I don't know the potentcy of Letaris but it doesn't bind to the B nor does (did) Encysives drug Thelin (Sitaxentan) which was pulled for safety (I forget what tox but sure could easily google). I heard Martin say its the safest ETA well 1 was pulled and the biggest selling one has a black box so everything is relative .
I don't know enough about the disease but what level of reduction is clinically meaningful? Having a letter from someone at FDA may be nice but for ETA's we've seen SPA's meet and not get approval (Encysive first 2 times around). And why doesn't/didn't GSK/Pfizer/Gilead pursue? On twitter he said that Actelion told him Tracleer wouldn't work. His theory is because it binds to A&B which could very well be (he certainly knows a lot more about the disease then me) but doesn't change the case for others. I think there may have been a few other ETA's developed too (none approved that I can think of) so its not as if its an empty field.