Triazole, imidazole, and thiazole-based compounds as potential agents against coronavirus

The expansion of the novel coronavirus known as SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2), COVID-19 (coronavirus disease 2019), or 2019-nCoV (2019 novel coronavirus) is a global concern over its pandemic potential. The need for therapeutic alternatives to stop this new pandemic is urgent. Nowadays, no efficacious therapy is available, and vaccines and drugs are underdeveloped to cure or prevent SARS-CoV-2 infections in many countries. Some vaccines candidates have been approved; however, a number of people are still skeptical of this coronavirus vaccines. Probably because of issues related to the quantity of the vaccine and a possible long-term side effects which are still being studied. The previous pandemics of infections caused by coronavirus, such as SARS-CoV in 2003, the Middle East respiratory syndrome coronavirus (MERS-CoV) in 2012, HCoV-229E, and HCoV-OC43 were described in the 1960 s, -HCoV-NL63 isolated in 2004, and HCoV-HKU1identified in 2005 prompted researchers to characterize many compounds against these viruses. Most of them could be potentially active against the currently emerging novel coronavirus. Five membered nitrogen heterocycles with a triazole, imidazole, and thiazole moiety are often found in many bioactive molecules such as coronavirus inhibitors. This present work summarizes to review the biological and structural studies of these compound types as coronavirus inhibitors.


Introduction
The recent unprecedented coronavirus pandemic caused a dramatic change in the days of humans that increases the scientific interest in coronaviruses globally. Coronaviruses (CoVs) belong to Nidovirales order, Coronaviridae family, and the subfamily of Coronavirinae contain 4 kinds: alphacoronavirus, betacoronavirus, gammacoronavirus, and deltacoronavirus (Fig. 1A) and are large (genome size 26-32 kb) [1], enveloped, positive-sense single-stranded ribonucleic acid (RNA) viruses that can infect both humans and animals. They are six human coronaviruses such as HCoV-229E, HCoV-OC43, HCoV-NL63, HCoV-HKU1, MERS (Middle East Respiratory Syndrome)-CoV, and SARS (severe acute respiratory syndrome)-CoV that are of zoonotic origin [2]. Coronaviruses (CoVs) are widespread in domestic and wild mammals, in various bird species and are the source of significant economic losses, particularly in pig and poultry farming [3]. Moreover, CoVs were found to be the causative agents of MERS-CoV and SARS-CoV. With the emergence of SARS-CoV in 2002 and the emergence of MERS-CoV in 2012 in the Arabian Peninsula, they have become important human pathogens. Among the six different coronaviruses are now described in humans, SARS-CoV and MERS-CoV are the most lethal, with 10% and 40% mortality rates, respectively. Additionally, these two viruses are the result of interspecies transmission. SARS-CoV was derived from the adaptation of a bat coronavirus, and MERS-CoV was transmitted to humans through contact with dromedaries themselves probably infected with a bat coronavirus. Now it is evidence that MERS-CoV can be transmitted person-to-person [4,5]. In 2019, a novel coronavirus (SARS-CoV-2) has been newly identified in humans, which caused millions of deaths from January 2020 to Mars 2021. As of Mars 09, 2021, the total number of confirmed SARS-CoV-2 infections worldwide is 117,907,247, deaths 2,614,842, and recovered 93,594,978 [6]. Coronavirus virions are enveloped and form spherical structures ( Fig. 1 B) [7].
The viral genome is made up of a single strand of positive RNA. Unfortunately, this SARS-CoV-2 spread with different RNA sequences and raised the question of drug efficiency against different Coronavirus variants [7]. To find new therapy against coronavirus, we focus on molecules containing scaffold triazole, imidazole, and thiazole as potential agents. These heterocycle and organic compounds are the most important and most active principles in medicinal chemistry. Heteroatoms including nitrogen [10,11], sulfur [12,13], oxygen [14], and others [15] containing in heterocyclic bioactive compounds are the main source of the reactivity of the target skeleton, the activity of the compounds, and interactions between target drugs. Triazoles (pyrrodiazoles) are a versatile class of heterocycle compounds. Many applications in drug discovery target triazole because it is considered as a good amide bioisostere that readily associates with biological targets via hydrogen bonding and dipole-dipole interactions [16].
For therapeutic options, these five-membered heterocycle scaffolds have as targets: coronavirus proteases such as 3C-like protease (3CLpro), papain-like protease (PLpro), and coronavirus helicase due to their essential implications for coronaviral replication. The main function of 3CLpro and PLpro is to process the viral polyprotein in a coordinated manner. SARS-CoV 3C like protease, plays an important role in viral polyprotein processing and controls the activity of the replicase complex [39]. The enzymatic activity of 3CLpro is an essential target for antiviral drug discovery against SARS-CoV and other coronavirus infections because of its importance for the viral life cycle [40]. In the literature, peptidomimetics were focused on 3CLpro inhibition. These compounds contain reaction groups, such as aldehydes, halo-methyl ketones, or Michael acceptors at the terminus with a covalent interaction with the active site Cys145 residue [41][42][43][44].
In this review, we focus on the current progress in the research and development of triazole, imidazole, and thiazole-containing molecule coronavirus inhibitors.

1,2,3-triazole
The major Coronavirus protease that cleaves the replicase polyproteins during viral replication is 3CLpro. This protease is targeted by numerous classes of protease inhibitors, such as zinc, mercury conjugates, C2-symmetric diols, peptidomimetic-α,β-unsaturated esters, ani- lides, N-phenyl-2-acetamide, biphenyl sulfone, glutamic acid, glutamine peptide with a trifluoromethyl ketone group, pyrimidinone, and pyrazole analog [45]. Benzotriazole showed as a new class of potent 3CLpro inhibitors. The most potent inhibitor among the benzotriazole esters is compound 1(IC 50 = 0.2 μM) [46]. The mode of inhibition was studied by docking experiments based on computer modeling (Autodock version 3.0.5) [47] for illustrating the binding compound with 3CLpro. Results indicated that the benzotriazole moiety was disposed of within the active site forming a pocket with Cys145, Ser144, and Gly143 [46]. Karypidou et al. reported that five 1,2,3-triazoles such as 2 (EC 50 = 8.95 μM), 3 (EC 50 = 9.45 μM), 4 (EC 50 = 9.455 μM), 5 (EC 50 = 8.90 μM), and 6 (EC 50 = 11.95 μM) ( Fig. 2-A) showed moderate activity against HcoV-229E but all are approximately 50 fold lower than the activity of the reference Urtica dioica agglutinin (UDA) (EC 50 = 0.2 μM) [48]. At the same time, using these triazoles through their structure-activity relationship (SAR), it was demonstrated that the active site is located within domains I and II, in which a catalytic dyad consisting of residues Cys145 and His41 [49]. The docking of bioactive derivatives of compounds 2 and 3 showed that they establish two HB interactions with Glu166 and His163, allowing also the position of the triazole ring in the proximity of the catalytic dyad [48]. Analyzed of compounds 4 and 5 gave a similar behavior, with the derivatives exhibiting antiviral activity, which able to establish two stable HB interactions within the catalytic site of 3CL pro. Compound 4 is anchored in the catalytic site through interaction with His163 and two interactions established with Glu166 and Thr25. Concerning the interactions of compound 5, it establishes stable HB contacts with both Glu166 and His163, while further anchoring to residue Gln189 [48].
Turlington et al. demonstrated potential non-covalent SARS-3CLpro inhibitors of the chemical class of triazoles [50]. Compounds 7 and 8 in (Fig. 2 A) were identified as a potential 3CLpro inhibitor with IC 50 = 6.2 μM and IC 50 = 4.11 μM respectively good selectivity versus PLpro (IC 50 > 60 μM) used as a control for cysteine-protease activity [50]. This study revealed through crystallography data of compound 8 and related triazoles, inhibition of 3CLpro via a novel mechanism of action and provide a new direction for additional noncovalent inhibitor design and refinement. Replacement of the Nmethyl pyrrole moiety with a thiophene ring and substitution on the acetamide moiety with isopropyl amide 9 and cyclobutylamide 10 ( Fig. 2 A) improved the inhibitory profile with IC 50 of 4.1 and 3.8 μM respectively [51]. Activity of 1-(4,5-dihydroxy-3-hydroxyme thylcyclopenten-2-enyl)-1H-1,2,3triazole-4-carboxylic acid amide (compound 11) against SARS-CoV was demonstrated with EC 50 = 47-μM [52]. Another kind of molecules, carbon quantum dots (CQDs) with a diameter below 10 nm and excellent water dispersion, are targeted for nanomedical applications because of a lack of visible signs of toxicity in animals [53]. Aleksandra and co-workers investigated the antiviral activity of CQDs for the treatment of HCoV-229E infection [54]. Results showed that the compound 12 concentration-dependent virus inactivation observed was estimated to EC 50 = 52 ± 8 µg mL −1 . Combination of mannose and compound 12 showed a complete loss of antiviral activity of the latter at low particle concentrations, with antiviral activity above 50 µg mL −1 . These highlight that boronic acid functions, where the mode of action is the selective and reversible formation of tetravalent complexes with cis-diols and thus glycan units [54], are interacting with HCoV-229E. Compound 12, considered as pseudo-lectin, targeted the S protein of HCoV-229E via a lectin-carbohydrate binding mechanism, similar to the oligomannose-specific lectin Griffithsin [55]. The presence of antiviral activity of the mannose saturated compound 12 might be due to the presence of the triazole scaffold on the particle's surface. Compound 13 bears no boronic acid function but a triazole scaffold, displays some antiviral activity, even though largely decreased compared to compound 12.
1,2,4-triazole 1,2,4-triazoles are structural isomers of 1,2,3-triazoles. They are versatile heterocyclic moieties with a broad spectrum of biological activities [56]. Triazole derivatives have shown as potential antiviral agents against the H1N1 influenza virus [57]. Zhao and coworkers showed that 1,2,4-triazole derivative had the most effective activity against influenza B [58]. Some triazole derivatives are shown to possess bioavailability [59]. Helicase inhibitors are attractive anticoronavirus treatment options thanks to the high homologous of helicases of various coronaviruses [60]. Supported by their modes of action, CoV-helicase inhibitors are often categorized into two groups. The primary one inhibits the unwinding and ATPase activity of SARS-CoV helicase, leading to inhibition of viral replication in vitro [61,62]. The second group of CoV-helicase inhibitors includes compounds that selectively inhibit the unwinding activity but not the ATPase activity of CoV helicase for instance compound 14. Adedeji et al. reported a variety limited of possible inhibitors of nsp13. It absolutely was shown that SSYA10-001 3-[(2-nitrophenyl)sulfanylmethyl]-4-prop-2-enyl-1 H-1,2,4-triazole-5-thione (14) (Fig. 2 A) a triazole derivative as an nsp13 non-inhibitor through the blockage of SARS-CoV and MERS-CoV replication but inhibited SARS-CoV via nsp13 with IC 50 = 6 μM [63]. The cleavage activity of 3CL-pro and PL-pro leads to the release of nonstructural proteins (nsp) which include nsp13 (NTPase/Helicase). It was supposed that also the compound 14 binding pocket of SARS-CoV nsp13 is preserved among different coronavirus helicases, revealing the discovery of broad-spectrum coronavirus inhibitors. The SARS-CoV nsp13 indeed, we noted high homology between SARS-CoV helicase and other coronavirus helicases (about 70%), which means that compound 14 may also inhibit several coronaviruses that cause human disease, including but not limited to HCoV-229E and HCoV-NL63. It was reported that SSYA10-001 is a potential inhibitor of viral replication in MERS-CoV replication [64]. Zaher et al. investigated the inhibition of helicase MERS-CoV enzymatic activity of newly halogenated 1,2,4-triazole derivatives against MERS-CoV [65]. Results showed that 4-amino-5-hydrazino-4H-1,2,4-triazole-3-thiol (15) inhibited the M-nsp 13 helicase and ATPase activity with an IC 50 value of 12.4 and 8.9 μmol L -1 respectively. Some halogenated 1,2,4-triazole derivatives showed significant anti-MERS-CoV activity. Compounds 16 and 17 (Fig. 2 B) were the foremost effective MERS-CoV helicase inhibitors with ATPase IC 50 values of 0.47 and 0.51 μmol L -1 respectively [65]. Besides, ortho-iodo derivative (compound 18) and orthochloro (compound 19) (Fig. 2B) derivatives were moderate inhibitors with ATPase IC 50 values of 2.73 and 3.9 μmol L -1 respectively. A known nonspecific antiviral, ribavirin has been used for treating SARS-CoV [66]. Ribavirin (compound 20) (Fig. 2B), is a guanine derivative, which is known for treating human coronavirus and respiratory syncytial virus (RSV) that has been evaluated in patients with SARS-CoV and MERS-CoV. However, it contains some side effects such as anemia, which must be severe at high doses [60] and its potency against SARS-CoV-2 is uncertain. Mechanistically, ribavirin inhibits RNA synthesis by viral RdRp also as well as inhibits mRNA capping [67]. The combination of interferon-α2b and ribavirin can reduce MERS-CoV replication in vitro and in vivo [68]. A typical cleavage recognition site is, therefore, (Ser, Ala)-(Val, Thr)-(Leu, Glu)-(Ser, Ala, Gly), which is conserved among all coronavirus 3CLpro [69]. These features were exploited for designing lead triazole compounds with a potential broad spectrum [70]. The molecular docking of compound 21 conformation revealed that it binds to the active site and forms similar key interactions throughout all five proteases (3D23, 2ZU2, 3V3M, 3TLO, and OC43). It had been shown that the 1,2,4-triazole scaffold formed a linker within the center of the structure and mediated the formation of a hydrogen bond between N2 and the backbone of Glu165/6, which formed a third hydrogen bond via the backbone carbonyl and ligand NH. HCoV-OC43 3CLpro exhibited a fourth hydrogen bond between N4 of the 1,2,4-triazole and Gln189. This latter is conserved in 3D23 and 3V3M structures, however, it was oriented towards the solvent-exposed exterior of the pocket. Concerning 2ZU2 and 3TLO, the Gln position is replaced by Pro188/9, which did not contribute to the formation of a bond [70]. 1,2,4-triazole analog (22) (Fig. 2B) exhibited moderate antiviral activity with EC 50 = 21 μM, selectivity index SI > 4.8 against SARS virus [52]. Takano et al. showed the activity of Itraconazole (ICZ) inhibiting type I FCoV (Feline coronavirus) infection [71]. It was demonstrated that type I FCoV is associated with cholesterol throughout the viral life cycle [72]. Previous studies demonstrated that U18666A, is a cholesterol transport inhibitor, inhibits type I FCoV infection [71]. Based on these results, U18666A has been identified as a drug target for type I FCoV [73,74]. The inhibition of intracellular cholesterol transport by ICZ was confirmed [75]. Itraconazoles induces cholesterol accumulation within the cytoplasm, but the mechanism has not been clarified [76,77]. Another study reported that ICZ induces cholesterol accumulation in the lysosome by acting on NPCI. For that, ICZ may inhibit type I FCoV replication via the identical mechanism as U18666A [75].
It was reported also that ICZ acts on the oxysterol-binding protein (OSBP) and inhibits the formation of viral replication organelles in enteroviruses [47]. However, it was supposed that the replication of coronavirus is associated with endoplasmic reticulum (ER)-derived structures referred to as double-membrane vesicles (DMVs) [78], but it's not clear that OSBP is involved during the formation of the viral replication organelle in coronavirus.

Imidazole
Imidazole and its derivatives are some of the most important bioactive compounds. The study of their biological activities can be done by theoretical or experimental methods. Many articles targeted the inhibition of coronavirus theoricatically (Table 1) [79]. Abdallah et al. reported this study theoretically the possibility of using twenty-eight bioactive imidazole compounds against coronavirus [63,80]. Results showed that compounds 23, 24, and 25 (Figs. [3][4][5] are the best inhibitors with the best docking score. The common characters between these compounds are aromatic rings with one or more heteroatoms such as O or F or N or Cl branched. There are different interactions between protein (active sites) and compounds such as hydrogen bonds, pi-pi interactions, electrostatic interactions, hydrophobic, and hydrophilic interactions. The more interactions with the active site, the more binding forces made to the protein, the more stable the complex formed between the protein and the inhibitor, and the more inhibition ability. The characters, size, and conformation of the amino acids may be important in the interactions between the inhibitor and active site throughout the substituted groups. Compound    Besides, the analogs might affect cellular pathways of nucleotide metabolism, which could also account for the toxicity associated with some compounds [81]. To investigate the role of the stereochemistry of the decahydroisoquinoline ring as well as different substitutions at the benzamide and imidazole moieties, Shimamoto and coworkers designed compounds 44-47 (Fig. 6). The X-ray structure of 3CLpro in complex with compound 44 inhibitor with IC 50 of 63 μM revealed that the decahydroisoquinoline scaffold gets into the S2 pocket, while the imidazole ring occupied S1 pocket (Fig. 7) [82].
The potential of an octahydroisochromene scaffold as a unique hydrophobic core to interact with the S2 pocket of the protease was investigated. The diastereoisomers (48-51) (Fig. 6) were tested and the results revealed that a specific configuration of compound 49 (1S, 3S), which could guarantee the correct position of the P1 imidazole and the aldehyde moiety within the protease active site. However, it has been demonstrated that the n-butyl chain at 1-position of the fused-ring system was also found to be important for hydrophobic interactions [83]. The compound 52 bearing imidazole showed a potent profile with IC 50 of 6.0 μM [51].

Thiazole
In the literature, among various biological activities, we have also anticoronavirus of compounds containing thiazole ring (compounds 53-85, Fig. 8). They reported that thiazolyl ketone-containing peptidic compounds as severe acute respiratory syndrome coronavirus protease (SARS-CoV 3CLpro) inhibitors with compound 53 as the most potent compound (Ki value of 2.20 μM and IC 50 of 9.5 μM) [80]. Konno et al. optimized the structure of compound 53 based on molecular docking studies using dimeric chymotrypsin-like proteases. Phenoxymethyl carbonyl substitution 54 showed a slight activity with an IC 50 of 6.8 μM. 4,5-dimethylthiazole (55) substitution at P1 0 increased the activity with IC 50 of 2 μM. Benzothiazole substitution (56 and 57) showed four-to five-fold more active with IC 50 of 1.7 μM and 2.3 μM respectively. These benzothiazoles substituted with electron-donating groups on the phenyl ring of P4 increased the activity (58-65) (Fig. 8) with IC 50 s of 0.6 μM. Replacing the phenyl group with pyridinyl 66 at P4 reduced the activity to IC 50 of 2.9 μM. Besides, replacement with phenylamino 67 recovered the activity to IC 50 of 1.5 μM. Increasing the chain length 68 reduced the activity to IC 50 of 7.5 μM. Substitution with polar groups did not increase the activity (69-71) (Fig. 8) IC 50s > 1600 μM, while substitution with hydrophobic chain increased solubility. Cyclic amide at P1 was found to be essential for activity, but its replacement with imidazolyl did not increase the activity (72-74) (Fig. 8) IC 50 s of 210 μM to 1600 μM) [84].  Apaydin et al. reported the synthesis and evaluation of a series of 1thia-4-azaspiro [4.5]decan-3-ones against human coronavirus [85]. They showed that seven compounds (76,77,78,79,80,81, and 82) (Fig. 8) were found to inhibit HCoV-229E replication. The most active compound was compound 81, with an EC 50 (Fig. 8) showed a promising potential SARS-CoV 3CLpro inhibitor with IC 50 of 4.3 μM with significant selectivity over four proteases (HAV 3Cpro, NS3pro, chymotrypsin, and papain) [86]. Tsai and co-workers used structurebased virtual screening to identify novel SARS-CoV 3CLpro inhibitors. The best performing derivative is shown with compound 84 with IC 50 value of 3 μM [87]. Zhang et al. exhibited close interactions between SARS-CoV main proteinase and inhibitors ( Table 2). The results showed that for ritonavir, the thiazole group (P1) and benzene group (P2) are inserted into S1 and S2 specificity pockets, while another benzene side chain (P3) might be too long to fit the substrate-binding pocket perfectly (Fig. 9) [88]. Ulusoy et al. evaluated a series of aryli-  Results revealed that some compounds like compound 85 ( Fig. 8) was the most active compound and inhibited FCoV with EC 50 of 7.5 μM [89].

Conclusion
This review was carried out to identify potential anti-SARS-CoV drug candidates from a known drug containing triazole, imidazole, and thiazole. The drug candidates presented in this study are not thorough and could be further broadened and examined for anticoronavirus activity. Combinations of experimental tests and structure-based docking simulations are used to identify therapies for viral infections. In some cases, the mechanism of action depends on the heterocycle scaffold. Mode of action of different compounds could be used as a basis for improving their properties and for developing and proposing specific anti-SARS-CoV-2 drugs.