Sanofi has acquired a 12% stake in the RNA interference specialist Alnylam Pharmaceuticals for $700 million as the partners expanded their agreement to develop treatments for rare genetic diseases.

The French drugmaker's Genzyme unit is paying $80 per share for the holding, which represents a 27% premium compared to the average price over the last 30 days. The expanded pact will also see Sanofi get more rights to patisiran, which is in Phase III for the treatment of  transthyretin (TTR)-familial amyloid polyneuropathy, a rare life-threatening disease that damages the nervous system.

Sanofi originally only had rights to the drug in Japan and the rest of the Asia-Pacific (where the disease "has a disproportionately high prevalence") and now will sell patisiran in the rest of the world, excluding North America and western Europe, where Alnylam will retain exclusive rights.

Sanofi is also getting rights to three other Alnylam products, including ALN-TTRsc, currently in Phase II for the treatment of familial amyloid cardiomyopathy.

David Meeker, Genzyme's chief executive, said the collaboration "is an important building block for our future. It strengthens our pipeline and provides us with the opportunity to meet the needs of patients with rare diseases around the world through our well-established global organisation". His counterpart at Alnylam, John Maraganore, added that the deal "is a game changer for…the advancement of RNAi therapeutics as a new class of genetic medicines".

Alnylam buys Sirna off Merck & Co for $175 million

The new pact was unveiled hours after Alnylam revealed that it is buying Merck & Co's RNAi subsidiary Sirna Therapeutics.

Alnylam is paying $175 million upfront, $25 million in cash and the rest in stock. Merck could also receive up to $115 million in milestone payments, plus single-digit royalties.

The deal represents a financial hit for the US drugs giant which acquired Sirna for $1.1 billion in October 2006. Merck Research Laboratories chief Iain Dukes said the sale "is consistent with our strategy to reduce emphasis on platform technologies and prioritise our R&D efforts."

He added that Merck will now "focus on product candidates capable of providing unambiguous promotable advantages to patients and payers".