The M-T hook structure is critical for design of HIV-1 fusion inhibitors

Heptad repeat 1-derived N peptide inhibitors improve broad-spectrum anti-HIV-1 activity

Background

HIV-1 N-peptide inhibitor (NPI) derived from N-terminal heptad-repeat region (HR1) of gp41 can target C-terminal heptad-repeat region (HR2) or the HR1 to interfere with the formation of endogenous six-helix bundle (6HB). However, the NPI is less active than the C-peptide inhibitor. In this study, we reported three HR1-derived NPIs designed by adding fusion peptide proximal region (FPPR) of gp41 or a trimeric motif MTQ into the N36 peptide and then evaluated their anti-HIV-1 activities.

Methods

Molecular modeling was performed using Swiss Model. The inhibitory activity of NPIs on HIV-1 was assessed by Env-pseudovirus infection assays and cell-cell fusion assays. Interaction between NPIs and HR2 peptides was evaluated by circular dichroism and Native PAGE.

Results

The three newly designed NPIs, FPPR-N36, MTQ-N36, and MTQ-FPPR-N36, exhibited higher anti-HIV-1 activity than N36. The stability of the coiled-coil core formed by three designed NPIs or the 6HB formed by C34 and these NPIs were significantly higher than those of corresponding monomer N36 or isoleucine zipper-engineered trimeric N36 (IZN36). The 50 % inhibitory concentrations (IC50) of MTQ-N36 against HIV-1 infection were at a nanomolar level, lower than those of other tested NPIs. The FPPR-N36 could also inhibit infection of HIV-1 strains that were resistant to N36 and IZN36.

Conclusions

The three newly designed NPIs had inhibitory activity against HIV-1 infection. Among them, MTQ-N36 exhibited a higher potential to inhibit HIV-1 entry than other peptides, and FPPR-N36 might be a promising candidate NPI for suppressing HIV-1 strains that are resistant to conventional NPIs.

Research Strategy for Short-peptide Fusion Inhibitors Based on 6-HB Core Structure against HIV-1: A Review

Acquired Immune Deficiency Syndrome (AIDS) is a devastating infectious disease caused by the Human Immunodeficiency Virus type 1 (HIV-1). Enfuvirtide (T20) is the first HIV-1 fusion inhibitor for marketing, which plays an important role in AIDS treatment. However, in the clinical application process, T20 has several drawbacks, such as a high level of development of drug resistance, a short half-life in vivo, and rapid renal clearance, which severely limits the clinical application. Therefore, the development of novel fusion inhibitors to address T20 shortcomings has long been the research hotspot. Short peptides have a long half-life through modification and a high barrier to drug resistance, which is expected to solve the current fusion inhibitors dilemma. In this paper, we summarized six emerging R&D strategies for short peptide-based fusion inhibitors against HIV-1. We hope that this review will provide fresh insights into the development of novel fusion inhibitors, as well as ideas for other viral fusion inhibitor discoveries based on the common membrane fusion 6-HB core structure.

Development of lipopeptide-based HIV-1/2 fusion inhibitors targeting the gp41 pocket site with a new design strategy

Emerging studies demonstrate that lipid conjugation is a vital strategy for designing peptide-based viral fusion inhibitors, and the so-called lipopeptides exhibit greatly improved antiviral activity. In the design of lipopeptides, a flexible linker between the peptide sequence and lipid molecule is generally required, mostly with a short polyethylene glycol or glycine-serine sequence. Very recently, we discovered that the helix-facilitating amino acid sequence "EAAAK" as a rigid linker is a more efficient method in the design of SARS-CoV-2 fusion inhibitory lipopeptides. In this study, we comprehensively characterized the functionalities of different linkers in HIV fusion inhibitors. A short-peptide inhibitor 2P23, which mainly targets the gp41 pocket site, was used as a design template, generating a group of cholesterol-modified lipopeptides. In the inhibition of HIV-1 infection, the lipopeptide inhibitors with a rigid linker were much superior than those with the flexible linkers, as indicated by LP-37 with the "EAAAK" linker and LP-39 with the repeated "EP" amino acid sequences. Both lipopeptides were very potent inhibitors of HIV-2 and simian immunodeficiency (SIV) either. Promisingly, LP-37 displayed high α-helicity, thermostability and binding ability to a target-mimic peptide, and it was metabolically stable when treated with temperature, proteolytic enzymes or human sera. Taken together, our studies have verified a universal strategy for designing viral fusion inhibitors and offered a novel HIV fusion inhibitor for drug development.

Characterization of novel HIV fusion-inhibitory lipopeptides with the M-T hook structure

Combination antiretroviral therapy (cART) has significantly improved the survival of HIV-infected individuals, but long-term treatment can cause side-effects and drug resistance; thus, the development of new antivirals is of importance. We previously identified an M-T hook structure and accordingly designed short-peptide fusion inhibitor 2P23, which mainly targets the gp41 pocket site and displays potent, broad-spectrum anti-HIV activity. In this study, we continuingly characterized the amino acid sequences of peptide and lipopeptide-based inhibitors containing the M-T hook residues. Among a group of lipopeptides, stearic acid (C18)-modified LP-25 and LP-29 exhibited greatly improved inhibitions against divergent HIV-1 subtypes and drug-resistant mutants. LP-25 and LP-29 were evaluated in rhesus macaques, and the ex vivo inhibition data demonstrated their potent, long-lasting in vivo anti-HIV activity, with LP-25 much better than LP-29. Both the lipopeptides displayed high α-helicity, thermostability and binding ability to a target-mimic peptide, and they were metabolically stable when treated with high temperature, proteolytic enzymes, human or monkey sera and human liver microsomes. Therefore, our studies have provided critical information for understanding the structure-activity relationship of HIV fusion inhibitors with the M-T hook structure and offered novel candidates for drug development.

A New Chimeric Antibody against the HIV-1 Fusion Inhibitory Peptide MT-C34 with a High Affinity and Fc-Mediated Cellular Cytotoxicity

Peptides from heptad repeat (HR1 and HR2) regions of gp41 are effective inhibitors of HIV-1 entry that block the fusion of viral and cellular membranes, but the generation of antibodies highly specific for these peptides is challenging. We have previously described a mouse hybridoma that recognizes MT-C34-related peptides derived from HR2. It was used for the selection of HIV-1-resistant CD4 lymphocytes engineered to express the MT-C34 peptide via a CRISPR/Cas9-mediated knock-in into the CXCR4 locus. In this study, we cloned variable domains of this antibody and generated a recombinant chimeric antibody (chAb) by combining it with the constant regions of the humanized antibody Trastuzumab. The new chAb displayed a high specificity and two-fold higher level of affinity than the parental mouse monoclonal antibody. In addition, chAb mediated up to 27-43% of the antibody-dependent cellular cytotoxicity towards cells expressing MT-C34 on their surface. The anti-MT-C34 chAb can be easily generated using plasmids available for the research community and can serve as a valuable tool for the detection, purification, and even subsequent elimination of HIV-1-resistant CD4 cells or CAR cells engineered to fight HIV-1 infection.

Rational Design of Sulfonyl-γ-AApeptides as Highly Potent HIV-1 Fusion Inhibitors with Broad-Spectrum Activity

The HIV-1 epidemic has significant social and economic implications for public health. Developing new antivirus drugs to eradicate drug resistance is still urgently needed. Herein, we demonstrated that sulfonyl-γ-AApeptides could be designed to mimic MTSC22EK, one potent HIV fusion inhibitor derived from CHR. The best two sequences revealed comparable activity to MTSC22EK in an authentic HIV-1 infection assay and exhibited broad-spectrum anti-HIV-1 activity to many HIV-1 clinical isolates. Furthermore, sulfonyl-γ-AApeptides show remarkable resistance to proteolysis and favorable permeability in PAMPA-GIT and PAMPA-BBB assays, suggesting that both sequences could control HIV-1 within the central nervous system and possess promising oral bioavailability. Mechanistic investigations suggest that these sulfonyl-γ-AApeptides function by mimicking the CHR of gp41 and tightly bind with NHR, thereby inhibiting the formation of the 6-HB structure necessary for HIV-1 fusion. Overall, our results suggest that sulfonyl-γ-AApeptides represent a new generation of anti-HIV-1 fusion inhibitors. Moreover, this design strategy could be adopted to modulate many of the PPIs.

Broad-spectrum anti-HIV activity and high drug resistance barrier of lipopeptide HIV fusion inhibitor LP-19

Lipopeptide-19, a HIV fusion inhibitor (LP-19), has showed potent anti-HIV activity. However, there is still limited information of the antiviral activity against different subtype clinical isolates and the drug resistance barrier of LP-19. Therefore, 47 HIV clinical isolates were selected for this study. The viral features were identified, in which 43 strains are CCR5 tropisms, and 4 strains are CCR5/CXCR4 tropisms, and there are 6 subtype B', 15 CRF01_AE, 14 CRF07_BC, 2 CRF08_BC and 10 URF strains. These 47 viruses were used to detected and analyze the inhibitory activities of LP-19. The results showed that the average 50% inhibitory concentration (IC₅₀) and 90% inhibitory concentration (IC₉₀) of LP-19 were 0.50 nM and 1.88 nM, respectively. The average IC₅₀ of LP-19 to B', CRF01_AE, CRF07_BC, CRF08_BC, and URF strains was 0.76 nM, 0.29 nM, 0.38 nM, 0.85 nM, and 0.44 nM, respectively. C34 and Enfuvirtide (T-20), two fusion inhibitors, were compared on the corresponding strains simultaneously. The antiviral activity of LP-19 was 16.7-fold and 86-fold higher than that of C34 and T-20. The antiviral activity of LP-19, C34, and T-20 were further detected and showed IC₅₀ was 0.15 nM, 1.02 nM, and 66.19 nM, respectively. IC₅₀ of LP-19 was about 7-fold and 441-fold higher compared to C34 and T-20 against HIV-1 NL4-3 strains. NL4-3 strains were exposed to increasing concentrations of LP-19 and C34 in MT-2 cell culture. The culture virus was sequenced and analyzed. The results showed that A243V mutation site identified at weeks 28, 32, 38, and 39 of the cell culture in the gp41 CP (cytoplasmic domain) region. NL4-3/A243V viruses containing A243V mutation were constructed. Comparing the antiviral activities of LP-19 against HIV NL4-3 to HIV strains (only 1.3-fold), HIV did not show drug resistance when LP-19 reached 512-fold of the initial concentration under the drug pressure for 39 weeks. This study suggests that LP-19 has broad-spectrum anti-HIV activity, and high drug resistance barrier.

In Vitro Selection and Characterization of HIV-1 Variants with Increased Resistance to LP-40, Enfuvirtide-Based Lipopeptide Inhibitor

In our previous work, we replaced the TRM (tryptophan-rich motif) of T20 (Enfuvirtide) with fatty acid (C16) to obtain the novel lipopeptide LP-40, and LP-40 displayed enhanced antiviral activity. In this study, we investigated whether the C16 modification could enhance the high-resistance barrier of the inhibitor LP-40. To address this question, we performed an in vitro simultaneous screening of HIV-1_NL4-3 resistance to T20 and LP-40. The mechanism of drug resistance for HIV-1 Env was further studied using the expression and processing of the Env glycoprotein, the effect of the Env mutation on the entry and fusion ability of the virus, and an analysis of changes to the gp41 core structure. The results indicate that the LP-40 activity is enhanced and that it has a high resistance barrier. In a detailed analysis of the resistance sites, we found that mutations in L33S conferred a stronger resistance, except for the well-recognized mutations in amino acids 36–45 of gp41 NHR, which reduced the inhibitory activity of the CHR-derived peptides. The compensatory mutation of eight amino acids in the CHR region (NDQEEDYN) plays an important role in drug resistance. LP-40 and T20 have similar resistance mutation sites, and we speculate that the same resistance profile may arise if LP-40 is used in a clinical setting.

Engineering T-Cell Resistance to HIV-1 Infection via Knock-In of Peptides from the Heptad Repeat 2 Domain of gp41

Previous studies suggest that short peptides from the heptad repeat 2 (HR2) domain of gp41 expressed on the cell surface are more potent inhibitors of HIV-1 entry than soluble analogs. However, their therapeutic potential has only been examined using lentiviral vectors. Here, we aimed to develop CRISPR/Cas9-based fusion inhibitory peptide knock-in (KI) technology for the generation and selection of HIV-1-resistant T cells. First, we embedded a series of HIV-1 fusion inhibitory peptides in CD52, the shortest glycosylphosphatidylinositol (GPI)-anchored protein, which efficiently delivers epitope tags to the cell surface and maintains a sufficient level of KI. Among the seven peptides tested, MT-C34, HP-23L, and 2P23 exhibited significant activity against both cell-free and cell-to-cell HIV-1 infection. The shed variant of MT-C34 provided insufficient protection against HIV-1 due to its low concentration in the culture medium. Using Cas9 plasmids or ribonucleoprotein electroporation and peptide-specific antibodies, we sorted CEM/R5 cells with biallelic KI of MT-C34 and 2P23 peptides at the CXCR4 locus. In combination, these peptides provided a higher level of protection than individual KI. By extending homology arms and cloning donor DNA into a plasmid containing signals for nuclear localization, we achieved KI of MT-C34 into the CXCR4 locus and HIV-1 proviral DNA at levels of up to 35% in the T-cell line and up to 4 to 5% in primary CD4 lymphocytes. Compared to lentiviral delivery, KI resulted in the higher MT-C34 surface expression and stronger protection of lymphocytes from HIV-1. Thus, we demonstrate that KI is a viable strategy for peptide-based therapy of HIV infection. IMPORTANCE HIV is a human lentivirus that infects CD4-positive immune cells and, when left untreated, manifests in the fatal disease known as AIDS. Antiretroviral therapy (ART) does not lead to viral clearance, and HIV persists in the organism as a latent provirus. One way to control infection is to increase the population of HIV-resistant CD4 lymphocytes via entry molecule knockout or expression of different antiviral genes. Peptides from the heptad repeat (HR) domain of gp41 are potent inhibitors of HIV-1 fusion, especially when designed to express on the cell surface. Individual gp41 peptides encoded by therapeutic lentiviral vectors have been evaluated and some have entered clinical trials. However, a CRISPR/Cas9-based gp41 peptide delivery platform that operates through concomitant target gene modification has not yet been developed due to low knock-in (KI) rates in primary cells. Here, we systematically evaluated the antiviral activity of different HR2 peptides cloned into the shortest carrier molecule, CD52. The resulting small-size transgene constructs encoding selected peptides, in combination with improvements to enhance donor vector nuclear import, helped to overcome precise editing restrictions in CD4 lymphocytes. Using KI into CXCR4, we demonstrated different options for target gene modification, effectively protecting edited cells against HIV-1.

Systematic Evaluation of Fluorination as Modification for Peptide-Based Fusion Inhibitors against HIV-1 Infection

With the emergence of novel viruses, the development of new antivirals is more urgent than ever. A key step in human immunodeficiency virus type 1 (HIV-1) infection is six-helix bundle formation within the envelope protein subunit gp41. Selective disruption of bundle formation by peptides has been shown to be effective; however, these drugs, exemplified by T20, are prone to rapid clearance from the patient. The incorporation of non-natural amino acids is known to improve these pharmacokinetic properties. Here, we evaluate a peptide inhibitor in which a critical Ile residue is replaced by fluorinated analogues. We characterized the influence of the fluorinated analogues on the biophysical properties of the peptide. Furthermore, we show that the fluorinated peptides can block HIV-1 infection of target cells at nanomolar levels. These findings demonstrate that fluorinated amino acids are appropriate tools for the development of novel peptide therapeutics.

A "Two-Birds-One-Stone" Approach toward the Design of Bifunctional Human Immunodeficiency Virus Type 1 Entry Inhibitors Targeting the CCR5 Coreceptor and gp41 N-Terminal Heptad Repeat Region

Previous studies have reported the stepwise nature of human immunodeficiency virus type 1 (HIV-1) entry and the pivotal role of coreceptor CCR5 and the gp41 N-terminal heptad repeat (NHR) region in this event. With this in mind, we herein report a dual-targeted drug compound featuring bifunctional entry inhibitors, consisting of a piperidine-4-carboxamide-based CCR5 antagonist, TAK-220, and a gp41 NHR-targeting fusion-inhibitory peptide, C34. The resultant chimeras were constructed by linking both pharmacophores with a polyethylene glycol spacer. One chimera, CP12TAK, exhibited exceptionally potent antiviral activity, about 40- and 306-fold over that of its parent inhibitors, C34 and TAK-220, respectively. In addition to R5-tropic viruses, CP12TAK also strongly inhibited infection of X4-tropic HIV-1 strains. These data are promising for the further development of CP12TAK as a new anti-HIV-1 drug. Results show that this strategy could be extended to the design of therapies against infection of other enveloped viruses.

HIV-1 Entry and Membrane Fusion Inhibitors

HIV-1 (human immunodeficiency virus type 1) infection begins with the attachment of the virion to a host cell by its envelope glycoprotein (Env), which subsequently induces fusion of viral and cell membranes to allow viral entry. Upon binding to primary receptor CD4 and coreceptor (e.g., chemokine receptor CCR5 or CXCR4), Env undergoes large conformational changes and unleashes its fusogenic potential to drive the membrane fusion. The structural biology of HIV-1 Env and its complexes with the cellular receptors not only has advanced our knowledge of the molecular mechanism of how HIV-1 enters the host cells but also provided a structural basis for the rational design of fusion inhibitors as potential antiviral therapeutics. In this review, we summarize our latest understanding of the HIV-1 membrane fusion process and discuss related therapeutic strategies to block viral entry.

Preparation and evaluation of amphipathic lipopeptide-loaded PLGA microspheres as sustained-release system for AIDS prevention

At present, AIDS drugs are typical inhibitors that cannot achieve permanent effects. Therefore, the research of blocking HIV infection is essential. Especially for people in the high-risk environment, long-term prevention is important, because HIV can easily infect cells once the drug is interrupted. However, there is still no long-acting AIDS prevention drug approved. Hence, the purpose of this study is to prepare a fusion inhibitor loaded poly(d, l-lactic-co-glycolic acid) (PLGA) microspheres as a sustained-release system for long-term AIDS prevention. As the HIV membrane fusion inhibitor (LP-98) used in this research is amphiphilic lipopeptide, W1/O/W2 double-emulsion method was chosen, and premix membrane emulsification technique was used for controlling the uniformity of particle size. Several process parameters that can impact drug loading efficiency were summarized: the concentration of LP-98 and PLGA, and the preparation condition of primary emulsion. Finally, the microspheres with high loading efficiency (>8%) and encapsulation efficiency (>90%) were successfully prepared under optimum conditions. Pharmacokinetic studies showed that LP-98-loaded microspheres were capable to continuously release for 24 days in rats. This research can promote the application of sustained-release microspheres in AIDS prevention, and the embedding technique used in this study can also provide references for the loading of other amphipathic drugs.

The Tryptophan-Rich Motif of HIV-1 gp41 Can Interact with the N-Terminal Deep Pocket Site: New Insights into the Structure and Function of gp41 and Its Inhibitors

Refolding of the HIV-1 gp41 N- and C-terminal heptad repeats (NHR and CHR, respectively) into a six-helix bundle (6-HB) juxtaposes viral and cellular membranes for fusion. The CHR-derived peptide T20 is the only clinically approved viral fusion inhibitor and has potent anti-HIV activity; however, its mechanism of action is not fully understood. In this study, we surprisingly found that T20 disrupted the α-helical conformation of the NHR-derived peptide N54 through its C-terminal tryptophan-rich motif (TRM) and that synthetic short peptides containing the TRM sequence, TRM8 and TRM12, disrupted the N54 helix in a dose-dependent manner. Interestingly, TRM8 efficiently interfered with the secondary structures of three overlapping NHR peptides (N44, N38, and N28) and interacted with N28, which contains mainly the deep NHR pocket-forming sequence, with high affinity, suggesting that TRM targeted the NHR pocket site to mediate the disruption. Unlike TRM8, the short peptide corresponding to the pocket-binding domain (PBD) of the CHR helix had no such disruptive effect, and the CHR peptide C34 could form a stable 6-HB with the NHR helix; however, addition of the TRM to the C terminus of C34 resulted in a peptide (C46) that destroyed the NHR helix. Although the TRM peptides alone had no anti-HIV activity and could not block the formation of 6-HB conformation, substitution of the TRM for the PBD in C34 resulted in a mutant inhibitor (C34TRM) with high binding and inhibitory capacities. Combined, the present data inform a new mode of action of T20 and the structure-function relationship of gp41.IMPORTANCE The HIV-1 Env glycoprotein mediates membrane fusion and is conformationally labile. Despite extensive efforts, the structural property of the native fusion protein gp41 is largely unknown, and the mechanism of action of the gp41-derived fusion inhibitor T20 remains elusive. Here, we report that T20 and its C-terminal tryptophan-rich motif (TRM) can efficiently impair the conformation of the gp41 N-terminal heptad repeat (NHR) coiled coil by interacting with the deep NHR pocket site. The TRM sequence has been verified to possess the ability to replace the pocket-binding domain of C34, a fusion inhibitor peptide with high anti-HIV potency. Therefore, our studies have not only facilitated understanding of the mechanism of action of T20 and developed novel HIV-1 fusion inhibitors but also provided new insights into the structural property of the prefusion state of gp41.

Mutations of Glu560 within HIV-1 Envelope Glycoprotein N-terminal heptad repeat region contribute to resistance to peptide inhibitors of virus entry

Background

Peptides corresponding to N- and C-terminal heptad repeat regions (HR1 and HR2, respectively) of gp41 can inhibit HIV-1 infection in a dominant negative manner by interfering with refolding of the viral HR1 and HR2 to form a six-helix bundle (6HB) that induces fusion between viral and host cell membranes. Previously, we found that HIV-1 acquired the mutations of Glu560 (E560) in HR1 of envelope (Env) to escape peptide inhibitors. The present study aimed to elucidate the critical role of position 560 in the virus entry and potential resistance mechanisms.

Results

The Glu560Lys/Asp/Gly (E560K/D/G) mutations in HR1 of gp41 that are selected under the pressure of N- and C-peptide inhibitors modified its molecular interactions with HR2 to change 6HB stability and peptide inhibitor binding. E560K mutation increased 6HB thermostability and resulted in resistance to N peptide inhibitors, but E560G or E560D as compensatory mutations destabilized the 6HB to reduce inhibitor binding and resulted in increased resistance to C peptide inhibitor, T20. Significantly, the neutralizing activities of all mutants to soluble CD4 and broadly neutralizing antibodies targeting membrane proximal external region, 2F5 and 4E10 were improved, indicating the mutations of E560 could regulate Env conformations through cross interactions with gp120 or gp41. The molecular modeling analysis of E560K/D/G mutants suggested that position 560 might interact with the residues within two potentially flexible topological layer 1 and layer 2 in the gp120 inner domain to apparently affect the CD4 utilization. The E560K/D/G mutations changed its interactions with Gln650 (Q650) in HR2 to contribute to the resistance of peptide inhibitors.

Conclusions

These findings identify the contributions of mutations of E560K/D/G in the highly conserved gp41 and highlight Env’s high degree of plasticity for virus entry and inhibitor design.

A Membrane-Anchored Short-Peptide Fusion Inhibitor Fully Protects Target Cells from Infections of Human Immunodeficiency Virus Type 1 (HIV-1), HIV-2, and Simian Immunodeficiency Virus

Emerging studies demonstrate that the antiviral activity of viral fusion inhibitor peptides can be dramatically improved when being chemically or genetically anchored to the cell membrane, where viral entry occurs. We previously reported that the short-peptide fusion inhibitor 2P23 and its lipid derivative possess highly potent antiviral activities against human immunodeficiency virus type 1 (HIV-1), HIV-2, and simian immunodeficiency virus (SIV). To develop a sterilizing or functional-cure strategy, here we genetically linked 2P23 and two control peptides (HIV-1 fusion inhibitor C34 and hepatitis B virus [HBV] entry inhibitor 4B10) with a glycosylphosphatidylinositol (GPI) attachment signal. As expected, GPI-anchored inhibitors were efficiently expressed on the plasma membrane of transduced TZM-bl cells and primarily directed to the lipid raft site without interfering with the expression of CD4, CCR5, and CXCR4. GPI-anchored 2P23 (GPI-2P23) completely protected TZM-bl cells from infections of divergent HIV-1, HIV-2, and SIV isolates as well as a panel of enfuvirtide (T20)-resistant mutants. GPI-2P23 also rendered the cells resistant to viral envelope-mediated cell-cell fusion and cell-associated virion-mediated cell-cell transmission. Moreover, GPI-2P23-modified human CD4⁺ T cells (CEMss-CCR5) fully blocked both R5- and X4-tropic HIV-1 isolates and displayed a robust survival advantage over unmodified cells during HIV-1 infection. In contrast, it was found that GPI-anchored C34 was much less effective in inhibiting HIV-2, SIV, and T20-resistant HIV-1 mutants. Therefore, our studies have demonstrated that genetically anchoring a short-peptide fusion inhibitor to the target cell membrane is a viable strategy for gene therapy of both HIV-1 and HIV-2 infections.IMPORTANCE Antiretroviral therapy with multiple drugs in combination can efficiently suppress HIV replication and dramatically reduce the morbidity and mortality associated with AIDS-related illness; however, antiretroviral therapy cannot eradiate the HIV reservoirs, and lifelong treatment is required, which often results in cumulative toxicities, drug resistance, and a multitude of complications, thus necessitating the development of sterilizing-cure or functional-cure strategies. Here, we report that genetically anchoring the short-peptide fusion inhibitor 2P23 to the cell membrane can fully prevent infections from divergent HIV-1, HIV-2, and SIV isolates as well as a panel of enfuvirtide-resistant mutants. Membrane-bound 2P23 also effectively blocks HIV-1 Env-mediated cell-cell fusion and cell-associated virion-mediated cell-cell transmission, renders CD4⁺ T cells nonpermissive to infection, and confers a robust survival advantage over unmodified cells. Thus, our studies verify a powerful strategy to generate resistant cells for gene therapy of both the HIV-1 and HIV-2 infections.

Dimeric C34 Derivatives Linked through Disulfide Bridges as New HIV-1 Fusion Inhibitors

C34, a 34-mer fragment peptide, is contained in the HIV-1 envelope protein gp41. A dimeric derivative of C34 linked through a disulfide bridge at its C terminus was synthesized and found to display potent anti-HIV activity, comparable with that of a previously reported PEGylated dimer of C34REG. The reduction in the size of the linker moiety for dimerization was thus successful, and this result might shed some light on the mechanism of the suppression of six-helix bundle formation by these C34 dimeric derivatives. Addition of a Gly-Cys(CH₂ CONH₂ )-Gly-Gly motif at the N-terminal position of a C34 monomeric derivative significantly increased the anti-HIV-1 activity. This moiety functions as a new pharmacophore, and this might provide a useful insight into the design of potent HIV-1 fusion inhibitors.

Conserved Residue Asn-145 in the C-Terminal Heptad Repeat Region of HIV-1 gp41 is Critical for Viral Fusion and Regulates the Antiviral Activity of Fusion Inhibitors

Entry of HIV-1 into target cells is mediated by its envelope (Env) glycoprotein composed of the receptor binding subunit gp120 and the fusion protein gp41. Refolding of the gp41 N- and C-terminal heptad repeats (NHR and CHR) into a six-helix bundle (6-HB) conformation drives the viral and cellular membranes in close apposition and generates huge amounts of energy to overcome the kinetic barrier leading to membrane fusion. In this study, we focused on characterizing the structural and functional properties of a single Asn-145 residue, which locates at the middle CHR site of gp41 and is extremely conserved among all the HIV-1, HIV-2, and simian immunodeficiency virus (SIV) isolates. By mutational analysis, we found that Asn-145 plays critical roles for Env-mediated cell-cell fusion and HIV-1 entry. As determined by circular dichroism (CD) spectroscopy and isothermal titration calorimetry (ITC), the substitution of Asn-145 with alanine (N145A) severely impaired the interactions between the NHR and CHR helices. Asn-145 was also verified to be important for the antiviral activity of CHR-derived peptide fusion inhibitors and served as a turn-point for the inhibitory potency. Intriguingly, Asn-145 could regulate the functionality of the M-T hook structure at the N-terminus of the inhibitors and displayed comparable activities with the C-terminal IDL anchor. Crystallographic studies further demonstrated the importance of Asn-145-mediated interhelical and intrahelical interactions in the 6-HB structure. Combined, the present results have provided valuable information for the structure-function relationship of HIV-1 gp41 and the structure-activity relationship of gp41-dependent fusion inhibitors.

Monotherapy with a low-dose lipopeptide HIV fusion inhibitor maintains long-term viral suppression in rhesus macaques

Combination antiretroviral therapy (cART) dramatically improves survival of HIV-infected patients, but lifelong treatment can ultimately result in cumulative toxicities and drug resistance, thus necessitating the development of new drugs with significantly improved pharmaceutical profiles. We recently found that the fusion inhibitor T-20 (enfuvirtide)-based lipopeptides possess dramatically increased anti-HIV activity. Herein, a group of novel lipopeptides were designed with different lengths of fatty acids, identifying a stearic acid-modified lipopeptide (LP-80) with the most potent anti-HIV activity. It inhibited a large panel of divergent HIV subtypes with a mean IC₅₀ in the extremely low picomolar range, being > 5,300-fold more active than T-20 and the neutralizing antibody VRC01. It also sustained the potent activity against T-20-resistant mutants and exhibited very high therapeutic selectivity index. Pharmacokinetics of LP-80 in rats and monkeys verified its potent and long-acting anti-HIV activity. In the monkey, subcutaneous administration of 3 mg/kg LP-80 yielded serum concentrations of 1,147 ng/ml after injection 72 h and 9 ng/ml after injection 168 h (7 days), equivalent to 42,062- and 330-fold higher than the measured IC₅₀ value. In SHIV infected rhesus macaques, a single low-dose LP-80 (3 mg/kg) sharply reduced viral loads to below the limitation of detection, and twice-weekly monotherapy could maintain long-term viral suppression.

T-20 is the only clinically approved viral fusion inhibitor, which is used in combination therapy for HIV-1 infection; however, it exhibits relatively low antiviral activity and easily induces drug resistance. Here we report a lipopeptide fusion inhibitor termed LP-80, which exhibits the most potent activity in inhibiting divergent HIV-1 subtypes. Especially, LP-80 has extremely potent and long-acting therapeutic efficacy with very low cytotoxicity, making it an ideal drug candidate for clinical use. Furthermore, LP-80 and its truncated versions can be used as important probes for exploiting the mechanisms of viral fusion and inhibition.

Sulfono-γ-AA modified peptides that inhibit HIV-1 fusion

The utilization of bioactive peptides in the development of highly selective and potent pharmacological agents for the disruption of protein-protein interactions is appealing for drug discovery. It is known that HIV-1 entry into a host cell is through a fusion process that is mediated by the trimeric viral glycoprotein gp120/41, which is derived from gp160 through proteolytic processing. Peptides derived from the HIV gp41 C-terminus have proven to be potent in inhibiting the fusion process. These peptides bind tightly to the hydrophobic pocket on the gp-41 N-terminus, which was previously identified as a potential inhibitor binding site. In this study, we introduce modified 23-residue C-peptides, 3 and 4, bearing a sulfono-γ-AA residue substitution and hydrocarbon stapling, respectively, which were developed for HIV-1 gp-41 N-terminus binding. Intriguingly, both 3 and 4 were capable of inhibiting envelope-mediated membrane fusion in cell-cell fusion assays at nanomolar potency. Our study reveals that sulfono-γ-AA modified peptides could be used for the development of more potent anti-HIV agents.

Structural and Functional Characterization of Membrane Fusion Inhibitors with Extremely Potent Activity against Human Immunodeficiency Virus Type 1 (HIV-1), HIV-2, and Simian Immunodeficiency Virus

T-20 (enfuvirtide) is the only membrane fusion inhibitor available for the treatment of viral infection; however, it has low anti-human immunodeficiency virus (anti-HIV) activity and a low genetic barrier for drug resistance. We recently reported that T-20 sequence-based lipopeptides possess extremely potent in vitro and in vivo efficacies (X. Ding, Z. Zhang, H. Chong, Y. Zhu, H. Wei, X. Wu, J. He, X. Wang, Y. He, 2017, J Virol 91:e00831-17, https://doi.org/10.1128/JVI.00831-17; H. Chong, J. Xue, Y. Zhu, Z. Cong, T. Chen, Y. Guo, Q. Wei, Y. Zhou, C. Qin, Y. He, 2018, J Virol 92:e00775-18, https://doi.org/10.1128/JVI.00775-18). Here, we focused on characterizing the structure-activity relationships of the T-20 derivatives. First, a novel lipopeptide termed LP-52 was generated with improved target-binding stability and anti-HIV activity. Second, a large panel of truncated lipopeptides was characterized, revealing a 21-amino-acid sequence core structure. Third, it was surprisingly found that the addition of the gp41 pocket-binding residues in the N terminus of the new inhibitors resulted in increased binding but decreased antiviral activities. Fourth, while LP-52 showed the most potent activity in inhibiting divergent HIV-1 subtypes, its truncated versions, such as LP-55 (25-mer) and LP-65 (24-mer), still maintained their potencies at very low picomolar concentrations; however, both the N- and C-terminal motifs of LP-52 played crucial roles in the inhibition of T-20-resistant HIV-1 mutants, HIV-2, and simian immunodeficiency virus (SIV) isolates. Fifth, we verified that LP-52 can bind to target cell membranes and human serum albumin and has low cytotoxicity and a high genetic barrier to inducing drug resistance.IMPORTANCE Development of novel membrane fusion inhibitors against HIV and other enveloped viruses is highly important in terms of the peptide drug T-20, which remains the only one for clinical use, even if it is limited by large dosages and resistance. Here, we report a novel T-20 sequence-based lipopeptide showing extremely potent and broad activities against HIV-1, HIV-2, SIV, and T-20-resistant mutants, as well as an extremely high therapeutic selectivity index and genetic resistance barrier. The structure-activity relationship (SAR) of the T-20 derivatives has been comprehensively characterized, revealing a critical sequence core structure and the target sites of viral vulnerability that do not include the gp41 pocket. The results also suggest that membrane-anchored inhibitors possess unique modes of action relative to unconjugated peptides. Combined, our series studies have not only provided drug candidates for clinical development but also offered important tools to elucidate the mechanisms of viral fusion and inhibition.

Design of Novel HIV-1/2 Fusion Inhibitors with High Therapeutic Efficacy in Rhesus Monkey Models

T-20 (enfuvirtide) is the only approved viral fusion inhibitor that is used for the treatment of human immunodeficiency virus type 1 (HIV-1) infection; however, it has relatively low antiviral activity and easily induces drug resistance. We recently reported a T-20-based lipopeptide fusion inhibitor (LP-40) showing improved anti-HIV activity (X. Ding et al., J Virol 91:e00831-17, 2017, https://doi.org/10.1128/JVI.00831-17). In this study, we designed LP-50 and LP-51 by refining the structure and function of LP-40. The two new lipopeptides showed dramatically enhanced secondary structure and binding stability and were exceptionally potent inhibitors of HIV-1, HIV-2, simian immunodeficiency virus (SIV), and chimeric simian-human immunodeficiency virus (SHIV), with mean 50% inhibitory concentrations (IC₅₀s) in the very low picomolar range. They also exhibited dramatically increased potencies in inhibiting a panel of T-20- and LP-40-resistant mutant viruses. In line with their in vitro data, LP-50 and LP-51 exhibited extremely potent and long-lasting ex vivo anti-HIV activities in rhesus monkeys: serum dilution peaks that inhibited 50% of virus infection were >15,200-fold higher than those for T-20 and LP-40. Low-dose, short-term monotherapy of LP-51 could sharply reduce viral loads to undetectable levels in acutely and chronically SHIV infected monkey models. To our knowledge, LP-50 and LP-51 are the most potent and broad HIV-1/2 and SIV fusion inhibitors, which can be developed for clinical use and can serve as tools for exploration of the mechanisms of viral entry and inhibition.IMPORTANCE T-20 remains the only membrane fusion inhibitor available for the treatment of viral infection, but its relatively low anti-HIV activity and genetic barrier for drug resistance have significantly limited its clinical application. Here we report two new lipopeptide-based fusion inhibitors (LP-50 and LP-51) showing extremely potent inhibitory activities against diverse HIV-1, HIV-2, SIV, and T-20-resistant variants. Promisingly, both inhibitors exhibited potent and long-lasting ex vivo anti-HIV activity and could efficiently suppress viral loads to undetectable levels in SHIV-infected monkey models. We believe that LP-50 and LP-51 are the most potent and broad-spectrum fusion inhibitors known to date and thus have high potential for clinical development.

Molecular mechanism of HIV-1 resistance to sifuvirtide, a clinical trial-approved membrane fusion inhibitor

Host cell infection with HIV-1 requires fusion of viral and cell membranes. Sifuvirtide (SFT) is a peptide-based HIV-1 fusion inhibitor approved for phase III clinical trials in China. Here, we focused on characterizing HIV-1 variants highly resistant to SFT to gain insight into the molecular resistance mechanism. Three primary substitutions (V38A, A47I, and Q52R) located at the inhibitor-binding site of HIV-1's envelope protein (Env) and one secondary substitution (N126K) located at the C-terminal heptad repeat region of the viral protein gp41, which is part of the envelope, conferred high SFT resistance and cross-resistance to the anti-HIV-1 drug T20 and the template peptide C34. Interestingly, SFT's resistance profile could be dramatically improved with an M-T hook structure-modified SFT (MTSFT) and with short-peptide inhibitors that mainly target the gp41 pocket (2P23 and its lipid derivative LP-19). We found that the V38A and Q52R substitutions reduce the binding stabilities of SFT, C34, and MTSFT, but they had no effect on the binding of 2P23 and LP-19; in sharp contrast, the A47I substitution enhanced fusion inhibitor binding. Furthermore, the primary resistance substitutions impaired Env-mediated membrane fusion and cell entry and changed the conformation of the gp41 core structure. Importantly, whereas the V38A and Q52R substitutions disrupted the N-terminal helix of gp41, a single A47I substitution greatly enhanced its thermostability. Taken together, our results provide crucial structural insights into the mechanism of HIV-1 resistance to gp41-dependent fusion inhibitors, which may inform the development of additional anti-HIV drugs.

Mechanism of HIV-1 Resistance to an Electronically Constrained α-Helical Peptide Membrane Fusion Inhibitor

SC29EK is an electronically constrained α-helical peptide HIV-1 fusion inhibitor that is highly effective against both wild-type and enfuvirtide (T20)-resistant viruses. In this study, we focused on investigating the mechanism of HIV-1 resistance to SC29EK by two approaches. First, SC29EK-escaping HIV-1 variants were selected and characterized. Three mutant viruses, which possessed two (N43K/E49A) or three (Q39R/N43K/N126K and N43K/E49A/N126K) amino acid substitutions in the N- and C-terminal repeat regions of gp41 were identified as conferring high resistance to SC29EK and cross-resistance to the first-generation (T20 and C34) and newly designed (sifuvirtide, MT-SC29EK, and 2P23) fusion inhibitors. The resistance mutations could reduce the binding stability of SC29EK, impair viral Env-mediated cell fusion and entry, and change the conformation of the gp41 core structure. Further, we determined the crystal structure of SC29EK in complex with a target mimic peptide, which revealed the critical intra- and interhelical interactions underlying the mode of action of SC29EK and the genetic pathway to HIV-1 resistance. Taken together, the present data provide new insights into the structure and function of gp41 and the structure-activity relationship (SAR) of viral fusion inhibitors.IMPORTANCE T20 is the only membrane fusion inhibitor available for treatment of viral infection, but it has relatively low anti-HIV activity and genetic barriers for resistance, thus calling for new drugs blocking the viral fusion process. As an electronically constrained α-helical peptide, SC29EK is highly potent against both wild-type and T20-resistant HIV-1 strains. Here, we report the characterization of HIV-1 variants resistant to SC29EK and the crystal structure of SC29EK. The key mutations mediating high resistance to SC29EK and cross-resistance to the first and new generations of fusion inhibitors as well as the underlying mechanisms were identified. The crystal structure of SC29EK bound to a target mimic peptide further revealed its action mode and genetic pathway to inducing resistance. Hence, our data have shed new lights on the mechanisms of HIV-1 fusion and its inhibition.

Structural Insights into the Mechanisms of Action of Short-Peptide HIV-1 Fusion Inhibitors Targeting the Gp41 Pocket

The deep hydrophobic pocket of HIV-1 gp41 has been considered a drug target, but short-peptides targeting this site usually lack potent antiviral activity. By applying the M-T hook structure, we previously generated highly potent short-peptide fusion inhibitors that specifically targeted the pocket site, such as MT-SC22EK, HP23L, and LP-11. Here, the crystal structures of HP23L and LP-11 bound to the target mimic peptide N36 demonstrated the critical intrahelical and interhelical interactions, especially verifying that the hook-like conformation was finely adopted while the methionine residue was replaced by the oxidation-less prone residue leucine, and that addition of an extra glutamic acid significantly enhanced the binding and inhibitory activities. The structure of HP23L bound to N36 with two mutations (E49K and L57R) revealed the critical residues and motifs mediating drug resistance and provided new insights into the mechanism of action of inhibitors. Therefore, the present data help our understanding for the structure-activity relationship (SAR) of HIV-1 fusion inhibitors and facilitate the development of novel antiviral drugs.

Efficient Fusion at Neutral pH by Human Immunodeficiency Virus gp41 Trimers Containing the Fusion Peptide and Transmembrane Domains

Human immunodeficiency virus (HIV) is membrane-enveloped, and an initial infection step is joining/fusion of viral and cell membranes. This step is catalyzed by gp41, which is a single-pass integral viral membrane protein. The protein contains an ∼170-residue ectodomain located outside the virus that is important for fusion and includes the fusion peptide (FP), N-helix, loop, C-helix, and viral membrane-proximal external region (MPER). The virion initially has noncovalent complexes between three gp41 ectodomains and three gp120 proteins. A gp120 contains ∼500 residues and functions to identify target T-cells and macrophages via binding to specific protein receptors of the target cell membrane. gp120 moves away from the gp41 ectodomain, and the ectodomain is thought to bind to the target cell membrane and mediate membrane fusion. The secondary and tertiary structures of the ectodomain are different in the initial complex with gp120 and the final state without gp120. There is not yet imaging of gp41 during fusion, so the temporal relationship between the gp41 and membrane structures is not known. This study describes biophysical and functional characterization of large gp41 constructs that include the ectodomain and transmembrane domain (TM). Significant fusion is observed of both neutral and anionic vesicles at neutral pH, which reflects the expected conditions of HIV/cell fusion. Fusion is enhanced by the FP, which in HIV/cell fusion likely contacts the host membrane, and the MPER and TM, which respectively interfacially contact and traverse the HIV membrane. Initial contact with vesicles is made by protein trimers that are in a native oligomeric state that reflects the initial complex with gp120 and also is commonly observed for the ectodomain without gp120. Circular dichroism data support helical structure for the N-helix, C-helix, and MPER and nonhelical structure for the FP and loop. Distributions of monomer, trimer, and hexamer states are observed by size-exclusion chromatography (SEC), with dependences on solubilizing detergent and construct. These SEC and other data are integrated into a refined working model of HIV/cell fusion that includes dissociation of the ectodomain into gp41 monomers followed by folding into hairpins that appose the two membranes, and subsequent fusion catalysis by trimers and hexamers of hairpins. The monomer and oligomer gp41 states may therefore satisfy dual requirements for HIV entry of membrane apposition and fusion.

Enfuvirtide (T20)-Based Lipopeptide Is a Potent HIV-1 Cell Fusion Inhibitor: Implications for Viral Entry and Inhibition

The peptide drug enfuvirtide (T20) is the only viral fusion inhibitor used in combination therapy for HIV-1 infection, but it has relatively low antiviral activity and easily induces drug resistance. Emerging studies demonstrate that lipopeptide-based fusion inhibitors, such as LP-11 and LP-19, which mainly target the gp41 pocket site, have greatly improved antiviral potency and in vivo stability. In this study, we focused on developing a T20-based lipopeptide inhibitor that lacks pocket-binding sequence and targets a different site. First, the C-terminal tryptophan-rich motif (TRM) of T20 was verified to be essential for its target binding and inhibition; then, a novel lipopeptide, termed LP-40, was created by replacing the TRM with a fatty acid group. LP-40 showed markedly enhanced binding affinity for the target site and dramatically increased inhibitory activity on HIV-1 membrane fusion, entry, and infection. Unlike LP-11 and LP-19, which required a flexible linker between the peptide sequence and the lipid moiety, addition of a linker to LP-40 sharply reduced its potency, implying different binding modes with the extended N-terminal helices of gp41. Also, interestingly, LP-40 showed more potent activity than LP-11 in inhibiting HIV-1 Env-mediated cell-cell fusion while it was less active than LP-11 in inhibiting pseudovirus entry, and the two inhibitors displayed synergistic antiviral effects. The crystal structure of LP-40 in complex with a target peptide revealed their key binding residues and motifs. Combined, our studies have not only provided a potent HIV-1 fusion inhibitor, but also revealed new insights into the mechanisms of viral inhibition.IMPORTANCE T20 is the only membrane fusion inhibitor available for treatment of viral infection; however, T20 requires high doses and has a low genetic barrier for resistance, and its inhibitory mechanism and structural basis remain unclear. Here, we report the design of LP-40, a T20-based lipopeptide inhibitor that has greatly improved anti-HIV activity and is a more potent inhibitor of cell-cell fusion than of cell-free virus infection. The binding modes of two classes of membrane-anchoring lipopeptides (LP-40 and LP-11) verify the current fusion model in which an extended prehairpin structure bridges the viral and cellular membranes, and their complementary effects suggest a vital strategy for combination therapy of HIV-1 infection. Moreover, our understanding of the mechanism of action of T20 and its derivatives benefits from the crystal structure of LP-40.

A Lipopeptide HIV-1/2 Fusion Inhibitor with Highly Potent In Vitro, Ex Vivo, and In Vivo Antiviral Activity

Peptides derived from the C-terminal heptad repeat (CHR) region of the human immunodeficiency virus type 1 (HIV-1) fusogenic protein gp41 are potent viral entry inhibitors, and currently, enfuvirtide (T-20) is the only one approved for clinical use; however, emerging drug resistance largely limits its efficacy. In this study, we generated a novel lipopeptide inhibitor, named LP-19, by integrating multiple design strategies, including an N-terminal M-T hook structure, an HIV-2 sequence, intrahelical salt bridges, and a membrane-anchoring lipid tail. LP-19 showed stable binding affinity and highly potent, broad, and long-lasting antiviral activity. In in vitro studies, LP-19 efficiently inhibited HIV-1-, HIV-2-, and simian immunodeficiency virus (SIV)-mediated cell fusion, viral entry, and infection, and it was highly active against diverse subtypes of primary HIV-1 isolates and inhibitor-resistant mutants. Ex vivo studies demonstrated that LP-19 exhibited dramatically increased anti-HIV activity and an extended half-life in rhesus macaques. In short-term monotherapy, LP-19 reduced viral loads to undetectable levels in acutely and chronically simian-human immunodeficiency virus (SHIV)-infected monkeys. Therefore, this study offers an ideal HIV-1/2 fusion inhibitor for clinical development and emphasizes the importance of the viral fusion step as a drug target.IMPORTANCE The peptide drug T-20 is the only viral fusion inhibitor in the clinic, which is used for combination therapy of HIV-1 infection; however, it requires a high dosage and easily induces drug resistance, calling for a new drug with significantly improved pharmaceutical profiles. Here, we have developed a short-lipopeptide-based fusion inhibitor, termed LP-19, which mainly targets the conserved gp41 pocket site and shows highly potent inhibitory activity against HIV-1, HIV-2, and even SIV isolates. LP-19 exhibits dramatically increased antiviral activity and an extended half-life in rhesus macaques, and it has potent therapeutic efficacy in SHIV-infected monkeys, highlighting its high potential as a new viral fusion inhibitor for clinical use.

A Helical Short-Peptide Fusion Inhibitor with Highly Potent Activity against Human Immunodeficiency Virus Type 1 (HIV-1), HIV-2, and Simian Immunodeficiency Virus

Human immunodeficiency virus type 2 (HIV-2) has already spread to different regions worldwide, and currently about 1 to 2 million people have been infected, calling for new antiviral agents that are effective on both HIV-1 and HIV-2 isolates. T20 (enfuvirtide), a 36-mer peptide derived from the C-terminal heptad repeat region (CHR) of gp41, is the only clinically approved HIV-1 fusion inhibitor, but it easily induces drug resistance and is not active on HIV-2. In this study, we first demonstrated that the M-T hook structure was also vital to enhancing the binding stability and inhibitory activity of diverse CHR-based peptide inhibitors. We then designed a novel short peptide (23-mer), termed 2P23, by introducing the M-T hook structure, HIV-2 sequences, and salt bridge-forming residues. Promisingly, 2P23 was a highly stable helical peptide with high binding to the surrogate targets derived from HIV-1, HIV-2, and simian immunodeficiency virus (SIV). Consistent with this, 2P23 exhibited potent activity in inhibiting diverse subtypes of HIV-1 isolates, T20-resistant HIV-1 mutants, and a panel of primary HIV-2 isolates, HIV-2 mutants, and SIV isolates. Therefore, we conclude that 2P23 has high potential to be further developed for clinical use, and it is also an ideal tool for exploring the mechanisms of HIV-1/2- and SIV-mediated membrane fusion.

IMPORTANCE

The peptide drug T20 is the only approved HIV-1 fusion inhibitor, but it is not active on HIV-2 isolates, which have currently infected 1 to 2 million people and continue to spread worldwide. Recent studies have demonstrated that the M-T hook structure can greatly enhance the binding and antiviral activities of gp41 CHR-derived inhibitors, especially for short peptides that are otherwise inactive. By combining the hook structure, HIV-2 sequence, and salt bridge-based strategies, the short peptide 2P23 has been successfully designed. 2P23 exhibits prominent advantages over many other peptide fusion inhibitors, including its potent and broad activity on HIV-1, HIV-2, and even SIV isolates, its stability as a helical, oligomeric peptide, and its high binding to diverse targets. The small size of 2P23 would benefit its synthesis and significantly reduce production cost. Therefore, 2P23 is an ideal candidate for further development, and it also provides a novel tool for studying HIV-1/2- and SIV-mediated cell fusion.

Rational improvement of gp41-targeting HIV-1 fusion inhibitors: an innovatively designed Ile-Asp-Leu tail with alternative conformations

Peptides derived from the C-terminal heptad repeat (CHR) of HIV gp41 have been developed as effective fusion inhibitors against HIV-1, but facing the challenges of enhancing potency and stability. Here, we report a rationally designed novel HIV-1 fusion inhibitor derived from CHR-derived peptide (Trp628~Gln653, named CP), but with an innovative Ile-Asp-Leu tail (IDL) that dramatically increased the inhibitory activity by up to 100 folds. We also determined the crystal structures of artificial fusion peptides N36- and N43-L6-CP-IDL. Although the overall structures of both fusion peptides share the canonical six-helix bundle (6-HB) configuration, their IDL tails adopt two different conformations: a one-turn helix with the N36, and a hook-like structure with the longer N43. Structural comparison showed that the hook-like IDL tail possesses a larger interaction interface with NHR than the helical one. Further molecular dynamics simulations of the two 6-HBs and isolated CP-IDL peptides suggested that hook-like form of IDL tail can be stabilized by its binding to NHR trimer. Therefore, CP-IDL has potential for further development as a new HIV fusion inhibitor, and this strategy could be widely used in developing artificial fusion inhibitors against HIV and other enveloped viruses.

Development of potent and long-acting HIV-1 fusion inhibitors

BACKGROUND

T20 (enfuvirtide) is the first approved HIV entry inhibitor and currently the only viral fusion inhibitor, but its low efficacy and genetic barrier to resistance significantly limit its application, calling for a next-generation drug.

DESIGN

On the basis of the M-T hook structure, we recently developed a short-peptide named HP23, which mainly targets the deep pocket site of gp41 and possesses highly potent antiviral activity. To improve the pharmaceutical properties of a peptide-based inhibitor, we modified HP23 by different classes of lipids including fatty acid, cholesterol, and sphingolipids. To avoid the potential problem of oxidation, the methionine residue in the M-T hook sequence of HP23 was replaced with leucine.

METHODS

Peptides were synthesized and their anti-HIV activity and biophysical properties were determined.

RESULTS

A group of lipopeptides were generated with greatly improved anti-HIV activity. Promisingly, a fatty acid-conjugated lipopeptide named LP-11 showed potent and broad inhibitory activity against diverse primary HIV-1 isolates and clinically drug-resistant mutants, and it had dramatically increased ex-vivo antiviral activity and extended half-life. Also, LP-11 displayed highly enhanced α-helicity and thermal stability, and it was physically stable under high temperature and humidity.

CONCLUSION

LP-11 has high potentials for clinical development and it can serve as an ideal tool for exploring the mechanisms of HIV-1 fusion and inhibition.

Glycosyl Phosphatidylinositol-Anchored C34 Peptide Derived From Human Immunodeficiency Virus Type 1 Gp41 Is a Potent Entry Inhibitor

Lipid rafts of the plasma membrane have been shown to be gateways for HIV-1 budding and entry. In nature, many glycosyl-phosphatidylinositol (GPI) anchored proteins are targeted to the lipid rafts. In the present study we constructed two fusion genes, in which C34 peptide or AVF peptide control was genetically linked with a GPI-attachment signal. Recombinant lentiviruses expressing the fusion genes were used to transduce TZM.bl and CEMss-CCR5 cells. Here, we show that with a GPI attachment signal both C34 and AVF are targeted to the lipid rafts through a GPI anchor. GPI-C34, but not GPI-AVF, in transduced TZM.bl cells efficiently blocks the infection of diverse HIV-1 strains of various subtypes. GPI-C34-transduced CEMss-CCR5 cells are totally resistant to HIV-1 infection. Importantly, maximum percentage of inhibition (MPI) by GPI-C34 is comparable to, if not higher than, a very high concentration of soluble C34. Potent blocking by GPI-C34 is likely due to its high local concentration, which allows GPI-C34 to efficiently bind to the prehairpin intermediate and prevent its transition to six helical bundle, thereby interfering with membrane fusion and virus entry. Our findings should have important implications in GPI-anchor-based therapy against HIV-1.

The C34 Peptide Fusion Inhibitor Binds to the Six-Helix Bundle Core Domain of HIV-1 gp41 by Displacement of the C-Terminal Helical Repeat Region

The conformational transition of the core domain of HIV-1 gp41 from a prehairpin intermediate to a six-helix bundle is responsible for virus-cell fusion. Several inhibitors which target the N-heptad repeat helical coiled-coil trimer that is fully accessible in the prehairpin intermediate have been designed. One such inhibitor is the peptide C34 derived from the C-heptad repeat of gp41 that forms the exterior of the six-helix bundle. Here, using a variety of biophysical techniques, including dye tagging, size-exclusion chromatography combined with multiangle light scattering, double electron-electron resonance EPR spectroscopy, and circular dichroism, we investigate the binding of C34 to two six-helix bundle mimetics comprising N- and C-heptad repeats either without (core(SP)) or with (core(S)) a short spacer connecting the two. In the case of core(SP), C34 directly exchanges with the C-heptad repeat. For core(S), up to two molecules of C34 bind the six-helix bundle via displacement of the C-heptad repeat. These results suggest that fusion inhibitors such as C34 can target a continuum of transitioning conformational states from the prehairpin intermediate to the six-helix bundle prior to the occurrence of irreversible fusion of viral and target cell membranes.

Genetic Pathway of HIV-1 Resistance to Novel Fusion Inhibitors Targeting the Gp41 Pocket

The peptide drug enfuvirtide (T20) is the only HIV-1 fusion inhibitor in clinical use, but it easily induces drug resistance, calling for new strategies for developing effective drugs. On the basis of the M-T hook structure, we recently developed highly potent short-peptide HIV-1 fusion inhibitors (MTSC22 and HP23), which mainly target the conserved gp41 pocket and possess high genetic barriers to resistance. Here, we focused on the selection and characterization of HIV-1 escape mutants of MTSC22, which revealed new resistance pathways and mechanisms. Two mutations (E49K and L57R) located at the inhibitor-binding site and two mutations (N126K and E136G) located at the C-terminal heptad repeat region of gp41 were identified as conferring high resistance either singly or in combination. While E49K reduced the C-terminal binding of inhibitors via an electrostatic repulsion, L57R dramatically disrupted the N-terminal binding of M-T hook structure and pocket-binding domain. Unlike E49K and N126K, which enhanced the stability of the endogenous viral six-helical bundle core (6-HB), L57R and E136G conversely destabilized the 6-HB structure. We also demonstrated that both primary and secondary mutations caused the structural changes in 6-HB and severely impaired the capability for HIV-1 entry. Collectively, our data provide novel insights into the mechanisms of short-peptide fusion inhibitors targeting the gp41 pocket site and help increase our understanding of the structure and function of gp41 and HIV-1 evolution.

IMPORTANCE

The deep pocket on the N-trimer of HIV-1 gp41 has been considered an ideal drug target because of its high degree of conservation and essential role in viral entry. Short-peptide fusion inhibitors, which contain an M-T hook structure and mainly target the pocket site, show extremely high binding and inhibitory activities as well as high genetic barriers to resistance. In this study, the HIV-1 mutants resistant to MTSC22 were selected and characterized, which revealed that the E49K and L57R substitutions at the inhibitor-binding site and the N126K and E136G substitutions at the C-terminal heptad repeat region of gp41 critically determine the resistance phenotype. The data provide novel insights into the mechanisms of action of the M-T hook structure-based fusion inhibitors which will help further our understanding of the structure-function relationship of gp41 and molecular pathways of HIV-1 evolution and eventually facilitate the development of new anti-HIV drugs.

Improved Pharmacological and Structural Properties of HIV Fusion Inhibitor AP3 over Enfuvirtide: Highlighting Advantages of Artificial Peptide Strategy

Enfuvirtide (T20), is the first HIV fusion inhibitor approved for treatment of HIV/AIDS patients who fail to respond to the current antiretroviral drugs. However, its clinical application is limited because of short half-life, drug resistance and cross-reactivity with the preexisting antibodies in HIV-infected patients. Using an artificial peptide strategy, we designed a peptide with non-native protein sequence, AP3, which exhibited potent antiviral activity against a broad spectrum of HIV-1 strains, including those resistant to T20, and had remarkably longer in vivo half-life than T20. While the preexisting antibodies in HIV-infected patients significantly suppressed T20’s antiviral activity, these antibodies neither recognized AP3, nor attenuated its anti-HIV-1 activity. Structurally different from T20, AP3 could fold into single-helix and interact with gp41 NHR. The two residues, Met and Thr, at the N-terminus of AP3 form a hook-like structure to stabilize interaction between AP3 and NHR helices. Therefore, AP3 has potential for further development as a new HIV fusion inhibitor with improved antiviral efficacy, resistance profile and pharmacological properties over enfuvirtide. Meanwhile, this study highlighted the advantages of artificially designed peptides, and confirmed that this strategy could be used in developing artificial peptide-based viral fusion inhibitors against HIV and other enveloped viruses.

The N-Terminal T-T Motif of a Third-Generation HIV-1 Fusion Inhibitor Is Not Required for Binding Affinity and Antiviral Activity

The highlighted next-generation HIV-1 fusion inhibitor peptide 1 is capped by two threonines. Here, we generated peptide 2 by deleting the T-T motif and compared their structural and antiviral properties. Significantly, two peptides showed similar helical and oligomeric states in solution, comparable binding affinities to the target, and no significant difference to inhibit HIV-1 fusion and infection. Also, the T-T motif was not associated with peptide 1 resistant mutations and its deletion did not affect peptide 1 against enfuvirtide-resistant HIV-1 mutants. The redundancy of the T-T motif was further verified by the model peptide C34 and short peptide inhibitors that mainly target the gp41 pocket, suggesting that the N-terminal T-T motif of peptide 1 could be removed or modified toward the development of new anti-HIV-1 drugs. Consistently, our data have verified that the M-T hook structure rather than the T-T motif is an efficient strategy for short peptide fusion inhibitors.

An artificial peptide-based HIV-1 fusion inhibitor containing M-T hook structure exhibiting improved antiviral potency and drug resistance profile

SUMMARY 

Aim: We previously designed an artificial HIV-1 fusion inhibitor, PBDtrp-m4HR. Here, we have added two amino acid residues that can form an M-T hook structure at its N-terminus, with the aim of improving its antiviral potency and drug-resistance profile. Methods: Peptides were synthesized and tested for their inhibitory activity on HIV-1 Env-mediated cell–cell fusion and infection by HIV-1 strains, including those resistant to T2635, the third generation HIV fusion inhibitor, as well as its binding affinity to the gp41 NHR-peptide N36. Results: MT-PBDtrp-m4HR exhibited improved inhibitory activity on HIV-1 infection and Env-mediated cell–cell fusion, displayed an improved drug-resistance profile and increased NHR-binding affinity. Conclusion: The added M-T hook is able to enhance or stabilize the interaction between MT-PBDtrp-m4HR and the viral gp41 NHR domain. Therefore, MT-PBDtrp-m4HR has potential to be further developed as a new HIV fusion inhibitor. The approach described in this study can also be used for designing artificial peptides against other enveloped viruses with class I membrane fusion proteins.

Identification and characterization of a subpocket on the N-trimer of HIV-1 Gp41: implication for viral entry and drug target

OBJECTIVE

Crystallographic studies of HIV-1 gp41 demonstrate a stable six-helix bundle (6-HB) folded by trimeric N and C-terminal heptad repeats (NHR and CHR), and a deep hydrophobic pocket (pocket-1) on the NHR helices (N-trimer); however, previous crystal structures of 6-HB core were determined by peptide fragments missing the downstream sequence of pocket-1; thus, the structural features of this site could not be observed.

DESIGN

We recently determined several 6-HB structures containing the pocket-1 and its downstream site. Here, we focused to investigate the structural features of N-trimer previously uncharacterized.

METHODS

Biophysical, biochemical and functional approaches were combined to characterize the downstream residues of pocket-1.

RESULTS

A subpocket (designated pocket-2) was visualized on the C-terminal portion of N-trimer, which is formed by a cluster of seven residues, including Leu587, Lys588 and Glu584 on one NHR helix and Tyr586, Val583, Ala582 and Arg579 of another NHR helix. Mutagenesis studies demonstrated that the pocket-2 residues play essential roles for HIV-1 Env-mediated cell entry and critically determine the antiviral activity of NHR-derived peptide fusion inhibitor T21. Further, the pocket-2 mutations dramatically impaired the thermostability and conformation of 6-HB structure and reduced the binding affinity of CHR-derived inhibitor HP23 that specifically targets the deep pocket-1.

CONCLUSION

These data have provided important information for the structure-function relationship of HIV-1 gp41 and for the development of antiviral entry inhibitors.

Mechanism of HIV-1 Resistance to Short-Peptide Fusion Inhibitors Targeting the Gp41 Pocket

The deep hydrophobic pocket on the N trimer of HIV-1 gp41 has been considered an ideal drug target. On the basis of the M-T hook structure, we recently developed short-peptide-based HIV-1 fusion inhibitors (MTSC22 and HP23), which mainly target the pocket site and possess highly potent antiviral activity. In this study, we focused on investigating their resistance pathways and mechanisms by escape HIV-1 mutants to SC22EK, a template peptide for MTSC22 and HP23. Two substitutions, E49K and N126K, located, respectively, at the N- and C-heptad repeat regions of gp41, were identified as conferring high resistance to the inhibitors targeting the pocket and cross-resistance to enfuvirtide (T20) and sifuvirtide (SFT). The underlying mechanisms of SC22EK-induced resistance include the following: (i) significantly reduced binding affinity of the inhibitors, (ii) dramatically enhanced interaction of the viral six-helix bundle, and (iii)severely damaged functionality of the viral Env complex. Our data have provided important information for the structure-function relationship of gp41 and the structure-activity relationship of viral fusion inhibitors.

IMPORTANCE

Enfuvirtide (T20) is the only HIV-1 fusion inhibitor in clinical use, but the problem of resistance significantly limits its use, calling for new strategies or concepts to develop next-generation drugs. On the basis of the M-T hook structure, short-peptide HIV-1 fusion inhibitors specifically targeting the gp41 pocket site exhibit high binding and antiviral activities. Here, we investigated the molecular pathway of HIV-1 resistance to the short inhibitors by selecting and mapping the escape mutants. The key substitutions for resistance and the underlying mechanisms have been finely characterized. The data provide important information for the structure-function relationship of gp41 and its inhibitors and will definitely help our future development of novel drugs that block gp41-dependent fusion.

Design of a highly potent HIV-1 fusion inhibitor targeting the gp41 pocket

OBJECTIVE

T20 (Enfuvirtide), which is a 36-residue peptide derived from the C-terminal heptad repeat (CHR) of gp41, is the only clinically available HIV-1 fusion inhibitor, but it easily induces drug resistance, which calls for next-generation drugs.

DESIGN

We recently demonstrated that the M-T hook structure can be used to design a short CHR peptide that specifically targets the conserved gp41 pocket rather than the T20-resistant sites. We attempted to develop more potent HIV-1 fusion inhibitors based on the structure-activity relationship of MT-SC22EK.

METHODS

Multiple biophysical and functional approaches were performed to determine the structural features, binding affinities and anti-HIV activities of the inhibitors.

RESULTS

The 23-residue peptide HP23, which mainly contains the M-T hook structure and pocket-binding sequence, showed a helical and trimeric state in solution. HP23 had dramatically improved binding stability and antiviral activity, and it was the most potent inhibitor of the M-T hook-modified and unmodified control peptides. More promisingly, HP23 was highly active in the inhibition of diverse HIV-1 subtypes, including T20 and MT-SC22EK resistant HIV-1 mutants, and it exhibited a high genetic barrier to the development of resistance.

CONCLUSION

Our studies delivered an ideal HIV-1 fusion inhibitor that specifically targeted the highly conserved gp41 pocket and possessed potent binding and antiviral activity. Furthermore, HP23 can serve as a critical tool to explore the mechanisms of HIV-1 fusion and inhibition.

C peptides as entry inhibitors for gene therapy

Peptides derived from the C-terminal heptad repeat 2 region of the HIV-1 gp41 envelope glycoprotein, so-called C peptides, are very potent HIV-1 fusion inhibitors. Antiviral genes encoding either membrane-anchored (ma) or secreted (iSAVE) C peptides have been engineered and allow direct in vivo production of the therapeutic peptides by genetically modified host cells. Membrane-anchored C peptides expressed in the HIV-1 target cells by T-cell or hematopoietic stem cell gene therapy efficiently prevent virus entry into the modified cells. Such gene-protection confers a selective survival advantage and allows accumulation of the genetically modified cells. Membrane-anchored C peptides have been successfully tested in a nonhuman primate model of AIDS and were found to be safe in a phase I clinical trial in AIDS patients transplanted with autologous gene-modified T-cells. Secreted C peptides have the crucial advantage of not only protecting genetically modified cells from HIV-1 infection, but also neighboring cells, thus suppressing virus replication even if only a small fraction of cells is genetically modified. Accordingly, various cell types can be considered as potential in vivo producer cells for iSAVE-based gene therapeutics, which could even be modified by direct in vivo gene delivery in future. In conclusion, C peptide gene therapeutics may provide a strong benefit to AIDS patients and could present an effective alternative to current antiretroviral drug regimens.

Peptide fusion inhibitors targeting the HIV-1 gp41: a patent review (2009 - 2014)

INTRODUCTION

As the first peptide HIV fusion inhibitor targeting gp41, enfuvirtide (T20) was approved by the US FDA in 2003 as a salvage therapy for HIV/AIDS patients who failed to respond to the then existing antiretroviral therapeutics. However, its clinical application is limited by its relatively low potency, low genetic barrier to drug resistance and short half-life. Therefore, it is essential to develop new peptide HIV fusion inhibitors with improved antiviral efficacy, drug-resistance profile and pharmaceutical properties.

AREAS COVERED

In this paper, we reviewed the patents, patent applications and related research articles for the development of new peptide fusion inhibitors targeting the HIV-1 gp41 published between 2009 and 2014.

EXPERT OPINION

To improve enfuvirtide's anti-HIV efficacy, drug-resistance profile, half-life and pharmaceutical properties, the best approaches include the addition of the pocket-binding domain (PBD) to the N-terminus of T20 and linking of the M-T hook to the N-terminus of PBD, as well as conjugation of cholesterol, serum albumin-binding motif or gp120-binding fragment with a PBD-containing C-terminal heptad repeat-peptide. Therefore, sifuvirtide from Tianjin FusoGen Pharmaceuticals, Inc., albuvirtide from Frontier Biotechnologies Co., Ltd., cholesterol-conjugated HIV fusion inhibitor from the Institute of Pathogen Biology, Chinese Academy of Medical Science, 2DLT, a bivalent HIV fusion inhibitor/inactivator, and an enfuvirtide/sifuvirtide combination regimen from the New York Blood Center may all have potential as next-generation HIV fusion inhibitors targeting gp41 for clinical use.

Molecular dynamics studies of the inhibitor C34 binding to the wild-type and mutant HIV-1 gp41: inhibitory and drug resistant mechanism

Mutations on NHR (N-terminal heptad repeat) associated with resistance to fusion inhibitor were observed. In addition, mutations on CHR (C-terminal heptad repeat) accompanied NHR mutations of gp41 are noted in many cases, like N43D/S138A double mutation. In this work, we explored the drug resistant mechanism of N43D mutation and the role of S138A second mutation in drug resistance. The binding modes of the wild type gp41 and the two mutants, N43D and N43D/S138A, with the HIV-1 fusion inhibitor C34, a 34-residue peptide mimicking CHR of gp41, were carried out by using molecular dynamics simulations. Based on the MD simulations, N43D mutation affects not only the stability of C34 binding, but also the binding energy of the inhibitor C34. Because N43D mutation may also affect the stable conformation of 6-HB, we introduced S138A second mutation into CHR of gp41 and determined the impact of this mutation. Through the comparative analysis of MD results of the N43D mutant and the N43D/S138A mutant, we found that CHR with S138A mutation shown more favorable affinity to NHR. Compelling differences in structures have been observed for these two mutants, particularly in the binding modes and in the hydrophobic interactions of the CHR (C34) located near the hydrophobic groove of the NHR. Because the conformational stability of 6-HB is important to HIV-1 infection, we suggested a hypothetical mechanism for the drug resistance: N43D single mutation not only impact the binding of inhibitor, but also affect the affinity between NHR and CHR of gp41, thus may reduce the rate of membrane fusion; compensatory mutation S138A would induce greater hydrophobic interactions between NHR and CHR, and render the CHR more compatible to NHR than inhibitors.

Hydrophobic mutations in buried polar residues enhance HIV-1 gp41 N-terminal heptad repeat-C-terminal heptad repeat interactions and C-peptides' anti-HIV activity

OBJECTIVE

To investigate the effect of mutations in a highly conserved buried polar area on the function of HIV-1 gp41.

DESIGN

During HIV-1 entry, a six helical bundle (6-HB) formation between the C-terminal and N-terminal heptad repeat (CHR and NHR) of gp41 provides energy for virus cell membrane fusion. In 6-HB, residues at a and d (a-d) positions of CHR directly interact with NHR and are buried. They are considered critical residues for 6-HB stability and for anti-HIV-1 activity of CHR-derived peptides (C-peptides). Most of a-d residues in CHR are hydrophobic, as buried hydrophobic residues facilitate protein stability. However, HIV-1 gp41 CHR contains a highly conserved polar area with four successive buried a-d polar residues: S649/Q652/N656/E659. We mutated these buried polar residues to hydrophobic residues, either Leu or Ile, and studied its effect on the gp41 NHR-CHR interactions and anti-HIV activities of the C-peptides.

METHODS

We measured the C-peptide mutants' ability to form 6-HB with NHR, thermal stability of the 6-HBs and C-peptides' inhibitory activity against both T20-sensitive and resistant HIV-1 strains.

RESULTS

All the mutated C-peptides retained their ability to form stable 6-HB with NHR and strongly inhibited HIV-1 replication. Strikingly, S649L and E659I mutations endow C-peptide with a significantly enhanced activity against T20-resistant HIV-1 strains.

CONCLUSION

The highly conserved buried a-d polar residues in HIV-1 gp41 CHR can be mutated as a means of developing new fusion inhibitors against drug-resistant HIV-1 strains. The concept can also be utilized to design fusion inhibitors against other viruses with similar mechanisms.

Two M-T hook residues greatly improve the antiviral activity and resistance profile of the HIV-1 fusion inhibitor SC29EK

Background

Peptides derived from the C-terminal heptad repeat (CHR) of HIV-1 gp41 such as T20 (Enfuvirtide) and C34 are potent viral fusion inhibitors. We have recently found that two N-terminal residues (Met115 and Thr116) of CHR peptides form a unique M-T hook structure that can greatly enhance the binding and anti-HIV activity of inhibitors. Here, we applied two M-T hook residues to optimize SC29EK, an electrostatically constrained peptide inhibitor with a potent anti-HIV activity.

Results

The resulting peptide MT-SC29EK showed a dramatically increased binding affinity and could block the six-helical bundle (6-HB) formation more efficiently. As expected, MT-SC29EK potently inhibited HIV-1 entry and infection, especially against those T20- and SC29EK-resistant HIV-1 variants. More importantly, MT-SC29EK and its short form (MT-SC22EK) suffered from the difficulty to induce HIV-1 resistance during the in vitro selection, suggesting their high genetic barriers to the development of resistance.

Conclusions

Our studies have verified the M-T hook structure as a vital strategy to design novel HIV-1 fusion inhibitors and offered an ideal candidate for clinical development.

HIV-1 envelope glycoprotein structure

The trimeric envelope glycoprotein of HIV-1, composed of gp120 and gp41 subunits, remains a major target for vaccine development. The structures of the core regions of monomeric gp120 and gp41 have been determined previously by X-ray crystallography. New insights into the structure of trimeric HIV-1 envelope glycoproteins are now coming from cryo-electron tomographic studies of the gp120/gp41 trimer as displayed on intact viruses and from cryo-electron microscopic studies of purified, soluble versions of the ectodomain of the trimer. Here, we review recent developments in these fields as they relate to our understanding of the structure and function of HIV-1 envelope glycoproteins.

Escape from human immunodeficiency virus type 1 (HIV-1) entry inhibitors

The human immunodeficiency virus (HIV) enters cells through a series of molecular interactions between the HIV envelope protein and cellular receptors, thus providing many opportunities to block infection. Entry inhibitors are currently being used in the clinic, and many more are under development. Unfortunately, as is the case for other classes of antiretroviral drugs that target later steps in the viral life cycle, HIV can become resistant to entry inhibitors. In contrast to inhibitors that block viral enzymes in intracellular compartments, entry inhibitors interfere with the function of the highly variable envelope glycoprotein as it continuously adapts to changing immune pressure and available target cells in the extracellular environment. Consequently, pathways and mechanisms of resistance for entry inhibitors are varied and often involve mutations across the envelope gene. This review provides a broad overview of entry inhibitor resistance mechanisms that inform our understanding of HIV entry and the design of new inhibitors and vaccines.