Fully automated one-pot radiosynthesis of O-(2-[18F]fluoroethyl)-L-tyrosine on the TracerLab FX(FN) module

State of the art procedures towards reactive [18F]fluoride in PET tracer synthesis

BACKGROUND

Positron emission tomography (PET) is a powerful, non-invasive preclinical and clinical nuclear imaging technique used in disease diagnosis and therapy assessment. Fluorine-18 is the predominant radionuclide used for PET tracer synthesis. An impressive variety of new 'late-stage' radiolabeling methodologies for the preparation of 18F-labeled tracers has appeared in order to improve the efficiency of the labeling reaction.

MAIN BODY

Despite these developments, one outstanding challenge into the early key steps of the process remains: the preparation of reactive [18F]fluoride from oxygen-18 enriched water ([18O]H2O). In the last decade, significant changes into the trapping, elution and drying stages have been introduced. This review provides an overview of the strategies and recent developments in the production of reactive [18F]fluoride and its use for radiolabeling.

CONCLUSION

Improved, modified or even completely new fluorine-18 work-up procedures have been developed in the last decade with widespread use in base-sensitive nucleophilic 18F-fluorination reactions. The many promising developments may lead to a few standardized drying methodologies for the routine production of a broad scale of PET tracers.

Development of an economical method to synthesize O-(2-[18 F]fluoroethyl)-L-tyrosine (18 FFET)

Positron emission tomography (PET) using O-(2-[18 F]fluoroethyl)-L-tyrosine ([18 F]FET) has shown great success in differentiating tumor recurrence from necrosis. In this study, we are reporting the experience of synthesis [18 F]FET by varying the concentration of TET precursor in different chemistry modules. TET precursor (2-10 mg) was used for the synthesis of [18 F]FET in an automated (MX Tracerlab) module (n = 6) and semiautomated (FX2N Tracerlab) module (n = 19). The quality control was performed for all the preparations. For human imaging, 220 ± 50 MBq of [18 F]FET was briefly injected into the patient to acquire PET-MR images. The radiochemical purity was greater than 95% for the final product in both modules. The decay corrected average yield was 10.7 ± 4.7% (10 mg, n = 3) and 8.2 ± 2.6% (2 mg, n = 3) with automated chemistry module and 36.7 ± 7.3% (8-10 mg, n = 12), 26.4 ± 3.1% (5-7 mg, n = 4), and 35.1 ± 3.8% (2-4 mg, n = 3) with semiautomated chemistry modules. The PET imaging showed uptake at the lesion site (SUVmax  = 7.5 ± 2.6) and concordance with the MR image. The [18 F]FET was produced with a higher radiochemical yield with 2.0 mg of the precursor with substantial yield and is suitable for brain tumor imaging.

[18F]FET-PET in children and adolescents with central nervous system tumors: does it support difficult clinical decision-making?

PURPOSE

Positron emission tomography (PET) with O-(2-[¹⁸F]fluoroethyl)-L-tyrosine ([¹⁸F]FET) is a well-established tool for non-invasive assessment of adult central nervous system (CNS) tumors. However, data on its diagnostic utility and impact on clinical management in children and adolescents are limited.

METHODS

Twenty-one children and young adults (13 males; mean age, 8.6 ± 5.2 years; range, 1-19 at initial diagnosis) with either newly diagnosed (n = 5) or pretreated (n = 16) CNS tumors were retrospectively analyzed. All patients had previously undergone neuro-oncological work-up including cranial magnetic resonance imaging. In all cases, [¹⁸F]FET-PET was indicated in a multidisciplinary team conference. The impact of PET imaging on clinical decision-making was assessed. Histopathology (n = 12) and/or clinical and imaging follow-up (n = 9) served as the standard of reference.

RESULTS

The addition of [¹⁸F]FET-PET to the available information had an impact on further patient management in 14 out of 21 subjects, with avoidance of invasive surgery or biopsy in four patients, biopsy guidance in four patients, change of further treatment in another five patients, and confirmation of diagnosis in one patient.

CONCLUSION

[¹⁸F]FET-PET may provide important additional information for treatment guidance in pediatric and adolescent patients with CNS tumors.

Use of multimodality imaging, histology, and treatment feasibility to characterize a transgenic Rag2-null rat model of glioblastoma

Many drugs that show potential in animal models of glioblastoma (GBM) fail to translate to the clinic, contributing to a paucity of new therapeutic options. In addition, animal model development often includes histologic assessment, but multiparametric/multimodality imaging is rarely included despite increasing utilization in patient cancer management. This study developed an intracranial recurrent, drug-resistant, human-derived glioblastoma tumor in Sprague-Dawley Rag2-Rag2 ^tm1Hera knockout rat and was characterized both histologically and using multiparametric/multimodality neuroimaging. Hybrid ¹⁸F-fluoroethyltyrosine positron emission tomography and magnetic resonance imaging, including chemical exchange saturation transfer (¹⁸F-FET PET/CEST MRI), was performed for full tumor viability determination and characterization. Histological analysis demonstrated human-like GBM features of the intracranially implanted tumor, with rapid tumor cell proliferation (Ki67 positivity: 30.5 ± 7.8%) and neovascular heterogeneity (von Willebrand factor VIII:1.8 to 5.0% positivity). Early serial MRI followed by simultaneous ¹⁸F-FET PET/CEST MRI demonstrated consistent, predictable tumor growth, with exponential tumor growth most evident between days 35 and 49 post-implantation. In a second, larger cohort of rats, ¹⁸F-FET PET/CEST MRI was performed in mature tumors (day 49 post-implantation) for biomarker determination, followed by evaluation of single and combination therapy as part of the model development and validation. The mean percentage of the injected dose per mL of ¹⁸F-FET PET correlated with the mean %CEST (r = 0.67, P < 0.05), but there was also a qualitative difference in hot spot location within the tumor, indicating complementary information regarding the tumor cell demand for amino acids and tumor intracellular mobile phase protein levels. Finally, the use of this glioblastoma animal model for therapy assessment was validated by its increased overall survival after treatment with combination therapy (temozolomide and idasanutlin) (P < 0.001). Our findings hold promise for a more accurate tumor viability determination and novel therapy assessment in vivo in a recently developed, reproducible, intracranial, PDX GBM.

Determination of Total Radiochemical Purity of [18F]FDG and [18F]FET by High-Performance Liquid Chromatography Avoiding TLC Method

The goal of this work was to present two high-performance liquid chromatography (HPLC) method that could be applied for the determination of the total radioactive purity of 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) and O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET). The separation of [18F]fluoride ions, [18F]FET and [18F]FET intermediate was accomplished on LiChrosper RP-18, 250 × 4 mm, 5 µm (Merck) analytical column. For mobile phase 10 mM potassium dihydrogen phosphate buffer at pH7 (A) and acetonitrile (B) was used: 0–2 min: 15% B; 2–12 min: 85% B; 12–15 min: 15% B, respectively. Analysis of [18F]FDG was performed using LiChrosper 100 NH2, 250 × 4.5 mm, 5 µm (Merck) analytical column. The initial mobile phase composition was 10 mM KH2PO4 buffer (pH7) and acetonitrile (15:85, v/v) and the acetonitrile ratio was decreased to 15% at 2 min after the sample injection and held for 5 min. Complete elution of [18F]fluoride ions from stationary phases could be achieved by adding 10 mg/mL K[19F]F to radioactive samples in a ratio 1:1 during the sample preparation. Recovery of [18F]fluoride ions ranged from 99.5 to 100.6%. The validation of the developed methods showed good results for linearity (r2 = 0.9981–0.9996), specificity (RS = 3.7–10.2), repeatability (%Area RSD% = 1.2–4.3%) and limit of quantitation (LOQ = 1.6–4.5 kBq). During the cross-validation similar radiochemical purity values were obtained by the novel HPLC methods and thin layer chromatography performed according to the recommendations of the Ph. Eur. monographs.

A generic gas chromatography method for determination of residual solvents in PET radiopharmaceuticals

A novel gas chromatography (GC) method for quantitation of volatile organic compounds (VOCs) in ¹⁸F- and ¹¹C-radiopharmaceuticals listed in the European Pharmacopoeia (Ph. Eur.) was proposed. Optimized chromatographic parameters were used for separation of ethanol, acetone, acetonitrile, tetrahydrofuran (THF), dibromomethane (DBM), 2-dimethylaminoethanol (deanol), N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) which could be detected in radioactive drug samples. The calculated peak resolutions (R_S) were higher than 2.0 at ethanol concentration of up to 11 m/m%. Reproducible results could be obtained using base deactivated fused silica wool as packing material of inlet liner. Validation parameters showed excellent linearity (r² ≥0.9998) in the range from 10 to at least 120% of concentration limit of solvents. The accuracy was determined as recovery of concentrations which ranged from 99.3% to 103.8%. Additionally, the relative standard deviation (RSD) of each solvent for inter-day and intra-day precision were in the range of 0.5-4.2% and 0.4-4.4%, respectively. The limit of quantitation (LOQ) for ethanol, acetone, acetonitrile, THF, DBM, deanol, DMF and DMSO was 0.48, 0.42, 0.43, 0.46, 4.35, 0.73, 0.68 and 0.50 mg/L, respectively. The developed procedure was successively applied for quantitation of ethanol, acetone, acetonitrile and deanol in radioactive drug samples of [¹¹C]methionine, [¹¹C]choline, 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) and O-(2-[¹⁸F]fluoroethyl)-L-tyrosine ([¹⁸F]FET). The proposed GC method applying flame ionization detection (FID) could be adapted in routine quality control of most frequently used positron emission tomography (PET) radiopharmaceuticals to perform the determination of residual solvents with analysis time of 12 min.

Economical droplet-based microfluidic production of [18F]FET and [18F]Florbetaben suitable for human use

Current equipment and methods for preparation of radiopharmaceuticals for positron emission tomography (PET) are expensive and best suited for large-scale multi-doses batches. Microfluidic radiosynthesizers have been shown to provide an economic approach to synthesize these compounds in smaller quantities, but can also be scaled to clinically-relevant levels. Batch microfluidic approaches, in particular, offer significant reduction in system size and reagent consumption. Here we show a simple and rapid technique to concentrate the radioisotope, prior to synthesis in a droplet-based radiosynthesizer, enabling production of clinically-relevant batches of [18F]FET and [18F]FBB. The synthesis was carried out with an automated synthesizer platform based on a disposable Teflon-silicon surface-tension trap chip. Up to 0.1 mL (4 GBq) of radioactivity was used per synthesis by drying cyclotron-produced aqueous [18F]fluoride in small increments directly inside the reaction site. Precursor solution (10 µL) was added to the dried [18F]fluoride, the reaction chip was heated for 5 min to perform radiofluorination, and then a deprotection step was performed with addition of acid solution and heating. The product was recovered in 80 µL volume and transferred to analytical HPLC for purification. Purified product was formulated via evaporation and resuspension or a micro-SPE formulation system. Quality control testing was performed on 3 sequential batches of each tracer. The method afforded production of up to 0.8 GBq of [18F]FET and [18F]FBB. Each production was completed within an hour. All batches passed quality control testing, confirming suitability for human use. In summary, we present a simple and efficient synthesis of clinically-relevant batches of [18F]FET and [18F]FBB using a microfluidic radiosynthesizer. This work demonstrates that the droplet-based micro-radiosynthesizer has a potential for batch-on-demand synthesis of 18F-labeled radiopharmaceuticals for human use.

A concentration-based microscale method for 18F-nucleophilic substitutions and its testing on the one-pot radiosynthesis of [18F]FET and [18F]fallypride

When applied to a radiosynthesis, a microscale approach can help to save precursor and improve yields. Thus, a 5-10 μL microscale method based on a concentration procedure was developed and applied to the radiosynthesis of [¹⁸F]FET and [¹⁸F]fallypride. In spite of using an amount of precursor ca. 100 times smaller, radiochemical yields were comparable or even higher than those reported in literature. Because of the very low reaction volumes, the possible effects of concentrated dose of activity and carrier fluoride were also investigated.

TSPO-PET and diffusion-weighted MRI for imaging a mouse model of infiltrative human glioma

BACKGROUND

Glioblastoma (GBM) is the most devastating brain tumor. Despite the use of multimodal treatments, most patients relapse, often due to the highly invasive nature of gliomas. However, the detection of glioma infiltration remains challenging. The aim of this study was to assess advanced PET and MRI techniques for visualizing biological activity and infiltration of the tumor.

METHODS

Using multimodality imaging, we investigated [18F]DPA-714, a radiotracer targeting the 18 kDa translocator protein (TSPO), [18F]FET PET, non-Gaussian diffusion MRI (apparent diffusion coefficient, kurtosis), and the S-index, a composite diffusion metric, to detect tumor infiltration in a human invasive glioma model. In vivo imaging findings were confirmed by autoradiography and immunofluorescence.

RESULTS

Increased tumor-to-contralateral [18F]DPA-714 uptake ratios (1.49 ± 0.11) were found starting 7 weeks after glioma cell implantation. TSPO-PET allowed visualization of glioma infiltration into the contralateral hemisphere 2 weeks earlier compared with the clinically relevant biomarker for biological glioma activity [18F]FET. Diffusion-weighted imaging (DWI), in particular kurtosis, was more sensitive than standard T2-weighted MRI to detect differences between the glioma-bearing and the contralateral hemisphere at 5 weeks. Immunofluorescence data reflect in vivo findings. Interestingly, labeling for tumoral and stromal TSPO indicates a predominant expression of TSPO by tumor cells.

CONCLUSION

These results suggest that advanced PET and MRI methods, such as [18F]DPA-714 and DWI, may be superior to standard imaging methods to visualize glioma growth and infiltration at an early stage.

Current trends in the use of O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET) in neurooncology

The diagnostic potential of PET using the amino acid analogue O-(2-[¹⁸F]fluoroethyl)-L-tyrosine ([¹⁸F]FET) in brain tumor diagnostics has been proven in many studies during the last two decades and is still the subject of multiple studies every year. In addition to standard magnetic resonance imaging (MRI), positron emission tomography (PET) using [¹⁸F]FET provides important diagnostic data concerning brain tumor delineation, therapy planning, treatment monitoring, and improved differentiation between treatment-related changes and tumor recurrence. The pharmacokinetics, uptake mechanisms and metabolism have been well described in various preclinical studies. The accumulation of [¹⁸F]FET in most benign lesions and healthy brain tissue has been shown to be low, thus providing a high contrast between tumor tissue and benign tissue alterations. Based on logistic advantages of F-18 labelling and convincing clinical results, [¹⁸F]FET has widely replaced short lived amino acid tracers such as L-[¹¹C]methyl-methionine ([¹¹C]MET) in many centers across Western Europe. This review summarizes the basic knowledge on [¹⁸F]FET and its contribution to the care of patients with brain tumors. In particular, recent studies about specificity, possible pitfalls, and the utility of [¹⁸F]FET PET in tumor grading and prognostication regarding the revised WHO classification of brain tumors are addressed.

Rapid, efficient, and economical synthesis of PET tracers in a droplet microreactor: application to O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET)

BACKGROUND

Conventional scale production of small batches of PET tracers (e.g. for preclinical imaging) is an inefficient use of resources. Using O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET), we demonstrate that simple microvolume radiosynthesis techniques can improve the efficiency of production by consuming tiny amounts of precursor, and maintaining high molar activity of the tracers even with low starting activity.

PROCEDURES

The synthesis was carried out in microvolume droplets manipulated on a disposable patterned silicon "chip" affixed to a heater. A droplet of [18F]fluoride containing TBAHCO3 was first deposited onto a chip and dried at 100 °C. Subsequently, a droplet containing 60 nmol of precursor was added to the chip and the fluorination reaction was performed at 90 °C for 5 min. Removal of protecting groups was accomplished with a droplet of HCl heated at 90 °C for 3 min. Finally, the crude product was collected in a methanol-water mixture, purified via analytical-scale radio-HPLC and formulated in saline. As a demonstration, using [18F]FET produced on the chip, we prepared aliquots with different molar activities to explore the impact on preclinical PET imaging of tumor-bearing mice.

RESULTS

The microdroplet synthesis exhibited an overall decay-corrected radiochemical yield of 55 ± 7% (n = 4) after purification and formulation. When automated, the synthesis could be completed in 35 min. Starting with < 370 MBq of activity, ~ 150 MBq of [18F]FET could be produced, sufficient for multiple in vivo experiments, with high molar activities (48-119 GBq/μmol). The demonstration imaging study revealed the uptake of [18F]FET in subcutaneous tumors, but no significant differences in tumor uptake as a result of molar activity differences (ranging 0.37-48 GBq/μmol) were observed.

CONCLUSIONS

A microdroplet synthesis of [18F]FET was developed demonstrating low reagent consumption, high yield, and high molar activity. The approach can be expanded to tracers other than [18F]FET, and adapted to produce higher quantities of the tracer sufficient for clinical PET imaging.

Facile fully automated radiosynthesis and quality control of O-(2-[18F]fluoroethyl)-l-tyrosine ([18F]FET) for human brain tumor imaging

O-(2-[¹⁸F]Fluoroethyl)-_l-tyrosine ([¹⁸F]FET) has become one of the most successful amino acid tracers for human brain tumor imaging with positron emission tomography (PET). Facile fully automated radiosynthesis and quality control (QC) of [¹⁸F]FET using our home-built automated multi-purpose ¹⁸F-radiosynthesis module are described. [¹⁸F]FET was produced in 75-80 min overall synthesis time with 20-25% radiochemical yield decay corrected to end of bombardment (EOB), based on H[¹⁸F]F. The radiochemical and enantiomeric purities were >99%, and the molar activity (A_m) was 189-411 GBq/μmol at EOB. The [¹⁸F]FET dose meets all QC criteria for clinical use, and is suitable for clinical PET study of brain tumor.

18F-fluoromisonidazole positron emission tomography may be applicable in the evaluation of colorectal cancer liver metastasis

BACKGROUND

Positron emission tomography (PET) imaging is a non-invasive functional imaging method used to reflect tumor spatial information, and to provide biological characteristics of tumor progression. The aim of this study was to focus on the application of ¹⁸F-fluoromisonidazole (FMISO) PET quantitative parameter of maximum standardized uptake value (SUVmax) ratio to detect the liver metastatic potential of human colorectal cancer (CRC) in mice.

METHODS

Colorectal liver metastases (CRLM) xenograft models were established by injecting tumor cells (LoVo, HT29 and HCT116) into spleen of mice, tumor-bearing xenograft models were established by subcutaneously injecting tumor cells in the right left flank of mice. Wound healing assays were performed to examine the ability of cell migration in vitro. ¹⁸F-FMISO uptake in CRC cell lines was measured by cellular uptake assay. ¹⁸F-FMISO-based micro-PET imaging of CRLM and tumor-bearing mice was performed and quantified by tumor-to-liver SUVmax ratio. The correlation between the ¹⁸F-FMISO SUVmax ratio, liver metastases number, hypoxia-induced factor 1α (HIF-1α) and serum starvation-induced glucose transporter 1 (GLUT-1) was evaluated using Pearson correlation analysis.

RESULTS

Compared with HT29 and HCT116, LoVo-CRLM mice had significantly higher liver metastases ratio and shorter median survival time. LoVo cells exhibited stronger migration capacity and higher radiotracer uptake compared with HT29 and HCT116 in in vitro. Moreover, ¹⁸F-FMISO SUVmax ratio was significantly higher in both LoVo-CRLM model and LoVo-bearing tumor model compared to models established using HT29 and HCT116. In addition, Pearson correlation analysis revealed a significant correlation between ¹⁸F-FMISO SUVmax ratio of CRLM mice and number of liver metastases larger than 0.5 cm, as well as between ¹⁸F-FMISO SUVmax ratio and HIF-1α or GLUT-1 expression in tumor-bearing tissues.

CONCLUSIONS

¹⁸F-FMISO parameter of SUVmax ratio may provide useful tumor biological information in mice with CRLM, thus allowing for better prediction of CRLM and yielding useful radioactive markers for predicting liver metastasis potential in CRC.

New fluoroethyl phenylalanine analogues as potential LAT1-targeting PET tracers for glioblastoma

The use of O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET) as a positron emission tomography (PET) tracer for brain tumor imaging might have some limitations because of the relatively low affinity for the L-type amino acid transporter 1 (LAT1). To assess the stereospecificity and evaluate the influence of aromatic ring modification of phenylalanine LAT1 targeting tracers, six different fluoroalkylated phenylalanine analogues were synthesized. After in vitro Ki determination, the most promising compound, 2-[18F]-2-fluoroethyl-L-phenylalanine (2-[18F]FELP), was selected for further evaluation and in vitro comparison with [18F]FET. Subsequently, 2-[18F]FELP was assessed in vivo and compared with [18F]FET and [18F]FDG in a F98 glioblastoma rat model. 2-[18F]FELP showed improved in vitro characteristics over [18F]FET, especially when the affinity and specificity for system L is concerned. Based on our results, 2-[18F]FELP is a promising new PET tracer for brain tumor imaging.

Noninvasive evaluation of 18F-FDG/18F-FMISO-based Micro PET in monitoring hepatic metastasis of colorectal cancer

This study aimed to explore the application of two radiotracers (¹⁸F-fluorodeoxyglucose (FDG) and ¹⁸F-fluoromisonidazole (FMISO)) in monitoring hepatic metastases of human colorectal cancer (CRC). Mouse models of CRC hepatic metastases were established by implantation of the human CRC cell lines LoVo and HT29 by intrasplenic injection. Wound healing and Transwell assays were performed to examine cell migration and invasion abilities. Radiotracer-based cellular uptake in vitro and micro-positron emission tomography imaging of liver metastases in vivo were performed. The incidence of liver metastases in LoVo-xenografted mice was significantly higher than that in HT29-xenografted ones. The SUVmax/mean values of ¹⁸F-FMISO, but not ¹⁸F-FDG, in LoVo xenografts were significantly greater than in HT29 xenografts. In vitro, LoVo cells exhibited stronger metastatic potential and higher radiotracer uptake than HT29 cells. Mechanistically, the expression of HIF-1α and GLUT-1 in LoVo cells and LoVo tumor tissues was remarkably higher than in HT29 cells and tissues. Linear regression analysis demonstrated correlations between cellular ¹⁸F-FDG/¹⁸F-FMISO uptake and HIF-1α/GLUT-1 expression in vitro, as well as between ¹⁸F-FMISO SUVmax and GLUT-1 expression in vivo. ¹⁸F-FMISO uptake may serve as a potential biomarker for the detection of liver metastases in CRC, whereas its clinical use warrants validation.

[18F]fluoroethyltyrosine-induced Cerenkov Luminescence Improves Image-Guided Surgical Resection of Glioma

The extent of surgical resection is significantly correlated with outcome in glioma; however, current intraoperative navigational tools are useful only in a subset of patients. We show here that a new optical intraoperative technique, Cerenkov luminescence imaging (CLI) following intravenous injection of O‑(2-[18F]fluoroethyl)-L-tyrosine (FET), can be used to accurately delineate glioma margins, performing better than the current standard of fluorescence imaging with 5-aminolevulinic acid (5-ALA). Methods: Rats implanted orthotopically with U87, F98 and C6 glioblastoma cells were injected with FET and 5-aminolevulinic acid (5-ALA). Positive and negative tumor regions on histopathology were compared with CL and fluorescence images. The capability of FET CLI and 5-ALA fluorescence imaging to detect tumor was assessed using receptor operator characteristic curves and optimal thresholds (CLIOptROC and 5-ALAOptROC) separating tumor from healthy brain tissue were determined. These thresholds were used to guide prospective tumor resections, where the presence of tumor cells in the resected material and in the remaining brain were assessed by Ki-67 staining. Results: FET CLI signal was correlated with signal in preoperative PET images (y = 1.06x - 0.01; p < 0.0001) and with expression of the amino acid transporter SLC7A5 (LAT1). FET CLI (AUC = 97%) discriminated between glioblastoma and normal brain in human and rat orthografts more accurately than 5-ALA fluorescence (AUC = 91%), with a sensitivity >92% and specificity >91%, and resulted in a more complete tumor resection. Conclusion: FET CLI can be used to accurately delineate glioblastoma tumor margins, performing better than the current standard of fluorescence imaging following 5-ALA administration, and is therefore a promising technique for clinical translation.

O-(2-18F-fluoroethyl)-l-tyrosine (18F-FET) uptake in insulinoma: first results from a xenograft mouse model and from human

INTRODUCTION

Herein we have evaluated the uptake of O-(2-¹⁸F-fluoroethyl)-l-tyrosine (¹⁸F-FET) in insulinoma in comparison with those of 6-¹⁸F-fluoro-3,4-dihydroxy-l-phenylalanine (¹⁸F-FDOPA) providing first data from both murine xenograft model and one patient with proved endogenous hyperinsulinemic hypoglycemia.

METHODS

Dynamic ¹⁸F-FET and carbidopa-assisted ¹⁸F-FDOPA PET were performed on tumor-bearing nude mice after subcutaneous injection of RIN-m5F murine beta cells and on a 30-year-old man with type-1 multiple endocrine neoplasia and hyperinsulinemic hypoglycemia defined by a positive fasting test.

RESULTS

Seven and three nude mice bearing a RIN-m5F insulinoma xenograft were respectively studied by ¹⁸F-FET and ¹⁸F-FDOPA μPET. Insulinoma xenograft was detected in all the imaged animals. Xenograft was characterized by an early but moderate increase of ¹⁸F-FET uptake followed by a slight decline of uptake intensity during the 20 min dynamic acquisition. Tumoral radiotracer peak intensity and the highest tumor-to-background contrast were reached about 5 minutes after ¹⁸F-FET iv. injection (mean SUV: 1.21 ± 0.10). The biodistribution of ¹⁸F-FET and ¹⁸F-FDOPA and their dynamic tumoral uptake profile and intensity were similar. In the examined patient, ¹⁸F-FDOPA and ¹⁸F-FET PET/CT showed one concordant focal area of well-defined increased uptake in the pancreatic tail corresponding to 11 mm histologically proved insulinoma. The SUV_max tumor to liver ratio was 1.5, 1.1 for ¹⁸F-FDOPA, 1.1, 1 for ¹⁸F-FET at early (0-5 min post injection) and delayed (5-20 min post injection) PET/CT acquisition, respectively. Despite the relatively low tumoral uptake intensity, insulinoma was clearly identified due to the low background in the pancreas. At the contrary, no ¹⁸F-FDOPA or ¹⁸F-FET tumoral uptake was revealed on whole-body PET/CT images performed about 30 min after radiotracer administration. Note of worth, the dynamic uptake pattern of ¹⁸F-FET and ¹⁸F-FDOPA were similar between human insulinoma and mice xenograft tumor.

CONCLUSION

¹⁸F-FET PET compared equally to ¹⁸F-FDOPA PET in a preclinical RIN-m5F murine model of insulinoma and in one patient with insulinoma-related hypoglycemia. However, in both cases, the tumoral uptake intensity was moderate and the tumor was only visible until 20 min after radiotracer injection. Hence, caution should be taken before asserting the translational relevance of our results in the clinical practices. However, the structural analogies between ¹⁸F-FET and ¹⁸F-FDOPA as well as the limited pancreatic uptake of ¹⁸F-FET in human, encourage evaluating ¹⁸F-FET as diagnostic radiotracer for insulinoma detection in further prospective studies involving large cohorts of patients.

Tumor-associated macrophages in glioblastoma multiforme-a suitable target for somatostatin receptor-based imaging and therapy?

Background

Glioblastoma multiforme (GBM) is the most common primary brain tumor in adults. Tumor-associated macrophages (TAM) have been shown to promote malignant growth and to correlate with poor prognosis. [1,4,7,10-tetraazacyclododecane-NN′,N″,N′″-tetraacetic acid]-d-Phe1,Tyr3-octreotate (DOTATATE) labeled with Gallium-68 selectively binds to somatostatin receptor 2A (SSTR2A) which is specifically expressed and up-regulated in activated macrophages. On the other hand, the role of SSTR2A expression on the cell surface of glioma cells has not been fully elucidated yet. The aim of this study was to non-invasively assess SSTR2A expression of both glioma cells as well as macrophages in GBM.

Methods

15 samples of patient-derived GBM were stained immunohistochemically for macrophage infiltration (CD68), proliferative activity (Ki67) as well as expression of SSTR2A. Anti-CD45 staining was performed to distinguish between resident microglia and tumor-infiltrating macrophages. In a subcohort, positron emission tomography (PET) imaging using ⁶⁸Ga-DOTATATE was performed and the semiquantitatively evaluated tracer uptake was compared to the results of immunohistochemistry.

Results

The amount of microglia/macrophages ranged from <10% to >50% in the tumor samples with the vast majority being resident microglial cells. A strong SSTR2A immunostaining was observed in endothelial cells of proliferating vessels, in neurons and neuropile. Only faint immunostaining was identified on isolated microglial and tumor cells. Somatostatin receptor imaging revealed areas of increased tracer accumulation in every patient. However, retention of the tracer did not correlate with immunohistochemical staining patterns.

Conclusion

SSTR2A seems not to be overexpressed in GBM samples tested, neither on the cell surface of resident microglia or infiltrating macrophages, nor on the surface of tumor cells. These data suggest that somatostatin receptor directed imaging and treatment strategies are less promising in GBM.

Targeting paraprotein biosynthesis for non-invasive characterization of myeloma biology

Purpose

Multiple myeloma is a hematologic malignancy originating from clonal plasma cells. Despite effective therapies, outcomes are highly variable suggesting marked disease heterogeneity. The role of functional imaging for therapeutic management of myeloma, such as positron emission tomography with 2-deoxy-2-[¹⁸F]fluoro-D-glucose (¹⁸F-FDG-PET), remains to be determined. Although some studies already suggested a prognostic value of ¹⁸F-FDG-PET, more specific tracers addressing hallmarks of myeloma biology, e.g. paraprotein biosynthesis, are needed. This study evaluated the amino acid tracers L-methyl-[¹¹C]-methionine (¹¹C-MET) and [¹⁸F]-fluoroethyl-L-tyrosine (¹⁸F-Fet) for their potential to image myeloma and to characterize tumor heterogeneity.

Experimental Design

To study the utility of ¹¹C-MET, ¹⁸F-Fet and ¹⁸F-FDG for myeloma imaging, time activity curves were compared in various human myeloma cell lines (INA-6, MM1.S, OPM-2) and correlated to cell-biological characteristics, such as marker gene expression and immunoglobulin levels. Likewise, patient-derived CD138⁺ plasma cells were characterized regarding uptake and biomedical features.

Results

Using myeloma cell lines and patient-derived CD138⁺ plasma cells, we found that the relative uptake of ¹¹C-MET exceeds that of ¹⁸F-FDG 1.5- to 5-fold and that of ¹⁸F-Fet 7- to 20-fold. Importantly, ¹¹C-MET uptake significantly differed between cell types associated with worse prognosis (e.g. t(4;14) in OPM-2 cells) and indolent ones and correlated with intracellular immunoglobulin light chain and cell surface CD138 and CXCR4 levels. Direct comparison of radiotracer uptake in primary samples further validated the superiority of ¹¹C-MET.

Conclusion

These data suggest that ¹¹C-MET might be a versatile biomarker for myeloma superior to routine functional imaging with ¹⁸F-FDG regarding diagnosis, risk stratification, prognosis and discrimination of tumor subtypes.

Synthesis and biological evaluation of O-[3-18F-fluoropropyl]-α-methyl tyrosine in mesothelioma-bearing rodents

Radiolabeled tyrosine analogs enter cancer cells via upregulated amino acid transporter system and have been shown to be superior to ¹⁸F-fluoro-2-deoxy-D-glucose (¹⁸F-FDG) in differential diagnosis in cancers. In this study, we synthesized O-[3-¹⁹F-fluoropropyl]-α-methyl tyrosine (¹⁹F-FPAMT) and used manual and automated methods to synthesize O-[3-¹⁸F-fluoropropyl]-α-methyl tyrosine (¹⁸F-FPAMT) in three steps: nucleophilic substitution, deprotection of butoxycarbonyl, and deesterification. Manual and automated synthesis methods produced ¹⁸F-FPAMT with a radiochemical purity >96%. The decay-corrected yield of ¹⁸F-FPAMT by manual synthesis was 34% at end-of-synthesis (88 min). The decay-corrected yield of ¹⁸F-FPAMT by automated synthesis was 15% at end-of-synthesis (110 min). ¹⁸F-FDG and ¹⁸F-FPAMT were used for in vitro and in vivo studies to evaluate the feasibility of ¹⁸F-FPAMT for imaging rat mesothelioma (IL-45). In vitro studies comparing ¹⁸F-FPAMT with ¹⁸F-FDG revealed that ¹⁸F-FDG had higher uptake than that of ¹⁸F-FPAMT, and the uptake ratio of ¹⁸F-FPAMT reached the plateau after being incubated for 60 min. Biodistribution studies revealed that the accumulation of ¹⁸F-FPAMT in the heart, lungs, thyroid, spleen, and brain was significantly lower than that of ¹⁸F-FDG. There was poor bone uptake in ¹⁸F-FPAMT for up to 3 hrs suggesting its in vivo stability. The imaging studies showed good visualization of tumors with ¹⁸F-FPAMT. Together, these results suggest that ¹⁸F-FPAMT can be successfully synthesized and has great potential in mesothelioma imaging.