Talipexole or pramipexole combinations with chloro-APB (SKF 82958) in MPTP-induced hemiparkinsonian monkeys

Advances in the Therapeutic Use of Non-Ergot Dopamine Agonists in the Treatment of Motor and Non-Motor Symptoms of Parkinson's Disease

Dopamine (DA) agonists, as an excellent dopamine replacement therapy for patients with early and advanced Parkinson's disease (PD), play a vital role in controlling motor and several nonmotor symptoms. Besides, the application of DA agonists may delay levodopa therapy and the associated risk of motor complications. Indeed, each DA agonist has unique pharmacokinetic and pharmacodynamic characteristics and therefore has different therapeutic efficacy and safety profile. The comorbidities, significant non-motor manifestations, concomitant medications, and clinical features of PD individuals should guide the selection of a specific DA agonist to provide a more patient-tailored treatment option. Thorough knowledge of DA agonists helps clinicians better balance clinical efficacy and side effects. Therefore, this review refers to recent English-written articles on DA agonist therapy for PD patients and summarizes the latest findings on non-ergot DA agonists as well as the advantages and disadvantages of each compound to help clinicians in the selection of a specific DA agonist. In addition, novel D1/D5 partial agonists and new formulations of DA agonists are also discussed.

Application of the concept of continuous dopaminergic stimulation for the management of Parkinson's disease

Continuous dopaminergic stimulation (CDS) is a prominent therapeutic concept for the treatment of Parkinson's disease (PD), which proposes that continuous brain dopamine-receptor stimulation, rather than intermittent doses of oral L-dopa, prevents or manages L-dopa-induced dyskinesias (LIDs). In the normal situation, dopaminergic neurons in the substantia nigra pars compacta fire tonically to keep the dopamine receptor stimulation at a steady-state level. But when the dopaminergic pathway is impaired, the dopamine receptor stimulation becomes intermittent or pulsatile. This pulsatile stimulation causes a series of gene and protein changes in striatal neurons, leading to alterations in the fi ring patterns of basal ganglia neurons that result in LIDs. Studies in animal models and clinical trials of PD have shown that approaches providing CDS, currently including patches, extended-release formulations of L-dopa or dopamine agonists, continuous delivery of apomorphine and duodenal L-dopa infusion, are associated with a decreased risk of LIDs. In this review, we summarize both preclinical and clinical evidence for the five methods that may provide CDS in theory and compare the advantages and disadvantages of these methods.

Blood-brain barrier transport of pramipexole, a dopamine D2 agonist

The blood-brain barrier (BBB) transport of pramipexole, a potent dopamine receptor agonist with high efficacy for Parkinson's disease, was mainly characterized using immortalized rat brain capillary endothelial cells (RBEC)1 as an in vitro BBB model. [(14)C]Pramipexole uptake by RBEC1 was dependent on temperature and pH, but not sodium ion concentration or membrane potential. The uptake was inhibited by several organic cations including pyrilamine. Mutual inhibition was observed between pramipexole and pyrilamine. In addition, [(14)C]pramipexole uptake was stimulated by preloading unlabeled pramipexole. RT-PCR analysis for organic cation transporters (rOCT1-3, rOCTN1-2) in RBEC1 was performed. The mRNA level of rOCTN2 was the highest, followed by rOCTN1, while expression of rOCT1, rOCT2 and rOCT3 was negligible. The brain uptake of [(14)C]pramipexole, which was measured by the in situ rat brain perfusion technique, was significantly inhibited by unlabeled pramipexole. These results suggest that pramipexole is, at least in part, transported across the BBB by an organic cation-sensitive transporter. The pramipexole transport in RBEC1 was pH-dependent, but sodium- and membrane potential-independent.

TRANSPORT OF THE DOPAMINE D2 AGONIST PRAMIPEXOLE BY RAT ORGANIC CATION TRANSPORTERS OCT1 AND OCT2 IN KIDNEY

We investigated the mechanism of renal tubular secretion of the dopamine D2 receptor agonist pramipexole in rats, focusing on organic cation transporters 1 and 2. The uptake of [14C]pramipexole by Xenopus oocytes injected with complementary RNA of either rat organic cation transporter (rOCT) 1 or rOCT2 was significantly higher than that by water-injected oocytes: the kinetic parameters, K(m) and V(max), of pramipexole uptake were 49.5 muM and 234 pmol/60 min/oocyte for rOCT1, and 16.9 microM and 12.8 pmol/60 min/oocyte for rOCT2. Pramipexole was taken up into kidney slices in a time- and concentration-dependent manner, and Eadie-Hofstee plots revealed the involvement of two saturable components. The kinetic parameters, K(m1) and V(max1), of the high-affinity component were 12.9 microM and 10.7 nmol/15 min/g kidney, respectively. The uptake of [14C]pramipexole by rOCT1, rOCT2, and kidney slices was inhibited by procainamide and corticosterone, which are selective inhibitors of rOCT1 and rOCT2, respectively. The IC50 values of procainamide and corticosterone for the uptake of [14C]pramipexole by rOCT1, rOCT2, and kidney slices were 7.7, 167.0, and 47.0 microM and 163.7, 10.7, and 47.7 microM, respectively. These results demonstrate that both rOCT1 and rOCT2 are involved in the renal uptake of pramipexole across the basolateral membrane of the proximal tubular epithelial cells.

[Pharmacological profiles and clinical effects of antiparkinsonian agent, pramipexole]

Pramipexole hydrochloride (pramipexole) is a nonergot dopamine D(2) agonist, and the S(-)enantiomer is used for the treatment of Parkinson's disease (PD). Pramipexole possessed the highest affinity with the D(3) subtype among the D(2) receptor subfamily members (D(2), D(3), D(4)), lacking affinity with the D(1) and D(5) subtype. Pramipexole ameliorated the motor disturbances in PD animal models, induced contralateral rotational behavior reflecting post-synaptic D(2) receptor stimulation in the striatum, and showed a variety of neuroprotective effects in vitro and in vivo experimental systems. The neuroprotective effects of pramipexole seemed to be derived from several mechanisms: stimulation of D(2) autoreceptor, stimulation of D(3) receptor, inhibition of oxidative reaction and following radical production, increase of Bcl-2 protein and inhibition of apoptotic cell death, and production of neurotrophic factor. Clinical efficacy of pramipexole both in monotherapy and combined use with L-DOPA were confirmed evaluating by UPDRS (Unified Parkinson's Disease Rating Scale) II (Activities of daily living) and III (Motor), in the results of clinical studies mainly performed in USA and European countries and partly in Japan. In addition, patients initially treated with pramipexole demonstrated reduction in problematic symptoms and in loss of striatal [(123)I]2beta-carboxymethoxy-3beta-(4-idodophenyl)tropan uptake, a marker of dopamine neuron degeneration, compared with those initially treated with L-DOPA.

Inhibitory effects of D2 agonists by striatal injection on excessive release of dopamine and hyperactivity induced by Bay K 8644 in rats

We investigated by means of behavioral and neurochemical studies the effects of either D(1) or D(2) agonist on excessive dopamine release and hyperactivity induced by the microinjection of Bay K 8644, and an L-type Ca(2+) channel stimulant, into the rat caudate putamen under a novel environmental condition. Hyperactivity (locomotor activity and rearing counts) and significant increases in extracellular dopamine levels induced by Bay K 8644 were concomitantly observed. D(1) agonist, SKF81297, administered into the caudate putamen did not block Bay K 8644-induced hyperactivity measured by monitoring both animal activity and increases in extracellular dopamine levels detected by microdialysis. Pretreatment with the D(2) agonists, bromocriptine, talipexole and pramipexole, into the caudate putamen significantly blocked Bay K 8644-induced hyperactivity for 45 min after Bay K 8644 administration, although the single administration of these agonists significantly potentiated locomotor activity and rearing behavior. Furthermore, these agonists significantly suppressed Bay K 8644-induced extracellular dopamine levels. Our results indicate that these D(2) agonists (1) act on postsynaptic neuronal D(2) receptors under conditions of normal or low dopamine release in the caudate putamen, and (2) act on presynaptic D(2) receptors (autoreceptors) when excessive levels of dopamine are released or hyperdopamine neuronal activity is induced. Consequently, the effect of D(2) agonists in the clinical treatment of Parkinson's disease may be due to stimulation of postsynaptic D(2) receptors rather than presynaptic autoreceptors.

The dopamine agonist pramipexole scavenges hydroxyl free radicals induced by striatal application of 6-hydroxydopamine in rats: an in vivo microdialysis study

Hydroxyl free radical production seems to play an important role in the pathogenesis of Parkinson's disease. In the present study, we investigated the dopamine agonists pramipexole and pergolide as well as the nitrone compound S-PBN (N-tert-butyl-alpha-(2-sulfophenyl)nitrone) to reduce hydroxyl radical formation. Microdialysis experiments were carried out in non-anaesthetized Wistar rats. Salicylate was incorporated into the perfusion fluid to measure indirectly hydroxyl radicals indicated by 2,3-dihydroxybenzoic acid (2,3-DHBA). Local perfusion with 0.2 or 2 nmol/2 microl/min 6-hydroxydopamine (6-OHDA) via the microdialysis probe significantly increased 2,3-DHBA levels 14-fold and 47-fold, respectively. Systemic application of either pergolide (0.05 mg/kg) or pramipexole (1 mg/kg) failed to significantly reduce 6-OHDA-induced hydroxyl radical production. In contrast, a 40 min pretreatment with pramipexole (2 and 10 nmol/2 microl/min via the probe) before onset of 6-OHDA perfusion, significantly attenuated 2, 3-DHBA levels compared with vehicle controls. S-PBN pretreatment (2 nmol/2 microl/min) was not effective to reduce 2,3-DHBA levels. In conclusion, pramipexole was able to reduce hydroxyl radical levels induced by 6-OHDA in vivo after local application. This property of pramipexole may be beneficial under conditions of enhanced hydroxyl radical formation in parkinsonian brains and may add to its well known dopamine D(2)-like receptor agonistic effects.

Pre-clinical studies of pramipexole: clinical relevance

This paper reviews the preclinical study of the novel dopamine agonist pramipexole and its use in early Parkinson's disease (PD). Emphasis will be given to those properties distinguishing this drug from other dopamine agonists, the relevance of the preclinical data to clinical trial results in early PD, and the putative neuroprotective properties of the compound. The conventional dopamine agonists are ergot-derived compounds that are most widely used as adjunctive therapies in advancing Parkinson's disease (PD). Examples of conventional agonists are bromocriptine and pergolide. Pramipexole is an aminobenzothiazole compound, recently introduced for the treatment of both early and advanced PD. Its nonergot structure may reduce the risk of side-effects, considered unique to ergot drugs, such as membranous fibrosis. Pramipexole is a full dopamine agonist with high selectivity for the D2 dopamine receptor family. This family includes the D2, D3 and D4 receptor subtypes. Pramipexole has a 5- to 7-fold greater affinity for the D3 receptor subtype with lower affinities for the D2 and D4 receptor subtypes. The drug has only minimal alpha2-adrenoceptor activity and virtually no other receptor agonism or antagonism. The optimal dopamine receptor activation for the safe and effective treatment of PD is not known. Findings in animal models and clinical studies indicate that activation of the postsynaptic D2 receptor subtype provides the most robust symptomatic improvement in PD. Given its pharmacological profile, it is not surprising that pramipexole was found to be effective in ameliorating parkinsonian signs in animal models. This therapeutic effect has been confirmed in clinical trials in both early and advanced PD. In early disease, it provides a clear reduction in the chief motor manifestations of PD and improved activities of daily living. Perhaps most striking is the large number of clinical trial patients who have remained on pramipexole monotherapy for many months. The majority of these subjects have been maintained on pramipexole for an excess of 24 months without requiring additional symptomatic treatment with levodopa. This is in contrast to the general clinical experience with older conventional agonists. Pramipexole also has a favourable pharmacokinetic profile. It is rapidly absorbed with peak levels appearing in the bloodstream within 2 h of oral dosing. It has a high absolute bioavailability of > 90% and can be administered without regard to meals. It has no significant effects on other antiparkinson drugs such as levodopa or selegiline. Its excretion is primarily renal and, thus, has little or no impact on hepatic cytochrome P450 enzymes or other related metabolic pathways. Pramipexole has also been theorized to have 'neuroprotectant' properties. Oxyradical generation is posited as a cause or accelerant of brain nigral cell death in PD. Pramipexole stimulates brain dopamine autoreceptors and reduces dopamine synthesis and turnover which may minimize oxidative stress due to dopamine metabolism. Furthermore, the compound has a low oxidation potential that may serve as an oxyradical scavenger in the PD brain. In summary, pramipexole is a new antiparkinson medication found to have unique dopamine agonist characteristics and putative neuroprotective properties.

Nicotine alone and in combination with L-DOPA methyl ester or the D(2) agonist N-0923 in MPTP-induced chronic hemiparkinsonian monkeys

Nicotine, the soluble methyl ester of L-DOPA, and the D(2) agonist N-0923 were given alone and in combination im to five hemiparkinsonian monkeys. Daily nicotine in doses of 32-320 micrograms/kg for 6 days each, surprisingly, had slight effects on motor activity. When combined with N-0923, nicotine did not further enhance its effects. However, L-DOPA methyl ester plus nicotine produced greater contraversive circling than L-DOPA methyl ester plus 0.9% NaCl. Similar effects were obtained on significant motor movements of both the affected (contralateral) and normal (ipsilateral) arm and hand. The results indicate that nicotine is synergistic with l-DOPA methyl ester, but not with the postsynaptic D(2) agonist N-0923.

The alleged dopamine D1 receptor agonist SKF 83959 is a dopamine D1 receptor antagonist in primate cells and interacts with other receptors

So far, no clear correlation has been found between the effects of dopamine D1 receptor agonists on motor behavior in primate models of Parkinson's disease and their ability to stimulate adenylate cyclase in rats, the benzazepine SKF 83959 (3-methyl-6-chloro-7,8-hydroxy-1-[3-methylphenyl]-2,3,4,5-tetrahydro-]H- 3-benzazepine) being the most striking example. Since this discrepancy might be attributed to: (A) the different species used to study these effects or (B) the interaction of SKF 83959 with other catecholamine receptors, the aims of this study were: (1) to study the ability of SKF 83959 to stimulate adenylate cyclase in cultured human and monkey glial cells equipped with dopamine D1 receptors and (2) to evaluate the affinity for and the functional interaction of SKF 83959 with other catecholamine receptors. Binding studies revealed that SKF 83959 displayed the highest affinity for the dopamine D1 receptor (pKi=6.72) and the alpha2-adrenoceptor (pKi=6.41) and moderate affinity for the dopamine D2 receptor and the noradrenaline transporter. In monkey and human cells, SKF 83959 did not stimulate cyclic adenosine monophosphate (cAMP) formation to a significant extent, but antagonized very potently the dopamine-induced stimulation of cAMP formation in both cell types. The compound stimulated basal dopamine outflow and inhibited depolarization-induced acetylcholine release only at concentrations > 10 microM. Finally, SKF 83959 concentration dependently increased electrically evoked noradrenaline release, indicating that it had alpha2-adrenoceptor blocking activity and interfered with the noradrenaline transporter. In conclusion, SKF 83959 is a potent dopamine D1 receptor and alpha2-adrenoceptor antagonist. Thus, the anti-parkinsonian effects of SKF 83959 in primates are not mediated by striatal dopamine D1 receptors coupled to adenylate cyclase in a stimulatory way.

Trihexyphenidyl potentiation of L-DOPA: reduced effectiveness three years later in MPTP-induced chronic hemiparkinsonian monkeys

The effects of a combination of trihexyphenidyl and L-DOPA methyl ester given i.m. were studied 3-5 years after MPTP induced hemiparkinsonism in five female adult Macaca nemistrina monkeys. Three years later, these studies were repeated to determine if the drug combination was equally effective. Although the combination of trihexyphenidyl and L-DOPA produced potentiation in both studies, 3 years later it was quantitatively less. This was due primarily to the reduced effectiveness of L-DOPA methyl ester in a dose of 12.5 mg/kg i.m. Even though the combination was less effective in subsequent years, the animals continued to show the same clinical signs of hemiparkinsonism. Reduced effectiveness of the drug combination does not appear to be due to a lessening of MPTP-induced hemiparkinsonism, but rather to the reduced effectiveness of L-DOPA.

Effects of talipexole on contraversive rotation and functional impairment in MPTP-induced chronic hemiparkinsonian monkeys

The effects of talipexole on functional motor improvement in comparison with contraversive circling were studied in five chronic (5-7 years post MPTP-lesioned) hemiparkinsonian Macaca nemestrina monkeys. Talipexole induced contraversive rotations in a dose of 32 microg/kg for about 1 hr after i.m. injection. Larger doses (56 and 100 microg/kg, i.m.) produced less effect due to sedation. Three different rating scales were used to assess functional improvement, including a clinical parkinsonism rating scale, volitional responses to fruit presentations, and number of significant hand movements. The optimal dose of talipexole was 32 microg/kg, i.m. Functional improvement by talipexole, including clinical parkinsonian rating scales and significant hand movements, as well as contraversive circling in hemiparkinsonian monkeys, confirm that this chronic animal model is useful in preclinical testing of drugs for the treatment of human parkinsonism.