The SARS-CoV-2 virus was first detected in a wholesale seafood market in Wuhan, Hubei province, China, in December 2019. The outbreak of the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) brought the world to a standstill, overwhelmed health facilities around the world and plunged global economies into a severe recession. Today, the world is still struggling to recover from the devastating impact of the pandemic, with new variants emerging. The SARS-CoV-2 virus causes severe pneumonia, a severe acute respiratory syndrome, leading to significant morbidity and mortality worldwide (Low et al. 2022; Yadav et al. 2021). The increased transmissibility, re-infection potential, disease severity and ongoing genetic mutations of SARS-CoV-2 have caused over 6.5 million deaths and 600 million cases worldwide, making it a public health emergency of international concern (Low et al. 2022). SARS-CoV-2 has been reported to cause mild to severe respiratory symptoms in immunocompromised patients and is asymptomatic in non-immunocompromised individuals (Weizhu et al. 2022). These observations underscore the need for continued testing, active monitoring, surveillance, control, antiviral treatment and, most importantly, vaccination. The evolution of SARS-CoV-2 with iteration on the spike glycoprotein gave rise to different variants of concern (VOC) such as alpha, beta, delta, gamma and omicron, which are currently variants under surveillance (VUM) (Mohsin and Mahmud 2022; Petersen et al. 2022). The emergence of the Omicron variant and its substrains BA.4 and BA.5, and other more immune-evasive strains such as BQ.1, BQ.1.1, BF.7, XBB and XBB.1, has drawn the attention of the World Health Organization (WHO), public health experts, scientists and governments to the urgency of alternative treatments (Offit 2023).
SARS-CoV-2 is a positive single-stranded RNA (+ ssRNA) spherically enveloped by a symmetric lipid bilayer nucleocapsid that also contains structural and non-structural proteins. The viral particle is composed of four structural proteins, the spike S, membrane M, envelope E and nucleocapsid N proteins (Nelson et al. 2020; Wong and Saier 2021). These structural proteins play essential roles in the host immune response, virion assembly, viral transcription, activation of the endoplasmic reticulum stress response and follow a signal sequence that facilitates their translocation to the endoplasmic reticulum. Non-structural proteins range from nsp1 to nsp10 and nsp12 to nsp16 (Thomas 2020; Wu et al. 2020). These nonstructural proteins perform a variety of functions, including protein cleavage, degradation of host mRNAs, viral replication-transcription, methylation to the 5' cap structure of viral mRNAs, and many others (Encinar et al. 2020; Yadav et al. 2021).
The SARS-CoV-2 life cycle involves viral cellular invasion, orchestrated by the spike S glycoprotein binding the host cell receptor ACE2 (angiotensin-converting enzyme 2) via its SARS-CoV-2 receptor binding domain of the S1 subunit to form the spike S glycoprotein-host cell receptor-ACE2 complex (Fehr et al. 2016; Nakagawa et al. 2018). This is followed by protease-mediated cleavage at the S1/S2 site by the transmembrane protease serine 2 (TMPRSS2), leading to membrane fusion, which facilitates viral nucleocapsid entry into the host cell cytosol (Li et al. 2003; Yadav et al. 2021). Following release of the viral nucleocapsid into the host cell cytosol, the polyproteins pp1a and pp1b produce 16 nsps through an autoproteolytic process, which subsequently forms the replication-transcription complex (RTC) for viral RNA synthesis, releasing multiple copies of viral RNA (Letko et al. 2020). The viral RNA (-ssRNA) serves as an intermediate template on which polymerase acts to produce multiple 5' nested sets of negative sense sgRNAs, which are subsequently used as templates to form a 3' nested set of positive sense sgRNAs, which then interact with the host ribosome to synthesise accessory and structural proteins for the SARS-CoV-2 (Kleine-Weber et al. 2018; Wang et al. 2020). This replication, post-translation mechanism and maturation steps in the cytosol of the host cell are tightly coordinated by the cysteine proteinase in nsp 5, more recently termed main proteinase (Mpro) or 3 cysteine-like proteinase (3CLpro) and papain-like protease (PLpro) (Cheng et al. 2021; V'Kovski et al. 2021). The main proteinase is a highly conserved 3-domain cysteine protease responsible for processing the C-terminus of nsp 4 to nsp 16 while activating RIG-I and MAVS through the K63-linked ubiquitination process, thereby allowing SARS-CoV-2 virus to escape neutralizing antibodies (Astuti et al. 2020). In contrast, the papain-like proteinase (PLpro), among other highly conserved multi-domain proteins, is designed to cleave ISGylated substrates (Naqvi et al. 2020; Weizhu et al. 2022). Notably, the immune escape mechanism orchestrated by nsp 3 via ubiquitin-like domain 1 depends on the ability of PLpro to target ISG15 modifications. Importantly, the immune escape mechanisms of nsp 3 and nsp 5 serve as attractive targets for antiviral drug design.
Recent studies have shown that in silico investigations are a rapid and cost-effective medium for screening plant-derived bioactive compounds with therapeutic activity using a computational biology approach to discover new drugs and inhibitors (small molecules or ligands) against potential molecular targets of the SARS-CoV-2 virus (Bultum et al. 2022). In some cases, certain drugs have been repurposed using a virtual screening approach to target SARS-CoV-2 viral proteases, with Paxlovid (nirmatrelvir-ritonavir), molnupiravir and remdesivir being approved by the Food and Drug Administration (FDA) for the treatment of hospitalised COVID-19 patients, particularly adults and children (Kushari et al. 2022; Umadevia et al. 2020). In addition, studies have demonstrated the potential of phytochemicals such as polyphenols (flavonoids), triterpenes and phytosterols, terpenoids, polysaccharides, capsaicinoids, carotenoids and tocopherols, alkaloids, saponins and glucosinolates, among others. quercetin, piperin, curcumin, reserpine, aloe-emodin, homonataloin and many others (Mujwar and Harwansh 2022; Rutwick Surya and Praveen 2021; Umadevia et al. 2020). Most of these phytochemicals possess medicinal properties such as antiviral, antioxidant, antimicrobial, anti-inflammatory, immunomodulatory and anticarcinogenic (Chtita et al. 2022; Prasanth et al. 2023).
The aim of this study was to investigate the antiviral potential of phytochemicals from Momordica balsamina. Momordica balsamina is a perennial plant, commonly known as balsam apple or African pumpkin, that grows in tropical Africa, India and Asia (Mashiane et al. 2021). It is a member of the Cucurbitaceae family, a creeper that grows on building walls and trees and is rich in medicinal and nutritious phytochemicals (Mashiane et al. 2021). In Africa, it is used in traditional medicine to treat diabetes (anti-hyperglycaemic and renoprotective), malaria (anti-malarial), reverse multidrug resistance in cancer (anticarcinogenic), and has antiviral, antibacterial, anti-inflammatory, antioxidant and analgesic effects, among others (Jibril et al. 2018; Kaur et al. 2013; Ludidia et al. 2019; Ramalhete et al. 2022). The whole vegetative parts of Momordica balsamina have been reported to have medicinal properties either as infusion, decoction, herbal tea or sauce (Managa et al. 2020; Subramaniam et al. 2017). In the present study, we evaluated six balsaminol sets (with different oxidation patterns), namely balsaminol A, balsaminol B, balsaminol C, balsaminol D, balsaminol E and balsaminol F from Momordica balsamina, as potential drugs against SARS-CoV-2 virus. These plant-derived phytoconstituents, along with the Food and Drug Administration (FDA) COVID-19 approved drugs Paxlovid (nirmatrelvir-ritonavir) and remdesivir as positive controls, were screened against 3 cysteine-like proteases (3CLpro) or major proteases, papain-like proteases (PLpro), spike S glycoprotein receptor binding domain and replicase polyprotein 1ab using molecular docking studies