Probably Aranesp, IMO. If so, the decision was presumably influenced by MRK’s plan to launch a branded knockoff of Aranesp in the US before the Aranesp process patent expires.
[From 8K filing]: Of the $188.0 million in milestone payments that the Company was eligible to receive under the 2006 Collaboration (if all milestones were achieved for the four product candidates), the milestone payments related to M249 totaled $10.0 million.
In other words, there are $178M in potential milestones remaining for Lovenox/EU, Copaxone, and M178, whose identity has not been disclosed.