For researchers and companies involved in clinical trials, this new approval process will probably change study designs. Whereas some companies might simply add pCR as an early measurement of benefit into their existing trials aimed at gauging survival benefit, others would be likely to design small initial studies aimed at pCR endpoints before convening larger, longer studies separately.

“You could have results in six months, and using a smaller study,” says pathologist William Symmans of the MD Anderson Cancer Center in Houston. “And you'd be bringing agents into this newly diagnosed population.”

But there will be no one-size-fits-all approach to approval—nor a specific percentage of patients achieving pCR that the FDA would like to see to grant approval, according to Prowell. Instead, the agency will manage decisions on a case-by-case basis.

Although the new rules would change some breast cancer studies, pCR is difficult to assess in cancers that are diffuse or more aggressive, Symmans says, so the move is not likely to lead to changes to other malignancies in the very near future. For now, he says, the data are sufficient to use pCR as an endpoint in only HER2-negative and triple-negative breast cancers. But researchers continue to study the potential of pCR as a marker of survival in other types of breast cancer and in colorectal and esophageal cancers, among others.

Researchers and the FDA both stress that before the final guidance is issued, an agreement must be reached on exact, standardized methods for how tissue samples are collected and analyzed for pCR. As press time, these issues of standardization were scheduled to be discussed at a public workshop at the FDA on 22 March and at a scientific panel convening 8 April at the annual meeting of the American Association for Cancer Research in Washington, DC, after which the FDA will move forward on its final guidance.

If the shift to pCR as a regulatory endpoint occurs, Symmans expects it would mean an increased involvement of pathologists—who analyze tissue specimens for evidence of cancer—in clinical trials. “I think there are going to be much more sophisticated, more data-driven refinements to the pCR endpoint in years to come,” he says. “But there's enough data already for the FDA to get going with using this as an endpoint, and I think it's going to be good for patients, and it's going to be good for drug development, and it's going to be good for clinical trials.”