TY - JOUR T1 - Variability in P-Glycoprotein Inhibitory Potency (IC<sub>50</sub>) Using Various in Vitro Experimental Systems: Implications for Universal Digoxin Drug-Drug Interaction Risk Assessment Decision Criteria JF - Drug Metabolism and Disposition JO - Drug Metab Dispos SP - 1347 LP - 1366 DO - 10.1124/dmd.112.050500 VL - 41 IS - 7 AU - Joe Bentz AU - Michael P. O’Connor AU - Dallas Bednarczyk AU - JoAnn Coleman AU - Caroline Lee AU - Johan Palm AU - Y. Anne Pak AU - Elke S. Perloff AU - Eric Reyner AU - Praveen Balimane AU - Marie Brännström AU - Xiaoyan Chu AU - Christoph Funk AU - Ailan Guo AU - Imad Hanna AU - Krisztina Herédi-Szabó AU - Kate Hillgren AU - Libin Li AU - Evelyn Hollnack-Pusch AU - Masoud Jamei AU - Xuena Lin AU - Andrew K. Mason AU - Sibylle Neuhoff AU - Aarti Patel AU - Lalitha Podila AU - Emile Plise AU - Ganesh Rajaraman AU - Laurent Salphati AU - Eric Sands AU - Mitchell E. Taub AU - Jan-Shiang Taur AU - Dietmar Weitz AU - Heleen M. Wortelboer AU - Cindy Q. Xia AU - Guangqing Xiao AU - Jocelyn Yabut AU - Tetsuo Yamagata AU - Lei Zhang AU - Harma Ellens Y1 - 2013/07/01 UR - http://dmd.aspetjournals.org/content/41/7/1347.abstract N2 - A P-glycoprotein (P-gp) IC50 working group was established with 23 participating pharmaceutical and contract research laboratories and one academic institution to assess interlaboratory variability in P-gp IC50 determinations. Each laboratory followed its in-house protocol to determine in vitro IC50 values for 16 inhibitors using four different test systems: human colon adenocarcinoma cells (Caco-2; eleven laboratories), Madin-Darby canine kidney cells transfected with MDR1 cDNA (MDCKII-MDR1; six laboratories), and Lilly Laboratories Cells—Porcine Kidney Nr. 1 cells transfected with MDR1 cDNA (LLC-PK1-MDR1; four laboratories), and membrane vesicles containing human P-glycoprotein (P-gp; five laboratories). For cell models, various equations to calculate remaining transport activity (e.g., efflux ratio, unidirectional flux, net-secretory-flux) were also evaluated. The difference in IC50 values for each of the inhibitors across all test systems and equations ranged from a minimum of 20- and 24-fold between lowest and highest IC50 values for sertraline and isradipine, to a maximum of 407- and 796-fold for telmisartan and verapamil, respectively. For telmisartan and verapamil, variability was greatly influenced by data from one laboratory in each case. Excluding these two data sets brings the range in IC50 values for telmisartan and verapamil down to 69- and 159-fold. The efflux ratio-based equation generally resulted in severalfold lower IC50 values compared with unidirectional or net-secretory-flux equations. Statistical analysis indicated that variability in IC50 values was mainly due to interlaboratory variability, rather than an implicit systematic difference between test systems. Potential reasons for variability are discussed and the simplest, most robust experimental design for P-gp IC50 determination proposed. The impact of these findings on drug-drug interaction risk assessment is discussed in the companion article (Ellens et al., 2013) and recommendations are provided. ER -