An analysis of laboratory variability and thresholds for human in vitro ADME/PK methods

Introduction Various in vitro methods are used to measure absorption, distribution, metabolism and excretion/pharmacokinetics (ADME/PK) of candidate drugs and predict and decide whether properties are clinically adequate.

Methods Objectives were to evaluate variability within and between laboratories for commonly used human in vitro ADME/PK methods and to explore whether reliable thresholds may be defined. The literature was searched for in vitro data for intrinsic metabolic clearance (hepatocyte CL_int), apparent intestinal permeability (Caco-2 P_app), efflux ratio (Caco-2 ER), solubility (S) and BCS-class, and corresponding clinical estimates. In vitro ADME/PK data for three example drugs (atenolol, diclofenac and gemfibrozil) were used to predict human in vivo ADME/PK and investigate whether these would pass a compound selection process.

Results and Conclusions Interlaboratory variability is considerable, especially for f_u, S, ER and BCS-classification, and on average about twice as high as intralaboratory variability. Approximate mean interlaboratory variability for CL_int, P_app, ER and f_u (3- to 3.5-fold) appears to be about 2- to 3-fold higher than corresponding interlaboratory variability. Mean and maximum interlaboratory range for CL_int, P_app, ER, f_u and S are approximately 5- to 100-fold and 50- to 4500-fold, respectively, with second largest range for f_u and largest range for S. For one drug, laboratories produced almost 1000-fold different CL_int • f_u-values. It appears difficult/impossible to set clear clinically useful thresholds, especially for CL_int, ER and S. Poor in vitro-in vivo consistency for S and BCS-classification and large portions of compounds out of reach for Caco-2 and conventional hepatocyte assays are evident. Predictions for reference compounds are consistent with inadequate in vivo ADME/PK. Ways to improve predictions and compound selection are suggested.