** Shares of Alphamab Oncology 9966.HK drop 11.8% to HK$8.07, their biggest intraday pct decline since May 12

** Stock hits the lowest since May 21, and on course to snap three straight sessions of gains, if current trend holds

** Alphamab Oncology says controlling shareholder plans to sell 14.6 mln existing shares at HK$8.05 each in share placement, reducing its stake to 31% from 32.5%

** Co says the controlling shareholder will use the proceeds to repay existing personal liabilities and debts, and the placement is not expected to have any impact on its business or operations

** The Hang Seng Biotech Index .HSHKBIO gains 1.3% and the Hang Seng Index .HSI climbs 1.2%

** Alphamab stock up 133.7%, YTD

(Reporting by Donny Kwok)

((donny.kwok@thomsonreuters.com))