Recent Developments on Novel Heterocyclic Compounds Thiadiazoles and Heterocyclic Compounds for COVID-19 Targets in Drug Discovery

This article mainly reviews the recent developments in the synthesis of novel 1,3,4-Thiadiazoles and also recent development of drug molecules for the treatment of COVID-19. The Thiadiazole heterocycle and its derivatives are the most important class of compounds among various heterocyclic compounds due to their wide range of biological activities and pharmaceutical importance. Thiadiazolemoeity exists as a principal structural pharmacopore in the combination of drug categories such as antimicrobial, anti-inﬂammatory, analgesic, antiepileptic, antiviral, antineoplastic, antitubercular agents. In this review article, we have attempted to show various synthetic procedures of thiadiazole derivatives along with their pharmacological activities and also the recent studies on Antiviral targets for the discovery of new drugs to combat Covid19 like viruses. Kothawade P, Bhalerao R, Kulkarni G et al. in 2017 synthesized the following1, 3, 4- thiadiazole derivatives and evaluated for its antidiabetic and antioxidant activity. The aryl carboxylic acid is treated with thiosemicarbazide in ethanol in the presence of catalytic quantity of sulfuric acid to obtain the respective thiadiazole.


INTRODUCTION
Heterocyclic compounds are the cyclic compounds possessing one or more atom(s) of other elements along with the carbon atoms in the ring system. Nitrogen, sulfur, oxygen are some of the most commonly used heteroatoms.
A number of thiadiazole-containing drugs are reported in the literature and marketed as shown below.

1,3,4-Thiadiazole and its Reactivity
Several researchers have used different synthetic strategies to obtain the desired target thiadiazole.
Some of `the synthetic methodologies are reviewed in this article and shown below [6-9].
Due to the low electron density at the carbon atoms in 1,3,4-thiadiazole such reactions as nitration, sulphonation, acetylation, halogenations normally do not take place.C-Acylation can be accomplished via rearrangement of intermediate Nacylthiadiazolium salts while radical halogenation can give chlorinated or brominated 2-halo-5substituted thiadiazoles.
Halo-substituted thiadiazoles are highly activated and react with a variety of nucleophiles. Carbonbased nucleophiles such as malonate have been used in the synthesis of 2-substituted thiadiazoles. When 2-chlorothiadiazole was treated with ethyl acetate in the presence of sodium hexamethyldisilazane (NaHMDS), the 5phenyl-1,3,4-thiadiazol-2-yl acetic ester was obtained as shown in the following reaction.
Reaction of various alkylating agents with unsubstituted and 5-substituted thiadiazoles yield 3-alkyl-1,3,4-thiadiazolium salts. These salts were deprotonated with ethoxide to produce carbenes which were then trapped with aromatic isocyanatesto yield spirocyclic compounds as mentioned below.
R--(CH2) 0 ; -(CH2) 4 ; -(CH2) 6 ; -(CH2) 8 ; -CH 2 C 6 H 4 CH 2 - Foroumadi et al synthesized a series of N-[5-(1methyl-5-nitro-2-imidazolyl)-1,3,4-thiadiazole-2yl] and N-[5-(nitrophenyl)-1,3,4-thiadiazole-2yl]piperazinylquinolone derivatives and evaluated for in vitro antibacterial activity against some gram-positive and Gram-negative bacteria. The antibacterial data revealed that all nitroimidazole derivatives showed interesting activity against tested gram-positive bacteria (MIC=0.008-0.03 µg/ml) while they did not show good activity against gram-negative organisms. All nitrophenyl analogues were inactive. Among all of the tested compounds (Ciprofloxacin derivative in nitroimidazole series) exhibited excellent activity and Trichophyton mentagrophytes. One compound with methyl substitution was found to be the most potent compound of the series by having MIC value of 6.25μg/ml on comparision with Ciprofloxacin and cyclopiroxolamine as standard drug. The good activity is attributed to the presence of pharmacologically active -CH3, OCH3, NH2, and 2,3-dichloro groups attached to phenyl group at position 6 of thethiadiazole ring. Results of antifungal screening showed that the presence of S-CH3 and S-C2H5 groups at position 4 of phenoxy group caused increased activity.

Fig. 1. SARS-Cov-2 Infection Cycle
Systemic peptide mapping on SARS-CoV allowed to discover the peptide CP-1, which binds with high affinity the heptad-repeat 2 region of S2 and interferes with the conformational changes leading to the 6-helix bundle formation and thus blocking Virus cell fusion process. Recently, a pan-corona virus fusion inhibitor lipopeptide (EK1C4) has been designed, targeting a structure formed by two specific regions in the S2 subunit.

Host-based Druggable Targets
In SARS-CoV-2, the angiotensin I converting enzyme 2 (ACE2) has been recently confirmed as the main virus receptor. Therefore, inhibition or modulation of ACE2 represents one of the proposed hostbased strategies for treatment of SARS CoV-2. ACE2 can be found in epithelial cells of lung, liver and testis. ACE2 was already known for mediating infection of the less pathogenic SARS-Co V, in particular, by recognizing the receptor-binding domain of the S protein (S1 RBD) with an helical region located in the peptidase domain.
The host cell surface transmembrane serine protease 2 (TMPRSS2) activates S protein present in the highly pathogenic human corona viruses SARS-CoV and MERS-CoV.1.
AAK1 and GAK inhibitors with therapeutic potential for COVID-19.
The two-pore channels (TPC1-3) regulate the conductance of sodium and calcium ions across cellular membranes. These are involved in the regulation of endolysosomal trafficking and Ebola entry in the host cell.

Clinical used drugs blockers of TPC2. A) Dopamine antagonists, B) SERMs
Therapeutic targeting of the innate immune response: It has been known that a cytokine storm results from an overreaction of the immune system in SARS and MERS patients. Clinical findings showed exuberant inflammatory responses during SARS-CoV-2 infection, further resulting in uncontrolled pulmonary inflammation, likely leading case fatality. The repurposing of hostbased therapeutics to control immune response may counterattack COVID-19 [38,39]. Some of these treatments include the use of recombinant IFN-and IFN-] as antiviralcytokines that inhibit viral replication in targeted cells. Some studies have reported that IFN-alone has more effect against SARS-CoV-2 than IFN-in vitro. In fact, combinations of IFN-and IFN-with other antivirals such as ribavirinand/ or lopinavir/ ritonavir (HIV treatment) have a synergistic effect in vitro and in animal models. In vitro and in vivo studies show the protective effect of Type III IFN 2L _3 treatment against SARS-CoV infection, that possibly in combination with IFN-I-could be an effective treatment for SARS-CoV-2. It has been shown recently that the potential benefits from low-dose corticosteroids treatment in SARS-CoV-2 critically ill patients. Another immunosuppressor, tocilizumab, an humanized monoclonal antibody against the interleukin IL-6 receptor reduces proinflammatory response in COVID-19 patients. Immunosuppressor treatments successfully used against other viruses, could be also used for COVID-19. These would include JAK inhibitors such as tofacitinib, baricitinib, and the recently approved upadacitinib, previously used in rheumatoid arthritisblinatumomab and HDAC inhibitors such as vorinostat or belinostat. Baricitinib is also a potent inhibitor of AAK1 and may also influence in decreasing viral infectivity, being a good candidate for clinical trials of COVID-19 [40][41][42][43][44][45][46].

Immunosuppression potential treatments. A) antiviral used in combination with IFN-α and IFN-β B) JAK inhibitors, C) HDAC inhibitors.
Anti-IFV-A agents with potential therapeutic effects on SARS-CoV-2.
Therapeutic options in clinical trials for COVID-19: Antivirals lopinavir/ritonavir have been recommended for clinical treatment for COVID-19. However, recent results from clinical trials do not confirm any benefit in hospitalized adult patients with severe COVID-19, but the combination of these two antivirals with interferon is found to be more promising for patients. Remdesivir is an adenosine analogue RdRp inhibitor with antiviral protection against SARS-CoV-2 and other viruses. Intravenous administration has been found to be efficacious in an American patient with COVID 19. Based on these results, Gilead Company provided the compound to China to perform the first clinical trials in SARS-CoV-2-infected individuals (Clinical trials NCT04257654/6). The FDA has approved the emergency use of this drug for COVID-19 patients with severe symptoms.Arbidol (umifenovir) is able to block viral fusion against influenza viruses. The antiviral activity of umifenovir against SARS-CoV-2 has been confirmed in vitro. First clinical data from patients with laboratory-confirmed COVID-19 points to a superior efficacy of umifenovir monotherapy to lopinavir/ ritonavit treatment. Also, other drugs used for influenza as Avigan (favipiravir) and Tamiflu (oseltamivir) have been used in patients.

CONCLUSIONS
In this report, we have described a brief review on the recent progress in the synthesis and biological activity studies of various Thiadiazole derivatives, an important class of heterocyclic compounds suitable as drug candidates for various drug targets.
Also focused in the present report, the structural druggable sites and determinants of antibodies efficacy against S Protein. The significance of structural spike(S) protein is remarkable due to its main role in the SARS-CoV-2 entry through the host receptor ACE2.Targetting human proteins is an excellent alternative and another promising way is the combination of available antiviral drugs acting through different targets in a multi-target strategy that has proven to increase efficacy during the recent current pandemic. It has been found during the recent pandemic that the repurposing of approved drugs is the only possibility to find a timely effective treatment. However, thecoronavirus outbreaks require thorough preparedness not only for the current situation but also for a future potential reemergence of novel coronaviruses. Hence it is of utmost importance to design drugs acting as pan-coronavirus antivirals or through a multitarget approach to avoid lack of effectiveness by viral mutation escape.

CONSENT
It is not applicable.

ETHICAL APPROVAL
It is not applicable.