On August 13, Genentech and the US Food and Drug Administration (FDA) disclosed that a notice had been sent to physicians relating to Genentech's antibody drug, Avastin. This episode gives a warning to other companies that similar drugs targeting the endothelium may raise safety concerns when used in combination therapy.

The drug, the first targeted angiogenesis inhibitor to reach the market, binds vascular endothelial growth factor (VEGF) and therefore prevents the formation of new blood vessels. It was approved in the US in February 2004 for the treatment of colon cancer in combination with a chemotherapy regimen that includes 5-fluorouracil (5-FU)(Nat. Biotechnol. 22, 371–374, 2004). Five months after that approval, Genentech issued a'Dear Doctor'letter, dated July 2004, which states that the company had discovered a twofold increase in serious arterial thromboembolic events in some colon cancer patients who received Avastin in conjunction with 5-FU compared to standard chemotherapy care. Genentech, in S. San Francisco, California, now estimates the overall risk of such events in all Avastin-treated patients at up to 5%.