Effect of long-term treatment with pramipexole or levodopa on presynaptic markers assessed by longitudinal [123I]FP-CIT SPECT and histochemistry

Genetic factors affecting dopaminergic deterioration during the premotor stage of Parkinson disease

To estimate dopaminergic dysfunction in patients with Parkinson disease (PD) during the premotor stage and to investigate the effect of genetic factors on the trajectories. Using longitudinal dopamine transporter single-photon emission computed tomography data from 367 sporadic PD (sPD), 72 LRRK2 (G2019S), and 39 GBA (N370S) PD patients in the Parkinson's Progression Markers Initiative (PPMI) study, we estimated the temporal trajectories of putaminal-specific binding ratios using an integrating function between baseline values and their annual change rates. In order to test reproducibility, we computed another trajectory for sPD using positron emission tomography data of 38 sPD patients at Gangnam Severance Hospital (GSH). Temporal trajectories of sPD were compared between the groups separated by age at onset (AAO) and polygenic load for common PD risk variants, and also compared with genetic PD. sPD patients in both the PPMI and GSH cohorts showed similar onset of dopaminergic degeneration around 10 years before motor onset. Early-onset PD patients exhibited later onset of degeneration and a faster decline in dopaminergic activity during the premotor period than late-onset patients. sPD patients with high polygenic load were associated with earlier onset and slower progression of dopaminergic dysfunction. Compared to the sPD and LRRK2 PD groups, GBA PD patients exhibited faster deterioration of dopaminergic function during the premotor stage. Dopaminergic dysfunction in PD appears to start about 10 years before motor onset. Genetic factors may be contributing to the heterogeneity of dopaminergic deterioration during the premotor stage.

Effect of polygenic load on striatal dopaminergic deterioration in Parkinson disease

OBJECTIVE

To investigate the effect of polygenic load on the progression of striatal dopaminergic dysfunction in patients with Parkinson disease (PD).

METHODS

Using data from 335 patients with PD in the Parkinson's Progression Markers Initiative (PPMI) database, we investigated the longitudinal association of PD-associated polygenic load with changes in striatal dopaminergic activity as measured by ¹²³I-N-3-fluoropropyl-2-β-carboxymethoxy-3β-(4-iodophenyl) nortropane (¹²³I-FP-CIT) SPECT over 4 years. PD-associated polygenic load was estimated by calculating weighted genetic risk scores (GRS) using 1) all available 27 PD-risk single nucleotide polymorphisms (SNPs) in the PPMI database (GRS1) and 2) 23 SNPs with minor allele frequency >0.05 (GRS2).

RESULTS

GRS1 and GRS2 were correlated with younger age at onset in patients with PD (GRS1, Spearman ρ = -0.128, p = 0.019; GRS2, Spearman ρ = -0.109, p = 0.047). Although GRS1 did not show an association with changes in striatal ¹²³I-FP-CIT availability, GRS2 was associated with a slower decline of striatal dopaminergic activity (interactions with disease duration in linear mixed model; caudate nucleus, estimate = 0.399, SE = 0.165, p = 0.028; putamen, estimate = 0.396, SE = 0.137, p = 0.016).

CONCLUSIONS

Our results suggest that genetic factors for PD risk may have heterogeneous effects on striatal dopaminergic degeneration, and some factors may be associated with a slower decline of dopaminergic activity. Composition of PD progression-specific GRS may be useful in predicting disease progression in patients.

Pharmacological treatment with L-DOPA may reduce striatal dopamine transporter binding in in vivo imaging studies

Numerous neurologic and psychiatric conditions are treated with pharmacological compounds, which lead to an increase of synaptic dopamine (DA) levels. One example is the DA precursor L-3,4-dihydroxyphenylalanine (L-DOPA), which is converted to DA in the presynaptic terminal. If the increase of DA concentrations in the synaptic cleft leads to competition with exogenous radioligands for presynaptic binding sites, this may have implications for DA transporter (DAT) imaging studies in patients under DAergic medication. This paper gives an overview on those findings, which, so far, have been obtained on DAT binding in human Parkinson's disease after treatment with L-DOPA. Findings, moreover, are related to results obtained on rats, mice or non-human primates. Results indicate that DAT imaging may be reduced in the striata of healthy animals, in the unlesioned striata of animal models of unilateral Parkinson's disease and in less severly impaired striata of Parkinsonian patients, if animal or human subjects are under acute or subchronic treatment with L-DOPA. If also striatal DAT binding is susceptible to alterations of synaptic DA levels, this may allow to quantify DA reuptake in analogy to DA release by assessing the competition between endogenous DA and the administered exogenous DAT radioligand.

The role of SPECT imaging of the dopaminergic system in translational research on Parkinson's disease

Imaging of the dopaminergic system with single photon emission computed tomography (SPECT), and particularly of the dopamine transporter (DAT) located in the striatum, is a well accepted tool in clinical practice to confirm or exclude loss of nigrostriatal dopamine (DA) neurons in patients suspected to suffer from Parkinson's disease (PD). SPECT techniques were developed successfully to image neurotransmitter systems, including the presynaptic DAT and postsynaptic dopamine D2/3 receptors, in rat and mouse models of PD. Here we review the results of preclinical SPECT studies of the dopaminergic system in rat and mouse models of PD. Initially, SPECT studies in animal models of PD were performed to validate that micro-SPECT is able to accurately assess parts of the dopaminergic system in small animals in-vivo. However, more recently, micro-SPECT DAT is increasingly used as a research tool to support the interpretation of human DAT SPECT studies in PD, including clinical trials examining the effects of potential neuroprotective drugs. Translational research with SPECT is an interesting development which may further increase our understanding of the pathophysiology and treatment of PD.

Safety analysis of 10 clinical trials and for 13 years after first approval of ioflupane 123I injection (DaTscan)

Ioflupane is an analog of cocaine that binds reversibly with high affinity to the dopamine transporter (DaT) protein, a marker for presynaptic terminals in dopaminergic nigrostriatal neurons. Ioflupane (123)I Injection is also known as DaTscan or DaTSCAN ((123)I-ioflupane is also called (123)I-2-β-carbomethoxy-3β-(4-iodophenyl)-N-(3-fluoropropyl)nortropane or (123)I-FP-CIT). The diagnostic efficacy of DaTscan has been described elsewhere. Here, we present a comprehensive analysis of the safety of DaTscan starting from initiation of clinical development through 13 y after the date of first market approval. Safety data in the sponsor's clinical development safety database from 10 completed DaTscan clinical trials were pooled, and postapproval experience was summarized from standardized aggregate safety reports submitted to regulatory agencies. A total of 1,180 clinical trial subjects (92% of 1,284 subjects planned to receive DaTscan in the clinical trials) received DaTscan. Percentages of subjects with adverse events by category were as follows: all (22%), considered at least possibly related to DaTscan by the investigator (4%), any severe (3%), headache (4%), nausea (2%), dizziness (2%), nasopharyngitis (1%), and injection site hematoma (1%). Four percent of subjects had at least 1 serious adverse event; 5 subjects (<1%) had serious adverse events that led to death. All serious adverse events, including those that led to death, were deemed by an expert clinician to be unrelated to DaTscan. An estimated half a million market doses of DaTscan (for single use) were administered from July 2000 through the July 2013 reporting period. In postapproval safety assessment, 1 death was reported 20 d after (and unrelated to) DaTscan administration. Two spontaneously reported serious adverse drug reactions (ADRs) and 32 spontaneously reported nonserious ADRs were submitted, approximately half of which are identified in labeling. Headache (in clinical trials) and injection site pain (postapproval) were the most commonly reported events or reactions. Although adverse events were reported for 1 in 5 clinical trial subjects, most were mild and considered unrelated to DaTscan administration. Severe events were uncommon, and no serious adverse event occurring in more than 1 subject was deemed related to DaTscan administration. In postapproval experience, the frequency of ADRs spontaneously reported was less than 1 per 10,000 doses administered. Comprehensive safety data show that DaTscan was well tolerated.

In vivo evaluation of the dopaminergic neurotransmission system using [123I]FP-CIT SPECT in 6-OHDA lesioned rats

The 6-hydroxydopamine (6-OHDA) rodent model of Parkinson's disease (PD) has been used to evaluate the nigrostriatal pathway. The aim of this work was to explore the relationship between the degree of 6-OHDA-induced dopaminergic degeneration and [(123)I]FP-CIT binding using single photon emission computed tomography (SPECT). Fourteen rats received a 6-OHDA injection (4 or 8 µg) into the left medial forebrain bundle. After 3 weeks, magnetic resonance imaging and scans with a small-animal SPECT system were performed. Finally, the nigrostriatal lesion was assessed by immunohistochemical analysis. Immunohistochemical analysis confirmed two levels of dopaminergic degeneration. Lesions induced by 6-OHDA diminished the ipsilateral [(123)I]FP-CIT binding by 61 and 76%, respectively. The decrease in tracer uptake between control and lesioned animals was statistically significant, as was the difference between the two 6-OHDA lesioned groups. Results concluded that [(123)I]FP-CIT SPECT is a useful technique to discriminate the degree of dopaminergic degeneration in a rat model of PD.

The role of preclinical SPECT in oncological and neurological research in combination with either CT or MRI

Preclinical imaging with SPECT combined with CT or MRI is used more and more frequently and has proven to be very useful in translational research. In this article, an overview of current preclinical research applications and trends of SPECT combined with CT or MRI, mainly in tumour imaging and neuroscience imaging, is given and the advantages and disadvantages of the different approaches are described. Today SPECT and CT systems are often integrated into a single device (commonly called a SPECT/CT system), whereas at present combined SPECT and MRI is almost always carried out with separate systems and fiducial markers to combine the separately acquired images. While preclinical SPECT/CT is most widely applied in oncology research, SPECT combined with MRI (SPECT/MRI when integrated in one system) offers the potential for both neuroscience applications and oncological applications. Today CT and MRI are still mainly used to localize radiotracer binding and to improve SPECT quantification, although both CT and MRI have additional potential. Future technology developments may include fast sequential or simultaneous acquisition of (dynamic) multimodality data, spectroscopy, fMRI along with high-resolution anatomic MRI, advanced CT procedures, and combinations of more than two modalities such as combinations of SPECT, PET, MRI and CT all together. This will all strongly depend on new technologies. With further advances in biology and chemistry for imaging molecular targets and (patho)physiological processes in vivo, the introduction of new imaging procedures and promising new radiopharmaceuticals in clinical practice may be accelerated.

Dopamine receptor agonists for Parkinson's disease

INTRODUCTION

Prolonged administration of l-3,4-dihydroxyphenylalanine (l-DOPA) for Parkinson's disease (PD) is hampered by motor complications related to the progressive incapacity of residual nigrostriatal neurons to properly utilize the drug. Direct stimulation of dopaminergic (DAergic) receptors with specific compounds (DA agonists) has, therefore, become an additional therapeutic tool for PD.

AREAS COVERED

DA agonists have considerable anti-parkinsonian symptomatic efficacy, although they are less potent than l-DOPA. This review summarizes pre-clinical and clinical data on DA agonists and their role in treating PD. Specific focus was put on second-generation, first-line non-ergolinic DA agonists and their motor, non-motor and putative neuroprotective effects. The anti-parkinsonian potential of recently developed DA agonists that reached Phase II and III clinical trials was also addressed.

EXPERT OPINION

DA agonists can be useful along the whole natural course of PD, as monotherapy in the initial phase or combined with l-DOPA in advanced PD. Extended-release formulations have been developed for second-generation DA agonists, which are better appreciated by patients. Neuroprotective properties have been proposed for DA agonists, based on pre-clinical studies, but never convincingly demonstrated in patients. New DA agonists, with better symptomatic efficacy and devoid of the side effects that characterize current compounds, are needed.