Primary Hypothesis: Brexpiprazole treatment will be associated with dose-dependent reduction in resting pupil diameter as a reflection of LC NE neuron target engagement in a group of subjects with PTSD. Secondary Hypothesis: Brexpiprazole therapy will be associated with a dosedependent decrease in CAPS-5 scores Tertiary Hypothesis: The pre-post treatment change in resting pupil diameter will be statistically significantly correlated with the pre-post change in CAPS-5 score.