HONG KONG (Reuters) - CStone Pharmaceuticals said on Wednesday a unit of Pfizer Inc PFE.N had agreed to buy a 9.9% stake for HK$1.55 billion ($200 million), as the Hong Kong-listed biopharmaceutical firm seeks to improve the commercialisation of its products. The news sent the Hong Kong-listed stock up as much as 39.6% to HK$12.98, the highest since October 2, 2019. The stock is on track for a fourth straight session of gains, and the best day since listing in February 2019. Shanghai-based CStone said the deal would allow it to focus on product development and strengthen its ability to commercialise CS1001 - an anti-PD-L1 monoclonal antibody. Pfizer Corporation had 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, which focuses on immuno-oncology medicines, said it had granted a Pfizer unit an exclusive licence to commercialise CS1001 in mainland China.