Cyanorona-20: The first potent anti-SARS-CoV-2 agent.

Explicit hindrance and blockade of the viral RNA-dependent RNA polymerase (RdRp) of SARS-CoV-2 is considered one of the most promising and efficient approaches for developing highly potent remedies for COVID-19. However, almost all of the reported viral RdRp inhibitors (either repurposed or new antiviral drugs) lack specific selectivity against the novel coronaviral RdRp and still at a beginning phase of advancement. Herein, I discovered and introduce a new pyrazine derivative, (E)-N-(4-cyanobenzylidene)-6-fluoro-3-hydroxypyrazine-2-carboxamide (cyanorona-20), as the first potent SARS-CoV-2 RdRp inhibitor with very high selectivity (209- and 45-fold more potent than favipiravir and remdesivir, respectively). This promising selective specific anti-COVID-19 compound is also deemed to be the first distinctive derivative of favipiravir. Cyanorona-20, the unprecedented nucleoside/nucleotide analog, was designed, synthesized, characterized, computationally studied, and biologically evaluated for its anti-COVID-19 actions (through a precise in vitro anti-COVID-19 assay). The results of the biological assay displayed that cyanorona-20 surprisingly exhibited very high and largely significant anti-COVID-19 activities (anti-SARS-CoV-2 EC50 = 0.45 μM), and, in addition, it could be also a very promising guide and lead compound for the design and synthesis of new anti-SARS-CoV-2 and anti-COVID-19 agents through structural modifications and further computational studies. Further appraisal for the improvement of cyanorona-20 medication is a prerequisite requirement in the coming days. In a word, the ascent of the second member (cyanorona-20 "Corona Antidote") of the novel and promising class of anti-COVID-19 pyrazine derivatives would drastically make a medical uprising in the pharmacotherapeutic treatment regimens and protocols of the recently-emerged SARS-CoV-2 infection and its accompanying COVID-19.

Introduction

In December 2019, a novel coronavirus (2019-nCoV), officially known as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2; Fig. 1 ), suddenly appeared in Wuhan (Wuhan City, Hubei Province, China) [1]. In spite of strict and rigorous containment measures, the transmission of this mysterious fatal viral microbe is ongoing resulting in the spread of the virus characteristic infection or disease, coronavirus disease 2019 (COVID-19), with its major signs and symptoms concentrated in the respiratory system of human (i.e., characterized by pulmonary infection in humans) [1], [2]. This outbreak of 2019-nCoV infection in China has spread across our Earth planet [1], [2]. The extreme efforts of multinational pharmaceutical companies, drug discovery research centers, and other related places have since focused on the search for effective medications and therapies able to destroy the virus, stop the viral replication, and/or counter the most severe and fatal effects of the disease [2], [3]. No specific antiviral agents have been officially and internationally approved for the successful treatment of COVID-19 to date [3].

Fig. 1

A diagrammatic representation of SARS-CoV-2 morphology and structure.

In this current status of the absence of any known efficient anti-COVID-19 therapy and, in addition, because of the intolerable situation of this public-health emergency of the COVID-19 pandemic, many researchers have proposed the repurposing of the known potent antiinfluenza drug favipiravir (it is a purine nucleic acid analog, which was officially approved for medical use in Japan since 2014; Fig. 2 ) to fight the novel COVID-19 [4], [5]. Preceding studies and investigations have identified the viral RNA-dependent RNA polymerase (RdRp) as a very potential drug target in COVID-19 therapy due to its highly significant and pivotal role in SARS-CoV-2 replication and transcription (i.e., in the virus life cycle), and, furthermore, this enzyme has a strategic feature and advantage of being absolutely absent in the coronavirus-uninfected human cells (i.e., viral RdRp is a drug target of extremely selective toxicity for SARS-CoV-2 particles) [5], [6], [7]. Favipiravir, as an antiviral agent, acts by selectively inhibiting viral RdRp (some other researches suggest that favipiravir, in addition of being a potent RdRp inhibitor, induces lethal RNA transversion mutations, thus producing a nonviable viral phenotype) [4], [8]. Favipiravir is a prodrug that is metabolized to its active form, favipiravir-ribofuranosyl-5′-triphosphate (favipiravir-RTP), mainly by the enzyme human hypoxanthine-guanine phosphoribosyltransferase (HGPRT) so as to stop the replication processes of the viral RNA genome (i.e., to inhibit the reproduction of the virus) [9]. However, many limitations have restricted the successful complete use of favipiravir as an efficient and potent anti-COVID-19 agent to date, e.g., reliable data regarding the in vitro SARS-CoV-2 inhibition are still unavailable [4], [5]; broad documented data regarding the in vivo SARS-CoV-2 inhibition and efficacy in preclinical animal studies are still unavailable [5], [10]; the few available animal experiments of favipiravir show the tendency and potential for teratogenic effects (there is a strong evidence that its use during pregnancy may cause harm to the fetus) [4]; favipiravir has not been shown to be effective in primary human airway cells [11]; lack of additional virus-toxic functional groups in favipiravir structure to augment its antiviral mechanism of action against the lethal and resistant SARS-CoV-2 [5], [12]; lipophilic/hydrophilic properties of favipiravir are not extremely balanced to achieve maximal bioavailability (specially in lungs) in humans suffering from that fatal COVID-19 [13]; expected binding affinities of active favipiravir-RTP molecule (as a viral RdRp inhibitor) to SARS-CoV-2 RdRp enzyme protein are not that great [12]; all data concerning its clinical use in human (mainly in Japan) are still mysterious and not that clear [4], [10], [13]; favipiravir as an anti-COVID-19 has not been used that much outside Japan till now (just in very few countries like China and Italy) [4]; favipiravir as an anti-COVID-19 drug is almost used only locally (off-label use) in Japan as its medical use as anti-COVID-19 has not been officially and internationally approved till now (the drug has not been approved as anti-COVID-19 by the WHO and the FDA, and it has not any complete or overall international scientific and medical consensus till now) [4], [13]; and many favipiravir published articles and papers have considerably irreproducible data and results [5], [10].

Fig. 2

Chemical structures of favipiravir (6-fluoro-3-hydroxypyrazine-2-carboxamide) and the newly-designed target compound cyanorona-20 ((E)-N-(4-cyanobenzylidene)-6-fluoro-3-hydroxypyrazine-2-carboxamide).

All these disadvantages and limitations of favipiravir anti-COVID-19 usage together with some importantly-needed additional properties in the favipiravir chemical structure to be a highly effective potent anti-SARS-CoV-2 drug (e.g., inhibiting and acting on many or more than one SARS-CoV-2 target; augmenting all the other expected modes of action on all the other possible viral protein targets, i.e., other than the main enzyme SARS-CoV-2 RdRp; overcoming the possibly-emerged resistance against favipiravir from the newly-mutated/-created coronaviruses; enhancing the distribution and bioavailability of the drug in the human respiratory system/lungs and the other nearer and target systems/organs; and achieving faster onset of action against this detrimental virus) have motivated me to start searching for a well-designed alternative or derivative of favipiravir to block SARS-CoV-2 RdRp very efficiently. The major aim is to add moieties, to favipiravir chemical structure, that have both polar (or hydrophilic) and nonpolar (or lipophilic) parts, viral replication-inhibiting activities, structure-stabilizing properties (mainly through resonance), biological compatibility with the human body living systems, and the smallest possible molecular volume/weight to fulfill all the previously-mentioned required characteristics together with solving the previously-mentioned issues and problems associated with favipiravir use. After extensive computational molecular modeling studies (including compound libraries screening and SARS-CoV-2 RdRp docking studies) together with my hypothetical intellectual suggestions, (E)-N-(4-cyanobenzylidene)-6-fluoro-3-hydroxypyrazine-2-carboxamide (cyanorona-20; Fig. 2), a new derivative of favipiravir, has been selected to perform this urgently-needed mission to save our planet from this destroying novel single-stranded RNA virus, SARS-CoV-2.

Cyanorona-20 is the 4-cyanobenzylidene derivative of favipiravir at the amino group, and is expected to be a prodrug that is metabolized inside the human body to its logically-predicted active nucleotide triphosphate form, cyanorona-20-ribofuranosyl-5′-triphosphate (cyanorona-20-RTP). The structural features of this newly-synthesized compound are optimal, as the structure of cyanorona-20 molecule totally obeys Lipinski's rule of five (Ro5), has extremely high druglikeness score, and has almost all the structural requirements (most of them have been mentioned above) for a compound to be a highly effective and potent anti-COVID-19 agent (i.e., SARS-CoV-2 RdRp inhibitor) [14], [15], [16]. The mechanism of interaction of cyanorona-20-RTP with the large molecule of SARS-CoV-2 RdRp has not been fully elucidated, but I hypothesize that cyanorona-20 in its in vivo active form may act through, at least, six different and synergistic major modes of action (i.e., through a very potent anti-COVID-19 multiaction or multiple activity; Fig. 3 ) [9], [11], [17], [18], [19], [20], as it may be misincorporated in a nascent SARS-CoV-2 RNA (thus preventing RNA strand elongation and viral proliferation), it may evade RNA proofreading by viral exoribonuclease (ExoN; thus causing a decrease in SARS-CoV-2 RNA production), it may competitively bind to conserved polymerase domains (thus preventing incorporation of mainly purine nucleotides for SARS-CoV-2 RNA replication and transcription), it may cause the SARS-CoV-2 RdRp to pause, it may induce an irreversible chain termination in the growing SARS-CoV-2 RNA, or it may induce lethal mutagenesis (thus, mainly, making the virus less effective and reducing its titer “viral titer”) during SARS-CoV-2 infection.

Fig. 3

A diagrammatic representation of cyanorona-20 major mechanism of anti-COVID-19 action.

Cyanorona-20, to the best of my knowledge (until the date of submitting this discovery article for publication), is the first bioactive derivative of favipiravir and also the first selective and potent direct-acting anti-COVID-19 agent. Cyanorona-20 fulfills and satisfies all the needed requirements to be an ideal anti-COVID-19 drug (i.e., to be a better antiviral and anti-COVID-19 drug than its parent compound favipiravir). For example, cyanorona-20 molecule has a virus-toxic cyano group (it may be also called a SARS-CoV-2 RdRp-destabilizing moiety, as it chemically causes a major steric clash with the SARS-CoV-2 RdRp molecule at some residues, this greatly helps in RdRp preliminary partial blockade and results in delayed chain termination in RNA synthesis which gives cyanorona-20 an extrapotency against the major resistance mechanisms that might be emerged by SARS-CoV-2 against favipiravir and most other classical potent antiviral nucleos(t)ide analogs [20]) (strong aliphatic polar group) and a resonance-stabilized benzene ring (in the benzylidene group) (a strong aromatic lipophilic moiety), both groups with a one-carbon-atom linker form a highly stable 4-cyanobenzylidene moiety (a SARS-CoV-2-toxic moiety which is extremely stabilized through strong resonance and inductive effects) which is not present in the parent favipiravir molecule, and adds an exceptional and excellent balanced lipophilic/hydrophilic properties along with electronic extrastability to the molecule (this makes the molecule more bioavailable and more biocompatible).

One of the interesting features of cyanorona-20 structure is its explicit ability to act as a multizincophore (zinc ionophores or zinc ion carriers, e.g., chloroquine [21], hydroxychloroquine [21], quercetin [22], epigallocatechin gallate [22], CoViTris2020 [23], and Taroxaz-104 [24], transport extracellular Zn2+ ions across the hydrophobic cell membranes to enter the living cell, and have been studied mainly for their antiviral activities, as they have been shown to effectively inhibit the replication of various viruses in vitro [25]). Zn2+ inhibits coronavirus RdRp activity (i.e., inhibits coronaviral replication and transcription) in vitro (Zn2+ ion is the only known elemental cofactor and ligand present in the crystal structure of SARS-CoV-2 RdRp and, thus, it has an extremely important role in controlling the activity of this COVID-19 RNA-synthesizing enzymatic machine) and, therefore, zinc ionophores have been shown to successfully block the replication process of coronaviruses intracellularly in cell cultures [26], [27], [28]. Based on this fact, inhibiting SARS-CoV-2 RdRp and its associated replication processes is one of the best strategies to inhibit and kill SARS-CoV-2 through designing and synthesizing (or searching for) molecules that have good zincophoric properties, and as much the molecule has more zincophoric centers in its structure as higher its zincophoric and related SARS-CoV-2-killing activities are. Cyanorona-20 is supposed, theoretically, to have about six zincophoric moieties or centers (four active nitrogen atoms and two active oxygen atoms), making it an ideal candidate to act as a potent zincophore (a multizincophoric anti-COVID-19 compound).

In this research paper (along with its Supporting Information file), I report the design, synthesis, characterization, computational studies, and anti-COVID-19 biological activities of this novel compound named cyanorona-20 (i.e., I report the discovery of cyanorona-20, the first bioactive derivative of favipiravir and the first selective/specific and potent anti-COVID-19 drug).

Results and discussion

Synthesis, chemistry, and stability of Cyanorona-20

Cyanorona-20 was successfully synthesized, as shown in Fig. 4 , in very good yields from its parent favipiravir via direct condensation with 4-cyanobenzaldehyde (equimolar amounts) in the presence of the strong dehydrating agent glacial acetic acid (gla. AcOH). The reaction could proceed either by conventional heating (with 85% yield) or under microwave irradiation (MWI) (with 96% yield). The structure of cyanorona-20 was characterized, elucidated, and confirmed through spectroscopic analyses (IR, 1H NMR, 13C NMR, and mass spectrometry) and microanalyses (elemental analysis for C, H, and N atoms). Spectral data and elemental analyses of the sample of this product were in an ideal and full agreement with the proposed structure of cyanorona-20 as presented in the Supporting Information file.

Fig. 4

Schematic representation of the conventional and microwave-assisted synthetic pathways of cyanorona-20 from favipiravir.

Cyanorona-20, quite like its parent compound favipiravir, is expected to be a tautomeric molecule [29]. According to the computational simulations studies, in almost all cases the molecule favors the enol-like tautomeric structure (the predominant form), which is substantially much more stable in the aqueous media as compared to the keto-like tautomeric structure as shown in Fig. 5 .

Fig. 5

Schematic representation of the tautomeric forms of cyanorona-20 molecule in aqueous solutions.

The Schiff base-like structure of cyanorona-20 molecule rendered me to accurately and extensively investigate and examine the stability behavior of this new compound (specially in aqueous media similar to those of the human body) using all the possible analytical and physicochemical methods. The final results of aqueous dissolution testing were excellent and sufficient as they showed that only less than 5% (as a maximum) of total cyanorona-20 amount undergoing hydrolysis to minor products and impurities after the maximum period of 3 months, and less than 50% of this 5% amount (i.e., less than 2.5% of total cyanorona-20 amount) undergoing hydrolysis to the parent favipiravir during the same 3-month period (see the representative HPLC chart, Chart S4, in the Supporting Information file), thus proving the good and adequate practical stability of cyanorona-20.

Antiviral anti-COVID-19 biological activity (in vitro assay) of Cyanorona-20

The results demonstrated in Table 1 explicitly revealed the extremely higher and surprising anti-COVID-19 effectiveness of the new compound cyanorona-20 (the most potent anti-SARS-CoV-2 compound) relative to that of each of the reference drugs. Among the five tested compounds, four compounds (cyanorona-20, remdesivir, HCl-arbidol-H2O, and favipiravir, respectively) were found to inhibit SARS-CoV-2 replication in Vero E6 cells with EC50 under 100 μM, while hydroxychloroquine sulfate was found to do that above 100 μM. Surprisingly, cyanorona-20 (EC50 = 0.45 μM) was found to be about 209 and 45 times more potent than favipiravir (EC50 = 94.09 μM) and remdesivir (EC50 = 20.17 μM), respectively, in anti-SARS-CoV-2 activity (in vitro). According to the assay, cyanorona-20 is expected to have very high clinical selectivity index (SI; SI = CC50/EC50) and safety margin (CC50 is much >100 μM), while, on the other hand, hydroxychloroquine sulfate is expected to have very narrow clinical therapeutic index (EC50 is just above 100 μM, CC50 = 93.06 μM). Cyanorona-20 is also having amazingly very small values of the concentration that causes 100% inhibition of the SARS-CoV-2 cytopathic effects in vitro (cyanorona-20 has the best CPEIC100 value, among all the five compounds tested, of 1.40 μM) and of the concentration that is required for 50% reduction in the number of SARS-CoV-2 RNA copies in vitro (cyanorona-20 has the best EC50 value, among all the five compounds tested, of 0.48 μM).

Table 1

Anti-COVID-19/antiviral activities (along with human/mammalian cells toxicities) of cyanorona-20 and the four reference drugs (favipiravir, remdesivir, HCl-arbidol-H2O, and hydroxychloroquine sulfate) against SARS-CoV-2 in Vero E6 cells.

Classification	Compound Name	CC50a (μM)	Inhibition of SARS-CoV-2 in vitro (μM)
			100% CPE Inhibitory Concentration (CPEIC100)b	50% Reduction in Infectious Virus (EC50)c	50% Reduction in Viral RNA Copy (EC50)d
Target Compound	Cyanorona-20	>100	1.40	0.45	0.48
Reference Compounds	Favipiravir	>100	98.82	94.09	>100
	Remdesivir	>100	22.50	20.17	23.88
	HCl-Arbidol-H2O	>100	81.52	64.20	68.42
	Hydroxychloroquine Sulfate	93.06	>100	>100	>100

CC50 or 50% cytotoxic concentration is the concentration of the tested compound that kills half the cells in an uninfected cell culture. CC50 was determined with serially-diluted compounds in Vero E6 cells at 48 h postincubation using CellTiter-Glow Luminescent Cell Viability Assay (Promega).

CPEIC100 or 100% CPE inhibitory concentration is the lowest concentration of the tested compound that causes 100% inhibition of the cytopathic effects (CPE) of SARS-CoV-2 virus in Vero E6 cells under increasing concentrations of the tested compound at 48 h postinfection. Compounds were serially 2-fold or 4-fold diluted from 100 μM concentration.

EC50 or 50% effective concentration is the concentration of the tested compound that is required for 50% reduction in infectious SARS-CoV-2 virus particles in vitro. EC50 is determined by infectious virus yield in culture supernatant at 48 h postinfection (log10 TCID50/mL).

EC50 or 50% effective concentration is the concentration of the tested compound that is required for 50% reduction in SARS-CoV-2 viral RNA copies in vitro. EC50 is determined by viral RNA copies number in culture supernatant at 48 h postinfection (log10 RNA copies/mL).

We should put into account that the three nucleoside/nucleotide analogs, cyanorona-20 (guanine analog), favipiravir (guanine analog), and remdesivir (adenosine analog), mainly undergo prior intracellular metabolic activation into their triphosphate forms by the host cellular enzymes (chiefly nucleoside kinases), which may differ among several cell types, thus evaluation of the actions of nucleos(t)ide analogs in primary human airway epithelial cells would undoubtedly facilitate the interpretation of the results. The metabolic activation would surely add additional anti-COVID-19 activities to the three drugs, and it would also successfully increase the clinical effectiveness of the three drugs. Interestingly, the four reference drugs (favipiravir, remdesivir, HCl-arbidol-H2O, and hydroxychloroquine sulfate) are currently undergoing extensive clinical trials, as anti-SARS-CoV-2/anti-COVID-19 agents, worldwide. The very high value of CC50 of cyanorona-20 indicates that cyanorona-20 would be expectedly well tolerated in the human body. The extremely minute value of anti-SARS-CoV-2 EC50 and the very high value of mammalian cells CC50 (i.e., the fantastically desirable and promising high value of SI) of cyanorona-20 indicate that this compound clearly favors the resistant RNA virus over DNA virus and mammalian cells, and this, in turn, expresses and proves the previously-expected selective specificity of this compound as anti-COVID-19 drug “Corona Antidote or Corona Killer” (see Introduction part). Using a combination formula (a mixture) of cyanorona-20 and remdesivir is a proposed choice, as it may have an exceptional and astonishing combinational synergistic anti-COVID-19 effect in further assays (in vivo) and clinical trials. Almost all the practical results concluded, here, in the antiviral anti-COVID-19 biological evaluation are consistent with the previous theoretical results extracted from the computational molecular and pharmacological predictions for the new compound cyanorona-20 and its four reference compounds.

Conclusions and future directions

Specific potent blockade of the novel SARS-CoV-2 RdRp has been validated as the most effective viable approach for targeted COVID-19 therapy and, therefore, all my efforts in our laboratories in 2020 were focused on designing and creating new drugs (specially, new potent derivatives of old drugs repurposed against COVID-19) targeting and efficiently inhibiting this important active viral polymerase. These efforts led to the discovery of an extremely promising selective specific and potent SARS-CoV-2 RdRp inhibitor, cyanorona-20 ((E)-N-(4-cyanobenzylidene)-6-fluoro-3-hydroxypyrazine-2-carboxamide), which dramatically inhibited SARS-CoV-2 RdRp with EC50 values of 0.45 and 0.48 μM (relatively and according to the used assay, cyanorona-20 is the most potent SARS-CoV-2 RdRp inhibitor till now), and interestingly presented about 209- and 45-fold anti-SARS-CoV-2 RdRp selectivity/potency more than favipiravir and remdesivir, respectively. On the other hand, the design and discovery of cyanorona-20 by the structural modification at the active amide moiety of the antiinfluenza favipiravir molecule opens the door for us to establish the first class of anti-COVID-19 agents (of the type “nucleoside analogs”), which will specifically comprise a series of pyrazine derivatives (beginning with the first two effective members, favipiravir and cyanorona-20) [30]. Prior extensive preparatory computational molecular calculations and studies showed that cyanorona-20 has the most ideal and the best balanced values of the pharmacokinetic and druglikeness descriptors (among the five evaluated drugs, cyanorona-20 and the four reference drugs, favipiravir, remdesivir, arbidol, and hydroxychloroquine) required to be an effectively potent anti-COVID-19 drug inside the human body [31], [32]. Computational molecular modeling analysis of the best inhibitory binding mode of the expected active metabolite of cyanorona-20 inside the human cell, cyanorona-20-RTP, showed that the added 4-cyanobenzylidene moiety interestingly increases the blocking affinity and potency at active and/or allosteric sites of the SARS-CoV-2 RdRp (binding free energy = −10.50 kcal/mol) when compared to those of the active metabolite of its parent favipiravir inside the human cell (which lacks this 4-cyanobenzylidene moiety), favipiravir-RTP (binding free energy = −8.40 kcal/mol). Surprisingly, cyanorona-20 and its active metabolite surpassed the four moderately to highly potent reference drugs and their active metabolites, respectively, in the values of almost all compared theoretical and practical anti-COVID-19 parameters, scores, and activities. If cyanorona-20 compound successfully passes the in vivo bioassays and preclinical/clinical trials with effectively significant results as anti-COVID-19 agent, a combination therapy (e.g., as an oral tablet or a nasal/oral prophylactic gel) with a second highly potent antiviral drug, such as remdesivir, will be a recommended possible choice for the COVID-19 treatment in the near future. In brief summary, in this new research paper, I discovered and reported the first derivative of favipiravir, cyanorona-20 “Corona Killer or Coronacide”, as a fantastic hit molecule (it can also be considered as the first potent anticoronaviral hybrid molecule of both potent antivirals favipiravir and remdesivir) with selective specific and potent successful inhibition against SARS-CoV-2 RdRp and, thus, the first known promising under-investigation candidate drug for the treatment of COVID-19.

Declaration of Competing Interest

The author declares that he has no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.