New generation of vaginal microbicides

The first generation of topical microbicides to be developed are designed to be applied to the vagina or rectum to prevent HIV transmission and are based on ”barrier“ methods that prevent HIV from gaining entry into cells. However, a newer generation of microbicides will make use of more sophisticated techniques to act directly on the virus or its surrounding cells and thus prevent infection.

The most advanced products
There are currently over thirty products under study. The most advanced products are first generation candidates that are based on a range of mechanisms:

• BufferGelTM is an acidic compound that would eventually act as a barrier to prevent entry of HIV into cells by mimicking the natural protection mechanisms found in the vagina;

• Pro2000, Carraguard® and cellulose sulphate are inhibitors that would prevent the virus which has entered the body from entering its target cells, thus preventing replication;

• SavvyTM is a surfactant that would destroy the virus before it comes into contact with human cells.

These products are being tested on HIV negative women in Phase IIb or Phase III trials, and the first reliable indicators based on efficacy testing results are expected to be available in 2007-2008.

Still some way to go
The 13th Conference on Retroviruses and Opportunistic Infections (CROI) held in Denver, U.S.A. last February reviewed and gave an update on other candidate microbicides which, in spite of being in the earlier phases of testing, will soon be in the research pipeline. John Moore of Cornell University Medical College in New York gave a plenary session review on ”Preventing HIV Transmission by Topical Microbicides,“ with a special emphasis on antiretroviral-based compounds.

SiRNA molecules to act on the cell
Efficacy testing results of UL29.2 were recently published in the journal Nature. Known as siRNA, this small molecule of interference RNA is capable of blocking the replication of the herpes simplex 2 virus (HSV2) when applied to the vagina of mice. RNA interference is an effective defence mechanism observed in both plants and invertebrates. A number of studies have assessed possible ways of using it to inhibit viral infections; in one of these studies the combination of siRNA with lipids was able to silence genes UL27 and UL29 in HSV-2 when applied to the vagina and the ectocervix (the outer part of the neck of the uterus close to the vagina) in female mice. This prevented HSV-2 infection in mice when they were exposed to the virus two hours after the application of the gel, and also halted progress of the infection in cases in which it occurred.

An interesting aspect is that this mechanism may offer lasting protection, since gene silencing has been observed to last up to nine days locally, although further studies are required to confirm this. More importantly, siRNA may also affect HIV gene expression in various positions. Therefore, this technology would be used for blocking several targets simultaneously.

Antiretrovirals to act on the virus or the cell
Topical microbicide gels containing reverse transcriptase inhibitors are also currently being tested. Tibotec’s candidate TMC-120, Biosyn’s UC-781 or Gilead’s tenofovir are believed to be safe, low-cost strategies that could easily be formulated in gel form. In spite of existing efficacy testing in non-human simian models, only a small number of trials have been carried out thus far, with tenofovir being the sole candidate that worked in this context. Moreover, existing models intended for topical testing of this kind of drug in simians exposed to SHIV (a simian virus modified to mimic HIV infection) need to be substantially improved to make them appropriate.

Several drugs that are capable of inhibiting HIV from entering or binding to human cells are currently being studied for their potential use as microbicides. For instance, PSCRANTES is a chemically modified chemokine that since 2004 has shown evidence of being effective in protecting macaque Rhesus monkeys from acquiring HIV through coreceptor CCR5. However, Moore pointed out that large amounts of the product were needed to achieve protection in the macaque models, which may result in a high market price.

Another candidate, CMPD 167, is a CCR5 inhibitor designed by the Merck pharmaceutical company and whose development in human models was halted, it originally being intended for use as an antiretroviral drug. CMPD 167 is currently being tested for topical use as a microbicide; it is believed to block T cell coreceptors and prevent the cell from attaching to the virus through a similar mechanism to that found with CCR5 inhibitors currently under development, namely maraviroc and vicriviroc. However, this candidate would not be effective in the case of exposure to viruses that use coreceptor CXCR4 to gain entry into the cell. Conversely, Bristol Myers Squibb’s BMS-378806, a candidate based on a similar concept, is designed to protect from viruses that use either CCR5 or CXCR4. BMS378806 works by binding to the gen gp120 found on the HIV cell coating and prevents it from binding to CD4. This would prevent the gene mutation that generally allows it to bind to CCR5 and CXCR4 co-receptors and enables the fusion of the virus with the cell. C52L is a peptide that is believed to inhibit virus-cell fusion in the same way. The candidate is similar to T-20, an antiretroviral of the fusion inhibitor class, and would act directly on gp41, another virus coating gene that enables contact with the cell even prior to gp120 intervention. It is also believed to work with CCR5 and CXCR4.

Although each of the three candidates were shown to prevent HIV infection in non-human simian models vaginally exposed to an SHIV version, the degree of protection was higher when used in a combined way (Veasey et al., 2005. Nature 438, 99-102): 21 out of 28 (75%) animals that received only one inhibitor were protected, in comparison with 16 out of 20 (80%) animals that received two inhibitors and all 3 (100%) animals that received the three compounds. The study revealed that all 9 animals that received a placebo gel were infected. Researchers aimed to evaluate the duration of protection and hence administered a triple combination at high daily doses over five days, while also exposing the animals daily to SHIV during the same five-day period and thirty minutes after the application of the product. The 10 animals of the control group were infected while 3 out of 5 animals (60%) were infected in the group that received microbicides.

It is likely that in the case of inhibitors, more than one product will be needed to achieve full efficacy, as is the case with combination therapies for HIV. Moreover, this would enable the microbicide to work on more strains or to create synergies that would avoid a number of undesired effects or even reduce the chances of transmission of single inhibitor resistant viruses. However, the appropriate approval and classification procedures to be adopted by regulatory agencies for this kind of product have yet to be identified. Nevertheless, although it would not make sense to market each of these microbicides separately only to test their effectiveness once combined, agencies should not delay approval of this strategy in order to begin a trial period.

Meanwhile, in the laboratory
Live bacteria or host bacteria to produce inhibitors

Host bacteria are the microflora commonly found within the body. Although little is known about this strategy the aim would be to find a way of genetically engineering these bacteria so that they can colonize the vagina and produce proteins and peptides capable of blocking HIV infection inside the genital tract, thereby offering lasting protection without affecting the vaginal environment. This promising concept nevertheless raises a number of concerns, since it would require the vagina to host colonies of live bacteria.

Marion Zibelli

References
”Preventing HIV Transmission by Topical Microbicides“, John Moore, Well Medical College, Cornell University, New York, U.S.A. (presentation is available in English at

www.retroconference.org/2006/data/files/retro2006_frameset.htm)

An siRNA-based microbicide protects mice from lethal herpes simplex virus 2 infection. Deborah Palliser et al. Nature 493, 89-94

EATN - European AIDS Treatment News, Volume 15, I – Spring 2006

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