The failure of a last-chance cancer drug to significantly improve survival rates in a clinical trial was a surprise and the manufacturer says it is trying to figure out why the product didn't do better.

Iressa (search) won Food and Drug Administration approval in 2003 for advanced lung cancer (search) patients who have exhausted standard therapy after studies showed it shrank tumors in some terminally ill patients.

But approval, based on preliminary studies, came with a requirement that the drug be further tested.

In a follow-up trial, it failed to produce an improvement that was statistically significant, prompting the manufacturer, AstraZeneca to stop promoting it and to advise doctors that it had failed to live up to expectations for unknown reasons.

AstraZeneca told an FDA advisory committee, meeting this week to review several cancer drugs, that it is conducting a detailed analysis of the trial.

"The failure to reach statistical significance for survival in the overall population was completely unexpected," the company said in its report to the committee.

It went on to say that Iressa does produce responses in tumors and that some groups of patients did see benefits, notably people of Asian descent and nonsmokers.

In a separate report, a study appearing this week in the Journal of the National Cancer Institute found a gene mutation involved in the development of some lung cancers that appears to occur only in nonsmokers.

That study, by doctors at the University of Texas Southwestern Medical Center, said gene mutation had increased sensitivity to Iressa. The same mutation also was found more commonly in women and patients from Japan.

While the FDA advisory committee was not scheduled to take any action on whether Iressa should remain on the market, the agency had asked the manufacturer to provide a report on its current studies of the drug.

AstraZeneca said it doesn't believe a decision on the commercial availability of the drug should be made until completion of the analysis of the trial, which it expects to finish by May or June. In the meantime, it has advised physicians about the lack of success and noted that alternative therapies are available.

However, the company said it intends to continue to make Iressa available "as an option for patients who are deemed appropriate."