In a late-stage trial, the drug helped people suffering from pulmonary arterial hypertension (PAH), a progressively worsening condition that can overburden the heart, to better tolerate physical exercise and improve heart function, Bayer said on Monday.

The results will be presented at this week's annual meeting of the American College of Chest Physicians (ACCP) in Atlanta.

PAH, the cause of which is unknown, affects only about 52 in a million people worldwide, according to Bayer.

Actelion generated 1.52 billion swiss francs ($1.65 billion) from its established PAH drug Tracleer last year.

Bayer declined to give a sales estimate for its product. Analysts on average saw 2017 sales from riociguat of $480 million before Monday's news, according to Thomson Reuters Pharma.

As part of the trial, doctors measured how far 443 participants could walk in six minutes before and after 12 weeks of either three-times-daily riociguat or a placebo pill.

Patients on riociguat improved their walking distance by 36 metres on average, while the control group showed no improvement from an initial 330-350 metres, Bayer said. The riociguat group also had better cardiovascular readings.

A similar improvement in walking distance was also achieved by adding riociguat to an existing Tracleer therapy, the study results showed.

Riociguat's side effects included headache, dizziness, swollen limbs and nausea but it was overall well tolerated, Bayer said.

A major downside of Tracleer is the risk of liver damage but Actelion is working on a new PAH drug, macitentan, as revenues from Tracleer decline. The Swiss biotech company said in April macitentan succeeded in slowing the disease and is due to present more data at this week's ACCP conference.

It is, however, facing growing competition from Gilead Sciences Inc's Letairis.

Bayer plans to file for marketing approval of riociguat in the United States, the European Union and Japan simultaneously in the first half of next year with a market launch possible in mid-2014.

($1 = 0.9229 Swiss francs)

(Reporting by Ludwig Burger and Frank Siebelt; Editing by Erica Billingham)

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