Jan 20 (Reuters) - Britain's GSK will buy RAPT Therapeutics for $2.2 billion, adding global rights to the U.S. firm's experimental food allergy drug ozureprubart to its respiratory and immunology portfolio, the drugmakers said on Tuesday.

RAPT Therapeutics shares surged 19.9% in overnight trading.

The move is GSK's first major acquisition under its new CEO Luke Miels as the company navigates U.S. tariffs and seeks new medicines to offset revenue declines from some top-selling drugs going off patent.

Investors are counting on Miels to help GSK hit its annual revenue target of more than 40 billion pounds ($54 billion) by 2031.

London-listed GSK will pay $58 per RAPT share, with an upfront investment of $1.9 billion. The deal will give GSK global rights to the ozureprubart programme, excluding mainland China, Macau, Taiwan and Hong Kong.

GSK shares fell about 1% in early trading to 1,802.5 pence per share.

PROMISING THERAPY

RAPT's ozureprubart is a lab-engineered therapy designed to prevent inflammation caused by allergic reactions. It works by targeting an antibody responsible for the immune reaction, and offers potential for a treatment with less frequent dosing than the current standard.

"Ozureprubart ... is consistent with our approach to acquire assets that address validated targets and where there is clear unmet medical need," GSK Chief Scientific Officer Tony Wood said in a statement.

On Tuesday GSK also said that Japan's Shionogi & Co would boost its stake in the group's majority-owned ViiV Healthcare after Pfizer's exit from the venture established in 2009.

GSK retains its 78.3% majority stake in ViiV, which is advancing a pipeline of long-acting injectable HIV treatments and prevention medicines, and will get a special dividend of $250 million as ViiV cancels the U.S. drugmaker's shares.

($1 = 0.7424 pounds)