Return of fear after successful exposure therapy for phobia is a common clinical challenge. A previous study on mice demonstrated that the persistent attenuation of remote fear memories can be achieved by combining histone deacetylase inhibitors (HDACis) with fear-memory retrieval prior to extinction training. To evaluate the translational potential of this approach, we conducted a randomized, double-blind placebo-controlled trial. Forty-eight individuals with DSM-IV spider phobia received either HDACi valproic acid (VPA, 500 mg) or a placebo prior to the retrieval of fear memory followed by exposure therapy in virtual reality. No significant group difference was found in terms of behavioral change on the behavioral approach test at 3 months follow-up and baseline (primary outcome). However, the VPA group displayed significantly reduced fear in two self-report questionnaires related to spider phobia (Fear of Spider Questionnaire; Spider Phobia Beliefs Questionnaire) as compared to the placebo group. No group differences were observed for psychophysiological indicators of fear. The favorable impact of a single administration of VPA in combination with fear-memory retrieval prior to exposure therapy suggests that it might be an effective way to enhance symptom improvement at the subjective level in the treatment of phobias. Further studies need to investigate the conditions under which an improvement on the physiological and behavioral levels can be achieved as well.

The authors have declared no competing interest.

NCT02789813

The study was funded by Research Fund Junior Researchers of the University of Basel and the Research Fund of the University Psychiatric Clinics Basel.

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The Ethics Committee of Northwest and Central Switzerland gave ethical approval for this work.

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