Preclinical profile of ITI-214, an inhibitor of phosphodiesterase 1, for enhancement of memory performance in rats

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.

Opportunities and perspectives of small molecular phosphodiesterase inhibitors in neurodegenerative diseases

Phosphodiesterase (PDE) is a superfamily of enzymes that are responsible for the hydrolysis of two second messengers: cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). PDE inhibition promotes the gene transcription by activating cAMP-response element binding protein (CREB), initiating gene transcription of brain-derived neurotrophic factor (BDNF). The procedure exerts neuroprotective profile, and motor and cognitive improving efficacy. From this point of view, PDE inhibition will provide a promising therapeutic strategy for treating neurodegenerative disorders. Herein, we summarized the PDE inhibitors that have entered the clinical trials or been discovered in recent five years. Well-designed clinical or preclinical investigations have confirmed the effectiveness of PDE inhibitors, such as decreasing Aβ oligomerization and tau phosphorylation, alleviating neuro-inflammation and oxidative stress, modulating neuronal plasticity and improving long-term cognitive impairment.

The perspective of cAMP/cGMP signaling and cyclic nucleotide phosphodiesterases in aortic aneurysm and dissection

Aortic aneurysm (AA) and dissection (AD) are aortic diseases caused primarily by medial layer degeneration and perivascular inflammation. They are lethal when the rupture happens. Vascular smooth muscle cells (SMCs) play critical roles in the pathogenesis of medial degeneration, characterized by SMC loss and elastin fiber degradation. Many molecular pathways, including cyclic nucleotide signaling, have been reported in regulating vascular SMC functions, matrix remodeling, and vascular structure integrity. Intracellular cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) are second messengers that mediate intracellular signaling transduction through activating effectors, such as protein kinase A (PKA) and PKG, respectively. cAMP and cGMP are synthesized by adenylyl cyclase (AC) and guanylyl cyclase (GC), respectively, and degraded by cyclic nucleotide phosphodiesterases (PDEs). In this review, we will discuss the roles and mechanisms of cAMP/cGMP signaling and PDEs in AA/AD formation and progression and the potential of PDE inhibitors in AA/AD, whether they are beneficial or detrimental. We also performed database analysis and summarized the results showing PDEs with significant expression changes under AA/AD, which should provide rationales for future research on PDEs in AA/AD.

Role of Phosphodiesterase 1 in the Regulation of Real-Time cGMP Levels and Contractility in Adult Mouse Cardiomyocytes

In mouse cardiomyocytes, the expression of two subfamilies of the calcium/calmodulin-regulated cyclic nucleotide phosphodiesterase 1 (PDE1)-PDE1A and PDE1C-has been reported. PDE1C was found to be the major subfamily in the human heart. It is a dual substrate PDE and can hydrolyze both 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP). Previously, it has been reported that the PDE1 inhibitor ITI-214 shows positive inotropic effects in heart failure patients which were largely attributed to the cAMP-dependent protein kinase (PKA) signaling. However, the role of PDE1 in the regulation of cardiac cGMP has not been directly addressed. Here, we studied the effect of PDE1 inhibition on cGMP levels in adult mouse ventricular cardiomyocytes using a highly sensitive fluorescent biosensor based on Förster resonance energy transfer (FRET). Live-cell imaging in paced and resting cardiomyocytes showed an increase in cGMP after PDE1 inhibition with ITI-214. Furthermore, PDE1 inhibition and PDE1A knockdown amplified the cGMP-FRET responses to the nitric oxide (NO)-donor sodium nitroprusside (SNP) but not to the C-type natriuretic peptide (CNP), indicating a specific role of PDE1 in the regulation of the NO-sensitive guanylyl cyclase (NO-GC)-regulated cGMP microdomain. ITI-214, in combination with CNP or SNP, showed a positive lusitropic effect, improving the relaxation of isolated myocytes. Immunoblot analysis revealed increased phospholamban (PLN) phosphorylation at Ser-16 in cells treated with a combination of SNP and PDE1 inhibitor but not with SNP alone. Our findings reveal a previously unreported role of PDE1 in the regulation of the NO-GC/cGMP microdomain and mouse ventricular myocyte contractility. Since PDE1 serves as a cGMP degrading PDE in cardiomyocytes and has the highest hydrolytic activities, it can be expected that PDE1 inhibition might be beneficial in combination with cGMP-elevating drugs for the treatment of cardiac diseases.

Ionocyte-Specific Regulation of Cystic Fibrosis Transmembrane Conductance Regulator

CFTR (cystic fibrosis transmembrane conductance regulator) is a tightly regulated anion channel that mediates chloride and bicarbonate conductance in many epithelia and in other tissues, but whether its regulation varies depending on the cell type has not been investigated. Epithelial CFTR expression is highest in rare cells called ionocytes. We studied CFTR regulation in control and ionocyte-enriched cultures by transducing bronchial basal cells with adenoviruses that encode only eGFP or FOXI1 (forkhead box I1) + eGFP as separate polypeptides. FOXI1 dramatically increased the number of transcripts for ionocyte markers ASCL3 (Achaete-Scute Family BHLH Transcription Factor 3), BSND, ATP6V1G3, ATP6V0D2, KCNMA1, and CFTR without altering those for secretory (SCGB1A1), basal (KRT5, KRT6, TP63), goblet (MUC5AC), or ciliated (FOXJ1) cells. The number of cells displaying strong FOXI1 expression was increased 7-fold, and there was no evidence for a broad increase in background immunofluorescence. Total CFTR mRNA and protein levels increased 10-fold and 2.5-fold, respectively. Ionocyte-enriched cultures displayed elevated basal current, increased adenylyl cyclase 5 expression, and tonic suppression of CFTR activity by the phosphodiesterase PDE1C, which has not been shown previously to regulate CFTR activity. The results indicate that CFTR regulation depends on cell type and identifies PDE1C as a potential target for therapeutics that aim to increase CFTR function specifically in ionocytes.

Evaluation of the acute oral toxicity and antipsychotic activity of a dual inhibitor of PDE1B and PDE10A in rat model of schizophrenia

Phosphodiesterase 1B (PDE1B) and PDE10A are dual-specificity PDEs that hydrolyse both cyclic adenosine monophosphate and cyclic guanosine monophosphate, and are highly expressed in the striatum. Several reports have suggested that PDE10A inhibitors may present a promising approach for the treatment of positive symptoms of schizophrenia, whereas PDE1B inhibitors may present a novel mechanism to modulate cognitive deficits. Previously, we have reported a novel dual inhibitor of PDE1B and PDE10A, compound 2 [(3-fluorophenyl)(2-methyl-2,3-dihydro-4H-benzo[b][1,4]oxazin-4-yl)methanone] which has shown inhibitory activity for human recombinant PDE1B and PDE10A in vitro. In the present study, the safety profile of compound 2 has been evaluated in rats in the acute oral toxicity study, as well as; the antipsychotic-like effects in the rat model of schizophrenia. Compound 2 was tolerated up to 1 g/kg when administered at a single oral dose. Additionally, compound 2 has strongly suppressed ketamine-induced hyperlocomotion, which presented a model for the positive symptoms of schizophrenia. It has also shown an ability to attenuate social isolation induced by chronic administration of ketamine and enhanced recognition memory of rats ​in the novel object recognition test. Altogether, our results suggest that compound 2 represents a promising therapy for the treatment of the three symptomatic domains of schizophrenia.

Recent developments of phosphodiesterase inhibitors: Clinical trials, emerging indications and novel molecules

The phosphodiesterase (PDE) enzymes, key regulator of the cyclic nucleotide signal transduction system, are long-established as attractive therapeutic targets. During investigation of trends within clinical trials, we have identified a particularly high number of clinical trials involving PDE inhibitors, prompting us to further evaluate the current status of this class of therapeutic agents. In total, we have identified 87 agents with PDE-inhibiting capacity, of which 85 interact with PDE enzymes as primary target. We provide an overview of the clinical drug development with focus on the current clinical uses, novel molecules and indications, highlighting relevant clinical studies. We found that the bulk of current clinical uses for this class of therapeutic agents are chronic obstructive pulmonary disease (COPD), vascular and cardiovascular disorders and inflammatory skin conditions. In COPD, particularly, PDE inhibitors are characterised by the compliance-limiting adverse reactions. We discuss efforts directed to appropriately adjusting the dose regimens and conducting structure-activity relationship studies to determine the effect of structural features on safety profile. The ongoing development predominantly concentrates on central nervous system diseases, such as schizophrenia, Alzheimer's disease, Parkinson's disease and fragile X syndrome; notable advancements are being also made in mycobacterial infections, HIV and Duchenne muscular dystrophy. Our analysis predicts the diversification of PDE inhibitors' will continue to grow thanks to the molecules in preclinical development and the ongoing research involving drugs in clinical development.

Nitric oxide/cGMP/CREB pathway and amyloid-beta crosstalk: From physiology to Alzheimer's disease

The nitric oxide (NO)/cGMP pathway has been extensively studied for its pivotal role in synaptic plasticity and memory processes, resulting in an increase of cAMP response element-binding (CREB) phosphorylation, and consequent synthesis of plasticity-related proteins. The NO/cGMP/CREB signaling is downregulated during aging and neurodegenerative disorders and is affected by Amyloid-β peptide (Aβ) and tau protein, whose increase and deposition is considered the key pathogenic event of Alzheimer's disease (AD). On the other hand, in physiological conditions, the crosstalk between the NO/cGMP/PKG/CREB pathway and Aβ ensures long-term potentiation and memory formation. This review summarizes the current knowledge on the interaction between the NO/cGMP/PKG/CREB pathway and Aβ in the healthy and diseased brain, offering a new perspective to shed light on AD pathophysiology. We will focus on the synaptic mechanisms underlying Aβ physiological interplay with cGMP pathway and how this balance is corrupted in AD, as high levels of Aβ interfere with NO production and cGMP molecular signaling leading to cognitive impairment. Finally, we will discuss results from preclinical and clinical studies proposing the increase of cGMP signaling as a therapeutic strategy in the treatment of AD.

Single doses of a highly selective inhibitor of phosphodiesterase 1 (lenrispodun) in healthy volunteers: a randomized pharmaco-fMRI clinical trial

Lenrispodun is a potent and highly selective inhibitor of phosphodiesterase (PDE) type 1, which is thought to prolong intracellular second messenger signaling within cortical and subcortical dopaminergic brain regions. This is the first study of a PDE1 inhibitor in healthy volunteers using behavioral and neuroimaging approaches to examine its effects on neural targets and to provide a safety and tolerability assessment. The primary objectives were to determine whether lenrispodun induces changes in BOLD fMRI signals in the inferior frontal gyrus (IFG) during the stop signal task, and the dorsal anterior insula (dAI) during the extinction phase of a fear conditioning/extinction task. Using a double-blind, placebo-controlled, within-subjects design, 26 healthy individuals (22 completed all fMRI sessions) received in random order a single oral dose of placebo, lenrispodun 1.0 milligram (mg) or lenrispodun 10.0 mg and completed several tasks in the scanner including the stop signal (n = 24) and fear conditioning/extinction tasks (n = 22). Prespecified region-of-interest analyses for the IFG and dAI were computed using linear mixed models. Lenrispodun induced increases in IFG activity during the stop signal task at 1.0 mg (Cohen's d = 0.63) but not 10.0 mg (Cohen's d = 0.07) vs. placebo. Lenrispodun did not induce changes in dAI activity during fear extinction at either dose. Exploratory outcomes revealed changes in cardiac interoception. Lenrispodun administration was well-tolerated. These results provide evidence that 1.0 mg lenrispodun selectively improved neural inhibitory control without altering fear extinction processing. Future investigations should determine whether lenrispodun improves inhibitory control in target populations such as individuals with attention deficit hyperactivity disorder. Trial registration: ClinicalTrials.gov identifier: NCT03489772.

Potential role of IP3/Ca2+ signaling and phosphodiesterases: Relevance to neurodegeneration in Alzheimer's disease and possible therapeutic strategies

Despite large investments by industry and governments, no disease-modifying medications for the treatment of patients with Alzheimer's disease (AD) have been found. The failures of various clinical trials indicate the need for a more in-depth understanding of the pathophysiology of AD and for innovative therapeutic strategies for its treatment. Here, we review the rational for targeting IP3 signaling, cytosolic calcium dysregulation, phosphodiesterases (PDEs), and secondary messengers like cGMP and cAMP, as well as their correlations with the pathophysiology of AD. Various drugs targeting these signaling cascades are still in pre-clinical and clinical trials which support the ideas presented in this article. Further, we describe different molecular mechanisms and medications currently being used in various pre-clinical and clinical trials involving IP3/Ca⁺² signaling. We also highlight various isoforms, as well as the functions and pharmacology of the PDEs broadly expressed in different parts of the brain and attempt to unravel the potential benefits of PDE inhibitors for use as novel medications to alleviate the pathogenesis of AD.

The Effects of Acute and Chronic Selective Phosphodiesterase 1 Inhibition on Smooth Muscle Cell-Associated Aging Features

Age-related cardiovascular diseases (CVDs) remain among the leading global causes of death, and vascular smooth muscle cell (VSMC) remodeling plays an essential role in its pathology. Reduced NO-cGMP pathway signaling is a major feature and pathogenic mechanism underlying vasodilator dysfunction. Recently, we identified phosphodiesterase (PDE) 1, an enzyme that hydrolyzes and inactivates the cyclic nucleotides cAMP and cGMP, and thereby provides a potential treatment target for restoring age-related vascular dysfunction due to aging of VSMC. Based on this hypothesis, we here tested the effects of PDE1 inhibition in a model of SMC-specific accelerated aging mice. SMC-KO and their WT littermates received either vehicle or the PDE1 inhibitor lenrispodun for 8 weeks. Vascular function was measured both in vivo (Laser Doppler technique) and ex vivo (organ bath). Moreover, we deployed UV irradiation in cell culture experiments to model accelerated aging in an in vitro situation. SMC-KO mice display a pronounced loss of vasodilator function in the isolated aorta, the cutaneous microvasculature, and mesenteric arteries. Ex vivo, in isolated vascular tissue, we found that PDE1 inhibition with lenrispodun improves vasodilation, while no improvement was observed in isolated aorta taken from mice after chronic treatment in vivo. However, during lenrispodun treatment in vivo, an enhanced microvascular response in association with upregulated cGMP levels was seen. Further, chronic lenrispodun treatment decreased TNF-α and IL-10 plasma levels while the elevated level of IL-6 in SMC-KO mice remained unchanged after treatment. PDE1 and senescence markers, p16 and p21, were increased in both SMC-KO aorta and cultured human VSMC in which DNA was damaged by ultraviolet irradiation. This increase was lowered by chronic lenrispodun. In contrast, lenrispodun increased the level of PDE1A in both situations. In conclusion, we demonstrated that PDE1 inhibition may be therapeutically useful in reversing aspects of age-related VSMC dysfunction by potentiating NO-cGMP signaling, preserving microvascular function, and decreasing senescence. Yet, after chronic treatment, the effects of PDE1 inhibition might be counteracted by the interplay between differential PDE1A and C expression. These results warrant further pharmacodynamic profiling of PDE enzyme regulation during chronic PDE1 inhibitor treatment.

PDE1 inhibitors: a review of the recent patent literature (2008-present)

INTRODUCTION

: PDE1 has been demonstrated to be a potential drug target for a variety of diseases, such as Alzheimer's disease and cardiovascular disease. In the past decades, numerous PDE1 inhibitors with structural diversities have been developed and patented by pharmaceutical companies, providing drug candidates for exploring novel disease indications of PDE1.

AREA COVERED

: This review aims to provide an overview of PDE1 inhibitors reported in patents from 2008 to present.

EXPERT OPINION

: Among current PDE1 inhibitors, only a few of them showed high selectivity over other PDEs, which might cause severe side effects in clinic. The development of highly selective PDE1 inhibitors is still the "top priority" in the following research. The selective recognition mechanism of PDE1 with inhibitors should be further elucidated by X-ray crystallography in order to provide evidences for the rational design of selective PDE1 inhibitors. In addition, PDE1 inhibitors should be applied in the different clinical indications beyond CNS diseases.

PDE1 Inhibition Modulates Cav1.2 Channel to Stimulate Cardiomyocyte Contraction

[Figure: see text].

Effects of phosphodiesterase-1 inhibitor on pulmonary vein electrophysiology and arrhythmogenesis

INTRODUCTION

Phosphodiesterase (PDE) isoform inhibitors have mechanical and electrical effects on the heart. Inhibition of PDE-1 enzymes is a novel strategy for treating heart failure. However, the electrophysiological effects of PDE-1 inhibition on the heart remain unclear. This study explored the effects of PDE-1 inhibition using ITI-214 on electrical activity in the pulmonary vein (PV), the most common trigger of atrial fibrillation, and investigated the underlying ionic mechanisms.

METHODS

Conventional microelectrodes or whole-cell patch clamps were employed to study the effects of ITI-214 (0.1-10 μM) on PV electrical activity, mechanical responses and ionic currents in isolated rabbit PV tissue specimens and isolated single PV cardiomyocytes.

RESULTS

ITI-214 at 1 μM and 10 μM (but not 0.1 μM) significantly reduced PV spontaneous beating rate (10 ± 2% and 10 ± 3%, respectively) and PV diastolic tension (11 ± 3% and 17 ± 3%, respectively). ITI-24 (1 μM) significantly reduced late sodium current (I_Na-Late ), L-type calcium current (I_Ca-L ) and the reverse mode of the sodium-calcium exchanger (NCX), but it did not affect peak sodium currents.

CONCLUSIONS

ITI-214 reduces PV spontaneous activity and PV diastolic tension by reducing I_Na-Late , I_Ca-L and NCX current. Considering its therapeutic potential in heart failure, targeting PDE-1 inhibition may provide a novel strategy for managing atrial arrhythmogenesis.

Acute Hemodynamic Effects and Tolerability of Phosphodiesterase-1 Inhibition With ITI-214 in Human Systolic Heart Failure

BACKGROUND

PDE1 (phosphodiesterase type 1) hydrolyzes cyclic adenosine and guanosine monophosphate. ITI-214 is a highly selective PDE1 inhibitor that induces arterial vasodilation and positive inotropy in larger mammals. Here, we assessed pharmacokinetics, hemodynamics, and tolerability of single-dose ITI-214 in humans with stable heart failure with reduced ejection fraction.

METHODS

Patients with heart failure with reduced ejection fraction were randomized 3:1 to 10, 30, or 90 mg ITI-214 single oral dose or placebo (n=9/group). Vital signs and electrocardiography were monitored predose to 5 hours postdose and transthoracic echoDoppler cardiography predose and 2-hours postdose.

RESULTS

Patient age averaged 54 years; 42% female, and 60% Black. Mean systolic blood pressure decreased 3 to 8 mm Hg (P<0.001) and heart rate increased 5 to 9 bpm (P≤0.001 for 10, 30 mg doses, RM-ANCOVA). After 4 hours, neither blood pressure or heart rate significantly differed among cohorts (supine or standing). ITI-214 increased mean left ventricular power index, a relatively load-insensitive inotropic index, by 0.143 Watts/mL²·10⁴ (P=0.03, a +41% rise; 5-71 CI) and cardiac output by 0.83 L/min (P=0.002, +31%, 13-49 CI) both at the 30 mg dose. Systemic vascular resistance declined with 30 mg (-564 dynes·s/cm-⁵, P<0.001) and 90 mg (-370, P=0.016). Diastolic changes were minimal, and no parameters were significantly altered with placebo. ITI-214 was well-tolerated. Five patients had mild-moderate hypotension or orthostatic hypotension recorded adverse events. There were no significant changes in arrhythmia outcome and no serious adverse events.

CONCLUSIONS

Single-dose ITI-214 is well-tolerated and confers inodilator effects in humans with heart failure with reduced ejection fraction. Further investigations of its therapeutic utility are warranted. Registration: URL: https://www.clinicaltrials.gov; Unique identifier: NCT03387215.

Selective Phosphodiesterase 1 Inhibition Ameliorates Vascular Function, Reduces Inflammatory Response, and Lowers Blood Pressure in Aging Animals

Diminished nitric oxide-cGMP-mediated relaxation plays a crucial role in cardiovascular aging, leading to decreased vasodilation, vascular hypertrophy and stiffening, and ultimately, cardiovascular dysfunction. Aging is the time-related worsening of physiologic function due to complex cellular and molecular interactions, and it is at least partly driven by DNA damage. Genetic deletion of the DNA repair enzyme ERCC1 endonuclease in Ercc1Δ/- mice provides us an efficient tool to accelerate vascular aging, explore mechanisms, and test potential treatments. Previously, we identified the cGMP-degrading enzyme phosphodiesterase 1 as a potential treatment target in vascular aging. In the present study, we studied the effect of acute and chronic treatment with ITI-214, a selective phosphodiesterase 1 inhibitor on vascular aging features in Ercc1Δ/- mice. Compared with wild-type mice, Ercc1Δ/- mice at the age of 14 weeks showed decreased reactive hyperemia, diminished endothelium-dependent and -independent responses of arteries in organ baths, carotid wall hypertrophy, and elevated circulating levels of inflammatory cytokines. Acute ITI-214 treatment in organ baths restored the arterial endothelium-independent vasodilation in Ercc1Δ/- mice. An 8-week treatment with 100 mg/kg per day ITI-214 improved endothelium-independent relaxation in both aorta and coronary arteries, at least partly restored the diminished reactive hyperemia, lowered the systolic and diastolic blood pressure, normalized the carotid hypertrophy, and ameliorated inflammatory responses exclusively in Ercc1Δ/- mice. These findings suggest phosphodiesterase 1 inhibition would provide a powerful tool for nitric oxide-cGMP augmentation and have significant therapeutic potential to battle arteriopathy related to aging. SIGNIFICANCE STATEMENT: The findings implicate the key role of phosphodiesterase 1 in vascular function and might be of clinical importance for the prevention of mortalities and morbidities related to vascular complications during aging, as well as for patients with progeria that show a high risk of cardiovascular disease.

Cellular context shapes cyclic nucleotide signaling in neurons through multiple levels of integration

Intracellular signaling with cyclic nucleotides are ubiquitous signaling pathways, yet the dynamics of these signals profoundly differ in different cell types. Biosensor imaging experiments, by providing direct measurements in intact cellular environment, reveal which receptors are activated by neuromodulators and how the coincidence of different neuromodulators is integrated at various levels in the signaling cascade. Phosphodiesterases appear as one important determinant of cross-talk between different signaling pathways. Finally, analysis of signal dynamics reveal that striatal medium-sized spiny neuron obey a different logic than other brain regions such as cortex, probably in relation with the function of this brain region which efficiently detects transient dopamine.

Advances in Cyclic Nucleotide Phosphodiesterase-Targeted PET Imaging and Drug Discovery

Cyclic nucleotide phosphodiesterases (PDEs) control the intracellular concentrations of cAMP and cGMP in virtually all mammalian cells. Accordingly, the PDE family regulates a myriad of physiological functions, including cell proliferation, differentiation and apoptosis, gene expression, central nervous system function, and muscle contraction. Along this line, dysfunction of PDEs has been implicated in neurodegenerative disorders, coronary artery diseases, chronic obstructive pulmonary disease, and cancer development. To date, 11 PDE families have been identified; however, their distinct roles in the various pathologies are largely unexplored and subject to contemporary research efforts. Indeed, there is growing interest for the development of isoform-selective PDE inhibitors as potential therapeutic agents. Similarly, the evolving knowledge on the various PDE isoforms has channeled the identification of new PET probes, allowing isoform-selective imaging. This review highlights recent advances in PDE-targeted PET tracer development, thereby focusing on efforts to assess disease-related PDE pathophysiology and to support isoform-selective drug discovery.

Phosphodiesterase as a Target for Cognition Enhancement in Schizophrenia

Schizophrenia is a severe mental disorder that affects more than 1% of the population worldwide. Dopamine system dysfunction and alterations in glutamatergic neurotransmission are strongly implicated in the aetiology of schizophrenia. To date, antipsychotic drugs are the only available treatment for the symptoms of schizophrenia. These medications, which act as D2-receptor antagonist, adequately address the positive symptoms of the disease, but they fail to improve the negative symptoms and cognitive impairment. In schizophrenia, cognitive impairment is a core feature of the disorder. Therefore, the treatment of cognitive impairment and the other symptoms related to schizophrenia remains a significant unmet medical need. Currently, phosphodiesterases (PDEs) are considered the best drug target for the treatment of schizophrenia since many PDE subfamilies are abundant in the brain regions that are relevant to cognition. Thus, this review aims to illustrate the mechanism of PDEs in treating the symptoms of schizophrenia and summarises the encouraging results of PDE inhibitors as anti-schizophrenic drugs in preclinical and clinical studies.

Role of phosphodiesterase 1 in the pathophysiology of diseases and potential therapeutic opportunities

Cyclic nucleotide phosphodiesterases (PDEs) are superfamily of enzymes that regulate the spatial and temporal relationship of second messenger signaling in the cellular system. Among the 11 different families of PDEs, phosphodiesterase 1 (PDE1) sub-family of enzymes hydrolyze both 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP) in a mutually competitive manner. The catalytic activity of PDE1 is stimulated by their binding to Ca²⁺/calmodulin (CaM), resulting in the integration of Ca²⁺ and cyclic nucleotide-mediated signaling in various diseases. The PDE1 family includes three subtypes, PDE1A, PDE1B and PDE1C, which differ for their relative affinities for cAMP and cGMP. These isoforms are differentially expressed throughout the body, including the cardiovascular, central nervous system and other organs. Thus, PDE1 enzymes play a critical role in the pathophysiology of diseases through the fundamental regulation of cAMP and cGMP signaling. This comprehensive review provides the current research on PDE1 and its potential utility as a therapeutic target in diseases including the cardiovascular, pulmonary, metabolic, neurocognitive, renal, cancers and possibly others.

Phosphodiesterase type 1 inhibition alters medial prefrontal cortical activity during goal-driven behaviour and partially reverses neurophysiological deficits in the rat phencyclidine model of schizophrenia

Positive modulation of cAMP signalling by phosphodiesterase (PDE) inhibitors has recently been explored as a potential target for the reversal of cognitive and behavioural deficits implicating the corticoaccumbal circuit. Previous studies show that PDE type 1 isoform B (PDE1B) inhibition may improve memory function in rodent models; however, the contribution of PDE1B inhibition to impulsivity, attentional and motivational functions as well as its neurophysiological effects have not been investigated. To address this, we recorded single unit activity in medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) in Lister Hooded rats treated with the PDE1B inhibitor Lu AF64386 and tested in the 5-choice serial reaction time task (5-CSRTT). We also asked whether PDE1B inhibition modulates neurophysiological deficits produced by subchronic phencyclidine (PCP) treatment, a rat pharmacological model of schizophrenia. Lu AF64386 significantly affected behavioural parameters consistent with a reduction in goal-directed behaviour, however without affecting accuracy. Additionally, it reduced mPFC neuronal activity. Pre-treatment with PCP did not affect behavioural parameters, however it significantly disrupted overall neuronal firing while increasing phasic responses to reward-predicting cues and disrupting mPFC-NAc cross-talk. The latter two effects were reversed by Lu AF64386. These findings suggest PDE1B inhibition may be beneficial in disorders implicating a dysfunction of the mPFC-NAc network.

Phosphodiesterase 1 Bridges Glutamate Inputs with NO- and Dopamine-Induced Cyclic Nucleotide Signals in the Striatum

The calcium-regulated phosphodiesterase 1 (PDE1) family is highly expressed in the brain, but its functional role in neurones is poorly understood. Using the selective PDE1 inhibitor Lu AF64196 and biosensors for cyclic nucleotides including a novel biosensor for cGMP, we analyzed the effect of PDE1 on cAMP and cGMP in individual neurones in brain slices from male newborn mice. Release of caged NMDA triggered a transient increase of intracellular calcium, which was associated with a decrease in cAMP and cGMP in medium spiny neurones in the striatum. Lu AF64196 alone did not increase neuronal cyclic nucleotide levels, but blocked the NMDA-induced reduction in cyclic nucleotides indicating that this was mediated by calcium-activated PDE1. Similar effects were observed in the prefrontal cortex and the hippocampus. Upon corelease of dopamine and NMDA, PDE1 was shown to down-regulate the D1-receptor mediated increase in cAMP. PDE1 inhibition increased long-term potentiation in rat ventral striatum, showing that PDE1 is implicated in the regulation of synaptic plasticity. Overall, our results show that PDE1 reduces cyclic nucleotide signaling in the context of glutamate and dopamine coincidence. This effect could have a therapeutic value for treating brain disorders related to dysfunctions in dopamine neuromodulation.

Cyclic Nucleotides Signaling and Phosphodiesterase Inhibition: Defying Alzheimer's Disease

Defects in brain functions associated with aging and neurodegenerative diseases benefit insignificantly from existing options, suggesting that there is a lack of understanding of pathological mechanisms. Alzheimer's disease (AD) is such a nearly untreatable, allied to age neurological deterioration for which only the symptomatic cure is available and the agents able to mould progression of the disease, is still far away. The altered expression of phosphodiesterases (PDE) and deregulated cyclic nucleotide signaling in AD has provoked a new thought of targeting cyclic nucleotide signaling in AD. Targeting cyclic nucleotides as an intracellular messenger seems to be a viable approach for certain biological processes in the brain and controlling substantial. Whereas, the synthesis, execution, and/or degradation of cyclic nucleotides has been closely linked to cognitive deficits. In relation to cognition, the cyclic nucleotides (cAMP and cGMP) have an imperative execution in different phases of memory, including gene transcription, neurogenesis, neuronal circuitry, synaptic plasticity and neuronal survival, etc. AD is witnessed by impairments of these basic processes underlying cognition, suggesting a crucial role of cAMP/cGMP signaling in AD populations. Phosphodiesterase inhibitors are the exclusive set of enzymes to facilitate hydrolysis and degradation of cAMP and cGMP thereby, maintains their optimum levels initiating it as an interesting target to explore. The present work reviews a neuroprotective and substantial influence of PDE inhibition on physiological status, pathological progression and neurobiological markers of AD in consonance with the intensities of cAMP and cGMP.

Advances in the Discovery of PDE10A Inhibitors for CNS-Related Disorders. Part 2: Focus on Schizophrenia

Schizophrenia is a debilitating mental disorder with relatively high prevalence (~1%), during which positive manifestations (such as psychotic states) and negative symptoms (e.g., a withdrawal from social life) occur. Moreover, some researchers consider cognitive impairment as a distinct domain of schizophrenia symptoms. The imbalance in dopamine activity, namely an excessive release of this neurotransmitter in the striatum and insufficient amounts in the prefrontal cortex is believed to be partially responsible for the occurrence of these groups of manifestations. Second-generation antipsychotics are currently the standard treatment of schizophrenia. Nevertheless, the existent treatment is sometimes ineffective and burdened with severe adverse effects, such as extrapyramidal symptoms. Thus, there is an urgent need to search for alternative treatment options of this disease. This review summarizes the results of recent preclinical and clinical studies on phosphodiesterase 10A (PDE10A), which is highly expressed in the mammalian striatum, as a potential drug target for the treatment of schizophrenia. Based on the literature data, not only selective PDE10A inhibitors but also dual PDE2A/10A, and PDE4B/10A inhibitors, as well as multifunctional ligands with a PDE10A inhibitory potency are compounds that may combine antipsychotic, precognitive, and antidepressant functions. Thus, designing such compounds may constitute a new direction of research for new potential medications for schizophrenia. Despite failures of previous clinical trials of selective PDE10A inhibitors for the treatment of schizophrenia, new compounds with this mechanism of action are currently investigated clinically, thus, the search for new inhibitors of PDE10A, both selective and multitarget, is still warranted.

Therapeutic Potential of Phosphodiesterase Inhibitors against Neurodegeneration: The Perspective of the Medicinal Chemist

Increasing human life expectancy prompts the development of novel remedies for cognitive decline: 44 million people worldwide are affected by dementia, and this number is predicted to triple by 2050. Acetylcholinesterase and N-methyl-d-aspartate receptors represent the targets of currently available drugs for Alzheimer’s disease, which are characterized by limited efficacy. Thus, the search for therapeutic agents with alternative or combined mechanisms of action is wide open. Since variations in 3′,5′-cyclic adenosine monophosphate, 3′,5′-cyclic guanosine monophosphate, and/or nitric oxide levels interfere with downstream pathways involved in memory processes, evidence supporting the potential of phosphodiesterase (PDE) inhibitors in contrasting neurodegeneration should be critically considered. For the preparation of this Review, more than 140 scientific papers were retrieved by searching PubMed and Scopus databases. A systematic approach was adopted when overviewing the different PDE isoforms, taking into account details on brain localization, downstream molecular mechanisms, and inhibitors currently under study, according to available in vitro and in vivo data. In the context of drug repurposing, a section focusing on PDE5 was introduced. Original computational studies were performed to rationalize the emerging evidence that suggests the role of PDE5 inhibitors as multi-target agents against neurodegeneration. Moreover, since such compounds must cross the blood–brain barrier and reach inhibitory concentrations in the central nervous system to exert their therapeutic activity, physicochemical parameters were analyzed and discussed. Taken together, literature and computational data suggest that some PDE5 inhibitors, such as tadalafil, represent promising candidates.

Metabolic and Pharmaceutical Aspects of Fluorinated Compounds

The applications of fluorine in drug design continue to expand, facilitated by an improved understanding of its effects on physicochemical properties and the development of synthetic methodologies that are providing access to new fluorinated motifs. In turn, studies of fluorinated molecules are providing deeper insights into the effects of fluorine on metabolic pathways, distribution, and disposition. Despite the high strength of the C-F bond, the departure of fluoride from metabolic intermediates can be facile. This reactivity has been leveraged in the design of mechanism-based enzyme inhibitors and has influenced the metabolic fate of fluorinated compounds. In this Perspective, we summarize the literature associated with the metabolism of fluorinated molecules, focusing on examples where the presence of fluorine influences the metabolic profile. These studies have revealed potentially problematic outcomes with some fluorinated motifs and are enhancing our understanding of how fluorine should be deployed.

Role of cyclic nucleotides and their downstream signaling cascades in memory function: Being at the right time at the right spot

A plethora of studies indicate the important role of cAMP and cGMP cascades in neuronal plasticity and memory function. As a result, altered cyclic nucleotide signaling has been implicated in the pathophysiology of mnemonic dysfunction encountered in several diseases. In the present review we provide a wide overview of studies regarding the involvement of cyclic nucleotides, as well as their upstream and downstream molecules, in physiological and pathological mnemonic processes. Next, we discuss the regulation of the intracellular concentration of cyclic nucleotides via phosphodiesterases, the enzymes that degrade cAMP and/or cGMP, and via A-kinase-anchoring proteins that refine signal compartmentalization of cAMP signaling. We also provide an overview of the available data pointing to the existence of specific time windows in cyclic nucleotide signaling during neuroplasticity and memory formation and the significance to target these specific time phases for improving memory formation. Finally, we highlight the importance of emerging imaging tools like Förster resonance energy transfer imaging and optogenetics in detecting, measuring and manipulating the action of cyclic nucleotide signaling cascades.

Cyclic nucleotide phosphodiesterases: New targets in the metabolic syndrome?

Metabolic diseases have a tremendous impact on human morbidity and mortality. Numerous targets regulating adenosine monophosphate kinase (AMPK) have been identified for treating the metabolic syndrome (MetS), and many compounds are being used or developed to increase AMPK activity. In parallel, the cyclic nucleotide phosphodiesterase families (PDEs) have emerged as new therapeutic targets in cardiovascular diseases, as well as in non-resolved pathologies. Since some PDE subfamilies inactivate cAMP into 5'-AMP, while the beneficial effects in MetS are related to 5'-AMP-dependent activation of AMPK, an analysis of the various controversial relationships between PDEs and AMPK in MetS appears interesting. The present review will describe the various PDE families, AMPK and molecular mechanisms in the MetS and discuss the PDEs/PDE modulators related to the tissues involved, thus supporting the discovery of original molecules and the design of new therapeutic approaches in MetS.

Inhibition of calcium-calmodulin-dependent phosphodiesterase (PDE1) suppresses inflammatory responses

A novel, potent, and highly specific inhibitor of calcium-calmodulin-dependent phosphodiesterases (PDE) of the PDE1 family, ITI-214, was used to investigate the role of PDE1 in inflammatory responses. ITI-214 dose-dependently suppressed lipopolysaccharide (LPS)-induced gene expression of pro-inflammatory cytokines in an immortalized murine microglial cell line, BV2 cells. RNA profiling (RNA-Seq) was used to analyze the impact of ITI-214 on the BV2 cell transcriptome in the absence and the presence of LPS. ITI-214 was found to regulate classes of genes that are involved in inflammation and cell migration responses to LPS exposure. The gene expression changes seen with ITI-214 treatment were distinct from those elicited by inhibitors of other PDEs with anti-inflammatory activity (e.g., a PDE4 inhibitor), indicating a distinct mechanism of action for PDE1. Functionally, ITI-214 inhibited ADP-induced migration of BV2 cells through a P2Y12-receptor-dependent pathway, possibly due to increases in the extent of cAMP and VASP phosphorylation downstream of receptor activation. Importantly, this effect was recapitulated in P2 rat microglial cells in vitro, indicating that these pathways are active in native microglial cells. These studies are the first to demonstrate that inhibition of PDE1 exerts anti-inflammatory effects through effects on microglia signaling pathways. The ability of PDE1 inhibitors to prevent or dampen excessive inflammatory responses of BV2 cells and microglia provides a basis for exploring their therapeutic utility in the treatment of neurodegenerative diseases associated with increased inflammation and microglia proliferation such as Parkinson's disease and Alzheimer's disease.

The importance of the nitric oxide-cGMP pathway in age-related cardiovascular disease: Focus on phosphodiesterase-1 and soluble guanylate cyclase

Among ageing-related illnesses, cardiovascular disease (CVD) remains the leading cause of morbidity and mortality causing one-third of all deaths worldwide. Ageing evokes a number of functional, pharmacological and morphological changes in the vasculature, accompanied by a progressive failure of protective and homeostatic mechanisms, resulting in target organ damage. Impaired vasomotor, proliferation, migration, antithrombotic and anti-inflammatory function in both the endothelial and vascular smooth muscle cells are parts of the vascular ageing phenotype. The endothelium regulates these functions by the release of a wide variety of active molecules including endothelium-derived relaxing factors such as nitric oxide, prostacyclin (PGI₂ ) and endothelium-derived hyperpolarization (EDH). During ageing, a functional decay of the nitric oxide pathway takes place. Nitric oxide signals to VSMC and other important cell types for vascular homeostasis through the second messenger cyclic guanosine monophosphate (cGMP). Maintenance of proper cGMP levels is an important goal in sustainment of proper vascular function during ageing. For this purpose, different components can be targeted in this signalling system, and among them, phosphodiesterase-1 (PDE1) and soluble guanylate cyclase (sGC) are crucial. This review focuses on the role of PDE1 and sGC in conditions that are relevant for vascular ageing.

A Novel Phosphodiesterase 1 Inhibitor DSR-141562 Exhibits Efficacies in Animal Models for Positive, Negative, and Cognitive Symptoms Associated with Schizophrenia

In our drug discovery program, we identified a novel orally available and brain-penetrant phosphodiesterase (PDE) 1 inhibitor, 3-methyl-7-(tetrahydro-2H-pyran-4-yl)-2-{[trans-4-(trifluoromethyl)cyclohexyl]-methoxy}imidazo[5,1-f][1,2,4]triazin-4(3H)-one (DSR-141562). In the present study, we characterized the preclinical profile of DSR-141562. This compound has preferential selectivity for predominantly brain-expressed PDE1B over other PDE1 family members, and high selectivity for the PDE1 family over other PDE families and 65 other tested biologic targets. Oral administration of DSR-141562 at 10 mg/kg slightly elevated the cGMP concentration, and it potently enhanced the increase of cGMP induced by a dopamine D₁ receptor agonist in mouse brains. The cGMP level in monkey cerebrospinal fluid was also elevated after treatment with DSR-141562 at 30 and 100 mg/kg and could be used as a translational biomarker. Since PDE1B is believed to regulate dopaminergic and glutamatergic signal transduction, we evaluated the effects of this compound using schizophrenia-related behavioral assays. DSR-141562 at 3-30 mg/kg potently inhibited methamphetamine-induced locomotor hyperactivity in rats, while it had only minimal effects on the spontaneous locomotor activity. Furthermore, DSR-141562 at 1-100 mg/kg did not induce any signs of catalepsy in rats. DSR-141562 at 0.3-3 mg/kg reversed social interaction and novel object recognition deficits induced by repeated treatment with an N-methyl-D-aspartate receptor antagonist, phencyclidine, in mice and rats, respectively. In common marmosets, DSR-141562 at 3 and 30 mg/kg improved the performance in object retrieval with detour tasks. These results suggest that DSR-141562 is a therapeutic candidate for positive, negative, and cognitive symptoms in schizophrenia. SIGNIFICANCE STATEMENT: This is the first paper showing that a phosphodiesterase 1 inhibitor is efficacious in animal models for positive and negative symptoms associated with schizophrenia. Furthermore, we demonstrated that this compound improved cognitive function in the common marmoset, a nonhuman primate.

Therapeutic targeting of 3',5'-cyclic nucleotide phosphodiesterases: inhibition and beyond

Phosphodiesterases (PDEs), enzymes that degrade 3',5'-cyclic nucleotides, are being pursued as therapeutic targets for several diseases, including those affecting the nervous system, the cardiovascular system, fertility, immunity, cancer and metabolism. Clinical development programmes have focused exclusively on catalytic inhibition, which continues to be a strong focus of ongoing drug discovery efforts. However, emerging evidence supports novel strategies to therapeutically target PDE function, including enhancing catalytic activity, normalizing altered compartmentalization and modulating post-translational modifications, as well as the potential use of PDEs as disease biomarkers. Importantly, a more refined appreciation of the intramolecular mechanisms regulating PDE function and trafficking is emerging, making these pioneering drug discovery efforts tractable.

Acute Enhancement of Cardiac Function by Phosphodiesterase Type 1 Inhibition

BACKGROUND

Phosphodiesterase type-1 (PDE1) hydrolyzes cAMP and cGMP and is constitutively expressed in the heart, although cardiac effects from its acute inhibition in vivo are largely unknown. Existing data are limited to rodents expressing mostly the cGMP-favoring PDE1A isoform. Human heart predominantly expresses PDE1C with balanced selectivity for cAMP and cGMP. Here, we determined the acute effects of PDE1 inhibition in PDE1C-expressing mammals, dogs, and rabbits, in normal and failing hearts, and explored its regulatory pathways.

METHODS

Conscious dogs chronically instrumented for pressure-volume relations were studied before and after tachypacing-induced heart failure (HF). A selective PDE1 inhibitor (ITI-214) was administered orally or intravenously±dobutamine. Pressure-volume analysis in anesthetized rabbits tested the role of β-adrenergic and adenosine receptor signaling on ITI-214 effects. Sarcomere and calcium dynamics were studied in rabbit left ventricular myocytes.

RESULTS

In normal and HF dogs, ITI-214 increased load-independent contractility, improved relaxation, and reduced systemic arterial resistance, raising cardiac output without altering systolic blood pressure. Heart rate increased, but less so in HF dogs. ITI-214 effects were additive to β-adrenergic receptor agonism (dobutamine). Dobutamine but not ITI-214 increased plasma cAMP. ITI-214 induced similar cardiovascular effects in rabbits, whereas mice displayed only mild vasodilation and no contractility effects. In rabbits, β-adrenergic receptor blockade (esmolol) prevented ITI-214-mediated chronotropy, but inotropy and vasodilation remained unchanged. By contrast, adenosine A_2B-receptor blockade (MRS-1754) suppressed ITI-214 cardiovascular effects. Adding fixed-rate atrial pacing did not alter the findings. ITI-214 alone did not affect sarcomere or whole-cell calcium dynamics, whereas β-adrenergic receptor agonism (isoproterenol) or PDE3 inhibition (cilostamide) increased both. Unlike cilostamide, which further enhanced shortening and peak calcium when combined with isoproterenol, ITI-214 had no impact on these responses. Both PDE1 and PDE3 inhibitors increased shortening and accelerated calcium decay when combined with forskolin, yet only cilostamide increased calcium transients.

CONCLUSIONS

PDE1 inhibition by ITI-214 in vivo confers acute inotropic, lusitropic, and arterial vasodilatory effects in PDE1C-expressing mammals with and without HF. The effects appear related to cAMP signaling that is different from that provided via β-adrenergic receptors or PDE3 modulation. ITI-214, which has completed phase I trials, may provide a novel therapy for HF.

Phosphodiesterase inhibitors say NO to Alzheimer's disease

Phosphodiesterases (PDEs) consisted of 11 subtypes (PDE1 to PDE11) and over 40 isoforms that regulate levels of cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP), the second messengers in cell functions. PDE inhibitors (PDEIs) have been attractive therapeutic targets due to their involvement in diverse medical conditions, e.g. cardiovascular diseases, autoimmune diseases, Alzheimer's disease (AD), etc. Among them; AD with a complex pathology is a progressive neurodegenerative disorder which affect mostly senile people in the world and only symptomatic treatment particularly using cholinesterase inhibitors in clinic is available at the moment for AD. Consequently, novel treatment strategies towards AD are still searched extensively. Since PDEs are broadly expressed in the brain, PDEIs are considered to modulate neurodegenerative conditions through regulating cAMP and cGMP in the brain. In this sense, several synthetic or natural molecules inhibiting various PDE subtypes such as rolipram and roflumilast (PDE4 inhibitors), vinpocetine (PDE1 inhibitor), cilostazol and milrinone (PDE3 inhibitors), sildenafil and tadalafil (PDE5 inhibitors), etc have been reported showing encouraging results for the treatment of AD. In this review, PDE superfamily will be scrutinized from the view point of structural features, isoforms, functions and pharmacology particularly attributed to PDEs as target for AD therapy.

Phosphodiesterase 1b (PDE1B) Regulates Spatial and Contextual Memory in Hippocampus

Augmentation of cyclic nucleotide signaling through inhibition of phosphodiesterase (PDE) activity has long been understood to enhance memory. Efforts in this domain have focused predominantly on PDE4, a cAMP-specific phosphodiesterase implicated in consolidation. But less is known about the function of other PDEs expressed in neuroanatomical regions critical to memory. The PDE1 isoforms are the only PDEs to regulate neuronal cAMP and cGMP levels in a Ca²⁺/Calmodulin (CaM) dependent manner. Here, we show that knock-down of PDE1B in hippocampus of adult mice enhances contextual and spatial memory without effect on non-cognitive behaviors. Pharmacological augmentation of memory in rats was observed with a selective inhibitor of PDE1 dosed before and immediately after training, but not with drug dosed either 1 h after training or before recall. Our data clearly demonstrate a role for the PDE1B isoforms as negative regulators of memory, and they implicate PDE1 in an early phase of consolidation, but not retrieval. Inhibition of PDE1B is a promising therapeutic mechanism for treating memory impairment.

The Phosphodiesterase 9 Inhibitor PF-04449613 Promotes Dendritic Spine Formation and Performance Improvement after Motor Learning

The cyclic nucleotide cGMP is an intracellular second messenger with important roles in neuronal functions and animals' behaviors. The phosphodiesterases (PDEs) are a family of enzymes that hydrolyze the second messengers cGMP and cAMP. Inhibition of phosphodiesterase 9 (PDE9), a main isoform of PDEs hydrolyzing cGMP, has been shown to improve learning and memory as well as cognitive function in rodents. However, the role of PDE9 in regulating neuronal structure and function in vivo remains unclear. Here we used in vivo two-photon microscopy to investigate the effect of a selective PDE9 inhibitor PF-04449613 on the activity and plasticity of dendritic spines of layer V pyramidal neurons in the mouse primary motor cortex. We found that administration of PF-04449613 increased calcium activity of dendrites and dendritic spines of layer V pyramidal neurons in mice under resting and running conditions. Chronic treatment of PF-04449613 over weeks increased dendritic spine formation and elimination under basal conditions. Furthermore, PF-04449613 treatment over 1-7 days increased the formation and survival of new spines as well as performance improvement after rotarod motor training. Taken together, our studies suggest that elevating the level of cGMP with the PDE9 inhibitor PF-04449613 increases synaptic calcium activity and learning-dependent synaptic plasticity, thereby contributing to performance improvement after learning. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 00: 000-000, 2018.

Vinpocetine halts ketamine-induced schizophrenia-like deficits in rats: impact on BDNF and GSK-3β/β-catenin pathway

There are increasing evidences supporting the involvement of oxidative stress and neuroinflammation in schizophrenia. Vinpocetine, a nootropic phosphodiesterase-1 inhibitor, was proven to possess anti-oxidant and anti-inflammatory potentials. This research aimed to reveal the likely protective features of vinpocetine against ketamine-induced schizophrenia-like deficits in rats. Additionally, the probable mechanisms contributing to this neuroprotection were also elucidated. Vinpocetine was given (20 mg/kg, i.p.) once a day for 14 days commencing 7 days before administrating ketamine (25 mg/kg i.p.). Risperidone was applied as a reference antipsychotic. Vinpocetine pre-treatment revealed a marked amendment in the hyperlocomotion, anxiety, and short-term memory deficits induced by ketamine in rats. In rats' hippocampus, ketamine induced a drastic increase in tissue levels of dopamine, lipid peroxidation, and pro-inflammatory cytokines along with a significant decrease in glutamate, GABA, SOD, and total anti-oxidant capacity. Also, ketamine induced a reduced level of BDNF together with the potentiation of GSK-3β/β-catenin pathway that led to the destruction of β-catenin. Pre-treatment of ketamine-challenged animals with vinpocetine significantly attenuated oxidative stress, inflammation, and neurotransmitter alterations. Vinpocetine also elevated BDNF expression and prevented ketamine-induced stimulation of the GSK-3β/β-catenin signaling. This research presents enlightenments into the role of vinpocetine in schizophrenia. This role may be accomplished through its effect on oxidative stress, inflammation as well as modulating BDNF and the GSK-3β/β-catenin pathway.

Targeting the dopamine D1 receptor or its downstream signalling by inhibiting phosphodiesterase-1 improves cognitive performance

Background and Purpose

Insufficient prefrontal dopamine 1 (D₁) receptor signalling has been linked to cognitive dysfunction in several psychiatric conditions. Because the PDE1 isoform B (PDE1B) is postulated to regulate D₁ receptor‐dependent signal transduction, in this study we aimed to elucidate the role of PDE1 in cognitive processes reliant on D₁ receptor function.

Experimental Approach

Cognitive performance of the D₁ receptor agonist, SKF38393, was studied in the T‐maze continuous alternation task and 5‐choice serial reaction time task. D₁ receptor/PDE1B double‐immunohistochemistry was performed using human and rat prefrontal brain sections. The pharmacological activity of the PDE1 inhibitor, ITI‐214, was assessed by measuring the increase in cAMP/cGMP in prefrontal brain tissue and its effect on working memory performance. Mechanistic studies on the modulation of prefrontal neuronal transmission by SKF38393 and ITI‐214 were performed using extracellular recordings in brain slices.

Key Results

SKF38393 improved working memory and attentional performance in rodents. D₁ receptor/PDE1B co‐expression was verified in both human and rat prefrontal brain sections. The pharmacological activity of ITI‐214 on its target, PDE1, was demonstrated by its ability to increase prefrontal cAMP/cGMP. In addition, ITI‐214 improved working memory performance. Both SKF38393 and ITI‐214 facilitated neuronal transmission in prefrontal brain slices.

Conclusion and Implications

We hypothesize that PDE1 inhibition improves working memory performance by increasing prefrontal synaptic transmission and/or postsynaptic D₁ receptor signalling, by modulating prefrontal downstream second messenger levels. These data, therefore, support the use of PDE1 inhibitors as a potential approach for the treatment of cognitive dysfunction.

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.

Novel Phosphodiesterase Inhibitors for Cognitive Improvement in Alzheimer's Disease

Alzheimer's disease (AD) is one of the greatest public health challenges. Phosphodiesterases (PDEs) are a superenzyme family responsible for the hydrolysis of two second messengers: cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Since several PDE subfamilies are highly expressed in the human brain, the inhibition of PDEs is involved in neurodegenerative processes by regulating the concentration of cAMP and/or cGMP. Currently, PDEs are considered as promising targets for the treatment of AD since many PDE inhibitors have exhibited remarkable cognitive improvement effects in preclinical studies and over 15 of them have been subjected to clinical trials. The aim of this review is to summarize the outstanding progress that has been made by PDE inhibitors as anti-AD agents with encouraging results in preclinical studies and clinical trials. The binding affinity, pharmacokinetics, underlying mechanisms, and limitations of these PDE inhibitors in the treatment of AD are also reviewed and discussed.

Investigational phosphodiesterase inhibitors in phase I and phase II clinical trials for Alzheimer's disease

INTRODUCTION

Phosphodiesterase (PDE) inhibitors improve signaling pathways in brain circuits by increasing intracellular cyclic adenosine monophosphate (cAMP) and/or cyclic guanosine monophosphate (cGMP). In the last decade, the first clinical studies investigating selective PDE inhibitors in Alzheimer's disease (AD) have been initiated, based on their positive effects on cognitive processes and neuroprotection in numerous animal studies. Areas covered: This article reviews the clinical studies investigating the pro-cognitive/neuroprotective effects of PDE inhibitors in patients with AD, as well as in age-associated memory impaired elderly and patients with mild cognitive impairment (MCI), the prodromal stage of AD. PDE inhibitors will also be discussed with respect to adverse effects including safety and tolerability. Expert opinion: The limited available data of clinical studies with PDE inhibitors tested in different populations of AD patients do not allow the drawing of any concrete conclusion yet. Currently, studies with a PDE3 (cilostazol) or PDE9 inhibitor (BI 409,306) are still ongoing in patients with MCI or AD, respectively. Studies with PDE4 inhibitors (HT-0712, roflumilast and BPN14770) in healthy elderly and elderly with age-associated memory impairments indicate that the optimum dose and/or inhibiting the most relevant PDE isoform hold great promise when tested in the appropriate population of patients with MCI or AD eventually.

Phosphodiesterase-1b (Pde1b) knockout mice are resistant to forced swim and tail suspension induced immobility and show upregulation of Pde10a

RATIONALE

Major depressive disorder is a leading cause of suicide and disability. Despite this, current antidepressants provide insufficient efficacy in more than 60% of patients. Most current antidepressants are presynaptic reuptake inhibitors; postsynaptic signal regulation has not received as much attention as potential treatment targets.

OBJECTIVES

We examined the effects of disruption of the postsynaptic cyclic nucleotide hydrolyzing enzyme, phosphodiesterase (PDE) 1b, on depressive-like behavior and the effects on PDE1B protein in wild-type (WT) mice following stress.

METHODS

Littermate knockout (KO) and WT mice were tested in locomotor activity, tail suspension (TST), and forced swim tests (FST). FST was also used to compare the effects of two antidepressants, fluoxetine and bupropion, in KO versus WT mice. Messenger RNA (mRNA) expression changes were also determined. WT mice underwent acute or chronic stress and markers of stress and PDE1B expression were examined.

RESULTS

Pde1b KO mice exhibited decreased TST and FST immobility. When treated with antidepressants, both WT and KO mice showed decreased FST immobility and the effect was additive in KO mice. Mice lacking Pde1b had increased striatal Pde10a mRNA expression. In WT mice, acute and chronic stress upregulated PDE1B expression while PDE10A expression was downregulated after chronic but not acute stress.

CONCLUSIONS

PDE1B is a potential therapeutic target for depression treatment because of the antidepressant-like phenotype seen in Pde1b KO mice.