EK1 Peptide: The Pan-Coronavirus Fusion Inhibitor
Coronavirus Fusion Inhibitor Peptides
0.19 micromolar IC50
EK1, a 36-amino-acid peptide, inhibited five distinct human coronaviruses with IC50 values between 0.19 and 0.62 micromolar in the first report of a true pan-coronavirus fusion inhibitor.
Xia et al., Science Advances, 2019
Xia et al., Science Advances, 2019
View as imageSeven coronaviruses are known to infect humans. Four cause common colds. Three have caused epidemics that killed hundreds of thousands of people. Vaccines target specific spike protein sequences and lose efficacy as viruses mutate. Monoclonal antibodies face the same problem. EK1 takes a different approach. It is a 36-amino-acid peptide that targets a conserved structural feature shared by all coronaviruses: the heptad repeat 1 (HR1) domain of the spike protein. By blocking the physical machinery of membrane fusion rather than a mutable surface epitope, EK1 inhibits every human coronavirus tested, from the common cold virus HCoV-OC43 to SARS-CoV-2 and its variants.[1] Its lipopeptide derivative EK1C4 achieves nanomolar potency and protects mice from lethal coronavirus challenge when delivered intranasally.[2] This article covers EK1's design, mechanism, potency data, lipopeptide derivatives, in vivo evidence, and the open questions that remain. For deeper treatments of related topics, see the cluster articles on antiviral peptides and SARS-CoV-2 and how defensins may neutralize coronavirus at the ACE2 receptor.
Key Takeaways
- EK1 inhibited five human coronaviruses (HCoV-229E, HCoV-NL63, HCoV-OC43, SARS-CoV, MERS-CoV) with IC50 values from 0.19 to 0.62 micromolar, and crystal structures confirmed stable binding to HR1 domains from all five viruses (Xia et al., Science Advances, 2019).[1]
- EK1C4, a cholesterol-conjugated lipopeptide derivative, achieved IC50 values of 1.3 nM against membrane fusion and 15.8 nM against pseudovirus infection, representing a 241-fold and 149-fold improvement over unmodified EK1 (Xia et al., Cell Research, 2020).[2]
- Intranasal EK1C4 protected mice from SARS-CoV-2 challenge both prophylactically and therapeutically, supporting nasal spray delivery for clinical use (Xia et al., Signal Transduction and Targeted Therapy, 2021).[3]
- EK1 and EK1C4 also inhibited HIV-1, HIV-2, and simian immunodeficiency virus, suggesting the HR1-targeting mechanism extends beyond coronaviruses (Zhu et al., Emerging Microbes & Infections, 2021).[4]
- A 25-hydroxycholesterol-modified variant, EK1P4HC, retained broad-spectrum activity against SARS-CoV-2 variants of concern and protected newborn mice from lethal HCoV-OC43 infection (Lan et al., Int J Mol Sci, 2021).[5]
- Over 60 peptide-based drugs have been approved by the FDA across therapeutic areas, but no peptide fusion inhibitor for coronaviruses has yet reached the market (Murugan et al., Advances in Experimental Medicine and Biology, 2021).[6]
How coronaviruses fuse with human cells
Every coronavirus carries a spike (S) protein on its surface. The spike is a trimeric glycoprotein that performs two sequential jobs: it binds a receptor on the host cell, and it then mediates fusion of the viral and cellular membranes. These are mechanically distinct steps, and EK1 targets only the second one.
After receptor binding triggers a conformational change, the S2 subunit of the spike protein exposes a fusion peptide that inserts into the host cell membrane. Two heptad repeat regions within S2, called HR1 and HR2, then fold toward each other. Three HR1 helices form a central coiled-coil trimer, and three HR2 helices pack into the grooves of that trimer, creating a structure called the six-helix bundle (6-HB). This 6-HB formation pulls the viral membrane and the host membrane together, forcing them to merge.[1]
This fusion mechanism is conserved across all coronaviruses. It is also conserved, with structural variations, across other enveloped viruses including HIV, influenza, Ebola, and paramyxoviruses. Enfuvirtide, the first peptide-based antiviral approved for clinical use (HIV-1, approved 2003), works by the same principle: it mimics the HR2 region and blocks 6-HB assembly. Kadam et al. (2017) demonstrated the same approach for influenza, designing cyclic peptides that blocked hemagglutinin-mediated fusion at nanomolar concentrations in mice.[7] EK1 applies this established strategy to the entire coronavirus family.
The HR1 domain is the key target because it is structurally constrained. Mutations in HR1 that disrupt the coiled-coil architecture would compromise the virus's ability to fuse, making resistance less likely than for receptor-binding domain mutations, which can change without eliminating cell entry.
How EK1 was designed
EK1 emerged from the laboratory of Shibo Jiang and Lu Lu at Fudan University. The team started with HR2P, a peptide derived from the HR2 domain of HCoV-OC43, one of the four common-cold coronaviruses. HR2P could inhibit OC43 but was weak against other coronaviruses. Through a systematic optimization campaign that introduced charged and hydrophobic residues at key positions along the peptide, the researchers identified a 36-amino-acid sequence that formed stable complexes with the HR1 domains of multiple divergent coronaviruses.[1]
The "EK" in the name comes from the introduction of glutamic acid (E) and lysine (K) residues at solvent-exposed positions, which form salt bridges that stabilize the alpha-helical structure. The "1" designates it as the lead candidate. X-ray crystallography confirmed that EK1 binds to the hydrophobic groove of HR1 trimers from HCoV-229E, HCoV-NL63, HCoV-OC43, SARS-CoV, and MERS-CoV, despite substantial sequence variation in the HR1 domains of these five viruses. The peptide tolerates this variation because it contacts conserved structural features of the coiled-coil rather than specific side chains.[1]
In cell-based assays, EK1 inhibited all five coronaviruses with IC50 values from 0.19 micromolar (HCoV-OC43) to 0.62 micromolar (HCoV-NL63). In an in vivo model, intranasal administration of EK1 protected mice from lethal HCoV-OC43 challenge, with protection lasting for hours after a single dose.[1]
EK1C4: lipid conjugation boosts potency 241-fold
EK1 at micromolar potency was a proof of concept. The next step was to increase its potency to levels compatible with clinical development. Xia et al. (2020) achieved this by conjugating a cholesterol group to the C-terminus of EK1 via a polyethylene glycol (PEG) linker, creating the lipopeptide EK1C4.[2]
The cholesterol moiety anchors EK1C4 in the cell membrane, positioning the peptide at the site where viral fusion occurs. This membrane-targeting strategy concentrates the inhibitor where it is needed rather than leaving it diluted in the extracellular fluid. The result was dramatic: EK1C4 inhibited SARS-CoV-2 spike-mediated membrane fusion with an IC50 of 1.3 nM and pseudovirus infection with an IC50 of 15.8 nM. These represent 241-fold and 149-fold improvements over unmodified EK1, respectively.[2]
Lipid conjugation is not a new idea in peptide antiviral design. Bulevirtide (Hepcludex), the peptide that blocks hepatitis B and D viral entry, uses a myristoyl (C14 fatty acid) group for the same purpose: anchoring the peptide near the membrane where viral entry occurs. The parallels are instructive. Both bulevirtide and EK1C4 are lipopeptide entry inhibitors that achieve low-nanomolar potency through membrane targeting, and both are administered locally (subcutaneous injection for bulevirtide, intranasal for EK1C4).[12]
EK1C4 retained broad-spectrum activity. It inhibited not only SARS-CoV-2 but also SARS-CoV, MERS-CoV, HCoV-229E, HCoV-NL63, and HCoV-OC43 at nanomolar concentrations. Crystal structures confirmed that the cholesterol conjugation did not alter how the peptide engaged the HR1 domain; it simply delivered more peptide to the membrane surface where spike proteins were undergoing conformational changes.[3]
Beyond cholesterol: alternative lipid modifications
The success of EK1C4 prompted exploration of other lipid conjugation strategies. Lan et al. (2021) conjugated 25-hydroxycholesterol (25-HC), a natural oxysterol with independent antiviral properties, to EK1. The resulting lipopeptide, EK1P4HC, combined two mechanisms: the fusion-inhibiting activity of EK1 and the membrane-disrupting antiviral activity of 25-HC. When tested separately, EK1 and 25-HC showed synergistic anti-SARS-CoV-2 activity, meaning their combination was more potent than either component alone.[5]
EK1P4HC was effective against SARS-CoV-2 and its variants of concern (Alpha, Beta, Gamma, Delta at the time of publication), as well as HCoV-OC43 and HCoV-229E. In a mouse model, EK1P4HC protected newborn mice from lethal HCoV-OC43 infection.[5] Whether 25-HC conjugation offers a clinical advantage over cholesterol conjugation (EK1C4) remains untested in head-to-head comparisons.
In vivo evidence and intranasal delivery
The most clinically relevant finding from EK1 research is that intranasal delivery works. Respiratory viruses enter through the nose and upper airways, and a nasal spray that blocks viral fusion at the site of initial infection represents a conceptually elegant prevention strategy.
Xia et al. (2021) reported preclinical evaluation of EK1C4 as a nasal spray. Intranasal administration of EK1C4 before SARS-CoV-2 challenge protected mice from infection (prophylactic use). Administration after challenge also reduced viral load (therapeutic use), though the protective effect was strongest when the peptide was delivered before or shortly after viral exposure.[3]
The nasal delivery route carries advantages specific to peptide drugs. Peptides are notoriously difficult to deliver orally because gastric enzymes destroy them and intestinal absorption is poor. Murugan et al. (2021) reviewed the landscape of peptide-based antiviral drugs and noted that while over 60 peptide drugs have received FDA approval, stability and bioavailability limitations remain central challenges for the class.[6] Intranasal delivery sidesteps these problems entirely: the peptide acts locally on the airway epithelium where coronaviruses attach and fuse, never needing to survive systemic circulation. This is the same logic that drives development of antiviral peptide nasal sprays against influenza.
The EK1C4 nasal spray has advanced to Phase III clinical trials, though published results from these trials are not yet available. The gap between promising mouse data and confirmed human efficacy remains substantial, and the pharmacokinetics of mucosal peptide delivery, including residence time, distribution across the nasal epithelium, and clearance rates, still need clinical characterization.
Activity beyond coronaviruses: HIV and SIV
An unexpected finding expanded EK1's potential scope. Zhu et al. (2021) demonstrated that EK1 and EK1C4 possessed potent inhibitory activity against HIV-1, HIV-2, and simian immunodeficiency virus (SIV).[4]
This cross-family activity is less surprising than it might seem. HIV's gp41 protein uses the same HR1/HR2 six-helix bundle mechanism to fuse with host cells. Enfuvirtide, the approved HIV fusion inhibitor, works by blocking gp41's 6-HB formation, just as EK1 blocks the coronavirus spike's 6-HB. What was unexpected was that a peptide optimized for coronavirus HR1 domains could also interact productively with HIV's gp41 HR1, given that the two proteins share minimal sequence identity.
The finding suggests that the structural geometry of the HR1 coiled-coil trimer, rather than its specific amino acid sequence, is what EK1 recognizes. If true, this geometric targeting could potentially extend to other enveloped viruses that use class I fusion proteins, including paramyxoviruses, Ebola, and influenza. Kadam et al. (2017) had already shown that peptidic fusion inhibitors targeting influenza hemagglutinin's stem region could achieve nanomolar inhibition in mice, establishing that the strategy translates across virus families.[7]
However, the practical significance of EK1's anti-HIV activity is uncertain. HIV treatment is dominated by small-molecule antiretrovirals with oral bioavailability, established resistance profiles, and decades of clinical data. A peptide requiring nasal or injectable delivery would need to offer a substantial advantage to compete. The anti-HIV finding is more useful as mechanistic validation than as a clinical development path.
Competing approaches to peptide-based coronavirus inhibition
EK1 is not the only peptide strategy targeting coronavirus entry. Several alternative approaches have shown promise, each with distinct strengths and limitations.
Extended HR2 peptides. Yang et al. (2022) at Stanford designed an unmodified peptide based on an extended region of the SARS-CoV-2 HR2 domain that achieved single-digit nanomolar inhibition of SARS-CoV-2 without lipid conjugation, chemical stapling, or any other modification. The extended peptide targeted a prehairpin intermediate of the spike protein and maintained activity against all major SARS-CoV-2 variants. Washout experiments showed prolonged inhibition, suggesting the peptide trapped the spike in a non-functional conformation.[8] This approach challenges the assumption that lipid conjugation is necessary for nanomolar potency and raises the question of whether simpler, unmodified peptides could be easier to manufacture at scale.
ACE2-derived stapled peptides. Quagliata et al. (2023) designed conformationally constrained helical peptides derived from the ACE2 receptor's alpha-1 helix, the region that directly contacts the spike receptor-binding domain (RBD). Triazole-stapled analogs showed micromolar antiviral activity, though a double-stapled variant lost activity, indicating that excessive rigidity can be counterproductive.[9] Stapled peptide technology is advancing rapidly for antiviral applications, with hydrocarbon staples, lactam bridges, and metal-coordination bonds all under investigation to improve peptide stability and cellular penetration.[10]
D-peptides. Feng et al. (2025) developed mirror-image D-peptides targeting the SARS-CoV-2 RBD and main protease using computational design. The best RBD-targeting D-peptide achieved submicromolar binding and blocked spike-ACE2 interaction. D-peptides resist proteolytic degradation entirely because mammalian enzymes cannot cleave the mirror-image backbone, potentially solving the stability problem without lipid conjugation.[11]
Defensins. Human innate immune peptides called defensins can neutralize coronaviruses through a different mechanism. Kudryashova et al. (2022) showed that the alpha-defensin HNP1 bound SARS-CoV-2 spike protein with submicromolar affinity, more than 20-fold stronger than its binding to serum albumin, and physically destabilized the spike to block infection.[13] Xu et al. (2021) confirmed that HNP1, HD5, and the theta-defensin analog RC-101 blocked SARS-CoV-2 entry in both pseudovirus and live virus assays.[14] Unlike EK1, defensins are not sequence-optimized fusion inhibitors; they work by destabilizing the spike protein's tertiary structure, a variant-agnostic mechanism. Whether airway defensin levels are sufficient for meaningful protection in vivo remains unresolved.
Each of these approaches addresses a different vulnerability in the coronavirus entry process. EK1's advantage is its established broad-spectrum data across the full coronavirus family and its advanced clinical development. Its limitation is the same one facing all peptide antivirals: delivery, cost, and the narrow therapeutic window of a locally acting drug.
What the evidence does not yet show
EK1's preclinical profile is strong, but critical gaps remain.
No published human efficacy data. Phase III clinical trials of EK1C4 nasal spray are reported to be underway, but no peer-reviewed results from human trials have been published. Mouse protection studies are encouraging but not predictive of human outcomes. The pharmacokinetics of intranasal peptide delivery in human airways, including mucosal residence time and distribution, are unknown from published data.
Resistance potential is theoretical. The argument that HR1-targeting fusion inhibitors face lower resistance pressure than RBD-targeting agents is structurally plausible but not validated in clinical practice. Enfuvirtide, the approved HIV fusion inhibitor, did encounter resistance mutations in gp41's HR1 domain in clinical use, though these mutations carried fitness costs for the virus. Whether coronaviruses could similarly develop EK1-resistance mutations under sustained drug pressure is unknown.
Manufacturing and stability. Peptide drugs are expensive to produce relative to small molecules. A 36-amino-acid peptide with a cholesterol conjugate (EK1C4) requires solid-phase peptide synthesis followed by chemical conjugation. Mashhadi et al. (2025) noted that while AI-driven design is accelerating peptide drug discovery, translational delivery platforms and manufacturing scalability remain bottlenecks for the antiviral peptide field as a whole.[15]
Duration of protection. Intranasal peptides are cleared from mucosal surfaces within hours. A nasal spray requiring multiple daily applications would face adherence challenges for prophylactic use. Whether lipid conjugation extends mucosal residence time sufficiently for once- or twice-daily dosing is not established.
Cold-chain requirements. Peptide drugs typically require refrigerated storage. A pandemic-response therapeutic ideally needs room-temperature stability for global distribution, and EK1C4's stability profile under distribution conditions has not been publicly characterized.
The fusion peptide as a vaccine and antibody target
A separate line of research focuses not on synthetic peptides that mimic HR2 but on the immune response to the spike's own fusion peptide (FP), a short hydrophobic segment within S2. Roederer et al. (2026) found that natural SARS-CoV-2 infection, but not mRNA vaccination alone, elicited antibodies targeting the FP region. A rare subset of donors produced FP-directed broadly neutralizing antibodies capable of neutralizing across the sarbecovirus subfamily.[16]
This finding connects to EK1 research in an important way. Both the FP-directed antibodies and EK1 work by targeting conserved S2 regions rather than the immunodominant but mutation-prone RBD. The discovery that the human immune system can, in rare cases, generate broadly neutralizing antibodies against the fusion machinery validates the biological premise underlying EK1: that the fusion apparatus is a viable, variant-resistant therapeutic target. It also suggests that next-generation vaccines designed to elicit FP-directed antibodies could complement peptide-based fusion inhibitors as part of a broader pandemic preparedness strategy involving antiviral peptides.
Where EK1 fits in pandemic preparedness
The COVID-19 pandemic revealed a structural gap in antiviral preparedness. Vaccines took months to develop, monoclonal antibodies lost efficacy as variants emerged, and small-molecule antivirals (remdesivir, Paxlovid) carried limitations of their own. A broad-spectrum, mutation-resistant antiviral that could be deployed rapidly against any coronavirus, including novel spillovers, would fill a role that no current drug occupies.
EK1 and its derivatives are positioned for this role. Because they target HR1-mediated fusion rather than RBD-mediated receptor binding, they should retain activity against novel coronaviruses with divergent RBDs. The original 2019 study demonstrated this by inhibiting five coronaviruses spanning both the alpha and beta genera.[1]
The concept of a pan-coronavirus prophylactic nasal spray is appealing. In a scenario where a novel coronavirus emerges, EK1C4 could theoretically be deployed immediately while vaccines are developed. This assumes that manufacturing can scale rapidly, that the nasal spray format provides sufficient protection, and that Phase III trials confirm the preclinical promise. None of these assumptions are yet proven.
Cyclotides, ultra-stable plant peptides that destroy viral envelopes through a distinct membrane-disruption mechanism, represent another broad-spectrum approach to antiviral peptide design. Both cyclotides and EK1 illustrate how peptide-based antivirals may complement vaccines and antibodies by targeting conserved viral features that are difficult to mutate away.
The Bottom Line
EK1 is a 36-amino-acid peptide that inhibits all tested human coronaviruses by targeting the conserved HR1 domain of the spike protein, preventing six-helix bundle formation and membrane fusion. Its lipopeptide derivative EK1C4 achieves nanomolar potency and protects mice intranasally. Phase III clinical trials are underway but no human efficacy data have been published. The evidence is strong at the preclinical level and the mechanism is sound, but the distance between mouse protection and a viable human therapeutic remains substantial.