Potent, broad-spectrum inhibition of human immunodeficiency virus type 1 by the CCR5 monoclonal antibody PRO 140

Antiretroviral therapy and drug resistance in human immunodeficiency virus type 2 infection

One to two million people worldwide are infected with the human immunodeficiency virus type 2 (HIV-2), with highest prevalences in West African countries, but also present in Western Europe, Asia and North America. Compared to HIV-1, HIV-2 infection undergoes a longer asymptomatic phase and progresses to AIDS more slowly. In addition, HIV-2 shows lower transmission rates, probably due to its lower viremia in infected individuals. There is limited experience in the treatment of HIV-2 infection and several antiretroviral drugs used to fight HIV-1 are not effective against HIV-2. Effective drugs against HIV-2 include nucleoside analogue reverse transcriptase (RT) inhibitors (e.g. zidovudine, tenofovir, lamivudine, emtricitabine, abacavir, stavudine and didanosine), protease inhibitors (saquinavir, lopinavir and darunavir), and integrase inhibitors (raltegravir, elvitegravir and dolutegravir). Maraviroc, a CCR5 antagonist blocking coreceptor binding during HIV entry, is active in vitro against CCR5-tropic HIV-2 but more studies are needed to validate its use in therapeutic treatments against HIV-2 infection. HIV-2 strains are naturally resistant to a few antiretroviral drugs developed to suppress HIV-1 propagation such as nonnucleoside RT inhibitors, several protease inhibitors and the fusion inhibitor enfuvirtide. Resistance selection in HIV-2 appears to be faster than in HIV-1. In this scenario, the development of novel drugs specific for HIV-2 is an important priority. In this review, we discuss current anti-HIV-2 therapies and mutational pathways leading to drug resistance.

CXCR4-derived synthetic peptides inducing anti-HIV-1 antibodies

Despite almost 30 years since the identification of the human immunodeficiency virus type I (HIV-1), development of effective AIDS vaccines has been hindered by the high mutability of HIV-1. The HIV-1 co-receptors CCR5 and CXCR4 are genetically stable, but viral proteins may mutate rapidly during the course of infection. CXCR4 is a seven transmembrane G protein-coupled receptor, possessing an N-terminal region (NT) and three extracellular loops (ECL1-3). Previous studies have shown that the CXCR4-ED-derived peptides inhibit the entry of HIV-1 by interacting with gp120, an HIV-1 envelope glycoprotein. In the present study, antigenicity of CXCR4-derived peptides has been investigated and the anti-HIV-1 effects of induced antisera have been assessed. It was found that CXCR4-ED-derived antigen molecules immunize mice, showing that the linear peptides have higher antigenicity than the cyclic peptides. The L1- and L2-induced antisera inhibited the HIV-1 entry significantly, while anti-N1 antibodies have no inhibitory activity. This study produced promising examples for the design of AIDS vaccines which target the human protein and can overcome mutability of HIV-1.

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.

Clinical development of monoclonal antibody-based drugs in HIV and HCV diseases

Today there are many licensed antiviral drugs, but the emergence of drug resistant strains sometimes invalidates the effects of the current therapies used in the treatment of infectious diseases. Compared to conventional antiviral drugs, monoclonal antibodies (mAbs) used as pharmacological molecules have particular physical characteristics and modes of action, and, therefore, they should be considered as a distinct therapeutic class. Despite being historically validated, antibodies may represent a novel tool for combatting infectious diseases. The current high cost of mAbs' production, storage and administration (by injection only) and the consequent obstacles to development are outweighed by mAbs' clinical advantages. These are related to a low toxicity combined with high specificity and versatility, which allows a specific antibody to mediate various biological effects, ranging from the virus neutralization mechanisms to the modulation of immune responses.This review briefly summarizes the recent technological advances in the field of immunoglobulin research, and the current status of mAb-based drugs in clinical trials for HIV and HCV diseases. For each clinical trial the available data are reported and the emerging conceptual problems of the employed mAbs are highlighted.This overview helps to give a clear picture of the efficacy and challenges of the mAbs in the field of these two infectious diseases which have such a global impact.

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.

Orthobunyavirus entry into neurons and other mammalian cells occurs via clathrin-mediated endocytosis and requires trafficking into early endosomes

La Crosse virus (LACV) is a leading cause of pediatric encephalitis and aseptic meningitis in the midwestern and southern United States, where it is considered an emerging human pathogen. No specific therapies or vaccines are available for LACV or any other orthobunyaviruses. Inhibition of LACV entry into cells is a potential target for therapeutic intervention, but this approach is limited by our current knowledge of the entry process. Here, we determined that clathrin-mediated endocytosis is the primary mechanism of orthobunyavirus entry and identified key cellular factors in this process. First, we demonstrated that LACV colocalized with clathrin shortly after infection in HeLa cells; we then confirmed the functional requirement of dynamin- and clathrin-mediated endocytosis for orthobunyavirus entry using several independent assays and, importantly, extended these findings to primary neuronal cultures. We also determined that macropinocytosis and caveolar endocytosis, both established routes of virus entry, are not critical for cellular entry of LACV. Moreover, we demonstrated that LACV infection is dependent on Rab5, which plays an important regulatory role in early endosomes, but not on Rab7, which is associated with late endosomes. These findings provide the first description of bunyavirus entry into cells of the central nervous system, where infection can cause severe neurological disease, and will aid in the design and development of antivirals and therapeutics that may be useful in the treatment of LACV and, more broadly, arboviral infections of the central nervous system.

Monoclonal antibody-based candidate therapeutics against HIV type 1

Treatment of HIV-1 infection has been highly successful with small molecule drugs. However, resistance still develops. In addition, long-term use can lead to toxicity with unpredictable effects on health. Finally, current drugs do not lead to HIV-1 eradication. The presence of the virus leads to chronic inflammation, which can result in increased morbidity and mortality after prolonged periods of infection. Monoclonal antibodies (mAbs) have been highly successful during the past two decades for therapy of many diseases, primarily cancers and immune disorders. They are relatively safe, especially human mAbs that have evolved in humans at high concentrations to fight diseases and long-term use may not lead to toxicities. Several broadly neutralizing mAbs (bnmAbs) against HIV-1 can protect animals but are not effective when used for therapy of an established infection. We have hypothesized that HIV-1 has evolved strategies to effectively escape neutralization by full-size antibodies in natural infections but not by smaller antibody fragments. Therefore, a promising direction of research is to discover and exploit antibody fragments as potential candidate therapeutics against HIV-1. Here we review several bnmAbs and engineered antibody domains (eAds), their in vitro and in vivo antiviral efficacy, mechanisms used by HIV-1 to escape them, and strategies that could be effective to develop more powerful mAb-based HIV-1 therapeutics.

Novel approaches to inhibit HIV entry

Human Immunodeficiency Virus (HIV) entry into target cells is a multi-step process involving binding of the viral glycoprotein, Env, to its receptor CD4 and a coreceptor-either CCR5 or CXCR4. Understanding the means by which HIV enters cells has led to the identification of genetic polymorphisms, such as the 32 base-pair deletion in the ccr5 gene (ccr5∆32) that confers resistance to infection in homozygous individuals, and has also resulted in the development of entry inhibitors-small molecule antagonists that block infection at the entry step. The recent demonstration of long-term control of HIV infection in a leukemic patient following a hematopoietic stem cell transplant using cells from a ccr5∆32 homozygous donor highlights the important role of the HIV entry in maintaining an established infection and has led to a number of attempts to treat HIV infection by genetically modifying the ccr5 gene. In this review, we describe the HIV entry process and provide an overview of the different classes of approved HIV entry inhibitors while highlighting novel genetic strategies aimed at blocking HIV infection at the level of entry.

Molecular mechanisms of HIV entry

Human immunodeficiency virus (HIV) entry is a complex and intricate process that facilitates delivery of the viral genome to the host cell. The only viral surface protein, Envelope (Env), is composed of a trimer of gp120 and gp41 heterodimers. It is essentially a fusion machine cloaked in a shroud of carbohydrate structures and variable loops of amino acids that enable it to evade the humoral immune response. For entry to occur gp120 sequentially engages the host protein CD4 and then one of two chemokine coreceptors, either CCR5 or CXCR4. CD4 binding facilitates exposure and formation of the coreceptor-binding site, and coreceptor binding then triggers the membrane fusion machinery in the gp41 subunit. Our understanding of HIV entry has led to the development of successful small molecule inhibitors for the clinical treatment of HIV infection as well as insights into viral tropism and pathogenesis.

Potential use of G protein-coupled receptor-blocking monoclonal antibodies as therapeutic agents for cancers

The therapeutic use of monoclonal antibodies (mAbs) is the fastest growing area of pharmaceutical development and has enjoyed significant clinical success since approval of the first mAb drug in1984. However, despite significant effort, there are still no approved therapeutic mAbs directed against the largest and most attractive family of drug targets: G protein-coupled receptors (GPCRs). GPCRs regulate essentially all cellular processes, including those that are fundamental to cancer pathology, such as proliferation, survival/drug resistance, migration, differentiation, tissue invasion, and angiogenesis. Many different GPCR isoforms are enhanced or dysregulated in multiple tumor types, and several GPCRs have known oncogenic activity. With approximately 350 distinct GPCRs in the genome, these receptors provide a rich landscape for the design of effective, targeted therapies for cancer, a uniquely heterogeneous disease family. While the generation of selective, efficacious mAbs has been problematic for these structurally complex integral membrane proteins, progress in the development of immunotherapeutics has been made by several independent groups. This chapter provides an overview of the roles of GPCRs in cancer and describes the current state of the art of GPCR-targeted mAb drugs.

Primary infection by a human immunodeficiency virus with atypical coreceptor tropism

The great majority of human immunodeficiency virus type 1 (HIV-1) strains enter CD4+ target cells by interacting with one of two coreceptors, CCR5 or CXCR4. Here we describe a transmitted/founder (T/F) virus (ZP6248) that was profoundly impaired in its ability to utilize CCR5 and CXCR4 coreceptors on multiple CD4+ cell lines as well as primary human CD4+ T cells and macrophages in vitro yet replicated to very high titers (>80 million RNA copies/ml) in an acutely infected individual. Interestingly, the envelope (Env) glycoprotein of this clade B virus had a rare GPEK sequence in the crown of its third variable loop (V3) rather than the consensus GPGR sequence. Extensive sequencing of sequential plasma samples showed that the GPEK sequence was present in virtually all Envs, including those from the earliest time points after infection. The molecularly cloned (single) T/F virus was able to replicate, albeit poorly, in cells obtained from ccr5Δ32 homozygous donors. The ZP6248 T/F virus could also infect cell lines overexpressing the alternative coreceptors GPR15, APJ, and FPRL-1. A single mutation in the V3 crown sequence (GPEK->GPGK) of ZP6248 restored its infectivity in CCR5+ cells but reduced its ability to replicate in GPR15+ cells, indicating that the V3 crown motif played an important role in usage of this alternative coreceptor. These results suggest that the ZP6248 T/F virus established an acute in vivo infection by using coreceptor(s) other than CCR5 or CXCR4 or that the CCR5 coreceptor existed in an unusual conformation in this individual.

Allosteric model of maraviroc binding to CC chemokine receptor 5 (CCR5)

Maraviroc is a nonpeptidic small molecule human immunodeficiency virus type 1 (HIV-1) entry inhibitor that has just entered the therapeutic arsenal for the treatment of patients. We recently demonstrated that maraviroc binding to the HIV-1 coreceptor, CC chemokine receptor 5 (CCR5), prevents it from binding the chemokine CCL3 and the viral envelope glycoprotein gp120 by an allosteric mechanism. However, incomplete knowledge of ligand-binding sites and the lack of CCR5 crystal structures have hampered an in-depth molecular understanding of how the inhibitor works. Here, we addressed these issues by combining site-directed mutagenesis (SDM) with homology modeling and docking. Six crystal structures of G-protein-coupled receptors were compared for their suitability for CCR5 modeling. All CCR5 models had equally good geometry, but that built from the recently reported dimeric structure of the other HIV-1 coreceptor CXCR4 bound to the peptide CVX15 (Protein Data Bank code 3OE0) best agreed with the SDM data and discriminated CCR5 from non-CCR5 binders in a virtual screening approach. SDM and automated docking predicted that maraviroc inserts deeply in CCR5 transmembrane cavity where it can occupy three different binding sites, whereas CCL3 and gp120 lie on distinct yet overlapped regions of the CCR5 extracellular loop 2. Data suggesting that the transmembrane cavity remains accessible for maraviroc in CCL3-bound and gp120-bound CCR5 help explain our previous observation that the inhibitor enhances dissociation of preformed ligand-CCR5 complexes. Finally, we identified residues in the predicted CCR5 dimer interface that are mandatory for gp120 binding, suggesting that receptor dimerization might represent a target for new CCR5 entry inhibitors.

HIV microbicides: state-of-the-art and new perspectives on the development of entry inhibitors

Since the discovery of HIV at the beginning of the 1980s, numerous efforts have been devoted to the search of an efficient vaccine. There are at least 25 drugs available for HIV treatment, but no cure is available. The observation that therapy for HIV disease is life long and that these drugs are associated with a number of side effects underlines the need for approaches aimed at preventing rather than treating infection. Additionally, the economic burden of treatment for the HIV infection occupies an increasing share of healthcare expenditure, making current practices likely to become difficult to sustain in the long run. Unfortunately, no effective vaccine for this disease is foreseeable in the near future. Microbicides could be an alternate way to build preventative approaches to HIV infection. Strategies based on preventing the virus from reaching its target cells seem to have some room for development and application. In this review we explore the state-of-the-art of available microbicides, focusing on HIV entry inhibitors. In addition, we discuss new compounds that show anti-HIV activity, which could be effective candidates.

Immunotherapeutic restoration in HIV-infected individuals

While the development of combined active antiretroviral therapy (cART) has dramatically improved life expectancies and quality of life in HIV-infected individuals, long-term clinical problems, such as metabolic complications, remain important constraints of life-long cART. Complete immune restoration using only cART is normally unattainable even in cases of sufficient plasma viral suppression. The need for immunologic adjuncts that complement cART remains, because while cART alone may result in the complete recovery of peripheral net CD4+ T lymphocytes, it may not affect the reservoir of HIV-infected cells. Here, we review current immunotherapies for HIV infection, with a particular emphasis on recent advances in cytokine therapies, therapeutic immunization, monoclonal antibodies, immune-modulating drugs, nanotechnology-based approaches and radioimmunotherapy.

Natural anti-CCR5 antibodies in HIV-infection and -exposure

Natural antibodies constitute a first-line of defence against pathogens; they may also play other roles in immune regulation and homeostasis, through their ability to bind host antigens, surface molecules and receptors. Natural anti-CCR5 antibodies can be decisive in preventing HIV infection in mucosal tissues and offer prompt and effective protection just at major sites of virus entry. Among natural anti-CCR5 antibodies, IgG and IgA to the ECL1 domain have been shown to block HIV effectively and durably without causing harm to the host. Their biological properties and their uncommon generation in subsets of HIV-infected and HIV-exposed individuals (so called ESN) will be introduced and discussed, with the aim at exploiting their potential in therapy and prevention.

Monoclonal CCR5 antibody for treatment of people with HIV infection

BACKGROUND

A monoclonal CCR5 antibody, PRO 140 (a humanised form of the PA14 antibody), inhibits CCR5-tropic (R5) HIV-1. This may be an effective new treatment for HIV-infected patients, with the potential to address the limitations of currently available therapies.

OBJECTIVES

We aimed to assess the efficacy and safety of PRO 140 for HIV-1-infected patients in randomised controlled trials (RCTs).

SEARCH STRATEGY

Databases including Cochrane's CENTRAL, PubMed, EMBASE and ISI Web of Knowledge, online trials registries and other sources were searched. The reference lists of related literature and presentations from major HIV/AIDS conferences were also screened.

SELECTION CRITERIA

RCTs and quasi-RCTs comparing PRO 140 with placebo or other antiretroviral drugs, or different doses for individuals infected with HIV.

DATA COLLECTION AND ANALYSIS

Two reviewers (L Li and TT Sun) independently screened all retrieved citations and selected relevant citations. Data were extracted independently by two authors (P Zhang and WQ Jia). Any disagreements when selecting studies and extracting data were adjudicated the review mentor (KH Yang). RevMan software was used for statistical analysis based an intention-to-treat analysis. Heterogeneity was examined using the I(2) statistic. I(2) estimates greater than 50% were regarded as moderate or high levels. According to the level of heterogeneity, either fixed or random effects models were used. If significant heterogeneity existed and the reasons could not be found, we reported the results qualitatively.

MAIN RESULTS

We included 2 trials comparing PRO 140 with placebo in adult patients with HIV infection. Our review indicates that PRO 140 may offer significant dose-dependent HIV-1 RNA suppression with tolerable side effects. PRO 140 2mg/Kg, 5 mg/Kg, 162mg weekly, 324 mg biweekly, 324 mg weekly showed statistically significant differences in the changes of HIV RNA level. Both HIV-1 RNA levels of PRO 140 2mg/Kg, 5mg/Kg on day 10 and PRO 140 162mg weekly, 324 mg biweekly, 324 mg weekly on day 22 were significantly reduced. PRO 140 0.5mg/Kg, 2mg/Kg, 5mg/Kg, 162 mg weekly; 324 mg biweekly; 324 mg weekly demonstrated greater antiviral response. Only PRO 140 324 mg weekly showed more patients with ≦400 copies/mL HIV-1 RNA. Only PRO 140 5 mg/Kg showed greater change in CD4(+) cell count on day 8. Headache, lymphadenopathy, diarrhoea, fatigue, and hypertension were reported to be the most frequent adverse events.

AUTHORS' CONCLUSIONS

Limited evidence from two small trials suggests that PRO 140 might demonstrate potent, dose-dependent, highly significant antiviral activity. The evidence is insufficient, so recommendations cannot yet be made. Larger, longer-term, double-blind RCTs are required to provide conclusive evidence.

New insights into the mechanisms whereby low molecular weight CCR5 ligands inhibit HIV-1 infection

CC chemokine receptor 5 (CCR5) is a G-protein-coupled receptor for the chemokines CCL3, -4, and -5 and a coreceptor for entry of R5-tropic strains of human immunodeficiency virus type 1 (HIV-1) into CD4(+) T-cells. We investigated the mechanisms whereby nonpeptidic, low molecular weight CCR5 ligands block HIV-1 entry and infection. Displacement binding assays and dissociation kinetics demonstrated that two of these molecules, i.e. TAK779 and maraviroc (MVC), inhibit CCL3 and the HIV-1 envelope glycoprotein gp120 binding to CCR5 by a noncompetitive and allosteric mechanism, supporting the view that they bind to regions of CCR5 distinct from the gp120- and CCL3-binding sites. We observed that TAK779 and MVC are full and weak inverse agonists for CCR5, respectively, indicating that they stabilize distinct CCR5 conformations with impaired abilities to activate G-proteins. Dissociation of [(125)I]CCL3 from CCR5 was accelerated by TAK779, to a lesser extent by MVC, and by GTP analogs, suggesting that inverse agonism contributes to allosteric inhibition of the chemokine binding to CCR5. TAK779 and MVC also promote dissociation of [(35)S]gp120 from CCR5 with an efficiency that correlates with their ability to act as inverse agonists. Displacement experiments revealed that affinities of MVC and TAK779 for the [(35)S]gp120-binding receptors are in the same range (IC(50) ∼6.4 versus 22 nm), although we found that MVC is 100-fold more potent than TAK779 for inhibiting HIV infection. This suggests that allosteric CCR5 inhibitors not only act by blocking gp120 binding but also alter distinct steps of CCR5 usage in the course of HIV infection.

In vitro selection and characterization of HIV-1 variants with increased resistance to sifuvirtide, a novel HIV-1 fusion inhibitor

Sifuvirtide, a novel fusion inhibitor against human immunodeficiency virus type I (HIV-1), which is more potent than enfuvirtide (T20) in cell culture, is currently under clinical investigation for the treatment of HIV-1 infection. We now report that in vitro selection of HIV-1 variants resistant to sifuvirtide in the presence of increasing concentrations of sifuvirtide has led to several specific mutations in the gp41 region that had not been previously reported. Many of these substitutions were confined to the N-terminal heptad repeat region at positions 37, 38, 41, and 43, either singly or in combination. A downstream substitution at position 126 (N126K) in the C-terminal heptad repeat region was also found. Site-directed mutagenesis studies have further identified the critical amino acid substitutions and combinations thereof in conferring the resistant genotypes. Furthermore, the mutant viruses demonstrated variable degrees of cross-resistance to enfuvirtide, some of which are preferentially more resistant to sifuvirtide. Impaired infectivity was also found for many of the mutant viruses. Biophysical and structural analyses of the key substitutions have revealed several potential novel mechanisms against sifuvirtide. Our results may help to predict potential resistant patterns in vivo and facilitate the further clinical development and therapeutic utility of sifuvirtide.

Phase 2a study of the CCR5 monoclonal antibody PRO 140 administered intravenously to HIV-infected adults

The anti-CCR5 antibody PRO 140 has shown potent and prolonged antiretroviral activity in subjects infected with CCR5-tropic (R5) HIV-1. Prior studies have examined single intravenous doses ranging up to 5 mg/kg of body weight or up to three subcutaneous doses ranging up to 324 mg. Here we report the results of a randomized, double-blind, placebo-controlled trial that examined the antiviral activity, tolerability, and pharmacokinetics of single 5-mg/kg and 10-mg/kg intravenous infusions of PRO 140 in 31 treated subjects. Eligibility criteria included HIV-1 RNA levels of >5,000 copies/ml, CD4(+) cell counts of >300/μl, no antiretroviral therapy for ≥12 weeks, and detection of only R5 HIV-1 in the original Trofile assay. Following poststudy testing with an enhanced-sensitivity Trofile assay, one subject treated with 10 mg/kg was reclassified as having dual/mixed-tropic virus at screening, and the data for that subject were censored from efficacy analyses. The mean maximum reduction of the HIV-1 RNA level from the baseline level was 1.8 log(10) units for both the 5-mg/kg and 10-mg/kg doses (P < 0.0001 relative to placebo). Viral loads reached their nadir at day 12 posttreatment and remained significantly (P < 0.01) reduced through day 29 for both PRO 140 dose groups. Treatment was generally well tolerated, with no dose-limiting toxicity being observed. Peak serum concentrations and overall exposures increased proportionally with dose. In summary, single 5-mg/kg and 10-mg/kg doses of PRO 140 exhibited potent, long-lived antiviral activity and were generally well tolerated. The findings further delineate the safety and antiviral properties of this novel, long-acting antiretroviral agent.

Production of pharmaceutical-grade recombinant aprotinin and a monoclonal antibody product using plant-based transient expression systems

Plants have been proposed as an attractive alternative for pharmaceutical protein production to current mammalian or microbial cell-based systems. Eukaryotic protein processing coupled with reduced production costs and low risk for mammalian pathogen contamination and other impurities have led many to predict that agricultural systems may offer the next wave for pharmaceutical product production. However, for this to become a reality, the quality of products produced at a relevant scale must equal or exceed the predetermined release criteria of identity, purity, potency and safety as required by pharmaceutical regulatory agencies. In this article, the ability of transient plant virus expression systems to produce a wide range of products at high purity and activity is reviewed. The production of different recombinant proteins is described along with comparisons with established standards, including high purity, specific activity and promising preclinical outcomes. Adaptation of transient plant virus systems to large-scale manufacturing formats required development of virus particle and Agrobacterium inoculation methods. One transient plant system case study illustrates the properties of greenhouse and field-produced recombinant aprotinin compared with an US Food and Drug Administration-approved pharmaceutical product and found them to be highly comparable in all properties evaluated. A second transient plant system case study demonstrates a fully functional monoclonal antibody conforming to release specifications. In conclusion, the production capacity of large quantities of recombinant protein offered by transient plant expression systems, coupled with robust downstream purification approaches, offers a promising solution to recombinant protein production that compares favourably to cell-based systems in scale, cost and quality.

Role of host cell factors in flavivirus infection: Implications for pathogenesis and development of antiviral drugs

The genus Flavivirus contains approximately 70 arthropod-borne enveloped RNA viruses many of which cause severe human and in some cases, animal disease. They include dengue virus, yellow fever virus, West Nile virus, Japanese encephalitis virus, and tick-borne encephalitis virus. Hundreds of thousands of deaths due to flavivirus infections occur each year, many of which are unpreventable due to lack of availability of appropriate vaccines and/or antiviral drugs. Flaviviruses exploit the cytoplasmic cellular machinery to facilitate propagation of infectious progeny virions. They engage in dynamic and antagonistic interactions with host cell membranes and biochemical processes. Following infection, the cells initiate various antiviral strategies to counteract viral invasion. In its defense, the virus has alternative strategies to suppress these host responses to infection. The fine balance between these interactions determines the outcome of the viral infection and disease progression. Published studies have revealed specific effects of flaviviruses on cellular processes, but the underlying mechanisms that determine the specific cytopathogenetic changes induced by different flaviviruses have not, as yet, been elucidated. Independently of the suppression of the type I IFN response which has been described in detail elsewhere, this review focuses on recent discoveries relating to alterations of host metabolism following viral infection. Such studies may contribute to new approaches to antiviral drug development. The role of host cellular factors will be examined in the context of protection and/or pathogenesis resulting from flavivirus infection, with particular emphasis on West Nile virus and dengue virus.

Anti-HIV-1 activity of weekly or biweekly treatment with subcutaneous PRO 140, a CCR5 monoclonal antibody

BACKGROUND

PRO 140 is a humanized CCR5 monoclonal antibody that has demonstrated potent antiviral activity when it is administered intravenously to adults infected with CCR5-tropic (R5) human immunodeficiency virus type 1 (HIV-1). This study is the first to evaluate subcutaneous administration.

METHODS

A randomized, double-blind, placebo-controlled study was conducted among 44 subjects with HIV-1 RNA levels of >5000 copies/mL, CD4(+) cell counts of >300 cells/microL, no receipt of antiretroviral therapy for >or=12 weeks, and only R5 HIV-1 detectable. Subjects received placebo, 162 mg of PRO 140, or 324 mg of PRO 140 weekly for 3 weeks or 324 mg of PRO 140 every other week for 2 doses by means of subcutaneous infusion. Subjects were monitored for 58 days for safety, antiviral effects, and PRO 140 serum concentrations.

RESULTS

Subcutaneous PRO 140 demonstrated potent and prolonged antiretroviral activity. Mean log(10) reductions in HIV-1 RNA level were 0.23, 0.99 (P=.009), 1.37 (P<.001), and 1.65 (P<.001) for the placebo, 162 mg weekly, 324 mg biweekly, and 324 mg weekly dose groups, respectively. Viral loads remained suppressed between successive doses. Treatment was generally well tolerated.

CONCLUSIONS

This trial demonstrates proof of concept for a monoclonal antibody administered subcutaneously in HIV-1 infected individuals. Subcutaneous PRO 140 offers the potential for significant dose-dependent HIV-1 RNA suppression and infrequent patient self-administration.

TRIAL REGISTRATION

ClinicalTrials.gov identifier: NCT00642707 .

CCR5: From Natural Resistance to a New Anti-HIV Strategy

The C-C chemokine receptor type 5 (CCR5) is a key player in HIV infection due to its major involvement in the infection process. Investigations into the role of the CCR5 coreceptor first focused on its binding to the virus and the molecular mechanisms leading to the entry and spread of HIV. The identification of naturally occurring CCR5 mutations has allowed scientists to address the CCR5 molecule as a promising target to prevent or limit HIV infection in vivo. Naturally occurring CCR5-specific antibodies have been found in exposed but uninfected people, and in a subset of HIV seropositive people who show long-term control of the infection. This suggests that natural autoimmunity to the CCR5 coreceptor exists and may play a role in HIV control. Such natural immunity has prompted strategies aimed at achieving anti-HIV humoral responses through CCR5 targeting, which will be described here.

Circular CCR5 peptide conjugates and uses thereof (WO2008074895)

BACKGROUND

Several new strategies targeting HIV infection aim to inhibit virus entry by blocking the chemokine receptor CCR5 used by macrophage tropic strains associated with early infection. The current application uses virus-like particles as a support to present CCR5 peptide antigens.

OBJECTIVES

The virus-like particle (VLP)-CCR5 composition aims to function as either a preventative and/or therapeutic vaccine inducing durable autoantibodies that can block CCR5 and prevent HIV entry or attenuate disease progression.

METHODS

The novelty of the current application lies in the chemical conjugation of circularised peptide antigens to VLPs, primarily the CCR5 N-terminal domain alone but also including the first extracellular loop (ECL-1). Immunised mice and rabbits generated antibodies that recognised native CCR5 and inhibited entry of pseudotype viruses bearing envelope glycoproteins from diverse primary strains in vitro.

RESULTS/CONCLUSIONS

Further work is required to assess the in vivo therapeutic potential of these CCR5 compositions. As therapeutic vaccines and/or preventative vaccines, the potential for selecting CXCR4 tropic virus populations associated with disease progression will need to be considered in addition to the broader consequences of targeting a cellular antigen involved in innate immunity.

Entry inhibitors in the treatment of HIV-1 infection

Infection of target cells by HIV is a complex, multi-stage process involving attachment to host cells and CD4 binding, coreceptor binding, and membrane fusion. Drugs that block HIV entry are collectively known as entry inhibitors, but comprise a complex group of drugs with multiple mechanisms of action depending on the stage of the entry process at which they act. Two entry inhibitors, maraviroc and enfuvirtide, have been approved for the treatment of HIV-1 infection, and a number of agents are in development. This review covers the entry inhibitors and their use in the management of HIV-1 infection. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, Vol 85, issue 1, 2010.

CCR5 monoclonal antibodies for HIV-1 therapy

PURPOSE OF REVIEW

To summarize emerging clinical and preclinical data pertaining to the use of CCR5 monoclonal antibodies (mAbs) as therapies for HIV-1 infection. The epitope specificity of CCR5 mAbs is discussed in relation to its critical impact on antiviral activity and CCR5 antagonism. We compare and contrast mAbs and small-molecule CCR5 antagonists in terms of their binding and antiviral properties. Two CCR5 mAbs have entered clinical testing and have successfully completed proof-of-concept studies in HIV-infected individuals, providing initial information on the potential therapeutic utility of these agents.

RECENT FINDINGS

New studies support the view that the most potent antiviral CCR5 mAbs recognize the second extracellular loop of CCR5 either exclusively or in combination with the amino terminus. Studies have revealed fundamental differences in how mAbs and small molecules bind CCR5 and inhibit HIV-1. CCR5 mAbs and small-molecule CCR5 antagonists have demonstrated consistent antiviral synergy and limited or no viral cross-resistance in independent studies. Single intravenous infusions of CCR5 mAbs significantly reduced HIV-1 RNA levels in infected individuals for 2-3 weeks without appreciable toxicity.

SUMMARY

CCR5 mAbs have demonstrated broad and potent antiviral activity in vitro. Clinical studies have established CCR5 mAbs as potent antiretroviral agents with prolonged activity following a single dose. CCR5 mAbs represent both a distinct class of CCR5 inhibitor and a novel approach to HIV-1 therapy.

HIV entry inhibitors and their potential in HIV therapy

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

Discovery of novel non-cyclam polynitrogenated CXCR4 coreceptor inhibitors

HIV cell fusion and entry have been validated as targets for therapeutic intervention against infection. Bicyclams were the first low-molecular-weight compounds to show specific interaction with CXCR4. The most potent bicyclam was AMD3100, in which the two cyclam moieties are tethered by a 1,4-phenylenebis(methylene) bridge. It was withdrawn from clinical trials owing to its lack of oral bioavailability and cardiotoxicity. We have designed a combinatorial library of non-cyclam polynitrogenated compounds by preserving the main features of AMD3100. At least two nitrogen atoms on each side of the p-phenylene moiety, one in the benzylic position and the other(s) in the heterocyclic system were maintained, and the distances between them were similar to the nitrogen atom distances in cyclam. A selection of diverse compounds from this library were prepared, and their in vitro activity was tested in cell cultures against HIV strains. This led to the identification of novel potent CXCR4 coreceptor inhibitors without cytotoxicity at the tested concentrations.

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.

Neutralization of HIV by antibodies

Antibodies can neutralize HIV-1 with potency and cross-reactivity that varies widely and is related but not correlated to their antigen-binding affinity. Therefore, in addition to measuring binding affinity, an evaluation of the antibody neutralizing activity in tissue cultures is important for development of antibody-based therapeutics, design of candidate vaccine immunogens, and understanding the mechanisms of virus entry, neutralization, and evasion of immune responses. The development of a standardized assay for measurement of the in vitro neutralizing activities of the antibody has remained a challenging goal in the last two decades. There are two types of widely used assays, which vary in details between different laboratories--assays based on cell line/pseudovirus and assays based on infection of peripheral blood mononuclear cells (PBMCs). Here we describe in detail the PBMC-based assay, which is more laborious but in our opinion represents a closer approximation of the in vivo situation. As with all other in vitro assays the results of such measurements are only an indication of the antibody potency in vivo, and animal studies and ultimately clinical trials are needed for the development of such antibodies as potential prophylactics and therapeutics.

Peptide inhibition of HIV-1: current status and future potential

More than 2 decades of intensive research has focused on defining replication mechanisms of HIV type 1 (HIV-1), the etiologic agent of AIDS. The delineation of strategies for combating this viral infection has yielded many innovative approaches toward this end. HIV-1 is a lentivirus in the family retroviridae that is relatively small with regard to both structure and genome size, having a diploid RNA genome of approximately 9 kb, with only three major genes and several gene products resulting from alternate splicing and translational frameshifting. Most marketed drugs for treating AIDS are inhibitors of HIV-1 reverse transcriptase or protease enzymes, but new targets include the integrase enzyme, cell surface interactions that facilitate viral entry, and also virus particle maturation and assembly. The emergence of drug-resistant variants of HIV-1 has been the main impediment to successful treatment of AIDS. Thus, there is a pressing need to develop novel treatment strategies targeting multiple stages of the virus life-cycle. Research efforts aimed at developing successful means for combating HIV-1 infection have included development of peptide inhibitors of HIV-1. This article summarizes past and current endeavors in the development of peptides that inhibit replication of HIV-1 and the role of peptide inhibitors in the search for new anti-HIV drugs.

CXCR4, inhibitors and mechanisms of action

In this review, the author discusses recent advances in anti-HIV inhibitors, targeting CXCR4, including natural and modified chemokines, peptides and organic compounds, their mechanisms of action, and the molecular process of virus invasion of immune cells. Peptides with strong anti-HIV activity exhibit several common features, such as electrostatic charges, cyclization, beta-turns and dimerization induced by a sulphide bond. Organic compounds, such as cyclams, display a unique metal-mediated mechanism in the binding process to its target CXCR4. Understanding of their mechanisms of action may be useful for the design of more effective drugs. Consecutive interactions of viral glycoprotein gp120 with CD4 and the co-receptor, CXCR4 or another co-receptor CCR5 on the cell surface leads to virus invasion into host cells. The molecular details of the binding between HIV glycoproteins and the co-receptors also provide a basis for anti-HIV therapy.

Closing two doors of viral entry: intramolecular combination of a coreceptor- and fusion inhibitor of HIV-1

We describe a novel strategy in which two inhibitors of HIV viral entry were incorporated into a single molecule. This bifunctional fusion inhibitor consists of an antibody blocking the binding of HIV to its co-receptor CCR5, and a covalently linked peptide which blocks envelope mediated virus-cell fusion. This novel bifunctional molecule is highly active on CCR5- and X4-tropic viruses in a single cycle assay and a reporter cell line with IC₅₀ values of 0.03–0.05 nM. We demonstrated that both inhibitors contribute to the antiviral activity. In the natural host peripheral blood mononuclear cells (PBMC) the inhibition of CXCR4-tropic viruses is dependant on the co-expression of CCR5 and CXCR4 receptors. This bifunctional inhibitor may offer potential for improved pharmacokinetic parameters for a fusion inhibitor in humans and the combination of two active antiviral agents in one molecule may provide better durability in controlling the emergence of resistant viruses.

Down-regulation of cell surface CXCR4 by HIV-1

Background

CXC chemokine receptor 4 (CXCR4), a member of the G-protein-coupled chemokine receptor family, can serve as a co-receptor along with CD4 for entry into the cell of T-cell tropic X4 human immunodeficiency virus type 1 (HIV-1) strains. Productive infection of T-lymphoblastoid cells by X4 HIV-1 markedly reduces cell-surface expression of CD4, but whether or not the co-receptor CXCR4 is down-regulated has not been conclusively determined.

Results

Infection of human T-lymphoblastoid cell line RH9 with HIV-1 resulted in down-regulation of cell surface CXCR4 expression. Down-regulation of surface CXCR4 correlated temporally with the increase in HIV-1 protein expression. CXCR4 was concentrated in intracellular compartments in H9 cells after HIV-1 infection. Immunofluorescence microscopy studies showed that CXCR4 and HIV-1 glycoproteins were co-localized in HIV infected cells. Inducible expression of HIV-1 envelope glycoproteins also resulted in down-regulation of CXCR4 from the cell surface.

Conclusion

These results indicated that cell surface CXCR4 was reduced in HIV-1 infected cells, whereas expression of another membrane antigen, CD3, was unaffected. CXCR4 down-regulation may be due to intracellular sequestering of HIV glycoprotein/CXCR4 complexes.

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.

Entry inhibitor-based microbicides are active in vitro against HIV-1 isolates from multiple genetic subtypes

Inhibitors of viral entry are under consideration as topical microbicides to prevent HIV-1 sexual transmission. Small molecules targeting HIV-1 gp120 (BMS-378806) or CCR5 (CMPD167), and a peptide fusion inhibitor (C52L), each blocks vaginal infection of macaques by a SHIV. A microbicide, however, must be active against multiple HIV-1 variants. We therefore tested BMS-C (a BMS-378806 derivative), CMPD167, C52L and the CXCR4 ligand AMD3465, alone and in combination, against 25 primary R5, 12 X4 and 7 R5X4 isolates from subtypes A-G. At high concentrations (0.1-1 microM), the replication of most R5 isolates in human donor lymphocytes was inhibited by >90%. At lower concentrations, double and triple combinations were more effective than individual inhibitors. Similar results were obtained with X4 viruses when AMD3465 was substituted for CMPD167. The R5X4 viruses were inhibited by combining AMD3465 with CMPD167, or by the coreceptor-independent compounds. Thus, combining entry inhibitors may improve microbicide effectiveness.

Peptide mimotopes selected with HIV-1-blocking monoclonal antibodies against CCR5 represent motifs specific for HIV-1 entry

CCR5 is a chemokine receptor that mediates entry of human immunodeficiency virus-1 (HIV-1). Two monoclonal antibodies (mAbs) that block HIV-1 entry, 3A9 and 5C7, were used to select peptide mimotopes of sequences on CCR5 from phage displayed peptide libraries. The selected mimotofpes comprised motifs at the N-terminus and on the first and third extracellular loops (ECL1 and ECL3) of CCR5. Amino acids in these motifs were exchanged for alanines by site-directed mutagenesis (sdm) in the cDNA for human CCR5. Ensuing effects on antibody binding to CCR5, cellular entry of HIV-1 and chemokine-induced signalling were analysed by transfection of mutant cDNAs into HEK293.CD4 cells. For both mAbs, fluorescence-activated cell sorting analysis was used to define overlapping conformational epitopes on CCR5 at the N-terminus, on ECL1 and ECL3. Mutation of the N-terminal motif 10YD11 prevented HIV-1 entry into transfected cells as judged by single round infection assays with R5 and R5X4 HIV-1 isolates, as did mutation of the motif 96FG97 in ECL1, whereas mutation of the motif 274RLD276 in ECL3 had only a minor effect. None of the motifs in CCR5 relevant to HIV-1 entry disrupted chemokine-induced signalling. Thus, peptide mimotopes of conformational contact sites of CCR5 with the paratope of mAbs 3A9 and 5C7 represent sites on CCR5 that are essential for HIV-1 entry. Structural knowledge of these mimotopes could help elucidate the nature of the interaction between CCR5 and HIV-1, and thus the derivation of specific inhibitors of entry of HIV-1 into susceptible cells without interference with chemokine signalling.

Microbicides and other topical agents in the prevention of HIV and sexually transmitted infections

According to information from UNAIDS, more than 42 million individuals are living with HIV worldwide. Most infected individuals live in developing countries where the availability of antiretroviral agents is still limited. As this pandemic is increasing largely through mucosal transmission, new methods of prevention are urgently needed. If available, agents that block HIV prior to or early after contact with mucosal epithelia would decrease the incidence of HIV infection and, therefore, potentially save millions of lives over the next few decades. Topically applied microbicides acting against HIV-1 can be subdivided into four subgroups, including agents directly inhibiting pathogens, agents acting on genital pH, agents blocking pathogen entry and replication inhibitors. In addition, microbicides might also allow fighting against other sexually transmitted infections, such as herpes simplex viruses. With concerted efforts directed towards developing efficient microbicides, topical anti-infective compounds may well become a new weapon against sexually transmitted infections, including HIV, in everyday clinical practice.

CCR5 small-molecule antagonists and monoclonal antibodies exert potent synergistic antiviral effects by cobinding to the receptor

A panel of four CCR5 monoclonal antibodies (mAbs) recognizing different epitopes on CCR5 was examined in CCR5-mediated cell-cell fusion assay, alone or in combination with a variety of small molecule CCR5 antagonists. Although no antagonism was observed between any of the CCR5 inhibitors, surprisingly potent synergy was observed between CCR5 mAbs and antagonists, and the synergistic activity was confirmed in other antiviral assays. Strong synergy was also observed between CCR5 inhibitors and the human immunodeficiency virus (HIV) fusion inhibitor enfuvirtide. There was no synergy observed between small molecule CCR5 inhibitors; however, potent synergy was observed between mAbs recognizing different parts of CCR5. In all synergistic combinations, greater synergy was achieved at higher percent inhibition levels. A negative correlation was found between the degree of synergy between the two classes of CCR5 inhibitors and the ability to compete each other for binding to the receptor. For example, the greatest synergy, observed between the mAb ROAb13 and the small molecule inhibitor maraviroc, did not interfere with binding to CCR5 for either inhibitor, whereas no synergy was found between mAb 45523 and maraviroc, which do compete for binding to CCR5. In addition, in contrast to a recent report, the CCR5 inhibitors tested here were found to inhibit the same stage of HIV entry. Based on the data presented here, we hypothesize that CCR5 inhibitors exert synergistic antiviral actions through a cobinding mechanism.

Microbicide drug candidates to prevent HIV infection

25 years after the first HIV/AIDS cases emerged in 1981, the disease continues to spread worldwide, with about 15 000 new infections every day. Although highly active antiretroviral therapy (HAART) has greatly reduced the rate of HIV infection, and the spread of the epidemic, this effect has largely been seen in developed countries. More than 90% of HIV-infected people live in developing countries, most of whom do not have access to this treatment. The development of efficient, widely available, and low-cost microbicides (gels and creams can be applied topically before sex) to prevent sexually transmitted HIV infections should be given high priority. We review different categories of microbicide drugs and lead compounds, their mechanism of action, current status of development, and progress in phase III trials.

The second extracellular loop of CCR5 contains the dominant epitopes for highly potent anti-human immunodeficiency virus monoclonal antibodies

Six mouse anti-human CCR5 monoclonal antibodies (mAbs) that showed potent antiviral activities were identified from over 26,000 mouse hybridomas. The epitopes for these mAbs were determined by using various CCR5 mutants, including CCR5/CCR2B chimeras. One mAb, ROAb13, was found to bind to a linear epitope in the N terminus of CCR5. Strikingly, the other five mAbs bind to epitopes derived from extracellular loop 2 (ECL2). The three most potent mAbs, ROAb12, ROAb14, and ROAb18, require residues from both the N-terminal (Lys171 and Glu172) and C-terminal (Trp190) halves of ECL2 for binding; two other mAbs, ROAb10 and ROAb51, which also showed potent antiviral activities, require Lys171 and Glu172 but not Trp190 for binding. Binding of the control mAb 2D7 completely relies on Lys171 and Glu172. Unlike 2D7, the novel mAbs ROAb12, ROAb14, and ROAb18 do not bind to the linear peptide 2D7-2SK. In addition, all three mAbs bind to monkey CCR5 (with Arg at position 171 instead of Lys); however, 2D7 does not. Since five of the six most potent CCR5 mAbs derived from the same pool of immunized mice require ECL2 as epitopes, we hypothesize that CCR5 ECL2 contains the dominant epitopes for mAbs with potent antiviral activities. These dominant epitopes were found in CCR5 from multiple species and were detected in large proportions of the total cell surface CCR5. mAbs recognizing these epitopes also showed high binding affinity. A homology model of CCR5 was generated to aid in the interpretation of these dominant epitopes in ECL2.

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.