A highly potent and stable pan-coronavirus fusion inhibitor as a candidate prophylactic and therapeutic for COVID-19 and other coronavirus diseases

Advance in peptide-based drug development: delivery platforms, therapeutics and vaccines

The successful approval of peptide-based drugs can be attributed to a collaborative effort across multiple disciplines. The integration of novel drug design and synthesis techniques, display library technology, delivery systems, bioengineering advancements, and artificial intelligence have significantly expedited the development of groundbreaking peptide-based drugs, effectively addressing the obstacles associated with their character, such as the rapid clearance and degradation, necessitating subcutaneous injection leading to increasing patient discomfort, and ultimately advancing translational research efforts. Peptides are presently employed in the management and diagnosis of a diverse array of medical conditions, such as diabetes mellitus, weight loss, oncology, and rare diseases, and are additionally garnering interest in facilitating targeted drug delivery platforms and the advancement of peptide-based vaccines. This paper provides an overview of the present market and clinical trial progress of peptide-based therapeutics, delivery platforms, and vaccines. It examines the key areas of research in peptide-based drug development through a literature analysis and emphasizes the structural modification principles of peptide-based drugs, as well as the recent advancements in screening, design, and delivery technologies. The accelerated advancement in the development of novel peptide-based therapeutics, including peptide-drug complexes, new peptide-based vaccines, and innovative peptide-based diagnostic reagents, has the potential to promote the era of precise customization of disease therapeutic schedule.

Development of Receptor-Integrated Magnetically Labeled Liposomes for Investigating SARS-CoV-2 Fusion Interactions

The impacts of highly pathogenic enveloped viruses, such as SARS-CoV-2, have turned scientific inquiry toward the fusion mechanisms responsible for viral pathogenesis and to seek cost-effective and adaptable strategies to mitigate future outbreaks. Current approaches for studying SARS-CoV-2 fusion include computational studies, pan-coronavirus viral inhibitors, and modified peptides and lipopeptides, along with various nanotechniques. Although these methodologies have illuminated the fusion mechanisms, they possess key limitations that prevent their widespread utility in outbreaks, including high financial or instrumental costs, operational proficiency, cytotoxicity, or viral specificity. This work measures changes in spin-spin T2 magnetic (transverse) relaxation times using a benchtop NMR instrument and introduces a bioanalytical approach to quickly quantify fusion interactions between the SARS-CoV-2 spike protein and liposome-coated iron oxide nanosensors (LIONs). Additionally, this study modifies the LION platform by appending the angiotensin-converting enzyme (ACE2) receptor, thereby creating LIONs-ACE2 that mimics the ACE2 host cell receptor targeted by SARS-CoV-2. Furthermore, SARS-CoV-2 fusion to other receptors reported to be involved is also examined. Environmental factors impacting fusion, such as calcium ion concentration, cholesterol composition, pH, neutralizing antibodies, and lower temperature, are investigated. Finally, molecular dynamics (MD) simulation studies reveal that the receptor binding domain (RBD) of the spike protein interacts more favorably with ACE2 than the lipid bilayer in the opened conformation, yet the closed conformation of RBD interacts with the bilayer with a similar energy as with ACE2. These findings reveal how the LION platform offers a customizable, fast-acting, inexpensive, and accessible mechanism for examining the fusion process of SARS-CoV-2 and other enveloped viruses.

Protein mimics of fusion core from SARS-CoV-1 can inhibit SARS-CoV-2 entry

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a member of the genus Betacoronavirus (subgenus Sarbecovirus) and shares significant genomic and phylogenetic similarities with severe acute respiratory syndrome coronavirus 1 (SARS-CoV-1). SARS-CoV-2 infection occurs through membrane fusion between the virus and host cell membranes, which is facilitated by the spike glycoprotein subunit 2 (S2). The folding of three heptad-repeat regions 1 (HR1) into a central trimeric core structure, along with the binding of three heptad-repeat regions 2 (HR2) in an antiparallel manner within the groove formed between the HR1 regions, which provides the driving force for membrane fusion. In this study, trimeric and monomeric six-helix bundles (6HB) were created by combining various truncations of the sequences from SARS-CoV-2 HR1 and HR2. In addition, monomeric five-helix bundles (5HB) were constructed using a similar method. Finally, we demonstrated a protein mimic, 5HB_V1 (from SARS-CoV-1), that exhibits activity in inhibiting SARS-CoV-2. These findings suggest a strategy to design monomeric 6HB and 5HB based on the SARS-CoV-2 fusion core: maintain the flanking sequences outside the α-helix region in HR2 and introduce point mutations to enhance hydrogen bonding between the helix bundles. The 5HB could be a target for designing new inhibitors against SARS-CoV-1 and SARS-CoV-2.

Development of potent pan-coronavirus fusion inhibitors with a new design strategy

Development of potent and broad-spectrum drugs against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) remains one of the top priorities, especially in the cases of the emergence of mutant viruses and inability of current vaccines to prevent viral transmission. In this study, we have generated a novel membrane fusion-inhibitory lipopeptide IPB29, which is currently under clinical trials; herein, we report its design strategy and preclinical data. First, we surprisingly found that IPB29 with a rigid linker between the peptide sequence and lipid molecule had greatly improved α-helical structure and antiviral activity. Second, IPB29 potently inhibited a large panel of SARS-CoV-2 variants including the previously and currently circulating viruses, such as Omicron XBB.5.1 and EG.5.1. Third, IPB29 could also cross-neutralize the bat- and pangolin-isolated SARS-CoV-2-related CoVs (RatG13, PCoV-GD, and PCoV-GX) and other human CoVs (SARS-CoV, MERS-CoV, HCoV-NL63, and HCoV-229E). Fourth, IPB29 administrated as an inhalation solution (IPB29-IS) in Syrian hamsters exhibited high therapeutic and preventive efficacies against SARS-CoV-2 Delta or Omicron variant. Fifth, the pharmacokinetic profiles and safety pharmacology of IPB29-IS were extensively characterized, providing data to support its evaluation in humans. In conclusion, our studies have demonstrated a novel design strategy for viral fusion inhibitors and offered an ideal drug candidate against SARS-CoV-2 and other coronaviruses.

Host factors of SARS-CoV-2 in infection, pathogenesis, and long-term effects

SARS-CoV-2 is the causative virus of the devastating COVID-19 pandemic that results in an unparalleled global health and economic crisis. Despite unprecedented scientific efforts and therapeutic interventions, the fight against COVID-19 continues as the rapid emergence of different SARS-CoV-2 variants of concern and the increasing challenge of long COVID-19, raising a vast demand to understand the pathomechanisms of COVID-19 and its long-term sequelae and develop therapeutic strategies beyond the virus per se. Notably, in addition to the virus itself, the replication cycle of SARS-CoV-2 and clinical severity of COVID-19 is also governed by host factors. In this review, we therefore comprehensively overview the replication cycle and pathogenesis of SARS-CoV-2 from the perspective of host factors and host-virus interactions. We sequentially outline the pathological implications of molecular interactions between host factors and SARS-CoV-2 in multi-organ and multi-system long COVID-19, and summarize current therapeutic strategies and agents targeting host factors for treating these diseases. This knowledge would be key for the identification of new pathophysiological aspects and mechanisms, and the development of actionable therapeutic targets and strategies for tackling COVID-19 and its sequelae.

Synergistic peptide combinations designed to suppress SARS-CoV-2

The SARS-CoV-2, responsible for the COVID-19 pandemic, poses a significant threat to global healthcare. Peptide and peptide-based inhibitors, known for their safety, efficacy, and selectivity, have recently emerged as promising candidates for treating late-developing viral infections. In this study, three peptides were selected to target different stages of viral invasion, specifically ACE2 and S protein binding, as well as membrane fusion. The objective was to assess their ability to impede the entry of the SARS-CoV-2 Spike pseudotyped virus. Our findings revealed that a combination of these three peptides demonstrated enhanced antiviral effects. This outcome substantiates the feasibility of developing effective peptide combinations to combat diseases related to SARS-CoV-2. Moreover, the three-peptide combinations, designed to target multiple aspects of SARS-CoV-2 viral entry, exhibited heightened viral inhibition and broad-spectrum antiviral properties.

Antiviral Protein-Protein Interaction Inhibitors

Continually repeating outbreaks of pathogenic viruses necessitate the construction of effective antiviral strategies. Therefore, the development of new specific antiviral drugs in a well-established and efficient manner is crucial. Taking into account the strong ability of viruses to change, therapies with diversified molecular targets must be sought. In addition to the widely explored viral enzyme inhibitor approach, inhibition of protein-protein interactions is a very valuable strategy. In this Perspective, protein-protein interaction inhibitors targeting HIV, SARS-CoV-2, HCV, Ebola, Dengue, and Chikungunya viruses are reviewed and discussed. Antibodies, peptides/peptidomimetics, and small molecules constitute three classes of compounds that have been explored, and each of them has some advantages and disadvantages for drug development.

Helix-based screening with structure prediction using artificial intelligence has potential for the rapid development of peptide inhibitors targeting class I viral fusion

The rapid development of drugs against emerging and re-emerging viruses is required to prevent future pandemics. However, inhibitors usually take a long time to optimize. Here, to improve the optimization step, we used two heptad repeats (HR) in the spike protein (S protein) of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) as a model and established a screening system for peptide-based inhibitors containing an α-helix region (SPICA). SPICA can be used to identify critical amino acid regions and evaluate the inhibitory effects of peptides as decoys. We further employed an artificial intelligence structure-prediction system (AlphaFold2) for the rapid analysis of structure-activity relationships. Here, we identified that critical amino acid regions, DVDLGD (amino acids 1163-1168 in the S protein), IQKEIDRLNE (1179-1188), and NLNESLIDL (1192-1200), played a pivotal role in SARS-CoV-2 fusion. Peptides containing these critical amino acid regions efficiently blocked viral replication. We also demonstrated that AlphaFold2 could successfully predict structures similar to the reported crystal and cryo-electron microscopy structures of the post-fusion form of the SARS-CoV-2 S protein. Notably, the predicted structures of the HR1 region and the peptide-based fusion inhibitors corresponded well with the antiviral effects of each fusion inhibitor. Thus, the combination of SPICA and AlphaFold2 is a powerful tool to design viral fusion inhibitors using only the amino-acid sequence of the fusion protein.

A stapled lipopeptide platform for preventing and treating highly pathogenic viruses of pandemic potential

The continued emergence of highly pathogenic viruses, which either thwart immune- and small molecule-based therapies or lack interventions entirely, mandates alternative approaches, particularly for prompt and facile pre- and post-exposure prophylaxis. Many highly pathogenic viruses, including coronaviruses, employ the six-helix bundle heptad repeat membrane fusion mechanism to achieve infection. Although heptad-repeat-2 decoys can inhibit viral entry by blocking six-helix bundle assembly, the biophysical and pharmacologic liabilities of peptides have hindered their clinical development. Here, we develop a chemically stapled lipopeptide inhibitor of SARS-CoV-2 as proof-of-concept for the platform. We show that our lead compound blocks infection by a spectrum of SARS-CoV-2 variants, exhibits mucosal persistence upon nasal administration, demonstrates enhanced stability compared to prior analogs, and mitigates infection in hamsters. We further demonstrate that our stapled lipopeptide platform yields nanomolar inhibitors of respiratory syncytial, Ebola, and Nipah viruses by targeting heptad-repeat-1 domains, which exhibit strikingly low mutation rates, enabling on-demand therapeutic intervention to combat viral outbreaks.

Repurposing Navitoclax to block SARS-CoV-2 fusion and entry by targeting heptapeptide repeat sequence 1 in S2 protein

Along with the long pandemic of COVID-19 caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has come the dilemma of emerging viral variants of concern (VOC), particularly Omicron and its subvariants, able to deftly escape immune surveillance and the otherwise protective effect of current vaccines and antibody drugs. We previously identified a peptide-based pan-CoV fusion inhibitor, termed as EK1, able to bind the HR1 region in viral spike (S) protein S2 subunit. This effectively blocked formation of the six-helix bundle (6-HB) fusion core and, thus, showed efficacy against all human coronaviruses (HCoVs). EK1 is now in phase 3 clinical trials. However, the peptide drug generally lacks oral availability. Therefore, we herein performed a structure-based virtual screening of the libraries of biologically active molecules and identified nine candidate compounds. One is Navitoclax, an orally active anticancer drug by inhibition of Bcl-2. Like EK1 peptide, it could bind HR1 and block 6-HB formation, efficiently inhibiting fusion and infection of all SARS-CoV-2 variants tested, as well as SARS-CoV and MERS-CoV, with IC50 values ranging from 0.5 to 3.7 μM. These findings suggest that Navitoclax is a promising repurposed drug candidate for development as a safe and orally available broad-spectrum antiviral drug to combat the current SARS-CoV-2 and its variants, as well as other HCoVs.

Current state-of-the-art and potential future therapeutic drugs against COVID-19

The novel coronavirus disease (COVID-19) continues to endanger human health, and its therapeutic drugs are under intensive research and development. Identifying the efficacy and toxicity of drugs in animal models is helpful for further screening of effective medications, which is also a prerequisite for drugs to enter clinical trials. Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) invades host cells mainly by the S protein on its surface. After the SARS-CoV-2 RNA genome is injected into the cells, M protein will help assemble and release new viruses. RdRp is crucial for virus replication, assembly, and release of new virus particles. This review analyzes and discusses 26 anti-SARS-CoV-2 drugs based on their mechanism of action, effectiveness and safety in different animal models. We propose five drugs to be the most promising to enter the next stage of clinical trial research, thus providing a reference for future drug development.

IgG Fc-Binding Peptide-Conjugated Pan-CoV Fusion Inhibitor Exhibits Extended In Vivo Half-Life and Synergistic Antiviral Effect When Combined with Neutralizing Antibodies

The peptide-based pan-coronavirus fusion inhibitor EK1 is in phase III clinical trials, and it has, thus far, shown good clinical application prospects against SARS-CoV-2 and its variants. To further improve its in vivo long-acting property, we herein developed an Fc-binding strategy by conjugating EK1 with human immunoglobulin G Fc-binding peptide (IBP), which can exploit the long half-life advantage of IgG in vivo. The newly engineered peptide IBP-EK1 showed potent and broad-spectrum inhibitory activity against SARS-CoV-2 and its variants, including various Omicron sublineages and other human coronaviruses (HCoVs) with low cytotoxicity. In mouse models, IBP-EK1 possessed potent prophylactic and therapeutic efficacy against lethal HCoV-OC43 challenge, and it showed good safety profile and low immunogenicity. More importantly, IBP-EK1 exhibited a significantly extended in vivo half-life in rhesus monkeys of up to 37.7 h, which is about 20-fold longer than that reported for EK1. Strikingly, IBP-EK1 displayed strong in vitro or ex vivo synergistic anti-HCoV effect when combined with monoclonal neutralizing antibodies, including REGN10933 or S309, suggesting that IBP-conjugated EK1 can be further developed as a long-acting, broad-spectrum anti-HCoV agent, either alone or in combination with neutralizing antibodies, to combat the current COVID-19 pandemic or future outbreaks caused by emerging and re-emerging highly pathogenic HCoVs.

A dePEGylated Lipopeptide-Based Pan-Coronavirus Fusion Inhibitor Exhibits Potent and Broad-Spectrum Anti-HIV-1 Activity without Eliciting Anti-PEG Antibodies

We previously identified a lipopeptide, EK1C4, by linking cholesterol to EK1, a pan-CoV fusion inhibitory peptide via a polyethylene glycol (PEG) linker, which showed potent pan-CoV fusion inhibitory activity. However, PEG can elicit antibodies to PEG in vivo, which will attenuate its antiviral activity. Therefore, we designed and synthesized a dePEGylated lipopeptide, EKL1C, by replacing the PEG linker in EK1C4 with a short peptide. Similar to EK1C4, EKL1C displayed potent inhibitory activity against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and other coronaviruses. In this study, we found that EKL1C also exhibited broad-spectrum fusion inhibitory activity against human immunodeficiency virus type 1 (HIV-1) infection by interacting with the N-terminal heptad repeat 1 (HR1) of viral gp41 to block six-helix bundle (6-HB) formation. These results suggest that HR1 is a common target for the development of broad-spectrum viral fusion inhibitors and EKL1C has potential clinical application as a candidate therapeutic or preventive agent against infection by coronavirus, HIV-1, and possibly other class I enveloped viruses.

HR121 targeting HR2 domain in S2 subunit of spike protein can serve as a broad-spectrum SARS-CoV-2 inhibitor via intranasal administration

The continuously emerging SARS-CoV-2 variants pose a great challenge to the efficacy of current drugs, this necessitates the development of broad-spectrum antiviral drugs. In the previous study, we designed a recombinant protein, heptad repeat (HR) 121, as a variant-proof vaccine. Here, we found it can act as a fusion inhibitor and demonstrated broadly neutralizing activities against SARS-CoV-2 and its main variants. Structure analysis suggested that HR121 targets the HR2 domain in SARS-CoV-2 spike (S) 2 subunit to block virus-cell fusion. Functional experiments demonstrated that HR121 can bind HR2 at serological-pH and endosomal-pH, highlighting its inhibition capacity when SARS-CoV-2 enters via either cellular membrane fusion or endosomal route. Importantly, HR121 can effectively inhibit SARS-CoV-2 and Omicron variant pseudoviruses entering the cells, as well as block authentic SARS-CoV-2 and Omicron BA.2 replications in human pulmonary alveolar epithelial cells. After intranasal administration to Syrian golden hamsters, it can protect hamsters from SARS-CoV-2 and Omicron BA.2 infection. Together, our results suggest that HR121 is a potent drug candidate with broadly neutralizing activities against SARS-CoV-2 and its variants.

Optimization, and biological evaluation of 3-O-β-chacotriosyl betulinic acid amide derivatives as novel small-molecule Omicron

SARS-CoV-2 Omicron viruses possess a high antigenic shift, and the approved anti-SARS-CoV-2 drugs are extremely limited, which makes the development of new antiviral drugs for the clinical treatment and prevention of SARS-CoV-2 outbreaks imperative. We have previously discovered a new series of markedly potent small-molecule inhibitors of SARS-CoV-2 virus entry, exampled by the hit compound 2. Here, we report a further study of bioisosteric replacement of the eater linker at the C-17 position of 2 with a variety of aromatic amine moieties, followed by a focused structure-activity relationship study, leading to the discovery of a series of novel 3-O-β-chacotriosyl BA amide derivatives as small-molecule Omicron fusion inhibitors with improved potency and selectivity index. Particularly, our medicinal chemistry efforts have resulted in a potent, and efficacious lead compound S-10 with appreciable pharmacokinetic properties, which exhibited broad-spectrum potency against Omicron and other variants with EC₅₀ values ranging from 0.82 to 5.45 μM. Mutagenesis studies confirmed that inhibition of Omicron viral entry was mediated by the direct interaction with S in the prefusion state. These results reveal that S-10 is suitable for further optimization as Omicron fusion inhibitors, with the potential to be developed as therapeutic agents for the treatment and control of SARS-CoV-2 ant its variants infections.

Targetable elements in SARS-CoV-2 S2 subunit for the design of pan-coronavirus fusion inhibitors and vaccines

The ongoing global pandemic of coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has caused devastating impacts on the public health and the global economy. Rapid viral antigenic evolution has led to the continual generation of new variants. Of special note is the recently expanding Omicron subvariants that are capable of immune evasion from most of the existing neutralizing antibodies (nAbs). This has posed new challenges for the prevention and treatment of COVID-19. Therefore, exploring broad-spectrum antiviral agents to combat the emerging variants is imperative. In sharp contrast to the massive accumulation of mutations within the SARS-CoV-2 receptor-binding domain (RBD), the S2 fusion subunit has remained highly conserved among variants. Hence, S2-based therapeutics may provide effective cross-protection against new SARS-CoV-2 variants. Here, we summarize the most recently developed broad-spectrum fusion inhibitors (e.g., nAbs, peptides, proteins, and small-molecule compounds) and candidate vaccines targeting the conserved elements in SARS-CoV-2 S2 subunit. The main focus includes all the targetable S2 elements, namely, the fusion peptide, stem helix, and heptad repeats 1 and 2 (HR1-HR2) bundle. Moreover, we provide a detailed summary of the characteristics and action-mechanisms for each class of cross-reactive fusion inhibitors, which should guide and promote future design of S2-based inhibitors and vaccines against new coronaviruses.

DRAVP: A Comprehensive Database of Antiviral Peptides and Proteins

Viruses with rapid replication and easy mutation can become resistant to antiviral drug treatment. With novel viral infections emerging, such as the recent COVID-19 pandemic, novel antiviral therapies are urgently needed. Antiviral proteins, such as interferon, have been used for treating chronic hepatitis C infections for decades. Natural-origin antimicrobial peptides, such as defensins, have also been identified as possessing antiviral activities, including direct antiviral effects and the ability to induce indirect immune responses to viruses. To promote the development of antiviral drugs, we constructed a data repository of antiviral peptides and proteins (DRAVP). The database provides general information, antiviral activity, structure information, physicochemical information, and literature information for peptides and proteins. Because most of the proteins and peptides lack experimentally determined structures, AlphaFold was used to predict each antiviral peptide's structure. A free website for users (http://dravp.cpu-bioinfor.org/, accessed on 30 August 2022) was constructed to facilitate data retrieval and sequence analysis. Additionally, all the data can be accessed from the web interface. The DRAVP database aims to be a useful resource for developing antiviral drugs.

Development of variant-proof severe acute respiratory syndrome coronavirus 2, pan-sarbecovirus, and pan-β-coronavirus vaccines

The newly emerged severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants with high transmission rates and striking immune evasion have posed a serious challenge to the application of current first-generation SARS-CoV-2 vaccines. Other sarbecoviruses, such as SARS-CoV and SARS-related coronaviruses (SARSr-CoVs), have the potential to cause outbreaks in the future. These facts call for the development of variant-proof SARS-CoV-2, pan-sarbecovirus or pan-β-CoV vaccines. Several novel vaccine platforms have been used to develop vaccines with broad-spectrum neutralizing antibody responses and protective immunity to combat the current SARS-CoV-2 and its variants, other sarbecoviruses, as well as other β-CoVs, in the future. In this review, we discussed the major target antigens and protective efficacy of current SARS-CoV-2 vaccines and summarized recent advances in broad-spectrum vaccines against sarbecoviruses and β-CoVs.

Pan-coronavirus fusion inhibitors to combat COVID-19 and other emerging coronavirus infectious diseases

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of the currently ongoing coronavirus disease 2019 (COVID-19) pandemic, has posed a serious threat to global public health. Recently, several SARS-CoV-2 variants of concern (VOCs) have emerged and caused numerous cases of reinfection in convalescent COVID-19 patients, as well as breakthrough infections in vaccinated individuals. This calls for the development of broad-spectrum antiviral drugs to combat SARS-CoV-2 and its VOCs. Pan-coronavirus fusion inhibitors, targeting the conserved heptad repeat 1 (HR1) in spike protein S2 subunit, can broadly and potently inhibit infection of SARS-CoV-2 and its variants, as well as other human coronaviruses. In this review, we summarized the most recent development of pan-coronavirus fusion inhibitors, such as EK1, EK1C4, and EKL1C, and highlighted their potential application in combating current COVID-19 infection and reinfection, as well as future emerging coronavirus infectious diseases.

Exploring Highly Conserved Regions of SARS-CoV-2 Spike S2 Subunit as Targets for Fusion Inhibition Using Chimeric Proteins

Since the beginning of the COVID-19 pandemic, considerable efforts have been made to develop protective vaccines against SARS-CoV-2 infection. However, immunity tends to decline within a few months, and new virus variants are emerging with increased transmissibility and capacity to evade natural or vaccine-acquired immunity. Therefore, new robust strategies are needed to combat SARS-CoV-2 infection. The viral spike composed of S1 and S2 subunits mediates viral attachment and membrane fusion to infect the host cell. In this process, interaction between the highly conserved heptad repeat 1 and 2 regions (HR1 and HR2) of S2 is crucial and for this reason; these regions are promising targets to fight SARS-CoV-2. Here, we describe the design and characterization of chimeric proteins that structurally imitate the S2 HR1 region in a trimeric coiled-coil conformation. We biophysically characterized the proteins and determined their capacity to bind the HR2 region, as well as their inhibitory activity of SARS-CoV-2 infection in vitro. HR1 mimetic proteins showed conformational heterogeneity and a propensity to form oligomers. Moreover, their structure is composed of subdomains with varied stability. Interestingly, the full HR1 proteins showed high affinity for HR2-derived peptides and SARS-CoV-2 inhibitory activity, whereas smaller proteins mimicking HR1 subdomains had a decreased affinity for their complementary HR2 region and did not inhibit the virus. The results provide insight into effective strategies to create mimetic proteins with broad inhibitory activity and therapeutic potential against SARS-CoV-2.

Sensitivity to Vaccines, Therapeutic Antibodies, and Viral Entry Inhibitors and Advances To Counter the SARS-CoV-2 Omicron Variant

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) keeps evolving and mutating into newer variants over time, which gain higher transmissibility, disease severity, and spread in communities at a faster rate, resulting in multiple waves of surge in Coronavirus Disease 2019 (COVID-19) cases. A highly mutated and transmissible SARS-CoV-2 Omicron variant has recently emerged, driving the extremely high peak of infections in almost all continents at an unprecedented speed and scale. The Omicron variant evades the protection rendered by vaccine-induced antibodies and natural infection, as well as overpowers the antibody-based immunotherapies, raising the concerns of current effectiveness of available vaccines and monoclonal antibody-based therapies. This review outlines the most recent advancements in studying the virology and biology of the Omicron variant, highlighting its increased resistance to current antibody-based therapeutics and its immune escape against vaccines. However, the Omicron variant is highly sensitive to viral fusion inhibitors targeting the HR1 motif in the spike protein, enzyme inhibitors, involving the endosomal fusion pathway, and ACE2-based entry inhibitors. Omicron variant-associated infectivity and entry mechanisms of Omicron variant are essentially distinct from previous characterized variants. Innate sensing and immune evasion of SARS-CoV-2 and T cell immunity to the virus provide new perspectives of vaccine and drug development. These findings are important for understanding SARS-CoV-2 viral biology and advances in developing vaccines, antibody-based therapies, and more effective strategies to mitigate the transmission of the Omicron variant or the next SARS-CoV-2 variant of concern.

A novel cyclic γ-AApeptide-based long-acting pan-coronavirus fusion inhibitor with potential oral bioavailability by targeting two sites in spike protein

The receptor-binding domain (RBD) in S1 subunit and heptad repeat 1 (HR1) domain in S2 subunit of SARS-CoV-2 spike (S) protein are the targets of neutralizing antibodies (nAbs) and pan-coronavirus (CoV) fusion inhibitory peptides, respectively. However, neither nAb- nor peptide-based drugs can be used orally. In this study, we screened a one-bead-two-compound (OBTC) cyclic γ-AApeptide library against SARS-CoV-2 S protein and identified a hit: S-20 with potent membrane fusion inhibitory activity, but moderate selectivity index (SI). After modification, one derivative, S-20-1, exhibited improved fusion inhibitory activity and SI (>1000). S-20-1 could effectively inhibit infection by pseudotyped and authentic SARS-CoV-2 and pseudotyped variants of concern (VOCs), including B.1.617.2 (Delta) and B.1.1.529 (Omicron), as well as MERS-CoV, SARS-CoV, HCoV-OC43, HCoV-229E, and HCoV-NL63. It could also inhibit infection of a pseudotyped SARS-related coronavirus WIV1 (SARSr-CoV-WIV1) from bats. Intranasal application of S-20-1 to mice before or after challenge with HCoV-OC43 or SARS-CoV-2 provided significant protection from infection. Importantly, S-20-1 was highly resistant to proteolytic degradation, had long half-life, and possessed favorable oral bioavailability. Mechanistic studies suggest that S-20-1 binds with high affinity to RBD in S1 and HR1 domain in S2 of SARS-CoV-2 S protein. Thus, with its pan-CoV fusion and entry inhibitory activity by targeting two sites in S protein, desirable half-life, and promising oral bioavailability, S-20-1 is a potential candidate for further development as a novel therapeutic and prophylactic drug against infection by SARS-CoV-2 and its variants, as well as future emerging and reemerging CoVs.

Macromolecular Viral Entry Inhibitors as Broad-Spectrum First-Line Antivirals with Activity against SARS-CoV-2

Inhibitors of viral cell entry based on poly(styrene sulfonate) and its core-shell nanoformulations based on gold nanoparticles are investigated against a panel of viruses, including clinical isolates of SARS-CoV-2. Macromolecular inhibitors are shown to exhibit the highly sought-after broad-spectrum antiviral activity, which covers most analyzed enveloped viruses and all of the variants of concern for SARS-CoV-2 tested. The inhibitory activity is quantified in vitro in appropriate cell culture models and for respiratory viral pathogens (respiratory syncytial virus and SARS-CoV-2) in mice. Results of this study comprise a significant step along the translational path of macromolecular inhibitors of virus cell entry, specifically against enveloped respiratory viruses.

A Modified Fibronectin Type III Domain-Conjugated, Long-Acting Pan-Coronavirus Fusion Inhibitor with Extended Half-Life

The coronavirus disease 2019 (COVID-19) pandemic caused by infection of SARS-CoV-2 and its variants has posed serious threats to global public health, thus calling for the development of potent and broad-spectrum antivirals. We previously designed and developed a peptide-based pan-coronavirus (CoV) fusion inhibitor, EK1, which is effective against all human CoVs (HCoV) tested by targeting the HCoV S protein HR1 domain. However, its relatively short half-life may limit its clinical use. Therefore, we designed, constructed, and expressed a recombinant protein, FL-EK1, which consists of a modified fibronectin type III domain (FN3) with albumin-binding capacity, a flexible linker, and EK1. As with EK1, we found that FL-EK1 could also effectively inhibit infection of SARS-CoV-2 and its variants, as well as HCoV-OC43. Furthermore, it protected mice from infection by the SARS-CoV-2 Delta variant and HCoV-OC43. Importantly, the half-life of FL-EK1 (30 h) is about 15.7-fold longer than that of EK1 (1.8 h). These results suggest that FL-EK1 is a promising candidate for the development of a pan-CoV fusion inhibitor-based long-acting antiviral drug for preventing and treating infection by current and future SARS-CoV-2 variants, as well as other HCoVs.

A Five−Helix−Based SARS−CoV−2 Fusion Inhibitor Targeting Heptad Repeat 2 Domain against SARS−CoV−2 and Its Variants of Concern

The prolonged duration of the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) pandemic has resulted in the continuous emergence of variants of concern (VOC, e.g., Omicron) and variants of interest (VOI, e.g., Lambda). These variants have challenged the protective efficacy of current COVID-19 vaccines, thus calling for the development of novel therapeutics against SARS-CoV-2 and its VOCs. Here, we constructed a novel fusion inhibitor-based recombinant protein, denoted as 5-Helix, consisting of three heptad repeat 1 (HR1) and two heptad repeat 2 (HR2) fragments. The 5-Helix interacted with the HR2 domain of the viral S2 subunit, the most conserved region in spike (S) protein, to block homologous six-helix bundle (6-HB) formation between viral HR1 and HR2 domains and, hence, viral S-mediated cell–cell fusion. The 5-Helix potently inhibited infection by pseudotyped SARS-CoV-2 and its VOCs, including Delta and Omicron variants. The 5-Helix also inhibited infection by authentic SARS-CoV-2 wild-type (nCoV-SH01) strain and its Delta variant. Collectively, our findings suggest that 5-Helix can be further developed as either a therapeutic or prophylactic to treat and prevent infection by SARS-CoV-2 and its variants.

A Palmitic Acid-Conjugated, Peptide-Based pan-CoV Fusion Inhibitor Potently Inhibits Infection of SARS-CoV-2 Omicron and Other Variants of Concern

Our previous studies have shown that cholesterol-conjugated, peptide-based pan-coronavirus (CoV) fusion inhibitors can potently inhibit human CoV infection. However, only palmitic acid (C16)-based lipopeptide drugs have been tested clinically, suggesting that the development of C16-based lipopeptide drugs is feasible. Here, we designed and synthesized a C16-modified pan-CoV fusion inhibitor, EK1-C16, and found that it potently inhibited infection by SARS-CoV-2 and its variants of concern (VOCs), including Omicron, and other human CoVs and bat SARS-related CoVs (SARSr-CoVs). These results suggest that EK1-C16 could be further developed for clinical use to prevent and treat infection by the currently circulating MERS-CoV, SARS-CoV-2 and its VOCs, as well as any future emerging or re-emerging coronaviruses.

Stapled Peptides Targeting SARS-CoV-2 Spike Protein HR1 Inhibit the Fusion of Virus to Its Cell Receptor

The pandemic of acute respiratory disease in 2019 caused by highly pathogenic and infectious SARS-CoV-2 has seriously endangered human public safety. The 6-HB (HR1-HR2 complex) formation occurring in the process of spike protein-mediated membrane fusion could serve as a conserved and potential target for the design of fusion inhibitors. Based on the HR2 domain of 6-HB, we designed and synthesized 32 stapled peptides using an all-hydrocarbon peptide stapling strategy. Owing to the improved proteolytic stability and higher helical contents, the optimized stapled peptides termed SCH2-1-20 and SCH2-1-27 showed better inhibitory activities against pseudo and authentic SARS-CoV-2 compared to the linear counterpart. Of note, SCH2-1-20 and SCH2-1-27 were proved to interfere with S protein-mediated membrane fusion. Structural modeling indicated similar binding modes between SCH2-1-20 and the linear peptide. These optimized stapled peptides could serve as potent fusion inhibitors in treating and preventing SARS-CoV-2, and the corresponding SAR could facilitate further optimization.