XTT formazan widely used to detect cell viability inhibits HIV type 1 infection in vitro by targeting gp41

Design, Synthesis, Docking Studies, and Biological Evaluation of Novel 2-Hydroxyacetophenone Derivatives as Anti-HIV-1 Agents

BACKGROUND

The persistence of HIV mutations and the existence of multidrug resistance have produced an opportunity for an array of innovative anti-HIV medicines with a variety of structures that target HIV key enzymes.

OBJECTIVE

The goal of this work was to find a new class of anti-HIV drugs founded on HIV integrase inhibitor pharmacophores.

METHODS

A novel class of 2-hydroxy acetophenone analogs featuring substituted benzamide or N-phenylthiourea groups was designed and synthesized based on the general pharmacophore of HIV-1 integrase inhibitors (INs).

RESULTS

Most of the synthesized analogs were found to be moderately active against the virus, with EC50 values ranging from 40 to 140 μM. Additionally, it was found that most of the compounds presented no considerable cytotoxicity (CC50 > 500 μΜ). The most potent compounds substituting with 4-fluorobenzamide (compound 7) and 4-methylbenzamide (compound 9) rings inhibited the HIV-1 replication by EC50 values of 40 and 45 μΜ, respectively. Docking studies using the crystallographic data available for PFV IN indicated that the Mg2+ coordination might be the possible mechanism of the anti-viral activity.

CONCLUSION

Our findings proved that the synthesized analogs may suggest a very good basis for the development of new anti-HIV-1 agents.

IgG Fc-binding motif-conjugated HIV-1 fusion inhibitor exhibits improved potency and in vivo half-life: Potential application in combination with broad neutralizing antibodies

The clinical application of conventional peptide drugs, such as the HIV-1 fusion inhibitor enfuvirtide, is limited by their short half-life in vivo. To overcome this limitation, we developed a new strategy to extend the in vivo half-life of a short HIV-1 fusion inhibitory peptide, CP24, by fusing it with the human IgG Fc-binding peptide (IBP). The newly engineered peptide IBP-CP24 exhibited potent and broad anti-HIV-1 activity with IC50 values ranging from 0.2 to 173.7 nM for inhibiting a broad spectrum of HIV-1 strains with different subtypes and tropisms, including those resistant to enfuvirtide. Most importantly, its half-life in the plasma of rhesus monkeys was 46.1 h, about 26- and 14-fold longer than that of CP24 (t1/2 = 1.7 h) and enfuvirtide (t1/2 = 3 h), respectively. IBP-CP24 intravenously administered in rhesus monkeys could not induce significant IBP-CP24-specific antibody response and it showed no obvious in vitro or in vivo toxicity. In the prophylactic study, humanized mice pretreated with IBP-CP24 were protected from HIV-1 infection. As a therapeutic treatment, coadministration of IBP-CP24 and normal human IgG to humanized mice with chronic HIV-1 infection resulted in a significant decrease of plasma viremia. Combining IBP-CP24 with a broad neutralizing antibody (bNAb) targeting CD4-binding site (CD4bs) in gp120 or a membrane proximal external region (MPER) in gp41 exhibited synergistic effect, resulting in significant dose-reduction of the bNAb and IBP-CP24. These results suggest that IBP-CP24 has the potential to be further developed as a new HIV-1 fusion inhibitor-based, long-acting anti-HIV drug that can be used alone or in combination with a bNAb for treatment and prevention of HIV-1 infection.

The anti-parasitic drug suramin potently inhibits formation of seminal amyloid fibrils and their interaction with HIV-1

Seminal amyloid fibrils are made up of naturally occurring peptide fragments and are key targets for the development of combination microbicides or antiviral drugs. Previously, we reported that the polysulfonic compound ADS-J1 is a potential candidate microbicide that not only inhibits HIV-1 entry, but also seminal fibrils. However, the carcinogenic azo moieties in ADS-J1 preclude its clinical application. Here, we screened several ADS-J1-like analogs and found that the antiparasitic drug suramin most potently inhibited seminal amyloid fibrils. Using various biochemical methods, including Congo red staining, CD analysis, transmission EM, viral infection assays, surface plasmon resonance imaging, and molecular dynamics simulations, we investigated suramin's inhibitory effects and its putative mechanism of action. We found that by forming a multivalent interaction, suramin binds to proteolytic peptides and mature fibrils, thereby inhibiting seminal fibril formation and blocking fibril-mediated enhancement of viral infection. Of note, suramin exhibited potent anti-HIV activities, and combining suramin with several antiretroviral drugs produced synergistic effects against HIV-1 in semen. Suramin also displayed a good safety profile for vaginal application. Moreover, suramin inhibited the semen-derived enhancer of viral infection (SEVI)/semen-mediated enhancement of HIV-1 transcytosis through genital epithelial cells and the subsequent infection of target cells. Collectively, suramin has great potential for further development as a combination microbicide to reduce the spread of the AIDS pandemic by targeting both viral and host factors involved in HIV-1 sexual transmission.

Computer-Aided Approaches for Targeting HIVgp41

Virus-cell fusion is the primary means by which the human immunodeficiency virus-1 (HIV) delivers its genetic material into the human T-cell host. Fusion is mediated in large part by the viral glycoprotein 41 (gp41) which advances through four distinct conformational states: (i) native, (ii) pre-hairpin intermediate, (iii) fusion active (fusogenic), and (iv) post-fusion. The pre-hairpin intermediate is a particularly attractive step for therapeutic intervention given that gp41 N-terminal heptad repeat (NHR) and C-terminal heptad repeat (CHR) domains are transiently exposed prior to the formation of a six-helix bundle required for fusion. Most peptide-based inhibitors, including the FDA-approved drug T20, target the intermediate and there are significant efforts to develop small molecule alternatives. Here, we review current approaches to studying interactions of inhibitors with gp41 with an emphasis on atomic-level computer modeling methods including molecular dynamics, free energy analysis, and docking. Atomistic modeling yields a unique level of structural and energetic detail, complementary to experimental approaches, which will be important for the design of improved next generation anti-HIV drugs.

Nucleic acid polymers inhibit duck hepatitis B virus infection in vitro

Nucleic acid polymers (NAPs) utilize the sequence-independent properties of phosphorothioate oligonucleotides (PS-ONs) to target protein interactions involved in viral replication. NAPs are broadly active against a diverse range of enveloped viruses that use type I entry mechanisms. The antiviral activity of NAPs against hepatitis B virus (HBV) infection was assessed in vitro in duck hepatitis B virus (DHBV)-infected primary duck hepatocytes (PDH). NAPs efficiently entered PDH in the absence of any transfection agent and displayed antiviral activity at concentrations of 0.01 to 10 μM, measured by their ability to prevent the intracellular accumulation of DHBV surface antigen, which was independent of their nucleotide sequence and was specifically dependent on phosphorothioation. Higher levels of antiviral activity were observed with NAPs 40 nucleotides in length or longer. The fully degenerate NAP (REP 2006) was active during DHBV infection or when added 12 h after infection. In contrast, an acidic-pH-sensitive NAP (REP 2031) that was broadly active against other viruses displayed antiviral activity when present during DHBV infection but no activity when added 12 h after infection, suggesting that NAPs exert their postentry effect in an acidic environment unique to DHBV infection. Both REP 2006 and REP 2031 displayed negligible cytotoxicity in PDH at concentrations of up to 10 μM, as assessed using an XTT [2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide] cytotoxicity assay. The antiviral activity of NAPs against DHBV in vitro was strictly dependent on their amphipathic character, suggesting that NAPs interact with amphipathic target(s) that are important for DHBV entry and postentry mechanisms required for infection.

The potent human immunodeficiency virus type 1 (HIV-1) entry inhibitor HR212 blocks formation of the envelope glycoprotein gp41 six-helix bundle

HR212, a recombinant protein composed of the heptad repeat, is a rationally designed human immunodeficiency virus type 1 (HIV-1) fusion inhibitor. This protein can be easily produced by Escherichia coli at a low cost. Previously, studies indicated that HR212 can efficiently inhibit the entry and replication of both laboratory and clinical HIV-1 strains, and this protein is more stable and less sensitive to proteinases than T20. The procedure of HIV-1 entry into the host cells can be divided into three main steps: gp120-CD4 interactions, coreceptor binding, and gp41 six-helix bundle formation and subsequent membrane fusion. The present study demonstrates that HR212 does not block gp120-CD4 binding or interfere with binding to the coreceptors CXCR4 and CCR5. Instead, HR212 efficiently blocks the six-helix bundle formation between peptides derived from the N-terminal heptad repeat (NHR) and the C-terminal heptad repeat (CHR) region of gp41. Fluorescence native polyacrylamide gel electrophoresis (FN-PAGE) indicated that HR212 could form a complex with peptide N36 to block gp41 fusogenic core formation. These results suggest that HR212 inhibits HIV-1 entry by targeting the NHR region of gp41. Therefore, HR212 can potentially be developed as a novel, high-efficiency, specific HIV-1 entry inhibitor.

Targeting HIV-1 gp41-induced fusion and pathogenesis for anti-viral therapy

HIV gp41 is a metastable protein whose native conformation is maintained in the form of a heterodimer with gp120. The non-covalently associated gp41/gp120 complex forms a trimer on the virus surface. As gp120 engages with HIV's receptor, CD4, and coreceptor, CXCR4 or CCR5, gp41 undergoes several conformational changes resulting in fusion between the viral and cellular membranes. Several lipophilic and amphiphilic domains have been shown to be critical in that process. While the obvious function of gp41 in viral entry is well-established its role in cellular membrane fusion and the link with pathogenesis are only now beginning to appear. Recent targeting of gp41 via fusion inhibitors has revealed an important role of this protein not only in viral entry but also in bystander apoptosis and HIV pathogenesis. Studies by our group and others have shown that the phenomenon of gp41-mediated hemifusion initiates apoptosis in bystander cells and correlates with virus pathogenesis. More interestingly, recent clinical evidence suggests that gp41 mutants arising after Enfuvirtide therapy are associated with CD4 cell increase and immunological benefits. This has in turn been correlated to a decrease in bystander apoptosis in our in vitro as well as in vivo assays. Although a great deal of work has been done to unravel HIV-1 gp41-mediated fusion mechanisms, the factors that regulate gp41-mediated fusion versus hemifusion and the mechanism by which hemifusion initiates bystander apoptosis are not fully understood. Further insight into these issues will open new avenues for drug development making gp41 a critical anti-HIV target both for neutralization and virus attenuation.

A natural theaflavins preparation inhibits HIV-1 infection by targeting the entry step: potential applications for preventing HIV-1 infection

Theaflavins are the major components of tea polyphenols in brewed black tea. We previously reported that theaflavin derivatives, such as TF3, inhibited HIV-1 entry by targeting gp41. However, it is difficult to purify the individual theaflavins and the purified compounds are highly unstable. To develop theaflavins as affordable anti-HIV-1 microbide for preventing HIV sexual transmission, we intended to use an economic natural preparation containing 90% of theaflavins (TFmix). Its antiviral activity against HIV-1 strains was evaluated in vitro using p24 production and luciferase assays. The mechanism by which TFmix inhibits HIV-1 infection was investigated using time-of-addition, cell-cell fusion and biophysical assays. The data suggested TFmix exhibited potent anti-HIV-1 activity on lab-adapted and primary HIV-1 strains with IC(50) less than 1.20 μM. It also effectively inhibited infection by T-20 resistant HIV-1 strains. The mechanism studies suggest that TFmix mainly inhibit the HIV-1 entry by targeting gp41 since it is effective in inhibiting gp41 six-helix bundle (6-HB) formation and HIV-1 envelope protein-mediated cell-cell fusion. TFmix could also inhibit HIV-1 reverse transcriptase (RT) activity, but the IC(50) is about 8-fold higher than that for inhibiting gp41 6-HB formation, suggesting RT is not a major target for TFmix. In conclusion, TFmix is an economic natural product preparation containing high content of theaflavins with potent anti-HIV-1 activity by targeting the viral entry step through the disruption of gp41 6-HB core structure. It has a potential to be developed as a safe and affordable topical microbicide for preventing sexual transmission of HIV.

Small molecule HIV entry inhibitors: Part II. Attachment and fusion inhibitors: 2004-2010

INTRODUCTION

The first US FDA approved HIV entry inhibitor drug Enfuvirdine belongs to the fusion inhibitor category. Earlier efforts in this area were focused on peptides and monoclonal antibodies; recently, the focus has shifted towards the development of small molecule HIV attachment and fusion inhibitors. They can be used for prophylactic purposes and also hold potential for the development of HIV microbicides.

AREAS COVERED

In a previous paper ('Small molecule HIV entry inhibitors: Part I'), we reviewed patents and patent applications for small molecule chemokine receptor antagonists from major pharmaceutical companies. In this paper, the development of small molecule HIV attachment and fusion inhibitors is discussed in detail. It covers patents and patent applications for small molecule HIV attachment and fusion inhibitors published between 2004 and 2010 and related literature with a focus on recent developments based on lead generation and lead modification.

EXPERT OPINION

To augment the potency of currently available antiretroviral drug combinations and to fight drug-resistant virus variants, more effective drugs which target additional steps in the viral replication cycle are urgently needed. HIV attachment and fusion processes are such targets. Inhibitors of these targets will provide additional options for the treatment of HIV drug-resistant strains. Small molecule HIV attachment inhibitors such as BMS-378806 and analogs from Bristol Myers Squibb, N-aryl piperidine derivatives from Propharmacon, and NBD-556 and NBD-557 from New York Blood Center may have potential as vaginal microbicidal agents and can be an economical alternative to monoclonal antibodies.

Development of peptide and small-molecule HIV-1 fusion inhibitors that target gp41

It has been 25 years since the development of the first efficient HIV-1/AIDS treatment. Scientists now know more about the HIV-1 infection life cycle, and more than 30 antiretroviral drugs have been developed, including HIV-1 fusion inhibitors. Fundamental work was begun in the early 1990s and led to the development of a novel class of anti-HIV-1 drugs, culminating in a peptide known as T20, which is currently the only HIV-1 fusion inhibitor approved by the US Food and Drug Administration. However, more work needs to be done to perfect the development of peptide and small-molecule HIV fusion inhibitors, particularly those that target gp41. Herein we present a brief overview of the development of this class of anti-HIV-1 drug by focusing on the achievements, challenges, and lessons learned. We cite hallmark studies of the past and comment on future drug development.

3-hydroxyphthalic anhydride-modified chicken ovalbumin exhibits potent and broad anti-HIV-1 activity: a potential microbicide for preventing sexual transmission of HIV-1

Heterosexual transmission is the primary route by which women acquire human immunodeficiency virus (HIV)/AIDS. Thus, development of woman-controlled topical microbicides for prevention of sexual transmission of HIV is urgently needed. Here we report that 3-hydroxyphthalic anhydride-modified chicken ovalbumin (HP-OVA) exhibits potent antiviral activity against a broad spectrum of human immunodeficiency virus type 1 (HIV-1) isolates with different genotypes and biotypes. Its antiviral activity is correlated with the percentages of the chemically modified and unmodified lysines and arginines in OVA. HP-OVA inhibits HIV-1 fusion and entry through multiple mechanisms of action, including (i) blocking gp120 binding to CD4 and (ii) interfering with gp41 six-helix bundle formation. Because of the widespread availability and established safety profile of OVA, HP-OVA has good potential to be developed as an effective, safe, and affordable microbicide for prevention of HIV sexual transmission.

Structure-based design, synthesis and biological evaluation of new N-carboxyphenylpyrrole derivatives as HIV fusion inhibitors targeting gp41

A new series of N-carboxyphenylpyrrole ligands were designed using GeometryFit based on an X-ray crystal structure of gp41. The synthesized ligands showed significant inhibitory activities against HIV gp41 6-helix bundle formation, HIV-1 mediated cell-cell fusion and HIV-1 replication.

ADS-J1 inhibits human immunodeficiency virus type 1 entry by interacting with the gp41 pocket region and blocking fusion-active gp41 core formation

We previously identified a small-molecule anti-human immunodeficiency virus type 1 (anti-HIV-1) compound, ADS-J1, using a computer-aided molecular docking technique for primary screening and a sandwich enzyme-linked immunosorbent assay (ELISA) as a secondary screening method. In the present study, we demonstrated that ADS-J1 is an HIV-1 entry inhibitor, as determined by a time-of-addition assay and an HIV-1-mediated cell fusion assay. Further mechanism studies confirmed that ADS-J1 does not block gp120-CD4 binding and exhibits a marginal interaction with the HIV-1 coreceptor CXCR4. However, ADS-J1 inhibited the fusion-active gp41 core formation mimicked by peptides derived from the viral gp41 N-terminal heptad repeat (NHR) and C-terminal heptad repeat (CHR), as determined by ELISA, native polyacrylamide gel electrophoresis, and circular dichroism analysis. Moreover, using a surface plasmon resonance assay, we found that ADS-J1 could bind directly to IQN17, a trimeric peptide containing the gp41 pocket region, resulting in the conformational change of IQN17 and the blockage of its interaction with a short D peptide, PIE7. The positively charged residue (K574) located in the gp41 pocket region is critical for the binding of ADS-J1 to NHR. These results suggest that ADS-J1 may bind to the viral gp41 NHR region through its hydrophobic and ionic interactions with the hydrophobic and positively charged resides located in the pocket region, subsequently blocking the association between the gp41 NHR and CHR regions to form the fusion-active gp41 core, thereby inhibiting HIV-1-mediated membrane fusion and virus entry.

New cholesterol-specific antibodies remodel HIV-1 target cells' surface and inhibit their in vitro virus production

The importance of membrane rafts in HIV-1 infection is still in the focus of interest. Here, we report that new monoclonal anticholesterol IgG antibodies (ACHAs), recognizing clustered membrane cholesterol (e.g., in lipid rafts), rearrange the lateral molecular organization of HIV-1 receptors and coreceptors in the plasma membrane of HIV-1 permissive human T-cells and macrophages. This remodeling is accompanied with a substantial inhibition of their infection and HIV-1 production in vitro. ACHAs promote the association of CXCR4 with both CD4 and lipid rafts, consistent with the decreased lateral mobility of CXCR4, while Fab fragments of ACHAs do not show these effects. ACHAs do not directly mask the extracellular domains of either CD4 or CXCR4 nor do they affect CXCR4 internalization. No significant inhibition of HIV production is seen when the virus is preincubated with the antibodies prior to infection. Thus, we propose that the observed inhibition is mainly due to the membrane remodeling induced by cholesterol-specific antibodies on the target cells. This, in turn, may prevent the proper spatio-temporal juxtaposition of HIV-1 glycoproteins with CD4 and chemokine receptors, thus negatively interfering with virus attachment/entry.

Potent D-peptide inhibitors of HIV-1 entry

During HIV-1 entry, the highly conserved gp41 N-trimer pocket region becomes transiently exposed and vulnerable to inhibition. Using mirror-image phage display and structure-assisted design, we have discovered protease-resistant D-amino acid peptides (D-peptides) that bind the N-trimer pocket with high affinity and potently inhibit viral entry. We also report high-resolution crystal structures of two of these D-peptides in complex with a pocket mimic that suggest sources of their high potency. A trimeric version of one of these peptides is the most potent pocket-specific entry inhibitor yet reported by three orders of magnitude (IC(50) = 250 pM). These results are the first demonstration that D-peptides can form specific and high-affinity interactions with natural protein targets and strengthen their promise as therapeutic agents. The D-peptides described here address limitations associated with current L-peptide entry inhibitors and are promising leads for the prevention and treatment of HIV/AIDS.

Conserved residue Lys574 in the cavity of HIV-1 Gp41 coiled-coil domain is critical for six-helix bundle stability and virus entry

The fusion-active HIV-1 gp41 core structure is a stable six-helix bundle (6-HB) formed by its N- and C-terminal heptad-repeat sequences (NHR and CHR). A highly conserved, deep hydrophobic cavity on the surface of the N-helical trimer is important for stability of the 6-HB and serves as an ideal target for developing anti-human immunodeficiency virus (HIV) fusion inhibitors. We have recently identified several small molecule HIV-1 fusion inhibitors that bind to the gp41 cavity through hydrophobic and ionic interactions and block the gp41 6-HB formation. Molecular docking analysis reveals that these small molecules fit inside the hydrophobic cavity and interact with positively charged residue Lys574 to form a conserved salt bridge. In this study, the functionality of Lys574 has been finely characterized by mutational analysis and biophysical approaches. We found that substitutions of Lys574 with non-conserved residues (K574D, K574E, and K574V) could completely abolish virus infectivity. With a set of wild-type and mutant N36 peptides derived from the NHR sequence as a model, we demonstrated that non-conservative Lys574 substitutions severely impaired the stability and conformation of 6-HBs as detected by circular dichroism spectroscopy, native polyacrylamide gel electrophoresis, and enzyme-linked immunosorbent assay. The binding affinity of N36 mutants bearing non-conservative Lys574 substitutions to the peptide C34 derived from the CHR sequence dramatically decreased as measured by isothermal titration calorimetry. These substitutions also significantly reduced the potency of N-peptides to inhibit HIV-1 infection. Collectively, these data suggest that conserved Lys574 plays a critical role in 6-HB formation and HIV-1 infectivity, and may serve as an important target for designing anti-HIV drugs.

Selection of a novel gp41-specific HIV-1 neutralizing human antibody by competitive antigen panning

The HIV envelope glycoprotein (Env) is composed of two non-covalently associated subunits: gp120 and gp41. Panning of phage-displayed antibody libraries against Env-based antigens has resulted mostly in selection of anti-gp120 antibodies. Native gp41 in the absence of gp120 is unstable. The use of gp41 fragments as antigens has resulted in selection of antibodies with only relatively modest neutralizing activity. To enhance selection of antibodies specific for gp41 in the context of the whole Env we developed a methodology termed competitive antigen panning (CAP). Using CAP, we identified a novel gp41-specific human monoclonal antibody (hmAb), m48, from an immune library derived from long-term nonprogressors with high titers of broadly cross-reactive neutralizing antibodies (bcnAbs). Selection of m48 was only successful using CAP and not through the conventional pre-incubation methodology. In assays based on spreading infection in peripheral blood mononuclear cells (PBMCs) m48 neutralized a panel of HIV-1 primary isolates from different clades more potently than the well-characterized broadly cross-reactive HIV-1-neutralizing antibodies IgG1 4E10 and Fab Z13. These results may have implications for the selection of novel gp41-specific bcnAbs and other antibodies, and for the development of HIV-1 inhibitors and vaccine immunogens.

Identification of a d-amino acid decapeptide HIV-1 entry inhibitor

Entry of human immunodeficiency virus type 1 (HIV-1) virion into host cells involves three major steps, each being a potential target for the development of entry inhibitors: gp120 binding to CD4, gp120-CD4 complex interacting with a coreceptor, and gp41 refolding to form a six-helix bundle. Using a D-amino acid decapeptide combinatorial library, we identified peptide dC13 as having potent HIV-1 fusion inhibitory activity, and effectively inhibiting infection by several laboratory-adapted and primary HIV-1 strains. While dC13 did not block binding of gp120 to CD4, nor disrupt the gp41 six-helix bundle formation, it effectively blocked the binding of an anti-CXCR4 monoclonal antibody and chemokine SDF-1alpha to CXCR4-expressing cells. However, because R5-using primary viruses were also neutralized, the antiviral activity of dC13 implies additional mode(s) of action. These results suggest that dC13 is a useful HIV-1 coreceptor antagonist for CXCR4 and, due to its biostability and simplicity, may be of value for developing a new class of HIV-1 entry inhibitors.

Evaluation of "credit card" libraries for inhibition of HIV-1 gp41 fusogenic core formation

Protein-protein interactions are of critical importance in biological systems, and small molecule modulators of such protein recognition and intervention processes are of particular interest. To investigate this area of research, we have synthesized small-molecule libraries that can disrupt a number of biologically relevant protein-protein interactions. These library members are designed upon planar motif, appended with a variety of chemical functions, which we have termed "credit-card" structures. From two of our "credit-card" libraries, a series of molecules were uncovered which act as inhibitors against the HIV-1 gp41 fusogenic 6-helix bundle core formation, viral antigen p24 formation, and cell-cell fusion at low micromolar concentrations. From the high-throughput screening assays we utilized, a selective index (SI) value of 4.2 was uncovered for compound 2261, which bodes well for future structure activity investigations and the design of more potent gp41 inhibitors.

Progress in identifying peptides and small-molecule inhibitors targeted to gp41 of HIV-1

During the last decade, a great number of activities have been geared in identifying newer targets for inhibiting HIV infection as well as understanding the targets for already identified anti-HIV-1 agents. The success in converting a proof-of-concept peptide T-20 (previously named DP-178), from the C-terminal heptad repeat (CHR) region of the envelope glycoprotein gp41 of HIV-1, to a drug named enfuvirtide was one of the phenomenal successes in HIV-1 drug discovery research that has been made in recent years. There were many reports of modifying peptides from the N-terminal heptad repeat and CHR regions with the objective of improving their activity. A few laboratories also reported the identification of small-molecule inhibitors that apparently bind to the hydrophobic cavity identified in the gp41 core structure and prevent the CHR peptide binding to the N-terminal heptad repeat peptide, thereby prevent the formation of the typical six-helix bundle, which has been thought to be necessary for the fusion between HIV and cell membranes.

Peptide, Peptidomimetic and Small-molecule Drug Discovery Targeting HIV-1 Host-cell Attachment and Entry through gp120, gp41, CCR5 and CXCR4+

This review highlights selected examples of peptide, peptidomimetic and small-molecule drug discovery targeting HIV-1 to advance novel anti-HIV pharmaceuticals that inhibit initial stages of the viral cycle; namely, attachment and entry. Some of these approaches have culminated in the development of peptide-based drugs, while other have exploited peptides as enabling tools toward the identification of small-molecule lead compounds. Both of these conceptually different approaches have facilitated lead optimization of molecules with complementary and often surprising anti-HIV pharmacological properties, supporting their role in pharmaceutical development. Furthermore, such molecules enabled mechanistic elucidation of viral attachment and entry and provided additional insights toward achieving the desired drug profile.

Inhibiting HIV fusion with a beta-peptide foldamer

Linear peptides derived from the HIV gp41 C-terminus (C-peptides), such as the 36-residue Fuzeon, are potent HIV fusion inhibitors. These molecules bind to the N-peptide region of gp41 and inhibit an intramolecular protein-protein interaction that powers fusion of the viral and host cell membranes. The N-peptide region contains a surface pocket that is occupied in the post-fusion state by three alpha-helical residues found near the gp41 C-terminus: Trp628, Trp631, and Ile635-the WWI epitope. Here, we describe a set of beta3-decapeptides (betaWWI-1-4) in which the WWI epitope is presented on one face of a short 14-helix stabilized by macrodipole neutralization and side chain-side chain salt bridges. betaWWI-1-4 bind in vitro to IZN17, a validated gp41 model, and inhibit syncytia formation in cell culture. Molecules lacking a complete WWI functional epitope neither bind IZN17 nor inhibit syncytia formation. These results provide evidence that short beta-peptide 14-helices can inhibit an intramolecular protein-protein interaction in vivo. Molecules related to betaWWI-1-4 could represent starting points for the development of highly potent inhibitors or antigens effective against HIV or other viruses, including SARS, Ebola, HRSV, and influenza, that employ common fusion mechanisms.

Inhibiting HIV fusion with a beta-peptide foldamer

Linear peptides derived from the HIV gp41 C-terminus (C-peptides), such as the 36-residue Fuzeon, are potent HIV fusion inhibitors. These molecules bind to the N-peptide region of gp41 and inhibit an intramolecular protein-protein interaction that powers fusion of the viral and host cell membranes. The N-peptide region contains a surface pocket that is occupied in the post-fusion state by three alpha-helical residues found near the gp41 C-terminus: Trp628, Trp631, and Ile635-the WWI epitope. Here, we describe a set of beta3-decapeptides (betaWWI-1-4) in which the WWI epitope is presented on one face of a short 14-helix stabilized by macrodipole neutralization and side chain-side chain salt bridges. betaWWI-1-4 bind in vitro to IZN17, a validated gp41 model, and inhibit syncytia formation in cell culture. Molecules lacking a complete WWI functional epitope neither bind IZN17 nor inhibit syncytia formation. These results provide evidence that short beta-peptide 14-helices can inhibit an intramolecular protein-protein interaction in vivo. Molecules related to betaWWI-1-4 could represent starting points for the development of highly potent inhibitors or antigens effective against HIV or other viruses, including SARS, Ebola, HRSV, and influenza, that employ common fusion mechanisms.

Combination of candidate microbicides cellulose acetate 1,2-benzenedicarboxylate and UC781 has synergistic and complementary effects against human immunodeficiency virus type 1 infection

The combination of two candidate microbicides, cellulose acetate 1,2-benzenedicarboxylate (CAP), a polymer that blocks human immunodeficiency virus type 1 (HIV-1) entry by targeting gp120 and gp41, and UC781, a tight-binding HIV-1 reverse transcriptase inhibitor (RTI), resulted in effective synergy for inhibition of MT-2 cell infection by HIV-1(IIIB), a laboratory-adapted virus strain. The 95% effective concentration values for the combination were reduced about 15- to 20-fold compared with those corresponding to the single compounds. The combination of CAP and UC781 is also synergistic in inhibiting infection of peripheral blood mononuclear cells by a primary HIV-1 isolate, 92US657. Combinations of CAP with other RTIs, such as efavirenz or zidovudine, also had significant synergistic effects on the inhibition of HIV-1 infection. In addition, CAP and UC781 had complementary effects against HIV-1 infection since (i) CAP inhibited infection by the UC781-resistant strain HIV-1(IIIB) A17 and (ii) pretreatment of MT-2 cells with UC781, but not CAP, abolished subsequent infection after removal of the compound. This suggests that the combination of CAP and UC781 represents a promising candidate microbicide for prevention of sexual transmission of HIV-1.

N-substituted pyrrole derivatives as novel human immunodeficiency virus type 1 entry inhibitors that interfere with the gp41 six-helix bundle formation and block virus fusion

A recently approved peptidic human immunodeficiency virus type 1 (HIV-1) fusion inhibitor, T-20 (Fuzeon; Trimeris Inc.), has shown significant promise in clinical application for treating HIV-1-infected individuals who have failed to respond to the currently available antiretroviral drugs. However, T-20 must be injected twice daily and is too expensive. Therefore, it is essential to develop orally available small molecule HIV-1 fusion inhibitors. By screening a chemical library consisting of "drug-like" compounds, we identified two N-substituted pyrroles, designated NB-2 and NB-64, that inhibited HIV-1 replication at a low micromolar range. The absence of the COOH group in NB-2 and NB-64 resulted in a loss of anti-HIV-1 activity, suggesting that this acid group plays an important role in mediating the antiviral activity. NB-2 and NB-64 inhibited HIV-1 fusion and entry by interfering with the gp41 six-helix bundle formation and disrupting the alpha-helical conformation. They blocked a d-peptide binding to the hydrophobic pocket on surface of the gp41 internal trimeric coiled-coil domain. Computer-aided molecular docking analysis has shown that they fit inside the hydrophobic pocket and that their COOH group interacts with a positively charged residue (K574) around the pocket to form a salt bridge. These results suggest that NB-2 and NB-64 may bind to the gp41 hydrophobic pocket through hydrophobic and ionic interactions and block the formation of the fusion-active gp41 core, thereby inhibiting HIV-1-mediated membrane fusion and virus entry. Therefore, NB-2 and NB-64 can be used as lead compounds toward designing and developing more potent small molecule HIV-1 fusion inhibitors targeting gp41.

Rapid and automated fluorescence-linked immunosorbent assay for high-throughput screening of HIV-1 fusion inhibitors targeting gp41

The human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp41 plays an important role in the virus entry. During the process of fusion between the viral and target cell membranes, the N- and C-terminal heptad repeat (HR) regions of the gp41 extracellular domain associate to form a 6-helical bundle, corresponding to the fusion-active gp41 core. Any compound that blocks the gp41 6-helix bundle formation between the N- and C-peptides, which are derived from the N- and C-terminal HR regions, respectively, may inhibit HIV-1 mediated membrane fusion. Based on this principle, we previously established a sandwich enzyme-linked immunosorbent assay (ELISA) for drug screening by using the N-peptide N36 and the C-peptide C34 and a monoclonal antibody (NC-1) which specifically recognizes the gp41 6-helix bundle. In the present study, a fluorescence-linked immunosorbent assay (FLISA) was developed by using fluorescein isothiocyanate (FITC)-conjugated C34 to replace C34 and by directly detecting fluorescence intensity instead of more complicated enzymatic reaction. Compared with the sandwich ELISA, this FLISA has similar sensitivity and specificity, but it is much more rapid, economic and convenient. Using an Integrated Robotic Sample Processing System, this assay has been applied for high-throughput screening of organic compounds on a large scale for HIV-1 fusion inhibitors targeting gp41.

Interaction between heptad repeat 1 and 2 regions in spike protein of SARS-associated coronavirus: implications for virus fusogenic mechanism and identification of fusion inhibitors

BACKGROUND

Studies on the fusion-inhibitory peptides derived from the heptad repeat 1 and 2 (HR1 and HR2) regions of the HIV-1 envelope glycoprotein gp41 provided crucial information on the viral fusogenic mechanism. We used a similar approach to study the fusogenic mechanism of severe-acute-respiratory-syndrome-associated coronavirus (SARS-CoV).

METHODS

We tested the inhibitory activity against infection of two sets of peptides corresponding to sequences of SARS-CoV spike protein HR1 and HR2 regions and investigated the interactions between the HR1 and HR2 peptides by surface plasmon resonance, sedimentation equilibration analysis, circular dichroism, native polyacrylamide-gel electrophoresis, size exclusion high-performance liquid chromatography, and computer-aided homology modelling and molecule docking analysis.

FINDINGS

One peptide, CP-1, derived from the HR2 region, inhibited SARS-CoV infection in the micromolar range. CP-1 bound with high affinity to a peptide from the HR1 region, NP-1. CP-1 alone had low alpha-helicity and self-associated to form a trimer in phosphate buffer (pH 7.2). CP-1 and NP-1 mixed in equimolar concentrations formed a six-helix bundle, similar to the fusogenic core structure of HIV-1 gp41.

INTERPRETATION

After binding to the target cell, the transmembrane spike protein might change conformation by association between the HR1 and HR2 regions to form an oligomeric structure, leading to fusion between the viral and target-cell membranes. At the prefusion intermediate state, CP-1 could bind to the HR1 region and interfere with the conformational changes, resulting in inhibition of SARS-CoV fusion with the target cells. CP-1 might be modifiable to increase its anti-SARS-CoV activity and could be further developed as an antiviral agent for treatment or prophylaxis of SARS-CoV infection.

Determination of the HIV-1 gp41 fusogenic core conformation modeled by synthetic peptides: applicable for identification of HIV-1 fusion inhibitors

Triggered by receptor binding of gp120, the human immunodeficiency virus type 1 (HIV-1) gp41 changes its conformation to a fusogenic six-helix bundle structure. In the present study, this core conformation modeled by the peptides derived from the gp41 N- and C-terminal heptad repeat regions was determined by fluorescence native polyacrylamide gel electrophoresis and size exclusion high-performance liquid chromatography (HPLC). Two previously described small molecule HIV-1 fusion inhibitors significantly blocked the six-helix bundle formation. It suggests that these biophysical techniques can be used in a novel way to study the conformational change of gp41 during virus entry into cells and to identify HIV-1 fusion inhibitors.