- Email: [email protected]
Oral pre-exposure prophylaxis for HIV prevention
Review
Oral pre-exposure prophylaxis for HIV prevention J. Gerardo Garcı´a-Lerma, Lynn Paxton, Peter H. Kilmarx and Walid Heneine Division of HIV/AIDS Prevention, National Center for HIV, Hepatitis, STD, and Prevention, Centers for Disease Control and Prevention, 1600 Clifton Road, Atlanta, GA 30329, USA
In the absence of an effective vaccine, HIV continues to spread worldwide, emphasizing the need for new biomedical interventions to limit its transmission. Appreciation of the challenges that HIV has to face to initiate an infection mucosally has spurred interest in evaluating the use of antiretroviral drugs to prevent infection. Recent animal studies using macaques or humanized mice models of mucosal transmission of SIV or HIV have shown that daily or intermittent pre-exposure prophylaxis (PrEP) with tenofovir disoproxil fumarate (TDF) and emtricitabine (FTC) can exploit early virus vulnerabilities and effectively prevent establishment of infection. These preclinical findings have fueled interest in evaluating the safety and efficacy of PrEP in humans. We provide an overview of the rationale behind PrEP and discuss the next steps in PrEP research, including the need to better define the ability of current drugs to reach and accumulate in mucosal tissues and protect cells that are primary targets during early HIV infection. Introduction The HIV/AIDS pandemic remains among our greatest public health challenges. In the current decade, the prevalence of HIV-1 infection has stabilized at 0.8% [1] worldwide. The overall number of people living with HIV is increasing as new infections continue to occur and AIDS deaths are prevented by increasingly available highly effective antiretroviral treatment (ART). Globally, there were an estimated 33.2 million people living with HIV infection or AIDS in 2007 [1]. In that year, the annual incidence of new HIV infections was an estimated 2.7 million, and there were an estimated 2.0 million HIVrelated deaths [1]. More than 96% of new infections were in low- and middle-income countries [2]. In the USA, the incidence has been relatively stable for the past decade [1], with an estimated 56,300 new cases in 2006. The ongoing high incidence of HIV infection and incomplete coverage with basic HIV prevention tools underscore the need for new, highly effective biomedical HIV interventions to complement existing prevention strategies (Box 1). In the absence of an effective vaccine, oral administration of antiretroviral drugs before HIV exposure (preexposure prophylaxis or PrEP) might be a reliable intervention to protect high-risk HIV-negative people from infection [3–6]. Mathematical models estimate that over the next 10 years an effective PrEP program could prevent Corresponding author: Garcı´a-Lerma, J.G. ([email protected])
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2.7–3.2 million of the 11 million new HIV-1 infections projected to occur in sub-Saharan Africa [7]. This potentially significant public health benefit requires a very high PrEP efficacy and can be lost or substantially reduced with a PrEP efficacy of <50%. Model simulations have also shown that an effective PrEP program could substantially reduce the incidence of HIV transmission in populations at high risk of infection in the USA [8]. PrEP is a proven concept for other infectious diseases such as malaria. Multiple lines of evidence suggest that it might also be a feasible chemoprevention strategy for preventing HIV among high-risk populations. Antiretroviral drugs effectively prevent HIV transmission at birth, during breastfeeding and after occupational exposure [9–11]. Studies in non-human primates have shown that daily oral administration of the reverse transcriptase (RT) inhibitors TDF and FTC, or a combination of FTC and TDF (Truvada), can prevent or delay transmission of simian or simian/human immunodeficiency virus (SIV/SHIV) in macaques in a dose-dependent manner [12–14]. These observations have fueled interest in evaluating the efficacy of daily PrEP in preventing HIV transmission in humans. Several clinical trials with TDF or Truvada are ongoing in high-risk populations. In this review, we discuss the rationale behind PrEP. We focus on the delicate balance of vulnerabilities of the virus and host during early infection and the existence of a narrow window of opportunity for interventions with antiretroviral drugs. We then provide an overview of drugs that are currently approved for treatment and are under consideration for PrEP in humans, in addition to existing preclinical animal model research with the most attractive drug candidates. We focus on oral PrEP (although antiretrovirals can also be provided topically by gels or other delivery mechanisms). This review highlights the promise of oral PrEP as a new biomedical strategy to prevent HIV transmission in humans. HIV transmission as a delicate balance of vulnerabilities of the virus and host The success of sexual transmission of HIV is dictated by the dynamic balance between virus and host vulnerabilities. An appreciation of the vulnerabilities and challenges that HIV must face to initiate an infection in the mucosa came from animal model studies of vaginal SIV infection (Figure 1; reviewed in [15]). These studies have consistently shown that, after penetration of SIV into the cervicovaginal epithelium, infection in cervicovaginal tissues
0165-6147/$ – see front matter . Published by Elsevier Ltd. doi:10.1016/j.tips.2009.10.009 Available online 4 December 2009
Review Box 1. Biomedical interventions to prevent HIV transmission Emerging biomedical interventions include male circumcision, HIV vaccines, microbicides, and antiretroviral drugs used either as HIV treatment to reduce infectiousness or as pre-exposure prophylaxis. Male circumcision. Shown to be efficacious in preventing femaleto-male transmission. Being implemented in countries where the prevalence of HIV is high and the prevalence of male circumcision is low. Vaccines: After the failure of a lead vaccine product in 2007, vaccine research is increasingly focused on basic science and discovery rather than on human clinical trials. The recent report of a modestly effective two-vaccine prime-boost vaccination strategy gives new hope to vaccine development, but the availability of a licensed, effective vaccine is still years away. Microbicides. Several early-generation microbicide products not found to be effective. A non-statistically significant trend toward protection was shown in one trial with the polyanionic binding inhibitor PRO 2000. Results of another, larger trial of PRO 2000 are anticipated in late 2009. Clinical trials of a promising TFV-based microbicide are also ongoing. Antiretroviral treatment of established HIV infection. Considered as a prevention intervention as it appears to decrease the likelihood of HIV transmission from the treated, HIV-infected individual to sex or needle-sharing partner(s). Oral pre-exposure prophylaxis. Nine PrEP trials using TDF or Truvada in different at-risk populations. Trials expected to reach completion from 2010 onwards.
during the first 1–3 days is limited to extremely small numbers of productively infected cells in rare foci [16–18]. This small local founder population of infected cells expands in the following days, possibly by accretion of new infections around the initial clusters [18]. Continuous expansion at the point of entry and dissemination of virus and infected cells through lymphatic drainage and the
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bloodstream establishes a sustainable infection in secondary lymphoid organs [16]. In fewer than two weeks, a very small founder population of productively infected cells at the portal of entry progresses to systemic infection with a burst of virus production and depletion of gut CD4+ T cells [15] (Figure 1). At this point, a robust virus-specific immune response can contain virus replication only to a certain degree [19]. Thus, the first days of infection at the mucosa when replication is limited to small clusters of infected cells are the periods of maximum virus vulnerability and, therefore, represent a ‘window of opportunity’ for HIV prevention interventions with vaccines, microbicides or antiretroviral drugs. The small founder population of infected cells seen during early SIV infection of macaques is a consequence of the effectiveness of the host in limiting virus access to susceptible target cells in the mucosa (Figure 1). This ‘barrier effect’ is best illustrated experimentally by the large virus dose (106–109 particles) that is generally needed to effectively infect macaques intravaginally after a single exposure [16]. On contact with the host, most of the virus inoculum is diluted and cleared in the cervicovaginal fluids or entrapped in cervical mucus. Inactivation by low pH and innate immunity further reduces the effective virus inoculum (reviewed in [20]). The multi-layered squamous or single-layered columnar epithelium from the cervicovaginal or rectal mucosa provides a barrier that greatly reduces exposure to susceptible target cells in the underlying stroma. A largely intact mucosa and a large proportion of defective particles incapable of establishing an infection have often been associated with restricted transmission of a homogenous virus population in macaques and humans [21,22]. However, host vulnerability to infection is
Figure 1. Model of mucosal HIV infection and virus vulnerabilities. SIV infection of the cervicovaginal epithelium of macaques is rapid: it can occur within 30–60 min of intravaginal exposure. Host restriction can efficiently limit the size of founder virus populations of infected cells. Early infection coincides with the period of maximum vulnerability of the virus and represents a major opportunity to prevent infection. If uncontrolled, infection expands by accretion and continuous seeding of viruses/infected cells to distal lymphoid tissue, resulting in a burst of virus production and depletion of gut CD4+ T cells. At this time, the balance of virus and host vulnerabilities is reversed; HIV is only partially vulnerable and can efficiently escape host immune responses; the host can control virus replication only to a certain degree.
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Review dynamic and can increase under certain conditions. For example, variations in hormone levels and thinning of the vaginal epithelium during the menstrual cycle usually coincide with periods of maximum susceptibility to infection [23]. Mechanical micro-abrasions of mucosal surfaces during intercourse or lesions due to concomitant genital ulcers can allow virus access to T cells, macrophages and dendritic cells in the lamina propria, can facilitate rapid virus dissemination to lymphatic tissues, and might result in transmission of multiple genetic variants [24,25]. These changes in vulnerability to infection are evident when examining the risks of heterosexual HIV transmission in humans. Whereas heterosexual transmissibility in stable couples without high-risk transmission co-factors can range from 1 to 10 transmissions per 1000 contacts, the presence of sexually transmitted infections or penile–anal contact can increase heterosexual infectivity to 1 to 3 transmissions per 10 contacts [26]. Mucosal HIV infection can be conceivably prevented by a rapid and efficient host immune response or by limiting the size of founder populations of infected cells to a theoretical threshold under which infection cannot be established. HIV vaccines have so far been unable to elicit protective immune responses, primarily because of the extreme genetic variability in circulating viral isolates and the high virus mutation rate that enables rapid escape from adaptive immune responses [27]. Anti-inflammatory agents that interfere with innate host responses and limit expansion of founder populations have shown promising results in macaques, although the existence of occult infections remains a possibility [18]. Antiretroviral drugs may conceivably block the establishment of founder populations of infected cells by specifically targeting the virus replicative cycle. Effective delivery of drugs at the portal of entry might therefore exploit the brief period of virus vulnerability and block HIV from establishing a persistent infection (Figure 2). As discussed below, the highly relevant macaque and humanized mice models of infection by SHIV and HIV have shown that daily and intermittent PrEP
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with antiretroviral drugs can take advantage of such vulnerabilities and effectively prevent infection. Antiretroviral drugs for oral PrEP Drug candidates for oral PrEP have largely been selected from currently approved drugs for treatment of individuals infected with HIV-1 because development of drugs exclusively for HIV prevention has not been pursued. Nevertheless, there are currently >30 drugs or drug combinations that are approved for treatment that target key steps in the HIV replication cycle, including inhibitors of virus entry, reverse transcription, integration and maturation [28]. Many desirable drug characteristics for PrEP overlap with those for treatment. These properties include good tolerability and safety, low pill burden, once-daily dosing, long half-life, high potency, and good resistance profiles that do not allow rapid emergence of resistance or broad cross-resistance with other drugs. Pre-integration drugs that prevent the establishment of infected cells are thought to be more suitable than post-integration drugs like protease inhibitors (although evidence to support this assumption is lacking). Additional pharmacokinetic properties that might be important for PrEP drugs targeting sexual transmission include the ability to rapidly reach and accumulate in genital and rectal tissues. In general, non-nucleoside RT inhibitors (e.g., nevirapine and efavirenz) or protease inhibitors (e.g., amprenavir, saquinavir and ritonavir) consistently achieve lower drug concentrations in the genital tract of males and females than in blood plasma [29–31]. High drug exposure in the female genital tract makes the entry inhibitor maraviroc a potentially attractive drug for PrEP [32]. Nucleoside and nucleotide RT inhibitors, including FTC, TDF, zidovudine (AZT) and lamivudine (3TC), also achieve concentrations in the genital tract that are 2–6-fold higher than in blood plasma [29–31,33–35]. However, the active drug of nucleoside and nucleotide analogs is the phosphorylated intracellular form and not the extracellular
Figure 2. Dynamic changes in the balance of vulnerabilities between virus and host. The success of sexual HIV transmission is dictated by the balance between vulnerabilities of the virus and host. High virus vulnerability during early (hours to 1–3 days) infection due to host restrictions (red line) can be lessened by altered host barriers (i.e. genital ulcer disease or mechanical micro-abrasions, blue line). Antiretroviral drugs may conceivably maintain virus vulnerability (green line) above a host vulnerability threshold and prevent establishment of an infection. Effective PrEP regimens should adequately penetrate mucosal tissues and protect all cell populations/subpopulations that are primary targets for early infection.
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drug measured in plasma or genital secretions. Limited data with TDF have shown that the high extracellular tenofovir (TFV) concentration seen in seminal plasma is also associated with high intracellular levels of TFV-diphosphate (TFV-DP) in seminal mononuclear cells. Intracellular FTC-triphosphate (FTC-TP) concentrations in seminal mononuclear cells were comparable with those seen in peripheral blood mononuclear cells [35,36]. Among the available candidate drugs for PrEP, only TDF and FTC are currently being evaluated in humans. They are administered as TDF alone or in combination with FTC (Truvada). There are many arguments for selecting FTC and TDF for human clinical trials. In contrast to other nucleoside RT inhibitors such as AZT and stavudine (d4T), both drugs are active in resting and activated T cells. The intracellular half-life of FTC-TP and TFV-DP is also very long (40 h to >100 h) thereby potentially extending their prophylactic activity if administered intermittently [37–39]. FTC and TDF are very potent, have a synergistic to additive effect in vitro, and a favorable safety and drug resistance profile [4,40]. FTC-resistant viruses containing
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the M184V mutation have a unique resistance pattern not shared by most RT inhibitors; if PrEP fails the antiviral activity of other nucleoside RT inhibitors may be maintained even in the presence of M184V. When combined with TDF, M184V and not the TDF-associated K65R mutation is the most frequent pathway of resistance to Truvada [41]. Both drugs are conveniently co-formulated in a once-daily pill. Preclinical research in animal models The potential use of antiretroviral drugs for HIV prophylaxis has been studied extensively in non-human primate models of mucosal and parenteral transmission of SIV or SHIV and, more recently, in a humanized mice model of vaginal HIV transmission (Table 1). Early work in macaques showing that high (20 mg/kg) doses of TFV administered subcutaneously could prevent parenteral SIV transmission was instrumental in establishing guidelines for post-exposure prophylaxis in humans by defining the timing of initiation and length of treatment that can provide protection from infection [42–44]. Indications that
Table 1. Available preclinical animal data with daily and intermittent PrEP Reference
Animal
Drugs and dose
Tsai et al. [44]
Long-tailed macaques
TFV, 20 mg/kg
Route of drug administration Subcutaneous
Van Rompay et al. [51]
Rhesus macaques
TFV, 30 mg/kg
Subcutaneous
Van Rompay et al. [46]
Rhesus macaques
TDF, 0.01 to 0.02 mg/kg
Oral
Multiple oral exposures to SIVmac251 (104 TCID50)
Van Rompay et al. [14]
Rhesus macaques
TDF, 10 mg/kg
Oral
Subbarao et al. [13]
Rhesus macaques
TDF, 22 mg/kg
Oral
Garcı´a-Lerma et al. [12]
Rhesus macaques
FTC, 20 mg/kg; TDF, 22 mg/kg; TFV, 22 mg/kg
Oral (Truvada) or subcutaneous (FTC, TFV)
Multiple oral exposures to SIVmac251 (105 TCID50) Repeat low-dose atraumatic rectal exposures to SHIV162p3 (10 TCID50) Repeat low-dose atraumatic rectal exposures to SHIV162p3 (10 TCID50)
Denton et al. [49]
Humanized mice
FTC, 3.5 mg TDF, 5.2 mg
Intraperitoneal
Garcı´a-Lerma et al. [52]
Rhesus macaques
FTC, 20 mg/kg TDF, 22 mg/kg
Oral
Virus exposure and dose Single intravenous exposure to SIVmne (103 TCID50) Oral, SIVmac251 (105 TCID50)
Single atraumatic intravaginal exposure to HIV-1JR-CSF (105 TCID50) Repeat low-dose atraumatic rectal exposures to SHIV162p3 (10 TCID50)
Interventions
Main findings
TFV initiated 48h prior to exposure and continued during 4 wks
Full protection
Two doses given 4h before and 24h after exposure TDF initiated 1 d before exposure and maintained during continuous virus inoculations; one additional dose after the last virus exposure Repeated cycles of daily TDF initiated 1–2 days before exposure Daily or weekly TDF
Full protection
Daily FTC (subcutaneous) Daily oral Truvada Daily or intermittent FTC/TFV (subcutaneous) Daily FTC/TDF initiated 48h prior to exposure and continued for 7 days
Several event-driven and exposure-independent intermittent modalities with Truvada (times relative to exposure): -22h/+2h, -3d/+2h, -7d/+2h, +2h/+26h, -2h/+24h,
No protection; low doses of TDF
Partial prophylactic efficacy; infection associated with low systemic drug exposures All controls animals infected after 1.5 exposures; 3 of 4 TDF-treated animals infected after 6 to 7 exposures Risk of infection reduced with human equivalent doses of FTC (4-fold) and Truvada (8-fold) No infection with FTC and a high dose of TFV (daily or intermittent) Full protection
High to moderate efficacy with 2 weekly humanequivalent doses of Truvada
TCID: tissue culture infectious dose.
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Review antiretrovirals administered before exposure could also prevent infection in a dose-dependent manner came from these and subsequent studies using different doses of TFV administered orally or subcutaneously, and from oral, rectal, or parenteral virus inoculations (Table 1) [14,44– 46]. We recently developed a repeat low-dose rhesus and pigtail macaque model of rectal and vaginal SHIV transmission that recapitulates several aspects of human transmission of HIV, and is well-suited for evaluating various PrEP strategies [13,47]. The virus dose is lower and more physiological than that used in conventional single highdose models [47,48]. The SHIV challenge dose contains an R5-tropic HIV-1SF162 envelope similar to naturally transmitted viruses. Virus exposures are repeated to mimic high-risk human exposures, thereby providing the opportunity to measure protection against multiple transmission events in each animal [12,13,22]. Using a rectal transmission model, we showed that higher drug doses and combination treatments might be more effective in preventing SHIV transmission than single drugs or lower doses [12,13]. At human-equivalent doses, daily FTC reduced the risk of infection by 3.8-fold whereas a nonstatistically significant trend towards delayed infection was seen in animals receiving daily TDF [12,13]. However, the combination of both drugs provided a nearly eightfold lower risk of infection with four of the six animals protected after 14 weekly rectal challenges [12]. These studies used a rectal transmission model. Although many biological similarities exist between rectal and vaginal HIV transmission, differences in the early events of mucosal infection or changes in drug exposure or susceptibility associated with the menstrual cycle and thinning of the epithelium are possible [23]. Evidence that Truvada might also protect against vaginal transmission has come from studies using humanized mice exposed vaginally to HIV. Even though 7/8 control mice became infected, none of the five animals receiving a high dose of Truvada were infected [49]. Some important observations of potential relevance to humans have been made from the analysis of PrEP breakthrough infections in macaques. First, two of the macaques infected in the studies mentioned above during daily PrEP with FTC or Truvada (one animal in each study) showed selection for drug-resistant viruses. In both macaques, the M184V mutation associated with FTC resistance was selected, thus reiterating the need to closely monitor for drug resistance if PrEP is implemented [12]. Second, PrEP breakthroughs during treatment with FTC and Truvada had reduced levels of virus replication during early infection compared with untreated control animals [12]. A reduction in early viremia in PrEP failures might contribute to a decrease in HIV-1 transmissibility at the population level and could add to the overall effectiveness of PrEP. Attenuated acute viremia might also reduce early CD4+ T cell depletion, help to preserve immune function, and attenuate the course of HIV infection [50]. Animal models have also been used to explore the efficacy of intermittent drug dosing with TDF or Truvada. Intermittent PrEP may be a more convenient strategy that can potentially be more cost-effective, reduce the risks of drug toxicities, increase adherence and minimize drug resistance emergence. In intermittent PrEP interventions, 78
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TDF or Truvada might be taken on the basis of exposure events or by following a weekly schedule independent of the time of exposure. FTC-TP and TFV-DP have long intracellular half-lives in humans [37–39] and can potentially achieve extended prophylactic activity when administered intermittently. Short-course prophylaxis with the long-acting drug nevirapine has successfully prevented HIV transmission at birth [11]. Studies in macaques showing protection from oral or rectal SIV/SHIV exposures by a two-dose subcutaneous regimen containing TFV or TFV/ FTC have provided the first proof-of-concept for intermittent PrEP [12,51]. However, the high drug doses and subcutaneous drug delivery might have overestimated efficacy in these studies. We recently showed, using humanequivalent doses of Truvada, that significant protection of macaques can be achieved with a single oral dose given up to 7 days before exposure, followed by a second dose 2 h after exposure [52]. We also found that exposure-driven prophylactic modalities initiated around the time of exposure maintained moderate protection. These studies are promising and help to identify potentially efficacious intermittent PrEP modalities with a wide window of protection. This broadens the possibilities for the development of more feasible and cost-effective antiretroviral prophylaxis strategies to prevent HIV transmission in humans. The animal studies mentioned above provided preclinical evidence that systemic daily or intermittent PrEP might potentially prevent HIV infection. The finding that daily PrEP with a combination of FTC and TDF was more efficient in macaques than single-drug PrEP prompted some of the clinical trials to be modified to evaluate Truvada instead of TDF. A combination of both drugs provides higher antiviral activity, increases the genetic barriers for resistance, and has a comparable convenience and safety profile. Nevertheless, only human clinical trials will show which strategy is more safe, efficacious, and cost-effective. Human clinical trials As of November 2009, nine PrEP trials enrolling >22,000 participants are at varying stages of completion (Table 2). The at-risk populations in these studies are men who have sex with men (MSM), heterosexual men and women, and intravenous drug users (IDUs). All trials are using TDFcontaining products, comprising TDF alone or in combination with FTC, and all are evaluating daily use with the exception of one pilot study of intermittent dosing. One trial (VOICE) is comparing oral TDF or TDF/FTC with topical PrEP with a TFV vaginal gel. Only one Phase-II study of TDF among 936 high-risk women in Ghana, Nigeria and Cameroon has been completed, and no differences in adverse events or grade-3 or -4 laboratory abnormalities were observed between placebo and TDF users. There were fewer infections in the TDF arm (two events versus six events in the placebo arm) even though the study was not of sufficient size or duration to examine the efficacy of tenofovir [53]. Current trials are projected to reach completion from 2010 onwards (Table 2). These trials will provide valuable information, including efficacy for the major routes of HIV transmission, the acceptability of daily dosing, the required frequency of safety monitoring, and an indication
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Table 2. Completed, ongoing, and planned clinical trials of oral HIV pre-exposure prophylaxis Study
Sponsor Product
Population
Countries
West Africa TDF Trial
FHI
Daily oral TDF
936 women
Daily oral TDF Daily oral TDF
400 MSM 2400 male and female IDU 1200 heterosexual men and women
Ghana, Cameroon, Nigeria US Thailand
Enrollment Status/projected started completion 2004 Completed in 2007 [54] 2005 Fully enrolled/2010 2005 98% enrolled/2010
Botswana
2007
Fully enrolled/2011
Peru, Ecuador, Brazil, US, Thailand, South Africa Kenya, Uganda
2007
Fully enrolled/2011
2008
Enrolling/2012
Kenya, Malawi, 2009 South Africa, Tanzania, Zambia Daily oral TDF 4,200 women Malawi, Uganda, 2009 Daily oral FTC/TDF South Africa, Zambia, Daily topical TFV gel Zimbabwe Daily oral FTC/TDF 150 at-risk discordant Kenya, Uganda 2009 Intermittent oral couples FTC/TDF
Enrolling/2012
US Extended TDF safety trial CDC CDC Bangkok TDF study TDF-2
CDC
iPrEX, UCSF
NIH, BMGF
Partners PrEP
BMGF
FEM-PrEP
FHI,
VOICE, MTN003
MTN, NIH
IAVI E001, E002 Phase I/II
IAVI
Daily oral FTC/TDF (switched from TDF in 2007) Daily oral FTC/TDF
Daily oral TDF Daily oral FTC/TDF Daily oral FTC/TDF
3000 MSM
3,900 discordant heterosexual couples 3,900 high-risk women
Enrolling/2013
Enrolling/2010
FHI, family health international; CDC, US Centers for Disease Control and Prevention; NIH, US National Institutes of Health; BMFG, Bill & Melissa Gates Foundation; MTN, Microbicide Trail Network; IAVI, International AIDS Vaccine Initiative; MSM, men who have sex with men; FTC, emtricitabine; TDF, tenofovir disoproxil fumarate; TFV, tenofovir
of the occurrence of HIV drug resistance among those who become infected while taking PrEP. It is also hoped that these trials will shed light on the utility of measuring drug levels in blood (plasma or mononuclear cells) or hair as indicators of adherence and drug exposure. However, given the long half-life of FTC-TP and TFV-DP, adherence rates may be difficult to assess. There are several areas that will require further research and ongoing surveillance if PrEP is to become part of an HIV prevention program. These include efficacy, drug adherence and drug resistance outside the rigorously controlled conditions of a clinical trial, the extent of any behavioral disinhibition or risk behavior change associated with PrEP, and the occurrence and extent of medication-sharing and black-market use. Next steps in PrEP research Although human trials will ultimately determine if oral PrEP can prevent HIV transmission, animal research has provided valuable information and can guide the next steps in the research and practice of PrEP. There are many unresolved issues that deserve further investigation. Additional research should explore if drug-resistant viruses have the potential to diminish PrEP efficacy. This is particularly relevant in areas with widespread access to antiretroviral drugs where transmission of drug-resistant viruses occurs at a relatively high (10–20%) frequency [54]. A comprehensive analysis of the pharmacology of antiretroviral drugs in genital secretions and tissues is also essential to define the best drug candidates and the most appropriate dosing strategies for PrEP. These studies should not only evaluate drug penetration in rectal and vaginal tissues, but also the degree of drug exposure in cells that are primary targets during early mucosal infection such as Langerhans cells, dendritic cells or activated/resting T cells [55]. A combination of in-vivo tissue pharmacokinetics and ex-vivo studies using explants
models of cervicovaginal and rectal tissues will shed light on potential cell populations/subpopulations that are not adequately protected by current PrEP regimens, and will help to define better drug regimens that may include, for instance, other drug classes such as entry inhibitors [56]. Defining the relationship between drug levels, drug dosing, and protection is also important. For instance, the threshold for levels of TFV-DP and FTC-TP that results in protection has not been defined in macaques or tissue explants. These studies may help to define good surrogate markers for adherence and efficacy of PrEP in humans. Special emphasis should be made in evaluating nextgeneration PrEP agents and long-acting drug formulations that may require less frequent dosing and could be taken independently of virus exposure. Concluding remarks Antiretroviral prophylaxis has been very effective in preventing mother-to-child transmission of HIV, and there is great hope that this strategy will prove effective against other routes of HIV transmission. Our understanding of the early events of mucosal infection points to rapid establishment of a small population of infected cells. This represents a window of high virus vulnerability, enabling antiretroviral drugs to block the establishment of a persistent infection. Many currently available drugs for treatment also have desirable characteristics for PrEP, but can differ in their ability to reach and accumulate in genital and rectal tissues, which may be critical to prevent sexual HIV transmission. Preclinical research in animal models of mucosal and parenteral transmission has provided proofof-principle of the efficacy of PrEP, and has informed the design of human clinical trials. These models provide tools to evaluate different PrEP regimens, including drugs from different classes and intermittent dosing, and may help to identify correlates of protection. Data from PrEP trials will 79
Review become available in the coming two years, and will help define the role of PrEP as an strategy to prevent HIV transmission. Disclaimer The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the Centers for Disease Control and Prevention. The use of trade names is for identification only and does not constitute endorsement by the US Department of Health and Human Services, the Public Health Service, or the Centers for Disease Control and Prevention. References 1 (2007) UNAIDS/WHO. AIDS epidemic update: December 2007. Geneva: UNAIDS (last accessed on Nov 2009) 2
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Trends in Pharmacological Sciences Vol.31 No.2 21 Keele, B.F. et al. (2008) Identification and characterization of transmitted and early founder virus envelopes in primary HIV-1 infection. Proc. Natl. Acad. Sci. U S A 105, 7552–7557 22 Keele, B.F. et al. (2009) Low-dose rectal inoculation of rhesus macaques by SIVsmE660 or SIVmac251 recapitulates human mucosal infection by HIV-1. J. Exp. Med. 206, 1117–1134 23 Wira, C.R. and Fahey, J.V. (2008) A new strategy to understand how HIV infects women: identification of a window of vulnerability during the menstrual cycle. AIDS 22, 1909–1917 24 Weiler, A.M. et al. (2008) Genital ulcers facilitate rapid viral entry and dissemination following intravaginal inoculation with cell-associated simian immunodeficiency virus SIVmac239. J. Virol. 82, 4154–4158 25 Haaland, R.E. et al. (2009) Inflammatory genital infections mitigate a severe genetic bottleneck in heterosexual transmission of subtype A and C HIV-1. PLoS Pathog. 5, e1000274 26 Powers, K.A. et al. (2008) Rethinking the heterosexual infectivity of HIV-1: a systematic review and meta-analysis. Lancet Infect. Dis. 8, 553–563 27 Walker, B.D. and Burton, D.R. (2008) Towards an HIV vaccine. Science 320, 760–764 28 US Food and Drug Administration (2009) Antiretroviral drugs used in the treatment of HIV infection. http://www.fda.gov/ForConsumers/ byAudience/ForPatientAdvocates/HIVandAIDSActivities/ ucm118915.htm (accessed 24 July 2009) 29 Dumond, J.B. et al. (2007) Antiretroviral drug exposure in the female genital tract: implications for oral pre- and post-exposure prophylaxis. AIDS 21, 1899–1907 30 Ghosn, J. et al. (2004) Penetration of enfuvirtide, tenofovir, efavirenz, and protease inhibitors in the genital tract of HIV-1-infected men. AIDS 18, 1958–1961 31 Min, S.S. et al. (2004) Protease inhibitor and nonnucleoside reverse transcriptase inhibitor concentrations in the genital tract of HIV-1infected women. J. Acquir. Immune Defic. Syndr. 37, 1577–1580 32 Dumond, J.B. et al. (2009) Maraviroc concentrates in the cervicovaginal fluid and vaginal tissue of HIV-negative women. J. Acquir. Immune Defic. Syndr. 51, 546–553 33 Pereira, A.S. et al. (1999) Nucleoside analogues achieve high concentrations in seminal plasma: relationship between drug concentration and virus burden. J. Infect. Dis. 180, 2039–2043 34 Taylor, S. et al. (2000) Concentrations of nevirapine, lamivudine and stavudine in semen of HIV-1-infected men. AIDS 14, 1979–1984 35 Vourvahis, M. et al. (2008) The pharmacokinetics and viral activity of tenofovir in the male genital tract. J. Acquir. Immune Defic. Syndr. 47, 329–333 36 Dumond, J.B. et al. (2008) Differential extracellular and intracellular concentrations of zidovudine and lamivudine in semen and plasma of HIV-1-infected men. J. Acquir. Immune Defic. Syndr. 48, 156–162 37 Hawkins, T. et al. (2005) Intracellular pharmacokinetics of tenofovir diphosphate, carbovir triphosphate, and lamivudine triphosphate in patients receiving triple-nucleoside regimens. J. Acquir. Immune Defic. Syndr. 39, 406–411 38 Pruvost, A. et al. (2005) Measurement of intracellular didanosine and tenofovir phosphorylated metabolites and possible interaction of the two drugs in human immunodeficiency virus-infected patients. Antimicrob. Agents Chemother. 49, 1907–1914 39 Wang, L.H. et al. (2004) Pharmacokinetic and pharmacodynamic characteristics of emtricitabine support its once daily dosing for the treatment of HIV infection. AIDS Res. Hum. Retroviruses 20, 1173– 1182 40 Borroto-Esoda, K. et al. (2006) In vitro evaluation of the anti-HIV activity and metabolic interactions of tenofovir and emtricitabine. Antivir. Ther. 11, 377–384 41 Johnson, M.A. et al. (2006) A once-daily lopinavir/ritonavir-based regimen provides noninferior antiviral activity compared with a twice-daily regimen. J. Acquir. Immune Defic. Syndr. 43, 153–160 42 Otten, R.A. et al. (2000) Efficacy of postexposure prophylaxis after intravaginal exposure of pig-tailed macaques to a human-derived retrovirus (human immunodeficiency virus type 2). J. Virol. 74, 9771–9775 43 Tsai, C.C. et al. (1998) Effectiveness of postinoculation (R)-9-(2phosphonylmethoxypropyl) adenine treatment for prevention of persistent simian immunodeficiency virus SIVmne infection depends
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45
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critically on timing of initiation and duration of treatment. J. Virol. 72, 4265–4273 Tsai, C.C. et al. (1995) Prevention of SIV infection in macaques by (R)-9-(2-phosphonylmethoxypropyl)adenine. Science 270, 1197– 1199 van Rompay, K.K. et al. (2001) Two low doses of tenofovir protect newborn macaques against oral simian immunodeficiency virus infection. J. Infect. Dis. 184, 429–438 van Rompay, K.K. et al. (2002) Topical adminsitration of low-dose tenofovir disoproxil fumarate to protect infant macaques against multiple oral exposures of low doses of simian immunodeficiency virus. J. Infect. Dis. 186, 1508–1513 Otten, R.A. et al. (2005) Multiple vaginal exposures to low doses of R5 simian-human immunodeficiency virus: strategy to study HIV preclinical interventions in nonhuman primates. J. Infect. Dis. 19, 164–173 Pilcher, C.D. et al. (2004) Brief but efficient: acute HIV infection and the sexual transmission of HIV. J. Infect. Dis. 189, 1785–1792 Denton, P.W. et al. (2008) Antiretroviral pre-exposure prophylaxis prevents vaginal transmission of HIV-1 in humanized BLT mice. PLoS Med 5, e16
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50 Gupta, S.B. et al. (2007) Estimating the benefit of an HIV-1 vaccine that reduces viral load set point. J. Infect. Dis. 195, 546–550 51 van Rompay, K.K. et al. (1998) Two doses of PMPA protect newborn macaques against oral simian immunodeficiency virus infection. AIDS 12, F79–83 52 Garcia-Lerma, J.G. et al. (2009) Prevention of rectal SHIV transmission in macaques by intermittent pre-exposure prophylaxis (iPrEP) with oral truvada, In 16th Conference on Retroviruses and Opportunistic Infections Montreal, Canada (February 8–11) (Abstract 47), 53 Peterson, L. et al. (2007) Tenofovir disoproxil fumarate for prevention of HIV infection in women: a phase 2, double-blind, randomized, placebo-controlled trial. PLoS Clin. Trials 2, e27 54 Booth, C.L. and Geretti, A.M. (2007) Prevalence and determinants of transmitted antiretroviral drug resistance in HIV-1 infection. J. Antimicrob. Chemother. 59, 1047–1056 55 Balzarini, J. et al. (2002) Metabolic activation of nucleoside and nucleotide reverse transcriptase inhibitors in dendritic and Langerhans cells. AIDS 16, 2159–2163 56 Veazey, R.S. et al. (2005) Protection of macaques from vaginal SHIV challenge by an orally delivered CCR5 inhibitor. Nat. Med. 11, 1293– 1294
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Oral pre-exposure prophylaxis for HIV prevention J. Gerardo Garcı´a-Lerma, Lynn Paxton, Peter H. Kilmarx and Walid Heneine Division of HIV/AIDS Prevention, National Center for HIV, Hepatitis, STD, and Prevention, Centers for Disease Control and Prevention, 1600 Clifton Road, Atlanta, GA 30329, USA
In the absence of an effective vaccine, HIV continues to spread worldwide, emphasizing the need for new biomedical interventions to limit its transmission. Appreciation of the challenges that HIV has to face to initiate an infection mucosally has spurred interest in evaluating the use of antiretroviral drugs to prevent infection. Recent animal studies using macaques or humanized mice models of mucosal transmission of SIV or HIV have shown that daily or intermittent pre-exposure prophylaxis (PrEP) with tenofovir disoproxil fumarate (TDF) and emtricitabine (FTC) can exploit early virus vulnerabilities and effectively prevent establishment of infection. These preclinical findings have fueled interest in evaluating the safety and efficacy of PrEP in humans. We provide an overview of the rationale behind PrEP and discuss the next steps in PrEP research, including the need to better define the ability of current drugs to reach and accumulate in mucosal tissues and protect cells that are primary targets during early HIV infection. Introduction The HIV/AIDS pandemic remains among our greatest public health challenges. In the current decade, the prevalence of HIV-1 infection has stabilized at 0.8% [1] worldwide. The overall number of people living with HIV is increasing as new infections continue to occur and AIDS deaths are prevented by increasingly available highly effective antiretroviral treatment (ART). Globally, there were an estimated 33.2 million people living with HIV infection or AIDS in 2007 [1]. In that year, the annual incidence of new HIV infections was an estimated 2.7 million, and there were an estimated 2.0 million HIVrelated deaths [1]. More than 96% of new infections were in low- and middle-income countries [2]. In the USA, the incidence has been relatively stable for the past decade [1], with an estimated 56,300 new cases in 2006. The ongoing high incidence of HIV infection and incomplete coverage with basic HIV prevention tools underscore the need for new, highly effective biomedical HIV interventions to complement existing prevention strategies (Box 1). In the absence of an effective vaccine, oral administration of antiretroviral drugs before HIV exposure (preexposure prophylaxis or PrEP) might be a reliable intervention to protect high-risk HIV-negative people from infection [3–6]. Mathematical models estimate that over the next 10 years an effective PrEP program could prevent Corresponding author: Garcı´a-Lerma, J.G. ([email protected])
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2.7–3.2 million of the 11 million new HIV-1 infections projected to occur in sub-Saharan Africa [7]. This potentially significant public health benefit requires a very high PrEP efficacy and can be lost or substantially reduced with a PrEP efficacy of <50%. Model simulations have also shown that an effective PrEP program could substantially reduce the incidence of HIV transmission in populations at high risk of infection in the USA [8]. PrEP is a proven concept for other infectious diseases such as malaria. Multiple lines of evidence suggest that it might also be a feasible chemoprevention strategy for preventing HIV among high-risk populations. Antiretroviral drugs effectively prevent HIV transmission at birth, during breastfeeding and after occupational exposure [9–11]. Studies in non-human primates have shown that daily oral administration of the reverse transcriptase (RT) inhibitors TDF and FTC, or a combination of FTC and TDF (Truvada), can prevent or delay transmission of simian or simian/human immunodeficiency virus (SIV/SHIV) in macaques in a dose-dependent manner [12–14]. These observations have fueled interest in evaluating the efficacy of daily PrEP in preventing HIV transmission in humans. Several clinical trials with TDF or Truvada are ongoing in high-risk populations. In this review, we discuss the rationale behind PrEP. We focus on the delicate balance of vulnerabilities of the virus and host during early infection and the existence of a narrow window of opportunity for interventions with antiretroviral drugs. We then provide an overview of drugs that are currently approved for treatment and are under consideration for PrEP in humans, in addition to existing preclinical animal model research with the most attractive drug candidates. We focus on oral PrEP (although antiretrovirals can also be provided topically by gels or other delivery mechanisms). This review highlights the promise of oral PrEP as a new biomedical strategy to prevent HIV transmission in humans. HIV transmission as a delicate balance of vulnerabilities of the virus and host The success of sexual transmission of HIV is dictated by the dynamic balance between virus and host vulnerabilities. An appreciation of the vulnerabilities and challenges that HIV must face to initiate an infection in the mucosa came from animal model studies of vaginal SIV infection (Figure 1; reviewed in [15]). These studies have consistently shown that, after penetration of SIV into the cervicovaginal epithelium, infection in cervicovaginal tissues
0165-6147/$ – see front matter . Published by Elsevier Ltd. doi:10.1016/j.tips.2009.10.009 Available online 4 December 2009
Review Box 1. Biomedical interventions to prevent HIV transmission Emerging biomedical interventions include male circumcision, HIV vaccines, microbicides, and antiretroviral drugs used either as HIV treatment to reduce infectiousness or as pre-exposure prophylaxis. Male circumcision. Shown to be efficacious in preventing femaleto-male transmission. Being implemented in countries where the prevalence of HIV is high and the prevalence of male circumcision is low. Vaccines: After the failure of a lead vaccine product in 2007, vaccine research is increasingly focused on basic science and discovery rather than on human clinical trials. The recent report of a modestly effective two-vaccine prime-boost vaccination strategy gives new hope to vaccine development, but the availability of a licensed, effective vaccine is still years away. Microbicides. Several early-generation microbicide products not found to be effective. A non-statistically significant trend toward protection was shown in one trial with the polyanionic binding inhibitor PRO 2000. Results of another, larger trial of PRO 2000 are anticipated in late 2009. Clinical trials of a promising TFV-based microbicide are also ongoing. Antiretroviral treatment of established HIV infection. Considered as a prevention intervention as it appears to decrease the likelihood of HIV transmission from the treated, HIV-infected individual to sex or needle-sharing partner(s). Oral pre-exposure prophylaxis. Nine PrEP trials using TDF or Truvada in different at-risk populations. Trials expected to reach completion from 2010 onwards.
during the first 1–3 days is limited to extremely small numbers of productively infected cells in rare foci [16–18]. This small local founder population of infected cells expands in the following days, possibly by accretion of new infections around the initial clusters [18]. Continuous expansion at the point of entry and dissemination of virus and infected cells through lymphatic drainage and the
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bloodstream establishes a sustainable infection in secondary lymphoid organs [16]. In fewer than two weeks, a very small founder population of productively infected cells at the portal of entry progresses to systemic infection with a burst of virus production and depletion of gut CD4+ T cells [15] (Figure 1). At this point, a robust virus-specific immune response can contain virus replication only to a certain degree [19]. Thus, the first days of infection at the mucosa when replication is limited to small clusters of infected cells are the periods of maximum virus vulnerability and, therefore, represent a ‘window of opportunity’ for HIV prevention interventions with vaccines, microbicides or antiretroviral drugs. The small founder population of infected cells seen during early SIV infection of macaques is a consequence of the effectiveness of the host in limiting virus access to susceptible target cells in the mucosa (Figure 1). This ‘barrier effect’ is best illustrated experimentally by the large virus dose (106–109 particles) that is generally needed to effectively infect macaques intravaginally after a single exposure [16]. On contact with the host, most of the virus inoculum is diluted and cleared in the cervicovaginal fluids or entrapped in cervical mucus. Inactivation by low pH and innate immunity further reduces the effective virus inoculum (reviewed in [20]). The multi-layered squamous or single-layered columnar epithelium from the cervicovaginal or rectal mucosa provides a barrier that greatly reduces exposure to susceptible target cells in the underlying stroma. A largely intact mucosa and a large proportion of defective particles incapable of establishing an infection have often been associated with restricted transmission of a homogenous virus population in macaques and humans [21,22]. However, host vulnerability to infection is
Figure 1. Model of mucosal HIV infection and virus vulnerabilities. SIV infection of the cervicovaginal epithelium of macaques is rapid: it can occur within 30–60 min of intravaginal exposure. Host restriction can efficiently limit the size of founder virus populations of infected cells. Early infection coincides with the period of maximum vulnerability of the virus and represents a major opportunity to prevent infection. If uncontrolled, infection expands by accretion and continuous seeding of viruses/infected cells to distal lymphoid tissue, resulting in a burst of virus production and depletion of gut CD4+ T cells. At this time, the balance of virus and host vulnerabilities is reversed; HIV is only partially vulnerable and can efficiently escape host immune responses; the host can control virus replication only to a certain degree.
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Review dynamic and can increase under certain conditions. For example, variations in hormone levels and thinning of the vaginal epithelium during the menstrual cycle usually coincide with periods of maximum susceptibility to infection [23]. Mechanical micro-abrasions of mucosal surfaces during intercourse or lesions due to concomitant genital ulcers can allow virus access to T cells, macrophages and dendritic cells in the lamina propria, can facilitate rapid virus dissemination to lymphatic tissues, and might result in transmission of multiple genetic variants [24,25]. These changes in vulnerability to infection are evident when examining the risks of heterosexual HIV transmission in humans. Whereas heterosexual transmissibility in stable couples without high-risk transmission co-factors can range from 1 to 10 transmissions per 1000 contacts, the presence of sexually transmitted infections or penile–anal contact can increase heterosexual infectivity to 1 to 3 transmissions per 10 contacts [26]. Mucosal HIV infection can be conceivably prevented by a rapid and efficient host immune response or by limiting the size of founder populations of infected cells to a theoretical threshold under which infection cannot be established. HIV vaccines have so far been unable to elicit protective immune responses, primarily because of the extreme genetic variability in circulating viral isolates and the high virus mutation rate that enables rapid escape from adaptive immune responses [27]. Anti-inflammatory agents that interfere with innate host responses and limit expansion of founder populations have shown promising results in macaques, although the existence of occult infections remains a possibility [18]. Antiretroviral drugs may conceivably block the establishment of founder populations of infected cells by specifically targeting the virus replicative cycle. Effective delivery of drugs at the portal of entry might therefore exploit the brief period of virus vulnerability and block HIV from establishing a persistent infection (Figure 2). As discussed below, the highly relevant macaque and humanized mice models of infection by SHIV and HIV have shown that daily and intermittent PrEP
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with antiretroviral drugs can take advantage of such vulnerabilities and effectively prevent infection. Antiretroviral drugs for oral PrEP Drug candidates for oral PrEP have largely been selected from currently approved drugs for treatment of individuals infected with HIV-1 because development of drugs exclusively for HIV prevention has not been pursued. Nevertheless, there are currently >30 drugs or drug combinations that are approved for treatment that target key steps in the HIV replication cycle, including inhibitors of virus entry, reverse transcription, integration and maturation [28]. Many desirable drug characteristics for PrEP overlap with those for treatment. These properties include good tolerability and safety, low pill burden, once-daily dosing, long half-life, high potency, and good resistance profiles that do not allow rapid emergence of resistance or broad cross-resistance with other drugs. Pre-integration drugs that prevent the establishment of infected cells are thought to be more suitable than post-integration drugs like protease inhibitors (although evidence to support this assumption is lacking). Additional pharmacokinetic properties that might be important for PrEP drugs targeting sexual transmission include the ability to rapidly reach and accumulate in genital and rectal tissues. In general, non-nucleoside RT inhibitors (e.g., nevirapine and efavirenz) or protease inhibitors (e.g., amprenavir, saquinavir and ritonavir) consistently achieve lower drug concentrations in the genital tract of males and females than in blood plasma [29–31]. High drug exposure in the female genital tract makes the entry inhibitor maraviroc a potentially attractive drug for PrEP [32]. Nucleoside and nucleotide RT inhibitors, including FTC, TDF, zidovudine (AZT) and lamivudine (3TC), also achieve concentrations in the genital tract that are 2–6-fold higher than in blood plasma [29–31,33–35]. However, the active drug of nucleoside and nucleotide analogs is the phosphorylated intracellular form and not the extracellular
Figure 2. Dynamic changes in the balance of vulnerabilities between virus and host. The success of sexual HIV transmission is dictated by the balance between vulnerabilities of the virus and host. High virus vulnerability during early (hours to 1–3 days) infection due to host restrictions (red line) can be lessened by altered host barriers (i.e. genital ulcer disease or mechanical micro-abrasions, blue line). Antiretroviral drugs may conceivably maintain virus vulnerability (green line) above a host vulnerability threshold and prevent establishment of an infection. Effective PrEP regimens should adequately penetrate mucosal tissues and protect all cell populations/subpopulations that are primary targets for early infection.
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drug measured in plasma or genital secretions. Limited data with TDF have shown that the high extracellular tenofovir (TFV) concentration seen in seminal plasma is also associated with high intracellular levels of TFV-diphosphate (TFV-DP) in seminal mononuclear cells. Intracellular FTC-triphosphate (FTC-TP) concentrations in seminal mononuclear cells were comparable with those seen in peripheral blood mononuclear cells [35,36]. Among the available candidate drugs for PrEP, only TDF and FTC are currently being evaluated in humans. They are administered as TDF alone or in combination with FTC (Truvada). There are many arguments for selecting FTC and TDF for human clinical trials. In contrast to other nucleoside RT inhibitors such as AZT and stavudine (d4T), both drugs are active in resting and activated T cells. The intracellular half-life of FTC-TP and TFV-DP is also very long (40 h to >100 h) thereby potentially extending their prophylactic activity if administered intermittently [37–39]. FTC and TDF are very potent, have a synergistic to additive effect in vitro, and a favorable safety and drug resistance profile [4,40]. FTC-resistant viruses containing
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the M184V mutation have a unique resistance pattern not shared by most RT inhibitors; if PrEP fails the antiviral activity of other nucleoside RT inhibitors may be maintained even in the presence of M184V. When combined with TDF, M184V and not the TDF-associated K65R mutation is the most frequent pathway of resistance to Truvada [41]. Both drugs are conveniently co-formulated in a once-daily pill. Preclinical research in animal models The potential use of antiretroviral drugs for HIV prophylaxis has been studied extensively in non-human primate models of mucosal and parenteral transmission of SIV or SHIV and, more recently, in a humanized mice model of vaginal HIV transmission (Table 1). Early work in macaques showing that high (20 mg/kg) doses of TFV administered subcutaneously could prevent parenteral SIV transmission was instrumental in establishing guidelines for post-exposure prophylaxis in humans by defining the timing of initiation and length of treatment that can provide protection from infection [42–44]. Indications that
Table 1. Available preclinical animal data with daily and intermittent PrEP Reference
Animal
Drugs and dose
Tsai et al. [44]
Long-tailed macaques
TFV, 20 mg/kg
Route of drug administration Subcutaneous
Van Rompay et al. [51]
Rhesus macaques
TFV, 30 mg/kg
Subcutaneous
Van Rompay et al. [46]
Rhesus macaques
TDF, 0.01 to 0.02 mg/kg
Oral
Multiple oral exposures to SIVmac251 (104 TCID50)
Van Rompay et al. [14]
Rhesus macaques
TDF, 10 mg/kg
Oral
Subbarao et al. [13]
Rhesus macaques
TDF, 22 mg/kg
Oral
Garcı´a-Lerma et al. [12]
Rhesus macaques
FTC, 20 mg/kg; TDF, 22 mg/kg; TFV, 22 mg/kg
Oral (Truvada) or subcutaneous (FTC, TFV)
Multiple oral exposures to SIVmac251 (105 TCID50) Repeat low-dose atraumatic rectal exposures to SHIV162p3 (10 TCID50) Repeat low-dose atraumatic rectal exposures to SHIV162p3 (10 TCID50)
Denton et al. [49]
Humanized mice
FTC, 3.5 mg TDF, 5.2 mg
Intraperitoneal
Garcı´a-Lerma et al. [52]
Rhesus macaques
FTC, 20 mg/kg TDF, 22 mg/kg
Oral
Virus exposure and dose Single intravenous exposure to SIVmne (103 TCID50) Oral, SIVmac251 (105 TCID50)
Single atraumatic intravaginal exposure to HIV-1JR-CSF (105 TCID50) Repeat low-dose atraumatic rectal exposures to SHIV162p3 (10 TCID50)
Interventions
Main findings
TFV initiated 48h prior to exposure and continued during 4 wks
Full protection
Two doses given 4h before and 24h after exposure TDF initiated 1 d before exposure and maintained during continuous virus inoculations; one additional dose after the last virus exposure Repeated cycles of daily TDF initiated 1–2 days before exposure Daily or weekly TDF
Full protection
Daily FTC (subcutaneous) Daily oral Truvada Daily or intermittent FTC/TFV (subcutaneous) Daily FTC/TDF initiated 48h prior to exposure and continued for 7 days
Several event-driven and exposure-independent intermittent modalities with Truvada (times relative to exposure): -22h/+2h, -3d/+2h, -7d/+2h, +2h/+26h, -2h/+24h,
No protection; low doses of TDF
Partial prophylactic efficacy; infection associated with low systemic drug exposures All controls animals infected after 1.5 exposures; 3 of 4 TDF-treated animals infected after 6 to 7 exposures Risk of infection reduced with human equivalent doses of FTC (4-fold) and Truvada (8-fold) No infection with FTC and a high dose of TFV (daily or intermittent) Full protection
High to moderate efficacy with 2 weekly humanequivalent doses of Truvada
TCID: tissue culture infectious dose.
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Review antiretrovirals administered before exposure could also prevent infection in a dose-dependent manner came from these and subsequent studies using different doses of TFV administered orally or subcutaneously, and from oral, rectal, or parenteral virus inoculations (Table 1) [14,44– 46]. We recently developed a repeat low-dose rhesus and pigtail macaque model of rectal and vaginal SHIV transmission that recapitulates several aspects of human transmission of HIV, and is well-suited for evaluating various PrEP strategies [13,47]. The virus dose is lower and more physiological than that used in conventional single highdose models [47,48]. The SHIV challenge dose contains an R5-tropic HIV-1SF162 envelope similar to naturally transmitted viruses. Virus exposures are repeated to mimic high-risk human exposures, thereby providing the opportunity to measure protection against multiple transmission events in each animal [12,13,22]. Using a rectal transmission model, we showed that higher drug doses and combination treatments might be more effective in preventing SHIV transmission than single drugs or lower doses [12,13]. At human-equivalent doses, daily FTC reduced the risk of infection by 3.8-fold whereas a nonstatistically significant trend towards delayed infection was seen in animals receiving daily TDF [12,13]. However, the combination of both drugs provided a nearly eightfold lower risk of infection with four of the six animals protected after 14 weekly rectal challenges [12]. These studies used a rectal transmission model. Although many biological similarities exist between rectal and vaginal HIV transmission, differences in the early events of mucosal infection or changes in drug exposure or susceptibility associated with the menstrual cycle and thinning of the epithelium are possible [23]. Evidence that Truvada might also protect against vaginal transmission has come from studies using humanized mice exposed vaginally to HIV. Even though 7/8 control mice became infected, none of the five animals receiving a high dose of Truvada were infected [49]. Some important observations of potential relevance to humans have been made from the analysis of PrEP breakthrough infections in macaques. First, two of the macaques infected in the studies mentioned above during daily PrEP with FTC or Truvada (one animal in each study) showed selection for drug-resistant viruses. In both macaques, the M184V mutation associated with FTC resistance was selected, thus reiterating the need to closely monitor for drug resistance if PrEP is implemented [12]. Second, PrEP breakthroughs during treatment with FTC and Truvada had reduced levels of virus replication during early infection compared with untreated control animals [12]. A reduction in early viremia in PrEP failures might contribute to a decrease in HIV-1 transmissibility at the population level and could add to the overall effectiveness of PrEP. Attenuated acute viremia might also reduce early CD4+ T cell depletion, help to preserve immune function, and attenuate the course of HIV infection [50]. Animal models have also been used to explore the efficacy of intermittent drug dosing with TDF or Truvada. Intermittent PrEP may be a more convenient strategy that can potentially be more cost-effective, reduce the risks of drug toxicities, increase adherence and minimize drug resistance emergence. In intermittent PrEP interventions, 78
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TDF or Truvada might be taken on the basis of exposure events or by following a weekly schedule independent of the time of exposure. FTC-TP and TFV-DP have long intracellular half-lives in humans [37–39] and can potentially achieve extended prophylactic activity when administered intermittently. Short-course prophylaxis with the long-acting drug nevirapine has successfully prevented HIV transmission at birth [11]. Studies in macaques showing protection from oral or rectal SIV/SHIV exposures by a two-dose subcutaneous regimen containing TFV or TFV/ FTC have provided the first proof-of-concept for intermittent PrEP [12,51]. However, the high drug doses and subcutaneous drug delivery might have overestimated efficacy in these studies. We recently showed, using humanequivalent doses of Truvada, that significant protection of macaques can be achieved with a single oral dose given up to 7 days before exposure, followed by a second dose 2 h after exposure [52]. We also found that exposure-driven prophylactic modalities initiated around the time of exposure maintained moderate protection. These studies are promising and help to identify potentially efficacious intermittent PrEP modalities with a wide window of protection. This broadens the possibilities for the development of more feasible and cost-effective antiretroviral prophylaxis strategies to prevent HIV transmission in humans. The animal studies mentioned above provided preclinical evidence that systemic daily or intermittent PrEP might potentially prevent HIV infection. The finding that daily PrEP with a combination of FTC and TDF was more efficient in macaques than single-drug PrEP prompted some of the clinical trials to be modified to evaluate Truvada instead of TDF. A combination of both drugs provides higher antiviral activity, increases the genetic barriers for resistance, and has a comparable convenience and safety profile. Nevertheless, only human clinical trials will show which strategy is more safe, efficacious, and cost-effective. Human clinical trials As of November 2009, nine PrEP trials enrolling >22,000 participants are at varying stages of completion (Table 2). The at-risk populations in these studies are men who have sex with men (MSM), heterosexual men and women, and intravenous drug users (IDUs). All trials are using TDFcontaining products, comprising TDF alone or in combination with FTC, and all are evaluating daily use with the exception of one pilot study of intermittent dosing. One trial (VOICE) is comparing oral TDF or TDF/FTC with topical PrEP with a TFV vaginal gel. Only one Phase-II study of TDF among 936 high-risk women in Ghana, Nigeria and Cameroon has been completed, and no differences in adverse events or grade-3 or -4 laboratory abnormalities were observed between placebo and TDF users. There were fewer infections in the TDF arm (two events versus six events in the placebo arm) even though the study was not of sufficient size or duration to examine the efficacy of tenofovir [53]. Current trials are projected to reach completion from 2010 onwards (Table 2). These trials will provide valuable information, including efficacy for the major routes of HIV transmission, the acceptability of daily dosing, the required frequency of safety monitoring, and an indication
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Table 2. Completed, ongoing, and planned clinical trials of oral HIV pre-exposure prophylaxis Study
Sponsor Product
Population
Countries
West Africa TDF Trial
FHI
Daily oral TDF
936 women
Daily oral TDF Daily oral TDF
400 MSM 2400 male and female IDU 1200 heterosexual men and women
Ghana, Cameroon, Nigeria US Thailand
Enrollment Status/projected started completion 2004 Completed in 2007 [54] 2005 Fully enrolled/2010 2005 98% enrolled/2010
Botswana
2007
Fully enrolled/2011
Peru, Ecuador, Brazil, US, Thailand, South Africa Kenya, Uganda
2007
Fully enrolled/2011
2008
Enrolling/2012
Kenya, Malawi, 2009 South Africa, Tanzania, Zambia Daily oral TDF 4,200 women Malawi, Uganda, 2009 Daily oral FTC/TDF South Africa, Zambia, Daily topical TFV gel Zimbabwe Daily oral FTC/TDF 150 at-risk discordant Kenya, Uganda 2009 Intermittent oral couples FTC/TDF
Enrolling/2012
US Extended TDF safety trial CDC CDC Bangkok TDF study TDF-2
CDC
iPrEX, UCSF
NIH, BMGF
Partners PrEP
BMGF
FEM-PrEP
FHI,
VOICE, MTN003
MTN, NIH
IAVI E001, E002 Phase I/II
IAVI
Daily oral FTC/TDF (switched from TDF in 2007) Daily oral FTC/TDF
Daily oral TDF Daily oral FTC/TDF Daily oral FTC/TDF
3000 MSM
3,900 discordant heterosexual couples 3,900 high-risk women
Enrolling/2013
Enrolling/2010
FHI, family health international; CDC, US Centers for Disease Control and Prevention; NIH, US National Institutes of Health; BMFG, Bill & Melissa Gates Foundation; MTN, Microbicide Trail Network; IAVI, International AIDS Vaccine Initiative; MSM, men who have sex with men; FTC, emtricitabine; TDF, tenofovir disoproxil fumarate; TFV, tenofovir
of the occurrence of HIV drug resistance among those who become infected while taking PrEP. It is also hoped that these trials will shed light on the utility of measuring drug levels in blood (plasma or mononuclear cells) or hair as indicators of adherence and drug exposure. However, given the long half-life of FTC-TP and TFV-DP, adherence rates may be difficult to assess. There are several areas that will require further research and ongoing surveillance if PrEP is to become part of an HIV prevention program. These include efficacy, drug adherence and drug resistance outside the rigorously controlled conditions of a clinical trial, the extent of any behavioral disinhibition or risk behavior change associated with PrEP, and the occurrence and extent of medication-sharing and black-market use. Next steps in PrEP research Although human trials will ultimately determine if oral PrEP can prevent HIV transmission, animal research has provided valuable information and can guide the next steps in the research and practice of PrEP. There are many unresolved issues that deserve further investigation. Additional research should explore if drug-resistant viruses have the potential to diminish PrEP efficacy. This is particularly relevant in areas with widespread access to antiretroviral drugs where transmission of drug-resistant viruses occurs at a relatively high (10–20%) frequency [54]. A comprehensive analysis of the pharmacology of antiretroviral drugs in genital secretions and tissues is also essential to define the best drug candidates and the most appropriate dosing strategies for PrEP. These studies should not only evaluate drug penetration in rectal and vaginal tissues, but also the degree of drug exposure in cells that are primary targets during early mucosal infection such as Langerhans cells, dendritic cells or activated/resting T cells [55]. A combination of in-vivo tissue pharmacokinetics and ex-vivo studies using explants
models of cervicovaginal and rectal tissues will shed light on potential cell populations/subpopulations that are not adequately protected by current PrEP regimens, and will help to define better drug regimens that may include, for instance, other drug classes such as entry inhibitors [56]. Defining the relationship between drug levels, drug dosing, and protection is also important. For instance, the threshold for levels of TFV-DP and FTC-TP that results in protection has not been defined in macaques or tissue explants. These studies may help to define good surrogate markers for adherence and efficacy of PrEP in humans. Special emphasis should be made in evaluating nextgeneration PrEP agents and long-acting drug formulations that may require less frequent dosing and could be taken independently of virus exposure. Concluding remarks Antiretroviral prophylaxis has been very effective in preventing mother-to-child transmission of HIV, and there is great hope that this strategy will prove effective against other routes of HIV transmission. Our understanding of the early events of mucosal infection points to rapid establishment of a small population of infected cells. This represents a window of high virus vulnerability, enabling antiretroviral drugs to block the establishment of a persistent infection. Many currently available drugs for treatment also have desirable characteristics for PrEP, but can differ in their ability to reach and accumulate in genital and rectal tissues, which may be critical to prevent sexual HIV transmission. Preclinical research in animal models of mucosal and parenteral transmission has provided proofof-principle of the efficacy of PrEP, and has informed the design of human clinical trials. These models provide tools to evaluate different PrEP regimens, including drugs from different classes and intermittent dosing, and may help to identify correlates of protection. Data from PrEP trials will 79
Review become available in the coming two years, and will help define the role of PrEP as an strategy to prevent HIV transmission. Disclaimer The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the Centers for Disease Control and Prevention. The use of trade names is for identification only and does not constitute endorsement by the US Department of Health and Human Services, the Public Health Service, or the Centers for Disease Control and Prevention. References 1
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Trends in Pharmacological Sciences Vol.31 No.2 21 Keele, B.F. et al. (2008) Identification and characterization of transmitted and early founder virus envelopes in primary HIV-1 infection. Proc. Natl. Acad. Sci. U S A 105, 7552–7557 22 Keele, B.F. et al. (2009) Low-dose rectal inoculation of rhesus macaques by SIVsmE660 or SIVmac251 recapitulates human mucosal infection by HIV-1. J. Exp. Med. 206, 1117–1134 23 Wira, C.R. and Fahey, J.V. (2008) A new strategy to understand how HIV infects women: identification of a window of vulnerability during the menstrual cycle. AIDS 22, 1909–1917 24 Weiler, A.M. et al. (2008) Genital ulcers facilitate rapid viral entry and dissemination following intravaginal inoculation with cell-associated simian immunodeficiency virus SIVmac239. J. Virol. 82, 4154–4158 25 Haaland, R.E. et al. (2009) Inflammatory genital infections mitigate a severe genetic bottleneck in heterosexual transmission of subtype A and C HIV-1. PLoS Pathog. 5, e1000274 26 Powers, K.A. et al. (2008) Rethinking the heterosexual infectivity of HIV-1: a systematic review and meta-analysis. Lancet Infect. Dis. 8, 553–563 27 Walker, B.D. and Burton, D.R. (2008) Towards an HIV vaccine. Science 320, 760–764 28 US Food and Drug Administration (2009) Antiretroviral drugs used in the treatment of HIV infection. http://www.fda.gov/ForConsumers/ byAudience/ForPatientAdvocates/HIVandAIDSActivities/ ucm118915.htm (accessed 24 July 2009) 29 Dumond, J.B. et al. (2007) Antiretroviral drug exposure in the female genital tract: implications for oral pre- and post-exposure prophylaxis. AIDS 21, 1899–1907 30 Ghosn, J. et al. (2004) Penetration of enfuvirtide, tenofovir, efavirenz, and protease inhibitors in the genital tract of HIV-1-infected men. AIDS 18, 1958–1961 31 Min, S.S. et al. (2004) Protease inhibitor and nonnucleoside reverse transcriptase inhibitor concentrations in the genital tract of HIV-1infected women. J. Acquir. Immune Defic. Syndr. 37, 1577–1580 32 Dumond, J.B. et al. (2009) Maraviroc concentrates in the cervicovaginal fluid and vaginal tissue of HIV-negative women. J. Acquir. Immune Defic. Syndr. 51, 546–553 33 Pereira, A.S. et al. (1999) Nucleoside analogues achieve high concentrations in seminal plasma: relationship between drug concentration and virus burden. J. Infect. Dis. 180, 2039–2043 34 Taylor, S. et al. (2000) Concentrations of nevirapine, lamivudine and stavudine in semen of HIV-1-infected men. AIDS 14, 1979–1984 35 Vourvahis, M. et al. (2008) The pharmacokinetics and viral activity of tenofovir in the male genital tract. J. Acquir. Immune Defic. Syndr. 47, 329–333 36 Dumond, J.B. et al. (2008) Differential extracellular and intracellular concentrations of zidovudine and lamivudine in semen and plasma of HIV-1-infected men. J. Acquir. Immune Defic. Syndr. 48, 156–162 37 Hawkins, T. et al. (2005) Intracellular pharmacokinetics of tenofovir diphosphate, carbovir triphosphate, and lamivudine triphosphate in patients receiving triple-nucleoside regimens. J. Acquir. Immune Defic. Syndr. 39, 406–411 38 Pruvost, A. et al. (2005) Measurement of intracellular didanosine and tenofovir phosphorylated metabolites and possible interaction of the two drugs in human immunodeficiency virus-infected patients. Antimicrob. Agents Chemother. 49, 1907–1914 39 Wang, L.H. et al. (2004) Pharmacokinetic and pharmacodynamic characteristics of emtricitabine support its once daily dosing for the treatment of HIV infection. AIDS Res. Hum. Retroviruses 20, 1173– 1182 40 Borroto-Esoda, K. et al. (2006) In vitro evaluation of the anti-HIV activity and metabolic interactions of tenofovir and emtricitabine. Antivir. Ther. 11, 377–384 41 Johnson, M.A. et al. (2006) A once-daily lopinavir/ritonavir-based regimen provides noninferior antiviral activity compared with a twice-daily regimen. J. Acquir. Immune Defic. Syndr. 43, 153–160 42 Otten, R.A. et al. (2000) Efficacy of postexposure prophylaxis after intravaginal exposure of pig-tailed macaques to a human-derived retrovirus (human immunodeficiency virus type 2). J. Virol. 74, 9771–9775 43 Tsai, C.C. et al. (1998) Effectiveness of postinoculation (R)-9-(2phosphonylmethoxypropyl) adenine treatment for prevention of persistent simian immunodeficiency virus SIVmne infection depends
Review
44
45
46
47
48 49
critically on timing of initiation and duration of treatment. J. Virol. 72, 4265–4273 Tsai, C.C. et al. (1995) Prevention of SIV infection in macaques by (R)-9-(2-phosphonylmethoxypropyl)adenine. Science 270, 1197– 1199 van Rompay, K.K. et al. (2001) Two low doses of tenofovir protect newborn macaques against oral simian immunodeficiency virus infection. J. Infect. Dis. 184, 429–438 van Rompay, K.K. et al. (2002) Topical adminsitration of low-dose tenofovir disoproxil fumarate to protect infant macaques against multiple oral exposures of low doses of simian immunodeficiency virus. J. Infect. Dis. 186, 1508–1513 Otten, R.A. et al. (2005) Multiple vaginal exposures to low doses of R5 simian-human immunodeficiency virus: strategy to study HIV preclinical interventions in nonhuman primates. J. Infect. Dis. 19, 164–173 Pilcher, C.D. et al. (2004) Brief but efficient: acute HIV infection and the sexual transmission of HIV. J. Infect. Dis. 189, 1785–1792 Denton, P.W. et al. (2008) Antiretroviral pre-exposure prophylaxis prevents vaginal transmission of HIV-1 in humanized BLT mice. PLoS Med 5, e16
Trends in Pharmacological Sciences
Vol.31 No.2
50 Gupta, S.B. et al. (2007) Estimating the benefit of an HIV-1 vaccine that reduces viral load set point. J. Infect. Dis. 195, 546–550 51 van Rompay, K.K. et al. (1998) Two doses of PMPA protect newborn macaques against oral simian immunodeficiency virus infection. AIDS 12, F79–83 52 Garcia-Lerma, J.G. et al. (2009) Prevention of rectal SHIV transmission in macaques by intermittent pre-exposure prophylaxis (iPrEP) with oral truvada, In 16th Conference on Retroviruses and Opportunistic Infections Montreal, Canada (February 8–11) (Abstract 47), 53 Peterson, L. et al. (2007) Tenofovir disoproxil fumarate for prevention of HIV infection in women: a phase 2, double-blind, randomized, placebo-controlled trial. PLoS Clin. Trials 2, e27 54 Booth, C.L. and Geretti, A.M. (2007) Prevalence and determinants of transmitted antiretroviral drug resistance in HIV-1 infection. J. Antimicrob. Chemother. 59, 1047–1056 55 Balzarini, J. et al. (2002) Metabolic activation of nucleoside and nucleotide reverse transcriptase inhibitors in dendritic and Langerhans cells. AIDS 16, 2159–2163 56 Veazey, R.S. et al. (2005) Protection of macaques from vaginal SHIV challenge by an orally delivered CCR5 inhibitor. Nat. Med. 11, 1293– 1294
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