Alnylam's drugs use RNA interference, or RNAi, technology. RNAi therapies work by turning off or silencing disease-causing genes.
Under terms of the exclusive alliance, Medicines will make a $25 million upfront cash payment to Alnylam. There is also the potential for up to $180 million in development and commercial payments if certain milestones are met.
Medicines is based in Parsippany, N.J. Alnylam is based in Cambridge, Mass.
Medicines' stock fell 42 cents to $30.11 in afternoon trading, while shares of Alnylam declined 57 cents, or 2.4 percent, to $23.67.