and to explore the factors that may have affected it. A total of 21 participants with unilateral severe chronic ICAD were included in the study. Of the 21 participants, 19 were men; the mean (SD) age was 52 (15) years. In 18 participants, we observed elevated ⁶⁸Ga-NOTA-PRGD2 uptake in the peri-infarct, subcortical, and periventricular regions of the lesioned side, with a higher ⁶⁸Ga-NOTA-PRGD2 SUV_max compared to that in the contralateral hemisphere (0.15 vs. 0.06, p=0.001). The ¹⁸F-FDG PET SUV_max was significantly lower on the lesioned side (11.28 vs. 13.92, p=0.001). Subgroup analyses revealed that the recent group (<6 months) had a higher lesion-to-contralateral region ratio SUV_max than the remote group (>6 months) (6.73 vs. 2.36, p<0.05). Our results provide molecular imaging evidence of angiogenesis in patients with severe chronic ICAD. Furthermore, the extent of angiogenesis in chronic ICAD may be affected by the post-qualified event time interval, and not by infarction itself or the severity of the arterial lesion.

Angiogenesis Imaging Using (68)Ga-RGD PET/CT: Therapeutic Implications

Angiogenesis imaging is important for diagnostic and therapeutic treatment of various malignant and nonmalignant diseases. The Arg-Gly-Asp (RGD) sequence has been known to bind with the αvβ3 integrin that is expressed on the surface of angiogenic blood vessels or tumor cells. Thus, various radiolabeled derivatives of RGD peptides have been developed for angiogenesis imaging. Among the various radionuclides, (68)Ga was the most widely studied for RGD peptide imaging because of its excellent nuclear physical properties, easy-to-label chemical properties, and cost-effectiveness owing to the availability of a (68)Ge-(68)Ga generator. Thus, various (68)Ga-labeled RGD derivatives have been developed and applied for preclinical and clinical studies. Clinical trials were performed for both malignant and nonmalignant diseases. Breast cancer, glioma, and lung cancer were malignant, and myocardial infarction, atherosclerosis, and moyamoya disease were nonmalignant among the investigated diseases. Further, these (68)Ga-labeled RGD derivatives could be applied to assess the effects of antiangiogenic treatment or theragnosis or both, of cancers. In conclusion, the angiogenesis imaging technology using (68)Ga-labeled RGD derivatives might be useful for the development of new therapeutic assessments, and for diagnostic and theragnostic applications.

Quantitative Evaluation of Tumor Early Response to a Vascular-Disrupting Agent with Dynamic PET

PURPOSE

The purpose of this study is to evaluate the early response of tumors to a vascular-disrupting agent (VDA) VEGF121/recombinant toxin gelonin (rGel) using dynamic [(18)F]FPPRGD2 positron emission tomography (PET) and kinetic parameter estimation.

PROCEDURES

Two tumor xenograft models: U87MG (highly vascularized) and A549 (moderately vascularized), were selected, and both were randomized into treatment and control groups. Sixty-minute dynamic PET scans with [(18)F]FPPRGD2 that targets to integrin αvβ3 were performed at days 0 (baseline), 1, and 3 since VEGF121/rGel treatment started. Dynamic PET-derived binding potential (BPND) and parametric maps were compared with tumor uptake (%ID/g) and the static PET image at 1 h after the tracer administration.

RESULTS

The growth of U87MG tumor was obviously delayed upon VEGF121/rGel treatment. A549 tumor was not responsive to the same treatment. BPND of treated U87MG tumors decreased significantly at day 1 (p < 0.05), and the difference was more significant at day 3 (p < 0.01), compared with the control group. However, the tracer uptake (%ID/g) derived from static images at 1-h time point did not show significant difference between the treated and control tumors until day 3. Little difference in tracer uptake (%ID/g) or BPND was found between treated and control A549 tumors. Considering the tracer retention in tumor and the slower clearance due to damaged tumor vasculature after treatment, BPND representing the actual specific binding portion appears to be more sensitive and accurate than the semiquantitative parameters (such as %ID/g) derived from static images to assess the early response of tumor to VDA treatment.

CONCLUSIONS

Quantitative analysis based on dynamic PET with [(18)F]FPPRGD2 shows advantages in distinguishing effective from ineffective treatment during the course of VEGF121/rGel therapy at early stage and is therefore more sensitive in assessing therapy response than static PET.

Investigation of injection dose and camera integration time on quantifying pharmacokinetics of a Cy5.5-GX1 probe with dynamic fluorescence imaging in vivo

The aim of this article is to investigate the influence of a tracer injection dose (ID) and camera integration time (IT) on quantifying pharmacokinetics of Cy5.5-GX1 in gastric cancer BGC-823 cell xenografted mice. Based on three factors, including whether or not to inject free GX1, the ID of Cy5.5-GX1, and the camera IT, 32 mice were randomly divided into eight groups and received 60-min dynamic fluorescence imaging. Gurfinkel exponential model (GEXPM) and Lammertsma simplified reference tissue model (SRTM) combined with a singular value decomposition analysis were used to quantitatively analyze the acquired dynamic fluorescent images. The binding potential (Bp) and the sum of the pharmacokinetic rate constants (SKRC) of Cy5.5-GX1 were determined by the SRTM and EXPM, respectively. In the tumor region, the SKRC value exhibited an obvious trend with change in the tracer ID, but the Bp value was not sensitive to it. Both the Bp and SKRC values were independent of the camera IT. In addition, the ratio of the tumor-to-muscle region was correlated with the camera IT but was independent of the tracer ID. Dynamic fluorescence imaging in conjunction with a kinetic analysis may provide more quantitative information than static fluorescence imaging, especially for a priori information on the optimal ID of targeted probes for individual therapy.

In vivo quantifying molecular specificity of Cy5.5-labeled cyclic 9-mer peptide probe with dynamic fluorescence imaging

We quantified molecular specificity of Cy5.5-GX1 in vivo with dynamic fluorescence imaging to better understand its kinetic properties. According to whether or not free GX1 was injected and when it was injected, twelve of BGC-823 xenografted mice were randomly divided into three groups and underwent a 60 minute dynamic fluorescence scanning. Combined with a principal-component analysis, the binding potential (Bp) of the probe was determined by both Logan graphical analysis with reference tissue model (GARTM) and Lammertsma simplified reference tissue model (SRTM). The sum of the pharmacokinetic rate constants (SKRC) was quantified by the Gurfinkel exponential model (GEXPM). Cy5.5-GX1 specifically targeted tumor both in vitro and in vivo. We obtained similar quantification results of Bp (GARTM Bp = 0.582 ± 0.2655, SRTM Bp = 0.618 ± 0.2923), and obtained a good linear relation between the Bp value and the SKRC value. Our results indicate that the SKRC value is more suitable for an early-stage kinetic data analysis, and the Bp value depicts kinetic characteristics under the equilibrium state. Dynamic fluorescence imaging in conjunction with various kinetic models are optimal tools to quantify molecular specificity of the Cy5.5-GX1 probe in vivo.

Preliminary Therapy Evaluation of (225)Ac-DOTA-c(RGDyK) Demonstrates that Cerenkov Radiation Derived from (225)Ac Daughter Decay Can Be Detected by Optical Imaging for In Vivo Tumor Visualization

The theranostic potential of ²²⁵Ac-based radiopharmaceuticals continues to increase as researchers seek innovative ways to harness the nuclear decay of this radioisotope for therapeutic and imaging applications. This communication describes the evaluation of ²²⁵Ac-DOTA-c(RGDyK) in both biodistribution and Cerenkov luminescence imaging (CLI) studies. Initially, La-DOTA-c(RGDyK) was prepared as a non-radioactive surrogate to evaluate methodologies that would contribute to an optimized radiochemical synthetic strategy and estimate the radioactive conjugate's affinity for α_vβ₃, using surface plasmon resonance spectroscopy. Surface plasmon resonance spectroscopy studies revealed the IC₅₀ and K_i of La-DOTA-c(RGDyK) to be 33 ± 13 nM and 26 ± 11 nM, respectively, and suggest that the complexation of the La³⁺ ion to the conjugate did not significantly alter integrin binding. Furthermore, use of this surrogate allowed optimization of radiochemical synthesis strategies to prepare ²²⁵Ac-DOTA-c(RGDyK) with high radiochemical purity and specific activity similar to other ²²⁵Ac-based radiopharmaceuticals. This radiopharmaceutical was highly stable in vitro. In vivo biodistribution studies confirmed the radiotracer's ability to target α_vβ₃ integrin with specificity; specificity was detected in tumor-bearing animals using Cerenkov luminescence imaging. Furthermore, tumor growth control was achieved using non-toxic doses of the radiopharmaceutical in U87mg tumor-bearing nude mice. To our knowledge, this is the first report to describe the CLI of α_vβ₃⁺ tumors in live animals using the daughter products derived from ²²⁵Ac decay in situ. This concept holds promise to further enhance development of targeted alpha particle therapy.

Clinical Application of Radiolabeled RGD Peptides for PET Imaging of Integrin αvβ3

Molecular imaging for non-invasive assessment of angiogenesisis is of great interest for clinicians because of the wide-spread application of anti-angiogenic cancer therapeutics. Besides, many other interventions that involve the change of blood vessel/tumor microenvironment would also benefit from such imaging strategies. Of the imaging techniques that target angiogenesis, radiolabeled Arg-Gly-Asp (RGD) peptides have been a major focus because of their high affinity and selectivity for integrin α_vβ₃--one of the most extensively examined target of angiogenesis. Since the level of integrin α_vβ₃ expression has been established as a surrogate marker of angiogenic activity, imaging α_vβ₃ expression can potentially be used as an early indicator of effectiveness of antiangiogenic therapy at the molecular level. In this review, we summarize RGD-based PET tracers that have already been used in clinical trials and intercompared them in terms of radiosynthesis, dosimetry, pharmacokinetics and clinical applications. A perspective of their future use in the clinic is also provided.

Fluorine-18 patents (2009-2015). Part 1: novel radiotracers

The most commonly utilized PET radionuclide is fluorine-18 ((18)F) because of its convenient half-life and excellent imaging properties. In this review, we present the first analysis of patents issued for radiotracers labeled with fluorine-18 (between 2009 and 2015), and provide perspective on current trends and future directions in PET radiotracer development.

Use of Labelled tLyP-1 as a Novel Ligand Targeting the NRP Receptor to Image Glioma

Background

Neuropilin (NRP) receptors are overexpressed in glioma tumor tissue, and therefore may be a potential target for imaging markers. We investigated whether labelled tLyP-1, an NRP targeting peptide, could be used as the targeting ligand for developing reagents for imaging glioma tumors.

Methods

The tLyP-1 peptide (CGNKRTR) was labeled with 5-carboxyfluorescein (FAM) or ¹⁸F-fluoride. A control peptide (MAQKTSH) was also labeled with FAM. The in vitro binding between FAM-tLyP-1 and U87MG cells and in vivo biodistribution of FAM-tLyP-1 in a U87MG glioblastoma xenograft model (nude mouse) were determined. The in vivo biodistribution of ¹⁸F-tLyP-1 was also determined by microPET/CT.

Results

In vitro, FAM-tLyP-1 was strongly taken up by U87MG cells at very low concentrations (1μM). In vivo, FAM-tLyP-1 accumulated in glioma (U87MG) tumors, but uptake was minimal in the normal brain tissue 1 h after administration. The distribution of FAM-tLyP-1 in the tumor tissue was consistent with expression of NRP1. The tumor/brain fluorescence intensity ratio in mice treated with FAM-tLyP-1 was significantly higher than the control FAM-labeled peptide 1 h after administration (3.44 ± 0.83 vs. 1.32 ± 0.15; t = 5.547, P = 0.001). Uptake of FAM-tLyP-1 in glioma tumors could be blocked by administering an excess of non-conjugated tLyP-1 peptide. [Lys4] tLyP-1 was labeled with ¹⁸F to synthesis a PET (¹⁸F-tLyP-1). MicroPET/CT imaging showed the tumor was visualized clearly with a high tumor/brain radiolabel ratio at 60 min (2.69 ± 0.52) and 120 min (3.11±0.25).

Conclusion

Taken together, our results suggest that tLyP-1 could be developed as a novel fluorescent or radio labelled tracer for imaging glioma.

Quantitative in vivo cell-surface receptor imaging in oncology: kinetic modeling and paired-agent principles from nuclear medicine and optical imaging

The development of methods to accurately quantify cell-surface receptors in living tissues would have a seminal impact in oncology. For example, accurate measures of receptor density in vivo could enhance early detection or surgical resection of tumors via protein-based contrast, allowing removal of cancer with high phenotype specificity. Alternatively, accurate receptor expression estimation could be used as a biomarker to guide patient-specific clinical oncology targeting of the same molecular pathway. Unfortunately, conventional molecular contrast-based imaging approaches are not well adapted to accurately estimating the nanomolar-level cell-surface receptor concentrations in tumors, as most images are dominated by nonspecific sources of contrast such as high vascular permeability and lymphatic inhibition. This article reviews approaches for overcoming these limitations based upon tracer kinetic modeling and the use of emerging protocols to estimate binding potential and the related receptor concentration. Methods such as using single time point imaging or a reference-tissue approach tend to have low accuracy in tumors, whereas paired-agent methods or advanced kinetic analyses are more promising to eliminate the dominance of interstitial space in the signals. Nuclear medicine and optical molecular imaging are the primary modalities used, as they have the nanomolar level sensitivity needed to quantify cell-surface receptor concentrations present in tissue, although each likely has a different clinical niche.

Pilot Prospective Evaluation of (18)F-Alfatide II for Detection of Skeletal Metastases

This pilot prospective evaluation study is to verify the efficiency of (18)F-Alfatide II, a specific PET imaging agent for integrin αvβ3, in detecting bone metastasis in human, with comparison to (18)F-FDG PET. Thirty recruited patients underwent (18)F-FDG and (18)F-alfatide II PET/CT successively within days. The final diagnosis of bone lesions was established based on the comprehensive assessment of all available data and clinical follow-up, which fall into four groups: osteolytic, osteoblastic, mixed and bone marrow. Visual analysis and quantification of SUVmax were performed to compare the detection sensitivity of (18)F-Alfatide II and (18)F-FDG PET. Eleven patients were found to have a total of 126 bone metastasis lesions. (18)F-Alfatide II PET can detect the bone metastatic lesions with good contrast and higher sensitivity (positive rate of 92%) than (18)F-FDG PET (77%). Especially, (18)F-Alfatide II PET showed superiority to (18)F-FDG PET in detecting osteoblastic (70% vs. 53%) and bone marrow metastatic lesions (98% vs. 77%). In conclusion, (18)F-Alfatide II PET/CT can be used to detect skeletal and bone marrow metastases, with nearly 100% sensitivity in osteolytic, mixed and bone marrow lesions. The sensitivity of (18)F-Alfatide II PET/CT in osteoblastic metastases is relatively low but still significantly higher than that of (18)F-FDG PET/CT. This pilot clinical study warrants the further application of (18)F-Alfatide II PET/CT in metastatic lesion detection, patient management and drug therapy response monitoring.

Comparison of three dimeric 18F-AlF-NOTA-RGD tracers

PURPOSE

RGD peptide-based radiotracers are well established as integrin αvβ3 imaging probes to evaluate tumor angiogenesis or tissue remodeling after ischemia or infarction. In order to optimize the labeling process and pharmacokinetics of the imaging probes, we synthesized three dimeric RGD peptides with or without PEGylation and performed in vivo screening.

PROCEDURES

Radiolabeling was achieved through the reaction of F-18 aluminum-fluoride complex with the cyclic chelator, 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA). Three imaging probes were synthesized as (18)F-AlF-NOTA-E[c(RGDfK)]2, (18)F-AlF-NOTA-PEG4-E[c(RGDfK)]2, and (18)F-AlF-NOTA-E[PEG4-c(RGDfk)]2. The receptor binding affinity was determined by competitive cell binding assay, and the stability was evaluated by mouse serum incubation. Tumor uptake and whole body distribution of the three tracers were compared through direct tissue sampling and PET quantification of U87MG tumor-bearing mice.

RESULTS

All three compounds remained intact after 120 min incubation with mouse serum. They all had a rapid and relatively high tracer uptake in U87MG tumors with good target-to-background ratios. Compared with the other two tracers, (18)F-AlF-NOTA-E[PEG4-c(RGDfk)]2 had the highest tumor uptake and the lowest accumulation in the liver. The integrin receptor specificity was confirmed by co-injection of unlabeled dimeric RGD peptide.

CONCLUSION

The rapid one-step radiolabeling strategy by the complexation of (18)F-aluminum fluoride with NOTA-peptide conjugates was successfully applied to synthesize three dimeric RGD peptides. Among the three probes developed, (18)F-AlF-NOTA-E[PEG4-c(RGDfk)]2 with relatively low liver uptake and high tumor accumulation appears to be a promising candidate for further translational research.

Targeting post-infarct inflammation by PET imaging: comparison of (68)Ga-citrate and (68)Ga-DOTATATE with (18)F-FDG in a mouse model

Imaging of inflammation early after myocardial infarction (MI) is a promising approach to the guidance of novel molecular interventions that support endogenous healing processes. (18)F-FDG PET has been used, but may be complicated by physiological myocyte uptake. We evaluated the potential of two alternative imaging targets: lactoferrin binding by (68)Ga-citrate and somatostatin receptor binding by (68)Ga-DOTATATE.

METHODS

C57Bl/6 mice underwent permanent coronary artery ligation. Serial PET imaging was performed 3 - 7 days after MI using (68)Ga-citrate, (68)Ga-DOTATATE, or (18)F-FDG with ketamine/xylazine suppression of myocyte glucose uptake. Myocardial perfusion was evaluated by (13)N-ammonia PET and cardiac geometry by contrast-enhanced ECG-gated CT.

RESULTS

Mice exhibited a perfusion defect of 30 - 40% (of the total left ventricle) with apical anterolateral wall akinesia and thinning on day 7 after MI. (18)F-FDG with ketamine/xylazine suppression demonstrated distinct uptake in the infarct region, as well as in the border zone and remote myocardium. The myocardial standardized uptake value in MI mice was significantly higher than in healthy mice under ketamine/xylazine anaesthesia (1.9 ± 0.4 vs. 1.0 ± 0.1). (68)Ga images exhibited high blood pool activity with no specific myocardial uptake up to 90 min after injection (tissue-to-blood contrast 0.9). (68)Ga-DOTATATE was rapidly cleared from the blood, but myocardial SUV was very low (0.10 ± 0.03).

CONCLUSION

Neither (68)Ga nor (68)Ga-DOTATATE is a useful alternative to (18)F-FDG for PET imaging of myocardial inflammation after MI in mice. Among the three tested approaches, (18)F-FDG with ketamine/xylazine suppression of cardiomyocyte uptake remains the most practical imaging marker of post-infarct inflammation.

PET radiopharmaceuticals for imaging integrin expression: tracers in clinical studies and recent developments

Noninvasive determination of integrin expression has become an interesting approach in nuclear medicine. Since the discovery of the first 18F-labeled cyclic RGD peptide as radiotracer for imaging integrin αvβ3 expression in vivo, there have been carried out enormous efforts to develop RGD peptides for PET imaging. Moreover, in recent years, additional integrins, including α5β1 and αvβ6 came into the focus of pharmaceutical radiochemistry. This review will discuss the tracers already evaluated in clinical trials and summarize the preliminary outcome. It will also give an overview on recent developments to further optimize the first-generation compounds such as [18F]Galacto-RGD. This includes recently developed 18F-labeling strategies and also new approaches in 68Ga-complex chemistry. Furthermore, the approaches to develop radiopharmaceuticals targeting integrin α5β1 and αvβ6 will be summarized and discussed.

Benefits of NOPO as chelator in gallium-68 peptides, exemplified by preclinical characterization of (68)Ga-NOPO-c(RGDfK)

The αvβ3-integrin addressing cyclic pentapeptide cyclo(RGDfK) was conjugated to NOPO, 1,4,7-triazacyclononane-1,4-bis[methylene(hydroxymethyl)phosphinic acid]-7-[methylene(2-carboxyethyl)phosphinic acid], a bifunctional chelator with exceptional gallium-68 labeling properties. NOPO-c(RGDfK) and its Ga(III) and Cu(II) complexes showed high affinity to αvβ3 integrin (IC50 = 0.94 ± 0.06, 1.02 ± 0.09, and 0.51 ± 0.06 nM, respectively). (68)Ga labeling of NOPO-c(RGDfK) in an automated GMP-compliant procedure was performed with near-quantitative radiochemical yield, using precursor amounts as low as 0.5 nmol (approximately 0.6 μg). (68)Ga-NOPO-c(RGDfK) was obtained with high purity (>99% by radio-HPLC/TLC) and, optionally, could be produced with specific activities up to 6 TBq/μmol. M21/M21L (human melanoma with high/low αvβ3 integrin expression) xenografted athymic CD-1 nude mice were used for biodistribution, in vivo stability studies, and PET imaging. (68)Ga-NOPO-c(RGDfK) showed rapid and specific uptake in M21 tumor xenografts (2.02 ± 0.34% ID/g at 60 min p.i.) and was found stable in vivo. Its high hydrophilicity is reflected by an octanol-water distribution coefficient (log D = -4.6) which is more than 1 order of magnitude lower compared to respective NOTA or DOTA analogues. As expected, (68)Ga-NOPO-c(RGDfK) thus showed fast renal clearance from nontargeted tissues. We conclude that NOPO might generally prove a useful means to improve renal clearance of corresponding radiopharmaceuticals by increasing the polarity of its bioconjugates. Favorable labeling properties render NOPO conjugates highly recommendable for reliable routine production of (68)Ga-radiopharmaceuticals in a clinical setting.

Evaluation of synovial angiogenesis in patients with rheumatoid arthritis using ⁶⁸Ga-PRGD2 PET/CT: a prospective proof-of-concept cohort study

Background

The study aimed to evaluate the use of positron emission tomography/computed tomography (PET/CT) with ⁶⁸Ga-PRGD2 as the tracer for imaging of synovial angiogenesis in patients with rheumatoid arthritis (RA).

Methods

Twenty untreated active patients with RA underwent ⁶⁸Ga-PRGD2 PET/CT and ¹⁸F-FDG PET/CT before treatment; two patients with osteoarthritis served as controls. Among the 20 patients with RA, 12 repeated the evaluations after 3-month treatment. The image findings were correlated with core variables of disease activity, including the clinical disease activity index (cDAI).

Results

Our findings demonstrated that ⁶⁸Ga-PRGD2 specifically accumulated in the synovia with active inflammation rich in neovasculature with high-level α_vβ₃-integrin expression, but not in the ¹⁸F-FDG-avid inflammatory lymph nodes. In patients with intense ¹⁸F-FDG uptake in muscles caused by arthritic pain, we observed that ⁶⁸Ga-PRGD2 PET/CT was better able to evaluate disease severity than ¹⁸F-FDG PET/CT. Both ⁶⁸Ga-PRGD2 accumulation and ¹⁸F-FDG uptake changed in response to therapeutic intervention, whereas the changes of ⁶⁸Ga-PRGD2, not ¹⁸F-FDG, significantly correlated with clinical measures of changes in the form of cDAI.

Conclusions

This is the first integrin imaging study conducted in patients with RA that preliminarily indicates the effectiveness of the novel method for evaluating synovial angiogenesis.

Clinical trial registration

This study has been registered online at NIH ClinicalTrial.gov (NCT01940926).

(18)F-alfatide II and (18)F-FDG dual-tracer dynamic PET for parametric, early prediction of tumor response to therapy

A single dynamic PET acquisition using multiple tracers administered closely in time could provide valuable complementary information about a tumor's status under quasiconstant conditions. This study aimed to investigate the utility of dual-tracer dynamic PET imaging with (18)F-alfatide II ((18)F-AlF-NOTA-E[PEG4-c(RGDfk)]2) and (18)F-FDG for parametric monitoring of tumor responses to therapy.

METHODS

We administered doxorubicin to one group of athymic nude mice with U87MG tumors and paclitaxel protein-bound particles to another group of mice with MDA-MB-435 tumors. To monitor therapeutic responses, we performed dual-tracer dynamic imaging, in sessions that lasted 90 min, starting with injection via the tail vein catheters with (18)F-alfatide II, followed 40 min later by (18)F-FDG. To achieve signal separation of the 2 tracers, we fit a 3-compartment reversible model to the time-activity curve of (18)F-alfatide II for the 40 min before (18)F-FDG injection and then extrapolated to 90 min. The (18)F-FDG tumor time-activity curve was isolated from the 90-min dual-tracer tumor time-activity curve by subtracting the fitted (18)F-alfatide II tumor time-activity curve. With separated tumor time-activity curves, the (18)F-alfatide II binding potential (Bp = k3/k4) and volume of distribution (VD) and (18)F-FDG influx rate ((K1 × k3)/(k2 + k3)) based on the Patlak method were calculated to validate the signal recovery in a comparison with 60-min single-tracer imaging and to monitor therapeutic response.

RESULTS

The transport and binding rate parameters K1-k3 of (18)F-alfatide II, calculated from the first 40 min of the dual-tracer dynamic scan, as well as Bp and VD correlated well with the parameters from the 60-min single-tracer scan (R(2) > 0.95). Compared with the results of single-tracer PET imaging, (18)F-FDG tumor uptake and influx were recovered well from dual-tracer imaging. On doxorubicin treatment, whereas no significant changes in static tracer uptake values of (18)F-alfatide II or (18)F-FDG were observed, both (18)F-alfatide II Bp and (18)F-FDG influx from kinetic analysis in tumors showed significant decreases. For therapy of MDA-MB-435 tumors with paclitaxel protein-bound particles, a significant decrease was observed only with (18)F-alfatide II Bp value from kinetic analysis but not (18)F-FDG influx.

CONCLUSION

The parameters fitted with compartmental modeling from the dual-tracer dynamic imaging are consistent with those from single-tracer imaging, substantiating the feasibility of this methodology. Even though no significant differences in tumor size were found until 5 d after doxorubicin treatment started, at day 3 there were already substantial differences in (18)F-alfatide II Bp and (18)F-FDG influx rate. Dual-tracer imaging can measure (18)F-alfatide II Bp value and (18)F-FDG influx simultaneously to evaluate tumor angiogenesis and metabolism. Such changes are known to precede anatomic changes, and thus parametric imaging may offer the promise of early prediction of therapy response.

Assessment of an elastin binding molecule for PET imaging of atherosclerotic plaques

Elastin is considered as a key player in human vascular diseases and it might contribute to the development of atherosclerosis. The elastin binding radiotracer, [(18)F]AlF-NOTA-EBM ([(18)F]2), was evaluated in a wild type mouse to determine its in vivo distribution and on human carotid atherosclerotic plaque tissues to assess its utility as a PET imaging agent for visualizing human atherosclerotic plaque lesions. The free ligand NOTA-EBM, which served as the precursor, was obtained in 25% chemical yield. The radiosynthesis of [(18)F]2 was accomplished by coordination of Al(18)F to NOTA-EBM in 8-13% decay corrected radiochemical yield (n = 7) and specific radioactivity of 59 ± 12 GBq/μmol. A dynamic in vivo PET scan in a healthy wild type mouse (C57BL/6) showed high accumulation of radioactivity in heart and lungs, organs reported to have high elastin content. Excretion of [(18)F]2 proceeded via the renal pathway and through the hepatobiliary system as indicated by a high uptake of radioactivity in the liver, intestines and gall bladder. In vitro autoradiography on human atherosclerotic plaque sections showed a heterogeneous distribution of [(18)F]2 with an elevated accumulation in stable and vulnerable atherosclerotic plaques compared to control samples of normal arteries. However, there was no statistical significance between the different plaque phenotypes and control samples. Competition experiments with 10.000-fold excess of free ligand NOTA-EBM resulted in a marked decrease of radioactivity accumulation, consistent with a target-specific ligand.

Radiofluorination using aluminum-fluoride (Al18F)

Targeted agents are increasingly used for treating cancer and other diseases, but patients may need to be carefully selected to maximize the potential for therapeutic benefit. One way to select patients is to bind an imaging radionuclide to a targeting agent of interest, so that its uptake in specific sites of disease can be visualized by positron-emission tomography (PET) or single-photon emission computed tomography.18F is the most commonly used radionuclide for PET imaging. Its half-life of approximately 2 h is suited for same-day imaging of many compounds that clear quickly from the body to allow visualization of uptake in the intended target. A significant impediment to its use, however, is the challenging coupling of 18F to a carbon atom of the targeting agent. Because fluorine binds to aluminum, we developed a procedure where the Al18F complex could be captured by a chelate, thereby greatly simplifying the way that imaging agents can be fluorinated for PET imaging. This article reviews our experience with this technology.

First experience of 18F-alfatide in lung cancer patients using a new lyophilized kit for rapid radiofluorination

(18)F-FPPRGD2, which was approved for clinical study recently, has favorable properties for integrin targeting and showed potential for antiangiogenic therapy and early response monitoring. However, the time-consuming multiple-step synthesis may limit its widespread applications in the clinic. In this study, we developed a simple lyophilized kit for labeling PRGD2 peptide ((18)F-AlF-NOTA-PRGD2, denoted as (18)F-alfatide) using a fluoride-aluminum complex that significantly simplified the labeling procedure.

METHODS

Nine patients with a primary diagnosis of lung cancer were examined by both static and dynamic PET imaging with (18)F-alfatide, and 1 tuberculosis patient was investigated using both (18)F-alfatide and (18)F-FDG imaging. Standardized uptake values were measured in tumors and other main organs at 30 min and 1 h after injection. Kinetic parameters were calculated by Logan graphical analysis. Immunohistochemistry and staining intensity quantification were performed to confirm the expression of integrin α(v)β(3).

RESULTS

Under the optimal conditions, the whole radiosynthesis including purification was accomplished within 20 min with a decay-corrected yield of 42.1% ± 2.0% and radiochemical purity of more than 95%. (18)F-alfatide PET imaging identified all tumors, with mean standardized uptake values of 2.90 ± 0.10. Tumor-to-muscle and tumor-to-blood ratios were 5.87 ± 2.02 and 2.71 ± 0.92, respectively.

CONCLUSION

(18)F-alfatide can be produced with excellent radiochemical yield and purity via a simple, 1-step, lyophilized kit. PET scanning with (18)F-alfatide allows specific imaging of αvβ3 expression with good contrast in lung cancer patients. This technique might be used for the assessment of angiogenesis and for planning and response evaluation of cancer therapies that would affect angiogenesis status and integrin expression levels.

One-step and one-pot-two-step radiosynthesis of cyclo-RGD-(18)F-aryltrifluoroborate conjugates for functional imaging

Arylboronates capture aqueous (18)F-fluoride in one step to afford a highly polar (18)F-labeled aryltrifluoroborate anion ((18)F-ArBF(3) (-)) that clears rapidly in vivo. To date however, there is little data to show that a ligand labeled with a prosthetic (18)F-ArBF(3) (-) will provide functional images. RGD, a high-affinity ligand for integrins that are present on the cell surface of numerous tumors, has been labeled in many formats with many different radionuclides, and as such represents a well-established ligand that can be used to evaluate new labeling methods. Herein we have labeled RGD with a prosthetic (18)F-ArBF(3) (-) via two approaches for the first time: 1) a RGD-boronate bioconjugate is directly labeled in one step and 2) an alkyne-modified arylborimidine is first converted to the corresponding (18)F-ArBF(3) (-) which is then conjugated to an RGD-azide via Cu(+)-mediated [2+3] dipolar cycloaddition in one pot over two steps. RGD-(18)F-ArBF(3) (-) bionconjugates were produced in reasonable radiochemical yields using low amounts of (18)F-fluoride anion (10-50 mCi). Despite relatively low specific activities, good tumor images are revealed in each case.