High-throughput screening uncovers a compound that activates latent HIV-1 and acts cooperatively with a histone deacetylase (HDAC) inhibitor

Discovery of 4-((3,4-dichlorophenyl)amino)-2-methylquinolin-6-ol derivatives as EGFR and HDAC dual inhibitors

In patients with non-small cell lung cancer (NSCLC), the standard therapy consists of selective tyrosine kinase inhibitors that target epidermal growth factor receptors (EGFR). Nonetheless, their clinical utility is primarily limited by the development of resistance to drugs. HDAC inhibitors have been shown in studies to reduce the level of EGFR that is expressed and downregulate the EGFR-induced phosphorylation of AKT and ERK. Therefore, dual inhibitors of EGFR and HDAC provide a potential approach as combination treatment synergistically inhibited the growth of NSCLC. Herein, we examined the EGFR inhibition effect of twenty compounds which designed and synthesized by us previously. Among them, compounds 12c and 12d exhibited powerful antiproliferative activity against the NCI-H1975 cell line with IC50 values of 0.48 ± 0.07 and 0.35 ± 0.02 μM, correspondingly. In cell-free kinase assays, both 12c and 12d demonstrated target-specific EGFR inhibition against wild type (EGFRwt). Furthermore, the expression of EGFR and phosphorylation of the EGF-induced pathways were significantly suppressed under the treatment of 12c and 12d. Besides, both histones H3 and H4 exhibited increased levels of acetylation following 12c and 12d treatment. The animal experiments shown that 12d could prevent the growth of tumor, inhibited the expression of EGFR and the phosphorylation levels of p70 S6K, AKT and p38 MAPK in vivo, and did not cause organ damage to the mice during the experiment. Overall, the results illustrated that compound 12c and 12d could serve as effective EGFR and HDAC dual inhibitors in NSCLC cells. Our work offers an alternative strategy for NSCLC therapy.

Medicinal Chemistry of Anti-HIV-1 Latency Chemotherapeutics: Biotargets, Binding Modes and Structure-Activity Relationship Investigation

The existence of latent viral reservoirs (LVRs), also called latent cells, has long been an acknowledged stubborn hurdle for effective treatment of HIV-1/AIDS. This stable and heterogeneous reservoir, which mainly exists in resting memory CD4⁺ T cells, is not only resistant to highly active antiretroviral therapy (HAART) but cannot be detected by the immune system, leading to rapid drug resistance and viral rebound once antiviral treatment is interrupted. Accordingly, various functional cure strategies have been proposed to combat this barrier, among which one of the widely accepted and utilized protocols is the so-called 'shock-and-kill' regimen. The protocol begins with latency-reversing agents (LRAs), either alone or in combination, to reactivate the latent HIV-1 proviruses, then eliminates them by viral cytopathic mechanisms (e.g., currently available antiviral drugs) or by the immune killing function of the immune system (e.g., NK and CD8+ T cells). In this review, we focuse on the currently explored small molecular LRAs, with emphasis on their mechanism-directed drug targets, binding modes and structure-relationship activity (SAR) profiles, aiming to provide safer and more effective remedies for treating HIV-1 infection.

HIV cure strategies: which ones are appropriate for Africa?

Although combination antiretroviral therapy (ART) has reduced mortality and improved lifespan for people living with HIV, it does not provide a cure. Patients must be on ART for the rest of their lives and contend with side effects, unsustainable costs, and the development of drug resistance. A cure for HIV is, therefore, warranted to avoid the limitations of the current therapy and restore full health. However, this cure is difficult to find due to the persistence of latently infected HIV cellular reservoirs during suppressive ART. Approaches to HIV cure being investigated include boosting the host immune system, genetic approaches to disable co-receptors and the viral genome, purging cells harboring latent HIV with latency-reversing latency agents (LRAs) (shock and kill), intensifying ART as a cure, preventing replication of latent proviruses (block and lock) and boosting T cell turnover to reduce HIV-1 reservoirs (rinse and replace). Since most people living with HIV are in Africa, methods being developed for a cure must be amenable to clinical trials and deployment on the continent. This review discusses the current approaches to HIV cure and comments on their appropriateness for Africa.

Polyphenol Extracts from Grape Seeds and Apple Can Reactivate Latent HIV-1 Transcription through Promoting P-TEFb Release from 7SK snRNP

The principal barrier for the eradication of HIV/AIDS is the virus latency. One of the effective strategies so called “shock and kill” is to use latency-reversing agents (LRAs) to activate the latent HIV reservoirs and then combine them with the highly active antiretroviral therapy (HAART) to eradicate the virus. However, most of the current LRAs are too toxic; therefore, they have not been used clinically. Our preliminary data indicated that polyphenols from grape seeds can activate HIV in latently infected Jurkat T cells. Owing to a lot of food containing polyphenols and based on a reasoning whether all of these kinds of polyphenols contain the latency-reversing function, in this study, we screened 22 fruits/vegetables to see whether polyphenols from these can reactivate latent HIV-1 transcription. We finally proved that the polyphenols from grape seeds, apple, pomegranate, and bilberry can reactivate latent HIV-1 transcription. The activation of which can be detected on the level of protein and mRNA. The activation of which is in a dose- and time-dependent manner, while the activated polyphenol extracts have the effects to stimulate Tat-independent HIV-1 transcription. The mechanism shows that polyphenol extracts from grape seeds and apple can stimulate P-TEFb's release from 7SK snRNP to induce HIV gene transcription. These results indicate that using a few food of high-content polyphenols as latent activators and combining HARRT may be of great use for the treatment of HIV/AIDS in the future.

HIV latency reversal agents: A potential path for functional cure?

Despite the advances in Human Immunodeficiency Virus (HIV) treatment, the cure for all HIV patients still poses a major challenge, which needs to be surpassed in the coming years. Among the strategies pursuing this aim, the 'kick-and-kill' approach, which involves the reactivation and elimination of a latent HIV reservoir that resides in some CD4⁺ T cells, appears promising. The first step of this approach requires the use of latency reversal agents (LRAs) that induce the reactivation of the latent virus. Although several classes of LRAs have been reported so far, some limitations of these compounds still need to be overcome before their clinical use. The complete exhaustion of the reservoir of latent virus will contribute to promote the second step of this approach, facilitating the elimination of the reactivated HIV. Therefore, potent, safe, and non-toxic LRAs are necessary to promote efficient elimination of the HIV-1 virus from its reservoir. In this review article, we focus on the promising LRAs that have been described in the literature over the past few years, highlighting the advantages and disadvantages of their use in the 'kick and kill' approach, thus opening a new avenue in the development of a potential cure.

Diversity of small molecule HIV-1 latency reversing agents identified in low- and high-throughput small molecule screens

The latency phenomenon produced by human immunodeficiency virus (HIV-1) prevents viral clearance by current therapies, and consequently development of a cure for HIV-1 disease represents a formidable challenge. Research over the past decade has resulted in identification of small molecules that are capable of exposing HIV-1 latent reservoirs, by reactivation of viral transcription, which is intended to render these infected cells sensitive to elimination by immune defense recognition or apoptosis. Molecules with this capability, known as latency-reversing agents (LRAs) could lead to realization of proposed HIV-1 cure strategies collectively termed "shock and kill," which are intended to eliminate the latently infected population by forced reactivation of virus replication in combination with additional interventions that enhance killing by the immune system or virus-mediated apoptosis. Here, we review efforts to discover novel LRAs via low- and high-throughput small molecule screens, and summarize characteristics and biochemical properties of chemical structures with this activity. We expect this analysis will provide insight toward further research into optimized designs for new classes of more potent LRAs.

An activator of G protein-coupled receptor and MEK1/2-ERK1/2 signaling inhibits HIV-1 replication by altering viral RNA processing

The ability of HIV-1 to evolve resistance to combined antiretroviral therapies (cARTs) has stimulated research into alternative means of controlling this infection. We assayed >60 modulators of RNA alternative splicing (AS) to identify new inhibitors of HIV-1 RNA processing-a segment of the viral lifecycle not targeted by current drugs-and discovered compound N-[4-chloro-3-(trifluoromethyl)phenyl]-7-nitro-2,1,3-benzoxadiazol-4-amine (5342191) as a potent inhibitor of both wild-type (Ba-L, NL4-3, LAI, IIIB, and N54) and drug-resistant strains of HIV-1 (IC50: ~700 nM) with no significant effect on cell viability at doses tested. 5342191 blocks expression of four essential HIV-1 structural and regulatory proteins (Gag, Env, Tat, and Rev) without affecting total protein synthesis of the cell. This response is associated with altered unspliced (US) and singly-spliced (SS) HIV-1 RNA accumulation (~60% reduction) and transport to the cytoplasm (loss of Rev) whereas parallel analysis of cellular RNAs revealed less than a 0.7% of host alternative splicing (AS) events (0.25-0.67% by ≥ 10-20%), gene expression (0.01-0.46% by ≥ 2-5 fold), and protein abundance (0.02-0.34% by ≥ 1.5-2 fold) being affected. Decreased expression of Tat, but not Gag/Env, upon 5342191 treatment was reversed by a proteasome inhibitor, suggesting that this compound alters the synthesis/degradation of this key viral factor. Consistent with an affect on HIV-1 RNA processing, 5342191 treatment of cells altered the abundance and phosphorylation of serine/arginine-rich splicing factor (SRSF) 1, 3, and 4. Despite the activation of several intracellular signaling pathways by 5342191 (Ras, MEK1/2-ERK1/2, and JNK1/2/3), inhibition of HIV-1 gene expression by this compound could be reversed by pre-treatment with either a G-protein α-subunit inhibitor or two different MEK1/2 inhibitors. These observations demonstrate enhanced sensitivity of HIV-1 gene expression to small changes in host RNA processing and highlights the potential of modulating host intracellular signaling as an alternative approach for controlling HIV-1 infection.

Dual effects of the novel ingenol derivatives on the acute and latent HIV-1 infections

The latent reservoir of HIV-1 in resting memory CD4⁺ T cells serves as a major barrier to curing HIV-1 infection. Reactivation of latent HIV-1 is proposed as a promising strategy for the clearance of the viral reservoirs. Because of the limitations of current latency reversal agents (LRAs), identification of new LRAs is urgently required. Here, we analyzed Euphorbia kansui extracts and obtained three ingenol derivative compounds named EK-1A, EK-5A and EK-15A. We found that ingenol derivatives can effectively reactivate latent HIV-1 at very low (nanomolar) concentrations in HIV latency model in vitro. Furthermore, ingenol derivatives exhibited synergy with other LRAs in reactivating latent HIV-1. We verified that EK-15A can promote latent HIV-1 reactivation in the ex vivo resting CD4⁺ T cells isolated from the peripheral blood of HIV-infected individuals on suppressive antiretroviral therapy. In addition, ingenol derivatives down-regulated the expression of cell surface HIV co-receptors CCR5 and CXCR4, therefore potentially preventing new infection of HIV-1. Our results indicated that the ingenol derivatives extracted from Euphorbia kansui have dual functions: reactivation of latent HIV-1 and inhibition of HIV-1 infection.

HIV "shock and kill" therapy: In need of revision

The implementation of antiretroviral therapy 23 years ago has rendered HIV infection clinically manageable. However, the disease remains incurable, since it establishes latent proviral reservoirs, which in turn can stochastically begin reproducing viral particles throughout the patient's lifetime. Viral latency itself depends in large part on the silencing environment of the infected host cell, which can be chemically manipulated. "Shock and kill" therapy intends to reverse proviral quiescence by inducing transcription with pharmaceuticals and allowing a combination of antiretroviral therapy, host immune clearance and HIV-cytolysis to remove latently infected cells, leading to a complete cure. Over 160 compounds functioning as latency-reversing agents (LRAs) have been identified to date, but none of the candidates has yet led to a promising functional cure. Furthermore, fundamental bioinformatic and clinical research from the past decade has highlighted the complexity and highly heterogeneous nature of the proviral reservoirs, shedding doubt on the "shock and kill" concept. Alternative therapies such as the HIV transcription-inhibiting "block and lock" strategy are therefore being considered. In this review we describe the variety of existing classes of LRAs, discuss their current drawbacks and highlight the potential for combinatorial "shocktail" therapies for potent proviral reactivation. We also suggest investigating LRAs with lesser-known mechanisms of action, and examine the feasibility of "block and lock" therapy.

Identification of Novel HIV-1 Latency-Reversing Agents from a Library of Marine Natural Products

Natural products originating from marine and plant materials are a rich source of chemical diversity and unique antimicrobials. Using an established in vitro model of HIV-1 latency, we screened 257 pure compounds from a marine natural product library and identified 4 (psammaplin A, aplysiatoxin, debromoaplysiatoxin, and previously-described alotaketal C) that induced expression of latent HIV-1 provirus in both cell line and primary cell models. Notably, aplysiatoxin induced similar levels of HIV-1 expression as prostratin but at up to 900-fold lower concentrations and without substantial effects on cell viability. Psammaplin A enhanced HIV-1 expression synergistically when treated in combination with the protein kinase C (PKC) activator prostratin, but not the histone deacetylase inhibitor (HDACi) panobinostat, suggesting that psammaplin A functions as a latency-reversing agent (LRA) of the HDACi class. Conversely, aplysiatoxin and debromoaplysiatoxin synergized with panobinostat but not prostratin, suggesting that they function as PKC activators. Our study identifies new compounds from previously untested marine natural products and adds to the repertoire of LRAs that can inform therapeutic “shock-and-kill”-based strategies to eliminate latent HIV-infected reservoirs.

Cardiac glycoside/aglycones inhibit HIV-1 gene expression by a mechanism requiring MEK1/2-ERK1/2 signaling

The capacity of HIV-1 to develop resistance to current drugs calls for innovative strategies to control this infection. We aimed at developing novel inhibitors of HIV-1 replication by targeting viral RNA processing—a stage dependent on conserved host processes. We previously reported that digoxin is a potent inhibitor of this stage. Herein, we identify 12 other cardiac glycoside/aglycones or cardiotonic steroids (CSs) that impede HIV growth in HIV-infected T cells from clinical patients at IC₅₀s (1.1–1.3 nM) that are 2–26 times below concentrations used in patients with heart conditions. We subsequently demonstrate that CSs inhibit HIV-1 gene expression in part through modulation of MEK1/2-ERK1/2 signaling via interaction with the Na⁺/K⁺-ATPase, independent of alterations in intracellular Ca²⁺. Supporting this hypothesis, depletion of the Na⁺/K⁺-ATPase or addition of a MEK1/2-ERK1/2 activator also impairs HIV-1 gene expression. Similar to digoxin, all CSs tested induce oversplicing of HIV-1 RNAs, reducing unspliced (Gag) and singly spliced RNAs (Env/p14-Tat) encoding essential HIV-1 structural/regulatory proteins. Furthermore, all CSs cause nuclear retention of genomic/unspliced RNAs, supporting viral RNA processing as the underlying mechanism for their disruption of HIV-1 replication. These findings call for further in vivo validation and supports the targeting of cellular processes to control HIV-1 infection.

Design, synthesis, and biological evaluation of AV6 derivatives as novel dual reactivators of latent HIV-1

The “shock and kill” strategy might be a promising therapeutic approach for HIV/AIDS due to the existence of latent viral reservoirs. A major challenge of the “shock and kill” strategy arises from the general lack of clinically effective latency-reversing agents (LRAs). The 2-methylquinoline derivative, antiviral 6 (AV6) has been reported to induce latent HIV-1 expression and act synergistically with a HDAC inhibitor VA to reverse HIV latency. We report herein the design and identification of AV6 analogues which possess the zinc-binding group of HDAC inhibitors and have dual acting mechanism for the reactivation of HIV-1 from latency. Evaluation of compounds for the reactivation of HIV-1 latency identified two excellent active compounds 12c and 12d. Further bioassays revealed that these two compounds reactivated latent HIV-1 through dual mechanism, the inhibition of HDACs and NFAT-required for early HIV-1 gene expression. Additionally, it was found that 12c and 12d could reactivate HIV-1 transcription by releasing P-TEFb from the inactive complex 7SK snRNP. At last, molecular docking identified their orientation and binding interactions at the active site of HDAC2. This experimental data suggests that 12c and 12d can be served as effective HIV-1 LRAs which can be taken up for further studies.

As dual-acting HIV LRAs, compounds 12c and 12d could activate latent HIV-1 via the NFAT-required mechanism and as histone deacetylase (HDAC) inhibitors.

Identification of benzazole compounds that induce HIV-1 transcription

Despite advances in antiretroviral therapy, HIV-1 infection remains incurable in patients and continues to present a significant public health burden worldwide. While a number of factors contribute to persistent HIV-1 infection in patients, the presence of a stable, long-lived reservoir of latent provirus represents a significant hurdle in realizing an effective cure. One potential strategy to eliminate HIV-1 reservoirs in patients is reactivation of latent provirus with latency reversing agents in combination with antiretroviral therapy, a strategy termed "shock and kill". This strategy has shown limited clinical effectiveness thus far, potentially due to limitations of the few therapeutics currently available. We have identified a novel class of benzazole compounds effective at inducing HIV-1 expression in several cellular models. These compounds do not act via histone deacetylase inhibition or T cell activation, and show specificity in activating HIV-1 in vitro. Initial exploration of structure-activity relationships and pharmaceutical properties indicates that these compounds represent a potential scaffold for development of more potent HIV-1 latency reversing agents.

A trichostatin A expression signature identified by TempO-Seq targeted whole transcriptome profiling

The use of gene expression signatures to classify compounds, identify efficacy or toxicity, and differentiate close analogs relies on the sensitivity of the method to identify modulated genes. We used a novel ligation-based targeted whole transcriptome expression profiling assay, TempO-Seq®, to determine whether previously unreported compound-responsive genes could be identified and incorporated into a broad but specific compound signature. TempO-Seq exhibits 99.6% specificity, single cell sensitivity, and excellent correlation with fold differences measured by RNA-Seq (R2 = 0.9) for 20,629 targets. Unlike many expression assays, TempO-Seq does not require RNA purification, cDNA synthesis, or capture of targeted RNA, and lacks a 3' end bias. To investigate the sensitivity of the TempO-Seq assay to identify significantly modulated compound-responsive genes, we derived whole transcriptome profiles from MCF-7 cells treated with the histone deacetylase inhibitor Trichostatin A (TSA) and identified more than 9,000 differentially expressed genes. The TSA profile for MCF-7 cells overlapped those for HL-60 and PC-3 cells in the Connectivity Map (cMAP) database, suggesting a common TSA-specific expression profile independent of baseline gene expression. A 43-gene cell-independent TSA signature was extracted from cMAP and confirmed in TempO-Seq MCF-7 data. Additional genes that were not previously reported to be TSA responsive in the cMAP database were also identified. TSA treatment of 5 cell types revealed 1,136 differentially expressed genes in common, including 785 genes not previously reported to be TSA responsive. We conclude that TSA induces a specific expression signature that is consistent across widely different cell types, that this signature contains genes not previously associated with TSA responses, and that TempO-Seq provides the sensitive differential expression detection needed to define such compound-specific, cell-independent, changes in expression.

RNA stability regulates human T cell leukemia virus type 1 gene expression in chronically-infected CD4 T cells

Regulation of expression of HTLV-1 gene products from integrated proviruses plays an important role in HTLV-1-associated disease pathogenesis. Previous studies have shown that T cell receptor (TCR)- and phorbol ester (PMA) stimulation of chronically infected CD4 T cells increases the expression of integrated HTLV-1 proviruses in latently infected cells, however the mechanism remains unknown. Analysis of HTLV-1 RNA and protein species following PMA treatment of the latently HTLV-1-infected, FS and SP T cell lines demonstrated rapid induction of tax/rex mRNA. This rapid increase in tax/rex mRNA was associated with markedly enhanced tax/rex mRNA stability while the stability of unspliced or singly spliced HTLV-1 RNAs did not increase. Tax/rex mRNA in the HTLV-1 constitutively expressing cell lines exhibited high basal stability even without PMA treatment. Our data support a model whereby T cell activation leads to increased HTLV-1 gene expression at least in part through increased tax/rex mRNA stability.

Modeling HIV-1 Latency in Primary T Cells Using a Replication-Competent Virus

HIV-1 latently infected cells in vivo can be found in extremely low frequencies. Therefore, in vitro cell culture models have been used extensively for the study of HIV-1 latency. Often, these in vitro systems utilize defective viruses. Defective viruses allow for synchronized infections and circumvent the use of antiretrovirals. In addition, replication-defective viruses cause minimal cytopathicity because they fail to spread and usually do not encode env or accessory genes. On the other hand, replication-competent viruses encode all or most viral genes and better recapitulate the nuances of the viral replication cycle. The study of latency with replication-competent viruses requires the use of antiretroviral drugs in culture, and this mirrors the use of antiretroviral treatment (ART) in vivo. We describe a model that utilizes cultured central memory CD4(+) T cells and replication-competent HIV-1. This method generates latently infected cells that can be reactivated using latency reversing agents in the presence of antiretroviral drugs. We also describe a method for the removal of productively infected cells prior to viral reactivation, which takes advantage of the downregulation of CD4 by HIV-1, and the use of a GFP-encoding virus for increased throughput.

PLGA-PEG Nanoparticles Coated with Anti-CD45RO and Loaded with HDAC Plus Protease Inhibitors Activate Latent HIV and Inhibit Viral Spread

Activating HIV-1 proviruses in latent reservoirs combined with inhibiting viral spread might be an effective anti-HIV therapeutic strategy. Active specific delivery of therapeutic drugs into cells harboring latent HIV, without the use of viral vectors, is a critical challenge to this objective. In this study, nanoparticles of poly(lactic-co-glycolic acid)-polyethylene glycol diblock copolymers conjugated with anti-CD45RO antibody and loaded with the histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA) and/or protease inhibitor nelfinavir (Nel) were tested for activity against latent virus in vitro. Nanoparticles loaded with SAHA, Nel, and SAHA + Nel were characterized in terms of size, surface morphology, zeta potential, entrapment efficiency, drug release, and toxicity to ACH-2 cells. We show that SAHA- and SAHA + Nel-loaded nanoparticles can target latently infected CD4(+) T-cells and stimulate virus production. Moreover, nanoparticles loaded with SAHA + NEL were capable of both activating latent virus and inhibiting viral spread. Taken together, these data demonstrate the potential of this novel reagent for targeting and eliminating latent HIV reservoirs.

Small Molecule Inhibitors of BAF; A Promising Family of Compounds in HIV-1 Latency Reversal

Persistence of latently infected cells in presence of Anti-Retroviral Therapy presents the main obstacle to HIV-1 eradication. Much effort is thus placed on identification of compounds capable of HIV-1 latency reversal in order to render infected cells susceptible to viral cytopathic effects and immune clearance. We identified the BAF chromatin remodeling complex as a key player required for maintenance of HIV-1 latency, highlighting its potential as a molecular target for inhibition in latency reversal. Here, we screened a recently identified panel of small molecule inhibitors of BAF (BAFi's) for potential to activate latent HIV-1. Latency reversal was strongly induced by BAFi's Caffeic Acid Phenethyl Ester and Pyrimethamine, two molecules previously characterized for clinical application. BAFi's reversed HIV-1 latency in cell line based latency models, in two ex vivo infected primary cell models of latency, as well as in HIV-1 infected patient's CD4 + T cells, without inducing T cell proliferation or activation. BAFi-induced HIV-1 latency reversal was synergistically enhanced upon PKC pathway activation and HDAC-inhibition. Therefore BAFi's constitute a promising family of molecules for inclusion in therapeutic combinatorial HIV-1 latency reversal.

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BAF complex inhibitors (BAFi's) activate latent HIV-1 in cell line models of latency.

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BAFi's in combination with HDAC inhibitors and PKC activators synergistically activate latent HIV-1.

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The BAFi's PYR and CAPE reverse HIV-1 latency in primary cell models of latency and in cells obtained from HIV-1 patients.

Access to Combination antiretroviral therapy (cART) has made HIV-1 infection a chronic disease. However, cART is not curative, as a small number of infected cells harboring silent virus with potential to renew the infection persist despite cART. Efforts to cure HIV-1 include activation of these latent cells, making them susceptible to immune clearance. Here we describe the activity of BAF inhibitors in HIV-1 activation in in vitro models of HIV-1 latency as well as in cells obtained from HIV-1 infected patient volunteers. Our data highlight the clinical potential of BAF inhibitors for inclusion in combinatorial therapy to reverse HIV-1 latency.

Anti-HIV Drug Discovery and Development: Current Innovations and Future Trends

The early effectiveness of combinatorial antiretroviral therapy (cART) in the treatment of HIV infection has been compromised to some extent by rapid development of multidrug-resistant HIV strains, poor bioavailability, and cumulative toxicities, and so there is a need for alternative strategies of antiretroviral drug discovery and additional therapeutic agents with novel action modes or targets. From this perspective, we first review current strategies of antiretroviral drug discovery and optimization, with the aid of selected examples from the recent literature. We highlight the development of phosphate ester-based prodrugs as a means to improve the aqueous solubility of HIV inhibitors, and the introduction of the substrate envelope hypothesis as a new approach for overcoming HIV drug resistance. Finally, we discuss future directions for research, including opportunities for exploitation of novel antiretroviral targets, and the strategy of activation of latent HIV reservoirs as a means to eradicate the virus.

Progress and challenges in the use of latent HIV-1 reactivating agents

Highly active antiretroviral therapy (HAART) can effectively suppress the replication of human immunodeficiency virus-1 (HIV-1) and block disease progression. However, chronic HIV-1 infection remains incurable due to the persistence of a viral reservoir, including the transcriptionally silent provirus in CD4(+) memory T cells and the sanctuary sites that are inaccessible to drugs. Reactivation and the subsequent elimination of latent virus through virus-specific cytotoxic effects or host immune responses are critical strategies for combating the disease. Indeed, a number of latency reactivating reagents have been identified through mechanism-directed approaches and large-scale screening, including: (1) histone deacetylase inhibitors (HDACi); (2) cytokines and chemokines; (3) DNA methyltransferase inhibitors (DNMTI); (4) histone methyltransferase inhibitors (HMTI); (5) protein kinase C (PKC) activators; (6) P-TEFb activators; and (7) unclassified agents, such as disulfram. They have proved to be efficacious in latent cell line models and CD4(+) T lymphocytes from HIV-1-infected patients. This review comprehensively summarizes the recent progress and relative challenges in this field.

HIV eradication: combinatorial approaches to activate latent viruses

The concept of eradication of the Human Immune Deficiency Virus (HIV) from infected patients has gained much attention in the last few years. While combination Anti-Retroviral Therapy (c-ART) has been extremely effective in suppressing viral replication, it is not curative. This is due to the presence of a reservoir of latent HIV infected cells, which persist in the presence of c-ART. Recently, pharmaceutical approaches have focused on the development of molecules able to induce HIV-1 replication from latently infected cells in order to render them susceptible to viral cytopathic effects and host immune responses. Alternative pathways and transcription complexes function to regulate the activity of the HIV promoter and might serve as molecular targets for compounds to activate latent HIV. A combined therapy coupling various depressors and activators will likely be the most effective in promoting HIV replication while avoiding pleiotropic effects at the cellular level. Moreover, in light of differences among HIV subtypes and variability in integration sites, the combination of multiple agents targeting multiple pathways will increase likelihood of therapeutic effectiveness and prevent mutational escape. This review provides an overview of the mechanisms that can be targeted to induce HIV activation focusing on potential combinatorial approaches.

HIV-1 latency: an update of molecular mechanisms and therapeutic strategies

The major obstacle towards HIV-1 eradication is the life-long persistence of the virus in reservoirs of latently infected cells. In these cells the proviral DNA is integrated in the host’s genome but it does not actively replicate, becoming invisible to the host immune system and unaffected by existing antiviral drugs. Rebound of viremia and recovery of systemic infection that follows interruption of therapy, necessitates life-long treatments with problems of compliance, toxicity, and untenable costs, especially in developing countries where the infection hits worst. Extensive research efforts have led to the proposal and preliminary testing of several anti-latency compounds, however, overall, eradication strategies have had, so far, limited clinical success while posing several risks for patients. This review will briefly summarize the more recent advances in the elucidation of mechanisms that regulates the establishment/maintenance of latency and therapeutic strategies currently under evaluation in order to eradicate HIV persistence.

Therapy for latent HIV-1 infection: the role of histone deacetylase inhibitors

Persistence of HIV-1 in latently infected CD4(+) T-cells prevents eradication in HIV-infected treated patients. Latency is characterized by a reversible silencing of transcription of integrated HIV-1. Several molecular mechanisms have been described which contribute to latency, including the establishment and maintenance of repressive chromatin on the HIV-1 promoter. Histone deacetylation is a landmark modification associated with transcriptional repression of the HIV-1 promoter and inhibition of histone deacetylase enzymes (HDACs) reactivates latent HIV-1. Here, we review the different HDAC inhibitors that have been studied in HIV-1 latency and their therapeutic potential in reactivating latent HIV-1.

An in-depth comparison of latent HIV-1 reactivation in multiple cell model systems and resting CD4+ T cells from aviremic patients

The possibility of HIV-1 eradication has been limited by the existence of latently infected cellular reservoirs. Studies to examine control of HIV latency and potential reactivation have been hindered by the small numbers of latently infected cells found in vivo. Major conceptual leaps have been facilitated by the use of latently infected T cell lines and primary cells. However, notable differences exist among cell model systems. Furthermore, screening efforts in specific cell models have identified drug candidates for "anti-latency" therapy, which often fail to reactivate HIV uniformly across different models. Therefore, the activity of a given drug candidate, demonstrated in a particular cellular model, cannot reliably predict its activity in other cell model systems or in infected patient cells, tested ex vivo. This situation represents a critical knowledge gap that adversely affects our ability to identify promising treatment compounds and hinders the advancement of drug testing into relevant animal models and clinical trials. To begin to understand the biological characteristics that are inherent to each HIV-1 latency model, we compared the response properties of five primary T cell models, four J-Lat cell models and those obtained with a viral outgrowth assay using patient-derived infected cells. A panel of thirteen stimuli that are known to reactivate HIV by defined mechanisms of action was selected and tested in parallel in all models. Our results indicate that no single in vitro cell model alone is able to capture accurately the ex vivo response characteristics of latently infected T cells from patients. Most cell models demonstrated that sensitivity to HIV reactivation was skewed toward or against specific drug classes. Protein kinase C agonists and PHA reactivated latent HIV uniformly across models, although drugs in most other classes did not.

Combined approaches for HIV cure

PURPOSE OF REVIEW

A serious effort has begun to develop therapies that may be capable of eradicating established HIV infection in man. Because of the biological complexity of HIV infection that persists despite potent antiretroviral therapy, it is widely believed that if such therapies can be developed they will involve complex, multimodality approaches. We highlight some of the recent studies in this effort.

RECENT FINDINGS

An inhibitor of histone deacetylase has been demonstrated to disrupt latency in man, and new histone deacetylase inhibitors have been identified. Other potential targets, such as histone methyltransferase, protein kinase C, and BRD4, have been recently studied. Model systems, both in primary cells and in animal models, are beginning to be validated. In the clinic, immune-based therapies to aid in the clearance of persistent infection are also being tested.

SUMMARY

It is too early to know what combination eradication therapies for HIV infection will look like in the future, but candidate therapies and model systems to perform preclinical validation are beginning to take shape.

Altering cell death pathways as an approach to cure HIV infection

Recent cases of successful control of human immunodeficiency virus (HIV) by bone marrow transplant in combination with suppressive antiretroviral therapy (ART) and very early initiation of ART have provided proof of concept that HIV infection might now be cured. Current efforts focusing on gene therapy, boosting HIV-specific immunity, reducing inflammation and activation of latency have all been the subject of recent excellent reviews. We now propose an additional avenue of research towards a cure for HIV: targeting HIV apoptosis regulatory pathways. The central enigma of HIV disease is that HIV infection kills most of the CD4 T cells that it infects, but those cells that are spared subsequently become a latent reservoir for HIV against which current medications are ineffective. We propose that if strategies could be devised which would favor the death of all cells which HIV infects, or if all latently infected cells that release HIV would succumb to viral-induced cytotoxicity, then these approaches combined with effective ART to prevent spreading infection, would together result in a cure for HIV. This premise is supported by observations in other viral systems where the relationship between productive infection, apoptosis resistance, and the development of latency or persistence has been established. Therefore we propose that research focused at understanding the mechanisms by which HIV induces apoptosis of infected cells, and ways that some cells escape the pro-apoptotic effects of productive HIV infection are critical to devising novel and rational approaches to cure HIV infection.

Therapeutics for HIV-1 reactivation from latency

Intensive combined antiretroviral therapy successfully suppresses HIV-1 replication and AIDS disease progression making infection manageable, but it is unable to eradicate the virus that persists in long-lived, drug-insensitive and immune system-insensitive reservoirs thus asking for life-long treatments with problems of compliance, resistance, toxicity and cost. These limitations and recent insights into latency mechanisms have fueled a renewed effort in finding a cure for HIV-1 infection. Proposed eradication strategies involve reactivation of the latent reservoir upon induction of viral transcription followed by the elimination of reactivated virus-producing cells by viral cytopathic effect or host immune response. Several molecules identified by mechanism-directed approaches or in large-scale screenings have been proposed as latency reversing agents. Some of them have already entered clinical testing in humans but with mixed or unsatisfactory results.

HIV-1 transcription and latency: an update

Combination antiretroviral therapy, despite being potent and life-prolonging, is not curative and does not eradicate HIV-1 infection since interruption of treatment inevitably results in a rapid rebound of viremia. Reactivation of latently infected cells harboring transcriptionally silent but replication-competent proviruses is a potential source of persistent residual viremia in cART-treated patients. Although multiple reservoirs may exist, the persistence of resting CD4+ T cells carrying a latent infection represents a major barrier to eradication. In this review, we will discuss the latest reports on the molecular mechanisms that may regulate HIV-1 latency at the transcriptional level, including transcriptional interference, the role of cellular factors, chromatin organization and epigenetic modifications, the viral Tat trans-activator and its cellular cofactors. Since latency mechanisms may also operate at the post-transcriptional level, we will consider inhibition of nuclear RNA export and inhibition of translation by microRNAs as potential barriers to HIV-1 gene expression. Finally, we will review the therapeutic approaches and clinical studies aimed at achieving either a sterilizing cure or a functional cure of HIV-1 infection, with a special emphasis on the most recent pharmacological strategies to reactivate the latent viruses and decrease the pool of viral reservoirs.

Targeting HIV latency: pharmacologic strategies toward eradication

The latent reservoir for HIV-1 in resting CD4(+) T cells remains a major barrier to HIV-1 eradication, even though highly active antiretroviral therapy (HAART) can successfully reduce plasma HIV-1 levels to below the detection limit of clinical assays and reverse disease progression. Proposed eradication strategies involve reactivation of this latent reservoir. Multiple mechanisms are believed to be involved in maintaining HIV-1 latency, mostly through suppression of transcription. These include cytoplasmic sequestration of host transcription factors and epigenetic modifications such as histone deacetylation, histone methylation and DNA methylation. Therefore, strategies targeting these mechanisms have been explored for reactivation of the latent reservoir. In this review, we discuss current pharmacological approaches toward eradication, focusing on small molecule latency-reversing agents, their mechanisms, advantages and limitations.

Antibody-based candidate therapeutics against HIV-1: implications for virus eradication and vaccine design

INTRODUCTION

The currently available anti-HIV-1 drugs can control the infection but do not eradicate the virus. Their long-term use can lead to side effects and resistance to therapy. Therefore, eradication of the virus has been a major goal of research. Biological therapeutics including broadly neutralizing monoclonal antibodies (bnAbs) are promising tools to reach this goal. They could also help design novel vaccine immunogens potentially capable of eliciting bnAbs targeting the HIV-1 envelope glycoproteins (Envs).

AREAS COVERED

We review HIV-1 bnAbs and their potential as candidate prophylactics and therapeutics used individually, in combination, or as bispecific fusion proteins. We also discuss their potential use in the 'activation-elimination' approach for HIV-1 eradication in infected patients receiving antiretroviral treatment as well as current vaccine design efforts based on understanding of interactions of candidate vaccine immunogens with matured bnAbs and their putative germline predecessors, and related antibody maturation pathways.

EXPERT OPINION

Exploration of HIV-1 bnAbs has provided and will continue to provide useful knowledge that helps develop novel types of biotherapeutics and vaccines. It is possible that bnAb-based candidate therapeutics could help eradicate HIV-1. Development of vaccine immunogens capable of eliciting potent bnAbs in humans remains a fundamental challenge.

Cyclin T1 and CDK9 T-loop phosphorylation are downregulated during establishment of HIV-1 latency in primary resting memory CD4+ T cells

P-TEFb, a cellular kinase composed of Cyclin T1 and CDK9, is essential for processive HIV-1 transcription. P-TEFb activity is dependent on phosphorylation of Thr186 in the CDK9 T loop. In resting CD4(+) T cells which are nonpermissive for HIV-1 replication, the levels of Cyclin T1 and T-loop-phosphorylated CDK9 are very low but increase significantly upon cellular activation. Little is known about how P-TEFb activity and expression are regulated in resting central memory CD4(+) T cells, one of the main reservoirs of latent HIV-1. We used an in vitro primary cell model of HIV-1 latency to show that P-TEFb availability in resting memory CD4(+) T cells is governed by the differential expression and phosphorylation of its subunits. This is in contrast to previous observations in dividing cells, where P-TEFb can be regulated by its sequestration in the 7SK RNP complex. We find that resting CD4(+) T cells, whether naïve or memory and independent of their infection status, have low levels of Cyclin T1 and T-loop-phosphorylated CDK9, which increase upon activation. We also show that the decrease in Cyclin T1 protein upon the acquisition of a memory phenotype is in part due to proteasome-mediated proteolysis and likely also to posttranscriptional downregulation by miR-150. We also found that HEXIM1 levels are very low in ex vivo- and in vitro-generated resting memory CD4(+) T cells, thus limiting the sequestration of P-TEFb in the 7SK RNP complex, indicating that this mechanism is unlikely to be a driver of viral latency in this cell type.

Prospects for treatment of latent HIV

Recent advances in antiretroviral therapy (ART) have drastically improved the quality of life for people with HIV infection. However, owing to the persistence of latent HIV in the presence of therapy, patients must remain on therapy indefinitely. Currently, the solution to the HIV pandemic rests on the prevention of new infections and many decades of ART for the steadily expanding number of people infected worldwide. ART is costly, requires ongoing medical care, and can have side effects, thereby preventing its universal availability. Therefore, to escape the ironic burdens of therapy, efforts have begun to develop treatments for latent HIV infection. Current approaches propose either complete eradication of infection or induction of a state of stringent control over viral replication without ART. This review will discuss these strategies in detail and their potential for clinical development.

The development of immune-modulating compounds to disrupt HIV latency

Antiretroviral therapy (ART) has proved highly effective in suppressing HIV-1 replication and disease progression. Nevertheless, ART has failed to eliminate the virus from infected individuals. The main obstacle to HIV-1 eradication is the persistence of cellular viral reservoirs. Therefore, the "shock-and-kill" strategy was proposed consisting of inducing HIV-1 escape from latency, in the presence of ART. This is followed by the elimination of reactivated, virus-producing cells. Immune modulators, including protein kinase C (PKC) activators, anti-leukemic drugs and histone deacetylase inhibitors (HDACis) have all demonstrated efficacy in the reactivation of latent virus replication. This review will focus on the potential use of these small molecules in the "shock and kill" strategy, the molecular basis for their action and the potential advantages of their immune-modulating activities.

Facts and fiction: cellular models for high throughput screening for HIV-1 reactivating drugs

A curative therapy for HIV-1 infection will have to include measures to eliminate the reservoir of latently HIV- 1 infected cells that allow the virus to persist despite otherwise successful therapy. To date, all efforts to deplete the latent reservoir by triggering viral reactivation have used preexisting drugs that are believed to potentially target molecular mechanisms controlling HIV-1 infection. These therapeutic attempts were not clinically successful. Only in the last few years have cellular models of latent HIV-1 infection suitable for high throughput screening been developed and concerted drug discovery efforts were initiated to discover new HIV-1 reactivating drugs. We here provide a historic overview about the development of cell models with latent HIV-1 infection that lend themselves to drug discovery. We provide an overview from the first reported latently infected cell lines to current in vitro models of latent HIV-1 infection in primary T cells, and compare their potential to be used in future large-scale drug screening efforts.

HIV reservoirs and strategies for eradication

Combination antiretroviral therapy (cART) has led to a reduction in morbidity and mortality in HIV-infected patients but therapy is lifelong and there is no cure for HIV. The major barriers to cure include HIV latency, which has been identified in different T-cell subsets, as well as persistence of HIV in anatomical reservoirs. We review recent developments in our understanding of the major reservoirs of HIV in patients on cART as well as how latency is established and maintained in T cells. Finally, we review the scientific rationale of and clinical experience with pharmacotherapeutic strategies aimed at eliminating latently infected cells.

Combination of biological screening in a cellular model of viral latency and virtual screening identifies novel compounds that reactivate HIV-1

Although highly active antiretroviral therapy (HAART) has converted HIV into a chronic disease, a reservoir of HIV latently infected resting T cells prevents the eradication of the virus from patients. To achieve eradication, HAART must be combined with drugs that reactivate the dormant viruses. We examined this problem in an established model of HIV postintegration latency by screening a library of small molecules. Initially, we identified eight molecules that reactivated latent HIV. Using them as templates, additional hits were identified by means of similarity-based virtual screening. One of those hits, 8-methoxy-6-methylquinolin-4-ol (MMQO), proved to be useful to reactivate HIV-1 in different cellular models, especially in combination with other known reactivating agents, without causing T-cell activation and with lower toxicity than that of the initial hits. Interestingly, we have established that MMQO produces Jun N-terminal protein kinase (JNK) activation and enhances the T-cell receptor (TCR)/CD3 stimulation of HIV-1 reactivation from latency but inhibits CD3-induced interleukin-2 (IL-2) and tumor necrosis factor alpha (TNF-α) gene transcription. Moreover, MMQO prevents TCR-induced cell cycle progression and proliferation in primary T cells. The present study documents that the combination of biological screening in a cellular model of viral latency with virtual screening is useful for the identification of novel agents able to reactivate HIV-1. Moreover, we set the bases for a hypothetical therapy to reactivate latent HIV by combining MMQO with physiological or pharmacological TCR/CD3 stimulation.

HDAC inhibitors in HIV

Combination antiretroviral therapy (cART) has led to a very substantial reduction in morbidity and mortality in HIV-infected patients; however, cART alone is unable to cure HIV and therapy is lifelong. Therefore, a new strategy to cure HIV is urgently needed. There is now a concerted effort from scientists, clinicians and funding agencies to identify ways to achieve either a functional cure (long-term control of HIV in the absence of cART) or a sterilizing cure (elimination of all HIV-infected cells). Multiple strategies aiming at achieving a cure for HIV are currently being investigated, including both pharmacotherapy and gene therapy. In this review, we will review the rationale as well as in vitro and clinical trial data that support the role of histone deacetylase inhibitors as one approach to cure HIV.