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The Food and Drug Administration has OK’d an Alnylam drug to treat a rare genetic disorder that can cause severe abdominal pain, making it the second medicine based on a Nobel prize-winning technique known as RNA interference to win regulatory approval.

RNA interference, or RNAi, lets scientists mute genes that aren’t functioning properly. Although scientists pioneered the technique decades ago, the FDA only approved the first RNAi-based drug, another of Alnylam’s, last year.

That drug, patisiran, costs up to $450,000 per year — but is subject to agreements with insurers that link the actual cost of the drug to how well individuals respond to it.

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The Cambridge biotech’s new drug is called givosiran and treats a condition called acute hepatic porphyria. Only half of the people in one trial for givosiran showed a complete elimination of the attacks that can be a symptom of acute hepatic porphyria. On average, people in that trial saw a 74 percent decrease in attacks over six months.

“The people who have been in the trials and the people that would benefit most from this drug are the people who have frequent attacks,” Dr. Karl Anderson, a porphyria specialist at the University of Texas Medical Branch who was involved in some of Alnylam’s clinical trials, told STAT in April.

Currently, Anderson said, the only treatment that can sometimes be used is weekly doses of hemin; hemin can’t be used to prevent attacks.

The vast majority of people with acute hepatic porphyria do not have frequent attacks; according to the National Organization for Rare Diseases, just 3-5 percent of people fall into that category.

In July, the company had estimated that 1,000 to 1,500 people could be eligible and interested in taking the drug.

Despite the relatively small number of eligible patients, some analysts were projecting peak sales of up to $600 million before Wednesday’s approval.

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The drug’s approval is a good sign for other companies developing drugs based on the RNAi technique. The drug is the first to use a type of sugar called GalNAc that allows more precise delivery of drugs to liver cells — which could, potentially, reduce serious side effects associated with the drug going elsewhere in the body.

In clinical trials, a significant proportion of people taking givosiran still experienced serious side effects, including chronic kidney disease, a high fever, and abnormal results on liver function tests.


Kate Sheridan can be reached at kate.sheridan@statnews.com.