Exelixis of the USA saw its shares slide more than 12% yesterday after it halted enrollment in a clinical trial of XL999, a drug for cancer, after cardiovascular side effects were seen in some patients.

XL999 is a spectrum selective kinase inhibitor (SSKI) that inhibits the activity of a number of cellular factors involved in the development and maintenance of blood vessels in tumours.

The Californian company said the programme could delayed for up to three months as it examines the side effects and determines whether they are drug-related. An analysis found that around 10% of patients suffered cardiovascular side effects during treatment with XL999.

There are six clinical trials in the Phase II programme, looking at the activity of the drug in treating renal cell carcinoma, colon, ovarian and non-small cell lung cancers, as well as acute myelogenous leukaemia and multiple myeloma. Patients with no cardiac effects will continue to receive the drug according to the protocols, but no new patients will be recruited while the matter is investigated, said Exelixis.

Exelixis also has seven other compounds in development, so the long-term impact of the news, even if XL999 has to be dropped, would be minimal. Its pipeline includes XL119 (becaterin), which is licensed to Helsinn and in Phase III trials for bile duct cancers; XL784 for diabetic kidney disease (Phase II); XL880 for kidney cancer (Phase II); XL647 for lung cancer (Phase II); as well as three Phase I programmes.