The U.S. health authorities issued Encysive Pharmaceuticals their fourth “approvable letter” in response to the drugmaker´s request for a formal dispute resolution on its NDA for experimental Thelin (sitaxsentan sodium), for the treatment of pulmonary arterial hypertension (PAH).

The FDA's reviewer agreed with the decision of the Division of Cardiovascular and Renal Products that, while the data suggested Thelin´s effectiveness, the evidence was not enough for approval.
The reviewer encouraged the company to conduct an additional study to demonstrate the drug's effectiveness in exercise capacity as measured by change in six-minute walk distance.

This is the fourth approvable letter emitted for Thelin. The company received its first letter in March, 2006, when the FDA said that another Phase III trial would be necessary to prove the drug met efficacy standards.

Encysive chose not to run the costly trial and has spent the last year and a half trying to change the FDA’s decision. After it received the third approvable letter, the company asked for formal dispute resolution, which has now been denied.

Thelin has already been approved in Europe. If eventually approved for the treatment of PAH in the U.S., Thelin would compete with Gilead Sciences´ Letairis (ambrisentan), which had sales for $ 45 million dollars in the first half of this year; and Actelion´s Tracleer (bosentan), with sales for $ 543 million dollars in the same period.

Another available drug is Pfizer´s Revatio, launched onto the market in June 2005, which has the same active ingredient as the company´s Viagra (sildenafil) but has not shown to be successful.