In vitro-in vivo extrapolation scaling factors for intestinal P-glycoprotein and breast cancer resistance protein: Part II. The impact of cross-laboratory variations of intestinal transporter relative expression factors on predicted drug disposition

Relative Expression Factors (REFs) are used to scale in vitro transporter kinetic data via In Vitro–In Vivo Extrapolation linked to Physiologically-Based Pharmacokinetic (IVIVE-PBPK) models to clinical observations. Primarily two techniques to quantify transporter protein expression are available, immunoblotting and LC-MS/MS. Literature collated Relative Expression Factors (REF) ranged from 0.4 to 5.1 and 1.1 to 90 for intestinal Pglycoprotein (P-gp) and Breast Cancer Resistance Protein (BCRP), respectively. The impact of using human jejunum-Caco-2 REFs, for P-gp (REFiP-gp) and BCRP (REFiBCRP), generated from the same samples, employing different proteomic methodologies from independent laboratories, on PBPK outcomes was assessed. A 5-fold decrease in REFiP-gp for a single oral dose of digoxin resulted in a 1.19- and 1.31-fold higher plasma AUC and Cmax, respectively. All generated REFiP-gp values led to simulated digoxin Cmax values within observed ranges; however, combining kinetic data generated from a different laboratory, with the 5-fold lower REFiP-gp, could not recover a digoxin-rifampicin DDI; emphasising the necessity to obtain transporter-specific kinetic estimates and REFs from the same in vitro system. For a theoretical BCRP compound, with absorption taking place primarily in the jejunum, a decrease in the REFiBCRP from 2.22 (University of Manchester) to 1.11 (Bertin Pharma) promoted proximal intestinal absorption, while delaying tmax 1.44-fold. Laboratory-specific differences in REF may lead to different IVIVE-PBPK outcomes. To understand the mechanisms underlying projected PK liabilities, it is important to assess the potential impact of bias on the generation of REFs on an inter-individual basis within a target population.