The novel phosphodiesterase 10A inhibitor THPP-1 has antipsychotic-like effects in rat and improves cognition in rat and rhesus monkey

Effects of PDE10A inhibitor MK-8189 in people with an acute episode of schizophrenia: A randomized proof-of-concept clinical trial

BACKGROUND

PDE10A inhibition represents a potential mechanism for treating schizophrenia. PDE10A inhibitors increase cyclic nucleotides in striatal neurons, thereby mimicking the effects of dopamine receptor D2 antagonists and D1 agonists. We evaluated the PDE10A inhibitor MK-8189 for treating schizophrenia.

METHODS

Randomized, double-blind, placebo and active-controlled, phase 2a, multicenter, inpatient trial in adults experiencing an acute episode of schizophrenia. Participants were randomized 2:2:1 to once-daily MK-8189 12 mg, placebo, or risperidone 6 mg (active control) for 4-weeks. The primary outcome was change-from-baseline in total score on the Positive and Negative Syndrome Scale (PANSS) at 4 weeks.

RESULTS

The number of treated participants was 90 for MK-8189, 89 for placebo, and 45 for risperidone. MK-8189 demonstrated a trend towards improvement versus placebo for change-from-baseline in PANSS total score after 4 weeks (difference = -4.7 [95 % CI: -9.8,0.5], P = 0.074). The active control risperidone was superior to placebo on PANNS total score (difference = -7.3 [95 % CI: -14.0,-0.6], P = 0.033), demonstrating assay sensitivity, while MK-8189 and risperidone did not significantly differ (difference = 2.6 [95 % CI: -4.0,9.2], P = 0.440). MK-8189 had a nominally significant effect on PANSS positive subscale score compared to placebo (difference = -2.2 [95 % CI: -3.8,-0.5], P = 0.011). Discontinuation of MK-8189 treatment due to an adverse event was low (<10 %). Extrapyramidal symptoms occurred with MK-8189 but were mostly mild and transient. Compared with placebo, MK-8189 reduced body weight while risperidone increased weight.

CONCLUSIONS

These findings suggest that PDE10A inhibition may produce antipsychotic effects and associated weight loss and that further trials with PDE10A inhibitors are warranted.

TRIAL REGISTRATION

Clinicaltrials.gov identifier: NCT03055338.

A Novel PDE10A Inhibitor for Tourette Syndrome and Other Movement Disorders

BACKGROUND

Tourette syndrome is a neurodevelopmental movement disorder involving basal ganglia dysfunction. PDE10A inhibitors modulate signaling in the striatal basal ganglia nuclei and are thus of interest as potential therapeutics in treating Tourette syndrome and other movement disorders.

METHODS

The preclinical pharmacology and toxicology, human safety and tolerability, and human PET striatal enzyme occupancy data for the PDE10A inhibitor EM-221 are presented.

RESULTS

EM-221 inhibited PDE10A with an in vitro IC50 of 9 pM and was >100,000 selective vs. other PDEs and other CNS receptors and enzymes. In rats, at doses of 0.05-0.50 mg/kg, EM-221 reduced hyperlocomotion and the disruption of prepulse inhibition induced by MK-801, attenuated conditioned avoidance, and facilitated novel object recognition, consistent with PDE10A's inhibition. EM-221 displayed no genotoxicity and was well tolerated up to 300 mg/kg in rats and 100 mg/kg in dogs. In single- and multiple-day ascending dose studies in healthy human volunteers, EM-221 was well tolerated up to 10 mg, with a maximum tolerated dose of 15 mg. PET imaging indicated that a PDE10A enzyme occupancy of up to 92.8% was achieved with a ~24 h half-life.

CONCLUSIONS

The preclinical and clinical data presented here support the study of EM-221 in phase 2 trials of Tourette syndrome and other movement disorders.

Emerging phosphodiesterase inhibitors for treatment of neurodegenerative diseases

Neurodegenerative diseases (NDs) cause progressive loss of neuron structure and ultimately lead to neuronal cell death. Since the available drugs show only limited symptomatic relief, NDs are currently considered as incurable. This review will illustrate the principal roles of the signaling systems of cyclic adenosine and guanosine 3',5'-monophosphates (cAMP and cGMP) in the neuronal functions, and summarize expression/activity changes of the associated enzymes in the ND patients, including cyclases, protein kinases, and phosphodiesterases (PDEs). As the sole enzymes hydrolyzing cAMP and cGMP, PDEs are logical targets for modification of neurodegeneration. We will focus on PDE inhibitors and their potentials as disease-modifying therapeutics for the treatment of Alzheimer's disease, Parkinson's disease, and Huntington's disease. For the overlapped but distinct contributions of cAMP and cGMP to NDs, we hypothesize that dual PDE inhibitors, which simultaneously regulate both cAMP and cGMP signaling pathways, may have complementary and synergistic effects on modifying neurodegeneration and thus represent a new direction on the discovery of ND drugs.

Discovery of MK-8189, a Highly Potent and Selective PDE10A Inhibitor for the Treatment of Schizophrenia

PDE10A is an important regulator of striatal signaling that, when inhibited, can normalize dysfunctional activity. Given the involvement of dysfunctional striatal activity with schizophrenia, PDE10A inhibition represents a potentially novel means for its treatment. With the goal of developing PDE10A inhibitors, early optimization of a fragment hit through rational design led to a series of potent pyrimidine PDE10A inhibitors that required further improvements in physicochemical properties, off-target activities, and pharmacokinetics. Herein we describe the discovery of an isomeric pyrimidine series that addresses the liabilities seen with earlier compounds and resulted in the invention of compound 18 (MK-8189), which is currently in Phase 2b clinical development for the treatment of schizophrenia.

Antiparkinsonian-like effects of CPL500036, a novel selective inhibitor of phosphodiesterase 10A, in the unilateral rat model of Parkinson's disease

Phosphodiesterase 10A (PDE10A), the enzyme which catalyzes hydrolysis of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), is located almost exclusively in striatal γ-amino-butyric acid (GABA)ergic medium spiny neurons (MSNs). Since dopaminergic deficiency in Parkinson's disease (PD) leads to functional imbalance of striatal direct and indirect output pathways formed by MSNs, PDE10A seems to be of special interest as a potential therapeutic target in PD. The aim of the present study was to examine the influence of 7-{5,8-dimethyl-[1,2,4]triazolo[1,5-a]pyrazin-2-yl}-2-phenylimidazo[1,2-a]pyrimidine (CPL500036), a novel selective inhibitor of PDE10A, on sensorimotor deficits and therapeutic effects of L-3,4-dihydroxyphenylalanine (L-DOPA) in hemiparkinsonian rats. Animals were unilaterally lesioned with 6-hydroxydopamine, and their sensorimotor deficits were examined in the stepping, cylinder, vibrissae and catalepsy tests. CPL500036 (0.1 and 0.3 mg/kg) was administered either acutely or chronically (2 weeks), alone or in combination with L-DOPA/benserazide (6 mg/kg/6 mg/kg). Acute treatment with CPL500036 reversed the lesion-induced impairments of contralateral forelimb use in the stepping and cylinder tests but did not influence deficits in the vibrissae test and the lesion-induced catalepsy. Moreover, CPL500036 did not diminish the therapeutic effects produced by acute and chronic treatment with L-DOPA in these tests. The present study suggests a potential use of CPL500036 as a co-treatment to L-DOPA in PD therapy.

Cognitive Deficit in Schizophrenia: From Etiology to Novel Treatments

Schizophrenia is a major mental illness characterized by positive and negative symptoms, and by cognitive deficit. Although cognitive impairment is disabling for patients, it has been largely neglected in the treatment of schizophrenia. There are several reasons for this lack of treatments for cognitive deficit, but the complexity of its etiology-in which neuroanatomic, biochemical and genetic factors concur-has contributed to the lack of effective treatments. In the last few years, there have been several attempts to develop novel drugs for the treatment of cognitive impairment in schizophrenia. Despite these efforts, little progress has been made. The latest findings point to the importance of developing personalized treatments for schizophrenia which enhance neuroplasticity, and of combining pharmacological treatments with non-pharmacological measures.

Effects of a novel M4 muscarinic positive allosteric modulator on behavior and cognitive deficits relevant to Alzheimer's disease and schizophrenia in rhesus monkey

Alzheimer's disease (AD) is a profoundly debilitating neurodegenerative disorder characterized most notably by progressive cognitive decline, but also agitation and behavioral disturbances that are extremely disruptive to patient and caregiver. Current pharmacological treatments for these symptoms have limited efficacy and significant side effects. We have recently reported the discovery of Compound 24, an M4 positive allosteric modulator (PAM) that is potent, highly selective, and devoid of cholinergic-like side effects in rats. In order to further evaluate the translatability of the effects of compound 24 in primates, here we describe the effect of Compound 24 on three behavioral and cognition assays in rhesus monkeys, the stimulant induced motor activity (SIMA) assay, the object retrieval detour task (ORD), and the visuo-spatial paired-associates learning (vsPAL) task. As far as we know, this is the first such characterization of an M4 PAM in non-human primate. Compound 24 and the clinical standard olanzapine attenuated amphetamine induced hyperactivity to a similar degree. In addition, Compound 24 demonstrated procognitive effects in scopolamine-impaired ORD and vsPAL, and these effects were of similar magnitude to donepezil. These findings suggest that M4 PAMs may be beneficial to diseases such as Alzheimer's disease and schizophrenia, which are marked by behavioral disturbances as well as deficits in cognitive function.

Papaverine, a Phosphodiesterase 10A Inhibitor, Ameliorates Quinolinic Acid-Induced Synaptotoxicity in Human Cortical Neurons

Phosphodiesterase-10A (PDE10A) hydrolyse the secondary messengers cGMP and cAMP, two molecules playing important roles in neurodevelopment and brain functions. PDE10A is associated to progression of neurodegenerative diseases like Alzheimer's, Parkinson's, Huntington's diseases, and a critical role in cognitive functions. The present study was undertaken to determine the possible neuroprotective effects and the associated mechanism of papaverine (PAP), a PDE10A isoenzyme inhibitor, against quinolinic acid (QUIN)-induced excitotoxicity using human primary cortical neurons. Cytotoxicity potential of PAP was analysed using MTS assay. Reactive oxygen species (ROS) and mitochondrial membrane potential were measured by DCF-DA and JC10 staining, respectively. Caspase 3/7 and cAMP levels were measured using ELISA kits. Effect of PAP on the CREB, BNDF and synaptic proteins such as SAP-97, synaptophysin, synapsin-I, and PSD-95 expression was analysed by Western blot. Pre-treatment with PAP increased intracellular cAMP and nicotinamide adenine dinucleotide (NAD⁺) levels, restored mitochondrial membrane potential (ΔΨm), and decreased ROS and caspase 3/7 content in QUIN exposed neurons. PAP up-regulated CREB and BDNF, and synaptic protein expression. In summary, these data indicate that PDE10A is involved in QUIN-mediated synaptotoxicity and its inhibition elicit neuroprotection by reducing the oxidative stress and protecting synaptic proteins via up-regulation of cAMP signalling cascade.

PDE10A Inhibitors-Clinical Failure or Window Into Antipsychotic Drug Action?

PDE10A, a phosphodiesterase that inactivates both cAMP and cGMP, is a unique signaling molecule in being highly and nearly exclusively expressed in striatal medium spiny neurons. These neurons dynamically integrate cortical information with dopamine-signaled value to mediate action selection among available behavioral options. Medium spiny neurons are components of either the direct or indirect striatal output pathways. Selective activation of indirect pathway medium spiny neurons by dopamine D2 receptor antagonists is putatively a key element in the mechanism of their antipsychotic efficacy. While PDE10A is expressed in all medium spiny neurons, studies in rodents indicated that PDE10A inhibition has behavioral effects in several key assays that phenocopy dopamine D2 receptor inhibition. This finding gave rise to the hypothesis that PDE10A inhibition also preferentially activates indirect pathway medium spiny neurons, a hypothesis that is consistent with electrophysiological, neurochemical, and molecular effects of PDE10A inhibitors. These data underwrote industry-wide efforts to investigate and develop PDE10A inhibitors as novel antipsychotics. Disappointingly, PDE10A inhibitors from 3 companies failed to evidence antipsychotic activity in patients with schizophrenia to the same extent as standard-of-care D2 antagonists. Given the notable similarities between PDE10A inhibitors and D2 antagonists, gaining an understanding of why only the latter class is antipsychotic affords a unique window into the basis for this therapeutic efficacy. With this in mind, we review the data on PDE10A inhibition as a step toward back-translating the limited antipsychotic efficacy of PDE10A inhibitors, hopefully to inform new efforts to develop better therapeutics to treat psychosis and schizophrenia.

Phosphodiesterase 10A Inhibitor Monotherapy Is Not an Effective Treatment of Acute Schizophrenia

BACKGROUND

Current treatments for psychotic symptoms associated with schizophrenia often provide inadequate efficacy with unacceptable adverse effects. Improved therapeutics have long been a goal of research. Preclinical testing suggests that phosphodiesterase 10A (PDE10A) inhibitors may provide a novel approach to treating psychosis associated with schizophrenia.

METHODS

The efficacy and safety of a highly selective PDE10A inhibitor, PF-02545920, was evaluated in a phase 2 multicenter, randomized, double-blind, placebo-controlled, parallel-group study. Eligible patients (18-65 years) with an acute exacerbation of schizophrenia were randomized 2:2:1:2 to PF-02545920 (5 or 15 mg every 12 hours [Q12H] titrated), risperidone (3 mg Q12H), or placebo for 28 days (n = 74:74:37:74). The primary objectives were to evaluate the efficacy of PF-02545920 using the Positive and Negative Syndrome Scale (PANNS) and safety/tolerability.

RESULTS

At day 28, PF-02545920 (either dose) was not significantly different from placebo for mean change from baseline in the PANNS total score (primary end point) or most other end points. Pharmacokinetics exposures seemed adequate for binding/inhibiting PDE10A enzyme. Risperidone was statistically different from placebo for the PANNS total score, demonstrating study sensitivity. Incidence rates for adverse events were similar among the groups. Both doses of PF-02545920 were generally well tolerated. Dystonia occurred in 1, 6, 0, and 3 patients in the PF-02545920 5 mg Q12H, PF-02545920 15 mg Q12H, risperidone, and placebo groups, respectively.

CONCLUSIONS

Neither dose of PF-02545920 was superior to placebo for the primary and most secondary end points. This indicates that PDE10A inhibition does not produce an antipsychotic effect in patients with acute exacerbation of schizophrenia.

Advances in Discovery of PDE10A Inhibitors for CNS-Related Disorders. Part 1: Overview of the Chemical and Biological Research

Phosphodiesterase 10A (PDE10A) is a double substrate enzyme that hydrolyzes second messenger molecules such as cyclic-3',5'-adenosine monophosphate (cAMP) and cyclic-3',5'-guanosine monophosphate (cGMP). Through this process, PDE10A controls intracellular signaling pathways in the mammalian brain and peripheral tissues. Pharmacological, biochemical, and anatomical data suggest that disorders in the second messenger system mediated by PDE10A may contribute to impairments in the central nervous system (CNS) function, including cognitive deficits as well as disturbances of behavior, emotion processing, and movement. This review provides a detailed description of PDE10A and the recent advances in the design of selective PDE10A inhibitors. The results of preclinical studies regarding the potential utility of PDE10A inhibitors for the treatment of CNS-related disorders, such as schizophrenia as well as Huntington's and Parkinson's diseases are also summarized.

Discovery of natural product inhibitors of phosphodiesterase 10A as novel therapeutic drug for schizophrenia using a multistep virtual screening

The major complaint that most of the schizophrenic patients' face is the cognitive impairment which affects the patient's quality of life. The current antipsychotic drugs treat only the positive symptoms without alleviating the negative or cognitive symptoms of the disease. In addition, the existing therapies are known to produce extrapyramidal side effects that affect the patient adherence to the treatment. PDE10A inhibitor is the new therapeutic approach which has been proven to be effective in alleviating the negative and cognitive symptoms of the disease. A number of PDE10A inhibitors have been developed, but no inhibitor has made it beyond the clinical trials so far. Thus, the present study has been conducted to identify a PDE10A inhibitor from natural sources to be used as a lead compound for the designing of novel selective PDE10A inhibitors. Ligand and structure-based pharmacophore models for PDE10A inhibitors were generated and employed for virtual screening of universal natural products database. From the virtual screening results, 37 compounds were docked into the active site of the PDE10A. Out of 37 compounds, three inhibitors showed the highest affinity for PDE10A where UNPD216549 showed the lowest binding energy and has been chosen as starting point for designing of novel PDE10A inhibitors. The structure-activity-relationship studies assisted in designing of selective PDE10A inhibitors. The optimization of the substituents on the phenyl ring resulted in 26 derivatives with lower binding energy with PDE10A as compared to the lead compound. Among these, MA 8 and MA 98 exhibited the highest affinity for PDE10A with binding energy (-10.90 Kcal/mol).

Cyclic nucleotide phosphodiesterases: potential therapeutic targets for alcohol use disorder

Alcohol use disorder (AUD), which combines the criteria of both alcohol abuse and dependence, contributes as an important causal factor to multiple health and social problems. Given the limitation of current treatments, novel medications for AUD are needed to better control alcohol consumption and maintain abstinence. It has been well established that the intracellular signal transduction mediated by the second messengers cyclic AMP (cAMP) and cyclic GMP (cGMP) crucially underlies the genetic predisposition, rewarding properties, relapsing features, and systemic toxicity of compulsive alcohol consumption. On this basis, the upstream modulators phosphodiesterases (PDEs), which critically control intracellular levels of cyclic nucleotides by catalyzing their degradation, are proposed to play a role in modulating alcohol abuse and dependent process. Here, we highlight existing evidence that correlates cAMP and cGMP signal cascades with the regulation of alcohol-drinking behavior and discuss the possibility that PDEs may become a novel class of therapeutic targets for AUD.

A Selective Phosphodiesterase 10A Inhibitor Reduces L-Dopa-Induced Dyskinesias in Parkinsonian Monkeys

BACKGROUND

Phosphodiesterase 10A is a member of the phosphodiesterase family whose brain expression is restricted to the striatum. Phosphodiesterase 10A regulates cyclic adenosine monophosphate and cyclic guanosine monophosphate, which mediate responses to dopamine receptor activation, and the levels of these cyclic nucleotides are decreased in experimental models of l-dopa-induced dyskinesia. The elevation of cyclic adenosine monophosphate/cyclic guanosine monophosphate levels by phosphodiesterase 10A inhibition may thus be targeted to reduce l-dopa-induced dyskinesia.

OBJECTIVES

The present study was aimed at determining the potential antidyskinetic effects of phosphodiesterase 10A inhibitors in a primate model of Parkinson's disease (PD). The experiments performed in this model were also intended to provide translational data for the design of future clinical trials.

METHODS

Five MPTP-treated macaques with advanced parkinsonism and reproducible l-dopa-induced dyskinesia were used. MR1916, a selective phosphodiesterase 10A inhibitor, at doses 0.0015 to 0.05 mg/kg, subcutaneously, or its vehicle (control test) was coadministered with l-dopa methyl ester acutely (predetermined optimal and suboptimal subcutaneous doses) and oral l-dopa chronically as daily treatment for 5 weeks. Standardized scales were used to assess motor disability and l-dopa-induced dyskinesia by blinded examiners. Pharmacokinetics was also examined.

RESULTS

MR1916 consistently reduced l-dopa-induced dyskinesia in acute tests of l-dopa optimal and suboptimal doses. Significant effects were present with every MR1916 dose tested, but the most effective was 0.015 mg/kg. None of the MR1916 doses tested affected the antiparkinsonian action of l-dopa at the optimal dose. The anti-l-dopa-induced dyskinesia effect of MR1916 (0.015 mg/kg, subcutaneously) was sustained with chronic administration, indicating that tolerance did not develop over the 5-week treatment. No adverse effects were observed after MR1916 administration acutely or chronically.

CONCLUSIONS

Results show that regulation of striatal cyclic nucleotides by phosphodiesterase 10A inhibition could be a useful therapeutic approach for l-dopa-induced dyskinesia, and therefore data support further studies of selective phosphodiesterase 10A inhibitors for PD therapy. © 2018 International Parkinson and Movement Disorder Society.

Inhibition of phosphodiesterases as a strategy to achieve neuroprotection in Huntington's disease

Huntington's disease (HD) is a fatal neurodegenerative condition, due to a mutation in the IT15 gene encoding for huntingtin. Currently, disease-modifying therapy is not available for HD, and only symptomatic drugs are administered for the management of symptoms. In the last few years, preclinical and clinical studies have indicated that pharmacological strategies aimed at inhibiting cyclic nucleotide phosphodiesterase (PDEs) may develop into a novel therapeutic approach in neurodegenerative disorders. PDEs are a family of enzymes that hydrolyze cyclic nucleotides into monophosphate isoforms. Cyclic nucleotides are second messengers that transduce the signal of hormones and neurotransmitters in many physiological processes, such as protein kinase cascades and synaptic transmission. An alteration in their balance results in the dysregulation of different biological mechanisms (transcriptional dysregulation, immune cell activation, inflammatory mechanisms, and regeneration) that are involved in neurological diseases. In this review, we discuss the action of phosphodiesterase inhibitors and their role as therapeutic agents in HD.

Phosphodiesterase inhibition and modulation of corticostriatal and hippocampal circuits: Clinical overview and translational considerations

The corticostriatal and hippocampal circuits contribute to the neurobiological underpinnings of several neuropsychiatric disorders, including Alzheimer's disease, Parkinson's disease and schizophrenia. Based on biological function, these circuits can be clustered into motor circuits, associative/cognitive circuits and limbic circuits. Together, dysfunctions in these circuits produce the wide range of symptoms observed in related neuropsychiatric disorders. Intracellular signaling in these circuits is largely mediated through the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) pathway with an additional role for the cyclic guanosine monophosphate (cGMP)/ protein kinase G (PKG) pathway, both of which can be regulated by phosphodiesterase inhibitors (PDE inhibitors). Through their effects on cAMP response element-binding protein (CREB) and Dopamine- and cAMP-Regulated PhosphoProtein MR 32 kDa (DARPP-32), cyclic nucleotide pathways are involved in synaptic transmission, neuron excitability, neuroplasticity and neuroprotection. In this clinical review, we provide an overview of the current clinical status, discuss the general mechanism of action of PDE inhibitors in relation to the corticostriatal and hippocampal circuits and consider several translational challenges.

TAK-063, a novel PDE10A inhibitor with balanced activation of direct and indirect pathways, provides a unique opportunity for the treatment of schizophrenia

The basal ganglia regulates motor, cognitive, and emotional behaviors. Dysfunction of dopamine system in this area is implicated in the pathophysiology of schizophrenia characterized by positive symptoms, negative symptoms, and cognitive deficits. Medium spiny neurons (MSNs) are principal output neurons of striatum in the basal ganglia. Similar to current antipsychotics with dopamine D₂ receptor antagonism or partial agonism, phosphodiesterase 10A (PDE10A) inhibitors activate indirect pathway MSNs, leading to the expectation of therapeutic potential for the treatment of psychosis. PDE10A inhibitors also activate direct pathway MSNs which may be associated with cognitive functions. These pathways have competing effects on antipsychotic-like activities and extrapyramidal symptoms in rodents. Therefore, careful consideration of activation pattern of these pathways by a PDE10A inhibitor is critical to produce potent efficacy and superior safety profiles. In this review, we outline the pharmacological profile of TAK-063, a novel PDE10A selective inhibitor. Our study revealed that off-rates of PDE10A inhibitors may characterize their pharmacological profiles via regulation of each MSN pathway. TAK-063, with a faster off-rate property, could provide a unique opportunity as a novel therapeutic approach to treatment of psychosis and cognitive deficits in schizophrenia. TAK-063 also has a therapeutic potential in other basal ganglia disorders.

In Vivo Imaging in Pharmaceutical Development and Its Impact on the 3Rs

It is well understood that the biopharmaceutical industry must improve efficiency along the path from laboratory concept to commercial product. In vivo imaging is recognized as a useful method to provide biomarkers for target engagement, treatment response, safety, and mechanism of action. Imaging biomarkers have the potential to inform the selection of drugs that are more likely to be safe and effective. Most of the imaging modalities for biopharmaceutical research are translatable to the clinic. In vivo imaging does not require removal of tissue to provide biomarkers, thus reducing the number of valuable preclinical subjects required for a study. Longitudinal imaging allows for quantitative intra-subject comparisons, enhancing statistical power, and further reducing the number of subjects needed for the evaluation of treatment effects in animal models. The noninvasive nature of in vivo imaging also provides a valuable approach to alleviate or minimize potential pain, suffering or distress.

The detour paradigm in animal cognition

In this paper, we review one of the oldest paradigms used in animal cognition: the detour paradigm. The paradigm presents the subject with a situation where a direct route to the goal is blocked and a detour must be made to reach it. Often being an ecologically valid and a versatile tool, the detour paradigm has been used to study diverse cognitive skills like insight, social learning, inhibitory control and route planning. Due to the relative ease of administrating detour tasks, the paradigm has lately been used in large-scale comparative studies in order to investigate the evolution of inhibitory control. Here we review the detour paradigm and some of its cognitive requirements, we identify various ecological and contextual factors that might affect detour performance, we also discuss developmental and neurological underpinnings of detour behaviors, and we suggest some methodological approaches to make species comparisons more robust.

Preclinical Characterization of the Phosphodiesterase 10A PET Tracer [(11)C]MK-8193

PURPOSE

A positron emission tomography (PET) tracer for the enzyme phosphodiesterase 10A (PDE10A) is desirable to guide the discovery and development of PDE10A inhibitors as potential therapeutics. The preclinical characterization of the PDE10A PET tracer [(11)C]MK-8193 is described.

PROCEDURES

In vitro binding studies with [(3)H]MK-8193 were conducted in rat, monkey, and human brain tissue. PET studies with [(11)C]MK-8193 were conducted in rats and rhesus monkeys at baseline and following administration of a PDE10A inhibitor.

RESULTS

[(3)H]MK-8193 is a high-affinity, selective PDE10A radioligand in rat, monkey, and human brain tissue. In vivo, [(11)C]MK-8193 displays rapid kinetics, low test-retest variability, and a large specific signal that is displaced by a structurally diverse PDE10A inhibitor, enabling the determination of pharmacokinetic/enzyme occupancy relationships.

CONCLUSIONS

[(11)C]MK-8193 is a useful PET tracer for the preclinical characterization of PDE10A therapeutic candidates in rat and monkey. Further evaluation of [(11)C]MK-8193 in humans is warranted.

Can Cyclic Nucleotide Phosphodiesterase Inhibitors Be Drugs for Parkinson's Disease?

Parkinson's disease (PD) has no known cure; available therapies are only capable of offering temporary, symptomatic relief to the patients. Varied therapeutic strategies that are clinically used for PD are pharmacological therapies including dopamine replacement therapies (with or without adjuvant), postsynaptic dopamine receptor stimulation, dopamine catabolism inhibitors and also anticholinergics. Surgical therapies like deep brain stimulation and ablative surgical techniques are also employed. Phosphodiesterases (PDEs) are enzymes that degrade the phosphodiester bond in the second messenger molecules, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). A number of PDE families are highly expressed in the striatum including PDE1-4, PDE7, PDE9 and PDE10. There are growing evidences to suggest that these enzymes play a critical role in modulating cAMP-mediated dopamine signalling at the postsynaptic region. Therefore, it is clear that PDEs, given the broad range of subtypes and their varied tissue- and region-specific distributions, will be able to provide a range of possibilities as drug targets. There is no phosphodiesterase inhibitor currently approved for use against PD. The development of small molecule inhibitors against cyclic nucleotide PDE is a particularly hot area of investigation, and a lot of research and development is geared in this direction with major players in the pharmaceutical industry investing heavily in developing such potential drug entities. This review, while critically assessing the existing body of literature on brain PDEs with particular interest in the striatum in the context of motor function regulation, indicates it is certainly likely that PDE inhibitors could be developed as therapeutic agents against PD.

Effects of a novel phosphodiesterase 10A inhibitor in non-human primates: A therapeutic approach for schizophrenia with improved side effect profile

Schizophrenia symptoms are associated with alterations in basal ganglia-cortical networks that include the cyclic nucleotides (cAMP/cGMP) signaling pathways. Phosphodiesterase 10A (PDE10A) inhibitors have been considered as therapeutic agents for schizophrenia because the regulation of cAMP and cGMP in the striatum by PDE10A plays an important role in the signaling mechanisms of the striatal-cortical network, and thereby in cognitive function. In the present study we assessed in non-human primates (NHPs) the effects of a novel PDE10A inhibitor (FRM-6308) that has demonstrated high potency and selectivity for human recombinant PDE10A in vitro. The behavioral effects of FRM-6308 in a dose range were determined in rhesus monkeys using a standardized motor disability scale for primates, motor tasks, and the "drug effects on the nervous system" (DENS) scale. The neuronal metabolic effects of FRM-6308 were determined with [(18)F]-fluorodeoxyglucose PET imaging. Results showed that FRM-6308 did not have any specific effects on the motor system at s.c. doses up to 0.32 mg/kg in NHPs, which induced a significant increase in the FDG-SUV in striatum (F 16.069, p < 0.05) and cortical (F 15.181, p < 0.05) regions. Higher doses induced sedation and occasional involuntary movements with clear development of tolerance after repeated exposures. These findings suggest that FRM-6308 has the adequate pharmacological profile to advance testing in clinical trials and demonstrate antipsychotic efficacy of PDE10A inhibition for the treatment of schizophrenia patients.

Dopamine D1 signaling involvement in the effects of the phosphodiesterase 10A inhibitor, PDM-042 on cognitive function and extrapyramidal side effect in rats

Inhibition of phosphodiesterase 10A (PDE10A) results in activation of a dopamine D₁ receptor-mediated direct pathway in addition to a dopamine D₂ receptor-mediated indirect pathway in the striatum. Therefore, PDE10A inhibitors could be novel therapeutics for schizophrenia, which differ from the currently available antipsychotics that directly block the dopamine D₂ receptor. Previously, we found that a novel PDE10A inhibitor, PDM-042, had antipsychotic-like activity similar to currently available antipsychotics and minimal cataleptic effects in rats. The purpose of the present study was to examine the pharmacological effects of PDM-042 on cognitive function and extrapyramidal side effect. In addition, we aimed to examine whether these effects were mediated by activation of dopamine D₁ signaling in rats. PDM-042 (1-3mg/kg) resulted in better discrimination of a novel object from a familiar one 48h after the acquisition trial, suggesting that PDM-042 increased object recognition memory. A dopamine D₁ receptor antagonist, SCH23390 (0.1mg/kg), significantly blocked the enhancement of the object recognition memory induced by PDM-042 (3mg/kg) without affecting the recognition index by itself. We also found that the cataleptic effect of PDM-042 (1mg/kg) was significantly enhanced by SCH23390 (0.01-0.03mg/kg). These results indicate that PDM-042 has the potential to increase object recognition memory and that the cognitive enhancing and cataleptic effects of PDM-042 are mediated at least by activation of dopamine D₁ signaling.

Detour Behavior of Mice Trained with Transparent, Semitransparent and Opaque Barriers

Detour tasks are commonly used to study problem solving skills and inhibitory control in canids and primates. However, there is no comparable detour test designed for rodents despite its significance for studying the development of executive skills. Furthermore, mice offer research opportunities that are not currently possible to achieve when primates are used. Therefore, the aim of the study was to translate the classic detour task to mice and to compare obtained data with key findings obtained previously in other mammals. The experiment was performed with V-shaped barriers and was based on the water escape paradigm. The study showed that an apparently simple task requiring mice to move around a small barrier constituted in fact a challenge that was strongly affected by the visibility of the target. The most difficult task involved a completely transparent barrier, which forced the mice to resolve a conflict between vision and tactile perception. The performance depended both on the inhibitory skills and on previous experiences. Additionally, all mice displayed a preference for one side of the barrier and most of them relied on the egocentric strategy. Obtained results show for the first time that the behavior of mice subjected to the detour task is comparable to the behavior of other mammals tested previously with free-standing barriers. This detailed characterization of the detour behavior of mice constitutes the first step toward the substitution of rodents for primates in laboratory experiments employing the detour task.

TAK-063, a PDE10A Inhibitor with Balanced Activation of Direct and Indirect Pathways, Provides Potent Antipsychotic-Like Effects in Multiple Paradigms

Phosphodiesterase 10A (PDE10A) inhibitors are expected to be novel drugs for schizophrenia through activation of both direct and indirect pathway medium spiny neurons. However, excess activation of the direct pathway by a dopamine D1 receptor agonist SKF82958 canceled antipsychotic-like effects of a dopamine D2 receptor antagonist haloperidol in methamphetamine (METH)-induced hyperactivity in rats. Thus, balanced activation of these pathways may be critical for PDE10A inhibitors. Current antipsychotics and the novel PDE10A inhibitor TAK-063, but not the selective PDE10A inhibitor MP-10, produced dose-dependent antipsychotic-like effects in METH-induced hyperactivity and prepulse inhibition in rodents. TAK-063 and MP-10 activated the indirect pathway to a similar extent; however, MP-10 caused greater activation of the direct pathway than did TAK-063. Interestingly, the off-rate of TAK-063 from PDE10A in rat brain sections was faster than that of MP-10, and a slower off-rate PDE10A inhibitor with TAK-063-like chemical structure showed an MP-10-like pharmacological profile. In general, faster off-rate enzyme inhibitors are more sensitive than slower off-rate inhibitors to binding inhibition by enzyme substrates. As expected, TAK-063 was more sensitive than MP-10 to binding inhibition by cyclic nucleotides. Moreover, an immunohistochemistry study suggested that cyclic adenosine monophosphate levels in the direct pathway were higher than those in the indirect pathway. These data can explain why TAK-063 showed partial activation of the direct pathway compared with MP-10. The findings presented here suggest that TAK-063's antipsychotic-like efficacy may be attributable to its unique pharmacological properties, resulting in balanced activation of the direct and indirect striatal pathways.

Behavioral and qEEG effects of the PDE10A inhibitor THPP-1 in a novel rhesus model of antipsychotic activity

RATIONALE

Much preclinical data, almost exclusively using rodent, supports the notion that phosphodiesterase 10A (PDE10A) inhibition may offer an alternative to the current standard of care in schizophrenia. However, concerns persist regarding the clinical translatability of these models for newer drug classes like PDE10A inhibitors.

OBJECTIVES

We therefore sought to characterize the clinical standard risperidone and the PDE10A inhibitor THPP-1 in nonhuman primate, both alone and when used as a combination therapy.

METHODS

THPP-1 and risperidone were tested in a novel rhesus model of stimulant-induced motor activity (SIMA) and in rhesus electroencephalography (EEG).

RESULTS

Consistent with rodent data, both THPP-1 and risperidone significantly attenuated the stimulant effects in SIMA when administered alone, though some differences were noted. Combination therapy with a low dose of risperidone produced significantly more robust effects. THPP-1 and risperidone also produced a marked reduction of wake cycle time and gamma frequency power in EEG. However, THPP-1 differed from risperidone by reducing spectral power of lower frequencies (delta).

CONCLUSIONS

SIMA results suggest that PDE10A inhibition produces antipsychotic-like effects in higher species, and that combination therapy with PDE10A inhibitors may produce more robust efficacy compared to monotherapies. EEG and qEEG results confirm that PDE10A inhibition does share some central signaling effects with clinically effective antipsychotics. The present combination therapy results may carry implications for the manner in which clinical testing of PDE10A inhibitors is conducted.

Pharmacological characterization of a novel potent, selective, and orally active phosphodiesterase 10A inhibitor, PDM-042 [(E)-4-(2-(2-(5,8-dimethyl-[1,2,4]triazolo[1,5-a]pyrazin-2-yl)vinyl)-6-(pyrrolidin-1-yl)pyrimidin-4-yl)morpholine] in rats: potential for the treatment of schizophrenia

Recently, we identified a novel phosphodiesterase 10A (PDE10A) inhibitor, PDM-042 ((E)-4-(2-(2-(5,8-dimethyl-[1,2,4]triazolo[1,5-a]pyrazin-2-yl)vinyl)-6-(pyrrolidin-1-yl)pyrimidin-4-yl)morpholine). PDM-042 showed potent inhibitory activities for human and rat PDE10A with IC ₅₀ values of less than 1 nmol/L and more than 1000-fold selectivity against other phosphodiesterases. Tritiated PDM-042, [³H]PDM-042, had high affinity for membranes prepared from rat striatum with a K_d value of 8.5 nmol/L. The specific binding of [³H]PDM-042 was displaced in a concentration-dependent manner by PDM-042 and another structurally unrelated PDE10A inhibitor, MP-10. In rat studies, PDM-042 showed excellent brain penetration (striatum/plasma ratio = 6.3), occupancy rate (86.6% at a dose of 3 mg/kg), and good oral bioavailability (33%). These data indicate that PDM-042 is a potent, selective, orally active, and brain-penetrable PDE10A inhibitor. In behavioral studies using rat models relevant to schizophrenia, PDM-042 significantly antagonized MK-801-induced hyperlocomotion (0.1-0.3 mg/kg) without affecting spontaneous locomotor activity and attenuated the conditioned avoidance response (CAR) (0.3-1 mg/kg). In tests for adverse effects, PDM-042 had a minimal effect on catalepsy, even at a much higher dose (10 mg/kg) than the minimal effective dose (0.3 mg/kg) in the CAR. Furthermore, PDM-042 had no effect on prolactin release or glucose elevation up to 3 mg/kg, while risperidone increased prolactin release and olanzapine enhanced glucose levels at doses near their efficacious ones in the CAR. Our results suggest that PDM-042 is a good pharmacological tool that can be used to investigate the role of PDE10A and may have therapeutic potential for the treatment of schizophrenia.

Phosphodiesterase regulation of alcohol drinking in rodents

Alcohol use disorders are chronically relapsing conditions characterized by persistent drinking despite the negative impact on one's life. The difficulty of achieving and maintaining sobriety suggests that current treatments fail to fully address the underlying causes of alcohol use disorders. Identifying additional pathways controlling alcohol consumption may uncover novel targets for medication development to improve treatment options. One family of proteins recently implicated in the regulation of alcohol consumption is the cyclic nucleotide phosphodiesterases (PDEs). As an integral component in the regulation of the second messengers cyclic AMP and cyclic GMP, and thus their cognate signaling pathways, PDEs present intriguing targets for pharmacotherapies to combat alcohol use disorders. As activation of cAMP/cGMP-dependent signaling cascades can dampen alcohol intake, PDE inhibitors may provide a novel target for reducing excessive alcohol consumption, as has been proposed for PDE4 and PDE10A. This review highlights preclinical literature demonstrating the involvement of cyclic nucleotide-dependent signaling in neuronal and behavioral responses to alcohol, as well as detailing the capacity of various PDE inhibitors to modulate alcohol intake. Together these data provide a framework for evaluating the potential utility of PDE inhibitors as novel treatments for alcohol use disorders.

Discovery of pyrazolopyrimidine phosphodiesterase 10A inhibitors for the treatment of schizophrenia

Herein, we present the identification of a novel class of pyrazolopyrimidine phosphodiesterase 10A (PDE10A) inhibitors. Beginning with a lead molecule (1) identified through a fragment-based drug discovery (FBDD) effort, lead optimization was enabled by rational design, X-ray crystallography, metabolic and off-target profiling, and fragment scaffold-hopping. We highlight the discovery of PyP-1, a potent, highly selective, and orally bioavailable pyrazolopyrimidine inhibitor of PDE10A. PyP-1 exhibits sub-nanomolar potency (PDE10A Ki=0.23nM), excellent pharmacokinetic (PK) and physicochemical properties, and a clean off-target profile. It displays dose-dependent efficacy in numerous pharmacodynamic (PD) assays that measure potential for anti-psychotic activity and cognitive improvement. PyP-1 also has a clean preclinical profile with respect to cataleptic potential in rats, prolactin secretion, and weight gain, common adverse events associated with currently marketed therapeutics. Further, PyP-1 displays in vivo preclinical target engagement as measured by PET enzyme occupancy in concert with [(11)C]MK-8193, a novel PDE10A PET tracer.

Synthesis of Fluorine-Containing Phosphodiesterase 10A (PDE10A) Inhibitors and the In Vivo Evaluation of F-18 Labeled PDE10A PET Tracers in Rodent and Nonhuman Primate

A series of fluorine-containing PDE10A inhibitors were designed and synthesized to improve the metabolic stability of [(11)C]MP-10. Twenty of the 22 new analogues had high potency and selectivity for PDE10A: 18a-j, 19d-j, 20a-b, and 21b had IC50 values <5 nM for PDE10A. Seven F-18 labeled compounds [(18)F]18a-e, [(18)F]18g, and [(18)F]20a were radiosynthesized by (18)F-introduction onto the quinoline rather than the pyrazole moiety of the MP-10 pharmacophore and performed in vivo evaluation. Biodistribution studies in rats showed ~2-fold higher activity in the PDE10A-enriched striatum than nontarget brain regions; this ratio increased from 5 to 30 min postinjection, particularly for [(18)F]18a-d and [(18)F]20a. MicroPET studies of [(18)F]18d and [(18)F]20a in nonhuman primates provided clear visualization of striatum with suitable equilibrium kinetics and favorable metabolic stability. These results suggest this strategy may identify a (18)F-labeled PET tracer for quantifying the levels of PDE10A in patients with CNS disorders including Huntington's disease and schizophrenia.

Synthesis and in vivo evaluation of novel quinoline derivatives as phosphodiesterase 10A inhibitors

A novel class of phosphodiesterase 10A (PDE10A) inhibitors with improved metabolic stability in mouse liver microsomes were designed and synthesized starting from 2-({4-[1-methyl-4-(pyridin-4-yl)-1H-pyrazol-3-yl]phenoxy}methyl)quinoline (MP-10). Replacement of the phenoxymethyl part of MP-10 with an oxymethyl phenyl unit led to the identification of 2-[4-({[1-methyl-4-(pyridin-4-yl)-1H-pyrazol-3-yl]oxy}methyl)phenyl]quinoline (14), which showed moderate PDE10A inhibitory activity with improved metabolic stability in mouse and human liver microsomes over MP-10. Compound 14 showed high concentrations in plasma and brain after intraperitoneal administration and dose-dependently attenuated the hyperlocomotion induced by phencyclidine in mice, and oral administration of 14 (0.1, 0.3 mg/kg) also improved visual-recognition memory impairment in mice.

Herbs to curb cyclic nucleotide phosphodiesterase and their potential role in Alzheimer's disease

Cyclic nucleotides viz., cAMP/cGMP has been well known to play important role in cellular function and deficiency in their levels has been implicated in the pathogenesis of various neurodegenerative disorders including Alzheimer's disease (AD). Phosphodiesterases (PDE) are the enzymes involved in the metabolism of cyclic nucleotides and the inhibition of phosphodiesterases is considered to be viable strategy to restore the level of cyclic nucleotides and their functions in the brain. Various synthetic PDE inhibitors had been used clinically for various disorders and also suggested to be useful candidates for treating neurological disorders. However, side effects of these synthetic PDE inhibitors have limited their use in clinical practice. Natural plant extracts or their bio-active compounds are considered to be safe and are widely acceptable. During the last decade, many plant extracts or their bio-active compounds were tested pre-clinically for PDE inhibitory activity and are reported to be equally potent in inhibiting PDE's, as that of synthetic compounds. The present review is aimed to discuss the potential plant extract/compounds with PDE inhibitory activity and critically discuss their potential role in Alzheimer's disease.

The PDE10A inhibitor MP-10 and haloperidol produce distinct gene expression profiles in the striatum and influence cataleptic behavior in rodents

Phosphodiesterase 10A (PDE10A) has garnered attention as a potential therapeutic target for schizophrenia due to its prominent striatal expression and ability to modulate striatal signaling. The present study used the selective PDE10A inhibitor MP-10 and the dopamine D2 antagonist haloperidol to compare effects of PDE10A inhibition and dopamine D2 blockade on striatopallidal (D2) and striatonigral (D1) pathway activation. Our studies confirmed that administration of MP-10 significantly elevates expression of the immediate early genes (IEG) c-fos, egr-1, and arc in rat striatum. Furthermore, we demonstrated that MP-10 induced egr-1 expression was distributed evenly between enkephalin-containing D2-neurons and substance P-containing D1-neurons. In contrast, haloperidol (3 mg/kg) selectively activated egr-1 expression in enkephalin neurons. Co-administration of MP-10 and haloperidol (0.5 mg/kg) increased IEG expression to a greater extent than either compound alone. Similarly, in a rat catalepsy assay, administration of haloperidol (0.5 mg/kg) or MP-10 (3-30 mg/kg) did not produce cataleptic behavior when dosed alone, but co-administration of haloperidol with MP-10 (3 and 10 mg/kg) induced cataleptic behaviors. Interestingly, co-administration of haloperidol with a high dose of MP-10 (30 mg/kg) failed to produce cataleptic behavior. These findings are important for understanding the neural circuits involved in catalepsy and suggest that the behavioral effects produced by PDE10A inhibitors may be influenced by concomitant medication and the level of PDE10A inhibition achieved by the dose of the inhibitor.

Discovery of [¹¹C]MK-8193 as a PET tracer to measure target engagement of phosphodiesterase 10A (PDE10A) inhibitors

Phosphodiesterase 10A (PDE10A) inhibition has recently been identified as a potential mechanism to treat multiple symptoms that manifest in schizophrenia. In order to facilitate preclinical development and support key proof-of-concept clinical trials of novel PDE10A inhibitors, it is critical to discover positron emission tomography (PET) tracers that enable plasma concentration/PDE10A occupancy relationships to be established across species with structurally diverse PDE10A inhibitors. In this Letter, we describe how a high-throughput screening hit was optimized to provide [(11)C]MK-8193 (8j), a PET tracer that supports the determination of plasma concentration/PDE10A occupancy relationships for structurally diverse series of PDE10A inhibitors in both rat and rhesus monkey.

Pharmacological evaluation of a novel phosphodiesterase 10A inhibitor in models of antipsychotic activity and cognition

In this study, we report the pharmacological effects of a novel PDE10A inhibitor, SEP-39. SEP-39 is a potent (1.0nM) inhibitor of human PDE10A in vitro, with good selectivity (>16000-fold) against other PDEs. In an in vivo occupancy study, the RO50 value was determined to be 0.7mg/kg (p.o.), corresponding to plasma and brain exposures of 28ng/mL and 43ng/g, respectively. Using microdialysis, we show that 3mg/kg (p.o.) SEP-39 significantly increased rat striatal cGMP concentrations. Furthermore, SEP-39 inhibits PCP-induced hyperlocomotion at doses of 1 and 3mg/kg (p.o.) corresponding to 59-86% occupancy. At similar doses in a catalepsy study, the time on the bar was increased but the maximal effect was less than that seen with haloperidol. In an EEG study, 3 and 10mg/kg (p.o.) SEP-39 suppressed REM intensity and increased the latency to REM sleep. We also demonstrate the procognitive effects of SEP-39 in the rat novel object recognition assay. These effects appear to require less PDE10A inhibition than the reversal of PCP-induced hyperlocomotion or EEG effects, as improvements in recognition index were seen at doses of 0.3mg/kg and above. Our data demonstrate that SEP-39 is a potent, orally active PDE10A inhibitor with therapeutic potential in a number of psychiatric indications.

Phosphodiesterase 10A inhibitors: analysis of US/EP patents granted since 2012

Phosphodiesterases are enzymes that metabolically inactivate the intracellular second messengers 3′,5′-cyclic adenosine and guanosine monophosphate contributing to the control of multiple biological processes. Among them, PDE10A has the most restricted distribution with high expression in striatal medium spiny neurons. Dysfunction of this key brain circuit has been associated with different psychiatric and neurodegenerative disorders. The unique role of PDE10A, together with its increased pharmacological characterization, have prompted enormous interest in investigating the potential of inhibitors of this enzyme as potential novel therapeutic agents This article reviews PDE10A related patents issued in the period 2012–2014 in the USA and Europe offering also a perspective on potential avenues for the future clinical development of phosphodiesterase 10A inhibitors.

A systems approach identifies networks and genes linking sleep and stress: implications for neuropsychiatric disorders

Sleep dysfunction and stress susceptibility are comorbid complex traits that often precede and predispose patients to a variety of neuropsychiatric diseases. Here, we demonstrate multilevel organizations of genetic landscape, candidate genes, and molecular networks associated with 328 stress and sleep traits in a chronically stressed population of 338 (C57BL/6J × A/J) F2 mice. We constructed striatal gene co-expression networks, revealing functionally and cell-type-specific gene co-regulations important for stress and sleep. Using a composite ranking system, we identified network modules most relevant for 15 independent phenotypic categories, highlighting a mitochondria/synaptic module that links sleep and stress. The key network regulators of this module are overrepresented with genes implicated in neuropsychiatric diseases. Our work suggests that the interplay among sleep, stress, and neuropathology emerges from genetic influences on gene expression and their collective organization through complex molecular networks, providing a framework for interrogating the mechanisms underlying sleep, stress susceptibility, and related neuropsychiatric disorders.

Schizophrenia drug discovery and development in an evolving era: are new drug targets fulfilling expectations?

Current therapeutics for schizophrenia, the typical and atypical antipsychotic class of drugs, derive their therapeutic benefit predominantly by antagonism of the dopamine D2 receptor subtype and have robust clinical benefit on positive symptoms of the disease with limited to no impact on negative symptoms and cognitive impairment. Driven by these therapeutic limitations of current treatments and the recognition that transmitter systems beyond the dopaminergic system in particular glutamatergic transmission contribute to the etiology of schizophrenia significant recent efforts have focused on the discovery and development of novel treatments for schizophrenia with mechanisms of action that are distinct from current drugs. Specifically, compounds selectively targeting the metabotropic glutamate receptor 2/3 subtype, phosphodiesterase subtype 10, glycine transporter subtype 1 and the alpha7 nicotinic acetylcholine receptor have been the subject of intense drug discovery and development efforts. Here we review recent clinical experience with the most advanced drug candidates targeting each of these novel mechanisms and discuss whether these new agents are living up to expectations.

PDE and cognitive processing: beyond the memory domain

Phosphodiesterase inhibitors (PDE-Is) enhance cAMP and/or cGMP signaling via reducing the degradation of these cyclic nucleotides. Both cAMP and cGMP signaling are essential for a variety of cellular functions and exert their effects both pre- and post-synaptically. Either of these second messengers relays and amplifies incoming signals at receptors on the cell surface making them important elements in signal transduction cascades and essential in cellular signaling in a variety of cell functions including neurotransmitter release and neuroprotection. Consequently, these processes can be influenced by PDE-Is as they increase cAMP and/or cGMP concentrations. PDE-Is have been considered as possible therapeutic agents to treat impaired memory function linked to several brain disorders, including depression, schizophrenia and Alzheimer's disease (AD). This review will, however, focus on the possible role of phosphodiesterases (PDEs) in cognitive decline beyond the memory domain. Here we will discuss the involvement of PDEs on three related domains: attention, information filtering (sensory- and sensorimotor gating) and response inhibition (drug-induced hyperlocomotion). Currently, these are emerging cognitive domains in the field of PDE research. Here we discuss experimental studies and the potential beneficial effects of PDE-I drugs on these cognitive domains, as effects of PDE-Is on these domains could potentially influence effects on memory performance. Overall, PDE4 seems to be the most promising target for all domains discussed in this review.