Short Communication In Vitro Investigation of Human UDP-Glucuronosyltransferase Isoforms Responsible for Tacrolimus Glucuronidation: Predominant Contribution of UGT1A4

Tacrolimus (Tacro) is a potent immunosuppressant and a central agent in the prevention of posttransplantation rejection. Tacro is characterized by a narrow therapeutic index and wide interindi-vidual pharmacokinetic fluctuation. The contribution of human UDP-glucuronosyltransferase (UGT) in its metabolism has not been extensively studied. In vitro metabolism studies support that the liver produced Tacro-glucuronide (Tacro-G) while its formation was minimal or undetectable in the presence of intestine and kidney microsomes. Among 16 human UGTs tested, UGT1A4 was the sole enzyme involved in Tacro-G formation. This conclusion is supported by the finding of inhibition with a specific substrate of UGT1A4 lamotrigine with Ki values similar for both human liver and UGT1A4 microsomes and the correlation with trifluoperazine-gluc-uronide formation by liver microsomes (rs 0.551; p 0.02). Formation of Tacro-G by liver samples varied among individuals (6.4-fold variation; n 16), and common nonsynonymous variants may contribute to this variability. In the human embryonic kidney 293 cellular model, no significant differences in enzyme kinetics could be revealed for UGT1A4*2 (P24T) and *3 (L48V), whereas the allozyme *4 (R11W) displayed a 2-fold higher velocity (p < 0.01) compared with the UGT1A4*1 enzyme preparation. In human liver samples, carriers of the UGT1A4 variants did not display statisti-cally different efficiency in Tacro-G formation compared with ho-mozygote for the reference genotype UGT1A4*1/*1. We conclude that UGT1A4 is the major isoform involved in Tacro glucuronida-tion, whereas additional studies are required to assess the contri-bution of UGT1A4 genetic factors in tacrolimus glucuronidation variability