The move marks first product opt-in since the companies formed an alliance in January last year to speed up and expand the development and commercialization of RNAi therapeutics globally.
Genzyme’s opt-in decision was based on encouraging clinical data from the phase 1 trial of ALN-AT3, including positive interim data.
ALN-AT3 is the third Alnylam product for which Genzyme exercised its opt-in right.
The first two products were exercised in early last year for patisiran and revusiran, investigational RNAi therapeutics to treat transthyretin-mediated amyloidosis.
With respect to ALN-AT3, Genzyme elected currently to opt into the program for its ROW rights.
Genzyme will pay 20% of worldwide development costs for regional programs where it will develop and commercialize in their ROW territory, such as ALN-AT3 as currently structured.
Alnylam is eligible to receive milestones totaling up to $75m per product for regional and co-develop/co-promote programs.
Genzyme president and CEO David Meeker said: "ALN-AT3 represents a unique and promising new approach for the potential treatment of hemophilia.
"We are excited to expand our clinical stage pipeline of treatments for patients with rare diseases, and to broaden our relationship with Alnylam."
ALN-AT3 is intended at correcting coagulation defects by knockdown of antithrombin (AT), which acts as a brake on the production of thrombin, a protein essential for the formation of a blood clot.
It is being evaluated in a Phase 1 study in people with moderate-to-severe hemophilia. A pivotal Phase 3 clinical trial is planned to commence in mid-2016.