Design and Synthesis of Novel and Selective Phosphodiesterase 2 (PDE2a) Inhibitors for the Treatment of Memory Disorders

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.

Insights into the selective mechanism of PDE2/9a inhibitors from silico aspects

The selective design of competitive enzyme inhibitors is an extremely difficult task but necessary work for certain types of systems, such as the phosphodiesterase (PDE) system addressed in this article. In the PDE family, PDE2A and PDE9 respectively target the central nervous system and heart failure, and share many conserved amino acids at their binding sites. Therefore, gaining a deep understanding of the selective mechanisms of PDE2A/9A is crucial for designing highly selective drugs. In this study, various computer-aided drug design (CADD) methods, including molecular docking, molecular dynamics simulations (MD), and binding free energy calculations, are employed to explore the selective mechanisms of PDE2A/9A. Overall, our research results indicate a selective design strategy for PDE2A, which involves incorporating hydrophobic or aromatic moieties into the molecular structure to better accommodate the hydrophobic pocket of PDE2A. Additionally, it is recommended to introduce functional groups capable of forming connections with selective residues, such as Phe830 and Gln812 for PDE2A, or Ala452 and Tyr424 for PDE9A, to enhance the selectivity of inhibitors targeting PDE2A/9A. This achievement is anticipated to pave the way for the development of innovative and selective small molecules targeting PDE2A/9A.Communicated by Ramaswamy H. Sarma.

Deciphering the Role of Peroxisome Proliferator-activated Receptor α and Phosphodiesterase Type 5 Targets in Alzheimer's Disease

The most prevalent cause of dementia is Alzheimer's disease (AD). Although the global AD rate is on a constant rise, medical research is yet to find a cure for this neurological condition. Current available therapeutic drugs for AD treatment only provide symptomatic alleviation. Therefore, it is essential to establish effective AD treatment strategies in addressing clinical needs. The development of disease-modifying treatments for use in the disease's early stages and the advancement of symptomatic drugs principally used in the disease's later stages are priorities in AD research. Given that the etiology of AD is difficult to comprehend, using a multimodal therapy intervention that targets molecular targets of AD-related degenerative processes is a practical strategy to change the course of AD progression. The current review article discussed PPAR-α (Peroxisome proliferator-activated receptor-α) and PDE5 (Phosphodiesterase type 5) targets with evidence for their preclinical and clinical importance. Furthermore, we support the targets with AD-related processes, functions, and remedial measures. A unique synergistic method for treating AD may involve the beneficial combinatorial targeting of these two receptors. Furthermore, we reviewed different PDE chemical families in this research and identified PDE5 inhibitors as one of the promising AD-related experimental and clinical disease-modifying medications. Lastly, we suggest jointly targeting these two pathways would be more beneficial than monotherapy in AD treatments.

Design and construction of novel pyridine-pyrimidine hybrids as selective COX-2 suppressors: anti-inflammatory potential, ulcerogenic profile, molecular modeling and ADME/Tox studies

NSAIDs represent a mainstay in pain and inflammation suppression, and their actions are mainly based on inhibiting COX-1 and COX-2 enzymes.Due to the adverse effects of these drugs, especially on the stomach and heart, scientists efforts have been directed to manufacture selective COX-2 without cardiovascular side effects and with minimal effects on the stomach. The cardiovascular side effects are thought to be related to the chemical composition rather than mechanism of action of these drugs.Novel pyridopyrimidines, 9a-j, were prepared and their chemical structures were confirmed by NMR, mass and IR Spectra, and elemental analysis. The effect of the 9a-j compounds on COX-1 and COX-2 was assessed and it was found that 2-hydrazino-5-(4-methoxyphenyl)-7-phenyl-3H-pyrido[2,3-d)pyrimidin-4-one (9d) was the most potent COX-2 inhibitor (IC50 = 0.54 uM) compared to celecoxib (IC50 = 1.11 uM) with selectivity indices of 6.56 and 5.12, respectively.The in vivo inhibition of paw edema of novel compounds 9a-j was measured using carrageenan-induced paw edema method, and that 2-hydrazino-5-(4-methoxyphenyl)-7-phenyl-3H-pyrido[2,3-d)pyrimidin-4-one (9d) showed the best inhibitory activity in comparison with the other compounds and celecoxib.The gastroprotective effect of the potent derivatives 9d, 9e, 9f, 9 g and 9h was investigated. 2-Hydrazino-5-(4-methoxyphenyl)-7-phenyl-3H-pyrido[2,3-d)pyrimidin-4-one (9d) and 7-(chlorophenyl)-hydrazino-5-(4-methoxyphenyl)-3H-pyrido[2,3-d)pyrimidin-4-one (9e) showed ulcer indices comparable to celecoxib (1 and 0.5 vs 0.5, respectively). Docking studies were carried out and they confirmed the mechanistic action of the designed compoundsCommunicated by Ramaswamy H. Sarma.

Design, synthesis, and biological evaluation of urolithin derivatives as potential phosphodiesterase II inhibitors

Urolithins are the gut microbiota metabolites of ellagitannins which are found in natural plants such as pomegranate, strawberry, and raspberry, and in nuts. Recently, several reports have clarified the underlying mechanism of urolithins in central nervous system inflammation. Therefore, urolithins have become potential therapeutic drug candidate molecules for central nervous system diseases. Derivatives 1–1d, 1–1f, 3–2a, and 3–2b of urolithin A, urolithin B, and methoxyurolithin A were found to have had significant inhibitory activity against phosphodiesterase II with IC50 values of 35.42, 39.96, 25.58, and 13.84 μM, respectively. Herein, we report the design and synthesis of urolithin derivatives along with a biological evaluation of their activity against phosphodiesterase II.

Phosphodiesterase 2A inhibition corrects the aberrant behavioral traits observed in genetic and environmental preclinical models of Autism Spectrum Disorder

Pharmacological inhibition of phosphodiesterase 2A (PDE2A), which catalyzes the hydrolysis of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), has recently been proposed as a novel therapeutic tool for Fragile X Syndrome (FXS), the leading monogenic cause of Autism Spectrum Disorder (ASD). Here, we investigated the role of PDE2A in ASD pathogenesis using two rat models that reflect one of either the genetic or environmental factors involved in the human disease: the genetic Fmr1-^Δexon 8 rat model and the environmental rat model based on prenatal exposure to valproic acid (VPA, 500 mg/kg). Prior to behavioral testing, the offspring was treated with the PDE2A inhibitor BAY607550 (0.05 mg/kg at infancy, 0.1 mg/kg at adolescence and adulthood). Socio-communicative symptoms were assessed in both models through the ultrasonic vocalization test at infancy and three-chamber test at adolescence and adulthood, while cognitive impairments were assessed by the novel object recognition test in Fmr1-^Δexon 8 rats (adolescence and adulthood) and by the inhibitory avoidance test in VPA-exposed rats (adulthood). PDE2A enzymatic activity in VPA-exposed infant rats was also assessed. In line with the increased PDE2A enzymatic activity previously observed in the brain of Fmr1-KO animals, we found an altered upstream regulation of PDE2A activity in the brain of VPA-exposed rats at an early developmental age (p < 0.05). Pharmacological inhibition of PDE2A normalized the communicative (p < 0.01, p < 0.05), social (p < 0.001, p < 0.05), and cognitive impairment (p < 0.001) displayed by both Fmr1-^Δexon 8 and VPA-exposed rats. Altogether, these data highlight a key role of PDE2A in brain development and point to PDE2A inhibition as a promising pharmacological approach for the deficits common to both FXS and ASD.

Synthesis and Biological Investigation of 1,2,4-Triazolo[4,3-a]azines as Potential HSF1 Inductors

Derivatives of fused 1,2,4-triazines containing heterocyclic and metallocene fragments were obtained by one-pot oxidative cyclization of heterocyclic hydrazones in the presence of hypervalent iodine(III) reagents. For 1,2,4-triazolo[4,3-a]azines, the ability to activate HSF1 was investigated. The obtained compounds were shown to increase the degree of HSF1 activation. It was shown that the 1,2,4-triazines can be used to induce Hsp70 expression and decrease the extent of mutant HTT aggregate formation.

Comparing the Performance of Different AMBER Protein Forcefields, Partial Charge Assignments, and Water Models for Absolute Binding Free Energy Calculations

Identifying chemical starting points is a vital first step in small molecule drug discovery and can take significant time and money. For this reason, computational approaches to virtual screening are of great interest as they can lower the cost and shorten timeframes. However, simple approaches such as molecular docking and pharmacophore screening are of limited accuracy and provide a low probability of success. Alchemical binding free energies represent a promising approach for virtual screening as they naturally incorporate the key effects of water molecules, protein flexibility, and binding entropy. However, the calculations are technically very challenging, with performance depending on the specific forcefield used. For this reason, it is important that the community has access to benchmark test sets to assess prediction accuracy. In this paper, we present an approach to alchemical binding free energies using OpenMM. We identify effective simulation parameters using an existing BRD4(1) test set and present two new benchmark sets (cMET and PDE2A) that can be used in the community for validation purposes. Our findings also highlight the effectiveness of some AMBER forcefields, in particular, AMBER ff15ipq.

Triazolopyrimidine Nuclei: Privileged Scaffolds for Developing Antiviral Agents with a Proper Pharmacokinetic Profile

Viruses are a continuing threat to global health. The lack or limited therapeutic armamentarium against some viral infections and increasing drug resistance issues make the search for new antiviral agents urgent. In recent years, a growing literature highlighted the use of triazolopyrimidine (TZP) heterocycles in the development of antiviral agents, with numerous compounds that showed potent antiviral activities against different RNA and DNA viruses. TZP core represents a privileged scaffold for achieving biologically active molecules, thanks to: i) the synthetic feasibility that allows to variously functionalize TZPs in the different positions of the nucleus, ii) the ability of TZP core to establish multiple interactions with the molecular target, and iii) its favorable pharmacokinetic properties. In the present review, after mentioning selected examples of TZP-based compounds with varied biological activities, we will focus on those antivirals that appeared in the literature in the last 10 years. Approaches used for their identification, the hit-to-lead studies, and the emerged structure-activity relationship will be described. A mention of the synthetic methodologies to prepare TZP nuclei will also be given. In addition, their mechanism of action, the binding mode within the biological target, and pharmacokinetic properties will be analyzed, highlighting the strengths and weaknesses of compounds based on the TZP scaffold, which is increasingly used in medicinal chemistry.

Inhibition of PDE2 and PDE4 synergistically improves memory consolidation processes

Phosphodiesterases (PDE) are the only enzymes that degrade cAMP and cGMP which are second messengers crucial to memory consolidation. Different PDE inhibitors have been developed and tested for their memory-enhancing potential, but the occurrence of side effects has hampered clinical progression. As separate inhibition of the PDE2 and PDE4 enzyme family has been shown to enhance memory, we investigated whether concurrent treatment with a PDE2 and PDE4 inhibitor can have synergistic effects on memory consolidation processes. We found that combined administration of PF-999 (PDE2 inhibitor) and roflumilast (PDE4 inhibitor) increases the phosphorylation of the AMPA receptor subunit GluR1 and induces CRE-mediated gene expression. Moreover, when combined sub-effective and effective doses of PF-999 and roflumilast were administered after learning, time-dependent forgetting was abolished in an object location memory task. Pharmacokinetic assessment indicated that combined treatment does not alter exposure of the individual compounds. Taken together, these findings suggest that combined PDE2 and PDE4 inhibition has synergistic effects on memory consolidation processes at sub-effective doses, which could therefore provide a therapeutic strategy with an improved safety profile.

Evaluation of the Structure-Activity Relationship of Microtubule-Targeting 1,2,4-Triazolo[1,5-a]pyrimidines Identifies New Candidates for Neurodegenerative Tauopathies

Studies in tau and Aβ plaque transgenic mouse models demonstrated that brain-penetrant microtubule (MT)-stabilizing compounds, including the 1,2,4-triazolo[1,5-a]pyrimidines, hold promise as candidate treatments for Alzheimer's disease and related neurodegenerative tauopathies. Triazolopyrimidines have already been investigated as anticancer agents; however, the antimitotic activity of these compounds does not always correlate with stabilization of MTs in cells. Indeed, previous studies from our laboratories identified a critical role for the fragment linked at C6 in determining whether triazolopyrimidines promote MT stabilization or, conversely, disrupt MT integrity in cells. To further elucidate the structure-activity relationship (SAR) and to identify potentially improved MT-stabilizing candidates for neurodegenerative disease, a comprehensive set of 68 triazolopyrimidine congeners bearing structural modifications at C6 and/or C7 was designed, synthesized, and evaluated. These studies expand upon prior understanding of triazolopyrimidine SAR and enabled the identification of novel analogues that, relative to the existing lead, exhibit improved physicochemical properties, MT-stabilizing activity, and pharmacokinetics.

The long and winding road of designing phosphodiesterase inhibitors for the treatment of heart failure

Cyclic nucleotide phosphodiesterases (PDEs) are a superfamily of enzymes known to play a critical role in the indirect regulation of several intracellular metabolism pathways through the selective hydrolysis of the phosphodiester bonds of specific second messenger substrates such as cAMP (3',5'-cyclic adenosine monophosphate) and cGMP (3',5'-cyclic guanosine monophosphate), influencing the hypertrophy, contractility, apoptosis and fibroses in the cardiovascular system. The expression and/or activity of multiple PDEs is altered during heart failure (HF), which leads to changes in levels of cyclic nucleotides and function of cardiac muscle. Within the cardiovascular system, PDEs 1-5, 8 and 9 are expressed and are interesting targets for the HF treatment. In this comprehensive review we will present a briefly description of the biochemical importance of each cardiovascular related PDE to the HF, and cover almost all the "long and winding road" of designing and discovering ligands, hits, lead compounds, clinical candidates and drugs as PDE inhibitors in the last decade.

[1,2,4]Triazolo[1,5-a]pyrimidine Phosphodiesterase 2A Inhibitors: Structure and Free-Energy Perturbation-Guided Exploration

We describe the hit-to-lead exploration of a [1,2,4]triazolo[1,5-a]pyrimidine phosphodiesterase 2A (PDE2A) inhibitor arising from high-throughput screening. X-ray crystallography enabled structure-guided design, leading to the identification of preferred substructural components. Further rounds of optimization used relative binding free-energy calculations to prioritize different substituents from the large accessible chemical space. The free-energy perturbation (FEP) calculations were performed for 265 putative PDE2A inhibitors, and 100 compounds were synthesized representing a relatively large prospective application providing unexpectedly active molecules with IC₅₀'s from 2340 to 0.89 nM. Lead compound 46 originating from the FEP calculations showed PDE2A inhibition IC₅₀ of 1.3 ± 0.39 nM, ∼100-fold selectivity versus other PDE enzymes, clean cytochrome P450 profile, in vivo target occupancy, and promise for further lead optimization.

The New Biomarker for Cervical Squamous Cell Carcinoma and Endocervical Adenocarcinoma (CESC) Based on Public Database Mining

To reconstruct the ceRNA biological network of cervical squamous cell carcinoma and endocervical adenocarcinoma (CESC) and to select an appropriate mRNA as a biomarker that could be used for CESC early diagnosis and prognosis evaluation. We downloaded CESC data from the TCGA public database, and statistical analysis was conducted with the R software to find out differential expressed genes encoding for lncRNAs, miRNAs, and mRNAs. The differentially expressed mRNAs (DEmRNAs) screened in the ceRNA network were analyzed for survival to find the mRNAs with significantly linked to the survival prognosis. These mRNAs were searched in the Pathological Atlas to identify the final appropriate mRNAs. Differential expression analysis revealed 773 lncRNAs, 94 miRNAs, and 2466 mRNAs. Survival analysis of DEmRNAs in the ceRNA network indicated that ADGRF4, ANXA8L1, HCAR3, IRF6, and PDE2A (P < 0.05) were negatively correlated with survival time. Verification of these six DEmRNAs in the Pathology Atlas indicated that PDE2A was a possible biomarker for CESC patients. PDE2A might be a biomarker for early diagnosis and prognosis evaluation of CESC patients, but due to the lack of available data, further studies may be needed for confirmation.

Radiosynthesis and Biological Investigation of a Novel Fluorine-18 Labeled Benzoimidazotriazine- Based Radioligand for the Imaging of Phosphodiesterase 2A with Positron Emission Tomography

A specific radioligand for the imaging of cyclic nucleotide phosphodiesterase 2A (PDE2A) via positron emission tomography (PET) would be helpful for research on the physiology and disease-related changes in the expression of this enzyme in the brain. In this report, the radiosynthesis of a novel PDE2A radioligand and the subsequent biological evaluation were described. Our prospective compound 1-(2-chloro-5-methoxy phenyl)-8-(2-fluoropyridin-4-yl)-3- methylbenzo[e]imidazo[5,1-c][1,2,4]triazine, benzoimidazotriazine (BIT1) (IC₅₀ PDE2A = 3.33 nM; 16-fold selectivity over PDE10A) was fluorine-18 labeled via aromatic nucleophilic substitution of the corresponding nitro precursor using the K[¹⁸F]F-K_2.2.2-carbonate complex system. The new radioligand [¹⁸F]BIT1 was obtained with a high radiochemical yield (54 ± 2%, n = 3), a high radiochemical purity (≥99%), and high molar activities (155–175 GBq/μmol, n = 3). In vitro autoradiography on pig brain cryosections exhibited a heterogeneous spatial distribution of [¹⁸F]BIT1 corresponding to the known pattern of expression of PDE2A. The investigation of in vivo metabolism of [¹⁸F]BIT1 in a mouse revealed sufficient metabolic stability. PET studies in mouse exhibited a moderate brain uptake of [¹⁸F]BIT1 with a maximum standardized uptake value of ~0.7 at 5 min p.i. However, in vivo blocking studies revealed a non-target specific binding of [¹⁸F]BIT1. Therefore, further structural modifications are needed to improve target selectivity.

Phosphodiesterase 5 inhibitors as novel agents for the treatment of Alzheimer's disease

Alzheimer's disease (AD), characterized by a progressive impairment of memory and cognition, is a major health problem in both developing and developed countries. Currently, no drugs can reverse the progression of AD. Phosphodiesterase 5 (PDE5) is a critical component of the cyclic guanosine monophosphate/protein kinase G (cGMP/PKG) signaling pathway in neurons, the inhibition of which has produced neuroprotective effects, and PDE5 inhibitors have recently been thought to be potential therapeutic agents for AD. In this paper, we summarized the outstanding progress that has been made in PDE5 inhibitors as anti-AD agents with encouraging results in animal studies, clinical trials and the investigations on the underlying mechanisms. The novel PDE5 inhibitors reported recently in the treatment of AD were also reviewed and discussed.

Synthesis and In Vitro Evaluation of 8-Pyridinyl-Substituted Benzo[e]imidazo[2,1-c][1,2,4]triazines as Phosphodiesterase 2A Inhibitors

Phosphodiesterase 2A (PDE2A) is highly expressed in distinct areas of the brain, which are known to be related to neuropsychiatric diseases. The development of suitable PDE2A tracers for Positron Emission Tomography (PET) would permit the in vivo imaging of the PDE2A and evaluation of disease-mediated alterations of its expression. A series of novel fluorinated PDE2A inhibitors on the basis of a Benzoimidazotriazine (BIT) scaffold was prepared leading to a prospective inhibitor for further development of a PDE2A PET imaging agent. BIT derivatives (BIT1–9) were obtained by a seven-step synthesis route, and their inhibitory potency towards PDE2A and selectivity over other PDEs were evaluated. BIT1 demonstrated much higher inhibition than other BIT derivatives (82.9% inhibition of PDE2A at 10 nM). BIT1 displayed an IC₅₀ for PDE2A of 3.33 nM with 16-fold selectivity over PDE10A. This finding revealed that a derivative bearing both a 2-fluoro-pyridin-4-yl and 2-chloro-5-methoxy-phenyl unit at the 8- and 1-position, respectively, appeared to be the most potent inhibitor. In vitro studies of BIT1 using mouse liver microsomes (MLM) disclosed BIT1 as a suitable ligand for ¹⁸F-labeling. Nevertheless, future in vivo metabolism studies are required.

1,2,4-Triazolo[1,5-a]pyrimidines in drug design

The 1,2,4-triazolo[1,5-a]pyrimidine (TP) heterocycle, in spite of its relatively simple structure, has proved to be remarkably versatile as evidenced by its use in many different applications reported over the years in different areas of drug design. For example, as the ring system of TPs is isoelectronic with that of purines, this heterocycle has been proposed as a possible surrogate of the purine ring. However, depending on the choice of substituents, the TP ring has also been described as a potentially viable bio-isostere of the carboxylic acid functional group and of the N-acetyl fragment of ε-N-acetylated lysine. In addition, the metal-chelating properties of the TP ring have also been exploited to generate candidate treatments for cancer and parasitic diseases. In the present review article, we discuss recent applications of the TP scaffold in medicinal chemistry, and provide an overview of its properties and methods of synthesis.

Design, synthesis of novel purin-6-one derivatives as phosphodiesterase 2 (PDE2) inhibitors: The neuroprotective and anxiolytic-like effects

Phosphodiesterase 2 (PDE2) has received much attention for the potential treatment of the central nervous system (CNS) disorders. Herein, based on the existing PDE2 inhibitors and their binding modes, a series of purin-6-one derivatives were designed, synthesized and evaluated for PDE2 inhibitory activities, which led to the discovery of the best compounds 6p and 6s with significant inhibitory potency (IC₅₀: 72 and 81 nM, respectively). Docking simulation was performed to insert compound 6s into the crystal structure of PDE2 at the active site to determine the binding mode. Furthermore, compound 6s significantly protected HT-22 cells against corticosterone-induced cytotoxicity and rescued corticosterone-induced decreases in cAMP and cGMP levels. It also produced anxiolytic-like effect in the elevated plus-maze test and exhibited favorable pharmacokinetic properties in vivo. These results might bring significant instruction for further development of potent PDE2 inhibitors.

Triazolopyrimidine and triazolopyridine scaffolds as TDP2 inhibitors

Tyrosyl-DNA phosphodiesterase 2 (TDP2) repairs topoisomerase II (TOP2) mediated DNA damages and causes cellular resistance to clinically used TOP2 poisons. Inhibiting TDP2 can potentially sensitize cancer cells toward TOP2 poisons. Commercial compound P10A10, to which the structure was assigned as 7-phenyl triazolopyrimidine analogue 6a, was previously identified as a TDP2 inhibitor hit in our virtual and fluorescence-based biochemical screening campaign. We report herein that the hit validation through resynthesis and structure elucidation revealed the correct structure of P10A10 (Chembridge ID 7236827) to be the 5-phenyl triazolopyrimidine regioisomer 7a. Subsequent structure-activity relationship (SAR) via the synthesis of a total of 47 analogues of both the 5-phenyl triazolopyrimidine scaffold (7) and its bioisosteric triazolopyridine scaffold (17) identified four derivatives (7a, 17a, 17e, and 17z) with significant TDP2 inhibition (IC₅₀ < 50 µM), with 17z showing excellent cell permeability and no cytotoxicity.

Synthesis and Biological Evaluation of Fused Tricyclic Heterocycle Piperazine (Piperidine) Derivatives As Potential Multireceptor Atypical Antipsychotics

Herein, a novel series of multireceptor ligands was developed as polypharmacological antipsychotic agents using the designed multiple ligand approach between dopamine receptors and serotonin receptors. Among them, compound 47 possessed unique pharmacological features, exhibiting high affinities for D₂, D₃, 5-HT_1A, 5-HT_2A, and 5-HT₆ receptors and low efficacy at the off-target receptors (5-HT_2C, histamine H₁, and adrenergic α₁ receptor). Compound 47 showed dose-dependent inhibition of apomorphine- and MK-801-induced motor behavior, and the conditioned avoidance response with low cataleptic effect. Moreover, compound 47 resulted nonsignificantly serum prolactin levels and weight gain change compared with risperidone. Additionally, compound 47 possessed a favorable pharmacokinetic profile with oral bioavailability of 58.8% in rats. Furthermore, compound 47 displayed procognition properties in a novel object recognition task in rats. Taken together, compound 47 may constitute a novel class of atypical antipsychotic drugs for schizophrenia.

Design, Synthesis, and Biological Evaluation of Orally Available First-Generation Dual-Target Selective Inhibitors of Acetylcholinesterase (AChE) and Phosphodiesterase 5 (PDE5) for the Treatment of Alzheimer's Disease

Through drug discovery strategies of repurposing and redeveloping existing drugs, a series of novel tadalafil derivatives were rationally designed, synthesized, and evaluated to seek dual-target AChE/PDE5 inhibitors as good candidate drugs for Alzheimer's disease (AD). Among these derivatives, 1p and 1w exhibited excellent selective dual-target AChE/PDE5 inhibitory activities and improved blood-brain barrier (BBB) penetrability. Importantly, 1w·Cit (citrate of 1w) could reverse the cognitive dysfunction of scopolamine-induced AD mice and exhibited an excellent effect on enhancing cAMP response element-binding protein (CREB) phosphorylation in vivo, a crucial factor in memory formation and synaptic plasticity. Moreover, the molecular docking simulations of 1w with hAChE and hPDE5A confirmed that our design strategy was rational. In summary, our research provides a potential selective dual-target AChE/PDE5 inhibitor as a good candidate drug for the treatment of AD, and it could also be regarded as a small molecule probe to validate the novel AD therapeutic approach in vivo.

Identification of a Potent, Highly Selective, and Brain Penetrant Phosphodiesterase 2A Inhibitor Clinical Candidate

Computational modeling was used to direct the synthesis of analogs of previously reported phosphodiesterase 2A (PDE2A) inhibitor 1 with an imidazotriazine core to yield compounds of significantly enhanced potency. The analog PF-05180999 (30) was subsequently identified as a preclinical candidate targeting cognitive impairment associated with schizophrenia. Compound 30 demonstrated potent binding to PDE2A in brain tissue, dose responsive mouse brain cGMP increases, and reversal of N-methyl-d-aspartate (NMDA) antagonist-induced (MK-801, ketamine) effects in electrophysiology and working memory models in rats. Preclinical pharmacokinetics revealed unbound brain/unbound plasma levels approaching unity and good oral bioavailability resulting in an average concentration at steady state (C_av,ss) predicted human dose of 30 mg once daily (q.d.). Modeling of a modified release formulation suggested that 25 mg twice daily (b.i.d.) could maintain plasma levels of 30 at or above targeted efficacious plasma levels for 24 h, which became part of the human clinical plan.

Discovery of a Novel Series of Pyrazolo[1,5-a]pyrimidine-Based Phosphodiesterase 2A Inhibitors Structurally Different from N-((1S)-1-(3-Fluoro-4-(trifluoromethoxy)phenyl)-2-methoxyethyl)-7-methoxy-2-oxo-2,3-dihydropyrido[2,3-b]pyrazine-4(1H)-carboxamide (TAK-915), for the Treatment of Cognitive Disorders

It has been hypothesized that selective inhibition of phosphodiesterase (PDE) 2A could potentially be a novel approach to treat cognitive impairment in neuropsychiatric and neurodegenerative disorders through augmentation of cyclic nucleotide signaling pathways in brain regions associated with learning and memory. Following our earlier work, this article describes a drug design strategy for a new series of lead compounds structurally distinct from our clinical candidate 2 (TAK-915), and subsequent medicinal chemistry efforts to optimize potency, selectivity over other PDE families, and other preclinical properties including in vitro phototoxicity and in vivo rat plasma clearance. These efforts resulted in the discovery of N-((1S)-2-hydroxy-2-methyl-1-(4-(trifluoromethoxy)phenyl)propyl)-6-methyl-5-(3-methyl-1H-1,2,4-triazol-1-yl)pyrazolo[1,5-a]pyrimidine-3-carboxamide (20), which robustly increased 3',5'-cyclic guanosine monophosphate (cGMP) levels in the rat brain following an oral dose, and moreover, attenuated MK-801-induced episodic memory deficits in a passive avoidance task in rats. These data provide further support to the potential therapeutic utility of PDE2A inhibitors in enhancing cognitive performance.

A systematic analysis of atomic protein-ligand interactions in the PDB

As the protein databank (PDB) recently passed the cap of 123 456 structures, it stands more than ever as an important resource not only to analyze structural features of specific biological systems, but also to study the prevalence of structural patterns observed in a large body of unrelated structures, that may reflect rules governing protein folding or molecular recognition. Here, we compiled a list of 11 016 unique structures of small-molecule ligands bound to proteins - 6444 of which have experimental binding affinity - representing 750 873 protein-ligand atomic interactions, and analyzed the frequency, geometry and impact of each interaction type. We find that hydrophobic interactions are generally enriched in high-efficiency ligands, but polar interactions are over-represented in fragment inhibitors. While most observations extracted from the PDB will be familiar to seasoned medicinal chemists, less expected findings, such as the high number of C-H···O hydrogen bonds or the relatively frequent amide-π stacking between the backbone amide of proteins and aromatic rings of ligands, uncover underused ligand design strategies.

Discovery of an Orally Bioavailable, Brain-Penetrating, in Vivo Active Phosphodiesterase 2A Inhibitor Lead Series for the Treatment of Cognitive Disorders

Herein, we describe the discovery of a potent, selective, brain-penetrating, in vivo active phosphodiesterase (PDE) 2A inhibitor lead series. To identify high-quality leads suitable for optimization and enable validation of the physiological function of PDE2A in vivo, structural modifications of the high-throughput screening hit 18 were performed. Our lead generation efforts revealed three key potency-enhancing functionalities with minimal increases in molecular weight (MW) and no change in topological polar surface area (TPSA). Combining these structural elements led to the identification of 6-methyl-N-((1R)-1-(4-(trifluoromethoxy)phenyl)propyl)pyrazolo[1,5-a]pyrimidine-3-carboxamide (38a), a molecule with the desired balance of preclinical properties. Further characterization by cocrystal structure analysis of 38a bound to PDE2A uncovered a unique binding mode and provided insights into its observed potency and PDE selectivity. Compound 38a significantly elevated 3',5'-cyclic guanosine monophosphate (cGMP) levels in mouse brain following oral administration, thus validating this compound as a useful pharmacological tool and an attractive lead for future optimization.

Discovery of Clinical Candidate N-((1S)-1-(3-Fluoro-4-(trifluoromethoxy)phenyl)-2-methoxyethyl)-7-methoxy-2-oxo-2,3-dihydropyrido[2,3-b]pyrazine-4(1H)-carboxamide (TAK-915): A Highly Potent, Selective, and Brain-Penetrating Phosphodiesterase 2A Inhibitor for the Treatment of Cognitive Disorders

Phosphodiesterase (PDE) 2A inhibitors have emerged as a novel mechanism with potential therapeutic option to ameliorate cognitive dysfunction in schizophrenia or Alzheimer's disease through upregulation of cyclic nucleotides in the brain and thereby achieve potentiation of cyclic nucleotide signaling pathways. This article details the expedited optimization of our recently disclosed pyrazolo[1,5-a]pyrimidine lead compound 4b, leading to the discovery of clinical candidate 36 (TAK-915), which demonstrates an appropriate combination of potency, PDE selectivity, and favorable pharmacokinetic (PK) properties, including brain penetration. Successful identification of 36 was realized through application of structure-based drug design (SBDD) to further improve potency and PDE selectivity, coupled with prospective design focused on physicochemical properties to deliver brain penetration. Oral administration of 36 demonstrated significant elevation of 3',5'-cyclic guanosine monophosphate (cGMP) levels in mouse brains and improved cognitive performance in a novel object recognition task in rats. Consequently, compound 36 was advanced into human clinical trials.

Efficient and regioselective one-step synthesis of 7-aryl-5-methyl- and 5-aryl-7-methyl-2-amino-[1,2,4]triazolo[1,5-a]pyrimidine derivatives

Two facile and efficient one-step procedures for the regioselective synthesis of 7-aryl-5-methyl- and 5-aryl-7-methyl-2-amino-[1,2,4]triazolo[1,5-a]pyrimidines.