Three private equity firms are rumoured to be considering bids for PRA International, just as the contract research organisation is said to be exploring a public offering.

KKR & Co, Cimven and Warburg Pincus are all interested in acquiring PRA, according to a Reuters report, which cites undisclosed sources familiar with the matter. A sale could raise as much as $1.3 billion, says the news agency.

The Raleigh, North Carolina-based CRO has been considering a number of options since current private equity owner Genstar Capital reportedly put the business up for sale in 2012.

Earlier this year PRA filed a draft registration statement with the US Securities & Exchange Commission (SEC) to gauge interest in an IPO without requiring any disclosure of confidential company information.

News of the move came after rival CRO Quintiles raised almost $950 million from its own IPO, and if it goes through would take PRA back into public ownership for the second time. A 2004 IPO raised around $100 million, but the company went back into private hands after Genstar bought it for $790 million in 2007.

Genstar subsequently tried to cash in on its investment by selling the clinical research specialist in 2011, but reportedly dropped the plan after failing to find a buyer that would meets its asking price.

At the time the IPO was being mooted, PRA said it would use the proceeds from the offering to reduce outstanding borrowings under its credit facilities, provide liquidity for existing shareholders, pay off certain fees and expenses, and to generate funds for working capital and general corporate purposes.

PRA has been growing rapidly in the last couple of years, with a particular focus on building its business into a truly global contract research player, and the company raised $410 million last year to fund new investments.

Armed with that war chest PRA has entered into a number of deals in recent months, including the formation of a joint venture with China's WuXi PharmaTech and the takeover of Eastern European CRO ClinStar.