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UPDATE 1-Alnylam RNAi drug reverses progress of rare fatal disease -study

(Adds share move)

Nov 2 (Reuters) - More than half of patients with a rare, rapidly progressing often fatal genetic disorder experienced significant improvement after being treated with an Alnylam Pharmaceutical Inc drug that uses gene silencing technology, according to results of a late stage study released on Thursday.

Alnylam shares rose more than 17 percent, to $142.16.

The drug, patisiran, was being tested against hereditary ATTR (hATTR) amyloidosis with polyneuropathy, which affects an estimated 50,000 people worldwide. It is being developed in partnership with Sanofi.

When Alnylam announced in September that the trial succeeded, its shares surged 51 percent. If approved, patisiran is forecast to reach annual sales of $1 billion by 2023, according to Thomson Reuters data.

More than 90 percent of those who received patisiran infusion every three weeks in the 225-patient trial experienced a halt in disease progression, and 56 percent showed an improvement in their condition after 18 months, the companies said.

On a neurological disease impairment scale, patisiran patients' scores on average declined by 6 points, indicating improvement, while scores of those who received a placebo worsened by 28 points, data presented at a medical meeting in Paris showed.

The mean 34-point difference was deemed highly statistically significant.

In addition, 51 percent of patisiran patients reported improvement on a quality of life scale, with a mean 6.7-point improvement at 18 months versus a 14.4-point worsening for placebo recipients. Patisiran patients also saw improvement in a 10-meter walking measure.

"Patients with hATTR amyloidosis... have a profound need for effective and safe treatment options," Dr. David Adams, the study's lead investigator, said in a statement. "As a clinician, it has been deeply rewarding to see the potential impact patisiran may have on the lives of hATTR patients."

The drug belongs to a new class of medicines that employ RNA interference, or RNAi, which prevents a defective gene from making disease-causing proteins.

If unchecked, the gene mutation causes amyloid proteins to accumulate in peripheral tissues of the nerves, gut and heart, causing damage to organs and tissue and a wide range of debilitating symptoms. Patients have a life expectancy of 2.5 to 15 years from symptom onset.

The companies plan to file for U.S. approval by year end and in Europe shortly after.

The most common patisiran side effects were peripheral edema and infusion-related reaction that led to one patient discontinuation. Of serious adverse side effects reported, only a case of diarrhea was deemed related to patisiran. (Reporting by Bill Berkrot; Editing by David Gregorio and Dan Grebler)