Complex Metabolism of the Novel Neurosteroid, Ganaxolone, in Humans: A Unique Challenge for Metabolites in Safety Testing Assessment

The human pharmacokinetics, metabolism, and excretion of [14C]-ganaxolone (GNX) were characterized in healthy male subjects (n = 8) following a single 300-mg (150 μCi) oral dose. GNX exhibited a short half-life of 4 hours in plasma, whereas total radioactivity had a half-life of 413 hours indicating extensive metabolism to long-lived metabolites. Identification of the major GNX circulating metabolites required extensive isolation and purification for liquid chromatography-tandem mass spectrometry analysis, together with in vitro studies, NMR spectroscopy, and synthetic chemistry support. This revealed that the major routes of GNX metabolism involved hydroxylation at the 16α-hydroxy position, stereoselective reduction of the 20-ketone to afford the corresponding 20α-hydroxysterol, and sulfation of the 3α-hydroxy group. This latter reaction yielded an unstable tertiary sulfate, which eliminated the elements of H2SO4 to introduce a double bond in the A ring. A combination of these pathways, together with oxidation of the 3β-methyl substituent to a carboxylic acid and sulfation at the 20α position, led to the major circulating metabolites in plasma, termed M2 and M17. These studies, which led to the complete or partial identification of no less than 59 metabolites of GNX, demonstrated the high complexity of the metabolic fate of this drug in humans and demonstrated that the major circulating products in plasma can result from multiple sequential processes that may not be easily replicated in animals or with animal or human in vitro systems. SIGNIFICANCE STATEMENT: Studies on the metabolism of [14C]-ganaxolone in humans revealed a complex array of products that circulated in plasma, the two major components of which were formed via an unexpected multi-step pathway. Complete structural characterization of these (disproportionate) human metabolites required extensive in vitro studies, along with contemporary mass spectrometry, NMR spectroscopy, and synthetic chemistry efforts, which served to underscore the limitations of traditional animal studies in predicting major circulating metabolites in man.