An improved radiosynthesis of [18F]AV-133: a PET imaging agent for vesicular monoamine transporter 2

Prediction of Parkinson's disease pathogenic variants using hybrid Machine learning systems and radiomic features

PURPOSE

In Parkinson's disease (PD), 5-10% of cases are of genetic origin with mutations identified in several genes such as leucine-rich repeat kinase 2 (LRRK2) and glucocerebrosidase (GBA). We aim to predict these two gene mutations using hybrid machine learning systems (HMLS), via imaging and non-imaging data, with the long-term goal to predict conversion to active disease.

METHODS

We studied 264 and 129 patients with known LRRK2 and GBA mutations status from PPMI database. Each dataset includes 513 features such as clinical features (CFs), conventional imaging features (CIFs) and radiomic features (RFs) extracted from DAT-SPECT images. Features, normalized by Z-score, were univariately analyzed for statistical significance by the t-test and chi-square test, adjusted by Benjamini-Hochberg correction. Multiple HMLSs, including 11 features extraction (FEA) or 10 features selection algorithms (FSA) linked with 21 classifiers were utilized. We also employed Ensemble Voting (EV) to classify the genes.

RESULTS

For prediction of LRRK2 mutation status, a number of HMLSs resulted in accuracies of 0.98 ± 0.02 and 1.00 in 5-fold cross-validation (80% out of total data points) and external testing (remaining 20%), respectively. For predicting GBA mutation status, multiple HMLSs resulted in high accuracies of 0.90 ± 0.08 and 0.96 in 5-fold cross-validation and external testing, respectively. We additionally showed that SPECT-based RFs added value to the specific prediction of of GBA mutation status.

CONCLUSION

We demonstrated that combining medical information with SPECT-based imaging features, and optimal utilization of HMLS can produce excellent prediction of the mutations status in PD patients.

Comparison of 18 F-DOPA and 18 F-DTBZ for PET/CT Imaging of Idiopathic Parkinson Disease

OBJECTIVE

The aim of this study was to compare 2 imaging tracers, 18 F-DOPA and 18 F-DTBZ, for PET/CT imaging in idiopathic Parkinson disease (PD).

METHODS

We recruited 32 PD patients and 12 healthy controls in this study. All subjects underwent both 18 F-DOPA and 18 F-DTBZ PET/CT, and the results were interpreted by visual analysis and semiquantitative analysis (specific uptake ratios [SURs]). A 1-way analysis of variance was used to compare the clinical data and the SURs among the patients at different stages. Regression analysis was performed to analyze the correlation between the SURs and the clinical data.

RESULTS

Among the PD patients, there were 7 patients in Hoehn and Yahr stage I, 14 patients in stage II, and 11 patients in stage III. Linear correlation was found in striatal SURs between the 2 tracers ( P < 0.05). In patients of early stages, the striatal SUR decrease percent of 2 tracers had no statistical difference (paired t test, P > 0.05). By initial visual analysis, all the patients were interpreted as positive with 18 F-DBTZ (6 unilaterally, 26 bilaterally), and 31 cases were regarded as positive with 18 F-DOPA (8 unilaterally, 23 bilaterally). After setting the upper limit of SUR images with the putamen SURs of healthy controls (SUR T ), all patients were interpreted as positive with both tracers ( 18 F-DTBZ: 5 unilaterally, 27 bilaterally; 18 F-DOPA: 4 unilaterally, 28 bilaterally).

CONCLUSION

18 F-DTBZ and 18 F-DOPA could reflect the same level of dopaminergic neuron degeneration for PD in early stages, and they have the consistent visual analysis results.

Diagnostic value of striatal 18F-FP-DTBZ PET in Parkinson’s disease

Background

18F-FP-DTBZ has been proven as a biomarker for quantifying the concentration of presynaptic vesicular monoamine transporter 2 (VMAT2). However, its clinical application is still limited.

Objectives

To evaluate the difference in dopaminergic integrity between patients with Parkinson’s disease (PD) and healthy controls (HC) using 18F-FP-DTBZ PET in vivo and to determine the diagnostic value of standardized uptake value ratios (SUVRs) using the Receiver Operating Characteristic (ROC) curve.

Methods

A total of 34 PD and 31 HC participants were enrolled in the PET/MR derivation cohort, while 89 PD and 18 HC participants were recruited in the PET/CT validation cohort. The Hoehn–Yahr Scale and the third part of the MDS-Unified Parkinson’s Disease Rating Scale (MDSUPDRS-III) were used to evaluate the disease staging and severity. All assessments and PET scanning were performed in drug-off states. The striatum was segmented into five subregions as follows: caudate, anterior dorsal putamen (ADP), anterior ventral putamen (AVP), posterior dorsal putamen (PDP), and posterior ventral putamen (PVP) using automatic pipeline built with the PMOD software (version 4.105). The SUVRs of the targeted subregions were calculated using the bilateral occipital cortex as the reference region.

Results

Regarding the diagnostic value, ROC curve and blind validation showed that the contralateral PDP (SUVR = 3.43) had the best diagnostic accuracy (AUC = 0.973; P &lt; 0.05), with a sensitivity of 97.1% (95% CI: 82.9–99.8%), specificity of 100% (95% CI: 86.3–100%), positive predictive value (PPV) of 100% (95% CI: 87.0–100%), negative predictive value (NPV) of 96.9% (95% CI: 82.0–99.8%), and an accuracy of 98.5% for the diagnosis of PD in the derivation cohort. Blind validation of 18F-FP-DTBZ PET imaging diagnosis was done using the PET/CT cohort, where participants with a SUVR of the PDP &lt;3.43 were defined as PD. Kappa test showed a consistency of 0.933 (P &lt; 0.05) between clinical diagnosis and imaging diagnosis, with a sensitivity of 98.9% (95% CI: 93.0–99.9%), specificity of 94.4% (95% CI: 70.6–99.7%), PPV of 98.9% (95% CI: 93.0–99.9%), NPV of 94.4% (95% CI: 70.6–99.7%), and a diagnostic accuracy of 98.1%.

Conclusions

Our results showed that an SUVR threshold of 3.43 in the PDP could effectively distinguish patients with PD from HC.

Synthesis, radiolabeling, and evaluation of a potent β-site APP cleaving enzyme (BACE1) inhibitor for PET imaging of BACE1 in vivo

The β-site APP-cleaving enzyme 1 (BACE1) plays important roles in the proteolytic processing of amyloid precursor protein, and can be regarded as an important target for the diagnosis and treatment of AD. This study aimed to report the synthesis and evaluation of an ¹⁸F-labeled 2-amino-3,4-dihydroquinazoline analog as a potential BACE1 radioligand. A fluoropropyl side chain was introduced to the phenyl of this 3,4-dihydroquinazoline scaffold to generate the radioligand. Our preliminary data indicated that although the 2-amino-3,4-dihydroquinazoline scaffold possessed favorable in-vitro properties as a PET ligand, its poor brain uptake hindered the in-vivo imaging of BACE1. Further investigation would be required to optimize the scaffold for the development of a blood-brain-barrier-permeable BACE1-targeted PET ligand.

A Purification Method of 18F-FP-(+)-DTBZ via Solid-Phase Extraction With Combined Cartridges

Background: To optimize [¹⁸F] 9-fluoropropyl-(+)-dihydrotetrabenazine (¹⁸F-FP-(+)-DTBZ) purification via solid-phase extraction (SPE) with combined cartridges to facilitate its widespread clinical application. Methods: A modified SPE purification method, employing Sep-Pak PS-2 and Sep-Pak C18 cartridges, was used for the preparation of ¹⁸F-FP-(+)-DTBZ. This method was compared to the purification method of high-pressure liquid chromatography (HPLC) and SPE with one cartridge, following quality control test and positron emission tomography (PET) imaging in healthy volunteers and patients with parkinsn's disease (PD). Results: A SPE purification method integrating Sep-Pak PS-2 and Sep-Pak C18 cartridges was implemented successfully. The retention time of ¹⁸F-FP-(+)-DTBZ purified by HPLC, SPE with Sep-Pak PS-2, SPE with Sep-Pak C18, and SPE with combined use of Sep-Pak PS-2 and Sep-Pak C18 cartridges was 8.7, 8.8, 8.7, and 8.9 min, respectively. Fewest impurity peak was detected in ¹⁸F-FP-(+)-DTBZ purified by the SPE with combined use of Sep-Pak PS-2 and Sep-Pak C18 cartridges. This modified SPE purification method provided a satisfactory radiochemical yield of 29 ± 1.8% with radiochemical purity >99% and shortened synthesis time to 27 min. The brain uptake of ¹⁸F-FP-(+)-DTBZ purified by the modified SPE was comparable to that purified by HPLC in both healthy volunteers and PD patients. Conclusions: A SPE method integrating Sep-Pak PS-2 and Sep-Pak C18 cartridges for purification of ¹⁸F-FP-(+)-DTBZ may be highly suited to automatic synthesis for routine clinical applications, as it provides excellent radiochemical purity, high yield as well as operational simplicity.

Molar activity of [18F]FP-(+)-DTBZ radiopharmaceutical: Determination and its effect on quantitative analysis of VMAT2 autoradiography

[¹⁸F]fluoropropyl-(+)-dihydrotetrabenazine ([¹⁸F]FP-(+)-DTBZ) is a rising positron tracer for imaging vesicular monoamine transporter II (VMAT2) in the central nervous system. The present work was to develop a novel chromatographic method capable of the molar activity (A_m) determination of [¹⁸F]FP-(+)-DTBZ. As a complement work of the A_m measurement, we also investigated the effect of A_m on the quantitative analysis of VMAT2 autoradiography with [¹⁸F]FP-(+)-DTBZ. The A_m determination was performed by high performance liquid chromatography (HPLC) using the non-radioactive standard (FP-(+)-DTBZ) for calibration plot of peak area against concentration. Based on this correlation, the A_m of [¹⁸F]FP-(+)-DTBZ was calculated and corrected to the end of synthesis. In the quantitative analysis of in vitro VMAT2 autoradiography, the striatum radioactivity uptake together with the uptake ratio of striatum versus cortex reduced along with the decrease of A_m and the increase of the FP-(+)-DTBZ content. Therefore, the A_m and the corresponding FP-(+)-DTBZ content have a significant effect on the quantitative analysis of VMAT2 autoradiography using [¹⁸F]FP-(+)-DTBZ.

An Efficient Automated Radiosynthesis and Bioactivity Confirmation of VMAT2 Tracer [18F]FP-(+)-DTBZ

PURPOSE

The aim of this study was to optimize the radiolabeling method of [¹⁸F]fluoropropyl-(+)-dihydrotetrabenazine ([¹⁸F]FP-(+)-DTBZ) to fulfill the demand of preclinical and clinical application.

PROCEDURES

Optimized labeling conditions were performed by altering the molar ratio of precursor to base (P/B), base species, solvents, reaction temperature, reaction time, and precursor concentration through manual radiosynthesis of [¹⁸F]FP-(+)-DTBZ. The conditions with the highest radiochemical yield (RCY) were applied to automated radiosynthesis, and the crude product was purified with a Sep-Pak Plus C18 cartridge. Quality control and stability of [¹⁸F]FP-(+)-DTBZ were carried out by HPLC. In vitro cellular uptake and blocking assays were conducted in human neuroblastoma cell line SH-SY5Y. In vivo imaging with small animal positron emission tomography (microPET) was performed with Sprague-Dawley rats.

RESULTS

Under the optimized conditions (P/K₂CO₃ = 1:8, heating at 120 °C for 3 min in dimethyl sulfoxide), an RCY of 88.7 % was obtained with 1.0 mg precursor. The optimized reaction conditions were successfully applied to an automated module and gave a high activity yield (AY) of 30-55 % in about 40 min with a > 99.0 % radiochemical purity (RCP) and a > 44.4 GBq/μmol molar activity (A_m). Stability test displayed that the RCP retained > 98.0 % in 8 h in saline and in phosphate buffer saline (PBS, pH 7.4). In vitro cellular uptake assay showed accumulation of [¹⁸F]FP-(+)-DTBZ in SH-SY5Y cells, which could be significantly inhibited by vesicular monoamine transporter 2 (VMAT2) inhibitor DTBZ. MicroPET images of rat brain displayed that the striatum showed the highest uptake with a standardized uptake value (SUV) of 3.91 ± 0.30 at ~ 70 min. Co-injection with DTBZ (1.0 mg/kg) resulted in a 75 % decrease of the striatal SUV, confirming the specificity of [¹⁸F]FP-(+)-DTBZ to VMAT2.

CONCLUSIONS

We obtained an optimized radiolabeling method of [¹⁸F]FP-(+)-DTBZ and successfully applied it to a commercial available module. The automated synthesis gave a high AY and RCP of [¹⁸F]FP-(+)-DTBZ with high and specific binding to VMAT2, facilitating its routine application for VMAT2 tracing.

Targeted pancreatic beta cell imaging for early diagnosis

Pancreatic beta cells are important in blood glucose level regulation. As type 1 and 2 diabetes are getting prevalent worldwide, we need to explore new methods for early detection of beta cell-related afflictions. Using bioimaging techniques to measure beta cell mass is crucial because a decrease in beta cell density is seen in diseases such as diabetes and thus can be a new way of diagnosis for such diseases. We also need to appraise beta cell purity in transplanted islets for type 1 diabetes patients. Sufficient amount of functional beta cells must also be determined before being transplanted to the patients. In this review, indirect imaging of beta cells will be discussed. This includes membrane protein on pancreatic beta cells whereby specific probes are designed for different imaging modalities mainly magnetic resonance imaging, positron emission tomography and fluorescence imaging. Direct imaging of insulin is also explored though probes synthesized for such function are relatively fewer. The path for successful pancreatic beta cell imaging is fraught with challenges like non-specific binding, lack of beta cell-restricted targets, the requirement of probes to cross multiple lipid layers to bind to intracellular insulin. Hence, there is an urgent need to develop new imaging techniques and innovative probing constructs in the entire imaging chain of bioengineering to provide early detection of beta cell-related pathology.

Synthesis of Tetrabenazine and Its Derivatives, Pursuing Efficiency and Selectivity

Tetrabenazine is a US Food and Drug Administration (FDA)-approved drug that exhibits a dopamine depleting effect and is used for the treatment of chorea in Huntington’s disease. Mechanistically, tetrabenazine binds and inhibits vesicular monoamine transporter type 2, which is responsible for importing neurotransmitters from the cytosol to the vesicles in neuronal cells. This transportation contributes to the release of neurotransmitters inside the cell to the synaptic cleft, resulting in dopaminergic signal transmission. The highly potent inhibitory activity of tetrabenazine has led to its advanced applications and in-depth investigation of prodrug design and metabolite drug discovery. In addition, the synthesis of enantiomerically pure tetrabenazine has been pursued. After a series of research studies, tetrabenazine derivatives such as valbenazine and deutetrabenazine have been approved by the US FDA. In addition, radioisotopically labeled tetrabenazine permits the early diagnosis of Parkinson’s disease, which is difficult to treat during the later stages of this disease. These applications were made possible by the synthetic efforts aimed toward the efficient and asymmetric synthesis of tetrabenazine. In this review, various syntheses of tetrabenazine and its derivatives have been summarized.

Rapid screening of nine unradiolabeled candidate compounds as PET brain imaging agents using cassette-wave microdosing and LC-MS/MS

The R&D of PET imaging agents is a complex system engineering, simplifying screening steps and increasing screening efficiency have become popular issues. The purpose of this study is to develop a new screening procedure using cassette-wave microdosing and LC-MS/MS to enhance the screening throughput of unradiolabeled candidate compounds as PET imaging agents. Nine compounds were divided into 3 sets and made into 3 cassettes. Fifteen rats were randomly divided into 3 groups, and every animal received three intravenous bolus injections at three different time points; the doses were at microdose levels. This dosing approach takes advantage of temporal and spatial differences and is likened to an input wave; therefore, this approach was named cassette-wave microdosing. The samples of different brain regions such as the hypothalamus, striatum, hippocampus, cortex, cerebellum and the remainder of the brain were detected by LC-MS/MS analysis. The research potential of the compounds as PET imaging agents is evaluated in terms of brain biodistribution data. The screening method is rapid, highly efficient, reliable and reduces animal usage. Additionally, it can shorten the evaluation process of radiopharmaceuticals and enhance the screening throughput of PET radiopharmaceuticals without the use of radioactive agents.

VMAT2 imaging agent, D6-[18F]FP-(+)-DTBZ: Improved radiosynthesis, purification by solid-phase extraction and characterization

OBJECTIVES

Recently, a deuterated tracer, D6-[¹⁸F]FP-(+)-DTBZ, 9-O-hexadeutero-3-[¹⁸F]fluoropropoxyl-(+)-dihydrotetrabenazine ([¹⁸F]9), targeting vesicular monoamine transporter 2 (VMAT2) in the central nervous system, was reported as a useful imaging agent for the diagnosis of Parkinson's disease (PD). The production of [¹⁸F]9 was optimized and simplified by using solid-phase extraction (SPE) purification.

METHODS

Three major nonradioactive impurities were synthesized and characterized. The preparation of [¹⁸F]9 was optimized by using different labeling conditions, and an SPE purification method was evaluated. The influence of chemical impurities in the final dose of [¹⁸F]9 was assessed by an in vitro binding assay, an assay of the in vivo biodistribution in mice, and ex vivo and in vitro autoradiography of brain sections.

RESULTS

Optimized fluorination conditions for [¹⁸F]9 were found - heating at 130 °C for 10 min in DMSO, and a high radiochemical yield and three major chemical impurities were observed. An SPE method involving a Sep-Pak® tC18 Plus Light cartridge with a two-step elution process was successfully implemented. This process gave a good radiochemical yield (38.7 ± 10.5%, decay corrected; radiochemical purity >99%) and low chemical impurities. An in vivo biodistribution study and autoradiography of brain sections showed that there was no significant difference between HPLC-purified and SPE-purified [¹⁸F]9.

CONCLUSION

A VMAT2 targeting imaging agent, D6-[¹⁸F]FP-(+)-DTBZ, [¹⁸F]9, was prepared by optimized labeling conditions and an easy SPE purification. This method offers a short preparation time and operational simplicity. In conjunction with PET imaging, this new VMAT2 agent might be a useful clinical tool for diagnosing PD.

The Parkinson's progression markers initiative (PPMI) - establishing a PD biomarker cohort

OBJECTIVE

The Parkinson's Progression Markers Initiative (PPMI) is an observational, international study designed to establish biomarker-defined cohorts and identify clinical, imaging, genetic, and biospecimen Parkinson's disease (PD) progression markers to accelerate disease-modifying therapeutic trials.

METHODS

A total of 423 untreated PD, 196 Healthy Control (HC) and 64 SWEDD (scans without evidence of dopaminergic deficit) subjects were enrolled at 24 sites. To enroll PD subjects as early as possible following diagnosis, subjects were eligible with only asymmetric bradykinesia or tremor plus a dopamine transporter (DAT) binding deficit on SPECT imaging. Acquisition of data was standardized as detailed at www.ppmi-info.org.

RESULTS

Approximately 9% of enrolled subjects had a single PD sign at baseline. DAT imaging excluded 16% of potential PD subjects with SWEDD. The total MDS-UPDRS for PD was 32.4 compared to 4.6 for HC and 28.2 for SWEDD. On average, PD subjects demonstrated 45% and 68% reduction in mean striatal and contralateral putamen Specific Binding Ratios (SBR), respectively. Cerebrospinal fluid (CSF) was acquired from >97% of all subjects. CSF (PD/HC/SWEDD pg/mL) α-synuclein (1845/2204/2141) was reduced in PD vs HC or SWEDD (P < 0.03). Similarly, t-tau (45/53) and p-tau (16/18) were reduced in PD versus HC (P < 0.01).

INTERPRETATION

PPMI has detailed the biomarker signature for an early PD cohort defined by clinical features and imaging biomarkers. This strategy provides the framework to establish biomarker cohorts and to define longitudinal progression biomarkers to support future PD treatment trials.

Developing a cassette microdosing approach to enhance the throughput of PET imaging agent screening

Cassette dosing is also known as N-in-One dosing: several compounds are simultaneously administrated to a single animal and then the samples are rapidly detected by LC-MS/MS. This approach is a successful strategy to enhance the efficiency of drug discovery and reduce animal usage. However, no report on the utility of the cassette approach in radiotracer discovery has appeared in the literature. This study designed a cassette microdose with LC-MS/MS method to enhance the throughput for screening radiopharmaceutical biodistribution in the rat brain directly. Three unradiolabeled compounds (FPBM FPBM2 and AV-133) were chosen as model drugs administrated intravenously to the rats as a cassette as opposed to discrete study. The rat brain biodistribution data, target localization, the differential uptake ratio (%ID/g) and the brain tissue-specific binding ratio were obtained by the LC-MS/MS analysis. These data matched very well with the values obtained by the standard radioactivity measurements. Moreover, no significant differences between discrete dosing and cassette dosing were observed. By circumventing the need for radiolabeled molecules, this method may be high-throughput and safe for the research and development of new PET imaging agents. The combination of cassette microdosing and LC-MS/MS would be a medium throughput screening tool at an early stage in the discovery/development process of PET imaging agents.

Effects of anesthetics on vesicular monoamine transporter type 2 binding to ¹⁸F-FP-(+)-DTBZ: a biodistribution study in rat brain

OBJECTIVES

The in vivo binding analysis of vesicular monoamine transporter type 2 (VMAT2) to radioligand has provided a means of investigating related disorders. Anesthesia is often inevitable when the investigations are performed in animals. In the present study, we tested effects of four commonly-used anesthetics: isoflurane, pentobarbital, chloral hydrate and ketamine, on in vivo VMAT2 binding to (18)F-FP-(+)-DTBZ, a specific VMAT2 radioligand, in rat brain.

METHODS

The transient equilibrium time window for in vivo binding of (18)F-FP-(+)-DTBZ after a bolus injection was firstly determined. The brain biodistribution studies under anesthetized and awake rats were then performed at the equilibrium time. Standard uptake values (SUVs) of the interest brain regions: the striatum (ST), hippocampus (HP), cortex (CX) and cerebellum (CB) were obtained; and ratios of tissue to cerebellum were calculated.

RESULTS

Isoflurane and pentobarbital did not alter distribution of (18)F-FP-(+)-DTBZ in the brain relative to the awake group; neither SUVs nor ratios of ST/CB and HP/CB were altered significantly. Chloral hydrate significantly increased SUVs of all the brain regions, but did not significantly alter ratios of ST/CB and HP/CB. Ketamine significantly increased SUVs of the striatum, hippocampus and cortex, and insignificantly increased the SUV of the cerebellum; consequently, ketamine significantly increased ratios of ST/CB and HP/CB.

CONCLUSIONS

It is concluded that in vivo VMAT2 binding to (18)F-FP-(+)-DTBZ are not altered by isoflurane and pentobarbital, but altered by chloral hydrate and ketamine. Isoflurane and pentobarbital may be promising anesthetic compounds for investigating in vivo VMAT2 binding. Further studies are warranted to investigate the interactions of anesthetics with VMAT2 binding potential with in vivo PET studies.

Cross-sectional and Test-Retest Characterization of PET with [(18)F]FP-(+)-DTBZ for β Cell Mass Estimates in Diabetes

Purpose

The vesicular monoamine transporter, type 2 (VMAT2) is expressed by insulin producing β cells and was evaluated as a biomarker of β cell mass (BCM) by positron emission tomography (PET) with [¹⁸F]fluoropropyl-dihydrotetrabenazine ([¹⁸F]FP-(+)-DTBZ).

Procedures

We evaluated the feasibility of longitudinal pancreatic PET VMAT2 quantification in the pancreas in two studies of healthy controls and patients with type 1 or 2 diabetes. VMAT2 binding potential (BP_ND) was estimated voxelwise using a reference tissue method in a cross-sectional study, followed by assessment of reproducibility using a test-retest paradigm. Metabolic function was evaluated by stimulated c-peptide measurements.

Results

Pancreatic BP_ND was significantly decreased in patients with type 1 diabetes relative to controls and the test-retest variability was 9.4 %.

Conclusions

Pancreatic VMAT2 content is significantly reduced in long-term diabetes patients relative to controls and repeat scans are sufficiently reproducible to suggest the feasibility clinically VMAT2 measurements in longitudinal studies of new onset diabetes.

Electronic supplementary material

The online version of this article (doi:10.1007/s11307-015-0888-7) contains supplementary material, which is available to authorized users.

Brain uptake of a non-radioactive pseudo-carrier and its effect on the biodistribution of [(18)F]AV-133 in mouse brain

INTRODUCTION

9-[(18)F]Fluoropropyl-(+)-dihydrotetrabenazine ([(18)F]AV-133) is a new PET imaging agent targeting vesicular monoamine transporter type II (VMAT2). To shorten the preparation of [(18)F]AV-133 and to make it more widely available, a simple and rapid purification method using solid-phase extraction (SPE) instead of high-pressure liquid chromatography (HPLC) was developed. The SPE method produced doses containing the non-radioactive pseudo-carrier 9-hydroxypropyl-(+)-dihydrotetrabenazine (AV-149). The objectives of this study were to evaluate the brain uptake of AV-149 by UPLC-MS/MS and its effect on the biodistribution of [(18)F]AV-133 in the brains of mice.

METHODS

The mice were injected with a bolus including [(18)F]AV-133 and different doses of AV-149. Brain tissue and blood samples were harvested. The effect of different amounts of AV-149 on [(18)F]AV-133 was evaluated by quantifying the brain distribution of radiolabelled tracer [(18)F]AV-133. The concentrations of AV-149 in the brain and plasma were analyzed using a UPLC-MS/MS method.

RESULTS

The concentrations of AV-149 in the brain and plasma exhibited a good linear relationship with the doses. The receptor occupancy curve was fit, and the calculated ED50 value was 8.165mg/kg. The brain biodistribution and regional selectivity of [(18)F]AV-133 had no obvious differences at AV-149 doses lower than 0.1mg/kg. With increasing doses of AV-149, the brain biodistribution of [(18)F]AV-133 changed significantly.

CONCLUSION

The results are important to further support that the improved radiolabelling procedure of [(18)F]AV-133 using an SPE method may be suitable for routine clinical application.

¹⁸F-FP-(+)-DTBZ positron emission tomography detection of monoaminergic deficient network in patients with carbon monoxide related parkinsonism

BACKGROUND AND PURPOSE

Although parkinsonism after carbon monoxide (CO) intoxication is well known, neurotransmitter deficient networks that are responsible for the severity of parkinsonism have rarely been systemically evaluated.

METHODS

Eighteen patients with CO-related parkinsonism and nine age- and sex-matched controls were enrolled for detailed neurological examinations, three-dimensional T1-weighted images, diffusion tensor imaging and (18)F-9-fluoropropyl-(+)-dihydrotetrabenzazine ((18)F-FP-(+)-DTBZ) positron emission tomography (PET). The structural analysis included voxel-based morphometry to assess grey matter atrophy and tract-based spatial statistics related to white matter involvement. For presynaptic monoaminergic assessment, volume of interest analysis in six subcortical regions and non-parametric voxel-wise comparison were performed on PET images with estimation of registration parameters from magnetic resonance images. All the imaging modalities were compared between the patients and controls. For the patients, a regression model for correlation with cognitive behaviour and Unified Parkinson's Disease Rating Scale (UPDRS) score was used.

RESULTS

In the patients, monoaminergic deficit networks were found in the caudate, anterior putamen, anterior insular, thalamus and anterior cingulate cortex. The UPDRS revealed significant correlations with the prefrontal white matter fractional anisotropy values and with the (18)F-FP-(+)-DTBZ uptake values in the caudate nucleus, insular, medial prefrontal and dorsomedial thalamus. The neuropsychiatric inventory score correlated with the (18)F-FP-(+)-DTBZ uptake values in the anterior cingulate cortex and dorsolateral prefrontal cortex.

CONCLUSIONS

Our study demonstrated monoaminergic deficits and white matter damage networks in CO-related parkinsonism that determined the severity of parkinsonism or behaviour changes. As the substantia nigra was spared, the monoaminergic topography of involvement suggests a different pathophysiology in CO-related parkinsonism.

PET radioligands for the vesicular transporters for monoamines and acetylcholine

The vesicular transporters for the monoamine and acetylcholine have been successfully targeted for the development of radioligands for human brain imaging. The vesicular monoamine transporter type 2 ligands are based on the structure of tetrabenazine, a known clinically used drug. In contrast, the radioligands for vesicular acetylcholine transporter are based on vesamicol, a toxic xenobiotic. The similarities and differences in the development of these two classes of radioligands are discussed.

Parkinson׳s disease-related modulation of functional connectivity associated with the striatum in the resting state in a nonhuman primate model

The goal of this study was to describe Parkinson׳s disease (PD)-related modulation of functional connectivity (FC) associated with the striatum in the resting state in a nonhuman primate model of early-stage PD. Weekly intravenous injections of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (0.5 mg/kg body weight) were performed until parkinsonian motor symptoms developed in four macaques. After 13 weeks of MPTP treatment, all monkeys displayed parkinsonian symptoms. During the course of the experiment, each animal underwent four magnetic resonance imaging scans and four positron emission tomography (PET) scans with the vesicular monoamine transporter 2 (VMAT2)-selective ligand 9-[(18)F] fluoropropyl-(+)-dihydrotetrabenazine, performed prior to the beginning of MPTP administration as well as after 4, 9, and 13 MPTP injections. The FC profile of the striatum was evaluated using a seed voxel correlation approach and post hoc region of interest analysis on resting-state functional magnetic resonance imaging data. The PET images were subjected to region of interest analysis to examine brain regional reductions in VMAT2 density in the PD model. Significant reductions in the connectivity pattern of the striatal regions were observed: limbic striatum and left hippocampus; caudate nucleus/associative and brain regions, including the right pre-supplementary motor area and bilateral dorsolateral prefrontal cortex; putamen/associative region and left inferior temporal gyrus or right orbital and medial prefrontal cortex; and putamen/motor and cortical structures, including the right superior temporal gyrus and bilateral postcentral gyrus. Subsequent PET studies showed the progressive loss of striatal VMAT2 in the striatum with the presentation of parkinsonism. Significant differences between the specific uptake ratio reductions in each striatal subdivision were not found. By using a long-term, low-dose MPTP-lesioned nonhuman primate model, this study demonstrated PD-related decreased corticostriatal FC in a resting state; moreover, altered sensorimotor integration was also found in early-stage PD.

Synthesis and biological evaluation of 10-(11) C-dihydrotetrabenazine as a vesicular monoamine transporter 2 radioligand

In this study, we synthesized a new carbon-11-labeled radiotracer, 10-(11) C-dihydrotetrabenazine (10-(11) C-DTBZ), and evaluated its potential as a vesicular monoamine transporter 2 (VMAT2) radioligand. The radiolabeled precursor 10-O-desmethyl-dihydrotetrabenazine (10-O-desmethyl-DTBZ) was prepared with a six-step reaction using 3-methoxy-4-benzyloxybenzaldehyde as starting material. 10-(11) C-DTBZ was synthesized by heating 1.0 mg of 10-hydroxy precursor and (11) C-methyl iodide in the presence of 0.3 mL of dimethyl sulfoxide and 4.0 µL of 3 N KOH at room temperature for 3 min. After purification by solid phase extraction using an alumina Sep-Pak cartridge, the final 10-(11) C-DTBZ product was obtained with a radiochemical purity of >99% and an uncorrected radiochemical yield of 18-26% (end of bombardment (EOB), n = 6). The overall synthesis time was approximately 20 min from the EOB to release of the product for quality control. Using small-animal positron emission tomography (microPET), the striatum of normal rats was found to exhibit symmetrical labeling (STR /STL  = 0.98 ± 0.05, n = 3) and the highest uptake of radioactivity (striatum/cerebellum, ST/CB = 2.89 ± 0.31 at 30-60 min, n = 3). In contrast, rats with 6-hydroxydopamine unilateral lesions yielded asymmetrical striatal images with a higher 10-(11) C-DTBZ concentration on the unlesioned side (STunlesioned /CB = 2.53 ± 0.18, at 30-60 min, n = 3) compared with the lesioned side (STlesioned /CB = 1.26 ± 0.10, n = 3). These results suggest that 10-(11) C-DTBZ may represent a promising PET radiotracer for imaging VMAT2.

Dopamine: PET Imaging and Parkinson Disease

This article discusses the current use of PET imaging in the evaluation of dopamine function in Parkinson disease (PD). The article reviews the major radioligands targeting dopaminergic systems in patients with parkinsonian disorders. The primary objective is to show the novel clinical applications of molecular imaging in the diagnosis and assessment of motor and nonmotor symptoms in PD.

Progressive loss of striatal dopamine terminals in MPTP-induced acute parkinsonism in cynomolgus monkeys using vesicular monoamine transporter type 2 PET imaging ([(18)F]AV-133)

The 1-methyl-4-phenyl-1,2,3,4-tetrahydropyridine (MPTP)-induced parkinsonism model, particularly in non-human primates, remains the gold-standard for studying the pathogenesis and assessing novel therapies for Parkinson's disease. However, whether the loss of dopaminergic neurons in this model is progressive remains controversial, mostly due to the lack of objective in vivo assessment of changes in the integrity of these neurons. In the present study, parkinsonism was induced in cynomolgus monkeys by intravenous administration of MPTP (0.2 mg/kg) for 15 days; stable parkinsonism developed over 90 days, when the symptoms were stable. Noninvasive positron emission tomographic neuroimaging of vesicular monoamine transporter 2 with 9-[(18)F] fluoropropyl-(+)-dihydrotetrabenazine ([(18)F]AV-133) was used before, and 15 and 90 days after the beginning of acute MPTP treatment. The imaging showed evident progressive loss of striatal uptake of [(18)F]AV-133. The dopaminergic denervation severity had a significant linear correlation with the clinical rating scores and the bradykinesia subscores. These findings demonstrated that [(18)F]AV-133 PET imaging is a useful tool to noninvasively evaluate the evolution of monoaminergic terminal loss in a monkey model of MPTP-induced parkinsonism.

f-[18F]fluoroethanol and 3-[18F]fluoropropanol: facile preparation, biodistribution in mice, and their application as nucleophiles in the synthesis of [18F]fluoroalkyl aryl ester and ether PET tracers

INTRODUCTION

2-[(18)F]Fluoroethoxy and 3-[(18)F]fluoropropoxy groups are common moieties in the structures of radiotracers used with positron emission tomography. The objectives of this study were (1) to develop an efficient one-step method for the preparation of 2-[(18)F]fluoroethanol (2-[(18)F]FEtOH) and 3-[(18)F]fluoropropanol (3-[(18)F]FPrOH); (2) to demonstrate the feasibility of using 2-[(18)F]FEtOH as a nucleophile for the synthesis of 2-[(18)F]fluoroethyl aryl esters and ethers; and (3) to determine the biodistribution profiles of 2-[(18)F]FEtOH and 3-[(18)F]FPrOH in mice.

METHODS

2-[(18)F]FEtOH and 3-[(18)F]FPrOH were prepared by reacting n-Bu4N[(18)F]F with ethylene carbonate and 1,3-dioxan-2-one, respectively, in diethylene glycol at 165°C and purified by distillation. 2-[(18)F]fluoroethyl 4-fluorobenzoate and 1-(2-[(18)F]fluoroethoxy)-4-nitrobenzene were prepared by coupling 2-[(18)F]FEtOH with 4-fluorobenzoyl chloride and 1-fluoro-4-nitrobenzene, respectively. Biodistribution and PET/CT imaging studies of 2-[(18)F]FEtOH and 3-[(18)F]FPrOH were performed in normal female Balb/C mice.

RESULTS

The preparation of 2-[(18)F]FEtOH and 3-[(18)F]FPrOH took 60 min, and their decay-corrected yields were 88.6 ± 2.0% (n = 9) and 65.6 ± 10.2% (n = 5), respectively. The decay-corrected yields for the preparation of 2-[(18)F]fluoroethyl 4-fluorobenzoate and 1-(2-[(18)F]fluoroethoxy)-4-nitrobenzene were 36.1 ± 5.4% (n = 3) and 27.7 ± 10.7% (n = 3), respectively. Imaging/biodistribution studies in mice using 2-[(18)F]FEtOH showed high initial radioactivity accumulation in all major organs followed by very slow clearance. On the contrary, by using 3-[(18)F]FPrOH, radioactivity accumulated in all major organs was cleared rapidly, but massive in vivo defluorination (31.3 ± 9.57%ID/g in bone at 1h post-injection) was observed.

CONCLUSIONS

Using 2-[(18)F]FEtOH/3-[(18)F]FPrOH as a nucleophile is a competitive new strategy for the synthesis of 2-[(18)F]fluoroethyl/3-[(18)F]fluoropropyl aryl esters and ethers. Our biodistribution data emphasize the importance of in vivo stability of PET tracers containing a 2-[(18)F]fluoroethyl or 3-[(18)F]fluoropropyl group due to high background and high bone uptake resulting from 2-[(18)F]FEtOH and 3-[(18)F]FPrOH, respectively. This is especially important for their aryl ester derivatives which are prone to in vivo hydrolysis.

Imaging of VMAT2 binding sites in the brain by (18)F-AV-133: the effect of a pseudo-carrier

OBJECTIVES

Recently, 9-[(18)F]fluoropropyl-(+)-dihydrotetrabenazine ((18)F-AV-133) was reported as a new vesicular monoamine transporter (VMAT2) imaging agent for diagnosis of Parkinson's disease (PD). To shorten the preparation of (18)F-AV-133 and to make it more widely available, we evaluated a simple, rapid purification with a solid-phase extraction method (SPE) using an Oasis HLB cartridge instead of high pressure liquid chromatography (HPLC). The SPE method produced doses containing a pseudo-carrier, 9-hydroxypropyl-(+)-dihydrotetrabenazine (AV-149).

METHODS

To test the possible side effects of this pseudo-carrier, comparative dynamic PET scans of the brains of normal monkeys (2 each) and uni-laterally 6-OH-dopamine-lesioned PD monkeys (2 each) were performed using (18)F-AV-133 doses prepared by either SPE (containing pseudo-carrier) or HPLC (containing no pseudo-carrier). Autoradiographs of post mortem monkey brain sections were evaluated to confirm the relative (18)F-AV-133 uptake in the PD monkey brains and the effects of the pseudo-carrier on VMAT2 binding.

RESULTS

The radiochemical purity of the (18)F-AV-133, whether prepared by SPE or by HPLC, was excellent (>99%). PET scans of normal and PD monkey brains showed an expected reduction of VMAT2 in the lesioned areas of the striatum. It was not affected by the presence of the pseudo-carrier, AV-149 (maximally 250 μg/dose). The reduced uptake in the striatum of the lesioned monkey brains was confirmed by autoradiography. Ex vivo inhibition studies of (18)F-AV-133 binding in rat brains, conducted with increasing amounts of AV-149, suggested that at the highest concentration (3.5mg/kg) the VMAT2 binding in the striatum was only moderately blocked (20% reduction).

CONCLUSIONS

The pseudo-carrier, AV-149, did not affect the (18)F-AV-133/PET imaging of VMAT2 binding sites in normal or uni-laterally lesioned monkey brains. The new streamlined SPE purification method will enable (18)F-AV-133 to be widely available for routine clinical application in determining changes in monoamine neurons for patient with movement disorders or other psychiatric illnesses.

PET imaging a MPTP-induced mouse model of Parkinson's disease using the fluoropropyl-dihydrotetrabenazine analog [18F]-DTBZ (AV-133)

Parkinson's disease (PD) is characterized by the loss of dopamine-producing neurons in the nigrostriatal system. Numerous researchers in the past have attempted to track the progression of dopaminergic depletion in PD. We applied a quantitative non-invasive PET imaging technique to follow this degeneration process in an MPTP-induced mouse model of PD. The VMAT2 ligand (18)F-DTBZ (AV-133) was used as a radioactive tracer in our imaging experiments to monitor the changes of the dopaminergic system. Intraperitoneal administrations of MPTP (a neurotoxin) were delivered to mice at regular intervals to induce lesions consistent with PD. Our results indicate a significant decline in the levels of striatal dopamine and its metabolites (DOPAC and HVA) following MPTP treatment as determined by HPLC method. Images obtained by positron emission tomography revealed uptake of (18)F-DTBZ analog in the mouse striatum. However, reduction in radioligand binding was evident in the striatum of MPTP lesioned animals as compared with the control group. Immunohistochemical analysis further confirmed PET imaging results and indicated the progressive loss of dopaminergic neurons in treated animals compared with the control counterparts. In conclusion, our findings suggest that MPTP induced PD in mouse model is appropriate to follow the degeneration of dopaminergic system and that (18)F-DTBZ analog is a potentially sensitive radiotracer that can used to diagnose changes associated with PD by PET imaging modality.

Serotonin and molecular neuroimaging in humans using PET

The serotonergic system is one of the most important modulatory neurotransmitter systems in the human brain. It plays a central role in major physiological processes and is implicated in a number of psychiatric disorders. Along with the dopaminergic system, it is also one of the phylogenetically oldest human neurotransmitter systems and one of the most diverse, with 14 different receptors identified up to this day, many of whose function remains to be understood. The system's functioning is even more diverse than the number of its receptors, since each is implicated in a number of different processes. This review aims at illustrating the distribution and summarizing the main functions of the serotonin (5-hydroxytryptamin, 5-HT) receptors as well as the serotonin transporter (SERT, 5-HTT), the vesicular monoamine transporter 2, monoamine oxidase type A and 5-HT synthesis in the human brain. Recent advances in in vivo quantification of these different receptors and enzymes that are part of the serotonergic system using positron emission tomography are described.

Study the effect of a pseudo-carrier on pharmacokinetics of 9-fluoropropyl-(+)-dihydrotetrabenazine in rat plasma by ultra-performance liquid chromatography-tandem mass spectrometry

To evaluate the effect of a pseudo-carrier (9-hydroxypropyl-(+)-dihydrotetrabenazine, AV-149) on pharmacokinetics of 9-fluoropropyl-(+)-dihydrotetrabenazine (AV-133), an ultra-performance liquid chromatography-tandem mass spectrometric (UPLC-MS/MS) method was developed and validated for the determination of AV-133 and AV-149 in rat plasma. AV-133 and AV-149 were extracted from plasma following protein precipitation. The chromatographic analysis was performed on an ACQUITY UPLC BEH™ C₁₈ column (50 mm x 2.1 mm x 1.7 μm) by a gradient elution. The mass spectrometer was operated in positive mode using electrospray ionization. The analytes were measured using the multiple reaction-monitoring mode (MRM). An external calibration was used, and the calibration curves were linear in the range of 1.00-800 ng/mL for AV-133 and AV-149. The accuracy ranged from 90.8% to 113.2% and the precision ranged from 2.7% to 9.9% for each analyte. The effect of a pseudo-carrier on pharmacokinetics of AV-133 was studied using the presented method.