Identification and Characterization of Oxymetazoline Glucuronidation in Human Liver Microsomes: Evidence for the Involvement of UGT1A9

The incubation of oxymetazoline, a nonprescription nasal decongestant, with human liver microsomes (HLMs) supplemented with uridine-5-diphosphoglucuronic acid (UDPGA) generated glucuronide metabolite as observed by LC/MS/MS. The uridine glucuronosyltransferases (UGTs) responsible for the O-glucuronidation of oxymetazoline remain thus far unidentified. The glucuronide formed in HLMs was identified by LC/MS/MS and characterized by one- and two-dimensional NMR to be the β-O-glucuronide of oxymetazoline. UGT screening with expressed UGTs identified UGT1A9 as the single UGT isoform catalyzing O-glucuronidation of oxymetazoline. Oxymetazoline O-glucuronidation by using HLMs was best fitted to the allosteric sigmoidal model. The derived S50 and Vmax values were 2.42 ± 0.40 mM and 8.69 ± 0.58 pmole/(min mg of protein), respectively, and maximum clearance (CLmax) was 3.61 L/min/mg. Oxymetazoline O-glucuronidation by using expressed UGT1A9 was best fitted to the substrate inhibition model. The derived Km and Vmax values were 2.53 ± 1.03 mM and 54.18 ± 16.92 pmole/(min mg of protein), respectively, and intrinsic clearance (CLint) was 21.41 L/(min mg). Our studies indicate that oxymetazoline is not glucuronidated at its nanomolar intranasal dose and thus is eliminated unchanged, because UGT1A9 would only contribute to its elimination at the toxic plasma concentrations.