4’-fluorouridine and its derivatives as potential COVID-19 oral drugs: a review

Introduction

Coronavirus disease 2019 (COVID-19) is a respiratory disease that emerged in December 2019. The World Health Organization (WHO) declared COVID-19 a pandemic, that is still ongoing, caused by the novel, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).¹ Coronaviruses are enclosed, positive-single-stranded large RNA viruses that can infect humans, as well as a variety of other animals. Tyrell and Bynoe,² who cultured viruses from patients with common colds, initially characterized coronaviruses in 1966. Coronaviruses attack the host respiratory system via the spike (S) glycoprotein, which has a receptor-binding domain (RBD) that mediates direct contact with a cellular receptor, angiotensin-converting enzyme 2 (ACE2). The S glycoprotein, which consists of two subunits, S1 and S2, is an inactive precursor that must be cleaved to mediate membrane fusion.³ Accordingly, the S1/S2 polybasic cleavage site is proteolytically cleaved by cellular cathepsin L and transmembrane protease serine 2 (TMPRSS2)^4,5 to facilitate viral entry.

COVID-19 was initially detected as a zoonotically transmitted disease in Wuhan, China and has since spread throughout the world and caused high burdens in society.^6,7 The disease has a broad spectrum of symptoms, ranging from mild and common symptoms, such as fever and cough, to the more severe presentations of pneumonia and even organ failure.⁸ Based on WHO COVID-19 Dashboard, more than 400 million cases of COVID-19 have been reported as of February 10, 2022, with over 5.7 million deaths.⁹ The causative agent, SARS-CoV-2, is easily transmitted through the air via respiratory droplets, direct contact with contaminated surfaces, and fecal–oral transmission,¹⁰ making it spread rapidly with a reproduction number (R0) of 2.69 and a case fatality rate (CFR) of 2.67.¹¹ Along with its rapid spread, it has a high mutation rate,^12,13 making the fight against this virus a race between drugs and vaccines and the virus itself.¹⁴

The repurposing of existing drugs is currently being extensively explored, as these drugs have the potential to significantly accelerate the prevention of viral spread and transmission, as well as the development of new COVID-19 therapies.^15,16 According to the results of the MOVe-OUT phase 3 trial, molnupiravir, a potent ribonucleoside analog that inhibits viral replication, would be the first oral treatment for COVID-19 if approved.¹⁷ This drug, which was previously known to have broad anti-influenza virus activity, has been shown to reduce the risk of hospitalization or death in COVID-19 patients.¹⁸ Molnupiravir exerts its effect by inserting itself into the viral genome during its synthesis by RNA-dependent RNA polymerase (RdRp).¹⁹ As a result, there are some concerns regarding its mechanism of action and its possible detrimental effects on human cells, which must be addressed prior to the administration of this drug to patients. It is considered that this drug may not only be cytotoxic to mammalian cells but may also be mutagenic.²⁰ Additionally, the possibility of viral genome mutations caused by antiviral drugs must also be given due consideration.

Amid concerns about the long-term effects of molnupiravir on host cells and possible viral mutations, there is rising hope in the emergence of 4′-fluorouridine (4′-FIU), whose mechanism of action involves the inhibition of RdRp activity. In one study, this drug was reported to inhibit the ability of respiratory syncytial viruses (RSV) to replicate in cells without impairing cell metabolism. Additionally, it inhibited SARS-CoV-2 and the agents of several other pandemic-potential viral infections, such as avian influenza.²¹ The promising benefits of 4′-FlU (or EIDD-02749) and its derivatives are highlighted in this review.

The SARS-CoV-2 genome

The genome of SARS-CoV-2 comprises a 30,000 nucleotide-long single-stranded positive-sense RNA, similar to the genomes of other human coronaviruses.^22,23 The genome is packed with viral nucleocapsid (N) proteins and surrounded by an envelope membrane composed of lipids and viral proteins: S (spike), M (membrane), and E (envelope) proteins.^24–26 The viral genome encodes a 7096-residue polyprotein with a variety of structural and non-structural proteins (NSPs). The large portion of the genomic content is composed of sequences encoding two non-structural proteins, followed by ORF1a, and ORF1ab, and subsequently the sequences encoding the structural proteins. ORFs1a and 1b encode the polyproteins pp1a and pp1ab, respectively, with the polyprotein pp1ab being encoded by the ribosomal frameshift mechanism of gene 1b. These polyproteins are then digested by virally encoded proteinases, which results in the production of 16 proteins that are highly conserved across all coronaviruses of the same family.²⁷ The viral genome has a guanine-cytosine (GC) content of 38%²⁸ and 11 protein-coding genes, with 12 expressed proteins. The hemagglutinin-esterase gene, which is found in several beta-coronaviruses, is absent in SARS-CoV-2.²³ The genomic arrangement of the ORFs is strikingly similar to that observed in SARS-CoV and Middle East respiratory syndrome coronavirus (MERS-CoV).^29–32 The ORFs are organized into replicase and protease genes, as well as genes encoding the important S, E, M, and N proteins, which are present in a regular 5′–3′ order and are important drug/vaccine targets. The products of these genes facilitate viral entry, fusion, and survival in host cells.^32,33 The SARS-CoV-2 genome, similar to the genomes of other human coronaviruses such as SARS-CoV and MERS-CoV, has a m7G-cap structure, with m7GpppA1, at the 5′ end, and an approximately 30–60 nucleotide long poly A tail at the 3′ end.³⁴

Based on the results of sequence alignment and phylogenetic analysis, SARS-CoV-2 is regarded as the newest member of the lineage B of the genus Betacoronavirus (β-CoV) in the family Coronaviridae of the order Nidovirales.^35,36 Due to differing epidemiological dynamics, SARS-CoV-2 is relatively more infectious than SARS-CoV and MERS-CoV. Other mammalian species may have served as intermediate or amplifying hosts, with eventual ecological isolation resulting in the acquisition of some or all of the mutations required for efficient human transmission.³⁷

Since its discovery, the SARS-CoV-2 genome has shown remarkable genomic variability.³⁸ In a recent study, 48,635 complete genomes of SARS-CoV-2 were compared with the reference Wuhan genome NC_045512.2 and were evaluated to exhibit an average of 7.23 mutations per sample.³⁹ The study demonstrated single nucleotide transitions to be the most common type of mutations prevalent in SARS-CoV-2 isolates globally.³⁹ Another study also found that 5775 distinct genomic variants of SARS-CoV-2 emerged compared to Wuhan genome NC_045512.2 with multiple mutations including in-frame deletions, non-coding deletions and insertions, and frameshift deletions and insertions.⁴⁰ The host RNA editing machinery, in which adenosine deaminase acts on RNA (ADAR deaminase (APOBEC) targets dsRNA for adenosine deamination to inosines (A-to-I) and apolipoprotein B mRNA editing enzyme catalytic subunit) deaminates cytosines to uracils (C-to-U) on ssRNA or ssDNA, and may play a role in the induction of the SARS-CoV-2 genome mutations and modifications found during viral infection.²²

While mutations in the viral genome are natural, during this pandemic, mutations in the SARS-CoV-2 genome have been a scourge. This is because of the numerous mutations that have enabled this virus to repeatedly evade antibody neutralization.^41,42 This is especially true when there are multiple mutations in the gene that encodes the S glycoprotein.⁴³ Special attention should be given to mutations in genes encoding non-structural proteins, as they may impair the effectiveness of antiviral drugs. Even though some variations are rare in specific regions, the C14408T and A23403G mutations on the Nsp12 and S proteins, respectively, have been found to be the most common worldwide and both induce missense mutations.⁴⁴ Accordingly, emerging variants of SARS-CoV-2 are very influential and pose a significant threat. Even if individuals have been exposed to SARS-CoV-2 previously, they can still become infected with other variants of SARS-CoV-2 and contract COVID-19, indicating that prior exposure to the virus does not guarantee complete immunity against the disease.⁴⁵

RdRp of SARS-CoV-2 as a potential drug target

The structural and non-structural proteins of SARS-CoV-2 are all potential drug/vaccine targets.²⁶ The main protease (M^pro), a key enzyme for the viral polyprotein proteolytic process, and RdRp, a key viral enzyme that is critical in mediating viral replication and transcription, are two attractive drug targets for SARS-CoV-2.^27,46,47

RdRp (Figure 1) catalyzes the replication of RNA from an RNA template. It catalyzes the production of complementary RNA strands for a given RNA template. It differs from the traditional DNA-dependent RNA polymerases, which are found in all species and catalyze the transcription of RNA from a DNA template.⁴⁸ RdRp is encoded in the genomes of the majority of RNA viruses that do not have a “DNA stage,” including SARS-CoV-2.^49,50 Structural modeling of RdRps from positive-sense RNA viruses (HCV and SARS-CoV-2) and negative-sense RNA viruses (i.e., influenza) have confirmed the differences between the two types of RdRps.⁵¹

Figure 1. The RNA-dependent RNA polymerase (RdRP) of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) modelled with MODELLER homology modelling and visualized in PyMol version 2.5.

Protein Data Bank (PDB) ID of RdRP: 7BV2.

RdRps are not required for the survival of eukaryotic cells. As a result, they have the potential to be used as therapeutic targets against viral infections. The replication of RNA from a template strand is prevented when RdRp is inhibited, although DNA-dependent RNA polymerases continue to function. While the RdRp of SARS-CoV-2 differs from the RdRp of SARS-CoV and MERS-COV²⁷ the RdRps of all three coronaviruses are highly conserved, suggesting that RdRp is a promising broad-spectrum antiviral target for coronaviruses.^52,53

4′-fluorouridine (4′-FIU) as a SARS-CoV-2 antiviral drug

Several vaccines have been developed to combat COVID-19, and worldwide vaccination is ongoing.⁵⁴ Alongside, several drugs against SARS-CoV-2 are also being investigated. Dexamethasone has been shown to significantly reduce the mortality rate of COVID-19 patients.⁵⁵ Remdesivir, an RdRp inhibitor,⁵⁶ has also been demonstrated to shorten recovery time in COVID-19 patients⁵⁷ and is currently licensed for clinical usage.⁵⁸ RdRp can use GS-441524 triphosphate (an active metabolite of remdesivir) as a substrate, thus causing premature chain termination and inhibition of viral replication. Remdesivir is a nucleotide analog that prevents further elongation of the RNA polynucleotide by covalently binding to and interfering with the termination of the nascent RNA either early or late, or by inhibiting further elongation of the RNA polynucleotide. As a result of the premature termination, nonfunctional RNA is produced, which is then degraded via the normal cellular functions of the body.^57,59

Apart from dexamethasone and remdesivir, molnupiravir has been reported to reduce the risk of hospitalization or mortality in at-risk patients by 50%.¹⁸ Molnupiravir is a highly effective ribonucleoside analog that prevents viral replication. 4′-FIU is another nucleotide analog that has been tested to have antiviral activity against SARS-CoV-2 by causing the stalling of RdRp.²¹ This compound, as well as its derivatives, has been shown to have antiviral activity against several viruses.^21,60,61 A novel oral drug with a mechanism of action distinct from that of molnupiravir is required, and this study aims to investigate 4′-FIU and its derivatives to potentially develop an oral drug that can be used in conjunction with or even as a replacement for molnupiravir.

4′-fluorouridine (4′-FIU) in general

Uridine, a ribonucleoside that forms nucleic acids, is a glycosylated pyrimidine analog of uracil that is attached by a –N1-glycosidic bond to a ribofuranose moiety.⁶² Uridine replaces thymidine in the RNA. Uridine analogs are often developed as antiviral drugs, especially for combating RNA viruses. The general hypothesis is that these uridine analogs interfere with or inhibit the replication of RNA viruses in host cells. One such example is the use of molnupiravir. It is a nucleoside analog that sometimes mimics the nucleoside cytidine and sometimes uridine and is currently awaiting approval as an oral COVID-19 treatment.⁶³ The possibility of 4′-fluorouridine being a new oral drug for COVID-19 with a different mechanism of action than molnupiravir is being investigated. It is a uridine nucleoside analog that contains uridine and fluorine. The 4′-FIU structure and its similarity to uridine described in Figure 2 were created using ChemDraw.⁶⁴ The antiviral activity of 4′-FlU has led to the suggestion that this is due to the small atomic radius and strong stereo-electronic effect of fluorine. This configuration may have an effect on the conformational flexibility of the backbone, thereby increasing metabolic stability.^21,65

Figure 2. Chemical structures of 4′-fluorouridine, uridine, 4′-fluorouridine-5′-monophosphate and 4′-fuorouridine-5′-Isobutyl ester.

Fluorouridine PubChem ID: 90233147 and uridine PubChem ID: 6029. The 4′-fluorouridine-5′-monophosphate and 4′-fuorouridine-5′-Isobutyl ester were created according to the 4′-fluorouridine structure (Created in ChemDraw Professional 17.0).

However, several studies have stated that 4′-FIU is quite unstable^66,67 compared to its derivatives.⁶⁷ 4′-FIU derivatives have been proven to have high enough stability for biological studies.⁶⁷ Examples of 4′-FIU derivatives are 4′-fluorouridine-5-monophosphate and 4′-fluorouridine 5′-O-triphospate.^61,66 Two patents, WO2019173602A1 and WO2021137913A2, showed that 4′-FIU, 4′-fluorouridine 5′-triphosphate (EIDD-02991) and 4′-fluorouridine-5′-isobutyl ester (EIDD-02947) (Figure 2) displayed antiviral activity against positive-sense RNA viruses and negative-sense RNA viruses by blocking the activity of viral RdRp.

Evidence from in vitro studies

A study revealed that 4′-FIU was active at submicromolar concentrations against paramyxovirus, rhabdovirus, measles virus (MeV), human parainfluenza virus type 3 (HPIV3), Sendai virus (SeV), vesicular stomatitis virus (VSV), and rabies virus (RabV).²¹ The compound rapidly accumulates intracellularly in human airway epithelial (HAE) cells, resulting in 3.42 nmol/million cells after 1 h of exposure. Additionally, the metabolic activities of tested cell lines such as Hep-2, Madin-Darby canine kidney (MDCK) cells, hamster kidney fibroblasts (BHK-T7), and human bronchial epithelium cells (BEAS-2B) when exposed with 500 μM 4′-FIU remains unaffected, implying that the antiviral effect is due to cytotoxicity.²¹

According to patent WO2019173602A1, 4′-FIU has a half maximal effective concentration (EC₅₀) of 1.86 μM in the Vero E6 cell line, cytotoxicity with a CC₅₀ of 380 μM, and stability in human plasma. CC₅₀ is the concentration of a drug required to achieve a 50% reduction in cell viability. A study found revealed that 4′-FIU has effective antiviral activity against strains of Arenaviridae and inhibits lymphocytic choriomeningitis virus (LCMV) with an EC₅₀ of 7.22 μM in the Vero E6 cell line.⁶⁰ This compound had a significant action against recombinant respiratory syncytial virus (RSV) A2-line19F and clinical RSV isolates, with EC₅₀ values ranging from 0.61 to 1.2 μM.⁶⁸ In cell-based minireplicon systems, 4′-FIU inhibited RSV RdRp complex activity during the initial mechanistic characterization. In a human tissue model, HAE cells, including ciliate and mucus-producing cells, with 4′-FIU added to the basolateral chamber after RSV infection, showed that this compound can reduce apical viral shedding with an EC₅₀ of 55 nM.⁶⁸ Confocal microscopy confirmed the efficacy of 4′-FIU at the basolateral position after the pseudostratified organization of the epithelium with tight junctions and the observation of a rare positive strain of RSV antigen.⁶⁸ In addition to RSV, 4′-FIU can also reduce paramyxovirus RdRp complex activity in cell-based minireplicon systems, and in a Nipah virus (NIV) minireplicon reporter test, 4′-FIU effectively suppressed the RdRp activity of NIV.²¹

Several studies on 4′-FIU derivatives have also revealed that they possess antiviral properties. The 4′-FIU 5′-O-triphosphate was shown to effectively inhibit the key hepatitis C virus (HCV) enzymes, nucleoside triphosphate (NTP)-dependent RNA polymerase NS5B, and NTP-dependent NTPase/helicase NS3. This compound successfully inhibited NTP-dependent RNA polymerase with an IC₅₀ value of 2 μM. For the NTP-dependent NTPase/helicase NS3, 4′-FIU 5′-O-triphosphate was found to be a substrate for the NTPase reaction and did not inhibit helicase activity, but this reaction was found to be slightly weaker than adenosine triphosphate (ATP).⁶¹

An in vitro study on 4′-FIU-5′-triphosphate showed that RSV RdRp recognized and incorporated 4′-FIU-5′-triphosphate in place of uridine triphosphate (UTP) during the synthesis of the genome. Full addition of cytidine triphosphate (CTP) and 4′-FIU caused limited elongation and not the full-length expected product, indicating that 4′-FIU delayed polymerase stalling.²¹ Another study showed that 4′-FIU derivatives potently inhibited HCV infection. The IC₅₀ of the 4′-FIU derivative in the polymerase NS5B assay was as low as 27 nM. Human DNA and RNA polymerase also showed little inhibition by the 4′-FIU derivative.⁶⁹

However, while several studies have shown that 4′-FIU derivatives have antiviral activity, they are different from 4′-FIU analogs. A study conducted to determine the antiviral activity of 4′-FIU analogs for the varicella-zoster virus (VZV) and human cytomegalovirus (HMCV) showed that there was no antiviral activity with an EC₅₀ value >100 μM.⁷⁰ Another study showed that SARS-CoV-2 was particularly sensitive to 4′-FIU, with EC₅₀ values ranging from 0.2 to 0.6 μM.²¹ Additionally, this study demonstrated that when 4′-FIU 5′-triphosphate was incorporated in place of UTP in SARS-CoV-2 RdRp, the enzyme did not immediately stall, but it stalled after multiple incorporations and was prominent when 4′-FIU 5′-triphosphate was incorporated. This is a distinctive mode of action in comparison to molnupiravir, which induces lethal viral mutagenesis once it is incorporated into the viral genomic RNA.²¹

Evidence from in vivo study

Fluorouridine has been shown to be effective at inhibiting biological activity, particularly cell division, in Tetrahymena pyriformis.⁷¹ After ingestion, compound 4′-FIU is further phosphorylated in the cell to form the active metabolite 4′-FIU (Figure 3A). In an in vivo study examining the efficacy of oral 4′-FIU against an early pandemic isolate (WA1) and SARS-CoV-2 alpha, gamma, and delta in the ferret model, which closely resembles uncomplicated human infection,⁷² peak plasma concentrations (Cmax) of 4′-FIU reached 34.8 and 63.3 M when delivered at 15 and 50 mg/kg, respectively, and overall exposure was 154 ± 27.6 and 413.1 ± 78.1 h× nmol/ml, indicating strong oral dose proportionality. For efficacy testing, this study recommended a single daily dose of 20 mg/kg body weight .²¹

Figure 3. The metabolism of A). 4′-fluorouridine, B). Favipiravir C). Remdesivir, and D). Molnupiravir in the body (Created in ChemDraw Professional 17.0).

Further research revealed that an intranasal infection of ferrets with 1×10⁵ plaque-forming units of SARS-CoV-2 resulted in viral shedding within a day and peaked 48 hours after infection.²¹ Treatment with 4′-FIU reduced the viral burden in nasal lavages by three orders of magnitude (WA1) to <50 PFU/ml within 12 hours of treatment commencement. Each of the three variants of concern (VOC) was extremely sensitive to 4′-FIU and remained below the detection level for 36 to 48 hours following the start of oral treatment. After 2.5 days of treatment, all animals had completely ceased shedding infectious particles.²¹ This single daily dose is superior to the recommended molnupiravir dose of twice daily.⁷³

Patent WO2019173602A1 demonstrated that 4′-FIU was extremely well tolerated in an in vivo study in mice. The compound was shown to reduce RSV load in the lungs, with the most effective dose being 5 mg/kg body weight. After day 5 post-infection, animals treated with 4′-FIU demonstrated a significant decrease in bioluminescence intensity in the lungs.²¹

4′fluorouridine (4′-FIU) antiviral activity compared to other antiviral drugs against SARS-CoV-2

Drug repositioning is the only feasible option for addressing the COVID-19 global challenge in the short term. Currently, only paxlovidis has been approved by the US Food and Drug Administration (FDA) for emergency use authorization (EUA) against SARS-CoV-2, although significant knowledge about the viral mechanism of infection and mode of replication has been gained over time. Adult and pediatric patients were shown to benefit from the use of paxlovid, a combination of nirmatrelvir and ritonavir tablets that were co-packaged for oral administration. Paxlovid was found to be effective in the treatment of mild-to-moderate COVID-19 in both adults and children. This medication is strongly recommended to be started as soon as possible following a COVID-19 diagnosis and within five days of symptom onset. It significantly reduced hospitalizations and deaths in patients with COVID-19 who are at a high risk of developing severe illness.⁷⁴

A few small-molecule antiviral drugs, such as remdesivir and favipiravir, have shown promising results in various advanced stages of clinical trials.⁷⁵ Favipiravir (FPV or T-705 or 6-fluoro-3-hydroxy-2-pyrazinecarboxamide) is a guanine analog that was synthesized by modifying a pyrazine analog. It is an antiviral agent that selectively and potently inhibits the RdRp of RNA viruses.⁷⁶ The compound is phosphorylated intracellularly to produce the active form, favipiravir-RTP (favipiravir ribo-furanosyl-5-triphosphate) (Figure 3B), which is recognized as a substrate by RdRp and inhibits the activity of RNA polymerase.⁷⁷

According to a previous study, the mechanism of action of FPV involves lethal mutagenesis.⁷⁸ Additionally, FPV demonstrated protection against RNA virus influenza in a mouse model⁷⁹ and Crimean-Congo hemorrhagic fever virus (CCHFV) in macaques. However, several cell lines showed reduced or missing selectivity against SARS-CoV-2.⁸⁰ It differs from 4′-FIU, which shows promising results both in vitro and in vivo.^21,60 Nevertheless, further studies are warranted to confirm this finding, in particular by employing cells that have different metabolic profiles. Additionally, FPV has been linked to an increased risk of teratogenicity and embryotoxicity⁸¹ and a poor pharmacokinetic (PK) profile.⁸²

A study stated that remdesivir is a promising antiviral agent for tackling SARS-CoV-2.⁵⁶ Remdesivir is an adenosine analog and a monophosphoramidate prodrug. Its activity involves the incorporation of its active form (remdesivir triphosphate, Figure 3C) into nascent viral RNA, halting RNA synthesis.⁵⁹ In vitro, a study revealed that remdesivir successfully suppressed SARS-CoV-2⁸³ and has now been granted approval.⁵⁸

Similar to remdesivir, 4′-FIU is also a nucleoside analog. While 4′-FIU is an oral medication, remdesivir is rarely used because of its intravenous delivery and is frequently recommended for patients with severe COVID-19 infection.⁸⁴ An orally accessible remdesivir analog may improve therapeutic benefit by allowing non-hospitalized patients to receive treatment sooner and can be taken orally at home. As a result, molnupiravir, another nucleoside analog, was developed and is currently approved in the United Kingdom,⁶³ and has already completed its phase III trial.¹⁸ Inside the body, molnupiravir is metabolized into molnupiravir-triphosphate (MTP) (Figure 3D). In contrast, molnupiravirtriphosphate prevents viral reproduction by generating more mutations than the virus can tolerate, a process known as lethal mutagenesis or catastrophe.^20,85 This raises concerns that it may have an effect on human genes; thus, additional research on its genetic safety is critical.⁸⁶

Similar to that of remdesivir, the mechanism of action of 4′-FIU as a potential oral drug for COVID-19 is by inhibiting RdRp to prevent viral RNA replication²¹ (Figure 4). The advantage is that 4′-FIU can be taken orally, whereas remdesivir must be administered intravenously, making 4′-FIU more convenient and an excellent choice for pre-hospitalized patients. Additionally, because 4′-FIU does not induce mutagenesis in the same way that molnupiravir does, concerns about the safety of molnupiravir may be addressed with 4′-FIU. In terms of efficacy, an in vivo study demonstrated that 4′-FIU is required only as a single daily dose,²¹ whereas molnupiravir is required as two daily doses.⁷³

Figure 4. The mechanism by which 4′-fluorouridine halts RNA-dependent RNA polymerase (RdRP) activity, thereby preventing viral replication (Created in Biorender).

Paxlovid, like molnupiravir, is administered orally, allowing COVID-19 patients to take it at home in the early stages of infection. Despite receiving EUA approval from the FDA, this drug is not approved for the prevention of COVID-19 before or after exposure, or for initiating therapy in people requiring hospitalization for severe or critical COVID-19 infection. Paxlovidis made up of two antiviral pills, nirmatrelvir and ritonavir. Nirmatrelvir inhibits the activity of the SARS-CoV-2 main protease (M^pro), an enzyme required for coronavirus replication. It is responsible for cleaving polyproteins at 11 cleavage sites, resulting in NSP4-9 and NSP12-15.⁸⁷ Ritonavir, on the other hand, inhibits CYP3A, a liver enzyme that is involved in the metabolism of a variety of drugs, including nirmatrelvir. Ritonavir reduces the body's damage to the active antivirals in paxlovid therapy, allowing it to remain at therapeutic levels for a longer period. The initial information stated that this effect has a significant impact on paxlovid's remarkable success in clinical trials based on the news. Paxlovid was administered twice daily for five days, for a total of 30 tablets. Paxlovid is not recommended for use for more than five consecutive days. In comparison to 4′-FIU, which is only required as a single daily dose,²¹ 4′-FIU may demonstrate greater efficacy. Paxlovid, in comparison to molnupiravir, does not exhibit DNA mutagenic interactions as reported by Pfizer’s press release. Pfizer also stated that when paxlovid is used in combination with other drugs metabolized by the CYP3A enzyme, the greatest risk is that the components of ritonavir will increase the effectiveness of other potentially harmful drugs, so this antiviral may not be suitable for everyone. Because of these unpredictable drug interactions, 4′-FIU may be a viable alternative for those who are unable to take paxvloid for medical reasons.

Conclusions

Numerous drug targets aimed at eradicating SARS-CoV-2 have been investigated as potential drug targets. One of them is RdRp, a viral RNA replication enzyme that operates directly on RNA templates. The drug 4′-fluorouridine can inhibit RdRp, a mechanism of action distinct from that of molnupiravir. This drug and its derivatives have the potential to be viable alternatives to the currently available oral COVID-19 treatment. Additionally, 4′-FIU may be used to address the genetic concerns associated with molnupiravir because it does not cause mutagenesis but causes RdRp to stall instead. A single daily dose of 4′-FIUis recommended as it has been shown to be effective. This may have some advantages over the twice daily dose of molnupiravir. However, additional in vitro and in vivo studies are required to address 4′-FIU as a COVID-19 oral drug. Currently, it is shown to have tremendous potential.

Data availability

No data are associated with this article.

Competing interests

No competing interests were disclosed.

Grant information

This study was partially funded by Lembaga Pengelola Dana Pendidikan (LPDP), managed by the Indonesian Science Fund (ISF) (Grant No: RISPRO/KI/B1/TKL/5/15448/2020) to Harapan Harapan.