In animal studies, microbicide gels and oral pre-exposure prophylaxis with antiretroviral drugs appear to provide protection against anal exposure to the monkey equivalent of HIV, according to two reports published in August.
However the studies show that two drugs are probably better than one, that patchy adherence greatly increases the risk of infection, and that even where pre-exposure prophylaxis (PrEP) does not prevent infection, it might result in substantially slower progression of HIV disease due to a blunting of the viral load peak that occurs soon after infection.
Full details of two animal-model studies of a microbicide gel (Cranage et al) and of oral PrEP (Garcia-Lerma et al) to prevent HIV infection via rectal exposure have been published in the journal PLoS Medicine.
Preliminary results from both studies were presented at the CROI Conference in Los Angeles in 2007 – see this report for the trial methodologies and results. These rectal results are also complemented by the results from a monkey study using tenofovir and FTC as a vaginal microbicide, presented recently at the Mexico World AIDS Conference.
Microbicide gels are now being developed containing antiretroviral drugs such as tenofovir that can be used in vaginal intercourse, and a large international trial is due to start shortly comparing a vaginal microbicide containing tenofovir with oral PrEP using either tenofovir or a combination of tenofovir and FTC (emtricitabine) (Truvada).
PrEP – the use of antiretroviral drugs to prevent HIV infection – is already being tested in five large international studies, including two in men who have sex with men, to determine how effectively it protects against anal or vaginal infection with HIV. Studies in men who have sex with men are comparing either tenofovir or Truvada with a placebo, and are due to report in 2009 and 2010 respectively.
However, there are still many unanswered questions about these two prevention technologies, and data from animal studies may provide important information that can help interpret the results of these studies, and design future studies too.
One area of uncertainty is whether tenofovir alone is potent enough to protect against infection. It is also unclear whether a topical microbicide containing an antiretroviral drug will be enough to protect against exposure, or whether a drug needs to be distributed throughout the body in order to prevent infection.
There are also questions about the degree of adherence that will be needed to PrEP, and the blood levels that need to be achieved in order to provide protection.
And if PrEP or an antiretroviral-containing microbicide does not protect against infection, does it have any effect on disease progression?
The published results of the two rectal studies, taken together, provide some tentative answers to these questions, although it should be noted that these are animal studies.
Single-drug microbicide or PrEP regimens may not be potent enough in the long term. The efficacy of a tenofovir gel in stopping a single rectal exposure to Simian Immunodeficiency Virus (SIV) was about 66%; the efficacy of oral prophylaxis using FTC (emtricitabine) to repeated exposure was about 33%. Using tenofovir and FTC together, however, substantially improved efficacy.
The amount of drug absorbed by the tissues and present in the plasma is crucial. In the case of the microbicide study no monkey with a total blood plasma concentration of tenofovir greater than 0.11% of the total drug dose administered became infected; every animal with a plasma concentration below this did. In the case of the PrEP study, oral dosing with the equivalent drug dose to a Truvada pill protected four out of six monkeys: increasing the tenofovir dose to 3-4 times the oral equivalent and injecting it subcutaneously protected six out of six monkeys, though this would probably be too toxic for humans. (Injecting tenofovir and FTC subcutaneously actually increased drug exposure very little over and above what would result from oral dosing; it was done for convenience reasons).
The ‘adherence’ of the monkeys to oral doses of tenofovir was good, ranging from one to seven doses missed (86%-98% adherence – the drugs were sprinkled on fruit). However both the monkeys that were infected had lower-than-average adherence; they only missed one dose each prior to infection, but missed a number of doses afterwards, which could have contributed to infection becoming established.
Both studies found that the majority of the animals that became infected with SIV despite receiving the microbicide or PrEP had a modified course of infection, developing substantially lower viral loads. The PrEP study found that infected animals exposed to PrEP had a peak viral level two logs (100-fold) lower than in infected animals in a control group. The microbicide study found that, out of three animals infected, one also had a viral load about two logs lower than average, which decreased spontaneously to undetectability; the other eventually developed a normal viral load, but after a substantial delay compared with control animals. This could translate into substantially slower progression to AIDS.
Related to this, the microbicide study found that the majority of the ‘exposed uninfected’ animals had immune responses to HIV in their T-cells (CD4 and CD8 cells). Cells in the blood (in three out of six animals and also in the one animal that remained uninfected on placebo) had significant immune responses to part of the viral shell, the Gag protein.
This presents opportunities, say the researchers, for using a combination microbicide and vaccination strategy that could combine antiretroviral drugs with immune enhancers to provoke such an immune response.
In contrast three out of four uninfected animals whose intestinal immune cells could be biopsied had immune responses to a whole range of SIV proteins. This indicates, say the researchers, that whole virus is being scooped up by dendritic cells in the gut but not infecting them; active infection does not become established until these cells then present virus to T-cells in the lymph nodes. This means that a topical microbicide has a narrow time window in which to work, as infecting virus gets ‘sequestered’ away rapidly into the bloodstream; certainly, in the microbicide study, the efficacy of microbicide given two hours after exposure was not greater than placebo.
The potential for oral PrEP to create HIV drug resistance is high. In the PrEP study, in which infected animals continued to receive antiretrovirals, two out of six infected monkeys developed the M184V FTC resistance mutation, even though one of them was also taking tenofovir, and they were the only two animals not to achieve an undetectable viral load.
In contrast the average peak blood plasma tenofovir levels seen in monkeys given the rectal microbicide was 116 nanograms (billionths of a gram) per millilitre (ng/ml) and the highest level was 181 ng/ml. This average level is nearly eight times the blood tenofovir levels seen in the vaginal study presented at the recent International AIDS Conference in Mexico, but even the highest reading is about 14 times lower than the 2500-7000 ng/ml of tenofovir needed to inhibit 50% of wild-type virus and unlikely to inhibit HIV replication enough to generate resistance.
Intermittent dosing of PrEP, around the time of sexual exposure, may be effective. This finding was not presented at CROI 2007 because it was an experiment conducted subsequent to the other dosing strategies in the Garcia-Lerma study. They gave six monkeys subcutaneous doses of FTC and high-dose tenofovir two hours before and 24 hours after exposure to SIV. No monkeys were infected after 14 weeks exposure. While this obviously mimics ‘weekend dosing’, it’s important to note that the dose of tenofovir used was too high for human use and also that two of the monkeys in this arm of the study had already received PrEP before and been uninfected and so could possibly (if the Cranage studies apply to PrEP too) have a degree of protective immune response to SIV.
In an accompanying editorial on the Cranage microbicide study (Hladik and Dezzutti), the commentators note that animal studies have not in the past been able to predict the failure of microbicides in large efficacy studies in humans. However, the results were encouraging.
They comment that further rectal microbicide animal-model studies should be conducted to measure efficacy against repeated exposure, both to virus and to the microbicide; that exposed uninfected animals who have developed T-cell responses to SIV should be rechallenged with SIV to find out if these responses remain protective; and that different combinations of antiretrovirals in microbicides should be tried, including possibly new agents such as CCR5 inhibitors, and their minimum effective concentrations carefully determined.
Cranage M et al. Prevention of SIV rectal transmission and priming of T cell responses in macaques after local pre-exposure application of tenofovir gel. PloS Medicine 5(8), online publication e 157. doi: 10.1371/journal.pmed.0050157. 2008. Read the study here.
Garcia-Lerma JG et al. Prevention of rectal SHIV transmission on macaques by daily or intermittent prophylaxis with emtricitabine and tenofovir. PloS Medicine 5(2):291-299. 2008. Read the study here.
Hladik F, Dezzutti CS. Can a topical microbicide prevent rectal HIV transmission? PloS medicine 5(8):1-3. Read this editorial here.