Human intrinsic clearance predicting

With this focus on CYP and fiver metabolism, most companies have established high throughput assays to measure compound stability in the presence of human (or preclinical species) fiver microsomes [49]. Disappearance of starting compound from an incubation with microsomes is monitored. Measurement at a single time point enables a rank-ordering of compounds for stability based on percent of parent compound remaining acquisition of data at multiple time points allows determination of half-life, intrinsic clearance, and extrapolation to a predicted in vivo clearance [50]. 

In a first step the scaling of intrinsic clearances determined in rat hepatocytes was compared to in vivo clearance. When taking account of non-linearity, the estimated hepatic metabolic clearance values were in reasonable agreement with observed total clearances, which ranged from 7 to 35 mL/min/kg, and it was considered reasonable to estimate the expected clearances in human by a similar scaling of human hepatocyte data. The error around the mean predicted human clearance was based on the variability seen in different batches of human hepatocytes. 

Obach RS. Prediction of human clearance of twenty-nine drugs from hepatic microsomal intrinsic clearance data an examination of in vitro half-life approach and nonspecific binding to microsomes. Drug Metab Dispos 1999 27 1350-1359. 

The binding to plasma or subcellular liver fraction can be taken into account for the prediction of human pharmacokinetic parameters either from preclinical and/or in vitro metabolism data (Obach et al. 1997 Mahmood 2000). Obach (1999) showed by comparison the in vivo investigated clearance values and clearance values projected from in vitro intrinsic clearance data of 29 drugs that the inclusion of blood and liver microsomes binding values gave the best agreement. 

Riley RJ, McGinnity DF, Austin RP. 2005. A unified model for predicting human hepatic, metabolic clearance from in vitro intrinsic clearance data in hepatocytes and microsomes. Drug Metab Dispos 33 1304—1311. 

Similar predictions of in vivo intrinsic clearance in the human from in vitro data have been produced by Ito et al. (20). The results are shown in Figure 30.12. The liver blood flow of about 1 niL/min g places the intrinsic clearances in perspective. These correlations show considerably more variability than do those for the rat. This reflects both methodologic difficulties and probably a large variability of enzyme activities within the human population. Also there seems to be a systematic underprediction for low-clearance drugs. This may reflect difficulties in measuring these low rates in vitro and/or extrahepatic metabolism. 

Thummelef al. (21) avoided the intrinsic variability of enzyme acctivity in the human population by predicting in vivo clearances from in vitro kinetic data in liver transplant patients. They were interested in using midazolam (MDZ) as a probe of CYP3A isoforms in the liver. Because biopsies were performed for the medical management of the transplant... 

Ekins S, Obach RS. Three dimensional-quantitative structure activity relationship computational approaches of prediction of human in vitro intrinsic clearance. / Pharmacol Exp Ther 2000 295 463-73. 

What are the strengths and weaknesses of these approaches The use of intrinsic clearance in vitro permits predictions between species for the particular enzyme/route of metabolism concerned. If humans have qualitatively different routes of metabolism for any particular compound, then this will weaken the predictive value of the in vitro observation. Similarly, allometric scaling works best for compounds with a high component of nonenzymatic elimination, such as our model compound with approximately 90% excretion as unchanged drug. This prediction weakens as variations in rates of enzymatic reactions become more important. The PK-PD modeling approaches use the existing in vivo data to calculate constants which can be applied to other in vivo data but does not, in its present form, link in vitro and in vivo data. 

Caco-2 cell monolayers or other cell lines as well as intrinsic clearance determination (half-lives) for structure/stability evaluation. The overall strategy for the industry is to use this screening tool as efficiently as possible, and not necessarily for all compounds in the chemical library. The standardised systems have a high throughput where thousands of chemical compounds can be tested in a short period of time and where the time for feedback into the projects is the most important success factor. Because of the importance of the measured values resulting from such assays for a forthcoming candidate drug selection, the importance of the assay for correct prediction of the human situation becomes evident. 

Because metabolic studies of pesticides have been extensively conducted in animals in vitro and in vivo, in vitrcj-in vivo correlation.s have been established in animals. By comparing in vitro data between humans and animals and assuming the in vifra-in vivo correlations observed in animals also exist in humans in a similar manner, one may be able to predict in vivo human metabolism of pesticides using in vitro human pesticide metabolism data. Therefore, the parameters obtained from the in vitro studies, such as /ffii (Michadis-Menten constant), (maximal reaction velocity), intrinsic clearance (inhibition... 

To predict hepatic clearance of Compound X in man, human in vitro intrinsic clearance could then be scaled to hepatic clearance, using a technique that had been validated in the rat. Ashfortt et al 1995Renal clearance is subject to an allometric relationship and can generally be scaled across species (see below). The predicted in vivo renal Cl for the rat (estimated by multiplying the predicted hepatic Cl by 9) may be scaled allometrically to... 

Bayliss MK, Bell JA, Jenner WN, Wilson K.Prediction of intrinsic clearance of loxtidine from kinetic studies in rat, dog and human hepatocytes. Biochem Soc Trans 1990 18 1198 1199. 

Naritomi Y, Terashita S, Kagayama A, Sugiyama Y. Utility of hepatocytes in predicting drug metabolism comparison of hepatic intrinsic clearance in rats and humans in vivo and in vitro. Drug Metab Dispos 2003 31 580-588. 

Reddy A, Heimbach T, Freiwald S, Smith D, Winters R, Michael S, Surendran N, Cai H. Validation of a semi-automated human hepatocyte assay for the determination and prediction of intrinsic clearance in discovery. J Pharm Biomed Anal... 

Sohlenius-Stembeck, A., Afeehus, L., Prussis, P., Neelissen, J., Hoogstraate, J., Johansson, J., Stembeck, J., and Petersson, C. (2010) Evaluation of the human prediction of clearance from hepatocyte and microsome intrinsic clearance for 52 drug compounds. Xenobiotica, 40, 637-649. 

Quantitative Structure Activity Relationship. Computational Approaches to Prediction of Human In Vitro Intrinsic Clearance. 

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