Repositioning: the fast track to new anti-malarial medicines?

BACKGROUND: Repositioning of existing drugs has been suggested as a fast track for developing new anti-malarial agents. The compound libraries of GlaxoSmithKline (GSK), Pfizer and AstraZeneca (AZ) comprising drugs that have undergone clinical studies in other therapeutic areas, but not achieved approval, and a set of US Food and Drug Administration (FDA)-approved drugs and other bio-actives were tested against Plasmodium falciparum blood stages. METHODS: Molecules were tested initially against erythrocytic co-cultures of P. falciparum to measure proliferation inhibition using one of the following methods: SYBR®I dye DNA staining assay (3D7, K1 or NF54 strains); [(3)H] hypoxanthine radioisotope incorporation assay (3D7 and 3D7A strain); or 4',6-diamidino-2-phenylindole (DAPI) DNA imaging assay (3D7 and Dd2 strains). After review of the available clinical pharmacokinetic and safety data, selected compounds with low µM activity and a suitable clinical profile were tested in vivo either in a Plasmodium berghei four-day test or in the P. falciparum Pf3D7(0087/N9) huSCID 'humanized' mouse model. RESULTS: Of the compounds included in the GSK and Pfizer sets, 3.8% (9/238) had relevant in vitro anti-malarial activity while 6/100 compounds from the AZ candidate drug library were active. In comparison, around 0.6% (24/3,800) of the FDA-approved drugs and other bio-actives were active. After evaluation of available clinical data, four investigational drugs, active in vitro were tested in the P. falciparum humanized mouse model: UK-112,214 (PAF-H1 inhibitor), CEP-701 (protein kinase inhibitor), CEP-1347 (protein kinase inhibitor), and PSC-833 (p-glycoprotein inhibitor). Only UK-112,214 showed significant efficacy against P. falciparum in vivo, although at high doses (ED90 131.3 mg/kg [95% CI 112.3, 156.7]), and parasitaemia was still present 96 hours after treatment commencement. Of the six actives from the AZ library, two compounds (AZ-1 and AZ-3) were marginally efficacious in vivo in a P. berghei model. CONCLUSIONS: Repositioning of existing therapeutics in malaria is an attractive proposal. Compounds active in vitro at µM concentrations were identified. However, therapeutic concentrations may not be effectively achieved in mice or humans because of poor bio-availability and/or safety concerns. Stringent safety requirements for anti-malarial drugs, given their widespread use in children, make this a challenging area in which to reposition therapy.

In vitro and in vivo activity of solithromycin (CEM-101) against Plasmodium species.

With the emergence of Plasmodium falciparum infections exhibiting increased parasite clearance times in response to treatment with artemisinin-based combination therapies, the need for new therapeutic agents is urgent. Solithromycin, a potent new fluoroketolide currently in development, has been shown to be an effective, broad-spectrum antimicrobial agent. Malarial parasites possess an unusual organelle, termed the apicoplast, which carries a cryptic genome of prokaryotic origin that encodes its own translation and transcription machinery. Given the similarity of apicoplast and bacterial ribosomes, we have examined solithromycin for antimalarial activity. Other antibiotics known to target the apicoplast, such as the macrolide azithromycin, demonstrate a delayed-death effect, whereby treated asexual blood-stage parasites die in the second generation of drug exposure. Solithromycin demonstrated potent in vitro activity against the NF54 strain of P. falciparum, as well as against two multidrug-resistant strains, Dd2 and 7G8. The dramatic increase in potency observed after two generations of exposure suggests that it targets the apicoplast. Solithromycin also retained potency against azithromycin-resistant parasites derived from Dd2 and 7G8, although these lines did demonstrate a degree of cross-resistance. In an in vivo model of P. berghei infection in mice, solithromycin demonstrated a 100% cure rate when administered as a dosage regimen of four doses of 100 mg/kg of body weight, the same dose required for artesunate or chloroquine to achieve 100% cure rates in this rodent malaria model. These promising in vitro and in vivo data support further investigations into the development of solithromycin as an antimalarial agent.

HIF prolyl hydroxylase inhibition increases cell viability and potentiates dopamine release in dopaminergic cells.

Hypoxia-inducible factor (HIF) controls the expression of genes that adapts the cellular condition to accommodate oxidative stress. The potential beneficial effect of HIF up-regulation in ischemia has recently gained interest substantiated by the known HIF-regulation of erythropoietin and other hypoxia accommodating genes. So far the perspectives for HIF up-regulation has been focused on anemia and ischemia related diseases but little information is available about the relevance of HIF biology for neurodegenerative disease like Parkinson's disease. We therefore sought out to characterize the effect of HIF-up-regulation on survival and dopamine homeostasis in dopaminergic cells. We used a low molecular weight HIF prolyl hydroxylase (HPH) inhibitor and lentiviral based shRNA knockdown of HPH subtypes as molecular tools to increase HIF protein level and downstream HIF-regulated genes. We show that HIF induction results in protection against oxidative stress in cellular models based on PC12 cells and LUHMES cells. In addition, HPH inhibition elevates tyrosine hydroxylase expression and activity, which causes increased dopamine synthesis and release in both PC12 cells and a primary rat ventral mesencephalic cell culture. All together these findings suggest that prolyl hydroxylases may represent novel targets for therapeutic intervention in disorders characterized by dopamine homeostasis dysregulation like Parkinson's disease.

Requirement of a dopaminergic neuronal phenotype for toxicity of low concentrations of 1-methyl-4-phenylpyridinium to human cells.

LUHMES cells are conditionally-immortalized non-transformed human fetal cells that can be differentiated to acquire a dopaminergic neuron-like phenotype under appropriate growth conditions. After differentiation by GDNF and cyclic adenosine monophosphate, LUHMES were sensitive to 1-methyl-4-phenylpyridinium (MPP(+)) toxicity at < or =5 microM, but resistant to the parental compound 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The high homogeneity and purity of the cultures allowed the detection of metabolic changes during the degeneration. Cellular ATP dropped in two phases after 24 and 48 h; cellular glutathione (GSH) decreased continuously, paralleled by an increase in lipid peroxidation. These events were accompanied by a time-dependent degeneration of neurites. Block of the dopamine transporter by GBR 12909 or mazindol completely abrogated MPP(+) toxicity. Inhibition of de novo dopamine synthesis by alpha-methyl-l-tyrosine or 3-iodo-l-tyrosine attenuated toxicity, but did not reduce the initial drop in ATP. Inhibition of mixed lineage kinases by CEP1347 completely prevented the MPP(+)-induced loss of viability and intracellular GSH, but failed to attenuate the initial drop of ATP. For the quantitative assessment of neurite degeneration, an automated imaging-based high content screening approach was applied and confirmed the findings made by pharmacological interventions in this study. Our data indicate that inhibition of mitochondrial ATP synthesis is not sufficient to trigger cell death in MPP(+)-treated LUHMES.

Inflammation in Parkinson's disease: causative or epiphenomenal?

Parkinson's disease (PD) is a neurodegenerative disorder characterized by a dramatic loss of dopaminergic neurons in the substantia nigra (SN). Several pathogenic mechanisms have been implicated in the demise of these cells, including dopamine-dependent oxidative stress, mitochondrial dysfunction, excitotoxicity, and proteasomal impairment. In recent years, the involvement of neuroinflammatory processes in nigral degeneration has gained increasing attention. Not only have activated microglia and increased levels of inflammatory mediators been detected in the striatum of PD patients, but a large body of animal studies points to a contributory role of inflammation in dopaminergic cell loss. For example, post-mortem examination of human subjects exposed to the parkinsonism-inducing toxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, revealed the presence of activated microglia decades after drug exposure, suggesting that even a brief pathogenic insult can induce an ongoing inflammatory response. Perhaps not surprisingly, nonsteroidal anti-inflammatory drugs have been shown to reduce the risk of developing PD. In the past few years, various pathways have come to light that could link neurodegeneration and microglial activation, finally ascribing a pathogenic trigger to the chronic inflammatory response characteristic of PD.

Vesicular monoamine transporter 2 regulates the sensitivity of rat dopaminergic neurons to disturbed cytosolic dopamine levels.

An abnormal accumulation of cytosolic dopamine resulting in reactive oxygen species and dopamine-quinone products may play an important role in the rather selective degeneration of substantia nigra pars compacta (SNc) dopaminergic neurons in Parkinson's disease. The neuronal-specific vesicular monoamine transporter (VMAT2), responsible for uptake of dopamine into vesicles, has been shown to play a central role both in intracellular dopamine homeostasis and sequestration of dopaminergic neurotoxins. Direct or indirect enhancement of VMAT2 activity could therefore have neuroprotective effects by decreasing cytosolic dopamine levels. Here, we demonstrate that transfection of VMAT2 in the dopaminergic cell line, PC12, increases intracellular dopamine content, augments potassium-induced dopamine release and attenuates cell death induced by the cytosolic dopamine enhancer, methamphetamine, suggesting an enhancement in vesicular dopamine storage. In rat ventral mesencephalic cultures highly enriched for dopaminergic neurons, lentiviral delivery of recombinant VMAT2 using a neuronal-specific promoter also resulted in elevated intracellular dopamine content and neurotransmitter release after depolarization. The opposite was seen after downregulation of VMAT2 using virally delivered shRNAs. Furthermore, using this VMAT2 knockdown model, we are the first to report a direct link between enhanced cytoplasmic dopamine levels, measured following mild permeabilization of the plasma membrane using digitonin, and neurite degeneration in primary dopaminergic neurons. In conclusion, our data support the hypothesis that an increase in vesicular sequestration of dopamine by modulation of VMAT2 activity could restore neuronal function and enhance dopaminergic cell survival in conditions of dysregulated dopamine homeostasis such as Parkinson's disease.

Safety, tolerability, pharmacokinetics, and activity of the novel long-acting antimalarial DSM265: a two-part first-in-human phase 1a/1b randomised study.

BACKGROUND: DSM265 is a novel antimalarial that inhibits plasmodial dihydroorotate dehydrogenase, an enzyme essential for pyrimidine biosynthesis. We investigated the safety, tolerability, and pharmacokinetics of DSM265, and tested its antimalarial activity. METHODS: Healthy participants aged 18-55 years were enrolled in a two-part study: part 1, a single ascending dose (25-1200 mg), double-blind, randomised, placebo-controlled study, and part 2, an open-label, randomised, active-comparator controlled study, in which participants were inoculated with Plasmodium falciparum induced blood-stage malaria (IBSM) and treated with DSM265 (150 mg) or mefloquine (10 mg/kg). Primary endpoints were DSM265 safety, tolerability, and pharmacokinetics. Randomisation lists were created using a validated, automated system. Both parts were registered with the Australian New Zealand Clinical Trials Registry, number ACTRN12613000522718 (part 1) and number ACTRN12613000527763 (part 2). FINDINGS: In part 1, 73 participants were enrolled between April 12, 2013, and July 14, 2015 (DSM265, n=55; placebo, n=18). In part 2, nine participants were enrolled between Sept 30 and Nov 25, 2013 (150 mg DSM265, n=7; 10 mg/kg mefloquine, n=2). In part 1, 117 adverse events were reported; no drug-related serious or severe events were reported. The most common drug-related adverse event was headache. The mean DSM265 peak plasma concentration (Cmax) ranged between 1310 ng/mL and 34 800 ng/mL and was reached in a median time (tmax) between 1·5 h and 4 h, with a mean elimination half-life between 86 h and 118 h. In part 2, the log10 parasite reduction ratio at 48 h in the DSM265 (150 mg) group was 1·55 (95% CI 1·42-1·67) and in the mefloquine (10 mg/kg) group was 2·34 (2·17-2·52), corresponding to a parasite clearance half-life of 9·4 h (8·7-10·2) and 6·2 h (5·7-6·7), respectively. The median minimum inhibitory concentration of DSM265 in blood was estimated as 1040 ng/mL (range 552-1500), resulting in a predicted single efficacious dose of 340 mg. Parasite clearance was significantly faster in participants who received mefloquine than in participants who received DSM265 (p<0·0001). INTERPRETATION: The good safety profile, long elimination half-life, and antimalarial effect of DSM265 supports its development as a partner drug in a single-dose antimalarial combination treatment. FUNDING: Wellcome Trust, UK Department for International Development, Global Health Innovative Technology Fund, Bill & Melinda Gates Foundation.

Progressive degeneration of human mesencephalic neuron-derived cells triggered by dopamine-dependent oxidative stress is dependent on the mixed-lineage kinase pathway.

Models of Parkinson's disease (PD) based on selective neuronal death have been used to study pathogenic mechanisms underlying nigral cell death and in some instances to develop symptomatic therapies. For validation of putative neuroprotectants, a model is desirable in which the events leading to neurodegeneration replicate those occurring in the disease. We developed a human in vitro model of PD based on the assumption that dysregulated cytoplasmic dopamine levels trigger cell loss in this disorder. Differentiated human mesencephalic neuron-derived cells were exposed to methamphetamine (METH) to promote cytoplasmic dopamine accumulation. In the presence of elevated iron concentrations, as observed in PD, increased cytosolic dopamine led to oxidative stress, c-Jun N-terminal kinase (JNK) pathway activation, neurite degeneration, and eventually apoptosis. We examined the role of the mixed-lineage kinases (MLKs) in this complex degenerative cascade by using the potent inhibitor 3,9-bis[(ethylthio)methyl]-K-252a (CEP1347). Inhibition of MLKs not only prevented FeCl2+/METH-induced JNK activation and apoptosis but also early events such as neurite degeneration and oxidative stress. This broad neuroprotective action of CEP1347 was associated with increased expression of an oxidative stress-response modulator, activating transcription factor 4. As a functional consequence, transcription of the cystine/glutamate and glycine transporters, cellular cystine uptake and intracellular levels of the redox buffer glutathione were augmented. In conclusion, this new human model of parkinsonian neurodegeneration has the potential to yield new insights into neurorestorative therapeutics and suggests that enhancement of cytoprotective mechanisms, in addition to blockade of apoptosis, may be essential for disease modulation.

A long-duration dihydroorotate dehydrogenase inhibitor (DSM265) for prevention and treatment of malaria.

Malaria is one of the most significant causes of childhood mortality, but disease control efforts are threatened by resistance of the Plasmodium parasite to current therapies. Continued progress in combating malaria requires development of new, easy to administer drug combinations with broad-ranging activity against all manifestations of the disease. DSM265, a triazolopyrimidine-based inhibitor of the pyrimidine biosynthetic enzyme dihydroorotate dehydrogenase (DHODH), is the first DHODH inhibitor to reach clinical development for treatment of malaria. We describe studies profiling the biological activity, pharmacological and pharmacokinetic properties, and safety of DSM265, which supported its advancement to human trials. DSM265 is highly selective toward DHODH of the malaria parasite Plasmodium, efficacious against both blood and liver stages of P. falciparum, and active against drug-resistant parasite isolates. Favorable pharmacokinetic properties of DSM265 are predicted to provide therapeutic concentrations for more than 8 days after a single oral dose in the range of 200 to 400 mg. DSM265 was well tolerated in repeat-dose and cardiovascular safety studies in mice and dogs, was not mutagenic, and was inactive against panels of human enzymes/receptors. The excellent safety profile, blood- and liver-stage activity, and predicted long half-life in humans position DSM265 as a new potential drug combination partner for either single-dose treatment or once-weekly chemoprevention. DSM265 has advantages over current treatment options that are dosed daily or are inactive against the parasite liver stage.

[Free radicals and ailing proteins--the culprits behind Parkinson disease?]. / Fria radikaler och sjuka proteiner--bovarna bakom Parkinsons sjukdom?

Parkinson's disease is one of the most common neurodegenerative diseases, and affects approximately 1% of the population over 65 years of age. Many different insults appear to be involved in the etiology of the disease, among them environmental toxins and mitochondrial dysfunction. During the past five years, mutations in five different genes have been linked to rare, familial forms of Parkinson's disease. One of the mutated proteins, alpha-synuclein is normally implicated in synaptic plasticity and vesicle function. Dysfunction of this protein might lead to increased cytoplasmic dopamine levels. Since cytoplasmic dopamine is readily prone to autooxidation and enzymatic degradation--processes which generate reactive oxygen species--failure to properly store dopamine into vesicles might lead to oxidative stress. Indeed, nigral tissue from idiopathic Parkinson's disease patients shows signs of oxidative damage. In this article we propose that dopamine-induced oxidative stress might be a common final pathway in the pathogenesis of the disease.

In vitro and in vivo characterization of the antimalarial lead compound SSJ-183 in Plasmodium models.

The objective of this work was to characterize the in vitro (Plasmodium falciparum) and in vivo (Plasmodium berghei) activity profile of the recently discovered lead compound SSJ-183. The molecule showed in vitro a fast and strong inhibitory effect on growth of all P. falciparum blood stages, with a tendency to a more pronounced stage-specific action on ring forms at low concentrations. Furthermore, the compound appeared to be equally efficacious on drug-resistant and drug-sensitive parasite strains. In vivo, SSJ-183 showed a rapid onset of action, comparable to that seen for the antimalarial drug artesunate. SSJ-183 exhibited a half-life of about 10 hours and no significant differences in absorption or exposure between noninfected and infected mice. SSJ-183 appears to be a promising new lead compound with an attractive antimalarial profile.

Challenges in antimalarial drug discovery.

Malaria is one of the most devastating diseases in the world, affecting almost 225 million people a year, and causing over 780,000 deaths, most of which are children under the age of 5 years. Following the recent call for the eradication of the disease, supported by the WHO, there has been increasing investment into antimalarial drug-discovery projects. These activities are aimed at generating the next generation of molecules focused on the treatment and transmission-blocking of Plasmodium falciparum and Plasmodium vivax endo- and exo-erythrocytic stages of the parasite. This article summarizes the current top-level thinking regarding the prosecution of such endeavors and the disease-specific considerations in project planning.

A randomized, double-blind, placebo-controlled study to investigate the safety, tolerability, and pharmacokinetics of single enantiomer (+)-mefloquine compared with racemic mefloquine in healthy persons.

Racemic mefloquine is a highly effective antimalarial whose clinical utility has been compromised by its association with neuropsychiatric and gastrointestinal side effects. It is hypothesized that the cause of the side effects may reside in the (-) enantiomer. We sought to compare the safety, tolerability and pharmacokinetic profile of (+)-mefloquine with racemic mefloquine in a randomized, ascending-dose, double-blind, active and placebo-controlled, parallel cohort study in healthy male and female adult volunteers. Although differing in its manifestations, both study drugs displayed a substantially worse tolerability profile compared with placebo. The systemic clearance was slower for (-)-mefloquine than (+)-mefloquine. Thus, (+)-mefloquine has a different safety and tolerability profile compared with racemic mefloquine but its global safety profile is not superior and replacement of the currently used antimalarial drug with (+)-mefloquine is not warranted.

Oxidative stress and inflammation in Parkinson's disease: is there a causal link?

Parkinson's disease (PD) is a neurodegenerative disorder characterized by a dramatic loss of dopaminergic neurons in the substantia nigra (SN). Among the many pathogenic mechanisms thought to contribute to the demise of these cells, dopamine-dependent oxidative stress has classically taken center stage due to extensive experimental evidence showing that dopamine-derived reactive oxygen species and oxidized dopamine metabolites are toxic to nigral neurons. In recent years, however, the involvement of neuro-inflammatory processes in nigral degeneration has gained increasing attention. Not only have activated microglia and increased levels of inflammatory mediators been detected in the striatum of deceased PD patients, but a large body of animal studies points to a contributory role of inflammation in dopaminergic cell loss. Recently, postmortem examination of human subjects exposed to the parkinsonism-inducing toxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), revealed the presence of activated microglia decades after drug exposure, suggesting that even a brief pathogenic insult can induce an ongoing inflammatory response. Perhaps not surprisingly, non-steroidal anti-inflammatory drugs (NSAIDs) have been shown to reduce the risk of developing PD. In the past few years, various pathways have come to light that could link dopamine-dependent oxidative stress and microglial activation, finally ascribing a pathogenic trigger to the chronic inflammatory response characteristic of PD.

Impaired dopamine storage resulting from alpha-synuclein mutations may contribute to the pathogenesis of Parkinson's disease.

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by the inability to initiate, execute and control movement. Neuropathologically, there is a striking loss of dopamine-producing neurons in the substantia nigra pars compacta, accompanied by depletion of dopamine in the striatum. Most forms of PD are sporadic, though in some cases familial inheritance is observed. In the late 1990s, two mutations in the alpha-synuclein gene were linked to rare, autosomal dominant forms of PD. Previously cloned from cholinergic vesicles of the Torpedo electric ray, alpha-synuclein is highly enriched in presynaptic nerve terminals and appears to be involved in synapse maintenance and plasticity. It is expressed ubiquitously in the brain, raising the important question of why dopaminergic neurons are primarily targeted in persons carrying mutations in alpha-synuclein. In this article, we review the current literature on alpha-synuclein and suggest a possible role for this protein in vesicle recycling via its regulation of phospholipase D2, its fatty acid-binding properties, or both. Exogenous application of dopamine, as well as redistribution of vesicular dopamine to the cytoplasm, can be toxic to dopaminergic neurons. Thus, impaired neurotransmitter storage arising from mutations in alpha-synuclein could lead to cytoplasmic accumulation of dopamine. The breakdown of this labile neurotransmitter in the cytoplasm could, in turn, promote oxidative stress and metabolic dysfunction, both of which have been observed in nigral tissue from PD patients.

Specific modulation of astrocyte inflammation by inhibition of mixed lineage kinases with CEP-1347.

Inflammatory conversion of murine astrocytes correlates with the activation of various MAPK, and inhibition of terminal MAPKs like JNK or p38 dampens the inflammatory reaction. Mixed lineage kinases (MLKs), a family of MAPK kinase kinases, may therefore be involved in astrocyte inflammation. In this study, we explored the effect of the MLK inhibitors CEP-1347 and CEP-11004 on the activation of murine astrocytes by either TNF plus IL-1 or by a complete cytokine mix containing additional IFN-gamma. The compounds blocked NO-, PG-, and IL-6 release with a median inhibitory concentration of approximately 100 nM. This activity correlated with a block of the JNK and the p38 pathways activated in complete cytokine mix-treated astrocytes. Although CEP-1347 did not affect the activation of NF-kappaB, it blocked the expression of cyclooxygenase-2 and inducible NO synthase at the transcriptional level. Quantitative transcript profiling of 17 inflammation-linked genes revealed a specific modulation pattern of astrocyte activation by MLK inhibition, for instance, characterized by up-regulation of the anti-stress factors inhibitor of apoptosis protein-2 and activated transcription factor 4, no effect on manganese superoxide dismutase and caspase-11, and down-regulation of major inflammatory players like TNF, GM-CSF, urokinase-type plasminogen activator, and IL-6. In conclusion, MLK inhibitors like CEP-1347 are highly potent astrocyte immune modulators with a novel spectrum of activity.

Targeted expression of BCL-2 attenuates MPP+ but not 6-OHDA induced cell death in dopaminergic neurons.

Neurodegenerative diseases such as Parkinson's disease exhibit complex features of cell death reflecting both the primary lesion as well as surrounding interconnected events. Because Bcl-2 family members are intimately involved in cell death processes, the present study used dopaminergic cultures from control, Bcl-2-overexpressing, or Bax-deficient genetically modified animals to determine the in situ effects of parkinsonism-inducing toxins. MPP(+)-mediated cell death was attenuated by Bcl-2 but did not require Bax. Accordingly, mutations or deletions within Bax heterodimerization domains, BH1, BH2, or BH3 had no effect on Bcl-2's ability to prevent cell death, whereas the cell-death suppressing BH4 domain did. Although both staurosporine and 6-OHDA induced apoptosis, overexpression of Bcl-2 only rescued cells from programmed cell death induced by staurosporine. Thus, differential cell death pathways are associated with these cytotoxic signals in primary models of Parkinson's disease.

Effect of mutant alpha-synuclein on dopamine homeostasis in a new human mesencephalic cell line.

Mutations in alpha-synuclein have been linked to rare, autosomal dominant forms of Parkinson's disease. Despite its ubiquitous expression, mutant alpha-synuclein primarily leads to the loss of dopamine-producing neurons in the substantia nigra. alpha-Synuclein is a presynaptic nerve terminal protein of unknown function, although several studies suggest it is important for synaptic plasticity and maintenance. The present study utilized a new human mesencephalic cell line, MESC2.10, to study the effect of A53T mutant alpha-synuclein on dopamine homeostasis. In addition to expressing markers of mature dopamine neurons, differentiated MESC2.10 cells are electrically active, produce dopamine, and express wild-type human alpha-synuclein. Lentivirus-induced overexpression of A53T mutant alpha-synuclein in differentiated MESC2.10 cells resulted in down-regulation of the vesicular dopamine transporter (VMAT2), decreased potassium-induced and increased amphetamine-induced dopamine release, enhanced cytoplasmic dopamine immunofluorescence, and increased intracellular levels of superoxide. These results suggest that mutant alpha-synuclein leads to an impairment in vesicular dopamine storage and consequent accumulation of dopamine in the cytosol, a pathogenic mechanism that underlies the toxicity of the psychostimulant amphetamine and the parkinsonian neurotoxin 1-methyl-4-phenylpyridinium. Interestingly, cells expressing A53T mutant alpha-synuclein were resistant to amphetamine-induced toxicity. Because extravesicular, cytoplasmic dopamine can be easily oxidized into reactive oxygen species and other toxic metabolites, mutations in alpha-synuclein might lead to Parkinson's disease by triggering protracted, low grade dopamine toxicity resulting in terminal degeneration and ultimately cell death.