Bayer has halted a Phase III study into its clot-prevention drug Xarelto after it showed no efficacy improvmenets over aspirin.

The trial was evaluating the drug against aspirin in patients with embolic stroke of undetermined source (ESUS).

While bleeding rates were low overall, an increase in bleeding was observed in the rivaroxaban arm compared to the low dose aspirin arm.

“Patients with ESUS currently have limited treatment options and the role of anticoagulants in this area remains uncertain. We will now analyse the data from NAVIGATE ESUS to better understand this outcome and its implications,” said Dr Joerg Moeller, Member of the Executive Committee of Bayer AG's Pharmaceutical Division and Head of Development.

Xarelto is already approved for a number of cardiovascular conditions, with its main profit-driver being the prevention of stroke and systemic embolism in adult patients with non-valvular atrial fibrillation (AF) and one or more risk factors. Bayer has been expecting the drug to continue its blockbuster rise, and after the halting of the ESUS trial stuck by its annual peak sale estimates of $5.3 billion.

The drug is jointly developed by Janssen.