Dopamine agonist monotherapy in Parkinson's disease and potential risk factors for dyskinesia: a meta-analysis of levodopa-controlled trials

Dose-response relationship of levodopa with dyskinesia in Parkinson's disease: A systematic review and meta-analysis

Despite existing evidence linking dyskinesia to levodopa, the primary treatment for Parkinson's, the dose-response relationship and risk factors remain uncertain. In this study, the risk for dyskinesia in patients with Parkinson's disease receiving levodopa was evaluated via meta-analysis and meta-regression approaches to examine dyskinesia risk factors more reliably and improve treatment strategies and patient care. The PubMed and Embase databases were searched to identify randomized controlled trials comparing levodopa with other anti-Parkinson's drugs published in English before June 31, 2023. The primary outcome was dyskinesia, and a risk of bias assessment was performed. In total, 24 studies met the inclusion criteria; 21 had a low risk of bias, and 3 had a high risk of bias. These studies included 4698 patients with Hoehn and Yahr Grade I-III Parkinson's disease. Our meta-analysis showed that the risk of dyskinesia was higher for levodopa than for other anti-Parkinson's drugs (odds ratio: 2.52 [95% confidence interval: 1.84-3.46]). Dyskinesia was not related to age (slope coefficient: 0.185 [0.095]; P = 0.061), disease duration (slope coefficient: 0.011 [0.018]; P = 0.566), or treatment duration (slope coefficient: 0.008 [0.007]; P = 0.216). The mean levodopa equivalent dose (slope coefficient: 0.004 [0.001]; P = 0.001) in the experimental group and the differences in drug doses between the experimental and control groups were correlated with the risk of dyskinesia. Results of randomized controlled trials supported an association between the levodopa dose and dyskinesia in patients with Parkinson's disease. Compared with levodopa users, users of other anti-Parkinson's drugs had a lower incidence of dyskinesia. Age, disease duration, and treatment duration were not correlated with dyskinesia. These findings suggest that anti-Parkinson's drugs other than levodopa, particularly in cases of early-stage Parkinson's disease, should be considered to reduce the risk of dyskinesia.

Ropinirole Cotreatment Prevents Perivascular Glial Recruitment in a Rat Model of L-DOPA-Induced Dyskinesia

Dopamine replacement therapy for Parkinson's disease is achieved using L-DOPA or dopamine D2/3 agonists, such as ropinirole. Here, we compare the effects of L-DOPA and ropinirole, alone or in combination, on patterns of glial and microvascular reactivity in the striatum. Rats with unilateral 6-hydroxydopamine lesions were treated with therapeutic-like doses of L-DOPA (6 mg/kg), an equipotent L-DOPA-ropinirole combination (L-DOPA 3 mg/kg plus ropinirole 0.5 mg/kg), or ropinirole alone. Immunohistochemistry was used to examine the reactivity of microglia (ionized calcium-binding adapter molecule 1, IBA-1) and astroglia (glial fibrillary acidic protein, GFAP), as well as blood vessel density (rat endothelial cell antigen 1, RECA-1) and albumin extravasation. L-DOPA monotreatment and L-DOPA-ropinirole cotreatment induced moderate-severe dyskinesia, whereas ropinirole alone had negligible dyskinetic effects. Despite similar dyskinesia severity, striking differences in perivascular microglia and astroglial reactivity were found between animals treated with L-DOPA vs. L-DOPA-ropinirole. The former exhibited a marked upregulation of perivascular IBA-1 cells (in part CD68-positive) and IBA-1-RECA-1 contact points, along with an increased microvessel density and strong perivascular GFAP expression. None of these markers were significantly upregulated in animals treated with L-DOPA-ropinirole or ropinirole alone. In summary, although ropinirole cotreatment does not prevent L-DOPA-induced dyskinesia, it protects from maladaptive gliovascular changes otherwise associated with this disorder, with potential long-term benefits to striatal tissue homeostasis.

Dopamine agonists versus levodopa monotherapy in early Parkinson's disease for the potential risks of motor complications: A network meta-analysis

BACKGROUND

Compared with levodopa, dopamine agonists (DAs) as initial treatment are associated with lower incidences of motor complications in early Parkinson's disease (PD). There is no strong evidence that a given DA is more potent in lower incidences of motor complications than another.

OBJECTIVE

We performed a network meta-analysis of levodopa versus DAs as monotherapy in early PD to access the risk of motor complications.

METHODS

Databases were searched up to June 2022 for eligible RCTs. Levodopa and four DAs (pramipexole, ropinirole, bromocriptine and pergolide) were investigated. The incidences of motor complications and efficacy, tolerability and safety outcomes were analyzed.

RESULTS

Nine RCTs (2112 patients) were included in the current study. The surface under the cumulative ranking curve (SUCRA) indicated that levodopa ranked first in the incidence of dyskinesia (0.988), followed by pergolide, pramipexole, ropinirole, and bromocriptine (0.704, 0.408, 0.240, 0.160). Pramipexole was least prone to wearing-off (0.109) and on-off fluctuation (0.041). Levodopa performed best in improvements of UPDRS-II, UPDRS-III, and UPDRS-II + III (0.925, 0.952, 0.934). Bromocriptine ranked first in total withdrawals and withdrawals due to adverse events (0.736, 0.751). Four DAs showed different adverse events profiles.

CONCLUSION

In the two non-ergot DAs, ropinirole is associated with a lower risk of dyskinesia while pramipexole is associated with lower risks of wearing-off and on-off fluctuations. Our research may facilitate head-to-head research, larger sample sizes, long following-up time RCTs to confirm the findings of this network meta-analysis.

Comparison of dyskinesia profiles after L-DOPA dose challenges with or without dopamine agonist coadministration

Many patients with Parkinson's disease (PD) experiencing l-DOPA-induced dyskinesia (LID) receive adjunct treatment with dopamine agonists, whose functional impact on LID is unknown. We set out to compare temporal and topographic profiles of abnormal involuntary movements (AIMs) after l-DOPA dose challenges including or not the dopamine agonist ropinirole. Twenty-five patients with PD and a history of dyskinesias were sequentially administered either l-DOPA alone (150% of usual morning dose) or an equipotent combination of l-DOPA and ropinirole in random order. Involuntary movements were assessed by two blinded raters prior and every 30 min after drug dosing using the Clinical Dyskinesia Rating Scale (CDRS). A sensor-recording smartphone was secured to the patients' abdomen during the test sessions. The two raters' CDRS scores were highly reliable and concordant with models of hyperkinesia presence and severity trained on accelerometer data. The dyskinesia time curves differed between treatments as the l-DOPA-ropinirole combination resulted in lower peak severity but longer duration of the AIMs compared with l-DOPA alone. At the peak of the AIMs curve (60-120 min), l-DOPA induced a significantly higher total hyperkinesia score, whereas in the end phase (240-270 min), both hyperkinesia and dystonia tended to be more severe after the l-DOPA-ropinirole combination (though reaching statistical significance only for the item, arm dystonia). Our results pave the way for the introduction of a combined l-DOPA-ropinirole challenge test in the early clinical evaluation of antidyskinetic treatments. Furthermore, we propose a machine-learning method to predict CDRS hyperkinesia severity using accelerometer data.

The complex molecular pharmacology of the dopamine D2 receptor: Implications for pramipexole, ropinirole, and rotigotine

With L-DOPA, dopamine agonists such as pramipexole, ropinirole and rotigotine constitute key therapeutic options for the management of motor symptoms of Parkinson's disease. These compounds exert their beneficial effect on motor behaviours by activating dopamine D₂-class receptors and thereby compensating for the declining dopaminergic transmission in the dorsal striatum. Despite a strong similarity in their mechanism of action, these three dopamine agonists present distinct clinical profiles, putatively underpinned by differences in their pharmacological properties. In this context, this review aims at contributing to close the gap between clinical observations and data from molecular neuropharmacology by exploring the properties of pramipexole, ropinirole and rotigotine from both the clinical and molecular perspectives. Indeed, this review first summarizes and compares the clinical features of these three dopamine agonists, and then explores their binding profiles at the different dopamine receptor subtypes. Moreover, the signalling profiles of pramipexole, ropinirole and rotigotine at the D₂ receptor are recapitulated, with a focus on biased signalling and the potential therapeutic implications. Overall, this review aims at providing a unifying framework of interpretation for both clinicians and fundamental pharmacologists interested in a deep understanding of the pharmacological properties of pramipexole, ropinirole and rotigotine.

Polymeric nanoparticles for dopamine and levodopa replacement in Parkinson's disease

As the world's population ages, the incidence of Parkinson's disease (PD), the second most common neurological ailment, keeps increasing. It is estimated that 1% of the global population over the age of 60 has the disease. The continuous loss of dopaminergic neurons and the concomitant brain depletion of dopamine levels represent the hallmarks of PD. As a result, current PD therapies primarily target dopamine or its precursor (levodopa). Therapeutic approaches that aim to provide an exogenous source of levodopa or dopamine are hindered by their poor bioavailability and the blood-brain barrier. Nevertheless, the fabrication of many polymeric nanoparticles has been exploited to deliver several drugs inside the brain. In addition to a brief introduction of PD and its current therapeutic approaches, this review covers novel polymeric nanoparticulate drug delivery systems exploited lately for dopamine and levodopa replacement in PD.

The Comparative Efficacy of Non-ergot Dopamine Agonist and Potential Risk Factors for Motor Complications and Side Effects From NEDA Use in Early Parkinson's Disease: Evidence From Clinical Trials

Background/Objectives

Non-ergot dopamine agonist (NEDA) are recommended as the first-line treatment for patients with early Parkinson's disease (PD) because of their efficacy in treating PD motor symptoms. However, systematic evaluations of the risk of motor complications induced by NEDA and risk factors potentially associated with motor complications are still lacking.

Methods

Medline, Embase, the Cochrane Central Register of Controlled Trials, and Web of Science were searched for potentially eligible randomized controlled trials. The incidence of motor complications (dyskinesia, motor fluctuations), impulsive-compulsive behaviors and adverse events and clinical disability rating scale (UPDRS) scores were evaluated using standard meta-analytic methods. Metaregression was conducted on the incidence of motor complications (dyskinesia) with treatment duration and NEDA dose as covariates.

Results

Patients treated with NEDA had significantly lower UPDRS total scores, motor scores and activity of daily living (ADL) scores than those receiving a placebo (weighted mean difference (WMD) −4.81, 95% CI −6.57 to −3.05; WMD −4.901, 95% CI −7.03 to −2.77; WMD −1.52, 95% CI −2.19 to −0.84, respectively). Patients in the NEDA and NEDA+open Levodopa (LD) groups had lower odds for dyskinesia than patients in the LD group (OR = 0.21, 95% CI: 0.15–0.29; OR = 0.31, 95% CI 0.24–0.42, respectively). Metaregressions indicated that the mean LD dose of the NEDA group increased, and the odds of developing dyskinesia increased (p = 0.012). However, the odds of developing dyskinesia in the NEDA group were not related to treatment duration (p = 0.308). PD patients treated with NEDA or NEDA+open LD had a lower risk of wearing-off implications than those treated with LD (all p < 0.05). No significant difference was found between the NEDA and placebo groups in impulsive-compulsive behavior development (p > 0.05). Patients in the NEDA group were more likely to suffer somnolence, edema, constipation, dizziness, hallucinations, nausea and vomiting than those in the placebo or LD group.

Conclusion

NEDA therapy reduces motor symptoms and improves ADLs in early PD. The odds of developing motor complications were lower with NEDA than with LD, and dyskinesia increased with increasing LD equivalent dose and was not influenced by NEDA treatment duration. Therefore, long-term treatment with an appropriate dosage of NEDA might be more suitable than LD for early PD patients.

Registration

PROSPERO CRD42021287172.

Liposomes: Novel Drug Delivery Approach for Targeting Parkinson's Disease

Parkinson's disease is one of the most severe progressive neurodegenerative disorders, having a mortifying effect on the health of millions of people around the globe. The neural cells producing dopamine in the substantia nigra of the brain die out. This leads to symptoms like hypokinesia, rigidity, bradykinesia, and rest tremor. Parkinsonism cannot be cured, but the symptoms can be reduced with the intervention of medicinal drugs, surgical treatments, and physical therapies. Delivering drugs to the brain for treating Parkinson's disease is very challenging. The blood-brain barrier acts as a highly selective semi-permeable barrier, which refrains the drug from reaching the brain. Conventional drug delivery systems used for Parkinson's disease do not readily cross the blood barrier and further lead to several side-effects. Recent advancements in drug delivery technologies have facilitated drug delivery to the brain without flooding the bloodstream and by directly targeting the neurons. In the era of Nanotherapeutics, liposomes are an efficient drug delivery option for brain targeting. Liposomes facilitate the passage of drugs across the blood-brain barrier, enhances the efficacy of the drugs, and minimize the side effects related to it. The review aims at providing a broad updated view of the liposomes, which can be used for targeting Parkinson's disease.

Improved functional and histochemical outcomes in l-DOPA plus tolcapone treated VMAT2-deficient mice

Parkinson disease is typically treated with L-3,4-dihydroxyphenylalanine (or levodopa) co-prescribed with concentration stabilizers to prevent undesired motor fluctuations. However, the beneficial role of the chronic combined therapy on disease progression has not been thoroughly explored. We hypothesized that tolcapone, a catechol-O-methyl-transferase inhibitor, co-administered with levodopa may offer beneficial long-term disease-modifying effects through its dopamine stabilization actions. Here, we followed vesicular monoamine transporter 2-deficient and wild-type mice treated twice daily per os with vehicle, levodopa (20 mg/kg), tolcapone (15 mg/kg) or levodopa (12.5 mg/kg) + tolcapone (15 mg/kg) for 17 weeks. We assessed open field, bar test and rotarod performances at baseline and every 4th week thereafter, corresponding to OFF-medication weeks. Finally, we collected coronal sections from the frontal caudate-putamen and determined the reactivity level of dopamine transporter. Vesicular monoamine transporter 2-deficient mice responded positively to chronic levodopa + tolcapone intervention in the bar test during OFF-periods. Neither levodopa nor tolcapone interventions offered significant improvements on their own. Similarly, chronic levodopa + tolcapone intervention was associated with partially rescued dopamine transporter levels, whereas animals treated solely with levodopa or tolcapone did not present this effect. Interestingly, 4-month progression of bar test scores correlated significantly with dopamine-transporter-label density. Overall, we observed a moderate functional and histopathological improvement effect by chronic dopamine replacement when combined with tolcapone in vesicular monoamine transporter 2-deficient mice. Altogether, chronic stabilization of dopamine levels by catechol-O-methyl-transferase inhibition, besides its intended immediate actions, arises as a potential long-term beneficial approach during the progression of Parkinson disease.

Analysis of Motor Complication and Relative Factors in a Cohort of Chinese Patients with Parkinson's Disease

Objective

Motor complications are common in Parkinson's disease (PD). The reported occurrence of motor complications varies across regions and races. The aim of our study was to describe the development of dyskinesias and motor fluctuations among Chinese PD patients and the relative risk factors.

Methods

In the current cross-sectional survey study, PD patients with motor fluctuations and dyskinesia were enrolled from March to November 2018 in Shaanxi province, a northwest area of China. Data were collected by the movement disorder specialists. A self-designed questionnaire was utilized during face-to-face interviews. In addition, the relevant factors of motor complications were analyzed by univariable and multivariable analyses.

Results

Of the166 PD patients recruited, 52 (31.33%) and 25 (15.06%) patients had motor fluctuations and dyskinesia, respectively, which occurred in 6.76 ± 3.77 and 8.61 ± 4.46 years after the onset of motor symptoms and 5.37 ± 3.33 and 6.80 ± 3.43 years after the treatment of levodopa therapy, respectively. Patients with motor fluctuations and dyskinesias had longer disease duration, younger onset age, higher Hoehn–Yahr stages and UPDRS III scores, higher daily levodopa dosage and levodopa equivalent daily dose (LEDD), and longer duration of levodopa treatment (P < 0.05). Bradykinesia-rigidity dominant patients had higher incidences of motor fluctuations (61.54% vs 38.46%) and dyskinesias (68.00% vs 32.00%) than tremor-dominant patients (P < 0.05). Results of the multivariate logistic regression analyses showed that the duration of levodopa therapy, age of the onset, and bradykinesia-rigidity dominant type were independent risk factors of motor fluctuations (P < 0.05). In addition, duration of disease and bradykinesia-rigidity dominant type were independent risk factors of dyskinesia (P < 0.05).

Conclusions

The rate of motor fluctuations was higher than dyskinesias in Chinese patients with Parkinson's disease. Patients with younger age onset, bradykinesia-rigidity dominant type, longer disease duration, and longer duration of levodopa therapy are more likely to develop motor complications.

Reciprocal cross-sensitization of D1 and D3 receptors following pharmacological stimulation in the hemiparkinsonian rat

In the majority of Parkinson's disease (PD) patients, long-term dopamine (DA) replacement therapy leads to dyskinesia characterized by abnormal involuntary movements (AIMs). There are various mechanisms of dyskinesia, such as the sensitization of striatal DA D1 receptors (D₁R) and upregulation of DA D3 receptors (D₃R). These receptors interact physically and functionally in D₁R-bearing medium spiny neurons to synergistically drive dyskinesia. However, the cross-receptor-mediated effects due to D₁R-D₃R cooperativity are still poorly understood. In pursuit of this, we examined whether or not pharmacological D₁R or D₃R stimulation sensitizes the dyskinetic response to the appositional agonist, a process known as cross-sensitization. First, we established D₁R-D₃R behavioral synergy in a cohort of 6-OHDA-lesioned female adult Sprague-Dawley rats. Then, in a new cohort, we tested for cross-sensitization in a between-subject design. Five groups received a sub-chronic regimen of either saline, the D₁R agonist SKF38393 (1.0 mg/kg), or the D₃R agonist PD128907 (0.3 mg/kg). For the final injection, each group received an acute injection of the other agonist. AIMs were monitored following each injection. Sub-chronic administration of both SKF38393 and PD128907 induced the development of dyskinesia. More importantly, cross-agonism tests revealed reciprocal cross-sensitization; chronic treatment with either SKF38393 or PD128907 induced sensitization to a single administration of the other agonist. This reciprocity was not marked by changes to either D₁R or D₃R striatal mRNA expression. The current study provides key behavioral data demonstrating the role of D3R in dyskinesia and provides behavioral evidence of D1R and D3R functional interactions.

Risk thresholds of levodopa dose for dyskinesia in Chinese patients with Parkinson's disease: a pilot study

Levodopa is widely used to treat Parkinson's disease (PD), and its long-term therapy may induce dyskinesia in a dose-dependent manner. However, the threshold dose with a relatively low risk for dyskinesia has not been determined. Demographic, clinical profiles and detailed information of dopaminergic drugs were recorded for 403 PD patients in treatment with levodopa. Variables were compared between dyskinesia and non-dyskinesia groups. Logistic regression analysis was used to assess the association between levodopa dose-related variables and dyskinesia. Receiver operating characteristic curve and decision tree classification model were used to investigate the cut-off value of levodopa dose to best separate the dyskinesia group from the non-dyskinesia group. Patients with dyskinesia tended to have a lower weight and age at onset, higher percentage of female and wearing-off, longer duration of disease and levodopa treatment, higher H-Y stage and MDS-UPDRS Part III score, and higher levodopa dose and levodopa equivalent dose than those without dyskinesia. After adjusted for demographical and clinical variables, levodopa dose-related factors (daily dose, cumulative dose, and weight-adjusted dose) were still associated with dyskinesia. Both the receiver operating characteristic and decision tree classification analysis indicated that patients who have taken levodopa dose ≤ 400 mg per day may be associated with a reduced risk for dyskinesia. In conclusion, we evaluated the thresholds of levodopa treatment with a relatively low risk for dyskinesia. These data should be considered for prevention and management of dyskinesia in patients with PD.

Regulation of dopamine neurotransmission from serotonergic neurons by ectopic expression of the dopamine D2 autoreceptor blocks levodopa-induced dyskinesia

Levodopa-induced dyskinesias (LID) are a prevalent side effect of chronic treatment with levodopa (L-DOPA) for the motor symptoms of Parkinson’s disease (PD). It has long been hypothesized that serotonergic neurons of the dorsal raphe nucleus (DRN) are capable of L-DOPA uptake and dysregulated release of dopamine (DA), and that this “false neurotransmission” phenomenon is a main contributor to LID development. Indeed, many preclinical studies have demonstrated LID management with serotonin receptor agonist treatment, but unfortunately, promising preclinical data has not been translated in large-scale clinical trials. Importantly, while there is an abundance of convincing clinical and preclinical evidence supporting a role of maladaptive serotonergic neurotransmission in LID expression, there is no direct evidence that dysregulated DA release from serotonergic neurons impacts LID formation. In this study, we ectopically expressed the DA autoreceptor D2R_s (or GFP) in the DRN of 6-hydroxydopamine (6-OHDA) lesioned rats. No negative impact on the therapeutic efficacy of L-DOPA was seen with rAAV-D2R_s therapy. However, D2R_s treated animals, when subjected to a LID-inducing dose regimen of L-DOPA, remained completely resistant to LID, even at high doses. Moreover, the same subjects remained resistant to LID formation when treated with direct DA receptor agonists, suggesting D2R_s activity in the DRN blocked dyskinesogenic L-DOPA priming of striatal neurons. In vivo microdialysis confirmed that DA efflux in the striatum was reduced with rAAV-D2R_s treatment, providing explicit evidence that abnormal DA release from DRN neurons can affect LID. This is the first direct evidence of dopaminergic neurotransmission in DRN neurons and its modulation with rAAV-D2R_s gene therapy confirms the serotonin hypothesis in LID, demonstrating that regulation of serotonergic neurons achieved with a gene therapy approach offers a novel and potent antidyskinetic therapy.

MTOR Pathway-Based Discovery of Genetic Susceptibility to L-DOPA-Induced Dyskinesia in Parkinson's Disease Patients

Dyskinesia induced by L-DOPA administration (LID) is one of the most invalidating adverse effects of the gold standard treatment restoring dopamine transmission in Parkinson's disease (PD). However, LID manifestation in parkinsonian patients is variable and heterogeneous. Here, we performed a candidate genetic pathway analysis of the mTOR signaling cascade to elucidate a potential genetic contribution to LID susceptibility, since mTOR inhibition ameliorates LID in PD animal models. We screened 64 single nucleotide polymorphisms (SNPs) mapping to 57 genes of the mTOR pathway in a retrospective cohort of 401 PD cases treated with L-DOPA (70 PD with moderate/severe LID and 331 with no/mild LID). We performed classic allelic, genotypic, and epistatic analyses to evaluate the association of individual or combinations of SNPs with LID onset and with LID severity after initiation of L-DOPA treatment. As for the time to LID onset, we found significant associations with SNP rs1043098 in the EIF4EBP2 gene and also with an epistatic interaction involving EIF4EBP2 rs1043098, RICTOR rs2043112, and PRKCA rs4790904. For LID severity, we found significant association with HRAS rs12628 and PRKN rs1801582 and also with a four-loci epistatic combination involving RPS6KB1 rs1292034, HRAS rs12628, RPS6KA2 rs6456121, and FCHSD1 rs456998. These findings indicate that the mTOR pathway contributes genetically to LID susceptibility. Our study could help to identify the most susceptible PD patients to L-DOPA in order to prevent the appearance of early and/or severe LID in a future. This information could also be used to stratify PD patients in clinical trials in a more accurate way.

Levodopa-induced dyskinesia: clinical features, incidence, and risk factors

Symptoms of Parkinson's disease have been controlled with levodopa for many years; however, motor complications consisting of wearing off of medication effect and dyskinesias tend to occur within a few years of starting levodopa. Motor complications can begin a few months after taking levodopa, with the average time to onset estimated to be 6.5 years. Dyskinesias can be troublesome and require intervention. Levodopa-induced dyskinesia can be composed of a variety of movement disorders including chorea, dystonia, ballism, myoclonus, and akathisia. Based on the clinical pattern, the most common dyskinesia is chorea and choreoathetosis. The clinical manifestations can be divided into three main categories based on their clinical movement patterns and the temporal correlation between the occurrence of dyskinesia and the levodopa dosing: on or peak-dose dyskinesias, biphasic dyskinesias, and Off dyskinesias. Severe cases of dyskinesia have been reported, with the extreme being dyskinesia-hyperpyrexia syndrome. The prevalence of LID has been reported in many studies, but the reported incidence varies. The rate of LID development is from 3 to 94%. The prevalence of LID mainly depends on age at onset, disease duration, and severity, and duration of levodopa therapy. Some of the risk factors for the development of dyskinesia are modifiable. Modifiable risk factors include levodopa dose and body weight. Non-modifiable risk factors include age, gender, duration of disease, clinical subtype, disease progression, disease severity, and genetic factors.

Levodopa-induced dyskinesia in Parkinson's disease: still no proof? A meta-analysis

We investigated whether there is a linear relationship between levodopa (LD) dose and treatment duration, and the development of levodopa-induced dyskinesia (LID) among patients with early untreated Parkinson's disease (PD). We performed a meta-analysis of randomized-controlled trials (RCTs) comparing LD monotherapy to any other antiparkinsonian treatment in early PD patients. Meta-regressions were conducted including as covariates the effects of LD dose, treatment duration, and age. We further proceeded in subgroup analyses based on the type of medications in the non-LD monotherapy (control) group and on whether patients in the control group received additional levodopa or not. Thirteen eligible RCTs were included, which revealed a significantly higher risk for dyskinesia in patients initially treated with LD monotherapy compared to any other treatment (OR = 2.82). None of the subsequent meta-regressions revealed any significant relationship with dose, treatment duration or age. Patients treated on LD monotherapy or MAOΙ plus LD were at a greater risk to develop LID than patients who received DA only or DA plus supplemental LD. The increased heterogeneity compromised the robustness of the results. The alleged correlation between LID and LD dose and treatment duration cannot be verified based on the data available so far. Well-designed, large-scale, long-term, RCTs on drug-naïve PD patients could allow the better comprehension of the pattern of the association between LID and LD treatment parameters.

The effect of istradefylline for Parkinson's disease: A meta-analysis

Adenosine A_2A receptor antagonists are an alternative treatment strategy for Parkinson's disease. Several randomized placebo controlled studies have tested the effect of A_2A receptor antagonist istradefylline, and more robust evidence has been acquired. This meta-analysis aimed to provide evidence for its efficacy and safety on patients with Parkinson's disease. After a systematic literature search, we calculated the pooled standardized mean difference and risk ratio for continuous and dichotomous variables, respectively. Further, sensitivity analyses were performed to confirm the effect estimated by meta-analyses. Publication bias was assessed by funnel plot and deviation of intercept. Six studies satisfied our inclusion criteria. Istradefylline (40 mg/day) decreased off time and improved motor symptoms of Parkinson's disease in homogeneous studies. Istradefylline at 20 mg/day decreased off time and improved motor symptoms, but heterogeneity was found in the analysis of the former among studies. There was a significant effect of istradefylline on dyskinesia in homogeneous studies. Publication bias, however, was observed in the comparison of dyskinesia. Other adverse events showed no significant difference. The present meta-analysis suggests that istradefylline at 40 mg/day could alleviate off time and motor symptoms derived from Parkinson's disease. Dyskinesia might be worsened, but publication bias prevents this from being clear.

Parkinson's Disease and development of levodopa induced motor complications: Influence of baseline features and first medical approach

Background

The introduction of levodopa in clinical practice represents a hallmark in the treatment of the neurodegenerative disease, Parkinson's Disease. However, levodopa induced motor complications, namely dyskinesias and motor fluctuations, develop in the majority of Parkinson's Disease patients.

Objective

to identify which Parkinson's Disease's, patient's and therapeutics' initial features are more associated with dyskinesias or motor fluctuations development.

Methods

Patients with diagnosed Parkinson's Disease attending neurology outpatient clinic at Centro Hospitalar São João were selected. For this observational study, data was retrospectively collected from patient's clinical records. A survival analysis model with univariate and multivariate regression analysis was used.

Results

87 patients with a mean of 72 ± 9.7 years were included. After a median follow-up of 6 (range 1-17) years, 35.6% patients developed dyskinesias; and with a median of 5 (range 1-16) years, 32.2% developed motor fluctuations. After multivariate analysis, the akinesia/rigidity subtype was found to have a higher risk of dyskinesias and motor fluctuations development. Age of onset ≤50 years was associated with motor fluctuations development.

Conclusion

In conclusion, our results suggest that Parkinson's Disease patients' initial characteristics, such as subtype or age of onset, are independently associated with the development of motor complications.

Advances in dopamine receptor agonists for the treatment of Parkinson's disease

INTRODUCTION

Dopamine agonists (DA) are a class of agents which directly stimulate dopamine receptors mimicking the endogenous neurotransmitter dopamine. At first used as adjunctive therapy in the advanced phases of the disease, over the years a significant role was found for DA monotherapy as a first approach in the initial stage of Parkinson's disease (PD). Several reviews have already reported efficacy and safety of DA in PD and differences between DA and levodopa. Therefore the objective of this review is to gather recent updates in DA therapy. A thorough knowledge of recent literature evidences, would help clinician in the management of treatment with DA.

AREAS COVERED

Our review investigates recent updates on DA therapy, the role of these compounds in controlling non-motor symptoms (NMS) as well as new formulations under clinical evaluation and newly emerged post-marketing safety considerations. A literature search has been performed using Medline and reviewing the bibliographies of selected articles.

EXPERT OPINION

DA represents a very important option in the treatment of PD, even though there are still some criticisms and unmet needs. A better knowledge of dopamine receptors could lead to identification of new compounds able to better balance clinical efficacy and side effects.

Initial treatment of Parkinson's disease in 2016: The 2000 consensus conference revisited

In 2000, a French consensus conference proposed guidelines for the treatment of Parkinson's disease (PD). Since then, new drugs have been concocted, new studies have been published and clinicians have become aware of some drug-induced adverse effects that were little known in the past. This has led us to reconsider the recommendations published 16 years ago. Thus, the aim of the present review is to present the recent data related to the different medications and non-pharmacological approaches available for PD, with a special focus on early-stage PD. Levodopa (LD), dopamine agonists (DAs), catechol-O-methyltransferase inhibitors (COMT-Is), anticholinergics, monoamine oxidase inhibitors (MAOB-Is) and amantadine have been considered, and their efficacy and safety for both motor as well as non-motor aspects are reported here. This has led to our proposal for a revised therapeutic strategy for the initiation of treatment in newly diagnosed PD patients, based on the available literature and the relative benefits/side effects balance.

Targeting the dopamine D3 receptor: an overview of drug design strategies

INTRODUCTION

Dopamine is a neurotransmitter widely distributed in both the periphery and the central nervous system (CNS). Its physiological effects are mediated by five closely related G protein-coupled receptors (GPCRs) that are divided into two major subclasses: the D1-like (D1, D5) and the D2-like (D2, D3, D4) receptors. D3 receptors (D3Rs) have the highest density in the limbic areas of the brain, which are associated with cognitive and emotional functions. These receptors are therefore attractive targets for therapeutic management.

AREAS COVERED

This review summarizes the functional and pharmacological characteristics of D3Rs, including the design and clinical relevance of full agonists, partial agonists and antagonists, as well as the capacity of these receptors to form active homodimers, heterodimers or higher order receptor complexes as pharmacological targets in several neurological and neurodegenerative disorders.

EXPERT OPINION

The high sequence homology between D3R and the D2-type challenges the development of D3R-selective compounds. The design of new D3R-preferential ligands with improved physicochemical properties should provide a better pharmacokinetic/bioavailability profile and lesser toxicity than is found with existing D3R ligands. It is also essential to optimize D3R affinity and, especially, D3R vs. D2-type binding and functional selectivity ratios. Developing allosteric and bitopic ligands should help to improve the D3R selectivity of these drugs. As most evidence points to the ability of GPCRs to form homomers and heteromers, the most promising therapeutic strategy in the future is likely to involve the application of heteromer-selective drugs. These selective ligands would display different affinities for a given receptor depending on the receptor partners within the heteromer. Therefore, designing novel compounds that specifically target and modulate D1R-D3R heteromers would be an interesting approach for the treatment of levodopa (L-DOPA)-induced dyskinesias.

Effects of L-dopa priming on cortical high beta and high gamma oscillatory activity in a rodent model of Parkinson's disease

Prolonged L-dopa treatment in Parkinson's disease (PD) often leads to the expression of abnormal involuntary movements known as L-dopa-induced dyskinesia. Recently, dramatic 80 Hz oscillatory local field potential (LFP) activity within the primary motor cortex has been linked to dyskinetic symptoms in a rodent model of PD and attributed to stimulation of cortical dopamine D1 receptors. To characterize the relationship between high gamma (70-110 Hz) cortical activity and the development of L-dopa-induced dyskinesia, cortical LFP and spike signals were recorded in hemiparkinsonian rats treated with L-dopa for 7 days, and dyskinesia was quantified using the abnormal involuntary movements (AIMs) scale. The relationship between high gamma and dyskinesia was further probed by assessment of the effects of pharmacological agents known to induce or modulate dyskinesia expression. Findings demonstrate that AIMs and high gamma LFP power increase between days 1 and 7 of L-dopa priming. Notably, high beta (25-35 Hz) power associated with parkinsonian bradykinesia decreased as AIMs and high gamma LFP power increased during priming. After priming, rats were treated with the D1 agonist SKF81297 and the D2 agonist quinpirole. Both dopamine agonists independently induced AIMs and high gamma cortical activity that were similar to that induced by L-dopa, showing that this LFP activity is neither D1 nor D2 receptor specific. The serotonin 1A receptor agonist 8-OH-DPAT reduced L-dopa- and DA agonist-induced AIMs and high gamma power to varying degrees, while the serotonin 1A antagonist WAY100635 reversed these effects. Unexpectedly, as cortical high gamma power increased, phase locking of cortical pyramidal spiking to high gamma oscillations decreased, raising questions regarding the neural substrate(s) responsible for high gamma generation and the functional correlation between high gamma and dyskinesia.

Pharmacological targeting of dopamine D3 receptors: Possible clinical applications of selective drugs

Dopamine D3 receptors have been pharmacologically engaged in humans since the development of the first antipsychotics and ergot-derivative dopamine (DA) agonists, even without knowing it. These agents were generally non-selective, developed primarily to target D2 receptors. In the last 10 years the understanding of the clinical implication of D3 receptors has been progressing also due to the identification of D3 gene polymorphisms, the use of more selective PET ligands such as [(11)C]-(+)-PHNO and the learning regarding the clinical use of the D3-preferential D2/D3 agonists ropinirole and pramipexole. A new specific neuroplasticity role of D3 receptor regarding dendrite arborisation outgrowth in dopaminergic neurons was also proposed to support, at least in part, the slowing of disease observed in subjects with Parkinson׳s Disease treated with DA agonists. Similar mechanisms could be at the basis of the antidepressant-like effects observed with DA agonists when co-administered with standard of care. Severe adverse event occurring with the use of anti-parkinsonian DA agonists in predisposed subjects, i.e., impulse control disorders, are now suggested to be putatively related to overactive D3 receptors. Not surprisingly, blockade of D3 receptors was proposed as treatment for addictive disorders, a goal that could be potentially achieved by repositioning buspirone, an anxiolytic drug with D3-preferential antagonistic features, or with novel selective D3 antagonists or partial agonists currently in development for schizophrenia. At the moment ABT-925 is the only selective D3 antagonist tested in schizophrenic patients in Phase II, showing an intriguing cognitive enhancing effects supported by preclinical data. Finally, exploratory pharmacogenetic analysis suggested that ABT-925 could be effective in a subpopulation of patients with a polymorphism on the D3 receptor, opening to a possible personalised medicine approach.

Modern treatment in Parkinson's disease, a personal approach

There are many guidelines available concerning the treatment of Parkinson's disease. Most of these advocate treating young-onset patients with a dopamine agonist and older patients with levodopa. The rationale behind this recommendation has its origins in the side effects associated with each of these drug classes: whilst levodopa leads to dyskinesia, which may not be relevant for patients with a limited life-expectancy, dopamine agonists have a much longer plasma half life which probably leads to more continuous dopamine receptor stimulation and thus decreases the occurrence and severity of dyskinesia. However, the side effects associated with the use of dopamine agonists, such as sleepiness, orthostatic problems, hallucinations and impulse control disorders are a drawback. In this overview, the hypothesis will be put forward that perhaps such a strict distinction is no longer needed. A new idea may be the early combination of levodopa with a dopamine agonist which would provide good clinical efficacy and, because of the relatively low doses involved, would reduce the side effects associated with both substances. MAO-B inhibitors may be a good option for early treatment and especially for patients who experience first motor fluctuations. Similarly, and particularly if a wearing-off symptom is present, COMT inhibitors smoothen and prolong the action of levodopa. More invasive escalation therapy comes into play when patients reach the advanced stages with problems of insufficient motor control, such as bradykinesia, rigidity and resting tremor, combined with on-time dyskinesia. The use of all oral and invasive treatment has to be individualized to gain a good motor and non-motor control and especially a good quality of life.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

Motor complications in an incident Parkinson's disease cohort

BACKGROUND AND PURPOSE

Levodopa treatment in Parkinson's disease (PD) causes motor fluctuations and dyskinesias, but few data describe their development or severity in unselected incident cohorts.

METHODS

Demographic, clinical, treatment, smoking, caffeine and alcohol data from 183 people with PD were gathered from the Parkinsonism Incidence in Northeast Scotland (PINE) study, a community-based, incident cohort. With Kaplan-Meier survival analysis and Cox regression modelling the development, and severity, of dyskinesias and motor fluctuations and which factors independently influenced their onset were assessed.

RESULTS

After a mean follow-up of 59 months, 39 patients (21.3%) developed motor fluctuations and 52 (28.4%) developed dyskinesias. Kaplan-Meier estimates of the probability of motor fluctuations and dyskinesias after 5 years of dopaminergic treatment were 29.2% [95% confidence interval (CI) 21.5%-38.8%] and 37.0% (95% CI 28.5%-47.1%) respectively. 19.8% developed motor fluctuations requiring treatment changes but only 4.0% (95% CI 1.5%-10.4%) developed dyskinesias requiring treatment changes by 5 years. Cumulative levodopa dose [hazard ratio (HR) 1.38 (95% CI 1.19-1.60)], female sex [HR 2.41 (1.19-4.89)] and younger age at diagnosis [HR 1.08 (1.04-1.11)] were independently associated with development of motor fluctuations. Cumulative levodopa dose [HR 1.23 (1.08-1.40)] and female sex [HR 2.51 (1.40-4.51)] were independently associated with dyskinesias. In exploratory analyses, moderate caffeine exposure was associated with fewer motor fluctuations, longer symptom duration with more dyskinesias, and tremor at diagnosis with higher rates of both complications.

CONCLUSIONS

In this community-based incident PD cohort, severe dyskinesias were rare. Cumulative levodopa dose was the strongest predictor of both dyskinesias and motor fluctuations.

Drug therapy for Parkinson’s disease: An update

Parkinson’s disease (PD) is the most common neurodegenerative movement disorder, affecting about 1% of the population above the age of 65. PD is characterized by a selective degeneration of the dopaminergic neurons of the substantia nigra pars compacta. This results in a marked loss of striatal dopamine and the development of the characteristic features of the disease, i.e., bradykinesia, rest tremor, rigidity, gait abnormalities and postural instability. Other types of neurons/neurotransmitters are also involved in PD, including cholinergic, serotonergic, glutamatergic, adenosine, and GABAergic neurotransmission which might have relevance to the motor, non-motor, neuropsychiatric and cognitive disturbances that occur in the course of the disease. The treatment of PD relies on replacement therapy with levodopa (L-dopa), the precursor of dopamine, in combination with a peripheral decarboxylase inhibitor (carbidopa or benserazide). The effect of L-dopa, however, declines over time together with the development of motor complications especially dyskinesia in a significant proportion of patients within 5 years of therapy. Other drugs include dopamine-receptor-agonists, catechol-O-methyltransferase inhibitors, monoamine oxidase type B (MAO-B) inhibitors, anticholinergics and adjuvant therapy with the antiviral drug and the N-methyl-D-aspartate glutamate receptor antagonist amantadine. Although, these medications can result in substantial improvements in parkinsonian symptoms, especially during the early stages of the disease, they are often not successful in advanced disease. Moreover, dopaminergic cell death continues over time, emphasizing the need for neuroprotective or neuroregenerative therapies. In recent years, research has focused on non-dopaminergic approach such as the use of A_2A receptor antagonists: istradefylline and preladenant or the calcium channel antagonist isradipine. Safinamide is a selective and reversible inhibitor of MAO-B, a glutamate receptor inhibitor as well as sodium and calcium channel blocker. Minocycline and pioglitazone are other agents which have been shown to prevent dopaminergic nigral cell loss in animal models of PD. There is also an evidence to suggest a benefit from iron chelation therapy with deferiprone and from the use of antioxidants or mitochondrial function enhancers such as creatine, alpha-lipoic acid, l-carnitine, and coenzyme Q10.

Dopamine agonists early monotherapy for the delay of development of levodopa-induced dyskinesias

Dyskinesias are common, often disabling motor complications emerging in Parkinson's disease following chronic levodopa treatment. Common views associate the development of dyskinesias both with progressive loss of striatal dopamine nerve terminals and with intermittent delivery of the short half-life levodopa. Thus, according to continuous dopaminergic stimulation theory, dopamine agonists having half-lifes longer than levodopa would minimize the risk of the development of dyskinesias. The article highlights some interesting aspects of the clinical trials testing dopamine agonists monotherapy as a strategy that can reduce the risk of motor complications, and raises some concerns in terms of their early use in Parkinson's disease treatment to prevent or delay dyskinesia. Finally, we emphasize the need for reconsideration of arguments against use of levodopa as a starting therapy for Parkinson's disease.

Effects of noradrenergic denervation by anti-DBH-saporin on behavioral responsivity to L-DOPA in the hemi-parkinsonian rat

Dopamine (DA) replacement with l-DOPA remains the most effective pharmacotherapy for motor symptoms of Parkinson's disease (PD) including tremor, postural instability, akinesia, and bradykinesia. Prolonged L-DOPA use frequently leads to deleterious side effects including involuntary choreic and dystonic movements known as L-DOPA induced dyskinesias (LID). DA loss in PD is frequently accompanied by concomitant noradrenergic (NE) denervation of the locus coeruleus (LC); however, the effects of NE loss on L-DOPA efficacy and LID remain controversial and are often overlooked in traditional animal models of PD. The current investigation examined the role of NE loss in L-DOPA therapy by employing the NE specific neurotoxin anti-DA-beta hydroxylase saporin (αDBH) in a rat model of PD. Rats received unilateral 6-hydroxydopamine lesions of the medial forebrain bundle to deplete nigral DA and intraventricular injection of vehicle (DA lesioned rats) or αDBH (DANE lesioned rats) to destroy NE neurons bilaterally. Results indicated that αDBH infusion drastically reduced NE neuron markers within the LC compared to rats that received vehicle treatment. Behaviorally, this loss did not alter the development or expression of L-DOPA- or DA agonist-induced dyskinesia. However, rats with additional NE lesions were less responsive to L-DOPA's pro-motor effects. Indeed, DANE lesioned animals rotated less and showed less attenuation of parkinsonian stepping deficits following high doses of L-DOPA than DA lesioned animals. These findings suggest that severe NE loss may reduce L-DOPA treatment efficacy and demonstrate that degradation of the NE system is an important consideration when evaluating L-DOPA effects in later stage PD.