The short of it is that, although RSR-13 has no toxicity and could therefore be given a conditional pass, the committee did not want to do so because it would have meant that further trials would be compromised since no doctor could ethically administer a placebo in a further trial.
The breast met subgroup, taken as a separate clinical in itself, would have most likely been approved. Makes you wonder if you could use the same trial to run two or more NDA's - one for all brain cancers, one for breast->brain, one for lung->brain, etc.
I have not heard of such an approach, so there must be a regulatory, if not scientific, reason why this has not been done.