UPDATE 2-Vertex hep C drug works in 24 weeks for many -study

* 92 pct of early responders cured after 24 weeks

* 88 pct early responder cure rate seen with 48 weeks

* Overall telaprevir cure rate 72 percent

* Shares fall .89 percent (Adds analyst comment, updates share price)

By Bill Berkrot

NEW YORK, Aug 10 (Reuters) - A study of Vertex Pharmaceuticals Inc's (VRTX.O) high profile experimental hepatitis C drug showed that the overwhelming majority of previously untreated patients who respond early to telaprevir can be cured in half the time of current therapy.

Of the so-called rapid viral responders, 92 percent of patients who received a total of 24 weeks of therapy with telaprevir plus the standard treatment of pegylated interferon and ribavirin achieved sustained virologic response, or SVR, which is considered tantamount to a cure.

The overall cure rate for all telaprevir patients was 72 percent, a touch below the 75 percent seen in an earlier Phase III trial. But most patients in whom the virus is cleared early may be able to complete treatment in half the time of current tough-to-tolerate therapy, the data showed.

After rising in pre-market trading, Vertex shares were off 0.89 percent on Nasdaq in a down market.

"Expectations were largely that it would be a successful trial, therefore there is not a major positive surprise in the results today," said Thomas Russo, an analyst for Robert W Baird.

"Obviously, it's improving the cure rate in patients in a big way," he added.

Telaprevir is widely expected to become a game changer in the treatment of the liver disease, and some analysts believe it could eventually capture annual sales of more than $4 billion once it is approved.

The lofty hopes for telaprevir are based on its ability to cure a far higher percentage of patients than standard drugs and its potential to cut treatment duration.

In the earlier Phase III trial of previously untreated patients interferon and ribavirin without telaprevir cured only 44 percent of patients. Those drugs must be taken for 48 weeks and often cause debilitating flu-like symptoms, leading many patients to discontinue their use.

The latest late-stage trial separated out patients in whom the virus was undetectable at both week 4 and after 12 weeks of treatment with the three-drug combination to see if there was any advantage to extending therapy with standard drugs beyond 24 weeks to the full 48 weeks in those patients.

Sixty-five percent of the 540 trial participants fell into the rapid responder category. In all cases telaprevir was part of the regimen for the first 12 weeks.

Among those rapid responders, there was an 88 percent cure rate with the full 48 weeks -- slightly less than with 24 weeks. But there was a lower relapse rate with the 48-week group -- 1.9 percent versus 5.7 percent with the 24-week regimen, or 3 patients versus 9.

"Patients who had a rapid response to telaprevir-based regimens at weeks 4 and 12 had a high likelihood of achieving a cure with 24 weeks of total treatment, which may provide important information to motivate people to continue therapy," Dr Kenneth Sherman, principal investigator of the trial, said in a statement.

The company next month is expected to release data from one more Phase III study called Realize -- this time in much tougher-to-treat patients who failed to be helped by prior therapy with standard drugs.

The three late-stage trials will form the basis for Vertex's application seeking approval of the drug that it plans to file with the U.S. Food and Drug Administration by the end of the year.

"So far they're two-for-two in the Phase III data roll-out," Russo said. "We would look for them to make it three-for-three with the Realize results next month.

"If anything, I believe there's some potential for that trial to surpass previous results in treatment failures in Phase II," Russo said.

Telaprevir, from a new class of hepatitis C treatments, is expected to compete with a similar drug being developed by Merck & Co(MRK.N) called boceprevir. But analysts have been virtually unanimous in their belief that telaprevir is the better of the two drugs.

Boceprevir led to an overall cure rate of 66 percent in a study recently released by Merck.

The safety and tolerability of telaprevir in the latest Vertex trial was similar to what was seen in the earlier late-stage study, the company said.

Adverse events, including rash and anemia, led to a discontinuation rate of nearly 7 percent of patients.

Vertex shares were down 33 cents at $36.67 in afternoon trade on Nasdaq. The broader Nasdaq Composite index was off by 1.7 percent. (Reporting by Bill Berkrot; Editing by Gary Hill, Dave Zimmerman and Gunna Dickson)