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

Carlyle topped private-equity firms including Blackstone Group, KKR & Co. and Hellman & Friedman that bid or expressed interest, said two of the people, who declined to be identified because the process was private. Most bids were between $33 a share and $38 a share, or as much as $4.3 billion, said the people.

A purchase in the middle of this range for PPD “would be an attractive deal for shareholders,” said David Windley, an analyst at Jefferies & Co. Going private might allow the drug research company “to explore more complex deal structures with clients,” he said.

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. The talks between Carlyle and PPD may still collapse.

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

The acquisition would be the largest for Carlyle since announcing the $3.9 billion purchase of telecommunications equipment maker CommScope.

Carlyle, which has announced deals valued at $10 billion in the last 12 months, is selling old investments and entering new ones as the company plans a formal process for its initial public offering. Carlyle announced today it would sell Insight Communications Co., a six-year investment, to Time Warner Cable for about $3 billion.

Pharmaceutical firms were not given the option of bidding on PPD, only private- equity funds were allowed to bid on the asset, said two people familiar with the matter.