In a third "approvable" letter for Encysive's pulmonary hypertension drug Thelin (sitaxsentan), FDA recommends the firm conduct an additional clinical study to demonstrate efficacy, the company announced June 15.

"FDA stated that Encysive's development program for Thelin did not demonstrate the evidence of effectiveness needed for approval. The FDA did note, however, that the Thelin development program provides some evidence that Thelin improves exercise tolerance in PAH," the Houston firm said in a statement.

"FDA 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," Encysive added.

"We believe we adequately addressed the issue raised by the FDA in the second approvable letter and we are deeply disappointed in their decision," CEO Bruce Given said.

Encysive responded in December to FDA's second approvable decision, which was issued in July (¹ Also see "Encysive Thelin Approval Could Come Days Ahead Of Gilead’s Ambrisentan" - Pink Sheet, 28 December, 2006.). That response was initially deemed incomplete due to formatting issues (² Also see "Encysive Hits Another Snag With Thelin" - Pink Sheet, 14 December, 2006.).

The news comes at a particularly competitive point in the pulmonary hypertension space, with Gilead receiving FDA approval the same day for its same-class endothelin receptor antagonist Letairis (ambrisentan) (see ³ Also see "Gilead Letairis Will Be Available Through LEAP Restricted Distribution Program" - Pink Sheet, 15 June, 2007.). The drugs were reviewed nearly back-to-back (⁴ Also see "FDA To Review Gilead’s Ambrisentan And Encysive’s Thelin Back-To-Back" - Pink Sheet, 16 February, 2007.).

While Encysive said it plans to meet with FDA as soon as possible to discuss the appropriate path forward with regard to sitaxsentan, the firm warned that the agency's decision may significantly impact its business. "The company may be required to make significant reductions in its infrastructure and workforce in the U.S. as a result of the decision by FDA. The company is reviewing its strategic alternatives and intends to communicate its plans at a later date," the firm explained.

Given added that "Encysive remains committed to Thelin in Europe, Australia and Canada, where it has been approved for sale." The product was cleared for PAH in Europe in August 2006 (⁵ Also see "Encysive’s Thelin Gets EU Nod Ahead Of U.S." - Pink Sheet, 11 August, 2006.).

- Lee Szilagyi (l.szilagyi@elsevier.com)

Thelin's Third "Approvable" Letter Recommends Additional Clinical Trial

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