Introduction
Mycophenolic acid (MPA) is a standard component of immunosuppressant
protocols in organ transplantation. Considerable variability of MPA
pharmacokinetics has attracted much attention and has been
comprehensively reviewed a number of times [e.g., 1-5]. A range of
“classical” factors interfere with exposure to MPA, including age,
body mass index (BMI), renal function, changes of gut microbiota,
reduced albumin levels, interactions with food, drug-drug interactions
at different levels (in particular with calcineurin inhibitors [CNI]
cyclosporine A [CsA] and tacrolimus, but also with other drugs) and
MPA formulation (immediate-release tablets of mycophenolate mofetil
[IR MMF] or enteric-coated [acid-resistant] tablets containing
MPA sodium salt [EC-MPS]) [1-5]. Orally administered MPA
undergoes complex processes that include prodrug activation (in the case
of MMF; by carboxylesterases, CES) in the intestinal cells and in the
liver; extensive biotransformation (around 90% of bioavailable
fraction) to an inactive 7-O-glucuronide (MPAG) mainly by the uridine
5’-diphospho-glucornosyltransferase (UGT) 1A9 in the liver (less so in
the kidney) with a minor contribution of other UGTs; less extensive
biotransformation by UGT2B7 (intestine, liver) to a biologically active
acyl-glucuronide (AcMPAG); minor biotransformation by cytochrome P450
enzymes CPY3A4 and CY3A5 (liver) to inactive 6-O-desmethyl MPA;
extensive albumin binding (in competition with MPAG); entero-hepatic
recirculation and, to a minor extent, active renal secretion of MPA and
MPAG [1-5]. MPA is a substrate of the efflux transporter multidrug
resistance protein 1 (MDR-1, encoded by ABCB1 ) (intestine), while
MPAG and AcMPAG are substrates to efflux transporter multidrug
resistance-associated protein 2 (MRP-2, encoded by ABCC2 ) and
influx organic anion transporter polypeptides, primarily OATP1B1 and 1B3
– these proteins move MPAG/AcMPAG in and out of the hepatocytes and
renal tubular cells [1-5].
A recent systematic review [4] identified 38 studies with different
designs, sampling populations and sample sizes, measured outcomes and
control of confounding, mainly in renal transplant recipients, assessing
relationship between several tens of single nucleotide polymorphisms
(SNPs) in 10 enzyme (UGT , CYP , CES families) and 6
transporter genes (ABCB1 , ABCC2 , SLCO1B1 ,SLCO1B3, SLCO2B1, ABCG2 ) and exposure to- or occurrence of
MPA-related adverse event. Those with at least 2 consistent reports
(in vivo or in vitro/in vivo ) about association
with MPA exposure/clinical effects and regardless of the number of
“negative” studies include: i) UGT1A9 c.-275T>A(rs6714486) and c.-2152C>T (rs17868320) variants (in
complete linkage disequilibrium, LD) result in increased enzyme activity
and lower exposure to MPA; ii) UGT2B7 802C>T(rs7439366) variants (or loci that are in LD) may also be relevant for
MPA clearance; iii) ABCB1 2677G>T/A (rs2032582),3435C>T (rs1045642) or 1236C>T(rs1128503) variant alleles (or haplotypes/diplotypes, since in LD)
increase the risk of adverse events; iv) in vitro , OATP1B1 with
the SLCO1B1*5 c.521T>C (rs4149056) polymorphism
shows reduced MPAG/AcMPAG uptake into hepatocytes. This might reduce
enterohepatic recirculation, and in one study, this SNP was associated
with a lower risk of MPA adverse events (no association in 6 other
studies, and further 5 failed to associate this SNP with MPA levels); v)SLCO1B3 c.334T>G (rs4149117) is in complete LD withSLCO1B3 c.699G>A (rs7311358). OATP1B3 with the
variant haplotype shows reduced MPAG up-take in vitro . In one
study, c.334T>G TT/TG patients had somewhat higher
MPA exposure vs. GG subjects (not observed in three further studies, and
one indicated just the opposite); vi) UGT1A9*3(c.98T>C , rs72551330) SNP results in reduced enzyme
activity. Prevalence of variant carriers is very low (≤3% in most of
the studies) [4,5]. In two studies, it was suggested that variant
carriers had lower exposure to MPA than wt subjects, but no association
between this SNP and MPA exposure/clearance was found in several other
studies [4,5]. vii) ABCC2 c.-24C>T (rs717620)
was reported associated with somewhat higher exposure to MPA, but the
opposite has also been reported; viii) so far, donors’ SNPs in renal
transplantation were rarely investigated – one study associated donor’sABCC2 1249G>A (rs2273697) with increased MPA
clearance [4].
In the present analysis we aimed to assess potential effect of an SNP in
the gene encoding breast cancer resistance protein (BCRP, ABCG2)ABCG2 c.421C>A (rs2231142; p.Q141K) on steady-state
exposure to MPA in stable renal transplant recipients. As reviewed
[4], four studies have failed to detect associations between this
SNP and exposure to MPA. Our motivation was based on the following: i)
ABCG2 is important for transmembrane transport of numerous drugs in the
intestine, liver and the kidney [6-9] andc.421C>A SNP results in reduced transporter activity
due to increased proteosomal degradation [9, 10]; ii) one study in
Japanese renal transplant patients suggested that ABCG2 participated in
pharmacokinetics of MPAG [11], and this may reflect on exposure to
MPA.