Mitochondrial complex I density is associated with IQ and cognition in cognitively healthy adults: an in vivo [18F]BCPP-EF PET study

BACKGROUND

Mitochondrial function plays a key role in regulating neurotransmission and may contribute to general intelligence. Mitochondrial complex I (MC-I) is the largest enzyme of the respiratory chain. Recently, it has become possible to measure MC-I distribution in vivo, using a novel positron emission tomography tracer [18F]BCPP-EF, thus, we set out to investigate the association between MC-I distribution and measures of cognitive function in the living healthy brain.

RESULTS

Analyses were performed in a voxel-wise manner and identified significant associations between [18F]BCPP-EF DVRCS-1 in the precentral gyrus and parietal lobes and WAIS-IV predicted IQ, WAIS-IV arithmetic and WAIS-IV symbol-digit substitution scores (voxel-wise Pearson's correlation coefficients transformed to Z-scores, thresholded at Z = 2.3 family-wise cluster correction at p < 0.05, n = 16). Arithmetic scores were associated with middle frontal and post-central gyri tracer uptake, symbol-digit substitution scores were associated with precentral gyrus tracer uptake. RAVLT recognition scores were associated with [18F]BCPP-EF DVRCS-1 in the middle frontal gyrus, post-central gyrus, occipital and parietal regions (n = 20).

CONCLUSIONS

Taken together, our findings support the theory that mitochondrial function may contribute to general intelligence and indicate that interindividual differences in MC-I should be a key consideration for research into mitochondrial dysfunction in conditions with cognitive impairment.

Synaptic Terminal Density Early in the Course of Schizophrenia: An In Vivo UCB-J Positron Emission Tomographic Imaging Study of SV2A

BACKGROUND

The synaptic hypothesis is an influential theory of the pathoetiology of schizophrenia (SCZ), which is supported by the finding that there is lower uptake of the synaptic terminal density marker [11C]UCB-J in patients with chronic SCZ than in control participants. However, it is unclear whether these differences are present early in the illness. To address this, we investigated [11C]UCB-J volume of distribution (VT) in antipsychotic-naïve/free patients with SCZ who were recruited from first-episode services compared with healthy volunteers.

METHODS

Forty-two volunteers (SCZ n = 21, healthy volunteers n = 21) underwent [11C]UCB-J positron emission tomography to index [11C]UCB-J VT and distribution volume ratio in the anterior cingulate, frontal, and dorsolateral prefrontal cortices; the temporal, parietal and occipital lobes; and the hippocampus, thalamus, and amygdala. Symptom severity was assessed in the SCZ group using the Positive and Negative Syndrome Scale.

RESULTS

We found no significant effects of group on [11C]UCB-J VT or distribution volume ratio in most regions of interest (effect sizes from d = 0.0-0.7, p > .05), with two exceptions: we found lower distribution volume ratio in the temporal lobe (d = 0.7, uncorrected p < .05) and lower VT/fp in the anterior cingulate cortex in patients (d = 0.7, uncorrected p < .05). The Positive and Negative Syndrome Scale total score was negatively associated with [11C]UCB-J VT in the hippocampus in the SCZ group (r = -0.48, p = .03).

CONCLUSIONS

These findings indicate that large differences in synaptic terminal density are not present early in SCZ, although there may be more subtle effects. When taken together with previous evidence of lower [11C]UCB-J VT in patients with chronic illness, this may indicate synaptic density changes during the course of SCZ.

Estimation of target occupancy in repeated dosing design studies using positron emission tomography: Biases due to target upregulation

Positron emission tomography (PET) has become indispensable in the quantification of target engagement by brain targeting medications. The relationship between the drug plasma concentration (or drug dose administered) and target occupancy determined during a PET occupancy study has provided valuable information for the assessment of novel pharmaceuticals in the early phases of drug development. Such information is also critical for the understanding of the mechanisms of action and side-effect profile of approved medication commonly used in the clinic. Occupancy studies conducted following repeated drug dosing (RD) can produce systematic differences from those conducted following single drug dose (SD), differences that have not been adequately explored. We have hypothesised that when differences are observed between RD and SD studies, they are related to changes in target density induced by repeated drug accumulation. We have developed a modified occupancy model to account for potential changes in target density and tested it on a sample dataset. We found that target upregulation can parsimoniously explain the differences in drug affinity estimated in SD and RD studies. Our findings have implications for the interpretation of RD occupancy data in the literature and the relationship between specific target occupancy levels and drug efficacy and tolerability.

Histamine-3 Receptor Availability and Glutamate Levels in the Brain: A PET-1H-MRS Study of Patients With Schizophrenia and Healthy Controls

BACKGROUND

The histamine-3 receptor (H3R) may have a role in cognitive processes through its action as a presynaptic heteroreceptor inhibiting the release of glutamate in the brain. To explore this, we examined anterior cingulate cortex (ACC) and striatum H3R availability in patients with schizophrenia and characterized their relationships with glutamate levels in corresponding brain regions.

METHODS

We employed a cross-sectional study, recruiting 12 patients with schizophrenia and 12 healthy volunteers. Participants underwent positron emission tomography using the H3R-specific radio ligand [11C]MK-8278, followed by proton magnetic resonance spectroscopy to measure glutamate levels, recorded as Glu and Glx. Based on existing literature, the ACC and striatum were selected as regions of interest.

RESULTS

We found significant inverse relationships between tracer uptake and Glu (r = -0.66, P = .02) and Glx (r = -0.62, P = .04) levels in the ACC of patients, which were absent in healthy volunteers (Glu: r = -0.19, P = .56, Glx: r = 0.10, P = .75). We also found a significant difference in striatal (F1,20 = 6.00, P = .02) and ACC (F1,19 = 4.75, P = .04) Glx levels between groups.

CONCLUSIONS

These results provide evidence of a regionally specific relationship between H3Rs and glutamate levels, which builds on existing preclinical literature. Our findings add to a growing literature indicating H3Rs may be a promising treatment target in schizophrenia, particularly for cognitive impairment, which has been associated with altered glutamate signaling.

The histamine system and cognitive function: An in vivo H3 receptor PET imaging study in healthy volunteers and patients with schizophrenia

BACKGROUND

The histamine-3 receptor (H3R) is an auto- and heteroreceptor that inhibits the release of histamine and other neurotransmitters. Post-mortem evidence has found altered H3R expression in patients with psychotic disorders, which may underlie cognitive impairment associated with schizophrenia (CIAS).

AIMS

We used positron emission tomography (PET) imaging to compare brain uptake of an H3R selective tracer between patients with schizophrenia and matched controls (healthy individuals). Regions of interest included the dorsolateral prefrontal cortex (DLPFC) and striatum. We explored correlations between tracer uptake and symptoms, including cognitive domains.

METHODS

A total of 12 patients and 12 matched controls were recruited to the study and were assessed with psychiatric and cognitive rating scales. They received a PET scan using the H3R-specific radioligand [11C]MK-8278 to determine H3R availability.

RESULTS

There was no statistically significant difference in tracer uptake between patients and controls in the DLPFC (t19 = 0.79, p = 0.44) or striatum (t21 = 1.18, p = 0.25). An exploratory analysis found evidence for lower volume of distribution in the left cuneus (pFWE-corrected = 0.01). DLPFC tracer uptake was strongly correlated with cognition in controls (trail making test (TMT) A: r = 0.77, p = 0.006; TMT B: rho = 0.74, p = 0.01), but not in patients (TMT A: r = -0.18, p = 0.62; TMT B: rho = -0.06, p = 0.81).

CONCLUSIONS

These findings indicate H3R in the DLPFC might play a role in executive function and this is disrupted in schizophrenia in the absence of major alterations in H3R availability as assessed using a selective radiotracer for H3R. This provides further evidence for the role of H3R in CIAS.

Neuroimaging in psychedelic drug development: past, present, and future

Psychedelic therapy (PT) is an emerging paradigm with great transdiagnostic potential for treating psychiatric disorders, including depression, addiction, post-traumatic stress disorder, and potentially others. 'Classic' serotonergic psychedelics, such as psilocybin and lysergic acid diethylamide (LSD), which have a key locus of action at the 5-HT2A receptor, form the main focus of this movement, but substances including ketamine, 3,4-Methylenedioxymethamphetamine (MDMA) and ibogaine also hold promise. The modern phase of development of these treatment modalities in the early 21st century has occurred concurrently with the wider use of advanced human neuroscientific research methods; principally neuroimaging. This can potentially enable assessment of drug and therapy brain effects with greater precision and quantification than any previous novel development in psychiatric pharmacology. We outline the major trends in existing data and suggest the modern development of PT has benefitted greatly from the use of neuroimaging. Important gaps in existing knowledge are identified, namely: the relationship between acute drug effects and longer-term (clinically-relevant) effects, the precise characterisation of effects at the 5-HT2A receptor and relationships with functional/clinical effects, and the possible impact of these compounds on neuroplasticity. A road-map for future research is laid out, outlining clinical studies which will directly address these three questions, principally using combined Positron Emission Tomography (PET) and Magnetic Resonance Imaging (MRI) methods, plus other adjunct techniques. Multimodal (PET/MRI) studies using modern PET techniques such as the 5-HT2A-selective ligand [11 C]Cimbi-36 (and other ligands sensitive to neuroplasticity changes) alongside MRI measures of brain function would provide a 'molecular-functional-clinical bridge' in understanding. Such results would help to resolve some of these questions and provide a firmer foundation for the ongoing development of PT.

Reward processing in schizophrenia and its relation to Mu opioid receptor availability and negative symptoms: A [11C]-carfentanil PET and fMRI study

BACKGROUND

Reward processing deficits are a core feature of schizophrenia and are thought to underlie negative symptoms. Pre-clinical evidence suggests that opioid neurotransmission is linked to reward processing. However, the contribution of Mu Opioid Receptor (MOR) signalling to the reward processing abnormalities in schizophrenia is unknown. Here, we examined the association between MOR availability and the neural processes underlying reward anticipation in patients with schizophrenia using multimodal neuroimaging.

METHOD

37 subjects (18 with Schizophrenia with moderate severity negative symptoms and 19 age and sex-matched healthy controls) underwent a functional MRI scan while performing the Monetary Incentive Delay (MID) task to measure the neural response to reward anticipation. Participants also had a [11C]-carfentanil PET scan to measure MOR availability.

RESULTS

Reward anticipation was associated with increased neural activation in a widespread network of brain regions including the striatum. Patients with schizophrenia had both significantly lower MOR availability in the striatum as well as striatal hypoactivation during reward anticipation. However, there was no association between MOR availability and striatal neural activity during reward anticipation in either patient or controls (Pearson's Correlation, controls df = 17, r = 0.321, p = 0.18, patients df = 16, r = 0.295, p = 0.24). There was no association between anticipation-related neural activation and negative symptoms (r = -0.120, p = 0.14) or anhedonia severity (social r = -0.365, p = 0.14 physical r = -0.120, p = 0.63).

CONCLUSIONS

Our data suggest reduced MOR availability in schizophrenia might not underlie striatal hypoactivation during reward anticipation in patients with established illness. Therefore, other mechanisms, such as dopamine dysfunction, warrant further investigation as treatment targets for this aspect of the disorder.

Effects of Kisspeptin on Sexual Brain Processing and Penile Tumescence in Men With Hypoactive Sexual Desire Disorder: A Randomized Clinical Trial

IMPORTANCE

The human physiological sexual response is crucial for reward, satisfaction, and reproduction. Disruption of the associated neurophysiological pathways predisposes to low sexual desire; the most prevalent psychological form is hypoactive sexual desire disorder (HSDD), which affects 8% of men but currently has no effective pharmacological treatment options. The reproductive neuropeptide kisspeptin offers a putative therapeutic target, owing to emerging understanding of its role in reproductive behavior.

OBJECTIVE

To determine the physiological, behavioral, neural, and hormonal effects of kisspeptin administration in men with HSDD.

DESIGN, SETTING, AND PARTICIPANTS

This double-blind, 2-way crossover, placebo-controlled randomized clinical trial was performed at a single academic research center in the UK. Eligible participants were right-handed heterosexual men with HSDD. Physiological, behavioral, functional magnetic resonance imaging (fMRI), and hormonal analyses were used to investigate the clinical and mechanistic effects of kisspeptin administration in response to visual sexual stimuli (short and long video tasks). The trial was conducted between January 11 and September 15, 2021, and data analysis was performed between October and November 2021.

INTERVENTIONS

Participants attended 2 study visits at least 7 days apart, in balanced random order, for intravenous infusion of kisspeptin-54 (1 nmol/kg/h) for 75 minutes or for administration of a rate-matched placebo.

MAIN OUTCOMES AND MEASURES

Changes in (1) brain activity on whole-brain analysis, as determined by fMRI blood oxygen level-dependent activity in response to visual sexual stimuli during kisspeptin administration compared with placebo, (2) physiological sexual arousal (penile tumescence), and (3) behavioral measures of sexual desire and arousal.

RESULTS

Of the 37 men randomized, 32 completed the trial. Participants had a mean (SD) age of 37.9 (8.6) years and a mean (SD) body mass index of 24.9 (5.4). On viewing sexual videos, kisspeptin significantly modulated brain activity in key structures of the sexual-processing network on whole-brain analysis compared with placebo (mean absolute change [Cohen d] = 0.81 [95% CI, 0.41-1.21]; P =  .003). Furthermore, improvements in several secondary analyses were observed, including significant increases in penile tumescence in response to sexual stimuli (by up to 56% more than placebo; mean difference = 0.28 units [95% CI, 0.04-0.52 units]; P = .02) and behavioral measures of sexual desire-most notably, increased happiness about sex (mean difference = 0.63 points [95% CI, 0.10-1.15 points]; P = .02).

CONCLUSIONS AND RELEVANCE

Collectively, this randomized clinical trial provides the first evidence to date showing that kisspeptin administration substantially modulates sexual brain processing in men with HSDD, with associated increases in penile tumescence and behavioral measures of sexual desire and arousal. These data suggest that kisspeptin has potential as the first pharmacological treatment for men with low sexual desire.

TRIAL REGISTRATION

isrctn.org Identifier: ISRCTN17271094.

OR17-5 Kisspeptin Increases Penile Tumescence and Sexual Brain Processing in Men with Hypoactive Sexual Desire Disorder

Background

Hypoactive Sexual Desire Disorder (HSDD) is associated with dysfunctional brain activation in regions governing sexual responses, resulting in a deficiency or absence of sexual desire with marked distress. It is of major clinical importance given it affects 8% of men with detrimental effects on quality of life, interpersonal relationships and fertility, but so far has no licensed treatment options. The reproductive neuropeptide kisspeptin offers a putative therapeutic target owing to its emerging role in modulating reproductive behaviour in animal models and healthy men. However, there are no studies examining its effects in HSDD. To address this, we performed the first clinical study of kisspeptin in men with HSDD.

Methods

We examined the effects of kisspeptin administration (vs placebo) on brain activity during short and long erotic video tasks using functional MRI in 32 men with HSDD (mean ±SEM age 37.9 ±1.5 y, BMI 24.9 ±1.0 kg/m2). The short video task used 20-second segments of erotic video with non-erotic video as control. During the long video task, participants viewed a continuous eight-minute erotic video. To provide functional and behavioural relevance for the associated fMRI brain responses during the long erotic video, simultaneous penile tumescence and subjective level of arousal were recorded. Participants also completed psychometric and behavioural questionnaires. Standard analysis methods were used for fMRI data from the short videos task, and the long videos task used regressors derived from the subjective arousal and penile tumescence data. The statistical threshold used for both was Z=2.3, p &lt; 0.05 (cluster-corrected).

Results

In response to visual erotic stimuli, kisspeptin administration significantly increased penile tumescence during the long video task compared to placebo, with kisspeptin increasing penile tumescence by 56% at six-minutes (p=0.002). In addition, kisspeptin increased participant-reported happiness about sex (p=0.02). During both video tasks, kisspeptin significantly modulated brain activity compared to placebo in key structures of the sexual-processing network, providing a mechanistic pathway for the increases in physiological and behavioural measures. In response to short erotic videos, kisspeptin enhanced left middle frontal gyrus and left anterior cingulate activity, and decreased activity in bilateral parahippocampus (all p&lt;0.05). During the long video task, kisspeptin enhanced right fusiform gyrus and bilateral visual cortex activity, and decreased left frontal pole, right posterior cingulate and bilateral precuneus activity (all &lt;0.05). Additionally, we observed positive correlations between the effects of kisspeptin on aforementioned brain activity and psychometric parameters of sexual desire and arousal (all p&lt;0.01).

Conclusion

Collectively, we demonstrate for the first time that kisspeptin administration in men with HSDD increases penile tumescence and psychometric measures of sexual desire and arousal by modulating sexual brain processing. Taken together, our data suggest that kisspeptin-based therapeutics may offer a novel, effective and much-needed clinical strategy for men with HSDD.

Presentation: Sunday, June 12, 2022 12:00 p.m. - 12:15 p.m.

Adenosine A2A receptor in schizophrenia: an in vivo brain PET imaging study

Adenosine A_2A receptors are highly enriched in the basal ganglia system, a region that is functionally implicated in schizophrenia. Preclinical evidence suggests a cross-regulation between adenosine A_2A and dopamine D₂ receptors in this region and that it is linked to the sensitization of the dopamine system. However, the relationship between A_2A receptor availability and schizophrenia has not been directly examined in vivo in patients with this disorder. To investigate, using positron emission tomography (PET), the availability of A_2A receptors in patients diagnosed with schizophrenia in comparison to matched healthy controls. A_2A receptor availability was measured using the PET tracer [¹¹C]SCH442416. Twelve male patients with chronic schizophrenia were compared to 13 matched healthy subjects. All patients were medicated with antipsychotics and none presented with any motor or extrapyramidal symptoms. Binding potential (BP_ND), a ratio measure between specific and non-specific tracer uptake, were compared between the groups for the caudate, putamen, accumbens and globus pallidum. There was no differences between A_2A receptor binding potential (BP_ND) of schizophrenia patients in the caudate (p = 0.16), putamen (p = 0.86), accumbens (p = 0.44) and globus pallidum (p = 0.09) to that of matched healthy subjects. There was also no significant correlation between [11C]SCH442416 binding and severity of psychotic symptoms (p = 0.2 to 0.82) or antipsychotic dosage (p = 0.13 to 0.34). By showing that A2A receptor availability in medicated patients with chronic male schizophrenia is not different than in healthy controls, this study does not support the primary role of this receptor in the pathogenesis of schizophrenia.

Brain Serotonin Release Is Reduced in Patients With Depression: A [11C]Cimbi-36 Positron Emission Tomography Study With a d-Amphetamine Challenge

BACKGROUND

The serotonin hypothesis of depression proposes that diminished serotonergic (5-HT) neurotransmission is causal in the pathophysiology of the disorder. Although the hypothesis is over 50 years old, there is no firm in vivo evidence for diminished 5-HT neurotransmission. We recently demonstrated that the 5-HT_2A receptor agonist positron emission tomography (PET) radioligand [¹¹C]Cimbi-36 is sensitive to increases in extracellular 5-HT induced by an acute d-amphetamine challenge. Here we applied [¹¹C]Cimbi-36 PET to compare brain 5-HT release capacity in patients experiencing a major depressive episode (MDE) to that of healthy control subjects (HCs) without depression.

METHODS

Seventeen antidepressant-free patients with MDE (3 female/14 male, mean age 44 ± 13 years, Hamilton Depression Rating Scale score 21 ± 4 [range 16-30]) and 20 HCs (3 female/17 male, mean age 32 ± 9 years) underwent 90-minute dynamic [¹¹C]Cimbi-36 PET before and 3 hours after a 0.5-mg/kg oral dose of d-amphetamine. Frontal cortex (main region of interest) 5-HT_2A receptor nondisplaceable binding was calculated from kinetic analysis using the multilinear analysis-1 approach with the cerebellum as the reference region.

RESULTS

Following d-amphetamine administration, frontal nondisplaceable binding potential (BP_ND) was significantly reduced in the HC group (1.04 ± 0.31 vs. 0.87 ± 0.24, p < .001) but not in the MDE group (0.97 ± 0.25 vs. 0.92 ± 0.22, not significant). ΔBP_ND of the MDE group was significantly lower than that of the HC group (HC: 15% ± 14% vs. MDE: 6.5% ± 20%, p = .041).

CONCLUSIONS

This first direct assessment of 5-HT release capacity in people with depression provides clear evidence for dysfunctional serotonergic neurotransmission in depression by demonstrating reduced 5-HT release capacity in patients experiencing an MDE.

Effects of Kisspeptin Administration in Women With Hypoactive Sexual Desire Disorder: A Randomized Clinical Trial

IMPORTANCE

Despite being the most common female sexual health complaint worldwide, current treatment options for hypoactive sexual desire disorder (HSDD) are limited in their safety and effectiveness. The hormone kisspeptin is a key endogenous activator of the reproductive hormonal axis with additional emerging roles in sexual and emotional behavior; however, its effects in women with HSDD are unknown.

OBJECTIVE

To test the hypothesis that kisspeptin enhances sexual and attraction brain processing in women with HSDD.

DESIGN, SETTING, AND PARTICIPANTS

This randomized clinical trial was double-masked and placebo controlled with a 2-way crossover. The trial was conducted in a university research setting in the UK from October 2020 to April 2021. Eligible participants were premenopausal women with HSDD. Functional neuroimaging, psychometric, and hormonal analyses were employed to investigate the effects of kisspeptin administration on brain processing, in response to erotic stimuli (erotic videos) and facial attraction (face images of varying attractiveness). Data were analyzed from May to December 2021.

INTERVENTIONS

A 75-minute intravenous infusion of kisspeptin-54 (1 nmol/kg/h) vs equivalent-rate placebo infusion.

MAIN OUTCOMES AND MEASURES

Blood oxygen level-dependent responses across the whole brain and regions of interest during kisspeptin vs placebo administration in response to erotic and facial attraction stimuli.

RESULTS

Of the 40 participants who were randomized, 32 women completed both kisspeptin and placebo visits, with a mean (SE) age of 29.2 (1.2) years. Kisspeptin administration resulted in modulations in sexual and facial attraction brain processing (deactivation of the left inferior frontal gyrus: Z max, 3.76; P = .01; activation of the right postcentral and supramarginal gyrus: Z max, 3.73; P < .001; deactivation of the right temporoparietal junction: Z max 4.08; P = .02). Furthermore, positive correlations were observed between kisspeptin-enhanced hippocampal activity in response to erotic videos, and baseline distress relating to sexual function (r = 0.469; P = .007). Kisspeptin's enhancement of posterior cingulate cortex activity in response to attractive male faces also correlated with reduced sexual aversion, providing additional functional significance (r = 0.476, P = .005). Kisspeptin was well-tolerated with no reported adverse effects.

CONCLUSIONS AND RELEVANCE

These findings lay the foundations for clinical applications for kisspeptin in women with HSDD.

TRIAL REGISTRATION

ISRCTN trial registry identifier: ISRCTN17271094.

Widespread cell stress and mitochondrial dysfunction occur in patients with early Alzheimer's disease

Cell stress and impaired oxidative phosphorylation are central to mechanisms of synaptic loss and neurodegeneration in the cellular pathology of Alzheimer's disease (AD). In this study, we quantified the in vivo expression of the endoplasmic reticulum stress marker, sigma 1 receptor (S1R), using [¹¹C]SA4503 positron emission tomography (PET), the mitochondrial complex I (MC1) with [¹⁸F]BCPP-EF, and the presynaptic vesicular protein SV2A with [¹¹C]UCB-J in 12 patients with early AD and in 16 cognitively normal controls. We integrated these molecular measures with assessments of regional brain volumes and cerebral blood flow (CBF) measured with magnetic resonance imaging arterial spin labeling. Eight patients with AD were followed longitudinally to estimate the rate of change of the physiological and structural pathology markers with disease progression. The patients showed widespread increases in S1R (≤ 27%) and regional reduction in MC1 (≥ -28%) and SV2A (≥ -25%) radioligand binding, brain volume (≥ -23%), and CBF (≥ -26%). [¹⁸F]BCPP-EF PET MC1 binding (≥ -12%) and brain volumes (≥ -5%) showed progressive reductions over 12 to 18 months, suggesting that they both could be used as pharmacodynamic indicators in early-stage therapeutics trials. Associations of reduced MC1 and SV2A and increased S1R radioligand binding with reduced cognitive performance in AD, although exploratory, suggested a loss of metabolic functional reserve with disease. Our study thus provides in vivo evidence for widespread, clinically relevant cellular stress and bioenergetic abnormalities in early AD.

Endogenous dopamine release in the human brain as a pharmacodynamic biomarker: evaluation of the new GPR139 agonist TAK-041 with [11C]PHNO PET

The use of positron emission tomography (PET) in early-phase development of novel drugs targeting the central nervous system, is well established for the evaluation of brain penetration and target engagement. However, when novel targets are involved a suitable PET ligand is not always available. We demonstrate an alternative approach that evaluates the attenuation of amphetamine-induced synaptic dopamine release by a novel agonist of the orphan G-protein-coupled receptor GPR139 (TAK-041). GPR139 agonism is a novel candidate mechanism for the treatment of schizophrenia and other disorders associated with social and cognitive dysfunction. Ten healthy volunteers underwent [¹¹C]PHNO PET at baseline, and twice after receiving an oral dose of d-amphetamine (0.5 mg/kg). One of the post-d-amphetamine scans for each subject was preceded by a single oral dose of TAK-041 (20 mg in five; 40 mg in the other five participants). D-amphetamine induced a significant decrease in [¹¹C]PHNO binding potential relative to the non-displaceable component (BP_ND) in all regions examined (16-28%), consistent with increased synaptic dopamine release. Pre-treatment with TAK-041 significantly attenuated the d-amphetamine-induced reduction in BP_ND in the a priori defined regions (putamen and ventral striatum: 26% and 18%, respectively). The reduction in BP_ND was generally higher after the 40 mg than the 20 mg TAK-041 dose, with the difference between doses reaching statistical significance in the putamen. Our findings suggest that TAK-041 enters the human brain and interacts with GPR139 to affect endogenous dopamine release. [¹¹C]PHNO PET is a practical method to detect the effects of novel drugs on the brain dopaminergic system in healthy volunteers, in the early stages of drug development.

Pharmacokinetics and brain σ1 receptor occupancy of MR309, a selective σ1 receptor antagonist

AIMS

Preclinical studies of MR309, a selective sigma-1 receptor (σ1R) antagonist, support a potential role in treating neuropathic pain. We report 2 studies that provide insight into the pharmacokinetics (PK) and brain σ1R binding of MR309.

METHODS

Steady-state PK of MR309 (400 mg once daily and 200 mg twice-daily [BID] for 10 days; EudraCT 2015-001818-99 [PK study]) and the relationship between MR309 plasma exposure and brain σ1R occupancy (EudraCT 2017-000670-11 [positron emission tomography study]) were investigated in healthy volunteers. Positron emission tomography using the σ1R ligand [11 C]SA4503 was conducted at baseline, and 2 and 8 hours after a single dose of MR309 (200-800 mg). The relationship between brain σ1R occupancy and MR309 exposure was explored using data-driven model fitting.

RESULTS

MR309 was well tolerated, brain σ1R occupancy ranged between 30.5 and 74.9% following single-dose MR309 (n = 7). MR309 BID provided a plasma PK profile with less fluctuation than once daily dosing (n = 16). MR309 200 mg BID yielded average steady state plasma concentrations between 2000 and 4000 ng/mL in the PK study, which corresponded to an estimated brain σ1R occupancy of 59-74%.

CONCLUSION

MR309 200 mg BID dose was below the 75% σ1R occupancy threshold expected to elicit maximal antinociceptive effect as observed in neuropathic pain models. Further investigations of MR309 for neuropathic pain will require higher brain σ1R occupancy, and establish the optimal dose by elucidating the clinical impact of a broad range of brain σ1R occupancy across different neuropathic pain indications.

Boosting the diagnostic power of amyloid-β PET using a data-driven spatially informed classifier for decision support

BACKGROUND

Amyloid-β (Aβ) PET has emerged as clinically useful for more accurate diagnosis of patients with cognitive decline. Aβ deposition is a necessary cause or response to the cellular pathology of Alzheimer's disease (AD). Usual clinical and research interpretation of amyloid PET does not fully utilise all information regarding the spatial distribution of signal. We present a data-driven, spatially informed classifier to boost the diagnostic power of amyloid PET in AD.

METHODS

Voxel-wise k-means clustering of amyloid-positive voxels was performed; clusters were mapped to brain anatomy and tested for their associations by diagnostic category and disease severity with 758 amyloid PET scans from volunteers in the AD continuum from the Alzheimer's Disease Neuroimaging Initiative (ADNI). A machine learning approach based on this spatially constrained model using an optimised quadratic support vector machine was developed for automatic classification of scans for AD vs non-AD pathology.

RESULTS

This classifier boosted the accuracy of classification of AD scans to 81% using the amyloid PET alone with an area under the curve (AUC) of 0.91 compared to other spatial methods. This increased sensitivity to detect AD by 15% and the AUC by 9% compared to the use of a composite region of interest SUVr.

CONCLUSIONS

The diagnostic classification accuracy of amyloid PET was improved using an automated data-driven spatial classifier. Our classifier highlights the importance of considering the spatial variation in Aβ PET signal for optimal interpretation of scans. The algorithm now is available to be evaluated prospectively as a tool for automated clinical decision support in research settings.

Glia Imaging Differentiates Multiple System Atrophy from Parkinson's Disease: A Positron Emission Tomography Study with [11 C]PBR28 and Machine Learning Analysis

BACKGROUND

The clinical diagnosis of multiple system atrophy (MSA) is challenged by overlapping features with Parkinson's disease (PD) and late-onset ataxias. Additional biomarkers are needed to confirm MSA and to advance the understanding of pathophysiology. Positron emission tomography (PET) imaging of the translocator protein (TSPO), expressed by glia cells, has shown elevations in MSA.

OBJECTIVE

In this multicenter PET study, we assess the performance of TSPO imaging as a diagnostic marker for MSA.

METHODS

We analyzed [¹¹ C]PBR28 binding to TSPO using imaging data of 66 patients with MSA and 24 patients with PD. Group comparisons were based on regional analysis of parametric images. The diagnostic readout included visual reading of PET images against clinical diagnosis and machine learning analyses. Sensitivity, specificity, and receiver operating curves were used to discriminate MSA from PD and cerebellar from parkinsonian variant MSA.

RESULTS

We observed a conspicuous pattern of elevated regional [¹¹ C]PBR28 binding to TSPO in MSA as compared with PD, with "hotspots" in the lentiform nucleus and cerebellar white matter. Visual reading discriminated MSA from PD with 100% specificity and 83% sensitivity. The machine learning approach improved sensitivity to 96%. We identified MSA subtype-specific TSPO binding patterns.

CONCLUSIONS

We found a pattern of significantly increased regional glial TSPO binding in patients with MSA. Intriguingly, our data are in line with severe neuroinflammation in MSA. Glia imaging may have potential to support clinical MSA diagnosis and patient stratification in clinical trials on novel drug therapies for an α-synucleinopathy that remains strikingly incurable. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Specific and non-specific binding of a tracer for the translocator-specific protein in schizophrenia: an [11C]-PBR28 blocking study

OBJECTIVE

The mitochondrial 18-kDa translocator protein (TSPO) is expressed by activated microglia and positron emission tomography enables the measurement of TSPO levels in the brain. Findings in schizophrenia have shown to vary depending on the outcome measure used and this discrepancy in TSPO results could be explained by lower non-displaceable binding (V_ND) in schizophrenia, which could obscure increases in specific binding. In this study, we have used the TSPO ligand XBD173 to block the TSPO radioligand [¹¹C]-PBR28 and used an occupancy plot to quantify V_ND in patients with schizophrenia.

METHODS

A total of 7 patients with a diagnosis of schizophrenia were recruited for this study. Each patient received two separate PET scans with [¹¹C]PBR28, one at baseline and one after the administration of the TSPO ligand XBD173. All patients were high-affinity binders (HABs) for the TSPO gene. We used an occupancy plot to quantify the non-displaceable component (V_ND) using 2TCM kinetic estimates with and without vascular correction. Finally we computed the V_ND at a single subject level using the SIME method.

RESULTS

All patients showed a global and generalized reduction in [¹¹C]PBR28 uptake after the administration of XBD173. Constraining the V_ND to be equal for all patients, the population V_ND was estimated to be 1.99 mL/cm³ (95% CI 1.90 to 2.08). When we used vascular correction, the fractional TSPO occupancy remained similar.

CONCLUSIONS

In schizophrenia patients, a substantial component of the [¹¹C]PBR28 signal represents specific binding to TSPO. Furthermore, the V_ND in patients with schizophrenia is similar to that previously reported in healthy controls. These results suggest that changes in non-specific binding between schizophrenia patients and healthy controls do not account for discrepant PET findings in this disorder.

Astrocyte reactivity with late-onset cognitive impairment assessed in vivo using 11C-BU99008 PET and its relationship with amyloid load

11C-BU99008 is a novel positron emission tomography (PET) tracer that enables selective imaging of astrocyte reactivity in vivo. To explore astrocyte reactivity associated with Alzheimer's disease, 11 older, cognitively impaired (CI) subjects and 9 age-matched healthy controls (HC) underwent 3T magnetic resonance imaging (MRI), 18F-florbetaben and 11C-BU99008 PET. The 8 amyloid (Aβ)-positive CI subjects had higher 11C-BU99008 uptake relative to HC across the whole brain, but particularly in frontal, temporal, medial temporal and occipital lobes. Biological parametric mapping demonstrated a positive voxel-wise neuroanatomical correlation between 11C-BU99008 and 18F-florbetaben. Autoradiography using 3H-BU99008 with post-mortem Alzheimer's brains confirmed through visual assessment that increased 3H-BU99008 binding localised with the astrocyte protein glial fibrillary acid protein and was not displaced by PiB or florbetaben. This proof-of-concept study provides direct evidence that 11C-BU99008 can measure in vivo astrocyte reactivity in people with late-life cognitive impairment and Alzheimer's disease. Our results confirm that increased astrocyte reactivity is found particularly in cortical regions with high Aβ load. Future studies now can explore how clinical expression of disease varies with astrocyte reactivity.

The relationship between synaptic density marker SV2A, glutamate and N-acetyl aspartate levels in healthy volunteers and schizophrenia: a multimodal PET and magnetic resonance spectroscopy brain imaging study

Glutamatergic excitotoxicity is hypothesised to underlie synaptic loss in schizophrenia pathogenesis, but it is unknown whether synaptic markers are related to glutamatergic function in vivo. Additionally, it has been proposed that N-acetyl aspartate (NAA) levels reflect neuronal integrity. Here, we investigated whether synaptic vesicle glycoprotein 2 A (SV2A) levels are related to glutamatergic markers and NAA in healthy volunteers (HV) and schizophrenia patients (SCZ). Forty volunteers (SCZ n = 18, HV n = 22) underwent [11C]UCB-J positron emission tomography and proton magnetic resonance spectroscopy (1H-MRS) imaging in the left hippocampus and anterior cingulate cortex (ACC) to index [11C]UCB-J distribution volume ratio (DVR), and creatine-scaled glutamate (Glu/Cr), glutamate and glutamine (Glx/Cr) and NAA (NAA/Cr). In healthy volunteers, but not patients, [11C]UCB-J DVR was significantly positively correlated with Glu/Cr, in both the hippocampus and ACC. Furthermore, in healthy volunteers, but not patients, [11C]UCB-J DVR was significantly positively correlated with Glx/Cr, in both the hippocampus and ACC. There were no significant relationships between [11C]UCB-J DVR and NAA/Cr in the hippocampus or ACC in healthy volunteers or patients. Therefore, an appreciable proportion of the brain 1H-MRS glutamatergic signal is related to synaptic density in healthy volunteers. This relationship is not seen in schizophrenia, which, taken with lower synaptic marker levels, is consistent with lower levels of glutamatergic terminals and/or a lower proportion of glutamatergic relative to GABAergic terminals in the ACC in schizophrenia.

Kisspeptin modulates gamma-aminobutyric acid levels in the human brain

Gamma-aminobutyric acid (GABA) is a key inhibitory neurotransmitter that has been implicated in the aetiology of common mood and behavioural disorders. By employing proton magnetic resonance spectroscopy in man, we demonstrate that administration of the reproductive neuropeptide, kisspeptin, robustly decreases GABA levels in the limbic system of the human brain; specifically the anterior cingulate cortex (ACC). This finding defines a novel kisspeptin-activated GABA pathway in man, and provides important mechanistic insights into the mood and behaviour-altering effects of kisspeptin seen in rodents and humans. In addition, this work has therapeutic implications as it identifies GABA-signalling as a potential target for the escalating development of kisspeptin-based therapies for common reproductive disorders of body and mind.

Acute acetate administration increases endogenous opioid levels in the human brain: A [11C]carfentanil molecular imaging study

INTRODUCTION

A recent study has shown that acetate administration leads to a fourfold increase in the transcription of proopiomelanocortin (POMC) mRNA in the hypothalamus. POMC is cleaved to peptides, including β-endorphin, an endogenous opioid (EO) agonist that binds preferentially to the µ-opioid receptor (MOR). We hypothesised that an acetate challenge would increase the levels of EO in the human brain. We have previously demonstrated that increased EO release in the human brain can be detected using positron emission tomography (PET) with the selective MOR radioligand [¹¹C]carfentanil. We used this approach to evaluate the effects of an acute acetate challenge on EO levels in the brain of healthy human volunteers.

METHODS

Seven volunteers each completed a baseline [¹¹C]carfentanil PET scan followed by an administration of sodium acetate before a second [¹¹C]carfentanil PET scan. Dynamic PET data were acquired over 90 minutes, and corrected for attenuation, scatter and subject motion. Regional [¹¹C] carfentanil BP_ND values were then calculated using the simplified reference tissue model (with the occipital grey matter as the reference region). Change in regional EO concentration was evaluated as the change in [¹¹C]carfentanil BP_ND following acetate administration.

RESULTS

Following sodium acetate administration, 2.5-6.5% reductions in [¹¹C]carfentanil regional BP_ND were seen, with statistical significance reached in the cerebellum, temporal lobe, orbitofrontal cortex, striatum and thalamus.

CONCLUSIONS

We have demonstrated that an acute acetate challenge has the potential to increase EO release in the human brain, providing a plausible mechanism of the central effects of acetate on appetite in humans.

Test-retest variability and reference region-based quantification of 18F-BCPP-EF for imaging mitochondrial complex I in the human brain

Mitochondrial complex I (MC-I) is an essential regulator of brain bioenergetics and can be quantified in the brain using PET radioligand ¹⁸F-BCPP-EF. Here we evaluate the test-retest reproducibility of ¹⁸F-BCPP-EF in humans, and assess the use of a non-invasive quantification method (standardised uptake value ratio - SUVR). Thirty healthy volunteers had a 90-min dynamic ¹⁸F-BCPP-EF scan with arterial blood sampling, five of which received a second scan to be included in the test-retest analysis. Time-activity curves (TAC) were analysed using multilinear analysis 1 (MA1) and the two-tissue compartment model (2TC) to estimate volumes of distribution (V_T). Regional SUVR-1 values were calculated from the 70 to 90-min TAC data using the centrum semiovale as a pseudo reference region, and compared to kinetic analysis-derived outcome measures. The mean absolute test-retest variability of V_T ranged from 12% to 18% across regions. Both DVR-1and SUVR-1 had improved test-retest variability in the range 2%-7%. SUVR-1 was highly correlated with DVR-1 (r² = 0.97, n = 30). In conclusion, ¹⁸F-BCPP-EF has suitable test-retest reproducibility and can be used to quantify MC-I in clinical studies.

The Effects of Kisspeptin on Brain Response to Food Images and Psychometric Parameters of Appetite in Healthy Men

CONTEXT

The hormone kisspeptin has crucial and well-characterized roles in reproduction. Emerging data from animal models also suggest that kisspeptin has important metabolic effects including modulation of food intake. However, to date there have been no studies exploring the effects of kisspeptin on brain responses to food stimuli in humans.

OBJECTIVE

This work aims to investigate the effects of kisspeptin administration on brain responses to visual food stimuli and psychometric parameters of appetite, in healthy men.

DESIGN

A double-blinded, randomized, placebo-controlled, crossover study was conducted.

PARTICIPANTS

Participants included 27 healthy, right-handed, eugonadal men (mean ± SEM: age 26.5 ± 1.1 years; body mass index 23.9 ± 0.4 kg/m2).

INTERVENTION

Participants received an intravenous infusion of 1 nmol/kg/h of kisspeptin or rate-matched vehicle over 75 minutes.

MAIN OUTCOME MEASURES

Measurements included change in brain activity on functional magnetic resonance imaging in response to visual food stimuli and change in psychometric parameters of appetite, during kisspeptin administration compared to vehicle.

RESULTS

Kisspeptin administration at a bioactive dose did not affect brain responses to visual food stimuli or psychometric parameters of appetite compared to vehicle.

CONCLUSIONS

This is the first study in humans investigating the effects of kisspeptin on brain regions regulating appetite and demonstrates that peripheral administration of kisspeptin does not alter brain responses to visual food stimuli or psychometric parameters of appetite in healthy men. These data provide key translational insights to further our understanding of the interaction between reproduction and metabolism.

Patterns of Mitochondrial TSPO Binding in Cerebral Small Vessel Disease: An in vivo PET Study With Neuropathological Comparison

Small vessel disease (SVD) is associated with cognitive impairment in older age and be implicated in vascular dementia. Post-mortem studies show proliferation of activated microglia in the affected white matter. However, the role of inflammation in SVD pathogenesis is incompletely understood and better biomarkers are needed. We hypothesized that expression of the 18 kDa translocator protein (TSPO), a marker of microglial activation, would be higher in SVD. Positron emission tomography (PET) was performed with the second-generation TSPO ligand [11C]PBR28 in 11 participants with SVD. TSPO binding was evaluated by a two-tissue compartment model, with and without a vascular binding component, in white matter hyperintensities (WMH) and normal-appearing white matter (NAWM). In post-mortem tissue, in a separate cohort of individuals with SVD, immunohistochemistry was performed for TSPO and a pan-microglial marker Iba1. Kinetic modeling showed reduced tracer volume and blood volume fraction in WMH compared with NAWM, but a significant increase in vascular binding. Vascular [11C]PBR28 binding was also increased compared with normal-appearing white matter of healthy participants free of SVD. Immunohistochemistry showed a diffuse increase in microglial staining (with Iba1) in sampled tissue in SVD compared with control samples, but with only a subset of microglia staining positively for TSPO. Intense TSPO staining was observed in the vicinity of damaged small blood vessels, which included perivascular macrophages. The results suggest an altered phenotype of activated microglia, with reduced TSPO expression, in the areas of greatest white matter ischemia in SVD, with implications for the interpretation of TSPO PET studies in older individuals or those with vascular risk factors.

Blunted endogenous opioid release following an oral dexamphetamine challenge in abstinent alcohol-dependent individuals

Addiction has been proposed as a 'reward deficient' state, which is compensated for with substance use. There is growing evidence of dysregulation in the opioid system, which plays a key role in reward, underpinning addiction. Low levels of endogenous opioids are implicated in vulnerability for developing alcohol dependence (AD) and high mu-opioid receptor (MOR) availability in early abstinence is associated with greater craving. This high MOR availability is proposed to be the target of opioid antagonist medication to prevent relapse. However, changes in endogenous opioid tone in AD are poorly characterised and are important to understand as opioid antagonists do not help everyone with AD. We used [11C]carfentanil, a selective MOR agonist positron emission tomography (PET) radioligand, to investigate endogenous opioid tone in AD for the first time. We recruited 13 abstinent male AD and 15 control participants who underwent two [11C]carfentanil PET scans, one before and one 3 h following a 0.5 mg/kg oral dose of dexamphetamine to measure baseline MOR availability and endogenous opioid release. We found significantly blunted dexamphetamine-induced opioid release in 5 out of 10 regions-of-interest including insula, frontal lobe and putamen in AD compared with controls, but no significantly higher MOR availability AD participants compared with HC in any region. This study is comparable to our previous results of blunted dexamphetamine-induced opioid release in gambling disorder, suggesting that this dysregulation in opioid tone is common to both behavioural and substance addictions.

DREADD Activation of Pedunculopontine Cholinergic Neurons Reverses Motor Deficits and Restores Striatal Dopamine Signaling in Parkinsonian Rats

The brainstem-based pedunculopontine nucleus (PPN) traditionally associates with motor function, but undergoes extensive degeneration during Parkinson's disease (PD), which correlates with axial motor deficits. PPN-deep brain stimulation (DBS) can alleviate certain symptoms, but its mechanism(s) of action remains unknown. We previously characterized rats hemi-intranigrally injected with the proteasomal inhibitor lactacystin, as an accurate preclinical model of PD. Here we used a combination of chemogenetics with positron emission tomography imaging for in vivo interrogation of discrete neural networks in this rat model of PD. Stimulation of excitatory designer receptors exclusively activated by designer drugs expressed within PPN cholinergic neurons activated residual nigrostriatal dopaminergic neurons to produce profound motor recovery, which correlated with striatal dopamine efflux as well as restored dopamine receptor 1- and dopamine receptor 2-based medium spiny neuron activity, as was ascertained with c-Fos-based immunohistochemistry and stereological cell counts. By revealing that the improved axial-related motor functions seen in PD patients receiving PPN-DBS may be due to stimulation of remaining PPN cholinergic neurons interacting with dopaminergic ones in both the substantia nigra pars compacta and the striatum, our data strongly favor the PPN cholinergic-midbrain dopaminergic connectome as mechanism for PPN-DBS's therapeutic effects. These findings have implications for refining PPN-DBS as a promising treatment modality available to PD patients.

Mitochondrial Complex 1, Sigma 1, and Synaptic Vesicle 2A in Early Drug-Naive Parkinson's Disease

BACKGROUND

Dysfunction of mitochondrial energy generation may contribute to neurodegeneration, leading to synaptic loss in Parkinson's disease (PD). The objective of this study was to find cross-sectional and longitudinal changes in PET markers of synaptic vesicle protein 2A, sigma 1 receptor, and mitochondrial complex 1 in drug-naive PD patients.

METHODS

Twelve early drug-naive PD patients and 16 healthy controls underwent a 3-Tesla MRI and PET imaging to quantify volume of distribution of [¹¹ C]UCB-J, [¹¹ C]SA-4503, and [¹⁸ F]BCPP-EF for synaptic vesicle protein 2A, sigma 1 receptor, and mitochondrial complex 1, respectively. Nine PD patients completed approximately 1-year follow-up assessments.

RESULTS

Reduced [¹¹ C]UCB-J volume of distribution in the caudate, putamen, thalamus, brain stem, and dorsal raphe and across cortical regions was observed in drug-naive PD patients compared with healthy controls. [¹¹ C]UCB-J volume of distribution was reduced in the locus coeruleus and substantia nigra but did not reach statistical significance. No significant differences were found in [¹¹ C]SA-4503 and [¹⁸ F]BCPP-EF volume of distribution in PD compared with healthy controls. Lower brain stem [¹¹ C]UCB-J volume of distribution correlated with Movement Disorder Society Unified Parkinson's Disease Rating Scale part III and total scores. No significant longitudinal changes were identified in PD patients at follow-up compared with baseline.

CONCLUSIONS

Our findings represent the first in vivo evidence of mitochondrial, endoplasmic reticulum, and synaptic dysfunction in drug-naive PD patients. Synaptic dysfunction likely occurs early in disease pathophysiology and has relevance to symptomatology. Mitochondrial complex 1 and sigma 1 receptor pathology warrants further investigations in PD. Studies in larger cohorts with longer follow-up will determine the validity of these PET markers to track disease progression. © 2020 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Kisspeptin enhances brain responses to olfactory and visual cues of attraction in men

Successful reproduction is a fundamental physiological process that relies on the integration of sensory cues of attraction with appropriate emotions and behaviors and the reproductive axis. However, the factors responsible for this integration remain largely unexplored. Using functional neuroimaging, hormonal, and psychometric analyses, we demonstrate that the reproductive hormone kisspeptin enhances brain activity in response to olfactory and visual cues of attraction in men. Furthermore, the brain regions enhanced by kisspeptin correspond to areas within the olfactory and limbic systems that govern sexual behavior and perception of beauty as well as overlap with its endogenous expression pattern. Of key functional and behavioral significance, we observed that kisspeptin was most effective in men with lower sexual quality-of-life scores. As such, our results reveal a previously undescribed attraction pathway in humans activated by kisspeptin and identify kisspeptin signaling as a new therapeutic target for related reproductive and psychosexual disorders.

Kisspeptin enhances brain processing in response to olfactory and visual cues of attraction and is most effective in men with lower sexual quality of life.

Advances in CNS PET: the state-of-the-art for new imaging targets for pathophysiology and drug development

PURPOSE

A limit on developing new treatments for a number of central nervous system (CNS) disorders has been the inadequate understanding of the in vivo pathophysiology underlying neurological and psychiatric disorders and the lack of in vivo tools to determine brain penetrance, target engagement, and relevant molecular activity of novel drugs. Molecular neuroimaging provides the tools to address this. This article aims to provide a state-of-the-art review of new PET tracers for CNS targets, focusing on developments in the last 5 years for targets recently available for in-human imaging.

METHODS

We provide an overview of the criteria used to evaluate PET tracers. We then used the National Institute of Mental Health Research Priorities list to identify the key CNS targets. We conducted a PubMed search (search period 1st of January 2013 to 31st of December 2018), which yielded 40 new PET tracers across 16 CNS targets which met our selectivity criteria. For each tracer, we summarised the evidence of its properties and potential for use in studies of CNS pathophysiology and drug evaluation, including its target selectivity and affinity, inter and intra-subject variability, and pharmacokinetic parameters. We also consider its potential limitations and missing characterisation data, but not specific applications in drug development. Where multiple tracers were present for a target, we provide a comparison of their properties.

RESULTS AND CONCLUSIONS

Our review shows that multiple new tracers have been developed for proteinopathy targets, particularly tau, as well as the purinoceptor P2X7, phosphodiesterase enzyme PDE10A, and synaptic vesicle glycoprotein 2A (SV2A), amongst others. Some of the most promising of these include 18F-MK-6240 for tau imaging, 11C-UCB-J for imaging SV2A, 11C-CURB and 11C-MK-3168 for characterisation of fatty acid amide hydrolase, 18F-FIMX for metabotropic glutamate receptor 1, and 18F-MNI-444 for imaging adenosine 2A. Our review also identifies recurrent issues within the field. Many of the tracers discussed lack in vivo blocking data, reducing confidence in selectivity. Additionally, late-stage identification of substantial off-target sites for multiple tracers highlights incomplete pre-clinical characterisation prior to translation, as well as human disease state studies carried out without confirmation of test-retest reproducibility.

Synaptic density marker SV2A is reduced in schizophrenia patients and unaffected by antipsychotics in rats

Synaptic dysfunction is hypothesised to play a key role in schizophrenia pathogenesis, but this has not been tested directly in vivo.  Here, we investigated synaptic vesicle glycoprotein 2A (SV2A) levels and their relationship to symptoms and structural brain measures using [11C]UCB-J positron emission tomography in 18 patients with schizophrenia and 18 controls. We found significant group and group-by-region interaction effects on volume of distribution (VT). [11C]UCB-J VT was significantly lower in the frontal and anterior cingulate cortices in schizophrenia with large effect sizes (Cohen's d = 0.8-0.9), but there was no significant difference in the hippocampus. We also investigated the effects of antipsychotic drug administration on SV2A levels in Sprague-Dawley rats using western blotting, [3H]UCB-J autoradiography and immunostaining with confocal microscopy, finding no significant effects on any measure. These findings indicate that there are lower synaptic terminal protein levels in schizophrenia in vivo and that antipsychotic drug exposure is unlikely to account for them.

Reduced mu opioid receptor availability in schizophrenia revealed with [11C]-carfentanil positron emission tomographic Imaging

Negative symptoms, such as amotivation and anhedonia, are a major cause of functional impairment in schizophrenia. There are currently no licensed treatments for negative symptoms, highlighting the need to understand the molecular mechanisms underlying them. Mu-opioid receptors (MOR) in the striatum play a key role in hedonic processing and reward function and are reduced post-mortem in schizophrenia. However, it is unknown if mu-opioid receptor availability is altered in-vivo or related to negative symptoms in schizophrenia. Using [¹¹C]-carfentanil positron emission tomography (PET) scans in 19 schizophrenia patients and 20 age-matched healthy controls, here we show a significantly lower MOR availability in patients with schizophrenia in the striatum (Cohen's d = 0.7), and the hedonic network. In addition, we report a marked global increase in inter-regional covariance of MOR availability in schizophrenia, largely due to increased cortical-subcortical covariance.

Imidazoline 2 binding sites reflecting astroglia pathology in Parkinson's disease: an in vivo11C-BU99008 PET study

Astroglia are multifunctional cells that regulate neuroinflammation and maintain homeostasis within the brain. Astroglial α-synuclein-positive cytoplasmic accumulations have been shown post-mortem in patients with Parkinson's disease and therefore astroglia may play an important role in the initiation and progression of Parkinson's disease. Imidazoline 2 binding sites are expressed on activated astroglia in the cortex, hippocampus, basal ganglia and brainstem; therefore, by measuring imidazoline 2 binding site levels we can indirectly evaluate astrogliosis in patients with Parkinson's disease. Here, we aimed to evaluate the role of astroglia activation in vivo in patients with Parkinson's disease using 11C-BU99008 PET, a novel radioligand with high specificity and selectivity for imidazoline 2 binding sites. Twenty-two patients with Parkinson's disease and 14 healthy control subjects underwent 3 T MRI and a 120-min 11C-BU99008 PET scan with volume of distribution (VT) estimated using a two-tissue compartmental model with a metabolite corrected arterial plasma input function. Parkinson's disease patients were stratified into early (n = 8) and moderate/advanced (n = 14) groups according to disease stage. In early Parkinson's disease, increased 11C-BU99008 VT uptake was observed in frontal (P = 0.022), temporal (P = 0.02), parietal (P = 0.026) and occipital (P = 0.047) cortical regions compared with healthy controls. The greatest 11C-BU99008 VT increase in patients with early Parkinson's disease was observed in the brainstem (52%; P = 0.018). In patients with moderate/advanced Parkinson's disease, loss of 11C-BU99008 VT was observed across frontal (P = 0.002), temporal (P < 0.001), parietal (P = 0.039), occipital (P = 0.024), and insula (P < 0.001) cortices; and in the subcortical regions of caudate (P < 0.001), putamen (P < 0.001) and thalamus (P < 0.001); and in the brainstem (P = 0.018) compared with healthy controls. In patients with Parkinson's disease, loss of 11C-BU99008 VT in cortical regions, striatum, thalamus and brainstem correlated with longer disease duration (P < 0.05) and higher disease burden scores, measured with Movement Disorder Society Unified Parkinson's Disease Rating Scale (P < 0.05). In the subgroup of patients with moderate/advanced Parkinson's disease, loss of 11C-BU99008 VT in the frontal (r = 0.79; P = 0.001), temporal (r = 0.74; P = 0.002) and parietal (r = 0.89; P < 0.001) cortex correlated with global cognitive impairment. This study demonstrates in vivo the role of astroglia in the initiation and progression of Parkinson's disease. Reactive astroglia observed early in Parkinson's disease could reflect a neuroprotective compensatory mechanisms and pro-inflammatory upregulation in response to α-synuclein accumulation. However, as the disease progresses and significant neurodegeneration occurs, astroglia lose their reactive function and such loss in the cortex has clinical relevance in the development of cognitive impairment.

Characterization of 3 PET Tracers for Quantification of Mitochondrial and Synaptic Function in Healthy Human Brain: 18F-BCPP-EF, 11C-SA-4503, and 11C-UCB-J

Mitochondrial complex 1 is involved in maintaining brain bioenergetics; σ-1 receptor responds to neuronal stress; and synaptic vesicle protein 2A reflects synaptic integrity. Expression of each of these proteins is altered in neurodegenerative diseases. Here, we characterize the kinetic behavior of 3 PET radioligands-¹⁸F-BCPP-EF, ¹¹C-SA-4503, and ¹¹C-UCB-J-for the measurement of mitochondrial complex 1, σ-1 receptor, and synaptic vesicle protein 2A, respectively, and determine appropriate analysis workflows for their application in future studies of the in vivo molecular pathology of these diseases. Methods: Twelve human subjects underwent dynamic PET scans with each radioligand, including associated arterial blood sampling. A range of kinetic models was investigated to identify an optimal kinetic analysis method for each radioligand and a suitable acquisition duration. Results: All 3 radioligands readily entered the brain and yielded heterogeneous uptake consistent with the known distribution of the targets. The optimal models determined for the regional estimates of volume of distribution were multilinear analysis 1 (MA1) and the 2-tissue-compartment model for ¹⁸F-BCPP-EF, MA1 for ¹¹C-SA-4503, and both MA1 and the 1-tissue-compartment model for ¹¹C-UCB-J. Acquisition times of 70, 80, and 60 min for ¹⁸F-BCPP-EF, ¹¹C-SA-4503, ¹¹C-UCB-J, respectively, provided good estimates of regional volume of distribution values. An effect of age was observed on ¹⁸F-BCPP-EF and ¹¹C-UCB-J signal in the caudate. Conclusion: These ligands can be assessed for their potential to stratify patients or monitor the progression of molecular neuropathology in neurodegenerative diseases.

Comparison of phosphodiesterase 10A and dopamine transporter levels as markers of disease burden in early Parkinson's disease

BACKGROUND

Recent work has shown loss of phosphodiesterase 10A levels in middle-stage and advanced treated patients with PD, which was associated with motor symptom severity.

OBJECTIVES

To assess phosphodiesterase 10A levels in early PD and compare with loss of dopamine transporter as markers of disease burden.

METHODS

Seventy-eight subjects were included in this study (17 early de novo, 15 early l-dopa-treated, 24 moderate-advanced l-dopa-treated patients with PD, and 22 healthy controls). All participants underwent [¹¹ C]IMA107 PET, [¹¹ C]PE2I PET, and 3-Tesla MRI scan.

RESULTS

Early de novo PD patients showed loss of [¹¹ C]IMA107 and of [¹¹ C]PE2I binding in caudate and putamen (P < 0.001); early l-dopa-treated PD patients showed additional loss of [¹¹ C]IMA107 in the caudate (P < 0.001; annual decline 3.6%) and putamen (P < 0.001; annual decline 2.8%), but loss of [¹¹ C]PE2I only in the putamen (P < 0.001; annual decline 6.8%). Lower [¹¹ C]IMA107 correlated with lower [¹¹ C]PE2I in the caudate (rho = 0.51; P < 0.01) and putamen (rho = 0.53; P < 0.01). Longer disease duration correlated with lower [¹¹ C]IMA107 in the caudate (rho = -0.72; P < 0.001) and putamen (rho = -0.48; P < 0.01), and with lower [¹¹ C]PE2I only in the putamen (rho = -0.65; P < 0.001). Higher burden of motor symptoms correlated with lower [¹¹ C]IMA107 in the caudate (rho = -0.42; P < 0.05) and putamen (rho = -0.41; P < 0.05), and with lower [¹¹ C]PE2I only in the putamen (rho = -0.69; P < 0.001).

CONCLUSION

Our findings demonstrate loss of phosphodiesterase 10A levels very early in the course of PD and is associated with the gradual and progressive increase of motor symptoms. Phosphodiesterase 10A imaging shows similar potential with dopamine transporter imaging to follow disease progression. © 2019 International Parkinson and Movement Disorder Society.

The role of phosphodiesterase 4 in excessive daytime sleepiness in Parkinson's disease

INTRODUCTION

Preclinical studies suggest a link between cAMP/PKA signalling, phosphodiesterase 4 (PDE4) expression and excessive daytime sleepiness (EDS). Here, we investigated in vivo the association between PDE4 expression and EDS in Parkinson's disease (PD) patients using [¹¹C]rolipram PET and MR imaging.

METHODS

Eighteen participants, 12 PD and 6 healthy controls, underwent one [¹¹C]rolipram PET and a multi-modal MRI scan. Probabilistic tractography was performed on subjects' diffusion data to functionally parcellate the striatum according with projections to limbic cortical areas. The severity of EDS was assessed using the Epworth Sleepiness Scale (ESS). To assess PDE4 expression in PD patients with EDS, the PD cohort was divided according to the presence (n = 5) or absence (n = 7) of EDS, defined using validated cut-off of score ≥10 on the ESS as score ≥10 on the ESS.

RESULTS

PD patients with EDS showed significantly increased [¹¹C]rolipram volume of distribution (V_T) in the caudate (P = 0.029), hypothalamus (P = 0.013), hippocampus (P = 0.036) and limbic striatum (P = 0.030) compared to patients without EDS. Furthermore, higher ESS scores correlated with increased [¹¹C]rolipram V_T in the caudate (r = 0.77; P = 0.003), hypothalamus (r = 0.84; P = 0.001), hippocampus (r = 0.81; P = 0.001) and limbic subdivisions of the striatum (r = 0.80; P = 0.003).

CONCLUSION

Our findings translate into humans preclinical data indicating that EDS is associated with elevated PDE4 in regions regulating sleep. The severity of EDS in PD was associated with elevated PDE4 expression; thus, suggesting a role of PDE4 in the pathophysiology of EDS in PD.

Cerebral serotonin transporter measurements with [11C]DASB: A review on acquisition and preprocessing across 21 PET centres

Positron Emission Tomography (PET) imaging has become a prominent tool to capture the spatiotemporal distribution of neurotransmitters and receptors in the brain. The outcome of a PET study can, however, potentially be obscured by suboptimal and/or inconsistent choices made in complex processing pipelines required to reach a quantitative estimate of radioligand binding. Variations in subject selection, experimental design, data acquisition, preprocessing, and statistical analysis may lead to different outcomes and neurobiological interpretations. We here review the approaches used in 105 original research articles published by 21 different PET centres, using the tracer [¹¹C]DASB for quantification of cerebral serotonin transporter binding, as an exemplary case. We highlight and quantify the impact of the remarkable variety of ways in which researchers are currently conducting their studies, while implicitly expecting generalizable results across research groups. Our review provides evidence that the foundation for a given choice of a preprocessing pipeline seems to be an overlooked aspect in modern PET neuroscience. Furthermore, we believe that a thorough testing of pipeline performance is necessary to produce reproducible research outcomes, avoiding biased results and allowing for better understanding of human brain function.

Disease-related patterns of in vivo pathology in Corticobasal syndrome

PURPOSE

To assess disease-related patterns of in vivo pathology in 11 patients with Corticobasal Syndrome (CBS) compared to 20 healthy controls and 33 mild cognitive impairment (MCI) patients due to Alzheimer's disease.

METHODS

We assessed tau aggregates with [18F]AV1451 PET, amyloid-β depositions with [18F]AV45 PET, and volumetric microstructural changes with MRI. We validated for [18F]AV1451 standardised uptake value ratio (SUVRs) against input functions from arterial metabolites and found that SUVRs and arterial-derived distribution volume ratio (DVRs) provide equally robust measures of [18F]AV1451 binding.

RESULTS

CBS patients showed increases in [18F]AV1451 SUVRs in parietal (P < 0.05) and frontal (P < 0.05) cortices in the affected hemisphere compared to healthy controls and in precentral (P = 0.008) and postcentral (P = 0.034) gyrus in the affected hemisphere compared to MCI patients. Our data were confirmed at the histopathological level in one CBS patient who underwent brain biopsy and showed sparse tau pathology in the parietal cortex co-localizing with increased [18F]AV1451 signal. Cortical and subcortical [18F]AV45 uptake was within normal levels in CBS patients. In parietal and frontal cortices of the most affected hemisphere we found also grey matter loss (P < 0.05), increased mean diffusivity (P < 0.05) and decreased fractional anisotropy (P < 0.05) in CBS patients compared to healthy controls and MCI patients. Grey matter loss and white matter changes in the precentral gyrus of CBS patients were associated with worse motor symptoms.

CONCLUSIONS

Our findings demonstrate disease-related patterns of in vivo tau and microstructural pathology in the absence of amyloid-β, which distinguish CBS from non-affected individuals and MCI patients.

Accuracy and reliability of [11C]PBR28 specific binding estimated without the use of a reference region

[¹¹C]PBR28 is a positron emission tomography radioligand used to examine the expression of the 18 kDa translocator protein (TSPO). TSPO is located in glial cells and can function as a marker for immune activation. Since TSPO is expressed throughout the brain, no true reference region exists. For this reason, an arterial input function is required for accurate quantification of [¹¹C]PBR28 binding and the most common outcome measure is the total distribution volume (V_T). Notably, V_T reflects both specific binding and non-displaceable binding. Therefore, estimates of specific binding, such as binding potential (e.g. BP_ND) and specific distribution volume (V_S) should theoretically be more sensitive to underlying differences in TSPO expression. It is unknown, however, if unbiased and accurate estimates of these outcome measures are obtainable for [¹¹C]PBR28. The Simultaneous Estimation (SIME) method uses time-activity-curves from multiple brain regions with the aim to obtain a brain-wide estimate of the non-displaceable distribution volume (V_ND), which can subsequently be used to improve the estimation of BP_ND and V_S. In this study we evaluated the accuracy of SIME-derived V_ND, and the reliability of resulting estimates of specific binding for [¹¹C]PBR28, using a combination of simulation experiments and in vivo studies in healthy humans. The simulation experiments, based on data from 54 unique [¹¹C]PBR28 examinations, showed that V_ND values estimated using SIME were both precise and accurate. Data from a pharmacological competition challenge (n = 5) showed that SIME provided V_ND values that were on average 19% lower than those obtained using the Lassen plot, but similar to values obtained using the Likelihood-Estimation of Occupancy technique. Test-retest data (n = 11) showed that SIME-derived V_S values exhibited good reliability and precision, while larger variability was observed in SIME-derived BP_ND values. The results support the use of SIME for quantifying specific binding of [¹¹C]PBR28, and suggest that V_S can be used in complement to the conventional outcome measure V_T. Additional studies in patient cohorts are warranted.

Quantification of human brain PDE4 occupancy by GSK356278: A [11C](R)-rolipram PET study

We characterized the relationship between the plasma concentration of the phospodiesterase (PDE)-4 inhibitor GSK356278 and occupancy of the PDE4 enzyme in the brain of healthy volunteers, using the positron emission tomography (PET) tracer [¹¹C](R)-rolipram. To this end, PET scans were acquired in eight male volunteers before and at 3 and 8 h after a single 14 mg oral dose of GSK356278. A metabolite-corrected arterial input function was used in conjunction with the dynamic PET emission data to estimate volumes of distribution (V_T) from a two-tissue compartment model. The administration of GSK356278 reduced [¹¹C](R)-rolipram whole brain V_T by 17% at 3 h post-dose (p = 0.01) and by 4% at 8 h post-dose. The mean plasma C_max was 42.3 ng/ml, leading to a PDE4 occupancy of 48% at T_max. The in vivo affinity of GSK356278 was estimated as EC₅₀ = 46 ± 3.6 ng/ml. We present the first report of a direct estimation of PDE4 blockade in the living human brain. In vivo affinity of GSK356278 for the PDE4, estimated in this early phase study, was combined with GSK356278 safety and tolerability data to decide on a therapeutic dose for future clinical development.

PDE10A and ADCY5 mutations linked to molecular and microstructural basal ganglia pathology

BACKGROUND

Striatal cyclic adenosine monophosphate activity modulates movement and is determined from the balance between its synthesis by adenylate cyclase 5 (ADCY5) and its degradation by phosphodiesterase 10A (PDE10A).

OBJECTIVE

We assessed the integrity of striatocortical pathways, in vivo, in 2 genetic hyperkinetic disorders caused by ADCY5 and PDE10A mutations.

METHODS

We studied 6 subjects with PDE10A and ADCY5 mutations using [¹¹ C]IMA107 PET, [¹²³ I]FP-CIT Single-photon emission computed tomography (SPECT) and multimodal MRI to investigate PDE10A and dopamine transporter availability, neuromelanin-containing neurons, and microstructural white and gray matter changes, respectively.

RESULTS

We found that PDE10A and ADCY5 mutations were associated with decreased PDE10A expression in the striatum and globus pallidus, decreased dopamine transporter expression in the striatum, loss of substantia nigra neuromelanin-containing neurons, and microstructural white and gray matter changes within the substantia nigra, striatum, thalamus, and frontoparietal cortices.

CONCLUSIONS

Our findings indicate an association between PDE10A and ADCY5 mutations and pre/postsynaptic molecular changes, substantia nigra damage, and white and gray matter changes within the striatocortical pathways. © 2018 International Parkinson and Movement Disorder Society.

Modulations of human resting brain connectivity by kisspeptin enhance sexual and emotional functions

BACKGROUND

Resting brain connectivity is a crucial component of human behavior demonstrated by disruptions in psychosexual and emotional disorders. Kisspeptin, a recently identified critical reproductive hormone, can alter activity in certain brain structures but its effects on resting brain connectivity and networks in humans remain elusive.

METHODS

We determined the effects of kisspeptin on resting brain connectivity (using functional neuroimaging) and behavior (using psychometric analyses) in healthy men, in a randomized double-blinded 2-way placebo-controlled study.

RESULTS

Kisspeptin's modulation of the default mode network (DMN) correlated with increased limbic activity in response to sexual stimuli (globus pallidus r = 0.500, P = 0.005; cingulate r = 0.475, P = 0.009). Furthermore, kisspeptin's DMN modulation was greater in men with less reward drive (r = -0.489, P = 0.008) and predicted reduced sexual aversion (r = -0.499, P = 0.006), providing key functional significance. Kisspeptin also enhanced key mood connections including between the amygdala-cingulate, hippocampus-cingulate, and hippocampus-globus pallidus (all P < 0.05). Consistent with this, kisspeptin's enhancement of hippocampus-globus pallidus connectivity predicted increased responses to negative stimuli in limbic structures (including the thalamus and cingulate [all P < 0.01]).

CONCLUSION

Taken together, our data demonstrate a previously unknown role for kisspeptin in the modulation of functional brain connectivity and networks, integrating these with reproductive hormones and behaviors. Our findings that kisspeptin modulates resting brain connectivity to enhance sexual and emotional processing and decrease sexual aversion, provide foundation for kisspeptin-based therapies for associated disorders of body and mind.

FUNDING

NIHR, MRC, and Wellcome Trust.

Evaluation of 11C-BU99008, a PET Ligand for the Imidazoline2 Binding Site in Human Brain

The imidazoline2 binding site (I2BS) is thought to be expressed in glia and implicated in the regulation of glial fibrillary acidic protein. A PET ligand for this target would be important for the investigation of neurodegenerative and neuroinflammatory diseases. 11C-BU99008 has previously been identified as a putative PET radioligand. Here, we present the first in vivo characterization of this PET radioligand in humans and assess its test-retest reproducibility. Methods: Fourteen healthy male volunteers underwent dynamic PET imaging with 11C-BU99008 and arterial sampling. Six subjects were used in a test-retest assessment, and 8 were used in a pharmacologic evaluation, undergoing a second or third heterologous competition scan with the mixed I2BS/α2-adrenoceptor drug idazoxan (n = 8; 20, 40, 60, and 80 mg) and the mixed irreversible monoamine oxidase type A/B inhibitor isocarboxazid (n = 4; 50 mg). Regional time-activity data were generated from arterial plasma input functions corrected for metabolites using the most appropriate model to derive the outcome measure VT (regional distribution volume). All image processing and kinetic analyses were performed in MIAKAT. Results: Brain uptake of 11C-BU99008 was good, with reversible kinetics and a heterogeneous distribution consistent with known I2BS expression. Model selection criteria indicated that the 2-tissue-compartment model was preferred. VT estimates were high in the striatum (105 ± 21 mL⋅cm-3), medium in the cingulate cortex (62 ± 10 mL⋅cm-3), and low in the cerebellum (41 ± 7 mL⋅cm-3). Test-retest reliability was reasonable. The uptake was dose-dependently reduced throughout the brain by pretreatment with idazoxan, with an average block across all regions of about 60% (VT, ∼30 mL⋅cm-3) at the highest dose (80 mg). The median effective dose for idazoxan was 28 mg. Uptake was not blocked by pretreatment with the monoamine oxidase inhibitor isocarboxazid. Conclusion:11C-BU99008 in human PET studies demonstrates good brain delivery, reversible kinetics, heterogeneous distribution, specific binding signal consistent with I2BS distribution, and good test-retest reliability.

First evaluation of PET-based human biodistribution and radiation dosimetry of 11C-BU99008, a tracer for imaging the imidazoline2 binding site

BACKGROUND

We measured whole body distribution of 11C-BU99008, a new PET biomarker for non-invasive identification of the imidazoline2 binding site. The purpose of this phase I study was to evaluate the biodistribution and radiation dosimetry of 11C-BU99008 in healthy human subjects.

METHODS

A single bolus injection of 11C-BU99008 (296 ± 10.5 MBq) was administered to four healthy subjects who underwent whole-body PET/CT over 120 min from the cranial vertex to the mid-thigh. Volumes of interest were drawn around visually identifiable source organs to generate time-activity curves (TAC). Residence times were determined from time-activity curves. Absorbed doses to individual organs and the whole body effective dose were calculated using OLINDA/EXM 1.1 for each subject.

RESULTS

The highest measured activity concentration was in the kidney and spleen. The longest residence time was in the muscle at 0.100 ± 0.023 h, followed by the liver at 0.067 ± 0.015 h and lungs at 0.052 ± 0.010 h. The highest mean organ absorbed dose was within the heart wall (0.028 ± 0.002 mGy/MBq), followed by the kidneys (0.026 ± 0.005 mGy/MBq). The critical organ was the heart wall. The total mean effective dose averaged over subjects was estimated to be 0.0056 ± 0.0004 mSv/MBq for an injection of 11C-BU99008.

CONCLUSIONS

The biodistribution of 11C-BU99008 has been shown here for the first time in humans. Our dosimetry data showed the total mean effective dose over all subjects was 0.0056 ± 0.0004 mSv/MBq, which would result in a total effective dose of 1.96 mSv for a typical injection of 350 MBq of 11C-BU99008. The effective dose is not appreciably different from those obtained with other 11C tracers.