Genentech

(DNA)

challenged conclusions made by a team of university researchers that its blockbuster cancer drug Avastin has been linked to a significantly increased risk of blood clots in the veins, according to a report.

The risk of the worrisome blood clots was 33% higher among those on Avastin compared with control group subjects in 15 clinical trials analyzed together, Shenhong Wu and his colleagues reported late Tuesday in the

Journal of the American Medical Association

, reports the

San Francisco Chronicle

.

Wu's study contradicts the conclusions from a pooled analysis of five studies by Genentech researchers, who reported in 2007 that the vein clots didn't rise significantly among subjects on Avastin.

Wu said his findings are not a reason to avoid taking Avastin, the newspaper said.

"Not in any way," said Wu, a researcher at the Stony Brook University Cancer Center in Stony Brook, N.Y. Rather, the study indicates that patients and their doctors should be watching carefully for symptoms of blood clots, which in severe cases can be fatal, he said in an interview with the

Chronicle

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. "For patients, it's important for them to recognize the risk."

Avastin, marketed by Genentech and Swiss drugmaker

Roche

, is designed to block tumor growth by preventing blood vessels from forming to nourish them. Combined with chemotherapy, it can extend the survival of people with certain forms of colon and lung cancer.

But the drug also is associated with serious side effects, including blood clots in the arteries. Other dangers, including hemorrhage and perforations of the bowel, are detailed on the Avastin label approved by the

Food and Drug Administration

in what is called a "black box warning," the FDA's highest-level alert, the newspaper reports.

Philippe Bishop, Genentech's head of clinical development for Avastin, said the results of Wu's analysis might have been skewed by a number of factors, including the different methods of reporting data in each of the 15 trials. But Bishop acknowledged that studies in general have found an incidence of venous clots averaging about two to three percentage points higher among those on Avastin compared with control groups, the

Chronicle

reports.

Such information is included in Avastin's label, he said. What Bishop disputes is that the data reveal a statistically significant difference in risk of 33%. No change in the drug's label is needed, he said, according to the newspaper.

This article was written by a staff member of TheStreet.com.