The Potential of Antiviral Peptides as COVID-19 Therapeutics

Abstract

To date, more than 17 million cases of coronavirus disease 2019 (COVID-19) and 900,000 deaths have been reported globally (World Health Organization, 2020). Major collaborative efforts focusing on the development of anti-SARS-CoV-2 vaccines and antiviral therapeutics are accelerated at an unprecedented rate. Various therapeutic agents quickly taken into clinical trials are largely based on existing drugs with non-specific antiviral activities or compounds pharmacologically speculated to be effective in enhancing the overall clinical outcome of COVID-19 patients. To name a few, these include the nucleoside inhibitor prodrug remdesivir (GS-5734), the antiparasitic drug ivermectin, the HIV protease inhibitor nelfinavir, the anti-inflammatory drug cepharanthine, the antimalarial drug hydroxychloroquine, the general steroid dexamethasone, and others (Caly et al., 2020; Ledford, 2020; Ohashi et al., 2020; Vanden Eynde, 2020; Wang Y. et al., 2020). However, only dexamethasone and remdesivir appear to confer promising treatment outcomes by reducing deaths or time to clinical improvement in the severe COVID-19 patients (Grein et al., 2020; Ledford, 2020; Wang Y. et al., 2020). The world is extremely desperate for a cure or prophylaxis against SARS-CoV-2 with the hope of saving more lives. Nevertheless, little has been described for antiviral peptides (AVPs) or alternatively known as antimicrobial peptides (AMPs) that possess antiviral activities. AVPs are a class of short (8–40 amino acids in length) polycationic antivirals with potent broad spectrum antiviral activities (Chang and Yang, 2013; Skalickova et al., 2015; Ahmed et al., 2019; Nyanguile, 2019; Sala et al., 2019; Vilas Boas et al., 2019). Interestingly, there are AVPs demonstrated to exert prophylactic and therapeutic effects against coronaviruses (CoVs). In this opinion paper, we describe the potential use of AVPs against COVID-19 based on the documented evidence against SARS-CoV2, SARS-CoV, MERS-CoV, SARS-related CoVs, and other respiratory viruses that shall warrant further development of this class of compounds in the face of the current pandemic threat. SARS-CoV-2 is an enveloped virus with the characteristic spike (S) glycoprotein to mediate viral entry via the cell surface receptor angiotensin-converting enzyme-2 (ACE2) (Zhou et al., 2020). The S1 subunit of S protein is the receptor-binding domain responsible for ACE2 binding (Xia et al., 2020b). Subsequently, a cellular serine protease TMPRSS2 is required for S protein priming, which induces proteolytic cleavage of S protein at S1/S2 and S2’ sites (Hoffmann et al., 2020). Following cleavage, both heptapeptide repeat 1 (HR1) and heptapeptide repeat 2 (HR2) regions of the S2 subunit of S protein interact to form the 6-helix bundle (6HB) fusion core (Liu et al., 2004; Kang et al., 2020). The formation of 6HB is critical in facilitating the viral membrane fusion process for viral entry into the host cell via endocytosis. During the late endosomal stage, endosomal acidification would induce virus-endosome membrane fusion, thereby leading to viral uncoating (viral RNA release) for the initiation of viral replication and infection (Du et al., 2009; Das et al., 2010; Letko et al., 2020). Notably, the S1 subunit of SARS-CoV-2 showed a 10- to 20-fold higher ACE2 binding affinity comparing to SARS-CoV, which may explain the higher transmissibility and infectivity of COVID-19 (Wrapp et al., 2020). Mucroporin-M1 (LFRLIKSLIKRLVSAFK) is a peptide analog designed with four residual mutations at positions G3R, P6K, G10K, and G11R from the parent peptide mucroporin (LFGLIPSLIGGLVSAFK) isolated from the venom of the scorpion Lychas mucronatus (Dai et al., 2008; Li et al., 2011). Such increase in cationicity enhanced the dipolar characteristic of the peptide helix, which contributed to the drastic increment in antiviral activity against SARS-CoV (50% virus infection, EC₅₀ of 14.46 µg/ml), influenza H5N1 (EC₅₀ of 2.10 µg/ml), and measles (EC₅₀ of 7.15 µg/ml) pseudoviruses (Li et al., 2011). Based on the observations that mucroporin-M1 directly interacted with measles viral particles and a greater antiviral effect with pre-treatment, mucroporin-M1 was proposed to serve the role of a molecular blocker, which must find its target before viral attachment to the host cells (Figure 1). Following peptide-virus binding, the strong electrostatic affinity of mucroporin-M1 could allow interaction and disruption of the viral envelope, thereby exerting a direct virucidal effect against SARS-CoV, MERS-CoV, and influenza H5N1 viruses (Li et al., 2011). Figure 1 Mechanism of actions of AVPs with potential anti-SARS-CoV-2 activities. Several AVPs target the key structural components of the virus to exert antiviral effects: mucroporin-M1 acts by disrupting viral envelope, HR2P-M2 targets the viral S protein-mediated fusion, EK1 and EK1C4 block the HR1 domain of viral S2 subunit, and P9 peptide inhibits late endosomal acidification and thus preventing viral RNA release. There are AVPs which confer antiviral protection to the host: RTD-1 is a potent antiviral immunomodulator to trigger protective immunity, and HD5 binds to and shields ACE2 from viral recognition and binding. EK1 (SLDQINVTFLDLEYEMKKLEEAIKKLEESYIDLKEL) is a pan-CoV fusion inhibitor designed by Xia and colleagues. This 36-residue peptide showed high cross-reactivity against all SARS-CoV, MERS-CoV, and three SARS-related CoVs from bats tested (Xia et al., 2019). As shown in Figure 1, EK1 acts by blocking the HR1 domain to disrupt the formation of the 6HB core, which causes inhibition of viral fusion entry into the host cell (Xia et al., 2020b). Intranasal application of EK1 further protected mice from pre- and post-challenges by the HCoV-OC43 alphacoronavirus and MERS-CoV challenges. Such dual therapeutic and prophylactic effects displayed by a single...