Aptalis, a specialty drug maker owned by TPG, has filed to raise up to $500 million through an IPO. The company has applied to trade under the APTA symbol on the NASDAQ. Formerly known as Axcan, the company has built itself a strong position in the North American and European gastroenterology and cystic fibrosis markets. TPG bought Axcan for $1.3 billion in 2008 and subsequently merged it with Eurand in 2011, renaming the combined entity Aptalis. Efforts to offload the company for more than $3 billion earlier this year didn't pan out.