May 13 (Reuters) - Merck ( MRK ) said on Monday it had

discontinued an experimental combination treatment testing a new

type of immunotherapy in patients with a severe form of skin

cancer after side effects led to high discontinuation.

This is the latest setback for the experimental drug,

vibostolimab, and the related promising new class of

immunotherapies called anti-TIGIT.

Merck ( MRK ) was testing vibostolimab along with Keytruda, its

top-selling cancer drug, in a late-stage study in patients with

resected high-risk melanoma.

An analysis showed the trial will not achieve the main goal

of statistically significant improvement in recurrence-free

survival as more side effects led to several patients

discontinuing the combination treatment versus those on Keytruda

alone, Merck ( MRK ) said.

The company will recommend patients receiving the

combination treatment be offered the Keytruda monotherapy.

Vibostolimab works by selectively binding itself to TIGIT, a

receptor on immune cells, to activate the immune system against

cancer cells.

Merck ( MRK ) said it still had an extensive clinical development

program evaluating the vibostolimab-Keytruda combination.

The combination treatment had earlier failed to

significantly slow disease progression in lung cancer patients

in a late-stage study last year.

Data from another trial in March last year had also showed

that it was less effective than a generic medicine called

docetaxel in some patients with non-small cell lung cancer.

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 seen

multiple setbacks.

Gilead and its partner Arcus Biosciences ( RCUS ) in January

stopped a late-stage trial involving their anti-TIGIT candidate

domvanalimab in a type of lung cancer. Gilead has announced a

$320 million investment in its biotech partner as it refocused

its anti-TIGIT push.