Current HIV Research - Volume 10, Issue 1, 2012

Most human immunodeficiency virus (HIV) infections are acquired during sexual contact, across the genital or rectal mucosal epithelium. At present, HIV preventive strategies such as behavioral and structural interventions (e.g., counseling and condom use) or pre-exposure prophylaxis (e.g., topical microbicides or the oral administration of antiretroviral drugs) seem to be the only effective and most indicated methods against the establishment of systemic HIV infection. The recent success of the CAPRISA 004 phase II clinical trial, using a tenofovir-based gel, as well as the iPreX trial, using oral TRUVADA®, not only provided the proof-of-concept for reverse transcription inhibitor (RTI)-based vaginal microbicides but also demonstrated the real possibility of using topical or oral pre-exposure prophylaxis (PrEP) to prevent sexual HIV transmission. Unfortunately, more recent failures in the FEMPreP and VOICE trial, using similar regimes, suggest that there is still room for improvement. Therefore, ongoing and future studies will be key in the development of novel potent and safe strategies to block HIV transmission.

Since the beginning of the AIDS pandemic, and following the discovery of the human immunodeficiency virus (HIV) as the etiological agent of the disease, it was clear that the virus gains access to the human host predominantly through the mucosal tissue after sexual exposure. As a consequence, the female genital tract (vaginal and cervical), as well as the rectal, penile, and oral mucosae have been extensively studied over the last thirty years towards a better understanding of - and to develop strategies to prevent - sexual HIV transmission. This review seeks to describe the biology of the events leading to HIV infection through the human mucosa and introduce some of the approaches attempted to prevent the sexual transmission of HIV.

Despite the identification of HIV-1 as the etiological agent responsible for AIDS nearly 30 years ago, a sterilizing vaccine capable of preventing transmission of the virus remains elusive. In response to struggles on the vaccine development front, significant effort has been devoted to preventing the transmission of HIV with alternative products, technologies, and strategies. One of the early alternative HIV prevention strategies was microbicides, which are topical products that can be used to prevent sexual transmission of HIV either vaginally or rectally. First generation microbicide products were designed to be simple gel formulations comprised of readily available active agents that were inexpensive and broadly active (i.e., non-specific). Unfortunately, despite the clinical investigation of multiple product concepts satisfying these requirements, none were shown to be efficacious in pivotal trials. More recently, microbicide and oral prevention strategies involving highly specific and potent anti-retroviral (ARV) drugs have shown to be efficacious in trials. Although building on these successes continues, these products have a number of issues including potential toxicity with long term use, selection of HIV resistance, and cost. Further, all of the original justifications for non-specific microbicide products remain valid. This review provides a brief history of non-specific microbicide development, outlines the evolution to, and limitations of, ARV based microbicides, and summarizes the current activity on non-specific microbicide product development.

Preventing the transmission of human immunodeficiency virus (HIV) is the main goal of numerous studies trying to develop an effective vaccine and microbicide agents. Here we review the use of antiretroviral drugs to inhibit viral entry as potential HIV microbicides. After the failure of nonoxynol-9 microbicide strategies shifted towards the use of compounds creating a physical barrier to virus attachment (e.g., surfactants) or inhibit the virus in the vaginal milieu (e.g., polyanions). These early, non-specific inhibitors showed promise in both in vitro and in vivo(non-human primates) studies but provided only modest protection from HIV transmission in clinical efficacy trials. The next generation of HIV entry microbicides was based on specifically blocking virus from entering host cells by targeting CD4 attachment, gp120 binding, and virus-cell membrane fusion events. Although protection from an SIV-HIV hybrid was evident in non-human primates treated and challenged in the vaginal cavity, none of these compounds have advanced to clinical trials as a microbicide. Here we will discuss the reasons for these failures, including the selection of drug resistant HIV variants, which raises questions as to the future of broadly effective microbicides based on HIV entry inhibitors. The outcome of continued research and potential efficacy trials on the next generation of entry inhibitors might reveal whether or not an effective entry microbicide can be developed.

The CAPRISA 004 study in South Africa has accelerated the development of vaginal and rectal microbicides containing antiretrovirals that target specific enzymes in the reproduction cycle of HIV, especially reverse transcriptase inhibitors (RTI). In this review we discuss the potential relevance of HIV-1 RTIs as microbicides, focusing in the nucleotide RTI tenofovir and six classes of nonnucleoside RTIs (including dapivirine, UC781, urea and thiourea PETTs, DABOs and a pyrimidinedione). Although tenofovir and dapivirine appear to be most advanced in clinical trials as potential microbicides, several issues remain unresolved, e.g., the importance of nonhuman primates as a “gatekeeper” for clinical trials, the emergence and spread of drug-resistant mutants, the combination of microbicides that target different phases of viral reproduction and the accessibility to microbicides in low-income countries. Thus, here we discuss the latest research on RTI as microbicides in the light of the continuing spread of the HIV pandemic from the point of view of medicinal chemistry, virological, and pharmaceutical studies.

Microbicides are products that can be applied to vaginal or rectal mucosal surfaces with the goal of preventing, or at least significantly reducing, the transmission of sexually transmitted infections including HIV-1. Despite more than two decades of HIV-1 research, there is still no efficacious HIV-1 vaccine, and the scientific community appears sceptical about the short or long-term feasibility of developing a vaccine that has the ability to induce sterilizing immunity against HIV-1. In this setting, microbicide research has been developed. Among the promising candidate microbicides, the integrase inhibitors are the most recently developed compounds. In fact, since the beginning reverse transcriptase, fusion and entry inhibitors were identified as possible HIV-specific candidate microbicides along with the non-specific topical microbicides. In the case of integrase inhibitors, only a few have demonstrated to block HIV-1 infection in models that mimic sexual transmission of the virus. These compounds have been tested in in vitro and ex vivo assays to determine their efficacy in pre- and/or post-exposure prophylactic settings. In particular, the naphthyridinecarboxyamide L-870,812 has been shown to block viral infection in pre- and post-exposure studies obtaining comparable results to the reverse transcriptase inhibitor PMPA. The purpose of this article is to provide an overview of integrase inhibitors as potential topical microbicides and their comparative evaluation with HIV-specific and non-specific microbicides.

The new generation of microbicide candidates is based on the use of antiretroviral (ARV) drugs. The first compounds tested were selected among ARV classes that block the early stages of viral replication cycle, either entry/fusion or reverse transcription. Recently, an additional class of ARVs acting on a later step of viral replication has entered the microbicide pipeline, the protease inhibitors (PIs). This class of ARVs has shown strong potency in highly active antiretroviral therapy (HAART) against human immunodeficiency-1 virus (HIV-1) and lower levels of induced resistance compared to other ARV classes. As candidate microbicides, PIs will have to follow the guiding principles of microbicides being effective, safe, user-friendly and affordable. Hence, aspects including antiviral potency, adverse effects in mucosal tissues, formulation, pharmacology at mucosal sites, emergence of resistance, will have to be considered in the design of a PI-based microbicide. In addition, PIs will have to be tested for their capacity to be used in combination-based microbicides with other ARV classes and in combination with other preventive strategies, such as HIV-1 vaccines. Microbicides containing HIV PIs could also be potentially used against other pathologies specific to the female and/or male genital tract or colorectum.

Tenofovir, a highly prescribed drug for the treatment of HIV/AIDS infections, has recently also shown its effectiveness as a potential topical microbicide drug in the prevention of HIV transmission. Here, we discuss the combination of tenofovir with various other antiretrovirals (ARV) highlighting the large class of carbohydrate-binding agents (CBAs) targeting the glycans on the viral envelope gp120 for their anti-HIV activity and their favorable combinatory potential. The tenofovir/CBA and several other ARV combinations consistently showed synergistic antiviral activities. Also combinations of other classes of ARV such as receptor (i.e. CD4, CXCR4 and CCR5) inhibitors, various monoclonal antibodies (mAbs) directed against the HIV envelope gp120 and HIV gp41 inhibitors were demonstrated to have synergistic anti-HIV effects. Moreover, certain antimetabolite drugs that show limited, if any, anti-HIV activity when administered as a single drug, can potentiate the antiviral activity of anti-HIV nucleoside analogues (NRTIs) by creating a beneficial metabolic and/or competitive advantage for the combined NRTIs. Thus, well-defined combinations of ARV may synergize and/or enhance the antiviral potency of the individual drugs and should be envisioned in the design of future microbicide studies. Recently, drugs such as tenofovir and acyclovir were demonstrated to be endowed with a dual (concomitant) antiviral (i.e. anti-HIV/HSV) activity in different in vitro, ex vivo and animal models. They also deserve special attention for their potential to prevent HIV transmission and to concomitantly suppress co-pathogens of HIV such as herpes viruses.

Despite advances in the treatment of HIV infection, heterosexual transmission of HIV remains high, and vaccines to prevent HIV acquisition have been unfruitful. Vaginal microbicides, on the other hand, have demonstrated considerable potential for HIV prevention, and a variety of compounds have been screened for their activity and safety as anti-HIV microbicides. Among these are the naturally occurring host defense peptides, small peptides from diverse lineages with intrinsic antiviral activity. Naturally occurring host defense peptides with anti-HIV activity are promising candidates for vaginal microbicide development. Their structural variance and accompanying mechanistic diversity provide a wide range of inhibitors whose antiviral activity can be exerted at nearly every stage of the HIV lifecycle. Additionally, peptide modification has been explored as a method for improving the anti-HIV activity of host defense peptides. Structure- and sequence-based alterations have achieved varying success in improving the potency and specificity of anti-HIV peptides. Overall, peptides have been discovered or engineered to inhibit HIV with therapeutic indices of >1000, encouraging their advancement toward clinical trials. Here we review the naturally occurring anti-HIV host defense peptides, demonstrating their breadth of mechanistic diversity, and exploring approaches to enhance and optimize their activity in order to expedite their development as safe and effective anti-HIV vaginal microbicides.

Here, we review the armamentarium on in vitro/ex vivo models of HIV sexual transmission and discuss how these models can be applied to study candidate microbicides.

There have been encouraging recent successes in the development of safe and effective topical microbicides to prevent vaginal or rectal HIV-1 transmission, based on the use of anti-retroviral drugs. However, much work remains to be accomplished before a microbicide becomes a standard element of prevention science strategies. Animal models should continue to play an important role in pre-clinical testing, with emphasis on safety, pharmacokinetic and efficacy testing.

The development of pre-exposure prophylactics or microbicide products for the reduction or elimination of the sexual transmission of HIV has numerous challenges or barriers to success. Historically traditional dosage forms such as gels have been developed in the field but more recently controlled release dosage forms such as vaginal rings and novel dosage forms such as polymeric thin films have been studied. Studies have begun to incorporate scientific strategies into the formulation design of microbicide products in order to develop safer and more effective products. In addition advanced drug delivery strategies to overcome barriers to delivery and specific drug targeting methods are being employed. In the present review, a comprehensive discussion of formulation efforts and novel delivery strategies in the field of microbicide product development is presented.

Throughout the world, and especially in countries comprising the developing world, women are now bearing the brunt of the HIV pandemic, with over 50% living with HIV infection primarily contracted through sexual transmission in monogamous relationships. Thus, effective chemical or physical means of preventing HIV transmission are urgently needed and in the absence of an approved and effective vaccine, microbicides have become the strategy of choice to provide women with the ability to prevent HIV transmission from their infected partners. Topical microbicides include agents specifically developed and formulated for use in either the vaginal or rectal environment to prevent the sexual transmission of infectious organisms, including pathogenic viruses, bacteria and fungi. Although a microbicide product will have many of the same properties as other anti-infective therapeutic agents and would be similarly developed through a defined preclinical program leading to human clinical trials, microbicide development bears its own challenges related to appropriate and informative preclinical investigation, formulation and delivery, and the complex biological environment in which the product must act, as well as the requirement to develop a product that is acceptable to the user. Following years of microbicide development and a series of unsuccessful human clinical trials, a preclinical microbicide development algorithm has been continuously evolving as greater understanding of the required properties of a successful microbicide are defined through laboratory and clinical experience. Herein, we discuss currently accepted practices required for the development of a successful microbicide product which will prevent cell-free and cell-associated virus transmission in the vaginal and rectal vaults.

Women account for half of new infections with HIV annually. Heterosexual transmission is the most common route of infection in resource limited settings (RLS). An effective microbicide would help decrease transmission of HIV and potentially enable women to have more control in sexual relationships. Research into microbicides is done predominantly in RLS. In addition, there will be different issues and challenges to consider with respect to rectal microbicide use in men. There exist several ethical issues around microbicide development and clinical research which we explore in this review. Respect for persons, including autonomy and protection of vulnerable populations, beneficence, and justice are explored as they relate to microbicide research. Improving standards of care in RLS, trial design, and informed consent are discussed in more detail. Special populations including pregnant women, adolescents, and men who have sex with men are considered in more detail. A multipronged approach to HIV prevention will be necessary to have an impact on HIV prevention. A continued discussion around ethical issues in regard to study design, interpretation of results and implementation of compounds brought to market will remain critically important.

As ‘proof of concept’ has now been well validated for topical microbicides, the progress has, appropriately, refined the questions of who, how, when and at what risk and cost. These are welcome challenges requiring intensified, cross-disciplinary responses. This is especially true in the areas of adherence and pharmacokinetic/pharmacodynamics (PK/PD) sampling and modeling to optimize preventive trials measuring “efficacy”, which is well known to reduce when measured as “effectiveness” in real-world use. Intensified exploratory and Phase 1 safety trials to investigate acceptability, adherence, PK and ex vivo efficacy with drug-exposed tissue biopsies/compartment fluids even though they are complex in design, management, assays and monitoring are moving forward As well, great strides in recent efforts in a variety of delivery formulations are promising. These current and future efforts will provide potential insights earlier that at Phase IIb or III in the development pipeline.