US regulators have again knocked back Janssen Research & Development's application to extend the label for the blood thinner Xarelto.

The US Food and Drug Administration has issued a second complete response letter to the firm's request to market the drug for the reduction of the risk of cardiovascular events in patients with acute coronary syndrome (ACS).

The company - a Johnson & Johnson group - filed the supplemental New Drug Application back in February last year, and received priority review designation from the FDA, but was subsequently turned down in June on fears over fatal bleeding.

Janssen later submitted additional data to support its application, but evidently this was not enough to address the regulator's concerns.

Details of the current CRL remain thin, but the issue seems to relate again to the pivotal ATLAS ACS 2 TIMI 51 trial and risk of bleeding.

Janssen said it is evaluating the letter and will respond to the agency's questions, but remains "confident in the robustness and results of the ATLAS ACS 2 TIMI 51 trial, evidenced by a significant reduction in cardiovascular events, including a clinically important decrease in cardiovascular death," according to vice president Christopher Nessel.

"While we saw an increase in major bleeding, there was no increase in fatal bleeding," he stressed.

Xarelto (rivaroxaban), which is also marketed by Bayer, has already won approval for six clinical uses in the US, but an approval for ACS would add significant ballast to sales, given that 1.2 million patients with ACS are hospitalised ever year in the country alone.

Sales of Xarelto, which was first launched in 2011, hit 322 million euros, rocketing 266% from the prior year.