Aug 8 (Reuters) - Merck ( MRK ) said on Thursday it had

discontinued a trial testing a combination immunotherapy

treatment in patients with a type of lung cancer after an

interim analysis showed the drug was unlikely to succeed.

The patients tested with a combination of its experimental

drug, vibostolimab, with Keytruda experienced a higher rate of

immune-related side effects, Merck ( MRK ) said, adding that patients

should stop ongoing treatment with the therapy.

The discontinuation marked yet another setback for

vibostolimab and the promising new class of immunotherapies

called anti-TIGIT to which it belongs.

Gilead Sciences ( GILD ), Roche and GSK are

among half a dozen drugmakers looking to grab a share of the

lucrative anti-TIGIT cancer drug market, but the field has

experienced multiple setbacks.

Vibostolimab works by selectively binding itself to TIGIT, a

receptor on immune cells, to activate the immune system against

cancer cells and prevent a misguided immune attack against

healthy cells.

Merck ( MRK ) was testing vibostolimab along with Keytruda as a

first-line treatment for patients with extensive-stage small

cell lung cancer in the late-stage study.

Last year, the combination therapy had failed to

significantly slow disease progression in lung cancer patients

who did not respond to previous treatments.

The drugmaker discontinued another late-stage study in May

testing the combination therapy in patients with a severe form

of skin cancer after side effects led to high discontinuation.

(Reporting by Leroy Leo in Bengaluru; Editing by Devika

Syamnath)