Determination of Human Hepatocyte Intrinsic Clearance for Slowly Metabolized Compounds: Comparison of a Primary Hepatocyte/Stromal Cell Co-culture with Plated Primary Hepatocytes and HepaRG. 
A key requirement in drug discovery is to accurately define intrinsic clearance (CL(int)) values of less than 1 µl/min/10(6) hepatocytes, which requires assays that allow for longer incubation time as a complement to suspended hepatocytes.  This study assessed the effectiveness of plated HepaRG cells, plated primary human hepatocytes (PHHs), and the HµREL human hepatocyte/stromal cell co-cultures for determination of low CL(int) values.  The investigation demonstrated that the systems were capable of providing statistically significant CL(int) estimations down to 0.2 µl/min/10(6) cells.  The HµREL assay provided a higher level of reproducibility, with repeat significant CL(int) values being defined in a minimum of triplicate consecutive assays for six of seven of the low CL(int) compounds compared with four of seven for PHHs and two of seven for HepaRG.  The assays were also compared with a suspension assay using drugs with higher CL(int) values and diverse enzymology.  The CL(int) values from the PHH and HµREL assays were similar to those defined by a hepatocyte suspension assay, indicating that they can be used interchangeably alongside a standard assay.  Finally, data from these two assays could also predict in vivo hepatic metabolic CL(int) to within 3-fold for greater than 70% of the compounds tested, with average fold errors (AFE) of 1.6 and 2.3, respectively, whereas the HepaRG data were predictive to within 3-fold for only 50% of compounds (AFE 2.9).  In summary, all systems have utility for low CL(int) determination, but the HµREL co-culture appears slightly superior regarding overall assay performance.