title: In vitro investigation of human UDP-glucuronosyltransferase isoforms responsible for tacrolimus glucuronidation: predominant contribution of UGT1A4.
abstract: 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 interindividual 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 K(i) values similar for both human liver and UGT1A4 microsomes and the correlation with trifluoperazine-glucuronide formation by liver microsomes (r(s) = 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 (P(24)T) and *3 (L(48)V), whereas the allozyme *4 (R(11)W) 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 statistically different efficiency in Tacro-G formation compared with homozygote for the reference genotype UGT1A4*1/*1. We conclude that UGT1A4 is the major isoform involved in Tacro glucuronidation, whereas additional studies are required to assess the contribution of UGT1A4 genetic factors in tacrolimus glucuronidation variability.
authors.affiliation: Pharmacogenomics Laboratory, Centre Hospitalier Universitaire de Québec Research Center, Québec, Canada.
authors.forename: Isabelle
authors.initials: I
authors.lastname: Laverdière
authors.forename: Patrick
authors.initials: P
authors.lastname: Caron
authors.forename: Mario
authors.initials: M
authors.lastname: Harvey
authors.forename: Éric
authors.initials: É
authors.lastname: Lévesque
authors.forename: Chantal
authors.initials: C
authors.lastname: Guillemette
mesh_terms: D020719:Glucuronides / Q000378:metabolism*; D014453:Glucuronosyltransferase / Q000378:metabolism*; D006801:Humans; D007166:Immunosuppressive Agents / Q000378:metabolism*; D066298:In Vitro Techniques; D013058:Mass Spectrometry; D016559:Tacrolimus / Q000378:metabolism*
chemical_list: D020719:Glucuronides; D007166:Immunosuppressive Agents; C019787:bilirubin glucuronoside glucuronosyltransferase; D014453:Glucuronosyltransferase; D016559:Tacrolimus