Isolation and characterization of human immunodeficiency virus type 1 resistant to the small-molecule CCR5 antagonist TAK-652

Monocyte-derived macrophages contribute to the deterioration of immunological liver injury in mice

BACKGROUND & AIMS

Autoimmune hepatitis (AIH) is characterized by hepatocyte destruction, leading to lymphocyte and macrophage accumulation in the liver. However, the specific mechanisms of how macrophages participate in the occurrence and development of AIH are still unclear. In this study, we investigated the effect of monocyte-derived macrophages on Con A-induced immunological liver injury in mice and we hypothesized that inhibition of CCR2 with the dual CCR2/5 inhibitor, cenicriviroc (CVC), would attenuate Con A-induced hepatitis in mice by reducing the recruitment of monocytes into the liver.

METHODS

Murine experimental AIH was established by concanavalin A (Con A) injection intravenously. Macrophages were depleted by injection of clodronate liposomes in Con A-treated mice. Moreover, inhibition of the CCR2/5 signaling pathway in Con A mice is achieved by CVC. Liver injury and infiltration of monocyte-derived macrophages were assessed by serum transaminase levels, histopathology, immunohistochemistry, flow cytometry, RT-qPCR, ELISA, TUNEL assay and dihydroethidium staining.

RESULTS

The number of macrophages in the mouse livers increased in the Con A-induced hepatitis mouse model, and flow cytometry showed a significant increase in the proportion of F4/80loCD11bhi monocyte-derived macrophages, while there was no significant change in the proportion of F4/80hiCD11blo Kupffer cells. After the depletion of liver macrophages by clodronate liposomes, the levels of serum ALT and AST, and the degree of liver tissue damage were alleviated in Con A-treated mice. Furthermore, Con A leaded an increase in the expression of a group of CC chemokines in mouse livers, and the elevation of CCL2 was prevented with the depletion of macrophages. Additionally, CVC reduced macrophage infiltration in the liver and ameliorated Con A-induced liver injury. Meanwhile, CVC reduced the apoptosis and oxidative damage of hepatocytes caused by Con A.

CONCLUSIONS

Our research demonstrates that there is an increase in monocyte-derived macrophages in the livers due to the monocyte infiltration resulted from the activation of the CCL2-CCR2 axis in Con A-induced liver injury mouse model. Pharmacological inhibition of CCR2 monocyte recruitment by CVC efficiently ameliorates the hepatic inflammation, indicating the therapeutic potential of CVC in patients with AIH.

An Artificial Peptide-Based Bifunctional HIV-1 Entry Inhibitor That Interferes with Viral Glycoprotein-41 Six-Helix Bundle Formation and Antagonizes CCR5 on the Host Cell Membrane

Human immunodeficiency virus type 1 (HIV-1) is characterized by high variability and drug resistance. This has necessitated the development of antivirals with a new chemotype and therapy. We previously identified an artificial peptide with non-native protein sequence, AP3, with the potential to inhibit HIV-1 fusion through targeting hydrophobic grooves on the N-terminal heptad repeat trimer of viral glycoprotein gp41. Here, a small-molecule HIV-1 inhibitor targeting chemokine coreceptor CCR5 on the host cell was integrated into the AP3 peptide, producing a novel dual-target inhibitor with improved activity against multiple HIV-1 strains including those resistant to the currently used anti-HIV-1 drug enfuvirtide. Its superior antiviral potency in comparison with the respective pharmacophoric moieties is in consonance with the dual binding of viral gp41 and host factor CCR5. Therefore, our work provides a potent artificial peptide-based bifunctional HIV-1 entry inhibitor and highlights the multitarget-directed ligands approach in the development of novel therapeutic anti-HIV-1 agents.

A "Two-Birds-One-Stone" Approach toward the Design of Bifunctional Human Immunodeficiency Virus Type 1 Entry Inhibitors Targeting the CCR5 Coreceptor and gp41 N-Terminal Heptad Repeat Region

Previous studies have reported the stepwise nature of human immunodeficiency virus type 1 (HIV-1) entry and the pivotal role of coreceptor CCR5 and the gp41 N-terminal heptad repeat (NHR) region in this event. With this in mind, we herein report a dual-targeted drug compound featuring bifunctional entry inhibitors, consisting of a piperidine-4-carboxamide-based CCR5 antagonist, TAK-220, and a gp41 NHR-targeting fusion-inhibitory peptide, C34. The resultant chimeras were constructed by linking both pharmacophores with a polyethylene glycol spacer. One chimera, CP12TAK, exhibited exceptionally potent antiviral activity, about 40- and 306-fold over that of its parent inhibitors, C34 and TAK-220, respectively. In addition to R5-tropic viruses, CP12TAK also strongly inhibited infection of X4-tropic HIV-1 strains. These data are promising for the further development of CP12TAK as a new anti-HIV-1 drug. Results show that this strategy could be extended to the design of therapies against infection of other enveloped viruses.

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.

Efficient construction of pyrrolo[1′,2′:1,2]azocino[4,5-c]quinolines via cascade cycloaddition and annulation reaction

Novel polysubstituted pyrrolo[1′,2′:1,2]azocino[4,5-c]quinolines were efficiently synthesized by a three-component reaction in refluxing methanol and a sequential domino annulation in refluxing acetonitrile.

CCR5 receptor antagonists in preclinical to phase II clinical development for treatment of HIV

INTRODUCTION

The chemokine receptor CCR5 has garnered significant attention in recent years as a target to treat HIV infection largely due to the approval and success of the drug Maraviroc. The side effects and inefficiencies with other first generation agents led to failed clinical trials, prompting the development of newer CCR5 antagonists. Areas covered: This review aims to survey the current status of 'next generation' CCR5 antagonists in the preclinical pipeline with an emphasis on emerging agents for the treatment of HIV infection. These efforts have culminated in the identification of advanced second-generation agents to reach the clinic and the dual CCR5/CCR2 antagonist Cenicriviroc as the most advanced currently in phase II clinical studies. Expert opinion: The clinical success of CCR5 inhibitors for treatment of HIV infection has rested largely on studies of Maraviroc and a second-generation dual CCR5/CCR2 antagonist Cenicriviroc. Although research efforts identified several promising preclinical candidates, these were dropped during early clinical studies. Despite patient access to Maraviroc, there is insufficient enthusiasm surrounding its use as front-line therapy for treatment of HIV. The non-HIV infection related development activities for Maraviroc and Cenicriviroc may help drive future interests.

Approved Antiviral Drugs over the Past 50 Years

Since the first antiviral drug, idoxuridine, was approved in 1963, 90 antiviral drugs categorized into 13 functional groups have been formally approved for the treatment of the following 9 human infectious diseases: (i) HIV infections (protease inhibitors, integrase inhibitors, entry inhibitors, nucleoside reverse transcriptase inhibitors, nonnucleoside reverse transcriptase inhibitors, and acyclic nucleoside phosphonate analogues), (ii) hepatitis B virus (HBV) infections (lamivudine, interferons, nucleoside analogues, and acyclic nucleoside phosphonate analogues), (iii) hepatitis C virus (HCV) infections (ribavirin, interferons, NS3/4A protease inhibitors, NS5A inhibitors, and NS5B polymerase inhibitors), (iv) herpesvirus infections (5-substituted 2'-deoxyuridine analogues, entry inhibitors, nucleoside analogues, pyrophosphate analogues, and acyclic guanosine analogues), (v) influenza virus infections (ribavirin, matrix 2 protein inhibitors, RNA polymerase inhibitors, and neuraminidase inhibitors), (vi) human cytomegalovirus infections (acyclic guanosine analogues, acyclic nucleoside phosphonate analogues, pyrophosphate analogues, and oligonucleotides), (vii) varicella-zoster virus infections (acyclic guanosine analogues, nucleoside analogues, 5-substituted 2'-deoxyuridine analogues, and antibodies), (viii) respiratory syncytial virus infections (ribavirin and antibodies), and (ix) external anogenital warts caused by human papillomavirus infections (imiquimod, sinecatechins, and podofilox). Here, we present for the first time a comprehensive overview of antiviral drugs approved over the past 50 years, shedding light on the development of effective antiviral treatments against current and emerging infectious diseases worldwide.

Reduced Baseline Sensitivity to Maraviroc Inhibition Among R5 HIV-1 Isolates From Individuals With Severe Immunodeficiency

Supplemental Digital Content is Available in the Text.

Incompatible Natures of the HIV-1 Envelope in Resistance to the CCR5 Antagonist Cenicriviroc and to Neutralizing Antibodies

Cenicriviroc is a CCR5 antagonist which prevents human immunodeficiency virus type 1 (HIV-1) from cellular entry. The CCR5-binding regions of the HIV-1 envelope glycoprotein are important targets for neutralizing antibodies (NAbs), and mutations conferring cenicriviroc resistance may therefore affect sensitivity to NAbs. Here, we used the in vitro induction of HIV-1 variants resistant to cenicriviroc or NAbs to examine the relationship between resistance to cenicriviroc and resistance to NAbs. The cenicriviroc-resistant variant KK652-67 (strain KK passaged 67 times in the presence of increasing concentrations of cenicriviroc) was sensitive to neutralization by NAbs against the V3 loop, the CD4-induced (CD4i) region, and the CD4-binding site (CD4bs), whereas the wild-type (WT) parental HIV-1 strain KKWT from which cenicriviroc-resistant strain KK652-67 was obtained was resistant to these NAbs. The V3 region of KK652-67 was important for cenicriviroc resistance and critical to the high sensitivity of the V3, CD4i, and CD4bs epitopes to NAbs. Moreover, induction of variants resistant to anti-V3 NAb 0.5γ and anti-CD4i NAb 4E9C from cenicriviroc-resistant strain KK652-67 resulted in reversion to the cenicriviroc-sensitive phenotype comparable to that of the parental strain, KKWT. Resistance to 0.5γ and 4E9C was caused by the novel substitutions R315K, G324R, and E381K in the V3 and C3 regions near the substitutions conferring cenicriviroc resistance. Importantly, these amino acid changes in the CCR5-binding region were also responsible for reversion to the cenicriviroc-sensitive phenotype. These results suggest the presence of key amino acid residues where resistance to cenicriviroc is incompatible with resistance to NAbs. This implies that cenicriviroc and neutralizing antibodies may restrict the emergence of variants resistant to each other.

Conformations and Conformational Processes of Hexahydrobenzazocines by NMR and DFT Studies

Conformational processes that occur in hexahydrobenzazocines have been studied with the (1)H and (13)C dynamic nuclear magnetic resonance (DNMR) spectroscopy. The coalescence effects are assigned to two different conformational processes: the ring-inversion of the ground-state conformations and the interconversion between two different conformers. The barriers for these processes are in the range of 42-52 and 42-43 kJ mol(-1), respectively. Molecular modeling on the density functional theory (DFT) level and the gauge invariant atomic orbitals (GIAO)-DFT calculations of isotropic shieldings and coupling constants for the set of low-energy conformations were compared with the experimental NMR data. The ground-state of all compounds in solution is the boat-chair (BC) conformation. The BC form adopts two different conformations because the nitrogen atom can be in the boat or chair parts of the BC structure. These two conformers are engaged in the interconversion process.

Resistance against inhibitors of HIV-1 entry into target cells

ABSTRACT 

HIV resistance against currently approved entry inhibitors, the chemokine receptor-5 (CCR5) antagonist maraviroc and the fusion inhibitor enfuvirtide (T-20), manifests in a complex manner that is distinct from the resistance patterns against other classes of antiretroviral drugs. Several attachment and fusion inhibitors are currently under various stages of development. Whereas CCR5 co-receptor antagonists have been widely studied until now, because patients who lack CCR5 are healthy and protected to some extent from HIV-infection, CXCR4-antagonist development has been slower, due to limited antiviral activity and potential toxicity given that CXCR4 may have essential cellular functions. Novel fusion inhibitor development is focusing on orally available small-molecule inhibitors that might replace T-20, which needs to be administered by subcutaneous injection.

The dual CCR5 and CCR2 inhibitor cenicriviroc does not redistribute HIV into extracellular space: implications for plasma viral load and intracellular DNA decline

OBJECTIVES

Cenicriviroc is a potent antagonist of the chemokine coreceptors 5 and 2 (CCR5/CCR2) and blocks HIV-1 entry. The CCR5 inhibitor maraviroc has been shown in tissue culture to be able to repel cell-free virions from the cell surface into extracellular space. We hypothesized that cenicriviroc might exhibit a similar effect, and tested this using clinical samples from the Phase IIb study 652-2-202, by measuring rates of intracellular DNA decline. We also monitored viral RNA levels in culture fluids.

METHODS

We infected PM-1 cells with CCR5-tropic HIV-1 BaL in the presence or absence of inhibitory concentrations of cenicriviroc (20 nM) or maraviroc (50 nM) or controls. Viral load levels and p24 were measured by ELISA, quantitative PCR and quantitative real-time reverse transcription PCR at 4 h post-infection. Frozen PBMC DNA samples from 30 patients with virological success in the Phase IIb study were studied, as were early and late reverse transcript levels. Docking studies compared binding between cenicriviroc/CCR5 and maraviroc/CCR5.

RESULTS

Unlike maraviroc, cenicriviroc did not cause an increase in the amount of virus present in culture fluids at 4 h compared with baseline. The use of cenicriviroc did, however, result in lower levels of intracellular viral DNA after 4 h. Structural modelling indicates that cenicriviroc binds more deeply than maraviroc to the hydrophobic pocket of CCR5, providing an explanation for the absence of viral rebound with cenicriviroc.

CONCLUSIONS

In contrast to maraviroc, cenicriviroc does not repel virus back into extracellular space. Differences in results may be due to superior binding of cenicriviroc to CCR5 compared with maraviroc.

Modeling the allosteric modulation of CCR5 function by Maraviroc

Maraviroc is a non-peptidic, low molecular weight CC chemokine receptor 5 (CCR5) ligand that has recently been marketed for the treatment of HIV infected individuals. This review discusses recent molecular modeling studies of CCR5 by homology to CXC chemokine receptor 4, their contribution to the understanding of the allosteric mode of action of the inhibitor and their potential for the development of future drugs with improved efficiency and preservation of CCR5 biological functions.

Selective and Dual Targeting of CCR2 and CCR5 Receptors: A Current Overview

The chemokine receptor 2 (CCR2) and chemokine receptor 5 (CCR5) are important mediators of leukocyte trafficking in inflammatory processes. The emerging evidence for a role of CCR2 and CCR5 receptors in human inflammatory diseases led to a growing interest in CCR2- and CCR5-selective antagonists. In this review, we focus on the recent development of selective CCR2/CCR5 receptor ligands and dual antagonists. Several compounds targeting CCR2, e.g., INCB8761 and MK0812, were developed as promising candidates for clinical trials, but failed to show clinical efficacy as presumed from preclinical models. The role of CCR5 receptors as the second co-receptor for the HIV-host cell fusion led to the development of various CCR5-selective ligands. Maraviroc is the first CCR5-targeting drug for the treatment of HIV-1 infections on the market. The role of CCR5 receptors in the progression of inflammatory processes fueled the use of CCR5 antagonists for the treatment of rheumatoid arthritis. Unfortunately, the use of maraviroc for the treatment of rheumatoid arthritis failed due to its inefficacy. Some of the ligands, e.g., TAK-779 and TAK-652, were also found to be dual antagonists of CCR2 and CCR5 receptors. The fact that CCR2 and CCR5 receptor antagonists contribute to the treatment of inflammatory diseases renders the development of dual antagonists as promising novel therapeutic strategy.

Structure and dynamics of the gp120 V3 loop that confers noncompetitive resistance in R5 HIV-1(JR-FL) to maraviroc

Maraviroc, an (HIV-1) entry inhibitor, binds to CCR5 and efficiently prevents R5 human immunodeficiency virus type 1 (HIV-1) from using CCR5 as a coreceptor for entry into CD4(+) cells. However, HIV-1 can elude maraviroc by using the drug-bound form of CCR5 as a coreceptor. This property is known as noncompetitive resistance. HIV-1(V3-M5) derived from HIV-1(JR-FLan) is a noncompetitive-resistant virus that contains five mutations (I304V/F312W/T314A/E317D/I318V) in the gp120 V3 loop alone. To obtain genetic and structural insights into maraviroc resistance in HIV-1, we performed here mutagenesis and computer-assisted structural study. A series of site-directed mutagenesis experiments demonstrated that combinations of V3 mutations are required for HIV-1(JR-FLan) to replicate in the presence of 1 µM maraviroc, and that a T199K mutation in the C2 region increases viral fitness in combination with V3 mutations. Molecular dynamic (MD) simulations of the gp120 outer domain V3 loop with or without the five mutations showed that the V3 mutations induced (i) changes in V3 configuration on the gp120 outer domain, (ii) reduction of an anti-parallel β-sheet in the V3 stem region, (iii) reduction in fluctuations of the V3 tip and stem regions, and (iv) a shift of the fluctuation site at the V3 base region. These results suggest that the HIV-1 gp120 V3 mutations that confer maraviroc resistance alter structure and dynamics of the V3 loop on the gp120 outer domain, and enable interactions between gp120 and the drug-bound form of CCR5.

HIV-1 antiretroviral drug therapy

The most significant advance in the medical management of HIV-1 infection has been the treatment of patients with antiviral drugs, which can suppress HIV-1 replication to undetectable levels. The discovery of HIV-1 as the causative agent of AIDS together with an ever-increasing understanding of the virus replication cycle have been instrumental in this effort by providing researchers with the knowledge and tools required to prosecute drug discovery efforts focused on targeted inhibition with specific pharmacological agents. To date, an arsenal of 24 Food and Drug Administration (FDA)-approved drugs are available for treatment of HIV-1 infections. These drugs are distributed into six distinct classes based on their molecular mechanism and resistance profiles: (1) nucleoside-analog reverse transcriptase inhibitors (NNRTIs), (2) non-nucleoside reverse transcriptase inhibitors (NNRTIs), (3) integrase inhibitors, (4) protease inhibitors (PIs), (5) fusion inhibitors, and (6) coreceptor antagonists. In this article, we will review the basic principles of antiretroviral drug therapy, the mode of drug action, and the factors leading to treatment failure (i.e., drug resistance).

Escape from human immunodeficiency virus type 1 (HIV-1) entry inhibitors

The human immunodeficiency virus (HIV) enters cells through a series of molecular interactions between the HIV envelope protein and cellular receptors, thus providing many opportunities to block infection. Entry inhibitors are currently being used in the clinic, and many more are under development. Unfortunately, as is the case for other classes of antiretroviral drugs that target later steps in the viral life cycle, HIV can become resistant to entry inhibitors. In contrast to inhibitors that block viral enzymes in intracellular compartments, entry inhibitors interfere with the function of the highly variable envelope glycoprotein as it continuously adapts to changing immune pressure and available target cells in the extracellular environment. Consequently, pathways and mechanisms of resistance for entry inhibitors are varied and often involve mutations across the envelope gene. This review provides a broad overview of entry inhibitor resistance mechanisms that inform our understanding of HIV entry and the design of new inhibitors and vaccines.

Entry inhibitors and their use in the treatment of HIV-1 infection

Entry of HIV into target cells is a complex, multi-stage process involving sequential attachment and CD4 binding, coreceptor binding, and membrane fusion. HIV entry inhibitors are a complex group of drugs with multiple mechanisms of action depending on the stage of the viral entry process they target. Two entry inhibitors are currently approved for the treatment of HIV-infected patients. Maraviroc, a CCR5 antagonist, blocks interactions between the viral envelope proteins and the CCR5 coreceptor. Enfuvirtide, a fusion inhibitor, disrupts conformational changes in gp41 that drive membrane fusion. A wide array of additional agents are in various stages of development. This review covers the entry inhibitors and their use in the treatment of HIV-infected patients.

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.

Molecular basis of human immunodeficiency virus type 1 drug resistance: overview and recent developments

The introduction of potent combination therapies in the mid-90s had a tremendous effect on AIDS mortality. However, drug resistance has been a major factor contributing to antiretroviral therapy failure. Currently, there are 26 drugs approved for treating human immunodeficiency virus (HIV) infections, although some of them are no longer prescribed. Most of the available antiretroviral drugs target HIV genome replication (i.e. reverse transcriptase inhibitors) and viral maturation (i.e. viral protease inhibitors). Other drugs in clinical use include a viral coreceptor antagonist (maraviroc), a fusion inhibitor (enfuvirtide) and two viral integrase inhibitors (raltegravir and elvitegravir). Elvitegravir and the nonnucleoside reverse transcriptase inhibitor rilpivirine have been the most recent additions to the antiretroviral drug armamentarium. An overview of the molecular mechanisms involved in antiretroviral drug resistance and the role of drug resistance-associated mutations was previously presented (Menéndez-Arias, L., 2010. Molecular basis of human immunodeficiency virus drug resistance: an update. Antiviral Res. 85, 210-231). This article provides now an updated review that covers currently approved drugs, new experimental agents (e.g. neutralizing antibodies) and selected drugs in preclinical or early clinical development (e.g. experimental integrase inhibitors). Special attention is dedicated to recent research on resistance to reverse transcriptase and integrase inhibitors. In addition, recently discovered interactions between HIV and host proteins and novel strategies to block HIV assembly or viral entry emerge as promising alternatives for the development of effective antiretroviral treatments.

5-(Perylen-3-yl)ethynyl-arabino-uridine (aUY11), an arabino-based rigid amphipathic fusion inhibitor, targets virion envelope lipids to inhibit fusion of influenza virus, hepatitis C virus, and other enveloped viruses

Entry of enveloped viruses requires fusion of viral and cellular membranes. Fusion requires the formation of an intermediate stalk structure, in which only the outer leaflets are fused. The stalk structure, in turn, requires the lipid bilayer of the envelope to bend into negative curvature. This process is inhibited by enrichment in the outer leaflet of lipids with larger polar headgroups, which favor positive curvature. Accordingly, phospholipids with such shape inhibit viral fusion. We previously identified a compound, 5-(perylen-3-yl)ethynyl-2'-deoxy-uridine (dUY11), with overall shape and amphipathicity similar to those of these phospholipids. dUY11 inhibited the formation of the negative curvature necessary for stalk formation and the fusion of a model enveloped virus, vesicular stomatitis virus (VSV). We proposed that dUY11 acted by biophysical mechanisms as a result of its shape and amphipathicity. To test this model, we have now characterized the mechanisms against influenza virus and HCV of 5-(perylen-3-yl)ethynyl-arabino-uridine (aUY11), which has shape and amphipathicity similar to those of dUY11 but contains an arabino-nucleoside. aUY11 interacted with envelope lipids to inhibit the infectivity of influenza virus, hepatitis C virus (HCV), herpes simplex virus 1 and 2 (HSV-1/2), and other enveloped viruses. It specifically inhibited the fusion of influenza virus, HCV, VSV, and even protein-free liposomes to cells. Furthermore, aUY11 inhibited the formation of negative curvature in model lipid bilayers. In summary, the arabino-derived aUY11 and the deoxy-derived dUY11 act by the same antiviral mechanisms against several enveloped but otherwise unrelated viruses. Therefore, chemically unrelated compounds of appropriate shape and amphipathicity target virion envelope lipids to inhibit formation of the negative curvature required for fusion, inhibiting infectivity by biophysical, not biochemical, mechanisms.

HIV-1 resistance to maraviroc conferred by a CD4 binding site mutation in the envelope glycoprotein gp120

Maraviroc (MVC) is a CCR5 antagonist that inhibits HIV-1 entry by binding to the coreceptor and inducing structural alterations in the extracellular loops. In this study, we isolated MVC-resistant variants from an HIV-1 primary isolate that arose after 21 weeks of tissue culture passage in the presence of inhibitor. gp120 sequences from passage control and MVC-resistant cultures were cloned into NL4-3 via yeast-based recombination followed by sequencing and drug susceptibility testing. Using 140 clones, three mutations were linked to MVC resistance, but none appeared in the V3 loop as was the case with previous HIV-1 strains resistant to CCR5 antagonists. Rather, resistance was dependent upon a single mutation in the C4 region of gp120. Chimeric clones bearing this N425K mutation replicated at high MVC concentrations and displayed significant shifts in 50% inhibitory concentrations (IC(50)s), characteristic of resistance to all other antiretroviral drugs but not typical of MVC resistance. Previous reports on MVC resistance describe an ability to use a drug-bound form of the receptor, leading to reduction in maximal drug inhibition. In contrast, our structural models on K425 gp120 suggest that this resistant mutation impacts CD4 interactions and highlights a novel pathway for MVC resistance.

Effects of sequence changes in the HIV-1 gp41 fusion peptide on CCR5 inhibitor resistance

A rare pathway of HIV-1 resistance to small molecule CCR5 inhibitors such as Vicriviroc (VCV) involves changes solely in the gp41 fusion peptide (FP). Here, we show that the G516V change is critical to VCV resistance in PBMC and TZM-bl cells, although it must be accompanied by either M518V or F519I to have a substantial impact. Modeling VCV inhibition data from the two cell types indicated that G516V allows both double mutants to use VCV-CCR5 complexes for entry. The model further identified F519I as an independent determinant of preference for the unoccupied, high-VCV affinity form of CCR5. From inhibitor-free reversion cultures, we also identified a substitution in the inner domain of gp120, T244A, which appears to counter the resistance phenotype created by the FP substitutions. Examining the interplay of these changes will enhance our understanding of Env complex interactions that influence both HIV-1 entry and resistance to CCR5 inhibitors.

V3 determinants of HIV-1 escape from the CCR5 inhibitors Maraviroc and Vicriviroc

HIV-1 develops resistance to CCR5 antagonists such as Maraviroc (MVC) and Vicriviroc (VVC) both in vitro and in vivo, with most changes arising in the gp120 V3 region. Both compounds bind to the same hydrophobic cavity in CCR5 in subtly different ways. Here, we investigated which V3 sequence changes are most associated with MVC and VVC resistance and how they affect the interaction between gp120 and the CCR5 NT. We found that VVC- and MVC-selected amino acid changes map to different V3 locations and involve residues that interact with the CCR5 NT in different ways. Changes in VVC-selected, but not MVC-selected, variants often involve charged residues. Although the overall V3 charge tends not to change, the introduction or removal of charged residues at specific positions affects the local electrostatic potential and could have structural and functional implications. In summary, VVC and MVC trigger the evolution of distinct HIV-1 resistance patterns in V3.

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.

Multiple CCR5 conformations on the cell surface are used differentially by human immunodeficiency viruses resistant or sensitive to CCR5 inhibitors

Resistance to small-molecule CCR5 inhibitors arises when HIV-1 variants acquire the ability to use inhibitor-bound CCR5 while still recognizing free CCR5. Two isolates, CC101.19 and D1/85.16, became resistant via four substitutions in the gp120 V3 region and three in the gp41 fusion peptide (FP), respectively. The binding characteristics of a panel of monoclonal antibodies (MAbs) imply that several antigenic forms of CCR5 are expressed at different levels on the surfaces of U87-CD4-CCR5 cells and primary CD4(+) T cells, in a cell-type-dependent manner. CCR5 binding and HIV-1 infection inhibition experiments suggest that the two CCR5 inhibitor-resistant viruses altered their interactions with CCR5 in different ways. As a result, both mutants became generally more sensitive to inhibition by CCR5 MAbs, and the FP mutant is specifically sensitive to a MAb that stains discrete cell surface clusters of CCR5 that may correspond to lipid rafts. We conclude that some MAbs detect different antigenic forms of CCR5 and that inhibitor-sensitive and -resistant viruses can use these CCR5 forms differently for entry in the presence or absence of CCR5 inhibitors.

Agonist-induced internalization of CC chemokine receptor 5 as a mechanism to inhibit HIV replication

The chemokine G protein-coupled receptor CC chemokine receptor 5 (CCR5) is used as an entry gate by CCR5-tropic and dual- or CCR5/CXC chemokine receptor 4-tropic strains of HIV to enter the human host cells. Thus, CCR5 antagonists (i.e., maraviroc) have been proven to be clinically effective by preventing the interaction between viral glycoprotein 120 and CCR5 and thus impeding viral entry into host cells. However, the emergence of HIV strains resistant to CCR5 antagonists has been reported in vitro and in vivo, where the virus has adapted to enter the cells via antagonist-bound CCR5. An alternative strategy that should obviate this mode of viral resistance would entail the ablation of the CCR5 portal for HIV entry from the cell surface through agonist-induced receptor internalization. Although this protective effect has been demonstrated clearly with natural CCR5 ligands, the chemoattractant properties of these chemokines have precluded them from further consideration in terms of drug development. Thus, we sought to explore the possibility of developing novel small molecules and selective CCR5 agonists devoid of eliciting chemotaxis. Indeed, the CCR5 agonists described herein were found to induce profound down-modulation of CCR5 (and not CXC chemokine receptor 4) from the cell surface and its sustained sequestration in the intracellular compartment without inducing chemotaxis in vitro. The bioactivity profile of these novel CCR5 agonists is exemplified by the compound (R)-2-(4-cyanophenyl)-N-(1-(1-(N,1-diphenylmethylsulfonamido)propan-2-yl)piperidin-4-yl)acetamide (ESN-196) that potently inhibits HIV-1 infection in human peripheral blood mononuclear cells and macrophages in vitro with potencies comparable to that of maraviroc and moreover demonstrates full activity against a maraviroc-resistant HIV-1 RU570 strain.

Resistance of a human immunodeficiency virus type 1 isolate to a small molecule CCR5 inhibitor can involve sequence changes in both gp120 and gp41

Here, we describe the genetic pathways taken by a human immunodeficiency virus type 1 (HIV-1) isolate, D101.12, to become resistant to the small molecule CCR5 inhibitor, vicriviroc (VCV), in vitro. Resistant D101.12 variants contained at least one substitution in the gp120 V3 region (H308P), plus one of two patterns of gp41 sequence changes involving the fusion peptide (FP) and a downstream residue: G514V+V535M or M518V+F519L+V535M. Studies of Env-chimeric and point-substituted viruses in peripheral blood mononuclear cells (PBMC) and TZM-bl cells showed that resistance can arise from the cooperative action of gp120 and gp41 changes, while retaining CCR5 usage. Modeling the VCV inhibition data from the two cell types suggests that D101.12 discriminates between high- and low-VCV affinity forms of CCR5 less than D1/85.16, a resistant virus with three FP substitutions.

Maraviroc: the first chemokine coreceptor 5 inhibitor

HIV viral entry occurs via viral interaction with host cell CD4 receptors and a second coreceptor, most commonly chemokine coreceptor (CCR)5. As a result, CCR5 antagonists have been developed to block HIV entry into CD4+ cells, thereby inhibiting viral replication. The first CCR5 inhibitor approved for use in the treatment of HIV was maraviroc. Maraviroc has been shown to be successful in reducing HIV replication in both antiretroviral treatment-experienced and treatment-naive populations. Since maraviroc is only efficacious against CCR5-tropic HIV virus, it is imperative to perform viral tropism testing prior to initiation of maraviroc. The currently available enhanced sensitivity Trofile™ assay (Monogram Biosciences, CA, USA) is the reference standard of tropism tests. Although it is highly sensitive, it remains a barrier to maraviroc use because it is expensive and has a long turnaround time. The development of simpler tropism assays may allow for more widespread use of maraviroc in the future. At present, maraviroc remains a highly useful drug in the management of HIV-infected persons infected with CCR5-tropic viruses.

Clinical use of CCR5 inhibitors in HIV and beyond

Since the discovery of CCR5 as a coreceptor for HIV entry, there has been interest in blockade of the receptor for treatment and prevention of HIV infection. Although several CCR5 antagonists have been evaluated in clinical trials, only maraviroc has been approved for clinical use in the treatment of HIV-infected patients. The efficacy, safety and resistance profile of CCR5 antagonists with a focus on maraviroc are reviewed here along with their usage in special and emerging clinical situations. Despite being approved for use since 2007, the optimal use of maraviroc has yet to be well-defined in HIV and potentially in other diseases. Maraviroc and other CCR5 antagonists have the potential for use in a variety of other clinical situations such as the prevention of HIV transmission, intensification of HIV treatment and prevention of rejection in organ transplantation. The use of CCR5 antagonists may be potentiated by other agents such as rapamycin which downregulate CCR5 receptors thus decreasing CCR5 density. There may even be a role for their use in combination with other entry inhibitors. However, clinical use of CCR5 antagonists may have negative consequences in diseases such as West Nile and Tick-borne encephalitis virus infections. In summary, CCR5 antagonists have great therapeutic potential in the treatment and prevention of HIV as well as future use in novel situations such as organ transplantation. Their optimal use either alone or in combination with other agents will be defined by further investigation.

Rigid amphipathic fusion inhibitors, small molecule antiviral compounds against enveloped viruses

Antiviral drugs targeting viral proteins often result in prompt selection for resistance. Moreover, the number of viral targets is limited. Novel antiviral targets are therefore needed. The unique characteristics of fusion between virion envelopes and cell membranes may provide such targets. Like all fusing bilayers, viral envelopes locally adopt hourglass-shaped stalks during the initial stages of fusion, a process that requires local negative membrane curvature. Unlike cellular vesicles, however, viral envelopes do not redistribute lipids between leaflets, can only use the energy released by virion proteins, and fuse to the extracellular leaflets of cell membranes. Enrichment in phospholipids with hydrophilic heads larger than their hydrophobic tails in the convex outer leaflet of vesicles favors positive curvature, therefore increasing the activation energy barrier for fusion. Such phospholipids can increase the activation barrier beyond the energy provided by virion proteins, thereby inhibiting viral fusion. However, phospholipids are not pharmacologically useful. We show here that a family of synthetic rigid amphiphiles of shape similar to such phospholipids, RAFIs (rigid amphipathic fusion inhibitors), inhibit the infectivity of several otherwise unrelated enveloped viruses, including hepatitis C and HSV-1 and -2 (lowest apparent IC(50) 48 nM), with no cytotoxic or cytostatic effects (selectivity index > 3,000) by inhibiting the increased negative curvature required for the initial stages of fusion.

Profile of maraviroc: a CCR5 antagonist in the management of treatment-experienced HIV patients

Maraviroc is the first and, so far, the only licensed representative of the class of chemokine receptor type 5 (CCR5) inhibitors used for the treatment of human immunodeficiency virus (HIV) infection. Its safety and efficacy were demonstrated in several clinical trials, and its use was approved in 2007 by the responsible authorities. Some specific issues are correlated with maraviroc and its use. It is the only drug in the antiretroviral armamentarium, which does not interact with the viral enzymes but with a human receptor. Hence, it is able to be long-term effective only if the infecting virus uses, exclusively, the CCR5 receptor. Occurrence and detection of the CCR5 tropism are some of the great challenges of maraviroc use in treatment-experienced patients. Although up to 80% of naive patients harbor CCR5-tropic virus, the occurrence of CXCR4 or other tropisms increases with the duration of HIV infection and treatment. Nonetheless, maraviroc is a potent medication for eligible patients and helps to improve the outcome of antiretroviral treatment (ART) of HIV infection.

A maraviroc-resistant HIV-1 with narrow cross-resistance to other CCR5 antagonists depends on both N-terminal and extracellular loop domains of drug-bound CCR5

CCR5 antagonists inhibit HIV entry by binding to a coreceptor and inducing changes in the extracellular loops (ECLs) of CCR5. In this study, we analyzed viruses from 11 treatment-experienced patients who experienced virologic failure on treatment regimens containing the CCR5 antagonist maraviroc (MVC). Viruses from one patient developed high-level resistance to MVC during the course of treatment. Although resistance to one CCR5 antagonist is often associated with broad cross-resistance to other agents, these viruses remained sensitive to most other CCR5 antagonists, including vicriviroc and aplaviroc. MVC resistance was dependent upon mutations within the V3 loop of the viral envelope (Env) protein and was modulated by additional mutations in the V4 loop. Deep sequencing of pretreatment plasma viral RNA indicated that resistance appears to have occurred by evolution of drug-bound CCR5 use, despite the presence of viral sequences predictive of CXCR4 use. Envs obtained from this patient before and during MVC treatment were able to infect cells expressing very low CCR5 levels, indicating highly efficient use of a coreceptor. In contrast to previous reports in which CCR5 antagonist-resistant viruses interact predominantly with the N terminus of CCR5, these MVC-resistant Envs were also dependent upon the drug-modified ECLs of CCR5 for entry. Our results suggest a model of CCR5 cross-resistance whereby viruses that predominantly utilize the N terminus are broadly cross-resistant to multiple CCR5 antagonists, whereas viruses that require both the N terminus and antagonist-specific ECL changes demonstrate a narrow cross-resistance profile.

Role of CXCR4 in HIV infection and its potential as a therapeutic target

The chemokine receptors CCR5 and CXCR4 are the two major coreceptors for HIV entry. Numerous efforts have been made to develop a new class of anti-HIV agents that target these coreceptors as an additional or alternative therapy to standard HAART. Among the CCR5 inhibitors developed so far, maraviroc is the first drug that has been approved by the US FDA for treating patients with R5 HIV-1. Although many CXCR4 inhibitors, some of which are highly active and orally bioavailable, have also been studied, they are still at preclinical stages or have been suspended during development. Importantly, the interaction between CXCR4 and its ligand SDF-1 is involved in various disease conditions, such as cancer cell metastasis, leukemia cell proliferation, rheumatoid arthritis and pulmonary fibrosis. Therefore, CXCR4 inhibitors have potential as novel therapeutics for the treatment of these diseases as well as HIV infection.

HIV-1 Entry, Inhibitors, and Resistance

Entry inhibitors represent a new class of antiretroviral agents for the treatment of infection with HIV-1. While resistance to other HIV drug classes has been well described, resistance to this new class is still ill defined despite considerable clinical use. Several potential mechanisms have been proposed: tropism switching (utilization of CXCR4 instead of CCR5 for entry), increased affinity for the coreceptor, increased rate of virus entry into host cells, and utilization of inhibitor-bound receptor for entry. In this review we will address the development of attachment, fusion, and coreceptor entry inhibitors and explore recent studies describing potential mechanisms of resistance.

HIV-1 resistance to CCR5 antagonists associated with highly efficient use of CCR5 and altered tropism on primary CD4+ T cells

We previously reported on a panel of HIV-1 clade B envelope (Env) proteins isolated from a patient treated with the CCR5 antagonist aplaviroc (APL) that were drug resistant. These Envs used the APL-bound conformation of CCR5, were cross resistant to other small-molecule CCR5 antagonists, and were isolated from the patient's pretreatment viral quasispecies as well as after therapy. We analyzed viral and host determinants of resistance and their effects on viral tropism on primary CD4(+) T cells. The V3 loop contained residues essential for viral resistance to APL, while additional mutations in gp120 and gp41 modulated the magnitude of drug resistance. However, these mutations were context dependent, being unable to confer resistance when introduced into a heterologous virus. The resistant virus displayed altered binding between gp120 and CCR5 such that the virus became critically dependent on the N' terminus of CCR5 in the presence of APL. In addition, the drug-resistant Envs studied here utilized CCR5 very efficiently: robust virus infection occurred even when very low levels of CCR5 were expressed. However, recognition of drug-bound CCR5 was less efficient, resulting in a tropism shift toward effector memory cells upon infection of primary CD4(+) T cells in the presence of APL, with relative sparing of the central memory CD4(+) T cell subset. If such a tropism shift proves to be a common feature of CCR5-antagonist-resistant viruses, then continued use of CCR5 antagonists even in the face of virologic failure could provide a relative degree of protection to the T(CM) subset of CD4(+) T cells and result in improved T cell homeostasis and immune function.

Different selection patterns of resistance and cross-resistance to HIV-1 agents targeting CCR5

OBJECTIVES

Identification of CCR5 as an antiretroviral target led to the development of several CCR5 antagonists in clinical trials and the approval of maraviroc. Evaluating the mechanism of drug resistance to CCR5 agents may have implications in the clinical development of this class of agents. We have analysed the resistance profile of two R5 HIV-1 strains [BaL and a clinical isolate (CI)] after long-term passage in cell culture in the presence of TAK-779, the first developed non-peptidic small molecule targeting CCR5.

METHODS

Genotypic and phenotypic tests were used to evaluate the resistance of virus isolated from cell culture in the presence of the CCR5 inhibitor TAK-779.

RESULTS

Mutations conferring resistance appeared in the gp120 sequence but were not confined to the V3 loop region, and both strains had a different mutation pattern. Recombination of the env gene of the BaL-derived resistant virus into the HIV-1 HXB2 wild-type backbone conferred resistance to TAK-779 and cross-resistance to maraviroc, with 63- and 11-fold changes in their EC(50) (50% effective concentration), respectively, together with an apparent reduction of the maximal plateau inhibition (MPI) of TAK-779 but not of maraviroc. Conversely, the resistant CI viruses showed an approximately 50% reduction in MPI for both TAK-779 and maraviroc.

CONCLUSIONS

We confirm that different pathways to the generation of CCR5 drug resistance/cross-resistance may occur that strongly depend on cell culture conditions, CCR5 availability and the genetic background of the HIV strain. Our study provides complementary information to understand the complexity of resistance to CCR5 antagonists.

Human immunodeficiency virus type 1 evasion of a neutralizing anti-V3 antibody involves acquisition of a potential glycosylation site in V2

It has been reported that the addition of a potential N-linked glycosylation site (PNGS) to the gp120 human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein provides protection against neutralizing antibodies (NAbs) by acting as a 'glycan shield'. In this study, we induced insertion of a PNGS into the V2 region of HIV-1(BaL) with the KD-247 anti-V3 neutralizing monoclonal antibody. In the presence of KD-247 (200 microg ml(-1)) at passage five, viruses with 3 aa mutations in the C2 (T240S and I283T) and V3 (T319A) regions expanded from pre-existing variants. After six passages with KD-247 (>300 microg ml(-1)), a PNGS emerged in the V2 region in addition to C2 (T240S) and V3 mutations (R315K and F317L). A variant with a PNGS insertion in V2, but no V3 mutations was sensitive to KD-247, whereas a clone with a V2 PNGS insertion and mutations in V3 demonstrated a high level of resistance to KD-247. Replication kinetic analysis revealed that the F317L mutation in V3 played a compensatory role for fitness-loss caused by the PNGS insertion in V2. The evading HIV-1 variant did not revert back to the wild-type virus after 14 passages without KD-247. These findings demonstrate that the virus with fitness-loss mutations can replicate equally as well as the wild-type virus to acquire some key mutations in the V3 stem and the C2 region, and the compensated variants containing PNGS do not revert back to the ancestral virus even in the absence of NAb.

Structure-function analysis of human immunodeficiency virus type 1 gp120 amino acid mutations associated with resistance to the CCR5 coreceptor antagonist vicriviroc

Vicriviroc (VCV) is a small-molecule CCR5 coreceptor antagonist currently in clinical trials for treatment of R5-tropic human immunodeficiency virus type 1 (HIV-1) infection. With this drug in development, identification of resistance mechanisms to VCV is needed to allow optimal outcomes in clinical practice. In this study we further characterized VCV resistance in a lab-adapted, VCV-resistant RU570 virus (RU570-VCV(res)). We show that K305R, R315Q, and K319T amino acid changes in the V3 loop, along with P437S in C4, completely reproduced the resistance phenotype in a chimeric ADA envelope containing the C2-V5 region from RU570 passage control gp120. The K305R amino acid change primarily impacted the degree of resistance, whereas K319T contributed to both resistance and virus infectivity. The P437S mutation in C4 had more influence on the relative degree of virus infectivity, while the R315Q mutation contributed to the virus concentration-dependent phenotypic resistance pattern observed for RU570-VCV(res). RU570-VCV(res) pseudovirus entry with VCV-bound CCR5 was dramatically reduced by Y10A, D11A, Y14A, and Y15A mutations in the N terminus of CCR5, whereas these mutations had less impact on entry in the absence of VCV. Notably, an additional Q315E/I317F substitution in the crown region of the V3 loop enhanced resistance to VCV, resulting in a stronger dependence on the N terminus for viral entry. By fitting the envelope mutations to a molecular model of a recently described docked N-terminal CCR5 peptide consisting of residues 2 to 15 in complex with HIV-1 gp120 CD4, potential new interactions in gp120 with the N terminus of CCR5 were uncovered. The cumulative results of this study suggest that as the RU570 VCV-resistant virus adapted to use the drug-bound receptor, it also developed an increased reliance on the N terminus of CCR5.

Two HIV-1 variants resistant to small molecule CCR5 inhibitors differ in how they use CCR5 for entry

HIV-1 variants resistant to small molecule CCR5 inhibitors recognize the inhibitor-CCR5 complex, while also interacting with free CCR5. The most common genetic route to resistance involves sequence changes in the gp120 V3 region, a pathway followed when the primary isolate CC1/85 was cultured with the AD101 inhibitor in vitro, creating the CC101.19 resistant variant. However, the D1/86.16 escape mutant contains no V3 changes but has three substitutions in the gp41 fusion peptide. By using CCR5 point-mutants and gp120-targeting agents, we have investigated how infectious clonal viruses derived from the parental and both resistant isolates interact with CCR5. We conclude that the V3 sequence changes in CC101.19 cl.7 create a virus with an increased dependency on interactions with the CCR5 N-terminus. Elements of the CCR5 binding site associated with the V3 region and the CD4-induced (CD4i) epitope cluster in the gp120 bridging sheet are more exposed on the native Env complex of CC101.19 cl.7, which is sensitive to neutralization via these epitopes. However, D1/86.16 cl.23 does not have an increased dependency on the CCR5 N-terminus, and its CCR5 binding site has not become more exposed. How this virus interacts with the inhibitor-CCR5 complex remains to be understood.

Human immunodeficiency virus type 1 (HIV-1) is the causative agent of AIDS. HIV-1 entry into target cells is triggered by the interaction of the viral envelope glycoproteins with a cell-surface receptor (CD4) and a co-receptor (CCR5), and culminates in fusion of the viral and cell membranes. Small molecule inhibitors that bind to CCR5 are a new class of drug for treating HIV-1-infected people. However, HIV-1 can evolve ways to become resistant to these compounds, by acquiring mutations that alter how its envelope glycoproteins (gp120-gp41) interact with CCR5. In this study, we investigated how two resistant viruses gained the ability to use the inhibitor-bound form of CCR5 through two different mechanisms. In the first virus, four amino acid substitutions in the V3 region of gp120 created an increased dependency on interactions with the CCR5 N-terminus. These changes altered the configuration of gp120, increasing the exposure of antibody epitopes in the V3 region and the CD4i epitope cluster associated with the CCR5 binding site. In contrast, the second virus, which became resistant via three sequence changes in the gp41 subunit, did not become more dependent on the CCR5 N-terminus and remained resistant to neutralization by antibodies against elements of the CCR5 binding site.

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.

Molecular basis of human immunodeficiency virus drug resistance: an update

Antiretroviral therapy has led to a significant decrease in human immunodeficiency virus (HIV)-related mortality. Approved antiretroviral drugs target different steps of the viral life cycle including viral entry (coreceptor antagonists and fusion inhibitors), reverse transcription (nucleoside and non-nucleoside inhibitors of the viral reverse transcriptase), integration (integrase inhibitors) and viral maturation (protease inhibitors). Despite the success of combination therapies, the emergence of drug resistance is still a major factor contributing to therapy failure. Viral resistance is caused by mutations in the HIV genome coding for structural changes in the target proteins that can affect the binding or activity of the antiretroviral drugs. This review provides an overview of the molecular mechanisms involved in the acquisition of resistance to currently used and promising investigational drugs, emphasizing the structural role of drug resistance mutations. The optimization of current antiretroviral drug regimens and the development of new drugs are still challenging issues in HIV chemotherapy. 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.

Inefficient entry of vicriviroc-resistant HIV-1 via the inhibitor-CCR5 complex at low cell surface CCR5 densities

HIV-1 variants resistant to small molecule CCR5 inhibitors such as vicriviroc (VVC) have modified Env complexes that can use both the inhibitor-bound and -free forms of the CCR5 co-receptor to enter target cells. However, entry via the inhibitor-CCR5 complex is inefficient in some, but not all, cell types, particularly cell lines engineered to express CCR5. We investigated the effect of increasing CCR5 expression, and hence the density of the inhibitor-CCR5 complex when a saturating inhibitor (VVC) concentration was present, by using 293-Affinofile cells, in which CCR5 expression is up-regulated by the transcriptional activator, ponasterone. When CCR5 expression was low, the resistant virus entered the target cells to a lesser extent when VVC was present than absent. However, at a higher CCR5 level, there was much less entry inhibition at a constant, saturating VVC concentration. We conclude that the relative decrease in entry of a VVC-resistant virus in some cell types results from its less efficient use of the VVC-CCR5 complex, and that increasing the CCR5 expression level can compensate for this inefficiency.