An independent data monitoring committee observed that patients taking the drug in the trial were at a possible increased risk for death and other severe adverse events as compared to patients in the placebo group.
The committee has however did not find any specific cause or common feature among the patients who died except that several of them appeared to have more serious and advanced underlying lung disease than the study population as a whole, Bayer said.
Bayer Pharmaceuticals executive committee member and head of global development Joerg Moeller said: "We understand that the need to terminate the study in PH-IIP is very disappointing for patients suffering from this disease, as well as for their doctors and healthcare providers.
"There is a significant unmet medical need for PH-IIP patients as there are no approved treatments, and finding an effective treatment remains a challenge."
Riociguat is a soluble guanylate cyclase (sGC) stimulator, developed by Bayer as an oral treatment to target an important molecular mechanism underlying PH.
sGC is an enzyme identified in the endothelial cells and the receptor for nitric oxide (NO).
Riociguat sensitizes sGC to endogenous NO by stabilizing the NO-sGC binding. In addition, it directly stimulates sGC through a different binding site, independently of NO.
Riociguat is approved in the US, the European Union and Japan, and several other countries for use in inoperable chronic thromboembolic pulmonary hypertension (CTEPH) or persistent or recurrent CTEPH after surgery, and in pulmonary arterial hypertension.