Rasagiline enhances l-DOPA-induced contralateral turning in the unilateral 6-hydroxydopamine-lesioned guinea-pig

The preferential nNOS inhibitor 7-nitroindazole and the non-selective one N(G)-nitro-L-arginine methyl ester administered alone or jointly with L-DOPA differentially affect motor behavior and monoamine metabolism in sham-operated and 6-OHDA-lesioned rats

Reciprocal interactions between nitrergic and dopaminergic systems play a key role in the control of motor behavior. In the present study, we performed a comparative analysis of motor behavior (locomotor activity, catalepsy, rotational behavior) and monoamine metabolism in the striatum and substantia nigra of unilaterally sham-operated and 6-OHDA-lesioned rats treated with the preferential neuronal nitric oxide synthase (nNOS) inhibitor 7-nitroindazole (7-NI) or the non-selective one N(G)-nitro-L-arginine methyl ester (L-NAME), alone or in combination with L-DOPA. Each NOS inhibitor given alone (50mg/kg) induced a distinct catalepsy 30 min after injection but only 7-NI impaired spontaneous locomotion after 10 min. In 6-OHDA-lesioned rats, chronic L-DOPA (25mg/kg) induced 2.5-h long contralateral rotations. 7-NI (30 and 50mg/kg) markedly reduced the intensity of L-DOPA-induced contralateral rotations while extending their duration until 4.5h whereas L-NAME (50 and 100mg/kg) only tended to attenuate their intensity without affecting the duration. 7-NI but not L-NAME significantly increased endogenous tissue DA levels in the nigrostriatal system of both sham-operated and 6-OHDA-lesioned rats. In L-DOPA-treated group, 7-NI significantly enhanced the L-DOPA-derived tissue DA content in this system and decreased the level of the intracellular DA metabolite DOPAC produced by monoamine oxidase (MAO). In contrast to 7-NI, L-NAME decreased markedly DA content and did not affect DOPAC level in the ipsilateral striatum. It means that the differences in 7-NI and L-NAME-mediated modulation of L-DOPA-induced behavioral and biochemical effects resulted not only from the inhibition of NOS activity but also from differences in their ability to inhibit MAO.

Animal models of Parkinson's disease: a source of novel treatments and clues to the cause of the disease

Animal models of Parkinson's disease (PD) have proved highly effective in the discovery of novel treatments for motor symptoms of PD and in the search for clues to the underlying cause of the illness. Models based on specific pathogenic mechanisms may subsequently lead to the development of neuroprotective agents for PD that stop or slow disease progression. The array of available rodent models is large and ranges from acute pharmacological models, such as the reserpine- or haloperidol-treated rats that display one or more parkinsonian signs, to models exhibiting destruction of the dopaminergic nigro-striatal pathway, such as the classical 6-hydroxydopamine (6-OHDA) rat and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse models. All of these have provided test beds in which new molecules for treating the motor symptoms of PD can be assessed. In addition, the emergence of abnormal involuntary movements (AIMs) with repeated treatment of 6-OHDA-lesioned rats with L-DOPA has allowed for examination of the mechanisms responsible for treatment-related dyskinesia in PD, and the detection of molecules able to prevent or reverse their appearance. Other toxin-based models of nigro-striatal tract degeneration include the systemic administration of the pesticides rotenone and paraquat, but whilst providing clues to disease pathogenesis, these are not so commonly used for drug development. The MPTP-treated primate model of PD, which closely mimics the clinical features of PD and in which all currently used anti-parkinsonian medications have been shown to be effective, is undoubtedly the most clinically-relevant of all available models. The MPTP-treated primate develops clear dyskinesia when repeatedly exposed to L-DOPA, and these parkinsonian animals have shown responses to novel dopaminergic agents that are highly predictive of their effect in man. Whether non-dopaminergic drugs show the same degree of predictability of response is a matter of debate. As our understanding of the pathogenesis of PD has improved, so new rodent models produced by agents mimicking these mechanisms, including proteasome inhibitors such as PSI, lactacystin and epoximycin or inflammogens like lipopolysaccharide (LPS) have been developed. A further generation of models aimed at mimicking the genetic causes of PD has also sprung up. Whilst these newer models have provided further clues to the disease pathology, they have so far been less commonly used for drug development. There is little doubt that the availability of experimental animal models of PD has dramatically altered dopaminergic drug treatment of the illness and the prevention and reversal of drug-related side effects that emerge with disease progression and chronic medication. However, so far, we have made little progress in moving into other pharmacological areas for the treatment of PD, and we have not developed models that reflect the progressive nature of the illness and its complexity in terms of the extent of pathology and biochemical change. Only when this occurs are we likely to make progress in developing agents to stop or slow the disease progression. The overarching question that draws all of these models together in the quest for better drug treatments for PD is how well do they recapitulate the human condition and how predictive are they of successful translation of drugs into the clinic? This article aims to clarify the current position and highlight the strengths and weaknesses of available models.

The 6-Hydroxydopamine model of parkinson’s disease

The neurotoxin 6-hydroxydopamine (6-OHDA) continues to constitute a valuable topical tool used chiefly in modeling Parkinson's disease in the rat. The classical method of intracerebral infusion of 6-OHDA involving a massive destruction of nigrostriatal dopaminergic neurons, is largely used to investigate motor and biochemical dysfunctions in Parkinson's disease. Subsequently, more subtle models of partial dopaminergic degeneration have been developed with the aim of revealing finer motor deficits. The present review will examine the main features of 6-OHDA models, namely the mechanisms of neurotoxin-induced neurodegeneration as well as several behavioural deficits and motor dysfunctions, including the priming model, modeled by this means. An overview of the most recent morphological and biochemical findings obtained with the 6-OHDA model will also be provided, particular attention being focused on the newly investigated intracellular mechanisms at the striatal level (e.g., A(2A) and NMDA receptors, PKA, CaMKII, ERK kinases, as well as immediate early genes, GAD67 and peptides). Thanks to studies performed in the 6-OHDA model, all these mechanisms have now been hypothesised to represent the site of pathological dysfunction at cellular level in Parkinson's disease.

Modification of L-DOPA pharmacological activity by MAO inhibitors

Dopamine behaves mainly as a MAO-A substrate in rodent brain, but selective inhibition of MAO-B results in an increased turning activity following L-DOPA administration in hemi-Parkinsonian rodents. Unilateral substantia nigra dopaminergic denervation results in serotonergic hyper-innervation which may increase the contribution of MAO-A in the denervated striatum. Possibly as a result of this, there was no change in striatal MAO-A activity when 95% of dopaminergic innervation was reduced by 6-hydroxydopamine, as assessed by apomorphine-induced turning activity. MAO-B as well as MAO-A may contribute to deamination of dopamine produced from L-DOPA.

Rasagiline (TVP-1012): a new selective monoamine oxidase inhibitor for Parkinson's disease

OBJECTIVE

This article reviews the chemistry, pharmacodynamics, pharmacokinetics, clinical efficacy, tolerability, drug-interaction potential, indications, dosing, and potential role of rasagiline mesylate, a new selective monoamine oxidase (MAO) type B (MAO-B) inhibitor, in the treatment of Parkinson's disease.

METHODS

A MEDLINE/PUBMED search (1986 through September 2006) was conducted to identify studies involving rasagiline written in English. Additional references were obtained from the bibliographies of these studies. All studies evaluating any aspect of rasagiline, including in vitro, in vivo (animal), and human studies, were reviewed.

RESULTS

Rasagiline mesylate was developed with the goal of producing a selective MAO-B inhibitor that is not metabolized to (presumed) toxic metabolites (eg, amphetamine and methamphetamine, which are byproducts of the metabolism of selegiline, another selective MAO-B inhibitor). In vitro and in vivo data have confirmed the drug's selectivity for MAO-B. Rasagiline is almost completely eliminated by oxidative metabolism (catalyzed by cytochrome P-450 [CYP] isozyme 1A2) followed by renal excretion of conjugated parent compound and metabolites. Drug clearance is sufficiently slow to allow once-daily dosing. Several studies have documented its efficacy as monotherapy for early-stage disease and as adjunctive therapy in L-dopa recipients with motor fluctuations. As monotherapy, rasagiline is well tolerated with an adverse-effect profile similar to that of placebo. As adjunctive therapy, it exhibits the expected adverse effects of dopamine excess, which can be ameliorated by reducing the L-dopa dosage. CYP1A2 inhibitors slow the elimination of rasagiline and mandate dosage reduction. Hepatic impairment has an analogous effect. The recommended dosage regimens for monotherapy and adjunctive therapy are 1 and 0.5 mg PO QD, respectively.

CONCLUSIONS

Despite the well-documented selectivity of rasagiline, the manufacturer recommends virtually all of the dietary (vis-à-vis tyramine) and drug restrictions of the nonselective MAO inhibitors. Although useful, selective MAO-B inhibitors have a limited role in Parkinson's disease. Of greater interest is the potential neuroprotective effect of rasagiline and its major metabolite, 1(R)-aminoindan, which may have great utility in a wide variety of neurodegenerative disorders of aging. In addition, bifunctional molecules combining selective MAO-B inhibition (based on the active moiety of rasagiline) with acetylcholinesterase inhibition or iron chelation may eventually be useful in Alzheimer's disease.

Rasagiline in the pharmacotherapy of Parkinson's disease--a review

Despite the current efficacious symptomatic approaches, the search is on for new therapies for Parkinson's disease that can control the cardinal symptoms of the disease (tremor, rigidity and bradykinesia), control/prevent motor complications induced by long-term levodopa, act on non-motor disease symptoms (dementia, dysautonomia, pain, insomnia, falls) and halt disease progression. Rasagiline is a monoamine oxidase-B inhibitor that has demonstrated efficacy against the cardinal symptoms of Parkinson's disease when used as monotherapy in early Parkinson's disease, and as an adjunct to levodopa in advanced disease stages. It reduces the duration and severity of poor symptom response episodes in fluctuating patients. Preliminary results allow discussion of putative effects of rasagiline on some non-motor signs and disease progression. This article outlines the evidence surrounding the efficacy and safety of rasagiline, and discusses its potential to address some of the currently unmet needs of Parkinson's disease therapy.

Clinical pharmacology of rasagiline: a novel, second-generation propargylamine for the treatment of Parkinson disease

Rasagiline is a novel second-generation propargylamine that irreversibly and selectively inhibits monoamine oxidase type B (MAO-B). For the management of Parkinson disease (PD), rasagiline is efficacious across the span of PD stages ranging from monotherapy in early disease to adjunctive treatment in patients with advancing disease and motor fluctuations. Rasagiline completely and selectively inhibits MAO-B with a potency 5 to 10 times greater than selegiline. Unlike the prototype propargylamine selegiline, which is metabolized to amphetamine derivatives, rasagiline is biotransformed to aminoindan, a non-amphetamine compound. Rasagiline is well tolerated with infrequent cardiovascular or psychiatric side effects, and at the recommended therapeutic dose of up to 1 mg once daily, tyramine restriction is unnecessary. In addition to MAO-B inhibition, the propargylamine chain also confers dose-related antioxidant and antiapoptotic effects, which have been associated with neuroprotection in multiple experimental models. Thus, in addition to symptomatic benefits, rasagiline offers the promise of clinically relevant neuroprotection.