Potential therapeutic use of the selective dopamine D1 receptor agonist, A-86929: an acute study in parkinsonian levodopa-primed monkeys

Predictive Value of Parkinsonian Primates in Pharmacologic Studies: A Comparison between the Macaque, Marmoset, and Squirrel Monkey

The 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned primate is the gold-standard animal model of Parkinson disease (PD) and has been used to assess the effectiveness of experimental drugs on dyskinesia, parkinsonism, and psychosis. Three species have been used in most studies-the macaque, marmoset, and squirrel monkey-the last much less so than the first two species; however, the predictive value of each species at forecasting clinical efficacy, or lack thereof, is poorly documented. Here, we have reviewed all the published literature detailing pharmacologic studies that assessed the effects of experimental drugs on dyskinesia, parkinsonism, and psychosis in each of these species and have calculated their predictive value of success and failure at the clinical level. We found that, for dyskinesia, the macaque has a positive predictive value of 87.5% and a false-positive rate of 38.1%, whereas the marmoset has a positive predictive value of 76.9% and a false-positive rate of 15.6%. For parkinsonism, the macaque has a positive predictive value of 68.2% and a false-positive rate of 44.4%, whereas the marmoset has a positive predictive value of 86.9% and a false-positive rate of 41.7%. No drug that alleviates psychosis in the clinic has shown efficacy at doing so in the macaque, whereas the marmoset has 100% positive predictive value. The small number of studies conducted in the squirrel monkey precluded us from calculating its predictive efficacy. We hope our results will help in the design of pharmacologic experiments and will facilitate the drug discovery and development process in PD.

Pharmacological strategies for the management of levodopa-induced dyskinesia in patients with Parkinson's disease

L-Dopa-induced dyskinesias (LID) are the most common adverse effects of long-term dopaminergic therapy in Parkinson's disease (PD). However, the exact mechanisms underlying dyskinesia are still unclear. For a long time, nigrostriatal degeneration and pulsatile stimulation of striatal postsynaptic receptors have been highlighted as the key factors for the development of LID. In recent years, PD models have revealed a wide range of non-dopaminergic neurotransmitter systems involved in pre- and postsynaptic changes and thereby contributing to the pathophysiology of LID. In the current review, we focus on therapeutic LID targets, mainly based on agents acting on dopaminergic, glutamatergic, serotoninergic, adrenergic, and cholinergic systems. Despite a large number of clinical trials, currently only amantadine and, to a lesser extent, clozapine are being used as effective strategies in the treatment of LID in clinical settings. Thus, in the second part of the article, we review the placebo-controlled trials on LID treatment in order to disentangle the changing scenario of drug development. Promising results include the extension of L-dopa action without inducing LID of the novel monoamine oxidase B- and glutamate-release inhibitor safinamide; however, this had no obvious effect on existing LID. Others, like the metabotropic glutamate-receptor antagonist AFQ056, showed promising results in some of the studies; however, confirmation is still lacking. Thus, to date, strategies of continuous dopaminergic stimulation seem the most promising to prevent or ameliorate LID. The success of future therapeutic strategies once moderate to severe LID occur will depend on the translation from preclinical experimental models into clinical practice in a bidirectional process.

D1 receptor agonist improves sleep-wake parameters in experimental parkinsonism

Both excessive daytime sleepiness (EDS) and rapid eye movement (REM) sleep deregulation are part of Parkinson's disease (PD) non-motor symptoms and may complicate dopamine replacement therapy. We report here that dopamine agonists act differentially on sleep architecture in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine macaque monkey. Continuous sleep and wake electroencephalographic monitoring revealed no effect of the selective dopamine D2 receptor agonist quinpirole on EDS, whereas the selective dopamine D1 receptor agonist SKF38393 efficiently alleviated EDS and restored REM sleep to baseline values. The present results question the relevance of abandoning D1 receptor agonist treatment in PD as it might actually improve sleep-related disorders.

The pharmacology of L-DOPA-induced dyskinesia in Parkinson's disease

L-3,4-Dihydroxyphenylalanine (L-DOPA) remains the most effective symptomatic treatment of Parkinson's disease (PD). However, long-term administration of L-DOPA is marred by the emergence of abnormal involuntary movements, i.e., L-DOPA-induced dyskinesia (LID). Years of intensive research have yielded significant progress in the quest to elucidate the mechanisms leading to the development and expression of dyskinesia and maintenance of the dyskinetic state, but the search for a complete understanding is still ongoing. Herein, we summarize the current knowledge of the pharmacology of LID in PD. Specifically, we review evidence gathered from postmortem and pharmacological studies, both preclinical and clinical, and discuss the involvement of dopaminergic and nondopaminergic systems, including glutamatergic, opioid, serotonergic, γ-aminobutyric acid (GABA)-ergic, adenosine, cannabinoid, adrenergic, histaminergic, and cholinergic systems. Moreover, we discuss changes occurring in transcription factors, intracellular signaling, and gene expression in the dyskinetic phenotype. Inasmuch as a multitude of neurotransmitters and receptors play a role in the etiology of dyskinesia, we propose that to optimally alleviate this motor complication, it may be necessary to develop combined treatment approaches that will target simultaneously more than one neurotransmitter system. This could be achieved via three ways as follows: 1) by developing compounds that will interact simultaneously to a multitude of receptors with the required agonist/antagonist effect at each target, 2) by targeting intracellular signaling cascades where the signals mediated by multiple receptors converge, and/or 3) to regulate gene expression in a manner that has effects on signaling by multiple pathways.

Anatomy of Graft-induced Dyskinesias: Circuit Remodeling in the Parkinsonian Striatum

The goal of researchers and clinicians interested in re-instituting cell based therapies for PD is to develop an effective and safe surgical approach to replace dopamine (DA) in individuals suffering from Parkinson's disease (PD). Worldwide clinical trials involving transplantation of embryonic DA neurons into individuals with PD have been discontinued because of the often devastating post-surgical side-effect known as graft-induced dyskinesia (GID). There have been many review articles published in recent years on this subject. There has been a tendency to promote single factors in the cause of GID. In this review, we contrast the pros and cons of multiple factors that have been suggested from clinical and/or preclinical observations, as well as novel factors not yet studied that may be involved with GID. It is our intention to provide a platform that might be instrumental in examining how individual factors that correlate with GID and/or striatal pathology might interact to give rise to dysfunctional circuit remodeling and aberrant motor output.

Striatal Signaling in L-DOPA-Induced Dyskinesia: Common Mechanisms with Drug Abuse and Long Term Memory Involving D1 Dopamine Receptor Stimulation

Parkinson’s disease is a common neurodegenerative disorder caused by the degeneration of midbrain substantia nigra dopaminergic neurons that project to the striatum. Despite extensive investigation aimed at finding new therapeutic approaches, the dopamine precursor molecule, 3,4-dihydroxyphenyl-l-alanine (l-DOPA), remains the most effective and commonly used treatment. However, chronic treatment and disease progression lead to changes in the brain’s response to l-DOPA, resulting in decreased therapeutic effect and the appearance of dyskinesias. l-DOPA-induced dyskinesia (LID) interferes significantly with normal motor activity and persists unless l-DOPA dosages are reduced to below therapeutic levels. Thus, controlling LID is one of the major challenges in Parkinson’s disease therapy. LID is the result of intermittent stimulation of supersensitive D1 dopamine receptors located in the very severely denervated striatal neurons. Through increased coupling to Gα_olf, resulting in greater stimulation of adenylyl-cyclase, D1 receptors phosphorylate DARPP-32, and other protein kinase A targets. Moreover, D1 receptor stimulation activates extracellular signal-regulated kinase and triggers a signaling pathway involving mammalian target for rapamycin and modifications of histones that results in changes in translation, chromatin modification, and gene transcription. In turn, sensitization of D1 receptor signaling causes a widespread increase in the metabolic response to D1 agonists and changes in the activity of basal ganglia neurons that correlate with the severity of LID. Importantly, different studies suggest that dyskinesias may share mechanisms with drug abuse and long term memory involving D1 receptor activation. Here we review evidence implicating D1 receptor signaling in the genesis of LID, analyze mechanisms that may translate enhanced D1 signaling into dyskinetic movements, and discuss the possibility that the mechanisms underlying LID are not unique to the Parkinson’s disease brain.

Dopamine receptor agonists for the treatment of early or advanced Parkinson's disease

Dopamine receptor agonists are indicated for the symptomatic treatment of early, moderate or advanced Parkinson's disease as well as for the reduction of levodopa-related motor complications. Ergolinic dopamine agonists, such as bromocriptine or pergolide, were initially developed and marketed, and then non-ergolinic dopamine agonists, such as pramipexole and ropinirole, were introduced, reducing the risk of drug-induced fibrotic reactions. Once-daily, controlled-release oral and transdermal formulations have been developed aiming at providing more stable 24-hour plasma drug concentrations and more convenient administration. A disease-modifying effect of dopamine agonists has not been demonstrated clinically. As with any other drug, dopamine agonists can also cause adverse drug reactions, which can be related or unrelated to dopaminergic hyperactivation. Dopaminergic reactions include nausea, hallucinations, confusion and orthostatic hypotension, among others, which were readily identified in pre-marketing clinical trials. During post-marketing surveillance, important adverse reactions were identified, such as daytime somnolence, impulse-control disorders and heart valve fibrosis. Other issues, including the efficacy of dopamine agonists for the treatment of non-motor symptoms, remain under evaluation.

The neuropharmacology of (-)-stepholidine and its potential applications

(-)-Stepholidine (SPD), a natural product isolated from the Chinese herb Stephania, possesses dopamine (DA) D1 partial agonistic and D2 antagonistic properties in the nigrostriatal and mesocorticolimbic DAergic pathways. These unique dual effects have suggested that SPD can effectively restore previously imbalanced functional linkage between D1 and D2 receptors under schizophrenic conditions, in which, SPD improves both the negative and positive symptoms of schizophrenia. SPD also relieves the motor symptoms of Parkinson's disease (PD) when co-administered with Levodopa. Furthermore, SPD exhibits neuroprotective effects through an antioxidative mechanism and slows down the progression of neuronal degeneration in the substantia nigra (SN) of PD patients and/or animal models. Therefore, SPD is a novel, natural compound with potentially therapeutic roles in the treatment of schizophrenia and/or PD.

Impact of Functional Age on the Use of Dopamine Agonists in Patients With Parkinson Disease

BACKGROUND

Parkinson disease (PD) is primarily a neurodegenerative disorder that affects as many as 1,500,000 people in the United States. It is predominantly a disease of the elderly, and special treatment challenges must be addressed in this patient population.

REVIEW SUMMARY

It has been generally accepted that PD patients over the age of 70 will have a shorter lifespan than younger patients and are at less risk for developing treatment-emergent complications by virtue of their lower exposure to medication over the course of the disease. Consequently, elderly patients are often treated aggressively when motor symptoms start to become disabling. Recent statistics, however, suggest that effective treatments for PD have helped to increase the longevity of PD patients, suggesting that elderly patients may, in fact, have sufficient medication exposure to raise concern over levodopa-induced motor complications. If this is the case, physicians should consider treating their elderly PD patients with dopamine agonists, which have several advantages over levodopa therapy, not the least of which is a lower prevalence of motor complications.

CONCLUSION

Dopamine agonists are safe and effective in elderly patients and offer numerous advantages when used as either monotherapy or adjunctive therapy, particularly in patients requiring long-term treatment.

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.

Subcutaneous Apomorphine

Apomorphine, a short-acting dopamine D1 and D2 receptor agonist, was the first dopamine receptor agonist used to treat Parkinson's disease. Subcutaneous apomorphine is currently used for the management of sudden, unexpected and refractory levodopa-induced 'off' states in fluctuating Parkinson's disease either as intermittent rescue injections or continuous infusions. Other indications include the challenge test for determining the dopaminergic responsiveness and finding the appropriate dose of the drug in intermittent subcutaneous administration. Except for a rapid on- and offset of the antiparkinsonian response with subcutaneous apomorphine, the magnitude and pattern of the motor response to single dose of subcutaneously administered apomorphine is qualitatively comparable to that of oral levodopa. Seventy-five percent of patients achieve a clinically significant improvement with a dose of apomorphine 4mg. The efficacy of intermittent subcutaneous apomorphine injections as an add-on to levodopa therapy in advanced Parkinson's disease was explored in one short-term, randomised, double-blind, placebo-controlled trial, one short-term and six long-term, open-label, uncontrolled studies, including a total of 195 patients. These studies provide evidence that this mode of administration was successful in aborting 'off' periods and improving Parkinson's disease motor scores, but tended to increase dyskinesias. No levodopa-sparing effect was observed. Eleven long-term, open-label, uncontrolled studies, including a total of 233 patients evaluated the efficacy of continuous subcutaneous apomorphine infusions in monotherapy or as an add-on to levodopa therapy in advanced Parkinson's disease. These studies proved that subcutaneous apomorphine infusions are successful in aborting 'off' periods, reducing dyskinesias and improving Parkinson's disease motor scores with the added benefit of a substantial levodopa-sparing effect. The apomorphine challenge test has at least 80% overall predictive ability to clinically diagnose Parkinson's disease across the different stages of the disease and parkinsonian syndromes. Similarly, those data also indicate that the apomorphine challenge test has a >80% ability to predict dopaminergic responsiveness across all stages of Parkinson's disease. Adverse events are usually mild and consist predominantly of cutaneous reactions and neuropsychiatric adverse effects. The incidence of adverse effects is higher in patients receiving continuous infusion than in those receiving intermittent pulsatile administration. Based on the results of these studies it is recommended that subcutaneous apomorphine either as intermittent injections or continuous infusions should be offered to any suitable Parkinson's disease patient who has difficulties in his/her management with conventional therapy. Low-dose levodopa therapy in combination with waking-day hours subcutaneous apomorphine infusion would probably be the most efficient treatment. Continuous subcutaneous apomorphine infusions should be evaluated before more invasive measures or neurosurgical interventions are contemplated.

Levodopa-induced response fluctuations in patients with Parkinson's disease: strategies for management

Fluctuations in response to levodopa in patients in the advanced stages of idiopathic Parkinson's disease occur frequently and are a difficult problem to treat. Patients who are treated with levodopa have an additional 10% risk of experiencing response fluctuations with each year of treatment: 50% of patients have this problem after 5 years of receiving levodopa therapy and almost 100% of patients after 10 years. The mechanisms by which response fluctuations occur are only partially understood and can be divided into three main types: (i) presynaptic neuronal degeneration leading to a lack of buffering of released levodopa, which is mainly related to wearing-off phenomena; (ii) postsynaptic changes in dopamine receptor sensitivity and number, partially caused by the presynaptic changes, which are clinically related to at-random response fluctuations; and (iii) pharmacokinetic and pharmacodynamic influences of exogenously administered dopaminergic agents. Several oral and parenteral treatment strategies are recommended to manage response fluctuations, such as optimisation of dopamine receptor agonist therapy in combination with a reduction of the levodopa load; use of slow-release levodopa formulations; use of catechol-O-methyltransferase inhibitors; an increase of levodopa dose frequency; use of high-dose amantadine; and intermittent or continuous use of apomorphine and/or levodopa. Continuous stimulation of dopamine receptors with dopaminergic agents is one of the crucial basic steps in the treatment of patients at an advanced stage of Parkinson's disease, and the preferential use of dopamine receptor agonists has proven to be successful in the prevention and treatment of response fluctuations.

The physiological basis of clinical deficits in Parkinson's disease

Despite the fact that Parkinson's disease (PD) is a relatively common neurological condition, the physiological derangements that result in its clinical features remain unclear. On combining findings from psychophysical, clinical and electrophysiological studies, an overriding theme is proposed that PD deficits are essentially quantitative rather than qualitative in nature. This may arise because the normal function of the basal ganglia is to activate neural processes selectively, providing appropriate diversion of "attentional" resources for decision-making aspects of motor tasks and appropriate "energising" of the executive aspects of such tasks. It is suggested that these concepts of attention, an idea stemming from psychophysical studies, and of energisation, which has derived from kinematic studies, may in fact reflect the same universal process of selective facilitation of particular processes and inhibition of others. In PD, without efficient facilitation, tasks may still be performed but less well than in normal individuals. Possible underlying mechanisms of basal ganglial function are discussed in the context of new findings on direct and indirect pathway actions and the role that oscillatory modulations may play in achieving selective facilitation is explored. Further investigation of disturbances of such mechanisms in PD may prove important in understanding the underlying pathophysiology of the condition.

Adrogolide HCl (ABT-431; DAS-431), a prodrug of the dopamine D1 receptor agonist, A-86929: preclinical pharmacology and clinical data

Adrogolide (ABT-431; DAS-431) is a chemically stable prodrug that is converted rapidly (<1 min) in plasma to A-86929, a full agonist at dopamine D1 receptors. In in vitro functional assays, A-86929 is over 400 times more selective for dopamine D1 than D2 receptors. In rats with a unilateral loss of striatal dopamine, A-86929 produces contralateral rotations that are inhibited by dopamine D1 but not by dopamine D2 receptor antagonists. Adrogolide improves behavioral disability and locomotor activity scores in MPTP-lesioned marmosets, a model of Parkinson's disease (PD), and shows no tolerance upon repeated dosing for 28 days. In PD patients, intravenous (i.v.) adrogolide has antiparkinson efficacy equivalent to that of L-DOPA with a tendency towards a reduced liability to induce dyskinesia. The adverse events associated with its use were of mild-to-moderate severity and included injection site reaction, asthenia, headache, nausea, vomiting, postural hypotension, vasodilitation, and dizziness. Adrogolide can also attenuate the ability of cocaine to induce cocaine-seeking behavior and does not itself induce cocaine-seeking behavior in a rodent model of cocaine craving and relapse. In human cocaine abusers, i.v. adrogolide reduces cocaine craving and other cocaine-induced subjective effects. The results of animal abuse liability studies indicate that adrogolide is unlikely to have abuse potential in man. Adrogolide has also been reported to reverse haloperidol-induced cognitive deficits in monkeys, suggesting that it may be an effective treatment for the cognitive dysfunction associated with aging and disease. Adrogolide undergoes a high hepatic "first-pass" metabolism in man after oral dosing and, as a result, has a low oral bioavailability (approximately 4%). This limitation may potentially be circumvented by oral inhalation formulations for intrapulmonary delivery that greatly increase the bioavailability of adrogolide. As the first full dopamine D1 receptor agonist to show efficacy in PD patients and to reduce the craving and subjective effects of cocaine in cocaine abusers, adrogolide represents an important tool in understanding the pharmacotherapeutic potential of dopamine D1 receptor agonists.

Effects of acute and repeated treatment with a novel dopamine D2 receptor ligand on L-DOPA-induced dyskinesias in MPTP monkeys

(S)-(-)-3-(3-(methylsulfonyl)phenyl)-1-propylpiperidine ((-)-OSU6162) is a phenylpiperidine derivative which exhibits low affinity to the dopamine D2 receptor in vitro. However, in vivo, positron emission tomography scanning studies show that the compound displaces the selective dopamine D2 receptor antagonist, raclopride. We have evaluated, in this study, the effect of (-)-OSU6162, on L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesias in a primate model of Parkinson's disease. Five 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated cynomolgus monkeys with a stable parkinsonian syndrome and reproducible dyskinesias to L-DOPA were used in this study. The monkeys were housed in observation cages equipped with an electronic motility monitoring system. They were injected subcutaneously (s.c.) with L-DOPA methyl ester (125 mg per animal) plus benserazide (50 mg per animal; L-DOPA/benserazide) alone or in combination with (-)-OSU6162 (1.0, 3.0, 6.0 or 10 mg/kg, s.c.). Subcutaneous injection of sterile saline was used as control. L-DOPA/benserazide increased locomotion and improved parkinsonism but also induced dyskinesias. Co-administration of (-)-OSU6162 with L-DOPA/benserazide produced a significant reduction in L-DOPA-induced dyskinesias. This improvement in L-DOPA-induced dyskinesias occurred mainly at the onset of the L-DOPA/benserazide effect as reflected by an increase in the duration of the "ON" state without dyskinesias up to 3.4 fold after (-)-OSU6162 co-administration as compared to L-DOPA/benserazide alone. The anti-dyskinetic effect of (-)-OSU6162 was maintained during 14 days and no tolerance to this effect was observed. Our data suggests that (-)-OSU6162 could be of significant clinical value to reduce L-DOPA-induced dyskinesias in fluctuating advanced Parkinson's disease patients.

The effects of central aromatic amino acid DOPA decarboxylase inhibition on the motor actions of L-DOPA and dopamine agonists in MPTP-treated primates

1. Endogenous L-DOPA may act as a neuromodulator contributing to the production of motor activity. We now investigate the effects of the centrally acting aromatic amino acid dopa decarboxylase (AADC) inhibitor NSD-1015 (3-hydroxybenzyl hydrazine) on the motor actions of L-DOPA and dopamine agonist drugs in MPTP treated common marmosets. 2. Pretreatment with NSD-1015 (10 - 50 mg kg(-1); i.p.) worsened baseline motor deficits in MPTP-treated common marmosets. Similarly, it abolished L-DOPA (5 - 18 mg kg(-1) s.c.) induced locomotor activity and reversal of disability. NSD-1015 pretreatment inhibited dopamine formation and elevated L-DOPA levels in plasma. 3. The increase in locomotor activity and improvement in disability produced by the administration of the D-1 agonist A-86929 (0.03 - 0. 04 mg kg(-1) s.c.) or the D-2 agonist quinpirole (0.05 - 0.3 mg kg(-1) i.p.) was abolished by NSD-1015 (25 mg kg(-1) i.p.) pretreatment. While the effects of a low dose combination of A-86929 (0.04 mg kg(-1) s.c.) and quinpirole (0.05 mg kg(-1) i.p.) were inhibited by NSD-1015 (25 mg kg(-1) i.p.), there was little effect on the action of a high dose combination of these drugs (0.08 mg kg(-1) A-86929 and 0.1 mg kg(-1) quinpirole). 4. Following central AADC inhibition with NSD-1015 (25 mg kg(-1) i.p.), locomotor behaviour induced by administration of high dose combinations of A-86929 (0.08 mg kg(-1) s.c.) and quinpirole (0.1 mg kg(-1) i.p.) was unaffected by L-DOPA (5 mg kg(-1) s.c.) pretreatment. 5. These results do not support a role for endogenous L-DOPA in spontaneous or drug induced locomotor activity. Rather, they strengthen the argument for the importance of endogenous dopaminergic tone in the motor actions of dopamine agonists.

Dopamine D1 receptor mRNA and receptor levels in the striatum of MPTP monkeys chronically treated with SKF-82958

The density of dopamine D1 receptor antagonist sites was measured by autoradiography and dopamine D1 receptor mRNA levels were measured by in situ hybridization in the striatum of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-exposed monkeys chronically treated with the dopamine D1 receptor agonist 6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benza zep ine hydrobromide (SKF-82958) administered in intermittent or continuous mode for a month. Normal and MPTP-exposed but otherwise untreated animals were used for comparison. Intermittent treatment with SKF-82958 relieved parkinsonian features and induced dyskinesias whereas given continuously this drug induced behavioral tolerance without dyskinesias. On the one hand, MPTP treatment tended to increase dopamine D1 receptor density in the putamen whereas treatment of MPTP monkeys with SKF-82958, intermittent or continuous, produced a significant increase compared to control animals. On the other hand, dopamine D1 receptor mRNA levels in the putamen appeared to decrease after MPTP lesion and agonist treatment as compared to dopamine D1 receptor density. In contrast, an apparent decrease in dopamine D1 receptor density and mRNA levels was observed in the nucleus accumbens of untreated MPTP monkeys whereas treatment of MPTP monkeys with SKF-82958, intermittent or continuous, produced a significant decrease compared to control animals. Thus, neither dyskinesias nor tolerance can be exclusively related to an increase or decrease in striatal dopamine D1 receptors, respectively.

Differentiation of dopamine agonists and their role in the treatment of Parkinson's disease

Since the pioneering work of Hornykiewicz and his colleagues, it has been recognized that dopaminergic cells die selectively in Parkinson's disease, and considerable improvement in symptoms can be achieved by administering levodopa, so that it may be converted to dopamine. However, levodopa has side-effects, and its duration of action is relatively brief. For these reasons, alternative approaches have been undertaken to stimulate the dopamine receptors. In particular, artificial agonists for dopamine receptors have been developed. The pioneer compound was bromocriptine, which stimulates the D2 family of receptors. Bromocriptine is an ergot derivative, and other compounds that are structurally related to ergot have been developed. In particular, lisuride and pergolide have been used for several years. Recently, an ergot derivative with an exceptionally long plasma half-life has been studied, cabergoline. Now there are also non-ergot derivatives that are D2 agonists, and are likely to have a role in the treatment of Parkinson's disease. Both ropinirole and pramipexole fall into this category, and each has been released in various countries for the treatment of Parkinson's disease. All of these compounds stimulate the D2 family of receptors, but they have varying actions on the D1 family of receptors. At present, there is no definite information on the role of the D1 family of receptors in either the therapeutic response to levodopa, or the development of adverse reactions. However, preliminary studies with a D1 agonist, ABT-431, are now in progress.

ABT-431, a D1 receptor agonist prodrug, has efficacy in Parkinson's disease

Studies in animal models show a selective D1 receptor agonist with full functional efficacy compared with dopamine to have antiparkinsonian efficacy of similar magnitude to levodopa, without the same propensity for inducing dyskinesia. To date, no such agent has been tested in humans. ABT-431 is the prodrug of A-86929, a full, selective D1 receptor agonist. Subjects (n = 14) with levodopa-responsive Parkinson's disease received five doses of ABT-431 (5, 10, 20, 30, and 40 mg) and one of placebo after a 12-hour levodopa holiday. Response was assessed by using the Unified Parkinson's Disease Rating Scale motor subsection. Dyskinesia was separately graded. ABT-431 showed efficacy significantly superior to placebo at doses of 10 mg and more, and of similar magnitude to that seen with levodopa. Dyskinesia was reduced in several patients after receiving ABT-431. There were no serious adverse events, the most common minor events being nausea and emesis, dizziness, and hypotension. Assuming that ABT-431 is not transformed in humans into an unknown active D2 metabolite, and remains selective for D1 receptors, it is the first dopamine D1 receptor agonist to demonstrate a full antiparkinsonian effect in patients with Parkinson's disease. These preliminary findings also suggest that it may exhibit a reduced tendency to provoke dyskinesia. The emergence of a well-tolerated D1 agonist should allow for the development of a better understanding of the relation between motor efficacy and dyskinesia in Parkinson's disease.

Dopamine agonists: what is the place of the newer compounds in the treatment of Parkinson's disease?

Three new dopamine agonists (cabergoline, pramipexole, ropinirole) have been put on to the market within the past months to treat patients with Parkinson's disease. Like any marketed dopamine agonists, the new compounds bind to the D2-like receptors. Pramipexole and ropinirole appear to be quite close drugs. Both are selective non ergot D2 (and preferentially D3) agonists, with an elimination half-life of 5 to 10 hours. Conversely, cabergoline is an ergot derivative, less selective for the D2 receptors, with a much longer elimination half-life (60 hours or more). In moderately advanced levodopa treated patients with Parkinson's disease and motor fluctuations, cabergoline, pramipexole and ropinirole all do significantly better than placebo in reducing UPDRS motor examination scores, time spent off and daily dose of levodopa. None of the 3 newer agonists proved to do significantly better than bromocriptine in this indication, at the cost of very similar adverse effects. In de novo levodopa naive patients, pramipexole and ropinirole did significantly better than placebo in short-term (few months) follow-up trials, at the cost again of classical dopaminergic adverse effects. Ropinirole was marginally more effective than bromocriptine, while its use induced the same risk of psychosis than the "old" reference agonist. Early treatment with cabergoline, compared with levodopa, in a long-term (5 year) study reduced the relative risk of developping motor complication by more than 50%. A similar study is presently on-going to compare ropinirole and levodopa. Clinical trials to assess putative neuroprotective effects are also on going with ropinirole and pramipexole. Up to now, the available clinical controlled data suggest that the newer dopamine agonists have very similar clinical effects with only minor superiority, if any, versus bromocriptine.

Antiparkinsonian effect of a new selective adenosine A2A receptor antagonist in MPTP-treated monkeys

BACKGROUND

Chronic treatment with L-3,4-dihydroxyphenylalanine (L-dopa) is often associated with motor side effects in PD patients. The search for new therapeutic approaches has led to study the role of other neuromodulators including adenosine. Among the four adenosine receptors characterized so far, the A2A subtype is distinctively present on striatopallidal output neurons containing enkephalin and mainly bearing dopamine (DA) D2 receptors (indirect pathway). Studies in DA-denervated rats suggest that blockade of adenosine A2A receptors might be used in PD.

OBJECTIVE

To evaluate the antiparkinsonian effect of a new selective adenosine A2A receptor antagonist, KW-6002, in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys.

METHODS

In the present study, we used six MPTP-exposed cynomolgus monkeys already primed and exhibiting L-dopa-induced dyskinesias to evaluate both the antiparkinsonian and dyskinetic effect upon challenge with two oral doses (60 and 90 mg/kg) of KW-6002 administered alone or in combination with L-dopa/benserazide (50/12.5 mg).

RESULTS

KW-6002 administered alone produced a dose-dependent antiparkinsonian response that reached the level of efficacy of L-dopa/benserazide but was less likely to reproduce dyskinesias in these animals. When co-administered, KW-6002 potentiated the effects of L-dopa/benserazide on motor activity (up to 30%) without affecting the dyskinetic response.

CONCLUSION

Adenosine A2A receptor antagonists have antiparkinsonian effects of their own with a reduced propensity to elicit dyskinesias. They might therefore be useful agents in the treatment of PD.

D1 receptor blockade improves L-dopa-induced dyskinesia but worsens parkinsonism in MPTP monkeys

OBJECTIVE

To determine whether dopamine (DA) D1 or DA D2 receptors are associated predominantly with the antiparkinsonian versus the dyskinetic effect of levodopa.

METHODS

The authors used four L-dopa-primed, dyskinetic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-exposed monkeys to test whether acute and selective blockade of the DA D1 receptor subtype, using SCH 23390 and NNC 01-112, could reduce L-dopa-induced dyskinesias without altering the relief of symptoms. Blockade of DA receptors using sulpiride (D2) and clozapine (D1-D2-like) was studied for comparison.

RESULTS

With the notable exception of the lowest dose of clozapine tested, coadministration of DA D1 or D2 antidopaminergic agents with L-dopa reduced the L-dopa-induced dyskinesias but also caused a return of parkinsonian disability. Prolonged latencies from intake of a single oral dose of L-dopa to turning "on," decreased duration of the "on" state, and a complete failure to induce benefit was also observed.

CONCLUSION

Low-dose clozapine could be an effective adjunct to reduce L-dopa-induced dyskinesias without altering the relief of parkinsonian symptoms. Interactions with many neurotransmitter systems may explain the better pharmacologic profile of clozapine, including DA D4 (rather than D1), serotonin, acetylcholine, and noradrenaline. Neither dyskinesias nor antiparkinsonian effects can be ascribed solely to the D2 or D1 receptor. Thus, some cooperation between the two receptors appears necessary for these behavioral effects.

Clinical pharmacology of dopamine agonists in Parkinson's disease

Oral levodopa is the most effective symptomatic treatment for Parkinson's disease. Dopamine agonists are useful adjuvants to levodopa in the pharmacotherapy of parkinsonian patients. Monotherapy with dopamine agonists in early Parkinson's disease has been advocated in order to delay the occurrence of complications associated with long term administration of levodopa. The use of dopamine agonists alone provides an adequate antiparkinsonian effect in only a minority of patients. In early stages of Parkinson's disease, dopamine agonists can produce a clinical response comparable with levodopa but, thereafter, their efficacy wanes. Early initiation of combination therapy with levodopa and dopamine agonists appears to reduce the severity and delay the appearance of the complications associated with long term administration of levodopa. Currently, dopamine agonists are most commonly used in combination with levodopa in patients in advanced stages of the disease who experience fluctuations of their motor symptoms. Despite their different pharmacodynamic and pharmacokinetic profiles, the ergot derivatives bromocriptine, lisuride and pergolide appear to be very similar in terms of their clinical efficacy. Continuous dopaminergic stimulation by parenteral infusion of water-soluble dopamine agonists such as apomorphine and lisuride can overcome motor fluctuations in advanced Parkinson's disease. Other dopamine agonists such as cabergoline, pramipexole and ropinirole are currently being studied. Further studies with these compounds will be required to determine their efficacy and adverse effects in comparison with the currently available orally active ergot agonists. It has been suggested that oxidative stress resulting from dopamine metabolism may be reduced by the administration of dopamine agonists. These drugs may therefore slow the rate of progression of Parkinson's disease. At present, however, there is no convincing clinical data to support a neuroprotective effect of dopamine agonists.