European Journal of Medicinal Chemistry

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European Journal of Medicinal Chemistry 207 (2020) 112795

Research paper
Synthesis and bioactivity of phenyl substituted furan and oxazole carboxylic acid derivatives as potential PDE4 inhibitors
Yinuo Lin a, 1, Wasim Ahmed a, 1, Min He a, 1, Xuwen Xiang a, Riyuan Tang b, c, **,
Zi-Ning Cui a, b, *
a State Key Laboratory for Conservation and Utilization of Subtropical Agro-bioresources, Integrative Microbiology Research Centre, Guangdong Province Key Laboratory of Microbial Signals and Disease Control, South China Agricultural University, Guangzhou, 510642, China
b Guangdong Laboratory for Lingnan Modern Agriculture, Guangzhou, 510642, China
c Department of Applied Chemistry, College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China

Article history:
Received 29 July 2020 Received in revised form 30 August 2020
Accepted 30 August 2020
Available online 4 September 2020

2-cyanoimino-1,3-thiazolidine Synthesis
PDE4 inhibitors Molecular simulation

a b s t r a c t

In this present study, a series of 5-phenyl-2-furan and 4-phenyl-2-oxazole derivatives were designed and synthesized as phosphodiesterase type 4 (PDE4) inhibitors. In vitro results showed that the synthesized compounds exhibited considerable inhibitory activity against PDE4B and blockade of LPS-induced TNF-a release. Among the designed compounds, Compound 5j exhibited lower IC50 value (1.4 mM) against PDE4 than parent rolipram (2.0 mM) in in vitro enzyme assay, which also displayed good in vivo activity in animal models of asthma/COPD and sepsis induced by LPS. Docking results suggested that introduction of methoxy group at para-position of phenyl ring, demonstrated good interaction with metal binding pocket domain of PDE4B, which was helpful to enhance inhibitory activity.
© 2020 Elsevier Masson SAS. All rights reserved.

1. Introduction

Physiological processes of animals, plants and microbes are regulated by secondary messengers. Cyclic adenosine mono- phosphate (cAMP) and cyclic guanosine monophosphate (cGMP) plays a cardinal regulatory role in airway epithelium, inflammatory cells, airway smooth muscle cells and immune cells in animal secondary messengers. Balance of these two important messengers (cAMP and cGMP) are disrupted by the group of enzymes called cyclic nucleotide phosphodiesterases (PDEs) [1,2]. These groups of enzymes consist of 11 contrasting families, segregated on the basis of their structures and properties during catalysis of secondary

* Corresponding author. State Key Laboratory for Conservation and Utilization of Subtropical Agro-bioresources, Integrative Microbiology Research Centre, Guang- dong Province Key Laboratory of Microbial Signals and Disease Control, South China Agricultural University, Guangzhou, 510642, China..

** Corresponding author. Guangdong Laboratory for Lingnan Modern Agriculture, Guangzhou, 510642, China.
E-mail addresses: [email protected] (R. Tang), [email protected] (Z.-N. Cui).
1 These authors contributed equally to this paper.messengers [3,4]. Out of these 11 different members, PDE4 is considered as one of the most important enzyme for targeting the cAMP, and the presence of PDE4 is also reported in immune cells.

PDE4 members target the catalysis of cAMP by hydrolysis of 30- phosphodiester bond resulting in an inactive 50-monophosphate [5] and thus inhibition of the PDE4 is contemplated an important enzymatic aspect to control it. PDE4 inhibition accelerates the increasing amount of cAMP which is helpful for physiological processes like airway muscle relaxation as well to stop the activa- tion of proinflammatory cell activation. Hence, there is an acute need to develop potential inhibitor of PDE4 that can act as anti- inflammatory agent. These types of chemical compounds can be further used for control of disease like asthma and pulmonary diseases [6,7]. In former times, researchers reported the development of novel in- hibitors (rolipram and piclamilast) as anti-inflammatory drugs in 1990s (Fig. 1). Inhibition of PDE4 elevates the cellular cAMP levels, due to which the activation of specific phosphorylation cascade, that inhibit the release of tumor necrosis factors-a (TNF-a), acti- vation of inflammatory cells and cytokines (interleukin-2, interleukin-12 and leukotriene B4).

Rolipram and piclamilast are well known for their high inhibi- tory activity against PDE4 and good curative effects, however, treatment with rolipram can cause nausea, sweating and other unbearable side effects for patients, as a structural analogue, piclamilast also contains aforementioned side effects, which limits their application. Hence, we have been focusing on the construction of new PDE4 inhibitor skeletons. As shown in our previous reports [8], 4-(3,4-dialkoxyphenyl) moiety was essential for PDE4 inhibi- tion where the catechol ether oxygens played an important role in binding to the enzyme. Further study shows that the modification of the 4-(3,4-dialkoxyphenyl) moiety to the 8-methoxyquinoline- 5-carboxamides (such as SCH 365351) could enhance the PDE4 inhibitory activity. Replacement of the amide moiety of SCH 365351 with five-membered heterocyclic rings (oxazole or furan ring) and the cyclization of imide moiety into pyrazole ring are both helpful to enhance PDE4 inhibitory activity. Further investigation found that simplification from 8-methoxyquinoline moiety to benzene ring was also beneficial to the enhancement of PDE4 inhibitory activity. Based on our previous findings described above, com- pounds 1 (pyrazole derivative containing furan ring) and 2 (pyr- azole derivative containing oxazole ring) was designed to show significant PDE4 inhibitor activity [8,9] (Fig. 1). With these two skeletons in hand, we wish to disclose here design and synthesis of three novel series of compounds 3 (2-alkyl-5-phenyl furan deriv- ative), 4 (2-carbonyl-5-phenyl furan derivative) and 5 (2-phenyl-4- carbonyl oxazole derivative) and speculated the effect of new introduced heterocyclic N-(thiazolidin-2-ylidene) cyanamide in the design. As demonstrating in our previous research, five membered heterocyclic moiety, forming hydrophobic interaction with protein, was essential for the bioactivity. A series of designed molecules derived from furan and oxazole scaffolds that are further inflated with superior PDE4 activity and selectively conducted in wet lab experiments and supported with the significant in vivo efficacy than rolipram.

2. Material and methods
2.1. General

Mass spectra were checked with a Bruker APEX IV spectrometer (Bruker, Fallanden, Switzerland). 1H NMR and 13C NMR spectra were measured on Bruker DPX600 (Bruker, Fallanden,Switzerland), while tetramethylsilane was used as an internal standard. Melting points were recorded with a Cole-Parmer melting point apparatus (ColeParmer, Vernon Hills, Illinois, USA). Elemental analyses were performed on a Vario EL elemental analyzer. Analytical thin-layer chromatography was carried out on silica gel 60 F254 plates. The promoter activity of hpa1 was checked by a FACS-Caliber flow cytometer (CytoFLEX USA). RNA concen- tration and purity were monitored using the Nanovue UV-Vs spectrophotometer (GE Healthcare Bio-Science, Sweden). The cDNA levels were quantified by Applied Biosystems 7500 Real-Time PCR System (Thermo,USA). The growth rates were recorded using a Bioscreen (Bioscreen, Finland).

2.2. Synthesis of title compounds 3, 4 and 5

2.2.1. General procedure for the synthesis of title compounds 3a-3k
According to the reported literature [10aec], Intermediates 5- substituted phenyl-2-furoic acid 7a-7k were prepared from substituted aniline by Meerwein arylation reaction, and the cor- responding 5-phenylfuran-2-carbonyl chloride were prepare using SOCl2 as solvent and reactant. A solution of 5-phenylfuran-2- carbonyl chloride (5.0 mmol, 1 equiv) was prepared in dry THF and cooled to 10 ◦C under nitrogen atmosphere. Sodium boro-
hydride (6.0 mmol, 1.2 equiv) was added to the THF solution and the reaction mixture was stirred for 2 h at 0 ◦C. The reaction was
quenched by adding 10% aqueous ammonium chloride solution. THF was distilled off and the residue was diluted by adding chlo- roform and water. The product was extracted with three portions of chloroform and the organic layers were combined, dried over anhydrous MgSO4, filtered and concentrated to get the crude alcohol. It was purified by column chromatography (silica gel, hexane and chloroform as eluents) to isolate a colorless liquid (80% yield). A mixture of (5-phenylfuran-2-yl) methanol (4.0 mmol) in dry dichloromethane (20 mL) and pyridine (4.0 mmol) was cooled in an ice bath. Solution of thionyl chloride (15.0 mmol) in dry dichloro- methane (10 mL) was added under N2 atmosphere, at such a rate to keep the temperature between 10 and 0 ◦C. After complete addition, the reaction mixture was stirred at room temperature for 2 h. Ice was added and the reaction mixture was stirred for further 5 min. A small amount of NaHCO3 was added to adjust pH 6.0. The organic layer was separated and dried over MgSO4. Filtration and evaporation of the solvent gave 2-(chloromethyl)-5-phenylfuran (40% yield). 2-cyanoimino-1,3-thiazolidine (2.0 mmol) and potassium car- bonate (3.0 mmol) were dissolved in 5 mL tetrahydrofuran and stirred at room temperature. To the solution, 2-(chloromethyl)-5- phenylfuran in acetonitrile was added dropwise. The reaction mixture was refluxed for 3 h. After the completion of the reaction, the solvent was evaporated under reduced pressure, water was added, and the mixture was extracted with ethyl acetate. The combined organic layer was washed with brine, dried over Na2SO4, and concentrated. The residue was purified by silica gel chroma- tography to give the corresponding product in 64% yield.

2.2.2. General procedure for the synthesis of title compounds 4a-4l
Intermediates 5-substituted phenyl-2-furoic acids 7a-7l were prepared from substituted aniline by Meerwein arylation reaction according to the reported literature [10aec]. Thionyl chloride (15 mL) was added to 0.01 mol of 5-substituted phenyl-2-furoic acid. The mixture was refluxed at 80 ◦C for 3 h. The reaction was monitored by periodic thin layer chromatography (TLC). When the reaction was completed, excess of thionyl chloride was removed under reduced pressure. The crude product was dissolved in 20 mL of anhydrous acetonitrile and added to the 10 mL of acetonitrile solution containing 2 mmol of 2-cyanoimino- 1,3-thiazolidine and 2 mmol of K2CO3. The mixture was stirred for 3e6 h at 75 ◦C, then acetonitrile was removed under reduced pressure. An excess of water was added and the reaction mixture was extracted three times with dichloromethane. After that, it was washed successively with 10% HCl, 10% NaHCO3 and water, dried over anhydrous MgSO4. The product was purified by column chromatography (40 250 mm) on silica gel using dichloro- methane and methanol (v/v 95:5) as the eluent to yield the title compounds.

2.2.3. General procedure for the synthesis of title compound 5a-5k
Intermediates ethyl 2-phenyloxazole-4-carboxylate were pre- pared following the reported literature [11], followed by the hy- drolysis of ester group to get 2-substituted phenyl-4-oxazole carboxylic acids as described below. Ethyl 2-phenyloxazole-4-carboxylate was dissolved in THF-H2O (1:1, 20 mL) and 2 M aqueous NaOH (2 equiv.) solution was added dropwise at 0 ◦C. Reaction mixture was stirred at room temperature for 2e4 h and monitored by TLC. Upon completion of reaction, THF was removed in vacuo and aqueous layer was washed with ethyl acetate and pH of aqueous layer was adjusted to 2 by the slow addition of 2 M aqueous HCl at 0 ◦C, which gives the precipitate at this stage. Solid was filtered, washed with water and dried in vacuo and used for the next step without further purification. Mixture of 2-phenyloxazole-4-carboxylic acid (1 equiv.) and thionyl chloride (5 mL) was refluxed for 1 h under nitrogen at- mosphere. Reaction was cooled to room temperature and excess of thionyl chloride was removed under vacuum. Crude mass was dissolved in acetonitrile, followed by the addition of K2CO3 (3 equiv.) and 2-cyanoimino-1,3-thiazolidine (1 equiv.) at room tem- perature. Reaction mixture was stirred for 3e6 h at room temper- ature and monitored by TLC. Upon completion of reaction, it was diluted with water and organic compounds were extracted with dichloromethane. Organic layer was washed with brine, dried over anhydrous Na2SO4, filtered and concentrated. Product was purified by silica gel (200e300 mesh) column chromatography using methanol/DCM solvents as an eluent to give the title compound 5 in moderate to good yield.

2.3. Bioassay

2.3.1. Assay of human PDE4 activity
Enzyme PDE4 was isolated from the sample as previously described method [10aec,12]. The enzyme was prepared from U937 cells which was derived from human monocytes, and was stored at 20 ◦C after preparation. Dilution of enzyme was done with triple distilled water containing bovine serum albumin and measurement of PDE4 activity was performed using this stored enzyme. The substrate solution was prepared by adding [3H] cAMP (300,000 dpm (5000 Bq)/assay) and 100 mmol/L cAMP solution to 100 mmol/L TriseHCl (pH 8.0) containing 5 mmol/L ethylene glycol-bis (b-aminoethyl ether) and O,O0-bis(2-aminoethyl) ethyl- eneglycol- N,N,N0,N0-tetraacetic acid. The substrate solution was mixed with the enzyme solution containing a test compound dis- solved in DMSO, and incubation was done for 30 min at 30 ◦C. Assays were performed in duplicate at different concentrations of each test compound.

2.3.2. Assay of TNF-a release
The blood is mixed with saline at a ratio of 1:1, and the pe- ripheral blood mononuclear cells (PBMCs) were isolated from buffy coats using Lymphoprep tubes [13]. The PBMCs were suspended in RPMI 1640 with 0.5% human serum albumin, pen/strep, and 2 mM L-glutamine at 5 × 105 cells/mL. The cells were pre-incubated with the test compounds in 96-well plates for 30 min and stimulated for 18 h with 1 mg/mL lipopolysaccharide. TNF-a concentration in the supernatants was measured by homogeneous time-resolved fluo- rescence resonance (TR-FRET). The assay is quantified by measuring fluorescence at 665 nm (proportional to TNF-a concentration) and 620 nm (control). Results are expressed as IC50 values (mM).

2.3.3. LPS induced sepsis for measurement of TNF-a inhibition in mice
The LPS induced sepsis model in mice was performed following the literature [14]. Female Swiss albino mice were selected ac- cording to the body weights, which were equivalent within each group. The mice were fasted for 20 h with free access to water and dosed for oral administration (po) with the test compounds sus- pended in vehicle containing 0.5% Tween 80 in 0.25% sodium salt of carboxymethyl cellulose. The control mice were performed the vehicle alone. After 30 min of oral dosing, the mice were injected into intraperitoneal cavity with 500 mg of lipopolysaccharide (Escherichia coli, LPS: B4 from Sigma) in phosphate buffer. Then the mice were bled via retro-orbital sinus puncture after 90 min of LPS administration. Serum samples were collected by centrifuging the blood samples at 4000 rpm for 20 min, which were stored over-night at 4 ◦C. Immediately, the serum samples were checked for TNF-a levels using commercial mouse TNF-a ELISA kit (Amersham Biosciences) and assay was carried out following the manufacturer instruction.

2.3.4. LPS induced neutrophilia model for asthma and COPD
LPS induced neutrophilia in Sprague Dawley rats was assessed according to the described protocol [15]. Male Sprague Dawley rats were acclimatized to laboratory conditions for one week prior to the experiment. According to the body weight, the rats were distributed to various groups randomly. Except normal group, all the rats were exposed to 100 mg/mL lipopolysaccharide (Escherichia coli, LPS: B4 from Sigma) for 40 min. The rats were dosed for oral administration (po) with the test compounds suspended in the vehicle containing 0.25% sodium salt of carboxymethylcellulose before half an hour of LPS exposure. Bronchoalveolar lavage (BAL) was performed 6 h after LPS exposure, total cell count and DLC (differential leukocyte count) were checked and compared with control.

2.4. Molecular docking

Molecular docking was performed on Surflex-Dock module of Sybyl 8.0 to know the exact active site of amino acids [10aec,16]. Crystal structure of PDE4B (PDB ID:1XMY) obtained from Protein Date Bank was used as the receptor for molecular docking study. The 3D structure of compounds 3j, 4j and 5j was drawn and opti- mized with SYBYL package. The docking procedure was started with the protomol generation, which was created using a ligand- based approach (native ligand for PDE4B structure). Proto threshold was set to 0.5 and proto bloat was kept at 0 as a default parameter. For docking, max conformation and max rotation values were 20 and 100, respectively. Pre-dock and post-dock energy minimization methods were also applied. Docking results were compared by the total score values. The pose with the higher total- score value was considered as the best one. After the end of mo- lecular docking, the interactions of the docked domain with ligand were analyzed.

3. Result and discussion
3.1. Chemistry

Based on our previous research work, we have designed three new and novel modified structures, 2,5-disubstituted furan (3 and 4) and 2,4-disubstituted oxazole (5) as shown in Fig. 1. We wish to analyze the effect of different heterocyclic rings, therefore thio- zolidine ring was selected to attach with furan or oxazole moieties, the effect of the reduction of amide to amine was also planned to analyze. Introduction of a heterocyclic ring 2-cyanoimino-1,3- thiazolidine was considered to be a new parameter of analysis.
In order to prepare the title compounds 3 and 4, intermediate 5- substituted phenyl-2-furoic acids 7a-7l were synthesized (Meer- wein arylation) following our previous reports [10aec] (Scheme 1). Obtained carboxylic acids was refluxed in SOCl2 for 1 h to form corresponding acid chlorides, which was coupled with N-(thiazo- lidin-2-ylidene) cyanamide in the presence of base K2CO3 in acetonitrile to furnish the title compounds 4a-4l (Scheme 1). In order to probe the influence of amine group instead of amide, structure 3 was designed which is the reduced form of amide 4. To synthesize alcohol precursor 8a-8k, reduction of carboxylic acid 7a-7k was first performed with lithium aluminium hydride, but due to low yield of the products, different methods were explored. Hence, carboxylic acid was converted into its corresponding acid chloride first by treatment with thionyl chloride and then it wasreduced with sodium borohydride in THF/DMF solvent at —10 to 0 ◦C. Alcohol was obtained in moderate to good yield by applying this reduction method. In order to couple this key intermediate with N-(thiazolidin-2-ylidene) cyanamide, alcohol was trans- formed into the corresponding chloride by reacting with thionyl chloride, and then nucleophilic substitution with N-(thiazolidin-2- ylidene) cyanamide in presence of K2CO3 in acetonitrile, title compounds 3a-3k were obtained. (Scheme 1). Following the reported literature, carboxylic acid precursors of title compounds 5a-5k were obtained by treating corresponding substituted formamides 9a-9k with ethyl 3-bromopyruvate [11], followed by the hydrolysis of esters with sodium hydroxide. With the carboxylic acid in hand, it was planned to couple with N- (thiazolidin-2-ylidene) cyanamide, hence, different coupling methods were tried such as i. EDCI, HOBt; ii. DCC, DMAP, but the yield was not satisfactory. The best result was obtained by treat- ment of carboxylic acids with thionyl chloride and then coupled with N-(thiazolidin-2-ylidene) cyanamide (Scheme 2). All the new compounds were characterized by 1H, 13C NMR and mass spectroscopy. The structures of compounds 3i, 4l and 5e were confirmed by X-ray single crystal diffraction and shown in Fig. 2. (See the supporting information data for details).

3.2. Biological evaluation and SAR studies

The in vitro activity of the inhibition of PDE4 and blockade of LPS induced TNF-a in human blood monolayer cells were listed in Table 1. Rolipram was used as positive marker during experiment. It was observed that inhibition activity was highly influenced by the substitution at different position of phenyl ring. Compounds with chloro substitution were found to be more active than unsub- stituted for PDE4B inhibition. Among the chloro substitution at different position, para substituted compound 3d (IC50 ¼ 15.7 mM) was found to be more effective than 3b (IC50 ¼ 101.6 mM) and 3c
(IC50 ¼ 62.8 mM). On comparison with same substituted at series 4 ¼and 5, compound 5d (IC50 3.6 mM) showed better activity. It was
observed that chloro substituents compounds showed better ac- tivity (TNF-a inhibition) than unsubstituted. Among these chloro substituted compounds, para substituted compound 5d showed Synthetic route to title compounds 3 and 4. Reagents and conditions: (a) i. NaNO2, HCl; ii. furan-2-carboxylic acid, cat. CuCl2, acetone, H2O; (b) i. SOCl2, reflux, 1h; ii. K2CO3, N-(thiazolidin-2-ylidene) cyanamide, CH3CN, rt, 3e6 h; (c) i. SOCl2, reflux, 1h; ii. NaBH4, THF, DMF, —10 to 0 ◦C; (d) i. SOCl2, Py, DCM, —10 to —5 ◦C; ii. K2CO3, N-(thiazolidin-2- ylidene) cyanamide, CH3CN, rt, 3e6 h. Synthetic route to title compounds 5a-5k. Reagents and conditions: (a) i. ethyl 3-bromopyruvate, NaHCO3, THF, 60 ◦C, 23 h; ii. (CF3CO)2O, THF, rt, 12 h; (b) 2 M aq. NaOH, THF, H2O, 0 ◦C to rt, 2e4 h; (c) i. SOCl2, reflux, 1 h; ii. K2CO3, N-(thiazolidin-2-ylidene) cyanamide, CH3CN, rt, 3e6 h.


Fig. 2. Single crystal structures of compounds 3i, 4l and 5e.

better results. Same pattern was observed for the fluoro substituted compounds 3g (IC50 18.9 mM) and little variation was found in comparison with para chloro substituted compound (3d). When the substituent was changed to bromo at para position, inhibition activity was further decreased (3i: IC50 ¼ 52.7 mM). In comparison
with series 3, 4 and 5, compound 5i (IC50 ¼ 46.7 mM) exhibit better
¼ ¼ ¼
activity than 3i (IC50 ¼ 52.7 mM) and 4i (IC50 ¼ 49.6 mM). Inhibition of TNF-a also followed the similar pattern and 5i (IC50 ¼ 131.2 mM) showed better activity than 3i (IC50 ¼ 156.2 mM) and 4i (IC50 ¼ 138.9 mM). 2, 6-Difluoro substituent was also analyzed and found that 5k (IC50 ¼ 16.3 mM) revealed modest activity against PDE4B in comparison with 3k (IC50 ¼ 45.7 mM) and 4k (IC50 ¼ 22.1 mM), also the same pattern was observed against TNF-a (5k: IC50 49.3 mM, 4k: IC50 60.1 mM, 3k: IC50 82.3 mM). When
the substituent at para position was changed from electron with- drawing to electron donating group, significance change was observed against PDE4B. IC50 values of 5h was found 9.2 mM in comparison to 3h and 4h (IC50 20.1 mM and 12.5 mM respectively). Among the para substitutions, methoxy substituted (electron
donating group) compound was found to be more promising. IC50 value of compound 5j was found 1.4 mM which was better than 3j (IC50 ¼ 9.6 mM) and 4j (IC50 ¼ 2.8 mM). Also the inhibition activity against TNF-a for compound 5j (IC50 11.8 mM) was better than any
other compound of series 3 and 4. We also speculated the selected in vitro active compounds for their PDE4B selectivity over PDE4D and it was concluded that compounds with all three series had obvious selectivity towards PDE4B. Compound 5j showed better selectivity (PDE4D/B 11.21) than reference rolipram and also with other tested compounds. Selected in vitro active compounds 3j, 4j and 5j were tested for LPS induced sepsis model for the measure- ment of TNF-a inhibition (in female Swiss Albino mice) and neu- trophilia inhibition for asthma and COPD (in male Sprague Dawley rats). Table 2 demonstrated the results of experiment including details such as oral dosages and number of animals grouped. The results showed that compound 5j revealed better inhibitory activity against TNF-a release (52.6%) and LPS induced neutrophilia inhi- bition (42.1%) than the positive control rolipram (45.3% and 36.8%), compound 3j (31.2% and 28.7%) and 4j (40.5% and 33.8%).

4. Summary

In the continuation of our effort towards the finding of new PDE4 inhibitors, herein, we designed and synthesized three novel series of compounds containing 2,5-disubstituted furan (3 and 4) and 2,4-disubstituted oxazole (5) moieties. Synthesized com- pounds were evaluated for in vitro activity against phosphodies- terase type 4 and TNF-a. Compounds 3j, 4j and 5j were found to show moderate to good inhibitory activity against PDE4 and TNF-a. Further these compounds were tested for in vivo activity in animal Interaction of PDE4B in complex with compound 3j (A, B), compound 4j (C, D) and compound 5j (E, F). The catalytic domain bound to 4j overlaid with rolipram (violet color) (G, H). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)models of asthma/COPD and sepsis induced by LPS in which com- pound 5j displayed good in vivo activity in animal models. A pri- mary SAR study revealed that incorporation of 2-cyanoimino-1,3- thiazolidine ring was effective and favored the inhibitory activity and substituent at para position of phenyl ring in the molecule had an important effect of inhibitory and it can be concluded that the effect of substitutions at para position in the structures were found to be crucial. The docking results demonstrated that title com- pounds interacted well with PDE4B protein by intermolecular hydrogen bonding (eCOeN, Gln443), hydrophobic interaction (thiazolidine Phe 446) and the metal coordination in the ligand-receptor complex (p-OMe, Zn2þ and Mg2þ). We believe that suchefforts will be beneficent in future towards the development of advanced and effective PDE4 inhibitors.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


We acknowledge the financial supports from the National Key Research and Development Program of China (2017YFD0200504), the National Natural Science Foundation of China (32072450, 31570122), the International Science and Technology
Cooperation Program in Guangdong (2020A0505100048), the Na- tional Key Project for Basic Research (973 Program, 2015CB150600).

Appendix A. Supplementary data

Supplementary data to this article can be found online at


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