A Comparative pO2 Probe and [18F]-Fluoro-Azomycinarabino-Furanoside ([18F]FAZA) PET Study Reveals Anesthesia-Induced Impairment of Oxygenation and Perfusion in Tumor and Muscle

Tumor hypoxia can be identified by [¹⁸F]FAZA positron emission tomography, or invasively using oxygen probes. The impact of anesthetics on tumor hypoxia remains controversial. The aim of this comprehensive study was to investigate the impact of isoflurane and ketamine/xylazine anesthesia on [¹⁸F]FAZA uptake and partial oxygen pressure (pO₂) in carcinoma and muscle tissue of air- and oxygen-breathing mice.

Methods

CT26 colon carcinoma-bearing mice were anesthetized with isoflurane (IF) or ketamine/xylazine (KX) while breathing air or oxygen (O₂). We performed 10 min static PET scans 1 h, 2 h and 3 h after [¹⁸F]FAZA injection and calculated the [¹⁸F]FAZA-uptake and tumor-to-muscle ratios (T/M). In another experimental group, we placed a pO₂ probe in the tumor as well as in the gastrocnemius muscle to measure the pO₂ and perfusion.

Results

Ketamine/xylazine-anesthetized mice yielded up to 3.5-fold higher T/M-ratios compared to their isoflurane-anesthetized littermates 1 h, 2 h and 3 h after [¹⁸F]FAZA injection regardless of whether the mice breathed air or oxygen (3 h, KX-air: 7.1 vs. IF-air: 1.8, p = 0.0001, KX-O₂: 4.4 vs. IF-O₂: 1.4, p < 0.0001). The enhanced T/M-ratios in ketamine/xylazine-anesthetized mice were mainly caused by an increased [¹⁸F]FAZA uptake in the carcinomas. Invasive pO₂ probe measurements yielded enhanced intra-tumoral pO₂ values in air- and oxygen-breathing ketamine/xylazine-anesthetized mice compared to isoflurane-anesthetized mice (KX-air: 1.01 mmHg, IF-air: 0.45 mmHg; KX-O₂ 9.73 mmHg, IF-O₂: 6.25 mmHg). Muscle oxygenation was significantly higher in air-breathing isoflurane-anesthetized (56.9 mmHg) than in ketamine/xylazine-anesthetized mice (33.8 mmHg, p = 0.0003).

Conclusion

[¹⁸F]FAZA tumor uptake was highest in ketamine/xylazine-anesthetized mice regardless of whether the mice breathed air or oxygen. The generally lower [¹⁸F]FAZA whole-body uptake in isoflurane-anesthetized mice could be due to the higher muscle pO₂-values in these mice compared to ketamine/xylazine-anesthetized mice. When performing preclinical in vivo hypoxia PET studies, oxygen should be avoided, and ketamine/xylazine-anesthesia might alleviate the identification of tumor hypoxia areals.     

An Intra-Individual Comparison of MRI, [18F]-FET and [18F]-FLT PET in Patients with High-Grade Gliomas

Objectives

Intra-individual spatial overlap analysis of tumor volumes assessed by MRI, the amino acid PET tracer [¹⁸F]-FET and the nucleoside PET tracer [¹⁸F]-FLT in high-grade gliomas (HGG).

Methods

MRI, [¹⁸F]-FET and [¹⁸F]-FLT PET data sets were retrospectively analyzed in 23 HGG patients. Morphologic tumor volumes on MRI (post-contrast T1 (cT1) and T2 images) were calculated using a semi-automatic image segmentation method. Metabolic tumor volumes for [¹⁸F]-FET and [¹⁸F]-FLT PETs were determined by image segmentation using a threshold-based volume of interest analysis. After co-registration with MRI the morphologic and metabolic tumor volumes were compared on an intra-individual basis in order to estimate spatial overlaps using the Spearman''s rank correlation coefficient and the Mann-Whitney U test.

Results

[¹⁸F]-FLT uptake was negative in tumors with no or only moderate contrast enhancement on MRI, detecting only 21 of 23 (91%) HGG. In addition, [¹⁸F]-FLT uptake was mainly restricted to cT1 tumor areas on MRI and [¹⁸F]-FLT volumes strongly correlated with cT1 volumes (r = 0.841, p<0.001). In contrast, [¹⁸F]-FET PET detected 22 of 23 (96%) HGG. [¹⁸F]-FET uptake beyond areas of cT1 was found in 61% of cases and [¹⁸F]-FET volumes showed only a moderate correlation with cT1 volumes (r = 0.573, p<0.001). Metabolic tumor volumes beyond cT1 tumor areas were significantly larger for [¹⁸F]-FET compared to [¹⁸F]-FLT tracer uptake (8.3 vs. 2.7 cm³, p<0.001).

Conclusion

In HGG [¹⁸F]-FET but not [¹⁸F]-FLT PET was able to detect metabolic active tumor tissue beyond contrast enhancing tumor on MRI. In contrast to [¹⁸F]-FET, blood-brain barrier breakdown seems to be a prerequisite for [¹⁸F]-FLT tracer uptake.     

[18F]DPA-714: Direct Comparison with [11C]PK11195 in a Model of Cerebral Ischemia in Rats

Purpose

Neuroinflammation is involved in several brain disorders and can be monitored through expression of the translocator protein 18 kDa (TSPO) on activated microglia. In recent years, several new PET radioligands for TSPO have been evaluated in disease models. [¹⁸F]DPA-714 is a TSPO radiotracer with great promise; however results vary between different experimental models of neuroinflammation. To further examine the potential of [¹⁸F]DPA-714, it was compared directly to [¹¹C]PK11195 in experimental cerebral ischaemia in rats.

Methods

Under anaesthesia, the middle cerebral artery of adult rats was occluded for 60 min using the filament model. Rats were allowed recovery for 5 to 7 days before one hour dynamic PET scans with [¹¹C]PK11195 and/or [¹⁸F]DPA-714 under anaesthesia.

Results

Uptake of [¹¹C]PK11195 vs [¹⁸F]DPA-714 in the ischemic lesion was similar (core/contralateral ratio: 2.84±0.67 vs 2.28±0.34 respectively), but severity of the brain ischemia and hence ligand uptake in the lesion appeared to vary greatly between animals scanned with [¹¹C]PK11195 or with [¹⁸F]DPA-714. To solve this issue of inter-individual variability, we performed a direct comparison of [¹¹C]PK11195 and [¹⁸F]DPA-714 by scanning the same animals sequentially with both tracers within 24 h. In this direct comparison, the core/contralateral ratio (3.35±1.21 vs 4.66±2.50 for [¹¹C]PK11195 vs [¹⁸F]DPA-714 respectively) showed a significantly better signal-to-noise ratio (1.6 (1.3–1.9, 95%CI) fold by linear regression) for [¹⁸F]DPA-714.

Conclusions

In a clinically relevant model of neuroinflammation, uptake for both radiotracers appeared to be similar at first, but a high variability was observed in our model. Therefore, to truly compare tracers in such models, we performed scans with both tracers in the same animals. By doing so, our result demonstrated that [¹⁸F]DPA-714 displayed a higher signal-to-noise ratio than [¹¹C]PK11195. Our results suggest that, with the longer half-life of [¹⁸F] which facilitates distribution of the tracer across PET centre, [¹⁸F]DPA-714 is a good alternative for TSPO imaging.     

[18F]DPA-714 as a biomarker for positron emission tomography imaging of rheumatoid arthritis in an animal model

Introduction

Rheumatoid arthritis (RA) is a chronic disease, affecting 0.5 to 1% of adults in industrialized countries, in which systemic inflammation and synovitis drive joint destruction. [¹⁸F]DPA-714 is a specific tracer of the 18 kDa translocator protein (TSPO), which is overexpressed on activated macrophages, and proposed as a biomarker of neuroinflammation. Today, diagnosis of patients with early inflammatory arthritis is limited by poor sensitivity and specificity. The present study aims to investigate the potential of [¹⁸F]DPA-714 to monitor in vivo inflammatory processes at a preclinical stage via positron emission tomography (PET).

Methods

RA was induced in Dark Agouti rats by subcutaneous injection of inactivated Mycobacterium tuberculosis. Development of arthritis clinical signs was investigated daily and the severity of the disease evaluated. Animals were imaged at the peak of inflammation using [¹⁸F]DPA-714 and a small-animal PET-CT tomograph.

Results

The first clinical signs appeared at 10 days post-injection, with a peak of inflammation at 20 days. At this time, PET-analyses showed a clear uptake of [¹⁸F]DPA-714 in swollen ankles, with mean values of 0.52 ± 0.18% injected dose (ID/cc) for treated (n = 11) and 0.19 ± 0.09 for non-treated (n = 6) rats. A good correlation between [¹⁸F]DPA-714’s uptake and swelling was also found. Immunohistochemistry showed an enhanced TSPO expression in hind paws, mainly co-localized with the macrophages specific antigen CD68 expressing cells.

Conclusion

These preliminary results demonstrate that the TSPO 18kDa specific radioligand [¹⁸F]DPA-714 is adapted for the study and follow-up of inflammation linked to RA in our experimental model, suggesting also a strong potential for clinical imaging of peripheral inflammation.     

Use of liposomes as carriers for radiation synovectomy

Using [^99mTc]pertechnetate as an aqueous space marker, the permeability of liposomes composed of seven different mixtures of distearoylphosphatidylcholine (DSPC) and sphingomyelin (SM) was determined. Liposomes containing 20–33% SM were the least permeable in the presence of rheumatoid synovial fluid. Following injection of ^99mTc-containing liposomes into the knee joints of rabbits, retention of ^99mTc in the knee was more than 200 times greater than following injection of nonencapsulated [^99mTc]pertechnetate. The knee clearance biologic half time of ^99mTe with DSPC/SM (4:1) liposomes was 64 h. Most of the activity that had leaked from the knee was not found in extra-articular tissues, suggesting rapid excretion. When DSPC/SM (4:1) liposomes were labeled with ¹¹¹In(oxine), a knee clearance biologic half time of greater than 1200 h was observed.     

Permeability of gloves used in nuclear medicine departments to [99mTc]-pertechnetate and [18F]-fluorodeoxyglucose: Radiation protection considerations

In order to evaluate the safety of the individual protection devices, the permeability of four different types of disposable gloves, commonly used in hospitals, was tested in relation to [^99mTc]-pertechnetate and to [¹⁸F]-fluorodeoxyglucose ([¹⁸F]-FDG).From these radiopharmaceutical solutions, a drop was deposited on the external surface of the glove which was opened and stretched with the external surface placed upward. The smear test technique permitted to evaluate the activity onto the inner surface of the glove at different times. The smear tests were measured in a well sodium iodide detector calibrated in efficiency for ^99mTc and ¹⁸F. The permeability was tested on ten samples of each type of gloves and was expressed as the ratio of the activity onto the inner surface at each time interval to the activity deposited on the external surface of the glove. For each type of gloves and for each sampling time, mean value, standard deviation and percentage coefficient of variation of permeability were evaluated.One type of gloves showed a low resistance to permeation of both radiopharmaceuticals, while another one only to pertechnetate. The other gloves were good performers.The results of this study suggest to test permeability for gloves used for handling radiopharmaceuticals, before their adoption in the clinical routine. This practice will provide a more careful service of radiation protection for nuclear medicine department staff.     

Radiosynthesis and preliminary evaluation of 4-[18F]fluoro-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide as a new positron emission tomography ligand for metabotropic glutamate receptor subtype 1

The purpose of this study was to develop 4-[¹⁸F]fluoro-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ([¹⁸F]FITM, [¹⁸F]4) as a new PET ligand for imaging metabotropic glutamate receptor subtype 1 (mGluR1). [¹⁸F]4 was synthesized by [¹⁸F]fluorination of a novel nitro precursor 3 with [¹⁸F]KF in the presence of Kryptofix 222. At the end of synthesis, 429-936 MBq (n = 8) of [¹⁸F]4 was obtained with >99% radiochemical purity and 204-559 GBq/μmol specific activity starting from 6.7 to 13.0 GBq of [¹⁸F]F⁻. The brain distribution of [¹⁸F]4 was determined by the in vitro and ex vivo autoradiography using rat brain sections. The in vitro and in vivo specific binding of [¹⁸F]4 to mGluR1 was detected in the cerebellum, thalamus, hippocampus, and striatum. These results suggest that [¹⁸F]4 is a promising PET ligand for the in vivo evaluation of mGluR1.     

A 99mTc generator based on the adsorption of [99Mo]molybdophosphate on hydrous manganese dioxide

The adsorption behaviour on hydrous manganese dioxide of ⁹⁹Mo in the form of molybdate and molybdophosphate and of ^99mTc in the form of pertechnetate was investigated at different solution acidities. The adsorption capacity of molybdenum is much higher for the molybdophosphate form. A ^99mTc-generator is suggested. This generator is based on the adsorption of [⁹⁹Mo]molybdophosphate on MnO₂, at an acidity of 0.5 N. ^99mTc is then eluted either by 0.9% NaCl or by methyl ethyl ketone. Radionuclidic, radiochemical and chemical purity tests of the eluates were performed and the present generator seems to offer some advantages over the traditional alumina generator.     

Lung scintigraphy with [123I]IMP in patients with pulmonary tuberculosis

Lung scintigraphy using N-isopropyl-p-[¹²³I]iodoamphetamine (IMP) was performed on 26 patients with pulmonary tuberculosis. Early (5 min after injection) and late images (4 h after injection) were obtained with a large-field γ-camera equipped with a digital computer. Lung scintigraphy using [^99mTc]MAA (MAA) was also done. Although early IMP images showed the same findings as [^99mTc]MAA images, a discrepancy between delayed IMP images and [^99mTc]MAA images was seen in some patients. Increment of activities seen in late images was demonstrated in most patients whose chest x-ray findings included exudative inflammatory changes. Uptake and clearance of IMP was considered to be affected by the active phase of pulmonary tuberculosis.     

Development of a Novel PET Tracer [18F]AlF-NOTA-C6 Targeting MMP2 for Tumor Imaging

Background and Objective

The overexpression of gelatinases, that is, matrix metalloproteinase MMP2 and MMP9, has been associated with tumor progression, invasion, and metastasis. To image MMP2 in tumors, we developed a novel ligand termed [¹⁸F]AlF-NOTA-C6, with consideration that: c(KAHWGFTLD)NH₂ (herein, C6) is a selective gelatinase inhibitor; Cy5.5-C6 has been visualized in many in vivo tumor models; positron emission tomography (PET) has a higher detection sensitivity and a wider field of view than optical imaging; fluorine-18 (¹⁸F) is the optimal PET radioisotope, and the creation of a [¹⁸F]AlF-peptide complex is a simple procedure.

Methods

C6 was conjugated to the bifunctional chelator NOTA (1, 4, 7-triazacyclononanetriacetic acid) for radiolabeling [¹⁸F]AlF conjugation. The MMP2-binding characteristics and tumor-targeting efficacy of [¹⁸F]AlF-NOTA-C6 were tested in vitro and in vivo.

Results

The non-decay corrected yield of [¹⁸F]AlF-NOTA-C6 was 46.2–64.2%, and the radiochemical purity exceeded 95%. [¹⁸F]AlF-NOTA-C6 was favorably retained in SKOV3 and PC3 cells, determined by cell uptake. Using NOTA-C6 as a competitive ligand, the uptake of [¹⁸F]AlF-NOTA-C6 in SKOV3 cells decreased in a dose-dependent manner. In biodistribution and PET imaging studies, higher radioactivity concentrations were observed in tumors. Pre-injection of C6 caused a marked reduction in tumor tissue uptake. Immunohistochemistry showed MMP2 in tumor tissues.

Conclusions

[¹⁸F]AlF-NOTA-C6 was easy to synthesize and has substantial potential as an imaging agent that targets MMP2 in tumors.     

[18F]FDG Accumulation in Early Coronary Atherosclerotic Lesions in Pigs

Objective

Inflammation is an important contributor to atherosclerosis progression. A glucose analogue ¹⁸F-fluorodeoxyglucose ([¹⁸F]FDG) has been used to detect atherosclerotic inflammation. However, it is not known to what extent [¹⁸F]FDG is taken up in different stages of atherosclerosis. We aimed to study the uptake of [¹⁸F]FDG to various stages of coronary plaques in a pig model.

Methods

First, diabetes was caused by streptozotocin injections (50 mg/kg for 3 days) in farm pigs (n = 10). After 6 months on high-fat diet, pigs underwent dual-gated cardiac PET/CT to measure [¹⁸F]FDG uptake in coronary arteries. Coronary segments (n = 33) were harvested for ex vivo measurement of radioactivity and autoradiography (ARG).

Results

Intimal thickening was observed in 16 segments and atheroma type plaques in 10 segments. Compared with the normal vessel wall, ARG showed 1.7±0.7 times higher [¹⁸F]FDG accumulation in the intimal thickening and 4.1±2.3 times higher in the atheromas (P = 0.004 and P = 0.003, respectively). Ex vivo mean vessel-to-blood ratio was higher in segments with atheroma than those without atherosclerosis (2.6±1.2 vs. 1.3±0.7, P = 0.04). In vivo PET imaging showed the highest target-to-background ratio (TBR) of 2.7. However, maximum TBR was not significantly different in segments without atherosclerosis (1.1±0.5) and either intimal thickening (1.2±0.4, P = 1.0) or atheroma (1.6±0.6, P = 0.4).

Conclusions

We found increased uptake of [¹⁸F]FDG in coronary atherosclerotic lesions in a pig model. However, uptake in these early stage lesions was not detectable with in vivo PET imaging. Further studies are needed to clarify whether visible [¹⁸F]FDG uptake in coronary arteries represents more advanced, highly inflamed plaques.     

99mTc(Cu)—mannitol complex: a new agent for dynamic renal function studies

Mannitol has been labelled with ^99mTc by using cuprous chloride as a reducing agent. Blood and kidney clearance of ^99mTc(Cu)-mannitol was slightly faster than that of ^99mTc(Sn)-DTPA in rat and maximum radioactivity ratio of kidneys to blood was 84.6 at 5 min. A comparative study of ^99mTc(Cu)-mannitol, ^99mTc(Sn)-DTPA was made in rabbits by taking serial images of kidneys and bladder with a γ camera. Results show superiority of ^99mTc(Cu)-mannitol over other agents for dynamic renal function studies.     

Synthesis and biological evaluation of novel F-18 labeled pyrazolo[1,5-a]pyrimidine derivatives: potential PET imaging agents for tumor detection

Two novel pyrazolo[1,5-a]pyrimidine derivatives, 7-(2-[¹⁸F]fluoroethylamino)-5-methylpyrazolo[1,5-a]pyrimidine-3-carbonitrile ([¹⁸F]FEMPPC, [¹⁸F]1) and N-(2-(3-cyano-5-methylpyrazolo[1,5-a]pyrimidin-7-ylamino)ethyl)-2-[¹⁸F]fluoro-4-nitrobenzamide ([¹⁸F]FCMPPN, [¹⁸F]2), have been designed and successively labeled with ¹⁸F by the nucleophilic substitution employing tosylate and nitryl as leaving groups, respectively. The radiochemical synthesis of both compounds was completed within 60 min with final high-performance liquid chromatography purification included. The corresponding radiochemical yields (without decay correction) were approximately 35% and 30%, respectively. Meanwhile, we compared the uptake characteristics of [¹⁸F]1 and [¹⁸F]2 with those of [¹⁸F]FDG and L-[¹⁸F]FET in S180 tumor cells. Furthermore, the tumor uptake of [¹⁸F]1 and [¹⁸F]2 was assessed in mice bearing S180 tumor and compared with [¹⁸F]FDG and L-[¹⁸F]FET in the same animal model. In vitro cell uptake studies showed [¹⁸F]1 had higher uptake than [¹⁸F]FDG, [¹⁸F]2 and L-[¹⁸F]FET over the 2 h period. In ex vivo biodistribution showed tumor/brain uptake ratios of [¹⁸F]2 were 12.35, 10.44, 8.69 and 5.13 at 15 min, 30 min, 60 min and 120 min post-injection, much higher than those of L-[¹⁸F]FET (2.43, 2.54, 2.93 and 2.95) and [¹⁸F]FDG (0.59, 0.61, 1.02 and 1.33) at the same time point. What’s more, the uptake of [¹⁸F]1 in tumor was 1.88, 4.37, 5.51, 2.95 and 2.88 at 5 min, 15 min, 30 min, 60 min and 120 min post-injection, respectively. There was a remarkable increasing trend before 30 min. The same trend was present for L-[¹⁸F]FET before 30 min and [¹⁸F]FDG before 60 min. Additionally, the tumor/brain uptake ratios of [¹⁸F]1 were superior to those of [¹⁸F]FDG at all the selected time points, the tumor/muscle and tumor/blood uptake ratios of [¹⁸F]1 at 30 min were higher than those of L-[¹⁸F]FET at the same time point. MicroPET image of [¹⁸F]1 administered into S180 tumor-bearing mouse acquired at 30 min post-injection illustrated that the uptake in S180 tumor was obvious. These results suggest that compound [¹⁸F]1 could be a new probe for PET tumor imaging.     

Metastatic melanoma imaging using a novel Tc-99m-labeled lactam-cyclized alpha-MSH peptide

The purpose of this study was to determine the metastatic melanoma imaging property of ^99mTc(EDDA)-HYNIC-Aoc-Nle-CycMSH_hex {hydrazinonicotinamide-8-aminooctanoic acid-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-CONH₂}. HYNIC-Aoc-Nle-CycMSH_hex was synthesized using fluorenylmethyloxy carbonyl (Fmoc) chemistry. The IC₅₀ value of HYNIC-Aoc-Nle-CycMSH_hex was 0.78?±?0.13?nM for B16/F10 melanoma cells. ^99mTc(EDDA)-HYNIC-Aoc-Nle-CycMSH_hex displayed significantly higher uptake (14.26?±?2.74 and 10.45?±?2.31%?ID/g) in B16/F10 metastatic melanoma-bearing lung than that in normal lung (0.90?±?0.15 and 0.53?±?0.14%?ID/g) at 2 and 4?h post-injection, respectively. B16/F10 pulmonary metastatic melanoma lesions were clearly visualized by SPECT/CT using ^99mTc(EDDA)-HYNIC-Aoc-Nle-CycMSH_hex as an imaging probe at 2?h post-injection, underscoring its potential as an imaging probe for metastatic melanoma detection.     

Evaluation of the novel folate receptor ligand [18F]fluoro-PEG-folate for macrophage targeting in a rat model of arthritis

Introduction

Detection of (subclinical) synovitis is relevant for both early diagnosis and monitoring of therapy of rheumatoid arthritis (RA). Previously, the potential of imaging (sub)clinical arthritis was demonstrated by targeting the translocator protein in activated macrophages using (R)-[¹¹C]PK11195 and positron emission tomography (PET). Images, however, also showed significant peri-articular background activity. The folate receptor (FR)-β is a potential alternative target for imaging activated macrophages. Therefore, the PET tracer [¹⁸F]fluoro-PEG-folate was synthesized and evaluated in both in vitro and ex vivo studies using a methylated BSA induced arthritis model.

Methods

[¹⁸F]fluoro-PEG-folate was synthesized in a two-step procedure. Relative binding affinities of non-radioactive fluoro-PEG-folate, folic acid and naturally circulating 5-methyltetrahydrofolate (5-Me-THF) to FR were determined using KB cells with high expression of FR. Both in vivo [¹⁸F]fluoro-PEG-folate PET and ex vivo tissue distribution studies were performed in arthritic and normal rats and results were compared with those of the established macrophage tracer (R)-[¹¹C]PK11195.

Results

[¹⁸F]fluoro-PEG-folate was synthesized with a purity >97%, a yield of 300 to 1,700 MBq and a specific activity between 40 and 70 GBq/µmol. Relative in vitro binding affinity for FR of F-PEG-folate was 1.8-fold lower than that of folic acid, but 3-fold higher than that of 5-Me-THF. In the rat model, [¹⁸F]fluoro-PEG-folate uptake in arthritic knees was increased compared with both contralateral knees and knees of normal rats. Uptake in arthritic knees could be blocked by an excess of glucosamine-folate, consistent with [¹⁸F]fluoro-PEG-folate being specifically bound to FR. Arthritic knee-to-bone and arthritic knee-to-blood ratios of [¹⁸F]fluoro-PEG-folate were increased compared with those of (R)-[¹¹C]PK11195. Reduction of 5-Me-THF levels in rat plasma to those mimicking human levels increased absolute [¹⁸F]fluoro-PEG-folate uptake in arthritic joints, but without improving target-to-background ratios.

Conclusions

The novel PET tracer [¹⁸F]fluoro-PEG-folate, designed to target FR on activated macrophages provided improved contrast in a rat model of arthritis compared with the accepted macrophage tracer (R)-[¹¹C]PK11195. These results warrant further exploration of [¹⁸F]fluoro-PEG-folate as a putative PET tracer for imaging (sub)clinical arthritis in RA patients.     

Assessment of radionuclide impurities in [18F]fluoromethylcholine ([18F]FMCH)

Purpose[¹⁸F]Fluoromethylcholine ([¹⁸F]FMCH) is a radiopharmaceutical used in positron emission tomography (PET) imaging for the study of prostate, breast, and brain tumors. It is usually synthesized in cyclotron facilities where ¹⁸F is produced by proton irradiation of [¹⁸O]H₂O through ¹⁸O(p,n)¹⁸F reaction. Due to the activation of target materials, the bombardment causes unwanted radionuclidic impurities in [¹⁸O]H₂O, that need to be removed during the radiopharmaceutical synthesis. Thus, the aim of this study is to quantify the radionuclide impurities in the ¹⁸F production process and in the synthesized [¹⁸F]FMCH, demonstrating the radionuclidic purity of this radiopharmaceutical.MethodsLong-lived radionuclide impurities were experimentally assessed using high-resolution gamma and liquid scintillation spectrometries, while short-lived impurities were monitored analyzing the decay curve of the irradiated [¹⁸O]H₂O with an activity calibrator. As spectrometric radionuclide library, a Geant4 Monte Carlo simulation of the ¹⁸F-target assembly was previously performed.Results³H, ^52,54Mn, ^56,57,58Co, ^95m,96Tc, ¹⁰⁹Cd, and ¹⁸⁴Re were found in the irradiated [¹⁸O]H₂O, but no radionuclide was found in the non-irradiated [¹⁸O]H₂O neither in the final [¹⁸F]FMCH solution with an activity concentration greater than the minimum detectable activity concentration. A total impurity activity <6.2 kBq was measured in the irradiated [¹⁸O]H₂O, whereas a [¹⁸F]FMCH radionuclide purity >99.9999998% was estimated. Finally, the decay curve of the irradiated [¹⁸O]H₂O revealed a very low maximum of ¹³N activity (<0.03% of ¹⁸F) even immediately after the end of bombardment.ConclusionsThis study demonstrated the radionuclidic purity of [¹⁸F]FMCH according to the EU Pharmacopeia.     

[18F]FLT and [18F]FDG PET for Non-invasive Treatment Monitoring of the Nicotinamide Phosphoribosyltransferase Inhibitor APO866 in Human Xenografts

Introduction

APO866 is a new anti-tumor compound inhibiting nicotinamide phosphoribosyltransferase (NAMPT). APO866 has an anti-tumor effect in several pre-clinical tumor models and is currently in several clinical phase II studies. 3′-deoxy-3′-[18F]fluorothymidine ([18F]FLT) is a tracer used to assess cell proliferation in vivo. The aim of this study was non-invasively to study effect of APO866 treatment on [18F]FLT and 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) uptake.

Methods

In vivo uptake of [18F]FLT and [18F]FDG in human ovary cancer xenografts in mice (A2780) was studied at various time points after APO866 treatment. Baseline [18F]FLT or [18F]FDG scans were made before treatment and repeated after 24 hours, 48 hours and 7 days. Tumor volume was followed with computed tomography (CT). Tracer uptake was quantified using small animal PET/CT. One hour after iv injection of tracer, static PET scans were performed. Imaging results were compared with Ki67 immunohistochemistry.

Results

Tumors treated with APO866 had volumes that were 114% (24 h), 128% (48 h) and 130% (Day 7) relative to baseline volumes at Day 0. In the control group tumor volumes were 118% (24 h), 145% (48 h) and 339% (Day 7) relative to baseline volumes Day 0. Tumor volume between the treatment and control group was significantly different at Day 7 (P = 0.001). Compared to baseline, [18F]FLT SUVmax was significantly different at 24 h (P<0.001), 48 h (P<0.001) and Day 7 (P<0.001) in the APO866 group. Compared to baseline, [18F]FDG SUVmax was significantly different at Day 7 (P = 0.005) in the APO866 group.

Conclusions

APO866 treatment caused a significant decrease in [18F]FLT uptake 24 and 48 hours after treatment initiation. The early reductions in tumor cell proliferation preceded decrease in tumor volume. The results show the possibility to use [18F]FLT and [18F]FDG to image treatment effect early following treatment with APO866 in future clinical studies.     

Limits of [18F]-FLT PET as a Biomarker of Proliferation in Oncology

Background

Non-invasive imaging biomarkers of cellular proliferation hold great promise for quantifying response to personalized medicine in oncology. An emerging approach to assess tumor proliferation utilizes the positron emission tomography (PET) tracer 3’-deoxy-3’[¹⁸F]-fluorothymidine, [¹⁸F]-FLT. Though several studies have associated serial changes in [¹⁸F]-FLT-PET with elements of therapeutic response, the degree to which [¹⁸F]-FLT-PET quantitatively reflects proliferative index has been continuously debated for more that a decade. The goal of this study was to elucidate quantitative relationships between [¹⁸F]-FLT-PET and cellular metrics of proliferation in treatment naïve human cell line xenografts commonly employed in cancer research.

Methods and Findings

[¹⁸F]-FLT-PET was conducted in human cancer xenograft-bearing mice. Quantitative relationships between PET, thymidine kinase 1 (TK1) protein levels and immunostaining for proliferation markers (Ki67, TK1, PCNA) were evaluated using imaging-matched tumor specimens. Overall, we determined that [¹⁸F]-FLT-PET reflects TK1 protein levels, yet the cell cycle specificity of TK1 expression and the extent to which tumors utilize thymidine salvage for DNA synthesis decouple [¹⁸F]-FLT-PET data from standard estimates of proliferative index.

Conclusions

Our findings illustrate that [¹⁸F]-FLT-PET reflects tumor proliferation as a function of thymidine salvage pathway utilization. Unlike more general proliferation markers, such as Ki67, [¹⁸F]-FLT PET reflects proliferative indices to variable and potentially unreliable extents. [¹⁸F]-FLT-PET cannot discriminate moderately proliferative, thymidine salvage-driven tumors from those of high proliferative index that rely primarily upon de novo thymidine synthesis. Accordingly, the magnitude of [¹⁸F]-FLT uptake should not be considered a surrogate of proliferative index. These data rationalize the diversity of [¹⁸F]-FLT-PET correlative results previously reported and suggest future best-practices when [¹⁸F]-FLT-PET is employed in oncology.     

Early [18F]FET-PET in Gliomas after Surgical Resection: Comparison with MRI and Histopathology

Background

The precise definition of the post-operative resection status in high-grade gliomas (HGG) is crucial for further management. We aimed to assess the feasibility of assessment of the resection status with early post-operative positron emission tomography (PET) using [¹⁸F]O-(2-[¹⁸F]-fluoroethyl)-L-tyrosine ([¹⁸F]FET).

Methods

25 patients with the suspicion of primary HGG were enrolled. All patients underwent pre-operative [¹⁸F]FET-PET and magnetic resonance imaging (MRI). Intra-operatively, resection status was assessed using 5-aminolevulinic acid (5-ALA). Imaging was repeated within 72h after neurosurgery. Post-operative [¹⁸F]FET-PET was compared with MRI, intra-operative assessment and clinical follow-up.

Results

[¹⁸F]FET-PET, MRI and intra-operative assessment consistently revealed complete resection in 12/25 (48%) patients and incomplete resection in 6/25 cases (24%). In 7 patients, PET revealed discordant findings. One patient was re-resected. 3/7 experienced tumor recurrence, 3/7 died shortly after brain surgery.

Conclusion

Early assessment of the resection status in HGG with [¹⁸F]FET-PET seems to be feasible.     

Synthesis and biological evaluation of anti-1-amino-2-[18F]fluoro-cyclobutyl-1-carboxylic acid (anti-2-[18F]FACBC) in rat 9L gliosarcoma

A new [¹⁸F] labeled amino acid anti-1-amino-2-[¹⁸F]fluoro-cyclobutyl-1-carboxylic acid 9 (anti-2-[¹⁸F]FACBC) was synthesized in 30% decay-corrected yield with high radiochemical purity over 99%. The cyclic sulfamidate precursor was very stable and highly reactive towards nucleophilic radiofluorination. Cell uptake assays with rat 9L gliosarcoma cells showed that [¹⁸F]9 was transported into tumor cells via multiple amino acid transport systems, including L and A systems. Biodistribution study in rats with intracranial 9L gliosarcoma tumors demonstrated that [¹⁸F]9 had a rapid and prolonged accumulation in tumors with 26:1 tumor to brain ratio at 120 min post-injection. In this model, [¹⁸F]9 is a potential PET tracer for brain tumor imaging.