Rectal microbicides are chemicals which are designed to be put up the anus into the rectum to kill off microbes that cause STIs when having anal sex with an infected partner. Ideally, they'd protect both the receptive and insertive partner.
Much more research is being done on vaginal microbicides, partly because of 'moral' issues (i.e. homophobia and general bigotry) but also because the vagina is a much easier place to get a microbicide to work.
Remember that being on the receiving end of anal sex with an HIV+ man is about fifteen times riskier than being on the receiving end of vaginal sex? Here are some reasons why:
Vagina: thick outer layer of the skin, about forty cells deep
Rectum: one cell layer deep, albeit with thicker cells than the vagina
Vagina: fewer CD4 cells near the surface to be infected by HIV
Rectum: complex and extensive immune structures under surface
Vagina: an enclosed 'pouch'
Rectum: a long tube, ultimately ending at the mouth. You don't need to go that far, but it's known that lube can be found two feet along, well into the colon, four hours after being placed in the rectum
So there are several trials of vaginal microbicides going on, but it looks like only one potential rectal product is even at a very early stage of trials.
It also doesn't help that understanding of HIV infection in the rectum is still not that well understood, and the rectum and colon are more fragile than the vagina, so there are increased safety issues (it's quite possible that using a vaginal microbicide might not work, and might be actively harmful!)
TMI: I was hoping to post this on (the UK's)Mother's Day...