Therapeutic potential of blocking HIV entry into cells: focus on membrane fusion inhibitors

Small-molecule inhibitors of dengue-virus entry

Flavivirus envelope protein (E) mediates membrane fusion and viral entry from endosomes. A low-pH induced, dimer-to-trimer rearrangement and reconfiguration of the membrane-proximal “stem" of the E ectodomain draw together the viral and cellular membranes. We found stem-derived peptides from dengue virus (DV) bind stem-less E trimer and mimic the stem-reconfiguration step in the fusion pathway. We adapted this experiment as a high-throughput screen for small molecules that block peptide binding and thus may inhibit viral entry. A compound identified in this screen, 1662G07, and a number of its analogs reversibly inhibit DV infectivity. They do so by binding the prefusion, dimeric E on the virion surface, before adsorption to a cell. They also block viral fusion with liposomes. Structure-activity relationship studies have led to analogs with submicromolar IC₉₀s against DV2, and certain analogs are active against DV serotypes 1,2, and 4. The compounds do not inhibit the closely related Kunjin virus. We propose that they bind in a previously identified, E-protein pocket, exposed on the virion surface and although this pocket is closed in the postfusion trimer, its mouth is fully accessible. Examination of the E-trimer coordinates (PDB 1OK8) shows that conformational fluctuations around the hinge could open the pocket without dissociating the trimer or otherwise generating molecular collisions. We propose that compounds such as 1662G07 trap the sE trimer in a “pocket-open" state, which has lost affinity for the stem peptide and cannot support the final “zipping up" of the stem.

Fusion of viral and cellular membranes is necessary to establish infection by an enveloped virus. This process is facilitated by rearrangement of protein(s) present on the virion surface in response to molecular cues from the compartment from which fusion occurs, such as low pH of an endosome. Dengue virus is an enveloped virus in the flavivirus family; its “E" (for envelope) protein is the fusion mediator. We previously showed that peptides derived from the membrane proximal “stem" of the E protein bind a form of E that represents a late-stage fusion intermediate. We used this assay to screen for small-molecule inhibitors that compete for stem-peptide association with E. We describe one such inhibitor and its analogs that block viral fusion. These inhibitors also block infectivity if added to dengue virus before infection. Withdrawing the inhibitor before fusion reverses the blockage. We propose that these small molecules bind a hydrophobic pocket on the virion surface and that the virus carries them into the endosome, where they prevent viral fusion by stabilizing an intermediate conformation of the E protein that cannot complete the fusion-promoting conformational change. Identification of these fusion inhibitors shows that viral entry is a possible target for anti-flavivirus drugs.

Examining potential therapies targeting myocardial fibrosis through the inhibition of transforming growth factor-beta 1

After injury, the heart undergoes a remodeling process consisting primarily of myocyte hypertrophy, apoptosis and interstitial fibrosis. Although initially beneficial, excess fibrosis gradually results in alteration of left ventricular properties and cardiac dysfunction. Transforming growth factor-beta 1 (TGF-beta(1)) is thought to be a primary mediator of fibrosis within the heart after injury. Currently, angiotensin II blockade is used to inhibit the actions of TGF-beta(1). However, recent studies indicate that angiotensin II blockade alone may not be sufficient to prevent TGF-beta(1)-induced fibrosis. Thus far, both in vivo and in vitro models have shown that direct TGF-beta(1) inhibition, NAPDH oxidase inhibitors, growth factors and hormonal treatment regimens targeting TGF-beta(1) may significantly reduce cardiac fibrosis after injury. This study attempts to underline these alternatives to angiotensin II blockade in combating TGF-beta(1)-induced cardiac dysfunction.

Design and characterization of an engineered gp41 protein from human immunodeficiency virus-1 as a tool for drug discovery

Two new proteins of approximately 70 amino acids in length, corresponding to an unnaturally-linked N- and C-helix of the ectodomain of the gp41 protein from the human immunodeficiency virus (HIV) type 1, were designed and characterized. A designed tripeptide links the C-terminus of the C-helix with the N-terminus of the N-helix in a circular permutation so that the C-helix precedes the N-helix in sequence. In addition to the artificial peptide linkage, the C-helix is truncated at its N-terminus to expose a region of the N-helix known as the "Trp-Trp-Ile" binding pocket. Sedimentation, crystallographic, and nuclear magnetic resonance studies confirmed that the protein had the desired trimeric structure with an unoccupied binding site. Spectroscopic and centrifugation studies demonstrated that the engineered protein had ligand binding characteristics similar to previously reported constructs. Unlike previous constructs which expose additional, shallow, non-conserved, and undesired binding pockets, only the single deep and conserved Trp-Trp-Ile pocket is exposed in the proteins of this study. This engineered version of gp41 protein will be potentially useful in research programs aimed at discovery of new drugs for therapy of HIV-infection in humans.

Enfuvirtide

Enfuvirtide is the first of a new class of drugs, the fusion inhibitors. It is a synthetic peptide which binds to the HIV glycoprotein 41 (gp41), blocking fusion of the viral and cellular membranes. HIV isolates with reduced susceptibility to enfuvirtide have been recovered from patients receiving enfuvirtide in combination with other antiretroviral agents. Enfuvirtide 90mg (subcutaneously, twice daily) in combination with optimised background (OB) antiretroviral therapy significantly reduced plasma HIV RNA levels compared with OB alone after treatment for 24 weeks in two randomised trials involving adults with advanced HIV infection. The antiviral efficacy of enfuvirtide was maintained through to 48 weeks. At 24 and 48 weeks, the increase from baseline in the CD4+ cell count was significantly greater for patients receiving enfuvirtide plus OB than for those receiving OB alone. Enfuvirtide 30 mg/m(2) or 60 mg/m(2) in combination with other antiretroviral agents reduced plasma HIV RNA levels and increased CD4+ cell counts in a small trial involving paediatric patients with HIV infection. Local injection-site reactions were common. Lymphadenopathy and pneumonia occurred more often in patients receiving enfuvirtide plus OB than in the control group. The incidence of most other events was similar in each group.

The therapeutic potential of CXCR4 antagonists in the treatment of HIV

Since the identification of the chemokine receptors CXCR4 and CCR5 as co-receptors for HIV-1 entry, several antagonists against these receptors have been synthesised. A highly selective CXCR4 antagonist, T22, and its downsized analogues T140 and TC14012, which inhibit X4-HIV-1 infection through their specific binding to CXCR4, have been identified. Besides T22 analogues, several other CXCR4 antagonists have been reported, such as AMD3100, ALX40-4C, KRH-1120 and AMD8664. Discovery of entry inhibitors, such as chemokine antagonists, may lead to the development of a new generation of antiHIV agents, since these inhibitors are thought to be useful for the clinical treatment of HIV-1-infected patients, especially at the late stage of treatment for AIDS patients developing multi-drug-resistant strains. In this review, recent research into CXCR4 antagonists in comparison with development of other antagonists is summarised.

The basis for HIV immunotherapeutic vaccines

The drug treatments introduced in recent years for HIV infection have enabled a marked reduction in morbidity and prolongation of life. These treatments, however, are often associated with acute and chronic toxicities, the development of resistant virus can limit their effectiveness, and they are too expensive and difficult to administer in most third world settings. A successful HIV immunotherapeutic vaccine has the potential to overcome these problems, and would be a valuable advance. The most promising approaches have induced the type of immune response found to correlate with reduced activity of HIV in man, especially cytotoxic T-cell responses, or have led to reduced HIV or SIV viral load and increased CD4 counts in non-human primates or man. The agents that have led to one or both of these effects have been selected for review, and include inactivated envelope depleted virus, recombinant envelope glycoprotein, DNA vaccines utilising HIV peptides or gene products, viral vectors, such as canarypox or attenuated vaccinia, with HIV core proteins. There are other approaches, such as alloimmunity, for which no candidate products yet exist, but which conceptually appear promising. Currently, however, only a few phase III studies of HIV therapeutic vaccines have been completed in man, and there has been a modest therapeutic effect. Further development of both existing and new candidates remains one of the key priorities in our fight against HIV.

1,3,4-Trisubstituted pyrrolidine CCR5 receptor antagonists. Part 1: discovery of the pyrrolidine scaffold and determination of its stereochemical requirements

A series of 1,3,4-trisubstituted pyrrolidines was discovered to have the ability to displace [(125)I]-MIP-1alpha from the CCR5 receptor expressed on Chinese hamster ovary (CHO) cell membranes. CCR5 activity was found to be dependent on the regiochemistry and the absolute stereochemistry of the pyrrolidine.

The potential for HIV fusion inhibition

In recent years, exciting developments in the understanding of the multi-step HIV-1 entry process have revealed potential approaches for therapeutic intervention that will compensate for the early disappointments of the soluble CD4 antigen approach. Although details of the HIV-1 fusion process are clearly complex, the proof-of-concept in the clinic provided by T-20 gives high hopes that new generation inhibitors of HIV fusion will be developed. This review highlights novel drug discovery technologies that are being employed in the search for such inhibitors.

HIV immunotherapeutic vaccines

New combinations of antiretrovirals have improved the quality of life and length of survival of patients with HIV infection and AIDS, but they have significant disadvantages. These include considerable toxicity, the development of drug resistance and expense. Successful immunotherapeutic vaccination against HIV would overcome these problems. None of the approaches that have been tried so far have shown a sufficient effect on HIV replication or on immunorestoration to merit their introduction to clinical practice. The most developed agent thus far is Remune, a gp120 depleted whole killed HIV-1 vaccine that has shown marked cytotoxic T lymphocyte responses when administered to man. CD4 count and HIV-1 viral load responses have occurred, but have so far been disappointing in their magnitude. Remune is entering Phase III trials in North America, Europe and the Far East, to determine clinical efficacy. Immunization using recombinant HIV envelope proteins, such as rgp160, for example with VaxSyn, have failed to produce a therapeutic response. Similarly, agents using HIV core antigens, such as p24VLP, have also failed to work. Hence, newer strategies have been tried. Recombinant canarypox vaccines like ALVAC 1452 and highly attenuated vaccinia virus vaccines, such as NYVAC, have been used in combination with HIV genes and peptides. Preliminary results suggest that they might reduce the HIV replication rate, but this needs confirming in larger clinical trials. DNA vaccination has produced encouraging results in monkeys, but the success has not yet been repeated in humans. Other strategies at an early stage include the exploitation of the protective alloimmune response in man. Outside the immunotherapeutic area, other promising new strategies that are being developed in parallel, include the fusion inhibitors, such as T-20. The potential benefits from a successful immunotherapeutic vaccine dictate that this area should, and will receive priority.