Shares in Actelion were on the rise this morning after it emerged last night that the US Food and Drug Administration had delayed approval of Encysive Pharmaceuticals’ Thelin, a drug which would compete with Actelion’s pulmonary artery hypertension treatment Tracleer.

The FDA said it would withhold approval of Thelin (sitaxsentan sodium), for the second time, pending the resolution of an unresolved issue with Encysive’s marketing application.

Shares in Encysive plummeted more than a third in after hours trading yesterday, while Actelion put on nearly 5% this morning as prospects for Tracleer (bosentan) in the remainder of 2006 improved.

Encysive said the matter raised by the FDA in its latest ‘approvable’ letter would not require any additional clinical trials, but is a matter of judgment that does require negotiations with the agency to see if agreement can be reached.

Sales of Tracleer were $335 million in the first half of this year, but had been expected to come under pressure in the second half on competition from Thelin as well as Pfizer’s Revatio (sildenafil) and Myogen’s ambrisentan, also due for approval in the USA before the end of the year.

Thelin has been tipped as a $200 million product at peak. A European Medicines Agency advisory panel recently recommended the drug for approval.