Frontiers in Clinical Drug Research - HIV: Volume 5 E-Book
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- Herausgeber: Bentham Science Publishers
- Kategorie: Fachliteratur
- Serie: Frontiers in Clinical Drug Research - HIV
- Sprache: Englisch
Frontiers in Clinical Drug Research – HIV is a book series that brings updated reviews to readers interested in learning about advances in the development of pharmaceutical agents for the treatment of acquired immune deficiency syndrome (AIDS) and other disorders associated with human immunodeficiency virus (HIV) infection. The scope of the book series covers a range of topics including the medicinal chemistry and pharmacology of natural and synthetic drugs employed in the treatment of AIDS (including HAART) and resulting complications, and the virology and immunological study of HIV and related viruses. Frontiers in Clinical Drug Research – HIV is a valuable resource for pharmaceutical scientists, clinicians and postgraduate students seeking updated and critically important information for developing clinical trials and devising research plans in HIV/AIDS research.
The fifth volume of this series features 5 chapters that cover these topics:
- Clinical Eradication of Latent HIV Reservoirs: Where Are We Now?
- HIV-1 Genotypic Drug Resistance Testing and Next-Generation Sequencing
- Current and Promising Multiclass Drug Regimens and Long-Acting Formulation Drugs in HIV Therapy
- Role of Nanotechnology in HIV Diagnosis and Prognosis
- Preventive and Therapeutic Features of Combination Therapy for HIV
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Seitenzahl: 420
Veröffentlichungsjahr: 2021
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PREFACE
The book series Frontiers in Clinical Drug Research-HIV presents important recent developments in the form of cutting-edge reviews written by eminent authorities in the field. The chapters in this 5th volume are mainly focused on different therapies, cell reservoirs of HIV-1, the combination of drugs and nanotechnology in the diagnosis and prognosis of HIV infection.
Wong and Jiang in Chapter 1 review therapeutic interventions for HIV that have entered preclinical and clinical trials. They also highlight their cure potentials and associated limitations. Liang et al., in Chapter 2 review the HIV-1 genotypic DR testing methods and focus on the main NGS platforms which are available for HIV-1 DR diagnosis. Chapter 3 by Vanangamudi et al., presents the currently available information on multiple drug combinations against HIV and the development of long-acting antiretroviral drugs. Chapter 4 by Wadhwani focuses on HIV pathogenesis and the role of nanotechnology in HIV diagnostics, drug delivery, and therapy. In the final chapter, Zaib et al. give an overview of the therapeutic drugs against HIV, their mechanism of action, their side effects as well as their recommended dosage.
I am grateful to all the eminent scientists for their excellent contributions. I also express my gratitude to the editorial staff, particularly Mr. Mahmood Alam (Editorial Director) and Ms. Fariya Zulfiqar (Manager Publications) for their hard work and persistent efforts.
List of Contributors
Clinical Eradication of Latent HIV Reservoirs: Where are we Now?
Lilly M. Wong1,Guochun Jiang1,2,*Abstract
Antiretroviral therapy (ART) is the leading therapeutic strategy for the suppression of HIV-1 (HIV) replication. However, ART is a life-long treatment with no effective sterilizing or functional cure of HIV. The main challenge with ART is its inability to eradicate HIV residing in the long-lived resting CD4+ T cells, otherwise known as the main latent HIV cellular reservoirs. HIV reservoirs are commonly found in various areas of the body: brain, liver, placenta, skin, GALT, and lymphoid tissues. Withdrawal of ART leads to the rapid rebound of viremia or progress into AIDS without re-treatment. Current clinical approaches such as “shock and kill,” “block and lock,” and gene editing exploit the molecular pathways of HIV latency for eradication or permanent suppression of the latent reservoirs. Novel pre-clinical or clinical approaches must take several limitations into consideration: dose-limiting toxicity, potency, and specificity. These limitations are the barriers to reservoir clearance. The “shock and kill” method employs latency reversal agents (LRAs), including histone deacetylase inhibitors (vorinostat, romidepsin, and panobinostat), PKC agonists (bryostatin-1, prostratin, ingenol, or Kansui), SMAC mimetics, STImulator of INterferon Gene (STING) agonists, and TLR agonists, for the disruption of HIV latency and subsequent eradication of latently infected cells. This is followed by immune clearance, including broadly neutralizing antibodies (bnAbs), therapeutic vaccines, or the use of immune checkpoint inhibitors (ICPi). LRAs have exhibited the ability to increase transcription. However, of the recognized LRAs, none have single-handedly reduced the reservoir, which underscores a potential need for combinational strategies. While some of these interventions have entered trials, repurposing our efforts towards a functional cure of HIV may also be productive. The “block and lock” method seeks permanent silencing of HIV transcriptional machinery through targets such as HIV protein Tat to possibly achieve remodeling of the epigenetic landscape at HIV LTR. Here, we review therapeutic interventions that have entered preclinical and pilot clinical trials and highlight their cure potentials and associated limitations. Prospective directions will be discussed for the development of these new therapeutics into drugs for the cure of HIV.
HIV PANDEMIC IN THE CURRENT SETTING
Human immunodeficiency virus type 1 (HIV-1), a retrovirus discovered in 1983, is the causative agent of acquired immune deficiency syndrome (AIDS). HIV transmission occurs through three routes: sexual intercourse, vertical (mother-to- child) transmission, and intravenous injection. Since the discovery of HIV, there have been 32.7 million deaths worldwide, and it affects more than 38 million individuals [1]. In the past few decades, the course of treatment for HIV infection has greatly improved through the establishment of antiretroviral therapy (ART). In 1985, the development of Retrovir® (zidovudine) by a collaboration between GlaxoSmithKline (GSK) and Samuel Broder demonstrated that HIV infection is manageable and survivable [2]. With the discovery of Retrovir®, seven distinct drug classes emerged with FDA approval: (1) nucleoside reverse transcriptase inhibitors (NRTIs), (2) nonnucleoside reverse transcriptase inhibitors (NNRTis), (3) protease inhibitors, (4) entry inhibitors, (5) integrase inhibitors, (6) coreceptor antagonists and (7) post-attachment inhibitors [3]. Of the seven drug classes, some HIV regimens exploit several drugs as a combinational strategy with the intention of targeting several mechanisms for HIV [4].
In 2019, only 66% (25.4 million out of 38 million individuals) of HIV-positive individuals had access to ART [1]. Individuals often deal with social stigma from the diagnosis of HIV/AIDS along with socioeconomic stress. These factors impact adherence, where pill fatigue and drug resistance can lead individuals to further complications or AIDS. Comorbidities can also introduce pharmacokinetic drug-drug interactions, a consequence often observed in aging populations [5]. ART has the ability to manage HIV infection effectively by suppressing HIV replication to minimal proviral loads (i.e., <50 copies/ml is the standard for the limit of detection) and latent HIV or possibly residual viral replication is responsible for the rebound of HIV when treatment is interrupted. Infected CD4+ T while cells turn to quiescence in the form of memory T cells, which harbor integrated HIV DNA, also called HIV provirus. These quiescent memory T cells have an estimated half-life of 44 months by quantitative viral outgrowth assay (QVOA) [6] or 48 months by intact proviral DNA assay (IPDA) [7]. Even with ART, it would require more than 73 years of treatment to eliminate the latent infection as the provirus remains undetectable by the immune system and continues to evade viral cytopathic effects (CPE); therefore, individuals must remain on treatment indefinitely in order to purge the reservoirs. In addition to the lifelong requirement of ART, treatment interruption poses another problem. Upon interruption of ART, the memory T cells harboring the stable proviral DNA can generate new replication and infections, which may be therapy-resistant [8]. Most HIV-positive individuals have a viral rebound, typically observed two weeks post-interruption due to the induction of productive HIV replication via cytokine induction encounters with latent proviruses [9]. These limitations reveal the need to develop therapeutic strategies that can effectively target the latent reservoirs.
Elite controllers, individuals who maintain silent viral reservoirs without intervention, make up less than 0.5% of HIV-infected individuals [10, 11]. For these HIV controllers, it indicates the potential for an HIV cure. Notably, only two individuals have been cured of HIV: the “Berlin Patient” and the “London Patient.” In 2009, the “Berlin Patient” (Timothy Ray Brown) was cured of HIV after receiving a hematopoietic stem cell (HSC) transplant from a donor containing an entry chemokine receptor (CCR5) mutation to prevent HIV infection of healthy cells (CCR5D32) [12]. The “Essen Patient” underwent an allogeneic HSCT, but upon ART interruption, rapid viral rebound occurred through the alternative entry coreceptor, CXCR4, via receptor switch [13]. After several unsuccessful attempts over the years with various candidates, the “London Patient” (Adam Castillejo) was announced free of HIV in 2019 after complications with grade 1 graft-versus-host disease (GVHD), cytomegalovirus (CMV), and Epstein-Barr virus (EBV) [14]. Unfortunately, HSCTs are limited in their benefits to provide a cure for HIV as HSCTs are hard to scale for populations due to difficulty in finding donor matches. In addition, the procedure has risks such as opportunistic infections and immunosuppression [15]. While the two individuals who are cured of HIV are a cause for celebration, the recurring success of an HIV cure through HSCTs indicates that we should move forward towards a sterilizing or functional cure through other methods for purposes of scalability and efficiency.
MOLECULAR MECHANISMS OF HIV LATENCY
Acute onset of HIV replication in susceptible target cells is followed by the rapid depletion of CD4+ T cells, which indicates the existence of HIV infection [16]. As a result of the rapid replication, high viral plasma loads are apparent [16]. There are three hallmarks for an HIV-infected individual: acute infection, clinical latency (also known as chronic HIV infection), and AIDS diagnosis [17]. Several genes are required for HIV replication and transcription in host immune cells to necessitate its survival; these genes include structural (gag, pol, and env), regulatory (tat and rev), and accessory (vif, vpr, and nef), which are flanked by two long terminal repeats (LTRs) at the 5’ and 3’ end. HIV transcription is further promoted through strict control of several mechanisms that target the Tat-TAR complex via viral host factors such as P-TEFb or cyclin T1 (CycT1)/cyclin-dependent kinase 9 (CDK9). During infection, clinical latency can be established once an individual undergoes an asymptomatic period, which generally lasts 8-10 years. Contributors to clinical latency are: a) low-level of HIV replication persistently depleting CD4+ T cell count via chronic immune activation and b) a degree (>50%) of replication-competent HIV-infected cells undergoing clonal expansion [18]. Without proper intervention such as ART to mitigate reservoir size, HIV will progress to the most severe stage of HIV infection coupled with opportunistic infections, AIDS [16].
HIV Latency
How is the latent reservoir established? Extensive research has investigated reservoir composition and stability; several groups have proposed several mechanisms for its establishment. The onset of latency at either early or late stage is dependent on the cell-cycle, integration landscape, and cellular state where HIV will continue to express or establish latency [19, 20].
Early Stage Latency
Early-stage latency is influenced by several factors in the chromatin environment through histone post-translational modifications (PTMs) such as histone acetylation, methylation, and crotonylation or through interference on transcriptional activity in cis, otherwise known as transcriptional interference (TI) [20]. Although the epigenetic landscape is not well-defined in the context of HIV, PTMs are pertinent to the development of latent reservoirs. With a heavy emphasis on the epigenetic environment, early-stage latency is seemingly influenced by the landscape through nucleosome positioning or RNA polymerase II transcriptional machinery [20]. PTMs modify the life cycle of HIV, including DNA, RNA, protein synthesis, and degradation, where induction and reduction of viral activity have previously been exhibited in several studies [21]. Lysine acetylation (Kac) was a heavily abundant PTM that was first discovered in 1964 by Allfrey and colleagues [22]. Transcription of HIV is activated when histone tails at HIV LTR are acetylated. In contrast, deacetylation of HIV is related to quescient HIV [23, 24]. Histone lysine acetylation is affected by inhibitors of histone deacetylases (HDACs) where latent HIV was reactivated. Accordingly many HDAC inhibitors have been developed to disrupt latent HIV [25-27], which have been extensively reviewed [28]. Similarly, lysine methylation, such H3K9 and H3K27 methylation, negatively regulate HIV transcription as repressors of HIV transcription for the establishment of HIV latency [21]. However, directly inhibiting histone methyltransferase by GSK343 had a very limited efficacy in the disruption of HIV latency unless it was included with other LRA such as HDACi or PKCa although it was initially shown that targeting EZH2 was able to reactivate latent HIV [29-31]. These observations indicate that histone methylation-mediated HIV latency may be highly stable or tends permanence. Lastly, histone lysine crotonylation (Kcr) is one of the latest epigenetic modifications to join the list in histone code. Kcr was previously detected through isotopic labeling in the core histones: linker histone H1, H2A, H2B, H3 and H4, where its presence was validated in nuclei and chromosomes via immunostaining [32]. Furthermore, Kcr is an evolutionary conserved epigenetic modification where it was linked to mouse embryonic fibroblast (MEF) cells and human HeLa cells [32]. Jiang and colleagues applied the findings of Kcr to cell models of HIV latency, peripheral blood mononuclear cells (PBMCs) from HIV-negative healthy donors and PBMCs from HIV-positive donors on ART to observe whether Kcr is applicable to HIV latency [33]. In their findings, Acyl-CoA Synthetase Short Chain Family Member 2 (ACSS2), provides crotonyl-CoA to histones via sodium crotonate (Na-Cro) at the HIV LTR where decrotonylation through ACSS2 suppression will reduce HIV transcription [33]. Furthermore, HIV reactivation was enhanced after crotonylation at the HIV LTR was primed through Na-Cro in vitro and ex vivo when vorinostat was administered afterwards [33]. In addition to these findings, disruption of lipid homeostasis has been reported in association to HIV infection where Jiang and colleagues reported the link of fatty acid metabolism to Kcr and HIV latency via the SIV-infected rhesus macaque model of AIDS where acute infection resulted in subsequently high levels of ACSS2 [33, 34]. These data indicate we should look towards modulating epigenetic environment for latency reversal.
HIV Transcription in the Context of Latency Establishment
While PTMs are increasingly relevant to establishment of HIV latency, it is also important to recognize other important factors for early stage latency. In particular, we must also look to transcriptional machinery for latency establishment. The HIV LTR is comprised of four elements: the promoter, enhancer, Tat activating region (TAR) and negative regulatory element (NRE). An enzymatic complex comprised of P-TEFb is responsible for Tat transactivation, which is mechanistically controlled by the 7SK complex where HIV transcription is enhanced during T cell activation and when protein expression or kinase activity is limited, it is a contributor of HIV latency in primary T cells [35-37]. P-TEFb is a positive elongation factor of HIV transcription where P-TEFb can induce transcriptional elongation or processive transcription of HIV, indicating an area of interest for developing anti-HIV compounds.
Similarly, HIV transcriptional initiation is tightly controlled through inducible transcription factors through factors such as nuclear factor of activated T cells (NFAT), Sp1 and NF-κB after they recruit to their consensus sites at HIV LTR. NF-κB is a classic example for the initiation of Tat transcription and can be enhanced by Sp1 and NFAT [36]. Members of the NF-κB family (NF-κB 1 (p50), NF-κB 2 (p52), p65 (RelA), RelB and c-Rel) are responsible for a broad range of functions through interactions with IκB proteins [38]. These functions are crucial for the inflammatory and immune response such as immune cell proliferation, differentiation and delicate control of apoptosis [38]. The latter is mediated by Inhibitor of APoptosis (IAP) protein family, in particular cIAP1 and cIAP2, and family of Tumor Necrosis Factor (TNF) proteins [39], which was just emerging as a new regulator of HIV transcription and latency [40].
Type I interferon (IFN) signaling is another aspect in the realm of HIV eradication. Discussion of IFN appears as early as 1957 where it was first described that IFN induction of IFN-stimulated genes (ISGs) is a therapeutic response to viral infection [41]. HIV has been reported as an inhibitor of type I and III IFN induction in three target groups: macrophages [42], CD4+ T cells [43] and dendritic cells [44], therefore, targeting IFN signaling remains an opportunity for antiviral therapy.
Moreover, it is important to discuss Toll-Like Receptors (TLRs) in the context of HIV. Various populations of immune cells including dendritic cells, macrophages and natural killer (NK) cells express TLRs both extracellularly (bacterial and fungal detection) and intracellularly (viral detection) [45]. Myeloid differentiation factor 88 (MyD88) is a protein utilized by almost all TLRs except for TLR 3 (which operates through TRIF) and is responsible for the activation of NF-κB for the innate and adaptive immune response, thereby making it favorable target for latency reversal [45].
NF-κB members are structurally similar where the members share a Rel homology domain (RHD), largely responsible for the functions of NF-κB [46]. During regulation of NF-κB activity, members of NF-κB are sequestered in the cytoplasm by IκB proteins. Dimerization by the ankyrin repeat domain, a 33-amino acid repeating motif, is responsible for the activation of NF-κB [47].
Two NF-κB pathways, canonical and noncanonical, have been linked to HIV transcription and latency where activation of the pathways is dependent on several features: stimulus for activation and its kinetics. Manipulation to NF-κB through chromatin remodeling or transcriptional interference helps to establish latency. Previous reports have shown that NF-κB is an essential component for HIV transcription and latency reversal after it binds to two κB consensus sites in several models including primary T cells [36]. Canonical NF-κB is activated by the phosphorylation of IκB. During quiescence, histone deacetylase is recruited to the HIV LTR and is bound by p50 homodimers. Once phosphorylation of IκB occurs, IκBα is degraded by the 26S proteosome where p65/p50 heterodimers can then translocate into the nucleus for activated transcription by Tat/P-TEFb interaction. When canonical NF-κB is impaired, HIV transcription is dormant, indicating that this signaling pathway is a prime target of anti-HIV compounds [47]. Unlike canonical NF-κB signaling, noncanonical NF-κB is governed by the cleavage of p100 into p52 after p100 is phosphorylated. Thereafter, p52/RelB heterodimer is formed and translocated into nucleus for active transcription of its target gene, such as HIV. The signaling pathways are also different in their kinetics where canonical NF-κB works quickly and independent of protein synthesis while noncanonical NF-κB works slowly and persistently, which is dependent of protein synthesis [40]. Recently, noncanonical NF-κB is particularly of interest due to its key characteristics as a longer-lasting signaling pathway and its specificity, which will be discussed later in this chapter.
There are three Sp1 binding sites at HIV LTR which are essential for the initiation of HIV transcription where Sp1 site III recruits p300 acetylatransferase to HIV LTR. This is distinct from NF-κB recruitment where its kappaB site I serves a stronger activating role than kappaB site II [48]. NF-κB (RelA/p50) further recruits p300 into HIV LTR to drive HIV transcription [49]. In contrast, p300 is disrupted from HIV LTR while chromatin repressors such as HDAC1, SUV391/HP1 by Sp1 or NF-κB recruit back to HIV LTR to establish its latency [24, 49, 50]. This is similarly observed in the role of AP4 protein during its regulation of HIV transcriptional repression [51]. Apparently, HIV evolves a common mechanism to hijack host factors for its Bright or Off mode by the recruitment of such positive and negative factors, thereby altering the sensitivity of viral gene expression to stochastic fluctuations in the Tat feedback loop [24, 48]. However, it is not clear why inhibiting any of these individual transcription factor can disrupt HIV latency unless these proteins form a super protein complex at the HIV LTR.
Late Stage Latency
A widely accepted mechanism of late stage latency is the effector-memory-transition (EMT) where active CD4+ T cells turn quiescent and is often a result of changes to the cellular state. The changes to the cellular state indicate that the cellular state is a major component in transition to latency [20]. The HIV genome has been detected in several memory cell subsets derived from several anatomical locations, making latent HIV highly challengeable to be disrupted in vivo [52]. Shan and colleagues reported that site of infection likely assisted the differentiation of EMT CD4+ T cells into effector memory (TEM), central memory (TCM) and transitional memory (TTM) cells where CXCR4, a coreceptor for HIV entry into CD4+ T cells, is prevalent [52]. In addition, their results also indicate that latency selectively occurs in EMT CD4+ T cells and demonstrate how latent reservoirs are formed [52]. These data together from early stage and late stage latency demonstrate the complexity of latent reservoir establishment and underscores the need in optimizing our understanding of latent infection.
CURE STRATEGIES
In the past three decades, HIV/AIDS research mostly limited its focus to optimization of ART, which has been thoroughly discussed before [53]. Here, we discuss theoretical cure strategies that bypass the limitations of ART.
Shock and Kill
A proof-of-concept study reported by Archin and colleagues demonstrated that eradication of HIV infection through pharmacological compounds, vorinostat in particular, is a possible strategy for HIV cure in resting CD4+ T cells ex vivo, otherwise known as “shock and kill” [27]. “Shock and kill” employs latency reversal agents (LRAs) to disrupt latent HIV through host-dependent mechanisms or through immune-mediated clearance.
To this extent, several classes of LRAs have been tested in pre-clinical and clinical studies to determine their potential as a sterilizing cure of HIV where the LRA in question activates the HIV LTR by altering the epigenetic environment, targeting receptors of HIV, or through nuclear factor-κB (NF-κB) signaling. LRAs from the class of inhibitors of histone deacetylases (HDACi) such as vorinostat, panbinostat and romidepsin have shown the ability to induce gene expression during clinical trials but have failed to reduce the reservoir size [26, 54, 55]. Other studies have invested efforts in protein kinase C (PKC) agonists, second mitochondrial-derived activator of caspases (SMAC) mimetics, STImulator of Interferon Gene (STING) agonists and TLR agonists.
Block and Lock
A strategy opposite to “shock and kill” has been proposed. A functional cure where therapeutic interventions target HIV transcriptional machinery for induction of a permanently silent state (“deep latency”), otherwise known as “block and lock” has been pre-clinically tested. In order to induce the silent state, approaches have been formed to target the epigenetic environment related to HIV latency. Practicality for the “block and lock” strategy is evidenced by the elite controllers [10, 56] and previously established presence of human endogenous retroviruses (HERVs). The human genome contain approximately 8% of HERVs which are residues (gag, pol and env) analogous to infectious retroviruses [57]. HERVs are quiescent until stimulated where much of its activity is achieved through epigenetic regulation such as DNA methylation [58]. Approaches for achieving deep latency include 1) epigenetic silencing or transcriptional gene silencing (TGS) and 2) RNA therapeutics.
Viral silencing appears in literature as early as the 1990’s. In 1993, Pätzold and colleagues applied an indolocarbazole, Gö 6976, to latently infected U937-derived cell line, U1, for inhibition of HIV replication through PKC- NF-κB signaling [59]. Results from Gö 6976 prompted Pätzold and colleagues to further investigate other indolocarbazoles. This led to the discovery of Gö 7716 and Gö 7775, which were highly-selective and potent PKC inhibitors at nanomolar levels in vitro [59]. Furthermore, p24 antigen and infectious viral particle production via known activators of HIV transcription (phorbol myristate acetate and TNF-α) was inhibited after indolocarbazoles were administered in vitro. In addition, cell proliferation was not reduced, eliminating the possibility of cytotoxicity for inhibition of HIV replication. Other studies include inhibitors of Tat (Ro 5-3335 and Ro 24-7429), NF-κB and anti-TNFα [60, 61]. Ro 24-7429 exhibited potential as a Tat inhibitor due to its tolerability in vivo, unfortunately, it did not control viral replication; therefore its continuation in study was discontinued [62, 63]. Targeting the Tat/TAR interaction directly inhibits the positive feedback loop for transcription to drive latency establishment; therefore, it remains a potential target for deep latency studies [63, 64]. These previous studies highlight the possibility to maintain the latent state of HIV infection.
However, questions remain—Is it enough to induce deep latency by solely inhibiting HIV Tat protein? Can epigenetic remodeling be achieved during the induction of deep latency? In other words, can deep latency be maintained? Will the treatment fail due to acquired drug resistance since Tat is an HIV protein? If a sustained silent state cannot be achieved, individuals with ART interruption would experience viral rebound. Transcription factors such as NF-κB and NFAT could be modulated through the available small molecules to avoid drug resistance or to enhance the induction of a permanently silent state similar to a combination therapy [58]. Although ART has become the gold standard for HIV-infected individuals, removing the lifelong daily regimen required for individuals is a large benefit. If a functional cure is achieved, it would allow individuals to cease ART with no viral rebound [58, 65].
Didehydro-cortistatin A
More recently, Mousseau and colleagues reported that an analog of Cortistatin A (CA), didehydro-Cortistatin A (dCA), is an inhibitor of Tat via Tat/TAR interaction [66] and cyclin-dependent kinase 8 (CDK8) [64]. dCA partially inhibited HIV replication in both acute and chronically infected cells with strikingly low concentrations. It was able to establish an almost permanent state of latency in multiple models of HIV latency along with primary CD4+ T cells from HIV-infected individuals [65]. Interestingly, delayed viral rebound after ART interruption was observed when dCA was administered to bone marrow-liver-thymus (BLT) humanized HIV mouse model in vivo [67]. Furthermore, dCA was also tested in SIVmac239 and SIVmac251-infected cell line models and CD4+ T cells isolated from rhesus macaques where dCA was shown to bind to SIV Tat domain for inhibition of Tat activity [68]. Although, dCA has been shown to inhibit Tat in preclinical assays, its limiting factor is the residual Tat-independent activity. Residual Tat-independent activity drives the promoter for transcriptional control of HIV, making the latent state partial to the effect of dCA. However, dCA may remain beneficial as it could prevent off target effects; thus, further studies may use dCA in combination therapies.
Triptolide
On the other hand, triptolide has gone under investigation for HIV inhibition. Triptolide is a traditional Chinese herbal medicine extract derived from Tripterygium wilfordii with anti-inflammatory and anti-cancer properties [69, 70] through its inhibitory properties of RNA polymerase I, II and III [71, 72]. The use of triptolide has been documented in cases of rheumatoid arthritis. Wan and colleagues tested triptolide as an anti-HIV drug in vitro [66]. Inhibition of HIV replication (97.9% reduction) through triptolide at nanomolar concentrations (5 nM) was observed in latently infected models (TZM-bl and Jurkat) as well as three strains (HIVNL4-3, HIVLAI and HIVBaL) of several donors of peripheral blood mononuclear cells. Triptolide’s cytotoxicity was also measured where the maximum non-toxic concentration in peripheral blood mononuclear cells was 4 μM while all tested nanomolar concentrations in latently infected cell lines did not present any cytotoxicity. The mechanism of triptolide is not well-known, however, Wan and colleagues revealed inhibitory activity directly on the Tat protein due to its ability to reduce Tat-GFP in HeLa-transfected cells with FLAG-tagged Tat. The mode-of-action for triptolide could be through proteasomal degradation of Tat. Wan and colleages eliminated the option of whether or not triptolide affects the NF-κB pathway by mutating two NF-κB binding sites in Jurkat cells. Tat transcription was not affected when the mutated Jurkats were treated with triptolide, thereby excluding the possibility of triptolide acting on the NF-κB signaling pathway. Furthermore, Tat mRNA was not altered when Tat-transfected HeLa cells were treated with and without triptolide, signaling that mediation of Tat protein expression occurs at either translation or degradation. Treatment with proteasomal inhibitor, MG132, subsequently resulted in 30% increase in Tat expression in cells treated with triptolide. Together, these data show triptolide’s mode-of-action to an extent through proteasomal degradation of Tat [66]. Despite its ability to inhibit HIV reactivation in cell lines of HIV latency, triptolide’s ability to globally inhibit transcription poses another risk in vivo. It is currently undergoing clinical trials to observe its potential as a therapeutic treatment for the “block and lock” strategy [NCT02219672; NCT03403569].
Gene Editing Technology for Disruption of Latent HIV
Modifications at the RNA and DNA level has become an attractive tool for HIV infection due to its specificity and potency, otherwise known as gene editing [73]. RNA interference (RNAi) is a standard defense mechanism used by eukaryotic organisms against pathogens where it was first discovered in Caenorhabditis elegans (C. elegans) in 1998 [74]. Double-stranded RNA is processed in small or short interfering RNAs (siRNAs) where it forms an RNA-induced silencing complex (RISC) for foreign RNA cleavage [74]. Several siRNAs and short hairpin RNAs (shRNAs) have been developed as therapeutic strategies for HIV where (1) may be used in combination with ART or (2) may be personalized to the individual [75]. RNAi is highly efficient for gene silencing which has been optimized through computational algorithms for high sequence specificity to minimize cellular toxicity [73].
Other gene-editing tools such as CRISPR/CRISPR-associated nuclease 9 (Cas9) [76], transcription activator-like nucleases (TALENS) [77], and zinc-finger nucleases (ZFNs) [78] are other potential tools for HIV treatment. As previous studies have shown that patients with the CCR5 mutation (CCR5D32) are cured of HIV, the aforementioned gene-editing tools were implemented in studies where site-specific double-stranded DNA breaks were applied via insertions and deletions at the LTR to generate therapeutic avenues for HIV cure. Transcription activator-like nucleases (TALENS) come from Xanthomonas bacteria from plants, which is comprised of a DNA-binding domain and a DNA-cleaving nuclease, FokI. TALENS have been used in several species of research with high success rate for specificity in human cells [79]. Pre-clinical tests showed that CD4+ T cells with CCR5 modification can proliferate and demonstrated a protective effect against HIV infection [78]. Zinc-finger nucleases (ZFNs) have been sought after in seven Phase 1/2 clinical trials with the first clinical trial from Tebas and colleagues [80]. Tebas and colleagues administered ZFN-modified autologous CD4 T cells to 12 individuals after ART interruption where HIV RNA and DNA decreased in most individuals where 1 individual’s HIV RNA level was undetectable [78], indicating that the administration of ZFNs in vivo is safe and has the ability to mimic the CCR5 modification [78].
CRISPR/Cas9
Although the news of ZFNs in vivo is exciting, some limitations of TALENS and ZFNs are that they are time-consuming and costly in addition to its difficulty in development and assessment in cellular assays. CRISPR/Cas9, the newest gene-editing technique, is cost-effective and more accurate with minimal off-target effects [81]. After its discovery in 1987 [82], CRISPR/Cas9 technology was rapidly developed in hematopoietic stem and progenitor cells (HSPCs) [83]. It was successfully tested at the U3 LTR region of 293T, HeLa and Jurkat cells by Ebina and colleagues for the suppression of HIV expression [84]. Through this breakthrough study, disruption of HIV was observed along with the ability to block latent HIV expression, indicating the potential for CRISPR/Cas9 to eradicate the virus [84]. A recent study by Yin and colleagues in 2017 demonstrated that proviral DNA in both Tg26 transgenic mice and HIV-infected BLT humanized mice can be excised for suppression of viral expression in vivo [85]. Taken together, these findings indicate the potential for CRISPR/Cas9 use in vivo where excision of HIV could be explored as a mechanism for suppression of HIV expression. Furthermore, CRISPR/Cas9 has also been used in studies to further the “shock and kill” strategy where NF-κB binding sites were targeted as these domains are significant to latency establishment. Zhang and colleagues utilized sgRNAs to modulate the HIV promoter at the U3 region where robust induction of HIV expression was observed in several cell models of HIV latency (Jurkat, TZM-bl and CHME5 microglial cells) [86]. Zhang and colleagues’ findings demonstrate that CRISPR/Cas9 technology could be used as an additional method of delivery for “block and lock” and “shock and kill” strategies in finding an HIV cure. To this extent, Saayman and colleagues generated sgRNAs for use in T cells and found similar results of induced HIV expression at the NF-κB binding sites [87] while Limsirichai and colleagues showed that a combination therapy of LRAs and CRISPR/Cas9 generates robust HIV reactivation [88].
Ultimately, further investigation is warranted as the current studies have not yet explored models of latency in primary T cells when ART is interrupted. Another concern of CRISPR/Cas9 is its off-target. Therefore, we continue to look towards development of gene editing technology for improved specificity and drug development for other therapeutic avenues.
DRUG DEVELOPMENT IN PRE-CLINICAL AND CLINICAL TRIALS
Latency reversal agents that have been tested in preclinical and/or clinical models are given in Fig. (1).
Fig. (1)) Latency reversal agents that have been tested in preclinical and/or clinical models.HDAC Inhibitors
Robust reactivation of HIV has been observed in vitro and ex vivo with inhibitors of histone deacetylase, an epigenetic modification (HDACi) associated with HIV latency. However, some latently infected cells are not inducible with a single dosage; therefore, dynamics of the latent reservoir must be kept in mind when administering LRAs [18]. Many of the clinical trials for latency reversal utilize inhibitors of histone deacetylation (vorinostat, romidepsin and panobinostat) [89-91]. One caveat is that there has not been any success in reservoir reduction in the
clinical trials with HDACi. While reservoir size remains unchallenged by HDACi alone, these studies preface the beginning of combination therapy with other potent LRAs.
Protein Kinase C Agonists
A family of protein kinase enzymes, protein kinase C (PKC), can be modulated for reactivation of HIV. Previous studies have found that phorbol esters such as phytohemagglutinin (PHA) and phorbol 12-myristate 13-acetate (PMA) reactivates latent HIV by PKC-canonical NF-κB signaling [92].
PKC agonists (PKCa) target PKC-canonical NF-κB signaling where IκBα is degraded to drive HIV transcription. During an inactive state, the pseudosubstrate (PS) region is sequestered. Ligands bound to the N-terminal domain of PKC activates the PKC pathway through phospholipase C (PLC) enzymes. Two messengers (inositol 1,4,5-trisphosphate and DAG) are generated after metabolism of phosphatidylinositol 4,5-bisphosphate. Messenger DAG binds with Ca2+, which subsequently phosphorylates IκB to govern the canonical NF-κB signaling pathway [47].
PKCa are mimetics of DAG for initiation of the PKC-canonical NF-κB signaling. There are three chemical families of PKCa which include phorbol esters such as PMA, prostratin and DPP, bryostatin-1, ingenols and its derivatives such as IngB, ingenol mebutate and Euphorbia kansui where these PKCa have been tested in several models of HIV latency, BLT humanized mice, and are undergoing clinical testing.
PMA, Prostratin and DPP
Phorbol esters have been shown to reactivate HIV from latency [93]. These phorbol esters include phorbol-13-myrisitate-12-acetate (PMA), prostratin (12-deoxyphorbol-13-acetate) and DPP (12-deoxyphorbol-13-phenylacetate). Administration of PMA and ionomycin (NFAT activator) increased HIV mRNA levels (148.8-fold) in resting CD4+ T cells from HIV-infected individuals on ART along with upregulated signaling downstream of the T cell receptor [94]. Although PMA/ionomycin have shown robust transcription, PMA is a tumor-promoting phorbol ester; therefore, its clinical use is limited [95].
More recently, prostratin was identified as another phorbol ester where it derives from Pimelea prostrata and Homalanthus nutans [96]. Prostratin from Homalanthus nutans has a history of use by Samoan healers as a course of treatment for hepatitis [96]. A consequence of plant extracts is that it is difficult to quantify the active ingredient. In particular, prostratin content varies between the sections of Homalanthus nutans [96]. Leaf and root tissues of Homalanthus nutans only generate 2.5 and 2.9 μg/g, respectively whereas the stem tissue can range from 0.2 to 52.6 μg/g [96]. Inconsistencies with quantitation provide another obstacle for utilizing prostratin in vivo. Limited quantities of prostratin prevent further investigation of its mode-of-action. Wender and colleagues reported a synthesis of prostratin from renewable resources to generate reliable quantities (milligrams to grams) of the compound for continued investigation of its clinical potential [97]. Interestingly, prostratin has failed to exhibit tumor-promoting effects like PMA, indicating that it is an inhibitor of oncogenic activity in an animal model (mouse skin) and furthermore, it inhibited the effect of PMA when PMA was administered as a pretreatment [98, 99]. Since its identification, prostratin has been tested in cell models of HIV latency in vitro [100], healthy donor PBMCs with three discrete tropisms and PBMCs isolated from HIV-infected individuals on ART [101], in primary human peripheral blood lymphocytes and within the SCID-hu (Thy/Liv) model [102]. In the tested models, prostratin elicited latent HIV reactivation with minimal effects on cellular activation, thereby preventing new infection [102]. With these findings, it renews the opportunity for prostratin as a candidate for clinical use.
DPP also exhibits non-tumor-promoting activity similar to prostratin. DPP presents a hydrophobic phenyl group whereas prostratin is missing this functional group while PMA contains an oxygenated hydrocarbon chain. It is suspected that its functional group sustains the active properties of DPP and PMA [103] where its potency in vitro is 20-40-fold more than prostratin [104] and has been assessed in PBMCs isolated from HIV-infected individuals on ART where the reactivation of latent HIV was 5-fold more than prostratin [103]. DPP’s ability to induce HIV expression makes it an attractive candidate like prostratin for the “shock and kill” strategy without inducing de novo HIV infection.
To this extent, Beans and colleagues generated prostratin analogs with modifications at the C13 position. The generated analogs had higher affinities for PKC and was tested in cell models of HIV latency in vitro where the analogs’ performance was greatly improved (20-fold more than DPP; 130-fold more than prostratin) [105]. It was also tested in resting CD4+ T cells where a 100-fold reactivation was observed [105]. More importantly, it maintained a minimal effect on cellular activation. The practical synthesis of prostratin analogs provides a scalable and efficient avenue for use in clinical applications.
Bryostatin-1
Macrocyclic lactones, a chemical family under PKCa, have exhibited pharmacologic properties in the reactivation of latent HIV. Macrocyclic lactones include approximately 20 bryostatins from Bugula neritina [106]. The original isolation of bryostatin from B. neritina only generated 18 grams of bryostatin-1 from 14 tons of source material in 1968, stressing the difficulty of its extraction and limiting its use for investigation [107]. Wender and colleagues reported a 29-step scalable synthesis of bryostatin-1 to generate several grams of bryostatin-1 and analogs [108]. Previously, syntheses of bryostatins ranges from 36-90 steps where bryostatin-1 previously required 57 steps in its synthesis [109].
Bryostatin-1 exhibited anti-tumor properties in vitro and in vivo [106], marking bryostatin-1 an appealing pharmacologic compound for clinical use like prostratin and DPP. Although it has gone through numerous phase I/II clinical trials, it has not reached the standard for phase III clinical trials but has shown potential in diseases such as Alzheimer’s disease [110] and more recently, HIV through its inhibitory effect on RNA polymerase II-phosphorylation [111]. Bryostatin-1 can dephosphorylate CDK2 where CDK2 has been previously linked to impaired Tat function [111]. Bryostatin-1 was shown to induce latent HIV expression without global T-cell activation in vitro and ex vivo [111]. Due to its ability to reactivate HIV without cellular activation in vitro and ex vivo, it entered a phase I clinical trial where it proved its safety in vivo but failed to induce HIV transcription at its provided low dosage. Its failure to reactivate latent HIV at the provided dosage implies that a higher dosage or combination therapy may be needed [112].
While bryostatin-1 was unable to reactivate HIV at the low dosage during the phase I clinical trial, it could also be that the naturally occurring bryostatins are not optimized for application in the “shock and kill” strategy. In addition, the low supply of bryostatin-1 deters further investigation of its use. Therefore, reformulation of bryostatins could provide another opportunity for HIV cure. Only 6% of naturally sourced materials are therapeutic agents in the realm of antivirals while the rest are synthesized and optimized from the original structure [113
