Treatment of advanced human immunodeficiency virus type 1 disease with the viral entry inhibitor PRO 542

Novel Bent Conformation of CD4 Induced by HIV-1 Inhibitor Indirectly Prevents Productive Viral Attachment

GSK3732394 is a multi-specific biologic inhibitor of HIV entry currently under clinical evaluation. A key component of this molecule is an adnectin (6940_B01) that binds to CD4 and inhibits downstream actions of gp160. Studies were performed to determine the binding site of the adnectin on CD4 and to understand the mechanism of inhibition. Using hydrogen-deuterium exchange with mass spectrometry (HDX), CD4 peptides showed differential rates of deuteration (either enhanced or slowed) in the presence of the adnectin that mapped predominantly to the interface of domains 2 and 3 (D2-D3). In addition, an X-ray crystal structure of an ibalizumab Fab/CD4(D1-D4)/adnectin complex revealed an extensive interface between the adnectin and residues on CD4 domains D2-D4 that stabilize a novel T-shaped CD4 conformation. A cryo-EM map of the gp140/CD4/GSK3732394 complex clearly shows the bent conformation for CD4 while bound to gp140. Mutagenic analyses on CD4 confirmed that amino acid F202 forms a key interaction with the adnectin. In addition, amino acid L151 was shown to be a critical indirect determinant of the specificity for binding to the human CD4 protein over related primate CD4 molecules, as it appears to modulate CD4's flexibility to adopt the adnectin-bound conformation. The significant conformational change of CD4 upon adnectin binding brings the D1 domain of CD4 in proximity to the host cell membrane surface, thereby re-orienting the gp120 binding site in a direction that is inaccessible to incoming virus due to a steric clash between gp160 trimers on the virus surface and the target cell membrane.

Synergistic Effect by Combining a gp120-Binding Protein and a gp41-Binding Antibody to Inactivate HIV-1 Virions and Inhibit HIV-1 Infection

Acquired immune deficiency syndrome (AIDS) has prevailed over the last 30 years. Although highly active antiretroviral therapy (HAART) has decreased mortality and efficiently controlled the progression of disease, no vaccine or curative drugs have been approved until now. A viral inactivator is expected to inactivate cell-free virions in the absence of target cells. Previously, we identified a gp120-binding protein, mD1.22, which can inactivate laboratory-adapted HIV-1. In this study, we have found that the gp41 N-terminal heptad repeat (NHR)-binding antibody D5 single-chain variable fragment (scFv) alone cannot inactivate HIV-1 at the high concentration tested. However, D5 scFv in the combination could enhance inactivation activity of mD1.22 against divergent HIV-1 strains, including HIV-1 laboratory-adapted strains, primary HIV-1 isolates, T20- and AZT-resistant strains, and LRA-reactivated virions. Combining mD1.22 and D5 scFv exhibited synergistic effect on inhibition of infection by divergent HIV-1 strains. These results suggest good potential to develop the strategy of combining a gp120-binding protein and a gp41-binding antibody for the treatment of HIV-1 infection.

Antivirals with common targets against highly pathogenic viruses

Historically, emerging viruses appear constantly and have cost millions of human lives. Currently, climate change and intense globalization have created favorable conditions for viral transmission. Therefore, effective antivirals, especially those targeting the conserved protein in multiple unrelated viruses, such as the compounds targeting RNA-dependent RNA polymerase, are urgently needed to combat more emerging and re-emerging viruses in the future. Here we reviewed the development of antivirals with common targets, including those against the same protein across viruses, or the same viral function, to provide clues for development of antivirals for future epidemics.

HIV-1 Entry and Prospects for Protecting against Infection

Human Immunodeficiency Virus type-1 (HIV-1) establishes a latent viral reservoir soon after infection, which poses a major challenge for drug treatment and curative strategies. Many efforts are therefore focused on blocking infection. To this end, both viral and host factors relevant to the onset of infection need to be considered. Given that HIV-1 is most often transmitted mucosally, strategies designed to protect against infection need to be effective at mucosal portals of entry. These strategies need to contend also with cell-free and cell-associated transmitted/founder (T/F) virus forms; both can initiate and establish infection. This review will discuss how insight from the current model of HIV-1 mucosal transmission and cell entry has highlighted challenges in developing effective strategies to prevent infection. First, we examine key viral and host factors that play a role in transmission and infection. We then discuss preventive strategies based on antibody-mediated protection, with emphasis on targeting T/F viruses and mucosal immunity. Lastly, we review treatment strategies targeting viral entry, with focus on the most clinically advanced entry inhibitors.

Innate Immune Response Against HIV-1

The innate immune system is comprised of both cellular and humoral players that recognise and eradicate invading pathogens. Therefore, the interplay between retroviruses and innate immunity has emerged as an important component of viral pathogenesis. HIV-1 infection in humans that results in hematologic abnormalities and immune suppression is well represented by changes in the CD4/CD8 T cell ratio and consequent cell death causing CD4 lymphopenia. The innate immune responses by mucosal barriers such as complement, DCs, macrophages, and NK cells as well as cytokine/chemokine profiles attain great importance in acute HIV-1 infection, and thus, prevent mucosal capture and transmission of HIV-1. Conversely, HIV-1 has evolved to overcome innate immune responses through RNA-mediated rapid mutations, pathogen-associated molecular patterns (PAMPs) modification, down-regulation of NK cell activity and complement receptors, resulting in increased secretion of inflammatory factors. Consequently, epithelial tissues lining up female reproductive tract express innate immune sensors including anti-microbial peptides responsible for forming primary barriers and have displayed an effective potent anti-HIV activity during phase I/II clinical trials.

Designed variants of ACE2-Fc that decouple anti-SARS-CoV-2 activities from unwanted cardiovascular effects

Angiotensin-converting enzyme 2 (ACE2) is the entry receptor for SARS-CoV-2, and recombinant ACE2 decoys are being evaluated as new antiviral therapies. We designed and tested an antibody-like ACE2-Fc fusion protein, which has the benefit of long pharmacological half-life and the potential to facilitate immune clearance of the virus. Out of a concern that the intrinsic catalytic activity of ACE2 may unintentionally alter the balance of its hormonal substrates and cause adverse cardiovascular effects in treatment, we performed a mutagenesis screening for inactivating the enzyme. Three mutants, R273A, H378A and E402A, completely lost their enzymatic activity for either surrogate or physiological substrates. All of them remained capable of binding SARS-CoV-2 and could suppress the transduction of a pseudotyped virus in cell culture. This study established new ACE2-Fc candidates as antiviral treatment for SARS-CoV-2 without potentially harmful side effects from ACE2's catalytic actions toward its vasoactive substrates.

Opening the HIV envelope: potential of CD4 mimics as multifunctional HIV entry inhibitors

PURPOSE OF REVIEW

Close to 2 million individuals globally become infected with HIV-1 each year and just over two-thirds will have access to life-prolonging antivirals. However, the rapid development of drug resistance creates challenges, such that generation of more effective therapies is not only warranted but a necessary endeavour. This review discusses a group of HIV-1 entry inhibitors known as CD4 mimics which exploit the highly conserved relationship between the HIV-1 envelope glycoprotein and the receptor, CD4.

RECENT FINDINGS

We review the structure/function guided evolution of these inhibitors, vital mechanistic insights that underpin broad and potent functional antagonism, recent evidence of utility demonstrated in animal and physiologically relevant in-vitro models, and current progress towards effective new-generation inhibitors.

SUMMARY

The current review highlights the promising potential of CD4 mimetics as multifunctional therapeutics.

Protein- and Peptide-Based Virus Inactivators: Inactivating Viruses Before Their Entry Into Cells

Infectious diseases caused by human immunodeficiency virus (HIV) and other highly pathogenic enveloped viruses, have threatened the global public health. Most antiviral drugs act as passive defenders to inhibit viral replication inside the cell, while a few of them function as gate keepers to combat viruses outside the cell, including fusion inhibitors, e.g., enfuvirtide, and receptor antagonists, e.g., maraviroc, as well as virus inactivators (including attachment inhibitors). Different from fusion inhibitors and receptor antagonists that must act in the presence of target cells, virus inactivators can actively inactivate cell-free virions in the blood, through interaction with one or more sites in the envelope glycoproteins (Envs) on virions. Notably, a number of protein- and peptide-based virus inactivators (PPVIs) under development are expected to have a better utilization rate than the current antiviral drugs and be safer for in vivo human application than the chemical-based virus inactivators. Here we have highlighted recent progress in developing PPVIs against several important enveloped viruses, including HIV, influenza virus, Zika virus (ZIKV), dengue virus (DENV), and herpes simplex virus (HSV), and the potential use of PPVIs for urgent treatment of infection by newly emerging or re-emerging viruses.

Development of Protein- and Peptide-Based HIV Entry Inhibitors Targeting gp120 or gp41

Application of highly active antiretroviral drugs (ARDs) effectively reduces morbidity and mortality in HIV-infected individuals. However, the emergence of multiple drug-resistant strains has led to the increased failure of ARDs, thus calling for the development of anti-HIV drugs with targets or mechanisms of action different from those of the current ARDs. The first peptide-based HIV entry inhibitor, enfuvirtide, was approved by the U.S. FDA in 2003 for treatment of HIV/AIDS patients who have failed to respond to the current ARDs, which has stimulated the development of several series of protein- and peptide-based HIV entry inhibitors in preclinical and clinical studies. In this review, we highlighted the properties and mechanisms of action for those promising protein- and peptide-based HIV entry inhibitors targeting the HIV-1 gp120 or gp41 and discussed their advantages and disadvantages, compared with the current ARDs.

Genetic Strategies for HIV Treatment and Prevention

Conventional HIV gene therapy approaches are based on engineering HIV target cells that are non-permissive to viral replication. However, expansion of gene-modified HIV target cells has been limited in patients. Alternative genetic strategies focus on generating gene-modified producer cells that secrete antiviral proteins (AVPs). The secreted AVPs interfere with HIV entry, and, therefore, they extend the protection against infection to unmodified HIV target cells. Since any cell type can potentially secrete AVPs, hematopoietic and non-hematopoietic cell lineages can function as producer cells. Secretion of AVPs from non-hematopoietic cells opens the possibility of using a genetic approach for HIV prevention. Another strategy aims at modifying cytotoxic T cells to selectively target and eliminate infected cells. This review provides an overview of the different genetic approaches for HIV treatment and prevention.

Engineering of Fc Fragments with Optimized Physicochemical Properties Implying Improvement of Clinical Potentials for Fc-Based Therapeutics

Therapeutic monoclonal antibodies and Fc-fusion proteins are successfully used in treatment of various diseases mainly including cancer, immune disease, and viral infection, which belong to the Fc-based therapeutics. In recent years, engineered Fc-derived antibody domains have also shown potential for Fc-based therapeutics. To increase the druggability of Fc-based therapeutic candidates, many efforts have been made in optimizing physicochemical properties and functions mediated by Fc fragment. The desired result is that we can simultaneously obtain Fc variants with increased physicochemical properties in vitro and capacity of mediating appropriate functions in vivo. However, changes of physicochemical properties of Fc may result in alternation of Fc-mediated functions and vice versa, which leads to undesired outcomes for further development of Fc-based therapeutics. Therefore, whether modified Fc fragments are suitable for achievement of expected clinical results or not needs to be seriously considered. Now, this question comes to be noticed and should be figured out to make better translation from the results of laboratory into clinical applications. In this review, we summarize different strategies on engineering physicochemical properties of Fc, and preliminarily elucidate the relationships between modified Fc in vitro and the subsequent therapeutic influence in vivo.

Delineating CD4 dependency of HIV-1: Adaptation to infect low level CD4 expressing target cells widens cellular tropism but severely impacts on envelope functionality

A hallmark of HIV-1 infection is the continuously declining number of the virus' predominant target cells, activated CD4+ T cells. With diminishing CD4+ T cell levels, the capacity to utilize alternate cell types and receptors, including cells that express low CD4 receptor levels such as macrophages, thus becomes crucial. To explore evolutionary paths that allow HIV-1 to acquire a wider host cell range by infecting cells with lower CD4 levels, we dissected the evolution of the envelope-CD4 interaction under in vitro culture conditions that mimicked the decline of CD4high target cells, using a prototypic subtype B, R5-tropic strain. Adaptation to CD4low targets proved to severely alter envelope functions including trimer opening as indicated by a higher affinity to CD4 and loss in shielding against neutralizing antibodies. We observed a strikingly decreased infectivity on CD4high target cells, but sustained infectivity on CD4low targets, including macrophages. Intriguingly, the adaptation to CD4low targets altered the kinetic of the entry process, leading to rapid CD4 engagement and an extended transition time between CD4 and CCR5 binding during entry. This phenotype was also observed for certain central nervous system (CNS) derived macrophage-tropic viruses, highlighting that the functional perturbation we defined upon in vitro adaptation to CD4low targets occurs in vivo. Collectively, our findings suggest that CD4low adapted envelopes may exhibit severe deficiencies in entry fitness and shielding early in their evolution. Considering this, adaptation to CD4low targets may preferentially occur in a sheltered and immune-privileged environment such as the CNS to allow fitness restoring compensatory mutations to occur.

Engineering broadly neutralizing antibodies for HIV prevention and therapy

A combination of advances spanning from isolation to delivery of potent HIV-specific antibodies has begun to revolutionize understandings of antibody-mediated antiviral activity. As a result, the set of broadly neutralizing and highly protective antibodies has grown in number, diversity, potency, and breadth of viral recognition and neutralization. These antibodies are now being further enhanced by rational engineering of their anti-HIV activities and coupled to cutting edge gene delivery and strategies to optimize their pharmacokinetics and biodistribution. As a result, the prospects for clinical use of HIV-specific antibodies to treat, clear, and prevent HIV infection are gaining momentum. Here we discuss the diverse methods whereby antibodies are being optimized for neutralization potency and breadth, biodistribution, pharmacokinetics, and effector function with the aim of revolutionizing HIV treatment and prevention options.

Cholesterol conjugation potentiates the antiviral activity of an HIV immunoadhesin

Immunoadhesins are engineered proteins combining the constant domain (Fc) of an antibody with a ligand-binding (adhesion) domain. They have significant potential as therapeutic agents, because they maintain the favourable pharmacokinetics of antibodies with an expanded repertoire of ligand-binding domains: proteins, peptides, or small molecules. We have recently reported that the addition of a cholesterol group to two HIV antibodies can dramatically improve their antiviral potency. Cholesterol, which can be conjugated at various positions in the antibody, including the constant (Fc) domain, endows the conjugate with affinity for the membrane lipid rafts, thus increasing its concentration at the site where viral entry occurs. Here, we extend this strategy to an HIV immunoadhesin, combining a cholesterol-conjugated Fc domain with the peptide fusion inhibitor C41. The immunoadhesin C41-Fc-chol displayed high affinity for Human Embryonic Kidney (HEK) 293 cells, and when tested on a panel of HIV-1 strains, it was considerably more potent than the unconjugated C41-Fc construct. Potentiation of antiviral activity was comparable to what was previously observed for the cholesterol-conjugated HIV antibodies. Given the key role of cholesterol in lipid raft formation and viral fusion, we expect that the same strategy should be broadly applicable to enveloped viruses, for many of which it is already known the sequence of a peptide fusion inhibitor similar to C41. Moreover, the sequence of heptad repeat-derived fusion inhibitors can often be predicted from genomic information alone, opening a path to immunoadhesins against emerging viruses.

Current progress in antiviral strategies

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Antiviral agents function as either viral targets or host factors.

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Virus-targeting antivirals (VTAs) function through a direct (DVTAs) or an indirect (InDVTAs) method in the viral life cycle.

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Host-targeting antivirals (HTAs) include reagents that target the host proteins that are involved in the viral life cycle.

The prevalence of chronic viral infectious diseases, such as human immunodeficiency virus (HIV), hepatitis C virus (HCV), and influenza virus; the emergence and re-emergence of new viral infections, such as picornaviruses and coronaviruses; and, particularly, resistance to currently used antiviral drugs have led to increased demand for new antiviral strategies and reagents. Increased understanding of the molecular mechanisms of viral infection has provided great potential for the discovery of new antiviral agents that target viral proteins or host factors. Virus-targeting antivirals can function directly or indirectly to inhibit the biological functions of viral proteins, mostly enzymatic activities, or to block viral replication machinery. Host-targeting antivirals target the host proteins that are involved in the viral life cycle, regulating the function of the immune system or other cellular processes in host cells. Here we review key targets and considerations for the development of both antiviral strategies.

HIV/AIDS pathogenesis and treatment options focusing on the viral entry inhibitors

The advent of highly active antiretroviral therapy (HAART) has revolutionized the treatment of HIV. A wide variety of antiretroviral agents are now available, allowing patients and physicians a choice of effective therapy. However, drug resistance and toxicities are emerging as major treatment challenges in the HAART era. The development of agents within existing and novel antiretroviral classes remains paramount in order to preserve the reduced morbidity and mortality we have come to expect from HAART use. This article details the development of the HIV epidemic, reviewing current and future treatment strategies, and concentrating upon the viral entry inhibitors.

Exceptionally potent and broadly cross-reactive, bispecific multivalent HIV-1 inhibitors based on single human CD4 and antibody domains

Soluble forms of the human immunodeficiency virus type 1 (HIV-1) primary receptor CD4 (soluble CD4 [sCD4]) have been extensively characterized for a quarter of a century as promising HIV-1 inhibitors, but they have not been clinically successful. By combining a protein cavity-filling strategy and the power of library technology, we identified an engineered cavity-altered single-domain sCD4 (mD1.22) with a unique combination of excellent properties, including broad and potent neutralizing activity, high specificity, stability, solubility, and affinity for the HIV-1 envelope glycoprotein gp120, and small molecular size. To further improve its neutralizing potency and breadth, we generated bispecific multivalent fusion proteins of mD1.22 with another potent HIV-1 inhibitor, an antibody domain (m36.4) that targets the coreceptor-binding site on gp120. The fusion proteins neutralized all HIV-1 isolates tested, with potencies about 10-, 50-, and 200-fold higher than those of the broadly neutralizing antibody VRC01, the U.S. FDA-approved peptide inhibitor T20, and the clinically tested sCD4-Fc fusion protein CD4-Ig, respectively. In addition, they exhibited higher stability and specificity and a lower aggregation propensity than CD4-Ig. Therefore, mD1.22 and related fusion proteins could be useful for HIV-1 prevention and therapy, including eradication of the virus.

Closing the door to human immunodeficiency virus

The pandemic of human immunodeficiency virus type one (HIV-1), the major etiologic agent of acquired immunodeficiency disease (AIDS), has led to over 33 million people living with the virus, among which 18 million are women and children. Until now, there is neither an effective vaccine nor a therapeutic cure despite over 30 years of efforts. Although the Thai RV144 vaccine trial has demonstrated an efficacy of 31.2%, an effective vaccine will likely rely on a breakthrough discovery of immunogens to elicit broadly reactive neutralizing antibodies, which may take years to achieve. Therefore, there is an urgency of exploring other prophylactic strategies. Recently, antiretroviral treatment as prevention is an exciting area of progress in HIV-1 research. Although effective, the implementation of such strategy faces great financial, political and social challenges in heavily affected regions such as developing countries where drug resistant viruses have already been found with growing incidence. Activating latently infected cells for therapeutic cure is another area of challenge. Since it is greatly difficult to eradicate HIV-1 after the establishment of viral latency, it is necessary to investigate strategies that may close the door to HIV-1. Here, we review studies on non-vaccine strategies in targeting viral entry, which may have critical implications for HIV-1 prevention.

HIV-1 entry inhibitors: recent development and clinical use

PURPOSE OF REVIEW

This review provides an overview of HIV-1 entry inhibitors, with a focus on drugs in the later stages of clinical development.

RECENT FINDINGS

Entry of HIV-1 into target cells involves viral attachment, co-receptor binding, and fusion. Antiretroviral drugs that interact with each step in the entry process have been developed, but only two are currently approved for clinical use. The small molecule attachment inhibitor BMS-663068 has shown potent antiviral activity in early phase studies, and phase 2b trials are currently underway. The postattachment inhibitor ibalizumab has shown antiviral activity in phase 1 and 2 trials; further studies, including subcutaneous delivery of drug to healthy individuals, are anticipated. The CCR5 antagonist maraviroc is approved for use in treatment-naïve and treatment-experienced patients. Cenicriviroc, a small-molecule CCR5 antagonist that also has activity as a CCR2 antagonist, has entered phase 2b studies. No CXCR4 antagonists are currently in clinical trials, but once daily, next-generation injectable peptide fusion inhibitors have entered human trials. Both maraviroc and ibalizumab are being studied for prevention of HIV-1 transmission and/or for use in nucleoside reverse transcriptase inhibitor-sparing antiretroviral regimens.

SUMMARY

Inhibition of HIV-1 entry continues to be a promising target for antiretroviral drug development.

Role of human CD4 D1D2 domain in HIV-1 infection

Broadly neutralizing antibodies and appropriate immunogens are critical for preexposure prophylaxis and therapeutic HIV vaccines. In this study, we aimed to explore effective antibodies against the genetically diverse HIV-1 strains by investigating the roles of human CD4 D1D2 domain and nonvariable immugens. The human CD4 D1D2 domain and the chimeric protein of mouse D1 domain/human D2 domain were expressed in Sf9 insect cells and purified by gel-filtration chromatography. The human CD4 D1D2 domain potently inhibited the infection of 77.8% HIV-1 pseudoviruses, including the clades AE, B' and BC, with less than 20 μg/mL of IC(50). pcDNA3.1-mhD1D2m and pcDNA3.1-mhD2m plasmids were used for the production of mouse anti-human CD4 polyclonal antibodies. The neutralizing activities of the polyclonal antibodies were determined by using pseudotyped HIV-1 viruses. The antibodies induced by plasmids containing human CD4 D1D2 domain were able to potently inhibit all pseudotyped HIV-1 strains. The antibodies from mhD1D2m-immunized mice also showed strong binding capacity to CD4 expressed on the surface of TZM-bl cells. The potent and broad inhibitory activity of antibodies against the human CD4 D1D2 domain may be used to develop effective passive immunization agent to control the spread of HIV infection.

A bivalent recombinant protein inactivates HIV-1 by targeting the gp41 prehairpin fusion intermediate induced by CD4 D1D2 domains

Background

Most currently approved anti-HIV drugs (e.g., reverse transcriptase inhibitors, protease inhibitors and fusion/entry inhibitors) must act inside or on surface of the target cell to inhibit HIV infection, but none can directly inactivate virions away from cells. Although soluble CD4 (sCD4) can inactivate laboratory-adapted HIV-1 strains, it fails to reduce the viral loads in clinical trials because of its low potency against primary isolates and tendency to enhance HIV-1 infection at low concentration. Thus, it is essential to design a better HIV inactivator with improved potency for developing new anti-HIV therapeutics that can actively attack the virus in the circulation before it attaches to and enter into the target cell.

Results

We engineered a bivalent HIV-1 inactivator, designated 2DLT, by linking the D1D2 domain of CD4 to T1144, the next generation HIV fusion inhibitor, with a 35-mer linker. The D1D2 domain in this soluble 2DLT protein could bind to the CD4-binding site and induce the formation of the gp41 prehairpin fusion-intermediate (PFI), but showed no sCD4-mediated enhancement of HIV-1 infection. The T1144 domain in 2DLT then bound to the exposed PFI, resulting in rapid inactivation of HIV-1 virions in the absence of the target cell. Beside, 2DLT could also inhibit fusion of the virus with the target cell if the virion escapes the first attack of 2DLT.

Conclusion

This bivalent molecule can serve as a dual barrier against HIV infection by first inactivating HIV-1 virions away from cells and then blocking HIV-1 entry on the target cell surface, indicating its potential for development as a new class of anti-HIV drug.

Candidate antibody-based therapeutics against HIV-1

Antibody-based therapeutics have been successfully used for the treatment of various diseases and as research tools. Several well characterized, broadly neutralizing monoclonal antibodies (bnmAbs) targeting HIV-1 envelope glycoproteins or related host cell surface proteins show sterilizing protection of animals, but they are not effective when used for therapy of an established infection in humans. Recently, a number of novel bnmAbs, engineered antibody domains (eAds), and multifunctional fusion proteins have been reported which exhibit exceptionally potent and broad neutralizing activity against a wide range of HIV-1 isolates from diverse genetic subtypes. eAds could be more effective in vivo than conventional full-size antibodies generated by the human immune system. Because of their small size (12∼15 kD), they can better access sterically restricted epitopes and penetrate densely packed tissue where HIV-1 replicates than the larger full-size antibodies. HIV-1 possesses a number of mechanisms to escape neutralization by full-size antibodies but could be less likely to develop resistance to eAds. Here, we review the in vitro and in vivo antiviral efficacies of existing HIV-1 bnmAbs, summarize the development of eAds and multispecific fusion proteins as novel types of HIV-1 inhibitors, and discuss possible strategies to generate more potent antibody-based candidate therapeutics against HIV-1, including some that could be used to eradicate the virus.

Protection against high-dose highly pathogenic mucosal SIV challenge at very low serum neutralizing titers of the antibody-like molecule CD4-IgG2

Passive transfer studies using monoclonal or polyclonal antibodies in the macaque model have been valuable for determining conditions for antibody protection against immunodeficiency virus challenge. Most studies have employed hybrid simian/human immunodeficiency virus (SHIV) challenge in conjunction with neutralizing human monoclonal antibodies. Passive protection against SIV, particularly the pathogenic prototype virus SIVmac239, has been little studied because of the paucity of neutralizing antibodies to this virus. Here, we show that the antibody-like molecule CD4-IgG2 potently neutralizes SIVmac239 in vitro. When administered by an osmotic pump to maintain concentrations given the short half-life of CD4-IgG2 in macaques, the molecule provided sterilizing immunity/protection against high-dose mucosal viral challenge to a high proportion of animals (5/7 at a 200 mg dose CD4-IgG2 and 3/6 at a 20 mg dose) at serum concentrations below 1.5 µg/ml. The neutralizing titers of such sera were predicted to be very low and indeed sera at a 1∶4 dilution produced no neutralization in a pseudovirus assay. Macaque anti-human CD4 titers did develop weakly at later time points in some animals but were not associated with the level of protection against viral challenge. The results show that, although SIVmac239 is considered a highly pathogenic virus for which vaccine-induced T cell responses in particular have provided limited benefit against high dose challenge, the antibody-like CD4-IgG2 molecule at surprisingly low serum concentration affords sterilizing immunity/protection to a majority of animals.

In vitro antiviral characteristics of HIV-1 attachment inhibitor BMS-626529, the active component of the prodrug BMS-663068

BMS-663068 is the phosphonooxymethyl prodrug of BMS-626529, a novel small-molecule attachment inhibitor that targets HIV-1 gp120 and prevents its binding to CD4(+) T cells. The activity of BMS-626529 is virus dependent, due to heterogeneity within gp120. In order to better understand the anti-HIV-1 spectrum of BMS-626529 against HIV-1, in vitro activities against a wide variety of laboratory strains and clinical isolates were determined. BMS-626529 had half-maximal effective concentration (EC(50)) values of <10 nM against the vast majority of viral isolates; however, susceptibility varied by >6 log(10), with half-maximal effective concentration values in the low pM range against the most susceptible viruses. The in vitro antiviral activity of BMS-626529 was generally not associated with either tropism or subtype, with few exceptions. Measurement of the binding affinity of BMS-626529 for purified gp120 suggests that a contributory factor to its inhibitory potency may be a relatively long dissociative half-life. Finally, in two-drug combination studies, BMS-626529 demonstrated additive or synergistic interactions with antiretroviral drugs of different mechanistic classes. These results suggest that BMS-626529 should be active against the majority of HIV-1 viruses and support the continued clinical development of the compound.

Cell-cell transmission enables HIV-1 to evade inhibition by potent CD4bs directed antibodies

HIV is known to spread efficiently both in a cell-free state and from cell to cell, however the relative importance of the cell-cell transmission mode in natural infection has not yet been resolved. Likewise to what extent cell-cell transmission is vulnerable to inhibition by neutralizing antibodies and entry inhibitors remains to be determined. Here we report on neutralizing antibody activity during cell-cell transmission using specifically tailored experimental strategies which enable unambiguous discrimination between the two transmission routes. We demonstrate that the activity of neutralizing monoclonal antibodies (mAbs) and entry inhibitors during cell-cell transmission varies depending on their mode of action. While gp41 directed agents remain active, CD4 binding site (CD4bs) directed inhibitors, including the potent neutralizing mAb VRC01, dramatically lose potency during cell-cell transmission. This implies that CD4bs mAbs act preferentially through blocking free virus transmission, while still allowing HIV to spread through cell-cell contacts. Thus providing a plausible explanation for how HIV maintains infectivity and rapidly escapes potent and broadly active CD4bs directed antibody responses in vivo.

HIV is known to spread both in a cell-free state and from cell to cell, however the relative importance of the cell-cell transmission mode in natural infection has not yet been resolved. Design of vaccines attempt to inhibit HIV entry into target cells as do engineered entry inhibitors used as therapeutics. While these agents are known to block the entry of cell-free HIV particles into cells, to what extent cell-cell transmission is vulnerable to such inhibition is unclear. Here we report that the activity of neutralizing antibodies and inhibitors during cell-cell transmission varies depending on their mode of action. A prominent class of neutralizing antibodies directed to the CD4 binding site on the virus envelope very efficiently blocks binding of the virus to its primary receptor on target cells, the CD4 molecule. These types of antibodies are elicited in natural infection and once isolated from infected individuals have shown to be highly potent. Why HIV still replicates in the presence of such potent antibodies remains unclear. Here we show that these CD4 binding site antibodies are dramatically less potent inhibitors of cell-cell transmission, and therefore act preferentially by blocking free virus transmission while allowing HIV to spread through cell-cell contact.

New approaches in the treatment of HIV/AIDS - focus on maraviroc and other CCR5 antagonists

Treatment of HIV-1 infection has produced dramatic success for many patients. Nevertheless, viral resistance continues to limit the efficacy of currently available agents in many patients. The CCR5 antagonists are a new class of antiretroviral agents that target a necessary coreceptor for viral entry of many strains of HIV-1. Recently, the first agent within this class, maraviroc, was approved by a number of regulatory agencies, including the Food and Drug Administration. Herein we review the role of the CCR5 receptor in HIV-1 infection and potential methods to target it in anti-HIV-1 therapy. We review the various categories of agents and discuss specific agents that have progressed to clinical study. We discuss in detail the recently approved, first in class CCR5 antagonist, maraviroc, and discuss aspects of resistance to CCR5 antagonism and the potential role of CCR5 antagonism in the management of HIV-1 infection.

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.

Potential use of rapamycin in HIV infection

The strong need for the development of alternative anti-HIV agents is primarily due to the emergence of strain-resistant viruses, the need for sustained adherence to complex treatment regimens and the toxicity of currently used antiviral drugs. This review analyzes proof of concept studies indicating that the immunomodulatory drug rapamycin (RAPA) possesses anti-HIV properties both in vitro and in vivo that qualifies it as a potential new anti-HIV drug. It represents a literature review of published studies that evaluated the in vitro and in vivo activity of RAPA in HIV. RAPA represses HIV-1 replication in vitro through different mechanisms including, but not limited, to down regulation of CCR5. In addition RAPA synergistically enhances the anti-HIV activity of entry inhibitors such as vicriviroc, aplaviroc and enfuvirtide in vitro. RAPA also inhibits HIV-1 infection in human peripheral blood leucocytes-SCID reconstituted mice. In addition, a prospective nonrandomized trial of HIV patient series receiving RAPA monotherapy after liver transplantation indicated significantly better control of HIV and hepatitis C virus (HCV) replication among patients taking RAPA monotherapy. Taken together, the evidence presented in this review suggests that RAPA may be a useful drug that should be evaluated for the prevention and treatment of HIV-1 infection.

Viral surface glycoproteins, gp120 and gp41, as potential drug targets against HIV-1: brief overview one quarter of a century past the approval of zidovudine, the first anti-retroviral drug

The first anti-HIV drug, zidovudine (AZT), was approved by the FDA a quarter of a century ago, in 1985. Currently, anti-HIV drug-combination therapies only target HIV-1 protease and reverse transcriptase. Unfortunately, most of these molecules present numerous shortcomings such as viral resistances and adverse effects. In addition, these drugs are involved in later stages of infection. Thus, it is necessary to develop new drugs that are able to block the first steps of viral life cycle. Entry of HIV-1 is mediated by its two envelope glycoproteins: gp120 and gp41. Upon gp120 binding to cellular receptors, gp41 undergoes a series of conformational changes from a non-fusogenic to a fusogenic conformation. The fusogenic core of gp41 is a trimer-of-hairpins structure in which three C-terminal helices pack against a central trimeric-coiled coil formed by three N-terminal helices. The formation of this fusogenic structure brings the viral and cellular membranes close together, a necessary condition for membrane fusion to occur. As gp120 and gp41 are attractive targets, the development of entry inhibitors represents an important avenue of anti-HIV drug therapy. The present review will focus on some general considerations about HIV, the main characteristics of gp120, gp41 and their inhibitors, with special emphasis on the advances of computational approaches employed in the development of bioactive compounds against HIV-1 entry process.

Potent strategy to inhibit HIV-1 by binding both gp120 and gp41

The development of an anti-HIV microbicide is critical in the fight against the spread of HIV. It is shown here that the covalent linking of compounds that bind gp120 with compounds that bind gp41 can inhibit HIV entry even more potently than individual inhibitors or noncovalent combinations. The most striking example involves griffithsin, a potent HIV inhibitor that binds to the surface of HIV gp120. While griffithsin inhibits HIV Env-mediated fusion in a CCR5-tropic cell-cell fusion assay with a 50% inhibitory concentration (IC(50)) of 1.31 ± 0.87 nM and the gp41-binding peptide C37 shows an IC(50) of 18.2 ± 7.6 nM, the covalently linked combination of griffithsin with C37 (Griff37) has an IC(50) of 0.15 ± 0.05 nM, exhibiting a potency 8.7-fold greater than that of griffithsin alone. Similarly, in CXCR4-tropic cell-cell fusion assays, Griff37 is 5.2-fold more potent than griffithsin alone. In viral assays, both griffithsin and Griff37 inhibit HIV replication at midpicomolar levels, but the linked compound Griff37 is severalfold more potent than griffithsin alone against both CCR5- and CXCR4-tropic virus strains. Another example of this strategy is the covalently linked combination of peptide C37 with a variant of the gp120-binding peptide CD4M33 (L. Martin et al., Nat. Biotechnol. 21:71-76, 2003). Also, nuclear magnetic resonance (NMR) spectra for several of these compounds are shown, including, to our knowledge, the first published NMR spectrum for griffithsin.

Recent advances in antiretroviral drugs

IMPORTANCE OF THE FIELD

Acquired immunodeficiency syndrome (AIDS) is one of the leading causes of death worldwide. Although the combination therapies of highly active antiretroviral therapy (HAART) have significantly contributed to virological suppression, improved immune function and quality of life, issues such as tolerability, drug-drug interactions and cross-resistance amongst members of a particular drug class still pose a significant barrier to long-term successful treatment. There is a constant need for newer anti HIV drugs with increased potency and improved pharmacokinetic properties either in the existing classes or drugs from new classes that target several new steps in HIV replication cycle.

AREAS COVERED IN THIS REVIEW

The authors have discussed newer antiretroviral drugs belonging to second-generation nucleoside analog reverse transcriptase inhibitors (amdoxovir, elvucitabine, apricitabine, racivir), non-nucleoside analog reverse transcriptase inhibitors (etravirine, rilpivirine), protease inhibitors (darunavir, tipranavir) as well as emerging new classes, i.e., fusion inhibitors (enfuvirtide, sifuvirtide), CCR5 inhibitors (maraviroc, vicriviroc, PRO 140, PRO 542), CD4-receptor inhibitors (ibalizumab), integrase inhibitors (raltegravir, elvitegravir, GSK-1349572), maturation inhibitors (bevirimat), cobicistat (pharmacoenhancer), lens epithelium-derived growth factor inhibitors and capsid assembly inhibitors.

WHAT THE READER WILL GAIN

The reader will gain an understanding of the mechanism of action, mechanism of resistance, stages of development and important clinical trials related to the newer antiretroviral drugs and future potential of these drugs.

TAKE HOME MESSAGE

The initial clinical trial data of these newer drugs are very encouraging for the long-term successful control of HIV in both treatment-naïve and treatment-experienced patients.

Role of complex carbohydrates in human immunodeficiency virus type 1 infection and resistance to antibody neutralization

Complex N-glycans flank the receptor binding sites of the outer domain of HIV-1 gp120, ostensibly forming a protective "fence" against antibodies. Here, we investigated the effects of rebuilding this fence with smaller glycoforms by expressing HIV-1 pseudovirions from a primary isolate in a human cell line lacking N-acetylglucosamine transferase I (GnTI), the enzyme that initiates the conversion of oligomannose N-glycans into complex N-glycans. Thus, complex glycans, including those that surround the receptor binding sites, are replaced by fully trimmed oligomannose stumps. Conversely, the untrimmed oligomannoses of the silent domain of gp120 are likely to remain unchanged. For comparison, we produced a mutant virus lacking a complex N-glycan of the V3 loop (N301Q). Both variants exhibited increased sensitivities to V3 loop-specific monoclonal antibodies (MAbs) and soluble CD4. The N301Q virus was also sensitive to "nonneutralizing" MAbs targeting the primary and secondary receptor binding sites. Endoglycosidase H treatment resulted in the removal of outer domain glycans from the GnTI- but not the parent Env trimers, and this was associated with a rapid and complete loss in infectivity. Nevertheless, the glycan-depleted trimers could still bind to soluble receptor and coreceptor analogs, suggesting a block in post-receptor binding conformational changes necessary for fusion. Collectively, our data show that the antennae of complex N-glycans serve to protect the V3 loop and CD4 binding site, while N-glycan stems regulate native trimer conformation, such that their removal can lead to global changes in neutralization sensitivity and, in extreme cases, an inability to complete the conformational rearrangements necessary for infection.

sCD4-17b bifunctional protein: extremely broad and potent neutralization of HIV-1 Env pseudotyped viruses from genetically diverse primary isolates

BACKGROUND

We previously described a potent recombinant HIV-1 neutralizing protein, sCD4-17b, composed of soluble CD4 attached via a flexible polypeptide linker to an SCFv of the 17b human monoclonal antibody directed against the highly conserved CD4-induced bridging sheet of gp120 involved in coreceptor binding. The sCD4 moiety of the bifunctional protein binds to gp120 on free virions, thereby enabling the 17b SCFv moiety to bind and block the gp120/coreceptor interaction required for entry. The previous studies using the MAGI-CCR5 assay system indicated that sCD4-17b (in concentrated cell culture medium, or partially purified) potently neutralized several genetically diverse HIIV-1 primary isolates; however, at the concentrations tested it was ineffective against several other strains despite the conservation of binding sites for both CD4 and 17b. To address this puzzle, we designed variants of sCD4-17b with different linker lengths, and tested the neutralizing activities of the immunoaffinity purified proteins over a broader concentration range against a large number of genetically diverse HIV-1 primary isolates, using the TZM-bl Env pseudotype assay system. We also examined the sCD4-17b sensitivities of isogenic viruses generated from different producer cell types.

RESULTS

We observed that immunoaffinity purified sCD4-17b effectively neutralized HIV-1 pseudotypes, including those from HIV-1 isolates previously found to be relatively insensitive in the MAGI-CCR5 assay. The potencies were equivalent for the original construct and a variant with a longer linker, as observed with both pseudotype particles and infectious virions; by contrast, a construct with a linker too short to enable simultaneous binding of the sCD4 and 17b SCFv moieties was much less effective. sCD4-17b displayed potent neutralizing activity against 100% of nearly 4 dozen HIV-1 primary isolates from diverse genetic subtypes (clades A, B, C, D, F, and circulating recombinant forms AE and AG). The neutralization breadth and potency were superior to what have been reported for the broadly neutralizing monoclonal antibodies IgG b12, 2G12, 2F5, and 4E10. The activity of sCD4-17b was found to be similar against isogenic virus particles from infectious molecular clones derived either directly from the transfected producer cell line or after a single passage through PBMCs; this contrasted with the monoclonal antibodies, which were less potent against the PMBC-passaged viruses.

CONCLUSIONS

The results highlight the extremely potent and broad neutralizing activity of sCD4-17b against genetically diverse HIV-1 primary isolates. The bifunctional protein has potential applications for antiviral approaches to combat HIV infection.

Evaluation of CD4-CD4i antibody architectures yields potent, broadly cross-reactive anti-human immunodeficiency virus reagents

The envelope glycoprotein of human immunodeficiency virus type 1 (HIV-1) has several adaptations that allow the virus to evade antibody neutralization. Nevertheless, a few broadly cross-reactive neutralizing antibodies as well as reagents containing portions of CD4, the HIV receptor, have demonstrated partial efficacy in suppressing viral replication. One type of reagent designed for improved HIV neutralization fuses the CD4 D1-D2 domains to the variable regions of an antibody recognizing the CD4-induced (CD4i) coreceptor binding site on the gp120 portion of the HIV envelope spike. We designed, expressed, purified, and tested the neutralization potencies of CD4-CD4i antibody reagents with different architectures, antibody combining sites, and linkers. We found that fusing CD4 to the heavy chain of the CD4i antibody E51 yields a bivalent reagent including an antibody Fc region that expresses well, is expected to have a long serum half-life, and has comparable or greater neutralization activity than well-known broadly neutralizing anti-HIV antibodies. A CD4 fusion with the anti-HIV carbohydrate antibody 2G12 also results in a potent neutralizing reagent with more broadly neutralizing activity than 2G12 alone.

HIV-1 entry inhibitors: an overview

PURPOSE OF REVIEW

To provide an overview of HIV-1 entry inhibitors, with a focus on chemokine receptor antagonists.

RECENT FINDINGS

Entry of HIV-1 into target cells is an ordered multistep process involving attachment, co-receptor binding, and fusion. Inhibitors of each step have been identified and shown to have antiviral activity in clinical trials. Phase 1-2 trials of monoclonal antibodies and small-molecule attachment inhibitors have demonstrated activity in HIV-1-infected patients, but none has progressed to later-phase clinical trials. The postattachment inhibitor ibalizumab has shown activity in phase 1 and 2 trials; further studies are anticipated. The CCR5 antagonists maraviroc (now been approved for clinical use) and vicriviroc (in phase 3 trials) have shown significant benefit in controlled trials in treatment-experienced patients; additional CCR5 antagonists are in various stages of clinical development. Targeting CXCR4 has proven to be more challenging. Although proof of concept has been demonstrated in phase 1-2 trials of two compounds, neither proved suitable for chronic administration. Little progress has been reported in developing longer acting or orally bioavailable fusion inhibitors.

SUMMARY

A CCR5 antagonist and a fusion inhibitor are approved for use as HIV-1 entry inhibitors. Development of drugs targeting other steps in HIV-1 entry is ongoing.

Novel targets for antiretroviral therapy: clinical progress to date

The advent of HIV-1 resistance to antiretroviral medications, the need for lifelong antiretroviral therapy (ART) for HIV-infected individuals, and the goal of minimizing ART-related adverse effects and toxicity all drive the need for new antiretroviral drugs. Two new classes of antiretroviral medications for HIV treatment, the CCR5 and integrase inhibitors, have recently been approved for use in patients in whom previous HIV treatment regimens have failed. These new agent classes are a welcome addition to other antiretroviral classes, which include nucleoside reverse transcriptase inhibitors, non-nucleoside reverse transcriptase inhibitors, protease inhibitors and fusion inhibitors. Maraviroc is a CCR5 co-receptor antagonist that blocks HIV binding to the CCR5 receptor, which is a CD4 co-receptor necessary for cell entry. It is approved for use in ART-experienced patients with CCR5-tropic HIV, and was found to significantly reduce HIV viral load and increase CD4+ cell count when combined with an optimized background ART regimen (OBR). Treatment failure with maraviroc has been described and is primarily associated with the presence of CXCR4-tropic virus. Vicriviroc is another CCR5 co-receptor antagonist that is in late clinical trials. Raltegravir is the first US FDA-approved HIV-1 integrase inhibitor. It is approved for use in ART-experienced patients and was found to significantly reduce HIV viral load and increase CD4+ cell counts compared with placebo in combination with an OBR. Raltegravir has also been studied in treatment-naive patients and was found to be non-inferior to an efavirenz-based regimen. Elvitegravir is another HIV-1 integrase inhibitor in clinical development. Other new antiretroviral agents in clinical development include PRO140, a monoclonal antibody against CCR5, and bevirimat, a maturation inhibitor that prevents late-stage gag polyprotein processing. A number of other drug targets, such as CCR5 co-receptor agonists, CXCR4 co-receptor antagonists, novel fusion inhibitors, and alternative antiretroviral strategies, such as immune stimulation and gene therapy, are under investigation.

HIV entry inhibitors and their potential in HIV therapy

This review discusses recent progress in the development of anti-HIV agents targeting the viral entry process. The three main classes (attachment inhibitors, co-receptor binding inhibitors, and fusion inhibitors) are further broken down by specific mechanism of action and structure. Many of these inhibitors are in advanced clinical trials, including the HIV maturation inhibitor bevirimat, from the authors' laboratories. In addition, the CCR5 inhibitor maraviroc has recently been FDA-approved. Possible roles for these agents in anti-HIV therapy, including treatment of virus resistant to current drugs, are also discussed.

Update on the development of HIV entry inhibitors

HIV fusion and entry are two steps in the viral lifecycle that can be targeted by several classes of antiviral drugs. The discovery of chemokines focused the attention on cellular co-receptors used by the virus for entering cells, and on the various steps of such processes that are subject to interactions with small molecules. Intense research has led to a wide range of effective compounds that are able to inhibit these initial steps of viral replication. All steps in the process of HIV entry into the cell may be targeted by specific compounds, grouped into three main classes (attachment inhibitors, co-receptor binding inhibitors and fusion inhibitors), which may be developed as novel antiretrovirals. Thus, several inhibitors of the gp120–CD4 interaction have been discovered (e.g., zintevir and BMS-378806). Small molecule chemokine receptor antagonists acting as HIV entry inhibitors have also been described recently, including those which interact with both the CXCR4 co-receptor (e.g., AMD3100, AMD3465, ALX40-4C, T22, T134 and T140) and CCR5 co-receptor antagonists (TAK-779, TAK-220, E913, AK-602 and NSC 651016 in clinical trials). Recently, a third family of antivirals started to be used clinically (in addition to reverse transcriptase and protease inhibitors), with the advent of enfuvirtide (T20), the first fusion inhibitor to be approved as an anti-HIV agent. Some of these compounds demonstrated in vitro synergism with other classes of antivirals, thus offering the rationale for their combination in therapies for HIV-infected individuals. Many HIV entry and fusion inhibitors are currently being investigated in controlled clinical trials, and a number of them are bioavailable as oral formulations. In 2007, the US FDA approved maraviroc as an anti-HIV agent. Maraviroc is the product of a medicinal chemistry effort initiated following identification of an imidazopyridine CCR5 ligand from a high-throughput screen of the Pfizer compound file. Maraviroc demonstrated potent antiviral activity against all CCR5-tropic HIV-1 viruses tested, including 43 primary isolates from various clades and diverse geographic origin. Maraviroc was active against 200 clinically derived HIV-1 envelope-recombinant pseudoviruses, 100 of which were derived from viruses resistant to existing drug classes. Furthermore, in October 2007, the FDA announced the approval of raltegravir for the treatment of HIV-1 infection as part of combination antiretroviral therapy in treatment-experienced patients with evidence of HIV-1 replication despite optimized background antiretroviral therapy. At present, raltegravir is the only drug in the integrase inhibitor class approved for clinical use. With the approval of raltegravir, oral agents targeting all three constitutive viral enzymes, reverse transcriptase, protease and integrase, are now represented in FDA-approved therapies.

Relationship of HIV-1 and SIV envelope glycoprotein trimer occupation and neutralization

Insights into the process of HIV-1 neutralization may assist rational vaccine design. Here, we compared antibody neutralization against the JR-FL primary isolate and trimer binding affinities judged by native PAGE. Monovalent Fab-trimer binding and neutralization showed a direct quantitative relationship, implying that neutralization begins as each trimer is occupied by one antibody. At saturation, three Fab or soluble CD4 molecules engaged each trimer. In contrast, a maximum of one soluble CD4 molecule bound to functional SIV trimers with a truncated a gp41 tail. Remarkably, soluble CD4 was found to trigger dramatic enhancement of this virus. Unlike Fabs, a quantitative correlation between JR-FL trimer binding and neutralization was unclear for some, but not all IgGs, as neutralization was markedly increased, but trimer affinity was largely unchanged. In addition, only one molecule of certain gp41-specific IgGs appeared to be able to bind each trimer. We discuss the implications of these findings in weighing the relative contributions of size, multivalent binding and other possible effects of IgGs to explain their increased potency.

Suppression of dualtropic human immunodeficiency virus type 1 by the CXCR4 antagonist AMD3100 is associated with efficiency of CXCR4 use and baseline virus composition

In a phase I/II evaluation of the CXCR4 antagonist AMD3100, human immunodeficiency virus RNA levels were significantly reduced in a single study subject who harbored CXCR4 (X4)-tropic virus, but not in subjects who harbored either dual/mixed (DM)-tropic or CCR5 (R5)-tropic virus (C. W. Hendrix et al., J. Acquir. Immune Defic. Syndr. 37:1253-1262, 2004). In this study, we analyzed the envelope clones of DM-tropic virus in baseline and treated virus populations from 14 subjects. Ten subjects exhibited significant reductions in CXCR4-mediated infectivity after 10 days of AMD3100 therapy relative to baseline (X4 suppressor group), while four subjects had no reduction of CXCR4-mediated infectivity (X4 nonsuppressor group). The baseline viruses of the X4 suppressor group infected CXCR4-expressing cells less efficiently than those of the X4 nonsuppressor group. Clonal analysis indicated that the baseline viruses from the X4 suppressor group contained a higher proportion of R5-tropic variants mixed with CXCR4-using variants, while the X4 nonsuppressor group was enriched for CXCR4-using variants. AMD3100 suppressed X4-tropic variants in all subjects studied, but not all dualtropic variants. Furthermore, dualtropic variants that used CXCR4 efficiently were suppressed by AMD3100, while dualtropic variants that used CXCR4 poorly were not. This study demonstrated that AMD3100 has the ability to suppress both X4-tropic and certain dualtropic variants in vivo. The suppression of CXCR4-using variants by AMD3100 is dependent on both the tropism composition of the virus population and the efficiency of CXCR4 usage of individual variants.

Maraviroc: a CCR5 antagonist

HIV requires binding to both the CD4 molecule and a coreceptor to enable entry into the cell. CCR5 is a chemokine receptor that is utilized as a coreceptor by the majority of virus in early asymptomatic HIV infection. Maraviroc is a novel small molecule CCR5 antagonist which, in Phase IIb/III clinical trials up to 48 weeks, has been shown to be efficacious as part of an optimized antiretroviral regimen against CCR5 tropic HIV-1 in treatment-experienced patients. A further trial has demonstrated its noninferiority to efavirenz in achieving a HIV viral load less than 400 copies/ml as part of highly active antiretroviral therapy in treatment-naive individuals. It has recently received regulatory approval for use in North America and Europe in treatment-experienced patients. With increasing use, the role of maraviroc in the treatment of HIV-infected patients will be more clearly defined.

Antibodies for HIV treatment and prevention: window of opportunity?

Monoclonal antibodies are routinely used as therapeutics in a number of disease settings and have thus also been explored as potential treatment for human immunodeficiency virus (HIV)-1 infection. Antibodies targeting viral antigens, and those directed to the cellular receptors, have been considered for use in prevention and therapy. For virus-targeted antibodies, attention has focused primarily on their neutralizing activity, but such antibodies also have the potential to exert antiviral effects via effector functions, such as antibody-dependent cellular cytotoxicity (ADCC), opsonization, or complement activation. Anti-cell antibodies act through occlusion or down-modulation of the viral receptors with notable impact in vivo, as recent trials have shown. This review summarizes the diverse specificities and modes of action of therapeutic antibodies against HIV-1 infection. Successes, challenges, and future opportunities of harnessing antibodies for therapy of HIV-1 infection are discussed.

Development of an HIV-1 reference panel of subtype B envelope clones isolated from the plasma of recently infected individuals

We have developed an HIV-1 reference panel of 20 subtype B envelope clones isolated from the plasma of recently infected individuals. It is widely accepted that a prophylactic vaccine against HIV-1 requires the development of novel immunogens that are capable of eliciting broadly protective neutralizing antibody responses. Historically, patient serum has been screened for such antibodies by assaying against laboratory strains, but these viruses typically have increased neutralization sensitivity compared with primary isolates. To create a more standardized and relevant assay system for vaccine evaluation, we have developed a panel of primary envelopes derived from the plasma of individuals with documented acute/early subtype B HIV-1 infection occurring between 2000 and 2004. The HIV-1 envelopes from this panel vary in mode of transmission, coreceptor tropism, fusogenicity, and overall sensitivity to neutralization. These envelope sequences represent a broad spectrum of subtype B genetic diversity with an average pairwise genetic distance of 12% and a range from 10% to 19%. This well-characterized HIV-1 envelope panel should be a valuable resource for optimizing and standardizing vaccine assessment and improving vaccine design.

Targeting protein–protein interactions for HIV therapeutics

The majority of current anti-HIV drugs target the viral reverse transcriptase or protease enzymes. However, enfuvirtide and maraviroc are drugs that have been US FDA approved recently and which function by inhibiting virus cell binding and entry which normally occurs through the interaction of the viral envelope protein with its cellular coreceptor. As HIV-1 utilizes many cellular cofactors during its replication cycle, there are a number of other protein–protein interactions that can serve as targets for anti-HIV drug development. In this review article we discuss the general method used to identify anti-HIV drugs that function through targeting protein–protein interactions. We also discuss the currently known cellular cofactors that may serve as targets in future drugs screens.