Mycophenolic acid pharmacokinetics and clinical outcomes in renal transplantation: effect of ABCC2 haplotype analysis and distribution into lymphocytes and kidney

Abstract

Mycophenolic acid (MPA) is an immunosuppressant commonly used to prevent rejection following renal transplantation. MPA prevents graft rejection through selective inhibition of inosine monophosphate dehydrogenase (IMPDH) enzyme in lymphocytes. It displays considerable inter-individual pharmacokinetic (PK) variability, and many factors, other than dose or total plasma concentrations, may impact on lymphocyte MPA concentrations. There is evidence that the multidrug resistance-associated protein 2 efflux transporter may influence MPA PK in plasma and affect its distribution into lymphocytes. Therefore, the major aim of this thesis was to investigate the influence of ABCC2 genetic polymorphisms as a determinant of inter-individual variability in MPA treatment response, as well as clinical outcomes. This thesis also set out to investigate intra-lymphocyte pharmacokineticpharmacodynamic analyses of MPA as predictors of graft rejection. The first major study of this thesis incorporated the development and validation of new LCMS/ MS methods for the quantification of MPA concentrations in human kidney biopsies (Chapter 2) taken as part of routine clinical procedures, and lymphocytes (Chapter 4), and applying these new methods in a pilot prospective clinical study of the utility of determining intra-renal and lymphocyte MPA concentrations as predictors of graft rejection. For the second major study (Chapter 3) of this thesis, recipients (n = 60) prescribed MMF, tacrolimus and prednisolone-based immunosuppression, and their donors (n = 46) were retrospectively genotyped for three common single nucleotide polymorphisms (SNPs) of ABCC2 (-24 C > T, 1249 G > A, 3972 C > T), haplotyped and patients were divided into wild-type, low or high ABCC2 expressor group. Whilst no relationship was observed between donor expressor status and MPA PK parameters, recipients with the high ABCC2 expressor haplotype (CAC, -24C/1249A/3972C, variant at 1249) had significantly higher MPA trough (C0) concentrations, presumably due to an increased enterohepatic recirculation of MPA, compared to recipients with low expressor haplotypes. The multivariate analyses indicated that MPA C0 concentrations were associated with recipient ABCC2 expressor phenotype, rejection incidence and peak panel reactive antibody. The third major study (Chapter 4) of this thesis investigated the concentrations of MPA and IMPDH activity in lymphocytes, as predictors of rejection in 48 renal transplant recipients. Lymphocyte MPA concentration was the only covariate independently associated with rejection incidence in the multivariate analysis, suggesting that lymphocyte concentrations may be the best indicator of MPA therapeutic efficacy compared to monitoring plasma concentrations alone. ROC curve analyses revealed that lymphocyte MPA concentration was a better predictor of severe (cellular/vascular) rejection compared to IMPDH activity or plasma concentrations. IMPDH activity was not a significant predictor of graft rejection, however, a single measurement of IMPDH activity at pre-dose may have accounted for the lack of significant relationship between IMPDH activity and rejection. In summary, ABCC2 genetic variability may be important in determining exposure to MPA and may have clinically significant impacts on MPA treatment response. Despite monitoring plasma concentrations to maintain MPA efficacy and minimise the consequences of overexposure, inter-individual differences in MPA plasma PK are still difficult to predict, but may be accounted for by directly measuring lymphocyte MPA concentrations.