Why was N-9 the first candidate microbicide?

The most thoroughly studied candidate microbicides have been based on the spermicide nonoxynol-9, also known as N-9. This is a detergent which was chosen for research on the basis of laboratory findings that it disrupted HIV and other sexually transmitted infections (STIs), even at very low doses, and because it was already in widespread use, including in lubricants for condoms. Animal studies confirmed that N-9 protects female monkeys against challenge with HIV-related viruses (Miller, Weber). Unfortunately, N-9 causes damage to human tissue, leading to inflammation and ulceration, which is dose related (Niruthisard).

Research into N-9 as a rectal microbicide has been minimal and where it has occurred, has produced even more worrying results than in trials as a vaginal microbicide: the use of nonoxynol-9 caused the rectal lining to slough off in both mice and humans, prompting a warning about the popular use of N-9-containing lubricants during anal sex. Far from protecting against HIV and other viral infections, N-9 leaves the rectum more susceptible to it (Phillips).

It was nonetheless hoped that low-dose N-9 products, used vaginally, might be able to protect against HIV without causing excess inflammation.

However, the results of the most extensive clinical trials carried out on any microbicide are clear-cut: N-9 increases HIV transmission to women who are at high risk of HIV when they use the product frequently, and appears to have no protective effect either against HIV or other STIs when used less frequently. There are better candidates available for evaluation, and the consensus is that future research should focus on those.

In the light of these findings, the use of N-9 based spermicides has also been questioned: current recommendations are set out separately:
http://www.aidsmap.com/web/pb1/eng/d5c786f8-1902-46ac-b6f4-fab410e36ec4.htm.