Wet and Wild
Microbicides are the wave of the sexual future.
by Vicki Burkitt

How can we stop sexual transmission of HIV? For decades we've promoted latex condoms and safer-sex messages, an imperfect combination that has left many pining for an alternative, especially women. With the development of microbicides–new agents that aim to kill or block HIV and other sexually transmitted diseases from the word go, that day is now closer.

At the inaugural Microbicides 2000 Conference in Alexandria, Virginia, more than 600 scientists, activists, and policy makers came together to discuss how microbicides (natural or chemical substances used in the vagina or rectum) can prevent HIV and other STDs, pregnancy, and perhaps one day allow HIV-positive couples to bear HIV-negative children.

"It has been at least 10 long years since we first began to talk about women's vulnerability to HIV, the barriers to condom use, and the urgent need for a prevention technology that women can control," says Geeta Rao Gupta, president of the International Center for Research on Women, a leader in the field. "Now there is finally a light at the end of the tunnel."

The momentum around microbicides stems from the devastating incidence of HIV infection in developing countries, where the virus is mainly transmitted through heterosexual contact. Of the estimated 33.6 million people living with HIV and AIDS (not to mention the 16.3 million who have already died), 23.3 million come from sub-Saharan Africa, and 55 percent are adult women. Another 6 million HIV-positive people live in South and Southeast Asia, nearly a third of them women. With more than 16,000 new infections every day, half of these occur in young people between 15 and 24 years old. In hard-hit countries like South Africa, for example, teenage girls are disproportionately affected compared with boys.

These numbers have risen steadily, due largely to the fact that many women lack access to condoms or are unable to negotiate their use with male partners. While we wait for a vaccine for HIV, there is a growing global demand for a cheap, easy-to-use microbicide. "We just can't afford to wait until the day we have a vaccine," says Peter Piot, executive director of the Joint United Nations Program on HIV/AIDS, emphasizing the importance of dialogue with politicians from affected countries. "Scientists will develop the best tools and the best interventions," he says, "[but] if the political will is not there, then we can forget it. It won't be applied."

The microbicide field has moved slowly but steadily, starting in 1990, when the Today sponge was developed. The first generation of products tested often contained the spermicide nonoxynol-9. Today at least 36 microbicides are in laboratory development, 20 in human safety studies and four in larger efficacy clinical trials. The mix includes some innovative, low-cost products.

Just as condoms revolutionized sexual behavior, so may microbicides. Ever thought of using rice plants to fight HIV? Seaweed? Read on.

Leading Contenders
First off the block is BufferGel, a spermicide and microbicide developed by ReProtect LLC, a Maryland biotechnology company. It stops sperm in its tracks by inactivating it; the sperm stops moving and can't fertilize the ovum. In early animal studies using mice, BufferGel blocked sexual transmission of herpes (HSV-2), and to a lesser extent, chlamydia. It also prevented papillomavirus infection in rabbits. BufferGel manages to do this by maintaining the natural acidic pH of the vagina while making the naturally alkaline semen more acidic. Human Phase I safety trials have been performed in the United States, Malawi, Zimbabwe, India and Thailand, where a total of 125 women were tested (there were no control groups in the trial). So far, BufferGel appears to be safe and nontoxic to the vaginal epithelium, the outer layer of the vaginal wall. Seventy percent of the trial participants said they would use the product if it became available.

Another contender is a weird, small protein called cyanovirin, which is isolated from a blue-green algae extract. At low concentrations, cyanovirin glues to specific sugars on diverse strains of HIV and doesn't let go, blocking cell fusion-entry of the virus into the host cell and its genes. Since fusion is an early step in HIV's life cycle, this approach is especially promising. Laboratory studies show that cyanovirin is stable, so it won't fall apart, or worse, mutate, and it doesn't harm host cells. "We've tried all the conventional ways to induce the emergence of a cynanovirin-resistant strain [of HIV]," says Dr. Michael Boyd, director of the Laboratory of Drug Discovery, Research and Development at the National Cancer Institute, which isolated the protein. "We haven't been able to do it, nor has any other lab."

Boyd believes cyanovirin could be formulated as a microbicide gel, or genetically engineered into friendly bacteria that normally live in our bodies, and implanted in the vagina. There, the introduced bacterial strain would grow happily, producing cyanovirin at concentrations that would hopefully inactivate HIV and help the body resist common problems like yeast infections caused by Candida.

Boyd's colleague at the University of Siena Medical School, Dr. Gianni Pozzi, a bacterial geneticist, is attempting just that. He started with a streptococcus bacteria commonly found in the mouth and engineered it to produce a candida-secreting toxin. In studies using mice, the modified bacteria prevented vaginal candida infection in three-fourths of the animals tested, and it thrived in the vagina. Says Pozzi, "We can treat vaginal candidiasis with [engineered] bacteria that express a candidacidal [toxic] molecule." Since vaginal yeast infections are often harder to treat in women with HIV, this is also good news.

Pozzi is also working to incorporate cyanovirin into human lactobacillus bacteria–the "good" bacteria found in the vagina. In a healthy individual, lactobacillus helps to maintain the natural balance of organisms found in the gut and colon, and in women, the vagina. If this novel approach works, he adds, "You will have the normal bacterium that is in every healthy vagina, [and it] will produce a microbicide."

If you think that's cosmic, how about genetically engineering rice to create human antibodies–or plantibodies? It's possible, but how? "We shoot beads that are coated with DNA [into rice cells]," explains Kristen Briggs, project leader for Epicyte Pharmaceutical in San Diego. "The gold particles burst the cell wall, and get into the cell, and a fraction of these cells take up the DNA into the chromosomes. So it's incorporated into the plant genome." That's easier said than done, since the modified cells need to be identified and cultured until they become rice plants. The plants then go out into a greenhouse where they cross-pollinate, seed is collected, and the plantibodies extracted.

Briggs's group has generated rice plants that produce human IgG or IgA, or plantibodies against the human herpes virus, HSV-2 (see Glossary). The antibodies block the virus in cell-culture experiments and prevent vaginal HSV-2 infection in mice. The California team also plans to determine which plantibodies are active in the vagina and rectum of monkeys, and in human secretions. Up to now, a big drawback to using human antibodies has been their high cost and low scale, which is why Briggs' rice plants are so appealing. "Plants offer about the only way that you can produce [antibodies] at the scale and the cost you need to make them available," she says.

Briggs hopes that plantibodies can be commercially harvested from rice fields, subject to FDA approval, of course. But what she really wants is a mix-and-match, multiantibody microbicide that will provide broad protection against the major STDs–including HIV–that could be used with or without a contraceptive.

Obstacles ahead
Despite the sci-fi feel to these approaches, the conference also highlighted how little we know about basic female and male reproductive biology. What constitutes a healthy vagina or rectum? Which markers, or signs, indicate infection, inflammation, or that an immune response is taking place? How does that differ from what we may see in blood? Do microbides harm the "good" bugs like lactobacillus that normally live in these places, or do they encourage "baddies" like candida to thrive? Only with this information can scientists assess which microbicides are safe for human trials.

Several researchers are also taking a closer look at the relationship between hormone levels and HIV. In women, estrogen levels fluctuate during the menstrual cycle, affecting the thickness or thinness of the vaginal wall, which is made up of various layers of cells. Less estrogen means a thinner wall. But what does that mean for HIV infection and transmission? What about post-menopausal women who have less estrogen than fertile women? Are they more susceptible? And how do women compare with men? Turning to the male reproductive system, more questions emerge: What about circumcised men versus uncircumcised men? Some African studies suggest that circumcision decreases the risk of exposure to HIV, while studies in Europe and the United states have provided no clear answers so far.

Rectal microbicides
The big buzz among insiders at the conference was that rectal, or anal, microbicides were finally on the agenda after being relegated to the "too-hard-to-sell" basket for so long. Although anal sex is practiced by heterosexuals and often serves as an alternative means of birth control, it carries the moral and social stigma that is still attached to homosexuality around the world. But as Alex Carballo-Dieguez, Ph.D. from the HIV Center for Clinical and Behavioral Studies in New York says, "We as scientists have the moral obligation to be sex affirmative." Since HIV is still singling out gay and bisexual men, there is also a public health imperative. Carballo-Dieguez emphasizes that microbicides should be tested in parallel–in the rectum and vagina. He also says there is a need to debunk several myths about the public demand for rectal microbicides.

Myth number one: Rectal microbicides are only of use to gay men. Michael Gross, senior scientist at the Statistical Center for HIV/AIDS Research and Prevention in Seattle, Washington, studies the acceptability of products for vaccine and microbicide trials and revealed that more women have anal sex than previously believed. In one study of 1,300 women at high risk for HIV infection, 32 percent reported that they had anal sex with their partner at least once. What's more, a subset of women were nine times more likely to reveal this information to a computer than face to face to a live person.

That comes as no surprise to Bethany Young Holt, an epidemiologist at the University of California at Berkeley's Center for Family and Community Health. She recently interviewed 22 heterosexual female and four male college students for a preliminary microbicide market study. The women, who rarely used condoms, talked about anal sex. "It happens a lot among heterosexual college students," says Holt, laughing. "These women weren't shy about it at all."

Myth number two: Gay men won't use rectal microbicides. Gross interviewed 35 monogamous white gay male couples enrolled in a Phase I safety study of nonoxynol-9 as a rectal microbicide. He found that 46 percent of the men said they would use the N-9 gel during anal sex if their partner was HIV-positive. That number was likely to go up if they had other sexual partners or don't know a partner's serostatus.

"Others feel that if only 50 percent would use a product, that would be pathetic," says Gross. "I would say if you took a population of men who weren't at obvious risk and close to 60 percent were using it, that is good news."

Actually, that might not be true for nonoxynol-9. Although N-9 is used in many spermicide creams, gels, and lubricants, its efficacy against HIV hasn't been proved. In some studies of women, very high doses of N-9 caused minor vaginal lesions that could increase exposure to HIV. In another study involving HIV–negative and–postive monogamous gay male couples, Connie Celum of the University of Washington studied the safety of N-9 in lubricants, which comprise 75 percent of the lube market.

During a 10-week period between 1995 and 1996, the couples agreed to wear condoms and remain "tops" (insertive sex) or "bottoms" (receptive sex). The N-9 lube product was inserted via applicator into the rectum. The result: "Bottoms" experienced rectal bloating with lube use, while "tops" experienced increased penile irritation, which resolved with time. Rectal biopsies of "bottoms" revealed that even before N-9 use, they had mild inflammation that increased with lubricant application. So anal sex might contribute to this effect. Another small study of four men by David Phillips at the Population Council in New York raises more alarm bells.

Here, lubricants containing N-9 were compared with lube containing PC515, a seaweed-derived product made of carageenan that blocks herpes infections, and with lube containing methylcellulose, an inactive thickener used in vaginal products. After 15 minutes, the N-9 lubricants had stripped sheets of epithelial cells from the rectum and exposed underlying tissue; the other products had not. Although the damage was repaired 10 hours later, these results worry Phillips, who plans to study more people. "There could be an increase in HIV transmission [during rectal sex]," he says of N-9-containing lubes.

Global challenges
Where do we go from here? Again, there are more questions than answers. What products should we push forward when research funds are limited? How will clinical trials be carried out–and where? If we enroll HIV-positive couples from developing countries, how are we going to support them? How do we ensure that everyone has access to microbicides? Who will be the financial backers and policy makers to make this possible?

"If we wait for the best product, we're not going to get there," says Helen Rees, director of the Reproductive Health Research Unit at the Chris Hani Baragwanath Hospital in Johannesburg, South Africa, and co-chair of the conference. "Sometimes the best is the enemy of the good." In her opinion, one echoed by many participants, it's better to move forward with various promising approaches than wait for a magic bullet. We have done that with HIV drugs and vaccines. Why not microbicides?

Vicki Burkitt is a freelance writer who reported on HIV evolution in our February/March issue.