This would fit with the prior expected completion date in 2016 per CT.gov - I'm assuming there was a DSMB for this trial that saw HR going uncomfortably above 1 leading to a decision to stop the trial. What is also strange to me though is that the halt comes about 2 years after the last patient enrolled in the trial. The 8k wording says no difference in PFS, TTP, or AE, though it's not clear if numerically there was a trend.
I remember looking at the smuc1 sub-group data (though I don't have the cite handy at the moment). Merck had looked at two bifurcation levels of smuc1 at ULN, which had a good HR, and let's say 1.5x ULN (don't recall exactly) which was much better. However I calculated that the treatment effect was entirelly driven by the >1.5x sub-group. Below that the HR was about 1, confining Stim benefit to roughly 20% of enrollment. Apparently Merck decided this was too large of a reduction in market size to be worth their while.