HONG KONG, Sept 30 (Reuters) - CStone Pharmaceuticals, which focuses on immuno-oncology medicines, said on Wednesday a unit of Pfizer Inc has agreed to buy 9.9% of the Hong Kong-listed biopharmaceutical firm for HK$1.55 billion ($200.01 million). Pfizer Corporation has agreed to buy 115.93 million new shares of China-based CStone at HK$13.37 apiece, representing 43.8% premium over the closing price of HK$9.30 on Tuesday. CStone will use the $200 million in proceeds from the share sale to fund development activities and strategic collaboration, the Hong Kong-listed firm said in a filing to the Hong Kong bourse. ($1 = 7.7497 Hong Kong dollars) (Reporting by Donny Kwok; Editing by Christian Schmollinger)