Benzothiazole Moiety and Its Derivatives as Antiviral Agents †

: A virus is a microorganism that uses the machinery of the host to multiply. At present, there are various species of viruses known to us that are dangerous for the health of human beings. One of such viruses has destroyed many lives nowadays and that is a coronavirus. Such other viruses like Human Immunodeﬁciency Virus, Poliovirus, etc., destroy one’s capability to survive normally. As science progresses, we invent many antiviral drugs as per the type of virus. There are many antiviral drugs available to treat viral infections. From them, benzothiazole derivatives are potent antiviral agents. Researchers continuously work on benzothiazole moiety to get more effective benzothiazole derivatives that can be used as antiviral agents. This review article gives information about various benzothiazole derivatives that are invented during 1980 to 2021, that act against the various viruses as antiviral agents, the structure–activity relationship of benzothiazole as an antiviral agent, various schemes to synthesize benzothiazole derivatives as an antiviral agent as well as includes various methods to evaluate the antiviral activity of novel synthetic compounds against speciﬁc viruses.


Introduction
Avirus is very tiny infectious agent that replicate only inside the living cells of an organism.Viruses infect all types of plants, animals and microorganisms also, like bacteria, etc. [1].Viral infections are considered to be one of the major threats to the health of human beings.Virus infections take place due to globalization and unexpected climate change [2].We are informed about just about 260 varieties of viruses, but the unknown varieties of viruses are responsible for 99.9% of total infection cases.These viruses come to the picture when they show some symptoms to the host [3].Despite of the development of many molecules as antiviral, they are unable to satisfy the requirement criteria to treat the viral infection and drug resistance of current viruses.That's why there is still a need for newer vaccines, diagnostic agents and antiviral molecules [4].Because, benzothiazole is such a versatile moiety, it shows many biological activities including antiviral effect against various species of viruses [5].Due to feasible physical and chemical properties of benzothiazole moiety, many researchers have tried to synthesize various benzothiazole derivatives that shows potent antiviral effects against various strain of viruses [6].
Structurally, benzothiazole is a fusion of two aryl rings; benzene and thiazole.As thiazole bears nitrogen and sulfur moiety, benzothiazole derivatives successfully binds to viruses and gives antiviral activity.Many articles are available for reference to develop newer antiviral agents, but this review article includes various novel synthesized antiviral compounds bearing benzothiazole moiety, the pathway to synthesize benzothiazole based antiviral agents, the structure-activity relationship of benzothiazole as antiviral agents and in vitro and in vivo methods to evaluate antiviral activity of novel synthetic compounds.That is why it is a unique article containing all the information to guide a researcher for synthesis of benzothiazole based antiviral agents.

Anti-Hepatitis C Virus Agents
HCV is an RNA virus with six genotypes.Hepatic fibrosis is the main symptom of HCV infection.The infection of HCV spreads worldwide.Global prevalence of HCV infection has been estimated as 2-3%, which is equal to 150-170 million people with infection

Anti-Hepatitis C Virus Agents
HCV is an RNA virus with six genotypes.Hepatic fibrosis is the mai HCV infection.The infection of HCV spreads worldwide.Global preva infection has been estimated as 2-3%, which is equal to 150-170 millio infection [9].A few benzothiazole derivatives are discovered that show an against HCV infection.
Using  on a homopolymeric RNA primer/template.The non-nucleoside inhibitor aurintricarboxylic acid (ATA) was used as a reference compound in the enzymatic assay.This compound has been shown to inhibit NS5B both in vitro and in replicon assays, through binding to the benzothiadizine allosteric pocket [10].
Girijavallabhan et al. have designed some HCV replication inhibitors bearing benzothiazole moiety.Compound (3) was screened for its HCV replication inhibition ability and the potent compound was found as an antiviral compound [11].All the structures of various antiviral agents those contain benzothiazole as a parent molecule are described in Figure 1.
Med. Sci.Forum 2021, 7, 9 3 of 7 (2) as potent HCV inhibitors.These compounds were screened for suppression of in vitro incorporation of [3H]-UTP by recombinant HCV RNA polymerase NS5BDC21 (genotype 1b) on a homopolymeric RNA primer/template.The non-nucleoside inhibitor aurintricarboxylic acid (ATA) was used as a reference compound in the enzymatic assay.This compound has been shown to inhibit NS5B both in vitro and in replicon assays, through binding to the benzothiadizine allosteric pocket [10].
Girijavallabhan et al. have designed some HCV replication inhibitors bearing benzothiazole moiety.Compound (3) was screened for its HCV replication inhibition ability and the potent compound was found as an antiviral compound [11].All the structures of various antiviral agents those contain benzothiazole as a parent molecule are described in Figure 1.

Anti-Herpes Virus Agents
Human cytomegalo virus is under the category of beta herpes virus.Once the person is infected with this particular virus, the virus remains in that person's body for their whole life.It does not affect a healthy human, but it shows symptoms in pregnant women or people with weak immunity [12].Then, the treatment becomes necessary.
The substituted benzothiazole derivatives (4) and ( 5) were prepared by reacting lactam with respective substituted bromobenzothiazole under Cu catalysis using modified Goldberg conditions by Alan et al. compounds ( 4) and ( 5) were tested against HCMV virus using modified ELISA technique.The compounds show potent antiviral activity against HCMV-Human Cytomegalovirus [13].

Anti-Dengue Virus Agents
The main cause of dengue is Flaviviridae virus with a carrier mosquito.This is a single-stranded RNA virus who infect almost 50 million people every year.Nowadays, no specific agents are available to treat dengue, but the effective molecules directly target the viral structural proteins [15].Various benzothiazole derivatives have been screened for their anti-dengue activity and some of them shows potent activity against dengue.Compound (7) was designed by Halim et al. for screening against dengue virus.Novel DENV NS-3 helicase inhibitor from zinc database was confirmed having antiviral activity against dengue virus by computational modelling techniques as well as in vitro and in vivo biological assays [16].

Anti-HIV Virus Agents
Human Immunodeficiency Virus is the most dangerous virus in the virus family.The resultant disorder is Auto Immunodeficiency Disorder.The current therapy against AIDS is based on six of categories drugs: nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs/NtRTIs); nonnucleoside reverse transcriptase inhibitors (NNRTIs); protease inhibitors (PIs); cell entry inhibitors [fusion inhibitors (FIs) and co-receptor inhibitors (CRIs)]; and integrase inhibitors (INIs) [17].HAART (Highly active anti-retroviral therapy) is beneficial, but it has a very high cost and severe toxicity.However, some benzothiazoles are analyzed for their anti-HIV activity, which may be at low cost and higher potency.

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Amine or amido linkage at the 2nd position of benzothiazole gives anticancer activity of compound.

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Second position of benzothiazole is active to attach substituents.

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Methyl group substitution at the 5th or 6th position of benzothiazole increases potency of antiviral compounds.

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Aryl moieties like pyrazole, pyridine, phenyl, imidazole, benzothiazole, thiazole, etc., at the 2nd position gives antiviral activity of the compound.Directly attached or through amine or amide linkage, aryl moiety at 2nd position of benzothiazole gives potent antiviral compounds.

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Fourth position of benzothiazole is also important for substitution in 2-aminobenzothiazoles to derive antiviral moieties.

Methods to Evaluate Antiviral Activity of Benzothiazole Derivatives
The antiviral activity of a novel synthetic compound can be measured by the four following methods in vitro.

Inhibition of Virus Induced Cytopathic Effect
A quantal assay can be used to determine effectiveness of those viruses that induce cytopathic effect, but do not cause plaque reduction.Here is the procedure to follow the assay method: Prepare a series of one fourth of cell cultures in culture tubes that are infected with a constant dose of 100 TCID50 (Median Tissue Culture Infectious Dose).The series should be kept at 37 • C for 1-2 h.Add antiviral agent with maintenance media to that series after completion of 1-2 h.Concentration range of antiviral agent should be from a minimal dose that does not show any antiviral activity to the maximum dose of inhibition.Virus induced cytopathic effects will be recorded each day until all the samples and blank cultures show cytopathic effects.ED50, that is, the 50% of the effective dose of antiviral drug, is the concentration that inhibits cytopathic effect in half of the culture tubes [21,22].

Plaque Reduction Assay
This method is applicable to all viruses that form plaque in suitable cells.This method is performed in corresponding cell monolayers infected with a constant concentration of virus depending on the size of monolayer.The monolayers are kept at 37 • C for 1-2 h and after those nutrients and antiviral agent with 1-2% methylcellulose is added.The infected culture is kept for rest for incubation for a respective period of time for different species of viruses.At the end of incubation period, all the cultures are stained for examination of plaque number.Plaque numbers in the culture without antiviral agent and with different concentration of antiviral agents are compared for results [23].

Virus Yield Reduction Assay
This method is used when long time determination of antiviral agent becomes necessary.Here, the cultures are infected with a fixed dose of virus and kept for 2 h at 37 • C then the unabsorbed viruses are removed by washing with Hanks' Balanced Salt Solution and the antiviral drug of various concentrations are added.After the incubation period, the cultures are tested for total virus yield.ED90 (Drug concentration 90% of the virus yield) in comparison with virus control is determined from dose response curves [24,25].

Assay Systems Based on Measurement of Specialized Functions and Viral Products
Certain viruses do not produce plaque or cytopathic effect, but perform some specific functions so we can use them as evaluation parameters.The specific functions could be hemagglutination, hemadsorption, extent of viral replication, reduction of virus specific polypeptides, synthesis of viral nucleic acids, reverse transcriptase activity, by dose response curve, etc., from these all measurements, one can derive ED50 value of antiviral agent and can compare it with virus control to evaluate antiviral activity of the compound [26,27].