The US Department of Justice has closed without charges its ten-month investigation of British-Swedish AstraZeneca revolving around the blood thinner Brilinta.

The probe into what the drugmaker regards as a $3.5 billion/year potential blockbuster was launched in October 2013 after questions were raised about results of an 18,000-patient clinical trial, which led to regulatory approval.  

Uncertainty about the outcome of the investigation, which experts said was highly unusual because it looked into the underlying data behind the granting of a license rather than possible marketing misconduct, had hurt sales.

Although AstraZeneca flagged up the promise of Brilinta in its defense against the unsuccessful $118 billion takeover bid by Pfizer earlier this year, the drug registered sales of only $216 million in the first half of 2014.

Results of the Plato study into Brilinta, first reported at a medical meeting in 2009, formed the basis of successful new drug applications in the US, Europe and other markets. The heart drug was launched in 2011.

Some aspects of the study, which relied heavily on patients recruited in Eastern Europe, have been criticized medical experts. In particular, analyses by two doctors reportedly suggested that patients who were monitored by AstraZeneca were claimed to have more beneficial effects than those monitored by an independent clinical research organization.

The company's 2023 estimate for Brilinta sales of $3.5 billion assumes that the blood-thinning drug works in a variety of other settings beyond its current indication for acute coronary syndrome.

A major development program by AstraZenca is now under way to prove the case, with a series of new clinical trials expected to report results every year between 2015 and 2018.

The product competes with Sanofi's now off-patent medicine Plavix, which is available as a cheaper generic.