Sitaxsentan

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Sitaxsentan or sitaxsentan sodium (actual INN: sitaxentan) (marketed as Thelin® by Encysive Pharmaceuticals) is a small molecule sodium salt that blocks the action of endothelin (ET) on the endothelin-A (ETA) receptor selectively (by a factor of 6000 compared to the ETB). It is a sulfonamide class endothelin receptor antagonist (ERA) and is undergoing Food and Drug Administration (FDA) review for treating pulmonary hypertension. The rationale for benefit compared to bosentan, a nonselective ET blocker, is negligible inhibition of the beneficial effects of ETB stimulation, such as nitric oxide production and clearance of ET from circulation. However, in clinical trials, the efficacy of sitaxsentan has been much the same as bosentan, but the liver toxicity has been better. Therefore sitaxsentan is expected to be marketed as a safer drug than bosentan, but not necessarily more effective.

Thelin has been approved for marketing in both the European Union (on 10 August, 2006), in Canada and in Australia (on 7 March, 2007). It is already marketed in Germany, The Netherlands, the United Kingdom, and Ireland. The medication is currently undergoing phase III clinical trials within the United States.

On the Prescription Drug User Fee Act (PDUFA) target action date of 24 March, 2006 the United States' FDA recommended an approvable status to Thelin® but said it would not yet approve the product. On July 24, 2006 Thelin received a second approvable letter stating that efficacy outcome issues raised in the context of the STRIDE-2 study were still unresolved. In July 2007, Encysive commenced a formal dispute resolution process in a preliminary meeting with the FDA.

Adverse effects
Adverse effects observed with Thelin® are class effects of endothelin receptor antagonists, and include :
 * liver enzyme abnormalities (increased ALT and AST)
 * headache
 * oedema
 * constipation
 * nasal congestion
 * upper respiratory tract infection
 * dizziness
 * insomnia
 * flushing.

Because Thelin® inhibits metabolism of warfarin, a decreased dose of warfarin is needed when co-administered with thelin. This is due to the fact that warfarin acts to prevent blood from clotting, and if it remains unmetabolized, it can continue to thin the blood.