Drug transporters are expressed in a number of tissues such as the intestine, liver, and kidney, and play key roles in drug absorption, distribution, and excretion. Variations in drug transporter gene expression significantly contribute to interindividual differences in drug responses. Epigenetic regulation of drug transporter genes has recently emerged as an important mechanism. Epigenetic regulation alters the expression of genes without changing DNA sequences. Epigenetic control mechanisms are associated with DNA methylation, histone modifications, and microRNAs. Herein we discuss recent advances in the study of the transcriptional and post-transcriptional mechanisms of drug transporters with a focus on epigenetic regulation. Areas covered: This review summarizes recent research on the epigenetic regulation of drug transporter genes, and highlights the importance of identifying novel biomarkers based on epigenetics for use in individualized drug therapy. Expert opinion: Researchers are actively attempting to elucidate the epigenetic mechanisms that control the expression of drug transporters, which affect the pharmacokinetics of drugs. Current evidence suggests that epigenetic changes play an important role in drug transporter function. A clearer understanding of epigenetic regulation in drug transporter genes will provide an insight into novel approaches to individualized drug therapy.

Filtered glucose is mostly reabsorbed by sodium-glucose cotransporter 2 (SGLT2) in the proximal tubules. SGLT2 is predominantly expressed in the human kidney. However, the regulatory mechanisms for gene expression in the human kidney remain unclear. We in this work elucidated the transcriptional regulatory mechanisms for the gene by nucleosome occupancy in the promoter region. Expressions of SGLT2 mRNA and protein were markedly weaker in human kidney-derived HK-2 cells than the human kidney. The nucleosome occupancy level in the promoter region was low in the kidney, but high in HK-2 cells. A treatment with a histone deacetylase inhibitor trichostatin A (TSA) decreased nucleosome occupancy in the promoter region and increased expression levels in HK-2 cells. The upregulation of expression by histone acetylation was accompanied by a higher binding frequency of hepatocyte nuclear factor (HNF) 1, a transcriptional modulator of in the human kidney, to the promoter region. The transfection of a HNF1 expression plasmid into HK-2 cells resulted in the upregulation of SGLT2 mRNA expression in the presence of TSA, but not in the treatment of dimethylsulfoxide as a control. Nucleosome occupancy in the promoter region was markedly higher in the liver and small intestine than the kidney. Our results indicate that tissue-specific nucleosome occupancy plays an important role in the regulation of gene expression via HNF1 binding at the promoter region.

Human pharmacokinetics (PK) profiles of monoclonal antibodies (mAbs) are usually predicted using non-human primates (NHP), but this comes with drawbacks in terms of cost and throughput. Therefore, we established a human PK profile prediction method using human neonatal Fc receptor (hFcRn) transgenic mice (TgM). We administered launched 13 mAbs to hFcRn TgM and measured the concentration in plasma using electro-chemiluminescence immunoassay. This was then used to calculate PK parameters and predict human PK profiles. The mAbs showed a bi-phased elimination pattern, and clearance (CL) (mL/d/kg) and distribution volume at steady state (V dss) (mL/kg) ranges were 11.0 to 131 and 110 to 285, respectively. There was a correlation in half-life at elimination phase (t 1/2β) between hFcRn TgM and humans for 10 mAbs showing CL of more than 80% in the elimination phase (R 2 0.714). Human t 1/2β was predicted using hFcRn TgM t 1/2β ; 9 out of 10 mAbs were within 2-fold the actual values, and all mAbs were within 3-fold. Regarding the predicted CL values, 7 out of 10 mAbs were within 2-fold the human values and all mAbs were within 3-fold. Furthermore, even on day 7 the predicted CL values of 8 out of 10 mAbs were within 2-fold the observed value, with all mAbs within 3-fold. These results suggest human PK profiles can be predicted using hFcRn TgM data. These methods can accelerate the development of antibody drugs while also reducing cost and improving throughput.

This study was performed to identify genetic polymorphisms in multidrug and toxin extrusion 2-K (MATE2-K, SLC47A2), a proton/ organic cation antiporter that plays a role in the transport of organic cations across the apical membrane in kidney epithelial cells into the urine, and to demonstrate their effects on MATE2-K functions in vitro. Four of the thirty single nucleotide polymorphisms (SNPs) we identified in three ethnic groups (Caucasian, African American, and Japanese) were novel [308C>G (P103R), c.487-8C>T, 818A>G (Y273C), and c.1018+14T>C]. The transport activities of the prototypical substrates, tetraethylammonium and metformin, for four nonsynonymous SNPs (P103R, P162L, G211V, and Y273C) were significantly different from those of the wild-type. In particular, transport activity was higher in P103R than in the wild-type, which is the first time elevated transport activity was demonstrated due to these coding SNPs. Kinetic analysis revealed that P103R had a higher V max value, whereas Y273C had a lower value than that in the wild-type. Cell surface protein expression levels were higher for P103R than for the wild-type, whereas Y273C expression was decreased. Immunofluorescence analysis revealed that the P103R protein was localized to the plasma membrane, whereas Y273C showed cytoplasmic localization. Therefore, the difference in transport activities between P103R and Y273C variants was suggested to be responsible for the different protein expression levels observed at the plasma membrane. Four nonsynonymous SNPs in this study showed relatively low allelic frequencies (0.5 to 2.1%), but these were associated with markedly reduced or increased MATE2-K function.

Evaluation of the binding and uptake of an antibody in liver non-parenchymal cells (NPC), including liver sinusoidal endothelial cells, is important for revealing its pharmacokinetic (PK) behavior, since NPC has important roles in eliminating an antibody from the blood via the Fc fragment of IgG receptor IIB (FcγRIIB). However, there is currently no in vitro quantitative assay using NPC. This study reports on the development of a cell-based assay for evaluating the binding and uptake of such an antibody using liver NPC of mice and monkeys. In mice, the FcγRIIB-expressing cells were identified in the CD146-positive and CD45-negative fraction by flow cytometry. A titration assay was performed to determine the PK parameters, and the obtained parameter was comparable to that determined by the fitting of the in vivo PK. This approach was also extended to NPC from monkeys. The concentration-dependent binding and uptake was measured to determine the PK parameters using monkey NPC, the FcγRIIB-expressing fraction of which was identified by CD31 and CD45. The findings presented herein demonstrate that the in vitro liver NPC assay using flow cytometry is a useful tool to determine the binding and uptake of biologics and to predict the PK.