ISSX 2013 - Aldehyde Oxidase

ISSX 2013 - Aldehyde Oxidase

Published:  29 September 2013

Selection of human liver S9 and cytosol fractions for evaluating clearance by aldehyde oxidase (AO): The impact of low versus high AO activity lots.

Aldehyde oxidase (AO) is a cytosolic enzyme present in the liver of humans and other mammals that catalyzes various oxidation and reduction reactions.  Biotransformation by AO is an important clearance mechanism for many drugs and drug candidates, with increasing importance in certain chemical spaces, and in some cases, such as zaleplon, Aldehyde oxidase metabolism leads to rapid in vivo clearance. Several publications have demonstrated the under-prediction of in vivo human clearance from in vitro clearance data, which are typically conducted with human liver subcellular fractions, such as S9 or cytosol.  Zientek and colleagues (2010)1 described a rank order approach, or ‘yard-stick’ approach, to categorize known AO substrates into low, medium or high clearance categories based on in vivo data.  With this approach, new drugs candidates can be evaluated in vitro in S9 or cytosol and the predicted in vivo clearance can be qualitatively ranked from low- to high-clearance.  These subcellular fractions, S9 and cytosol, are commercially available from multiple sources and in many formats (individuals and pools of various sizes), which causes variation in AO activity. 
 
Because of the necessity to scale AO clearance with a rank-order approach, the present study set forth to determine which human liver S9 and cytosol lots (individual or pooled) can be utilized to predict in vivo AO clearance once threshold values are determined with appropriate probe drugs.  Therefore, this study evaluates the impact of low versus high AO activity in human liver S9 and cytosol preparations on the prediction of scaled clearance for AO substrates with the ‘yard-stick’ approach.