Ibalizumab: an anti-CD4 monoclonal antibody for the treatment of HIV-1 infection

New insights into protein-protein interaction modulators in drug discovery and therapeutic advance

Protein-protein interactions (PPIs) are fundamental to cellular signaling and transduction which marks them as attractive therapeutic drug development targets. What were once considered to be undruggable targets have become increasingly feasible due to the progress that has been made over the last two decades and the rapid technological advances. This work explores the influence of technological innovations on PPI research and development. Additionally, the diverse strategies for discovering, modulating, and characterizing PPIs and their corresponding modulators are examined with the aim of presenting a streamlined pipeline for advancing PPI-targeted therapeutics. By showcasing carefully selected case studies in PPI modulator discovery and development, we aim to illustrate the efficacy of various strategies for identifying, optimizing, and overcoming challenges associated with PPI modulator design. The valuable lessons and insights gained from the identification, optimization, and approval of PPI modulators are discussed with the aim of demonstrating that PPI modulators have transitioned beyond early-stage drug discovery and now represent a prime opportunity with significant potential. The selected examples of PPI modulators encompass those developed for cancer, inflammation and immunomodulation, as well as antiviral applications. This perspective aims to establish a foundation for the effective targeting and modulation of PPIs using PPI modulators and pave the way for future drug development.

New Therapies and Strategies to Curb HIV Infections with a Focus on Macrophages and Reservoirs

More than 80 million people worldwide have been infected with the human immunodeficiency virus (HIV). There are now approximately 39 million individuals living with HIV/acquired immunodeficiency syndrome (AIDS). Although treatments against HIV infection are available, AIDS remains a serious disease. Combination antiretroviral therapy (cART), also known as highly active antiretroviral therapy (HAART), consists of treatment with a combination of several antiretroviral drugs that block multiple stages in the virus replication cycle. However, the increasing usage of cART is inevitably associated with the emergence of HIV drug resistance. In addition, the development of persistent cellular reservoirs of latent HIV is a critical obstacle to viral eradication since viral rebound takes place once anti-retroviral therapy (ART) is interrupted. Thus, several efforts are being applied to new generations of drugs, vaccines and new types of cART. In this review, we summarize the antiviral therapies used for the treatment of HIV/AIDS, both as individual agents and as combination therapies, and highlight the role of both macrophages and HIV cellular reservoirs and the most recent clinical studies related to this disease.

Surface patches induce nonspecific binding and phase separation of antibodies

Nonspecific interactions are a key challenge in the successful development of therapeutic antibodies. The tendency for nonspecific binding of antibodies is often difficult to reduce by rational design, and instead, it is necessary to rely on comprehensive screening campaigns. To address this issue, we performed a systematic analysis of the impact of surface patch properties on antibody nonspecificity using a designer antibody library as a model system and single-stranded DNA as a nonspecificity ligand. Using an in-solution microfluidic approach, we find that the antibodies tested bind to single-stranded DNA with affinities as high as KD = 1 µM. We show that DNA binding is driven primarily by a hydrophobic patch in the complementarity-determining regions. By quantifying the surface patches across the library, the nonspecific binding affinity is shown to correlate with a trade-off between the hydrophobic and total charged patch areas. Moreover, we show that a change in formulation conditions at low ionic strengths leads to DNA-induced antibody phase separation as a manifestation of nonspecific binding at low micromolar antibody concentrations. We highlight that phase separation is driven by a cooperative electrostatic network assembly mechanism of antibodies with DNA, which correlates with a balance between positive and negative charged patches. Importantly, our study demonstrates that both nonspecific binding and phase separation are controlled by the size of the surface patches. Taken together, these findings highlight the importance of surface patches and their role in conferring antibody nonspecificity and its macroscopic manifestation in phase separation.

Non-specificity as the sticky problem in therapeutic antibody development

Antibodies are highly potent therapeutic scaffolds with more than a hundred different products approved on the market. Successful development of antibody-based drugs requires a trade-off between high target specificity and target binding affinity. In order to better understand this problem, we here review non-specific interactions and explore their fundamental physicochemical origins. We discuss the role of surface patches - clusters of surface-exposed amino acid residues with similar physicochemical properties - as inducers of non-specific interactions. These patches collectively drive interactions including dipole-dipole, π-stacking and hydrophobic interactions to complementary moieties. We elucidate links between these supramolecular assembly processes and macroscopic development issues, such as decreased physical stability and poor in vivo half-life. Finally, we highlight challenges and opportunities for optimizing protein binding specificity and minimizing non-specificity for future generations of therapeutics.

Immunization with HIV-1 trimeric SOSIP.664 BG505 or Founder Virus C (FVCEnv) covalently complexed to two-domain CD4S60C elicits cross-clade neutralizing antibodies in New Zealand white rabbits

Background: An ongoing challenge in HIV-1 vaccine research is finding a novel HIV-1 envelope glycoprotein (Env)-based immunogen that elicits broadly cross-neutralizing antibodies (bnAbs) without requiring complex sequential immunization regimens to drive the required antibody affinity maturation. Previous vaccination studies have shown monomeric Env and Env trimers which contain the GCN4 leucine zipper trimerization domain and are covalently bound to the first two domains of CD4 (2dCD4^S60C) generate potent bnAbs in small animals. Since SOSIP.664 trimers are considered the most accurate, conformationally intact representation of HIV-1 Env generated to date, this study further evaluated the immunogenicity of SOSIP.664 HIV Env trimers (the well characterized BG505 and FVC_Env) covalently complexed to 2dCD4^S60C. Methods: Recombinant BG505 SOSIP.664 and FVC_Env SOSIP.664 were expressed in mammalian cells, purified, covalently coupled to 2dCD4^S60C and antigenically characterized for their interaction with HIV-1 bnAbs. The immunogenicity of BG505 SOSIP.664-2dCD4^S60C and FVC_Env SOSIP.664-2dCD4^S60C was investigated in New Zealand white rabbits and compared to unliganded FVC_Env and 2dCD4^S60C. Rabbit sera were tested for the presence of neutralizing antibodies against a panel of 17 pseudoviruses. Results: Both BG505 SOSIP.664-2dCD4^S60C and FVC_Env SOSIP.664-2dCD4^S60C elicited a potent, HIV-specific response in rabbits with antibodies having considerable potency and breadth (70.5% and 76%, respectively) when tested against a global panel of 17 pseudoviruses mainly composed of harder-to-neutralize multiple clade tier-2 pseudoviruses. Conclusion: BG505 SOSIP.664-2dCD4^S60C and FVC_EnvSOSIP.664-2dCD4^S60C are highly immunogenic and elicit potent, broadly neutralizing antibodies, the extent of which has never been reported previously for SOSIP.664 trimers. Adding to our previous results, the ability to consistently elicit these types of potent, cross-neutralizing antibody responses is dependent on novel epitopes exposed following the covalent binding of Env (independent of sequence and conformation) to 2dCD4^S60C. These findings justify further investment into research exploring modified open, CD4-bound Env conformations as novel vaccine immunogens.

Novel Antiretroviral Therapeutic Strategies for HIV

When the first cases of HIV infection appeared in the 1980s, AIDS was a deadly disease without any therapeutic alternatives. Currently, there is still no cure for most cases mainly due to the multiple tissues that act as a reservoir for this virus besides the high viral mutagenesis that leads to an antiretroviral drug resistance. Throughout the years, multiple drugs with specific mechanisms of action on distinct targets have been approved. In this review, the most recent phase III clinical studies and other research therapies as advanced antiretroviral nanodelivery systems will be here discussed. Although the combined antiretroviral therapy is effective in reducing viral loading to undetectable levels, it also presents some disadvantages, such as usual side effects, high frequency of administration, and the possibility of drug resistance. Therefore, several new drugs, delivery systems, and vaccines have been tested in pre-clinical and clinical trials. Regarding drug delivery, an attempt to change the route of administration of some conventional antiretrovirals has proven to be successful and surpassed some issues related to patient compliance. Nanotechnology has brought a new approach to overcoming certain obstacles of formulation design including drug solubility and biodistribution. Overall, the encapsulation of antiretroviral drugs into nanosystems has shown improved drug release and pharmacokinetic profile.

CNS Neurotoxicity of Antiretrovirals

The development of novel antiretroviral treatments has led to a significant turning point in the fight against HIV. Although therapy leads to virologic suppression and prolonged life expectancies, HIV-associated neurocognitive disorder (HAND) remains prevalent. While various hypotheses have been proposed to explain this phenomenon, a growing body of literature explores the neurotoxic effects of antiretroviral therapy. Research to date brings into question the potential role of such medications in neurocognitive and neuropsychiatric impairment seen in HIV-positive patients. This review highlights recent findings and controversies in cellular, molecular, and clinical neurotoxicity of antiretrovirals. It explores the pathogenesis of such toxicity and relates it to clinical manifestations in each medication class. The concept of accelerated aging in persons living with HIV (PLWH) as well as potential treatments for HAND are also discussed. Ultimately, this article hopes to educate clinicians and basic scientists about the neurotoxic effects of antiretrovirals and spur future scientific investigation into this important topic. Graphical Abstract.

Common Nodes of Virus-Host Interaction Revealed Through an Integrated Network Analysis

Viruses are one of the major causes of acute and chronic infectious diseases and thus a major contributor to the global burden of disease. Several studies have shown how viruses have evolved to hijack basic cellular pathways and evade innate immune response by modulating key host factors and signaling pathways. A collective view of these multiple studies could advance our understanding of virus-host interactions and provide new therapeutic perspectives for the treatment of viral diseases. Here, we performed an integrative meta-analysis to elucidate the 17 different host-virus interactomes. Network and bioinformatics analyses showed how viruses with small genomes efficiently achieve the maximal effect by targeting multifunctional and highly connected host proteins with a high occurrence of disordered regions. We also identified the core cellular process subnetworks that are targeted by all the viruses. Integration with functional RNA interference (RNAi) datasets showed that a large proportion of the targets are required for viral replication. Furthermore, we performed an interactome-informed drug re-purposing screen and identified novel activities for broad-spectrum antiviral agents against hepatitis C virus and human metapneumovirus. Altogether, these orthogonal datasets could serve as a platform for hypothesis generation and follow-up studies to broaden our understanding of the viral evasion landscape.

Ibalizumab

PURPOSE OF REVIEW

Antiretroviral options for patients infected with multiclass resistant HIV-1 warrant the development of new agents with unique mechanisms of action and modes of delivery. Here we review one such agent, ibalizumab, a parenteral CD4 postattachment inhibitor that has demonstrated efficacy as part of combination antiretroviral therapy in the treatment of HIV-1.

RECENT FINDINGS

In a phase III clinical trial in HIV-infected participants with multiclass antiretroviral drug resistance, the intravenous administration of ibalizumab led to declines in plasma HIV-1 RNA more than 0.5 log in 83% of participants at 1 week. An optimized background antiretroviral regimen was then added, and plasma HIV-1 RNA became less than 50 copies/ml in 43% of participants at 24 weeks. Adverse effects of ibalizumab were uncommon and generally low grade. Ibalizumab was approved by the US Food and Drug Administration on March 16, 2018, under the trade name Trogarzo.

SUMMARY

Ibalizumab has demonstrated both safety and efficacy in the treatment of HIV-1 infection. Its primary use will be in the setting of multidrug resistant virus as part of combination antiretroviral therapy. Further enhancements of ibalizumab to prolong its clearance and broaden its activity are in development.

Investigational drugs in HIV: Pros and cons of entry and fusion inhibitors (Review)

Despite the profound changes and improvements reached in the field of HIV treatment, tolerability and adherence to highly active antiretroviral therapy remains a challenge. Furthermore, multi-experienced patients could take advantage of drugs with different mechanisms of action to combat the spread of resistance to actual therapy. For these reasons identification of new HIV drugs is crucial. Among all the molecules that at present are under investigation, entry and fusion inhibitors pose an interesting class owing to their peculiar characteristics, including prevention of entry of the virus into the human cells. In this study, we reviewed articles, clinical trials, and conference communications about all the drugs under investigation belonging to the class of entry and fusion inhibitors that are at least in phase I clinical trials.

Where are we with injectables against HIV infection and what are the remaining challenges?

INTRODUCTION

Drug adherence has been a recurring issue in the field of HIV treatment, and low treatment adherence is typically associated with emergence of drug resistance, treatment failure and increased risks of transmission. Injectable antiretroviral drugs offer a unique opportunity to counter this issue for the treatment of HIV-positive individuals. In addition, injectables offer a remarkable opportunity to reduce new HIV infections, if applied in the context of both treatment-as-prevention and pre-exposure prophylaxis. Areas covered: Researchers and drug companies are developing long-acting agents that possess long biological half-life and excellent pharmacokinetic profiles that can be administered intramuscularly, intravenously, or subcutaneously. These long-acting injectables are categorized as drugs that target different steps of HIV replication cycle or monoclonal antibodies that target HIV entry. Expert commentary: Injectables against HIV have the potential to revolutionize the fight against HIV by facilitating both treatment and prevention in a wide variety of clinical settings. Several challenges remain including the identification of potent two-drug combinations of drugs that can be formulated as injectables, and thorough drug-drug interaction studies with a broad variety of medications. Finally we believe that the healthcare benefits of injectables will require regulatory changes to allow self-injection before they reach their full potential.

Ibalizumab Targeting CD4 Receptors, An Emerging Molecule in HIV Therapy

The HIV infection is responsible for the most devastating global pandemic of the last century. More than 39 million people have died of HIV/AIDS since 1981. The development of the antiretroviral (ARV) treatment begins with the discovery of zidovudine a nucleoside reverse transcriptase inhibitor. This breakthrough was followed by other ARV drug classes and representatives. Presently, HIV treatment employs 27 ARV representatives belonging to five different classes. Despite the proven benefits of ARV treatment and its long-term control of the HIV infection, there is an increasing concern about the numerous adverse effects and resistance to current ARV drugs. Therefore, the new HIV treatment strategies focus on the development of new ARV agents with a high genetic barrier to resistance and low toxicity. Monoclonal antibodies (MAbs) belong to a new drug class with encouraging results in the treatment of cancer, autoimmune disorders and most recently against HIV infection. The advantages of using MAbs for HIV treatment are related to their antiviral effect, lack of toxicity, good resistance profile, additional synergy with other ARV drug classes and ability to restore CD4 T-cell responses. The current article is a short summary of ibalizumab, an anti-CD4 monoclonal antibody that interferes with HIV viral entry. Current studies on ibalizumab have underlined its antiviral potential, minimal adverse effects, and lack of crossed resistance with other ARV agents thus supporting its further therapeutic use in multidrug resistant HIV-infected patients.

Discovery and Characterization of a Novel CD4-Binding Adnectin with Potent Anti-HIV Activity

A novel fibronectin-based protein (Adnectin) HIV-1 inhibitor was generated using in vitro selection. This inhibitor binds to human CD4 with a high affinity (3.9 nM) and inhibits viral entry at a step after CD4 engagement and preceding membrane fusion. The progenitor sequence of this novel inhibitor was selected from a library of trillions of Adnectin variants using mRNA display and then further optimized for improved antiviral and physical properties. The final optimized inhibitor exhibited full potency against a panel of 124 envelope (gp160) proteins spanning 11 subtypes, indicating broad-spectrum activity. Resistance profiling studies showed that this inhibitor required 30 passages (151 days) in culture to acquire sufficient resistance to result in viral titer breakthrough. Resistance mapped to the loss of multiple potential N-linked glycosylation sites in gp120, suggesting that inhibition is due to steric hindrance of CD4-binding-induced conformational changes.

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.

Antiviral Therapy by HIV-1 Broadly Neutralizing and Inhibitory Antibodies

Human immunodeficiency virus type 1 (HIV-1) infection causes acquired immune deficiency syndrome (AIDS), a global epidemic for more than three decades. HIV-1 replication is primarily controlled through antiretroviral therapy (ART) but this treatment does not cure HIV-1 infection. Furthermore, there is increasing viral resistance to ART, and side effects associated with long-term therapy. Consequently, there is a need of alternative candidates for HIV-1 prevention and therapy. Recent advances have discovered multiple broadly neutralizing antibodies against HIV-1. In this review, we describe the key epitopes on the HIV-1 Env protein and the reciprocal broadly neutralizing antibodies, and discuss the ongoing clinical trials of broadly neutralizing and inhibitory antibody therapy as well as antibody combinations, bispecific antibodies, and methods that improve therapeutic efficacy by combining broadly neutralizing antibodies (bNAbs) with latency reversing agents. Compared with ART, HIV-1 therapeutics that incorporate these broadly neutralizing and inhibitory antibodies offer the advantage of decreasing virus load and clearing infected cells, which is a promising prospect in HIV-1 prevention and treatment.

New Antiretroviral Treatment for HIV

The Joint United Nations Programme on HIV/AIDS (UNAIDS) has set the global goal of ending the AIDS world epidemic by 2030. In order to end this epidemic they have established a 90-90-90 goal to be achieved by 2020, which may be problematic, especially in low- and middle-income countries. This goal includes 90% of individuals with HIV globally being diagnosed, on treatment, and virologically suppressed. Based on global estimates from 2014-2015, approximately 36.9 million individuals are living with HIV. Of those, 53% have been diagnosed with HIV, 41% are on antiretroviral therapy (ART), and 32% have viral suppression with <1000 copies/ml. Comprehensive approaches are needed to improve the number of people living with HIV (PLWH) who are diagnosed, linked, and engaged in care. Once PLWH are retained in care, treatment is key to both HIV prevention and transmission. The development and advancement of new ART is necessary to assist in reaching these goals by improving safety profiles, decreasing pill burden, improving quality of life and life expectancy, and creating new mechanisms to overcome resistance. The focus of this review is to highlight and review data for antiretroviral agents recently added to the market as well as discuss agents in various stages of development (new formulations and mechanisms of action).

History and Practice: Antibodies in Infectious Diseases

Antibodies and passive antibody therapy in the treatment of infectious diseases is the story of a treatment concept which dates back more than 120 years, to the 1890s, when the use of serum from immunized animals provided the first effective treatment options against infections with Clostridium tetani and Corynebacterium diphtheriae. However, after the discovery of penicillin by Fleming in 1928, and the subsequent introduction of the much cheaper and safer antibiotics in the 1930s, serum therapy was largely abandoned. However, the broad and general use of antibiotics in human and veterinary medicine has resulted in the development of multi-resistant strains of bacteria with limited to no response to existing treatments and the need for alternative treatment options. The combined specificity and flexibility of antibody-based treatments makes them very valuable tools for designing specific antibody treatments to infectious agents. These attributes have already caused a revolution in new antibody-based treatments in oncology and inflammatory diseases, with many approved products. However, only one monoclonal antibody, palivizumab, for the prevention and treatment of respiratory syncytial virus, is approved for infectious diseases. The high cost of monoclonal antibody therapies, the need for parallel development of diagnostics, and the relatively small markets are major barriers for their development in the presence of cheap antibiotics. It is time to take a new and revised look into the future to find appropriate niches in infectious diseases where new antibody-based treatments or combinations with existing antibiotics, could prove their value and serve as stepping stones for broader acceptance of the potential for and value of these treatments.

Long-acting antiviral agents for HIV treatment

PURPOSE OF REVIEW

Long-acting antiretroviral (ARV) agents are currently under development for the treatment of chronic HIV infection. This review focuses on data recently produced on injectable ARVs for patients living with HIV/AIDS and on the patients' perspectives on the use of these agents.

RECENT FINDINGS

Crystalline nanoparticle formulations of the nonnucleoside reverse transcriptase inhibitor rilpivirine (TMC278) and of the HIV-1 integrase strand transfer inhibitor cabotegravir (GSK1265744) have progressed into phase II clinical trials as injectable maintenance therapy for patients living with HIV/AIDS with an undetectable viral load.

SUMMARY

Phase II studies evaluating the coadministration of rilpivirine and cabotegravir intramuscularly to HIV-infected individuals with an undetectable viral load are currently underway. Rilpivirine and cabotegravir are characterized by different mechanisms of action against HIV and a favorable drug interaction profile, providing a rationale for coadministration. The high potency and low daily dosing requirements of oral cabotegravir and rilpivirine facilitate long-acting formulation development. Intramuscular dosing is preceded by an oral lead-in phase to assess safety and tolerability in individual participants. In addition to assessing the safety of injectable therapies in ongoing studies, it will be important to evaluate whether differences in drug adherence between injectable and oral therapies lead to different virologic outcomes, including rates of virologic failure and the emergence of resistance. Long-acting formulations may be associated with challenges, such as the management of adverse effects with persistent drug concentrations and the risk of virologic resistance, as drug concentrations decline following discontinuation.

Expanding the antimalarial toolkit: Targeting host-parasite interactions

Recent successes in malaria control are threatened by drug-resistant Plasmodium parasites and insecticide-resistant Anopheles mosquitoes, and first generation vaccines offer only partial protection. New research approaches have highlighted host as well as parasite molecules or pathways that could be targeted for interventions. In this study, we discuss host–parasite interactions at the different stages of the Plasmodium life cycle within the mammalian host and the potential for therapeutics that prevent parasite migration, invasion, intracellular growth, or egress from host cells, as well as parasite-induced pathology.

Basigin is a druggable target for host-oriented antimalarial interventions

Zenonos et al. report the development of a new therapeutic for P. falciparum malaria. A recombinant chimeric antibody targeting basigin—a receptor essential for erythrocyte invasion—inhibited parasite invasion and rapidly cleared an established blood-stage infection in vivo.

Plasmodium falciparum is the parasite responsible for the most lethal form of malaria, an infectious disease that causes a large proportion of childhood deaths and poses a significant barrier to socioeconomic development in many countries. Although antimalarial drugs exist, the repeated emergence and spread of drug-resistant parasites limit their useful lifespan. An alternative strategy that could limit the evolution of drug-resistant parasites is to target host factors that are essential and universally required for parasite growth. Host-targeted therapeutics have been successfully applied in other infectious diseases but have never been attempted for malaria. Here, we report the development of a recombinant chimeric antibody (Ab-1) against basigin, an erythrocyte receptor necessary for parasite invasion as a putative antimalarial therapeutic. Ab-1 inhibited the PfRH5-basigin interaction and potently blocked erythrocyte invasion by all parasite strains tested. Importantly, Ab-1 rapidly cleared an established P. falciparum blood-stage infection with no overt toxicity in an in vivo infection model. Collectively, our data demonstrate that antibodies or other therapeutics targeting host basigin could be an effective treatment for patients infected with multi-drug resistant P. falciparum.

Intra-spike crosslinking overcomes antibody evasion by HIV-1

Antibodies developed during HIV-1 infection lose efficacy as the viral spike mutates. We postulated that anti-HIV-1 antibodies primarily bind monovalently because HIV's low spike density impedes bivalent binding through inter-spike crosslinking, and the spike structure prohibits bivalent binding through intra-spike crosslinking. Monovalent binding reduces avidity and potency, thus expanding the range of mutations permitting antibody evasion. To test this idea, we engineered antibody-based molecules capable of bivalent binding through intra-spike crosslinking. We used DNA as a "molecular ruler" to measure intra-epitope distances on virion-bound spikes and construct intra-spike crosslinking molecules. Optimal bivalent reagents exhibited up to 2.5 orders of magnitude increased potency (>100-fold average increases across virus panels) and identified conformational states of virion-bound spikes. The demonstration that intra-spike crosslinking lowers the concentration of antibodies required for neutralization supports the hypothesis that low spike densities facilitate antibody evasion and the use of molecules capable of intra-spike crosslinking for therapy or passive protection.

Neutralization properties of simian immunodeficiency viruses infecting chimpanzees and gorillas

Broadly cross-reactive neutralizing antibodies (bNabs) represent powerful tools to combat human immunodeficiency virus type 1 (HIV-1) infection. Here, we examined whether HIV-1-specific bNabs are capable of cross-neutralizing distantly related simian immunodeficiency viruses (SIVs) infecting central (Pan troglodytes troglodytes) (SIVcpzPtt) and eastern (Pan troglodytes schweinfurthii) (SIVcpzPts) chimpanzees (n = 11) as well as western gorillas (Gorilla gorilla gorilla) (SIVgor) (n = 1). We found that bNabs directed against the CD4 binding site (n = 10), peptidoglycans at the base of variable loop 3 (V3) (n = 5), and epitopes at the interface of surface (gp120) and membrane-bound (gp41) envelope glycoproteins (n = 5) failed to neutralize SIVcpz and SIVgor strains. In addition, apex V2-directed bNabs (n = 3) as well as llama-derived (heavy chain only) antibodies (n = 6) recognizing both the CD4 binding site and gp41 epitopes were either completely inactive or neutralized only a fraction of SIVcpzPtt strains. In contrast, one antibody targeting the membrane-proximal external region (MPER) of gp41 (10E8), functional CD4 and CCR5 receptor mimetics (eCD4-Ig, eCD4-Ig(mim2), CD4-218.3-E51, and CD4-218.3-E51-mim2), as well as mono- and bispecific anti-human CD4 (iMab and LM52) and CCR5 (PRO140, PRO140-10E8) receptor antibodies neutralized >90% of SIVcpz and SIVgor strains with low-nanomolar (0.13 to 8.4 nM) potency. Importantly, the latter antibodies blocked virus entry not only in TZM-bl cells but also in Cf2Th cells expressing chimpanzee CD4 and CCR5 and neutralized SIVcpz in chimpanzee CD4(+) T cells, with 50% inhibitory concentrations (IC50s) ranging from 3.6 to 40.5 nM. These findings provide new insight into the protective capacity of anti-HIV-1 bNabs and identify candidates for further development to combat SIVcpz infection.

IMPORTANCE

SIVcpz is widespread in wild-living chimpanzees and can cause AIDS-like immunopathology and clinical disease. HIV-1 infection of humans can be controlled by antiretroviral therapy; however, treatment of wild-living African apes with current drug regimens is not feasible. Nonetheless, it may be possible to curb the spread of SIVcpz in select ape communities using vectored immunoprophylaxis and/or therapy. Here, we show that antibodies and antibody-like inhibitors developed to combat HIV-1 infection in humans are capable of neutralizing genetically diverse SIVcpz and SIVgor strains with considerable breadth and potency, including in primary chimpanzee CD4(+) T cells. These reagents provide an important first step toward translating intervention strategies currently developed to treat and prevent AIDS in humans to SIV-infected apes.

Passive Immunization

Whereas active immunity refers to the process of exposing the individual to an antigen to generate an adaptive immune response, passive immunity refers to the transfer of antibodies from one individual to another. Passive immunity provides immediate but short-lived protection, lasting several weeks up to 3 or 4 months. Passive immunity can occur naturally, when maternal antibodies are transferred to the fetus through the placenta or from breast milk to the gut of the infant. It can also be produced artificially, when antibody preparations derived from sera or secretions of immunized donors or, more recently, different antibody producing platforms are transferred via systemic or mucosal route to nonimmune individuals. Passive immunization has recently become an attractive approach because of the emergence of new and drug-resistant microorganisms, diseases that are unresponsive to drug therapy and individuals with an impaired immune system who are unable to respond to conventional vaccines. This chapter addresses the contributions of natural and artificial acquired passive immunity in understanding the concept of passive immunization. We will mainly focus on administration of antibodies for protection against various infectious agents entering through mucosal surfaces.

Nef neutralizes the ability of exosomes from CD4+ T cells to act as decoys during HIV-1 infection

Nef is an HIV-1 accessory protein that promotes viral replication and pathogenesis. A key function of Nef is to ensure sustained depletion of CD4 and MHC-I molecules in infected cells by inducing targeting of these proteins to multivesicular bodies (MVBs), and ultimately to lysosomes for degradation. Nef also affects cellular secretory routes promoting its own secretion via exosomes. To better understand the effects of Nef on the exocytic pathway, we investigated whether this viral factor modifies the composition of exosomes released by T lymphocytes. We showed that both CD4 and MHC-I molecules are secreted in exosomes from T cells and that the expression of Nef reduces the amount of these proteins in exosomes. To investigate the functional role for this novel activity of Nef, we performed in vitro HIV-1 infection assays in the presence of distinct populations of exosomes. We demonstrated that exosomes released by CD4⁺ T cells, but not CD4⁻ T cells, efficiently inhibit HIV-1 infection in vitro. Because CD4 is the main receptor for HIV-1 infection, these results suggest that CD4 molecules displayed on the surface of exosomes can bind to envelope proteins of HIV-1 hindering virus interaction with target cells and infection. Importantly, CD4-depleted exosomes released by CD4⁺ T cells expressing Nef have a reduced capacity to inhibit HIV-1 infection in vitro. These results provide evidence that Nef promotes HIV-1 infection by reducing the expression of CD4 in exosomes from infected cells, besides the original role of Nef in reducing the CD4 levels at the cell surface.

Insight into the modified Ibalizumab-human CD4 receptor interactions: using a computational binding free energy approach

Antibody drugs are very useful tools for the treatment of many chronic diseases. Recently, however, patients and doctors have encountered the problem of drug resistance. How to improve the affinity of antibody drugs has therefore become a pressing issue. Ibalizumab is a humanized monoclonal antibody that binds human CD4, the primary receptor for human immunodeficiency virus type 1. This study investigates the mutation residues of the complementarity determining regions of Ibalizumab. We propose using the wild and mutations of Ibalizumab-human CD4 receptor complex structures, molecular dynamics techniques, alanine-scanning mutagenesis calculations and solvated interaction energies methods to predict the binding free energy of the Ibalizumab-human CD4 receptor complex structures. This work found that revealed three key positions (31th, 32th and 33th in HCDR-1) of the residues may play an important role in Ibalizumab-human CD4 receptor complex interactions. Therefore, bioengineering substitutions of the three key positions and increasing number of intermolecular interactions (HCDR-1 of Ibalizumab/human CD4 receptor) might improve the binding affinities of this complex structure.

Functions of Antibodies

Antibodies can impact pathogens in the presence or in the absence of effector cells or effector molecules such as complement, and experiments can often sort out with precision the mechanisms by which an antibody inhibits a pathogen in vitro . In addition, in vivo models, particularly those engineered to knock in or knock out effector cells or effector molecules, are excellent tools for understanding antibody functions. However, it is highly likely that multiple antibody functions occur simultaneously or sequentially in the presence of an infecting organism in vivo . The most critical incentive for measuring antibody functions is to provide a basis for vaccine development and for the development of therapeutic antibodies. In this respect, some functions, such as virus neutralization, serve to inhibit the acquisition of a pathogen or limit its pathogenesis. However, antibodies can also enhance replication or contribute to pathogenesis. This review emphasizes those antibody functions that are potentially beneficial to the host. In addition, this review will focus on the effects of antibodies on organisms themselves, rather than on the toxins the organisms may produce.

Long-acting injectable antiretrovirals for HIV treatment and prevention

PURPOSE OF REVIEW

Long-acting antiretroviral (ARV) drugs may improve adherence to therapy and extend opportunities for therapeutic or prophylactic intervention to underserved patient populations. This review focuses on recent advances in the development of small molecule long-acting injectable ARV agents.

RECENT FINDINGS

The need for combination ART and physicochemical and dosing limitations of current ARV drugs impede attempts to redevelop them as long-acting injectable formulations. However, the intrinsic properties of rilpivirine, a nonnucleoside reverse transcriptase inhibitor, and GSK1265744, an HIV-1 integrase strand transfer inhibitor, have enabled crystalline nanoparticle formulations to progress to clinical trials.

SUMMARY

Investigational long-acting injectable nanoformulations of rilpivirine and GSK1265744 are clinical-stage development candidates. Complementary pharmacologic properties of both agents - different mechanisms of action, resistance profiles, metabolic pathways, lack of drug interactions and low daily oral doses - offer the potential for combination use. Phase I studies of the pharmacokinetics and safety of each long-acting formulation alone and in combination indicate that a monthly dosing regimen is possible for HIV treatment. An ongoing phase IIb trial of oral GSK1265744 and oral rilpivirine is evaluating this two-drug regimen for maintenance of virologic suppression; results will inform future studies using the injectable formulations. Additional preclinical and clinical studies indicate a potential use of each agent for HIV pre-exposure prophylaxis.

Activity of the HIV-1 attachment inhibitor BMS-626529, the active component of the prodrug BMS-663068, against CD4-independent viruses and HIV-1 envelopes resistant to other entry inhibitors

BMS-626529 is a novel small-molecule HIV-1 attachment inhibitor active against both CCR5- and CXCR4-tropic viruses. BMS-626529 functions by preventing gp120 from binding to CD4. A prodrug of this compound, BMS-663068, is currently in clinical development. As a theoretical resistance pathway to BMS-663068 could be the development of a CD4-independent phenotype, we examined the activity of BMS-626529 against CD4-independent viruses and investigated whether resistance to BMS-626529 could be associated with a CD4-independent phenotype. Finally, we evaluated whether cross-resistance exists between BMS-626529 and other HIV-1 entry inhibitors. Two laboratory-derived envelopes with a CD4-independent phenotype (one CXCR4 tropic and one CCR5 tropic), five envelopes from clinical isolates with preexisting BMS-626529 resistance, and several site-specific mutant BMS-626529-resistant envelopes were examined for their dependence on CD4 for infectivity or susceptibility to BMS-626529. Viruses resistant to other entry inhibitors (enfuvirtide, maraviroc, and ibalizumab) were also examined for susceptibility to BMS-626529. Both CD4-independent laboratory isolates retained sensitivity to BMS-626529 in CD4(-) cells, while HIV-1 envelopes from viruses resistant to BMS-626529 exhibited no evidence of a CD4-independent phenotype. BMS-626529 also exhibited inhibitory activity against ibalizumab- and enfuvirtide-resistant envelopes. While there appeared to be some association between maraviroc resistance and reduced susceptibility to BMS-626529, an absolute correlation cannot be presumed, since some CCR5-tropic maraviroc-resistant envelopes remained sensitive to BMS-626529. Clinical use of the prodrug BMS-663068 is unlikely to promote resistance via generation of CD4-independent virus. No cross-resistance between BMS-626529 and other HIV entry inhibitors was observed, which could allow for sequential or concurrent use with different classes of entry inhibitors.

Antibody conjugation approach enhances breadth and potency of neutralization of anti-HIV-1 antibodies and CD4-IgG

Broadly neutralizing antibodies PG9 and PG16 effectively neutralize 70 to 80% of circulating HIV-1 isolates. In this study, the neutralization abilities of PG9 and PG16 were further enhanced by bioconjugation with aplaviroc, a small-molecule inhibitor of virus entry into host cells. A novel air-stable diazonium hexafluorophosphate reagent that allows for rapid, tyrosine-selective functionalization of proteins and antibodies under mild conditions was used to prepare a series of aplaviroc-conjugated antibodies, including b12, 2G12, PG9, PG16, and CD4-IgG. The conjugated antibodies blocked HIV-1 entry through two mechanisms: by binding to the virus itself and by blocking the CCR5 receptor on host cells. Chemical modification did not significantly alter the potency of the parent antibodies against nonresistant HIV-1 strains. Conjugation did not alter the pharmacokinetics of a model IgG in blood. The PG9-aplaviroc conjugate was tested against a panel of 117 HIV-1 strains and was found to neutralize 100% of the viruses. PG9-aplaviroc conjugate IC50s were lower than those of PG9 in neutralization studies of 36 of the 117 HIV-1 strains. These results support this new approach to bispecific antibodies and offer a potential new strategy for combining HIV-1 therapies.

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.

Treating HIV-1 Infection: What Might the Future Hold?

Advances in antiretroviral combination therapy lasting the past two decades have transformed HIV-1 infection from a fatal disease into a chronic medical condition that in many cases does not compromise life quality. There are 25 different antiretroviral agents available currently, allowing for patient-centered, individualized management of HIV-1 infection, and ongoing progress in HIV-1 virology and antiretroviral pharmacology is likely to expand treatment options further in the future. Nevertheless, antiretroviral therapy continues to have limitations, including insufficient immunological reconstitution, selection of drug resistance, ongoing abnormal immune activation despite effective suppression of HIV-1 viremia, and the inability to target latently infected cells that are responsible for long-term viral persistence. Owing to these shortcomings, the theoretical ability of antiretroviral therapy to extend life expectancy to normal levels is not realized in many cases. Strategies to address these limitations are a matter of active ongoing research and will be summarized in this article.

HIV inhibition by CD4 and CCR5-derived glycopeptides

We have previously proposed a method by which natural antibodies can be redirected toward a known pathogen. We could show that CD4-derived peptides coupled to the galα1,3gal sugar moiety, a specificity held by natural antibodies, were able to neutralize HIV. Importantly, the antibody-peptide-antigen complexes activated the innate immune system through the Fc parts of the natural antibody. We now tested these peptides for their effectiveness on primary isolates and included sequence variations to increase their binding affinity. In addition, we evaluated three new CCR5-derived peptides. All peptides were tested for neutralization of six primary HIV-1 isolates. When testing three of the previously published glycopeptides we found that 10 to 100 times higher concentrations were needed to achieve the same neutralization of primary isolates. We found that the modifications of the CD4 glycopeptides modestly improved the neutralization of HIV-1. The modified CD4 and the CCR5 glycopeptides neutralized HIV-1 strains from different patients and of different subtypes. Notably, the combination of CD4 and CCR5 glycopeptides enhanced the neutralization potential as compared to the single peptides. A combination of CD4- and CCR5-galα1,3gal-linked peptides redirected natural antibodies to neutralize primary isolates of HIV-1, although less efficiently than laboratory-adapted strains. This might represent a new and valuable tool to block the entry of HIV into susceptible cells.

Antiretroviral drugs: critical issues and recent advances

Human immunodeficiency virus (HIV) infection is now recognized as a chronic illness. Although the success of highly active antiretroviral therapy is beyond question, several issues still persist. Since the drugs cannot eradicate the virus, cure is not yet possible, and patients have to maintain a lifelong adherence with the risk of toxic effects, drug-drug interactions and drug resistance. A clear understanding of the viral replication and its interaction with host cell factors has led to the development of a large number of effective antiretroviral drugs (ARVs). New drugs in the existing class such as apricitabine, elvucitabine and etravirine have shown promising results against HIV isolates resistant to first line drugs. These drugs have offered a new choice for patients with drug resistant disease. However, the impact of their long term use on safety is yet to be assessed. Novel drugs with unique mechanism of action such as CD4 receptor attachment inhibitors, maturation inhibitors, pharmacokinetic enhancers, capsid assembly inhibitors and lens epithelium derived growth factor inhibitors are still under development. Currently, ARVs, especially tenofovir and emtricitabine, are also being evaluated for prevention of sexual transmission of HIV-1. The initial results of an HIV prevention trial network are encouraging and have recommended the use of ARVs for pre-exposure prophylaxis. Thus, ARVs form the key component of HIV prevention and treatment strategy. This article discusses the challenges associated with HIV-1 treatment and updates several major advances in the development of ARVs.

Monoclonal antibodies in infectious diseases: clinical pipeline in 2011

Of the more than 20 monoclonal antibodies (mAbs) generated to combat infectious diseases (ID) that are in clinical development in 2011, most are in phase 1 or 2 and are directed against either viruses or bacterial toxins. Several high-profile anti-ID mAbs have recently failed in clinical trials. Despite the advancement in recombinant engineering technologies, anti-ID mAbs have yet to deliver on their promise as "magic bullets," especially against nosocomial infections. A paradigm shift in favor of developing mAb combinations, which act synergistically with each other or with small molecule drugs, may be required to move the field forward.

Application and interpretation of free and total drug measurements in the development of biologics

Ligand-binding assays are used to determine concentration levels of biopharmaceuticals in biological matrices. The whole molecule does not serve as a basis for quantification, but subregions are captured and detected by specific binding critical reagents that have been produced for the sole purpose of bioanalysis. An assay can be designed to measure the free or the total analyte. Depending on the format of the assay, different observations and interpretations could be deemed. In the case studies presented in this article, the same serum samples were subjected to analysis in parallel by two different assay formats. In three out of the four cases presented, the results and the data interpretation were remarkably different. Therefore, it is essential for the bioanalyst to communicate to other stake-holders, such as toxicologists and pharmacokineticists, what the assay detects and measures for plausible data interpretation and implication.

HIV envelope: challenges and opportunities for development of entry inhibitors

The HIV envelope proteins glycoprotein 120 (gp120) and glycoprotein 41 (gp41) play crucial roles in HIV entry, therefore they are of extreme interest in the development of novel therapeutics. Studies using diverse methods, including structural biology and mutagenesis, have resulted in a detailed model for envelope-mediated entry, which consists of multiple conformations, each a potential target for therapeutic intervention. In this review, the challenges, strategies and progress to date for developing novel entry inhibitors directed at disrupting HIV gp120 and gp41 function are discussed.