UGT1A1 and UGT1A3 activity and inhibition in human liver and intestinal microsomes and a recombinant UGT system under similar assay conditions using selective substrates and inhibitors

1. In vitro enzyme kinetics and inhibition data was compared for UGT1A1 and UGT1A3 isoforms under similar assay conditions using human liver microsomes (HLM), human intestinal microsomes (HIM) and recombinant UGT (rUGT) enzyme systems. 2. UGT1A1 catalysed beta-estradiol 3-beta-D-glucuronide formation showed allosteric sigmoidal kinetics in all enzyme systems; while UGT1A3 catalysed CDCA 24-acyl-beta-D-glucuronide formation exhibited Michaelis-Menten kinetics in HLM, substrate inhibition kinetics in HIM and rUGT systems. Corresponding K-m or S-50 concentrations of beta-estradiol and CDCA were employed in the respective UGT inhibition studies. 3. Atazanavir inhibited the production of beta-estradiol 3-beta-D-glucuronide with IC50 values of 0.54 mu M and 0.16 mu M in HLM and rUGT1A1, respectively. But its inhibition potential was not observed in HIM, indicating potential cross-talk with other high-affinity intestinal UGT isozymes. On the other hand, zafirlukast, a pan UGT inhibitor, exhibited moderate inhibition in HIM with an IC50 value of 16.70 mu M. Lithocholic acid, inhibited the production of CDCA 24-acyl-beta-D-glucuronide with IC50 values of 1.68, 1.84, and 12.42 mu M in HLM, rUGT1A3, and HIM, respectively. 4. These results indicated that HLM, HIM, and rUGTs may be used as complementary in vitro systems to evaluate hepatic and intestinal UGT mediated DDIs at the screening stage.