Have you done any of the looking at historical rates of ORR in such a highly refractory population?
If you are not counting GNMBP status then I can find very similar trials, and that is were I guessed 5% from.
I have no clue at all about how G+ selection would affect this. I do see cites that it is associated with outcomes (G+ has worse outcomes), but no sites on ORR.
EDIT: The Eribulan P3 trial was close in patient pool, and the comparator arm ORR was 5%.