Rectal microbicides that protect against HIV transmission via anal sex are a bigger technical challenge than vaginal ones. The rectal lining is more delicate than the vaginal, so safety has been an issue; research has shown that many of the gel formulations used in lubricants damage rectal cells and may actually enhance HIV transmission.
But the biggest challenge is coverage. A rectal microbicide has to cover a considerably greater area of mucous membrane as the gut is an open-ended tube; devices such as rings cannot be used and CAT scans have shown that semen may travel a considerable way up into the colon after anal sex.
Since a lot of gay men use enemas and douches, it seemed logical to try and make a microbicide formulated as an enema, and scientists have been developing one for at least five years. The issue has been getting the enema formulation right. Standard lubricants are hyper-osmolar: this means they suck the water out of rectal tissue cells, which not only causes irritation but means the HIV drug in the gel has to fight against a current, so to speak, going the other way.
Researchers at the second HIV Research for Prevention Conference (HIVR4P) in Chicago presented the first results on tissue concentration and potential anti-HIV efficacy in monkey studies of a tenofovir-containing gel that is hypo-osmolar. This means that the cells absorb water from the gel and therefore actively transport the drug it contains through the cellular membrane.
François Villinger of the University of Louisiana presented results for four different enema formulations. Two were iso-osmolar, meaning they contained the same balance of salts as cells do; this means their effect on the direction of water transport between enema and cell is neutral. Two were hypo-osmolar. They tested two concentrations of tenofovir, at 1.76 and 5.28 milligrams per millilitre (mg/ml).
The researchers gave monkeys a simple dose of the enema and measured concentrations of tenofovir in their blood and in rectal tissue biopsies an hour, a day and three days after the dose. They also took cells from biopsies at these points and cultivated them in a lab dish with monkey-HIV (SIV) to see if they could be infected – a so-called explant test.
With both doses of the hypo-osmolar formulation, drug levels in cells reached their peak faster than with the iso-osmolar formulation.
The 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 formulations, though the difference in the levels in cells was not significant at three days after dose. An important consideration in tenofovir is whether the concentration of the prodrug in blood, tenofovir disoproxil fumarate (TDF) which is the version actually dosed, correlates with the biologically active, metabolised version of the drug that has to be inside cells, tenofovir diphosphate (TFV-DP), and the levels correlated at all doses. The concentrations of TFV-DP specifically in CD4 cells, which are the important cells to get into, were five times higher with the larger hypo-osmolar dose, one hour after dosing, than they were with any other formulation. There was no damage observed to rectal tissues with any formulation either.
In the HIV prevention assays, the hypo-osmolar high dose formulation was highly effective in preventing HIV infection by two different strains of SIV
Cells taken from biopsies one hour after microbicide dosing were completely protected from HIV infection and in cells taken 24 hours after dosing, biopsies taken from two out of six monkeys were infected compared with all of them with other microbicide doses. The issue of how long semen stays in the gut and can infect cells is important here, as it dictates how long the microbicide needs to be effective for.
Nonetheless, there are promising results, showing that it is possible to make an enema microbicide that causes cells to actively absorb drug, and human studies are planned.
Villinger F et al. Hypo-osmolar Formulation of TFV Enemas Promotes Uptake and Transformation of TFV to TFV-DP in Tissues and Prevents SHIV/SIV Infection. HIV Research for Prevention (HIVR4P) 2016 conference, Chicago, abstract OA04.03, 2016.