Influence of Nonsynonymous Polymorphisms of UGT1A8 and
UGT2B7 Metabolizing Enzymes on the Formation of Phenolic and
Acyl Glucuronides of Mycophenolic Acid
Olivier Bernard, Jelena Tojcic, Kim Journault, Louis Perusse, and Chantal Guillemette
Canada Research Chair in Pharmacogenomics, Oncology and Molecular Endocrinology Research Center, CHUL Research
Center, Faculty of Pharmacy (O.B., J.T., K.J., C.G.) and Division of Kinesiology, Department of Preventive Medicine (L.P.),
Laval University, Quebec, Canada
Received April 19, 2006; accepted June 20, 2006
Mycophenolic acid (MPA) is the active metabolite of mycopheno-
late mofetil (MMF), a standard immunosuppressive drug approved
for clinical use in the prevention of acute allograft rejection after
organ transplantation. This study examines the role of the genetic
variants of UDP-glucuronosyltransferase (UGT) 1A8 and 2B7 en-
zymes involved in the formation of the primary metabolite of MPA,
the inactive phenolic glucuronide (MPAG), and the reactive acyl
glucuronide (AcMPAG). The first exon of UGT1A8 was first rese-
quenced in the region encoding for the substrate binding domain in
254 Caucasians and 41 African Americans. Eight nonsynonymous
changes were observed and led to the following amino acid sub-
stitutions: S43L, H53N, S126G, A144V, A173G, A231T, T240A, and C277Y.
Thirteen haplotypes were inferred, comprising only two previously
described alleles, namely, UGT1A8*2 (A173G) and UGT1A8*3
(C277Y). Upon stable expression in human embryonic kidney 293
cells, the UGT1A8*3 (C277Y), *5 (G173A240), *7 (A231T), *8 (S43L), and
*9 (N53G) proteins were associated with the most profound de-
creases in the formation of MPAG and AcMPAG, indicating that
these amino acids are critical for substrate binding and enzyme
function. Altogether, the low-activity UGT1A8 enzymes are carried
by 2.8 to 4.8% of the population. The variant of the UGT2B7 protein
(UGT2B7*2 Y268), the main enzyme involved in the formation of
AcMPAG, demonstrated a catalytic efficiency comparable with
that of UGT2B7*1 (H268). In conclusion, although the common
UGT2B7*2 variant is predicted to have limited impact, several
UGT1A8 variants identified may potentially account for the large
interindividual variance in MMF pharmacokinetics and deserve
further clinical investigations.
Mycophenolate mofetil (MMF; Cellcept, Hoffmann-La Roche,
Nutley, NJ), an immunosuppressive drug, is approved for clinical use
in the prevention of acute allograft rejection after organ transplanta-
tion, as well as hematopoietic stem cell transplantation (Sollinger,
1995; Bullingham et al., 1998; Cohn et al., 1999). Mycophenolic acid
(MPA), its active metabolite, is a selective inhibitor of inosine mono-
phosphate dehydrogenase (IMPDH). The metabolism of MPA in-
volves mainly its conjugation by UDP-glucuronosyltransferase (UGT)
enzymes, yielding two glucuronide conjugates; namely, the major
derivative MPAG and the minor metabolite AcMPAG (Bullingham et
al., 1998; Shipkova et al., 2001b). MPAG has no inhibitory effects on
IMPDH and is the major urinary excretion product of MPA (Bulling-
ham et al., 1996; Schutz et al., 1999). In contrast, AcMPAG may be
biologically active by inhibiting IMPDH and leukocyte proliferation,
and by inducing cytokine release (Schutz et al., 1999; Wieland et al.,
2000; Shipkova et al., 2001b). A relationship between plasma levels
of MPA and clinical outcomes in transplant patients has been dem-
onstrated (Hale et al., 1998; van Gelder et al., 1999; Oellerich et al.,
2000; Weber et al., 2002). Besides, AcMPAG has been suggested to
be involved in some of the toxicities experienced by patients receiving
MMF, including neutropenia and gastrointestinal disorders (Wieland
et al., 2000; Maes et al., 2002). Therefore, factors affecting the extent
of MPA glucuronidation are likely to be clinically significant.
Recently, UGT1A9 has been identified as the main enzyme in-
volved in the hepatic formation of MPAG (Bernard and Guillemette,
2004). This enzyme was predicted to be the key determinant of
MPAG formation in vivo because the metabolism of MPA takes place
mainly in the liver (Bowalgaha and Miners, 2001; Shipkova et al.,
2001a; Bernard and Guillemette, 2004). Functional genetic variants
within the UGT1A9 gene have been uncovered recently by our group
(Villeneuve et al., 2003; Girard et al., 2004). In human liver micro-
somes, the presence of the variants ?275A?T and ?2152T?C of the
UGT1A9 promoter region were associated with a 2.3-fold higher
hepatic expression of UGT1A9 and a 2.1-fold increased glucuronida-
This work was supported by the Canadian Institutes of Health Research
(MOP-42392) and Canada Research Chair Program (C.G.). O.B. is the recipient of
a studentship award from the Fonds de la Recherche en Sante ´ du Que ´bec. C.G.
is the chairholder of the Canada Research Chair in Pharmacogenomics.
Part of this work has been presented at the 13th North American Meeting of
the International Society for the Study of Xenobiotics, 2005 October 23–27th,
informationcan befound at
ABBREVIATIONS: MMF, mycophenolate mofetil; MPA, mycophenolic acid; IMPDH, inosine monosphospate dehydrogenase; UGT, UDP-
glucuronosyltransferase; MPAG, mycophenolic acid phenolic glucuronide; AcMPAG, mycophenolic acid acyl glucuronide; SNP, single-nucleotide
polymorphism; PCR, polymerase chain reaction; HEK, human embryonic kidney; CLint, intrinsic clearance.
DRUG METABOLISM AND DISPOSITION
Copyright © 2006 by The American Society for Pharmacology and Experimental Therapeutics
DMD 34:1539–1545, 2006
Vol. 34, No. 9
Printed in U.S.A.
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Address correspondence to: Chantal Guillemette, Canada Research Chair in
Pharmacogenomics, Pharmacogenomics Laboratory, CHUL Research Center,
T3-48, 2705 Boul. Laurier, QC, G1V 4G2, Canada. E-mail: chantal.guillemette@
MPA GLUCURONIDATION BY UGT1A8 AND UGT2B7 VARIANTS