HIV prevention in women: next steps

Nanoformulations of Rilpivirine for Topical Pericoital and Systemic Coitus-Independent Administration Efficiently Prevent HIV Transmission

Vaginal HIV transmission accounts for the majority of new infections worldwide. Currently, multiple efforts to prevent HIV transmission are based on pre-exposure prophylaxis with various antiretroviral drugs. Here, we describe two novel nanoformulations of the reverse transcriptase inhibitor rilpivirine for pericoital and coitus-independent HIV prevention. Topically applied rilpivirine, encapsulated in PLGA nanoparticles, was delivered in a thermosensitive gel, which becomes solid at body temperature. PLGA nanoparticles with encapsulated rilpivirine coated the reproductive tract and offered significant protection to BLT humanized mice from a vaginal high-dose HIV-1 challenge. A different nanosuspension of crystalline rilpivirine (RPV LA), administered intramuscularly, protected BLT mice from a single vaginal high-dose HIV-1 challenge one week after drug administration. Using transmitted/founder viruses, which were previously shown to establish de novo infection in humans, we demonstrated that RPV LA offers significant protection from two consecutive high-dose HIV-1 challenges one and four weeks after drug administration. In this experiment, we also showed that, in certain cases, even in the presence of drug, HIV infection could occur without overt or detectable systemic replication until levels of drug were reduced. We also showed that infection in the presence of drug can result in acquisition of multiple viruses after subsequent exposures. These observations have important implications for the implementation of long-acting antiretroviral formulations for HIV prevention. They provide first evidence that occult infections can occur, despite the presence of sustained levels of antiretroviral drugs. Together, our results demonstrate that topically- or systemically administered rilpivirine offers significant coitus-dependent or coitus-independent protection from HIV infection.

When taken consistently, PrEP has been shown to reduce the risk of HIV infection by up to 92% in people who are at high risk. However, PrEP is much less effective if it is not taken consistently. To improve adherence to the drug regimen, several new drug delivery systems, that include novel gel formulations and long-acting delivery systems, are being evaluated. In this manuscript, we used BLT humanized mice, an in vivo model of vaginal HIV transmission, to evaluate two novel delivery systems for HIV prevention. In the first approach, we combined the highly efficient encapsulation of antiretroviral drugs into nanoparticles with a thermosensitive gel that remains liquid at room temperature and solidifies at body temperature. Our results showed that this delivery system provided significant protection from HIV vaginal infection. In a second approach, we evaluated a long-acting nanoparticle formulation for coitus-independent protection from HIV acquisition. Our results showed that a single injection of the long-acting antiviral drug also resulted in reduced HIV infection. However, protection was not complete and transmission was concealed by a significant delay in the onset of plasma viremia that could result in superinfection by two different viruses administered up to four weeks apart.

Vaginal deployment and tenofovir delivery by microbicide gels

Gels are one of the soft material platforms being evaluated to deliver topically acting anti-HIV drugs (microbicides) to the vaginal environment. For each drug, its loaded concentration, gel properties and applied volume, and frequency of dosing can be designed to optimize PK and, thence, PD. These factors also impact user sensory perceptions and acceptability. Deterministic compartmental modeling of vaginal deployment and drug delivery achieved by test gels can help delineate how multiple parameters characterizing drug, vehicle, vaginal environment, and dosing govern details of PK and PD and also gel leakage from the canal. Such microbicide delivery is a transport process combining convection, e.g., from gel spreading along the vaginal canal, with drug diffusion in multiple compartments, including gel, mucosal epithelium, and stroma. The present work builds upon prior models of gel coating flows and drug diffusion (without convection) in the vaginal environment. It combines and extends these initial approaches in several key ways, including: (1) linking convective drug transport due to gel spreading with drug diffusion and (2) accounting for natural variations in dimensions of the canal and the site of gel placement therein. Results are obtained for a leading microbicide drug, tenofovir, delivered by three prototype microbicide gels, with a range of rheological properties. The model includes phosphorylation of tenofovir to tenofovir diphosphate (which manifests reverse transcriptase activity in host cells), the stromal concentration distributions of which are related to reference prophylactic values against HIV. This yields a computed summary measure related to gel protection ("percent protected"). Analyses illustrate tradeoffs amongst gel properties, drug loading, volume and site of placement, and vaginal dimensions, in the time and space history of gel distribution and tenofovir transport to sites of its anti-HIV action and concentrations and potential prophylactic actions of tenofovir diphosphate therein.

Understanding the Profile of Tuberculosis and Human Immunodeficiency Virus Coinfection: Insights from Expanded HIV Surveillance at a Tuberculosis Facility in Durban, South Africa

Background. Expanded HIV surveillance in TB patients forms part of the World Health Organization framework for strategic collaborative activity. Surveillance helps understand the epidemiology of the local dual epidemic and enables design of a tailored response to these challenges. Methods. We conducted an observational, cross-sectional study of anonymous unlinked HIV testing for 741 consecutive TB suspects attending an urban TB facility during a seven-week period in 2008. Results. A total of 512 patients were found to have TB. The mean age was 35.7 years, and 63% were male. The prevalence of HIV was 72.2% (95% CI: 68.2-75.9) in all TB cases, 69.8% (95% CI: 65.3-74.2) in pulmonary tuberculosis (PTB), 81.6% (95% CI: 72.9-90.3) in extrapulmonary disease, and 66.8% (95% CI: 60.7-72.9) in those without TB disease. HIV prevalence in TB patients was higher in females than males and in younger age groups (18-29 years). The sex ratio of PTB patients correlated with the sex ratio of the prevalence of HIV in the respective age groups (P < 0.05). Conclusion. The use of a rapid HIV test performed on sputum anonymously provides an opportunity for HIV surveillance in this high-burdened setting, which has the potential to lend valuable insight into the coepidemics.

Measuring dilution of microbicide gels with optical imaging

We present a novel approach for measuring topical microbicide gel dilution using optical imaging. The approach compares gel thickness measurements from fluorimetry and multiplexed low coherence interferometry in order to calculate dilution of a gel. As a microbicide gel becomes diluted at fixed thickness, its mLCI thickness measurement remains constant, while the fluorimetry signal decreases in intensity. The difference between the two measurements is related to the extent of gel dilution. These two optical modalities are implemented in a single endoscopic instrument that enables simultaneous data collection. A preliminary validation study was performed with in vitro placebo gel measurements taken in a controlled test socket. It was found that change in slope of the regression line between fluorimetry and mLCI based measurements indicates dilution. A dilution calibration curve was then generated by repeating the test socket measurements with serial dilutions of placebo gel with vaginal fluid simulant. This methodology can provide valuable dilution information on candidate microbicide products, which could substantially enhance our understanding of their in vivo functioning.

Multicompartmental pharmacokinetic model of tenofovir delivery by a vaginal gel

Background

Trials of a vaginal Tenofovir gel for pre-exposure prophylaxis (PrEP) for HIV have given conflicting results. Knowledge of concentrations of Tenofovir and its active form Tenofovir diphosphate, at putative sites of anti-HIV functioning, is central to understanding trial outcomes and design of products and dosage regimens. Topical Tenofovir delivery to the vaginal environment is complex, multivariate and non-linear; determinants relate to drug, vehicle, dosage regimen, and environment. Experimental PK methods cannot yield mechanistic understanding of this process, and have uncontrolled variability in drug sampling. Mechanistic modeling of the process could help delineate its determinants, and be a tool in design and interpretation of products and trials.

Methods and Findings

We created a four-compartment mass transport model for Tenofovir delivery by a gel: gel, epithelium, stroma, blood. Transport was diffusion-driven in vaginal compartments; blood concentration was time-varying but homogeneous. Parameters for the model derived from in vitro and in vivo PK data, to which model predictions gave good agreement. Steep concentration gradients occurred in stroma ≤8 hours after gel release. Increasing epithelial thickness delayed initial TFV delivery to stroma and its decline: t_max increased but AUC at 24 hours was not significantly altered. At 24 and 48 hours, stromal concentrations were 6.3% and 0.2% of C_max. Concentrations in simulated biopsies overestimated stromal concentrations, as much as ∼5X, depending upon time of sampling, biopsy thickness and epithelial thickness.

Conclusions

There was reasonably good agreement of model predictions with clinical PK data. Conversion of TFV to TFV-DP was not included, but PK data suggest a linear relationship between them. Thus contrasts predicted by this model can inform design of gels and dosage regimens in clinical trials, and interpretation of PK data. This mass transport based approach can be extended to TFV conversion to TFV-DP, and to other drugs and dosage forms.

Microbicides: still a long road to success

The development of efficient microbicides, the topically applied compounds that protect uninfected individuals from acquiring HIV-1, is a promising strategy to contain HIV-1 epidemics. Such microbicides should of course possess anti-HIV-1 activity, but they should also act against other genital pathogens, which facilitate HIV-1 transmission. The new trend in microbicide strategy is to use drugs currently used in HIV-1 therapy. The success of this strategy is mixed so far and is impaired by our limited knowledge of the basic mechanisms of HIV-1 transmission as well as by the inadequacy of the systems in which microbicides are tested in preclinical studies.

Factors Important to the Prioritization and Development of Successful Topical Microbicides for HIV-1

Significant advancements in topical microbicide development have occurred since the prevention strategy was first described as a means to inhibit the sexual transmission of HIV-1. The lack of clinical efficacy of the first generation microbicide products has focused development attention on specific antiretroviral agents, and these agents have proven partially successful in human clinical trials. With greater understanding of vaginal and rectal virus infection, replication, and dissemination, better microbicide products and delivery strategies should result in products with enhanced potency. However, a variety of development gaps exist which relate to product dosing, formulation and delivery, and pharmacokinetics and pharmacodynamics which must be better understood in order to prioritize microbicide products for clinical development. In vitro, ex vivo, and in vivo models must be optimized with regard to these development gaps in order to put the right product at the right place, at the right time, and at the right concentration for effective inhibition of virus transmission. As the microbicide field continues to evolve, we must harness the knowledge gained from unsuccessful and successful clinical trials and development programs to continuously enhance our preclinical development algorithms.

A drug evaluation of 1% tenofovir gel and tenofovir disoproxil fumarate tablets for the prevention of HIV infection

INTRODUCTION

More than a million people acquire HIV infection annually. Pre-exposure prophylaxis (PrEP) using antiretrovirals is currently being investigated for HIV prevention. Oral and topical formulations of tenofovir have undergone preclinical and clinical testing to assess acceptability, safety and effectiveness in preventing HIV infection.

AREAS COVERED

The tenofovir drug development pathway from compound discovery, preclinical animal model testing and human testing were reviewed for safety, tolerability and efficacy. Tenofovir is well tolerated and safe when used both systemically or applied topically for HIV prevention. High drug concentrations at the site of HIV transmission and concomitant low systemic drug concentrations are achieved with vaginal application. Coitally applied gel may be the favored prevention option for women compared with the tablets, which may be more suitable for prevention in men and sero-discordant couples. However, recent contradictory effectiveness outcomes in women need to be better understood.

EXPERT OPINION

Emerging evidence has brought new hope that antiretrovirals can potentially change the course of the HIV epidemic when used as early treatment for prevention, as topical or oral PrEP. Although some trial results appear conflicting, behavioral factors, adherence to dosing and pharmacokinetic properties of the different tenofovir formulations and dosing approaches offer plausible explanations for most of the variations in effectiveness observed in different trials.

Unraveling the divergent results of pre-exposure prophylaxis trials for HIV prevention

Although the balance of recent evidence supports the efficacy of antiretroviral (ARV)-based pre-exposure prophylaxis (PrEP) against HIV-1 infection, recent negative trial results are perplexing. Of seven trials with available HIV endpoints, three different products have been tested: tenofovir 1% vaginal gel, oral tenofovir disoproxil fumarate (TDF) tablets, and TDF/emtricitabine tablets. Six of these trials were conducted exclusively in sub-Saharan Africa; all found the products to be well tolerated, and four demonstrated effectiveness. Furthermore, the HIV Prevention Trial Network (HPTN) 052 trial recently confirmed that antiretroviral treatment leads to 96% reduction in transmission to HIV-negative partners in HIV-serodiscordant couples. These results, along with human and animal data, provide substantial evidence for the efficacy of antiretroviral-based HIV prevention. Yet assessment of oral TDF/emtricitabine in the FEM-PrEP study and of oral and vaginal tenofovir in the Microbicide Trial Network (MTN)-003 trial (VOICE) was stopped for futility. How do we make sense of these discrepant results? We believe that adherence is a key factor, although it cannot be the only factor. Expanding upon a recent editorial in the Lancet, we discuss the impact of suboptimal product adherence on PrEP efficacy in the context of variable drug concentration at the exposure site, integrity of the vaginal epithelium, and the role of acute infection.

Implementing microbicides in low-income countries

The magnitude of the global human immunodeficiency virus (HIV) epidemic is determined by women from lower income countries, specifically sub-Saharan Africa. Microbicides offer women who are unable to negotiate safe sex practices a self-initiated HIV prevention method. Of note, is its potential to yield significant public health benefits even with relatively conservative efficacy, coverage and user adherence estimates, making microbicides an effective intervention to invest scarce healthcare resources. Existing healthcare delivery systems provide an excellent opportunity to identify women at highest risk for infection and to also provide an access point to initiate microbicide use. Innovative quality improvement approaches, which strengthen existing sexual reproductive health services and include HIV testing, and linkages to care and treatment services, provide an opportunity to lay the foundations for wide-scale provision of microbicides. The potential to enhance health outcomes in women and infants and potentially affect rates of new HIV infection may soon be realised.

Combinatorial prevention of HIV transmission in women: the case for a vaginal microbicide

Women are now becoming pivotal in the epidemiological spread of HIV infection throughout the world, especially in developing countries, where heterosexual transmission accounts for more than 80% of all new HIV infections. Recently, significant but partial successes have occurred in the field of HIV prevention, including male circumcision, preventive HIV vaccines, vaginal microbicides and oral pre-exposure prophylaxis, and there is increasingly widespread access to antiretroviral treatment. However, none of the currently available tools for HIV intervention are sufficiently effective, particularly for women, and all require further development. Among all biomedical approaches, microbicides could hold the greatest hope of curtailing AIDS worldwide, especially if used by women in Africa. Research for an efficacious microbicide constitutes a priority in the global agenda to prevent HIV infection. Finally, the combination of existing partially effective strategies for HIV prevention should be promoted, scaled-up and evaluated.

Preexposure prophylaxis for HIV prevention

Reducing the incidence of HIV remains one of our greatest public health challenges. However, there is growing optimism that preexposure prophylaxis (PrEP) could have a major impact on preventing incident HIV infection. Recently presented data on the use of oral PrEP in men who have sex with men (MSM) have provided proof-of-principle for this strategy. Additional clinical trials are evaluating whether PrEP provides similar protection to risk groups other than MSM, such as heterosexual persons and injection drug users. Still unanswered questions include optimal dosing strategies, long-term safety, maximizing adherence and minimizing costs, addressing drug resistance in the face of PrEP failure, optimizing access, and assessing effects on risk behavior. Future implementation will be guided by the results of clinical trials in progress. This article provides a review of the data on the potential strengths and limitations of PrEP as an HIV prevention strategy, identifies challenges to implementation of this approach, and outlines knowledge gaps.

One percent tenofovir applied topically to humanized BLT mice and used according to the CAPRISA 004 experimental design demonstrates partial protection from vaginal HIV infection, validating the BLT model for evaluation of new microbicide candidates

Recent iPrEx clinical trial results provided evidence that systemic preexposure prophylaxis (PrEP) with emtricitabine (FTC) and tenofovir disoproxil fumarate (TDF) can partially prevent rectal HIV transmission in humans. Similarly, we have previously demonstrated that systemic administration of the same FTC-TDF combination efficiently prevented rectal transmission in humanized bone marrow/liver/thymus (BLT) mice. The CAPRISA 004 trial recently demonstrated that topical application of the tenofovir could partially prevent vaginal HIV-1 transmission in humans. To further validate the usefulness of the BLT mouse model for testing HIV prevention strategies, we evaluated the topical administration of tenofovir as used in CAPRISA 004 to prevent vaginal HIV transmission in BLT mice. Our results demonstrate that vaginally administered 1% tenofovir significantly reduced HIV transmission in BLT mice (P = 0.002). Together with the results obtained after systemic antiretroviral PrEP, these topical inhibitor data serve to validate the use of humanized BLT mice to evaluate both systemic and topical inhibitors of HIV transmission. Based on these observations, we tested six additional microbicide candidates for their ability to prevent vaginal HIV transmission: a C-peptide fusion inhibitor (C52L), a membrane-disrupting amphipathic peptide inhibitor (C5A), a trimeric d-peptide fusion inhibitor (PIE12-Trimer), a combination of reverse transcriptase inhibitors (FTC-TDF), a thioester zinc finger inhibitor (TC247), and a small-molecule Rac inhibitor (NSC23766). No protection was seen with the Rac inhibitor NSC23766. The thioester compound TC247 offered partial protection. Significant protection was afforded by FTC-TDF, and complete protection was offered by three different peptide inhibitors tested. Our results demonstrate that these effective topical inhibitors have excellent potential to prevent vaginal HIV transmission in humans.