Safety and efficacy of perampanel in advanced Parkinson's disease: a randomized, placebo-controlled study

Targeting Striatal Glutamate and Phosphodiesterases to Control L-DOPA-Induced Dyskinesia

A large body of work during the past several decades has been focused on therapeutic strategies to control L-DOPA-induced dyskinesias (LIDs), common motor complications of long-term L-DOPA therapy in Parkinson's disease (PD). Yet, LIDs remain a clinical challenge for the management of patients with advanced disease. Glutamatergic dysregulation of striatal projection neurons (SPNs) appears to be a key contributor to altered motor responses to L-DOPA. Targeting striatal hyperactivity at the glutamatergic neurotransmission level led to significant preclinical and clinical trials of a variety of antiglutamatergic agents. In fact, the only FDA-approved treatment for LIDs is amantadine, a drug with NMDAR antagonistic actions. Still, novel agents with improved pharmacological profiles are needed for LID therapy. Recently other therapeutic targets to reduce dysregulated SPN activity at the signal transduction level have emerged. In particular, mechanisms regulating the levels of cyclic nucleotides play a major role in the transduction of dopamine signals in SPNs. The phosphodiesterases (PDEs), a large family of enzymes that degrade cyclic nucleotides in a specific manner, are of special interest. We will review the research for antiglutamatergic and PDE inhibition strategies in view of the future development of novel LID therapies.

Parkinson's disease, epilepsy, and amyotrophic lateral sclerosis-emerging role of AMPA and kainate subtypes of ionotropic glutamate receptors

Ionotropic glutamate receptors (iGluRs) mediate the majority of excitatory neurotransmission and are implicated in various neurological disorders. In this review, we discuss the role of the two fastest iGluRs subtypes, namely, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate receptors, in the pathogenesis and treatment of Parkinson's disease, epilepsy, and amyotrophic lateral sclerosis. Although both AMPA and kainate receptors represent promising therapeutic targets for the treatment of these diseases, many of their antagonists show adverse side effects. Further studies of factors affecting the selective subunit expression and trafficking of AMPA and kainate receptors, and a reasonable approach to their regulation by the recently identified novel compounds remain promising directions for pharmacological research.

Effect of Serum Perampanel Concentration on Sporadic Amyotrophic Lateral Sclerosis Progression

BACKGROUND AND PURPOSE

To clarify the effect of perampanel (PER) on sporadic amyotrophic lateral sclerosis (sALS) progression, the relationship between the changes in Revised Amyotrophic Lateral Sclerosis Functional Rating Scale (ALSFRS-R) scores and serum PER concentrations was investigated.

METHODS

12 patients with sALS from our hospital who agreed to participate and completed the PER for sALS randomized phase 2 study were included. After completing the study, we retrospectively obtained serum PER concentration data from the patients. Based on their mean PER concentrations, we divided the patients who had been taking PER into two groups: four patients with a mean PER concentration of ≥400 ng/mL were assigned to the H group, and three with a mean PER concentration of <400 ng/mL were assigned to the L group. The control group consisted of five patients who had been taking a placebo. We obtained the ALSFRS-R scores of each patient at 36 and 48 weeks after randomization. The differences in ALSFRS-R scores at baseline (0 weeks) and each subsequent week were used in the analysis.

RESULTS

At 48 weeks, there were no differences in the degree of deterioration of the bulbar, upper and lower limb, and respiratory ALSFRS-R subscores and total ALSFRS-R score. However, at 36 weeks, the bulbar subscore was significantly lower in the H group than in the control group (p=0.032).

CONCLUSIONS

Because high PER concentrations may exacerbate bulbar symptoms in patients with sALS, serum PER measurements may be beneficial when patients with sALS are taking PER.

The safety of perampanel in different disorders and doses: A meta-analysis

PURPOSE

To investigate the safety of perampanel in different disorders and doses.

METHODS

Embase, the Cochrane Library, Medline, and ClinicalTrials.gov were searched from inception to July 2022 for randomized controlled trials (RCTs). The meta-analysis was performed by using Review Manager 5.3 and R 4.2.1 software.

RESULTS

A total of 17 RCTs with 5711 subjects were included in the final analysis. The double-blind treatment phase was from 12 weeks to 48 weeks. Our results showed that 11 adverse events (aggression, ataxia, balance disorder, dizziness, fall, fatigue, irritability, rash, somnolence, vertigo, and weight increase) were statistically significantly associated with perampanel, and 4 of them (ataxia, dizziness, fatigue, and somnolence) showed a clear dose-response relationship. Psychiatric adverse events occurred most frequently among serious treatment-emergent adverse events (TEAEs). At 8 mg/day, seven adverse events (aggression, balance disorder, dizziness, fatigue, irritability, vertigo, and weight increase) occurred more frequently in patients with epilepsy than in patients with other disorders, whereas dose discontinuation rates due to adverse events were lower in patients with epilepsy than in patients with other disorders.

CONCLUSION

The safety profile of perampanel is dependent on diseases and dose. The risk of adverse events was statistically significantly higher, with doses exceeding 4 mg/day. Despite a higher risk of adverse events, patients with epilepsy had a lower perampanel discontinuation rate than patients with other disorders.

Kainate Receptor Antagonists: Recent Advances and Therapeutic Perspective

Since the 1990s, ionotropic glutamate receptors have served as an outstanding target for drug discovery research aimed at the discovery of new neurotherapeutic agents. With the recent approval of perampanel, the first marketed non-competitive antagonist of AMPA receptors, particular interest has been directed toward 'non-NMDA' (AMPA and kainate) receptor inhibitors. Although the role of AMPA receptors in the development of neurological or psychiatric disorders has been well recognized and characterized, progress in understanding the function of kainate receptors (KARs) has been hampered, mainly due to the lack of specific and selective pharmacological tools. The latest findings in the biology of KA receptors indicate that they are involved in neurophysiological activity and play an important role in both health and disease, including conditions such as anxiety, schizophrenia, epilepsy, neuropathic pain, and migraine. Therefore, we reviewed recent advances in the field of competitive and non-competitive kainate receptor antagonists and their potential therapeutic applications. Due to the high level of structural divergence among the compounds described here, we decided to divide them into seven groups according to their overall structure, presenting a total of 72 active compounds.

Glutamate modulation for the treatment of levodopa induced dyskinesia: a brief review of the drugs tested in the clinic

Levodopa is the standard treatment for Parkinson's disease, but its use is marred by the emergence of dyskinesia, for which treatment options remain limited. Here, we review the glutamatergic modulators that were assessed for their antidyskinetic potential in clinical trials, including N-methyl-D-aspartate (NMDA) antagonists, agonists at the glycine-binding site on NMDA receptors, metabotropic glutamate (mGlu) 4 agonists, mGlu₅ antagonists, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) antagonists and glutamate release inhibitors. Several agents that were investigated are not selective for their targets, raising uncertainty about the extent to which glutamatergic modulation contributed to their effects. Except for amantadine, the use of glutamatergic modulators for the treatment of dyskinesia in Parkinson's disease remains largely investigational, with promising results obtained with mGlu₅ negative allosteric modulation.

Amantadine in the treatment of Parkinson's disease and other movement disorders

The efficacy of amantadine in the symptomatic treatment of patients with Parkinson's disease, discovered serendipitously more than 50 years ago, has stood the test of time and the drug is still commonly used by neurologists today. Its pharmacological actions are unique in combining dopaminergic and glutamatergic properties, which account for its dual effect on parkinsonian signs and symptoms and levodopa-induced dyskinesias. Furthermore, amantadine has additional and less well-defined pharmacological effects, including on anticholinergic and serotonergic activity. Evidence from randomised controlled trials over the past 5 years has confirmed the efficacy of amantadine to treat levodopa-induced dyskinesias in patients with Parkinson's disease, and clinical studies have also provided support for its potential to reduce motor fluctuations. Other uses of amantadine, such as in the treatment of drug-induced parkinsonism, atypical parkinsonism, Huntington's disease, or tardive dyskinesia, lack a strong evidence base. Future trials should examine its role in the management of motor and non-motor symptoms in patients with early Parkinson's disease and those with other movement disorders.

An Open-Label Phase 2a Study to Evaluate the Safety and Tolerability of Perampanel in Cervical Dystonia

Background

Cervical dystonia (CD) is the most common focal isolated dystonia. Preclinical studies report that AMPA-selective glutamate receptor antagonists improve dystonia. Perampanel is a clinically available, AMPA receptor antagonist that has shown efficacy and safety in epilepsy.

Objectives

To determine safety and tolerability of perampanel in CD.

Methods

We performed a phase 2a, open-label, multicenter study to evaluate tolerability and safety of perampanel in CD. Included subjects had primary CD; those on botulinum toxin were 8 weeks post last injection. All subjects received perampanel 2 mg/day, titrated 2 mg weekly over 6 weeks, to maximum 12 mg/day; maintenance phase was 4 weeks, ending at week 10. Primary endpoints included tolerability, defined as ability to remain on perampanel for the maintenance period, at any dose level and safety, determined from adverse events (AEs) collected at each visit. Secondary exploratory endpoints included Toronto Western Spasmodic Torticollis Rating Scale (TWSTRS), quality of life (cervical dystonia impact profile [CIDP]-58) and Clinical Global Impression of change (CGI).

Results

CD participants (n = 25) were recruited. Eight subjects withdrew; 4 because of AEs, 3 for other reasons and 1 lost to follow up. One subject tolerated 12 mg/day. Eight subjects (30.8%) tolerated 2 mg, whereas 19.2% tolerated 4 mg/day, and 15.4% tolerated 6 mg or 8 mg/day. All subjects experienced AEs. The most common AEs were dizziness, imbalance, and irritability. Exploratory endpoints of TWSTRS showed some improved pain scores and CIDP-58 improved sleep.

Conclusions

Tolerability to perampanel was variable in CD subjects. Lower doses would be considered for future studies in this population.

Perampanel Inhibits α-Synuclein Transmission in Parkinson's Disease Models

BACKGROUND

The intercellular transmission of pathogenic proteins plays a key role in the clinicopathological progression of neurodegenerative diseases. Previous studies have demonstrated that this uptake and release process is regulated by neuronal activity.

OBJECTIVE

The objective of this study was to examine the effect of perampanel, an antiepileptic drug, on α-synuclein transmission in cultured cells and mouse models of Parkinson's disease.

METHODS

Mouse primary hippocampal neurons were transduced with α-synuclein preformed fibrils to examine the effect of perampanel on the development of α-synuclein pathology and its mechanisms of action. An α-synuclein preformed fibril-injected mouse model was used to validate the effect of oral administration of perampanel on the α-synuclein pathology in vivo.

RESULTS

Perampanel inhibited the development of α-synuclein pathology in mouse hippocampal neurons transduced with α-synuclein preformed fibrils. Interestingly, perampanel blocked the neuronal uptake of α-synuclein preformed fibrils by inhibiting macropinocytosis in a neuronal activity-dependent manner. We confirmed that oral administration of perampanel ameliorated the development of α-synuclein pathology in wild-type mice inoculated with α-synuclein preformed fibrils.

CONCLUSION

Modulation of neuronal activity could be a promising therapeutic target for Parkinson's disease, and perampanel could be a novel disease-modifying drug for Parkinson's disease. © 2021 International Parkinson and Movement Disorder Society.

Overview of Therapeutic Drugs and Methods for the Treatment of Parkinson's Disease

Parkinson's Disease (PD) is a neurodegenerative disease involving degeneration of dopaminergic neurons of the nigrostriatal pathways. Over the past decades, most of the medications for the treatment of PD patients have been used to modulate dopamine concentrations in the basal ganglia. This includes levodopa and its inhibitory metabolizing enzymes. In addition to modulating dopamine concentrations in the brain, there are D2-like dopamine receptor agonists that mimic the action of dopamine to compensate for the deficit in dopamine found in PD patients. Muscarinic antagonists' drugs are used rarely due to some side effects. Monoamine oxidase inhibitors are among the first in line, and are considered popular drugs that reduce the metabolism of dopamine in PD patients. Furthermore, we discussed in this review the existence of certain glutamate receptor antagonists for the treatment of PD. Alternatively, we further discussed the potential therapeutic role of adenosine (2A) receptor antagonists, such as tozadenant and istradefylline in the treatment of PD. We also discussed the important role of serotonin1A receptor agonist, adrenergic autoreceptors (α2) antagonists and calcium channel blockers in the treatment of PD. Finally, neurotrophic factors, such as glial cell line-derived neurotrophic growth factor and brain-derived neurotrophic factor are considered the primary factors for neuroprotection in PD.

Roles of Glutamate Receptors in Parkinson's Disease

Parkinson's disease is a progressive neurodegenerative disorder resulting from the degeneration of pigmented dopaminergic neurons in the substantia nigra pars compacta. It induces a series of functional modifications in the circuitry of the basal ganglia nuclei and leads to severe motor disturbances. The amino acid glutamate, as an excitatory neurotransmitter, plays a key role in the disruption of normal basal ganglia function regulated through the interaction with its receptor proteins. It has been proven that glutamate receptors participate in the modulation of neuronal excitability, transmitter release, and long-term synaptic plasticity, in addition to being related to the altered neurotransmission in Parkinson's disease. Therefore, they are considered new targets for improving the therapeutic strategies used to treat Parkinson's disease. In this review, we discuss the biological characteristics of these receptors and demonstrate the receptor-mediated neuroprotection in Parkinson's disease. Pharmacological manipulation of these receptors during anti-Parkinsonian processes in both experimental studies and clinical trials are also summarized.

Receptor Ligands as Helping Hands to L-DOPA in the Treatment of Parkinson's Disease

Levodopa (LD) is the most effective drug in the treatment of Parkinson’s disease (PD). However, although it represents the “gold standard” of PD therapy, LD can cause side effects, including gastrointestinal and cardiovascular symptoms as well as transient elevated liver enzyme levels. Moreover, LD therapy leads to LD-induced dyskinesia (LID), a disabling motor complication that represents a major challenge for the clinical neurologist. Due to the many limitations associated with LD therapeutic use, other dopaminergic and non-dopaminergic drugs are being developed to optimize the treatment response. This review focuses on recent investigations about non-dopaminergic central nervous system (CNS) receptor ligands that have been identified to have therapeutic potential for the treatment of motor and non-motor symptoms of PD. In a different way, such agents may contribute to extending LD response and/or ameliorate LD-induced side effects.

Perampanel: Does it have broad-spectrum potential?

This article reviews the profile of perampanel, a novel noncompetitive α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptor antagonist, and its role as a potential broad-spectrum antiepileptic drug in the treatment of epilepsy. For this narrative review, data were collected using specified search criteria. Articles reporting the evidence for perampanel's efficacy from preclinical models, phase 3 clinical studies, observational studies, and descriptive evidence were included. AMPA receptors play a key role in mediating the action of glutamate at the excitatory synapse. Preclinical research showed the AMPA receptor blockade to constitute a promising target for antiepileptic drug therapy. In animal models, perampanel proved to be protective against seizures and reduce seizure severity and duration. Four phase-3 randomized controlled trials (3 in patients with focal seizures and one in primary generalized tonic-clonic seizures in idiopathic generalized epilepsy) have been completed. In focal (partial) onset seizures, perampanel (4, 8, and 12 mg) significantly reduced seizure frequency per 28 days (23.3%-28.8% vs 12.8%; P < .01) and responder rates (≥50% reduction in seizures) (28.5%-35.3% vs 19.3%; P < .05) compared with placebo. In primary generalized tonic-clonic seizures, perampanel 8 mg resulted in greater reduction in seizure frequency per 28 days (-76.5% vs -38.4%; P < .0001) and responder rate (64.2% vs 39.5%; P = .0019) than placebo. The efficacy, safety, and tolerability of perampanel have been reproduced in real-world clinical practice, and the agent has been shown to be effective in other epilepsy syndromes. Perampanel is a potentially broad-spectrum antiepileptic drug with a novel mechanism of action that may be a useful addition for patients with epilepsy with various seizure types. The availability of novel antiepileptic drugs for epilepsy treatment enables more individualized treatment for these patients.

Current Nondopaminergic Therapeutic Options for Motor Symptoms of Parkinson's Disease

Objective:

The aim of this study was to summarize recent studies on nondopaminergic options for the treatment of motor symptoms in Parkinson's disease (PD).

Data Sources:

Papers in English published in PubMed, Cochrane, and Ovid Nursing databases between January 1988 and November 2016 were searched using the following keywords: PD, nondopaminergic therapy, adenosine, glutamatergic, adrenergic, serotoninergic, histaminic, and iron chelator. We also reviewed the ongoing clinical trials in the website of clinicaltrials.gov.

Study Selection:

Articles related to the nondopaminergic treatment of motor symptoms in PD were selected for this review.

Results:

PD is conventionally treated with dopamine replacement strategies, which are effective in the early stages of PD. Long-term use of levodopa could result in motor complications. Recent studies revealed that nondopaminergic systems such as adenosine, glutamatergic, adrenergic, serotoninergic, histaminic, and iron chelator pathways could include potential therapeutic targets for motor symptoms, including motor fluctuations, levodopa-induced dyskinesia, and gait disorders. Some nondopaminergic drugs, such as istradefylline and amantadine, are currently used clinically, while most such drugs are in preclinical testing stages. Transitioning of these agents into clinically beneficial strategies requires reliable evaluation since several agents have failed to show consistent results despite positive findings at the preclinical level.

Conclusions:

Targeting nondopaminergic transmission could improve some motor symptoms in PD, especially the discomfort of dyskinesia. Although nondopaminergic treatments show great potential in PD treatment as an adjunct therapy to levodopa, further investigation is required to ensure their success.

Deconstructing behavioral neuropharmacology with cellular specificity

Behavior has molecular, cellular, and circuit determinants. However, because many proteins are broadly expressed, their acute manipulation within defined cells has been difficult. Here, we combined the speed and molecular specificity of pharmacology with the cell type specificity of genetic tools. DART (drugs acutely restricted by tethering) is a technique that rapidly localizes drugs to the surface of defined cells, without prior modification of the native target. We first developed an AMPAR antagonist DART, with validation in cultured neuronal assays, in slices of mouse dorsal striatum, and in behaving mice. In parkinsonian animals, motor deficits were causally attributed to AMPARs in indirect spiny projection neurons (iSPNs) and to excess phasic firing of tonically active interneurons (TANs). Together, iSPNs and TANs (i.e., D2 cells) drove akinesia, whereas movement execution deficits reflected the ratio of AMPARs in D2 versus D1 cells. Finally, we designed a muscarinic antagonist DART in one iteration, demonstrating applicability of the method to diverse targets.

Pharmacokinetics and brain uptake study of novel AMPA receptor antagonist perampanel in SD rats using a validated UHPLC-QTOF-MS method

Perampanel (PER) is a novel AMPA receptor antagonist for antiepileptic therapy and is prospective for the treatment of other neurological disorders. A highly sensitive and rapid UHPLC-QTOF-MS method was developed for the quantification of PER in plasma/brain homogenate of SD rat with alogliptin as an internal standard (IS). Chromatographic separation was carried out on an Acquity UPLC HSS Cyano column (100mm×2.1mm, 1.8μm) using gradient mobile phase consisting of 0.1% formic acid and acetonitrile at a flow rate of 0. 4mL/min. Sample preparation was carried out by a simple protein precipitation method. The mass spectrometric analysis of target ions at [M+H]⁺m/z 350.1288 for PER and m/z 340.1779 for IS was monitored with extracted ion chromatography. The developed analytical method meets the US-FDA and EMA bioanalytical guidelines and was found to be precise, accurate, selective and rugged. It exhibited good sensitivity (0.4ng/mL) and linearity over a range of 0.4-400ng/mL in both the bio-matrices. The method was successfully applied to pharmacokinetics and brain uptake study of PER after oral administration to SD rats. The study results showed PER has penetrated the blood-brain barrier, brain to plasma ratio (Kp) was found to be 0.62±0.05 and its rapidly eliminated from the brain.

Levodopa treatment and dendritic spine pathology

Parkinson's disease (PD) is a neurodegenerative disorder associated with the progressive loss of nigrostriatal dopaminergic neurons. Levodopa is the most effective treatment for the motor symptoms of PD. However, chronic oral levodopa treatment can lead to various motor and nonmotor complications because of nonphysiological pulsatile dopaminergic stimulation in the brain. Examinations of autopsy cases with PD have revealed a decreased number of dendritic spines of striatal neurons. Animal models of PD have revealed altered density and morphology of dendritic spines of neurons in various brain regions after dopaminergic denervation or dopaminergic denervation plus levodopa treatment, indicating altered synaptic transmission. Recent studies using rodent models have reported dendritic spine head enlargement in the caudate‐putamen, nucleus accumbens, primary motor cortex, and prefrontal cortex in cases where chronic levodopa treatment following dopaminergic denervation induced dyskinesia‐like abnormal involuntary movement. Hypertrophy of spines results from insertion of alpha‐amino‐2,3‐dihydro‐5‐methyl‐3‐oxo‐4‐isoxazolepropanoic acid receptors into the postsynaptic membrane. Such spine enlargement indicates hypersensitivity of the synapse to excitatory inputs and is compatible with a lack of depotentiation, which is an electrophysiological hallmark of levodopa‐induced dyskinesia found in the corticostriatal synapses of dyskinetic animals and the motor cortex of dyskinetic PD patients. This synaptic plasticity may be one of the mechanisms underlying the priming of levodopa‐induced complications such as levodopa‐induced dyskinesia and dopamine dysregulation syndrome. Drugs that could potentially prevent spine enlargement, such as calcium channel blockers, N‐methyl‐D‐aspartate receptor antagonists, alpha‐amino‐2,3‐dihydro‐5‐methyl‐3‐oxo‐4‐isoxazolepropanoic acid receptor antagonists, and metabotropic glutamate receptor antagonists, are candidates for treatment of levodopa‐induced complications in PD. © 2017 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Nondopaminergic treatments for Parkinson's disease: current and future prospects

Parkinson's disease is primarily caused by dysfunction of dopaminergic neurons, however, nondopaminergic (ND) systems are also involved. ND targets are potentially useful to reduce doses of levodopa or to treat nonlevodopa-responsive symptoms. Recent studies have investigated the role of ND drugs for motor and nonmotor symptoms. Adenosine A2A receptor antagonists, mixed inhibitors of sodium/calcium channels and monoamine oxidase-B have recently been found to improve motor fluctuations. N-methyl-d-aspartate receptor antagonists and serotonin 5HT1B receptor agonists demonstrated benefit in levodopa-induced dyskinesia. Conversely, studies using antiepileptic drugs and adrenoreceptor antagonist had conflicting results. Moreover, metabotropic glutamate receptor antagonists also failed to improve symptoms. The current review summarizes the most recent findings on ND drugs over the last 2 years.

[The Competence Network Parkinson (CNP)]

The Competence Network Parkinson (CNP) is a research infrastructure for disease-oriented translational and clinical research in the field of Parkinson syndromes (PS). It was initiated in 1999 and funded until 2008 by the German Ministry for Education and Research (BMBF). The CNP created a highly frequented website with information on PS for the general public and for experts. The CNP designed and established one of the first electronic internet-based data entry systems (secuTrial®) - fulfilling the legal standards of data safety and security - a material bank for genetic research on Parkinson's disease (PD), implemented and investigated new methods for early diagnosis of PD and related atypical PS including in vivo dopamine transporter imaging (DAT SPECT), established the German Parkinson Study Group (GPS-Pharma) with 40 certified trial centres for pharmacotherapeutical trials and the German interdisciplinary Parkinson Study Group (neurology and neurosurgery) for deep brain stimulation (GPS-DBS), and carried out several pharmacoeconomic and health care studies on PD in Germany. Sustainability of the infrastructure CNP has in part been achieved in form of the GPS-Pharma and the GPS-DBS, as well as in the German Study Group on REM Sleep Behaviour Disorder (RBD), a prodromal phase of PD. Part of the CNP activities, such as genetic research and research on cohorts of PD patients, have been incorporated into the German Center for Neurodegenerative Disorders (DZNE). Furthermore, topics such as health care research are funded within projects of the EU research program. The article describes problems in setting up a competence network from scratch and contains recommendations how to avoid them in the future.

Dyskinesias and impulse control disorders in Parkinson's disease: From pathogenesis to potential therapeutic approaches

Dopaminergic treatment in Parkinson's disease (PD) reduces the severity of motor symptoms of the disease. However, its chronic use is associated with disabling motor and behavioral side effects, among which levodopa-induced dyskinesias (LID) and impulse control disorders (ICD) are the most common. The underlying mechanisms and pathological substrate of these dopaminergic complications are not fully understood. Recently, the refinement of imaging techniques and the study of the genetics and molecular bases of LID and ICD indicate that, although different, they could share some features. In addition, animal models of parkinsonism with LID have provided important knowledge about mechanisms underlying such complications. In contrast, animal models of parkinsonism and abnormal impulsivity, although useful regarding some aspects of human ICD, do not fully resemble the clinical phenotype of ICD in patients with PD, and until now have provided limited information. Studies on animal models of addiction could complement the previous models and provide some insights into the background of these behavioral complications given that ICD are regarded as behavioral addictions. Here we review the most relevant advances in relation to imaging, genetics, biochemistry and pharmacological interventions to treat LID and ICD in patients with PD and in animal models with a view to better understand the overlapping and unique maladaptations to dopaminergic therapy that are associated with LID and ICD.

Treatment of the later stages of Parkinson's disease - pharmacological approaches now and in the future

The problems associated with the pharmacological treatment of the later stages of Parkinson's disease (PD) remain those seen over many years. These centre on a loss of drug effect ('wearing off') with disease progression, the occurrence of dyskinesia, notably with L-dopa use and the appearance of non-motor symptoms that are largely refractory to dopaminergic medication. Treatment strategies in late PD have been dominated by the use of drug combinations and the subtle manipulation of drug dosage. However, change is occurring as the understanding of the basis of motor complications and fluctuations and non-motor symptoms improves. New pharmacological options are expanding with the advent of longer acting versions of existing dopaminergic drugs, new drug delivery systems and the introduction of non-dopaminergic agents able to manipulate motor function both within the basal ganglia and in other brain regions. Non-dopaminergic agents are also being investigated for the treatment of dyskinesia and for the relief of non-motor symptoms. However, while therapy continues to improve, the treatment of late stage PD remains problematic with non-motor symptoms dominating the unmet need in this patient group.

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.

Targeting glutamatergic synapses in Parkinson's disease

Parkinson's disease (PD) is characterized by progressive degeneration of dopaminergic neurons of the substantia nigra and dramatic motor and cognitive impairments. The current knowledge indicates that the strength of glutamatergic signals from the cortex to the striatum is regulated during the progression of the disease. The efficacy of ionotropic glutamate receptors to modulate synaptic transmission in the striatum indicates that modulation of the activity of these receptors may represent a key target to rescue the altered neurotransmission in PD. Preclinical and clinical studies suggest that agents targeting ionotropic glutamate receptors may ameliorate the motor symptoms of PD as well as to reduce the onset of levodopa-induced dyskinetic motor behaviour.

Advances in non-dopaminergic treatments for Parkinson's disease

Since the 1960's treatments for Parkinson's disease (PD) have traditionally been directed to restore or replace dopamine, with L-Dopa being the gold standard. However, chronic L-Dopa use is associated with debilitating dyskinesias, limiting its effectiveness. This has resulted in extensive efforts to develop new therapies that work in ways other than restoring or replacing dopamine. Here we describe newly emerging non-dopaminergic therapeutic strategies for PD, including drugs targeting adenosine, glutamate, adrenergic, and serotonin receptors, as well as GLP-1 agonists, calcium channel blockers, iron chelators, anti-inflammatories, neurotrophic factors, and gene therapies. We provide a detailed account of their success in animal models and their translation to human clinical trials. We then consider how advances in understanding the mechanisms of PD, genetics, the possibility that PD may consist of multiple disease states, understanding of the etiology of PD in non-dopaminergic regions as well as advances in clinical trial design will be essential for ongoing advances. We conclude that despite the challenges ahead, patients have much cause for optimism that novel therapeutics that offer better disease management and/or which slow disease progression are inevitable.

Emerging therapies for Parkinson's disease: from bench to bedside

The prevalence of Parkinson's disease (PD) increases with age and is projected to increase in parallel to the rising average age of the population. The disease can have significant health-related, social, and financial implications not only for the patient and the caregiver, but for the health care system as well. While the neuropathology of this neurodegenerative disorder is fairly well understood, its etiology remains a mystery, making it difficult to target therapy. The currently available drugs for treatment provide only symptomatic relief and do not control or prevent disease progression, and as a result patient compliance and satisfaction are low. Several emerging pharmacotherapies for PD are in different stages of clinical development. These therapies include adenosine A2A receptor antagonists, glutamate receptor antagonists, monoamine oxidase inhibitors, anti-apoptotic agents, and antioxidants such as coenzyme Q10, N-acetyl cysteine, and edaravone. Other emerging non-pharmacotherapies include viral vector gene therapy, microRNAs, transglutaminases, RTP801, stem cells and glial derived neurotrophic factor (GDNF). In addition, surgical procedures including deep brain stimulation, pallidotomy, thalamotomy and gamma knife surgery have emerged as alternative interventions for advanced PD patients who have completely utilized standard treatments and still suffer from persistent motor fluctuations. While several of these therapies hold much promise in delaying the onset of the disease and slowing its progression, more pharmacotherapies and surgical interventions need to be investigated in different stages of PD. It is hoped that these emerging therapies and surgical procedures will strengthen our clinical armamentarium for improved treatment of PD.

Therapeutic potential of targeting glutamate receptors in Parkinson's disease

Glutamate plays a complex role in many aspects of Parkinson's disease including the loss of dopaminergic neurons, the classical motor symptoms as well as associated non-motor symptoms and the treatment-related side effect, L-DOPA-induced dyskinesia. This widespread involvement opens up possibilities for glutamate-based therapies to provide a more rounded approach to treatment than is afforded by current dopamine replacement therapies. Beneficial effects of blocking postsynaptic glutamate transmission have already been noted in a range of preclinical studies using antagonists of NMDA receptors or negative allosteric modulators of metabotropic glutamate receptor 5 (mGlu5), while positive allosteric modulators of mGlu4 in particular, although at an earlier stage of investigation, also look promising. This review addresses each of the key features of Parkinson's disease in turn, summarising the contribution glutamate makes to that feature and presenting an up-to-date account of the potential for drugs acting at ionotropic or metabotropic glutamate receptors to provide relief. Whilst only a handful of these have progressed to clinical trials to date, notably NMDA and NR2B antagonists against motor symptoms and L-DOPA-induced dyskinesia, with mGlu5 negative allosteric modulators also against L-DOPA-induced dyskinesia, the mainly positive outcomes of these trials, coupled with supportive preclinical data for other strategies in animal models of Parkinson's disease and L-DOPA-induced dyskinesia, raise cautious optimism that a glutamate-based therapeutic approach will have significant impact on the treatment of Parkinson's disease.

Perampanel

Each month, subscribers to The Formulary Monograph Service receive 5 to 6 well-documented monographs on drugs that are newly released or are in late phase 3 trials. The monographs are targeted to Pharmacy & Therapeutics Committees. Subscribers also receive monthly 1-page summary monographs on agents that are useful for agendas and pharmacy/nursing in-services. A comprehensive target drug utilization evaluation/medication use evaluation (DUE/MUE) is also provided each month. With a subscription, the monographs are sent in print and are also available on-line. Monographs can be customized to meet the needs of a facility. A drug class review is now published monthly with The Formulary Monograph Service. Through the cooperation of The Formulary, Hospital Pharmacy publishes selected reviews in this column. For more information about The Formulary Monograph Service.Through the cooperation of The Formulary, Hospital Pharmacy publishes selected reviews in this column. For more information about The Formulary Monograph Service, call The Formulary at 800-322-4349. The April 2013 monograph topics are alogliptin, crofelemer, lomitapide, ponatinib, and sumatriptan iontophoretic transdermal. The DUE/MUE is on alogliptin.

AMPA receptor inhibitors for the treatment of epilepsy: the role of perampanel

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in the postsynaptic membrane are involved in fast excitatory signaling in the brain and their activation may lead to the firing of action potentials. Talampanel and perampanel were the first noncompetitive AMPA receptor antagonists to be tested as add-on drugs in patients with refractory partial seizures, and were found to be effective in improving seizure control. Due to an unfavorable kinetic and tolerability profile, talampanel clinical development in the field of epilepsy was discontinued early while perampanel has been recently approved in Europe and the USA as adjunctive therapy for adults with partial seizures with or without secondary generalization. The recommended perampanel starting dose is 2 mg/day once daily, which can be increased up to the recommended maintenance dose of 4-8 mg/day. Increments should be of 2 mg/day and based on clinical response and tolerability. Titration should be performed at 1-week intervals or at lower speed and a 12-mg daily dose should be considered after careful evaluation. To date, no serious and/or idiosyncratic adverse effects have been associated with this agent. Most frequently reported adverse effects are dizziness, ataxia, aggression, irritability, vertigo, somnolence, fatigue, headache and gait disturbance. Weight increase is the only non-neurological adverse effects associated with perampanel.

Levodopa-induced dyskinesias in Parkinson's disease: emerging treatments

Parkinson's disease therapy is still focused on the use of L-3,4-dihydroxyphenylalanine (levodopa or L-dopa) for the symptomatic treatment of the main clinical features of the disease, despite intensive pharmacological research in the last few decades. However, regardless of its effectiveness, the long-term use of levodopa causes, in combination with disease progression, the development of motor complications termed levodopa-induced dyskinesias (LIDs). LIDs are the result of profound modifications in the functional organization of the basal ganglia circuitry, possibly related to the chronic and pulsatile stimulation of striatal dopaminergic receptors by levodopa. Hence, for decades the key feature of a potentially effective agent against LIDs has been its ability to ensure more continuous dopaminergic stimulation in the brain. The growing knowledge regarding the pathophysiology of LIDs and the increasing evidence on involvement of nondopaminergic systems raises the possibility of more promising therapeutic approaches in the future. In the current review, we focus on novel therapies for LIDs in Parkinson's disease, based mainly on agents that interfere with glutamatergic, serotonergic, adenosine, adrenergic, and cholinergic neurotransmission that are currently in testing or clinical development.

Metabotropic glutamate receptors for Parkinson's disease therapy

Excessive glutamatergic signalling within the basal ganglia is implicated in the progression of Parkinson's disease (PD) and inthe emergence of dyskinesia associated with long-term treatment with L-DOPA. There is considerable research focus on the discovery and development of compounds that modulate glutamatergic signalling via glutamate receptors, as treatments for PD and L-DOPA-induced dyskinesia (LID). Although initial preclinical studies with ionotropic glutamate receptor antagonists showed antiparkinsonian and antidyskinetic activity, their clinical use was limited due to psychiatric adverse effects, with the exception of amantadine, a weak N-methyl-d-aspartate (NMDA) antagonist, currently used to reduce dyskinesia in PD patients. Metabotropic receptor (mGlu receptor) modulators were considered to have a more favourable side-effect profile, and several agents have been studied in preclinical models of PD. The most promising results have been seen clinically with selective antagonists of mGlu5 receptor and preclinically with selective positive allosteric modulators of mGlu4 receptor. The growing understanding of glutamate receptor crosstalk also raises the possibility of more precise modulation of glutamatergic transmission, which may lead to the development of more effective agents for PD.

Ascertainment bias in dementias: a secondary to tertiary centre analysis in Central Italy and conceptual review

BACKGROUND AND AIMS

Ascertainment bias (AB) indicates a bias of an evaluation centre in estimating the prevalence/incidence of a disease due to the specific expertise of the centre. The aim of our study was to evaluate classification of different types of dementia in new cases appearing in secondary and tertiary centres, in order to evidence possible occurrence of AB in the various (secondary to tertiary) dementia centres.

METHODS

To assess the mechanism of AB, the rates of new cases of the different forms of dementia reported by different centres were compared. The centres involved in the study were 11 hospital-based centres including a tertiary centre, located in the University Department of Clinical Neurology. The tertiary centre is endowed with state-of-the-art diagnostic facilities and its scientific production is prominently focused on dementia with Lewy bodies (DLB) thus suggesting the possible occurrence of a bias. Four main categories of dementia were identified: Alzheimer's disease (AD), DLB, fronto-temporal dementia (FTD), vascular dementia (VaD), with other forms in a category apart. The classification rate of new cases of dementia in the tertiary centre was compared with rates reported by secondary centres and rates of recoding were calculated during a follow-up of 2 years.

RESULTS

The study classified 2,042 newly diagnosed cases of dementia in a population of 1,370,000 inhabitants of which 315,000 were older than 65. AD was categorized in 48-52 % of cases, DLB in 25-28 %, FTD in 2-4 % and VaD in 17-28 %. During the 2-year follow-up the diagnosis was re-classified in 40 patients (3 %). The rate of recoding was 5 % in the tertiary centre, 2-8 % in referrals from secondary to tertiary centre, 2-10 % in recodings performed in secondary centres and addressed to tertiary centre. Recoding or percentages of new cases of AD or DLB were not different in the comparison between secondary or between secondary and tertiary centres. FTD and VaD were instead significantly recoded.

CONCLUSION

The results of the study suggest that in a homogeneous area, AB is not interfering with diagnosis of AD or DLB.

The adverse event profile of perampanel: meta-analysis of randomized controlled trials

BACKGROUND AND PURPOSE

To identify adverse events (AEs) significantly associated with perampanel treatment in double-blind clinical studies (RCTs). Serious AEs, study withdrawals due to AEs and dose-effect responses of individual AEs were also investigated.

METHODS

All placebo controlled, double-blind RCTs investigating therapeutic effects of oral perampanel were searched. AEs were assessed for their association with perampanel after exclusion of synonyms, rare AEs and non-assessable AEs. Risk difference (RD) was used to evaluate the association of any AE (99% confidence intervals) and withdrawals or serious AEs (95% confidence intervals) with perampanel.

RESULTS

Nine RCTs (five in pharmacoresistant epilepsy and four in Parkinson's disease) were included in our study. Almost 4000 patients had been recruited, 2627 of whom were randomized to perampanel and treated with drug doses of 0.5 mg/day (n = 68), 1 mg/day (n = 65), 2 mg/day (n = 753), 4 mg/day (n = 1017), 8 mg/day (n = 431) or 12 mg/day (n = 293). Serious AEs were not significantly associated with perampanel treatment. The experimental drug was significantly associated with an increased risk of AE-related study withdrawals at 4 mg/day [RD (95% confidence interval) 0.03 (0.00, 0.06)] and 12 mg/day [RD (95% confidence interval) 0.13 (0.07, 0.18)]. Of 15 identified AEs, five (dizziness, ataxia, somnolence, irritability and weight increase) were found to be significantly associated with perampanel and one (seizure worsening) was significantly associated with placebo.

CONCLUSIONS

Vestibulocerebellar AEs (dizziness, ataxia), sedative effects (somnolence), irritability and weight increase were significantly associated with perampanel treatment.

Targeting glutamate receptors to tackle the pathogenesis, clinical symptoms and levodopa-induced dyskinesia associated with Parkinson's disease

The appearance of levodopa-induced dyskinesia (LID) and ongoing degeneration of nigrostriatal dopaminergic neurons are two key features of Parkinson's disease (PD) that current treatments fail to address. Increased glutamate transmission contributes to the motor symptoms in PD, to the striatal plasticity that underpins LID and to the progression of neurodegeneration through excitotoxic mechanisms. Glutamate receptors have therefore long been considered as potential targets for pharmacological intervention in PD, with emphasis on either blocking activation of 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid (AMPA), N-methyl-D-aspartate (NMDA) or excitatory metabotropic glutamate (mGlu) 5 receptors or promoting the activation of group II/III mGlu receptors. Following a brief summary of the role of glutamate in PD and LID, this article explores the current status of pharmacological studies in pre-clinical rodent and primate models through to clinical trials, where applicable, that support the potential of glutamate-based therapeutic interventions. To date, AMPA antagonists have shown good efficacy against LID in rat and primate models, but the failure of perampanel to lessen LID in clinical trials casts doubt on the translational potential of this approach. In contrast, antagonists selective for NR2B-containing NMDA receptors were effective against LID in animal models and in small-scale clinical trials, though observed adverse cognitive effects need addressing. So far, mGlu5 antagonists or negative allosteric modulators (NAMs) look set to become the first introduced for tackling LID, with AFQ-056 reported to exhibit good efficacy in phase II clinical trials. NR2B antagonists and mGlu5 NAMs may subsequently prove to also be effective disease-modifying agents if their protective effects in rat and primate models of PD, respectively, are replicated in the next stages of investigation. Finally, group III mGlu4 agonists or positive allosteric modulators (PAMs), although in the early pre-clinical stages of investigation, are showing good efficacy against motor symptoms, neurodegeneration and LID. It is anticipated that the recent development of mGlu4 PAMs with improved systemic bioavailability will facilitate progression of these agents into the primate model of PD where their potential can be further explored.

Perampanel in Parkinson disease fluctuations: a double-blind randomized trial with placebo and entacapone

OBJECTIVES

Perampanel is a selective and noncompetitive α-amino-3-hydroxy-5-methylisoxazole propionic acid-type glutamate receptor antagonist that improves motor symptoms in animal models of Parkinson disease (PD). The aim of this study was to assess the efficacy and tolerability of perampanel in L-dopa-treated patients with moderately severe PD and motor fluctuations using an active comparator study design.

METHODS

This was a prospective, randomized, double-blind, 3-arm, parallel-group, controlled study assessing the effects of perampanel (4 mg/d), placebo, or entacapone (200 mg with each dose of L-dopa) in 723 L-dopa-treated patients with PD with "OFF" problems. The primary outcome measure was the change from baseline in mean total daily OFF time based on diaries. Secondary end points included change from baseline in Unified Parkinson's Disease Rating Scale part II while OFF, Unified Parkinson's Disease Rating Scale part III while "ON," and mean total daily ON time without dyskinesias or with nontroublesome dyskinesias.

RESULTS

In total, 480 patients (66.4%) completed the study, which was terminated early after negative results of 2 other large placebo-controlled studies became available. Perampanel was not superior to placebo on any efficacy end point, whereas entacapone was superior to placebo on the primary end point (P = 0.034) and most secondary outcomes. Perampanel was generally well tolerated.

CONCLUSIONS

Perampanel (4 mg/d) was well tolerated but did not have a clinically significant effect in improving motor symptoms of L-dopa-treated patients with moderately advanced PD and motor fluctuations. These patients did respond to the active comparator, entacapone, confirming the validity of the findings despite the early termination of the study.

New pharmacological avenues for the treatment of L-DOPA-induced dyskinesias in Parkinson's disease: targeting glutamate and adenosine receptors

INTRODUCTION

Parkinson's disease (PD) therapy is still centered on the use of L-3,4-dihydroxyphenylalanine (L-DOPA), which is hampered by numerous side effects, including abnormal involuntary movements known as L-DOPA-induced dyskinesias (LIDs). LIDs are the result of pre- and postsynaptic changes at the corticostriatal level, induced by chronic and pulsatile stimulation of striatal dopaminergic receptors. These changes impact on synaptic plasticity and involve also selected, nondopaminergic receptors expressed by striatal projection neurons.

AREAS COVERED

Among nondopaminergic receptors, glutamate receptors - NMDA and mGluR5 subtypes in particular - and adenosine A(2A) receptors are those most likely involved in LIDs. The aim of the present review is to summarize results of studies undertaken with specific antagonists of these receptors, first conducted in animal models of LIDs, which in selected cases have been translated into clinical trials.

EXPERT OPINION

Selected antagonists of glutamate and adenosine receptors have been proposed as anti-dyskinetic agents. Promising results have been obtained in preclinical investigations and in initial clinical trials, but long-term safety, tolerability and efficacy studies in patients are still required. The current development of novel antagonists, including tools able to act on receptor mosaics, may provide innovative tools for LIDs management in the next future.

Perampanel, an AMPA antagonist, found to have no benefit in reducing "off" time in Parkinson's disease

BACKGROUND

Perampanel is a selective, noncompetitive α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor antagonist. Two multicenter randomized, double-blind, placebo-controlled, parallel-group phase III studies assessed the efficacy and safety of adjunctive perampanel in patients with Parkinson's disease and motor fluctuations.

METHODS

In both phase III studies (301 and 302), levodopa-treated patients were randomized and treated with once-daily oral placebo (n = 504), perampanel 2 mg (n = 509), or perampanel 4 mg (n = 501). The primary end point was change in daily "off" time from baseline. The treatment period was 30 weeks in study 301 and 20 weeks in study 302.

RESULTS

For any efficacy end point, perampanel 2 or 4 mg was not superior to placebo. Perampanel was well tolerated up to 4 mg/day.

CONCLUSIONS

Perampanel failed to significantly improve motor symptoms versus placebo. There was also no effect on the duration or disability of levodopa-induced dyskinesia.

Nondopaminergic treatments for Parkinson’s disease

SUMMARY The main treatment strategy for Parkinson’s disease (PD) is focused on dopamine replacement. However, PD is no longer seen purely as a disease of the dopaminergic system, as the pathological processes involve neurodegeneration and altered neurotransmission of several nondopaminergic systems that are involved in both motor and nonmotor features of the disease. This article reviews current and experimental nondopaminergic pharmacological approaches to treatments for PD with a focus on motor symptoms, treatments of L-dopa-induced motor complications and treatments of nonmotor symptoms including mood disorders, cognition, psychosis and autonomic problems.

Milestones in Parkinson's disease therapeutics

In the mid-1980s, the treatment of Parkinson's disease was quite exclusively centered on dopatherapy and was focusing on dopamine systems and motor symptoms. A few dopamine agonists and a monoamine oxidase B inhibitor (selegiline) were used as adjuncts in advanced Parkinson's disease. In the early 2010s, levodopa remains the gold standard. New insights into the organization of the basal ganglia paved the way for deep brain stimulation, especially of the subthalamic nucleus, providing spectacular improvement of drug-refractory levodopa-induced motor complications. Novel dopamine agonists (pramipexole, ropinirole, rotigotine), catecholmethyltransferase inhibitors (entacapone), and monoamine oxidase B inhibitors (rasagiline) have also been developed to provide more continuous oral delivery of dopaminergic stimulation in order to improve motor outcomes. Using dopamine agonists early, before levodopa, proved to delay the onset of dyskinesia, although this is achieved at the price of potentially disabling daytime somnolence or impulse control disorders. The demonstration of an antidyskinetic effect of the glutamate antagonist amantadine opened the door for novel nondopaminergic approaches of Parkinson's disease therapy. More recently, nonmotor symptoms (depression, dementia, and psychosis) have been the focus of the first randomized controlled trials in this field. Despite therapeutic advances, Parkinson's disease continues to be a relentlessly progressive disorder leading to severe disability. Neuroprotective interventions able to modify the progression of Parkinson's disease have stood out as a failed therapeutic goal over the last 2 decades, despite potentially encouraging results with compounds like rasagiline. Newer molecular targets, new animal models, novel clinical trial designs, and biomarkers to assess disease modification have created hope for future therapeutic interventions.

Glutamate signaling in the pathophysiology and therapy of schizophrenia

Glutamatergic neurotransmission, particularly through the N-methyl-d-aspartate (NMDA) receptor, has drawn attention for its role in the pathophysiology of schizophrenia. This paper reviews the neurodevelopmental origin and genetic susceptibility of schizophrenia relevant to NMDA neurotransmission, and discusses the relationship between NMDA hypofunction and different domains of symptom in schizophrenia as well as putative treatment modality for the disorder. A series of clinical trials and a meta-analysis which compared currently available NMDA-enhancing agents suggests that glycine, d-serine, and sarcosine are more efficacious than d-cycloserine in improving the overall psychopathology of schizophrenia without side effect or safety concern. In addition, enhancing glutamatergic neurotransmission via activating the AMPA receptor, metabotropic glutamate receptor or inhibition of d-amino acid oxidase (DAO) is also reviewed. More studies are needed to determine the NMDA vulnerability in schizophrenia and to confirm the long-term efficacy, functional outcome, and safety of these NMDA-enhancing agents in schizophrenic patients, particularly those with refractory negative and cognitive symptoms, or serious adverse effects while taking the existing antipsychotic agents.