I don't recall the company stating that the first lookin is event driven.
In P3s one normally talks about early looks with the possibility of halting the trial for efficacy. If such, it would virtually certainly be event based (for a primamry endpoint of OS).
The only other possibility I could imagine is that the first look is some sort of safety and/or futility test required by the FDA due to the P2 "Fargoism". A safety look would be based on enrollment. But this seams unlikely for a few reasons. First, why would PPHM even mention it? Second, a planned safety look would seam to require an enrollment pause.
But the above paragraph is all speculation. I would assume a simple event based look, and if not it is something weird and not likely to benefit anybody.
EDIT: I posted this before seeing itsabouttime's statement that it is event based.