The study found that patients who received a combination of Rituxan and standard chemotherapy in a large clinical trial did not live longer than patients who received chemotherapy alone.
However, analysts regarded the study, funded by the National Cancer Institute, as inconclusive because the clinical trial was not designed to directly compare the two treatment regimens.
In a conference call held after the stock market close, Genentech executives also cautioned against drawing conclusions. Gwen Fyfe, vice president for hematology and oncology, said the study wasn't yet complete, so the results could change.
"We think the data is immature," she said.
Rituxan, co-marketed by Genentech and Idec Pharmaceuticals Corp., had sales last year of $1.16 billion and is among the best-selling cancer drugs. Rituxan is a key profit driver for South San Francisco-based Genentech and is approved only as a treatment for indolent non-Hodgkin's lymphoma, which is incurable.
But the aggressive, fast-moving form of the disease -- although curable in 30% to 40% of cases when treated with chemotherapy -- is more prevalent, and thus represents a significant market for Genentech and San Diego-based Idec.
Last year, a study financed by Hoffmann-La Roche Inc., which markets Rituxan in Europe and owns 58.4% of Genentech, found that a combination of Rituxan and chemotherapy improved survival of patients with fast-moving non-Hodgkin's lymphoma. That study encouraged physicians to order significant off-label use of Rituxan.
Patients in the Hoffmann-La Roche trial received more doses of Rituxan than patients in the new study.
The study was designed to show whether patients would benefit from so-called maintenance therapy with Rituxan. But added doses of Rituxan induced longer remissions only in patients who started on chemotherapy alone.
And the study found that maintenance therapy did not improve survival.
Results of the study were posted on the Web site of the American Society of Hematology on Tuesday. It is scheduled to be presented at the society's medical conference in San Diego next month.
Genentech's shares rose $1 to $82 on Tuesday on the New York Stock Exchange. The shares benefited from speculation that the company's experimental cancer drug Avastin might be approved without a Food and Drug Administration panel review.
Idec's shares fell $1.94 to $33.26 on Nasdaq.