Quintiles, the biggest provider of testing services to drugmakers, filed to raise $600 million in an initial public offering.

Morgan Stanley, Barclays Plc and JPMorgan Chase & Co. are leading the offering, according to a regulatory filing Friday. The company didn’t specify a price range or a date for the proposed sale, and the amount is a placeholder that may change.

But a public offering would be the second for the Durham company. The clinical research organization went public in 1994, only to be taken private again in 2003 when founder and then CEO Dennis Gillings and others bought out stockholders. 

Boston-based Bain and TPG of Fort Worth, Texas, own about 23 percent each of Quintiles, while affiliates of London-based 3i Group Plc hold 15 percent and Singapore state-owned asset manager Temasek Holdings Pte owns almost 10 percent, the filing shows. Gillings, who stepped aside from the CEO post last year and became chairman, owns about 24 percent of the stock with affiliates.

The private equity firms bought Quintiles in 2008 in a transaction that valued the Durham, North Carolina-based company at about $3.8 billion, a person familiar with the matter said last year.

The owners are taking the company public as U.S. stocks trade near a five-year high. Better-than-expected profits among listed companies and renewed investor appetite for equities have helped fuel gains for buyout-backed companies that have completed IPOs this year such as Norwegian Cruise Line Holdings Ltd. and Boise Cascade Co.

Gillings founded Quintiles more than three decades ago while he was a biostatistics professor at the University of North Carolina at Chapel Hill. While he was still teaching he also performed consulting services for drug firms, at times including his students in the work. The company that became Quintiles started in a trailer on the UNC campus. Now a global player with operations in about 100 countries, Quintiles is the largest firm among   clinical research organizations and the company generated revenue of $3.7 billion in 2012 and net income of $177.5 million, according to the filing. Quintiles employs more than 27,000 employees worldwide.

Quintiles and its owners may offer stock in the IPO, according to the prospectus, and proceeds will be used to repay debt. Long-term borrowings at Quintiles totaled about $2.42 billion as of Dec. 31, while cash and equivalents amounted to $567.7 million. 

(Bloomberg News and The AP contributed to this report)