Recent progress in antiretrovirals--lessons from resistance

A Combined approach of Pharmacophore Modeling, QSAR Study, Molecular Docking and in silico ADME/Tox prediction of 4-Arylthio & 4- Aryloxy-3- Iodopyridine-2(1H)-one analogs to identify potential Reverse Transcriptase inhibitor: Anti-HIV agents

BACKGROUND

Reverse transcriptase is an important therapeutic target to treat AIDS caused by the Human Immunodeficiency Virus (HIV). Despite many effective anti-HIV drugs, reverse transcriptase (RT) inhibitors remain the cornerstone of the drug regimen to treat AIDS. In the present work, we have expedited the use of different computational modules and presented an easy, cost-effective and high throughput screening method to identify potential reverse transcriptase inhibitors.

METHODS

A congeneric series of 4-Arylthio & 4-Aryloxy-3- Iodopyridine-2(1H)-one analogs having anti-HIV activity were subjected to structure-based 2D, 3D QSAR, Pharmacophore Modeling, and Molecular Docking to elucidate the structural properties required for the design of potent HIV-RT inhibitors. Prediction of preliminary Pharmacokinetic and the Drug Likeliness profile was performed for these compounds by in silico ADME study.

RESULTS

The 2D and 3D- QSAR models were developed by correlating two and three-dimensional descriptors with activity (pIC50) by sphere exclusion method and k-nearest neighbor molecular field analysis approach, respectively. The significant 2D- QSAR model developed by Partial Least Square associated with the Sphere Exclusion method (PLS-SE) having r2 and q2 values 0.9509 and 0.8038 respectively. The 3D-QSAR model by Step Wise variable selection method (SW-kNN MFA) is more significant which has a cross-validated squared correlation coefficient q2= 0.8509 and a non-cross-validated correlation coefficient pred_r2= 0.8102. The pharmacophore hypothesis was developed which comprised 5 features includes 3 aliphatic regions (Ala), 1 H-bond donor (HDr) and 1 H-bond acceptor (HAc). Docking studies of the selected inhibitors with the active site of reverse transcriptase enzyme showed hydrogen bond and π - π interaction with LYS-101, LYS-103, TYR- 181, TYR-188 and TRP-229 residues present at the active site. All the candidates with good bioavailability and ADMET drug likeliness properties.

CONCLUSION

The results of the present work provide more useful information and important structural insights for the discovery, design of novel and potent reverse transcriptase inhibitors with high therapeutic windows in the future.

Neoflavonoids as Inhibitors of HIV-1 Replication by Targeting the Tat and NF-κB Pathways

Twenty-eight neoflavonoids have been prepared and evaluated in vitro against HIV-1. Antiviral activity was assessed on MT-2 cells infected with viral clones carrying the luciferase reporter gene. Inhibition of HIV transcription and Tat function were tested on cells stably transfected with the HIV-LTR and Tat protein. Seven 4-phenylchromen-2-one derivatives showed HIV transcriptional inhibitory activity but only the phenylchrome-2-one 10 inhibited NF-κB and displayed anti-Tat activity simultaneously. Compounds 10, 14, and 25, inhibited HIV replication in both targets at concentrations <25 μM. The assays of these synthetic 4-phenylchromen-2-ones may aid in the investigation of some aspects of the anti-HIV activity of such compounds and could serve as a scaffold for designing better anti-HIV compounds, which may lead to a potential anti-HIV therapeutic drug.

Prevalence of Transmitted Drug-Resistance Mutations and Polymorphisms in HIV-1 Reverse Transcriptase, Protease, and gp41 Sequences Among Recent Seroconverters in Southern Poland

Background

Monitoring of drug resistance-related mutations among patients with recent HIV-1 infection offers an opportunity to describe current patterns of transmitted drug resistance (TDR) mutations.

Material/Methods

Of 298 individuals newly diagnosed from March 2008 to February 2014 in southern Poland, 47 were deemed to have recent HIV-1 infection by the limiting antigen avidity immunoassay. Proviral DNA was amplified and sequenced in the reverse transcriptase, protease, and gp41 coding regions. Mutations were interpreted according to the Stanford Database algorithm and/or the International Antiviral Society USA guidelines. TDR mutations were defined according to the WHO surveillance list.

Results

Among 47 patients with recent HIV-1 infection only 1 (2%) had evidence of TDR mutation. No major resistance mutations were found, but the frequency of strains with ≥1 accessory resistance-associated mutations was high, at 98%. Accessory mutations were present in 11% of reverse transcriptase, 96% of protease, and 27% of gp41 sequences. Mean number of accessory resistance mutations in the reverse transcriptase and protease sequences was higher in viruses with no compensatory mutations in the gp41 HR2 domain than in strains with such mutations (p=0.031).

Conclusions

Despite the low prevalence of strains with TDR mutations, the frequency of accessory mutations was considerable, which may reflect the history of drug pressure among transmitters or natural viral genetic diversity, and may be relevant for future clinical outcomes. The accumulation of the accessory resistance mutations within the pol gene may restrict the occurrence of compensatory mutations related to enfuvirtide resistance or vice versa.

HIV-1 gag: an emerging target for antiretroviral therapy

The advances made in the treatment of HIV-1 infection represent a major success of modern biomedical research, prolonging healthy life and reducing virus transmission. There remain, however, many challenges relating primarily to side effects of long-term therapy and the ever-present danger of the emergence of drug-resistant strains. To counter these threats, there is a continuing need for new and better drugs, ideally targeting multiple independent steps in the HIV-1 replication cycle. The most successful current drugs target the viral enzymes: protease (PR), reverse transcriptase (RT), and integrase (IN). In this review, we outline the advances made in targeting the Gag protein and its mature products, particularly capsid and nucleocapsid, and highlight possible targets for future pharmacological intervention.

Natural polymorphisms and unusual mutations in HIV-1 protease with potential antiretroviral resistance: a bioinformatic analysis

Background

The correlations of genotypic and phenotypic tests with treatment, clinical history and the significance of mutations in viruses of HIV-infected patients are used to establish resistance mutations to protease inhibitors (PIs). Emerging mutations in human immunodeficiency virus type 1 (HIV-1) protease confer resistance to PIs by inducing structural changes at the ligand interaction site. The aim of this study was to establish an in silico structural relationship between natural HIV-1 polymorphisms and unusual HIV-1 mutations that confer resistance to PIs.

Results

Protease sequences isolated from 151 Mexican HIV-1 patients that were naïve to, or subjected to antiretroviral therapy, were examined. We identified 41 unrelated resistance mutations with a prevalence greater than 1%. Among these mutations, nine exhibited positive selection, three were natural polymorphisms (L63S/V/H) in a codon associated with drug resistance, and six were unusual mutations (L5F, D29V, L63R/G, P79L and T91V). The D29V mutation, with a prevalence of 1.32% in the studied population, was only found in patients treated with antiretroviral drugs. Using in silico modelling, we observed that D29V formed unstable protease complexes when were docked with lopinavir, saquinavir, darunavir, tipranavir, indinavir and atazanavir.

Conclusions

The structural correlation of natural polymorphisms and unusual mutations with drug resistance is useful for the identification of HIV-1 variants with potential resistance to PIs. The D29V mutation likely confers a selection advantage in viruses; however, in silico, presence of this mutation results in unstable enzyme/PI complexes, that possibly induce resistance to PIs.

New findings in cleavage sites variability across groups, subtypes and recombinants of human immunodeficiency virus type 1

Background

Polymorphisms at cleavage sites (CS) can influence Gag and Pol proteins processing by the viral protease (PR), restore viral fitness and influence the virological outcome of specific antiretroviral drugs. However, data of HIV-1 variant-associated CS variability is scarce.

Methods

In this descriptive research, we examine the effect of HIV-1 variants on CS conservation using all 9,028 gag and 3,906 pol HIV-1 sequences deposited in GenBank, focusing on the 110 residues (10 per site) involved at 11 CS: P17/P24, P24/P2, P2/P7, P7/P1, P1/P6^gag, NC/TFP, TFP/P6^pol, P6^pol/PR, PR/RT^p51, RT^p51/RT^p66 and RT^p66/IN. CS consensus amino acid sequences across HIV-1 groups (M, O, N, P), group M 9 subtypes and 51 circulating recombinant forms (CRF) were inferred from our alignments and compared to the HIV-1 consensus-of-consensuses sequence provided by GenBank.

Results

In all HIV-1 variants, the most conserved CS were PR/RT^p51, RT^p51/RT^p66, P24/P2 and RT^p66/IN and the least P2/P7 and P6^pol/PR. Conservation was significantly lower in subtypes vs. recombinants in P2/P7 and TFP/P6^pol and higher in P17/P24. We found a significantly higher conservation rate among Group M vs. non-M Groups HIV-1. The late processing sites at Gag (P7/P1) and GagPol precursors (PR/RT^p51) presented a significantly higher conservation vs. the first CS (P2/P7) in the 4 HIV-1 groups. Here we show 52 highly conserved residues across HIV-1 variants in 11 CS and the amino acid consensus sequence in each HIV-1 group and HIV-1 group M variant for each 11 CS.

Conclusions

This is the first study to describe the CS conservation level across all HIV-1 variants and 11 sites in one of the largest available sequence HIV-1 dataset. These results could help other researchers for the future design of both novel antiretroviral agents acting as maturation inhibitors as well as for vaccine targeting CS.

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.

Immunogenicity of sequences around HIV-1 protease cleavage sites: potential targets and population coverage analysis for a HIV vaccine targeting protease cleavage sites

Developing an effective preventative vaccine against HIV-1 has proved to be a great challenge. The classical and proven vaccine approach has failed so far or produced a modest effect, new approaches are needed. In this study we evaluated the immunogenicity of the sequences around the protease cleavage sites (PCS) and the population coverage of a vaccine targeting HIV-1 PCS. The sequence conservation was evaluated by comparing entropy score of sequences around PCS with Gag and Pol. The immunogenicity of sequences around the 12 PCS (+10/-10 amino acids) was analyzed by identifying epitopes of HLA class I alleles in PCS region using four approaches: (1) identification of previously reported HLA class I allele epitopes around PCS region; (2) screening and validating epitopes of 8 HLA class I alleles common to most world populations using iTopia Epitope Discovery system and IFN-γ ELISpot assays; (3) screening of 151 patients of Pumwani cohort for PBMC IFN-γ ELISPOT responses to the subtype A and D consensus around PCS region; and (4) prediction of HLA alleles with epitopes around the PCS using NetMHCpan. Population coverage was calculated using the web-based analysis tool of the Immune Epitope Database based on HLA class I genotype frequencies from dbMHC database. The results showed that many HLA class I alleles have multiple epitopes in the 12 PCS regions, indicating sequence immunogenicity around PCS. Multiple epitopes of many HLA class I alleles common to >95% world populations have been identified around the 12 PCS region. Targeting these sites is a feasible vaccine approach.

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.

Quantitative prediction of integrase inhibitor resistance from genotype through consensus linear regression modeling

BACKGROUND

Integrase inhibitors (INI) form a new drug class in the treatment of HIV-1 patients. We developed a linear regression modeling approach to make a quantitative raltegravir (RAL) resistance phenotype prediction, as Fold Change in IC50 against a wild type virus, from mutations in the integrase genotype.

METHODS

We developed a clonal genotype-phenotype database with 991 clones from 153 clinical isolates of INI naïve and RAL treated patients, and 28 site-directed mutants.We did the development of the RAL linear regression model in two stages, employing a genetic algorithm (GA) to select integrase mutations by consensus. First, we ran multiple GAs to generate first order linear regression models (GA models) that were stochastically optimized to reach a goal R2 accuracy, and consisted of a fixed-length subset of integrase mutations to estimate INI resistance. Secondly, we derived a consensus linear regression model in a forward stepwise regression procedure, considering integrase mutations or mutation pairs by descending prevalence in the GA models.

RESULTS

The most frequently occurring mutations in the GA models were 92Q, 97A, 143R and 155H (all 100%), 143G (90%), 148H/R (89%), 148K (88%), 151I (81%), 121Y (75%), 143C (72%), and 74M (69%). The RAL second order model contained 30 single mutations and five mutation pairs (p < 0.01): 143C/R&97A, 155H&97A/151I and 74M&151I. The R2 performance of this model on the clonal training data was 0.97, and 0.78 on an unseen population genotype-phenotype dataset of 171 clinical isolates from RAL treated and INI naïve patients.

CONCLUSIONS

We describe a systematic approach to derive a model for predicting INI resistance from a limited amount of clonal samples. Our RAL second order model is made available as an Additional file for calculating a resistance phenotype as the sum of integrase mutations and mutation pairs.

Antiviral agents: structural basis of action and rational design

During the last 30 years, significant progress has been made in the development of novel antiviral drugs, mainly crystallizing in the establishment of potent antiretroviral therapies and the approval of drugs inhibiting hepatitis C virus replication. Although major targets of antiviral intervention involve intracellular processes required for the synthesis of viral proteins and nucleic acids, a number of inhibitors blocking virus assembly, budding, maturation, entry or uncoating act on virions or viral capsids. In this review, we focus on the drug discovery process while presenting the currently used methodologies to identify novel antiviral drugs by using a computer-based approach. We provide examples illustrating structure-based antiviral drug development, specifically neuraminidase inhibitors against influenza virus (e.g. oseltamivir and zanamivir) and human immunodeficiency virus type 1 protease inhibitors (i.e. the development of darunavir from early peptidomimetic compounds such as saquinavir). A number of drugs in preclinical development acting against picornaviruses, hepatitis B virus and human immunodeficiency virus and their mechanism of action are presented to show how viral capsids can be exploited as targets of antiviral therapy.

Recombinant rabbit single-chain antibodies bind to the catalytic and C-terminal domains of HIV-1 integrase protein and strongly inhibit HIV-1 replication

The human immunodeficiency virus type 1 (HIV-1) integrase (IN) protein plays an important role during the early stages of the retroviral life cycle and therefore is an attractive target for therapeutic intervention. We immunized rabbits with HIV-1 IN protein and developed a combinatorial single-chain variable fragment (scFv) library against IN. Five different scFv antibodies with high binding activity and specificity for IN were identified. These scFvs recognize the catalytic and C-terminal domains of IN and block the strand-transfer process. Cells expressing anti-IN-scFvs were highly resistant to HIV-1 replication due to an inhibition of the integration process itself. These results provide proof-of-concept that rabbit anti-IN-scFv intrabodies can be designed to block the early stages of HIV-1 replication without causing cellular toxicity. Therefore, these anti-IN-scFvs may be useful agents for "intracellular immunization"-based gene therapy strategies. Furthermore, because of their epitope binding characteristics, these scFvs can be used also as new tools to study the structure and function of HIV-1 IN protein.

3-Phenylcoumarins as inhibitors of HIV-1 replication

We have synthesized fourteen 3-phenylcoumarin derivatives and evaluated their anti-HIV activity. Antiviral activity was assessed on MT-2 cells infected with viral clones carrying the luciferase gene as reporter. Inhibition of HIV transcription and Tat function were tested on cells stably transfected with the HIV-LTR and Tat protein. Six compounds displayed NF-κB inhibition, four resulted Tat antagonists and three of them showed both activities. Three compounds inhibited HIV replication with IC₅₀ values < 25 µM. The antiviral effect of the 4-hydroxycoumarin derivative 19 correlates with its specific inhibition of Tat functions, while compound 8, 3-(2-chlorophenyl)coumarin, seems to act through a mechanism unrelated to the molecular targets considered in this research.

HIV type 1 Gag as a target for antiviral therapy

The Gag proteins of HIV-1 are central players in virus particle assembly, release, and maturation, and also function in the establishment of a productive infection. Despite their importance throughout the replication cycle, there are currently no approved antiretroviral therapies that target the Gag precursor protein or any of the mature Gag proteins. Recent progress in understanding the structural and cell biology of HIV-1 Gag function has revealed a number of potential Gag-related targets for possible therapeutic intervention. In this review, we summarize our current understanding of HIV-1 Gag and suggest some approaches for the development of novel antiretroviral agents that target Gag.

Does antiretroviral therapy improve HIV-associated cognitive impairment? A quantitative review of the literature

The development of antiretroviral therapy (ART) has dramatically improved survival for those living with human immunodeficiency virus (HIV), but whether ART improves cognitive functioning remains unclear. The aim of the present review was to examine systematically the extent to which ART improves cognition among individuals with HIV using meta-analytic methods. Twenty-three studies were included in the quantitative review. ART was associated with modest improvements in attention (mean d = .17; p < .001; 95% confidence interval [CI], .09/.25), executive function (mean d = .18; p < .001; 95% CI, .10/.26), and motor function (mean d = .24; p < .001; 95% CI, .16/.32). ART did not improve language, verbal memory, visual memory or visuospatial function. The extent to which cognition improved was correlated with the change in CD4 cell count following ART, suggesting a link between cognitive outcome and immune system integrity. Together, the present findings indicate that the neuropsychological test performance of most HIV patients taking ART is comparable to those not taking ART. Development of pharmaceutical treatments and rehabilitation strategies that target the cognitive effects of HIV infection is needed.

New insights into HIV assembly and trafficking

Assembly and release of human immunodeficiency virus type 1 (HIV-1) particles is mediated by the viral Gag polyprotein precursor. Gag is synthesized in the cytosol and rapidly translocates to membrane to orchestrate particle production. The cell biology of HIV-1 Gag trafficking is currently one of the least understood aspects of HIV-1 replication. In this review, we highlight the current understanding of the cellular machinery involved in Gag trafficking and virus assembly.

HIV-1 maturation inhibitor bevirimat stabilizes the immature Gag lattice

Maturation of nascent virions, a key step in retroviral replication, involves cleavage of the Gag polyprotein by the viral protease into its matrix (MA), capsid (CA), and nucleocapsid (NC) components and their subsequent reorganization. Bevirimat (BVM) defines a new class of antiviral drugs termed maturation inhibitors. BVM acts by blocking the final cleavage event in Gag processing, the separation of CA from its C-terminal spacer peptide 1 (SP1). Prior evidence suggests that BVM binds to Gag assembled in immature virions, preventing the protease from accessing the CA-SP1 cleavage site. To investigate this hypothesis, we used cryo-electron tomography to examine the structures of (noninfectious) HIV-1 viral particles isolated from BVM-treated cells. We find that these particles contain an incomplete shell of density underlying the viral envelope, with a hexagonal honeycomb structure similar to the Gag lattice of immature HIV but lacking the innermost, NC-related, layer. We conclude that the shell represents a remnant of the immature Gag lattice that has been processed, except at the CA-SP1 sites, but has remained largely intact. We also compared BVM-treated particles with virions formed by the mutant CA5, in which cleavage between CA and SP1 is also blocked. Here, we find a thinner CA-related shell with no visible evidence of honeycomb organization, indicative of an altered conformation and further suggesting that binding of BVM stabilizes the immature lattice. In both cases, the observed failure to assemble mature capsids correlates with the loss of infectivity.

EASY-HIT: HIV full-replication technology for broad discovery of multiple classes of HIV inhibitors

HIV replication assays are important tools for HIV drug discovery efforts. Here, we present a full HIV replication system (EASY-HIT) for the identification and analysis of HIV inhibitors. This technology is based on adherently growing HIV-susceptible cells, with a stable fluorescent reporter gene activated by HIV Tat and Rev. A fluorescence-based assay was designed that measures HIV infection by two parameters relating to the early and the late phases of HIV replication, respectively. Validation of the assay with a panel of nine reference inhibitors yielded effective inhibitory concentrations consistent with published data and allowed discrimination between inhibitors of early and late phases of HIV replication. Finer resolution of the effects of reference drugs on different steps of HIV replication was achieved in secondary time-of-addition assays. The EASY-HIT assay yielded high Z' scores (>0.9) and signal stabilities, confirming its robustness. Screening of the LOPAC(1280) library identified 10 compounds (0.8%), of which eight were known to inhibit HIV, validating the suitability of this assay for screening applications. Studies evaluating anti-HIV activities of natural products with the EASY-HIT technology led to the identification of three novel inhibitory compounds that apparently act at different steps of HIV-1 replication. Furthermore, we demonstrate successful evaluation of plant extracts for HIV-inhibitory activities, suggesting application of this technology for the surveillance of biological extracts with anti-HIV activities. We conclude that the EASY-HIT technology is a versatile tool for the discovery and characterization of HIV inhibitors.

Virus maturation as a new HIV-1 therapeutic target

Development of novel therapeutic targets against HIV-1 is a high research priority owing to the serious clinical consequences associated with acquisition of resistance to current antiretroviral drugs. The HIV-1 structural protein Gag represents a potential new therapeutic target as it plays a central role in virus particle production yet is not targeted by any of the antiretroviral drugs approved at present. The Gag polyprotein precursor multimerizes to form immature particles that bud from the infected cell. Concomitant with virus release, the Gag precursor undergoes proteolytic processing by the viral protease to generate the mature Gag proteins, which include capsid (CA). Once liberated from the Gag polyprotein precursor, CA molecules interact to reassemble into a condensed conical core, which organizes the viral RNA genome and several viral proteins to facilitate virus replication in the next round of infection. Correct Gag proteolytic processing and core assembly are therefore essential for virus infectivity. In this review, we discuss new strategies to inhibit maturation by targeting proteolytic cleavage sites in Gag or CA-CA interactions required for core formation. The identification and development of lead maturation inhibitors are highlighted.

Novel approaches to inhibiting HIV-1 replication

Considerable success has been achieved in the treatment of HIV-1 infection, and more than two-dozen antiretroviral drugs are available targeting several distinct steps in the viral replication cycle. However, resistance to these compounds emerges readily, even in the context of combination therapy. Drug toxicity, adverse drug-drug interactions, and accompanying poor patient adherence can also lead to treatment failure. These considerations make continued development of novel antiretroviral therapeutics necessary. In this article, we highlight a number of steps in the HIV-1 replication cycle that represent promising targets for drug discovery. These include lipid raft microdomains, the RNase H activity of the viral enzyme reverse transcriptase, uncoating of the viral core, host cell machinery involved in the integration of the viral DNA into host cell chromatin, virus assembly, maturation, and budding, and the functions of several viral accessory proteins. We discuss the relevant molecular and cell biology, and describe progress to date in developing inhibitors against these novel targets. 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.

Synthesis and anti-HIV-1 integrase activity of modified dinucleotides

The synthesis of a series of thirty-eight new modified dinucleotides and dinucleotide conjugate analogues of d-(5')ApC(3') is described. The inhibitory activity of these compounds toward HIV-1 integrase was examined in enzymatic assays using the natural dinucleotide as a reference. Among the compounds, a perylene-dinucleotide conjugate has shown a two micromolar anti-integrase activity due to the presence of both the intercalator and the dinucleotide.

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.

Proteasome inhibitors block HIV-1 replication by affecting both cellular and viral targets

HIV-1 has proved to be notoriously difficult to tackle despite the availability of more than 20 clinically approved drugs. The majority of these drugs, however, target viral genes and their continued use will select for drug-resistant strains. Since NF-kappaB signaling is critical for viral replication, we wanted to investigate the effect of proteasome inhibitors on viral gene expression. We herein demonstrate that proteasome and NF-kappaB inhibitors effectively shut down transcription from the HIV-1 LTR-promoter. We further show that replication of HIV-1 in PBMC was severely compromised following treatment with proteasome inhibitors alone or in combination with other antiretroviral drugs. Finally, incubation of PBMC with these drugs reduced expression of IL-2 inducible T cell kinase (Itk), a Tec-family kinase, recently shown to be required for HIV-1 replication. These results suggest that proteasome inhibitors suppress LTR-promoter activity by interfering with cellular targets required for viral replication.

Analysis of binding parameters of HIV-1 integrase inhibitors: correlates of drug inhibition and resistance

This study undertook an exploratory data analysis of the binding parameters of HIV-1 integrase inhibitors. The study group involved inhibitors in preclinical development from the diketo acid, pyrroloquinoline and naphthyridine carboxamide families and the most advanced inhibitors Raltegravir and Elvitegravir. Distinct differences were observed in the energetics of binding between the studied classes of inhibitors that also correlated with drug resistant patterns. Quantitative-property-activity-relationships correlated experimental IC(50) values to the binding energy and the logarithm of the partition coefficient between n-octanol and water (clogP). The approach followed here serves as an improved basis for the development of 'second generation' integrase inhibitors.

Impact of human immunodeficiency virus type 1 resistance to protease inhibitors on evolution of resistance to the maturation inhibitor bevirimat (PA-457)

The maturation inhibitor bevirimat [3-O-(3',3'dimethysuccinyl)betulinic acid; BVM; also known as PA-457 or DSB] potently inhibits human immunodeficiency virus type 1 (HIV-1) replication by blocking protease (PR)-mediated cleavage at the junction between capsid (CA) and spacer peptide 1 (SP1) in Gag. We previously isolated a panel of single-amino-acid substitutions that confer resistance to BVM in vitro (C. S. Adamson, S. D. Ablan, I. Boeras, R. Goila-Gaur, F. Soheilian, K. Nagashima, F. Li, K. Salzwedel, M. Sakalian, C. T. Wild, and E. O. Freed, J. Virol. 80:10957-10971, 2006). The BVM resistance mutations cluster at or near the CA-SP1 cleavage site. Because BVM likely will be used clinically in patients harboring viruses resistant to PR inhibitors (PIs), in this study we evaluated the interplay between a PI-resistant (PIR) PR and the BVM resistance mutations in Gag. As expected, the PIR mutations had no effect on inhibition by BVM; however, we observed general processing defects and a slight delay in viral replication in Jurkat T cells associated with the PIR mutations, even in the absence of compound. When combined, most BVM resistance and PIR mutations acted additively to impair viral replication, particularly in the presence of BVM. The BVM-resistant mutant SP1-A1V was an exception, as it supported robust replication in the context of either wild-type (WT) or PIR PR, even at high BVM concentrations. Significantly, the emergence of BVM resistance was delayed in the context of the PIR PR, and the SP1-A1V mutation was acquired most frequently with either WT or PIR PR. These results suggest that resistance to BVM is less likely to emerge in patients who have failed PIs than in patients who are PI naive. We predict that the SP1-A1V substitution is the most likely to emerge in vivo, as this mutant replicates robustly independently of PR mutations or BVM. These findings offer insights into the effect of PIR mutations on the evolution of BVM resistance in PI-experienced patients.

Insight into the mechanisms of aminoglycoside derivatives interaction with HIV-1 entry steps and viral gene transcription

In recent years, based on peptide models of HIV-1 RNA binding, NMR structures of Tat-responsive element-ligand complexes and aminoglycoside-RNA interactions, and HIV-1 Tat structure, we have designed and synthesized aminoglycoside-arginine conjugates (AACs) and aminoglycoside poly-arginine conjugates (APACs), to serve as Tat mimetics. These novel molecules inhibit HIV-1 infectivity with 50% effective concentration values in the low micromolar range, the most potent compounds being the hexa-arginine-neomycin B and nona-D-arginine-neomycin conjugates. Importantly, these compounds, in addition to acting as Tat antagonists, inhibit HIV-1 infectivity by blocking several steps in HIV-1 cell entry. The AACs and APACs inhibit HIV-1 cell entry by interacting with gp120 at the CD4-binding site, by interacting with CXCR4 at the binding site of the CXCR4 mAb 12G5, and apparently by interacting with transient structures of the ectodomain of gp41. In the current review, we discuss the mechanisms of anti-HIV-1 activities of these AACs, APACs and other aminoglycoside derivatives in detail. Targeting several key processes in the viral life cycle by the same compound not only may increase its antiviral efficacy, but more importantly, may reduce the capacity of the virus to develop resistance to the compound. AACs and APACs may thus serve as leading compounds for the development of multitargeting novel HIV-1 inhibitors.