Occupancy of pramipexole (Sifrol) at cerebral dopamine D2/3 receptors in Parkinson's disease patients

The role of neuroimaging in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder that affects millions of people worldwide. Two hallmarks of PD are the accumulation of alpha-synuclein and the loss of dopaminergic neurons in the brain. There is no cure for PD, and all existing treatments focus on alleviating the symptoms. PD diagnosis is also based on the symptoms, such as abnormalities of movement, mood, and cognition observed in the patients. Molecular imaging methods such as magnetic resonance imaging (MRI), single-photon emission computed tomography (SPECT), and positron emission tomography (PET) can detect objective alterations in the neurochemical machinery of the brain and help diagnose and study neurodegenerative diseases. This review addresses the application of functional MRI, PET, and SPECT in PD patients. We provide an overview of the imaging targets, discuss the rationale behind target selection, the agents (tracers) with which the imaging can be performed, and the main findings regarding each target's state in PD. Molecular imaging has proven itself effective in supporting clinical diagnosis of PD and has helped reveal that PD is a heterogeneous disorder, which has important implications for the development of future therapies. However, the application of molecular imaging for early diagnosis of PD or for differentiation between PD and atypical parkinsonisms has remained challenging. The final section of the review is dedicated to new imaging targets with which one can detect the PD-related pathological changes upstream from dopaminergic degeneration. The foremost of those targets is alpha-synuclein. We discuss the progress of tracer development achieved so far and challenges on the path toward alpha-synuclein imaging in humans.

Study protocol for a randomised placebo-controlled trial of pramipexole in addition to mood stabilisers for patients with treatment resistant bipolar depression (the PAX-BD study)

BACKGROUND

Treatment Resistant Bipolar Depression (TRBD) is a major contributor to the burden of disease associated with Bipolar Disorder (BD). Treatment options for people experiencing bipolar depression are limited to three interventions listed by National Institute for Health and Care: lamotrigine, quetiapine and olanzapine, of which the latter two are often not well tolerated. The majority of depressed people with BD are therefore prescribed antidepressants despite limited efficacy. This demonstrates an unmet need for additional interventions. Pramipexole has been shown to improve mood symptoms in animal models of depression, in people with Parkinson's Disease and two proof of principle trials of pramipexole for people with BD who are currently depressed.

METHODS

The PAX-BD study, funded by the United Kingdom (UK) National Institute for Health Research, aims to extend previous findings by assessing the efficacy, safety and health economic impact of pramipexole in addition to mood stabilisers for patients with TRBD. A randomised, double-blind, placebo controlled design is conducted in a naturalistic UK National Health Service setting. An internal pilot study to examine feasibility and acceptability of the study design is included. Participants with TRBD are screened from National Health Service secondary care services in up to 40 mental health trusts in the UK, with the aim of recruiting approximately 414 participants into a pre-randomisation phase to achieve a target of 290 randomised participants. Primary safety and efficacy measures are at 12 weeks following randomisation, with follow up of participants to 52 weeks. The primary outcome is depressive symptoms as measured by Quick Inventory for Depressive Symptomatology - Self Report. Secondary outcomes include changes in anxiety, manic symptoms, tolerability, acceptability, quality of life and cost-effectiveness. Outcome measures are collected remotely using self-report tools implemented online, and observer-rated assessments conducted via telephone. ANCOVA will be used to examine the difference in rating scale scores between treatment arms, and dependent on compliance in completion of weekly self-report measures. A mixed effects linear regression model may also be used to account for repeated measures.

TRIAL REGISTRATION

ISRCTN72151939. Registered on 28 August 2019, http://www.isrctn.com/ISRCTN72151939 Protocol Version: 04-FEB-2021, Version 9.0.

Symptoms of Medication Withdrawal in Parkinson's Disease: Considerations for Informed Consent in Patient-Oriented Research

INTRODUCTION

Dopamine medication withdrawal in Parkinson's disease (PD) is commonly employed in clinical practice and can be required for participation in research studies. When asked to withdraw from medications, participants often enquire as to what symptoms they should expect.

OBJECTIVES

This study sought to improve the informed consent process by identifying patient-reported symptoms when dopamine treatment is withheld. We also sought to provide clinical guidance regarding the extent of these symptoms and consider participant willingness to undergo these assessments.

METHODS

Participants were recruited from community-based PD programs and support groups in Nashville, Tennessee, USA. A patient-based questionnaire determined the frequency and severity of motor and nonmotor symptoms. The questionnaire also assessed whether patients would be willing to abstain from medication at a future date and under what circumstances.

RESULTS

A total of 31/90 participants reported willingness to withdraw from dopaminergic medications for clinical or research purposes. Tremor, walking, and balance were the most common motor symptoms that worsened during this time. Sleep dysfunction, constipation, and tremor were noted as the most severe symptoms. Of note, 10% of participants indicated that they would not be willing to go off medications again, suggesting that a minority of patients find this to be most discomforting. When prompted for a reason why participants would be willing to come off of their medications again, "for clinical purposes" was selected the most.

CONCLUSIONS

Study teams should list these symptoms in the applications to their institutional review board and in the informed consent to provide guidance for participants.

Dopamine receptor density and white mater integrity: 18F-fallypride positron emission tomography and diffusion tensor imaging study in healthy and schizophrenia subjects

Dopaminergic dysfunction and changes in white matter integrity are among the most replicated findings in schizophrenia. A modulating role of dopamine in myelin formation has been proposed in animal models and healthy human brain, but has not yet been systematically explored in schizophrenia. We used diffusion tensor imaging and ¹⁸F-fallypride positron emission tomography in 19 healthy and 25 schizophrenia subjects to assess the relationship between gray matter dopamine D₂/D₃ receptor density and white matter fractional anisotropy in each diagnostic group. AFNI regions of interest were acquired for 42 cortical Brodmann areas and subcortical gray matter structures as well as stereotaxically placed in representative white matter areas implicated in schizophrenia neuroimaging literature. Welch's t-test with permutation-based p value adjustment was used to compare means of z-transformed correlations between fractional anisotropy and ¹⁸F-fallypride binding potentials in hypothesis-driven regions of interest in the diagnostic groups. Healthy subjects displayed an extensive pattern of predominantly negative correlations between ¹⁸F-fallypride binding across a range of cortical and subcortical gray matter regions and fractional anisotropy in rostral white matter regions (internal capsule, frontal lobe, anterior corpus callosum). These patterns were disrupted in subjects with schizophrenia, who displayed significantly weaker overall correlations as well as comparatively scant numbers of significant correlations with the internal capsule and frontal (but not temporal) white matter, especially for dopamine receptor density in thalamic nuclei. Dopamine D₂/D₃ receptor density and white matter integrity appear to be interrelated, and their decreases in schizophrenia may stem from hyperdopaminergia with dysregulation of dopaminergic impact on axonal myelination.

Molecular imaging of schizophrenia: Neurochemical findings in a heterogeneous and evolving disorder

The past four decades have seen enormous efforts placed on a search for molecular markers of schizophrenia using positron emission tomography (PET) and single photon emission computed tomography (SPECT). In this narrative review, we cast a broad net to define and summarize what researchers have learned about schizophrenia from molecular imaging studies. Some PET studies of brain energy metabolism with the glucose analogue FDGhave have shown a hypofrontality defect in patients with schizophrenia, but more generally indicate a loss of metabolic coherence between different brain regions. An early finding of significantly increased striatal trapping of the dopamine synthesis tracer FDOPA has survived a meta-analysis of many replications, but the increase is not pathognomonic of the disorder, since one half of patients have entirely normal dopamine synthesis capacity. Similarly, competition SPECT studies show greater basal and amphetamine-evoked dopamine occupancy at post-synaptic dopamine D_2/3 receptors in patients with schizophrenia, but the difference is likewise not pathognomonic. We thus propose that molecular imaging studies of brain dopamine indicate neurochemical heterogeneity within the diagnostic entity of schizophrenia. Occupancy studies have established the relevant target engagement by antipsychotic medications at dopamine D_2/3 receptors in living brain. There is evidence for elevated frontal cortical dopamine D₁ receptors, especially in relation to cognitive deficits in schizophrenia. There is a general lack of consistent findings of abnormalities in serotonin markers, but some evidence for decreased levels of nicotinic receptors in patients. There are sparse and somewhat inconsistent findings of reduced binding of muscarinic, glutamate, and opioid receptors ligands, inconsistent findings of microglial activation, and very recently, evidence of globally reduced levels of synaptic proteins in brain of patients. One study reports a decline in histone acetylase binding that is confined to the dorsolateral prefrontal cortex. In most contexts, the phase of the disease and effects of past or present medication can obscure or confound PET and SPECT findings in schizophrenia.

Classics in Chemical Neuroscience: Pramipexole

Pramipexole was first manufactured by Pharmacia and Upjohn in July 1997 under the United States brand names of Mirapex and Mirapex ER. Pramipexole is classified as a nonergoline aminobenzothiazole compound that selectively agonizes the dopamine D₂-like receptor subfamily, which includes the D₂, D₃, and D₄ receptor subtypes. Pramipexole is a unique compound in its therapeutic potential because it has D₃-preferring properties. The D₃ receptor target has implications in both motor and psychiatric symptoms of Parkinson's disease, restless leg syndrome, and bipolar and unipolar depression. Currently, pramipexole is approved to treat signs and symptoms of idiopathic Parkinson's disease and moderate to severe symptoms of primary restless leg syndrome. Parkinson's disease is characterized by tremor, bradykinesia, rigidity, gait disorders, and a disturbance of posture due to a decrease in dopamine stores in the substantia nigra with the consequent presence of Lewy bodies. Restless leg syndrome is a neurologic sensorimotor disorder characterized by a compelling urge to move the body/limb to relieve this uncomfortable sensation. In this Review, we will discuss the synthesis, drug metabolism, pharmacology, adverse effects, history, and the importance of pramipexole to neuroscience and describe its role in therapy.

The role of dopamine pharmacotherapy and addiction-like behaviors in Parkinson's disease

Addictions involve a spectrum of behaviors that encompass features of impulsivity and compulsivity, herein referred to as impulsive-compulsive spectrum disorders (ICSDs). The etiology of ICSDs likely involves a complex interplay among neurobiological, psychological and social risk factors. Neurobiological risk factors include the status of the neuroanatomical circuits that govern ICSDs. These circuits can be altered by disease, as well as exogenous influences such as centrally-acting pharmacologics. The 'poster child' for this scenario is Parkinson's disease (PD) medically managed by pharmacological treatments. PD is a progressive neurodegenerative disease that involves a gradual loss of dopaminergic neurons largely within nigrostriatal projections. Replacement therapy includes dopamine receptor agonists that directly activate postsynaptic dopamine receptors (bypassing the requirement for functioning presynaptic terminals). Some clinically useful dopamine agonists, e.g., pramipexole and ropinirole, exhibit high affinity for the D2/D3 receptor subtypes. These agonists provide excellent relief from PD motor symptoms, but some patients exhibit debilitating ICSD. Teasing out the neuropsychiatric contribution of PD-associated pathology from the drugs used to treat PD motor symptoms is challenging. In this review, we posit that modern clinical and preclinical research converge on the conclusion that dopamine replacement therapy can mediate addictions in PD and other neurological disorders. We provide five categories of evidences that align with this position: (i) ICSD prevalence is greater with D2/D3 receptor agonist therapy vs PD alone. (ii) Capacity of dopamine replacement therapy to produce addiction-like behaviors is independent of disease for which the therapy is being provided. (iii) ICSD-like behaviors are recapitulated in laboratory rats with and without PD-like pathology. (iv) Behavioral pathology co-varies with drug exposure. (v) ICSD Features of ICSDs are consistent with agonist pharmacology and neuroanatomical substrates of addictions. Considering the underpinnings of ICSDs in PD should not only help therapeutic decision-making in neurological disorders, but also apprise ICSDs in general.

Efficacy of pramipexole combined with levodopa for Parkinson's disease treatment and their effects on QOL and serum TNF- levels

PURPOSE

To investigate the efficacy of combining the dopamine receptor agonist pramipexole with levodopa for Parkinson's disease (PD) treatment and to measure their effects on quality of life and tumor necrosis factor (TNF)-α levels in PD patients.

BASIC PROCEDURE

In total, 160 PD patients who were admitted to our hospital were equally randomized into a control treatment group (levodopa alone) and the study group (pramipexole combined with levodopa). Both groups were treated for 12 weeks.

FINDINGS

After treatment, scores from the Unified Parkinson's Disease Rating Scales (1-3), the Hamilton Depression Scale, and the Parkinson's Disease Questionnaire (PDQ-39) were significantly decreased in both groups, whereas Mini-Mental State Examination scores were significantly increased. After treatment, the study group had significantly lower scores for all scales except the Mini-Mental State Examination, for which those who received combined treatment had significantly higher scores than the control group. The incidence of adverse reactions was significantly lower in the study group than in the control group. Furthermore, after treatment, serum TNF-α levels were significantly decreased in both groups compared with pre-treatment levels.

CONCLUSION

Pramipexole combined with levodopa relieved PD symptoms and improved the quality of life of PD patients, potentially by suppressing serum TNF-α levels.

Potential Effect of Prolonged Sevoflurane Anesthesia on the Kinetics of [11C]Raclopride in Non-human Primates

Purpose

Positron emission tomography (PET) in non-human primates (NHP) is commonly performed under anesthesia, with sevoflurane being a widely used inhaled anesthetic. PET measurement in NHP can be repeated, and a difference in radioligand kinetics has previously been observed between the first and second PET measurement on the same day using sevoflurane anesthesia. In this study, we evaluated the effect of prolonged sevoflurane anesthesia on kinetics and binding potential (BP_ND) of [¹¹C]raclopride in NHP.

Procedures

Three cynomolgus monkeys underwent two to three PET measurements with [¹¹C]raclopride under continuous sevoflurane anesthesia on the same day. The concentration of sevoflurane was adjusted according to the general conditions and safety parameters of the NHP. Time to peak (TTP) radioactivity in the striatum was estimated from time-activity curves (TACs). The BP_ND in the striatum was calculated by the simplified reference tissue model using the cerebellum as reference region.

Results

In each NHP, the TTP became shorter in the later PET measurements than in the first one. Across all measurements (n = 8), concentration of sevoflurane correlated with TTP (Spearman’s ρ = − 0.79, p = 0.03), but not with BP_ND (ρ = − 0.25, p = 0.55).

Conclusions

These data suggest that sevoflurane affects the shape of TACs but has no evident effect on BP_ND in consecutive PET measurements.

Nigrostriatal and Mesolimbic D2/3 Receptor Expression in Parkinson's Disease Patients with Compulsive Reward-Driven Behaviors

The nigrostriatal and mesocorticolimbic dopamine networks regulate reward-driven behavior. Regional alterations to mesolimbic dopamine D_2/3 receptor expression are described in drug-seeking and addiction disorders. Parkinson's disease (PD) patients are frequently prescribed D₂-like dopamine agonist (DAgonist) therapy for motor symptoms, yet a proportion develop clinically significant behavioral addictions characterized by impulsive and compulsive behaviors (ICBs). Until now, changes in D_2/3 receptor binding in both striatal and extrastriatal regions have not been concurrently quantified in this population. We identified 35 human PD patients (both male and female) receiving DAgonist therapy, with (n = 17) and without (n = 18) ICBs, matched for age, disease duration, disease severity, and dose of dopamine therapy. In the off-dopamine state, all completed PET imaging with [¹⁸F]fallypride, a high affinity D₂-like receptor ligand that can measure striatal and extrastriatal D_2/3 nondisplaceable binding potential (BP_ND). Striatal differences between ICB+/ICB- patients localized to the ventral striatum and putamen, where ICB+ subjects had reduced BP_ND In this group, self-reported severity of ICB symptoms positively correlated with midbrain D_2/3 receptor BP_ND Group differences in regional D_2/3 BP_ND relationships were also notable: ICB+ (but not ICB-) patients expressed positive correlations between midbrain and caudate, putamen, globus pallidus, and amygdala BP_NDs. These findings support the hypothesis that compulsive behaviors in PD are associated with reduced ventral and dorsal striatal D_2/3 expression, similar to changes in comparable behavioral disorders. The data also suggest that relatively preserved ventral midbrain dopaminergic projections throughout nigrostriatal and mesolimbic networks are characteristic of ICB+ patients, and may account for differential DAgonist therapeutic response.SIGNIFICANCE STATEMENT The biologic determinants of compulsive reward-based behaviors have broad clinical relevance, from addiction to neurodegenerative disorders. Here, we address biomolecular distinctions in Parkinson's disease patients with impulsive compulsive behaviors (ICBs). This is the first study to image a large cohort of ICB+ patients using positron emission tomography with [18F]fallypride, allowing quantification of D_2/3 receptors throughout the mesocorticolimbic network. We demonstrate widespread differences in dopaminergic networks, including (1) D2-like receptor distinctions in the ventral striatum and putamen, and (2) a preservation of widespread dopaminergic projections emerging from the midbrain, which is associated with the severity of compulsive behaviors. This clearly illustrates the roles of D_2/3 receptors and medication effects in maladaptive behaviors, and localizes them specifically to nigrostriatal and extrastriatal regions.

[18F]fallypride characterization of striatal and extrastriatal D2/3 receptors in Parkinson's disease

Parkinson's disease (PD) is characterized by widespread degeneration of monoaminergic (especially dopaminergic) networks, manifesting with a number of both motor and non-motor symptoms. Regional alterations to dopamine D2/3 receptors in PD patients are documented in striatal and some extrastriatal areas, and medications that target D2/3 receptors can improve motor and non-motor symptoms. However, data regarding the combined pattern of D2/3 receptor binding in both striatal and extrastriatal regions in PD are limited. We studied 35 PD patients off-medication and 31 age- and sex-matched healthy controls (HCs) using PET imaging with [18F]fallypride, a high affinity D2/3 receptor ligand, to measure striatal and extrastriatal D2/3 nondisplaceable binding potential (BPND). PD patients completed PET imaging in the off medication state, and motor severity was concurrently assessed. Voxel-wise evaluation between groups revealed significant BPND reductions in PD patients in striatal and several extrastriatal regions, including the locus coeruleus and mesotemporal cortex. A region-of-interest (ROI) based approach quantified differences in dopamine D2/3 receptors, where reduced BPND was noted in the globus pallidus, caudate, amygdala, hippocampus, ventral midbrain, and thalamus of PD patients relative to HC subjects. Motor severity positively correlated with D2/3 receptor density in the putamen and globus pallidus. These findings support the hypothesis that abnormal D2/3 expression occurs in regions related to both the motor and non-motor symptoms of PD, including areas richly invested with noradrenergic neurons.

Antidopaminergic medication in healthy subjects provokes subjective and objective mental impairments tightly correlated with perturbation of biogenic monoamine metabolism and prolactin secretion

OBJECTIVES

Off-label prescription of antipsychotics to patients without psychotic symptoms has become a routine matter for many psychiatrists and also some general practitioners. Nonetheless, little is known about the possibly detrimental effects of antidopaminergic medications on general psychopathology, subjective mental state, or a possible association with physiological parameters in nonpsychotic individuals.

METHODS

In this randomized, single-blinded study, groups of healthy volunteers (n=18) received low doses of reserpine, aripiprazole, haloperidol, or placebo on 7 successive days. Relevant physiological parameters (plasma prolactin, concentrations of catecholamine metabolites in plasma, and 24-hour urine) and each subject's mental state (Positive and Negative Syndrome Scale, Hamilton Rating Scale for Depression, visual analogue scale, Beck Depression Inventory II) were assessed at the start and end of the trial.

RESULTS

Of the three active treatments, only reserpine caused a significant increase in some plasma- and urine-catecholamine metabolites, but all three medications evoked objective and subjective changes in general psychopathology scores, which correlated with individual increases in plasma homovanillic acid concentrations. Both objective and subjective impairments were significantly more pronounced in the subgroup with greatest increase of plasma prolactin. Subjects experiencing the most pronounced side effects under haloperidol, which compelled them to drop out, showed significantly higher prolactin concentration increases than those who tolerated haloperidol well.

CONCLUSION

We found consistent associations between altered markers of dopamine transmission and several objective and subjective mental impairments in healthy volunteers after 1 week's treatment with antidopaminergic medications. These findings should draw attention to a more intensive risk-benefit evaluation in cases of off-label prescription of antipsychotic medications.