The seminal paper on the CYP2D6 Activity Score (AS) was first published ten years ago and, since its introduction in 2008, it has been widely accepted in the field of pharmacogenetics. This scoring system facilitates the translation of highly complex CYP2D6 diplotype data into a patient’s phenotype to guide drug therapy and is at the core of all CYP2D6 gene/drug pair guidelines issued by the Clinical Pharmacogenetics Implementation Consortium (CPIC). The AS, however, only explains a portion of the variability observed among individuals and ethnicities. In this review, we provide an overview of sources in addition to CYP2D6 genotype that contribute to the variability in CYP2D6-mediated drug metabolism and discuss other factors, genetic and non-genetic, that likely contribute to the observed variability in CYP2D6 enzymatic activity.

The age-dependent absolute protein abundance of carboxylesterase (CES) 1 and CES2 in human liver was investigated and applied to predict infant pharmacokinetics (PK) of oseltamivir. The CES absolute protein abundance was determined by liquid chromatography-tandem mass spectrometry proteomics in human liver microsomal and cytosolic fractions prepared from tissue samples obtained from 136 pediatric donors and 35 adult donors. Two surrogate peptides per protein were selected for the quantification of CES1 and CES2 protein abundance. Purified CES1 and CES2 protein standards were used as calibrators, and the heavy labeled peptides were used as the internal standards. In hepatic microsomes, CES1 and CES2 abundance (in picomoles per milligram total protein) increased approximately 5-fold (315.2 vs. 1664.4) and approximately 3-fold (59.8 vs. 174.1) from neonates to adults, respectively. CES1 protein abundance in liver cytosol also showed age-dependent maturation. Oseltamivir carboxylase activity was correlated with protein abundance in pediatric and adult liver microsomes. The protein abundance data were then used to model in vivo PK of oseltamivir in infants using pediatric physiologically based PK modeling and incorporating the protein abundance–based ontogeny function into the existing pediatric Simcyp model. The predicted pediatric area under the curve, maximal plasma concentration, and time for maximal plasma concentration values were below 2.1-fold of the clinically observed values, respectively.

Limited understanding of species differences in kidney transporters is a critical knowledge gap for prediction of drug-induced acute kidney injury, drug interaction, and pharmacokinetics in humans. Here, we report protein abundance data of nineteen transporters in the kidney cortex across five species (human, monkey, dog, rat, and mouse). In general, the abundance of all the 19 membrane transporters was higher in preclinical species as compared to human except for MDR1, OCT3 and OCTN1. In nonhuman primate, the total abundance of 12 transporters for which absolute data were available, was 2.1-fold higher (p value = 0.025) relative to human but the % distribution of these transporters was identical in both species. MRP4, OCTN2, OAT2, Na+K+ATPase, MRP3, SGLT2, OAT1, MRP1, MDR1, OCT2 were expressed differently with cross-species variability of 8.2, 7.4, 6.1, 5.9, 5.4, 5.2, 4.1, 3.3 and 2.8-fold, respectively. Sex differences were only significant in rodents and dog. High protein-protein correlation was observed in OAT1 versus MRP2/MRP4 as well as OCT2 versus MATE1 in human and monkey. The cross-species and sex-dependent protein abundance data are important for animal to human scaling of drug clearance as well as for mechanistic understanding of kidney physiology and derisking of kidney toxicity for new therapeutic candidates in drug development. SIGNIFICANCE STATEMENT Prediction of drug-induced acute kidney injury, drug interaction, and pharmacokinetics in humans from animal data requires understanding of cross-species abundance of drug transporters in drug disposition relevant tissues, e.g., kidney, liver and intestine. We quantified protein abundance data of nineteen transporters in the kidney cortex across human, monkey, dog, rat, and mouse. Significant quantitative differences were observed in transporter abundance across species. Further, few transporters were only detected in specific species, e.g., OCT3 in human and BCRP in rodents. Moreover, several proteins (OAT1 versus MRP2/MRP4 and OCT2 versus MATE1) showed significant protein-protein correlations. These quantitative protein abundance data are important for mechanistic understanding of kidney toxicity and prediction of human renal drug clearance from animal data.