A Non-Classical Neuroactive Steroid Exhibiting Potent, Efficacious GABA_A Receptor Agonism and NMDA Receptor...

Neuroactive steroids modulate GABA_A and NMDA receptors allosterically, typically requiring specific structural features for their activity. In this study, we characterize YX84, a novel neuroactive steroid bearing a 3β sulfate and p-trifluoroacetylbenzyl alcohol attached in an ether linkage to a hydroxyl group at steroid carbon 17. This compound and similar analogues exhibit an atypical pharmacological profile, with three distinct actions at GABA_A receptors. First, YX84 is a full agonist, with EC₅₀ near 1 μM and comparable efficacy to GABA at GABA_A receptors in native hippocampal neurons. It presents as a full agonist relative to GABA at α4/δ subunit-containing receptors. Second, YX84 acts as a slow-onset, potent positive allosteric modulator (PAM) of GABA_A receptors at concentrations below those that gate a response. Finally, YX84 exhibits rapid desensitizing and/or blocking kinetics; voltage dependence is consistent with a contribution of channel block. Structure-activity relationship analyses reveal that both functional groups are essential for gating activity, while classical requirements such as carbon 3 hydroxyl stereoselectivity and carbon 5 reduction are dispensable. YX84 also modestly inhibits NMDA receptor currents, suggesting weak negative allosteric modulation. Behavioral assays show that intraperitoneal administration of YX84 (30 mg/kg) does not impair sensorimotor function, unlike allopregnanolone. These findings identify YX84 as a structurally distinct neuroactive steroid with dual receptor activity and favorable behavioral tolerability, offering a promising scaffold for therapeutic development targeting excitatory/inhibitory imbalance in neuropsychiatric disorders if pharmacokinetic considerations can be overcome.