There were two approved ERA that are selective to the A receptor (Tracleer targets both). One got pulled for toxicity. @DPDResearch posted a link to an interesting medscape article on tox with an ERA he wasn't sure if it was a class effect or not but thought being to B caused fluid retention. Considering Letairis is still on the market I think they are reasonably safe at a low dose.
Pipex probably had as low a market cap and had an orphan focus plus their first compound should have been a sure thing but looking at their filings it was not a company I would have invested in (and thankfully didn't). I don't think Martin's company is a Pipex. I am not sold on the Ligand compound either though I'd like to see human data.