Human intrinsic clearance

First, if the compound was cleared mainly by hepatic metabolism in the animal species tested and if human hepatocytes in vitro suggest the same will be true in humans, then the measured hepatocyte clearance may be used in a process called in vitro/in vivo scaling (20, pp. 207-228) to provide an estimate of the human intrinsic clearance. The application of Equation 5 then gives an estimate of the human systemic clearance. Second, the animal PK parameters of CL and Vj can be subjected to allometric scaling (20, pp. 207-228) whereby the PK parameter is related to a measurable allometric variable such as body mass, body surface area, heart rate, and so forth. (21) by fitting these parameter-variable pairs for several species to an empirical power equation of the form... 

G. 75 drugs with log human intrinsic clearance unbound drugs [132]... 

Rat hepatocyte intrinsic clearance Human microsome intrinsic clearance Rat IV clearance... 

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]. 

Fig. 3 Human liver microsome apparent intrinsic clearance (Clint,app) vs clogD. Open squares and filled triangles represent two different chemical series (series A and B, respectively)...

The equation can be solved for intrinsic clearance (Clj) based upon systemic clearance (Clj) obtained after i.v. administration and hepatic blood flow (Q) in the test species. Intrinsic clearance in man can then be estimated based upon relative in vitro microsomal stabibty and the equation solved to provide an estimate for human systemic clearance. Hence this approach combines aUometry (by considering differences in organ blood flow) and species-specific differences in metabolic clearance. 

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. 

Fig. 10.12. (a) Experimental human liver microsome stability (FILM %Rem 1 . ) vs. calculated Log of solubility (c LogS). (b) Experimental human liver microsome stability expressed as apparent intrinsic clearance (FILM CL(int) xL/min/mg) vs. calculated Log of solubility (c LogS). A threshold value of cLogS > -3 and -4.0 is required for stability in FILM based on %R and intrinsic clearance, respectively. 

Cytochrome b5 affects the kinetics of drug metabolism by certain CYP enzymes hence, coexpression of this microsomal hemoprotein (together with NADPH-CYP reductase) can affect the catalytic efficiency of certain recombinant CYP enzymes (76,109). For example, the presence of cytochrome b5 tends to increase Fmax for reactions catalyzed by CYP3 A4, whereas it tends to decrease Km for reactions catalyzed by CYP2E1. In both cases, cytochrome b5 increases Vmax/Km, which is a measure of in vitro intrinsic clearance. The fact that some commercially available recombinant CYP enzymes are expressed with cytochrome b5 while others are not complicates the interpretation of results of studies performed with recombinant human CYP enzymes. 

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. 

To make use of the data gathered from in vitro systems, there must be a system for scaling up from what is observed in vitro to what is expected to happen in humans. This process is called in vitro-in vivo extrapolation (IVIVE) and involves the use of calculations which utilize scaling factors to extrapolate from in vitro to in vivo. Methods for IVIVE were first described more than 25 years ago by Rane et al., who used in vitro intrinsic clearance data collected from in vitro experiments on... 

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. 

Table 20 M3 and M2 binding affinities and intrinsic clearance (rat and human) of select hydantoin derivatives...

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. 

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. 

---