Determine if brexpiprazole treatment will be associated with a dose-dependent reduction in resting pupil diameter as a reflection of locus coeruleus (LC) norepinephrine (NE) neuron target engagement in a group of subjects with PTSD. All subjects will be evaluated by physical examination, ECG, standard blood chemistry, hematologic labs, toxicology testing, and urinalysis. Results of these studies must demonstrate a lack of clinically significant abnormalities prior to enrollment. Subjects will need to satisfy DSM-5 criteria for PTSD and receive a CAPS-5 score of 40 or greater on testing for study enrollment. Resting pupil diameter during pupillometric evaluation after two weeks on each treatment will serve as the primary outcome measure. This will be compared in the treatment groups using mixed effects repeated measures models to evaluate if there is a significant difference in pupil size among the treatments studied. As a secondary analysis this approach will be used to evaluate whether there is treatment effect on total CAPS-5 score. Lastly, the investigators will compute correlations between pupil size and CAPS-5 scores.