Encysive’s stock closed down a dramatic 49% at $4.60 yesterday, as investors shrank back on news that US approval of the group’s pulmonary arterial hypertension drug, Thelin (sitaxsentan sodium), is subject to a delay.

The US Food and Drug Administration has issued an approvable letter for the product, asking for more data before full clearance can be considered. And although Encysive is remaining relatively tight-lipped over the exact nature of the FDA’s request, it is feared that additional clinical studies may have to be carried out, which could take a considerable amount of time to complete.

This delay will substantially eat into - if not totally wipe out - the first-to-market advantage Thelin was expected to enjoy over its closest rival, Myogen’s ambrisentan, which was granted a priority review by US regulators earlier this month. Shares in the firm climbed almost 4% to $36.36 on Encysive’s misfortune, as the prospect of getting ambrisentan onto the shelves first fuelled investor excitement.

The news also allows Swiss biotechnology group Actelion, which sells the PAH Tracleer (bosentan), a little breather. The company has been preparing itself for battle with Revatio (sildenafil), the recently-approved PAH offering from Pfizer which contains the same active ingredient as its erectile disorder drug Viagra, as well as Thelin and ambrisentan, should they gain approval.