WASHINGTON (AP) -- It would be a powerful tool for women: a gel or cream they could apply just before sex to protect against AIDS or other sexual infections, without having to get the man to agree to a condom.

Scientists have spent years hunting such a "topical microbicide" with little luck so far, although they have some early candidates that might fight the AIDS virus.

But Penn State University researchers announced a surprise: an experimental microbicide that promises to protect against both HIV and a sexually transmitted virus that causes deadly cervical cancer.

It is a common ingredient in shampoos and toothpastes, so researchers already know it is safe; it also is cheap.

Many chemicals are being tested for anti-HIV action. After all, the AIDS virus is spreading most rapidly in heterosexual women.

But Penn State microbiologist Mary Howett's discovery of the first chemical that also destroyed human papilloma virus, or HPV, has researchers intrigued. The National Institutes of Health cautiously hopes that first-stage testing of sodium dodecyl sulfate could begin in a small number of women later this year.

"We're excited about the broad-spectrum nature," said Dr. Penny Hitchcock, NIH's microbicide chief. The HPV protection is particularly intriguing because "it's been a lot harder for us to find anything that works with that."

"But we're cautious," she stressed, because researchers have lots of work _ including turning the ingredient into a usable vaginal gel _ before initial testing could begin.

Human papillomavirus is a common sexually transmitted infection. About one in four women is estimated to have a strain of HPV.

Most HPV is symptomless, but some strains go on to cause cervical cancer that kills 5,000 American women and 250,000 women worldwide every year. (The global toll is so high because developing countries cannot afford Pap smears, tests that let most American women discover cervical abnormalities before they grow into actual cancer.)

"If you could eliminate (HPV), you could potentially prevent all those cancers," Howett explained.

Microbicides are gels or films that women would insert into the vagina before sex to protect against various sexually transmitted diseases. So far, none has panned out. A study just found that the popular spermicide nonoxynol-9, long thought the best hope for an anti-HIV gel, does not protect after all.

Scientists now are studying whether other spermicides or chemicals could work instead, and the NIH is spending millions to help.

One approach in early testing is an acid-buffer gel. Semen is very alkaline, raising the pH level of a woman's vagina enough to help acid-averse HIV infect more easily. The gel would maintain a vagina's natural acidity in hopes of killing HIV.

But until now, no experimental microbicides have blocked the tougher cervical cancer virus.

In the February journal Antimicrobial Agents and Chemotherapy, Howett reported that in test-tube studies, the ingredient SDS inactivated the AID and herpes viruses. Like other experimental microbicides, SDS is a detergent that dissolves the fatty coating that holds those viruses together.

The cervical cancer virus is coated with tightly packed proteins that mere detergents cannot dissolve. But SDS also is a "denaturing" agent _ and it picked those proteins apart.

After it worked in test tubes, Howett injected human cells with the cancer virus or with SDS-treated virus and then implanted those cells inside mice. The treated human cells grew normally with no evidence of HPV infection. The untreated cells grew into genital warts.

It will take years to prove whether SDS will work in the bedroom as well as the lab. Still, Howett calls SDS "a major step toward ... producing a practical, nontoxic, inexpensive, discreet product" to protect women's sexual health.

Copyright 1999 The Associated Press. All rights reserved.