Researchers announced last week that an experimental vaccine, developed by Merck, was highly effective in preventing infections by two strains of the virus that causes cervical cancer.

Women who received the vaccine during a 2-year study were protected against precancerous lesions caused by two strains of human papillomavirus (HPV).

"The findings are dramatic and clear cut," says Dr. Douglas Lowy of NCI's Center for Cancer Research (CCR), who co-developed the original technology on which the vaccine is based. "In the first year and a half after the vaccinations, there was apparently complete protection against lesions caused by the two strains in the vaccine."

The vaccine, called Gardasil, targets HPV types 16 and 18, which cause about 70 percent of cervical cancers. HPV is transmitted sexually and causes almost all cases of cervical cancer. The disease kills more than 200,000 women each year, including many in developing countries.

Merck plans to submit Gardasil to the FDA for approval by year's end using data from the study, which involved more than 10,000 women. The results were presented at the annual meeting of the Infectious Diseases Society of America in San Francisco.

"You couldn't ask for better data than these," says Dr. John Schiller of NCI's CCR, who along with Dr. Lowy developed the underlying technology for the vaccine. "If all goes well, we could have a vaccine by the end of next summer."

The vaccine also protected against the two HPV types (6 and 11) that cause 90 percent of genital warts. These strains can lead to false-positives on cervical cancer screening tests, such as Pap tests, and cause unwarranted fears about cancer in some women.

"The vaccine includes the four types because we thought it was important to provide broad coverage against the array of biologically important HPV types," says Dr. Mark Feinberg of Merck's Vaccine Division.

When Gardasil was administered in 3 doses over 6 months, the protection was 100 percent for a period of 1.5 years. The long-term efficacy of the vaccine is still not known, and follow-up studies will be required to answer this question.

The researchers emphasize that women who get the vaccine should still follow guidelines for cervical cancer screening. At least a third of the cervical cancers are caused by strains not targeted by the vaccine.

A Phase III clinical trial is underway in Costa Rica to test an HPV 16/18 vaccine manufactured by GlaxoSmithKline (GSK). The researchers, led by Dr. Allan Hildesheim of NCI's Division of Cancer Epidemiology and Genetics, plan to enroll up to 7,000 women and follow them for at least 4 years.

The vaccines developed by Merck and GSK are based on "virus-like particles," the technology created by Drs. Lowy and Schiller. They discovered that a single HPV protein could form noninfectious, virus-like "shells" and trigger protective antibody responses when given as part of a vaccine.

Because the vaccine is preventive, the most cost-effective time to give it would likely be before women become sexually active. In the United States, recommendations about when to give the vaccine would be made by the Advisory Committee on Immunization Practices at the Centers for Disease Control and Prevention.

If approved, Gardasil would be the second approved cancer prevention vaccine after the hepatitis B vaccine, which protects against some forms of liver cancer.

By Edward R. Winstead