M. Harwood, M. Zhang, S. Pathak, I. Gardner, S. Neuhoff
Simcyp Ltd. (a Certara company), Sheffield, UK
A combination of in vitro transporter kinetic data from cell monolayers and expression-based scaling factors are employed to predict the in vivo impact of intestinal transporters, an In Vitro-In Vivo Extrapolation (IVIVE) approach within Physiologically-Based Pharmacokinetic (PBPK) models. At present, the region-specific intestinal transporter expression levels are described within IVIVE-PBPK models based on relative expression approaches obtained from PCR or immunoblotting. In recent years, there has been an increase in absolute transporter expression measurements using quantitative proteomic approaches. The aim of this study was to compile available region-specific transporter expression data to enable robust values to be used for scaling purposes in IVIVE-PBPK modelling. This is the first in-depth analysis reporting region-specific transporter expression obtained via absolute; relative or a combination of both approaches and collated in a database. Original articles quantifying intestinal transporter expression were retrieved on PubMed using the following search terms <Human>, <Intestinal>, <Transporter>, <Absolute>, <Relative>, <Protein>, <Expression>, <Abundance>, <Proteomics>. In total 44 articles were found of which 28 were of sufficient quality to be used in the meta-analysis with >1300 measurements of transporter expression included. Studies were excluded where quantification took place in; subjects of ethnicities other than Caucasians; disease tissue; subjects <18 years old; duplicate samples between studies; a matrix that was not a total membrane matrix; pooled matrices or cDNA. The GetData Graph Digitizer was used to obtain numerical values from graphical representations of data in publications. The donors demographic data was collected where available, and the weighted mean absolute and relative expression in each of 8 gut segments including 7 small intestinal (duodenum, 2 jejunum, and 4 ileum segments) and the colon was calculated for 16 transporters (6 solute carriers (SLC) and 10 ATP-dependent (ABC) transporters). Relative expression data was normalised to the expression level in the proximal jejunum. The highest jejunal transporter abundances were found for SLC15A1 (PepT1) with 3.69 pmol/mg total membrane protein (Coefficient of Variation (CV) 62%, n=11 samples, 3 studies), with the highest abundance ABC transporter being ABCC2 (MRP2) with 0.86 pmol/mg (CV 58%, n=11 samples, 3 studies). Transporters displaying significant region-specific expression included, SLC10A2 (IBAT/ASBT) which showed 18-fold greater expression in the terminal ileum compared to the proximal jejunum, MRP3 showed >2-fold higher expression in the duodenum and colon, while SLC22A4 (OCTN1) showed >2-fold higher expression in colon compared to the proximal jejunum. SLC22A1 (OCT1) showed >2-fold lower expression in colon compared to proximal jejunum. For all other evaluated transporters, the region-specific expression was relatively uniform across the segments with no transporter displaying expression greater or less than 2-fold higher than in proximal jejunum. This is the first systematic analysis of intestinal regional transporter abundances. It can be expected to be useful for IVIVE for Caucasians. Similar analyses are required for other ethnicities and disease models.