LOS ANGELES - A biotech billionaire struck a $500 million deal Wednesday to buy the Los Angeles Times, ending the paper's quarrelsome relationship with its Chicago-based overseers and bringing it under local ownership for the first time in 18 years. The agreement between Los Angeles medical entrepreneur Dr. Patrick Soon-Shiong and Tronc represents the latest instance of a rich, civic-minded individual purchasing a newspaper from a big corporation. The South African-born Soon-Shiong, 65, amassed his fortune in part by developing a cancer drug in 1991.