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1-Phenyl-N-(benzothiazol-2-yl)methanimine derivatives as Middle East respiratory syndrome coronavirus inhibitors

Middle East respiratory syndrome coronavirus (MERS-CoV) poses a serious threat to human health, and currently there are no effective or specific therapies available to treat it. Herein a series of 1-phenyl-N-(benzothiazol-2-yl)methanimine derivatives with inhibitory activity against MERS-CoV are described. The compound 4f with a 50% inhibition concentration value of 0.09 μM is a promising inhibitor that warrants further evaluation, towards the development of potential anti-MERS-CoV drugs.


Introduction
Middle East respiratory syndrome coronavirus (MERS-CoV) is a novel zoonotic virus in the Middle East. 1,2 It rst appeared in Saudi Arabia in June 2012, 3 then spread to the rest of the Middle East and South Korea. 4 MERS-CoV is primarily transmitted from animals to humans, 5 but it can reportedly spread from human to human through close contact. 6,7 It can cause severe respiratory disease, with symptoms of fever, coughing, and dyspnea, and may contribute to severe gastrointestinal diseases and renal failure. 8 As at 03 June 2020, the World Health Organization had reported 2494 laboratory-conrmed cases of MERS-CoV infection, including 858 conrmed deaths (an approximately 35% fatality rate) in over 27 countries.
Viral genomic studies indicate that MERS-CoV is an enveloped virus with positive-sense single-stranded RNA that belongs to lineage C in the genus Betacoronavirus of the family Coronaviridae under the Nidovirales order. 3 It is the sixth coronavirus known to infect humans, and the rst known lineage C Betacoronavirus associated with human infection. 37,38 The MERS-CoV genome is approximately 30 kilobases in size and contains more than 10 open reading frames (ORFs). 39 The large replicase ORF1a and ORF1b produce polyproteins PP1a and PP1ab, which are subsequently cleaved into 15 or 16 nonstructural proteins. [40][41][42] The region downstream of ORF1b encodes at least four main structural proteins: spike (S), membrane (M), envelope (E), and nucleocapsid (N).
The S protein serves as the critical surface-located trimeric glycoprotein of MERS-CoV and induces entry into host cells, 43 and it is composed of subunits S1 and S2. The receptor-binding domain on the S1 subunit attaches to dipeptidyl peptidase 4, which is a well-known target for diabetes. [44][45][46][47][48][49] The conformation of the S2 subunit then changes and a six-helix bundle fusion core is formed, which enables the formation of the viral envelope, facilitating tight host cell membrane binding for fusion and subsequent host cell entry. 50 Thus the S protein plays a pivotal role in mediating the entry of the virus into target host cells.
According to previous reports, MERS-CoV pseudovirus expressing S protein, which allows for single-cycle infection in cells expressing dipeptidyl peptidase 4, can be used to rapidly screen MERS-CoV entry inhibitors. 51 In the current study, novel small-molecule inhibitors of entry events of pseudovirus expressing S protein were identied via high-throughput screening technology using a cell-based assay. compound libraries. Using this approach, compound 1a was identied as a modest entry inhibitor (Fig. 1B) with a 50% inhibition concentration (IC 50 ) of 0.73 mM (Fig. 1C). Using 1a as a hit compound, a series of 1-phenyl-N-(benzothiazol-2-yl) methanimine derivatives were designed and synthesized, and structure-activity relationships and their inhibitory potencies against MERS-S pseudovirus were investigated.

Results and discussion
Four major regions were partitioned for diversication; the benzothiazole portion (blue), the linker region (pink), the imine portion (yellow), and the phenyl portion (green) (Fig. 1B). Compounds 1a-1g were synthesized rst as outlined in Scheme 1. Commercially available aryl bromides were used as the starting material, and the corresponding intermediate amines 3a-3d were prepared using the Suzuki-Miyaura cross-coupling reaction. 53 Compounds 1a-1e were generated using the reaction of an intermediate amine with the corresponding aromatic aldehyde in the presence of AcOH as catalyst. 54 Compound 1g was generated by the reaction of 3a with 2-hydroxy-3methoxybenzoyl chloride. Compound 1f was generated by a reduction reaction of 1a with NaBH 4 . 55 Their IC 50 values against MERS-S pseudovirus and 50% cytotoxic concentration (CC 50 ) values are summarized in Table 1. The Schiff base structure of hit compound 1a was conrmed by comprehensive spectral data. The 1 H NMR spectrum depicted a sharp singlet peak at d 8.67 parts per million (ppm) indicating the presence of azomethine proton (-CH]N-). Phenolic-OH proton and methoxy protons exhibited singlet peaks at d 13.44 and d 3.94 ppm respectively. A multiplet signal at d 6.87-8.14 ppm was assigned to the aromatic protons. The 13 C nuclear magnetic resonance spectrum of 1a revealed corresponding peaks in the range of d 114.2-166.1 ppm which were assigned to azomethine (-CH]N-) carbon and aryl carbons. A peak at d 55.2 ppm was identical to the methoxy carbon. The 1 H-1 H COSY spectrum demonstrated chemical shis of three protons on ring C at d 6.87-7.06 ppm, and two protons near the nitrogen atom side on ring B at d 7.36-7.41 ppm. Correlation between the azomethine proton and the two protons near the nitrogen atom side on ring B was observed in the 1 H-1 H NOSEY correlation spectrum, indicating that the imine moiety of compound 1a is Egeometry. The HRMS (ESI) spectrum of 1a depicted a signal at m/z 383.0823, indicating a [M + Na] + signal with the molecular formula C 21 H 16 N 2 NaO 2 S, which is concordant with the structure of 1a. Characteristic spectra and data are shown in the ESI. † Compounds with benzoxazole (1b) or benzimidazole (1c) motifs lost inhibitory activity, as did the compound without Table 1 Inhibitory activity of 1a-1g against MERS-S pseudovirus a IC 50 : 50% inhibitory concentration (mM). b CC 50 : 50% cytotoxic concentration (mM). c HRP: peptide mimics of heptad repeat domain in the MERS spike protein, which is responsible for conformational change during spike protein-mediated viral-host membrane fusion.
00 >100 a IC 50 : 50% inhibitory concentration (mM). b CC 50 : 50% cytotoxic concentration (mM). c HRP: peptide mimics of heptad repeat domain in the MERS spike protein, which is responsible for conformational change during spike protein-mediated viral-host membrane fusion. a phenyl group B (1e) in the linker region. Modest inhibitory activity was evident when the phenyl group B was replaced with a pyridyl group (1d). When the imine portion was reduced or replaced with amide (1f and 1g), inhibitory activity was markedly reduced. Modication was then focused on the phenyl ring C of the compound, and derivatives 1h-1m were designed and synthesized in a similar way as 1a. Their IC 50 values on MERS-S pseudovirus and CC 50 values are summarized in Table 2.
All the derivatives demonstrated inhibitory activities with IC 50 values ranging from 0.20 mM to 0.57 mM, and no cell toxicity. Either changing the position of the hydroxyl group and methoxy group or the absence of these groups increased the activity slightly, the compounds containing 3-OH were more potent. Changing the phenyl ring C to a pyridine ring did not elicit signicant changes in inhibitory activity.
Given the results of 1h-1m, further structural extensions were investigated in the phenyl ring C area. A series of vemember aromatic heterocycles were introduced to replace the phenyl ring C. Derivatives 1o and 1r, substituted by a 3-thienyl group, exhibited marked inhibitory activity with respective IC 50 values of 0.14 mM and 0.62 mM. The introduction of a nitro group on the thiophene motif caused a slight reduction in inhibitory activity. Compound 1q exhibited modest inhibitory capacity, indicating that the substitution mode inuenced activity. None of the compounds in this series exhibited any cytotoxicity.
Based on the above results, while retaining the 3-thienyl group at ring C position a structure-activity relationship investigation was conducted in which structural extensions at the benzothiazole part of this compound were assessed. As shown in Scheme 2, commercially available aryl bromides were used as starting materials, and the corresponding intermediate amines 3f-3u were prepared using Suzuki-Miyaura crosscoupling or a nucleophilic substitution reaction. The nal compounds 4a-4p were readily generated via a similar method that has been previously outlined in Scheme 1. Their IC 50 values against MERS-S pseudovirus and their CC 50 values are summarized in Table 3.
Replacement of a benzothiazole unit (4a-4d) abolished the inhibitory activity (Table 3), further conrming that the benzothiazole motif played a pivotal role in inhibitory capacity. An atom of uorine-the most electronegative of the halogenswas then introduced at different benzothiazole positions, and the derivatives obtained (4e-4h) exhibited diverse inhibitory capacity. Introduction of a uorine atom at the 6-position of the phenyl ring A (4f) increased inhibition activity, with an IC 50 value of 0.09 mM, and introduction of a uorine atom at another position resulted in modest inhibitory potency. The introduction of other groups such as alkoxy, alkyl, amino, and carboxyl groups (4i-4n) reduced inhibitory activity. The reduction in inhibitory capacity is most likely due to the steric effect of the bulky group. To increase the exibility of the compounds, heteroatom was introduced between the benzothiazole group and phenyl ring B to obtain 4o and 4p, but unfortunately inhibitory activity was abolished. The CC 50 values of these compounds were greater than 100 mM, indicating no toxicity or at least lower toxicity to uninfected cells. The results of the study suggest that compound 4f has potential for the subsequent development of novel anti-MERS-CoV agents.

Conclusion
In summary, a series of new 1-phenyl-N-(benzothiazol-2-yl) methanimine derivatives was developed as MERS-CoV entry inhibitors via modication of a hit compound, which was obtained by screening an orthogonal cocktail library composed of 48 000 compounds. All of the compounds were assessed for their capacity to inhibit the entry of pseudovirus expressing MERS-CoV S protein. Structure-activity relationships indicated that benzothiazole was the best option for region A of the structure, and the presence of a phenyl ring B and an imine portion favored the retention of inhibitory activity against MERS-CoV. The introduction of a 6-member aromatic ring or a 3-thiophene ring at ring C position also favored inhibitory activity. Remarkably, compound 4f, with an IC 50 value of 0.09 mM, was more potent than the initial hit compound. In addition, none of these types of structures exhibited cytotoxicity. The present study demonstrates an approach for the structural modication of hit compounds, and a novel skeleton for new anti-MERS-CoV drug research. Further research on this series of derivatives is ongoing.

Chemistry
Commercial reagents and solvents were purchased commercial suppliers and used without further purication. All nonaqueous reactions were under a nitrogen atmosphere and all non-aqueous reaction vessels were oven-dried. Flash column chromatography was performed using Qingdao Ocean silica gel (200-300) with the indicated eluents. All nal compounds were characterized by their NMR and HRMS spectra, unless stated otherwise. 1 H NMR spectra were recorded at a spectrometer frequency of 400 MHz, and 13 C NMR spectra at 100 MHz on Bruker Avance 400. Chemical shis are reported in d (ppm) using signals of tetramethylsilane (TMS) as the internal standard. High-resolution mass spectra (HRMS) were measured on a Bruker ESI-TOF high-resolution mass spectrometer. Melting points (mp) were uncorrected and were recorded on a Buchi B-54 melting point apparatus.
General procedure for the synthesis of compounds 3a-3d and 3f-3s 2a-2q (1 mmol), the corresponding pinacol borate (1.2 mmol) and Pd(dppf)Cl 2 $CH 2 Cl 2 (0.05 mmol) were suspended in DMF (6 mL) in the presence of 2 M K 2 CO 3 (2 mL) and stirred at 70 C overnight under nitrogen. The reaction mixture was diluted with H 2 O (40 mL) and organics were extracted with EtOAc (3Â 30 mL). The combined organic layers were washed with brine (30 mL), dried over MgSO 4 , ltered and concentrated in vacuo to afford crude mixture. The crude mixture was then puried by ash column chromatography in a mixture of petroleum ether and ethyl acetate to yield the pure products 3a-3d and 3f-3s.

Synthesis of 4-(benzo[d]thiazol-2-ylthio)aniline (3u)
4-Aminobenzenethiol (2 mmol) and 2a (2 mmol) was dissolved in MeCN (5 mL) followed by the addition of K 2 CO 3 (2.4 mmol) and stirred at reux for 3 h. Upon completion, the reaction was allowed to cold to room temperature. The reaction mixture was diluted with H 2 O (40 mL) and organics were extracted with EtOAc (3Â 30 mL). The combined organic layers were washed with brine (30 mL), dried over MgSO 4 , ltered and concentrated in vacuo to afford crude mixture. The crude mixture was then puried by ash column chromatography in a mixture of petroleum ether and ethyl acetate to yield the pure product 3u. White solid; yield 50%. 1  General procedure for the synthesis of compounds 1a-1d, 1h-1u and 4a-4p The catalytic amount of acetic acid was added to a solution of 3a-3d, 3f-3u (0.44 mmol) and the corresponding aromatic aldehyde (0.53 mmol) in MeOH (4 mL), the mixture was stirred at reux for 2 h. The resulting precipitate was isolated by suction ltration and washed with MeOH (8 mL) to give the pure products 1a-1d, 1h-1u and 4a-4p.   4 Hz, 2H, ArH). 13 29 At 72 h post-transfection, supernatants were harvested and ltered through 0.45 mm lter, the ltered supernatant was further concentrated 10 folds by ultracentrifugation with 100 kDa cut-off, aliquoted and stored at À80 C as stock vial. The p24 level of pseudovirus stock solution was determined by ELISA kit according to the manual (ab218268, Abcam).

Compound library and HTS screening
Compound library samples were orthogonally pooled as mixtures of 10 compounds per well at 2 mg mL À1 each, with duplicate representation for each compound. This bidirectional orthogonal pooling strategy allows for greater screening efficiency and throughput for large compound libraries. The pooling of compound library was prepared as described previously. 52 Test compounds were freshly prepared from initial dimethyl sulfoxide (DMSO) stocks, 80 cocktails per plate. Huh-7 cells were then trypsinized, counted and seeded at a nal concentration of 104 cells per well. Plates were placed overnight in a CO 2 incubator. The following day, samples were then added with an automatic liquid handler (FX from Beckman-Coulter). Finally, the pseudotyped virus, thawed and diluted immediately prior to use in cell culture medium, was added under a BSL-2 hood using Hydra (Thermo Fisher Scientic). The nal concentration of DMSO in all wells was maintained at 2%. The plates were incubated at 37 C in a humidied CO 2 incubator for 48 h. Bright-Glo substrate (Promega) was added directly to each well and cell lysis was allowed to proceed in the dark for 5 min.
Cytotoxic effect of compounds was assayed by MTT method which evaluated the reduction product of MTT by cellular succinate dehydrogenase as an indicator of cell liability. Toxicity was calculated as 100% minus percentage of OD @570 readout each well against the control well with no drug, 100% À OD @570 (compound)/OD @570 (control) Â 100%.
Luciferase activity was measured using the Envision microplate luminometer (PerkinElmer). Inhibition was calculated as 100% minus percentage of luciferase readout each well against the control well with no drug, 100% À Luc (compound)/Luc (control) Â 100%.

Measurement of IC 50 and CC 50
For determining CC 50 and IC 50 , compounds were serially diluted and assayed as described for the HTS screening, CC 50 and IC 50 were determined by regression analysis using 4 parameter regression.

Conflicts of interest
There are no conicts of interest to declare.