Abstract
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.
UGT1A1 catalysed β-estradiol 3-β-D-glucuronide formation showed allosteric sigmoidal kinetics in all enzyme systems; while UGT1A3 catalysed CDCA 24-acyl-β-D-glucuronide formation exhibited Michaelis–Menten kinetics in HLM, substrate inhibition kinetics in HIM and rUGT systems. Corresponding Km or S50 concentrations of β-estradiol and CDCA were employed in the respective UGT inhibition studies.
Atazanavir inhibited the production of β-estradiol 3-β-D-glucuronide with
values of 0.54 µM and 0.16 µM 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
value of 16.70 µM. Lithocholic acid, inhibited the production of CDCA 24-acyl-β-D-glucuronide with
values of 1.68, 1.84, and 12.42 µM in HLM, rUGT1A3, and HIM, respectively.
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.
Disclosure statement
The authors report no declarations of interest.