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Human immunodeficiency virus (HIV), the causative agent of acquired immunodeficiency syndrome (AIDS), has become one of the most costly and deadly epidemics in human history. Once thought of as a death sentence, HIV infection has become a chronically manageable disease. Cc-chemokine receptor-5 (CCR5) is known as a main co-receptor in human immunodeficiency virus-1 (HIV-1) infection. So, it could be a target for inhibition of HIV- 1 entry into CD 4+ immune cells. Many studies showed homozygote individual with 32bp deletion in CCR5 gene had nature resistance to HIV-1. Berlin and Boston patients transplanted with allogeneic hematopoietic stem cell (HSC) and demonstrated effective cure for HIV-1 infection. Additionally, developments in the use of lentiviral vectors and targeted nucleases have opened the doors of precision medicine and enabled new treatment methodologies to combat HIV infection through targeted ablation or down-regulation of CCR5 expression. And the chemokine receptor CCR5 has garnered significant attention in recent years as a target to treat HIV infection largely due to the approval and success of the drug Maraviroc.
Fig. 1. Global population distribution of HIV overlaid with gene therapy clinical trials
CCR5-edited gene therapies for HIV cure
HIV entry requires interaction of the viral particle with two surface receptors found on the target cell. The primary receptor for viral entry is the surface marker CD4. This receptor is an essential molecule in the immune system, and is responsible for eliciting adaptive immunity against invading pathogens. Once HIV successfully binds to CD4, it then requires a secondary co-receptor to complete the infection process (Fig. 1). There are two known co-receptors for the virus: the dominant co-receptor for most HIV isolates is CCR5 (“R5-tropic viruses”), whereas the secondary alternative co-receptor is CXCR4 (“X4-tropic viruses”).
Fig. 2. The interactions between HIV particle and cell surface receptors during virus entry
Multiple CCR5 loss-of-function approaches could generate an HIV infection-resistant immune system and/or contribute to eradication of latently infected cells. The most direct method is to mimic the conditions of the Berlin Patient, where an infected individual receives an allogeneic HCT from a matched CCR5Δ32 donor. However, this method presents several significant problems. Instead of allogeneic transplantation, recent advances in the field of gene therapy have presented a method for generating a functional knock-out of CCR5 expression from a patient’s own cells (“autologous transplantation”). Various “designer nuclease” platforms, which can be targeted to specific genomic loci, take a patient’s own cells and engineer them to resemble a CCR5Δ32 phenotype. This would functionally recreate the Belin Patient’s case without the deleterious side effects of GvHD and the requirement for an HLA-matched donor. With the discovery and development of designer nuclease platforms, such as Zinc Finger Nucleases (ZFNs) and clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9, this concept of engineering viral resistance in a patient’s own cells is becoming a reality.
To date, clinical trials have been initiated using gene therapy approaches to knock-out CCR5 expression in human patients. In ongoing clinical trials, this approach is combined with shRNA-dependent down-regulation of CCR5 expression, providing two lines of defense against viral fusion, and targeting viruses that use either CCR5 or CXCR4 co-receptors.Two such protocols are actively enrolling patients (NCT02343666 and NCT02378922). One of these studies, which began recruiting patients in 2016, uses a similar combination of CCR5 shRNA and C46 expression, but also includes a chemo-selectable gene (MGMTP140K) to enable an in vivo titration of modified cell numbers after infusion (NCT02343666). Another ongoing study, scheduled to end in 2017, looks at the combination of CD4+ and HSPC modified cells with a dual expressing lentiviral vector (Cal-1) encoding CCR5 shRNA and mC46 (NCT01734850).
Development of CCR5 antagonist for treatment of HIV
Following the demonstration of the importance of the CCR5 receptor for HIV-1 entry, and based on the fact that G protein-coupled receptors have historically been tractable to potent, selective, low dose small-molecule drugs, Pfizer Global Research and Development (Sandwich, UK) initiated a CCR5 antagonist discovery program. An intensive medicinal chemistry program to optimize binding potency against the receptor, antiviral activity, absorption, and pharmacokinetics, and selectivity against human cellular targets resulted in the identification of MVC (formerly UK-427857) as a promising candidate for further development. Mechanism-of-action studies confirmed that MVC blocked the binding of virus gp120 to CCR5 and acted as a slow-offset functional antagonist of CCR5. MVC had no adverse effects in cell-based cytotoxicity studies, was highly selective for CCR5, and was predicted to have human pharmacokinetics consistent with once-daily (QD) or twice-daily (BID) dosing.
The success of MVC in patients has set a high standard for next generation CCR5 entry inhibitors as both Vicriviroc and Apliviroc failed in the clinic mainly due to lack of efficacy. However, the inadequacies of hepatotoxicity and use in only treatment experienced patients place CCR5 antagonists in a secondary role in the treatment of HIV. The on-going experimental and clinical research on CCR5 antagonists for HIV treatment has resulted in several significant second-generation agents with novel structural features, and dual chemokine activity. These follow-up leads based on MVC and the other second-generation CCR5 compounds have failed to produce a clinical candidate or failed to reach advanced stage clinical trials with MVC itself remaining superior. Surprisingly, the only CCR5 antagonist currently in advanced clinical trials for the treatment of chronic HIV infection is Cenicriviroc (TAK-652, 51). In addition to CCR5, Cenicriviroc also binds to CCR2 and may confer immunologic, cardiovascular and metabolic benefits in HIV infected individuals who experience low levels of immune activation during prolonged HAART treatment. The favorable outcome of these efforts may also resurrect the therapeutic and commercial interest in research and development activities to find newer and more efficacious CCR5 antagonists with expanded utility.
Since the 2008 report documenting the Berlin Patient as the first known HIV cure, the scientific field has aggressively pursued replicating this success. Public and private funding agencies worldwide have invested billions of dollars to advance research aimed at making such a treatment broadly applicable to a greater number of patients. Thanks to the significant financial and personnel investment worldwide into gene therapy approaches for HIV cure, tremendous advancements have also been made in related HIV fields such as better understanding latency and characterizing viral reservoirs. The future outlook for both HIV treatment and potential cure efforts is getting brighter. Until the day when these technologies fulfill their potential in creating a broadly applicable and permanent HIV cure, the scientific field and the community as a whole await the second individual to be cured of HIV, and then millions more.
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