Additional Analyses Do Not Implicate Opioid Signaling in the Antidepressant Effects of Ketamine.

Dear Editors, We appreciate the comments from Bandeira et al.1 regarding our study, “Antidepressant efficacy of ketamine plus naltrexone for major depression comorbid with alcohol use disorder: a randomized controlled trial.”2 The interactive therapeutic effects of ketamine and naltrexone appear to be complex, and we appreciate the opportunity to address nuances of the interactive effects of these drugs that could not be developed in our initial report. Bandeira et al. note that the pharmacokinetics of the single depot injection of extended-release naltrexone used in our study may produce 2 peaks in naltrexone plasma levels, one that emerges roughly 2 hours after administration and another that develops 2-3 days after administration and persists for more than 1 month. Bandeira et al. noted the numerical blunting of the antidepressant effects of ketamine after the first ketamine infusion when naltrexone levels were at their early peak. We did not include post hoc comparisons to test this hypothesis in our initial report because the lack of significance in the overall analysis failed to meet the statistical threshold for significance that would justify those post hoc analyses. Nonetheless, the group comparison of Montgomery–Åsberg Depression Rating Scale (MADRS) scores at the first post-ketamine time point did not reach the threshold for statistical significance (P = .12). The trend-level interaction was primarily driven by higher MADRS scores in the midazolam group compared to both ketamine groups (all P < .04) observed 1 week after the last ketamine infusion (Fig. 4 in the original manuscript). A second reanalysis of the data was also performed using baseline MADRS levels as a model covariate. As in the original model, the group-by-visit interaction approached significance (P = .06). Baseline-adjusted levels following infusion were numerically higher in the ketamine plus naltrexone group compared to the ketamine plus saline group at a threshold that, again, did not reach statistical significance (P = .06). As in the original analysis, the interaction was primarily driven by higher MADRS scores among patients receiving midazolam compared to both ketamine groups (all P < .02) 1 week following the last ketamine infusion. Supporting this observation, if the data from the last visit are dropped from the overall analysis, even trend-level significance is lost (P = .55). Thus, it does not appear that the trend-level group difference was explained by the numerically lower MADRS scores in the group receiving ketamine and saline relative to the group receiving ketamine plus naltrexone after the first ketamine infusion. We finally note that all analyses were tested at the 2-sided alpha = .05 threshold. Had even a modest adjustment for multiple testing been applied, none of the above effects would survive correction for Type I error. Taken together, our study does not provide evidence that meets the threshold for significance to indicate that there is a significant interaction of ketamine and naltrexone on the antidepressant effects of ketamine. Nonetheless, the trend-level findings may merit continued study. Bandeira et al. also suggest that plasma concentrations of intramuscular (IM) naltrexone fell below the threshold needed to block opioid effects during weekly ketamine infusions. It is not clear which form of opioid signaling (μ, δ, κ) and which threshold of blockade they refer to, making it difficult to precisely respond to this question. Naltrexone blocks all 3 forms of opioid signaling with greatest potency at μ opioid receptors and less potency at δ and κ opioid receptors.3 The FDA recommends administering IM naltrexone monthly due to persisting plasma naltrexone levels.4 A 300-mg dose of IM naltrexone has been shown to block mu-opioid-mediated drug effects for 28 days.5 Thus, we suspect that the naltrexone dose that we employed was adequate to test the experimental hypothesis. However, higher naltrexone doses may merit further investigation. Bandeira et al. also raised the possibility that injection-related discomfort may have unblinded naltrexone effects, influencing our study results. The predicted impact of naltrexone unblinding is not clear. Patients who expected that naltrexone would block ketamine effects, might show greater reductions in ketamine efficacy when they believed that they received naltrexone. This might have amplified the apparent naltrexone signal in our study. Bandeira et al. wonder why the remission rates in our study of patients with comorbid major depression and alcohol use disorder exceeded typical response rates in studies of treatment-resistant depression. We hypothesize that alcohol withdrawal symptoms may have contributed to depression severity in our patients. This hypothesis would be consistent with the efficacy of both benzodiazepines and N-methyl-D-aspartate (NMDA) glutamate receptor antagonists for reducing withdrawal-related depressed mood.6 Bandeira et al. note that our study did not combine a psychological intervention with the study pharmacological interventions. We appreciate their suggestion that specific interactions between medication and psychotherapy might be relevant clinically, and we look forward to seeing future research addressing this issue. Bandeira et al. raise the possibility that reduced endogenous opioid receptor function in alcohol use disorder may have blunted the naltrexone response in our study. We cannot address this issue directly. However, alcohol use disorder-related downregulation of opioid receptors has been attributed to the ability of repeated ethanol consumption to produce sustained elevations of endogenous opioid release.7 We note that chronic opioid exposure also downregulates signaling via opioid receptors, but it enhances the response to antagonists, like naltrexone.8 Finally, Bandeira et al. query whether sex differences in the study groups contributed to differing findings across studies. However, as the authors note, neither Jelen et al. nor our study was sufficiently powered to examine sex effects. Indeed, our sample included only 13 (22%) female subjects distributed equally across the 3 treatment arms. A sensitivity analysis restricted to males, produced similar patterns of effects including a trend-level (P = .07) treatment-by-visit interaction and a non-significant comparison of ketamine plus naltrexone to ketamine plus saline MADRS following the first ketamine infusion (P = .39). Nonetheless, we agree that well-powered trials should be conducted in the future to examine for potential sex-specific differences. In summary, our additional analyses do not support the hypothesis that endogenous opioid signaling activated by ketamine mediates its antidepressant effects. We acknowledge that there was a statistically non-significant blunting of the initial antidepressant effects of ketamine by the combination of ketamine and naltrexone. We caution against over-interpreting this non-significant numerical difference. We apply this same caution to the statistically more robust late-emerging potentiation of the antidepressant effects of ketamine by naltrexone in these same patients. Thus, it is possible that there is a subtle contribution of endogenous opioid signaling to the antidepressant effects of ketamine that was beyond the capacity of our study to detect. This work was supported by a grant award (5I01CX001379) from the Department of Veterans Affairs. Dr. G.Y. was also supported by CSR&D Research Career Development Transition Award (5IK4CX002091) from the Department of Veterans Affairs. The views expressed are those of the authors, and not necessarily those of the U.S. Department of Veterans Affairs. The authors alone are responsible for the content and writing of this letter. Dr. Krystal has served as a consultant for Aptinyx, Inc.; Biogen, Idec, MA; Bionomics, Limited (Australia); Boehringer Ingelheim International; Clearmind Medicine, Inc.; Cybin IRL (Ireland Limited Company); Enveric Biosciences; Epiodyne, Inc.; EpiVario, Inc.; Janssen Research & Development; Jazz Pharmaceuticals, Inc.; Otsuka America Pharmaceutical, Inc.; Perception Neuroscience, Inc.; Praxis Precision Medicines, Inc.; Spring Care, Inc.; Sunovion Pharmaceuticals, Inc. Dr. Krystal has served as a scientific advisory board member for: Biohaven Pharmaceuticals; BioXcel Therapeutics, Inc. (Clinical Advisory Board); Cerevel Therapeutics, LLC; Delix Therapeutics, Inc.; Eisai, Inc.; EpiVario, Inc.; Freedom Biosciences, Inc.; Jazz Pharmaceuticals, Inc.; Neumora Therapeutics, Inc.; Neurocrine Biosciences, Inc.; Novartis Pharmaceuticals Corporation; Praxis Precision Medicines, Inc.; PsychoGenics, Inc.; Tempero Bio, Inc.; Terran Biosciences, Inc. In the past 3 years, Dr. Krystal has the following patents: John Krystal, Godfrey Pearlson, Stephanie O’Malley, Marc Potenza, Fabrizio Gasparini, Baltazar Gomez-Mancilla, Vincent Malaterre. Mavoglurant in treating gambling and gaming disorders. He is an inventor on patents licensed by Yale University to Janssen Pharmaceuticals, Biohaven Pharmaceuticals, Spring Health, Freedom Biosciences, and Novartis Pharmaceuticals. Dr. Krystal has stock or stock options from Biohaven Pharmaceuticals, Catego Therapeutics, Clearmind Medicine, Inc.; Damona Pharmaceuticals, EpiVario, Inc., Freedom Biosciences, Neumora Therapeutics, Rest Therapeutics, Spring Health, Tempero Bio, Inc., Terran Biosciences, and Tetricus, Inc. Other authors declare no conflict of interest to report. The data that support the findings of this article are available from the corresponding author upon reasonable request.