In vitro-in vivo scaling of CYP-mediated metabolic clearance using a relative activity factor (RAF) approach

Quantitative prediction of hepatic clearance is a key element in predicting the human pharmacokinetic profile in the nonclinical stages. In the present study, we focused on the major cytochrome P450 (P450) isoforms (CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP3A4) and tested a relative activity factor (RAF) method to quantitatively predict in vivo hepatic intrinsic clearance (CLh,int,vivo) and fraction metabolized (fm) by the P450 isoforms directly from an in vitro recombinant P450 system. We selected multiple probe substrates for CYP1A2 (caffeine, tizanidine, phenacetin), CYP2C9 ((S)-acenocoumarol, glimepiride, lornoxicam, tolbutamide, (S)-warfarin), CYP2C19 ((S)-lansoprazole, omeprazole, pantoprazole), CYP2D6 (desipramine, metoprolol, nebivolol, tolterodine), and CYP3A4 (alprazolam, felodipine, midazolam, nisoldipine, sildenafil, triazolam) to calculate the representative RAF value for each P450 isoform based on the in vivo-to-in vitro clearance ratio of the multiple probe substrates. The most pronounced substrate dependency of the RAF values was noted for CYP3A4 (2698 [alprazolam] to 19073 [nisoldipine] pmol P450/kg). Using the geometric mean of the RAF values for each isoform, a within 3-fold prediction of the CLh,int,vivo was obtained for all the 11 test drugs, except glibenclamide, which is a known substrate of hepatic uptake transporters. The fm values of the responsible P450 isoform(s) could be well predicted for mexiletine, tamsulosin, risperidone, celecoxib, and glibenclamide. This simple, practical RAF method can be one of the useful nonclinical methods to estimate the CLh,int,vivo and fm mediated by the major P450 isoforms, which would promote earlier understanding of the impact of genetic polymorphisms and drug-drug interactions on the human pharmacokinetics of the substrate compounds. SIGNIFICANCE STATEMENT: The relative activity factor method has been used for extrapolating in vitro clearance from recombinant systems to liver microsomes, but this study utilized this method to predict in vivo hepatic clearance and fraction metabolized values. By applying relative activity factor values obtained from multiple probe substrates, this study was able to quantitatively predict the in vivo clearances mediated by CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4. This simple, practical method will help optimize metabolic clearances via the major cytochrome P450 isoforms in the nonclinical stages.