Denmark's Novo Nordisk could see the U.S. launch of its biggest drug hope, a new long-acting insulin, delayed until 2018 while it conducts more tests to satisfy regulators, it said on Wednesday.

The world's biggest insulin producer was dealt a major blow in February when the U.S. Food and Drug Administration (FDA) refused to approve the drug, called Tresiba, and instead asked for extra tests to assess potential heart risks.

On Wednesday, a clarification of the expected timetable for the drug's progress disappointed analysts, some of whom said the launch could be even later than Novo's estimates.

"This points to a potential approval in 2017 at the absolute earliest, with 2021 on a worst-case scenario," Jefferies said in a note to clients.

The setback for Tresiba, also known as degludec, is good news for rival makers of insulin medicines, notably France's Sanofi, whose Lantus product is threatened by Novo's newer ultra-long-lasting treatment.

The big concern for Novo's investors, though, is that a lengthy delay in getting Tresiba launched in the world's biggest drugs market will undermine its ability to stay ahead of rivals such as Sanofi and Eli Lilly.

Novo said it expected to start cardio-vascular trials with Tresiba within one year. Data to support an interim analysis would be available after two to three years, the company said.

"Assuming that you will have a response from the FDA of the refiling within about half a year, then it (the launch) should be more in the three, maybe five year (timeframe) in the worst case scenario," finance chief Jesper Brandgaard said.

That would point to a launch of Tresiba in 2016 at the best and in 2018 at the worst.

Analysts had been expecting Tresiba and Ryzodeg, another of Novo's insulins, to be selling around $2.8 billion a year by 2017, according to consensus forecasts compiled by Thomson Reuters Pharma. Those numbers look set to fall sharply, since the U.S. market made up more than half of the forecast.

Brandgaard also said the cost of the cardiovascular trials was expected to be $200-300 million over four to five years.

Ups sales view

Novo also nudged up its 2013 sales forecast, the second time in three months, after first-quarter sales and operating profit beat expectations.

The profit rise was driven by a higher-than-expected 14% increase in sales of modern insulins, and a 35% rise in sales of diabetes drug Victoza, which was however slightly below forecasts.

Novo said it now expected 2013 sales growth in local currencies of 9-11%, against 8-11% previously.

It kept its forecast for operating profit growth of around 10%, also in local currencies.

Some analysts had hoped for bigger upgrades.

"Seen in the light of the strong performance in the first quarter, full year expectations look conservative and might disappoint some investors," said Nordea in a note.

Novo made a first-quarter operating profit of 7.6 billion crowns on sales of 20 billion.