The way the study doctors explained the problem to me is:

1) Without biomarkers, they are not sure if the study participant has PD. (This is especially true with recently diagnosed applicants)

2)Things like an unexplained low white blood cell count bring up safety and liability concerns.

3)Some applicants UPDRS (motor) score normally fluctuate too much to detect a clinically important difference due to a study medication.

4)A lot of study participants that think they are getting the placebo drop out.