Alfaxalone

Alfaxalone (3-alpha-hydroxy-5-alpha-pregnane-11,20-dione) is a synthetic neuroactive steroid with potent general anesthetic properties mediated principally through positive allosteric modulation and, at higher concentrations, direct activation of gamma-aminobutyric acid type A (GABAA) receptors. First synthesized at the Glaxo UK Pharmacology Department in the late 1960s and introduced in 1971 as the principal active component of the intravenous anesthetic formulation Althesin (CT-1341), alfaxalone was administered in an estimated three million human anesthetic procedures across the United Kingdom, Europe, and other jurisdictions before its withdrawal from human clinical use in 1984 owing to anaphylactoid reactions attributed to the Cremophor EL solubilizing vehicle rather than to the active steroid itself. The compound was subsequently reformulated in 2-hydroxypropyl-beta-cyclodextrin (HPCD) by Jurox Animal Health and re-entered clinical practice as Alfaxan, a veterinary injectable anesthetic registered for induction and maintenance of general anesthesia in dogs and cats in Australia, New Zealand, the United Kingdom, continental Europe, the United States, and other jurisdictions. The HPCD formulation is devoid of the histamine-releasing properties of Cremophor EL and produces rapid-onset, short-duration, non-cumulative anesthesia with a cardiovascular safety profile superior to propofol in published comparative studies.

Alfaxalone binds at the transmembrane beta-plus/alpha-minus subunit interface of GABAA receptors, a site structurally characterized by X-ray crystallography and cryo-electron microscopy of alpha1-beta3-gamma2 receptor assemblies. At sub-micromolar concentrations the compound potentiates GABA-evoked chloride conductance (positive allosteric modulation); at concentrations exceeding approximately one micromolar it directly gates the chloride channel in the absence of GABA (direct agonism). The composite dose-dependent pharmacology produces a continuum from anxiolysis and sedation through surgical anesthesia. Unlike the endogenous neurosteroid allopregnanolone and unlike classical progestogens, alfaxalone has no measurable activity at glucocorticoid, mineralocorticoid, or progesterone nuclear hormone receptors, a selectivity that simplifies its pharmacological profile and eliminates endocrine confounding in chronic or repeated dosing.

A second molecular activity, characterized by in vitro studies from the Goodchild laboratory, is activation of the human pregnane X receptor (PXR) with greater efficacy than allopregnanolone. PXR activation drives transcription of brain-derived neurotrophic factor (BDNF) and other neuroprotective gene targets, providing a mechanistic basis for the observation in a double-blind randomized clinical trial that alfaxalone total intravenous anesthesia preserves postoperative serum mature BDNF levels and cognitive function relative to propofol and propofol-sevoflurane comparator arms (Serrao and Goodchild, 2022).

Pharmacokinetics across species are dominated by rapid hepatic biotransformation. In dogs, plasma clearance after intravenous bolus is approximately 59 mL/kg/min (approaching hepatic blood flow), terminal elimination half-life is approximately 25 minutes, and volume of distribution is approximately 2.4 L/kg. In cats, clearance is approximately 25 mL/kg/min, half-life approximately 45 minutes, and volume of distribution approximately 1.8 L/kg. In the Phase 1 human trial of Phaxan (alfaxalone formulated in sulfobutylether-beta-cyclodextrin), plasma clearance was high and equal to hepatic blood flow, with rapid onset and offset of anesthesia comparable to propofol. The compound does not accumulate on repeated bolus dosing or continuous rate infusion at clinical doses, supporting its use for total intravenous anesthesia.

Drawbridge Pharmaceuticals has advanced the human formulation (Phaxan) through Phase 1 dose-finding (Goodchild et al., 2019), a Phase 1c randomized double-blind comparison with propofol (Monagle et al., 2015), and a Phase 3 pilot study in hip arthroplasty (Serrao and Goodchild, 2022). Results demonstrate fast-onset, short-duration anesthesia with cognitive recovery comparable to propofol, less cardiovascular depression, less airway obstruction, and no pain on injection. The compound is not currently approved for human use in any jurisdiction. Additional research applications include anticonvulsant activity in refractory status epilepticus models, anxiolytic and sedative-hypnotic effects at sub-anesthetic doses, and neuroprotective activity through the PXR-BDNF pathway.

This monograph documents the chemistry, synthesis, and stereochemistry of alfaxalone; the GABAA receptor pharmacology in structural and electrophysiological detail; the pregnane X receptor neuroprotective mechanism; comprehensive pharmacokinetics across species; the clinical and veterinary evidence base; sourcing and quality verification for research applications; reconstitution and handling; stack-interaction considerations; the adverse-event and safety signal; and a structured comparative assessment of five alternative intravenous anesthetic or neurosteroid agents (propofol, etomidate, ketamine, brexanolone, ganaxolone) against alfaxalone on five competency standards.