Phenylethylidenehydrazine (PEH) is a novel GABA transaminase (GABA-T) inhibitor and low-activity monoamine oxidase inhibitor, serving as a key metabolite of the antidepressant phenelzine. PEH plays a central role in the elevation of GABA levels in the brain, a primary mechanism contributing to phenelzine’s anxiolytic effects. This is evidenced by the lack of significant anxiolytic activity by N-acetyl-phenelzine, a potent MAO inhibitor devoid of GABAergic effects. PEH increases GABA availability by inhibiting GABA-T, enhancing its inhibitory action in the central nervous system. This mechanism may provide therapeutic benefits, including anticonvulsant properties and potential efficacy in managing anxiety disorders.

In rodent studies, PEH has been shown to significantly elevate GABA concentrations, surpassing the 200% threshold associated with effective anxiolytic effects. Research indicates a ceiling effect for GABA-T inhibition by PEH and phenelzine at approximately 50%, even at high doses.

PEH has been observed to dramatically increase whole-brain levels of alanine and tyrosine, similar to phenelzine. Evidence suggests these amino acid elevations are primarily mediated by PEH as an active metabolite, though the implications of these effects remain a subject of ongoing research.

PEH is a very promising candidate, and could set a new standard for developing future anxiolytic agents, thanks to its unique GABAergic activity and absence of known addictive properties. However, further studies are needed to fully understand its potential applications.