FRANKFURT European and U.S. drug regulators are looking into whether a defective blood clotting test device affected a trial involving Bayer's top-selling anti-clotting drug Xarelto, hitting the German drugmaker's shares.
The study compared Xarelto, also known as rivaroxaban, with older drug warfarin for preventing strokes in patients with a type of irregular heartbeat common among the elderly.
"Due to the defect it is now thought that the INR (international normalised ratio) device may have impacted the clotting results in some patients in the warfarin group," the European Medicines Agency (EMA) said, adding it would publish results of its investigation when complete.
EMA Executive Director Guido Rasi said he would like to see independent research confirming the benefits of Xarelto, given the controversy.
"It would be nice to have some independent study carried out to give confidence in the use of this medicine," he told reporters. "Having said that, I would expect that quite soon we will have a final opinion."
An INR device measures how quickly blood starts clotting. The Alere INRatio and INRatio2 PT/INR Monitor system was recalled in December 2014 after giving falsely low test results.
Bayer and Duke University's Clinical Research Institute, which conducted some of the trial work and which used the device in question, said they had each concluded from fresh analyses that the faulty device did not have an effect on the overall trial results.
"The findings from the (secondary) analysis are consistent with the results from the original trial and do not alter the conclusions," Duke said in a statement.
Bayer chalked up 1.68 billion euros ($1.8 billion) in revenue from Xarelto in 2014 and 1.60 billion in the first nine months of 2015, up 38 percent from a year earlier, making it Bayer's best-selling drug.
The investigation into the device hit Bayer's shares, which were 1.7 percent lower by 1530 GMT, although analysts said the impact would likely be limited.
"Given that ... probably hundreds of different devices were used to assess the control group patients, a single defective device is unlikely to have a statistically relevant effect on the overall outcome," Kepler Cheuvreux's Fabian Wenner said.
The U.S. Food and Drug Administration said it was aware of concerns and was reviewing relevant data.
"Bayer is working closely with health authorities to address any questions they may have," a spokeswoman said. "Study after study continues to confirm that Xarelto is performing as expected with a positive benefit-risk profile."
The investigation was first reported by German daily Handelsblatt.
Xarelto, jointly developed and marketed with Bayer's partner Johnson & Johnson, competes with Bristol Myers-Squibb and Pfizer's Eliquis. Bayer said at the end of October that Xarelto has a 34 percent market share globally.
($1 = 0.9158 euros)
(Additional reporting by Victoria Bryan, Till Weber and Ben Hirschler)