By Ransdell Pierson and Bill Berkrot

NEW YORK (Reuters) - Abbott Laboratories Inc said on Friday that it could have a shorter duration combination therapy for hepatitis C on the market in 2015 with annual sales potential of about $2 billion.

Significantly, its combination therapy does not include tough-to-tolerate interferon, which causes flu-like symptoms that lead many patients with the serious liver disease to stop or delay treatment.

Abbott said that in very small mid-stage trials, a combination of two of its experimental medicines and two other drugs were showing the potential for cure rates as high as 90 percent with as little as 12 weeks of therapy. The shortest duration current patients can hope for is 24 weeks.

Abbott discussed its hepatitis C trials and other experimental medicines at a meeting with investors and analysts in New York just two days after announcing it would spin off its prescription drugs business into a separate company.

"While early, these results are unprecedented in that very high cure rates are being achieved ... with only 12 weeks of interferon-free therapy," said Richard Gonzalez, who will head the new company.

The data presented by Abbott were culled from a pair of trials with a total of just 44 patients. Far larger trials would be needed to confirm the efficacy and safety of the combination therapy.

JPMorgan analyst Michael Weinstein was taking a cautious stance on the potential for Abbott's approach to hepatitis C.

"It's still early and there are a lot of other companies working in this space," he said.

One such company, Pharmasset Inc, has seen its shares soar in recent months over promising data for its own experimental interferon-free hepatitis C regimen. But Pharmasset shares fell as much as 20 percent after the Abbott presentation and were down 12 percent in afternoon trade.

"The market was expecting Pharmasset to be the first all-oral combination regimen for hepatitis C," said Brian Skorney, an analyst for Brean Murray, Carret & Co.

He believes the Pharmasset sell-off was an overreaction, given how little data Abbott has provided.

"I'd be interested to see what the safety profile will be for the four drugs," he said of the Abbott combination. "Pharmasset's is two drugs."

If the Abbott data were to hold up in much larger trials, its therapy could eclipse the new drugs from Vertex Pharmaceuticals Inc and Merck & Co that have been hailed as breakthroughs for achieving far-higher cure rates with the potential for shorter treatment durations than the older standard regimen of pegylated interferon and ribavirin, which had to be taken for 48 weeks.

Both the Merck and Vertex drugs must be taken along with interferon and ribavirin. Vertex, in its pivotal clinical trials, saw cure rates as high as 80 percent for its Incivek, while Merck's Vicrelis cured 66 percent of patients. Neither of the new regimens is taken for less than 24 weeks.

Shares of Vertex, which has just the one drug on the market, fell almost 3 percent.

The two Phase II trials of previously untreated patients combined the antiviral drug ritonavir with Abbott's ABT-450 -- a protease inhibitor from the same class as the Vertex and Merck drugs -- one of two Abbott polymerase inhibitors, ABT-333 or ABT-072, and ribavirin.

All patients who remained in the studies achieved an early virologic response at 12 weeks, meaning the virus was undetectable in the blood, Abbott said, with no virus rebounds occurring during therapy.

Of the 10 patients so far who were tested 24 weeks after completing the 12 weeks of therapy, nine had achieved a sustained virologic response, or SVR, the company said.

"In other words, these patients were cured with a 12-week course of therapy without interferon," Gonzalez said.

The regimen has been well tolerated to date, the company said, without discussing side effect details.

Hepatitis C, which if untreated can lead to cirrhosis, liver cancer and the need for a transplant, infects an estimated 4 million Americans, and 180 million worldwide.

Abbott plans to present more detailed data on these and other trials next year. It said the U.S. Food and Drug Administration had given fast track status to the regimen.

With fast-track status a company can submit data from clinical trials on a rolling basis as it becomes available rather than waiting until pivotal Phase III studies are complete before submitting the results to the FDA.

Abbott shares were off 0.4 percent at $53.86.

(Reporting by Ransdell Pierson and Bill Berkrot, editing by Gerald E. McCormick)