J. O. Cardoso,1 R. V. Oliveira,2 J. Lu,1 Z. Desta1; 1Indiana University, Indianapolis, IN, 2Federal University of São Carlos, São Carlos, Brazil

BACKGROUND: Montelukast is recommended as a selective in vitro and in vivo probe of CYP2C8 activity. We further evaluated CYPs and UGTs involved in montelukast metabolism.
METHODS: Kinetics for formation of motelukast oxidative and glucuronidated metabolites was determined by incubating montelukast at 37°C for 30 min with human liver microsomes (HLMs, 0.25 mg/ml) and cofactors (a NADPH-generating system or UDPGA). Alamethicin-activated HLMs was used for the UGT assays. Montelukast and its oxidative metabolites were assayed using LC-MS/MS.
.RESULTS: Average kinetic parameters for the formation of four previously known oxidative and two new conjugated metabolites in 2 HLMs are shown in the Table. Glucuronidation of montelukast accounted for 83% of the total in vitro CLint. The major oxidative metabolite, 1,2 diol, accounts for 81% of the total oxidative in vitro CLint, and kinetic, correlation and inhibition analyses in HLMs and experiments in 11 expressed CYPs indicate major involvement of CYP2C9 and CYP2C8 in its formation: CYP3A4 catalyzed 21(R)- and 21(S)-hydroxylation of montelukast.
CONCLUSION: Our data suggest that the in vitro and probably in vivo utility of montelukast as a probe of CYP2C8 activity may be compromised due to major role of UGTs and involvement of multiple CYPs in its metabolism.
Table: Kinetic parameters for the metabolism of montelukast by CYPs and UGTs in HLMs
MetabolitesVmax (pmol/min/mg protein)Km (µM)Vmax/Km (µL/min/mg protein)
Montelukast 1,2 diol6.140.3219.65
Acyl-β-D Glucuronide (M1)151.101.36108.71