Preventive technologies: antiretroviral and vaccine development

14 July 2016. Related: Pipeline report, Antiretrovirals, HIV prevention and transmission, Vaccines and microbicides.

Tim Horn and Richard Jefferys, TAG

With the continued rollout and implementation of global, national, and regional HIV incidence targets and timelines, one thing has become abundantly clear: reducing HIV rates below endemic and epidemic levels in all vulnerable populations and subpopulations everywhere in the world will require not only a monumental scale-up of care and antiretroviral therapy for those living with the virus, but also fierce commitment to primary biomedical prevention. This isn’t simply rhetoric, but rather a public health mandate that is supported by a growing body of epidemiological and other scientific data. [1, 2, 3, 4]

However, only a fraction of adolescents and adults vulnerable to HIV are accessing one on the most important evidence-based additions to the prevention toolbox: coformulated tenofovir disoproxil fumarate and emtricitabine (Truvada; TDF/FTC) as a pre-exposure prophylaxis (PrEP). In the U.S. alone, where TDF/FTC has been approved as PrEP since July 2012, of the 1.2 million adults with indications for PrEP – a likely conservative estimate from the U.S. Centers for Disease Control and Prevention (CDC) – only an estimated 4% have used it, even briefly. [5, 6] For PrEP to have a population-level effect in the U.S., however, a substantial increase in PrEP uptake will be required: 40% use among high-risk men who have sex with men, 10% use among people who inject drugs, and 10% use among high-risk heterosexuals would be in the absence of any improvements in clinical care engagement and viral load suppression rates among people living with HIV, necessary to prevent approximately 48,000 new infections between 2015 and 2020. [7]

Significant barriers to PrEP uptake exist, with the most egregious examples being achingly slow product registration and national health plan inclusion – the high cost of TDF/FTC, along with limited cost-effectiveness data, are considerable factors [8] – in many high-, middle-, and low-income countries. Even where PrEP has been approved, myriad access challenges exist. Examples in the U.S. include restrictions in CDC and other federal agency funds to pay for TDF/FTC; reluctance to expand Medicaid in many states, particularly those with high HIV prevalence and incidence estimates; and lags in awareness of PrEP and best screening, prescribing, and monitoring practices among primary care providers. Implementation strategies to overcome these structural hindrances, on top of myriad social and behavioral barriers, will be critical for PrEP’s success.

Success also depends on expanding the toolbox of biomedical prevention modalities that can extend the very high level of adherence-dependent protection associated with oral TDF/FTC to individuals and populations with unique safety, dosing, and affordability needs. [9] Additional oral antiretroviral regimens, including maraviroc (MVC)- and tenofovir alafenamide (TAF)-based combinations, are in various stages of development. Although no generic contenders have yet entered clinical trials, the scientific and economic basis for fast-track evaluations of TDF (which loses its patent at the end of 2017) and 3TC can potentially hasten global scale-up of oral PrEP before coformulated TDF/FTC’s patent expires in 2021.

Interest in long-acting (LA) products also continues to grow. An injectable nanoformulation of cabotegravir is now entering its efficacy phase of development. Another intriguing contender, much further back in the pipeline, is MK-8591, Merck’s nucleoside reverse transcriptase inhibitor, which may allow for injectable dosing separated by several months (see The Antiretroviral Pipeline).

Researchers and advocates for women’s health are also working to wrap their heads around long-awaited data from ASPIRE and the Ring Study, two phase III investigations of the dapivirine intravaginal ring reported at the 2016 Conference on Retroviruses and Opportunistic Infections (CROI) earlier this year in Boston and reviewed in detail below. Both trials suggest that the dapivirine ring has the potential to be at least moderately effective, but that significant utilization challenges exist, particularly for young sub-Saharan African women – a highly vulnerable population in dire need of an acceptable primary prevention option.

Also inching forward is a cache of other products for vaginal and rectal administration: gels, tablets, and films, along with an array of other intravaginal rings that combine drug and hormonal products for broad-spectrum antiviral and contraceptive protection.

Passive immunization involves the infusion of antibodies that are capable of inhibiting a large swathe of HIV variants, and represents another potential biomedical prevention option that is now being vigorously pursued. The launching of the first efficacy trials of the approach in adults was announced recently. [10] Known as the antibody-mediated prevention (AMP) studies, they will test bimonthly infusions of the VRC01 antibody in populations that are at high risk of HIV infection.

Inducing protective antibody responses with a vaccine remains a stubborn challenge for researchers, but progress continues in the pre-clinical realm, and there is hope that candidates capable of leading the immune system down the road toward the generation of effective antibodies will enter clinical trials in the next year or two. [11, 12]

In the interim, the major news for the HIV vaccine field is that the first efficacy trial in nearly a decade will begin later this year. [13] The trial, HIV Vaccine Trials Network (HVTN) 702, is designed to try and duplicate or improve on the slight, but statistically significant, 31.2% reduction in risk of HIV acquisition that was observed in RV144, a large-scale evaluation of a prime-boost vaccine regimen that was conducted in Thailand and reported results in 2009. [14] HVTN 702 will take place in South Africa and, if all goes according to plan, results are anticipated by 2020.

Antiretrovirals for Prevention

ACTG, AIDS Clinical Trials Group; CGN, carrageenan; EI, entry inhibitor; FTC, emtricitabine; HC, hormonal contraceptive; HPTN, HIV Prevention Trials Network; IM, intramuscular; IPM, International Partnership for Microbicides; MVC, maraviroc; NNRTI, non-nucleoside reverse transcriptase inhibitor; NRTI, nucleoside reverse transcriptase inhibitor; NtRTI, nucleotide reverse transcriptase inhibitor; PI, protease inhibitor; TAF, tenofovir alafenamide fumarate; ZA, zinc acetate

Oral Formulations

With scale-up initiatives to bolster TDF/FTC awareness and utilization where it is approved as PrEP under way – along with ongoing efforts to see that the coformulation is registered and covered by national health programs in other countries – two additional oral products continue to make their way down the biomedical prevention pipeline.

The advantages of these compounds – which include ViiV’s MVC and Gilead’s TAF plus FTC – as PrEP remain unclear. Possibilities include improved markers of renal and bone safety relative to TDF-inclusive regimens. Although kidney and bone problems remain uncommon and mild, and are almost always reversible following drug cessation among long-term TDF/FTC PrEP users in clinical trial and demonstration project cohorts, new oral compounds may prove to be useful for those with other risk factors (e.g., underlying renal insufficiency, baseline bone mineral deficiency, concomitant use of nephrotoxic or bone mineral-depleting medications, and advancing age). [15, 16, 17, 18, 19, 20]

MVC (Selzentry)

CCR5-tropic HIV-HIV that utilizes the CCR5 coreceptor on CD4 cells to gain entry and establish infection – is responsible for more than 95% of new sexually transmitted infections of the virus. [21, 22] Thus, there has been interest in studying the CCR5 antagonist MVC for its potential use as PrEP. Not only might MVC be associated with a reduced risk of adverse events, its unique mechanism of blockading cellular rather than viral protein function may also be associated with a reduced risk of developing drug resistance.

Preliminary data from the phase II NEXT-PrEP study (HPTN 069/ACTG A5305) suggest that MVC is well tolerated and is potentially efficacious as PrEP in transgender women and men who have sex with men (TG/MSM), although it is likely more effective as a component of a multi-drug regimen. [23] Results from the study’s cohort of 200 women vulnerable to HIV infection enrolled in NEXT-PrEP will be reported at the 2016 International AIDS Conference in Durban.

The TG/MSM cohort comprises 406 participants randomized to receive either MVC alone, MVC plus FTC, MVC plus TDF, or standard-of-care TDF/FTC. All participants had a history of condomless anal intercourse with at least one HIV-positive or unknown serostatus man within 90 days of study entry. The median age at baseline was 30 years; 2% were transgender, 28% black/African American, 22% Latino, and 62% white.

There were 34 sexually transmitted infections (STIs) diagnosed among 31 participants (8%) at study entry. During the 48-week follow-up period, 115 additional STIs were diagnosed.

Adherence rates, based on plasma drug concentrations in a random subset of 160 participants across the four study groups, were 83% at week 24 and 77% at week 48.

There were 67 grade 3–4 adverse events, with no differences in occurrence rates or severity among the four study arms. Hypophosphatemia (17%) and upper respiratory tract infections (11%) were the most common grade 2 or higher adverse events. Other than increased creatinine being documented in 1% of study volunteers in the MVC/FTC group, renal safety results were limited. Additional safety data, including bone mineral density findings, are forthcoming.

Five new HIV infections were documented during the study, for an annual incidence rate of 1.4% (95% confidence interval (CI): 0.8%–2.3%). Four of the five infections occurred in participants in the MVC monotherapy group; one occurred in the MVC/TDF group, but this participant had undetectable plasma drug concentrations at every study visit.

One of the participants in the MVC monotherapy group who became infected had a plasma drug concentration at the time of seroconversion (145 ng/mL) that was significantly above the median pre-dose state of 32 ng/mL expected in individuals with 100% adherence. His MVC plasma concentrations were highly variable prior to HIV detection at week 16, however, indicating that adherence was inconsistent. The second and third participant in the MVC monotherapy group to become infected had suboptimal drug concentrations on their documented seroconversion dates: 6.7 and 0.7 ng/mL; both individuals also had a history of drug concentrations consistent with poor adherence. The fourth participant in the MVC group to become infected had undetectable drug concentrations at the time of seroconversion and throughout the duration of the study.

Adherence observations aside, the imbalance in new infections among those in the MVC monotherapy group raises questions about the potential for stand-alone MVC as PrEP and the likely need for MVC-inclusive combinations to ensure efficacy.

A substudy of NEXT-PrEP underscores this concern. [24] It was conducted to explore MVC’s association with increased gut-associated lymphoid tissue (GALT) T cell activation as well as increased CCR5 expression [25, 26] – observations in HIV-positive people that would be undesirable in HIV-negative individuals who are already vulnerable to the virus – and to determine whether MVC is associated with suppression of colorectal explant HIV infection.

There were no significant differences in CD4+ cell activation phenotypes between baseline and week 24 or 48 samples collected from the 55 TG/MSM participants selected at random from the four study groups. Nor were there increases in CCR5 phenotype in the substudy. And although significant viral suppression was seen between baseline and week 24 with all of the PrEP study regimens in the explant infection study, no significant suppression was seen at week 48 in samples from substudy participants receiving MVC alone.

The reason(s) for MVC’s diminished suppression of explant HIV infection are not clear. The study investigators noted that a limitation of explant research is that MVC disassociates, or loses its affinity for, tissue in culture – findings that don’t necessarily mirror what happens in vivo. Pharmacokinetic (PK) and pharmacodynamics (PD) data are pending and may help to explain the lack of efficacy.

The next steps for MVC’s development as oral PrEP, or a component thereof, are currently being discussed.

TAF and FTC

Like TDF, TAF is a prodrug formulation of tenofovir. Unlike TDF, which is converted in the blood to the active drug tenofovir diphosphate (TFV-DP) and then taken up into cells, TAF is primarily metabolized and converted to TFV-DP inside of cells. Using a much lower dose (25 mg), TAF achieves plasma tenofovir levels that are roughly 90% lower, but intracellular concentrations that are approximately four- to sevenfold higher. [27, 28] The reduced systemic exposure has the potential for fewer renal- and bone-related toxicities compared with TDF. TAF’s low-milligram dosing also has the potential for reduced generic production costs and, ultimately, greater affordability versus TDF/FTC in low-income countries. Hence, TAF coformulated with FTC – approved as Descovy in the U.S. and other countries, but indicated only for the treatment of HIV infection – is also being eyed as an alternative to Truvada.

Results from CDC evaluations of TAF plus FTC in rhesus macaques that were rectally challenged with simian-human immunodeficiency virus (SHIV) – a model and methodology that were used previously to establish the potential efficacy of TDF and TDF/FTC as PrEP – were reported at CROI 2016. [29] The first of the two-part study established TAF 1.5 mg/kg as the macaque dose required to achieve intracellular concentrations that were comparable to those typically seen in humans receiving TAF 25 mg. The second part of the study treated 12 macaques with either TAF or placebo and then challenged them weekly for up to 19 weeks.

None of the TAF-treated macaques were infected after 19 exposures – 100% protection, whereas the previous macaque studies of TDF/FTC suggested 94% protection after 14 SHIV exposures – and all six of the placebo-treated macaques were infected by their tenth exposure to the virus. Of note, macaque rectal concentrations of TFV-DP were lower with TAF than those of the macaques previously treated with TDF.

Some questions about the interchangeability of TAF for TDF as PrEP have arisen following a second report at CROI 2016, this time from a study evaluating concentrations of TFV and TFV-DP in mucosal tissues from eight HIV-negative women who received a single dose of TAF 25 mg. [30] Consistent with PK data used to support TAF’s use as a treatment, the plasma levels of TFV were 19-fold lower and peripheral blood mononuclear cell (PBMC) levels of TFV-DP were ninefold higher than those seen following single-dose TDF 300 mg dosing in an earlier study.

The investigators hypothesized that TFV-DP levels following TAF administration would also be significantly higher in cervical, vaginal, and rectal cells compared with TDF. Conversely, intracellular concentrations in biopsied tissues proved to be significantly lower: twofold in cervicovaginal samples and 13-fold in rectal samples. And, compared with TDF, TAF administration resulted in a higher percentage of tissue samples with undetectable drug levels: 63% of the rectal and 75% of genital tract samples had TFV and TFV-DP concentrations below the level of detection.

Making heads or tails of the macaque and human tissue studies is especially difficult in light of the fact that pharmacologic correlates of protection and surrogate/biologic markers of prophylactic efficacy have not yet been fully validated. The research teams from both studies agree that additional study will be necessary before any conclusions about the efficacy of F/TAF can be made, particularly in comparison with a coformulated regimen that has not only been proven to be highly effective and well tolerated as PrEP, but will be coming off of patent in five years.

Additional investigations aimed at determining the pharmacology of TAF in mucosal tissues are expected, as are large-scale registrational trials conducted by Gilead Sciences.

Of additional interest are extended-release implants containing TAF. The Oak Crest Institute for Science (Monrovia, California) published encouraging animal PK data from a study of a subdermal delivery system similar to that used for removable contraceptive rods (e.g., Norplant). [31] Also in early development is a biodegradable thin-film polymer device containing TAF that can be administered subcutaneously. [32]

Long-Acting (LA) Formulations

Improving the acceptability of PrEP is one approach to strengthening adherence rates among populations at risk for HIV infection. Particular focus is being placed on the development of LA nanosuspension formulations of antiretrovirals with PrEP potential, which may allow for doses that are separated by weeks or months. The drug furthest along the development path is LA cabotegravir (CAB LA), ViiV Healthcare’s integrase strand transfer inhibitor (and dolutegravir analog) entering phase III trials. On a less certain course is a LA injectable version of rilpivirine (RPV LA), Janssen’s NNRTI.

Cabotegravir LA

Long-awaited preliminary data from the phase IIa ÉCLAIR trial were reported at CROI 2016. [33] The study, designed to assess safety and tolerability, randomized 127 HIV-negative men between 18 and 65 years of age and at low risk of acquiring HIV at screening to either CAB (N = 106) or placebo (N = 21). For the first four weeks of the trial, oral CAB (30 mg) or placebo were administered, followed by a seven-day washout period. The injection phase began at week five and ended at week 41, with CAB LA 800 mg or saline being administered via IM injections during visits on weeks 5, 17, and 29. The study also included a follow-up phase, for a total of 81 weeks.

The median age at baseline was 30 years. Approximately 57% were white, 33% were black/African American, and 14% were Hispanic/Latino. Approximately 76% in the placebo group, compared with 85% in the CAB group, listed sex with other men as their risk factor for HIV (compared with heterosexual contact for 29% and 21%, and occupational exposure for 5% and 2%, respectively).

Adverse events leading to withdrawal during the oral phase (N = 7) included three events of neutropenia, three events of increasing creatine phosphokinase (CPK), and one event of fatigue – all of which occurred in the CAB group. For participants who entered the injection phase, a similar proportion (93% for CAB LA, 95% for placebo) completed all three injection cycles. Injection intolerability led to withdrawal in 4% of CAB LA participants. One participant in the placebo group seroconverted and subsequently withdrew from the study.

The number of grade 2–4 adverse events in the CAB group was higher than in the placebo group during the injection phase (80% for CAB LA, 48% for placebo). Grade 2 events in the injection phase that were not related to injection site pain included fever (7% versus 0%, respectively), injection site itching (6% versus 0%, respectively), and injection site swelling (6% versus 0%, respectively).

CAB PK data throughout each 12-week dosing interval were reported. Results showed trough concentrations to be lower than the prespecified ideal (fourfold higher than the protein-adjusted 90% inhibitory concentration [4× PA-IC90] of 0.664 mg/mL) at the end of the dosing intervals in approximately two-thirds of participants; 15% to 31% had trough concentrations below 1× PA-IC90 at the end of the dosing intervals. On the basis of these findings, a new dosing strategy of 600 mg IM injections every eight weeks has been selected for CAB LA’s continued development.

Two seroconversions were reported: one in the placebo group at week 23 and one in the CAB LA group at week 53-24 weeks after the participant’s final injection. The participant in the CAB group who ultimately seroconverted had no detectable CAB in blood plasma at week 53 and was one of the individuals who had CAB trough concentrations below 1× PA-IC90 on two occasions during the injection phase of the study.

A second phase IIa trial, HIV Prevention Trials Network study 077 (HPTN 077), is currently enrolling approximately 176 HIV-negative volunteers – 60% of the participants will be women – in the U.S., South America, and sub-Saharan Africa. [34] It is designed similarly to ÉCLAIR, with its primary objectives being safety, tolerability, and acceptability assessments in participants at low-to-minimal risk of HIV infection.

Currently in the final stages of development is HPTN 083, a phase IIb/III head-to-head safety and efficacy trial of CAB LA versus oral TDF/FTC. [35] In step 1 of the trial, lasting five weeks, participants will receive oral TDF/FTC or oral CAB 30 mg daily, depending on the randomization. In step 2, participants will receive a daily oral placebo plus active CAB LA 3 mL injections at two time points four weeks apart and every eight weeks thereafter, or active daily oral TDF/FTC plus placebo injections, for up to 180 weeks. In step 3, to cover the prolonged PK “tail” associated with CAB LA dosing, all participants will receive daily oral TDF/FTC for approximately one year, starting no later than eight weeks after the last injection.

HPTN 083 trial has a planned enrollment of 4,500 TG/MSM individuals 18 years of age and older who are at high risk for sexually acquiring HIV infection. The estimated study completion date is June 2020.

Rilpivirine LA

Encouraging phase I results from the SSAT 040 study evaluating the PK of RPV LA in plasma, the genital tract in women, and the rectum in men were published in 2014. [36] Later that year, however, preliminary data from the MWRI-01 phase I study suggested that RPV LA’s activity in rectal versus cervicovaginal tissues may differ considerably. [37] Although RPV levels following single 600 mg and 1,200 mg (2 × 600 mg) doses were higher in vaginal fluids versus rectal fluids, rectal tissues were found to have twice the concentration of RPV compared with vaginal tissues. In fact, rectal cell explants were fully resistant to HIV nearly two months after the 1,200 mg RPV LA injections were given, whereas vaginal and cervical cell explants appeared to be no better protected from HIV following either dose of the drug.

A more recent study characterized the concentrations of RPV needed to prevent HIV infection in mucosal tissue. [38] Although rectal tissue RPV PK appeared to be sufficient to block HIV infection – concentrations were approximately fivefold higher than what would be required to suppress viral infection – 2.5-fold more drug was needed in female genital tissue to demonstrate similar inhibition. These data, the authors noted, support the explant findings from MWRI-01, in which HIV infection was suppressed in rectal tissue, but not in cervicovaginal tissues.

Still under way, with an expected completion date of October 2017, is HPTN 076, a phase II safety and acceptability evaluation of RPV LA, compared with placebo, in approximately 132 HIV-negative women between 18 and 45 years of age in Cape Town, South Africa; Harare, Zimbabwe; Newark, New Jersey; and Bronx, New York. [39] The women have been randomized (2:1) to receive either daily oral rilpivirine 25 mg or placebo for four weeks. In the absence of any safety signals, the participants will receive either 1,200 mg RPV LA (2 mL IM injections in both gluteal muscles) or placebo every eight weeks for a total of six injections.

Based on the conflicting PK and explant infection data reported to date, compounded by the formulation’s need for cold-chain storage, RPV LA is not expected to move into phase III trials for PrEP. [40]

Microbicides: Intravaginal Rings

With a growing body of data suggesting that antiretroviral-based prevention modalities are effective for women who are vulnerable to HIV infection, provided that adherence levels that are consistent with protection can be achieved, there has been considerable interest in more user-friendly and longer acting technologies. Polymeric intravaginal rings (IVRs), similar to those used to control the release of estrogens or progestogens that provide contraceptive protection, are one such technology and are currently in various stages of development.

Dapivirine

The most clinically advanced candidate is a silicone elastomer IVR containing 25 mg dapivirine (TMC120), a NNRTI licensed to the International Partnership for Microbicides (IPM) by Janssen Sciences Ireland UC. Data from two registrational trials, the Microbicide Trials Network’s ASPIRE study (MTN-020) and the International Partnership for Microbicides’ Ring Study (IPM 027), were reported at CROI 2016, with the final ASPIRE results being simultaneously published in the New England Journal of Medicine. [41, 42, 43]

ASPIRE, a phase III trial conducted at sites in Malawi, South Africa, Uganda, and Zimbabwe, randomized 2,629 HIV-negative women between 18 and 45 years of age to receive the dapivirine IVR or a matching placebo IVR, which were self-inserted and removed once a month for a year. The Ring Study, a phase II/III evaluation at six South African and one Ugandan site, compared the dapivirine IVR to a placebo IVR, inserted once every month over 24 months, in 1,959 HIV-negative women between 18 and 45.

Results from both studies, highlighted in table 2, suggest that the dapivirine IVR is safe and moderately effective at reducing incident HIV in African women. HIV infection rates were reduced by approximately one-third overall, with greater protection occurring in both trials among women 22 years of age and older: 56% in ASPIRE and 37% in the Ring Study, with little to no protection among women 21 years of age and younger.

In ASPIRE, women 22 years of age and older appeared to use the ring more consistently. In the Ring Study, there was a trend toward higher efficacy with more consistent use. In discussing the ASPIRE results, principal investigator Jared Baeten, MD, of the University of Washington reiterated that strong relationships between adherence and HIV protection are expected in all biomedical prevention studies. However, unlike post hoc PK analyses from oral TDF/FTC PrEP trials, which have demonstrated strong adherence is associated with protection approaching 100%, inferences regarding maximum possible protection using the dapivirine IVR are not yet possible. Further analyses are required to define whether there is a threshold of dapivirine IVR use for protection against HIV, and to ferret out both behavioral and possible biologic effects that may have contributed to the lack of HIV protection in the youngest women in both studies.

Additional dapivirine IVR results reported at CROI 2016 included data from a phase IIa PK, safety, adherence, and acceptability study in 96 postmenopausal women in the U.S (MTN-024/IMP 031). [44, 45] The women – the mean age at baseline was 56.8 years; 66% were white and 31% were black/African American, 61% had a primary sexual partner, and 66% were sexually active – were randomized 3:1 to monthly IVRs containing dapivirine 25 mg or placebo for 12 weeks.

IVRs were safe and well tolerated. Only two women chose not to continue using the rings as a result of adverse events. There was no difference in the number of women with related grade 2 or higher adverse events in the two groups (8% versus 13% in the dapivirine and placebo groups, respectively), and no statistical significantly differences in grade 3 or higher adverse events (6% versus 0%, respectively). One grade 3 adverse event (vaginal pain) was deemed to be related to study product. Adherence rates were also high, with most women (99%) saying the IVR was very easy/easy to use, 96% indicating that it never interfered with daily activities, 91% reporting they either liked or very much liked the IVR, and 65% preferring VR to condoms.

Two cost-effectiveness analyses were also reported at CROI 2016. [46, 47] According to one analysis using a deterministic HIV transmission model of South Africa, the dapivirine IVR could avert 125,000–175,000, 265,000–364,000, or 427,000–588,000 infections at 25%, 50%, and 75% efficacy, respectively, from 2017–2050 under the different counterfactual scenarios.46 This represents 1.1% to 7.0% of total HIV infections in this period at corresponding cost-effectiveness of US$1,000 to US$1,300, US$370 to US$520, and US$160 to US$260 per disability-adjusted life year (DALY) averted.

As for next steps, two open-level evaluations of the dapivirine IVR are planned. MTN-025, the HIV Open-Label Prevention Extension (HOPE) trial, is an ASPIRE follow-on study to assess continued safety and adherence, and is scheduled to begin this summer. IPM hopes to conduct its own open-label extension follow-on study to provide former Ring Study participants with the dapivirine IVR.

IPM plans to submit the dossier of dapivirine IVR evidence – ASPIRE and the Ring Study are only a part of an extensive research portfolio (table 3) – required for licensure to regulatory agencies beginning in early 2017.

Source: Microbicide Trials Network, International Partnership for Microbicides

Microbicide Gels

The future of vaginal microbicides remains uncertain following the disappointing data from both the FACTS 001 and VOICE studies evaluating 1% tenofovir gel. [48, 49] Additional data are anticipated, however. These include results from the phase IIIb CAPRISA 008 – currently being analyzed by CONRAD investigators – an open-label follow-on study of the phase IIb CAPRISA 004 trial to collect additional safety and data and to evaluate the feasibility and effectiveness of providing 1% tenofovir gel to HIV-negative women through family planning clinics in KwaZulu-Natal, South Africa. [50]

Although adherence, rather than potency, was believed to be the primary factor associated with poor efficacy in the FACTS 001 and VOICE studies, a number of gel-based microbicides containing alternative compounds – dapivirine, MVC, and a broad-spectrum coformulation of MIV-150, zinc acetate, and carrageenan (see below) – are at various stages of early development. Several of these products, in addition to a reduced-glycerin 1% tenofovir gel, are also being evaluated for rectal use and protection.

Reduced-Glycerin 1% Tenofovir Gel

The reduced-glycerin 1% tenofovir gel (RG 1% TFV), developed by CONRAD, has an improved osmolarity profile, meaning that it contains fewer sugars and salts relative to epithelial cells, and therefore prevents tissues from purging too much water. This may in turn prevent damage to the structural integrity of the rectum’s lining and help to minimize the gastrointestinal side effects documented in earlier 1% tenofovir gel studies. [51, 52]

MTN-017 is a phase II safety and acceptability study of RG 1% TFV in 195 HIV-negative MSM and transgender women in the U.S., Peru, Thailand, and South Africa. [53] It was designed as a follow-up trial to the phase I MTN-007 rectal study, the data from which were published last year. [54]

The study employed a crossover design that required all study participants to cycle through three eight-week regimens: RG 1% TFV used daily, RG 1% TFV used episodically before and after receptive anal intercourse, and TDF/FTC taken orally daily.

Most side effects from study products in MTN-017 were minor, with roughly a third of participants experiencing grade 2 or higher adverse events using the three regimens: 34% for TDF/FTC, 33% for daily RG 1% TFV, and 30% for episodic RG 1% TFV.

Overall, participants were highly adherent in MTN-017, which was defined as >80% of expected doses taken and assessed by convergence scoring of daily texts and study product returns. Episodic RG 1% TFV and daily oral TDF/FTC dosing were associated with >80% adherence among 93% and 94% of study participants, respectively, compared with 83% adherence to daily RG 1% TFV dosing. Using qualitative plasma PK testing, the investigators also noted that 94.3% TFV-positive samples were associated with oral TDF/FTC, compared with 80.2% with daily RG 1% TFV dosing.

With respect to acceptability, participants reported they would be less likely to use the daily RG 1% TFV regimen (odds ratio [OR]: 0.38, P < 0.001), but would be just as likely to use the episodic RG 1% TFV regimen (OR: 0.70, P = 0.23), for HIV protection compared with oral TDF/FTC dosing. Overall liking of the regimens also favored oral TDF/FTC compared with either daily (OR: 0.28, P < 0.28) or episodic (OR: 0.37, P < 0.002) RG 1% TFV dosing.

Additional data from MTN-017, including quantitative PK testing data to shed additional light on adherence in the trial, are forthcoming.

PC-1005

The Population Council is developing PC-1005, a combination gel containing the NNRTI MIV-150, zinc acetate, and carrageenan. PC-1005 potentially offers protection not just against HIV, but also against herpes simplex virus-2 (HSV-2) and human papillomavirus.

Phase I safety, PK, acceptability, and adherence data were presented at CROI 2016. [55] The trial enrolled 25 HIV-negative women between 19 and 44 years of age. Following a three-day open label evaluation of PC-1005 in five participants, 20 women were randomized to apply PC-1005 4 mL or placebo once daily for 14 days.

Seventeen women completed the randomized phase of the trial (two were lost to follow up and one withdrew before dosing). There were no severe adverse events or early discontinuations because of adverse events. MIV-150 was absorbed systemically at low levels and there was measurable HIV and HPV activity in cervicovaginal lavages (CVLs). Acceptability was also high: 94% of participants reported a willingness to use the gel in the future.

Additional data presented at CROI 2016 indicate that PC-1005 inhibits HIV and HSV-2 infection in cervical explants in a dose-dependent manner. [56]

Contraceptive-Inclusive Multipurpose Prevention Technologies

As has been well documented in the development of oral PrEP and microbicides, there is a need for biomedical prevention options that protect against not just HIV (and other STIs), but also unwanted pregnancies. For women vulnerable to both, the development of multipurpose prevention technologies (MPTs) is of tremendous interest.

Products currently in preclinical development can be categorized as either LA or on-demand. LA MPTs include IVRs, and on-demand products include gels that can be used around the time of intercourse.

LA MPTs that are currently in phase I studies include IVRs containing levonorgestrel – a synthetic progestogen that has been studied and used extensively, and is therefore considered to be suitable for formulation in matrix IVRs – combined with either tenofovir or dapivirine. Data from CONRAD’s phase I evaluation of the safety, PK, and acceptability of its IVR containing tenofovir and levonorgestrel are being analyzed. [57] A phase I safety and PK study of IPM’s matrix IVR containing dapivirine and levorgestrel is still being finalized in partnership with the Microbicide Trials Network.

Preventive Vaccines, Passive Immunization, and Antibody Gene Transfer

Passive Immunization/Antibody Gene Transfer

HIV is notorious for its ability to evade antibody responses, and for a long period the number of antibodies known to be capable of significantly inhibiting the virus could be counted on one hand. But the landscape has changed dramatically in recent years as a result of technological advances that have allowed the identification of an ever-growing list of antibodies that can neutralize a broad array of HIV isolates from multiple different clades. Many of these broadly neutralizing antibodies (bNAbs) are extremely potent, meaning that they can neutralize the virus even when present at relatively low concentrations. [58]

The burgeoning armamentarium of bNAbs has spurred researchers to develop and manufacture candidates for testing in people, both in the prevention and treatment contexts. The furthest along is VRC01, a bNAb that was discovered toward the end of the last decade by scientists at the Dale & Betty Bumpers Vaccine Research Center (VRC) at the U.S. National Institutes of Health. [59] Phase I trials showed favorable safety and pharmacokinetic profiles, [60] leading to the recent initiation of the AMP studies, two large-scale phase IIb efficacy evaluations that represent collaborations between the HVTN and the HPTN:

• HVTN 704/HPTN 085 will enroll approximately 2,700 men and transgender people who have sex with men at sites in Brazil, Peru, and the U.S.
• HVTN 703/HPTN 081 will enroll approximately 1,500 sexually active women at sites in Botswana, Kenya, Malawi, Mozambique, South Africa, Tanzania, and Zimbabwe.

Participants will be randomly assigned to receive either placebo or VRC01 at one of two doses: 30 mg/kg or 10 mg/kg. Infusions are scheduled every eight weeks. The primary endpoints are safety and efficacy, with secondary analyses including assessments of VRC01 levels, markers of protection, and antibody effector functions. If the trials proceed as expected, results are likely to become available around 2022.

It is currently unclear whether VRC01 might be developed commercially if significant efficacy is demonstrated. Since VRC01 was discovered, several other broader and more potent bNAbs have been identified, and certain dual combinations have been reported to neutralize over 98% of circulating HIV isolates from multiple clades [61] – theoretically, at least, these newer bNAbs may be better candidates for advancing toward possible licensure.

Delivery is another issue for passive immunization approaches, and ongoing work is aiming to produce LA bNAb formulations that would be amenable to subcutaneous injection (as opposed to the inconvenient method of inpatient intravenous infusion being used in the AMP studies). A phase I trial of a LA version of VRC01, VRC01LS, was started earlier this year. [62]

The question of whether the expense of manufacturing bNAb may be an impediment to making passive immunization a real-world prevention option has been the subject of some debate. In a presentation at the Cent Gardes HIV vaccine conference last October, John Mascola from the VRC noted that the current cost to manufacture a bNAb is about US$100 per gram, which equates to US$1,200 per year for a bimonthly 30 mg/kg dose administered to a 70 kg adult. However, according to Mascola, progress in improving potency, half-life, and manufacturing efficiency could conceivably bring this figure down to as low as US$10 per person per year. [63]

Antibody gene transfer, also known as vectored immunoprophylaxis, is a potential alternate method of bNAb delivery that is similar to gene therapy. The approach utilizes adeno-associated virus (AAV) vectors that are modified to encode genes for producing bNAbs. When injected into muscle tissue, the AAV vectors can act as factories for the persistent generation of bNAbs. A first human trial of an AAV vector encoding the bNAb PG9 began in the UK in 2014, but results have not yet been published. [64] The VRC is aiming to start a trial of an AAV vector encoding another bNAb, VRC07, before the end of the year. [65, 66]

HIV Vaccines

For the past several years, TAG’s annual Pipeline Report has been covering the incremental progress toward the next round of HIV vaccine efficacy trials. This year represents a milestone because, on May 18, it was announced that the first of these trials, HVTN 702, is scheduled to begin before the end of the year.

HVTN 702 is the culmination of a huge amount of work conducted by the Pox-Protein Public-Private Partnership (P5), a collaboration involving the HVTN, the Bill & Melinda Gates Foundation, Novartis Vaccines and Diagnostics, Sanofi Pasteur, the South African Medical Research Council, the U.S. Military HIV Research Program, and the U.S. National Institute of Allergy and Infectious Diseases (NIAID) Division of AIDS.

The goal of P5 is to duplicate or improve on the results of the RV144 trial in Thailand, which demonstrated a significant 31.2% reduction in the risk of HIV acquisition being associated with receipt of a prime-boost vaccine regimen consisting of a canarypox vector encoding HIV antigens (vCP1521) and an HIV envelope protein (AIDSVAX). Notably, vaccine efficacy appeared to be higher after one year of follow up in RV144, at around 60%, but subsequently waned – this has led to the incorporation of additional booster immunizations in the design of HVTN 702. Versions of the vaccines used in RV144 have also been developed and tailored to the prevalent clade C HIV that is circulating in South Africa, where HVTN 702 will take place.

The trial will enroll a total of approximately 5,400 men and women between the ages of 18 and 35 years who are at risk for HIV infection at 15 sites in South Africa. Participants will be randomized to receive placebo immunizations or ALVAC vCP2438 plus a boost consisting of two clade C HIV gp120 proteins in MF59 adjuvant. The ALVAC vector is administered alone at baseline and after one month, and then in combination with the gp120 boost at months 3, 6, and 12 (in RV144, the final boost was at month 6).

The decision to begin HVTN 702 was based on results from an ongoing study in South Africa, HVTN 100, which tested the same vaccine regimen to establish whether it induced HIV-specific antibody and CD4+ T cell immune responses comparable or superior to those that were associated with protection from HIV infection in RV144. The specific criteria included:

• Prevalence of binding antibodies to clade C gp120 antigens in the vaccine that approach 90%
• Prevalence of V1V2 antibodies to clade gp70 scaffold antigens of >57% at week 28
• CD4+ T cell responses to HIV Env of ~60%

Based on the results of RV144, the above immune responses would predict vaccine efficacy of at least 50% at two years of follow up. [67] The data from HVTN 100 have not yet been presented publicly, but the announcement that HVTN 702 has been given the green light indicates that all of the immune response criteria were met.

In terms of anticipating results, three interim analyses of HVTN 702 are planned, two in 2018 and one in 2019, which will assess whether the trial should continue or be stopped early as a result of either evidence of efficacy or futility (a finding that, if the trial were to proceed, it would not be able show a difference between vaccines and placebo). If the trial proceeds, the final efficacy analysis will take place in 2020.

Although it is hoped that HVTN 702 may confirm and extend the results of RV144, there are also some reasons to be cautious about expectations. The study population in South Africa is at a higher risk for HIV infection than was the case for the Thai participants in RV144, and there is evidence in the original trial that higher HIV risk is associated with lower or absent vaccine efficacy.

In addition, background levels of immune activation and inflammation have been reported to be higher on the African continent than other geographic locales, [68, 69] and this is likely linked to higher rates of HIV transmission as a result of the virus’s predilection for targeting activated immune cells. In a microbicide trial conducted in South African women, genital tract inflammation was found to be a significant correlate of risk for HIV acquisition. [70, 71] It is not known whether inflammation or other factors (such as coinfections) could act as countervailing forces capable of undermining otherwise protective vaccine-induced immune responses, but this is one of the questions that HVTN 702 has the potential to address.

In addition to the P5 program, another large collaborative effort (involving the International AIDS Vaccine Initiative, the Beth Israel Deaconess Medical Center, the U.S. Military HIV Research Program, the Ragon Institute, NIAID, and Crucell Holland B.V., one of the Janssen Pharmaceutical Companies of Johnson & Johnson) is continuing to test various adenovirus and modified Vaccinia Ankara strain (MVA) vectors, as well as HIV Env protein boosts, with the goal of starting efficacy trials of selected vector/protein prime-boost combinations toward the end of this decade. [72, 73, 74, 75]

An array of other HIV vaccine candidates loiter in the waiting room of early-stage clinical testing, uncertain whether they’ll ever be called on to pass through the doorway to further development. Results from efficacy trials such as HVTN 702 should help to shed additional light on what types of immune responses are associated with protection against HIV infection, which in turn should help to identify which of the various vaccine concepts in the pipeline have promise.

Relatively few clinical trials of new experimental HIV vaccines have begun over the past year, but among them is a candidate that has been in the works for many years. The creation of George Lewis and colleagues at Robert Gallo’s Institute for Human Virology in Maryland, the vaccine is comprised of elements of both the HIV gp120 protein and the human CD4 protein. The goal is to generate immune responses to HIV antigens that are typically only briefly exposed as the virus binds to the CD4 protein on target cells, and the vaccine has shown some success in macaque challenge studies. [76] There have been concerns that including parts of CD4 could result in the induction of autoimmune responses, but this was not observed in a recently published macaque study [77], and a dose-escalation trial is now underway in humans.

The 2015 Pipeline Report mentioned plans to study a new oral, probiotic-based HIV vaccine developed by Jean-Marie Andrieu, which had shown a surprising degree of protective efficacy in the SIV/macaque model [78] and drawn considerable press coverage. [79] Unfortunately, an attempt to confirm the macaque results by the laboratory of Guido Silvestri at Emory University produced very disparate findings, with no evidence of protection documented. [80] Andrieu and colleagues believe that the use of macaques of Indian rather than Chinese origin may explain the divergent outcomes because of differing immune response genes, [81] but it seems likely that further research will be needed before the candidate can advance into clinical trials.

Conclusion

Several antiretroviral-based modalities, along with numerous active and passive immunization strategies, continue to make their way down the HIV biomedical prevention pipeline. Although there have been no registrational approval filings for any primary prevention products other than coformulated TDF/FTC anywhere in the world, immunologic, antiviral activity, PK/PD, safety, and efficacy data from clinical trials of new products continue to accumulate.

Testaments to this include entire abstract sessions dedicated to biomedical prevention at longstanding HIV congresses. In fact, the bulk of data reported in this year’s Pipeline Report chapter come from standing-room-only presentations at CROI in Boston earlier this year. There is also the biennial HIV Research for Prevention (HIVR4P) conference, the second of which will be held in Chicago in October.

However, with the advancement of several antiretroviral compounds, biologics, and vaccines, as well as new delivery technologies – compounded by the real-world scale up of TDF/FTC PrEP as a cornerstone of primary prevention services – a number of product registration challenges are becoming increasingly apparent.

One issue is how to incorporate PrEP into background standard-of-care options in vaccine and prevention-based immunotherapy clinical trials. In HVTN 702, for example, South African study participants will receive referrals to local programs where they may obtain TDF/FTC, as opposed to active provision of PrEP as a component of prevention services (e.g., free condoms and lubricant, counseling, and access to STI testing and treatment). This is similar to the standard-of-care approach being employed in the VRC01 AMP Study (HVTN 704/HPTN 085), although in that case U.S. participants have access to a specific referral program that allows their primary care provider to offer TDF/FTC PrEP free of charge.

It has been argued that TDF/FTC should be offered through these trials themselves. [82] This is, however, a difficult issue to wrestle with, as active provision of PrEP may substantially increase the person-years of follow-up required – and, with it, the study’s population size and expense – to reach the statistically sound number of seroconversion events needed for efficacy analyses. Indeed, the local Institutional Review Boards and both local and global Community Advisory Boards responsible for reviewing and approving both HVTN 702 and the AMP Study appear to have found the practice of referring participants to external sources of PrEP to be acceptable, at least at the current time.

A second related issue involves registrational trial methodologies that are necessarily rigorous in their design, yet feasible for the sponsors of new biomedical prevention candidates – a large number of which are not-for-profit programs that are dependent on finite public and philanthropic support. A major factor influencing study designs is the ethical principle of beneficence, which requires the abandonment of placebo comparisons and the inclusion of proven interventions, such as oral TDF/FTC, in control groups. Regulatory agencies, however, still want proof that an experimental PrEP regimen is more effective than placebo. This in turn requires reliable background incidence estimates, which have repeatedly proven to be difficult to come by in PrEP clinical trials. Also required are many person-years of follow up – and, by extension, extremely large, long, and expensive clinical trials – to yield the number of seroconversions necessary for standard non-inferiority comparisons, particularly with a highly efficacious regimen such as TDF/FTC. [83, 84]

Close attention to these issues, particularly as an increasing number of products enter phase II and III stages of development, is critical. A stringent, but amenable, regulatory climate is necessary to ensure the availability of necessary safety, efficacy, and acceptability data, without being prohibitively costly and ultimately deterring critical investments by product sponsors, particularly those heavily dependent on limited public and philanthropic funding.

Acknowledgments

The authors wish to thank Jeremiah Johnson for his research contributions and Simon Collins for his review of the antiretroviral section of this chapter.

References

1. Okano JT, Robbins D, Palk L, Gerstoft J, Obel N, Blower S. Testing the hypothesis that treatment can eliminate HIV: a nationwide, population-based study of the Danish HIV epidemic in men who have sex with men. Lancet Infect Dis. 2016 May 9. doi: 10.1016/S1473-3099(16)30022-6. [Epub ahead of print]
2. Jones A, Cremin I, Abdullah F, et al. Transformation of HIV from pandemic to low-endemic levels: a public health approach to combination prevention. Lancet. 2014 Jul 19;384(9939):272–9. doi: 10.1016/S0140-6736(13)62230-8.
3. Powers KA, Kretzschmar ME, Miller WC, Cohen MS. Impact of early-stage HIV transmission on treatment as prevention. Proc Natl Acad Sci U S A. 2014 Nov 11;111(45):15867–8. doi: 10.1073/pnas.1418496111.
4. Kretzchmar ME, Schim van der Loeff MF, Birrell PJ, De Angeleis D, Coutinho RA. Prospects of elimination on HIV with test-and-treat strategy. Proc Natil Acad Sci USA. 2013 Sep 24;110(39):15538–43. doi: 10.1073/pnas.1301801110.
5. Smith DK, Van Handel M, Wolitski RJ, et al. Vital signs: estimated percentages and numbers of adults with indications for preexposure propylaxis to prevent HIV acquisition – United States, 2015. MMWR Morb Mortal Wkly Rep. 2015 Nov 27;64(46):1291–5. doi: 10.15585/mmwr.mm6446a4.
6. Bush S, Magnuson D, Rawlings MK, Hawkins T, McCallister S, Mera Giler R. Racial characteristics of FTC/TDF for pre-exposure prophylaxis (PrEP) users in the US. Paper presented at: 2016 ASM Microbe; 2016 June 16–20; Boston, MA. Available from:
   http://www.asmmicrobe.org/images/OralAbstractsUpdated.pdf. (Access 2016 June 24)
7. Yaylali E, Farnham PG, Jacobson E, et al. Impact of improving HIV care and treatment and initiating PrEP in the U.S., 2015–2020 (Abstract 10151). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 2016 February 22–25; Boston, MA. Available from:
   http://www.croiconference.org/sessions/impact-improving-hiv-care-and-treatment-and-initiating-prep-us-2015-2020. (Accessed 2016 May 16)
8. European Centre for Disease Prevention and Control. ECDC meeting report. Pre-exposure prophylaxis in the EU/EEA: challenges and opportunities. Stockholm 27–28 April 2016. Solna, Sweden: European Centre for Disease Prevention and Control; 2016 June 2.
   http://www.eatg.org/gallery/174250/PrEP%20in%20the%20EU-EEA%20-%20challenges%20and%20opportunities.pdf. (Accessed 2016 June 7)
9. Fonner VA, Dalglish SL, Kennedy CE, et al. Effectiveness and safety of oral HIV pre-exposure prophylaxis (PrEP) for all populations: A systematic review and meta-analysis. AIDS. 2016 May 5. doi: 10.1097/QAD.0000000000001145. [Epub ahead of print]
10. The National Institute of Allergy and Infectious Diseases (NIAID) (Press Release). NIH Launches Large Clinical Trials of Antibody-Based HIV Prevention. 2016 April 7. Available from:
    https://www.niaid.nih.gov/news/newsreleases/2016/Pages/AMP-studies-launch.aspx. (Accessed 2016 May 2)
11. Jardine JG, Ota T, Sok D, et al. HIV-1 VACCINES. Priming a broadly neutralizing antibody response to HIV-1 using a germline-targeting immunogen. Science. 2015 Jul 10;349(6244):156–61. doi: 10.1126/science.aac5894.
12. The Scripps Research Institute (Press Release). New Findings in Humans Provide Encouraging Foundation for Upcoming AIDS Vaccine Clinical Trial. 2016 March 24. Available from:
    https://www.scripps.edu/news/press/2016/20160324schief.html. (Accessed 2016 May 2)
13. The National Institute of Allergy and Infectious Diseases (NIAID) (Press Release). Large-Scale HIV Vaccine Trial to Launch in South Africa. 2016 May 18. Available from:
    http://www.niaid.nih.gov/news/newsreleases/2016/Pages/HVTN702.aspx. (Accessed 2016 May 19)
14. Rerks-Ngarm S, Pitisuttithum P, Nitayaphan S, et al. Vaccination with ALVAC and AIDSVAX to prevent HIV-1 infection in Thailand. N Engl J Med. 2009 Dec 3;361(23):2209–20. doi: 10.1056/NEJMoa0908492.
15. Mugwanya K, Baeten J, Celum C, et al. Low risk of proximal tubular dysfunction associated with emtricitabine-tenofovir disoproxil fumarate pre-exposure prophylaxis in men and women. J Infect Dis. 2016 Mar 29. doi: 10.1093/infdis/jiw125. [Epub ahead of print]
16. Gandhi M, Glidden DV, Liu AY, et al. Higher cumulative TFV/FTC levels in PrEP associated with decline in renal function (Abstract 866). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/higher-cumulative-tfvftc-levels-prep-associated-decline-renal-function-0. (Accessed 2016 May 16)
17. Liu AY, Vittinghoff E, Anderson PL, et al. Changes in renal function associated with TDF/FTC PrEP use in the US Demo Project (Abstract 867). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/changes-renal-function-associated-tdfftc-prep-use-us-demo-project-0. (Accessed 2016 May 16)
18. Liu AY, Vittinghoff E, Sellmeyer DE, et al. Bone mineral density in HIV-negative men participating in a tenofovir pre-exposure prophylaxis randomized clinical trial in San Francisco. PLoS One. 2011;6(8):e23688. doi: 10.1371/journal.pone.0023688.
19. Mulligan K, Glidden DV, Anderson PL, et al. Preexposure Prophylaxis Initiative Study Team. Effects of Emtricitabine/Tenofovir on Bone Mineral Density in HIV-Negative Persons in a Randomized, Double-Blind, Placebo-Controlled Trial. Clin Infect Dis. 2015 Aug 15;61(4):572–80. doi: 10.1093/cid/civ324.
20. Grant R, Mulligan K, McMahan V, et al. Recovery of bone mineral density after stopping oral HIV preexposure prophylaxis (Abstract 48LB). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/recovery-bone-mineral-density-after-stopping-oral-hiv-preexposure-prophylaxis. (Accessed 2016 May 16)
21. Meng G, Wei X, Wu X, et al. Primary intestinal epithelial cells selectively transfer R5 HIV-1 to CCR5+ cells. Nat Med. 2002;8:150–6. doi: 10.1038/nm0202-150.
22. Moore JP, Kitchen SG, Pugach P, Zack JA. The CCR5 and CXCR4 coreceptors–central to understanding the transmission and pathogenesis of human immunodeficiency virus type 1 infection. AIDS Res Hum Retroviruses. 2004 Jan;20(1):111–26. doi: 10.1089/088922204322749567.
23. Gulick R, Wilkin TJ, Chen Y, et al. HPTN 069/ACTG 5305: phase II study of maraviroc-based regimens for HIV PrEP in MSM (Abstract 103). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/hptn-069actg-5305-phase-ii-study-maraviroc-based-regimens-hiv-prep-msm. (Accessed 2016 May 16)
24. McGowan I, Nikiforov A, Young A, et al. PrEP impact on T-cell activation and explant infection: HPTN 069/ACTG 5305 substudy (Abstract 104). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/prep-impact-t-cell-activation-and-explant-infection-hptn-069actg-5305-substudy. (Accessed 2016 May 16)
25. Hunt PW, Shulman NS, Hayes TL, et al. The immunologic effects of maraviroc intensification in treated HIV-infected individuals with incomplete CD4+ T-cell recovery: a randomized trial. Blood. 2013 Jun 6;121(23):4635–46. doi: 10.1182/blood-2012-06-436345.
26. Samri A, Mouilot G, Psomas C, et al. Maraviroc intensification in HIV-infected patients induces increased CCR5 expression on T cells (Abstract 309). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/maraviroc-intensification-hiv-infected-patients-induces-increased-ccr5-expression-t-cells. (Accessed 2016 May 16)
27. Markowitz M, Zolopa A, Squares K, et al. Phase I/II study of the pharmacokinetics, safety and antiretroviral activity of tenofovir alafenamide, a new prodrug of the HIV reverse transcriptase inhibitor tenofovir, in HIV-infected adults. J Antimicrob Chemother. 2014 May;69(5):1362–9. doi: 10.1093/jac/dkt53.
28. Ruane P, DeJesus E, Berger D, et al. Antiviral activity, safety, and pharmacokinetics/pharmacodynamics of tenofovir alafenamide as 10-day monotherapy in HIV-1-positive adults. J Acquir Immune Defic Syndr. 2013 Aug 1;63(4):449–55. doi: 10.1097/QAI.0b013e3182965d45.
29. Massud I, Mitchell J, Babusis D, et al. Chemoprophylaxis with oral FTC/TAF protects macaques from rectal SHIV infection (Abstract 107). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/chemoprophylaxis-oral-ftctaf-protects-macaques-rectal-shiv-infection. (Accessed 2016 May 16)
30. Garrett KL, Cottrell ML, Prince HM, et al. Concentrations of TFV and TFVdp in female mucosal tissues after a single dose of TAF (Abstract 102LB). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/concentrations-tfv-and-tfvdp-female-mucosal-tissues-after-single-dose-taf. (Accessed 2016 May 16)
31. Gunawardana M, Remedios-Chan M, Miller CS, et al. Pharmacokinetics of long-acting tenofovir alafenamide (GS-7340) subdermal implant for HIV prophylaxis. Antimicrob Agents Chemother. 2015 Jul;59(7):3913–9. doi: 10.1128/AAC.00656-15.
32. Schlesinger E, Johengen D, Johnson L, et al. A long-acting biodegradable subcutaneous implant for tenofovir alafenamide fumarate HIV PrEP (Abstract 879). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/long-acting-biodegradable-subcutaneous-implant-tenofovir-hiv-prep. (Accessed 2016 May 16)
33. Markowitz M, Frank I, Grant R, et al. ECLAIR: Phase 2A safety and PK study of cabotegravir LA in HIV-uninfected men (Abstract 106). Paper presented at: Conference on Retroviruses and Opportunistic Infections (CROI); 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/%C3%A9clair-phase-2a-safety-and-pk-study-cabotegravir-la-hiv-uninfected-men.
34. ClinicalTrials.gov [Internet]. Bethesda (MD): National Library of Medicine (U.S.). 2000. Identifier NCT02178800, Evaluating the safety, tolerability, and pharmacokinetics of an investigational, injectable HIV medicine (GSK1265744) in HIV-uninfected adults. 2014 June 27 (cited 2016 May 16). Available from:
    https://clinicaltrials.gov/ct2/show/NCT02178800.
35. ClinicalTrials.gov [Internet]. Bethesda (MD): National Library of Medicine (U.S.). 2000. Identifier NCT02720094, Safety and efficacy study of injectable cabotegravir compared to daily oral tenofovir disoproxil fumarate/emtricitabine (TDF/FTC), for pre-exposure prophylaxis in HIV-uninfected cisgender men and transgender women who have sex with men. 2016 March 21 (cited 2016 May 16). Available from:
    https://clinicaltrials.gov/ct2/show/NCT02720094.
36. Jackson AGA, Else LJ, Mesquita PMM, et al. A compartmental pharmacokinetic evaluation of long-acting rilpivirine in HIV-negative volunteers for pre-exposure prophylaxis. Clinical Pharmacology & Therapeutics. 2014 Sep;96(3):314–23. doi: 10.1038/clpt.2014.118.
37. McGowan I, Siegel A, Duffil K, et al. A Phase 1 Open Label Safety, Acceptability, Pharmacokinetic, and Pharmacodynamic Study of Intramuscular TMC278 LA (the MWRI-01 Study) (Abstract OA27.06 LB). Paper presented at: HIV Research for Prevention (HIV R4P); 28–31 October 2014; Cape Town, South Africa. Available from:
    http://online.liebertpub.com/doi/full/10.1089/aid.2014.5131a.abstract.
38. Dezzutti CS, Else LJ, Yandura SE, et al. Distinct pharmacodynamics anctivity of rilpivirine in mucosal explant tissue (Abstract 874). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/distinct-pharmacodynamic-activity-rilpivirine-mucosal-explant-tissue. (Accessed 2016 May 16)
39. ClinicalTrials.gov [Internet]. Bethesda (MD): National Library of Medicine (U.S.). 2000. Identifier NCT02165202, Phase II safety and acceptability of an investigational product, TMC278LA, for pre-exposure prophylaxis; 2014 May 21 (cited 2016 May 16).
    https://clinicaltrials.gov/ct2/show/NCT02165202.
40. McGowan I. The promise and challenge of sustained delivery of PrEP (Abstract 71). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 22–25 October 2016; Boston, MA. Available from:
    http://www.croiconference.org/sessions/promise-and-challenges-sustained-delivery-prep. (Accessed 2016 May 16)
41. Nel A, Kapiga S, Bekker LG, et al. Safety and efficacy of dapivirine vaginal ring for HIV-1 prevention in African women (Abstract 110LB). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections (CROI); 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/safety-and-efficacy-dapivirine-vaginal-ring-hiv-1-prevention-african-women. (Accessed 2016 May 16)
42. Baeten JM, Palanee-Phillips T, Brown ER, et al. A phase III trial of the dapivirine vaginal ring for HIV-1 prevention in women (Abstract 109LB). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections (CROI); 2016 February 22–26; Boston, MA. Available from:
    http://www.croiconference.org/sessions/phase-iii-trial-dapivirine-vaginal-ring-hiv-1-prevention-women. (Accessed 2016 May 16)
43. Baeten JM, Palanee-Phillips T, Brown ER, et al. Use of a Vaginal Ring Containing Dapivirine for HIV-1 Prevention in Women. N Engl J Med. 2016 Feb 22. doi: 10.1056/NEJMoa1506110 [Epub ahead of print]
44. Chen BA, Housley C, Salata A, et al. Safety and pharmacokinetics of dapivirine vaginal rings in postmenopausal US women (Abstract 872). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/safety-and-pharmacokinetics-dapivirine-vaginal-rings-postmenopausal-us-women. (Accessed 2016 May 16)
45. Van der Straten A, Laborde N, Cheng H, et al. Adherence and acceptability of dapivirine vaginal ring in postmenopausal US women (Abstract 873). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/adherence-and-acceptability-dapivirine-vaginal-ring-postmenopausal-us-women. (Accessed 2016 May 16)
46. Glaubius R, Penrose KJ, Hood G, Parikh UM, Abbas U. Dapivirine vaginal ring preexposure prophylaxis for HIV prevention in South Africa (Abstract 1057). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/dapivirine-vaginal-ring-preexposure-prophylaxis-hiv-prevention-south-africa. (Accessed 2016 May 16)
47. Smith J, Harris K, Garnett G, Van Damme L, Hallett T. Cost-effectiveness of the intravaginal dapivirine ring: a modeling analysis (Abstract 1058). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/cost-effectiveness-intravaginal-dapivirine-ring-modeling-analysis. (Accessed 2016 May 16)
48. Rees H, Delany-Moretlwe S, Lombard C, et al.FACTS 001 phase III trial of pericoital tenofovir 1% gel for HIV prevention in women (Abstract 26LB); 22nd Conference on Retroviruses and Opportunistic Infections; 2015 February 23–26; Seattle, WA. Available from:
    http://www.croiconference.org/sessions/facts-001-phase-iii-trial-pericoital-tenofovir-1-gel-hiv-prevention-women. (Accessed 2016 May 16)
49. Marrazzo JM, Ramjee G, Richardson BA, et al. Tenofovir-based preexposure prophylaxis for HIV infection among African women. N Engl J Med. 2015 Feb 5;372(6):509–18. doi:10.1056/NEJMoa1402269.
50. ClinicalTrials.gov [Internet]. Bethesda (MD): National Library of Medicine (U.S.). 2012. Identifier NCT01691768, Implementation effectiveness and safety of tenofovir gel provision through family planning services; 2012 July 5 (cited 2016 May 16).
    https://clinicaltrials.gov/ct2/show/NCT01691768.
51. Dezzutti CS, Rohan LC, Wang L, et al. Reformulated tenofovir gel for use as a dual compartment microbicide. J Antimicrob Chemother. 2012 Sep;67(9):2139–42. doi: 10.1093/jac/dks173.
52. Anton PA, Cranston RD, Kashuba A. RMP-02/MTN-006: a phase 1 rectal safety, acceptability, pharmacokinetic, and pharmacodynamics study of tenofovir 1% gel compared with oral tenofovir disoproxil fumarate. AIDS Res Hum Retroviruses. 2012 Nov;28(11):1412–21. doi: 10.1089/AID.2012.0262.
53. Cranston R, Lama J, Richardson BA, et al. MTN-017: rectal phase 2 extended safety and acceptability study of 1% tenofovir gel (Abstract 108LB). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/mtn-017-rectal-phase-2-extended-safety-and-acceptability-study-1-tenofovir-gel. (Accessed 2016 May 16)
54. Mcgowan I, Cranston RD, Duffill K, et al. A Phase 1 Randomized, Open Label, Rectal Safety, Acceptability, Pharmacokinetic, and Pharmacodynamic Study of Three Formulations of Tenofovir 1% Gel (the CHARM-01 Study). PLoS One. 2015 May 5;10(5):e0125363. doi: 10.1371/journal.pone.0125363.
55. Friedland BA, Hoosley C, Gehret Plagianos M, et al. A first-in-human trial of PC-1005 (MIV-150 and zinc acetate in a carrageenan gel (Abstract 875). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/first-human-trial-pc-1005-miv-150-and-zinc-acetate-carrageenan-gel. (Accessed 2016 May 16)
56. Villegas G, Zhang S, Mizenina O, et al. CVLs from women vaginally dosed with PC-1005 inhibit mucosal HIV-1 and HSV-2 ex vivo (Abstact 876). Paper presented at: 2016 Conference on Retroviruses and Opportunistic Infections; 2016 February 22–25; Boston, MA. Available from:
    http://www.croiconference.org/sessions/cvls-women-vaginally-dosed-pc-1005-inhibit-mucosal-hiv-1-and-hsv-2-ex-vivo. (Accessed 2016 May 16)
57. ClinicalTrials.gov [Internet]. Bethesda (MD): National Library of Medicine (U.S.). 2012. Identifier NCT02235662, Phase I one-month safety, PK, PD, and acceptability study of IVR release TFV and LNG or TFV alone; 2014 July 14 (cited 2016 May 16).
    https://clinicaltrials.gov/ct2/show/NCT02235662.
58. Burton DR, Hangartner L. Broadly Neutralizing Antibodies to HIV and Their Role in Vaccine Design. Annu Rev Immunol. 2016 May 20;34:635–59. doi: 10.1146/annurev-immunol-041015-055515.
59. Wu X, Yang ZY, Li Y, et al. Rational design of envelope identifies broadly neutralizing human monoclonal antibodies to HIV-1.Science. 2010 Aug 13;329(5993):856–61. doi: 10.1126/science.1187659. Epub 2010 Jul 8.
60. Ledgerwood JE, Coates EE, Yamshchikov G, et al. Safety, pharmacokinetics and neutralization of the broadly neutralizing HIV-1 human monoclonal antibody VRC01 in healthy adults. Clin Exp Immunol. 2015 Dec;182(3):289–301. doi: 10.1111/cei.12692. Epub 2015 Sep 24.
61. Barouch DH. Broadly neutralizing antibodies for HIV-1 eradication strategies (Abstract 67). Paper presented at: 2015 Conference on Retroviruses and Opportunistic Infections; 2015 February 23–26; Seattle, WA. Available from:
    http://www.croiwebcasts.org/console/player/25642?mediaType=audio&.
62. Xencor, Inc. (Press Release). National Institutes of Health Initiates Phase 1 Trial of VRC01LS Anti-HIV Therapeutic Antibody Using Xencor’s Xtend Fc Technology. 2016 January 7. Available from:
    http://www.prnewswire.com/news-releases/national-institutes-of-health-initiates-phase-1-trial-of-vrc01ls-anti-hiv-therapeutic-antibody-using-xencors-xtend-fc-technology-300200802.html. (Accessed 2016 May 2)
63. Mascola J. Overview of HIV Antibodies for Prevention and Treatment. Presented at: Cent Gardes HIV Vaccines Conference; Veyrier-du-Lac, France; 2015 October 25–27. Available from:
    http://webcasts.vaccineenterprise.org/console/player/29041?mediaType=audio&.
64. Brown J, Excler JL, Kim JH. New prospects for a preventive HIV-1 vaccine. J Virus Erad. 2015 Apr;1(2):78–88.
65. Saunders KO, Wang L, Joyce MG, et al. Broadly Neutralizing Human Immunodeficiency Virus Type 1 Antibody Gene Transfer Protects Nonhuman Primates from Mucosal Simian-Human Immunodeficiency Virus Infection. J Virol. 2015 Aug;89(16):8334–45. doi: 10.1128/JVI.00908-15.
66. Graham G. Passive Immunization for Prevention and Treatment of HIV. Presented at: Cent Gardes HIV Vaccines Conference; Veyrier-du-Lac, France; 2015 October 25–27. Available from:
    http://webcasts.vaccineenterprise.org/console/player/29042?mediaType=audio&.
67. Gray G. HIV vaccines trials: Advancing into Efficacy. Presented at: Cent Gardes HIV Vaccines Conference; Veyrier-du-Lac, France; 2015 October 25–27. Available from:
    http://webcasts.vaccineenterprise.org/console/player/29051?mediaType=audio&.
68. Clerici M, Butto S, Lukwiya M, et al. Immune activation in Africa is environmentally-driven and is associated with upregulation of CCR5. Italian-Ugandan AIDS Project. AIDS. 2000 Sep 29;14(14):2083–92.
69. Cohen CR, Moscicki AB, Scott ME, et al. Increased levels of immune activation in the genital tract of healthy young women from sub-Saharan Africa. AIDS. 2010 Aug 24;24(13):2069–74. doi: 10.1097/QAD.0b013e32833c323b.
70. Masson L, Passmore JA, Liebenberg LJ, et al. Genital inflammation and the risk of HIV acquisition in women. Clin Infect Dis. 2015 Jul 15;61(2):260–9. doi: 10.1093/cid/civ298.
71. Passmore JA. Genital mucosal inflammation and HIV prevention: barriers to protection. Presented at: Cent Gardes HIV Vaccines Conference; Veyrier-du-Lac, France; 2015 October 25–27. Available from:
    http://webcasts.vaccineenterprise.org/console/player/29049?mediaType=audio&.
72. Baden LR, Karita E, Mutua G, et al. Assessment of the Safety and Immunogenicity of 2 Novel Vaccine Platforms for HIV-1 Prevention: A Randomized Trial. Ann Intern Med. 2016 Mar 1;164(5):313–22. doi: 10.7326/M15-0880.
73. Barouch DH, Alter G, Broge T, et al. Protective efficacy of adenovirus/protein vaccines against SIV challenges in rhesus monkeys. Science. 2015 Jul 17;349(6245):320–4. doi: 10.1126/science.aab3886.
74. Johnson & Johnson (Press Release). Investigational HIV Vaccine Regimen Shows Encouraging Results in Non-Human Primates. 2015 July 2. Available from:
    http://www.jnj.com/news/all/Investigational-HIV-Vaccine-Regimen-Shows-Encouraging-Results-in-Non-Human-Primates. (Accessed 2016 May 2)
75. Tomaka FL. HIV Prophylactic Vaccine Overview Development Program. Presented at: AVAC Vaccines in Vivo: Advances in AIDS Vaccine Research Webinar. 2015 May 18. Available from:
    http://www.avac.org/event/vaccines-vivo-advances-aids-vaccine-research.
76. Fouts TR, Bagley K, Prado IJ, et al. Balance of cellular and humoral immunity determines the level of protection by HIV vaccines in rhesus macaque models of HIV infection. Proc Natl Acad Sci U S A. 2015 Mar 3;112(9):E992–9. doi: 10.1073/pnas.1423669112.
77. Schwartz JA, Prado I, Misamore J, et al. An HIV gp120-CD4 Immunogen Does Not Elicit Autoimmune Antibody Responses in Cynomolgus Macaques. Clin Vaccine Immunol. 2016 May 18. doi:10.1128/CVI.00115-16. [Epub ahead of print]
78. Lu W, Chen S, Lai C, Guo W, Fu L, Andrieu JM. Induction of CD8+ regulatory T cells protects macaques against SIV challenge. Cell Rep. 2012 Dec 27;2(6):1736–46. doi: 10.1016/j.celrep.2012.11.016.
79. Nguyen TC. “Researcher has a radical idea for a drinkable, probiotic HIV vaccine.” Washington Post. 2014 September 10. Available from:
    https://www.washingtonpost.com/news/innovations/wp/2014/09/10/researcher-has-a-radical-idea-for-a-drinkable-probiotic-hiv-vaccine/. (Accessed 2016 May 2)
80. Silvestri G. Inactivated AT-2 treated SIVmac239 and Lactobacillus plantarum (LP) as a candidate AIDS vaccine. Presented at: Cent Gardes HIV Vaccines Conference; Veyrier-du-Lac, France; 2015 October 25–27. Available from:
    http://webcasts.vaccineenterprise.org/console/player/29037?mediaType=audio&.
81. Lu W, Chen S, Lai C, et al. Suppression of HIV Replication by CD8(+) Regulatory T-Cells in Elite Controllers. Front Immunol. 2016 Apr 18;7:134. doi: 10.3389/fimmu.2016.00134. eCollection 2016.
82. Cowan EA, Macklin R. Is preexposure prophylaxis ready for prime time use in HIV prevention research? AIDS. 2014 Jan 28;28(3):293–5. doi: 10.1097/QAD.0000000000000055.
83. Dunn DT, Glidden DV. Statistical issues in trials of preexposure prophylaxis. Curr Opin HIV AIDS. 2016 Jan;11(1):116–21. doi: 10.1097/COH.0000000000000218.
84. Donnell D, Hughes JP, Wang L, Chen YQ, Fleming TR. Study design considerations for evaluating efficacy of systemic preexposure prophylaxis interventions. J Acquir Immune Defic Syndr. 2013 Jul;63 Suppl 2:S130–4. doi: 10.1097/QAI.0b013e3182986fac.