Current evidence for the role of rapid-acting antidepressants (RAAD) in bipolar depression? A perspective and plan for action

After decades of limited progress in depression treatment, recent advancements have sparked renewed interest in developing novel antidepressants, particularly rapid-acting antidepressants (RAADs). Despite these promising developments, there remains a significant gap in research on bipolar depression. While several antipsychotics have been investigated for their efficacy in bipolar depression due to the reduced risk of mania induction, research on RAADs, such as (es)ketamine, remains scarce despite their demonstrated safety and effectiveness. This review gives an overview of current developments in RAADs in the context of bipolar disorder. Both published studies as well as phase II, III and IV studies on bipolar depression (based on clinicaltrials.gov) are reviewed in this work. The following RAAD substance classes have been or are currently investigated as possible treatments for bipolar depression: NMDA antagonists and indirect AMPA agonists (ketamine, esketamine, riluzole, felbamate), GABA-A activators or positive allosteric modulators (zuranolone, pregnenolone, PEA), psychedelics (psilocybin, 5-MeO-DMT), muscarine receptor antagonists (scopolamine), and kappa-opioid receptor antagonists (navacaprant). Other than the well-established efficacy and safety of (es)ketamine in treating bipolar depression, there has been little research effort in the treatment of bipolar depression. Recent research into RAADs demonstrates the growing field of novel mechanisms of action in the pharmacological treatment of bipolar depression. However, there is an urgent need for well-controlled clinical studies on RAADs in bipolar depression to expand treatment options and improve outcomes for millions of affected individuals worldwide.