Updated: Sep 19, 2021
SARS-CoV-2 is a positive-sense RNA β-coronavirus, enclosing a capped polyadenylated 30 kb genome, which is the largest among RNA viruses (). SARS-CoV-2 binds to the ACE2 enzyme on the surface of the target host cell by way of its outer spike protein (S) (). The receptor-binding domain (RBD) on the S1 subunit interacts with the peptidase domain of ACE2. After partitioning into the host membrane, sequential enzymatic cleavages ultimately lead to the release of the viral genome into the cell ().
The development of successful vaccines has been a priority in the pharmaceutical and scientific community (). However, the time between the initial SARS-CoV-2 outbreak in December 2019 until the pharmaceutical companies' began vaccine distribution spanned over a year (). During this period, two million people have died worldwide, according to the World Health Organization (WHO). Moreover, the increasing mutations detected in the S protein have raised concerns that virus evolution might outpace vaccine rollout and the time needed to reach herd immunity (). Additionally, while vaccines are the mainstay for halting the pandemic, it remains critical to develop therapeutics to treat patients and reduce the disease burden.
The drug ivermectin (IVM) has recently been shown to inhibit replication of SARS-CoV-2 in cell cultures (). IVM is a widely used drug, known best for its antiparasitic properties in both veterinary and human medicine. It was first discovered in the 1970s by microbiologist Satoshi Omura and parasitologist William Campbell (). Fifty years later, this same drug is suddenly at the forefront of the race against the current pandemic, namely via its unintentional inhibition of nuclear transport. It is important to understand and elucidate the ‘journey’ of how IVM emerged as a therapeutic agent against SARS-CoV-2, to follow this precedent and encourage repurposing available drugs for an increasing number of diseases. As such, we aim to highlight essential steps and components in the SARS-CoV-2 lifecycle, the significance of the nucleocapsid protein, the anecdotal evidence that hinted at its potential as an anti-viral drug, and its molecular mechanism of action. Finally, we summarize the real-time results of current clinical trials.
The Putative Role of the Nucleocapsid Protein Within the Nucleus
It was previously shown that viral proteins that enter the nucleus might suppress host genes related to the anti-viral response, leading ultimately to increased pathogenicity (25). This may also be the case with SARS-CoV-2, as in vitro studies indicated that the SARS-CoV-2 NP could interact with dsDNA, possibly due to its high positive charge and the negative charge of DNA (26). Although the exact activity of the SARS-CoV-2 N within the nucleus has not been fully characterized, the previous examination of several coronaviruses Ns can offer insight (24).
The N of the coronavirus infectious bronchitis virus (IBV) was detected not only in the cytoplasm but also within the nucleolus. Nucleolus targeting was also shown with the SARS-CoV-1 N (27). It is important to note that the presence of N in the nucleus was indispensable for the replication of IBV, highlighting that cytosolic activity was not sufficient. In another related coronavirus, mouse hepatitis virus (MHV), nuclear proteins were also implicated in its replication. MHV N was specifically detected in the nucleolus, which itself is formed during the interphase of the cell cycle and allows the formation of ribosomal RNA (rRNA) and ribosomal subunits. The reason for N targeting of the nucleolus is not entirely understood. However, it is possible that N associates with rRNAs, to ‘reserve’ their use for translation of sub-genomic RNA. It was also shown in vitro that N transfection into cells resulted in multi-nucleate cells, indicating the delay of cytokinesis (24). This would provide favorable and prolonged conditions for the virus intracellularly to continue to synthesize its genome and sub-genome, translate its proteins, and enable sufficient virion packaging. Moreover, N is proposed to dampen the host cell’s antiviral transcriptional response within the nucleus (). Nevertheless, confirming the presence of SARS-CoV-2 N in the nucleolus and understanding its role would elucidate the pathogenicity of this virus.
N is an essential component of newly formed virions as it ensures a proper ‘delivery’ of the replicated viral RNA genome within the developing envelope (28, 29). Moreover, it is essential for proper viral RNA-dependent RNA polymerase activity, as demonstrated in Influenza A (29). As such, targeting the activity of N would offer a potent antiviral activity against SARS-CoV-2. N was shown to be an effective anti-viral target against Influenza A. One of the useful properties of N is its numerous binding sites, which have been shown to accommodate various drugs (29, 30). For example, compounds that can target the tail-loop binding pocket abrogate N oligomerization, while the compound F66 binds to the RNA-binding groove of the protein and is associated with improved survival in animal models infected with Influenza A (29). Figure 1 illustrates how the N of SARS-CoV-2 facilitates virus replication and mitigates the host cell response, thus further strengthening its position as a promising target of anti-viral drugs.
The importance of the SARS-CoV-2 nucleocapsid protein (N). The N exerts numerous functions that facilitate viral replication while mitigating the host cell response. Owing to its NLS motifs, the protein retains a relatively high positive charge, compared to the N of other coronavirus clades. This enhances its transport into the nucleus where it may silence host anti-viral genes while sequestering ribosomal subunits, possibly for viral mRNA translation, as demonstrated with the N of other related viruses. Moreover, the N is important for stabilizing the interaction between the viral mRNA and nsp3 protein, which facilitates genome replication. In addition, it tethers the newly emerged viral RNA to the viral envelope, ultimately allowing for its encapsulation and formation of new viral progeny. Given these features and its abundance in the infected cell, it would be a promising drug target against SARS-CoV-2.
Soon after IVM emerged as a potential therapeutic agent, clinical trials on COVID-19 patients ensued. However, the available published data and ongoing clinical trials, which are summarized in Table 1, do not provide a clear and uniform understanding of the effect of IVM on COVID-19 patients. This is mainly due to small sample sizes (n=12-203) and the lack of information specifying when exactly IVM is administered after testing positive for SARS-CoV-2 (46, 52–54). It is important to highlight how soon after testing positive the patient receives IVM, in addition to the degree of COVID-19 severity, to understand if the effect of the drug is dependent on time and symptom severity. Additionally, several studies are retrospective in which investigators examined past COVID-19 patients who were prescribed IVM, without proper placebo control groups (46, 53). Moreover, most of the studies utilize the antiparasitic effective dose for IVM (0.2 mg/kg body weight), which is substantially less than the equivalent in vitro dose of IVM used against SARS-CoV-2, 53, 54). Nevertheless, the available data does indicate that IVM may be effective against COVID-19.
Credited to Front. Immunol.