In metabolic clearance

The drawback of this approach is that it is essentially empirical, and does not allow for differences in metabolic clearance between the species, i.e., it assumes that clearance is proportional to blood flow. This works well for compounds that are highly extracted in the liver, and/or where passive renal clearance is the major pathway [5, 68]. An approach for compounds that are actively secreted into the urine has also been proposed [69], although the precise values of some of the physiological scaling factors have been questioned [70]. 

When considering the Hkely pharmacokinetic profile of a novel compound in man, it is important to recognize the variability that may be encountered in the cHnical setting. Animal pharmacokinetic studies are generally conducted in inbred animal colonies that tend to show minimal inter-subject variabiHty. The human population contains a diverse genetic mix, without the additional variability introduced by age, disease states, environmental factors and co-medications. Hence any estimate of pharmacokinetic behaviour in man must be tempered by the expected inherent variability. For compounds with high metabolic clearance (e. g. midazolam), inter-individual variability in metabolic clearance can lead to greater than 10-fold variation in oral clearance and hence systemic exposure [1]. 

Species Scaling Incorporating Differences in Metabolic Clearance... 

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. 

A threefold difference in metabolic clearance as assessed in vitro. 

Walle T, Walle U, Cowart TD, Conradi EC. 1989 Pathway selective sex differences in metabolic clearance of propranolol in human subjects. Clin. Pharm. Ther. 46(3) 257-263. 

Two distinct bases for these types of effects may be distinguished pharmacokinetic and pharmacodynamic. Pharmacokinetic-based toxic effects are due to an increase in the concentration of the compound or active metabolite at the target site. This may be due to an increase in the dose, altered metabolism or saturation of elimination processes for example. An example is the increased hypotensive effect of debrisoquine in poor metabolizers where there is a genetic basis for a reduction in metabolic clearance of the drug (see Chapter 5). 

Comparison of metabolic pathways across species which can in turn be used to explain pharmacologic or toxicologic differences between species Justification of species for preclinical safety and pharmacokinetics studies Help identify enzyme systems involved in metabolic clearance Support clinical development... 

As a class, the chiral NSAiDs display a wide range of stereoselectivity in plasma concentrations in humans (Table 1, Fig. 7). This is mostly due to variations in metabolic clearances of the enantiomers. Flurbiprofen and ketoprofen possess S R AUC ratios of close to one (Table 1) [246,247]. In contrast, drugs such as fenoprofen, etodolac, and ketorolac possess S R ratios that differ substantially from unity (Table 1) [248-250]. In all cases, concentrations of NSAID eutomer exceed those of distomer. Etodolac and ketorolac provide notable exceptions to this commonality, in that concentrations of distomer are much higher than those of the eutomer. 

In 12 healthy subjects, sulfinpyrazone 200 mg given every 6 hours for one week, increased the clearance of a single 1-g dose of paracetamol by 23%. There was a 26% increase in metabolic clearance of the glucuronide... 

Pharmacokinetic and pharmacodynamic differences between drug isomers present another important issue relating to drug metabolism. Individual enantiomers of drugs administered as racemates show different pharmacokinetic profiles due to differences in metabolic clearance rates and binding affinities to blood plasma proteins [34]. 

Data from both in vivo and in vitro systems showed PbTx-3 to have an intermediate extraction ratio, indicating in vivo clearance of PbTx-3 was equally dependent upon liver blood flow and the activity of toxin-metabolizing enzymes. Studies on the effects of varying flow rates and metabolism on the total body clearance of PbTx-3 are planned. Finally, comparison of in vivo metabolism data to those derived from in vitro metabolism in isolated perfused livers and isolated hepatocytes suggested that in vitro systems accurately reflect in vivo metabolic processes and can be used to predict the toxicokinetic parameters of PbTx-3. 

PBPK models have also been used to explain the rate of excretion of inhaled trichloroethylene and its major metabolites (Bogen 1988 Fisher et al. 1989, 1990, 1991 Ikeda et al. 1972 Ramsey and Anderson 1984 Sato et al. 1977). One model was based on the results of trichloroethylene inhalation studies using volunteers who inhaled 100 ppm trichloroethylene for 4 horns (Sato et al. 1977). The model used first-order kinetics to describe the major metabolic pathways for trichloroethylene in vessel-rich tissues (brain, liver, kidney), low perfused muscle tissue, and poorly perfused fat tissue and assumed that the compartments were at equilibrium. A value of 104 L/hour for whole-body metabolic clearance of trichloroethylene was predicted. Another PBPK model was developed to fit human metabolism data to urinary metabolites measured in chronically exposed workers (Bogen 1988). This model assumed that pulmonary uptake is continuous, so that the alveolar concentration is in equilibrium with that in the blood and all tissue compartments, and was an expansion of a model developed to predict the behavior of styrene (another volatile organic compound) in four tissue groups (Ramsey and Andersen 1984). 

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]. 

GIT, is considered to be lost from the absorption site, as is metabolic clearance and sequestration in various cell types and membranes (72,14). It is clear from Scheme I that the relative rates of the various processes will define the bioavailable fraction of the dose and understanding those factors which control pulmonary absorption kinetics is obviously the key to enhancing bioavailability via the lung. In a recent book (75) the molecular dependence of lung binding and metabolism was considered alongside the parallel processes of absorption, clearance and dissolution in the lung (14). Some key features of this work will be repeated as it relates to the systemic delivery of polypeptides. 

Instead of using the oral bioavailability of a drug, one can attempt to correlate PM values with permeability coefficients generated from in situ perfused intestinal preparations. Here, one eliminates the complexities of liver metabolism, clearance, and formulation variables. Recently, this type of in vitro-in situ correlation has been conducted using the model peptides (described previously in Section V.B.2). The permeabilities of these model peptides were determined using a perfused rat intestinal preparation which involved cannulation of the mesenteric vein (Kim et al., 1993). With this preparation, it was possible to measure both the disappearance of the peptides from the intestinal perfusate and the appearance of the peptides in the mesenteric vein. Thus, clearance values (CLapp) could be calculated for each peptide. Knowing the effective surface area of the perfused rat ileum, the CLapp values could be converted to permeability coefficients (P). When the permeability coefficients of the model peptides were plotted as a function of the lipophilicity of the peptides, as measured by partition coefficients in octanol-water, a poor correlation (r2 = 0.02) was observed. A better correlation was observed between the permeabilities of these peptides and the number of potential hydrogen bonds the peptide can make with water (r2 = 0.56,... 

Lave, T., Coassolo, P., Reigner, B., Prediction of hepatic metabolic clearance based on interspecies allometric scaling techniques and in vitro-in vivo correlations, Clin. Pharmacokinet. 1999, 36, 211-321 and references cited therein. 

Ketoconazole (a potent inhibitor of CYP3A4) has been shown to increase the oral bioavailability of cyclosporin from 22 to 56% [50]. This consisted of a 1.8-fold decrease in systemic clearance combined with a 4.9-fold decrease in oral clearance. The authors estimated that hepatic extraction was decreased only 1.15-fold, whereas the oral bioavailability increased 2.6-fold and the observation was attributed to decreased intestinal metabolism. Erythromycin was also shown to increase the oral bioavailability of cyclosporin A 1.7-fold, while pre-treatment with rifampin (an inducer of CYP3A4) decreased oral bioavailability of cyclosporin from 27% to 10% due to a 4.2-fold increase in oral clearance but only a 1.2-fold increase in systemic clearance. Floren et al. [51] have also shown that ketoconazole can double the oral bioavailability of tacrolimus in man by inhibiting gut wall CYP3A4. 

R. J., In vitro analysis of human drug glucuronidation and prediction of in vivo metabolic clearance, J. Pharmacol. Exp. Ther. 2002, 301, 382-390. 

An additional example of a bioavailability-predicted absorption plot is shown for a series of calcium antagonists (Fig. 19.8). Again there is considerable scatter in the data, and the four compounds - felodipine, nisoldipine, diltiazem, and verapamil -are predicted to be much better absorbed than was actually observed. Some of these compounds are known to undergo rapid first-pass metabolic clearance, and are also P-gp inhibitors or substrates (diltiazem and felodipine are P-gp substrates nicardipine and nitrendipine are P-gp inhibitors [25] verapamil is a P-gp inhibitor), and this might contribute to the scatter obtained in the graph. 

This model integrates existing in vitro data, such as Caco-2 permeability (Papp) and metabolic stability in liver S9 or microsomes, to estimate bioavailability as being either low, medium, or high. Oral bioavailability predictions for not only humans but also other species can be made by using the metabolic stability values of drugs in liver microsomal enzyme preparations from that species. A premise of this model is that metabolic clearance is more important than renal or biliary clearance in determining bioavailability. However, despite the lack of in vitro renal... 

Obach et al. [27] proposed a model to predict human bioavailability from a retrospective study of in vitro metabolism and in vivo animal pharmacokinetic (PK) data. While their model yielded acceptable predictions (within a factor of 2) for an expansive group of compounds, it relied extensively on in vivo animal PK data for interspecies scaling in order to estimate human PK parameters. Animal data are more time-consuming and costly to obtain than are permeability and metabolic clearance data hence, this approach may be limited to the later stages of discovery support when the numbers of compounds being evaluated are fewer. 

Deltour L, Foglio MH, Deuster G. Metabolic deficiencies in alcohol dehydrogenase Adhl, Adh3, and Adh4 null mutant mice Overlapping roles of Adhl and Adh4 in ethanol clearance and metabolism of retinol to retinoic acid. J Biol Chem 1999 274 16796-16801. 

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