Recombinant CD4-IgG2 in human immunodeficiency virus type 1-infected children: phase 1/2 study. The Pediatric AIDS Clinical Trials Group Protocol 351 Study Team

Pediatric immunotherapy and HIV control

PURPOSE OF REVIEW

Highlighting opportunities/potential for immunotherapy by understanding dynamics of HIV control during pediatric HIV infection with and without antiretroviral therapy (ART), as modeled in Simian immunodeficiency virus (SIV) and Simian-human immunodeficiency virus (SHIV)-infected rhesus macaques and observed in clinical trials. This review outlines mode of transmission, pathogenesis of pediatric HIV, unique aspects of the infant immune system, infant macaque models and immunotherapies.

RECENT FINDINGS

During the earliest stages of perinatal HIV infection, the infant immune system is characterized by a unique environment defined by immune tolerance and lack of HIV-specific T cell responses which contribute to disease progression. Moreover, primary lymphoid organs such as the thymus appear to play a distinct role in HIV pathogenesis in children living with HIV (CLWH). Key components of the immune system determine the degree of viral control, targets for strategies to induce viral control, and the response to immunotherapy. The pursuit of highly potent broadly neutralizing antibodies (bNAbs) and T cell vaccines has revolutionized the approach to HIV cure. Administration of HIV-1-specific bNAbs, targeting the highly variable envelope improves humoral immunity, and T cell vaccines induce or improve T cell responses such as the cytotoxic effects of HIV-1-specific CD8+ T cells, both of which are promising options towards virologic control and ART-free remission as evidenced by completed and ongoing clinical trials.

SUMMARY

Understanding early events during HIV infection and disease progression in CLWH serves as a foundation for predicting or targeting later outcomes by harnessing the immune system's natural responses. The developing pediatric immune system offers multiple opportunities for specific long-term immunotherapies capable of improving quality of life during adolescence and adulthood.

In vivo affinity maturation of the HIV-1 Env-binding domain of CD4

Many human proteins have been repurposed as biologics for clinical use. These proteins have been engineered with in vitro techniques that improve affinity for their ligands. However, these approaches do not select against properties that impair efficacy such as protease sensitivity or self-reactivity. Here we engineer the B-cell receptor of primary murine B cells to express a human protein biologic without disrupting their ability to affinity mature. Specifically, CD4 domains 1 and 2 (D1D2) of a half-life enhanced-HIV-1 entry inhibitor CD4-Ig (CD4-Ig-v0) were introduced into the heavy-chain loci of murine B cells, which were then adoptively transferred to wild-type mice. After immunization, transferred B cells proliferated, class switched, affinity matured, and efficiently produced D1D2-presenting antibodies. Somatic hypermutations found in the D1D2-encoding region of engrafted B cells improved binding affinity of CD4-Ig-v0 for the HIV-1 envelope glycoprotein (Env) and the neutralization potency of CD4-Ig-v0 by more than ten-fold across a global panel of HIV-1 isolates, without impairing its pharmacokinetic properties. Thus, affinity maturation of non-antibody protein biologics in vivo can guide development of more effective therapeutics.

Soluble CD4 and low molecular weight CD4-mimetic compounds sensitize cells to be killed by anti-HIV cytotoxic immunoconjugates

IMPORTANCE

HIV infection can be effectively treated to prevent the development of AIDS, but it cannot be cured. We have attached poisons to anti-HIV antibodies to kill the infected cells that persist even after years of effective antiviral therapy. Here we show that the killing of infected cells can be markedly enhanced by the addition of soluble forms of the HIV receptor CD4 or by mimics of CD4.

Broadly Effective ACE2 Decoy Proteins Protect Mice from Lethal SARS-CoV-2 Infection

As severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants have been causing increasingly serious drug resistance problem, development of broadly effective and hard-to-escape anti-SARS-CoV-2 agents is an urgent need. Here, we describe further development and characterization of two SARS-CoV-2 receptor decoy proteins, ACE2-Ig-95 and ACE2-Ig-105/106. We found that both proteins had potent and robust in vitro neutralization activities against diverse SARS-CoV-2 variants, including BQ.1 and XBB.1, that are resistant to most clinically used monoclonal antibodies. In a stringent lethal SARS-CoV-2 infection mouse model, both proteins lowered the lung viral load by up to ~1,000-fold, prevented the emergence of clinical signs in >75% animals, and increased the animal survival rate from 0% (untreated) to >87.5% (treated). These results demonstrate that both proteins are good drug candidates for protecting animals from severe COVID-19. In a head-to-head comparison of these two proteins with five previously described ACE2-Ig constructs, we found that two constructs, each carrying five surface mutations in the ACE2 region, had partial loss of neutralization potency against three SARS-CoV-2 variants. These data suggest that extensively mutating ACE2 residues near the receptor binding domain (RBD)-binding interface should be avoided or performed with extra caution. Furthermore, we found that both ACE2-Ig-95 and ACE2-Ig-105/106 could be produced to the level of grams per liter, demonstrating the developability of them as biologic drug candidates. Stress condition stability testing of them further suggests that more studies are required in the future to improve the stability of these proteins. These studies provide useful insight into critical factors for engineering and preclinical development of ACE2 decoys as broadly effective therapeutics against diverse ACE2-utilizing coronaviruses. IMPORTANCE Engineering soluble ACE2 proteins that function as a receptor decoy to block SARS-CoV-2 infection is a very attractive approach to creating broadly effective and hard-to-escape anti-SARS-CoV-2 agents. This article describes development of two antibody-like soluble ACE2 proteins that broadly block diverse SARS-CoV-2 variants, including Omicron. In a stringent COVID-19 mouse model, both proteins successfully protected >87.5% animals from lethal SARS-CoV-2 infection. In addition, a head-to-head comparison of the two constructs developed in this study with five previously described ACE2 decoy constructs was performed here. Two previously described constructs with relatively more ACE2 surface mutations were found with less robust neutralization activities against diverse SARS-CoV-2 variants. Furthermore, the developability of the two proteins as biologic drug candidates was also assessed here. This study provides two broad anti-SARS-CoV-2 drug candidates and useful insight into critical factors for engineering and preclinical development of ACE2 decoys as broadly effective therapeutics against diverse ACE2-utilizing coronaviruses.

David versus goliath: ACE2-Fc receptor traps as potential SARS-CoV-2 inhibitors

Anti-SARS-CoV-2 monoclonal antibodies and vaccines have shown improvement in lowering viral burden and hospitalization. However, emerging SARS-CoV-2 variants contain neutralizing antibody-escape mutations. Therefore, several reports have suggested the administration of recombinant angiotensin-converting enzyme 2 (rACE2) as a soluble receptor trap to block SARS-CoV-2 infection and limit viral escape potential. Several strategies have been implemented to enhance the efficacy of rACE2 as a therapeutic agent. Fc fusions have been used to improve pharmacokinetics and boost the affinity and avidity of ACE2 decoys for the virus spike protein. Furthermore, the intrinsic catalytic activity of ACE2 can be eliminated by introducing point mutations on the catalytic site of ACE2 to obtain an exclusive antiviral activity. This review summarizes different evolution platforms that have been used to enhance ACE2-Fc (i.e., immunoadhesins) as potential therapeutics for the current pandemic or future outbreaks of SARS-associated betacoronaviruses.

Anti-HIV Aptamers: Challenges and Prospects

Human Immunodeficiency Virus (HIV) infection continues to be a significant health burden in many countries around the world. Current HIV treatment through a combination of different antiretroviral drugs (cART) effectively suppresses viral replication, but drug resistance and crossresistance are significant challenges. This has prompted the search for novel targets and agents, such as nucleic acid aptamers. Nucleic acid aptamers are oligonucleotides that attach to the target sites with high affinity and specificity. This review provides a target-by-target account of research into anti-HIV aptamers and summarises the challenges and prospects of this therapeutic strategy, specifically in the unique context of HIV infection.

Bispecific Anti-HIV Immunoadhesins That Bind Gp120 and Gp41 Have Broad and Potent HIV-Neutralizing Activity

We have constructed bispecific immunoglobulin-like immunoadhesins that bind to both the HIV-envelope glycoproteins: gp120 and gp41. These immunoadhesins have N terminal domains of human CD4 engrafted onto the N-terminus of the heavy chain of human anti-gp41 mAb 7B2. Binding of these constructs to recombinant Env and their antiviral activities were compared to that of the parental mAbs and CD4, as well as to control mAbs. The CD4/7B2 constructs bind to both gp41 and gp140, as well as to native Env expressed on the surface of infected cells. These constructs deliver cytotoxic immunoconjugates to HIV-infected cells, but not as well as a mixture of 7B2 and sCD4, and opsonize for antibody-mediated phagocytosis. Most surprisingly, given that 7B2 neutralizes weakly, if at all, is that the chimeric CD4/7B2 immunoadhesins exhibit broad and potent neutralization of HIV, comparable to that of well-known neutralizing mAbs. These data add to the growing evidence that enhanced neutralizing activity can be obtained with bifunctional mAbs/immunoadhesins. The enhanced neutralization activity of the CD4/7B2 chimeras may result from cross-linking of the two Env subunits with subsequent inhibition of the pre-fusion conformational events that are necessary for entry.

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.

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.

Engineered ACE2 receptor traps potently neutralize SARS-CoV-2

An essential mechanism for severe acute respiratory syndrome coronavirus 1 (SARS-CoV-1) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection begins with the viral spike protein binding to the human receptor protein angiotensin-converting enzyme II (ACE2). Here, we describe a stepwise engineering approach to generate a set of affinity optimized, enzymatically inactivated ACE2 variants that potently block SARS-CoV-2 infection of cells. These optimized receptor traps tightly bind the receptor binding domain (RBD) of the viral spike protein and prevent entry into host cells. We first computationally designed the ACE2-RBD interface using a two-stage flexible protein backbone design process that improved affinity for the RBD by up to 12-fold. These designed receptor variants were affinity matured an additional 14-fold by random mutagenesis and selection using yeast surface display. The highest-affinity variant contained seven amino acid changes and bound to the RBD 170-fold more tightly than wild-type ACE2. With the addition of the natural ACE2 collectrin domain and fusion to a human immunoglobulin crystallizable fragment (Fc) domain for increased stabilization and avidity, the most optimal ACE2 receptor traps neutralized SARS-CoV-2-pseudotyped lentivirus and authentic SARS-CoV-2 virus with half-maximal inhibitory concentrations (IC50s) in the 10- to 100-ng/mL range. Engineered ACE2 receptor traps offer a promising route to fighting infections by SARS-CoV-2 and other ACE2-using coronaviruses, with the key advantage that viral resistance would also likely impair viral entry. Moreover, such traps can be predesigned for viruses with known entry receptors for faster therapeutic response without the need for neutralizing antibodies isolated from convalescent patients.

Immunoglobulin G2 Antibody as a Potential Target for COVID-19 Vaccine

The global threat of COVID-19 is continued with no commercially available vaccine or drug yet. While the application of convalescent therapy is usually beneficial, for critically ill patients, the detrimental effect associated with some antibodies is also reported. The immunoglobulin G (IgG) antibody in response to severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection is described, albeit the lack of defining whether the difference in subclasses has a beneficial or detrimental role. IgG2 has limited ability to activate innate immune cells and complement-mediated inflammation, which have been inversely described in SARS-CoV-2 pathogenesis. The expansion of IgG2 is promoted by interferon γ (IFN-γ); however, there is a low level of IFN-γ in COVID-19 patients. Therefore, this review describes the importance of targeting IgG2, with IFN-γ in minimizing the SARS-CoV-2 associated inflammation, and may provide insight into the design of vaccine or antibody-based therapies to COVID-19 disease.

Engineered ACE2 receptor traps potently neutralize SARS-CoV-2

An essential mechanism for SARS-CoV-1 and -2 infection begins with the viral spike protein binding to the human receptor protein angiotensin-converting enzyme II (ACE2). Here we describe a stepwise engineering approach to generate a set of affinity optimized, enzymatically inactivated ACE2 variants that potently block SARS-CoV-2 infection of cells. These optimized receptor traps tightly bind the receptor binding domain (RBD) of the viral spike protein and prevent entry into host cells. We first computationally designed the ACE2-RBD interface using a two-stage flexible protein backbone design process that improved affinity for the RBD by up to 12-fold. These designed receptor variants were affinity matured an additional 14-fold by random mutagenesis and selection using yeast surface display. The highest affinity variant contained seven amino acid changes and bound to the RBD 170-fold more tightly than wild-type ACE2. With the addition of the natural ACE2 collectrin domain and fusion to a human Fc domain for increased stabilization and avidity, the most optimal ACE2 receptor traps neutralized SARS-CoV-2 pseudotyped lentivirus and authentic SARS-CoV-2 virus with half-maximal inhibitory concentrations (IC50) in the 10-100 ng/ml range. Engineered ACE2 receptor traps offer a promising route to fighting infections by SARS-CoV-2 and other ACE2-utilizing coronaviruses, with the key advantage that viral resistance would also likely impair viral entry. Moreover, such traps can be predesigned for viruses with known entry receptors for faster therapeutic response without the need for neutralizing antibodies isolated or generated from convalescent patients.

Therapeutic strategies in an outbreak scenario to treat the novel coronavirus originating in Wuhan, China

A novel coronavirus (2019-nCoV) originating in Wuhan, China presents a potential respiratory viral pandemic to the world population. Current efforts are focused on containment and quarantine of infected individuals. Ultimately, the outbreak could be controlled with a protective vaccine to prevent 2019-nCoV infection. While vaccine research should be pursued intensely, there exists today no therapy to treat 2019-nCoV upon infection, despite an urgent need to find options to help these patients and preclude potential death. Herein, I review the potential options to treat 2019-nCoV in patients, with an emphasis on the necessity for speed and timeliness in developing new and effective therapies in this outbreak. I consider the options of drug repurposing, developing neutralizing monoclonal antibody therapy, and an oligonucleotide strategy targeting the viral RNA genome, emphasizing the promise and pitfalls of these approaches. Finally, I advocate for the fastest strategy to develop a treatment now, which could be resistant to any mutations the virus may have in the future. The proposal is a biologic that blocks 2019-nCoV entry using a soluble version of the viral receptor, angiotensin-converting enzyme 2 (ACE2), fused to an immunoglobulin Fc domain (ACE2-Fc), providing a neutralizing antibody with maximal breath to avoid any viral escape, while also helping to recruit the immune system to build lasting immunity. The ACE2-Fc therapy would also supplement decreased ACE2 levels in the lungs during infection, thereby directly treating acute respiratory distress pathophysiology as a third mechanism of action. The sequence of the ACE2-Fc protein is provided to investigators, allowing its possible use in recombinant protein expression systems to start producing drug today to treat patients under compassionate use, while formal clinical trials are later undertaken. Such a treatment could help infected patients before a protective vaccine is developed and widely available in the coming months to year(s).

Therapeutic strategies in an outbreak scenario to treat the novel coronavirus originating in Wuhan, China

A novel coronavirus (2019-nCoV) originating in Wuhan, China presents a potential respiratory viral pandemic to the world population. Current efforts are focused on containment and quarantine of infected individuals. Ultimately, the outbreak could be controlled with a protective vaccine to prevent 2019-nCoV infection. While vaccine research should be pursued intensely, there exists today no therapy to treat 2019-nCoV upon infection, despite an urgent need to find options to help these patients and preclude potential death. Herein, I review the potential options to treat 2019-nCoV in patients, with an emphasis on the necessity for speed and timeliness in developing new and effective therapies in this outbreak. I consider the options of drug repurposing, developing neutralizing monoclonal antibody therapy, and an oligonucleotide strategy targeting the viral RNA genome, emphasizing the promise and pitfalls of these approaches. Finally, I advocate for the fastest strategy to develop a treatment now, which could be resistant to any mutations the virus may have in the future. The proposal is a biologic that blocks 2019-nCoV entry using a soluble version of the viral receptor, angiotensin-converting enzyme 2 (ACE2), fused to an immunoglobulin Fc domain, providing a neutralizing antibody with maximal breath to avoid any viral escape, while also helping to recruit the immune system to build lasting immunity. The sequence of the ACE2-Fc protein is provided to investigators, allowing its possible use in recombinant protein expression systems to start producing drug today to treat patients under compassionate use, while formal clinical trials are later undertaken. Such a treatment could help infected patients before a protective vaccine is developed and widely available in the coming months to year(s).

Identification of Human Anti-HIV gp160 Monoclonal Antibodies That Make Effective Immunotoxins

The envelope (Env) glycoprotein of HIV is the only intact viral protein expressed on the surface of both virions and infected cells. Env is the target of neutralizing antibodies (Abs) and has been the subject of intense study in efforts to produce HIV vaccines. Therapeutic anti-Env Abs can also exert antiviral effects via Fc-mediated effector mechanisms or as cytotoxic immunoconjugates, such as immunotoxins (ITs). In the course of screening monoclonal antibodies (MAbs) for their ability to deliver cytotoxic agents to infected or Env-transfected cells, we noted disparities in their functional activities. Different MAbs showed diverse functions that did not correlate with each other. For example, MAbs against the external loop region of gp41 made the most effective ITs against infected cells but did not neutralize virus and bound only moderately to the same cells that they killed so effectively when they were used in ITs. There were also differences in IT-mediated killing among transfected and infected cell lines that were unrelated to the binding of the MAb to the target cells. Our studies of a well-characterized antigen demonstrate that MAbs against different epitopes have different functional activities and that the binding of one MAb can influence the interaction of other MAbs that bind elsewhere on the antigen. These results have implications for the use of MAbs and ITs to kill HIV-infected cells and eradicate persistent reservoirs of HIV infection.

IMPORTANCE

There is increased interest in using antibodies to treat and cure HIV infection. Antibodies can neutralize free virus and kill cells already carrying the virus. The virus envelope (Env) is the only HIV protein expressed on the surfaces of virions and infected cells. In this study, we examined a panel of human anti-Env antibodies for their ability to deliver cell-killing toxins to HIV-infected cells and to perform other antiviral functions. The ability of an antibody to make an effective immunotoxin could not be predicted from its other functional characteristics, such as its neutralizing activity. Anti-HIV immunotoxins could be used to eliminate virus reservoirs that persist despite effective antiretroviral therapy.

Design and In Vivo Characterization of Immunoconjugates Targeting HIV gp160

The envelope (Env) glycoprotein of HIV is expressed on the surface of productively infected cells and can be used as a target for cytotoxic immunoconjugates (ICs), in which cell-killing moieties, including toxins, drugs, or radionuclides, are chemically or genetically linked to monoclonal antibodies (MAbs) or other targeting ligands. Such ICs could be used to eliminate persistent reservoirs of HIV infection. We have found that MAbs which bind to the external loop of gp41, e.g., MAb 7B2, make highly effective ICs, particularly when used in combination with soluble CD4. We evaluated the toxicity, immunogenicity, and efficacy of the ICs targeted with 7B2 in mice and in simian-human immunodeficiency virus-infected macaques. In the macaques, we tested immunotoxins (ITs), consisting of protein toxins bound to the targeting agent. ITs were well tolerated and initially efficacious but were ultimately limited by their immunogenicity. In an effort to decrease immunogenicity, we tested different toxic moieties, including recombinant toxins, cytotoxic drugs, and tubulin inhibitors. ICs containing deglycosylated ricin A chain prepared from ricin toxin extracted from castor beans were the most effective in killing HIV-infected cells. Having identified immunogenicity as a major concern, we show that conjugation of IT to polyethylene glycol limits immunogenicity. These studies demonstrate that cytotoxic ICs can target virus-infected cells in vivo but also highlight potential problems to be addressed.

IMPORTANCE

It is not yet possible to cure HIV infection. Even after years of fully effective antiviral therapy, a persistent reservoir of virus-infected cells remains. Here we propose that a targeted conjugate consisting of an anti-HIV antibody bound to a toxic moiety could function to kill the HIV-infected cells that constitute this reservoir. We tested this approach in HIV-infected cells grown in the lab and in animal infections. Our studies demonstrated that these immunoconjugates are effective both in vitro and in test animals. In particular, ITs constructed with the deglycosylated A chain prepared from native ricin were the most effective in killing cells, but their utility was blunted because they provoked immune reactions that interfered with the therapeutic effects. We then demonstrated that coating of the ITs with polyethylene glycol minimized the immunogenicity, as has been demonstrated with other protein therapies.

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.

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.

Bifunctional CD4-DC-SIGN fusion proteins demonstrate enhanced avidity to gp120 and inhibit HIV-1 infection and dissemination

Early stages of mucosal infection are potential targets for HIV-1 prevention. CD4 is the primary receptor in HIV-1 infection whereas DC-SIGN likely plays an important role in HIV-1 dissemination, particularly during sexual transmission. To test the hypothesis that an inhibitor simultaneously targeting both CD4 and DC-SIGN binding sites on gp120 may provide a potent anti-HIV strategy, we designed constructs by fusing the extracellular CD4 and DC-SIGN domains together with varied arrangements of the lengths of CD4, DC-SIGN and the linker. We expressed, purified and characterized a series of soluble CD4-linker-DC-SIGN (CLD) fusion proteins. Several CLDs, composed of a longer linker and an extra neck domain of DC-SIGN, had enhanced affinity for gp120 as evidenced by molecular-interaction analysis. Furthermore, such CLDs exhibited significantly enhanced neutralization activity against both laboratory-adapted and primary HIV-1 isolates. Moreover, CLDs efficiently inhibited HIV-1 infection in trans via a DC-SIGN-expressing cell line and primary human dendritic cells. This was further strengthened by the results from the human cervical explant model, showing that CLDs potently prevented both localized and disseminated infections. This is the first time that soluble DC-SIGN-based bifunctional proteins have demonstrated anti-HIV potency. Our study provides proof of the concept that targeting both CD4 and DC-SIGN binding sites on gp120 represents a novel antiviral strategy. Given that DC-SIGN binding to gp120 increases exposure of the CD4 binding site and that the soluble forms of CD4 and DC-SIGN occur in vivo, further improvement of CLDs may render them potentially useful in prophylaxis or therapeutics.

SAXS data analysis and modeling of tetravalent neutralizing antibody CD4-IgG2 -/+ HIV-1 gp120 revealed that first two gp120 bind to the same Fab arm

This communication describes SAXS data based global structures of tetravalent antibody CD4-IgG2 and its dimeric to pentameric complexes with gp120s. Comparison of models brought forth that while the two CD4s grafted on each arm remain tightly packed in the unliganded antibody, they enable binding of first two gp120s preferentially to the same Fab arm in an asymmetric manner. Retention of residues in the CD4-Fab linker earlier reasoned to enable bi-fold collapse of gp120-bound soluble CD4, and observed asymmetry of the (CD4-IgG2)/(gp120)(2) complex suggest that encoded flexibility in CD4-Fab linker is a critical structure-function factor for this broad spectrum neutralizing antibody.

Therapies for coronaviruses. Part I of II -- viral entry inhibitors

BACKGROUND

Severe acute respiratory syndrome (SARS) coronavirus emerged fleetingly in the winter of 2002 and again in the winter of 2003, resulting in the infection of ~8,000 people and the death of ~800. The identification of the putative natural reservoir suggests that a re-emergence is possible. The functions of many coronaviral proteins have now been elucidated, resulting in many novel approaches to therapy.

OBJECTIVE

To review anticoronaviral therapies based on inhibition of viral entry into the host cell and to cast light on promising approaches and future developments.

METHOD

The published literature, in particular patent publications, is searched for relevant documents. The information is organized and critiqued.

RESULTS/CONCLUSION

The approaches to combating coronaviral infections are built on the foundation of antivirals against other viruses and the fundamental insights gained by dissection of the coronaviral lifecycle. These approaches include the prevention of viral entry, reviewed here, and interference with the intracellular lifecycle of the virus in the infected cell, reviewed next. Of the viral-entry inhibitors, monoclonal antibodies have demonstrated efficacy, clinical application in other viral infections, and the potential to impact a future epidemic. Moreover, combinations of monoclonal antibodies have been shown to have a broader spectrum of antiviral activity.

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.

Selection, characterization and application of new RNA HIV gp 120 aptamers for facile delivery of Dicer substrate siRNAs into HIV infected cells

The envelope glycoprotein of human immunodeficiency virus (HIV) consists of an exterior glycoprotein (gp120) and a trans-membrane domain (gp41) and has an important role in viral entry into cells. HIV-1 entry has been validated as a clinically relevant anti-viral strategy for drug discovery. In the present work, several 2'-F substituted RNA aptamers that bind to the HIV-1(BaL) gp120 protein with nanomole affinity were isolated from a RNA library by the SELEX (Systematic Evolution of Ligands by EXponential enrichment) procedure. From two of these aptamers we created a series of new dual inhibitory function anti-gp120 aptamer-siRNA chimeras. The aptamers and aptamer-siRNA chimeras specifically bind to and are internalized into cells expressing HIV gp160. The Dicer-substrate siRNA delivered by the aptamers is functionally processed by Dicer, resulting in specific inhibition of HIV-1 replication and infectivity in cultured CEM T-cells and primary blood mononuclear cells (PBMCs). Moreover, we have introduced a 'sticky' sequence onto a chemically synthesized aptamer which facilitates attachment of the Dicer substrate siRNAs for potential multiplexing. Our results provide a set of novel inhibitory agents for blocking HIV replication and further validate the use of aptamers for delivery of Dicer substrate siRNAs.

Inhibitors of viral entry

The entry of viruses into target cells involves a complex series of sequential steps, with opportunities for inhibition at every stage. Entry inhibitors exert their biological properties by inhibiting protein-protein interactions either within the viral envelope (Env) glycoproteins or between viral Env and host-cell receptors. The nature of resistance to entry inhibitors also differs from compounds inhibiting enzymatic targets due to their different modes of action and the relative variability in Env sequences both temporally and between patients. Two drugs that target HIV-1 entry, enfuvirtide and maraviroc, are now licensed for treatment of HIV-1 infection. The efficacy of these drugs validates entry as a point of intervention in viral life cycles and, in the context of HIV treatment, contributes to the growing armamentarium of antivirals which, in multidrug combinations, can effectively inhibit viral replication and prevent disease progression.

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.

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.

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.

Antitumor activities of TEM8-Fc: an engineered antibody-like molecule targeting tumor endothelial marker 8

Tumor endothelial marker 8 (TEM8) was discovered as a cell membrane protein that is predominantly expressed in tumor endothelium and identified as a receptor for anthrax toxin. We developed an antibody-like molecule that consists of the protective antigen (PA)-binding domain of human TEM8 linked to the Fc portion of human immunoglobulin G1 (TEM8-Fc). This engineered protein bound to PA in a divalent cation-dependent manner and efficiently protected J774A.1 macrophage-like cells against anthrax toxin challenge in a dose-dependent manner. TEM8-Fc suppressed the growth and metastasis of xenograft human tumors in athymic nude mice (control versus 10 mg/kg TEM8-Fc, mean tumor weight: LS-180, 1.72 versus 0.16 g, difference = 1.56 g, 95% confidence interval [CI] = 0.96 to 2.16 g; P<.001; MCF-7, 1.12 versus 0.08 g, difference = 1.04 g, 95% CI = 0.77 to 1.31 g; P<.001; HepG2, 1.28 versus 0.35 g, difference = 0.93 g, 95% CI = 0.60 to 1.25 g; P<.001). Furthermore, TEM8 interacted with the M2 isoenzyme of pyruvate kinase (M2-PK), which has an important role in tumor growth and metastasis. TEM8-Fc is a novel therapeutic antibody-like agent in the management of solid tumors that may act by trapping M2-PK.

Novel inhibitors of the early steps of the HIV-1 life cycle

Considerable advances have been made on compounds that are active as inhibitors of HIV entry and fusion. The discovery of chemokines a few years ago focused the attention on coreceptor inhibitors in addition to fusion and attachment blockers. During the last 5 years, there has been an intense research activity from both private companies and academic institutions to find effective compounds that are capable of inhibiting the initial steps in the HIV life cycle. Some of the presented compounds demonstrated in vitro synergism, thus there is the rationale of their combined use in HIV-infected individuals. Many entry and fusion inhibitors of HIV are being investigated in controlled clinical trials and there are a number of them that are bioavailable as oral formulations. This is an essential feature for an extended use of these compounds with the purpose of ameliorating patients' adherence to medications; therefore, preventing the development of drug resistance. Among the many compounds that are being investigated, some are in the preclinical arena and others are more advanced in development stages. Overall, the main aim is to establish the action of these compounds on the immune system (e.g., the balance of the system after shutting off CCR5 or CXCR4 coreceptors) and the possible burden of unexplained side effects. This review focuses on the recent developments in this field with a particular attention on promising compounds in preclinical and clinical trials.

Anti-HIV agents targeting the interaction of gp120 with the cellular CD4 receptor

Perhaps one of the most effective approaches to prevent and inhibit viral infections is to block host cell receptors that are used by viruses to gain cell entry. Major advances have been made over the past decade in the understanding of the molecular mechanism of HIV entry into target cells. A crucial step in this entry process is the interaction of the external HIV envelope glycoprotein, gp120, with the cellular CD4 receptor molecule. This binding step represents a potential target for new antiviral agents, and current efforts to develop safe and effective HIV entry inhibitors are focused on natural ligands and/or monoclonal antibodies that interfere with gp120/CD4 interaction. Also, small synthetic compounds obtained either by high-throughput screening of large compound libraries or by structure-guided rational design have recently entered the antiretroviral arena. In this review, the anti-HIV activity of novel entry inhibitors targeting gp120/CD4 interaction is outlined, and special attention is given to the cyclotriazadisulfonamide compounds, which are the most specific CD4-targeted antiviral drugs described so far.

The appealing story of HIV entry inhibitors : from discovery of biological mechanisms to drug development

Current therapeutic intervention in HIV infection relies upon 20 different drugs. Despite the impressive efficacy shown by these drugs, we are confronted with an unexpected frequency of adverse effects, such as mitochondrial toxicity and lipodystrophy, and resistance, not only to individual drugs but to entire drug classes.Thus, there is now a great need for new antiretroviral drugs with reduced toxicity, increased activity against drug-resistant viruses and a greater capacity to reach tissue sanctuaries of the virus. Two different HIV molecules have been selected as targets of drug inhibition so far: reverse transcriptase and protease. Drugs that target the interactions between the HIV envelope and the cellular receptor complex are a 'new entry' into the scenario of HIV therapy and have recently raised great interest because of their activity against multidrug-resistant viruses. There are several compounds that are at different developmental stages in the pipeline to counter HIV entry, among them: (i) the attachment inhibitor dextrin-2-sulfate; (ii) the inhibitors of the glycoprotein (gp) 120/CD4 interaction PRO 542, TNX 355 and BMS 488043; (iii) the co-receptor inhibitors subdivided in those targeting CCR5 (SCH 417690 [SCH D], UK 427857 GW 873140, PRO 140, TAK 220, AMD 887) and those targeting CXCR4 (AMD 070, KRH 2731); and (iv) the fusion inhibitors enfuvirtide (T-20) and tifuvirtide (T-1249). The story of the first of these drugs, enfuvirtide, which has successfully completed phase III clinical trials, has been approved by the US FDA and by the European Medicines Agency, and is now commercially available worldwide, is an example of how the knowledge of basic molecular mechanisms can rapidly translate into the development of clinically effective molecules.

Emerging drug targets for antiretroviral therapy

Current targets for antiretroviral therapy (ART) include the viral enzymes reverse transcriptase and protease. The use of a combination of inhibitors targeting these enzymes can reduce viral load for a prolonged period and delay disease progression. However, complications of ART, including the emergence of viruses resistant to current drugs, are driving the development of new antiretroviral agents targeting not only the reverse transcriptase and protease enzymes but novel targets as well. Indeed, enfuvirtide, an inhibitor targeting the viral envelope protein (Env) was recently approved for use in combination therapy in individuals not responding to current antiretroviral regimens. Emerging drug targets for ART include: (i) inhibitors that directly or indirectly target Env; (ii) the HIV enzyme integrase; and (iii) inhibitors of maturation that target the substrate of the protease enzyme. Env mediates entry of HIV into target cells via a multistep process that presents three distinct targets for inhibition by viral and cellular-specific agents. First, attachment of virions to the cell surface via nonspecific interactions and CD4 binding can be blocked by inhibitors that include cyanovirin-N, cyclotriazadisulfonamide analogues, PRO 2000, TNX 355 and PRO 542. In addition, BMS 806 can block CD4-induced conformational changes. Secondly, Env interactions with the co-receptor molecules can be targeted by CCR5 antagonists including SCH-D, maraviroc (UK 427857) and aplaviroc (GW 873140), and the CXCR4 antagonist AMD 070. Thirdly, fusion of viral and cellular membranes can be inhibited by peptides such as enfuvirtide and tifuvirtide (T 1249). The development of entry inhibitors has been rapid, with an increasing number entering clinical trials. Moreover, some entry inhibitors are also being evaluated as candidate microbicides to prevent mucosal transmission of HIV. The integrase enzyme facilitates the integration of viral DNA into the host cell genome. The uniqueness and specificity of this reaction makes integrase an attractive drug target. However, integrase inhibitors have been slow to reach clinical development, although recent contenders, including L 870810, show promise. Inhibitors that target viral maturation via a unique mode of action, such as PA 457, also have potential. In addition, recent advances in our understanding of cellular pathways involved in the life cycle of HIV have also identified novel targets that may have potential for future antiretroviral intervention, including interactions between the cellular proteins APOBEC3G and TSG101, and the viral proteins Vif and p6, respectively. In summary, a number of antiretroviral agents in development make HIV entry, integration and maturation emerging drug targets. A multifaceted approach to ART, using combinations of inhibitors that target different steps of the viral life cycle, has the best potential for long-term control of HIV infection. Furthermore, the development of microbicides targeting HIV holds promise for reducing HIV transmission events.

CADA, a novel CD4-targeted HIV inhibitor, is synergistic with various anti-HIV drugs in vitro

OBJECTIVE

To evaluate the anti-HIV-1 activity of the cyclotriazadisulfonamide CADA against primary isolates in vitro and the combination of CADA with approved anti-HIV drugs for potential synergy.

METHODS

Peripheral blood mononuclear cells (PBMC) were treated with CADA and infected with 16 different clinical isolates. After 8 days of infection, the median inhibitory concentration (IC50) was calculated from the p24 viral antigen content in the supernatant. MT-4 cells were infected with HIV-1NL4.3 and then cultured with CADA or other antiretroviral drugs (i.e., several reverse transcriptase, protease and entry inhibitors), alone and in combination. After 4 days, IC50 was determined for the various drugs in replicate assays. Analysis of combined effects was performed using the median effect principle (CalcuSyn; Biosoft).

RESULTS

The entry inhibitor CADA exerted a potent and consistent anti-HIV-1 activity against a wide range of R5, R5/X4 and X4 primary isolates in PBMC. From the two-drug studies, combination indices showed synergy between CADA and reverse transcriptase inhibitors (zidovudine, stavudine, lamivudine, zalcitabine, didanosine, abacavir, tenofovir, nevirapine, delavirdine and efavirenz), and protease inhibitors (lopinavir, saquinavir, indinavir, nelfinavir, amprenavir and ritonavir). In addition, the combination of CADA with the gp41 fusion inhibitor T-20 (enfuvirtide), the CXCR4 antagonist AMD3100 and the gp120-specific interacting plant lectins from Galanthus nivalis (GNA) and Hippeastrum hybrid (HHA) also resulted in a synergistic inhibition.

CONCLUSIONS

Compounds that can specifically downmodulate the CD4 receptor in PBMC have broad-spectrum anti-HIV activity against primary isolates and act synergistically when used in conjunction with currently available antiretroviral drugs. They deserve further study as potential candidate anti-HIV drugs.

Molecular biology of the human immunodeficiency virus: current and future targets for intervention

Inhibitors of the reverse transcriptase and protease enzymes of the human immunodeficiency virus form the backbone of current antiretroviral therapy. However, other therapeutic targets have been identified in studies of interactions between HIV and its target cells. This review presents an overview of HIV replication, emphasizing recently recognized cellular and viral molecules that may be exploited in future pharmacological approaches to prevent and treat infection with HIV.

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

Viral entry inhibitors represent an emerging mode of therapy for human immunodeficiency virus type 1 (HIV-1) infection. PRO 542 (CD4-immunoglobulin G2) is a tetravalent CD4-immunoglobulin fusion protein that broadly neutralizes primary HIV-1 isolates. PRO 542 binds to the viral surface glycoprotein gp120 and blocks attachment and entry of virus into CD4(+) cells. Previously, PRO 542 demonstrated antiviral activity without significant toxicity when tested at single doses ranging to 10 mg/kg. In this study, 12 HIV-infected individuals were treated with 25-mg/kg single-dose PRO 542 and then monitored for safety, antiviral effects, and PRO 542 pharmacokinetics for 6 weeks. The study examined two treatment cohorts that differed in the extent of HIV-1 disease progression. PRO 542 at 25 mg/kg was well tolerated and demonstrated a serum half-life of 3 days. Statistically significant acute reductions in HIV-1 RNA levels were observed across all study patients, and greater antiviral effects were observed in the cohort of patients with more advanced HIV-1 disease. In advanced disease (HIV-1 RNA > 100,000 copies/ml; CD4 lymphocytes < 200 cells/mm(3)), PRO 542 mediated an 80% response rate and statistically significant approximately 0.5 log(10) mean reductions in viral load for 4 to 6 weeks posttreatment. Similar findings were obtained in an analysis of all (n = 11) advanced disease patients treated to date with single doses of PRO 542 ranging from 1 to 25 mg/kg. In addition, a significant correlation was observed between antiviral effects observed in vivo and viral susceptibility to PRO 542 in vitro. The findings support continued development of PRO 542 for salvage therapy of advanced HIV-1 disease.

HIV-1 entry and its inhibition

Entry of HIV-1 virions into cells is a complex and dynamic process carried out by envelope (Env) glycoproteins on the surface of the virion that promote the thermodynamically unfavorable fusion of highly stable viral and target cell membranes. Insight gained from studies of the mechanism of viral entry allowed insight into the design of novel inhibitors of HIV-1 entry, several of which are now in clinical trials. This review highlights the mechanism by which viral and cellular proteins mediate entry of HIV-1 into permissive cells, with an emphasis on targeting this process in the design of novel therapies that target distinct steps of the entry process, including antagonizing receptor binding events and blocking conformational changes intimately involved in membrane fusion.

Specific CD4 down-modulating compounds with potent anti-HIV activity

Despite the availability of the current clinically approved anti-HIV drugs, new classes of effective antiviral agents are still urgently needed to combat AIDS. A promising approach for drug development and vaccine design involves targeting research on HIV-1 entry, a multistep process that comprises viral attachment, coreceptor interactions, and fusion. Determination of the viral entry process in detail has enabled the design of specific agents that can inhibit each step in the HIV entry process. Therapeutic agents that interfere with the binding of the HIV envelope glycoprotein gp120 to the CD4 receptor (e.g., PRO 542, PRO 2000, and CV-N) or the coreceptors CCR5 and CXCR4 (e.g., SCH-C and AMD3100) are briefly outlined in this review. The anti-HIV activity of cyclotriazadisulfonamides, a novel class of compounds with a unique mode of action by down-modulating the CD4 receptor in lymphocytic and monocytic cells, is especially highlighted. On the basis of the successful results of T-20, the first approved entry inhibitor, the development of effective antiretrovirals that block HIV entry will certainly be further encouraged.

Biochemical and genetic characterizations of a novel human immunodeficiency virus type 1 inhibitor that blocks gp120-CD4 interactions

BMS-378806 is a recently discovered small-molecule human immunodeficiency virus type 1 (HIV-1) attachment inhibitor with good antiviral activity and pharmacokinetic properties. Here, we demonstrate that the compound targets viral entry by inhibiting the binding of the HIV-1 envelope gp120 protein to cellular CD4 receptors via a specific and competitive mechanism. BMS-378806 binds directly to gp120 at a stoichiometry of approximately 1:1, with a binding affinity similar to that of soluble CD4. The potential BMS-378806 target site was localized to a specific region within the CD4 binding pocket of gp120 by using HIV-1 gp120 variants carrying either compound-selected resistant substitutions or gp120-CD4 contact site mutations. Mapping of resistance substitutions to the HIV-1 envelope, and the lack of compound activity against a CD4-independent viral infection confirm the gp120-CD4 interactions as the target in infected cells. BMS-378806 therefore serves as a prototype for this new class of antiretroviral agents and validates gp120 as a viable target for small-molecule inhibitors.

A small molecule HIV-1 inhibitor that targets the HIV-1 envelope and inhibits CD4 receptor binding

BMS-378806 is a recently discovered small molecule HIV-1 inhibitor that blocks viral entrance to cells. The compound exhibits potent inhibitory activity against a panel of R5-(virus using the CCR5 coreceptor), X4-(virus using the CXCR4 coreceptor), and R5/X4 HIV-1 laboratory and clinical isolates of the B subtype (median EC50 of 0.04 microM) in culture assays. BMS-378806 is selective for HIV-1 and inactive against HIV-2, SIV and a panel of other viruses, and exhibits no significant cytotoxicity in the 14 cell types tested (concentration for 50% reduction of cell growth, >225 microM). Mechanism of action studies demonstrated that BMS-378806 binds to gp120 and inhibits the interactions of the HIV-1 envelope protein to cellular CD4 receptors. Further confirmation that BMS-378806 targets the envelope in infected cells was obtained through the isolation of resistant variants and the mapping of resistance substitutions to the HIV-1 envelope. In particular, two substitutions, M426L and M475I, are situated in the CD4 binding pocket of gp120. Recombinant HIV-1 carrying these two substitutions demonstrated significantly reduced susceptibility to compound inhibition. BMS-378806 displays many favorable pharmacological traits, such as low protein binding, minimal human serum effect on anti-HIV-1 potency, good oral bioavailability in animal species, and a clean safety profile in initial animal toxicology studies. Together, the data show that BMS-378806 is a representative of a new class of HIV inhibitors that has the potential to become a valued addition to our current armamentarium of antiretroviral drugs.

Multiple interactions across the surface of the gp120 core structure determine the global neutralization resistance phenotype of human immunodeficiency virus type 1

Resistance to neutralization is an important characteristic of primary isolates of human immunodeficiency virus type 1 (HIV-1) that relates to the potential for successful vaccination to prevent infection and use of immunotherapeutics for treatment of established infection. In order to further elucidate mechanisms responsible for neutralization resistance, we studied the molecular mechanisms that determine the resistance of the primary virus isolate of the strain HIV-1 MN to neutralization by soluble CD4 (sCD4). As is the case for the global neutralization resistance phenotype, sCD4 resistance depended upon sequences in the amino-terminal heptad repeat region of gp41 (HR1), as well as on multiple functional interactions within the envelope complex. The functional interactions that determined the resistance included interactions between the variable loop 1 and 2 (V1/V2) region and sequences in or near the CD4 binding site (CD4bs) and with the V3 loop. Additionally, the V3 loop region was found to interact functionally with sequences in the outer domain of gp120, distant from the CD4bs and coreceptor-binding site, as well as with a residue thought to be located centrally in the coreceptor-binding site. These and previous results provide the basis for a model by which functional signals that determine the neutralization resistance, high-infectivity phenotype depend upon interactions occurring across the surface of the gp120 core structure and involving variable loop structures and gp41. This model should be useful in efforts to define epitopes that may be important for primary virus neutralization.

HIV-1 entry and entry inhibitors as therapeutic agents

Defining the mechanisms of HIV-1 entry has enabled the rational design of strategies aimed at interfering with the process. This article delineates what is currently understood about HIV-1 entry, as a window through which to understand what will likely be the next major group of antiretroviral therapeutics. These exciting new approaches offer the promise of adding viral entry to reverse transcription and protein processing as steps to block in the viral life cycle. Several principles learned with other antiretroviral drugs are sure to be valid for entry antagonists, whereas other considerations may be unique to this group of agents. There is no agent to which HIV-1 has not been able to acquire resistance and this is likely to remain the case. Multiple rounds of viral replication are required to generate the genetic diversity that forms the basis of resistance. Combination therapy in which replication is maximally suppressed will remain a cornerstone of treatment with entry inhibitors, as with other agents. Furthermore, the coreceptor specificity of some entry and fusion inhibitors argues that combinations will likely be needed to broaden the effective range of susceptible viral variants. Finally, the targeting of multiple steps within the entry process has the potential for synergy. The fusion inhibitor T20 and CXCR4 antagonist AMD3100 are synergistic in vitro at blocking infection of PBMC with clinical isolates [115] and T20 combined with the CD4 inhibitor PRO 542 have synergistic in vitro effects, with more than 10-fold greater inhibition of R5, X4, and R5X4 strains than either agent alone [116]. Entry antagonists raise other, unique issues. As discussed previously, the theoretic concern exists that blocking CCR5 could enhance the emergence of CXCR4-using variants and possibly accelerate disease. So far, in vitro selection for variants resistant to the CCR5 antagonist SCH-C in PBMC (which express both CCR5 and CXCR4) has resulted in mutants that were resistant to the blocker but still used CCR5. Alternatively, because many HIV-1 strains have the capacity to use several other chemokine or orphan receptors for entry, blocking both CCR5 and CXCR could lead to a variant that uses one of these other molecules in place of the principal coreceptors, although data in vitro so far suggest that this is unlikely [13,14]. This new class of antiviral drugs offers great promise but also novel concerns, and careful analysis of viruses that arise with their use in vivo is essential.

New antiretroviral drugs

Despite the availability of 16 antiretroviral drugs approved for the treatment of HIV infection, current combination regimens present challenges. Newer antiretroviral drugs are needed to improve convenience, reduce toxicity and, of particular importance, to provide antiretroviral activity against viral strains resistant to the currently available antiretroviral agents. Candidate drugs with novel properties are in development in the two currently available drug classes: HIV reverse transcriptase inhibitors (nucleoside analogs, non-nucleoside analogs and nucleotide analogs) and HIV protease inhibitors (PI). Investigational nucleoside analog reverse transcriptase inhibitors (nRTI) include emtricitabine (FTC) and amdoxovir (DAPD), and investigational non-nucleoside reverse transcriptase inhibitors (NNRTI) include DPC 083 and TMC 125. New protease inhibitors under investigation include atazanavir (BMS-232 632), tipranavir, and TMC 114. In addition, newer agents with novel mechanisms of action such as HIV entry inhibitors (that inhibit the three steps of HIV entry: CD4 attachment, chemokine receptor binding and membrane fusion) and HIV integrase inhibitors are under investigation. Investigational entry inhibitors include PRO 542 (a CD4 attachment inhibitor), Schering C (a chemokine receptor inhibitor), enfuvirtide (T-20) and T-1249, inhibitors of membrane fusion. Investigational HIV integrase inhibitors include S-1360. Continued progress in the treatment of HIV disease will result from the development of new antiretroviral drugs.