Antiparkinsonian activity of talipexole in MPTP-treated monkeys: in combination with L-dopa and as chronic treatment

Pharmacokinetics of l-dopa in plasma and extracellular fluid of striatum in common marmosets

L-3, 4-Dihydroxyphenylalanine (L-dopa) has been widely used for the treatment of Parkinson's disease (PD), but its pharmacokinetics in the striatum have hardly been investigated, especially in primates. In this study, we examined the concentration of L-dopa in plasma and in the extracellular fluid (ECF) of the striatum in common marmosets using microdialysis and high-performance liquid chromatography (HPLC) with electrochemical detection. With a clinically therapeutic dosage of L-dopa/benserazide (20/5 mg/kg, p.o.), the t(max) of L-dopa was 30 min in plasma and 60-90 min in ECF of striatum. Mean C(max) was 20.3 microM in plasma and 442.9 nM in ECF of striatum, which is about 2.2% of that in plasma. The L-dopa concentration in ECF is much lower than those previously applied during in vivo studies for L-dopa toxicity.

Effects of acute administration of DA agonists on locomotor activity: MPTP versus neonatal intracerebroventricular 6-OHDA treatment

The effects of several dopamine (DA) receptor agonists upon locomotor activity on adult MPTP-treated mice and postnatal 6-hydroxydopamine- (6-OHDA-) treated rats were assessed in ten experiments. C57 BL/6 mice were administered 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP, 2 x 40 mg/kg, s.c., 24-hr interval between injections) at 5-months-age, while 1-day-old male Wistar rat pups were given intracisternal 6-OHDA (50 mg, once following desipramine, 25 mg/kg). MPTP-treated mice were tested 4-5 weeks following MPTP injections whereas neonatal 6-OHDA rats were tested at 3-months-age. Locomotor activity was measured in respective activity test chambers following acute administration of DA receptor agonists. In MPTP-treated mice, apomorphine failed to elevate locomotor activity but instead further exacerbated (1.0 and 3.0 mg/kg, s.c.) the hypokinesia of these animals while inducing marked increases in control mice. Cabergoline (0.3 mg/kg, s.c.) and bromocriptine (3.0 mg/kg, s.c.) caused dose-specific elevations of locomotion in MPTP and control mice but suppressed activity at the highest doses. Quinpirole (0.2 mg/kg) and 7-hydroxydipropylaminotetralin (7-OH-DPAT; 300 nmole/kg) increased locomotion in hypokinesic MPTP-treated mice; in control mice, activity was elevated by quinpirole (0.2 and 0.7 mg/kg) and 7-OH-DPAT (100 and 300 nmole/kg), while higher doses suppressed activity. Neither SKF 38393 (1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol) nor FCE 23884 [4-(9,10-didehydro-6-methylergolin-8 beta-yl) methyl-piperazine-2,6-dione] affected locomotor activity. Apomorphine (0.3, 1.0 and 2.0 mg/kg), bromocriptine (3.0 mg/kg) and cabergoline (1.0 mg/kg) stimulated locomotion in sham-operated rats, and to a greater extent in the 6-OHDA-treated rats. Higher dose cabergoline (3.0 mg/kg) induced increased activity of similar extent in sham controls and 6-OHDA treated rats. Activity-enhancing effects of quinpirole (0.2, 0.7 and 2.1 mg/kg) in sham rats were attenuated in 6-OHDA treated rats. Both SKF 38393 (10 mg/kg) and FCE 23884 (0.3 and 1.0 mg/kg) induced locomotor activity increases in 6-OHDA, but not sham, rats. Finally, 7-OH-DPAT (1200 mg/kg) enhanced activity in 6-OHDA rats vs. shams. The effects of the DA agonists are discussed with regard to the putative antihypokinesic effects in MPTP mice and DA-receptor supersensitivity effects in neonatal 6-OHDA rats, pertaining to their more-or-less selective subreceptor profiles.

Antiparkinsonian drugs and their neuroprotective effects

In Parkinson's disease, while dopamine (DA) replacement therapy, such as with L-DOPA (levodopa), improves the symptoms, it does not inhibit the degeneration of DA neurons in the substantia nigra. Numerous studies have suggested that both endogenous and environmental neurotoxins and oxidative stress may participate in this disease, but the detailed mechanisms are still unclear. Recent genetic studies in familial Parkinson's disease and parkinsonism have shown several gene mutations. This new information regarding its pathogenesis offers novel prospects for effective strategies involving the neuroprotection of vulnerable DA neurons. This review summarizes current findings regarding the pathogenesis and antiparkinsonian drugs, and discusses their possibilities of targets to develop novel neuroprotective drugs.

Effects of anti-Parkinsonian drugs on neurobehavioural changes induced by bilateral 6-hydroxydopamine lesions in rats

1. Effects of anti-Parkinsonian drugs on neurobehavioural changes induced by bilateral lesions of dopaminergic neurons were investigated in rats. 2. Dopaminergic neurons in rats were lesioned bilaterally by injection of 6-hydroxydopamine (6-OHDA; 8 micrograms) into the medial forebrain bundle at the level of the posterolateral hypothalamus. As a result, a decrease in locomotor activity and marked catalepsy and prolongation of grasping time were observed. 3. Levodopa, talipexole, bromocriptine and theophylline dose-dependently antagonized the decrease in locomotor activity induced by bilateral 6-OHDA lesions. These drugs also showed antagonistic effects on the appearance of catalepsy and prolongation of grasping time induced by bilateral 6-OHDA lesions. In contrast, trihexyphenidyl showed no antagonizing effect on the neurobehavioural changes induced by 6-OHDA lesions at any concentration tested. 4. Combined treatment with levodopa and talipexole antagonized the neurobehavioural changes induced by bilateral 6-OHDA lesions, whereas no marked changes were observed when either drug was administered separately. The same findings were noted with the simultaneous use of either levodopa (2 mg/kg) and theophylline (2 mg/kg) or talipexole (0.005 mg/kg) and theophylline (2 mg/kg). 5. These results indicate that this model may be useful for estimating the effects of drugs in the treatment of Parkinson's disease.

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.

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

The effects of two predominant dopamine D2-like receptor agonists, talipexole (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo [4,5-d]-azepine dihydrochloride, B-HT 920 CL2) and pramipexole (S(-)2-amino-4,5,6,7-tetrahydro-6-propyl-aminobenzothiazole dihydrochloride, SND 919 CL2Y), were studied alone and in combination with the selective dopamine D1-like receptor agonist chloro-APB ((+/-)6-chloro-7-8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-benz azepine hydrobromide, SKF 82958) in five chronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) lesioned hemiparkinsonian Macaca nemestrina monkeys. Talipexole induced contraversive rotation in a dose-dependent manner up to 32 microg/kg, i.m. Talipexole was more potent than pramipexole (10 vs. 32 microg/kg, i.m.), but pramipexole was more efficacious in producing contraversive rotational behavior and significant hand movements in the afflicted limb. Larger doses of chloro-APB also produced contraversive rotation. Combinations of each dopamine D2-like receptor agonist in a median effective dose with chloro-APB (23.4 and 74.8 microg/kg, i.m.) had synergistic effects, producing either addition or potentiation, depending upon the dose used. The effects noted with these combinations were less than the effect of a large dose (100 microg/kg) of pramipexole. Talipexole, in the largest dose studied (100 microg/kg, i.m.), produced sedation which was not seen with the same dose of pramipexole. No significant extrapyramidal side effects were noted with either agent.

Anti-tremor activity of talipexole produced by selective dopamine D2 receptor stimulation in cynomolgus monkeys with unilateral lesions in the ventromedial tegmentum

The anti-tremor activity of talipexole (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo[4,5-d]azepine dihydrochloride, B-HT 920 CL2, Domin), a non-ergot dopamine D2 receptor agonist which possesses alpha 2-adrenoceptor agonistic and 5-HT3 receptor antagonistic properties, was examined in monkeys with a unilateral lesion in the ventromedial tegmentum. Talipexole dose dependently suppressed the tremor and had ED50 values of 34 micrograms/kg s.c. and 84 micrograms/kg p.o. The anti-tremor effect of talipexole occurred at much lower doses than that of an ergot dopamine receptor agonist, bromocriptine (2-bromo-alpha-ergocryptine mesilate, ED50; 2.5 mg/kg s.c.), and talipexole acted synergistically in combination with L-DOPA (3,4-dihydroxyphenylalanine). In ventromedial tegmentum-lesioned monkeys, anti-tremor doses of talipexole did not cause emetic behavior, but had sedative effects. Conversely, monkeys given bromocriptine exhibited oral movement, salivation and vomiting when anti-tremor effects were observed, but not marked sedative behavior at any of the doses investigated. During repeated administration of talipexole (a daily dose of 50 micrograms/kg s.c. for 21 days), the extent and duration of the anti-tremor effect did not change, but those of the sedative effect decreased gradually. The anti-tremor effect of talipexole was significantly suppressed by sulpiride, but not by SCH 23390 (7-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-ol) or yohimbine, while the sedative effect was inhibited by sulpiride and yohimbine. The main metabolites of talipexole had no anti-tremor or sedative effects. These results indicate that talipexole exerts its anti-tremor activity via selective dopamine D2 receptor stimulation.