[en] Pridopidine is an investigational drug for Huntington disease (HD). Pridopidine was originally thought to act as a dopamine stabilizer. However, pridopidine shows highest affinity to the sigma-1 receptor (S1R) and enhances neuroprotection via the S1R in preclinical studies. Using [${}^{18}$F] fluspidine and [${}^{18}$F] fallypride PET, the purpose of this study was to assess in vivo target engagement/receptor occupancy of pridopidine to the S1R and dopamine D2/D3 receptor (D2/D3R) at clinical relevant doses in healthy volunteers (HVs) and as proof-of-concept in a small number of patients with HD. Using [${}^{18}$F] fluspidine PET (300 MBq, 0–90 min), 11 male HVs (pridopidine 0.5 to 90 mg; six dose groups) and three male patients with HD (pridopidine 90 mg) were investigated twice, without and 2 h after single dose of pridopidine. Using [${}^{18}$F] fallypride PET (200 MBq, 0–210 min), four male HVs were studied without and 2 h following pridopidine administration (90 mg). Receptor occupancy was analyzed by the Lassen plot. S1R occupancy as function of pridopidine dose (or plasma concentration) in HVs could be described by a three-parameter Hill equation with a Hill coefficient larger than one. A high degree of S1R occupancy (87% to 91%) was found throughout the brain at pridopidine doses ranging from 22.5 to 90 mg. S1R occupancy was 43% at 1 mg pridopidine. In contrast, at 90 mg pridopidine, the D2/D3R occupancy was only minimal (~ 3%). Our PET findings indicate that at clinically relevant single dose of 90 mg, pridopidine acts as a selective S1R ligand showing near to complete S1R occupancy with negligible occupancy of the D2/D3R. The dose S1R occupancy relationship suggests cooperative binding of pridopidine to the S1R. Our findings provide significant clarification about pridopidine’s mechanism of action and support further use of the 45-mg twice-daily dose to achieve full and selective targeting of the S1R in future clinical trials of neurodegenerative disorders.

[en] Depression is a common symptom in patients suffering from Parkinson's disease (PD) and markedly reduces their quality of life. As post-mortem studies have shown, its presence may reflect extensive cell loss in the midbrain and brainstem with imbalances in monoaminergic neurotransmitters. However, in vivo evidence of specific monoaminergic deficits in depressed PD patients is still sparse. Therefore, we studied PD patients with depression (PD+D) and without depression (PD-D) using high-resolution single-photon emission computed tomography (SPECT) and the monoamine transporter marker [¹²³I]FP-CIT. A magnetic resonance imaging-based region-of-interest analysis was applied to quantify the specific-to-nondisplaceable [¹²³I]FP-CIT binding coefficient V₃'' in the striatum, thalamus and midbrain/brainstem regions. PD+D patients had significantly lower V₃'' compared with PD-D patients in the striatum (p<0.001), thalamus (p=0.002), and midbrain/brainstem (p=0.025). Only PD+D patients without selective serotonin reuptake inhibitor (SSRI) treatment showed lower thalamic and midbrain V₃'' than controls (p<0.001, p=0.029). In a small sub-group of SSRI-treated PD+D patients neither thalamic V₃'' nor midbrain/brainstem V₃'' differed from those in PD-D patients (p=0.168, p=0.201) or controls (p=0.384, p=0.318). Our data indicate that depression in PD is associated with a more pronounced loss of striatal dopamine transporter availability that is most likely secondary to increased dopaminergic degeneration. In addition, depressed PD patients have a lower availability of midbrain/brainstem monoamine transporters than nondepressed PD patients. These findings provide in vivo evidence in support of the known post-mortem data demonstrating more extensive nerve cell loss in PD with depression and indicate that SPECT imaging can help to identify pathophysiological changes underlying nonmotor symptoms in this common movement disorder. (orig.)

[en] The term of neurodegenerative diseases covers a heterogeneous group of disorders that are distinguished by progressive degeneration of the structure and function of the nervous system such as dementias, movement disorders, motor neuron disorders, as well as some prion disorders. In recent years, a paradigm shift started for the diagnosis of neurodegenerative diseases, for which successively clinical testing is supplemented by biomarker information. In research scenarios, it was even proposed recently to substitute the current syndromic by a biological definition of Alzheimer’s diseases. PET examinations with various radiotracers play an important role in providing non-invasive biomarkers and co-morbidity information in neurodegeneration. Information on co-morbidity, e.g. Aβ plaques and Lewy-bodies or Aβ plaques in patients with aphasia or the absence of Aβ plaques in clinical AD patients are of interest to expand our knowledge about the pathogenesis of different phenotypically defined neurodegenerative diseases. Moreover, this information is also important in therapeutic trials targeting histopathological abnormalities.

The aim of this review is to present an overview of the currently available radiotracers for imaging neurodegenerative diseases in research and in routine clinical settings. In this context, we also provide a short summary of the most frequent neurodegenerative diseases from a nuclear medicine point of view, their clinical and pathophysiological as well as nuclear imaging characteristics, and the resulting need for new radiotracers.

[en] Complementing clinical findings with those generated by biomarkers - such as β-amyloid-targeted positron emission tomography (PET) imaging - has been proposed as a means of increasing overall accuracy in the diagnosis of Alzheimer's disease (AD). Florbetaben ([¹⁸F]BAY 94-9172) is a novel β-amyloid PET tracer currently in global clinical development. We present the results of a proof of mechanism study in which the diagnostic efficacy, pharmacokinetics, safety and tolerability of florbetaben were assessed. The value of various quantitative parameters derived from the PET scans as potential surrogate markers of cognitive decline was also investigated. Ten patients with mild-moderate probable AD (DSM-IV and NINCDS-ADRDA criteria) and ten age-matched (≥ 55 years) healthy controls (HCs) were administered a single dose of 300 MBq florbetaben, which contained a tracer mass dose of < 5 μg. The 70-90 min post-injection brain PET data were visually analysed by three blinded experts. Quantitative assessment was also performed via MRI-based, anatomical sampling of predefined volumes of interest (VOI) and subsequent calculation of standardized uptake value (SUV) ratios (SUVRs, cerebellar cortex as reference region). Furthermore, single-case, voxelwise analysis was used to calculate individual ''whole brain β-amyloid load''. Visual analysis of the PET data revealed nine of the ten AD, but only one of the ten HC brains to be β-amyloid positive (p = 0.001), with high inter-reader agreement (weighted kappa ≥ 0.88). When compared to HCs, the neocortical SUVRs were significantly higher in the ADs (with descending order of effect size) in frontal cortex, lateral temporal cortex, occipital cortex, anterior and posterior cingulate cortices, and parietal cortex (p = 0.003-0.010). Voxel-based group comparison confirmed these differences. Amongst the PET-derived parameters, the Statistical Parametric Mapping-based whole brain β-amyloid load yielded the closest correlation with the Mini-Mental State Examination scores (r = -0.736, p < 0.001), following a nonlinear regression curve. No serious adverse events or other safety concerns were seen. These results indicate florbetaben to be a safe and efficacious β-amyloid-targeted tracer with favourable brain kinetics. Subjects with AD could be easily differentiated from HCs by both visual and quantitative assessment of the PET data. The operator-independent, voxel-based analysis yielded whole brain β-amyloid load which appeared valuable as a surrogate marker of disease severity. (orig.)

[en] Modulation of the immune system by the CNS may involve serotonergic regulation via the brain serotonin transporters (SERT). This regulation may be disturbed in patients with CNS disorders including multiple sclerosis (MS). Central serotonergic mechanisms have not been investigated in MS by in vivo imaging. The objective of the study was to assess the availability of SERT in antidepressant-naive patients with MS by means of PET. Included in this study were 23 patients with MS and 22 matched healthy volunteers who were investigated with PET and the SERT-selective marker [¹¹C]DASB, and distribution volume ratios were determined. Clinical assessment of the patients included the expanded disability status scale, the MS fatigue scale Wuerzburger Erschoepfungsinventar bei MS (WEIMuS) and the Beck Depression Inventory (BDI). The PET data were analysed with both volume-of-interest and voxel-based analyses to determine regional SERT availability. Patients had lower SERT availability in the cingulate cortex, the thalamus and the insula, and increased availability in the orbitofrontal cortex. Patients with relapsing/remitting MS tended to have lower SERT in the hippocampus, whereas patients with primary progressive disease showed increased SERT availability in prefrontal regions. There was a positive correlation between SERT availability in the insula and both depression and fatigue scores (r = 0.56 vs. BDI, p = 0.02; r = 0.49 vs. WEIMuS, p = 0.05). Serotonergic neurotransmission in MS patients is altered in limbic and paralimbic regions as well as in the frontal cortex that this appears to contribute to psychiatric symptoms of MS. (orig.)

[en] The role of dopamine D_1-type receptor (D_1R)-expressing neurons in the regulation of motivated behavior and reward prediction has not yet been fully established. As a prerequisite for future research assessing D_1-mediated neuronal network regulation using simultaneous PET/MRI and D_1R-selective ["1"1C]SCH23390, this study investigated the stability of central D_1R measurements between two independent PET/MRI sessions under baseline conditions. Thirteen healthy volunteers (7 female, age 33 ± 13 yrs) underwent 90-min emission scans, each after 90-s bolus injection of 486 ± 16 MBq ["1"1C]SCH23390, on two separate days within 2-4 weeks using a PET/MRI system. Parametric images of D_1R distribution volume ratio (DVR) and binding potential (BP_N_D) were generated by a multi-linear reference tissue model with two parameters and the cerebellar cortex as receptor-free reference region. Volume-of-interest (VOI) analysis was performed with manual VOIs drawn on consecutive transverse MRI slices for brain regions with high and low D_1R density. The DVR varied from 2.5 ± 0.3 to 2.9 ± 0.5 in regions with high D_1R density (e.g. the head of the caudate) and from 1.2 ± 0.1 to 1.6 ± 0.2 in regions with low D_1R density (e.g. the prefrontal cortex). The absolute variability of the DVR ranged from 2.4% ± 1.3% to 5.1% ± 5.3%, while Bland-Altman analyses revealed very low differences in mean DVR (e.g. 0.013 ± 0.17 for the nucleus accumbens). Intraclass correlation (one-way, random) indicated very high agreement (0.93 in average) for both DVR and BP_N_D values. Accordingly, the absolute variability of BP_N_D ranged from 7.0% ± 4.7% to 12.5% ± 10.6%; however, there were regions with very low D_1R content, such as the occipital cortex, with higher mean variability. The test-retest reliability of D_1R measurements in this study was very high. This was the case not only for D_1R-rich brain areas, but also for regions with low D_1R density. These results will provide a solid base for future joint PET/MRI data analyses in stimulation-dependent mapping of D_1R-containing neurons and their effects on projections in neuronal circuits that determine behavior. (orig.)

[en] [¹¹C]DASB is currently the most frequently used highly selective radiotracer for visualization and quantification of central SERT. Its use, however, is hampered by the short half-life of ¹¹C, the moderate cortical test-retest reliability, and the lack of quantifying endogenous serotonin. Labelling with ¹⁸F allows in principle longer acquisition times for kinetic analysis in brain tissue and may provide higher sensitivity. The aim of our study was to firstly use the new highly SERT-selective ¹⁸F-labelled fluoromethyl analogue of (+)-McN5652 ((+)-[¹⁸F]FMe-McN5652) in humans and to evaluate its potential for SERT quantification. The PET data from five healthy volunteers (three men, two women, age 39 ± 10 years) coregistered with individual MRI scans were semiquantitatively assessed by volume-of-interest analysis using the software package PMOD. Rate constants and total distribution volumes (V_T) were calculated using a two-tissue compartment model and arterial input function measurements were corrected for metabolite/plasma data. Standardized uptake region-to-cerebellum ratios as a measure of specific radiotracer accumulation were compared with those of a [¹¹C]DASB PET dataset from 21 healthy subjects (10 men, 11 women, age 38 ± 8 years). The two-tissue compartment model provided adequate fits to the data. Estimates of total distribution volume (V_T) demonstrated good identifiability based on the coefficients of variation (COV) for the volumes of interest in SERT-rich and cortical areas (COV V_T <10%). Compared with [¹¹C]DASB PET, there was a tendency to lower mean uptake values in (+)-[¹⁸F]FMe-McN5652 PET; however, the standard deviation was also somewhat lower. Altogether, cerebral (+)-[¹⁸F]FMe-McN5652 uptake corresponded well with the known SERT distribution in humans. The results showed that (+)-[¹⁸F]FMe-McN5652 is also suitable for in vivo quantification of SERT with PET. Because of the long half-life of ¹⁸F, the widespread use within a satellite concept seems feasible. (orig.)

[en] Although the mechanisms by which the central noradrenaline (NA) system influences appetite and controls energy balance are quite well understood, its relationship to changes in body weight remains largely unknown. The main goal of this study was to further clarify whether the brain NA system is a stable trait or whether it can be altered by dietary intervention. We aimed to compare central NA transporter (NAT) availability in ten obese, otherwise healthy individuals with a body mass index (BMI) of 42.4 ± 3.7 kg/m² (age 34 ± 9 years, four women) and ten matched non-obese, healthy controls (BMI 23.9 ± 2.5 kg/m², age 33 ± 10 years, four women) who underwent PET with the NAT-selective radiotracer (S,S)-[¹¹C]O-methylreboxetine (MRB) before and 6 months after dietary intervention. MRI-based individual volume-of-interest analyses revealed an increase in binding potential (BP_ND) in the insula and the hippocampus of obese individuals, which correlated well with changes in BMI (-3.3 ± 5.3%; p = 0.03) following completion of the dietary intervention. Furthermore, voxel-wise regression analyses showed that lower BP_ND in these regions, but also in the midbrain and the prefrontal cortex, at baseline was associated with higher achieved weight loss (e.g., hippocampal area R ² = 0.80; p < 0.0001). No changes were observed in non-obese controls. These first longitudinal interventional data on NAT availability in highly obese individuals indicate that the central NA system is modifiable. Our findings suggest that NAT availability before intervention could help predict the amount and success of weight loss in obese individuals and help adjust treatment options individually by allowing prediction of the benefit of a dietary intervention. (orig.)

[en] The role of the central serotonin (5-hydroxytryptamine, 5-HT) system in feeding has been extensively studied in animals with the 5-HT family of transporters (5-HTT) being identified as key molecules in the regulation of satiety and body weight. Aberrant 5-HT transmission has been implicated in the pathogenesis of human obesity by in vivo positron emission tomography (PET) and single-photon emission computed tomography (SPECT) imaging techniques. However, results obtained thus far from studies of central 5-HTT availability have been inconsistent, which is thought to be brought about mainly by the low number of individuals with a high body mass index (BMI) previously used. The aim of this study was therefore to assess 5-HTT availability in the brains of highly obese otherwise healthy individuals compared with non-obese healthy controls. We performed PET using the 5-HTT selective radiotracer ["1"1C] DASB on 30 highly obese (BMI range between 35 and 55 kg/m"2) and 15 age- and sex-matched non-obese volunteers (BMI range between 19 and 27 kg/m"2) in a cross-sectional study design. The 5-HTT binding potential (BP_N_D) was used as the outcome parameter. On a group level, there was no significant difference in 5-HTT BP_N_D in various cortical and subcortical regions in individuals with the highest BMI compared with non-obese controls, while statistical models showed minor effects of age, sex, and the degree of depression on 5-HTT BP_N_D. The overall finding of a lack of significantly altered 5-HTT availability together with its high variance in obese individuals justifies the investigation of individual behavioral responses to external and internal cues which may further define distinct phenotypes and subgroups in human obesity. (orig.)

[en] The brain noradrenaline (NA) system plays an important role in the central nervous control of energy balance and is thus implicated in the pathogenesis of obesity. The specific processes modulated by this neurotransmitter which lead to obesity and overeating are still a matter of debate. We tested the hypothesis that in vivo NA transporter (NAT) availability is changed in obesity by using positron emission tomography (PET) and S,S-["1"1C]O-methylreboxetine (MRB) in twenty subjects comprising ten highly obese (body mass index BMI > 35 kg/m"2), metabolically healthy, non-depressed individuals and ten non-obese (BMI < 30 kg/m"2) healthy controls. Overall, we found no significant differences in binding potential (BP_N_D) values between obese and non-obese individuals in the investigated brain regions, including the NAT-rich thalamus (0.40 ± 0.14 vs. 0.41 ± 0.18; p = 0.84) though additional discriminant analysis correctly identified individual group affiliation based on regional BP_N_D in all but one (control) case. Furthermore, inter-regional correlation analyses indicated different BP_N_D patterns between both groups but this did not survive testing for multiple comparions. Our data do not find an overall involvement of NAT changes in human obesity. However, preliminary secondary findings of distinct regional and associative patterns warrant further investigation. (orig.)