Genentech’s cancer drug Avastin was linked to a significantly increased risk of blood clots in the veins, according to an analysis of earlier studies. Patients who received the drug were 33 percent more likely to have blood clots develop in their veins than those who didn’t receive it, Dr. Shenhong Wu and his colleagues reported late Tuesday in the Journal of the American Medical Association.

Contrary to other studies by Genentech researchers that have said that the vein clots did not rise significantly the risk of vein clots among patients taking Avastin, this new study indicates that doctors and their patients should be watching carefully for symptoms of blood clots. Doctors should monitor their patients for signs of vein clots and use blood thinners in those most likely to develop clots, said Wu and his colleagues.

Many cancer patients already have an elevated risk of blood clots in the body's veins, known as venous thromboembolism. Venous thrombotic embolism (VTE) is a dangerous condition because the blood clot forming in the veins can break loose and travel through the circulatory system to the right side of the heart. The blood clot could easily become dangerous if it manages to travel to the lungs, where it becomes a pulmonary embolism, which can be life threatening or even fatal.

A spokesman for the drugmaker said the drug’s label already warns about the risk of blood clots. The label also mentions the fact that some people taking Avastin and chemotherapy have had blood clots in the veins.

Avastin, marketed by Genentech of South San Francisco and the Swiss drugmaker Roche, slows the formation of blood vessels that supply tumours. The drug is based on anti-angiogenesis mechanism designed to combat cancer by preventing the formation of these blood vessels that can be dangerous. The mechanism is simple: the drug interferes with the blood supply of a tumour, cutting its ability to grow and spread in the body.

Avastin is presently Genentech’s best sold drug, managing to reach sales of $704 million in the third quarter. Genentech shares dropped 20 cents to close at $80.63 before the results of the study were made public.

Study authors say more research needs to be done to evaluate specific cancer risk in patients taking Avastin combined with other therapies and to see if the risk increases when the drug is combined with other types of drugs or therapies. A previous study published last year in the Journal of Cancer Institute, found an increased risk of blood clots in arteries, not in veins.

In October, the results of a late-stage clinical trial into the benefits of combining Genentech’s cancer drug Avastin (bevacizumab) to the drug Tarceva (erlotinib) did not show an improvement in overall survival with the Avastin-Tarceva combination compared with Tarceva alone. However, the combination treatment showed clear evidence of clinical activity, with improvements in progression-free survival (PFS) and response rate.