Merck to buy Harpoon Therapeutics for $680 million

(Reuters) -Merck & Co said on Monday it would buy cancer drug developer Harpoon Therapeutics for about $680 million, reinforcing its oncology portfolio with immunotherapies.

The U.S. drugmaker has been doubling down on building future revenue streams as its blockbuster immunotherapy, Keytruda, inches closer to losing key patents, opening the doors for competition from biosimilars.

Merck has offered $23 per share in cash for California-based Harpoon, representing a 118% premium to the stock's last close on Friday.

Shares of Harpoon more than doubled premarket on Monday after deal talks were first reported by Bloomberg.

Harpoon has two immunotherapies in early stage development - one for a type of lung cancer and the other for multiple myeloma.

Immunotherapies harness the power of the body's immune system to treat people suffering from cancer and work by directing the patient's T cells to recognize and attack the cancer.

(Reporting by Sriparna Roy and Bhanvi Satija in Bengaluru; Editing by Sriraj Kalluvila)