Long-duration effect and the postsynaptic compartment: study using a dopamine agonist with a short half-life

Motivational Vigor in Parkinson's Disease Requires the Short and Long Duration Response to Levodopa

BACKGROUND

Impaired movement vigor (bradykinesia) is a cardinal feature of Parkinson's disease (PD) and hypothesized to result from abnormal motivational processes-impaired motivation-vigor coupling. Dopamine replacement therapy (DRT) improves bradykinesia, but the response to DRT is multifaceted, comprising a short-duration response (SDR) and a long-duration response (LDR) only manifesting with chronic treatment. Prior experiments assessing motivation-vigor coupling in PD used chronically treated subjects, obscuring the roles of the SDR and LDR.

METHODS

To disambiguate the SDR and LDR, 11 de novo PD subjects (6 male [M]:5 female [F]; mean age, 67) were studied before treatment, after an acute levodopa (l-dopa) dose, and in both the practical "off" (LDR) and "on" (LDR + SDR) states after chronic stable treatment. At each visit, subjects were characterized with a standard battery including the Movement Disorder Society-Sponsored Revision of the Unified Parkinson's Disease Rating Scale (MDS-UPDRS) and an incentivized joystick task to assess motor performance in response to varying rewards.

RESULTS

l-Dopa induced a robust SDR and LDR, with further improvement in the combined SDR + LDR state. At baseline, after acute treatment (SDR), and after LDR induction, subjects did not exhibit the normal increase in movement speed with increasing reward. Only in the combined SDR + LDR state was there restoration of motivation-vigor coupling.

CONCLUSIONS

Although consistent with prior results in chronically treated PD subjects, the significant improvement in motor performance observed with the SDR and LDR suggests that bradykinesia is not solely secondary to deficient modulation of motivational processes. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Adopting the Rumsfeld approach to understanding the action of levodopa and apomorphine in Parkinson's disease

Dopaminergic therapies dominate the treatment of the motor and non-motor symptoms of Parkinson's disease (PD) but there have been no major advances in therapy in many decades. Two of the oldest drugs used appear more effective than others-levodopa and apomorphine-but the reasons for this are seldom discussed and this may be one cause for a lack of progress. This short review questions current thinking on drug action and looks at whether adopting the philosophy of ex-US Secretary of State Donald Rumsfeld reveals 'unknown' aspects of the actions of levodopa and apomorphine that provide clues for a way forward. It appears that both levodopa and apomorphine have a more complex pharmacology than classical views would suggest. In addition, there are unexpected facets to the mechanisms through which levodopa acts that are either forgotten as 'known unknowns' or ignored as 'unknown unknowns'. The conclusion reached is that we may not know as much as we think about drug action in PD and there is a case for looking beyond the obvious.

Why do 'OFF' periods still occur during continuous drug delivery in Parkinson's disease?

Continuous drug delivery (CDD) is used in moderately advanced and late-stage Parkinson’s disease (PD) to control motor and non-motor fluctuations (‘OFF’ periods). Transdermal rotigotine is indicated for early fluctuations, while subcutaneous apomorphine infusion and levodopa-carbidopa intestinal gel are utilised in advanced PD. All three strategies are considered examples of continuous dopaminergic stimulation achieved through CDD. A central premise of the CDD is to achieve stable control of the parkinsonian motor and non-motor states and avoid emergence of ‘OFF’ periods. However, data suggest that despite their efficacy in reducing the number and duration of ‘OFF’ periods, these strategies still do not prevent ‘OFF’ periods in the middle to late stages of PD, thus contradicting the widely held concepts of continuous drug delivery and continuous dopaminergic stimulation. Why these emergent ‘OFF’ periods still occur is unknown. In this review, we analyse the potential reasons for their persistence. The contribution of drug- and device-related involvement, and the problems related to site-specific drug delivery are analysed. We propose that changes in dopaminergic and non-dopaminergic mechanisms in the basal ganglia might render these persistent ‘OFF’ periods unresponsive to dopaminergic therapy delivered via CDD.

Adaptive changes in striatal projection neurons explain the long duration response and the emergence of dyskinesias in patients with Parkinson's disease

Neuronal activity in the brain is tightly regulated. During operation in real time, for instance, feedback and feedforward loops limit excessive excitation. In addition, cell autonomous processes ensure that neurons’ average activity is restored to a setpoint in response to chronic perturbations. These processes are summarized as homeostatic plasticity (Turrigiano in Cold Spring Harb Perspect Biol 4:a005736–a005736, 2012). In the basal ganglia, information is mainly transmitted through disinhibition, which already constraints the possible range of neuronal activity. When this tightly adjusted system is challenged by the chronic decline in dopaminergic neurotransmission in Parkinson’s disease (PD), homeostatic plasticity aims to compensate for this perturbation. We here summarize recent experimental work from animals demonstrating that striatal projection neurons adapt excitability and morphology in response to chronic dopamine depletion and substitution. We relate these cellular processes to clinical observations in patients with PD that cannot be explained by the classical model of basal ganglia function. These include the long duration response to dopaminergic medication that takes weeks to develop and days to wear off. Moreover, dyskinesias are considered signs of excessive dopaminergic neurotransmission in Parkinson’s disease, but they are typically more severe on the body side that is more strongly affected by dopamine depletion. We hypothesize that these clinical observations can be explained by homeostatic plasticity in the basal ganglia, suggesting that plastic changes in response to chronic dopamine depletion and substitution need to be incorporated into models of basal ganglia function. In addition, better understanding the molecular mechanism of homeostatic plasticity might offer new treatment options to avoid motor complications in patients with PD.

Therapeutic Strategies to Treat or Prevent Off Episodes in Adults with Parkinson's Disease

Parkinson's disease is a chronic, neurodegenerative disease, which manifests with a mixture of motor, cognitive and behavioural symptoms. Levodopa is the most effective antiparkinsonian treatment to date, although chronic use engenders a mixture of complications in a substantial proportion of patients. Amongst these is the occurrence of episodes of worsening symptoms-'off' phenomena. These episodes can manifest with either motor or non-motor symptoms or a combination of these features and have been found to have profound impacts on patients' quality of life. Although preventative measures are poorly evidenced, avoiding excessive total daily levodopa intake in selected populations that are deemed to be of a higher risk for developing these episodes warrants further exploration. Methods to improve levodopa bioavailability and delivery to the brain are currently available and are of value in addressing these episodes once they have become established. These include modifications to levodopa formulations as well as the use of complimentary agents that improve levodopa bioavailability. The deployment of device-assisted approaches is a further dimension that can be considered in addressing these debilitating episodes. This review summarises the clinical manifestations of 'off' phenomena and the current approaches to treat them. Although we briefly discuss clinical advances on the horizon, the predominant focus is on existing, established treatments.

Parkinson's disease and translational research

Parkinson’s disease (PD) is diagnosed when patients exhibit bradykinesia with tremor and/or rigidity, and when these symptoms respond to dopaminergic medications. Yet in the last years there was a greater recognition of additional aspects of the disease including non-motor symptoms and prodromal states with associated pathology in various regions of the nervous system. In this review we discuss current concepts of two major alterations found during the course of the disease: cytoplasmic aggregates of the protein α-synuclein and the degeneration of dopaminergic neurons. We provide an overview of new approaches in this field based on current concepts and latest literature. In many areas, translational research on PD has advanced the understanding of the disease but there is still a need for more effective therapeutic options based on the insights into the basic biological phenomena.

Dopamine agonist treatment increases sensitivity to gamble outcomes in the hippocampus in de novo Parkinson's disease

BACKGROUND

Parkinson's disease is associated with severe nigro-striatal dopamine depletion, leading to motor dysfunction and altered reward processing. We previously showed that drug-naïve patients with Parkinson's disease had a consistent attenuation of reward signalling in the mesolimbic and mesocortical system. Here, we address the neurobiological effects of dopaminergic therapy on reward sensitivity in the mesolimbic circuitry, and how this may contribute to neuropsychiatric symptoms.

OBJECTIVES

We tested the hypothesis that (1) dopaminergic treatment would restore the attenuated, mesolimbic and mesocortical responses to reward; and (2) restoration of reward responsivity by dopaminergic treatment would predict motor performance and the emergence of impulse control symptoms.

METHODS

In 11 drug-naïve Parkinson patients, we prospectively assessed treatment-induced changes in reward processing before, and eight weeks after initiation of monotherapy with dopamine agonists. They were compared to 10 non-medicated healthy controls who were also measured longitudinally. We used whole-brain functional magnetic resonance imaging at 3 Tesla to assess the reward responsivity of the brain to monetary gains and losses, while participants performed a simple consequential gambling task.

RESULTS

In patients, dopaminergic treatment improved clinical motor symptoms without significantly changing task performance. Dopamine agonist therapy induced a stronger reward responsivity in the right hippocampus with higher doses being less effective. None of the patients developed impulse control disorders in the follow-up period of four years.

CONCLUSIONS

Short-term treatment with first-ever dopaminergic medication partially restores deficient reward-related processing in the hippocampus in de novo Parkinson's disease.

A systematic review on the clinical experience with melevodopa/carbidopa fixed combination in patients with Parkinson disease

INTRODUCTION

We have performed a systematic literature review to evaluate the current evidence of the pharmacokinetic (PK), efficacy and safety profile of oral melevodopa/carbidopa fixed combination in the management of motor fluctuations in patients with Parkinson disease (PD).

EVIDENCE ACQUISITION

Search has been started from common libraries and was then restricted to articles of studies in humans with melevodopa/carbidopa as fixed combination. Abstracts of international congresses have been also explored. The search has led to the identification of one PK study and five efficacy/safety studies that included more 743 PD patients overall, 488 of which treated with melevodopa/carbidopa effervescent tablets (Sirio®, Chiesi Farmaceutici S.p.A., Parma, Italy).

EVIDENCE SYNTHESIS

Melevodopa/carbidopa has a more rapid absorption, less apparent drug accumulation, less inter-patient variability and more effective LD delivery after the early morning and early afternoon dose compared to standard LD. Although they differed in used dose regimens, duration of treatment and endpoints, the results of the efficacy/safety studies suggest that treatment with melevodopa/carbidopa may determine improvements in motor fluctuations compared to treatment with standard LD/CD formulations, with no increased risk of adverse effects or LD-induced dyskinesias.

CONCLUSIONS

Data from literature have shown that melevodopa/carbidopa in a highly soluble formulation as effervescent oral tablets is effective in improving control of motor complications in PD patients compared to conventional levodopa. Further research is needed to confirm this evidence in large controlled trials and to explore if melevodopa/carbidopa may have a potential role in the earlier phases of the disease or in special PD populations.

The spectrum of "off" in Parkinson's disease: What have we learned over 40 years?

The terms "on" and "off" were used by Marsden and his contemporaries over 40 years ago to describe times when Parkinson's disease patients experienced good motor function ("on") and immobility ("off"). Yet there remains no published consensus definition of "off", leading clinicians and patients to develop individualized impressions of "off" determinations. In this paper, we first discuss the evolution of the terminology and understanding of "off" states since Marsden's time, which now include non-motor as well as motor symptoms. We then review pathophysiology and risk factors for the development of "off" states as well as tools to detect the "off" state, before proposing a practical definition of "off" for consideration. A common, practical definition of the "off" state could improve clinical recognition of "off" symptoms and lead to significant benefit for patients.

Calcium, mitochondrial dysfunction and slowing the progression of Parkinson's disease

Parkinson's disease is characterized by progressively distributed Lewy pathology and neurodegeneration. The motor symptoms of clinical Parkinson's disease (cPD) are unequivocally linked to the degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNc). Several features of these neurons appear to make them selectively vulnerable to factors thought to cause cPD, like aging, genetic mutations and environmental toxins. Among these features, Ca²⁺ entry through Cav1 channels is particularly amenable to pharmacotherapy in early stage cPD patients. This review outlines the linkage between these channels, mitochondrial oxidant stress and cPD pathogenesis. It also summarizes considerations that went into the design and execution of the ongoing Phase 3 clinical trial with an inhibitor of these channels - isradipine.

A novel design of a Phase III trial of isradipine in early Parkinson disease (STEADY-PD III)

Objective

To describe the rationale for a novel study design and baseline characteristics of a disease‐modifying trial of isradipine 10 mg daily in early Parkinson disease (PD).

Methods

STEADY‐PDIII is a 36‐month, Phase 3, parallel group, placebo‐controlled study of the efficacy of isradipine 10 mg daily in 336 participants with early PD as measured by the change in the Unified Parkinson Disease Rating Scale (UPDRS) Part I‐III score in the practically defined ON state. Secondary outcome measures include clinically meaningful measures of disability progression in early PD: (1) Time to initiation and utilization of dopaminergic therapy; (2) Time to onset of motor complications; (3) Change in nonmotor disability. Exploratory measures include global measures of functional disability, quality of life, change in the ambulatory capacity, cognitive function, and pharmacokinetic analysis. Rationale for the current design and alternative design approaches are discussed.

Results

The entire cohort of 336 participants was enrolled at 55 Parkinson Study Group sites in North America. The percentage of male participants were 68.5% with a mean age of 61.9 years (sd 9.0), mean Hoehn and Yahr stage of 1.7 (sd 0.5), mean UPDRS total of 23.1 (sd 8.6), and MoCA of 28.1 (sd 1.4).

Interpretation

STEADY‐PD III has a novel and innovative design allowing for the determination of longer duration benefits on clinically relevant outcomes in a relatively small cohort on top of the benefit derived from symptomatic therapy. Baseline characteristics are similar to those in previously enrolled de novo PD trials. This study represents a unique opportunity to evaluate the potential impact of a novel therapy to slow progression of PD disability and provide clinically meaningful benefits.

The role of neuroplasticity in dopaminergic therapy for Parkinson disease

Dopamine replacement is a mainstay of therapeutic strategies for Parkinson disease (PD). The motor response to therapy involves an immediate improvement in motor function, known as the short-duration response (SDR), followed by a long-duration response (LDR) that develops more slowly, over weeks. Here, we review evidence in patients and animal models suggesting that dopamine-dependent corticostriatal plasticity, and retention of such plasticity in the absence of dopamine, are the mechanisms underlying the LDR. Conversely, experience-dependent aberrant plasticity that develops slowly under reduced dopamine levels could contribute substantially to PD motor symptoms before initiation of dopamine replacement therapy. We place these findings in the context of the role of dopamine in basal ganglia function and corticostriatal plasticity, and provide a new framework suggesting that therapies that enhance the LDR could be more effective than those targeting the SDR. We further propose that changes in neuroplasticity constitute a form of disease modification that is distinct from prevention of degeneration, and could be responsible for some of the unexplained disease-modifying effects of certain therapies. Understanding such plasticity could provide novel therapeutic approaches that combine rehabilitation and pharmacotherapy for treatment of neurological and psychiatric disorders involving basal ganglia dysfunction.

Concomitant short- and long-duration response to levodopa in the 6-OHDA-lesioned rat: a behavioural and molecular study

The long-duration response (LDR) is a sustained improvement in parkinsonism due to chronic levodopa therapy and lasts after discontinuation of treatment. We have investigated the molecular changes that underlie the LDR in rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion. Animals were treated for 22 days with levodopa or saline. Forelimb akinesia was evaluated prior and following a test dose of levodopa. Rotational behaviour was weekly evaluated. Levodopa induced an improvement in the parkinsonian limb akinesia that lasted for 48 h after withdrawal. A shortening in the duration of rotational behaviour was observed. After 3 days of washout, levodopa treatment maintained elevated striatal preproenkephalin mRNA expression, also inducing an increase in preprodynorphin (PDyn) and dopamine D-3 receptor mRNAs, but without any modification of the adenosine A(2A) mRNA expression induced by 6-OHDA. Levodopa reversed the lesion-induced increase in the expression of cytochrome oxidase mRNA in the subthalamic nucleus and glutamate decarboxylase mRNA in the pars reticulata of the substantia nigra. After 7 days of levodopa washout, the molecular markers show a decline in the basal ganglia evolving towards the parkinsonian state, being statistically significant for the striatal PDyn mRNA. This study characterizes the concomitant presence of the short-duration response and LDR to levodopa in the 6-OHDA model of parkinsonism and shows that the molecular changes induced by levodopa in the basal ganglia are not permanent and that this reversal after levodopa washout may be responsible for the gradual motor deterioration that characterize the LDR.

End-of-dose deterioration in non ergolinic dopamine agonist monotherapy of Parkinson’s disease

The study was designed to investigate the possible occurrence of "wearing-off" (WO) during dopamine agonist (DA) monotherapy. Sixty patients with "de novo" idiopathic PD were randomised into one of two DA monotherapy branches to receive oral ropinirole at 15 mg per day, or pramipexole at 2.1 mg per day. DA doses could be increased in the following two years but levodopa could not be added until the study ended. WO was assessed by self-evaluation charts confirmed by a blinded observation of a 30% or greater deterioration in the Unified Parkinson's Disease Rating Scale (UPDRS) motor score. Proc Mixed and Kaplan-Meier curves evaluated treatment variables as a function of time. T-tests were used to compare post-hoc variables reclassified according to WO occurrence. Thirty patients received ropinirole, and 30 pramipexole monotherapy. Eighteen patients (30%) experienced "wearing-off" 15-21 months after beginning monotherapy. No differences were observed between treatments. WO phenomena was observed 3.4+/-0.3 hours after intake of the morning or afternoon dose and consisted of UPDRS score worsening by 11.1+/-2.1 points (69-111% more than "on" score). Statistical evaluation gave evidence of differences between patients who experienced WO and those who did not: UPDRS motor scores obtained at admission to the study were higher (by 3.4+/-0.2 points, p=0.01 t-test) and DA doses at 6-12 months were higher in fluctuating patients. UPDRS motor scores deteriorated, however. similarly and there were no differences, in UPDRS scores recorded in ON conditions, between fluctuating and non-fluctuating patients at the end of the study. Our findings provide evidence of WO phenomena in patients with early PD receiving non-ergolinic DA monotherapy.

The levodopa wearing-off phenomenon in Parkinson’s disease: pharmacokinetic considerations

Levodopa is the most efficacious treatment in the management of Parkinson's disease. Unfortunately, chronic use of traditional levodopa/dopa decarboxylase inhibitor formulations is associated with the development of complications, such as wearing-off and dyskinesia. In an attempt to avoid these complications, some physicians delay the introduction of levodopa or employ levodopa-sparing strategies; however, these strategies are frequently suboptimal for patients. As most patients require the superior efficacy of levodopa during the course of their disease, an appreciation of the changing response to levodopa over time and an understanding of the pharmacokinetic principles underlying the development of complications such as wearing-off is essential in the long-term management of the patient.

Pharmacokinetic and Pharmacodynamic Changes During the First Four Years of Levodopa Treatment in Parkinson’s Disease

The purpose of this analysis is to describe how levodopa pharmacokinetic and pharmacodynamic parameters change over the first 4 years of long-term levodopa treatment in patients with Parkinson's disease. Twenty previously untreated Parkinsonian patients were admitted to the general clinical research center (GCRC) for 4 days at the beginning of long-term levodopa therapy and 6, 12, 24 and 48 months later. On each GCRC admission, patients received a 2 hr IV infusion of levodopa on day 1 and day 4 with no oral levodopa between the infusions. After the first GCRC admission patients were treated with oral levodopa dosed for optimal control of Parkinsonism. Motor function was measured by finger tapping rate. A pharmacokinetic-pharmacodynamic model incorporating 3 effect compartments was used to fit the individual plasma levodopa concentrations and tapping rates. Motor function before the first levodopa infusion (E0(1)) improved over the first 20 months and subsequently returned to the initial baseline at the start of the study. A similar pattern was seen in motor function before the second infusion (E0(2)) after the 3 days levodopa withdrawal, with a decline predicted to fall below the initial baseline at the start of the study by 6 years. Eight patients showed an increase in maximum tapping rate with levodopa (E(max)) approaching a steady state after 16 months. Ten patients showed an increase in E(max) with a peak at 31 months. One patient showed a linear decrease and another patient did not change over the 48 months. Longitudinal progress models were used to describe the time course of pharmacokinetic and pharmacodynamic parameters over 4 years. Peak treatment benefit, defined as the difference between E(max) and E0(1) or E0(2) (D(max)1 or D(max)2), increased with time particularly after the 3-day levodopa withdrawal. Deterioration of pre-dose motor function (E0) as disease progresses coupled with a greater amplitude of response due to levodopa (D(max)) could be a key factor contributing to motor fluctuations associated with long-term levodopa treatment.

Novel pharmacological strategies for motor complications in Parkinson’s disease

In advanced Parkinson's disease, the combination of disease progression and levodopa therapy leads to the development of motor problems complicating the therapeutic response, known as motor response complications. The nonphysiological, pulsatile stimulation produced by most currently available dopaminergic therapies triggers a complicated series of responses resulting in the dysregulation of glutamate receptors and many other neurotransmitter systems on striatal neurons. Although a number of novel compounds that provide a more continuous dopaminergic stimulation are becoming available, no practical way to accomplish this in a truly physiological manner currently exists. Novel strategies for pharmacological intervention with the use of nondopaminergic treatments, with drugs targeting selected transmitter receptors expressed on striatal neurons appear more promising. These include NMDA or AMPA antagonists, or drugs acting on 5-hydroxytryptamine subtype 2A, alpha2-adrenergic, adenosine A2A and cannabinoid CB1 receptors. Future strategies may also target pre- and postsynaptic components that regulate firing pattern, like synaptic vesicle proteins, or nonsynaptic gap junction communication mechanisms, or drugs with actions at the signal transduction systems that modulate the phosphorylation state of NMDA receptors. These new therapeutic strategies, alone or in combination, hold the promise of providing effective control or reversal of motor response complications.

Dopamine agonists in Parkinson’s disease

Levodopa (LD), the immediate precursor of dopamine, is the most effective agent in the treatment of Parkinson's disease (PD). While quite successful in treating the primary motor deficits of PD, most patients eventually develop LD-related motor fluctuation, dyskinesias and other adverse effects associated with chronic LD therapy. There is also concern that LD is neurotoxic, although this has not been demonstrated in any in vivo studies. Dopamine agonists (DAs) have been shown to be about as effective as LD in symptomatic treatment of mild-to-moderate PD. In addition, there is a lower tendency to develop motor fluctuations and dyskinesias with DA treatment than after initiation of therapy with LD. Furthermore, there is preclinical and clinical data to suggest a slowing of neurodegeneration with DAs. The adverse effects of DAs are similar to those experienced with LD, except that the ergot agents are associated with a small risk of tissue fibrosis not noted with the non-ergot DAs.

Arguments for the use of dopamine receptor agonists in clinical and preclinical Parkinson's disease

On the basis of experimental studies which have demonstrated deleterious effects of L-DOPA (L-3,4-dihydroxyphenylalanine) in vivo and in vitro, it has been suggested that L-DOPA itself may contribute to the progression of Parkinson's disease. This hypothesis is, for many clinicians, the rationale for postponing the employment of and reducing the applied dosage of L-DOPA and for beginning therapy with dopamine receptor agonists or the monoamine oxidase type B (MAO-B) inhibitor selegiline. Furthermore, clinical studies have demonstrated that early treatment with dopamine receptor agonists is associated with a lower incidence of motor fluctuations and dyskinesia. Dopamine receptor agonists exert their symptomatic effect by directly activating dopamine receptors, bypassing the presynaptic synthesis of dopamine and the degenerating nigro-striatal dopaminergic system. They can thus also be of benefit late in the therapy of the disorder. In addition, the pharmacological profile of dopamine receptor agonists suggests a possible neuroprotective effect. This paper reviews briefly the pharmacology of dopamine receptor agonists and basic knowledge concerning the dopamine receptor stimulation which underlies their therapeutic effect. Preclinical approaches for demonstrating neuroprotective effects and their clinical relevance are also discussed.

Prevention and treatment of motor fluctuations

The symptoms of Parkinson's disease can become increasingly difficult to control as the disease advances, particularly with the development of motor complications, such as end-of-dose wearing-off and dyskinesias, following long-term therapy. At this stage, the patient is frequently referred to a Parkinson's disease specialist for advice on the management of their disease. In this review we provide an overview of the Parkinson's disease specialist's strategies for coping with such problems. This includes strategy to prevent or delay motor fluctuations and the concept of the long duration response. The paper also includes establishing the optimum and most rational levodopa treatment schedule, improving levodopa absorption, use of COMT-inhibition, the addition of oral dopaminergic agonists, and the use of subcutaneous injections or infusions of apomorphine or lisuride. Finally, we highlight the increasing importance of treatment strategies that stimulate dopamine receptors in a more continuous, less pulsatile manner as a way of reducing the risk of developing treatment-associated motor complications.

Apomorphine infusion and the long-duration response to levodopa in advanced Parkinson's disease

The authors investigated the long-duration response to levodopa in advanced Parkinson's disease. Eight patients with advanced Parkinson's disease disabled by severe ON/OFF fluctuations treated by chronic daytime subcutaneous apomorphine infusion with supplemental oral levodopa were studied. On day 1, oral levodopa was withdrawn at 4:00 pm and on the following morning subcutaneous apomorphine infusion was continued at the same rate without levodopa therapy. While receiving apomorphine alone, seven of the eight patients turned ON, and their usual dyskinesias returned. The ON phase persisted for 60 to 100 minutes (mean, 185.7 minutes) but then, despite continued, constant-rate apomorphine infusion to stabilize plasma levels, switched to an OFF phase. The authors conclude that the clinical effect of apomorphine is sustained by levodopa long-duration response. This effect is probably the result of postsynaptic mechanisms. In patients with advanced Parkinson's disease, the long-duration response to levodopa is present although slightly diminished.

Motor response to levodopa and the evolution of motor fluctuations in the first decade of treatment of Parkinson's disease

Thirty-four patients with Parkinson's disease were followed for a mean period of 8 years from the time of initiation of levodopa medication. Levodopa response was charted from the starting point of pharmacological treatment to give a longitudinal point of view of the changes that evolve as the disease progresses. Objective measurements of the motor response to levodopa test-doses were made at approximately three yearly intervals. Motor fluctuations developed in 58% of the patient group after a mean treatment period of 35 months. Dyskinesia developed in parallel with fluctuations but appeared on average 7 months before symptomatic wearing-off effects of levodopa doses. The patients with motor fluctuations had significantly better responses to levodopa. By contrast, nonfluctuators were more prone to develop increasing midline motor disability affecting speech, gait and balance. Comparison of test-dose and pretreatment scores suggested that a substantial long-duration response to levodopa remains after many years of treatment, and that lateralized motor deficits show a stronger long duration response than midline ones. Motor fluctuations are a consequence of disease progression but their early development is, on balance, associated with better long-term functional ability because these patients have the greater capacity to respond to pharmacological treatment.

Evolution of the response to levodopa during the first 4 years of therapy

The short-duration response, long-duration response, and dyskinetic response to levodopa change during long-term levodopa therapy. How these responses evolve, and which changes contribute to the emergence of motor fluctuations, remain unclear. We studied 18 subjects with Parkinson's disease before they began levodopa therapy and after 6, 12, 24, and 48 months of long-term levodopa therapy. The responses to 2-hour levodopa infusions after overnight and after 3 days of levodopa withdrawal were studied from 6 months onward. The mean magnitude of the short-duration response and the long-duration response measured after overnight without antiparkinsonian medications did not change during the 4 years. However, after 3 days without levodopa, it was apparent that the short-duration-response magnitude was progressively increasing (p < 0.0001) and that the long-duration response was decaying more rapidly (p = 0.0004). The short-duration-response magnitude at 4 years was inversely related to the long-duration-response magnitude (p = 0.022), suggesting that the long-duration response was one determinant of the short-duration-response magnitude. Dyskinesia increased progressively in severity during the study (p = 0.013). The duration of the short-duration response and dyskinesia did not change during the 4 years. Subject reports of motor fluctuations tended to be associated with a large short-duration response (p = 0.054). We suggest that a larger long-duration response, rather than a shortened one, is more important to the development of fluctuations. Improving the baseline or practical-off motor function to reduce the magnitude of the short-duration response may be a strategy to treat fluctuations.