Benoxacor is enantioselectively metabolized by microsomes and cytosol from the human liver

Benoxacor is a safener added to current-use herbicide formulations to protect the target crop from herbicidal toxicity. It is an emerging environmental contaminant that has been detected in surface waters, raising the possibility of human exposure via drinking water. Because it is not subject to the same regulations as active pesticide ingredients, its metabolism and toxicity in humans have not been studied. Here we investigate the enantioselective metabolism of benoxacor in human subcellular fractions. Pooled human liver microsomes (pHLM) and cytosol (pHLC) were incubated with racemic benoxacor for up to 30-min. Gas chromatographic analyses were used to measure the enantioselective depletion of benoxacor. pHLMs with and without nicotinamide adenine dinucleotide phosphate (NADPH, co-factor for cytochrome P450 enzymes [CYPs]) and pHLC with glutathione (GSH, co-factor for glutathione S-transferases [GSTs]) metabolized benoxacor. These results demonstrate that microsomal CYPs, microsomal carboxylesterase (CESs), and cytosolic GSTs metabolize benoxacor. Females were predicted to have a higher clearance of benoxacor by GSTs than males. Male and female pHLM incubations with NADPH showed enrichment of the first eluting benoxacor enantiomer (E1-benoxacor). pHLM incubations without NADPH and pHLC incubations with GSH showed an enrichment of the second eluting enantiomer of benoxacor (E2-benoxacor). Our results indicate that human hepatic microsomal and cytosolic enzymes enantioselectively metabolize benoxacor, a fact that needs to be considered when investigating human exposures and toxicities of benoxacor.