Pfizer is voluntarily withdrawing Thelin in the European Union, Canada and Australia and discontinuing clinical studies of the treatment for pulmonary arterial hypertension worldwide over concerns about injury to the liver.

The drugs giant said it is taking this action “in the interest of patient safety” based on a review of emerging information from clinical trials and post-marketing reports. Pfizer noted that while liver toxicity is a known complication of the class of drugs to which Thelin (sitaxentan) belongs, ie endothelin-1 blockers, “a new potentially life-threatening idiosyncratic risk of liver injury” has been observed.

The company added that “given the availability of alternate treatments”, notably Actelion’s blockbuster PAH drug Tracleer (bosentan), it has concluded that the overall benefit of Thelin no longer outweighs the risk. Patients receiving Thelin should be transitioned to appropriate therapies as soon as safely possible.

The European Medicines Agency noted that the action was taken following information on two cases of fatal liver injury. Since its initial marketing authorisation, in 2006, Thelin has been contra-indicated in patients with mild to severe hepatic impairment (Child-Pugh Class A-C) and elevated aminotransferases prior to initiation of treatment.

Pfizer got hold of the drug when it acquired Encysive Pharmaceuticals in 2008 for around $195 million. Thelin, which was rejected three times by regulators in the USA, has not been a big earner and worldwide sales for the first nine months of the year reached just $44.4 million.