Novo Nordisk has suffered a huge blow after US regulators refused to approve its new insulin Tresiba.

The Danish drugmaker saw its shares slide as much as 15% after it emerged that the US Food and Drug Administration demanded a new trial looking at the cardiovascular effects of the long-acting insulin, substantially delaying its potential entry onto the market.

Novo said it does not expect to be able to provide the requested data during 2013, and failure to win approval of Tresiba (insulin degludec) in the all-important US market at this time could really impede on the firm's ability to hit its financial targets, rocking shareholder confidence.

Bernstein analyst Tim Anderson of Bernstein has reportedly said that in the best-case scenario the drug won't be on the US market until 2015, and that is assuming the new trial can gather enough patients quickly and the FDA requires only partial data, according to Reuters.

The FDA's decision has come as somewhat of a shock, given that, despite concerns over cardiovascular safety issues, its advisory panel actually supported approval of Tresiba back in November.

The panel unanimously recommended that a cardiovascular outcomes trial should be conducted, but said this could be done post-approval.

Approvals in Europe, Japan

This was followed by green lights in Europe for the drug and sister product Ryzodeg (insulin degludec/insulin aspart) in Europe last month (they are also available in Japan).

Analysts had been expecting the drug to quickly hit blockbuster status and rake in billions of dollars a year for Novo, but given the FDA's reluctance, this is now in doubt. 

Novo is hoping that Tresiba will challenge Sanofi's Lantus, the world's biggest-selling insulin product Lantus (insulin glargine), as its studies have shown that it demonstrated a significantly lower risk of overall and nocturnal hypoglycaemia, while successfully achieving equivalent reductions in HbA1c.

Its uncertain future in the US will now give Lantus some breathing space, however, news which helped nudge Sanofi's shares up 3% by close on the New York Stock Exchange last night.