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.