Is This Enema The Next PrEP?

For most people, getting an enema may not be the highlight of the week, and the use of enemas  before or after anal sex has been linked with higher risks of getting sexually transmitted infections, including HIV. But a new study may change all of that and transform enemas into part of a well-balanced sexual health plan. Recent studies of enema microbicides that protect against HIV transmission have shown promising results in monkeys, and human studies are planned in the near future.

Microbial enemas were developed in part because of the difficulty people have in maintaining adherence to daily HIV prevention protocols. In 2014 the National Institutes of Health awarded $21 million to researchers at eight institutes to further the development of an HIV-prevention enema. 

At the time, Craig W. Hendrix of John Hopkins University told the press, “An antimicrobial enema solution goes straight to the Achilles’ heel of current HIV prevention — poor adherence to daily oral pills for pre-exposure prophylaxis." 

In its most basic form, the post-sex enema needs to kill any HIV released during the preceeding sexual activity: in other words, it needs to act as a post-exposure prophylaxis (PEP).  But, researchers also dream of the enema providing protection from additional exposures to HIV in the following day(s). Enemas were seen as a perfect delivery system because over 50 percent of gay and bisexual men have acknowledged using them before or following anal sex. 

So how does the PrEP enema work? First, you need to understand the complexity of the human rectum and its relationship with semen. After all, in most cases that someone contracts HIV anally it's through semen entering the bloodstream via the rectal lining, which is more delicate and prone to tear than vaginal tissue.

You probably think the of the rectum as a type of self-contained area not unlike the vagina. But actually, the rectum and anus are just the lowest points in an open-ended tube that runs from your mouth through your gut to your back door. Combine that with the fact that semen is alive and programmed to swim upwards in search of a reproductive target, and you get the potential that semen released during anal sex can travel inward and upward into your body. Because of this, semen can reach areas in the colon more susceptible to infection. (Contrast that with semen released during oral sex, which may be swallowed but will be killed by stomach acids.)  

The researchers funded by NIH are testing various concentrations of tenofovir to determine the most effective solution strength. But scientists still needed to develop a better delivery system, one that could provide protection farther into the anal cavity while avoiding damage to the rectal lining.

It all may come down the type of gel lubricant used. Some gel formulations have actually been shown to damage rectal cells and increase risk. But the new gel formulations is hypo-osomlar, rather than hyper-osmolar (which is in standard lubricants). The difference is everything: lubricants that are hyper-osmolar suck water out of rectal tissue cells, which causes irritation and forces the drug to fight to travel upstream. Meanwhile, with hypo-osmolar lubricants, the cells absorb water from the gel and can transport the drug it contains up the cellular membrane. In other words, it travels on and upward potentially killing the HIV virus throughout the lower gut.

In order for the gel to do its job, scientists need to see how long the drug stays in a person's system after an enema. Testing the drug on monkeys, they discovered that a higher dose of the hypo-osmolar formulation produced drug concentrations in both blood and inside cells that were five to eleven times higher than any of the other gel formulations tested.

Results, shared publicly at the October 2016 HIV Research for Prevention conference, show that cells taken from biopsies one hour after microbicide dosing were totally protected from HIV. After 24 hours, two out of six monkeys were infected with the simian version of HIV, compared with all the monkeys on a lower microbicide dosing becoming SIV-positive.

The ultimate conundrom now is determining how long semen stays in the intestinal system after having anal sex. This will dictate how long the microbicide needs to stay effective. Once drug trials determine the optimal microbicide dosing, it's clear they'll want to pair it with a hypo-osmolar lubricant.