Biological evaluation of 3-[18F]fluoro-α-methyl-d-tyrosine (d-[18F]FAMT) as a novel amino acid tracer for positron emission tomography

Objective

3-[¹⁸F]Fluoro-α-methyl-l-tyrosine (l-[¹⁸F]FAMT) is a useful amino acid tracer for positron emission tomography (PET) imaging of malignant tumors. Because d-amino acids are not well distributed in non-target organs and are rapidly excreted in urine, the d-isomer of [¹⁸F]FAMT could allow clear PET imaging of tumors early after administration. In this study, we prepared 3-[¹⁸F]fluoro-α-methyl-d-tyrosine (d-[¹⁸F]FAMT) and evaluated its usefulness.

Methods

d-[¹⁸F]FAMT was synthesized according to the method for preparation of l-[¹⁸F]FAMT. The in vitro and in vivo stability of [¹⁸F]FAMT were evaluated by high-performance liquid chromatography. Cellular uptake of [¹⁸F]FAMT was evaluated using LS180 colon adenocarcinoma cells. Biodistribution studies were performed in LS180 tumor-bearing mice, and the tumors were imaged using a small-animal PET scanner.

Results

The radiolabeling yield of d-[¹⁸F]FAMT was approximately 10 %, similar to that of l-[¹⁸F]FAMT. Over 95 % of d-[¹⁸F]FAMT remained intact in mice until 60 min after administration. d-[¹⁸F]FAMT was gradually taken up by the LS180 cells. Tumor uptake of d-[¹⁸F]FAMT was competitively inhibited by pretreatment with α-methyl-l-tyrosine, a selective substrate for the system l-amino acid transporter 1 (LAT1), suggesting the involvement of LAT1 in tumor uptake of d-[¹⁸F]FAMT. In biodistribution studies, d-[¹⁸F]FAMT showed rapid clearance from the blood, marked accumulation and retention in the tumor, and lower accumulation in non-target organs, especially kidney and pancreas, compared to l-[¹⁸F]FAMT. The amount of d-[¹⁸F]FAMT in the tumor was also reduced, and tumor-to-blood ratio and tumor-to-muscle ratio of d-[¹⁸F]FAMT were similar to those of l-[¹⁸F]FAMT at every time point. PET imaging with d-[¹⁸F]FAMT did not provide a clear image of the tumor early after administration. However, d-[¹⁸F]FAMT provided higher tumor-to-background contrast than l-[¹⁸F]FAMT.

Conclusions

d-[¹⁸F]FAMT showed rapid blood clearance, low accumulation in non-target organs, and tumor-selective imaging compared with l-[¹⁸F]FAMT. Thus, d-[¹⁸F]FAMT could potentially serve as a novel PET tracer for imaging malignant tumors.     

The translocator protein ligand [18F]DPA-714 images glioma and activated microglia in vivo

Purpose

In recent years there has been an increase in the development of radioligands targeting the 18-kDa translocator protein (TSPO). TSPO expression is well documented in activated microglia and serves as a biomarker for imaging neuroinflammation. In addition, TSPO has also been reported to be overexpressed in a number of cancer cell lines and human tumours including glioma. Here we investigated the use of [¹⁸F]DPA-714, a new TSPO positron emission tomography (PET) radioligand to image glioma in vivo.

Methods

We studied the uptake of [¹⁸F]DPA-714 in three different rat strains implanted with 9L rat glioma cells: Fischer (F), Wistar (W) and Sprague Dawley (SD) rats. Dynamic [¹⁸F]DPA-714 PET imaging, kinetic modelling of PET data and in vivo displacement studies using unlabelled DPA-714 and PK11195 were performed. Validation of TSPO expression in 9L glioma cell lines and intracranial 9L gliomas were investigated using Western blotting and immunohistochemistry of brain tissue sections.

Results

All rats showed significant [¹⁸F]DPA-714 PET accumulation at the site of 9L tumour implantation compared to the contralateral brain hemisphere with a difference in uptake among the three strains (F?>?W?>?SD). The radiotracer showed high specificity for TSPO as demonstrated by the significant reduction of [¹⁸F]DPA-714 binding in the tumour after administration of unlabelled DPA-714 or PK11195. TSPO expression was confirmed by Western blotting in 9L cells in vitro and by immunohistochemistry ex vivo.

Conclusion

The TSPO radioligand [¹⁸F]DPA-714 can be used for PET imaging of intracranial 9L glioma in different rat strains. This preclinical study demonstrates the feasibility of employing [¹⁸F]DPA-714 as an alternative radiotracer to image human glioma.     

Imaging for metabotropic glutamate receptor subtype 1 in rat and monkey brains using PET with [18F]FITM

Purpose

In this study, we evaluate the utility of 4-[¹⁸F]fluoro-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ([¹⁸F]FITM) as a positron emission tomography (PET) ligand for imaging of the metabotropic glutamate receptor subtype 1 (mGluR1) in rat and monkey brains.

Methods

In vivo distribution of [¹⁸F]FITM in brains was evaluated by PET scans with or without the mGluR1-selective antagonist (JNJ16259685). Kinetic parameters of monkey PET data were obtained using the two-tissue compartment model with arterial blood sampling.

Results

In PET studies in rat and monkey brains, the highest uptake of radioactivity was in the cerebellum, followed by moderate uptake in the thalamus, hippocampus and striatum. The lowest uptake of radioactivity was detected in the pons. These uptakes in all brain regions were dramatically decreased by pre-administration of JNJ16259685. In kinetic analysis of monkey PET, the highest volume of distribution (V _T ) was detected in the cerebellum (V _T ?=?11.5).

Conclusion

[¹⁸F]FITM has an excellent profile as a PET ligand for mGluR1 imaging. PET with [¹⁸F]FITM may prove useful for determining the regional distribution and density of mGluR1 and the mGluR1 occupancy of drugs in human brains.     

Early detection and longitudinal monitoring of experimental primary and disseminated melanoma using [18F]ICF01006, a highly promising melanoma PET tracer

Purpose

Here, we report a new and rapid radiosynthesis of ¹⁸F-N-[2-(diethylamino)ethyl]-6-fluoro-pyridine-3-carboxamide ([¹⁸F]ICF01006), a molecule with a high specificity for melanotic tissue, and its evaluation in a murine model for early specific detection of pigmented primary and disseminated melanoma.

Methods

[¹⁸F]ICF01006 was synthesized using a new one-step bromine-for-fluorine nucleophilic heteroaromatic substitution. Melanoma models were induced by subcutaneous (primary tumour) or intravenous (lung colonies) injection of B16BL6 melanoma cells in C57BL/6J mice. The relevance and sensitivity of positron emission tomography (PET) imaging using [¹⁸F]ICF01006 were evaluated at different stages of tumoural growth and compared to ¹⁸F-fluorodeoxyglucose ([¹⁸F]FDG).

Results

The fully automated radiosynthesis of [¹⁸F]ICF01006 led to a radiochemical yield of 61?% and a radiochemical purity >99?% (specific activity 70–80?GBq/μmol; total synthesis time 42?min). Tumours were visualized before they were palpable as early as 1?h post-injection with [¹⁸F]ICF01006 tumoural uptake of 1.64?±?0.57, 3.40?±?1.47 and 11.44?±?2.67 percentage of injected dose per gram of tissue (%ID/g) at days 3, 5 and 14, respectively. [¹⁸F]ICF01006 PET imaging also allowed detection of melanoma pulmonary colonies from day 9 after tumour cell inoculation, with a lung radiotracer accumulation correlated with melanoma invasion. At day 21, radioactivity uptake in lungs reached a value of 5.23?±?2.08 %ID/g (versus 0.41?±?0.90 %ID/g in control mice). In the two models, comparison with [¹⁸F]FDG showed that both radiotracers were able to detect melanoma lesions, but [¹⁸F]ICF01006 was superior in terms of contrast and specificity.

Conclusion

Our promising results provide further preclinical data, reinforcing the excellent potential of [¹⁸F]ICF01006 PET imaging for early specific diagnosis and follow-up of melanin-positive disseminated melanoma.     

[18F]FDG-PET/CT in patients affected by retroperitoneal fibrosis: a bicentric experience

Purpose

The aim of our study was to assess the feasibility and usefulness of 2-[¹⁸F]-fluoro-2-deoxy-d-glucose positron emission tomography computed tomography ([¹⁸F]FDG-PET/CT) in patients affected by retroperitoneal fibrosis.

Materials and methods

We retrospectively evaluated 25 patients studied in two centers: 18 underwent [¹⁸F]FDG-PET/CT as initial evaluation, three during follow-up, three during steroid therapy, and one to re-evaluate the disease. Among the group who underwent initial evaluation, ten underwent a second [¹⁸F]FDG-PET/CT after steroid therapy.

Results

[¹⁸F]FDG-PET/CT was positive in 18 patients and negative in seven. Among the ten patients who underwent a second study after steroid therapy, six showed complete metabolic response, three partial response, and one no significant maximum standardized uptake value (SUV_max) reduction.

Conclusion

Our preliminary results show that [¹⁸F]FDG-PET/CT is feasible and suitable for evaluating retroperitoneal fibrosis and is useful in assessing therapy response. Larger studies are desirable to confirm these findings and to determine the appropriate position of [¹⁸F]FDG-PET/CT in the diagnostic flow chart for this disease.     

Synthesis and biological evaluation of [18F]tetrafluoroborate: a PET imaging agent for thyroid disease and reporter gene imaging of the sodium/iodide symporter

Purpose

The human sodium/iodide symporter (hNIS) is a well-established target in thyroid disease and reporter gene imaging using gamma emitters ¹²³I-iodide, ¹³¹I-iodide and ^99mTc-pertechnetate. However, no PET imaging agent is routinely available. The aim of this study was to prepare and evaluate ¹⁸F-labelled tetrafluoroborate ([¹⁸F]TFB) for PET imaging of hNIS.

Methods

[¹⁸F]TFB was prepared by isotopic exchange of BF ₄ ^? with [¹⁸F]fluoride in hot hydrochloric acid and purified using an alumina column. Its identity, purity and stability in serum were determined by HPLC, thin-layer chromatography (TLC) and mass spectrometry. Its interaction with NIS was assessed in vitro using FRTL-5 rat thyroid cells, with and without stimulation by thyroid-stimulating hormone (TSH), in the presence and absence of perchlorate. Biodistribution and PET imaging studies were performed using BALB/c mice, with and without perchlorate inhibition.

Results

[¹⁸F]TFB was readily prepared with specific activity of 10 GBq/mg. It showed rapid accumulation in FRTL-5 cells that was stimulated by TSH and inhibited by perchlorate, and rapid specific accumulation in vivo in thyroid (SUV?=?72 after 1 h) and stomach that was inhibited 95% by perchlorate.

Conclusion

[¹⁸F]TFB is an easily prepared PET imaging agent for rodent NIS and should be evaluated for hNIS PET imaging in humans.     

Regional distribution and kinetics of [18F]fluciclovine (anti-[18F]FACBC), a tracer of amino acid transport, in subjects with primary prostate cancer

Purpose

[¹⁸F]Fluciclovine (anti-[¹⁸F]FACBC) is a synthetic amino acid developed for PET assessment of the anabolic component of tumour metabolism in clinical routine. This phase 1 trial evaluated the safety, tracer stability and uptake kinetics of [¹⁸F]fluciclovine in patients.

Methods

Six patients with biopsy-proven prostate cancer were investigated with 3-T MRI and PET/CT. All underwent dynamic [¹⁸F]fluciclovine PET/CT of the pelvic area for up to 120 min after injection of 418?±?10 MBq of tracer with simultaneous blood sampling of radioactivity. The kinetics of uptake in tumours and normal tissues were evaluated using standardized uptake values (SUVs) and compartmental modelling.

Results

Tumour deposits as defined by MRI were clearly visualized by PET. Urine excretion was minimal and normal tissue background was low. Uptake of [¹⁸F]fluciclovine in tumour from the blood was rapid and the tumour-to-normal tissue contrast was highest between 1 and 15 min after injection with a 65 % reduction in mean tumour uptake at 90 min after injection. A one-compartment model fitted the tracer kinetics well. Early SUVs correlated well with both the influx rate constant (K ₁) and the volume of distribution of the tracer (V _T). There were no signs of tracer metabolite formation. The product was well tolerated in all patients without significant adverse events.

Conclusion

[¹⁸F]Fluciclovine shows high uptake in prostate cancer deposits and appears safe for use in humans. The production is robust and the formulation stable in vivo. An early imaging window seems to provide the best visual results. SUV measurements capture most of the kinetic information that can be obtained from more advanced models, potentially simplifying quantification in future studies.     

[11C]PIB, [18F]FDG and MR imaging in patients with mild cognitive impairment

Purpose

Cortical glucose metabolism, brain amyloid β accumulation and hippocampal atrophy imaging have all been suggested as potential biomarkers in predicting which patients with mild cognitive impairment (MCI) will convert to Alzheimer’s disease (AD). The aim of this study was to compare the prognostic ability of [¹¹C]PIB PET, [¹⁸F]FDG PET and quantitative hippocampal volumes measured with MR imaging in predicting conversion to AD in patients with MCI.

Methods

The study group comprised 29 patients with MCI who underwent [¹¹C]PIB PET and MR imaging. Of these, 22 also underwent [¹⁸F]FDG PET. All subjects were invited back for clinical evaluation after 2 years.

Results

During the follow-up time 17 patients had converted to AD while 12 continued to meet the criteria for MCI. The two groups did not differ in age, gender or education level, but the converter group tended to have lower MMSE and Word List learning than the nonconverter group. High [¹¹C]PIB retention in the frontotemporal regions and anterior and posterior cingulate (p?<?0.05) predicted conversion to AD. Also reduced [¹⁸F]FDG uptake in the left lateral temporal cortex (LTC) predicted conversion (p?<?0.05), but quantitative hippocampal volumes did not (p?>?0.1). In receiver operating characteristic (ROC) analysis the measurements that best predicted the conversion were [¹¹C]PIB retention in the lateral frontal cortex and [¹⁸F]FDG uptake in the left LTC. Both PET methods resulted in good sensitivity and specificity and neither was significantly superior to the other.

Conclusion

The findings indicate that [¹¹C]PIB and [¹⁸F]FDG are superior to hippocampal volumes in predicting conversion to AD in patients with MCI.     

Pharmacokinetics of [18F]flutemetamol in wild-type rodents and its binding to beta amyloid deposits in a mouse model of Alzheimer’s disease

Purpose

The aim of this study was to investigate the potential of [¹⁸F]flutemetamol as a preclinical PET tracer for imaging β-amyloid (Aβ) deposition by comparing its pharmacokinetics to those of [¹¹C]Pittsburgh compound B ([¹¹C]PIB) in wild-type Sprague Dawley rats and C57Bl/6N mice. In addition, binding of [¹⁸F]flutemetamol to Aβ deposits was studied in the Tg2576 transgenic mouse model of Alzheimer’s disease.

Methods

[¹⁸F]Flutemetamol biodistribution was evaluated using ex vivo PET methods and in vivo PET imaging in wild-type rats and mice. Metabolism and binding of [¹¹C]PIB and [¹⁸F]flutemetamol to plasma proteins were analysed using thin-layer chromatography and ultrafiltration methods, respectively. Radiation dose estimates were calculated from rat ex vivo biodistribution data. The binding of [¹⁸F]flutemetamol to Aβ deposits was also studied using ex vivo and in vitro autoradiography. The location of Aβ deposits in the brain was determined with thioflavine S staining and immunohistochemistry.

Results

The pharmacokinetics of [¹⁸F]flutemetamol resembled that of [¹¹C]PIB in rats and mice. In vivo studies showed that both tracers readily entered the brain, and were excreted via the hepatobiliary pathway in both rats and mice. The metabolism of [¹⁸F]flutemetamol into radioactive metabolites was faster than that of [¹¹C]PIB. [¹⁸F]Flutemetamol cleared more slowly from the brain than [¹¹C]PIB, particularly from white matter, in line with its higher lipophilicity. Effective dose estimates for [¹¹C]PIB and [¹⁸F]flutemetamol were 2.28 and 6.65?μSv/MBq, respectively. Autoradiographs showed [¹⁸F]flutemetamol binding to fibrillar Aβ deposits in the brain of Tg2576 mice.

Conclusion

Based on its pharmacokinetic profile, [¹⁸F]flutemetamol showed potential as a PET tracer for preclinical imaging. It showed good brain uptake and was bound to Aβ deposits in the brain of Tg2576 mice. However, its high lipophilicity might complicate the analysis of PET data, particularly in small-animal imaging.     

Biodistribution and radiation dosimetry of the 18?kDa translocator protein (TSPO) radioligand [18F]FEDAA1106: a human whole-body PET study

Purpose

[¹⁸F]FEDAA1106 is a recently developed positron emission tomography (PET) radioligand for in vivo quantification of the 18?kDa translocator protein [TSPO or, as earlier called, the peripheral benzodiazepine receptor (PBR)]. TSPO imaging is expected to be useful for the clinical evaluation of neuroinflammatory diseases. The aim of this study was to provide dosimetry estimates for [¹⁸F]FEDAA1106 based on human whole-body PET measurements.

Methods

PET scans were performed for a total of 6.6?h after the injection of 183.8?±?9.1?MBq of [¹⁸F]FEDAA1106 in six healthy subjects. Regions of interest were drawn on coronal images. Estimates of the absorbed doses of radiation were calculated using the OLINDA software.

Results

Peak uptake was largest in lungs, followed by liver, small intestine, kidney, spleen and other organs. Peak values of the percent injected dose (%ID) at a time after radioligand injection were calculated for the lungs (27.1%ID at 0.2?h), liver (21.1%ID at 0.6?h), small intestine (10.4%ID at 6.3?h), kidney (4.9%ID at 1.8?h) and spleen (4.6%ID at 0.6?h). The largest absorbed dose was found in the spleen (0.12?mSv/MBq), followed by kidneys (0.094?mSv/MBq). The calculated mean effective dose was 0.036?mSv/MBq.

Conclusion

Based on the distribution and dose estimates, the estimated radiation burden of [¹⁸F]FEDAA1106 is moderately higher than that of [¹⁸F]fluorodeoxyglucose (FDG). In clinical studies, the administered activity of this radioligand ought to be adjusted in line with regional regulations. This result would be helpful for further clinical TSPO imaging studies.     

Whole-body MR-DWIBS vs. [18F]-FDG-PET/CT in the study of malignant tumors: a retrospective study

Purpose

Our aim was to assess the overall diagnostic accuracy of magnetic resonance diffusion-weighted whole-body imaging with background signal suppression (MR-DWIBS) compared with ([¹⁸F]-fluorodeoxyglucose (FDG)-positron emission tomography (PET)/computed tomography (CT), considered the reference standard of whole-body tumour imaging modalities, in a series of consecutive patients with malignant tumour.

Materials and methods

Thirty-eight patients diagnosed with a malignant tumour over a 4-month period were enrolled in this retrospective, observational study. PET/CT and MR-DWIBS images were reviewed in double-blind manner by a nuclear medicine physician and radiologists with 4 years experience. Lesion size, standard uptake value (SUV) and apparent diffusion coefficient (ADC) were measured and calculated for each lesion.

Results

The qualitative analysis of MR-DWIBS and [¹⁸F]-FDG-PET/CT showed that two patients were negative at both techniques. MR-DWIBS was positive in 36 patients, 34 of whom were positive and two negative at [¹⁸F]-FDG-PET/CT, respectively. Two hundred and fifty-five lesions were identified by MR-DWIBS and 184 by [¹⁸F]-FDG-PET/CT, which was a significative discordance. Correlation between SUV and ADC of lesions positive at both techniques was not statistically significant. The mean difference between lesion size in [¹⁸F]-FDG-PET/CT and MR-DWIBS was not statistically significant. No correlation was found between glucose metabolism and water motion.

Conclusions

MR-DWIBS may be used to evaluate localisation of parenchymal neoplasms but is less efficacious in characterising lymph-node and skeletal lesions. [¹⁸F]-FDG-PET/CT remains the best whole-body technique to identify lymph-node and skeletal lesions, but its limitation is identifying tumours with low glucose metabolism as in mucinous neoplasms. MR-DWIBS evaluation must be integrated with morphological images to increase MR diagnostic accuracy.     

Imaging of amyloid deposition in human brain using positron emission tomography and [18F]FACT: comparison with [11C]PIB

Purpose

The characteristic neuropathological changes in Alzheimer’s disease (AD) are deposition of amyloid senile plaques and neurofibrillary tangles. The ¹⁸F-labeled amyloid tracer, [¹⁸F]2-[(2-{(E)-2-[2-(dimethylamino)-1,3-thiazol-5-yl]vinyl}-1,3-benzoxazol-6-yl)oxy]-3-fluoropropan-1-ol (FACT), one of the benzoxazole derivatives, was recently developed. In the present study, deposition of amyloid senile plaques was measured by positron emission tomography (PET) with both [¹¹C]Pittsburgh compound B (PIB) and [¹⁸F]FACT in the same subjects, and the regional uptakes of both radiotracers were directly compared.

Methods

Two PET scans, one of each with [¹¹C]PIB and [¹⁸F]FACT, were performed sequentially on six normal control subjects, two mild cognitive impairment (MCI) patients, and six AD patients. The standardized uptake value ratio of brain regions to the cerebellum was calculated with partial volume correction using magnetic resonance (MR) images to remove the effects of white matter accumulation.

Results

No significant differences in the cerebral cortical uptake were observed between normal control subjects and AD patients in [¹⁸F]FACT studies without partial volume correction, while significant differences were observed in [¹¹C]PIB. After partial volume correction, the cerebral cortical uptake was significantly larger in AD patients than in normal control subjects for [¹⁸F]FACT studies as well as [¹¹C]PIB. Relatively lower uptakes of [¹¹C]PIB in distribution were observed in the medial side of the temporal cortex and in the occipital cortex as compared with [¹⁸F]FACT. Relatively higher uptake of [¹¹C]PIB in distribution was observed in the frontal and parietal cortices.

Conclusion

Since [¹⁸F]FACT might bind more preferentially to dense-cored amyloid deposition, regional differences in cerebral cortical uptake between [¹¹C]PIB and [¹⁸F]FACT might be due to differences in regional distribution between diffuse and dense-cored amyloid plaque shown in the autoradiographic and histochemical assays of postmortem AD brain sections.     

In vivo imaging of neuroinflammation: a comparative study between [18F]PBR111, [11C]CLINME and [11C]PK11195 in an acute rodent model

Purpose

The key role of neuroinflammation in acute and chronic neurological disorders has stimulated the search for specific radiotracers targeting the peripheral benzodiazepine receptor (PBR)/18 kDa translocator protein (TSPO), a hallmark of neuroinflammation. Here we evaluate the new radiotracer for positron emission tomography (PET) [¹⁸F]PBR111 in a rodent model of acute inflammation and compare it with [¹¹C]CLINME, an ¹¹C-labelled tracer of the same chemical family, and with the isoquinolinic carboxamide [¹¹C]PK11195.

Methods

We studied radiometabolites by HPLC, in vitro binding by autoradiography and in vivo brain kinetics as well as in vivo specificity of binding using PET imaging.

Results

We show that this radiotracer has a high in vitro specificity for PBR/TSPO versus central benzodiazepine receptors, as reflected by the drastic reduction of its binding to target tissue by addition of PK11195 or PBR111, while addition of flumazenil does not affect binding. Only intact [¹⁸F]PBR111 is detected in brain up to 60 min after i.v. injection, and PET imaging shows an increased uptake in the lesion as compared to the contralateral side as early as 6 min after injection. Administration of an excess of PK11195 and PBR111, 20 min after [¹⁸F]PBR111 administration, induces a rapid and complete displacement of [¹⁸F]PBR111 binding from the lesion. Modelling of the PET data using the simplified reference tissue model showed increased binding potential (BP) in comparison to [¹¹C]PK11195.

Conclusion

[¹⁸F]PBR111 is a metabolically stable tracer with a high specific in vitro and in vivo binding to TSPO. In addition, considering the longer half-life of ¹⁸F over ¹¹C, these results support [¹⁸F]PBR111 as a promising PET tracer of the PBR/TSPO for neuroinflammation imaging.     

[<Superscript>18</Superscript>F]FP-(+)-DTBZ PET study in a lactacystin-treated rat model of Parkinson disease

Objective

Lactacystin has been used to establish rodent models of Parkinson disease (PD), with cerebral α-synuclein inclusions. This study evaluated the uptake of [¹⁸F]9-fluoropropyl-(+)-dihydrotetrabenazine ([¹⁸F]FP-(+)-DTBZ), a vesicular monoamine transporter type 2 (VMAT2)-targeting radiotracer, through positron emission tomography (PET) in lactacystin-treated rat brains.

Methods

Adult male Sprague–Dawley rats were randomly treated with a single intracranial dose of lactacystin (2 or 5 μg) or saline (served as the sham control) into the left medial forebrain bundle. A 30-min static [¹⁸F]FP-(+)-DTBZ brain PET scan was performed following an intravenous [¹⁸F]FP-(+)-DTBZ dose (approximately 22 MBq) in each animal at 2 and 3 weeks after lactacystin treatment. Upon completing the last PET scans, the animals were killed, and their brains were dissected for ex vivo autoradiography (ARG) and immunohistochemical (IHC) staining of tyrosine hydroxylase (TH) as well as VMAT2.

Results

Both the 2- and 5-μg lactacystin-treated groups exhibited significantly decreased specific [¹⁸F]FP-(+)-DTBZ uptake in the ipsilateral striata (I-ST) at 2 weeks (1.51 and 1.16, respectively) and 3 weeks (1.36 and 1.00, respectively) after lactacystin treatment, compared with the uptake in the corresponding contralateral striata (C-ST) (3.48 and 3.08 for the 2- and 5-μg lactacystin-treated groups, respectively, at 2 weeks; 3.36 and 3.11 for the 2- and 5-μg lactacystin-treated groups, respectively, at 3 weeks) and the sham controls (3.34–3.53). Lactacystin-induced decline in I-ST [¹⁸F]FP-(+)-DTBZ uptake was also demonstrated through ex vivo ARG, and the corresponding dopaminergic neuron damage was confirmed by the results of TH- and VMAT2-IHC studies.

Conclusions

In this PD model, lactacystin-induced dopaminergic terminal damage in the ipsilateral striatum could be clearly visualized through in vivo [¹⁸F]FP-(+)-DTBZ PET imaging. This may serve as a useful approach for evaluating the effectiveness of new treatments for PD.

     

Comparison of the binding characteristics of [18F]THK-523 and other amyloid imaging tracers to Alzheimer��s disease pathology

Purpose

Extensive deposition of senile plaques and neurofibrillary tangles in the brain is a pathological hallmark of Alzheimer??s disease (AD). Although several PET imaging agents have been developed for in vivo detection of senile plaques, no PET probe is currently available for selective detection of neurofibrillary tangles in the living human brain. Recently, [¹⁸F]THK-523 was developed as a potential in vivo imaging probe for tau pathology. The purpose of this study was to compare the binding properties of [¹⁸F]THK-523 and other amyloid imaging agents, including PiB, BF-227 and FDDNP, to synthetic protein fibrils and human brain tissue.

Methods

In vitro radioligand binding assays were conducted using synthetic amyloid ??₄₂ and K18??K280-tau fibrils. Nonspecific binding was determined by the addition of unlabelled compounds at a concentration of 2???M. To examine radioligand binding to neuropathological lesions, in vitro autoradiography was conducted using sections of AD brain.

Results

[¹⁸F]THK-523 showed higher affinity for tau fibrils than for A?? fibrils, whereas the other probes showed a higher affinity for A?? fibrils. The autoradiographic analysis indicated that [¹⁸F]THK-523 accumulated in the regions containing a high density of tau protein deposits. Conversely, PiB and BF-227 accumulated in the regions containing a high density of A?? plaques.

Conclusion

These findings suggest that the unique binding profile of [¹⁸F]THK-523 can be used to identify tau deposits in AD brain.     

Novel electrophilic synthesis of 6-[¹⁸F]fluorodopamine and comprehensive biological evaluation

Purpose

6-[¹⁸F]Fluorodopamine (4-(2-aminoethyl)-5-[¹⁸F]fluorobenzene-1,2-diol, 6-[¹⁸F]FDA) is a tracer for imaging sympathetically innervated tissues. Previous electrophilic labelling methods produced 6-[¹⁸F]FDA with low specific radioactivity (SA) which has limited its wider use. Our aim was to employ electrophilic labelling and increase the SA to around 15?GBq/μmol. We also sought to determine an extensive biodistribution pattern for 6-[¹⁸F]FDA in rats in order to thoroughly identify tissues with dense sympathetic innervation that were specifically labelled with 6-[¹⁸F]FDA. In addition, to investigate the safety profile of 6-[¹⁸F]FDA in larger animals, we performed in vivo studies in pigs.

Methods

6-[¹⁸F]FDA was synthesised using high SA electrophilic [¹⁸F]F₂ as the labelling reagent. Biodistribution and metabolism of 6-[¹⁸F]FDA was determined ex vivo in rats, and in vivo studies were done in pigs.

Results

6-[¹⁸F]FDA was synthesised with 2.6?±?1.1% radiochemical yield. The total amount of purified 6-[¹⁸F]FDA was 663?±?291?MBq at the end of synthesis (EOS). SA, decay corrected to EOS, was 13.2?±?2.7?GBq/μmol. Radiochemical purity exceeded 99.0%. Specific uptake of 6-[¹⁸F]FDA was demonstrated in heart, lung, pancreas, adrenal gland, lower large intestine (LLI), eye, thyroid gland, spleen and stomach tissue. 6-[¹⁸F]FDA in rat plasma declined rapidly, with a half-life of 2?min, indicating fast metabolism. In vivo PET studies in pigs confirmed the tracer could be used safely without pharmacological effects.

Conclusion

6-[¹⁸F]FDA was synthesised with good radiopharmaceutical quality and yields high enough for several human PET studies. The SA of 6-[¹⁸F]FDA was improved by 50- to 500-fold compared to previous electrophilic methods. Uptake of 6-[¹⁸F]FDA was specific in various peripheral organs, indicating that 6-[¹⁸F]FDA PET can be used to investigate sympathoneural functions beyond cardiac studies when higher specific uptake is achieved.     

PET imaging of the brain serotonin transporters (SERT) with N,N-dimethyl-2-(2-amino-4-[18F]fluorophenylthio)benzylamine (4-[18F]-ADAM) in humans: a preliminary study

Purpose

The aim of this study was to assess the feasibility of using 4-[¹⁸F]-ADAM as a brain SERT imaging agent in humans.

Methods

Enrolled in the study were 19 healthy Taiwanese subjects (11 men, 8 women; age 33?±?9?years). The PET data were semiquantitatively analyzed and expressed as specific uptake ratios (SUR) and distribution volume ratios (DVR) using the software package PMOD. The SUR and DVR of 4-[¹⁸F]-ADAM in the raphe nucleus (RN), midbrain (MB), thalamus (TH), striatum (STR) and prefrontal cortex (PFC) were determined using the cerebellum (CB) as the reference region.

Results

4-[¹⁸F]-ADAM bound to known SERT-rich regions in human brain. The order of the regional brain uptake was MB (RN) > TH > STR > PFC > CB. The DVR (n?=?4, t*?=?60?min) in the RN, TH, STR and PFC were 3.00?±?0.50, 2.25?±?0.45, 2.05?±?0.31 and 1.40?±?0.13, respectively. The optimal time for imaging brain SERT with 4-[¹⁸F]-ADAM was 120?C140?min after injection. At the optimal imaging time, the SURs (n?=?15) in the MB, TH, STR, and PFC were 2.25?±?0.20, 2.28?±?0.20, 2.12?±?0.18 and 1.47?±?0.14, respectively. There were no significant differences in SERT availability between men and women (p?<?0.05).

Conclusion

The results of this study showed that 4-[¹⁸F]-ADAM was safe for human studies and its distribution in human brain appeared to correlate well with the known distribution of SERT in the human brain. In addition, it had high specific binding and a reasonable optimal time for imaging brain SERT in humans. Thus, 4-[¹⁸F]-ADAM may be feasible for assessing the status of brain SERT in humans.     

Thymidine phosphorylase influences [18F]fluorothymidine uptake in cancer cells and patients with non-small cell lung cancer

Purpose

Thymidine phosphorylase (TP), a key enzyme in the pyrimidine nucleoside salvage pathway, catalyses the reversible phosphorylation of thymidine, thereby generating thymine and 2-deoxy-D-ribose-1-phosphate. By regulating the levels of endogenous thymidine, TP may influence [¹⁸F]fluorothymidine ([¹⁸F]FLT) uptake. We investigated the effect of TP activity on [¹⁸F]FLT uptake by tumours.

Methods

Uptake of [³H]FLT and [³H]thymidine ([³H]Thd) and the activities of TP, thymidine kinase 1 (TK1), and equilibrative nucleoside transporter 1 (ENT1) were determined in exponentially growing A431, A549, HT29, HOP92, ACHN, and SKOV3 cells in the presence or absence of tipiracil hydrochloride, a TP inhibitor. Eighty-five non-small cell lung cancer tissues from a patient cohort that was previously studied with [¹⁸F]FLT positron emission tomography (PET) were retrieved and subjected to immunohistochemical analysis of TP expression. Factors that affected the maximum standardised uptake value (SUVmax) of [¹⁸F]FLT-PET were identified by multiple linear regression analysis.

Results

A431 cells had the highest TP activity; A549 and HT29 cells had moderate TP activity; and ACHN, SKOV3, and HOP92 cells had little detectable TP activity. Cell lines with high TP activity took up more [³H]FLT than [³H]Thd, whereas cells with little TP activity took up more [³H]Thd than [³H]FLT. In cells with high TP activity, TP inhibition decreased [³H]FLT uptake and increased [³H]Thd uptake. However, TP inhibition had no effect on ACHN, SKOV3, and HOP92 cells. TP inhibition did not change TK1 or ENT1 activity, but did increase the intracellular level of thymidine. The SUVmax of [¹⁸F]FLT was affected by three independent factors: Ki-67 expression (P?<?0.001), immunohistochemical TP score (P?<?0.001), and tumour size (P?=?0.015).

Conclusions

TP activity influences [¹⁸F]FLT uptake, and may explain preferential uptake of [¹⁸F]FLT over [³H]Thd. These results provide important insights into the biology of [¹⁸F]FLT as a proliferation marker.     

Early [<Superscript>18</Superscript>F]florbetaben and [<Superscript>11</Superscript>C]PiB PET images are a surrogate biomarker of neuronal injury in Alzheimer’s disease

Purpose

[¹⁸F]FDG is a commonly used neuronal injury biomarker for early and differential diagnosis of dementia. Typically, the blood supply to the brain is closely coupled to glucose consumption. Early uptake of the Aβ tracer [¹¹C]PiB on PET images is mainly determined by cerebral blood flow and shows a high correlation with [¹⁸F]FDG uptake. Uptake data for ¹⁸F-labelled Aβ PET tracers are, however, scarce. We investigated the value of early PET images using the novel Aβ tracer [¹⁸F]FBB in the diagnosis of Alzhimers disease (AD).

Methods

This retrospective analysis included 22 patients with MCI or dementia who underwent dual time-point PET imaging with either [¹¹C]PiB (11 patients) or [¹⁸F]FBB (11 patients) in routine clinical practice. Images were acquired 1 – 9 min after administration of both tracers and 40 – 70 min and 90 – 110 min after administration of [¹¹C]PiB and [¹⁸F]FBB, respectively. The patients also underwent [¹⁸F]FDG brain PET imaging. PET data were analysed visually and semiquantitatively. Associations between early Aβ tracer uptake and dementia as well as brain atrophy were investigated.

Results

Regional visual scores of early Aβ tracer and [¹⁸F]FDG PET images were significantly correlated (Spearman’s ρ?=?0.780, P?<?0.001). Global brain visual analysis revealed identical results between early Aβ tracer and [¹⁸F]FDG PET images. In a VOI-based analysis, the early Aβ tracer data correlated significantly with the [¹⁸F]FDG data (r?=?0.779, P?<?0.001), but there were no differences between [¹⁸F]FBB and [¹¹C]PiB. Cortical SUVRs in regions typically affected in AD on early Aβ tracer and [¹⁸F]FDG PET images were correlated with MMSE scores (ρ?=?0.458, P?=?0.032, and ρ?=?0.456, P?=?0.033, respectively). A voxel-wise group-based search for areas with relatively higher tracer uptake on early Aβ tracer PET images compared with [¹⁸F]FDG PET images revealed a small cluster in the midbrain/pons; no significant clusters were found for the opposite comparison.

Conclusion

Early [¹⁸F]FBB and [¹¹C]PiB PET brain images are similar to [¹⁸F]FDG PET images in AD patients, and these tracers could potentially be used as biomarkers in place of [¹⁸F]FDG. Thus, Aβ tracer PET imaging has the potential to provide biomarker information on AD pathology and neuronal injury. The potential of this approach for supporting the diagnosis of AD needs to be confirmed in prospective studies in larger cohorts.