Merck & Co. sold $4.25 billion of bonds in a four-part offering, its biggest ever, to finance the acquisition of rival drugmaker Schering-Plough Corp., Bloomberg News reported. The maker of asthma treatment Singulair is taking advantage of investor demand for pharmaceutical-company debt to finance its acquisition, even as the rally in corporate debt shows signs of stalling, Guy Lebas, chief economist at Janney Montgomery Scott LLC in Philadelphia, told the news service. Merck's sale is the biggest non-financial U.S. corporate bond offering since Midland, Mich.-based Dow Chemical Co.'s three-part, $6 billion issue on May 7, Bloomberg data show.