ZURICH (Reuters) - Swiss drugmaker Roche’s shares dropped on Wednesday after it told U.S.-based haemophilia advocacy groups that five patients treated with its medicine Hemlibra had died, while maintaining that the therapy was not the cause of the deaths.

The logo of Swiss drugmaker Roche is seen at its headquarters in Basel, Switzerland February 1, 2018. REUTERS/Arnd Wiegmann

The shares slipped 2 percent by 0850 GMT -- double the decline in the European healthcare sector -- as news of the patient deaths took the shine off a separate Japanese court ruling in Roche’s favour in a patent dispute over Hemlibra with rival drugmaker Shire.

The Hemophilia Federation of America (HFA) and the National Hemophilia Foundation (NHF) groups issued alerts on Tuesday after Roche’s Genentech unit informed them of the deaths of patients with the genetic disease that stops blood from clotting properly.

“To date, five adults taking Hemlibra have passed away,” Roche said in a statement to Reuters. “For all, the treating physician or investigator’s assessment was that the cause of death was unrelated to Hemlibra.”

One of the deaths was reported in 2017, in which a patient experiencing a bleeding event refused a blood transfusion for religious reasons.

One adult who was taking part in a compassionate-use programme died in 2016, Roche said on Wednesday. Another died in 2017, while two have died this year.

Roche did not give the specific causes of those deaths in its statement.

There have also been several cases of adverse events -- thrombotic microangiopathy, or damage to blood vessels in vital organs -- that emerged during clinical trials. Roche has blamed these on other medicines used to treat bleeding events.

Analysts have said Hemlibra, which costs roughly $450,000 per year, has potential to hit $5 billion in annual sales by muscling in on older drugs from rivals like Shire. But risks remain, as highlighted by scrutiny of these latest deaths.

“With very limited information, it is unclear if there are additional safety concerns with Hemlibra at this point,” wrote Jefferies analyst David Steinberg.

“However, it’s a possibility that heightened concerns -- which arose after the initial death... -- could re-emerge.”

With Hemlibra, Roche has expanded its haematology franchise beyond blood cancers and is banking on the new medicine to help make up for falling revenue from patent-expired oncology drugs Rituxan, Avastin and Herceptin.

But its incursion into a rare disease with a first-of-its-kind drug that doctors are still trying to understand also brings challenges, including intense scrutiny from the community of 20,000 U.S. patients.

They are well organised after many sufferers died in the 1980s when they contracted HIV from contaminated blood products.

“HFA recognizes that there has been robust discussion on social media as the community has been made aware of this and other adverse events surrounding Hemlibra,” the group said. “We will continue to monitor the situation and share information as we learn more.”