Promising Positron Emission Tomography Tracer Research Articles

Synthesis and Preclinical Evaluation of [18F]AlF-NOTA-Asp2-PEG2-Folate as a Novel Folate-Receptor-Targeted Tracer for PET Imaging.

Recently, the folate receptor (FR) has become an exciting target for the diagnosis of FR-positive malignancies. Nevertheless, suboptimal invivo pharmacokinetic properties, particularly high uptake in the renal and hepatobiliary systems, are important limiting factors for the clinical translation of most FR-based radiotracers. In this study, we developed a novel 18F-labeled FR-targeted positron emission tomography (PET) tracer [18F]AlF-NOTA-Asp2-PEG2-Folate modified with a hydrophilic linker (-Asp2-PEG2) to optimize its pharmacokinetic properties and conducted a comprehensive preclinical assessment. The [18F]AlF-NOTA-Asp2-PEG2-Folate was manually synthesized within 30 min with a non-decay-corrected radiochemical yield of 16.3 ± 2.0% (n = 5). Among KB cells, [18F]AlF-NOTA-Asp2-PEG2-Folate exhibited high specificity and affinity for FR. PET/CT imaging and biodistribution experiments in KB tumor-bearing mice showed decent tumor uptake (1.7 ± 0.3% ID/g) and significantly decreased uptake in kidneys and liver (22.2 ± 2.1 and 0.3 ± 0.1% ID/g at 60 min p.i., respectively) of [18F]AlF-NOTA-Asp2-PEG2-Folate, compared to the known tracer [18F]AlF-NOTA-Folate (78.6 ± 5.1 and 5.3 ± 0.5 % ID/g at 90 min p.i., respectively). The favorable properties of [18F]AlF-NOTA-Asp2-PEG2-Folate, including its efficient synthesis, decent tumor uptake, relatively low renal uptake, and rapid clearance from most normal organs, portray it as a promising PET tracer for FR-positive tumors.

Two Novel [68Ga]Ga-Labeled Radiotracers Based on Metabolically Stable [Sar11]RM26 Antagonistic Peptide for Diagnostic Positron Emission Tomography Imaging of GRPR-Positive Prostate Cancer.

Gastrin releasing peptide receptor (GRPR) is overexpressed in prostate cancer (PC-3) and can be used for diagnostic purposes. We herein present the design and preclinical evaluation of two novel NOTA/NODAGA-containing peptides suitable for labeling with the positron emission tomography (PET) radionuclide Ga-68. These analogs are based on the previously reported GRPR-antagonist DOTAGA-PEG2-[Sar11]RM26, developed for targeted radiotheraostic applications. Both NOTA-PEG2-[Sar11]RM26 and NODAGA-PEG2-[Sar11]RM26 were successfully labeled with Ga-68 and evaluated in vitro and in vivo using PC-3 cell models. Both, [68Ga]Ga-NOTA-PEG2-[Sar11]RM26 and [68Ga]Ga-NODAGA-PEG2-[Sar11]RM26 displayed high metal-chelate stability in phosphate buffered saline and against the EDTA-challenge. The two [68Ga]Ga-labeled conjugates demonstrated highly GRPR-mediated uptake in vitro and in vivo and exhibited a slow internalization over time, typical for radioantagonistis. The [natGa]Ga-loaded peptides displayed affinity in the low nanomole range for GRPR in competition binding experiments. The new radiotracers demonstrated biodistribution profiles suitable for diagnostic imaging shortly after administration with fast background clearance. Their high tumor uptake (13 ± 1 and 15 ± 3% IA/g for NOTA and NODAGA conjugates, respectively) and high tumor-to-blood ratios (60 ± 10 and 220 ± 70, respectively) 3 h pi renders them promising PET tracers for use in patients. Tumor-to-normal organ ratios were higher for [68Ga]Ga-NODAGA-PEG2-[Sar11]RM26 than for the NOTA-containing counterpart. The performance of the two radiopeptides was further supported with the PET/CT images. In conclusion, [68Ga]Ga-NODAGA-PEG2-[Sar11]RM26 is a promising PET imaging tracer for visualization of GRPR-expressing lesions with high imaging contrast shortly after administration.

Preclinical evaluation of 68Ga-labeled peptide CK2 for PET imaging of NRP-1 expression in vivo.

Neuropilin-1 (NRP-1) is a multifunctional protein involved in a variety of biological processes such as angiogenesis, tumorigenesis and immunomodulation. It was usually overexpressed in many cancer cell lines and correlated with poor prognosis of breast cancer. Positron emission tomography (PET) is an advanced imaging technique for detecting the function and metabolism of tumor-associated molecules in real time, dynamically, quantitatively and noninvasively. To improve the level of early diagnosis and evaluate the prognosis of breast cancer, an NRP-1 targeting peptide-based tracer [68Ga]Ga-NOTA-PEG4-CK2 was designed to sensitively and specifically detect the NRP-1 expression in vivo via PET imaging. In silico modeling and microscale thermophoresis (MST) assay were carried out to design the NRP-1 targeting peptide NOTA-PEG4-CK2, and it was further radiolabeled with 68Ga to prepare the tracer [68Ga]Ga-NOTA-PEG4-CK2. The radiochemical yield (RCY), radiochemical purity (RCP), molar activity (Am), lipid-water partition coefficient (Log P) and stability of [68Ga]Ga-NOTA-PEG4-CK2 were assessed. The targeting specificity of the tracer for NRP-1 was investigated by in vitro cellular uptake assay and in vivo PET imaging as well as blocking studies. The sensitivity of the tracer in monitoring the dynamic changes of NRP-1 expression induced by chemical drug was also investigated in vitro and in vivo. Ex vivo biodistribution, autoradiography, western blot, and immunofluorescence staining were also performed to study the specificity of [68Ga]Ga-NOTA-PEG4-CK2 for NRP-1. [68Ga]Ga-NOTA-PEG4-CK2 was designed and synthesized with high RCY (> 98%), high stability (RCP > 95%) and high affinity to NRP-1 (KD = 25.39 ± 1.65nM). In vitro cellular uptake assay showed that the tracer [68Ga]Ga-NOTA-PEG4-CK2 can specifically bind to NRP-1 positive cancer cells MDA-MB-231 (1.04 ± 0.04% at 2h) rather than NRP-1 negative cancer cells NCI-H1299 (0.43 ± 0.05%). In vivo PET imaging showed the maximum tumor uptake of [68Ga]Ga-NOTA-PEG4-CK2 in MDA-MB-231 xenografts (4.16 ± 0.67%ID/mL) was significantly higher than that in NCI-H1299 xenografts (1.03 ± 0.19%ID/mL) at 10min post injection, and the former exhibited higher tumor-to-muscle uptake ratio (5.22 ± 0.18) than the latter (1.07 ± 0.27) at 60min post injection. MDA-MB-231 xenografts pretreated with nonradioactive precursor NOTA-PEG4-CK2 showed little tumor uptake of [68Ga]Ga-NOTA-PEG4-CK2 (1.67 ± 0.38%ID/mL at 10min post injection). Both cellular uptake assay and PET imaging revealed that NRP-1 expression in breast cancer MDA-MB-231 could be effectively suppressed by SB-203580 treatment and can be sensitively detected by [68Ga]Ga-NOTA-PEG4-CK2. Ex vivo analysis also proved the high specificity and sensitivity of [68Ga]Ga-NOTA-PEG4-CK2 for NRP-1 expression in MDA-MB-231 xenografts. A promising NRP-1 targeting PET tracer [68Ga]Ga-NOTA-PEG4-CK2 was successfully prepared. It showed remarkable specificity and sensitivity in monitoring the dynamic changes of NRP-1 expression. Hence, it could provide valuable information for early diagnosis of NRP-1 relevant cancers and evaluating the prognosis of cancer patients.

A facile synthesis of precursor for the σ-1 receptor PET radioligand [18 F]FTC-146 and its radiofluorination.

The σ-1 receptor is a non-opioid transmembrane protein involved in various human pathologies including neurodegenerative diseases, inflammation, and cancer. The previously published ligand [18 F]FTC-146 is among the most promising tools for σ-1 molecular imaging by positron emission tomography (PET), with a potential for application in clinical diagnostics and research. However, the published six- or four-step synthesis of the tosyl ester precursor for its radiosynthesis is complicated and time-consuming. Herein, we present a simple one-step precursor synthesis followed by a one-step fluorine-18 labeling procedure that streamlines the preparation of [18 F]FTC-146. Instead of a tosyl-based precursor, we developed a one-step synthesis of the precursor analog AM-16 containing a chloride leaving group for the SN 2 reaction with 18 F-fluoride. 18 F-fluorination of AM-16 led to a moderate decay-corrected radiochemical yield (RCY = 7.5%) with molar activity (Am ) of 45.9GBq/μmol. Further optimization of this procedure should enable routine radiopharmaceutical production of this promising PET tracer.

Next generation fibroblast activation protein (FAP) targeting PET tracers - The tetrazine ligation allows an easy and convenient way to 18F-labeled (4-quinolinoyl)glycyl-2-cyanopyrrolidines

Small-molecular fibroblast activation protein inhibitor (FAPI)-based tracer have been shown to be promising Positron Emission Tomography (PET) 68Ga-labeled radiopharmaceuticals to image a variety of tumors including pancreatic, breast, and colorectal cancers, among others. In this study, we developed a novel 18F-labeled FAPI derivative. [18F]6 was labeled using a synthon approach based on the tetrazine ligation. It showed subnanomolar affinity for the FAP protein and a good selectivity profile against known off-target proteases. Small animal PET studies revealed high tumor uptake and good target-to-background ratios. [18F]6 was excreted via the liver. Overall, [18F]6 showed promising characteristics to be used as a PET tracer and could serve as a lead for further development of halogen-based theranostic FAPI radiopharmaceuticals.

Development and Initial Assessment of [18F]OP-801: a Novel Hydroxyl Dendrimer PET Tracer for Preclinical Imaging of Innate Immune Activation in the Whole Body and Brain.

Innate immune activation plays a critical role in the onset and progression of many diseases. While positron emission tomography (PET) imaging provides a non-invasive means to visualize and quantify such immune responses, most available tracers are not specific for innate immune cells. To address this need, we developed [18F]OP-801 by radiolabeling a novel hydroxyl dendrimer that is selectively taken up by reactive macrophages/microglia and evaluated its ability to detect innate immune activation in mice following lipopolysaccharide (LPS) challenge. OP-801 was radiolabeled in two steps: [18F]fluorination of a tosyl precursor to yield [18F]3-fluoropropylazide, followed by a copper-catalyzed click reaction. After purification and stability testing, [18F]OP-801 (150-250 μCi) was intravenously injected into female C57BL/6 mice 24 h after intraperitoneal administration of LPS (10 mg/kg, n=14) or saline (n=6). Upon completing dynamic PET/CT imaging, mice were perfused, and radioactivity was measured in tissues of interest via gamma counting or autoradiography. [18F]OP-801 was produced with >95% radiochemical purity, 12-52 μCi/μg specific activity, and 4.3±1.5% decay-corrected yield. Ex vivo metabolite analysis of plasma samples (n=4) demonstrated high stability in mice (97±3% intact tracer >120 min post-injection). PET/CT images of mice following LPS challenge revealed higher signal in organs known to be inflamed in this context, including the liver, lung, and spleen. Gamma counting confirmed PET findings, showing significantly elevated signal in the same tissues compared to saline-injected mice: the liver (p=0.009), lung (p=0.030), and spleen (p=0.004). Brain PET/CT images (summed 50-60 min) revealed linearly increasing [18F]OP-801 uptake in the whole brain that significantly correlated with murine sepsis score (r=0.85, p<0.0001). Specifically, tracer uptake was significantly higher in the brain stem, cortex, olfactory bulb, white matter, and ventricles of LPS-treated mice compared to saline-treated mice (p<0.05). [18F]OP-801 is a promising new PET tracer for sensitive and specific detection of activated macrophages and microglia that warrants further investigation.

Clinical Radiosynthesis and Translation of [18F]OP-801: A Novel Radiotracer for Imaging Reactive Microglia and Macrophages.

Positron emission tomography (PET) is a powerful tool for studying neuroinflammatory diseases; however, current PET biomarkers of neuroinflammation possess significant limitations. We recently reported a promising dendrimer PET tracer ([18F]OP-801), which is selectively taken up by reactive microglia and macrophages. Here, we describe further important characterization of [18F]OP-801 in addition to optimization and validation of a two-step clinical radiosynthesis. [18F]OP-801 was found to be stable in human plasma for 90 min post incubation, and human dose estimates were calculated for 24 organs of interest; kidneys and urinary bladder wall without bladder voiding were identified as receiving the highest absorbed dose. Following optimization detailed herein, automated radiosynthesis and quality control (QC) analyses of [18F]OP-801 were performed in triplicate in suitable radiochemical yield (6.89 ± 2.23% decay corrected), specific activity (37.49 ± 15.49 GBq/mg), and radiochemical purity for clinical imaging. Importantly, imaging mice with tracer (prepared using optimized methods) 24 h following the intraperitoneal injection of liposaccharide resulted in the robust brain PET signal. Cumulatively, these data enable clinical translation of [18F]OP-801 for imaging reactive microglia and macrophages in humans. Data from three validation runs of the clinical manufacturing and QC were submitted to the Food and Drug Administration (FDA) as part of a Drug Master File (DMF). Subsequent FDA approval to proceed was obtained, and a phase 1/2 clinical trial (NCT05395624) for first-in-human imaging in healthy controls and patients with amyotrophic lateral sclerosis is underway.

Imaging the TGFβ type I receptor in pulmonary arterial hypertension

Transforming growth factor β (TGFβ) activity is perturbed in remodelled pulmonary vasculature of patients with pulmonary arterial hypertension (PAH), cancer, vascular diseases and developmental disorders. Inhibition of TGFβ, which signals via activin receptor-like kinase 5 (ALK5), prevents progression and development of experimental PAH. The purpose of this study was to assess two ALK5 targeting positron emission tomography (PET) tracers ([11C]LR111 and [18F]EW-7197) for imaging ALK5 in monocrotaline (MCT)- and Sugen/hypoxia (SuHx)-induced PAH. Both tracers were subjected to extensive in vitro and in vivo studies. [11C]LR111 showed the highest metabolic stability, as 46 ± 2% of intact tracer was still present in rat blood plasma after 60 min. In autoradiography experiments, [11C]LR111 showed high ALK5 binding in vitro compared with controls, 3.2 and 1.5 times higher in SuHx and MCT, respectively. In addition, its binding could be blocked by SB431542, an adenosine triphosphate competitive ALK5 kinase inhibitor. However, [18F]EW-7197 showed the best in vivo results. 15 min after injection, uptake was 2.5 and 1.4 times higher in the SuHx and MCT lungs, compared with controls. Therefore, [18F]EW-7197 is a promising PET tracer for ALK5 imaging in PAH.

68Ga-Labeled Cystine Knot Peptide Targeting Integrin αvβ6 for Lung Cancer PET Imaging.

Integrin αvβ6 has been considered as a promising biomarker for lung cancer, and its expression is often related to poor prognosis. An αvβ6-binding cystine knot peptide R01-MG was previously engineered and validated. Here, we developed a positron emission tomography (PET) probe of R01-MG for imaging αvβ6-positive lung cancer. Cystine knot peptide R01-MG was synthesized through solid-phase peptide synthesis chemistry and radiolabeled with 68Ga after being conjugated with 1,4,7,10-tetraazacyclododecane-N,N',N″,N‴-tetraacetic acid (DOTA). The stability of 68Ga-DOTA-R01-MG was analyzed in phosphate-buffered saline (PBS) (pH 7.4) and fetal bovine serum (FBS). The cell uptake assay of the probe was evaluated using αvβ6-positive (A549 and H1975) and αvβ6-negative (H1299) lung cancer cell lines. In addition, small animal PET imaging and biodistribution studies of 68Ga-DOTA-R01-MG were performed in αvβ6-positive and αvβ6-negative lung cancer models. Our study showed that 68Ga-DOTA-R01-MG exhibited excellent stability in PBS and FBS. Small animal PET imaging and biodistribution data revealed that 68Ga-DOTA-R01-MG displayed rapid and good tumor uptake in animal models with αvβ6-positive lung cancer, and the probe was rapidly cleared from the normal tissues, resulting in good tumor-to-normal tissue contrasts. Meanwhile, no obvious tumor uptake of 68Ga-DOTA-R01-MG was observed in animal models with αvβ6-negative lung cancer, demonstrating specific binding of the probe to integrin αvβ6. In conclusion, 68Ga-DOTA-R01-MG has great potential to be a promising PET tracer for imaging αvβ6-positive lung cancer.

18F]BTK-1: A Novel Positron Emission Tomography Tracer for Imaging Bruton's Tyrosine Kinase.

Bruton's tyrosine kinase (BTK) is a key component of B cell receptor (BCR) signaling, and as such a critical regulator of cell proliferation and survival. Aberrant BCR signaling is important in the pathogenesis of various B cell malignancies and autoimmune disorders. Here, we describe the development of a novel positron emission tomography (PET) tracer for imaging BTK expression and/or occupancy by small molecule therapeutics. Radiochemistry was carried out by reacting the precursor with [18F]fluoride on a GE FX-FN TracerLab synthesis module to produce [18F]BTK-1 with a 6% decay-corrected radiochemical yield, 100 ± 6GBq/µmol molar activity, and a radiochemical purity of 99%. Following intravenous administration of [18F]BTK-1 (3.63 ± 0.59MBq, 0.084 ± 0.05µg), 60-min dynamic images were acquired in two xenograft models: REC-1, an efficacious mantle cell lymphoma model, and U87MG, a non-efficacious glioblastoma model. Subsequent studies included vehicle, pretreatment (10min prior to tracer injection), and displacement (30min post-tracer injection) studies with different reversible BTK inhibitors to examine BTK binding. Human radiation dosimetry was estimated based on PET imaging in healthy rats. Uptake of [18F]BTK-1 was significantly higher in BTK expressing REC-1 tumors than non-BTK expressing U87MG tumors. Administration of BTK inhibitors prior to tracer administration blocked [18F]BTK-1 binding in the REC-1 tumor model consistent with [18F]BTK-1 binding to BTK. The predicted effective dose in humans was 0.0199 ± 0.0007mSv/MBq. [18F]BTK-1 is a promising PET tracer for imaging of BTK, which could provide valuable information for patient selection, drug dose determination, and improving our understanding of BTK biology in humans.

Development of a Promising 18F-Radiotracer for PET Imaging Legumain Activity In Vivo.

Legumain has been found overexpressed in several cancers, which serves as an important biomarker for cancer diagnosis. In this research, a novel fluorine-18 labeled radioactive tracer [18F]SF-AAN targeting legumain was designed and synthesized for positron emission tomography (PET) imaging. Nonradioactive probe [19F]SF-AAN was obtained through chemical and solid phase peptide synthesis. After a simple one-step 18F labeling, the radiotracer [18F]SF-AAN was obtained with a high radiochemical conversion rate (>85%) and radiochemical purity (99%) as well as high molar activity (12.77 ± 0.50 MBq/nmol). The targeting specificity of [18F]SF-AAN for detecting legumain activity was investigated systematically in vitro and in vivo. In vitro cellular uptake assay showed that the uptake of [18F]SF-AAN in legumain-positive MDA-MB-468 cells was twice as much as that in legumain-negative PC-3 cells at 4 h. In vivo PET imaging revealed that the tumor uptake of [18F]SF-AAN in MDA-MB-468 tumor-bearing mice was about 2.7 times of that in PC-3 tumor-bearing mice at 10 min post injection. The experimental results indicated that [18F]SF-AAN could serve as a promising PET tracer for detecting the legumain expression sensitively and specifically, which would be beneficial for the diagnosis of legumain-related diseases.

68Ga-WRWWWW Is a Potential Positron Emission Tomography Probe for Imaging Inflammatory Diseases by Targeting Formyl Peptide Receptor 2.

Inflammation plays a significant role in many physiological and pathological processes. Molecular imaging could provide functional as well as anatomical information for visualizing various inflammatory diseases. Advancements in imaging tracers for inflammation would improve the accuracy of diagnosis and monitoring, thus facilitating patient care. The positron emission tomography (PET) imaging tracer, 68Ga-labeled antagonist peptide Trp-Arg-Trp-Trp-Trp-Trp (WRWWWW, WRW4), targets formyl peptide receptor 2 (FPR2), which is in turn widely distributed in a variety of tissues and is associated with many inflammatory diseases. In the current study, we aimed to investigate the potential of 68Ga-WRW4 for detecting and monitoring inflammatory lesions in mice. We established an inflammation mouse model by the intramuscular injection of turpentine oil into the left thigh. WRW4 was labeled with 68Ga with an overall radiochemical yield >90% and radiochemical purity >99%. 68Ga-WRW4 uptake in inflamed muscle peaked on day 2 (1.14 ± 0.01 percentage of the injected dose per gram of tissue (%ID/g)) and the uptake ratio of inflammatory/normal muscle also reached a maximum (12.36 ± 2.35). Strong PET signals were detected in the left thigh at 60 min after the injection of 68Ga-WRW4 in experimental mice, but weak or no signals were detected in mice in the blocking and control groups. 68Ga-WRW4 uptake was in agreement with the dynamics of immune cell infiltration during the inflammatory reaction. These results suggest that 68Ga-WRW4 is a promising PET tracer suitable for the noninvasive detection of FPR2 expression and for monitoring inflammatory activity in inflammation-bearing mice.

Tumor Microenvironment Responsive "Head-to-Foot" Self-Assembly Nanoplatform for Positron Emission Tomography Imaging in Living Subjects.

Sensitivity and specificity of molecular probes are two important factors in determining the accuracy of cancer diagnosis or the efficacy of cancer treatment. However, the development of probes with high sensitivity and strong specificity still poses many challenges. Herein, we report an 18F-labeled smart tracer ([18F]1) targeting cancer-associated biotin receptor (BR) and self-assembling into nanoparticles in response to intracellular glutathione. The tracer [18F]1 selectively targeted BR-positive cancer cells A549 and Hela and formed nanoparticles through self-assembly with an average diameter of 138.2 ± 16.3 nm. The character of self-assembly into nanoparticles enhanced the uptake and extended the retention of probe [18F]1 in the target tissue and hence improved the quality of positron emission tomography (PET) images. Thus, [18F]1 is a promising PET tracer for accurately detecting BR-positive cancers. Moreover, the tumor microenvironment responsive "head-to-foot" self-assembly nanoplatform is particularly attractive for development of other smart molecular probes.

68Ga]Ga-4HMSA a promising new PET tracer for imaging inflammation

BackgroundImaging diagnosis of inflammation has been challenging for many years. Inflammation imaging agents commonly used in nuclear medicine, such as [67Ga]Ga-citrate and 2-deoxy-2-[18F]fluoro-d-glucose ([18F]FDG) showed some limitations. The identification of a radiotracer with high specificity and low radiation dose is clinically important. With the commercialization of 68Ge/68Ga generators and the high 68Ga cyclotron production capacity, the study of 68Ga-based tracer for inflammation has increased and shown good potential. In the present work, we report the synthesis of 4HMSA, a new acyclic chelator, and its first investigation for 68Ga complexation and as a new positron emission tomography (PET) imaging agent of inflammation in comparison to [68Ga]Ga-citrate.ResultsThe present experimental studies have shown that the novel [68Ga]Ga-4HMSA is stable allowing imaging of inflammation in a preclinical model of adjuvant- and pathogen-based inflammation involving intraplantar injection of complete Freund’s adjuvant (CFA). We also found that [68Ga]Ga-4HMSA displayed similar uptakes in the inflamed paw than [68Ga]Ga-citrate, which are superior compared to those of contralateral (non-injected) paws at days 1–3 from PET imaging. [68Ga]Ga-citrate accumulated in the upper body of the animal such as the liver, lungs and the heart, whereas the [68Ga]Ga-4HMSA revealed low uptakes in the majority of the organs and was cleared relatively rapidly from blood circulation through the kidneys and bladder.ConclusionThe results highlight the potential of [68Ga]Ga-4HMSA as an interesting alternative to [68Ga]Ga-citrate for inflammation imaging by PET. The new PET tracer also offers additional advantages than [68Ga]Ga-citrate in term of dosimetry and lower overall background activity.

In vivo Imaging of Cannabinoid Type 2 Receptors: Functional and Structural Alterations in Mouse Model of Cerebral Ischemia by PET and MRI

PurposeStroke is one of the most prevalent vascular diseases. Non-invasive molecular imaging methods have the potential to provide critical insights into the temporal dynamics and follow alterations of receptor expression and metabolism in ischemic stroke. The aim of this study was to assess the cannabinoid type 2 receptor (CB2R) levels in transient middle cerebral artery occlusion (tMCAO) mouse models at subacute stage using positron emission tomography (PET) with our novel tracer [18F]RoSMA-18-d6 and structural imaging by magnetic resonance imaging (MRI).ProceduresOur recently developed CB2R PET tracer [18F]RoSMA-18-d6 was used for imaging neuroinflammation at 24 h after reperfusion in tMCAO mice. The RNA expression levels of CB2R and other inflammatory markers were analyzed by quantitative real-time polymerase chain reaction using brain tissues from tMCAO (1 h occlusion) and sham-operated mice. [18F]fluorodeoxyglucose (FDG) was included for evaluation of the cerebral metabolic rate of glucose (CMRglc). In addition, diffusion-weighted imaging and T2-weighted imaging were performed for anatomical reference and delineating the lesion in tMCAO mice.ResultsmRNA expressions of inflammatory markers TNF-α, Iba1, MMP9 and GFAP, CNR2 were increased to 1.3–2.5 fold at 24 h after reperfusion in the ipsilateral compared to contralateral hemisphere of tMCAO mice, while mRNA expression of the neuronal marker MAP-2 was markedly reduced to ca. 50 %. Reduced [18F]FDG uptake was observed in the ischemic striatum of tMCAO mouse brain at 24 h after reperfusion. Although higher activity of [18F]RoSMA-18-d6 in ex vivo biodistribution studies and higher standard uptake value ratio (SUVR) were detected in the ischemic ipsilateral compared to contralateral striatum in tMCAO mice, the in vivo specificity of [18F]RoSMA-18-d6 was confirmed only in the CB2R-rich spleen.ConclusionsThis study revealed an increased [18F]RoSMA-18-d6 measure of CB2R and a reduced [18F]FDG measure of CMRglc in the ischemic striatum of tMCAO mice at subacute stage. [18F]RoSMA-18-d6 might be a promising PET tracer for detecting CB2R alterations in animal models of neuroinflammation without neuronal loss.

Preclinical Evaluation of [11C]YC-72-AB85 for In Vivo Visualization of Heat Shock Protein 90 in Brain and Cancer with Positron Emission Tomography.

Aberrant Hsp90 has been implied in cancer and neurodegenerative disorders. The development of a suitable Hsp90 Positron emission tomography (PET) probe can provide in vivo quantification of the expression levels of Hsp90 as a biomarker for diagnosis and follow-up of cancer and central nervous system (CNS) disease progression. In this respect, [11C]YC-72-AB85 was evaluated as an Hsp90 PET probe in B16.F10 melanoma bearing mice and its brain uptake was determined in rats and nonhuman primate. In vitro binding of [11C]YC-72-AB85 to tissue slices of mouse B16.F10 melanoma, PC3 prostate carcinoma, and rodent brain was evaluated using autoradiography. Biodistribution of [11C]YC-72-AB85 was evaluated in healthy and B16.F10 melanoma mice. In vivo brain uptake was assessed by μPET studies in rats and a rhesus monkey. In vitro binding was deemed Hsp90-specific by blocking studies with heterologous Hsp90 inhibitors onalespib and SNX-0723. Saturable Hsp90 binding was observed in brain, tumor, blood, and blood-rich organs in mice. In combined pretreatment and displacement studies, reversible and Hsp90-specific binding of [11C]YC-72-AB85 was observed in rat brain. Dynamic μPET brain scans in baseline and blocking conditions in a rhesus monkey indicated Hsp90-specific binding. [11C]YC-72-AB85 is a promising PET tracer for in vivo visualization of Hsp90 in tumor and brain. Clear differences of Hsp90 binding to blood and blood-rich organs were observed in tumor vs control mice. Further, we clearly demonstrate, for the first time, binding to a saturable Hsp90 pool in brain of rats and a rhesus monkey.

Automated Radiosynthesis, Quality Control, and Biodistribution of Ga-68 Pentixafor: First Indian Experience.

Background:Chemokine receptor CXCR4 is overexpressed in more than 27 different human tumors that make it a promising target in oncology. Ga-68 Pentixafor is the most promising positron emission tomography tracer for imaging CXCR4 receptors; hence, the present study was carried out to optimize the radiosynthesis of Ga-68-Pentixafor using fully automated method and the quality control (QC) checks were performed before being used as a clinical product. We also studied the normal biodistribution pattern of Ga-68-pentixafor intended for the use in variety of malignancies.Materials and Methods:We optimized the automated radio-synthesis of Ga-68 Pentixafor under good manufacturing practice conditions. A total of 62 productions were carried out in a span of 4 years. Extensive QC tests were performed to check for potency, identity, efficacy, and stability of the tracer. Biodistribution of Ga-68 Pentixafor was investigated in a healthy volunteer to determine normal range of standardized uptake valuemaximum (SUVmax) values in various organs.Results:The radiotracer was prepared successfully in 57/62 productions with radiochemical purity of >99%. Mean radiolabelling efficiency of 73.1% ± 7.7% (n = 57) was obtained with synthesis time approximatively of 34 min. The radiolabeled complex showed no signs of dissociation up to 4 h at the room temperature. Ga-68 Pentixafor upon incubation with human serum was found to be stable at 37°C for 4 h. The highest normal organ uptake was seen in urinary bladder (SUVmean = 146.0), spleen (SUVmean = 6.80) followed by kidneys (SUVmean = 4.99).Conclusion:Using the automated radiosynthesis, Ga-68 Pentixafor exhibited good radiolabelling efficiency with excellent in vitro and in vivo stability and favorable biodistribution showing clinical applicability of the tracer.

Para-chloro-2-[18F]fluoroethyl-etomidate: A promising new PET radiotracer for adrenocortical imaging.

Introduction: [11C]Metomidate ([11C]MTO), the methyl ester analogue of etomidate, was developed as a positron emission tomography (PET) radiotracer for adrenocortical tumours and has also been suggested for imaging in primary aldosteronism (PA). A disadvantage of [11C]MTO is the rather high non-specific binding in the liver, which impacts both visualization and quantification of the uptake in the right adrenal gland. Furthermore, the short 20-minute half-life of carbon-11 is a logistic challenge in the clinical setting.Objectives: The aim of this study was to further evaluate the previously published fluorine-18 (T1/2=109.5 min) etomidate analogue, para-chloro-2-[18F]fluoroethyl etomidate; [18F]CETO, as an adrenal PET tracer.Methods: In vitro experiments included autoradiography on human and cynomolgus monkey (non-human primate, NHP) tissues and binding studies on adrenal tissue from NHPs. In vivo studies with [18F]CETO in mice, rats and NHP, using PET and CT/MRI, assessed biodistribution and binding specificity in comparison to [11C]MTO.Results: The binding of [18F]CETO in the normal adrenal cortex, as well as in human adrenocortical adenomas and adrenocortical carcinomas, was shown to be specific, both in vitro (in humans) and in vivo (in rats and NHP) with an in vitro Kd of 0.66 nM. Non-specific uptake of [18F]CETO in NHP liver was found to be low compared to that of [11C]MTO.Conclusions: High specificity of [18F]CETO to the adrenal cortex was demonstrated, with in vivo binding properties qualitatively surpassing those of [11C]MTO. Non-specific binding to the liver was significantly lower than that of [11C]MTO. [18F]CETO is a promising new PET tracer for imaging of adrenocortical disease and should be evaluated further in humans.

11C-metomidate PET in the diagnosis of adrenal masses and primary aldosteronism: a review of the literature.

Adrenal masses are commonly encountered in clinical practice, many of whom are incidental. Identifying malignancy, and excess hormone production is essential for appropriate management. Biochemical workup and imaging tests (dedicated adrenal CT and/or MRI) are used to determine the likelihood of excessive hormone function and malignancy, respectively. However, imaging cannot provide information about function and biochemical workup cannot localize the source. Furthermore, in primary aldosteronism, adrenal vein sampling, the gold standard for lateralization, has important limitations such as the technical expertise required, the elevated costs, and potential complications. Over the last decades, there has been a renewed interest in alternative noninvasive imaging techniques that provide information about adrenal function without the need for invasive procedures. In this review, we will evaluate the evidence and the potential role of 11C-metomidate as a promising positron emission tomography (PET) tracer in clinical practice. A review of the English literature for articles describing the use of the tracer 11C-metomidate in adrenal disorders. A total of 12 studies were included in the systematic review, which altogether addressed the use of 11C-metomidate in adrenal masses and the application of this tracer in primary aldosteronism. 11C-metomidate, a selective inhibitor of 11-β-hydroxylase, demonstrated a high specificity for adrenocortical tissue. In addition, 11C-metomidate is correlated with this enzyme activity making it a potentially useful PET tracer for the identification primary aldosteronism, in addition to detection of adrenocortical masses.

Hypoxia PET Imaging with [18F]-HX4-A Promising Next-Generation Tracer.

Hypoxia—a common feature of the majority of solid tumors—is a negative prognostic factor, as it is associated with invasion, metastasis and therapy resistance. To date, a variety of methods are available for the assessment of tumor hypoxia, including the use of positron emission tomography (PET). A plethora of hypoxia PET tracers, each with its own strengths and limitations, has been developed and successfully validated, thereby providing useful prognostic or predictive information. The current review focusses on [18F]-HX4, a promising next-generation hypoxia PET tracer. After a brief history of its development, we discuss and compare its characteristics with other hypoxia PET tracers and provide an update on its progression into the clinic. Lastly, we address the potential applications of assessing tumor hypoxia using [18F]-HX4, with a focus on improving patient-tailored therapies.