Created by: Carter Nattrass, Matthew Fernandes, Bisman Singh, Sehyun Cheon, Jessica Xing and Lamisa Syed

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the novel virus that is currently driving the coronavirus disease 2019 (COVID-19) pandemic. SARS-CoV-2 is a highly transmissible virus that has caused 380 million infections and approximately 5 million deaths worldwide (WHO, 2022). COVID-19 is an upper respiratory illness that causes a range of pathogenicity among individuals of all ages (Vanshylla et al., 2022). Most commonly, symptoms can range from fever and flu-like symptoms (Cevik et al., 2020). On the other hand, severe illness can present as shortness of breath, chest pain and severe pneumonia, which can potentially lead to hospitalization and death (Cevik et al., 2020). The rapid production of COVID-19 vaccines has allowed for the significant mitigation of severe illness and hospitalizations upon infection (Vanshylla et al., 2022). However, those who are unvaccinated or are immunocompromised are unable to mount a robust immune response against the virus and are at risk for developing severe illness (Vanshylla et al., 2022). As well, the current variant of concern (VOC) is the B.1.1.529 variant, also known as the Omicron variant (PHO, 2021). This variant poses a new threat to public health as these mutations can escape the immune protection provided by the current vaccines (Vanshylla et al., 2022). As such, it is critical for scientists and clinicians to study and expand upon current therapeutic targets against SARS-CoV-2 to further develop efficacious interventions.

Figure 1. A simplified overview of the genes found within the SARS-CoV2 Genome (Jungreis et al., 2021). (Image adapted from Jungreis et al., 2021).

The SARS-CoV-2 RNA genome is a positive strand made up of 29,903 nucleotides which is currently known to encode for 30 mature proteins (Jungreis et al., 2021). It contains many open reading frames (ORFs) in the genome as well (Jungreis et al., 2021). ORFs are stretches of codons anywhere in the genome which begin and end with start and stop codons (Jungreis et al., 2021). Some of these ORFs are protein coding regions which are translated into functional proteins responsible towards the transmission, replication, immune avoidance, and fitness of the virus (Jungreis et al., 2021). ORF1a and ORF1b span two thirds of the genome starting at the 5’ end (Jungreis et al., 2021). ORF1a is translated into polyprotein pp1a which matures into non-structural proteins (nsp) 1-11 (Jungreis et al., 2021). ORF1ab represents a programmed frameshift of four codons at the end of ORF1a which leads to the translation of both ORF1a and ORF1b (Jungreis et al., 2021). ORF1ab is translated into polyprotein pp1ab which matures into nsps 12-16 (Jungreis et al., 2021). The functional domains for some nsps have been discovered which include 3C-like cysteine proteinase for nsp5, RNA-dependent RNA polymerase for most of nsp12, nidovirus RdRp-associated nucleotidyltransferase at the N-terminus of nsp12, helicase for nsp13, and exonuclease for nsp14 (Jungreis et al., 2021). In general, nsps 1-11 regulate gene expression while nsps 12-16 are involved in replication (Jungreis et al., 2021). Four proteins which are conserved across coronaviruses are encoded by the final third portion of the genome (Jungreis et al., 2021). They consist of the S, E, M, and N proteins (Jungreis et al., 2021). S is for the spike protein found on the surface of the virus which is made up of S1 and S2 where S1 aids in attaching to the angiotensin-converting enzyme 2 (ACE2) receptors while S2 aids in the entry of the virus, E is for the envelope protein, M is for the membrane glycoprotein, and N is for the nucleocapsid phosphoprotein (Jungreis et al., 2021). Also, the final third portion of the genome includes several other protein coding ORFs which have been identified such as: ORFs 3a, 3c, 6, 7a, 7b, 8, and 9b (Jungreis et al., 2021).

Figure 2. SARS-CoV-2 and its key structural proteins (adapted from Mittal et al., 2020).

SARS-CoV-2 belongs to the β coronavirus family (Mittal et al., 2020). Coronaviruses are large, enveloped, positive sense, single stranded RNA viruses that have spike-like projections of glycoproteins on their surface that give them a crown-like appearance when viewed under an electron microscope, hence their name (Mittal et al., 2020). The genome of SARS-CoV-2 encodes for both structural and nonstructural proteins. The structural proteins consist of the membrane (M), envelope (E), spike (S), and nucleocapsid (N) proteins; these structural proteins allow for host infection, membrane fusion, viral assembly, morphogenesis, and release of virus particles (Mittal et al., 2020). There are also various nonstructural proteins such as: 3-chymotrypsin-like protease, papain-like protease, and RNA-dependent RNA polymerase (RdRp), that allow for viral replication and transcription (Huang et al., 2020; Mittal et al., 2020).

Envelope (E): The E protein is the smallest structural protein and is found in the viral membrane (Mittal et al., 2020). After viral entry, the E protein localizes to the endoplasmic reticulum and Golgi complex in the host cells (Mittal et al., 2020). The E protein is thought to facilitate virus-like particle formation (Mittal et al., 2020).

Membrane (M): The M glycoprotein, also located in the viral membrane, is the most abundant structural protein (around one hundred times more common than the E protein) (Mittal et al., 2020). The M protein plays a major role in the intracellular formation of virus particles (Mousavizadeh & Ghasemi, 2021). The M protein also interacts with other structural proteins and promotes membrane curvature during virion budding (Mariano et al., 2020).

Nucleocapsid (N): The N protein is responsible for packaging the viral genome RNA (gRNA) into helical ribonucleoprotein (RNP) complexes (Mittal et al., 2020). The N protein is also thought to be involved in viral genome replication by guiding the viral genome to the replication-transcription complex (Mariano et al., 2020).

The E, M, and N proteins are all involved in virus-like particle formation and viral assembly to varying degrees (Mittal et al., 2020).

Spike (S): SARS-Cov-2 is covered with homotrimeric S proteins that protrude from the viral envelope (Mittal et al., 2020). These proteins are key facilitators of viral entry into the host cell (Mittal et al., 2020). S proteins are clove-shaped, type-I transmembrane proteins that consist of 3 segments: a large ectodomain, a transmembrane anchor, and an intracellular tail (Mittal et al., 2020). The ectodomain of S proteins are made of the S1 subunit, where the receptor-binding domain (RBD) is found, and the membrane-fusion (S2) subunit (Mittal et al., 2020). S proteins are common among all coronaviruses and are the main inducers of neutralizing antibodies; therefore, much focus is put on these proteins when developing vaccines and antiviral drugs (Mousavizadeh & Ghasemi, 2021).

Figure 3. RBD position and spike protein attachment to ACE2 receptor (adapted from Jackson et al., 2021).

The S protein on the virion surface is present as a trimer, with three receptor-binding S1 heads sitting on top of a trimeric membrane fusion S2 stalk (Shang et al., 2020). The S1 and S2 subunits have unique functions in the viral entry process. The S1 subunit binds the ACE2 receptor using the RBD, which mediates viral attachment (Jackson et al., 2021). The S2 subunit anchors the S protein to the virion membrane and contains a fusion peptide and other machinery needed to facilitate membrane fusion (Jackson et al., 2021). S proteins exist in two conformations: closed and open, based on the position of their RBD (Khare et al., 2021). The RBD can switch between a standing-up (open) or lying-down (closed) position for receptor binding and immune evasion, respectively (Shang et al., 2020). The prefusion S protein trimer, present on the envelope of assembled SARS-CoV-2 viruses, fluctuates between a three RBD-down (closed) conformation and a one RBD-up (open) conformation, which can bind to the ACE2 receptor and initiate the viral entry process (Jackson et al., 2021; Mariano et al., 2020). Overall, the S protein mediates receptor recognition and membrane fusion by binding to the ACE2 receptor on the host cell, allowing the virus to enter and infect cells (Yan et al., 2020; Mousavizadeh & Ghasemi, 2021).

Figure 4. Mutations present within the spike protein of SARS-CoV2 variants (Araf et al., 2022). The red lines indicate the location of these mutations (Araf et al., 2022). NTD is the N-terminal domain, RBD is the receptor binding domain, HR2 is the heptapeptide repeat sequence, and TM is the transmembrane domain (Araf et al., 2022). S1 and S2 are the two subunits of the spike protein (Araf et al., 2022). (Image adapted from Araf et al., 2022).

The genetic diversity of the genome is central to the pathogenesis of SARS-CoV-2 as a result of the random mutations and recombination events (Bano et al., 2022). It has a high mutation rate compared to other RNA viruses at 8 x 10^-4 nucleotides/genome (Bano et al., 2022). The mutations within the spike protein have been concerning as this may increase its affinity to the ACE2 receptor, thus increasing the pathogenesis (Bano et al., 2022). This has been observed from the many variants which have arisen throughout the course of the pandemic. Each of these variants are present within the same lineage, but can be distinguished through their mutations that contribute to its transmissibility and severity of causing disease (Tao et al., 2021). Increased transmissibility is observed from the high reproduction rate or outcompeting other variants while severity of disease is analyzed based on hospitalizations (Tao et al., 2021). Although there are many variants, the following variants covered are those which were concerning within certain areas due to their unique characteristics.

To start, the Alpha variant arose to be the most dominant variant in the second quarter of 2021 within the U.S. as well as Europe (Tao et al., 2021). It included mutations N501Y and P681H to the receptor binding domain (RBD) as well as deletions at 69-70 and 144 in the N-terminal domain (NTD) of the spike protein (Tao et al., 2021). In addition, mutations D3L, R203K and G204R in the nucleocapsid as well as mutations in nsp6 were also present in this variant (Tao et al., 2021). From this, the transmissibility and mortality were each increased by 50% (Tao et al., 2021). Secondly, the Beta variant was most prevalent between October 2020 and January 2021 within South Africa (Tao et al., 2021). It consisted of mutations K417N, E484K and N501Y to the RBD as well as 5 mutations to the NTD (Tao et al., 2021). This resulted in increasing its transmissibility by 50% in comparison to the previous lineage (Tao et al., 2021). Thirdly, the Gamma variant was most prevalent in Brazil with a large infection rate (Tao et al., 2021). In June 2021, it accounted for a large proportion of cases in South American and Caribbean countries (Tao et al., 2021). It is characterized by N501Y, E484K and K417T mutations to the RBD as well as 5 mutations to the NTD (Tao et al., 2021). In particular, one mutation in the NTD has been observed to affect the ability of neutralizing antibodies targeting the NTD (Tao et al., 2021). In comparison to previous variants, there was a 3-4 times increase in virus levels as well as a 1.1-1.8 increase in mortality (Tao et al., 2021). Next, the Delta variant arose from the surging cases in India in early 2021 (Tao et al., 2021). It consisted of mutations L452R and T478K within the RBD, mutations in ORF3 and ORF7a, mutation P681R to the proximal furin cleavage site, mutations to the nucleocapsid gene, mutations in ORF1a and ORF1b, as well as mutations in the spike protein’s NTD and S2 domains (Tao et al., 2021). The Delta variant was highly infectious as it was able to spread to 54 other countries such as the UK and USA where it replaced the alpha variant (Tao et al., 2021). Lastly, the Omicron variant is the latest novel variant of concern as of November 26, 2021 (Araf et al., 2022). It was first observed within South Africa and has since been widespread across many countries (Araf et al., 2022). Omicron is the most highly mutated variant of concern with 18,261 mutations (Araf et al., 2022). Of these mutations, 30 are found within the spike protein (Araf et al., 2022). The RBD consists of most of the mutations to the spike protein and 11 are found within the NTD (Araf et al., 2022). These mutations have been observed to contribute towards the increased transmissibility and resistance to immunity conferred through the vaccine (Araf et al., 2022). With new variants on the rise, it is crucial to study new mutations which have the potential to increase the fitness of the virus (Guo et al., 2021). This is observed with mutations at positions 367, 364, 436, 614 which have been used to test the effectiveness of future therapies such as monoclonal antibodies (Guo et al., 2021).

Upon virus infection, the human body has two immune responses against a pathogen. The first response comes from the innate immune system which uses broad pathogen recognition to identify and clear an infection (Ashkar, 2020). If the innate immune defenses fail to clear the infection, the second line of immune defense called the adaptive immune system is activated. The adaptive immune response initiates a specific response against the pathogen using a humoral response (antibody mediated) or cellular response (T cell mediated) (Ashkar, 2020). In the humoral response, B lymphocytes (B cells) produce and secrete antibodies (Abs), which are proteins that can bind to specific regions (antigens) on pathogens and activate antiviral responses. Resting B cells have membrane bound antibodies, and upon binding to the antigen, the B cells are activated (Lichty, 2020a). The activated B cell can then secrete multiple antibodies against the pathogen into the blood system (Graham, 2021). Activated B cells can become effector B cells that produce many antibodies against the pathogen or form memory B cells that recognize the virus antigen upon re-infection which allows for long term protection (Lichty, 2020a). Upon the binding of a foreign antigen to the B-cell antigen receptor, the activation and differentiation of B cells into antibody-secreting plasma cells is initiated (Mahmuda et al., 2017). The subsequent antibodies produced then proceed to protect the host in one of three ways:

1. Opsonization: Antibodies coat the pathogen so that they may be recognized by accessory cells and phagocytosed (Forthal, 2014).
2. Complement Activation: Antibodies activate the complement system which enhances the overall efficacy of the immune system (Forthal, 2014).
3. Neutralization: Antibodies bind to the pathogen in a manner that prevents or greatly reduces its capacity to invade host cells (Forthal, 2014).

For fighting SARS-CoV-2 infection, neutralizing antibodies (nAbs) such as monoclonal antibodies (mAbs) have been approved by the Food and Drug Administration (FDA) for emergency use authorization (EUA) in severe COVID-19 cases such as for unvaccinated individuals and immunocompromised patients (CDC, 2022). These nAbs are characterized by their ability to bind to the virus in a manner that prevents the virus from attaching to host cell surface receptors and subsequently invading that cell (Lichty, 2020b). As they can be designed to possess high levels of specificity and binding affinity, these monoclonal antibodies serve to be an effective and efficient therapy for a wide range of ailments (Mahmuda et al., 2017). For example, neutralizing antibodies, such as IgA and IgG, are commonly used to protect mucosal lung surfaces from upper respiratory infections (CDC, 2022). As SARS-CoV-2 continues to mutate, scientists are also studying broadly neutralizing Abs against the virus which can target SARS-CoV-2’s highly conserved structures (Cameroni et al., 2021).

As antibodies are characterized by high levels of specificity and affinity, scientists have developed a method to engineer antibodies in vitro so that they may be used to efficiently target the desired antigen (Mahmuda et al., 2017). These monoclonal antibodies are typically derived from the clonal expansion of unique antibody-secreting plasma cells (Mahmuda et al., 2017). They are designed to express monovalent affinity so that they may selectively bind to a single epitope (Sadeghalvad & Rezaei, 2021).

Since their conception, a wide variety of monoclonal antibodies have been developed, each with its own unique mechanism of function. For example, neutralizing monoclonal antibodies (nAbs) prevent viruses from attaching to the host cell surface receptors (Lichty, 2020b). The nAbs can aggregate on the surface of the virus and inhibit the virus surface proteins’ ability to bind and enter host cells (Lichty, 2020b).

Figure 5. Simplified schematic representation of a monoclonal antibody molecule (from Janeway et al., 2001).

Regardless of their function, monoclonal antibodies are all characterized by their ability to recognize the epitope of a single antigen, thereby providing a high degree of specificity and binding affinity (Sadeghalvad & Rezaei, 2021). They are composed of four polypeptide chains; two heavy chains and two light chains with both variable and constant regions (Sadeghalvad & Rezaei, 2021). The polypeptide chains are stabilized by covalent disulfide bonds, creating a resultant “Y” structure (Sadeghalvad & Rezaei, 2021). mAbs consists of a variable fragment antigen-binding (Fab) region, which is responsible for the specific binding of the antibody to the antigen. They also have a constant fragment crystallizable (Fc) region, which determines the functional properties of the antibody and mediates biological activities (Sadeghalvad & Rezaei, 2021) Complementarity-determining regions (CDRs), the areas on the variable chains where the antibody binds to the antigen, are also crucial to antibody diversity.

Monoclonal antibody (mAb) therapy has been previously used for a wide range of viral infections and thus, current research and development of anti-SARS-CoV-2 mAbs show great potential. In particular, human monoclonal antibodies that neutralize SARS-CoV-2 and its variants provide an attractive treatment strategy (Kumar et al., 2021). }

Figure 6. Representation of the 4 classes of SARS-CoV-2 RBD dependent mAbs. Antibody variable heavy chain region (sky blue) and light chain variable region (magenta) are marked (from Kumar et al., 2018).

Currently, the majority of neutralizing mAbs against SARS-CoV-2 either target the RBD or the NTD of the spike glycoprotein (Kumar et al., 2021). Since the S protein exists in different conformations based on the position of RBD protein, RBD-specific neutralizing mAbs can be categorized into 4 major classes (I, II, III, and IV) that are established by the epitope recognition and binding mode (Kumar et al., 2021). Class I and II mAbs recognize up and up+down RBD conformations respectively and bind to the receptor-binding motif (RBM) region of the RBD (Kumar et al., 2021). As this region is responsible for the primary contact with host ACE2 to initiate the entry of the virus, mAbs that block this RBM–ACE2 interaction are considered “ACE2 blockers.” (Kumar et al., 2021). Similarly, the class III mAbs block ACE2 binding sites and can interact with adjacent RBD protomers by recognizing both the up and down RBD conformation (Kumar et al., 2021). Lastly, the class IV mAbs bind to conserved regions in RBD or RBD in “up” conformation only (Kumar et al., 2021).

Currently, eight SARS-CoV-2 RBD-specific mAbs have been approved by the FDA under an EUA to treat COVID-19 non-hospitalized patients at high risk of severe illness (Kumar et al., 2021). Within 10 days of symptoms onset, the therapeutic mAbs have been administered in a range of 0.5 g to 1.2 g per dose or as a 2.4 g cocktail (Kumar et al., 2021). Varying dosages between 1.2 to 8 g have been tested with no observable dose-dependent effects (Kumar et al., 2021). In fact, they have demonstrated high efficacy in trials with a reduction of 70% to 85% in hospitalization or death and are currently being used for intravenous administration (Kumar et al., 2021). Intramuscular or subcutaneous administration testing is underway to facilitate larger access by overcoming the requirement of hospital settings (Kumar et al., 2021).

(Voskuil et al., 2020)

• mAbs express high specificity and affinity to a single epitope
• Limited off target effects/ cross reactivity
• Large quantities of identical antibodies can be produced
• Batch-batch homogeneity

(Tsumoto et al., 2019)

• Small changes in epitope structure can affect their function and binding affinity
• High cost of production limits the widespread use of mAbs
• Long production time (4-6 months)

Broadly neutralizing antibodies (bnAbs) are integral to consider when targeting multiple variants of a virus with a single intervention (Kumar et al., 2018). bnAbs have primarily been investigated in the context of human immunodeficiency virus (HIV) (Liu et al., 2020). The HIV genome is prone to mutations which creates difficulties in therapeutic targeting (Liu et al., 2020). When viral structures mutate, the antigens change, which prevent existing Ab therapies to bind and neutralize the virus (Kumar et al., 2018). As such, scientists are investigating bnAbs to target highly conserved regions of the virus, which avoid antigenic shifts, and thus can be consistently targeted (Cameroni et al., 2021). BnAbs have been created against conserved regions on HIV-1’s surface envelope protein (Liu et al., 2020). This was investigated using high-throughput neutralization assays which identified bnAbs that had high binding efficacy against conserved epitopes in HIV-1 (Liu et al. 2019). After successful in vitro and in vivo preclinical models, HIV-1 bnAbs entered phase I clinical trials (Liu et al., 2020). The results from one phase I trial found that the bnAb VRC01 against a HIV receptor binding protein was safe in toxicity studies and successfully neutralized various HIV strains (Liu et al., 2020). However, bnAb treatment is not completely curative as clinical trial results report limited long term effects with rebound viraemia (viral replication in the blood) and short half-lives (Kumar et al., 2018; Liu et al., 2020). Overall, the study of bnAbs in the field of HIV therapeutics can set the foundation for the experimental design and therapeutic targeting in other infectious disease fields, including SARS-CoV-2 studies.

Cameroni et al. (2021) recently discovered bnAbs against the Omicron variant, the most recent variant of concern for Sars-CoV-2. The Omicron variant contains 30 amino acid mutations in the S protein which the current vaccines and therapies are based on (Araf et al., 2022). Thus, determining alternative strategies to neutralize infection from the Omicron variant is needed. Using pseudovirus neutralization screens, the Cameroni team identified six bnAbs that retained neutralizing ability in and outside of the RBD out of 44 monoclonal antibodies (Figure 7):

• RBD Site I: S2K146, S2X324, and S2N28
• RBD Site II: S2X259
• Site IV (outside the RBD): sotrovimab
• Site V: S2H97

Figure 7. Visualization of four bnAb colored targets, or epitopes, on the SARS-CoV-2 Spike protein (gray). (Cameroni et al., 2021).

The six monoclonal antibodies were also cross-reactive against other SARS-CoV-2 variants (Cameroni et al., 2021). This indicated that these antigenic areas may have been evolutionary conserved among SARS-CoV-2 variants, thereby establishing therapeutic targets for future bnAbs, antiviral therapies and vaccines (Cameroni et al., 2021).

• Can target multiple variants of concern
• Can reduce production cost (Ding et al., 2021)

• Has a half life up to 10 days (short term protection) (Liu et al., 2020)
• Risk of rebound viraemia (Liu et al., 2020)

Small molecules are small organic compounds with a molecular weight of <900 Dalton which make up about 90% of pharmaceutical drugs(Nagaraju & Bonavida, 2019). These drugs can easily infiltrate through the cell membrane and interact with targeted molecules inside the cell which makes them an attractive option to use where biologics cannot reach(Nagaraju & Bonavida, 2019). Another benefit of these molecules is that they are affordable as they can be administered orally and are convenient to use when needed as opposed to other treatment types which require the patient to visit a doctor. These molecules are a great option as they can greatly reduce the workload at hospitals, especially now due to the COVID-19 pandemic (Nagaraju & Bonavida, 2019).

Small molecules are a great option as a potential therapeutic for COVID-19 as they can target the virus protein in multiple ways(Mengist et al., 2021; Kabinger et al., 2021). Some effective techniques which have seen some results are main protease inhibitors and nucleoside analogs targeting RNA-dependent RNA polymerase (RdRp)(Mengist et al., 2021; Kabinger et al., 2021). The main protease of SARS-CoV-2 is a potential drug target because it is responsible for the maturation of itself and other important polyproteins (Mengist et al., 2021). Whereas the nucleoside analogs stop the virus protein from replicating by targeting RdRp and causing widespread mutations ultimately blocking transcription(Kabinger et al., 2021).

Two important and recent drugs that have been approved by the FDA for EUA are Paxlovid by Pfizer which is given along with Ritonavir (U.S. Food and Drug Administration, 2021a) and Molnupiravir by Merck and Ridgeback (U.S. Food and Drug Administration, 2021b). Paxlovid uses the main protease inhibitor technique and Molnupiravir is a nucleoside analog that targets RdRp increasing viral RNA mutations which then stalls polymerase activity and ultimately blocks transcription. Third drug that has been approved for over a year by the FDA (U.S. Food and Drug Administration, n.d.) that also uses the same technique is Remdesivir (Kokic et al., 2021).

Another potential small molecule therapeutic treatment is the destabilization of the ACE2-RBD complex which prevents the virus protein from entering and infecting the cell. Nilotinib and SSAA09E2 are two promising molecules that have high binding affinities with druggable pocket of the ACE2-RBD complex (Razizadeh et al., 2021).

Combination therapy involves the usage of more than one medication to treat a patient (Akinbolade et al. 2021). Adult SARS-CoV-2 patients are treated with either monotherapy or combination therapy depending on the disease severity (National Institute of Health, 2022). Hospitalized patients in the early stages are treated with antiviral drug remdesivir monotherapy but patients in later stages are treated with a combination of immunomodulatory drug dexamethasone, remdesivir, and a second immunomodulatory drug depending on the progress of SARS-CoV-2 and the rate of disease progression (National Institute of Health, 2022).

Combination therapies are a small part of therapies undergoing clinical trials and most that have or are undergoing clinical trials have failed to meet their endpoints in hospitalized patients (Akinbolade et al. 2021). Combination therapies listed above are the few rare cases that successfully met their therapeutic endpoints (National Institute of Health, 2022; Akinbolade et al., 2021). Clinical trials involving remdesivir combination therapies commonly involved the immunomodulatory drugs to target the viral replication stage and the hyperactive immune response respectively (Akinbolade et al. 2021). Remdesivir was administered along baricitinib which showed significant therapeutic benefit compared to only remdesivir (Akinbolade et al. 2021). However, other combinations such as tocilizumab, bamlanivimab, hyperimmune immunoglobulin, interferon beta-1a failed to meet therapeutic endpoints (Akinbolade et al. 2021). Other ongoing clinical trials seek to replace remdesivir with similar antivirals that achieves faster viral clearance with similar mode of mechanism, such as lopinavir/ritonavir and favipiravir (Akinbolade et al. 2021).

Monoclonal antibodies (mAb) for SARS-CoV-2 treatment are undergoing clinical trials, as monotherapy or as part of combination therapy. mAb used for treating SARS-CoV-2 patients clinically can be categorized into two: blocking mAb and neutralizing mAb. Blocking mAb inhibits IL-6 to prevent the cytokine storm caused by elevated IL-6 levels (Akinbolade et al. 2021). Previously approved mAbs such as sarilumab and tocilizumab are repurposed and used in combination with dexamethasone under the emergency use authorization (EUA) by U.S. Food and Drug Administration (FDA) (Akinbolade et al. 2021; National Institute of Health, 2022). Baricitinib inhibits janus kinase 1 and 2 (JAK1, JAK2) which is used to treat rheumatoid arthritis was repurposed as part of a combination treatment with remdesivir (Akinbolade et al. 2021). Baricitinib/remdesivir combination resulted in shorter recovery time compared to remdesivir monotherapy in the ACTT-2 trial and was granted EUA (National Institute of Health, 2022; Akinbolade et al. 2021).

Neutralizing mAb against the spike glycoprotein of SARS-CoV-2 were developed to prevent viral entry through human angiotensin-converting enzyme 2 receptor (hACE2) (Akinbolade et al. 2021). Bamlanivimab is an IgG1 mAb that targets the spike protein of SARS-CoV-2. Bamlanivimab monotherapy was granted EUA by the FDA as early clinical trials showed a decrease in mortality but later revoked as bamlanivimab monotherapy resulted in an increase in resistant strains of SARS-CoV-2 (National Institute of Health, 2022; (Akinbolade et al. 2021). A subsequent clinical trial BLAZE-1 showed that bamlanivimab monotherapy showed no significant difference, however bamlanivimab/etesevimab combination therapy showed a significant difference compared to placebo (National Institute of Health, 2022). The FDA authorized the use of REGN-COV2 under EUA, which is a combination therapy consisting of casirivimab and imdevimab which bind to distinct epitopes of the receptor binding domain (RBD) of the spike protein of SARS-CoV-2 (National Institute of Health, 2022; (Akinbolade et al. 2021). The usage of two neutralizing mAbs that target non-overlapping regions is to prevent the rise of resistance to this treatment (Akinbolade et al. 2021). When administered, REGN-COV2 resulted in a large decrease in viral load and hospitalization compared to placebo groups (National Institute of Health, 2022). However, due to the surge in prevalence of the omicron variant, both bamlanivimab/etesevimab and REGN-COV2 had their EUA revoked as the mAbs were completely ineffective against the omicron variant (National Institute of Health, 2022).

In conclusion, the severity of contracting COVID-19 has prompted the response in developing numerous therapies. The potential therapies covered include monoclonal antibodies, broadly neutralizing antibodies, and small molecules. Additionally, combination therapies have been used in some circumstances depending on how severe the disease is. Through improved understanding of the pathogenesis and important target sites of SARS-CoV-2, new and effective therapies can be developed. This is explored in the study by Kim et al. which aims in developing a monoclonal antibody therapy for SARS CoV-2. The CT-P59 mAb was studied for its ability in neutralizing SARS-CoV-2 by binding to the receptor binding motif (RBM) in the RBD of the spike protein (Kim et al., 2021). This therapy can be used in conjunction with small molecules to be a potential therapeutic used to treat COVID-19 in the future (Kim et al., 2021).

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