Jan 30 (Reuters) - Merck ( MRK ) plans to stop a

late-stage study testing its drug to treat pulmonary arterial

hypertension (PAH) ahead of time based on strong efficacy shown

in previous studies, the drugmaker said on Thursday.

The therapy, branded Winrevair, is already approved for

treating the rare condition which causes high blood pressure in

the lungs, leading to symptoms such as shortness of breath,

chest pain and dizziness.

Analysts expect Winrevair to become a multi-billion-dollar

product for Merck ( MRK ), which has been beefing up its portfolio of

cardiovascular drugs to counter a looming hit to sales of its

top-selling cancer treatment Keytruda from close copies.

Merck's ( MRK ) decision was partly based on interim data from

another late-stage study, named ZENITH, in which Winrevair

helped significantly reduce the risk of death in PAH patients.

"Based on the strong, positive interim efficacy data from

the ZENITH trial, as well as the totality of available Winrevair

data, we concluded that it would not be ethical to continue the

HYPERION study," said Eliav Barr, chief medical officer at

Merck ( MRK ).

The current study, named HYPERION, enrolled about 300 newly

diagnosed patients PAH patients who were randomly chosen to

receive Winrevair along with a background PAH therapy or placebo

and background PAH therapy.

The trial participants would be able to continue receiving

Winrevair through a separate open-label extension study, Merck ( MRK )

said, adding that it has discussed the decision with the U.S.

Food and Drug Administration.

Winrevair has brought in sales of $219 million since its

launch in March. Analysts expect the drug to bring in $1.5

billion in sales this year, according to LSEG data.

The therapy is also approved in the U.K., European Union and

30 other markets.