TOKYO (Dow Jones)--Shionogi & Co. (4507.TO) said Wednesday it and GlaxoSmithKline PLC (GSK) will start a phase-3 clinical study on their HIV infection treatment S/GSK1349572 following favorable results from second stage phase-2 trials. S/GSK1349572 is for HIV-infected patients who have never been treated with anti-HIV drugs or haven't responded well to Merck & Co.'s (MRK) Isentress, which belongs to the same treatment class. -By Kazuhiro Shimamura, Dow Jones Newswires; 813-6269-2790; [email protected] (END) Dow Jones Newswires July 21, 2010 05:47 ET (09:47 GMT)