There's no way to know the broader picture, but I can tell you beyond any doubt that it affects my wife in a very calming way, so either it or one of its metabolites is getting through.
That brings up a point to think about. As Ron has pointed out ocassionally,
the BBB is leaking like a sieve in PD. It would seem that things that would not cross the BBB in a healthy person just might in a PWP. So, if something looks promising but there is the barrier problem, don't make the assumption that it is useless to try.