Hai-Bin Luo

Design, Synthesis, and Evaluation of Multitarget-Directed Resveratrol Derivatives for the Treatment of Alzheimer’s Disease

A series of multitarget-directed resveratrol derivatives was designed and synthesized for the treatment of Alzheimers disease (AD). In vitro studies indicated that most of the target compounds exhibit significant inhibition of self-induced β-amyloid (Aβ) aggregation and Cu(II)-induced Aβ1-42 aggregation and acted as potential antioxidants and biometal chelators. In particular, compounds 5d and 10d are potential lead compounds for AD therapy (5d, IC50 = 7.56 μM and 10d, IC50 = 6.51 μM for self-induced Aβ aggregation; the oxygen radical absorbance capacity assay using fluorescein (ORAC-FL) values are 4.72 and 4.70, respectively). Moreover, these compounds are capable of disassembling the highly structured Aβ fibrils generated by self- and Cu(II)-induced Aβ aggregation. Furthermore, 5d crossed the blood-brain barrier (BBB) in vitro and did not exhibit any acute toxicity in mice at doses of up to 2000 mg/kg. Taken together, the data indicate that 5d is a very promising lead compound for AD.

Synthesis and evaluation of multi-target-directed ligands against Alzheimer's disease based on the fusion of donepezil and ebselen.

A novel series of compounds obtained by fusing the cholinesterase inhibitor donepezil and the antioxidant ebselen were designed as multi-target-directed ligands against Alzheimers disease. An in vitro assay showed that some of these molecules did not exhibit highly potent cholinesterase inhibitory activity but did have various other ebselen-related pharmacological effects. Among the molecules, compound 7d, one of the most potent acetylcholinesterase inhibitors (IC50 values of 0.042 μM for Electrophorus electricus acetylcholinesterase and 0.097 μM for human acetylcholinesterase), was found to be a strong butyrylcholinesterase inhibitor (IC50 = 1.586 μM), to possess rapid H2O2 and peroxynitrite scavenging activity and glutathione peroxidase-like activity (ν0 = 123.5 μM min(-1)), and to be a substrate of mammalian TrxR. A toxicity test in mice showed no acute toxicity at doses of up to 2000 mg/kg. According to an in vitro blood-brain barrier model, 7d is able to penetrate the central nervous system.

Identification of novel phosphodiesterase-4D inhibitors prescreened by molecular dynamics-augmented modeling and validated by bioassay.

Phosphodiesterase-4D (PDE4D) has been proved to be a potential therapeutic target against strokes. In the present study, a procedure of integrating pharmacophore, molecular docking, molecular dynamics (MD) simulations, binding free energy calculations, and finally validation with bioassay was developed and described to search for novel PDE4D inhibitors from the SPECS database. Among the 29 compounds selected by our MD-augmented strategy, 15 hits were found with IC50 between 1.9 and 50 μM (a hit rate of 52%) and 6 potent hits showed IC50 less than 10 μM, which suggested that MD simulations can explore the intermolecular interactions of PDE4D-inhibitor complexes more precisely and thus significantly enhanced the hit rate of this screening. The effective and efficient integrated procedures described in this study could be readily applied to screening studies toward other drug targets.

The Molecular Basis for the Selectivity of Tadalafil toward Phosphodiesterase 5 and 6: A Modeling Study

Great attention has been paid to the clinical significance of phosphodiesterase 5 (PDE5) inhibitors, such as sildenafil, tadalafil, and vardenafil widely used for erectile dysfunction. However, sildenafil causes side effects on visual functions since it shows similar potencies to inhibit PDE5 and PDE6, whereas tadalafil gives a high selectivity of 1020-fold against PDE6. Till now, their molecular mechanisms of selectivity of PDE5 versus PDE6 have remained unknown in the absence of the crystal structure of PDE6. In order to elucidate its isoform-selective inhibitory mechanism, a 3D model of PDE6 was constructed by homology modeling, and its interaction patterns with tadalafil plus sildenafil were exploited by molecular docking, molecular dynamics (MD) simulations, and binding free energy calculations. The present work reveals that tadalafil exhibits a less negative predicted binding free energy of -35.21 kcal/mol with PDE6 compared with the value of -41.12 kcal/mol for PDE5, which suggests that tadalafil prefers PDE5 rather than PDE6 and confers a high selectivity for PDE5 versus PDE6. The binding free energy results for tadalafil were consistent with external bioassay studies (IC50 = 5100 and 5 nM toward PDE6 and PDE5, respectively). Two important residues from the Q2 pockets (Val782 and Leu804 in PDE5 and their corresponding Val738 and Met760 in PDE6) were further identified to account for the high selectivity of tadalafil for PDE5 versus PDE6. These findings have shed light on the continuous puzzle of why sildenafil (IC50 = 74 and 6 nM toward PDE6 and PDE5, respectively) causes visual disorders because of its poor selectivity but tadalafil does not. In addition, the homology model of PDE6 can be used to design more potent and selective second-generation PDE5 inhibitors with less inhibitory potency against PDE6.

Discovery of novel PDE9 inhibitors capable of inhibiting Aβ aggregation as potential candidates for the treatment of Alzheimer's disease.

Recently, phosphodiesterase-9 (PDE9) inhibitors and biometal-chelators have received much attention as potential therapeutics for the treatment of Alzheimer’s disease (AD). Here, we designed, synthesized, and evaluated a novel series of PDE9 inhibitors with the ability to chelate metal ions. The bioassay results showed that most of these molecules strongly inhibited PDE9 activity. Compound 16 showed an IC50 of 34 nM against PDE9 and more than 55-fold selectivity against other PDEs. In addition, this compound displayed remarkable metal-chelating capacity and a considerable ability to halt copper redox cycling. Notably, in comparison to the reference compound clioquinol, it inhibited metal-induced Aβ1-42 aggregation more effectively and promoted greater disassembly of the highly structured Aβ fibrils generated through Cu2+-induced Aβ aggregation. These activities of 16, together with its favorable blood-brain barrier permeability, suggest that 16 may be a promising compound for treatment of AD.

Discovery of Novel Phosphodiesterase-2A Inhibitors by Structure-Based Virtual Screening, Structural Optimization, and Bioassay

Phosphodiesterase-2A (PDE2A) is a potential therapeutic target for treatment of Alzheimers disease and pulmonary hypertension. However, most of the current PDE2A inhibitors have moderate selectivity over other PDEs. In the present study, we described the discovery of novel PDE2A inhibitors by structure-based virtual screening combining pharmacophore model screening, molecular docking, molecular dynamics simulations, and bioassay validation. Nine hits out of 30 molecules from the SPECS database (a hit rate of 30%) inhibited PDE2A with affinity less than 50 μM. Optimization of compound AQ-390/10779040 (IC50 = 4.6 μM) from the virtual screening, which holds a novel scaffold of benzo[cd]indol-2(1H)-one among PDE inhibitors, leads to discovery of a new compound LHB-8 with a significant improvement of inhibition (IC50 = 570 nM). The modeling studies demonstrated that LHB-8 formed an extra hydrogen bond with Asp808 and a hydrophobic interaction with Thr768, in addition to the common interactions with Gln859 and Phe862 of PDE2A. The novel scaffolds discovered in the present study can be used for rational design of PDE2A inhibitors with high affinity.

Discovery of Novel Pyrazolopyrimidinone Derivatives as Phosphodiesterase 9A Inhibitors Capable of Inhibiting Butyrylcholinesterase for Treatment of Alzheimer’s Disease

Discovery of multitarget-directed ligands (MTDLs), targeting different factors simultaneously to control the complicated pathogenesis of Alzheimers disease (AD), has become an important research area in recent years. Both phosphodiesterase 9A (PDE9A) and butyrylcholinesterase (BuChE) inhibitors could participate in different processes of AD to attenuate neuronal injuries and improve cognitive impairments. However, research on MTDLs combining the inhibition of PDE9A and BuChE simultaneously has not been reported yet. In this study, a series of novel pyrazolopyrimidinone-rivastigmine hybrids were designed, synthesized, and evaluated in vitro. Most compounds exhibited remarkable inhibitory activities against both PDE9A and BuChE. Compounds 6c and 6f showed the best IC50 values against PDE9A (6c, 14 nM; 6f, 17 nM) together with the considerable inhibition against BuChE (IC50, 6c, 3.3 μM; 6f, 0.97 μM). Their inhibitory potencies against BuChE were even higher than the anti-AD drug rivastigmine. It is worthy mentioning that both showed moderate selectivity for BuChE over acetylcholinesterase (AChE). Molecular docking studies revealed their binding patterns and explained the influence of configuration and substitutions on the inhibition of PDE9A and BuChE. Furthermore, compounds 6c and 6f exhibited negligible toxicity, which made them suitable for the further study of AD in vivo.

Ab Initio QM/MM Study Shows a Highly Dissociated SN2 Hydrolysis Mechanism for the cGMP-Specific Phosphodiesterase-5.

Phosphodiesterases (PDEs) are the sole enzymes hydrolyzing the important second messengers cGMP and cAMP and have been identified as therapeutic targets for several diseases. The most successful examples are PDE5 inhibitors (i.e., sildenafil and tadalafil), which have been approved for the treatment of male erectile dysfunction and pulmonary hypertension. However, the side effects mostly due to nonselective inhibition toward other PDE isoforms, set back the clinical usage of PDE5 inhibitors. Until now, the exact catalytic mechanism of the substrate cGMP by PDE5 is still unclear. Herein, the first computational study on the catalytic hydrolysis mechanism of cGMP for PDE5 (catalytic domain) is performed by employing the state-of-the-art ab initio quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) simulations. Our simulations show a SN2 type reaction procedure via a highly dissociated transition state with a reaction barrier of 8.88 kcal/mol, which is quite different from the previously suggested hydrolysis mechanism of cAMP for PDE4. Furthermore, the subsequent ligand exchange and the release of the product GMP have also been investigated by binding energy analysis and MD simulations. It is deduced that ligand exchange would be the rate-determining step of the whole reaction, which is consistent with many previous experimental results. The obtained mechanistic insights should be valuable for not only the rational design of more specific inhibitors toward PDE5 but also understanding the general hydrolysis mechanism of cGMP-specific PDEs.

Novel Phosphodiesterase Inhibitors for Cognitive Improvement in Alzheimer’s Disease

Alzheimers 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.

Discovery and Optimization of Chromeno[2,3-c]pyrrol-9(2H)-ones as Novel Selective and Orally Bioavailable Phosphodiesterase 5 Inhibitors for the Treatment of Pulmonary Arterial Hypertension

Phosphodiesterase 5 (PDE5) inhibitors have been used as clinical agents to treat erectile dysfunction and pulmonary arterial hypertension (PAH). Herein, we detail the discovery of a novel series of chromeno[2,3-c]pyrrol-9(2H)-one derivatives as selective and orally bioavailable inhibitors against phosphodiesterase 5. Medicinal chemistry optimization resulted in 2, which exhibits a desirable inhibitory potency of 5.6 nM with remarkable selectivity as well as excellent pharmacokinetic properties and an oral bioavailability of 63.4%. In addition, oral administration of 2 at a dose of 5.0 mg/kg caused better pharmacodynamics effects on both mPAP (mean pulmonary artery pressure) and RVHI (index of right ventricle hypertrophy) than sildenafil citrate at a dose of 10.0 mg/kg. These activities along with its reasonable druglike properties, such as human liver microsomal stability, cytochrome inhibition, hERG inhibition, and pharmacological safety, indicate that 2 is a potential candidate for the treatment of PAH.