In vitro autoradiography and in vivo evaluation in cynomolgus monkey of [18F]FE-PE2I, a new dopamine transporter PET radioligand

Positron Emission Tomography Imaging of Synaptic Dysfunction in Parkinson's Disease

Parkinson's disease (PD) is one of the most common neurodegenerative diseases with a complex pathogenesis. Aggregations formed by abnormal deposition of alpha-synuclein (αSyn) lead to synapse dysfunction of the dopamine and non-dopamine systems. The loss of dopaminergic neurons and concomitant alterations in non-dopaminergic function in PD constitute its primary pathological manifestation. Positron emission tomography (PET), as a representative molecular imaging technique, enables the non-invasive visualization, characterization, and quantification of biological processes at cellular and molecular levels. Imaging synaptic function with PET would provide insights into the mechanisms underlying PD and facilitate the optimization of clinical management. In this review, we focus on the synaptic dysfunction associated with the αSyn pathology of PD, summarize various related targets and radiopharmaceuticals, and discuss applications and perspectives of PET imaging of synaptic dysfunction in PD.

A review of the neurotransmitter system associated with cognitive function of the cerebellum in Parkinson's disease

The dichotomized brain system is a concept that was generalized from the 'dual syndrome hypothesis' to explain the heterogeneity of cognitive impairment, in which anterior and posterior brain systems are independent but partially overlap. The dopaminergic system acts on the anterior brain and is responsible for executive function, working memory, and planning. In contrast, the cholinergic system acts on the posterior brain and is responsible for semantic fluency and visuospatial function. Evidence from dopaminergic/cholinergic imaging or functional neuroimaging has shed significant insight relating to the involvement of the cerebellum in the cognitive process of patients with Parkinson's disease. Previous research has reported evidence that the cerebellum receives both dopaminergic and cholinergic projections. However, whether these two neurotransmitter systems are associated with cognitive function has yet to be fully elucidated. Furthermore, the precise role of the cerebellum in patients with Parkinson's disease and cognitive impairment remains unclear. Therefore, in this review, we summarize the cerebellar dopaminergic and cholinergic projections and their relationships with cognition, as reported by previous studies, and investigated the role of the cerebellum in patients with Parkinson's disease and cognitive impairment, as determined by functional neuroimaging. Our findings will help us to understand the role of the cerebellum in the mechanisms underlying cognitive impairment in Parkinson's disease.

Microsome Mediated in Vitro Metabolism: A Convenient Method for the Preparation of the PET Radioligand Metabolite [18F]FE-PE2I-OH for Translational Dopamine Transporter Imaging

Undesired radiometabolites can be detrimental to the development of positron emission tomography (PET) radioligands. Methods for quantifying radioligand metabolites in brain tissue include ex vivo studies in small animals or labeling and imaging of the radiometabolite(s) of interest. The latter is a time- and resource-demanding process, which often includes multistep organic synthesis. We hypothesized that this process could be replaced by making use of liver microsomes, an in vitro system that mimics metabolism. In this study, rat liver microsomes were used to prepare radiometabolites of the dopamine transporter radioligand [18F]FE-PE2I for in vitro imaging using autoradiography and in vivo imaging using PET in rats and nonhuman primates. The primary investigated hydroxy-metabolite [18F]FE-PE2I-OH ([18F]2) was obtained in a 2% radiochemical yield and >99% radiochemical purity. In vitro and in vivo imaging demonstrated that [18F]2 readily crossed the blood-brain barrier and bound specifically and reversibly to the dopamine transporter. In conclusions, the current study demonstrates the potential of liver microsomes in the production of radiometabolites for translational imaging studies and radioligand discovery.

Sertraline treatment influences [18F]FE-PE2I PET imaging for Parkinsonism

BACKGROUND

The dopamine transporter (DaT) PET ligand [¹⁸F]FE-PE2I is used to aid the diagnosis of Parkinson's disease. After encountering four patients with a history of daily sertraline use, who all showed atypical findings on [¹⁸F]FE-PE2I PET, we suspected that the selective serotonin reuptake inhibitor (SSRI), sertraline, might interfere with the results and lead to globally reduced striatal [¹⁸F]FE-PE2I binding due to sertraline's high affinity for DaT.

METHODS

We rescanned the four patients with [¹⁸F]FE-PE2I PET after a 5-day sertraline pause. Sertraline plasma concentration was estimated based on body weight and dose, and specific binding ratios (SBR) in caudate nucleus, known to be more preserved in Parkinson's, were used to estimate the effect on tracer binding. Comparison was made to a patient with [¹⁸F]FE-PE2I PET before and after a 7-day Modafinil pause.

RESULTS

We found a significant effect of sertraline on caudate nucleus SBR (p = 0.029). The effect showed a linear dose-dependent relationship that corresponds to a reduction in SBR by 0.32 or 0.44 for a 75 kg male or a 65 kg female, respectively, taking a daily dose of 50 mg sertraline.

CONCLUSION

Sertraline is one of the most commonly used antidepressants and in contrast to other SSRI's, sertraline show high affinity for DaT. We recommend that sertraline treatment is taken into account when patients are undergoing [¹⁸F]FE-PE2I PET especially in patients showing apparent globally reduced PE2I binding. If tolerable, pausing of the sertraline treatment should be considered, especially for doses above 50 mg/day.

Longitudinal DAT changes measured with [18F]FE-PE2I PET in patients with Parkinson's disease; a validation study

BACKGROUND

Dopamine transporter (DAT) PET provides higher resolution than DAT SPECT and opportunity for integrated imaging with MRI. The radioligand [¹⁸F]FE-PE2I is highly selective for the DAT, and PET measurements with this radioligand have good reliability and repeatability in patients with non-advanced Parkinson's disease.

OBJECTIVES

To validate [¹⁸F]FE-PE2I PET as measurement tool of longitudinal DAT changes in patients with Parkinson's disease.

METHODS

Thirty-seven subjects with Parkinson's disease (Hoehn and Yahr stage < 3) were included in a longitudinal PET study with [¹⁸F]FE-PE2I. DAT availability (BP_ND) in the caudate nucleus, putamen, sensorimotor striatum, and substantia nigra, was estimated with parametric imaging using Logan graphical analysis and cerebellum as reference region. For comparison with DAT-SPECT literature, sample size calculations for disease intervention studies were made.

RESULTS

Baseline and follow-up PET data (interval: 2.3 ± 0.5 years) were available for 25 patients (9 females, 16 males). Median age was 64.7 years (range 46-76); symptom duration: 3 years (0.25-14); Hoehn and Yahr stage (H&Y): 1 (1-2). Annualized DAT decline and effect size were: -8.5 ± 6.6 % and 1.08 for caudate nucleus; -7.1 ± 6.1 % and 1.02 for putamen; -8.3 ± 8.5 % and 0.99 for sensorimotor striatum; -0.11 ± 9.3 % and 0.11 for substantia nigra. The estimated minimum sample size needed for a treatment trial using [¹⁸F]FE-PE2I PET as imaging marker is 2-3 times lower than is reported in literature on [¹²³I]FP-CIT SPECT.

CONCLUSIONS

Longitudinal [¹⁸F]FE-PE2I PET measurements in non-advanced PD demonstrate a striatal DAT decline consistent with previous SPECT and PET studies. No obvious changes of DAT availability were observed in the substantia nigra, indicating perhaps slower progression or compensatory changes. The effect sizes were numerically larger than reported in the literature for other DAT radioligands, suggesting that [¹⁸F]FE-PE2I might detect smaller DAT changes, and can be well used as progression marker in clinical trials.

Mangosteen Pericarp Extract Supplementation Boosts Antioxidant Status via Rebuilding Gut Microbiota to Attenuate Motor Deficit in 6-OHDA-Induced Parkinson's Disease

Oxidative stress and gut dysbiosis have been known to precede Parkinson's disease (PD). An antioxidant-rich product, mangosteen pericarp (MP), has the ability to counterbalance excessive free radicals and the imbalanced gut microbiota composition, suggesting the MP's capacity to delay PD progression. In this study, we explored the effects of two doses of MP extract in a unilateral 6-hydroxydopamine (6-OHDA)-induced PD rat model. We revealed that the 8-week supplementation of a low dose (LMP) and a high dose of the MP extract (HMP) improved motor function, as observed in decreased contralateral rotation, improved time spent on rod, and higher dopamine binding transporter (DAT) in the substantia nigra pars compacta (SNc). The MP extract, especially the HMP, also increased antioxidant-related gene expressions, restored muscle mitochondrial function, and remodeled fecal microbiota composition, which were followed by reduced reactive oxygen species levels in brain and inflammation in plasma. Importantly, bacterial genera Sutterella, Rothia, and Aggregatibacter, which were negatively correlated with antioxidant gene expressions, decreased in the HMP group. It is imperative to note that in addition to directly acting as an antioxidant to reduce excessive free radicals, MP extract might also increase antioxidant state by rebuilding gut microbiota, thereby enhanced anti-inflammatory capacity and restored mitochondrial function to attenuate motor deficit in 6-OHDA-induced PD-like condition. All in all, MP extract is a potential candidate for auxiliary therapy for PD.

[18F]FE-PE2I PET is a feasible alternative to [123I]FP-CIT SPECT for dopamine transporter imaging in clinically uncertain parkinsonism

BACKGROUND

Dopamine transporter (DAT) imaging of striatum is clinically used in Parkinson's disease (PD) and neurodegenerative parkinsonian syndromes (PS) especially in the early disease stages. The aim of the present study was to evaluate the diagnostic performance of the recently developed tracer for DAT imaging [¹⁸F]FE-PE2I PET/CT to the reference standard [¹²³I]FP-CIT SPECT.

METHODS

Ninety-eight unselected patients referred for DAT imaging were included prospectively and consecutively and evaluated with [¹⁸F]FE-PE2I PET/CT and [¹²³I]FP-CIT SPECT on two separate days. PET and SPECT scans were categorized independently by two blinded expert readers as either normal, vascular changes, or mixed. Semiquantitative values were obtained for each modality and compared regarding effect size using Glass' delta.

RESULTS

Fifty-six of the [¹²³I]FP-CIT SPECT scans were considered abnormal (52 caused by PS, 4 by infarctions). Using [¹⁸F]FE-PE2I PET/CT, 95 of the 98 patients were categorized identically to SPECT as PS or non-PS with a sensitivity of 0.94 [0.84-0.99] and a specificity of 1.00 [0.92-1.00]. Inter-reader agreement for [¹⁸F]FE-PE2I PET with a kappa of 0.97 [0.89-1.00] was comparable to the agreement for [¹²³I]FP-CIT SPECT of 0.96 [0.76-1.00]. Semiquantitative values for short 10-min reconstructions of [¹⁸F]FE-PE2I PET/CT were comparable to longer reconstructions. The effect size for putamen/caudate nucleus ratio was significantly increased using PET compared to SPECT.

CONCLUSIONS

The high correspondence of [¹⁸F]FE-PE2I PET compared to reference standard [¹²³I]FP-CIT SPECT establishes [¹⁸F]FE-PE2I PET as a feasible PET tracer for clinical use with favourable scan logistics.

Role of Dopamine Transporter in the Relationship Between Plasma Cortisol and Cognition

OBJECTIVE

Cortisol is associated with cognition in both healthy individuals and patients with neuropsychiatric disorders. Regarding the effects of cortisol on the dopamine system and the association between dopamine transporter (DAT) and cognition, DAT might be a central target linking cortisol and cognition. This study explored the role of striatal DAT in the cortisol-cognition relationship.

METHODS

We recruited 33 patients with carbon monoxide poisoning and 33 age- and sex-matched healthy controls. All participants underwent cognitive assessments of attention, memory, and executive function. Single-photon emission computed tomography with 99mTc-TRODAT was used to determine striatal DAT availability. Plasma cortisol, tumor necrosis factor α, and interleukin-10 levels were measured using enzyme-linked immunosorbent assays.

RESULTS

Compared with healthy controls, patients with carbon monoxide poisoning had lower cognitive performance, bilateral striatal DAT availability, and plasma tumor necrosis factor-α levels and higher cortisol and interleukin-10 levels. In all participants, plasma cortisol level and bilateral striatal DAT availability were negatively and positively related to cognition, respectively, including memory and executive function with β from -0.361 (95% confidence interval [CI] = -0.633 to -0.090) to 0.588 (95% CI = 0.319 to 0.858). Moreover, bilateral striatal DAT mediated the cortisol-cognition relationship with indirect effects from -0.067 (95% CI = -0.179 to -0.001) to -0.135 (95% CI = -0.295 to -0.024). The cytokine levels did not influence the mediation effects.

CONCLUSIONS

This is the first study to demonstrate that striatal DAT mediates the cortisol-cognition relationship. Future studies are needed to comprehensively evaluate the role of the dopamine system in cortisol-cognition associations and treatment implications.

Effect of DL-Methylephedrine on Dopamine Transporter Using Positron Emission Tomography With [18F]FE-PE2I

RATIONALE

Since ephedrine has a dopamine transporter (DAT) inhibitory effect similar to amphetamine, dl-methylephedrine, a derivative of ephedrine, is considered to have the characteristics of a central nervous system stimulant due to the DAT inhibitory effect. For example, the World Anti-Doping Agency categorizes dl-methylephedrine as a stimulant in the prohibited list for competitions. Assuming to have the same effect as ephedrine, the urinary concentration of dl-methylephedrine is regulated below 10 μg/mL, as is ephedrine. However, the extent to which dl-methylephedrine affects brain function is not yet fully understood.

OBJECTIVES

The purpose of this study was to evaluate DAT occupancy by a single oral administration of a daily dose of dl-methylephedrine using positron emission tomography (PET) with [¹⁸F]FE-PE2I to characterize its stimulatory effect on the central nervous system.

METHODS

Nine healthy male volunteers were enrolled in the study. The experiments were designed as a placebo-controlled randomized double-blind crossover comparative study. After the first PET scan in a drug-free state, the second and third PET scans were performed with randomized dosing at 60 mg of dl-methylephedrine or placebo. The plasma and urine concentrations of dl-methylephedrine were measured just before and after the PET scans, respectively.

RESULTS

Mean urine and plasma concentrations of dl-methylephedrine were 13.9 μg/mL and 215.2 ng/mL, respectively. Mean DAT occupancy in the caudate was 4.4% for dl-methylephedrine and 1.2% for placebo. Mean DAT occupancy in the putamen was 3.6% for dl-methylephedrine and 0.5% for placebo. There was no significant difference of DAT occupancies between the groups.

CONCLUSION

In this study, the urinary concentration of dl-methylephedrine (13.9 μg/mL) was higher than the prohibited reference value (10.0 μg/mL), and there was no significant difference in DAT occupancy between dl-methylephedrine and placebo. These findings suggest that a clinical daily dose of dl-methylephedrine may exceed the doping regulation value according to urine concentration; however, it was considered that at least the central excitatory effect mediated by DAT inhibition was not observed at the daily dose of dl-methylephedrine.

The Cerebellar Dopaminergic System

In the central nervous system (CNS), dopamine (DA) is involved in motor and cognitive functions. Although the cerebellum is not been considered an elective dopaminergic region, studies attributed to it a critical role in dopamine deficit-related neurological and psychiatric disorders [e.g., Parkinson's disease (PD) and schizophrenia (SCZ)]. Data on the cerebellar dopaminergic neuronal system are still lacking. Nevertheless, biochemical studies detected in the mammalians cerebellum high dopamine levels, while chemical neuroanatomy studies revealed the presence of midbrain dopaminergic afferents to the cerebellum as well as wide distribution of the dopaminergic receptor subtypes (DRD1-DRD5). The present review summarizes the data on the cerebellar dopaminergic system including its involvement in associative and projective circuits. Furthermore, this study also briefly discusses the role of the cerebellar dopaminergic system in some neurologic and psychiatric disorders and suggests its potential involvement as a target in pharmacologic and non-pharmacologic treatments.

Simplified quantification of [18F]FE-PE2I PET in Parkinson's disease: Discriminative power, test-retest reliability and longitudinal validity during early peak and late pseudo-equilibrium

Quantification of dopamine transporter (DAT) availability with [¹⁸F]FE-PE2I PET enables the detection of presynaptic dopamine deficiency and provides a potential progression marker for Parkinson`s disease (PD). Simplified quantification is feasible, but the time window of short acquisition protocols may have a substantial impact on the reliability of striatal binding estimates. Dynamic [¹⁸F]FE-PE2I PET data of cross-sectional (33 PD patients, 24 controls), test-retest (9 patients), and longitudinal (12 patients) cohorts were used to assess the variability and reliability of specific binding ratios (SBR) measured during early peak and late pseudo-equilibrium. Receiver operating characteristics area under the curve (PD vs. controls) was high for early (0.996) and late (0.991) SBR. Early SBR provided more favourable effect size, absolute variability, and standard error of measurement than late SBR (caudate: 1.29 vs. 1.23; 6.9% vs. 9.8%; 0.09 vs. 0.20; putamen: 1.75 vs. 1.67; 7.7% vs. 14.0%; 0.08 vs. 0.17). The annual percentage change was comparable for both time windows (-7.2%-8.5%), but decline was significant only for early SBR. Whereas early and late [¹⁸F]FE-PE2I PET acquisitions have similar discriminative power to separate PD patients and controls, the early peak equilibrium acquisition can be recommended if [¹⁸F]FE-PE2I is used to measure longitudinal changes of DAT availability.

Multi-Atlas MRI-Based Striatum Segmentation for 123I-FP-CIT SPECT (DAT-SPECT) Compared With the Bolt Method and SPECT-Atlas-Based Segmentation Method Toward the Accurate Diagnosis of Parkinson's Disease/Syndrome

Aims: This study aimed to analyze the performance of multi-atlas MRI-based parcellation for ¹²³I-FP-CIT SPECT (DAT-SPECT) in healthy volunteers. The proposed method was compared with the SPECT-atlas-based and Bolt methods. ¹⁸F-FE-PE2I-PET (DAT-PET) was used as a reference.

Methods: Thirty healthy subjects underwent DAT-SPECT, DAT-PET, and 3D-T1WI-MRI. We calculated the striatum uptake ratio (SUR/SBR), caudate uptake ratio (CUR), and putamen uptake ratio (PUR) for DAT-SPECT using the multi-atlas MRI-based method, SPECT-atlas-based method, and Bolt method. In the multi-atlas MRI-based method, the cerebellum, occipital cortex, and whole-brain were used as reference regions. The correlation of age with DAT-SPECT activity and the correlations of SUR/SBR, CUR, and PUR between DAT-SPECT and DAT-PET were calculated by each of the three methods.

Results: The correlation between age and SUR/SBR for DAT-SPECT based on the multi-atlas MRI-based method was comparable to that based on the SPECT-atlas-based method (r = −0.441 to −0.496 vs. −0.488). The highest correlation between DAT-SPECT and DAT-PET was observed using the multi-atlas MRI-based method with the occipital lobe defined as the reference region compared with the SPECT-atlas-based and Bolt methods (SUR, CUR, and PUR: 0.687, 0.723, and 0.676 vs. 0.698, 0.660, and 0.616 vs. 0.655).

Conclusion: Multi-atlas MRI-based parcellation with the occipital lobe defined as the reference region was at least comparable to the clinical methods.

Synthesis and Biological Evaluation of [18F]FECNT-d4 as a Novel PET Agent for Dopamine Transporter Imaging

PURPOSE

The dopamine transporter (DAT) is a marker of the occurrence and development of Parkinson's disease (PD) and other diseases with nigrostriatal degeneration. 2β-Carbomethoxy-3β-(4-chlorophenyl)-8-(2-[¹⁸F]-fluoroethyl)nortropane ([¹⁸F]FECNT), an ¹⁸F-labelled tropane derivative, was reported to be a useful positron-emitting probe for DAT. However, the rapid formation of brain-penetrating radioactive metabolites is an impediment to the proper quantitation of DAT in PET studies with [¹⁸F]FECNT. Deuterium-substituted analogues have presented better in vivo stability to reduce metabolites. This study aimed to synthesize a deuterium-substituted DAT radiotracer, [¹⁸F]FECNT-d₄, and to make a preliminary investigation of its properties as a DAT tracer in vivo.

PROCEDURES

The ligand [¹⁸F]FECNT-d₄ was obtained by one-step radiolabelling reaction. The lipophilicity was measured by the shake-flask method. Binding properties of [¹⁸F]FECNT-d₄ were estimated by in vitro binding assay, biodistribution, and microPET imaging in rats. In vivo stability of [¹⁸F]FECNT-d₄ was estimated by radio-HPLC.

RESULTS

[¹⁸F]FECNT-d₄ was synthesized at an average activity yield of 46 ± 17 % (n = 15) and the molar activity was 67 ± 12 GBq/μmol. The deuterated tracer showed suitable lipophilicity and the ability to penetrate the blood-brain barrier (brain uptake of 1.72 % ID at 5 min). [¹⁸F]FECNT-d₄ displayed a high binding affinity for DAT comparable to that of [¹⁸F]FECNT in rat striatum homogenates. Biodistribution results in normal rats showed that [¹⁸F]FECNT-d₄ exhibited a higher ratio of the target to non-target (striatum/cerebellum) at 15 min post administration (5.00 ± 0.44 vs 3.84 ± 0.24 for [¹⁸F]FECNT-d₄ vs [¹⁸F]FECNT). MicroPET imaging studies of [¹⁸F]FECNT-d₄ in normal rats showed that the ligand selectively localized to DAT-rich striatal regions and the accumulation could be blocked with DAT inhibitor. Furthermore, in the unilateral PD model rat, a significant reduction of the signal was found in the lesioned side relative to the unlesioned side. Striatal standardized uptake value of [¹⁸F]FECNT-d₄ remained ~4.02 in the striatum between 5 and 20 min, whereas that of [¹⁸F]FECNT fell rapidly from 4.11 to 2.95. Radio-HPLC analysis of the plasma demonstrated better in vivo stability of [¹⁸F]FECNT-d₄ than [¹⁸F]FECNT.

CONCLUSION

The deuterated compound [¹⁸F]FECNT-d₄ may serve as a promising PET imaging agent to assess DAT-related disorders.

Fully automated radiosynthesis of [18F]LBT999 on TRACERlab FXFN and AllinOne modules, a PET radiopharmaceutical for imaging the dopamine transporter in human brain

Background

Fluorine labelled 8-((E)-4-fluoro-but-2-enyl)-3β-p-tolyl-8-aza-bicyclo[3.2.1]octane-2β-carboxylic acid methyl ester ([¹⁸F]LBT999) is a selective radioligand for the in vivo neuroimaging and quantification of the dopamine transporter by Positron Emission Tomography (PET). [¹⁸F]LBT999 was produced on a TRACERlab FXFN for the Phase I study but for Phase III and a potent industrial production transfer, production was also implemented on an AllinOne (AIO) system requiring a single use cassette. Both production methods are reported herein.

Results

Automation of [¹⁸F]LBT999 radiosynthesis on FXFN was carried out in 35% yield (decay-corrected) in 65 min (n = 16), with a radiochemical purity higher than 99% and a molar activity of 158 GBq/μmol at the end of synthesis. The transfer to the AIO platform followed by optimizations allowed the production of [¹⁸F]LBT999 in 32.7% yield (decay-corrected) within 48 min (n = 5), with a radiochemical purity better than 98% and a molar activity above 154 GBq/μmol on average at the end of synthesis. Quality controls of both methods met the specification for clinical application.

Conclusion

Both modules allow efficient and reproducible radiosynthesis of [¹⁸F]LBT999 with good radiochemical yields and a reasonable synthesis time. The developments made on AIO, such as its ability to meet pharmaceutical criteria and to more easily comply with GMP requirements, make it an optimal approach for the potent industrial production of [¹⁸F]LBT999 and future wider use.

Interaction of dopamine transporter and metabolite ratios underpinning the cognitive dysfunction in patients with carbon monoxide poisoning: A combined SPECT and MRS study

Cognitive dysfunction has been reported in patients with carbon monoxide (CO) poisoning. However, the underpinning mechanism remained unclear. This study examined dopamine transporter (DAT) and metabolite ratios concurrently and their relationships with cognitive dysfunction in CO poisoning. Eighteen suicide attempters with charcoal burning which results in CO poisoning and 18 age- and gender- matched normal controls were recruited. A battery of cognitive assessments including attention, memory, and executive function was administered. Each participant received one single photon emission computed tomography with ^99mTc-TRODAT for measuring striatal DAT availability and proton magnetic resonance spectroscopy to determine N-acetyl aspartate/creatine (NAA/Cr), choline-containing compounds/creatine (Cho/Cr) and myo-inositol/creatine (mI/Cr) in the left parietal white matter and mid-occipital gray matter (OGM). CO poisoning patients had significant impairments in memory and executive function. Compared to normal, CO poisoning patients had lower striatal DAT availability, lower NAA/Cr levels in both regions and higher Cho/Cr levels in both regions. In CO poisoning patients, the altered left striatal DAT availability and Cho/Cr level in OGM were significantly associated with executive dysfunction in the expected directions. Moreover, there was a significant interaction between these two imaging indices on their relationships with executive dysfunction and combination of them could adequately predict executive dysfunction in more CO poisoning cases than either alone. The current results suggested that both alterations in DAT availability and metabolite ratios might play crucial roles in executive dysfunction in CO poisoning. This research also highlights the importance of multimodal imaging approaches for studying neurotoxicity effects.

Dealing with PET radiometabolites

Abstract

Positron emission tomography (PET) offers the study of biochemical, physiological, and pharmacological functions at a cellular and molecular level. The performance of a PET study mostly depends on the used radiotracer of interest. However, the development of a novel PET tracer is very difficult, as it is required to fulfill a lot of important criteria. PET radiotracers usually encounter different chemical modifications including redox reaction, hydrolysis, decarboxylation, and various conjugation processes within living organisms. Due to this biotransformation, different chemical entities are produced, and the amount of the parent radiotracer is declined. Consequently, the signal measured by the PET scanner indicates the entire amount of radioactivity deposited in the tissue; however, it does not offer any indication about the chemical disposition of the parent radiotracer itself. From a radiopharmaceutical perspective, it is necessary to quantify the parent radiotracer’s fraction present in the tissue. Hence, the identification of radiometabolites of the radiotracers is vital for PET imaging. There are mainly two reasons for the chemical identification of PET radiometabolites: firstly, to determine the amount of parent radiotracers in plasma, and secondly, to rule out (if a radiometabolite enters the brain) or correct any radiometabolite accumulation in peripheral tissue. Besides, radiometabolite formations of the tracer might be of concern for the PET study, as the radiometabolic products may display considerably contrasting distribution patterns inside the body when compared with the radiotracer itself. Therefore, necessary information is needed about these biochemical transformations to understand the distribution of radioactivity throughout the body. Various published review articles on PET radiometabolites mainly focus on the sample preparation techniques and recently available technology to improve the radiometabolite analysis process. This article essentially summarizes the chemical and structural identity of the radiometabolites of various radiotracers including [¹¹C]PBB3, [¹¹C]flumazenil, [¹⁸F]FEPE2I, [¹¹C]PBR28, [¹¹C]MADAM, and (+)[¹⁸F]flubatine. Besides, the importance of radiometabolite analysis in PET imaging is also briefly summarized. Moreover, this review also highlights how a slight chemical modification could reduce the formation of radiometabolites, which could interfere with the results of PET imaging.

Graphical abstract

Usefulness of PET With [18F]LBT-999 for the Evaluation of Presynaptic Dopaminergic Neuronal Loss in a Clinical Environment

Purpose: The density of the neuronal dopamine transporter (DAT) is directly correlated with the presynaptic dopaminergic system injury. In a first study, we evaluated the brain distribution and kinetics of [¹⁸F]LBT-999, a DAT PET radioligand, in a group of eight healthy subjects. Taking into account the results obtained in healthy volunteers, we wanted to evaluate whether the loss of presynaptic striatal dopaminergic fibers could be estimated, under routine clinical conditions, using [¹⁸F]LBT-999 and a short PET acquisition.

Materials and methods: Six patients with Parkinson's disease (PD) were compared with eight controls. Eighty-nine minutes of dynamic PET following an intravenous injection of [¹⁸F]LBT-999 were acquired. Using regions of interest for striatal nuclei, substantia nigra (SN), cerebellum, and occipital cortex, defined over each T1 3D MRI, time–activity curves (TACs) were obtained. From TACs, binding potential (BP_ND) using the simplified reference tissue model and distribution volume ratios (DVRs) using Logan graphical analysis were calculated. Ratios obtained for a 10-min image, acquired between 30 and 40 min post-injection, were also calculated. Cerebellum activity was used as non-specific reference region.

Results: In PD patients and as expected, striatal uptake was lower than in controls which is confirmed by BP_ND, DVR, and ratios calculated for both striatal nuclei and SN, significantly inferior in PD patients compared with controls (p < 0.001).

Conclusions: PET with [¹⁸F]LBT-999 could be an alternative to assess dopaminergic presynaptic injury in a clinical environment using a single 10 min acquisition.

Reliability of dopamine transporter PET measurements with [18F]FE-PE2I in patients with Parkinson's disease

Background

Reliable quantification of dopamine transporter (DAT), a biomarker for Parkinson’s disease (PD), is essential for diagnostic purposes as well as for evaluation of potential disease-modifying treatment. Due to degeneration of dopaminergic neurons and thus lower expected radioligand binding to DAT, higher measurement variability in PD patients might be expected than earlier reproducibility results in healthy controls. Therefore, we aimed to examine the test-retest properties of [¹⁸F]FE-PE2I-PET in PD patients.

Methods

Nine patients with PD (Hoehn and Yahr stage < 3) were included (men/women 6/3; mean age 65.2 ± 6.8 years). Each patient underwent two [¹⁸F]FE-PE2I-PET measurements within 7–28 days. The outcome measure was non-displaceable binding potential generated using wavelet-aided parametric imaging with cerebellum as reference region. We assessed test-retest performance using estimates of reliability and repeatability. Regions for primary analysis were caudate, putamen, ventral striatum, and substantia nigra. Exploratory analysis was performed for functional subdivisions of the striatum. We also compared the more vs. less affected side.

Results

[¹⁸F]FE-PE2I showed absolute variability estimates of 5.3–7.6% in striatal regions and 11% in substantia nigra and ICCs of 0.74–0.97 (median 0.91). The absolute variability for functional striatal subdivisions was 6.0–9.6% and ICCs of 0.76–0.91 (median 0.91). The less affected substantia nigra exhibited greater consistency than the more affected side. According to power calculations based on the current sample size, DAT changes of 5–11% in the striatum and 28% in the substantia nigra can be detected with a power of 0.8 (p < 0.0125).

Conclusion

DAT-PET measurements with [¹⁸F]FE-PE2I in PD patients showed good repeatability and reliability. The slightly lower reliability in the substantia nigra in patients may be explained by lower DAT density and smaller anatomical size. Power calculations suggest that [¹⁸F]FE-PE2I PET is a suitable marker for longitudinal DAT decline in PD.

Trial registration

EudraCT 2017-003327-29

Low dopamine transporter binding in the nucleus accumbens in geriatric patients with severe depression

AIM

Dysfunction of dopaminergic neurons in the central nervous system is considered to be related to major depressive disorder (MDD). Especially, MDD in geriatric patients is characterized by anhedonia, which is assumed to be associated with reduced dopamine neurotransmission in the reward system. Dopamine transporter (DAT) is considered to reflect the function of the dopamine nerve system. However, previous DAT imaging studies using single photon emission computed tomography or positron emission tomography (PET) have shown inconsistent results. The radioligand [¹⁸ F]FE-PE2I for PET enables more precise evaluation of DAT availability. Hence, we aimed to evaluate the DAT availability in geriatric patients with MDD using [¹⁸ F]FE-PE2I.

METHODS

Eleven geriatric patients with severe MDD and 27 healthy controls underwent PET with [¹⁸ F]FE-PE2I, which has high affinity and selectivity for DAT. Binding potentials (BP_ND ) in the striatum (caudate and putamen), nucleus accumbens (NAc), and substantia nigra were calculated. BP_ND values were compared between MDD patients and healthy controls.

RESULTS

MDD patients showed significantly lower DAT BP_ND in the NAc (P = 0.009), and there was a trend of lower BP_ND in the putamen (P = 0.032) compared to controls.

CONCLUSION

We found low DAT in the NAc and putamen in geriatric patients with severe MDD, which could be related to dysregulation of the reward system.

Electroconvulsive therapy decreases striatal dopamine transporter binding in patients with depression: A positron emission tomography study with [18F]FE-PE2I

Electroconvulsive therapy (ECT) is an effective treatment for major depression. Previous studies suggested that dopaminergic neurotransmission plays a crucial role in the mechanism of the action of ECT. Since dopamine transporters (DAT) regulate extracellular dopamine concentration, DAT represents an interesting target for the study of the mechanism of action of ECT. Eight inpatients (7 patients with major depressive disorder and 1 patient with bipolar disorder with a DSM-IV diagnosis) received a series of 7-15(11.3±5.2) bilateral ECT sessions.The severity of symptoms was assessed using the 21-item Hamilton Depression Rating Scale (HDRS) and Clinical Global Impression-Severity (CGI-S). All patients were examined with [¹⁸F]FE-PE2I positron emission tomography (PET) at pre-ECT, after the 10^th ECT, and at post-ECT. Striatal DAT-binding potential (BP_ND) of all patients was reduced, with an average change ratio of DAT-BP_ND of -13.1±5.6%. In the 2 cases with 15 ECT sessions, the ratio change of DAT-BP_ND after the 15th ECT was larger than that after the 10th ECT. Also, HDRS and CGI-S were reduced. These results indicate that the dopamine nervous system is part of themechanism of action of ECT.

EANM practice guideline/SNMMI procedure standard for dopaminergic imaging in Parkinsonian syndromes 1.0

Purpose

This joint practice guideline or procedure standard was developed collaboratively by the European Association of Nuclear Medicine (EANM) and the Society of Nuclear Medicine and Molecular Imaging (SNMMI). The goal of this guideline is to assist nuclear medicine practitioners in recommending, performing, interpreting, and reporting the results of dopaminergic imaging in parkinsonian syndromes.

Methods

Currently nuclear medicine investigations can assess both presynaptic and postsynaptic function of dopaminergic synapses. To date both EANM and SNMMI have published procedural guidelines for dopamine transporter imaging with single photon emission computed tomography (SPECT) (in 2009 and 2011, respectively). An EANM guideline for D2 SPECT imaging is also available (2009). Since the publication of these previous guidelines, new lines of evidence have been made available on semiquantification, harmonization, comparison with normal datasets, and longitudinal analyses of dopamine transporter imaging with SPECT. Similarly, details on acquisition protocols and simplified quantification methods are now available for dopamine transporter imaging with PET, including recently developed fluorinated tracers. Finally, [¹⁸F]fluorodopa PET is now used in some centers for the differential diagnosis of parkinsonism, although procedural guidelines aiming to define standard procedures for [¹⁸F]fluorodopa imaging in this setting are still lacking.

Conclusion

All these emerging issues are addressed in the present procedural guidelines for dopaminergic imaging in parkinsonian syndromes.

The Effect of Sertoli Cells on Xenotransplantation and Allotransplantation of Ventral Mesencephalic Tissue in a Rat Model of Parkinson's Disease

Intra-striatal transplantation of fetal ventral mesencephalic (VM) tissue has a therapeutic effect on patients with Parkinson’s disease (PD). Sertoli cells (SCs) possess immune-modulatory properties that benefit transplantation. We hypothesized that co-graft of SCs with VM tissue can attenuate rejection. Hemi-parkinsonian rats were generated by injecting 6-hydroxydopamine into the right medial forebrain bundle of Sprague Dawley (SD) rats. The rats were then intrastriatally transplanted with VM tissue from rats or pigs (rVM or pVM), with/without a co-graft of SCs (rVM+SCs or pVM+SCs). Recovery of dopaminergic function and survival of the grafts were evaluated using the apomorphine-induced rotation test and small animal-positron emission tomography (PET) coupled with [¹⁸F] DOPA or [¹⁸F] FE-PE2I, respectively. Immunohistochemistry (IHC) examination was used to determine the survival of the grafted dopaminergic neurons in the striatum and to investigate immune-modulatory effects of SCs. The results showed that the rVM+SCs and pVM+SCs groups had significantly improved drug-induced rotational behavior compared with the VM alone groups. PET revealed a significant increase in specific uptake ratios (SURs) of [¹⁸F] DOPA and [¹⁸F] FE-PE2I in the grafted striatum of the rVM+SCs and pVM+SCs groups as compared to that of the rVM and pVM groups. SC and VM tissue co-graft led to better dopaminergic (DA) cell survival. The co-grafted groups exhibited lower populations of T-cells and activated microglia compared to the groups without SCs. Our results suggest that co-graft of SCs benefit both xeno- and allo-transplantation of VM tissue in a PD rat model. Use of SCs enhanced the survival of the grafted dopaminergic neurons and improved functional recovery. The enhancement may in part be attributable to the immune-modulatory properties of SCs. In addition, [¹⁸F]DOPA and [¹⁸F]FE-PE2I coupled with PET may provide a feasible method for in vivo evaluation of the functional integrity of the grafted DA cell in parkinsonian rats.

PET imaging of Dopamine Transporter with [18F]LBT-999: initial evaluation in healthy volunteers

BACKGROUND

To evaluate in healthy human brain the distribution, uptake, and kinetics of [18F]LBT-999, a PET ligand targeting the dopamine transporter, to assess its ability to explore dopaminergic innervation, using a shorter protocol, more convenient for patients than currently with [123I]ioflupane.

METHODS

After intravenous injection of [18F]LBT-999, 8 healthy subjects (53-80y) underwent a dynamic PET-scan. Venous samples were concomitantly obtained for metabolites analysis. Time activity curves (TACs) were generated for several ROIs (caudate, putamen, occipital cortex, substantia nigra and cerebellum). Cerebellum was used as reference region to calculate binding potentials (BPND).

RESULTS

No adverse events or detectable pharmacological effects were reported. [18F]LBT-999 PET revealed a good cerebral distribution, with an intense and symmetric uptake in both putamen and caudate (BPND of 6.75±1.17 and 6.30±1.17, respectively), without other brain abnormal tracer accumulation. Regional TACs showed a plateau from the maximal uptake, 20min pi, to the end of the acquisition for both caudate and putamen, whereas uptake in substantia nigra decreased progressively. A faster clearance and lowest BPND values were observed in both cortex and cerebellum. Ratios to the cerebellum exhibit value of about 3 in substantia nigra, close to 10 for both caudate and putamen, and remained around the value of 1 in cortex. The parent fraction of [18F]LBT-999 in plasma was 80%, 60% and 45% at 15, 30 and 45 min pi, respectively.

CONCLUSIONS

These findings support the usefulness of [18F]LBT-999 for a quantitative clinical evaluation of presynaptic dopaminergic innervation.

In Vitro Ligand Binding Kinetics Explains the Pharmacokinetics of [18F]FE-PE2I in Dopamine Transporter PET Imaging

Two of the most popular positron emission tomography (PET) tracers, [¹¹C]PE2I and [¹⁸F]FE-PE2I, used to quantify dopamine transporters (DAT), display dissimilar kinetic behavior in in vivo assays. This difference can be explained by comparing values of kinetic rate constants, which characterize interaction of these tracers with DAT sites in vitro. At the same time, this kinetic analysis showed that the overall binding mechanism is similar for these two tracers and includes a fast step of complex formation followed by a slow isomerization step of this complex. Comparison with previous PE2I data revealed that isomerization of the DAT complex with PE2I occurs three times faster than in the case of FE-PE2I, which leads to the slower onset of peak specific binding of the former tracer in the DAT-rich regions. Therefore, ligands with slower isomerization on-rate, including [¹⁸F]FE-PE2I, seem to be better tracers in vivo, and their properties can be predicted in vitro.

Ventral striatal dopamine transporter availability is associated with lower trait motor impulsivity in healthy adults

Impulsivity is a transdiagnostic feature of a range of externalizing psychiatric disorders. Preclinical work links reduced ventral striatal dopamine transporter (DAT) availability with heightened impulsivity and novelty seeking. However, there is a lack of human data investigating the relationship between DAT availability, particularly in subregions of the striatum, and the personality traits of impulsivity and novelty seeking. Here we collected PET measures of DAT availability (BP_ND) using the tracer ¹⁸F-FE-PE2I in 47 healthy adult subjects and examined relations between BP_ND in striatum, including its subregions: caudate, putamen, and ventral striatum (VS), and trait impulsivity (Barratt Impulsiveness Scale: BIS-11) and novelty seeking (Tridimensional Personality Questionnaire: TPQ-NS), controlling for age and sex. DAT BP_ND in each striatal subregion showed nominal negative associations with total BIS-11 but not TPQ-NS. At the subscale level, VS DAT BP_ND was significantly associated with BIS-11 motor impulsivity (e.g., taking actions without thinking) after correction for multiple comparisons. VS DAT BP_ND explained 13.2% of the variance in motor impulsivity. Our data demonstrate that DAT availability in VS is negatively related to impulsivity and suggest a particular influence of DAT regulation of dopamine signaling in VS on acting without deliberation (BIS motor impulsivity). While needing replication, these data converge with models of ventral striatal functions that emphasize its role as a key interface linking motivation to action.

Dopamine transporter imaging with [18F]FE-PE2I PET and [123I]FP-CIT SPECT-a clinical comparison

Background

Dopamine transporter (DAT) imaging may be of diagnostic value in patients with clinically suspected parkinsonian disease. The purpose of this study was to compare the diagnostic performance of DAT imaging with positron emission computed tomography (PET), using the recently developed, highly DAT-selective radiopharmaceutical [¹⁸F]FE-PE2I (FE-PE2I), to the commercially available and frequently used method with [¹²³I]FP-CIT (FP-CIT) single-photon emission computed tomography (SPECT) in early-stage idiopathic parkinsonian syndrome (PS).

Methods

Twenty-two patients with a clinical de novo diagnosis of PS and 28 healthy controls (HC) participating in an on-going clinical trial of FE-PE2I were analyzed in this study. Within the trial protocol, participants are clinically reassessed 2 years after inclusion. A commercially available software was used for automatic calculation of FP-CIT-specific uptake ratio (SUR). MRI-based volumes of interest combined with threshold PET segmentation were used for FE-PE2I binding potential relative to non-displaceable binding (BP_ND) quantification and specific uptake value ratios (SUVR).

Results

PET with FE-PE2I revealed significant differences between patients with a clinical de novo diagnosis of PS and healthy controls in striatal DAT availability (p < 0.001), with excellent accuracy of predicting dopaminergic deficit in early-stage PS. The effect sizes were calculated for FE-PE2I BP_ND (Glass’s Δ = 2.95), FE-PE2I SUVR (Glass’s Δ = 2.57), and FP-CIT SUR (Glass’s Δ = 2.29). The intraclass correlation (ICC) between FE-PE2I BP_ND FP-CIT SUR was high in the caudate (ICC = 0.923), putamen (ICC = 0.922), and striatum (ICC = 0.946), p < 0.001. Five of the 22 patients displayed preserved striatal DAT availability in the striatum with both methods. At follow-up, a non-PS clinical diagnosis was confirmed in three of these, while one was clinically diagnosed with corticobasal syndrome. In these patients, FE-PE2I binding was also normal in the substantia nigra (SN), while significantly reduced in the remaining patients. FE-PE2I measurement of the mean DAT availability in the putamen was strongly correlated with BP_ND in the SN (R = 0.816, p < 0.001). Olfaction and mean putamen DAT availability was correlated using both FE-PE2I BP_ND and FP-CIT SUR (R ≥ 0.616, p < 0.001).

Conclusion

DAT imaging with FE-PE2I PET yields excellent basic diagnostic differentiation in early-stage PS, at least as good as FP-CIT SPECT.

Optimal Acquisition Time Window and Simplified Quantification of Dopamine Transporter Availability Using 18F-FE-PE2I in Healthy Controls and Parkinson Disease Patients

¹⁸F-(E)-N-(3-iodoprop-2-enyl)-2β-carbofluoroethoxy-3β-(4'methylphenyl)nortropane (¹⁸F-FE-PE2I) is a newly developed dopamine transporter (DAT) PET radioligand. Full quantification methods rely on dynamic acquisition of ¹⁸F-FE-PE2I, but in a clinical setting a simplified protocol is preferable. The aims of this study were to identify the optimal acquisition time window for ¹⁸F-FE-PE2I and to validate the specific binding ratio (SBR) as a simplified quantification method.

METHODS

Ten Parkinson disease (PD) patients and 10 controls were included. Ninety-three-min dynamic PET measurements with ¹⁸F-FE-PE2I were conducted using the high-resolution research tomograph (HRRT). The dynamic measurement was also smoothed to the resolution of a clinical PET system (HR). Regions of interest for the caudate, putamen, ventral striatum, substantia nigra (SN), and cerebellum were manually drawn on coregistered MR images. The outcome measure was the SBR, and the gold standard was the binding potential obtained with wavelet-aided parametric imaging (WAPI BP_ND). The cerebellum was used as a reference region. In a preliminary analysis, SBR was computed for 8 time windows (SBR_dyn). Linear regression analysis and Bland-Altman plots were used to select the optimal acquisition time window. An average image from the selected time window was created, from which new SBR values (SBR calculated on the average image on the HRRT and SBR calculated on the average image on the simulated HR images) were calculated and compared with WAPI BP_ND The effect size was calculated.

RESULTS

SBR_dyn values for the time window between 16.5 and 42 min correlated best with WAPI BP_ND (r² = 0.98, P < 0.001). Significant correlations (P < 0.001) were observed between SBR_HR and WAPI-BP_ND (r² = 0.95 in controls and 0.97 in PD patients). In the striatum, SBR_HR values were 37% lower than BP_ND in controls, 29% in PD patients, whereas in the SN the underestimation was 22% in controls and 15% in PD patients. Similar effect sizes for BP_ND and SBR_HR were found in the caudate (0.6), putamen (1.7 and 1.4), ventral striatum (0.7), and SN (0.5 and 0.4).

CONCLUSION

A single ¹⁸F-FE-PE2I acquisition between 16.5 and 42 min provides the best outcome measure for simplified DAT quantification. Despite underestimation of the BP_ND, the SBR can be used in a clinical setting as a valid quantification method for DAT using ¹⁸F-FE-PE2I, because it provides differentiation similar to BP_ND between controls and PD patients.

Principal Component Analysis of Multimodal Neuromelanin MRI and Dopamine Transporter PET Data Provides a Specific Metric for the Nigral Dopaminergic Neuronal Density

The loss of dopaminergic (DA) neurons in the substantia nigra (SN) is a major pathophysiological feature of patients with Parkinson's disease (PD). As nigral DA neurons contain both neuromelanin (NM) and dopamine transporter (DAT), decreased intensities in both NM-sensitive MRI and DAT PET reflect decreased DA neuronal density. This study demonstrates that a more specific metric for the nigral DA neuronal density can be derived with multimodal MRI and PET. Participants were 11 clinically diagnosed PD patients and 10 age and gender matched healthy controls (HCs). Two quantities, the NM-related index (R_NM) and the binding potential of the radiotracer [¹⁸F]FE-PE2I to DAT (BP_ND) in SN, were measured for each subject using MRI and PET, respectively. Principal component analysis (PCA) was applied to the multimodal data set to estimate principal components. One of the components, PC_P, corresponds to a basis vector oriented in a direction where both BP_ND and R_NM increase. The ability of BP_ND, R_NM and PC_P to discriminate between HC and PD groups was compared. Correlation analyses between the motor score of the unified Parkinson's disease rating scale and each metric were also performed. PC_P, BP_ND and R_NM for PD patients were significantly lower than those for HCs (F = 16.26, P<0.001; F = 6.05, P = 0.008; F = 7.31, P = 0.034, respectively). The differential diagnostic performance between the HC and PD groups as assessed by the area under the receiver-operating characteristic curve was best for PC_P (0.94, 95% CI: 0.66–1.00). A significant negative correlation was found between the motor severity score and PC_p (R = -0.70, P<0.001) and R_NM (R = -0.52, P = 0.015), but not for BP_ND (R = -0.36, P = 0.110). PCA of multimodal NM-sensitive MRI and DAT PET data provides a metric for nigral DA neuronal density that will help illuminate the pathophysiology of PD in SN. Further studies are required to explore whether PCA is useful for other parkinsonian syndromes.

Characterization of tau positron emission tomography tracer [18F]AV-1451 binding to postmortem tissue in Alzheimer's disease, primary tauopathies, and other dementias

INTRODUCTION

Aggregation of tau is a hallmark of many neurodegenerative diseases, and tau imaging with positron emission tomography (PET) may allow early diagnosis and treatment monitoring. We assessed binding of the PET tracer [¹⁸F]AV-1451 in a range of dementias.

METHODS

Phosphorimaging was used to quantify binding to postmortem brain tissue from 33 patients with different, histopathologically characterized, neurodegenerative dementias.

RESULTS

[¹⁸F]AV-1451 showed high specific binding in cases with Alzheimer's disease (AD), moderate binding in Pick's disease and frontotemporal dementia with parkinsonism-17, and low but displaceable binding in corticobasal degeneration, progressive supranuclear palsy, non-tau proteinopathies, and in controls without pathology. Tracer binding did not correlate with tau load within disease groups.

DISCUSSION

[¹⁸F]AV-1451 binds to tau in AD, and some other tauopathies. However, evidence for a non-tau binding site and lack of correlation between tracer binding and antibody staining suggest that reliable quantification of tau load with this tracer is problematic.

PET imaging of dopamine transporters with [(18)F]FE-PE2I: Effects of anti-Parkinsonian drugs

PURPOSE

This study aimed to assess the striatal [(18)F]FE-PE2I binding and the immunohistochemical stain of tyrosine hydroxylase (TH) in the striatum, and to evaluate the effects of therapeutic drugs on [(18)F]FE-PE2I binding.

METHODS

Dynamic PET/CT of [(18)F]FE-PE2I was performed in Parkinson's disease (PD) rat models, induced by the unilateral injection of 6-OHDA into the striatum. A simplified reference tissue model method was used to calculate the striatal binding potential (striatal BPND). Each of the four normal rats was pretreated with pramipexole, amantadine, and escitalopram 30 min before [(18)F]FE-PE2I injection. The effect of L-DOPA combined with benserazide was assessed in the normal and PD rats.

RESULTS

The BPND was significantly lower in the lesioned striatum than in the striatum of the normal rats. After the pretreatment with pramipexole, amantadine, and escitalopram, the values of the striatal BPND did not differ from those of the controls. The pretreatment with L-DOPA/benserazide, however, significantly reduced the striatal BPND. The striatal BPND of the PD rats with L-DOPA/benserazide pretreatment was not different from that of the same PD rats with placebo treatment.

CONCLUSION

[(18)F]FE-PE2I may be used as a radioligand for the in-vivo imaging of the DAT. In the normal rats, [(18)F]FE-PE2I binding is unaffected by pramipexole, amantadine, and escitalopram. L-DOPA/benserazide does not affect the striatal [(18)F]FE-PE2I binding in PD rats.

Evaluation of semi-quantitative method for quantification of dopamine transporter in human PET study with ¹⁸F-FE-PE2I

OBJECTIVES

Positron emission tomography (PET) with ¹⁸F-FE-PE2I is useful for investigating the function of dopamine transporter, and kinetics of ¹⁸F-FE-PE2I could be described by standard two-tissue compartment model (2CM) using plasma input function. In this study, we investigated the feasibility of semi-quantitative methods for estimating binding potential (BPND) and transporter occupancy to shorten the scan period and to reduce the effect of statistical noise on quantitative outcomes using computer simulation and human PET studies with ¹⁸F-FE-PE2I.

METHODS

In the simulations, time-activity curves (TACs) for the putamen with a wide range of BPND were generated. In these TACs, BPNDs were estimated by standardized uptake value ratio (SUVR) using various integration intervals and the simplified reference tissue model (SRTM) with the cerebellum as reference region, and reduction of BPND assuming transporter occupancy by antipsychotics was calculated from BPND obtained from TACs with various BPND values. These estimates were evaluated by comparison with those of 2CM. In human studies with normal volunteers, BPNDs were estimated in the caudate and putamen using SUVR and SRTM with the cerebellar reference region, and compared with BPND by standard 2CM.

RESULTS

In the simulations, BPND estimated by SUVR with late time frames and SRTM showed linear correlation with those by 2CM, although the estimates by SUVR were overestimated and affected by the cerebral blood flow as BPND became higher. As for transporter occupancy, SRTM showed higher linearity with 2CM and less effect of statistical noise than the SUVR method. In human studies, BPND by SRTM and SUVR with late time frames showed good correlation with BPND by 2CM.

CONCLUSIONS

Although SRTM is more reliable than the SUVR method for BPND and occupancy estimation, SUVR using late time frames has the potential to provide practical indices of BPND and occupancy with a shorter scan period.

In vivo activity of modafinil on dopamine transporter measured with positron emission tomography and [¹⁸F]FE-PE2I

Modafinil, a wake-promoting drug used to treat narcolepsy, is a dopamine transporter inhibitor and is said to have very low abuse liability; this, however, is still up for debate. We conducted a dopamine transporter (DAT) occupancy study with modafinil (200 or 300 mg) in ten healthy volunteers using positron emission tomography (PET) with [¹⁸F]FE-PE2I, a new PET radioligand with high affinity and selectivity for the dopamine transporter, to characterize its relation to abuse liability. Mean striatal DAT occupancies were 51.4% at 200 mg and 56.9% at 300 mg. There was a significant correlation between occupancy and plasma concentration, indicating dose dependency of DAT inhibition by modafinil in the striatum, and especially in the nucleus accumbens. This study showed that DAT occupancy by modafinil was close to that of methylphenidate, indicating that modafinil may be near the same level as methylphenidate in relation to abuse liability in terms of dopaminergic transmission.

Age-related decline in dopamine transporter in human brain using PET with a new radioligand [¹⁸F]FE-PE2I

OBJECTIVE

Dopamine transporter (DAT) density is considered as a marker of pre-synaptic function. Numerous neuroimaging studies have consistently demonstrated an age-related decrease in DAT density in normal human brain. However, the precise degree of the regional decline is not yet clear. The purpose of this study was to evaluate the effect of the normal aging process on DAT densities in human-specific brain regions including the substantia nigra and thalamus using positron emission tomography (PET) with [(18)F]FE-PE2I, a new PET radioligand with high affinity and selectivity for DAT.

METHODS

Thirty-six healthy volunteers ranging in age from 22 to 80 years were scanned with PET employing [(18)F]FE-PE2I for measuring DAT densities. Region of interest (ROI)-based analysis was used, and ROIs were manually defined for the caudate, putamen, substantia nigra, thalamus, and cerebellar cortex. DAT binding was quantified using a simplified reference tissue model, and the cerebellum was used as reference region. Estimations of binding potential in the caudate, putamen, substantia nigra, and thalamus were individually regressed according to age using simple regression analysis. Estimates of DAT loss per decade were obtained using the values from the regression slopes.

RESULTS

There were 7.6, 7.7, and 3.4% per-decade declines in DAT in the caudate, putamen, and substantia nigra, respectively. By contrast, there was no age-related decline of DAT in the thalamus.

CONCLUSIONS

[(18)F]FE-PE2I allowed reliable quantification of DAT, not only in the caudate and putamen but also in the substantia nigra. From the results, we demonstrated the age-related decline in the caudate and putamen as reported in previous studies, and additionally for those in the substantia nigra for the first time.

Application of feedback-controlled bolus plus infusion (FC-B/I) method for quantitative PET imaging of dopamine transporters with [(18)F]β-CFT-FE in conscious monkey brain

The competitive inhibition of dopamine transporters (DAT) with cocaine, a specific DAT inhibitor, was evaluated with a feedback-controlled bolus plus infusion (FC-B/I) method using animal positron emission tomography (PET) in the living brain of conscious monkey. 2β-Carbomethoxy-3β-(4-fluorophenyl)-8-(2-[(18)F]fluoroethyl) nortropane ([(18)F]β-CFT-FE; Harada et al. [2004] Synapse 54:37-45) was used for this study because it provided specific, fast, and reversible kinetic properties to DAT in the striatum. In FC-B/I method, the real-time image reconstruction was started just after intravenous bolus injection of [(18)F]β-CFT-FE to generate a time-activity curve in the striatum, and the infusion rate was adjusted to achieve an equilibrium state of the striatal radioactivity concentrations by means of a feedback-control algorithm. The first equilibrium state in the brain was reached within 20 min after the infusion start. Intravenous administration of cocaine at the doses of 0.02, 0.1, and 0.5 mg/kg shifted the equilibrium radioactivity level to the second equilibrium state in a dose-dependent manner, while no significant alterations was observed in the cerebellum. The present results demonstrated that the combined use of FC-B/I method and PET probe with fast kinetics like [(18)F]β-CFT-FE could be useful to assess the occupancy of drugs in the living brain with PET.

Synthesis and evaluation of novel tropane derivatives as potential PET imaging agents for the dopamine transporter

A novel series of tropane derivatives containing a fluorinated tertiary amino or amide at the 2β position was synthesized, labeled with the positron-emitter fluorine-18 (t(1/2)=109.8 min), and tested as potential in vivo dopamine transporter (DAT) imaging agents. The corresponding chlorinated analogs were prepared and employed as precursors for radiolabeling leading to the fluorine-18-labeled derivatives via a one-step nucleophilic aliphatic substitution reaction. In vitro binding results showed that the 2β-amino compounds 6b, 6d and 7b displayed moderately high affinities to DAT (K(i)<10nM). Biodistribution studies of [(18)F]6b and [(18)F]6d showed that the brain uptakes in rats were low. This is likely due to their low lipophilicities. Further structural modifications of these tropane derivatives will be needed to improve their in vivo properties as DAT imaging agents.

[18F]CFT synthesis and binding to monoamine transporters in rats

BACKGROUND

We present the electrophilic synthesis of [18F]2β-carbomethoxy-3β-(4-fluoro)tropane [[18F]CFT] and the pharmacological specificity and selectivity of [18F]CFT for monoamine transporters in the brain and peripheral organs of rats. The human radiation dose is extrapolated from the animal data.

METHODS

[18F]CFT was synthesized by electrophilic fluorination of a stannylated precursor by using post-target-produced [18F]F2 as a fluorinating agent. The ex vivo 18F-activity biodistribution of [18F]CFT in the brain of rats was studied by autoradiography. The binding of [18F]CFT to the monoamine transporters was studied using in vivo blocking experiments with dopamine transporter [DAT], norepinephrine transporter [NET], or serotonin transporter [SERT] inhibitors. In vivo animal positron emission tomography was used as a comparative method to determine tracer kinetics. Human radiation dose was assessed using OLINDA software.

RESULTS

The radiochemical yield of [18F]CFT from the initial [18F]F-, decay corrected to the end of bombardment, was 3.2 ± 1.0%. The specific activity [SA] was 14.5 ± 3.4 GBq/μmol, decay corrected to the end of synthesis. Radiochemical purity exceeded 99%. DAT-specific binding was found in the striatum, locus coeruleus, and pancreas. NET-specific binding was found in the locus coeruleus. SERT-specific binding was not found in any of the studied organs. Effective dose equivalent [EDE] estimated for the standard human model was 12.8 μSv/MBq. Effective dose [ED] was 9.17 μSv/MBq.

CONCLUSIONS

Post-target-produced high-SA [18F]F2 was used to incorporate18F directly into the phenyl ring of [18F]CFT. The final product had high radiochemical and chemical purities and a high SA for DAT and NET studies in vivo. In periphery, [18F]CFT showed a specific uptake in the pancreas. EDE and ED corresponded well with other18F-radioligands.

Simplified quantification and whole-body distribution of [18F]FE-PE2I in nonhuman primates: prediction for human studies

INTRODUCTION

[(18)F]FE-PE2I is a promising dopamine transporter (DAT) radioligand. In nonhuman primates, we examined the accuracy of simplified quantification methods and the estimates of radiation dose of [(18)F]FE-PE2I.

METHODS

In the quantification study, binding potential (BP(ND)) values previously reported in three rhesus monkeys using kinetic and graphical analyses of [(18)F]FE-PE2I were used for comparison. BP(ND) using the cerebellum as reference region was obtained with four reference tissue methods applied to the [(18)F]FE-PE2I data that were compared with the kinetic and graphical analyses. In the whole-body study, estimates of adsorbed radiation were obtained in two cynomolgus monkeys.

RESULTS

All reference tissue methods provided BP(ND) values within 5% of the values obtained with the kinetic and graphical analyses. The shortest imaging time for stable BP(ND) estimation was 54 min. The average effective dose of [(18)F]FE-PE2I was 0.021 mSv/MBq, similar to 2-deoxy-2-[(18)F]fluoro-d-glucose.

CONCLUSIONS

The results in nonhuman primates suggest that [(18)F]FE-PE2I is suitable for accurate and stable DAT quantification, and its radiation dose estimates would allow for a maximal administered radioactivity of 476 MBq in human subjects.

Advancement in PET quantification using 3D-OP-OSEM point spread function reconstruction with the HRRT

PURPOSE

Image reconstruction including the modelling of the point spread function (PSF) is an approach improving the resolution of the PET images. This study assessed the quantitative improvements provided by the implementation of the PSF modelling in the reconstruction of the PET data using the High Resolution Research Tomograph (HRRT).

METHODS

Measurements were performed on the NEMA-IEC/2001 (Image Quality) phantom for image quality and on an anthropomorphic brain phantom (STEPBRAIN). PSF reconstruction was also applied to PET measurements in two cynomolgus monkeys examined with [(18)F]FE-PE2I (dopamine transporter) and with [11C]MNPA (D2 receptor), and in one human subject examined with [11C]raclopride (D2 receptor).

RESULTS

PSF reconstruction increased the recovery coefficient (RC) in the NEMA phantom by 11-40% and the grey to white matter ratio in the STEPBRAIN phantom by 17%. PSF reconstruction increased binding potential (BP (ND)) in the striatum and midbrain by 14 and 18% in the [18F]FE-PE2I study, and striatal BP (ND) by 6 and 10% in the [11C]MNPA and [11C]raclopride studies.

CONCLUSION

PSF reconstruction improved quantification by increasing the RC and thus reducing the partial volume effect. This method provides improved conditions for PET quantification in clinical studies with the HRRT system, particularly when targeting receptor populations in small brain structures.