Effect Size of Brexpiprazole for PTSD
The standardized effect size of brexpiprazole added to sertraline for PTSD is modest, with a placebo-subtracted difference of approximately -5.6 points on the CAPS-5 scale (95% CI: -8.79 to -2.38), representing a clinically meaningful but moderate improvement in PTSD symptoms.
Evidence from Clinical Trials
The effect size comes from the most robust Phase 3 trial data available. In the primary efficacy study, brexpiprazole (2-3 mg/day) combined with sertraline (150 mg/day) demonstrated a least-squares mean change of -19.2 points on the CAPS-5 total score at week 10, compared to -13.6 points for sertraline plus placebo, yielding a difference of -5.59 points (P < .001) 1.
A second Phase 3 trial using flexible dosing showed similar results, with brexpiprazole plus sertraline achieving a -16.4 point reduction versus -11.4 points for sertraline alone, producing a difference of -5.08 points (95% CI: -8.96 to -1.20; P = .011) 2.
However, a third Phase 3 trial using fixed-dose brexpiprazole (2 mg or 3 mg) failed to separate from sertraline plus placebo, with no significant differences observed at week 10 3. This inconsistency highlights important limitations in the evidence base.
Meta-Analytic Findings
A systematic review and meta-analysis of four RCTs (981 patients) confirmed the modest effect size, showing:
- CAPS-5 improvement: Mean difference of -3.77 points (95% CI: -6.79 to -0.75; P = 0.01) 4
- CGI-S scores: No statistically significant difference (MD -0.15; 95% CI -0.33 to 0.04; P = 0.12) 4
The fact that global clinical impression scores did not significantly improve despite CAPS-5 changes suggests the clinical meaningfulness of this effect size remains debatable.
Contextualizing the Effect Size
To put this in perspective:
- The CAPS-5 total score ranges from 0-80 points
- A 5-6 point difference represents approximately 6-8% additional improvement beyond sertraline monotherapy
- This is considerably smaller than the effect sizes typically seen with first-line trauma-focused psychotherapies
- The effect is statistically significant but falls in the "small to moderate" range by conventional standards
Critical Limitations
The evidence base has significant weaknesses:
- One of three Phase 3 trials was negative 3
- High placebo response rates across all trials (sertraline alone produced 11-18 point CAPS-5 reductions)
- Brexpiprazole monotherapy showed no efficacy for PTSD 2, indicating the effect depends entirely on combination with an SSRI
- FDA rejected brexpiprazole for PTSD indication despite supportive data 5
Safety Trade-offs
The modest efficacy gains come with measurable adverse effects:
- Weight gain: Mean difference of 1.47 kg (95% CI: 0.84 to 2.09; P < 0.01) 4
- Somnolence: 2.39-fold increased risk (95% CI: 1.03 to 5.57; P = 0.04) 4
- Akathisia occurred in 13.3% with brexpiprazole monotherapy 2
Clinical Bottom Line
The standardized effect size of brexpiprazole augmentation is approximately 0.3-0.4 standard deviations based on the CAPS-5 data, which translates to a "small to moderate" clinical effect. While statistically significant in two of three Phase 3 trials, this represents only incremental benefit beyond sertraline monotherapy, with added metabolic and sedation risks. The inconsistent trial results and FDA rejection underscore that this is not a robust or reliable augmentation strategy for PTSD.