Genentech has updated the labelling for its blockbuster cancer drug Avastin to include a warning that it can cause a rare brain condition that can lead to seizures, blindness and other visual and neurological disturbances, as well as high blood pressure.

The condition, known as reversible posterior leukoencephalopathy syndrome, is rare and has affected in less than 0.1% of patients treated with Avastin (bevacizumab), which is approved to treat colorectal cancer and under review as a treatment for breast and non small cell lung cancers.

The US Food and Drug Administration (FDA) said on its website that the onset of symptoms has been reported to occur from 16 hours to one year after starting Avastin.

A letter published in the New England Journal of Medicine in March described two patients who developed RPLS whilst on Avastin therapy. Both recovered from the condition, which can be diagnosed using magnetic resonance imaging.

Meanwhile, the updated label also includes warnings about a link between Avastin and the development of nasal septum perforations, holes in the cartilage separating the nostrils. Neither warning is expected to have a material impact on the sales potential for Avastin, which analysts have predicted could bring in anything between $4 billion and $8 billion a year at its peak.

Genentech booked $821 million from sales of Avastin in the first half of this year, and it is among the company’s fastest-growing products. But efforts to extend the indications of the drug to include breast cancer were blocked earlier this month after the FDA asked for a ‘substantial’ update on the safety and efficacy of the drug in this indication.

Genentech also suffered a disappointment earlier this year when a clinical trial failed to support Avastin’s efficacy in pancreatic cancer

Outside the USA, Avastin is sold by Genentech’s majority shareholder Roche, which reported $1.13 billion in revenues from the drug in the first half of 2006.