Recent developments on PET radiotracers for TSPO and their applications in neuroimaging

The 18 kDa translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor, is predominately localized to the outer mitochondrial membrane in steroidogenic cells. Brain TSPO expression is relatively low under physiological conditions, but is upregulated in response to glial cell activation. As the primary index of neuroinflammation, TSPO is implicated in the pathogenesis and progression of numerous neuropsychiatric disorders and neurodegenerative diseases, including Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD), multiple sclerosis (MS), major depressive disorder (MDD) and obsessive compulsive disorder (OCD). In this context, numerous TSPO-targeted positron emission tomography (PET) tracers have been developed. Among them, several radioligands have advanced to clinical research studies. In this review, we will overview the recent development of TSPO PET tracers, focusing on the radioligand design, radioisotope labeling, pharmacokinetics, and PET imaging evaluation. Additionally, we will consider current limitations, as well as translational potential for future application of TSPO radiopharmaceuticals. This review aims to not only present the challenges in current TSPO PET imaging, but to also provide a new perspective on TSPO targeted PET tracer discovery efforts. Addressing these challenges will facilitate the translation of TSPO in clinical studies of neuroinflammation associated with central nervous system diseases.

Serotonergic dysregulation is linked to sleep problems in Parkinson's disease

Introduction

Sleep disturbances are common non-motor symptoms in Parkinson's disease (PD). Experimental studies suggest involvement of the serotonergic system in the regulation of sleep and arousal. Using [¹¹C]DASB positron emission tomography, a marker of serotonin transporter availability, we investigated whether sleep dysfunction is associated with serotonergic dysfunction in PD.

Methods

We studied 14 PD patients with sleep dysfunction, 14 PD without sleep dysfunction, and 12 healthy controls. Groups were matched for age, disease duration, severity of motor symptoms, daily intake of levodopa equivalent units, body-mass-index, depression and fatigue. [¹¹C]DASB non-displaceable binding potential (BP_ND) was calculated for regions with a role in the regulation of sleep and arousal.

Results

[¹¹C]DASB BP_ND was reduced by 32–49% in PD patients with sleep dysfunction, and 14–25% in PD without sleep dysfunction, compared to healthy controls. PD patients with sleep dysfunction had lower [¹¹C]DASB BP_ND in caudate (P < 0.01), putamen (P < 0.001), ventral striatum (P < 0.001), thalamus (P < 0.05), hypothalamus (P < 0.001) and raphe nuclei (P < 0.01), compared to PD without sleep dysfunction. Higher severity of sleep symptoms (assessed with Parkinson Disease Sleep Scale) correlated with lower [¹¹C]DASB binding in caudate (r = 0.77; P < 0.001), putamen (r = 0.84; P < 0.001), ventral striatum (r = 0.86; P < 0.001), thalamus (r = 0.79; P < 0.001), hypothalamus (r = 0.90; P < 0.001) and raphe nuclei (r = 0.83; P < 0.001).

Conclusions

Our findings demonstrate that sleep dysfunction in PD is associated with reduced serotonergic function in the midbrain raphe, basal ganglia and hypothalamus. Strategies to increase serotonin levels in the brain could be a promising approach to treat sleep dysfunction in PD, and may also have relevance in other neurodegenerative disorders.

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Reduced PET [¹¹C]DASB binding in Parkinson patients with sleep disturbances

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PD with sleep disturbances shows loss of serotonin in the striatum, raphe and hypothalamus.

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Loss of serotonin correlated with severity of sleep symptoms in PD patients.

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Serotonergic dysfunction could contribute to pathophysiology of sleep disturbances.

Chronic exposure to dopamine agonists affects the integrity of striatal D2 receptors in Parkinson's patients

We aimed to investigate the integrity and clinical relevance of striatal dopamine receptor type-2 (D₂R) availability in Parkinson's disease (PD) patients. We studied 68 PD patients, spanning from early to advanced disease stages, and 12 healthy controls. All participants received one [¹¹C]raclopride PET scan in an OFF medication condition for quantification of striatal D₂R availability in vivo. Parametric images of [¹¹C]raclopride non-displaceable binding potential were generated from the dynamic [¹¹C]raclopride scans using implementation of the simplified reference tissue model with cerebellum as the reference tissue. PET data were interrogated for correlations with clinical data related to disease burden and dopaminergic treatment. PD patients showed a mean 16.7% decrease in caudate D₂R and a mean 3.5% increase in putaminal D₂R availability compared to healthy controls. Lower caudate [¹¹C]raclopride BP_ND correlated with longer PD duration. PD patients on dopamine agonist treatment had 9.2% reduced D₂R availability in the caudate and 12.8% in the putamen compared to PD patients who never received treatment with dopamine agonists. Higher amounts of lifetime dopamine agonist therapy correlated with reduced D₂Rs availability in both caudate and putamen. No associations between striatal D₂R availability and levodopa treatment and dyskinesias were found. In advancing PD the caudate and putamen D₂R availability are differentially affected. Chronic exposure to treatment with dopamine agonists, but no levodopa, suppresses striatal D₂R availability, which may have relevance to output signaling to frontal lobes and the occurrence of executive deficits, but not dyskinesias.

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D₂R in caudate and putamen are differentially affected in PD.

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Loss of D₂R in caudate correlates with longer disease duration.

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Dopamine agonists treatment, but not levodopa, suppresses caudate and putamen D₂Rs.

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No association between striatal D₂R availability, levodopa treatment and dyskinesia.