A widely used spermicide may increase a woman's risk of contracting human papillomavirus from a sex partner, a study in mice suggests. On the other hand, a thickening agent in many vaginal lubricants sold commercially impedes the virus' ability to infect female mice via their genital tracts—even in the presence of the spermicide.

It remains to be seen whether the findings will translate to people, cautions study coauthor Jeffrey N. Roberts, a human papillomavirus (HPV) researcher at the National Cancer Institute in Bethesda, Md. But the results "raise the intriguing possibility that you could formulate [the two compounds] as a spermicide that would prevent HPV infection," he says.

Nonoxynol-9, the most widely used spermicide, has come under scrutiny because its strong detergent properties—which make it lethal to sperm—irritate the lining of the genital tract of some women.

Roberts and his colleagues tested mice to establish whether that damage might facilitate HPV passage.

First, they abraded the genital tracts of some female mice with a tiny brush. A genetically modified version of HPV, applied to the damaged tissue 6 hours later, infected cells lining the tract. The virus' modification made it traceable. Mice with unabraded genital tissue largely resisted infection.

Next, the researchers inserted nonoxynol-9 into the unabraded genital tracts of a second group of mice. Six hours later, they exposed the animals to the modified HPV. Five times as much HPV infected the mice as it did after abrasion, the researchers report in the July Nature Medicine.

"Nonoxynol-9 seems to compromise the barrier effect of the cells lining the genital tract while not inactivating the virus in any way," says Roberts.

When nonoxynol-9 and HPV were inserted simultaneously into mice with unabraded genital tracts, no infection occurred. That suggests that "it might take some time" for the detergent properties of nonoxynol-9 to disrupt the layers of cells lining the genital tract, Roberts says.

"The data presented are really very convincing," says immunologist W. Martin Kast of the University of Southern California in Los Angeles. "This leads you to think there is a chance the virus might do something similar in human beings."

The scientists conducted a parallel set of experiments with carrageenan, a seaweed derivative used to thicken vaginal lubricants. It had shown signs of inhibiting HPV in lab tests (SN: 7/22/06, p. 62).

In mice, carrageenan fended off HPV infection almost completely after the animals underwent genital-tract abrasion or exposure to nonoxynol-9.

There are more than 100 HPV strains. Some cause genital warts, while others can lead to cervical cancer. A vaccine called Gardasil prevents infection by two wart-causing and two cancer-causing HPV strains (SN: 10/15/05, p. 243; SN: 11/20/04, p. 332). Roberts and his colleagues used one of the latter strains, HPV-16, in the current study.

Kast says that researchers need to test whether carrageenan would complement Gardasil. In the new study, carrageenan blocked infection by HPV-16 and showed signs of inhibiting two cancer-causing strains that the vaccine doesn't address.

Meanwhile, the current findings "make the case that carrageenan is a useful compound to add to nonoxynol-9" to offset its detrimental effects, Kast says.

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