Carlyle Group is in exclusive talks to buy Pharmaceutical Product Development Inc., a clinical research company, said five people with knowledge of the discussions.

Other private equity funds that bid or expressed interest in the business included Blackstone Group LP, KKR & Co. and Hellman & Friedman LLC, said two of the people, who declined to be identified because the process was private. Most bids were between $33 and $38 a share, or as much as $4.3 billion, said the people. PPD’s stock closed at $26.60 on Aug. 12 on the Nasdaq Stock Market, valuing the company at about $3 billion.

Carlyle is talking to other private-equity firms about joining it in an offer for the company, one person said. Rival bidders were informed last week that another firm had been given exclusivity to negotiate with PPD, said another person. Morgan Stanley is advising PPD, according to the people.

Spokesmen for Carlyle, Morgan Stanley, Blackstone and KKR declined to comment. Hellman & Friedman and PPD didn’t immediately return calls seeking comment.