I read the abstract from the link Imtesty provided and I think "attenuated" plaque is referring to plaque that is changed to a more stable form of plaque which is thought to be less prone to rupture thus causing a cardiovascular event. ie the plaque has been attenuated.

If I recall correctly, after some of the post hoc analysis was revealed RVX had some IVUS slides in the presentations that showed a positive change in the plaque composition in the Apabetalone treated trial population. Positive because, we were told at the time, these changes made the plaque more stable. 

Just my recollection, please correct me if I am incorrect.