Interindividual variance of cytochrome P450 forms in human hepatic microsomes: correlation of individual forms with xenobiotic metabolism and implications in risk assessment

Radical induction theory of ulcerative colitis

To propose a new pathogenesis called Radical Induction to explain the genesis and progression of ulcerative colitis (UC). UC is an inflammatory bowel disease. Colonic inflammation in UC is mediated by a buildup of white blood cells (WBCs) within the colonic mucosal lining; however, to date there is no answer for why WBCs initially enter the colonic mucosa to begin with. A new pathogenesis termed “Radical Induction Theory” is proposed to explain this and states that excess un-neutralized hydrogen peroxide, produced within colonic epithelial cells as a result of aberrant cellular metabolism, diffuses through cell membranes to the extracellular space where it is converted to the highly damaging hydroxyl radical resulting in oxidative damage to structures comprising the colonic epithelial barrier. Once damaged, the barrier is unable to exclude highly immunogenic fecal bacterial antigens from invading the normally sterile submucosa. This antigenic exposure provokes an initial immune response of WBC infiltration into the colonic mucosa. Once present in the mucosa, WBCs are stimulated to secrete toxins by direct exposure to fecal bacteria leading to mucosal ulceration and bloody diarrhea characteristic of this disease.

Estimation of interindividual variation in oxidative metabolism of dichloromethane in human volunteers

A modified version of the original physiologically based pharmacokinetic (PBPK) model by Andersen et al. (1987) has been developed and used in conjunction with previously published human kinetic data for dichloromethane (DCM) metabolism and to assess interindividual variability in the rate of oxidative metabolism. Time-course data for 13 volunteers (10 males, 3 females) exposed to one or more concentrations of DCM (50 ppm, 100 ppm, 150 ppm, or 200 ppm) for 7.5h were used to optimize the maximal rate of hepatic metabolism (V(maxC)) through the cytochrome P450 pathway for each individual. DCM breath and blood concentrations were used, along with carboxyhemoglobin concentrations in blood and carbon monoxide (CO) concentrations in exhaled breath, to estimate the model parameters. Significant improvements in model fit were achieved when extrahepatic oxidative metabolism of DCM was added to the model structure. The 13 individual V(maxC) values ranged from 7.1 to 23.6 mg/h/kg0.7 and appeared to be bimodally distributed. The distribution was not sex related and may be related to differential CYP2E1 induction. A comparison of the observed variation in V(maxC) values to other estimates of variability in the rate of oxidative metabolism and human CYP2E1 activity suggest a relatively narrow range in human hepatic activity toward DCM.

SHORT-TERM INHIBITORY EFFECTS OF NITRIC OXIDE ON CYTOCHROME P450-MEDIATED DRUG METABOLISM: TIME DEPENDENCY AND REVERSIBILITY PROFILES IN ISOLATED PERFUSED RAT LIVERS

Nitric oxide (NO) is implicated as a mediator in the decreased catalytic activities of cytochrome P450 (P450) enzymes during inflammation or infection. Here, we examined the time course and the reversibility of the NO effect on P450s using isolated perfused rat livers. Livers were perfused at a constant rate with the NO donor sodium nitroprusside (SNP) for 0.5 or 1 h, followed by washout periods of 0 to 2.5 h. At the end of perfusion, microsomes were prepared and analyzed for P450 activities and other metabolic markers. Whereas 0.5 h of NO exposure caused an irreversible decline (approximately 30%) in total P450 content, a greater decline after 1 h of NO (approximately 55%) was mostly (approximately 30%) reversible, a pattern identical to that observed for the microsomal heme content. NO exposure also caused an enzyme-selective and time-dependent decline in P450 activities. Whereas the pattern of decline and reversibility of activities were qualitatively similar for CYP3A2, 2C11, 2E1, and 1A1/2, they differed for 2B1/2 and 2D1 in that the decline in the activity was delayed (1 h) for 2B1/2 and not observed for 2D1. This may be attributed to the accessibility of heme or cysteine thiolate and/or the presence/reactivity of critical cysteinyl amino acid residues in various P450 enzymes. Additionally, for most enzymes, the activity showed a biphasic decline, one within 1 h of SNP perfusion and another after 2 h of washout. This was associated with an identical biphasic decline in the microsomal free thiols, presumably due to the rapid and slow reaction of NO and peroxynitrite, respectively, with critical P450 thiols. The short-term effects of NO on P450 are time-dependent and enzyme-selective, with both reversible and irreversible mechanisms.

The metabolic rate constants and specific activity of human and rat hepatic cytochrome P-450 2E1 toward toluene and chloroform

Chloroform (CHCl3) is a near-ubiquitous environmental contaminant, a by-product of the disinfection of drinking water sources and a commercially important compound. Standards for safe exposure have been established based on information defining its toxicity, which is mediated by metabolites. The metabolism of CHCl3 is via cytochrome P-450 2E1 (CYP2E1)-mediated oxidation to phosgene, which is known to obey a saturable mechanism. CYP2E1 is a highly conserved form, expressed in all mammalian systems studied, and is responsible for the metabolism of a great many low-molecular-weight (halogenated) compounds. However, the Michaelis-Menten rate constants for CHCl3 oxidation have not been derived in vitro, and the specific activity of CYP2E1 toward CHCl3 has not been reported. In this investigation with microsomal protein (MSP), apparent Vmax values of 27.6 and 28.3 nmol/h/mg MSP and apparent K(m) values of 1 and 0.15 microM in rats and human organ donors, respectively, were demonstrated. The specific activity of CYP2E1 toward CHCl3 in rats and humans was 5.29 and 5.24 pmol/min/pmol CYP2E1, respectively. Toluene metabolism to benzyl alcohol (BA), another CYP2E1-dependent reaction, was also highly dependent on CYP2E1 content in humans, and was more efficient than was CHCl3 metabolism. The specific activity of human CYP2E1 toward toluene metabolism in human MSP was 23 pmol/min/pmol CYP2E1. These results demonstrate that differences in CYP2E1 content of MSP among individuals and between species are largely responsible for observed differences in toluene and CHCl3 metabolism in vitro.

Hepatic Disposition and Effects of Nitric Oxide Donors: Rapid and Concentration-Dependent Reduction in the Cytochrome P450-Mediated Drug Metabolism in Isolated Perfused Rat Livers

Various mechanisms, including high levels of cytokines and nitric oxide (NO), have been proposed as mediators for inflammation-induced cytochrome 450 down-regulation. However, the contribution of each of these mediators to the observed effects is controversial. We used an isolated perfused rat liver (IPRL) model to test the direct effects of NO donors on CYP450 down-regulation in the absence of cytokines or other confounding in vivo factors. Our hypothesis was that NO rapidly and concentration-dependently decreases CYP450 activities in IPRL. Livers were perfused (60 min) with 50 to 500 microM sodium nitroprusside (SNP) or 100 to 500 microM isosorbide dinitrate (ISDN) as NO donors, and the perfusate and biliary disposition of SNP, ISDN, and generated nitrate/nitrite (NO(x)) were determined. Additionally, at the end of perfusion, catalytic activities and protein levels of various cytochrome isoenzymes were measured. Both SNP and ISDN exhibited linear hepatic disposition with extraction ratios of approximately 0.30 and 0.50, respectively. Furthermore, although in small amounts, both NO donors and NO(x) were found in the bile. Except for CYP2D1, the catalytic activities of all the studied isoenzymes were substantially (up to 85%) decreased by both NO donors. However, the apoprotein levels of isoenzymes remained largely unchanged. Additionally, the inhibitory effects of NO donors were concentration-dependent, with the concentrations of SNP producing one-half of maximum inhibition being in the order of 2C11 > 2B1/2 > 2E1 = 3A2 > 1A1/2. These studies indicate that the effects of NO on the down-regulation of cytochrome 450 catalytic activity are rapid, concentration-dependent, and isoenzyme-selective.

Drug interactions with angiotensin receptor blockers: a comparison with other antihypertensives

The ever-increasing introduction of new therapeutic agents means that the potential for drug interactions is likely to escalate. Numerous different classes of drugs are currently used to treat hypertension. The angiotensin receptor blockers offer one of the newest approaches to the management of patients with high blood pressure. Compared with other classes of antihypertensive agents, the angiotensin receptor blockers appear overall to have a low potential for drug interactions, but variations within the class have been detected. Losartan and irbesartan have a greater affinity for cytochrome p450 (CYP) isoenzymes and, thus, are more likely to be implicated in drug interactions. There is pharmacokinetic evidence to suggest that such interactions could have a clinical impact. Candesartan cilexetil, valsartan and eprosartan have variable but generally modest affinity and telmisartan has no affinity for any of the CYP isoenzymes. In vitro studies and pharmacokinetic/pharmacodynamic evaluation can provide evidence for some interactions, but only a relatively small number of drug combinations are usually studied in this way. The absence of any pharmacokinetic evidence of drug interaction, however, should not lead to complacency. Patients should be made aware of possible interactions, especially involving the concurrent use of over-the-counter products, and it may be prudent for all patients receiving antihypertensive treatment to be monitored for possible drug interactions at their regular check-ups. The physician can help by prescribing agents with a low potential for interaction, such as angiotensin receptor blockers.

The impact of cytochrome P450 2E1-dependent metabolic variance on a risk-relevant pharmacokinetic outcome in humans

Risk assessments include assumptions about sensitive subpopulations, such as the fraction of the general population that is sensitive and the extent that biochemical or physiological attributes influence sensitivity. Uncertainty factors (UF) account for both pharmacokinetic (PK) and pharmacodynamic (PD) components, allowing the inclusion of risk-relevant information to replace default assumptions about PK and PD variance (uncertainty). Large numbers of human organ donor samples and recent advances in methods to extrapolate in vitro enzyme expression and activity data to the intact human enable the investigation of the impact of PK variability on human susceptibility. The hepatotoxicity of trichloroethylene (TCE) is mediated by acid metabolites formed by cytochrome P450 2E1 (CYP2E1) oxidation, and differences in the CYP2E1 expression are hypothesized to affect susceptibility to TCE's liver injury. This study was designed specifically to examine the contribution of statistically quantified variance in enzyme content and activity on the risk of hepatotoxic injury among adult humans. We combined data sets describing (1) the microsomal protein content of human liver, (2) the CYP2E1 content of human liver microsomal protein, and (3) the in vitro Vmax for TCE oxidation by humans. The 5th and 95th percentiles of the resulting distribution (TCE oxidized per minute per gram liver) differed by approximately sixfold. These values were converted to mg TCE oxidized/h/kg body mass and incorporated in a human PBPK model. Simulations of 8-hour inhalation exposure to 50 ppm and oral exposure to 5 micro g TCE/L in 2 L drinking water showed that the amount of TCE oxidized in the liver differs by 2% or less under extreme values of CYP2E1 expression and activity (here, selected as the 5th and 95th percentiles of the resulting distribution). This indicates that differences in enzyme expression and TCE oxidation among the central 90% of the adult human population account for approximately 2% of the difference in production of the risk-relevant PK outcome for TCE-mediated liver injury. Integration of in vitro metabolism information into physiological models may reduce the uncertainties associated with risk contributions of differences in enzyme expression and the UF that represent PK variability.

Relationship between genetic polymorphism of cytochrome P450IIE1 and fatty liver

AIM: To study the correlation between genetic polymorphism of cytochrome P450IIE1 (CYPIIE1) and fatty liver.

METHODS: Peripheral blood mononuclear cells were collected in 56 patients with fatty liver, 26 patients without fatty liver and 20 normal controls. Then PCR-RFLP was used to analyze genetic polymorphism of CYPIIE1 in monocytes on the region of Pst I and Rsa I.

RESULTS: The frequency of homozygotic C1 gene in patients with alcoholic fatty liver (28.6%), obese fatty liver (38.5%), or diabetic fatty liver (33.3%) was significantly lower than that of the corresponding patients without fatty liver (100%, 100% and 80% respectively), while the frequency of C2 genes, including C1/C2 and C2/C2, was significantly higher (71.4%/0%, 61.5%/0%, and 66.7%/20%) (P < 0.01). The frequency distribution of the above genes of non-fatty liver patients (100%/0, 100%/0, and 80%/20%) was not significantly different from that of the normal controls (85%/15%) (P > 0.05).

CONCLUSION: The genetic polymorphism of CYPIIE1 on the position of Pst I and Rsa I is related to the susceptibility of fatty liver. Besides, C2 gene may play a key role in the pathogenesis of fatty liver.

Comparative analysis of CYP3A expression in human liver suggests only a minor role for CYP3A5 in drug metabolism

To study mechanisms behind the interindividual variability in CYP3A expression and the relative contribution of the different CYP3A enzymes to the overall CYP3A activity, we have analyzed CYP3A4, CYP3A5, CYP3A43, and PXR mRNA and CYP3A4 and CYP3A5 protein expression, catalytic activity, and polymorphism in the CYP3A5 gene in a panel of 46 Caucasian human livers. Protein quantification was performed by Western blotting using enzyme-specific antibodies directed to the C termini of CYP3A4 or CYP3A5, and carrier protein-coupled peptides as standards. The mRNA levels were determined by quantitative real-time PCR. CYP3A activity was measured by analysis of the rate of testosterone 6beta-hydroxylation. A correlation existed between all CYP3A and PXR mRNA transcripts measured. The interindividual variability in CYP3A4 and CYP3A5 mRNA expression was higher than that of CYP3A protein and activity. The CYP3A5 protein was expressed at quantifiable levels in 5 (10.9%) of the livers. Four of those were heterozygous for the CYP3A5*1 allele and had CYP3A5 protein at a mean level of 17% of that of total CYP3A, whereas one liver sample was from a CYP3A5*3 homozygote individual having lower amounts of CYP3A5. In total, CYP3A5 only contributed 2% of the overall CYP3A protein among all samples. In conclusion, our data indicate that CYP3A4, CYP3A5, CYP3A43, and PXR hepatic mRNA expression correlate, indicating common regulatory features, and that the contribution of CYP3A5 to hepatic drug metabolism in Caucasians is insignificant.

Involvement of CYP3A4, CYP2C8, and CYP2D6 in the metabolism of (R)- and (S)-methadone in vitro

To clarify the oxidative metabolism of methadone (R)- and (S)-enantiomers, the depletion of parent (R)- and (S)-methadone and the formation of racemic 2-ethylidene-1,5-dimethyl-3,3-diphe-nylpyrolidine were studied using human liver microsomes and recombinant cytochrome P450 enzymes. Based on studies with isoform-selective chemical inhibitors and expressed enzymes, CYP3A4 was the predominant enzyme involved in the metabolism of (R)-methadone. However, it has different stereoselectivity toward (R)- and (S)-methadone. In recombinant CYP3A4, the metabolic clearance of (R)-methadone was about 4-fold higher than that of (S)-methadone. CYP2C8 is also involved in the metabolism of methadone, but its contribution to the metabolism of (R)-methadone was smaller than that of CYP3A4. But for the metabolism of (S)-methadone, the roles of CYP2C8 and CYP3A4 appeared equal. Although CYP2D6 is involved in the metabolism of (R)- and (S)-methadone, its role was smaller compared with CYP3A4 and CYP2C8. Using clinically relevant concentrations of ketoconazole (1 microM, selective CYP3A4 inhibitor), trimethoprim (100 microM, selective CYP2C8 inhibitor), and paroxetine (5 microM, potent CYP2D6 inhibitor), these inhibitors decreased the hepatic metabolism of (R)-[(S)-]methadone by 69% (47%), 22% (51%), and 41% (77%), respectively. However, inhibition of the metabolism of (R)- and (S)-methadone by paroxetine was due to inhibition not only of CYP2D6, but also CYP3A4 and, to a minor extent, CYP2C8. The present in vitro findings indicated that CYP3A4, CYP2C8, and CYP2D6 are all involved in the stereoselective metabolism of methadone (R)- and (S)-enantiomers. These data suggest that coadministration of inhibitors of CYP3A4 and CYP2C8 may produce clinically significant drug-drug interactions with methadone.

Metabolism of the soil and groundwater contaminants, ethylene dibromide and trichloroethylene, by the tropical leguminous tree, Leuceana leucocephala

Ethylene dibromide (EDB; dibromoethane) and trichloroethylene (TCE) are hazardous environmental pollutants. The use of plants to treat polluted sites and groundwater, termed phytoremediation, requires plants that can both effectively remove the pollutant as well as grow in the climatic region of the site. In this paper, we report that the tropical leguminous tree, Leuceana leucocephala var. K636, is able to take up and metabolize EDB and TCE. The plants were grown in sterile hydroponic solution without its symbiont, Rhizobium. EDB and TCE were both metabolized by the plant, as indicated by the formation of bromide ion from EDB and trichloroethanol from TCE. Each plant organ was independently capable of debromination of EDB. L. leucocephala is being used to treat perched groundwater as part of a remedial alternative to address an accidental EDB spill in Hawaii. Bromide levels of plant tissues from the trees grown in the phytoremediation treatment cells at the Hawaii Site were elevated, indicating uptake and degradation of brominated compounds in the trees. This report is the first evidence of a tropical tree effectively metabolizing these common organic pollutants.

Microsomal cytochrome P450 levels and activities of isolated rat livers perfused with albumin

PURPOSE

We recently showed that the perfusion of isolated rat livers with perfusates containing bovine serum albumin (BSA) would significantly stimulate the release of tumor necrosis factor (TNF)-alpha. Here, we hypothesize that BSA-induced increase in the release of TNF-alpha, and possibly other cytokines, would affect cytochrome P450 (CYP)-mediated drug metabolism.

METHODS

Rat livers were perfused ex vivo for 1, 2, or 3 h with a physiologic buffer containing or lacking 1% BSA (n = 4-5/group). At the end of perfusion, liver microsomes were prepared and analyzed for their total CYP, CYP2E1, CYP3A2, and CYP2C11 protein contents and the activities of cytochrome c reductase, CYP2E1, CYP3A2, CYP2C11, CYP2E1, CYP2D1, CYP1A1, and CYP2B1/2. In addition, the concentrations of various cytokines and nitric oxide were quantified in the outlet perfusate.

RESULTS

In the absence of BSA, the perfusate levels of all measured cytokines and nitric oxide were low. However, when the perfusate contained BSA, the levels of TNF-alpha, interleukin-6, and nitric oxide increased significantly (p < 0.005). Perfusion of the livers for 3 h with the BSA-containing perfusate resulted in significant (p < 0.05) decreases in the total CYP (41%), CYP2E1 (59%), CYP3A2 (68%), and CYP2C11 (50%) protein contents and activities of cytochrome c reductase (31%), CYP2E1 (66%), CYP3A2 (54%), and CYP2G11 (51%). In contrast, perfusion of livers for 1 or 2 h with the BSA perfusate did not have any significant effect on CYP-mediated metabolism. The CYP1A2, CYP2D1, and CYP2B1/2 activities were not affected by BSA, regardless of perfusion time.

CONCLUSION

Addition of BSA to perfusates, which is a routine practice in isolated rat liver studies, can reduce CYP-mediated drug metabolism by a mechanism independent of protein-binding effect.

Incorporating human interindividual biotransformation variance in health risk assessment

The protection of sensitive individuals within a population dictates that measures other than central tendencies be employed to estimate risk. The refinement of human health risk assessments for chemicals metabolized by the liver to reflect data on human variability can be accomplished through (1) the characterization of enzyme expression in large banks of human liver samples, (2) the employment of appropriate techniques for the quantification and extrapolation of metabolic rates derived in vitro, and (3) the judicious application of physiologically based pharmacokinetic (PBPK) modeling. While in vitro measurements of specific biochemical reactions from multiple human samples can yield qualitatively valuable data on human variance, such measures must be put into the perspective of the intact human to yield the most valuable predictions of metabolic differences among humans. For quantitative metabolism data to be the most valuable in risk assessment, they must be tied to human anatomy and physiology, and the impact of their variance evaluated under real exposure scenarios. For chemicals metabolized in the liver, the concentration of parent chemical in the liver represents the substrate concentration in the Michaelis Menten description of metabolism. Metabolic constants derived in vitro may be extrapolated to the intact liver, when appropriate conditions are met. Metabolic capacity Vmax; the maximal rate of the reaction) can be scaled directly to the concentration of enzyme (or enzyme fraction) contained in the liver. Several environmental, genetic and lifestyle factors can influence the concentration of cytochrome P450 forms (CYP) in the liver by affecting either (1) the extent to which the CYP forms are expressed in the endoplasmic reticulum of the cell (isolated as the microsomal fraction from tissue homogenates), or (2) the expression of microsomal protein in intact liver tissue. Biochemically sound measures of the hepatic distribution of xenobiotic metabolizing enzymes among humans, based on expression of the enzymes within microsomal protein and the distribution of microsomal protein among intact livers, can be combined with metabolic constants derived in vitro to generate values consistent with those employed in PBPK models. When completed, the distribution (and bounds) of Vmax values can be estimated and included in PBPK models. Exercising such models under plausible exposure scenarios will demonstrate the extent to which human interindividual enzyme variance can influence parameters (i.e., the detoxication of a toxic chemical through metabolism) that may influence risk. In this article, we describe a methodology and conditions which must exist for such an approach to be successful.

Susceptibility to the ototoxic properties of toluene is species specific

Toluene is the most widely used industrial solvent. It has been shown to be ototoxic in mice and rats, and to increase permanent threshold shift in conjunction with exposure to noise. Chinchillas are widely used for studying noise effects on the cochlea. The present study was initiated to study toluene and noise interaction in chinchillas. Thirty-three chinchillas were exposed to a 95 dBA 500 Hz octave band noise plus 2000 ppm toluene, 8 or 12 h per day for 10 days. Auditory function was estimated using the auditory brainstem response (ABR) to tones between 500 Hz and 16 kHz. There was no effect on the ABR of toluene alone. Noise alone produced a threshold shift. There was no interaction of noise and toluene on the ear. The present study suggests that chinchillas are markedly less susceptible to the ototoxic effect of toluene than mice and rats. A working hypothesis as to the species differences was that chinchilla liver was able to detoxify the toluene. Hepatic microsomes from chinchillas, rats and humans were tested for their ability to convert toluene to the more water-soluble compound - benzyl alcohol. Chinchilla livers were found to contain more of the P450 enzymes CYP2E1 and CYP2B than rats or humans. In addition, the data show that the P450 enzymes are more active in chinchillas than in rats and humans. In conclusion, the results suggest that rats and mice are a more appropriate model for human toluene ototoxicity. However, chinchillas may provide a valuable model for investigating how ototoxic agents can be detoxified to less damaging compounds.

Genetic polymorphisms in assessing interindividual variability in delivered dose

Increasing sophistication in methods used to account for human variability in susceptibility to toxicants has been one of the success stories in the continuing evolution of risk assessment science. Genetic polymorphisms have been suggested as an important contributor to overall human variability. Recently, data on polymorphisms in metabolic enzymes have been integrated with physiologically based pharmacokinetic (PBPK) modeling as an approach to determining the resulting overall variability. We present an analysis of the potential contribution of polymorphisms in enzymes modulating the disposition of four diverse compounds: methylene chloride, warfarin, parathion, and dichloroacetic acid. Through these case studies, we identify key uncertainties likely to be encountered in the use of polymorphism data and highlight potential simplifying assumptions that might be required to test the hypothesis that genetic factors are a substantive source of human variability in susceptibility to environmental toxicants. These uncertainties include (1) the relative contribution of multiple enzyme systems, (2) the extent of induction/inhibition through coexposure, (3) allelic frequencies of major ethnic groups, (4) the absence of chemical-specific data on the kinetic parameters for the different allelic forms of key enzymes, (5) large numbers of low-frequency alleles, and (6) uncertainty regarding differences between in vitro and in vivo kinetic data. Our effort sets the stage for the acquisition of critical data and further integration of polymorphism data with PBPK modeling as a means to quantitate population variability.

Chloral hydrate for progressive myoclonus epilepsy: a new look at an old drug

This study demonstrates that chloral hydrate can be used to control daytime myoclonic exacerbations. It reports on four patients with progressive myoclonus epilepsy--three with Unverricht-Lündborg disease (EPM1) and one with progressive external ophthalmoplegia (PEO)--all of whom were taking more than one antiepileptic drug. Response to the liquid formulation was faster than response to the capsule and was preferred by the patients. The unusual feature was less than expected sedation or development of tolerance even at daily doses above 500 mg administered for years. Because chloral hydrate helped to improve quality of life, it should be made available to patients with progressive myoclonus epilepsy as adjunctive therapy. Recent evidence of interactions with various excitatory and inhibitory amino acid neurotransmitter-operated ion channels as a mechanism of action may provide insight into altered neurotransmission in progressive myoclonus epilepsy.

Application of in vitro biotransformation data and pharmacokinetic modeling to risk assessment

The adverse biological effects of toxic substances are dependent upon the exposure concentration and the duration of exposure. Pharmacokinetic models can quantitatively relate the external concentration of a toxicant in the environment to the internal dose of the toxicant in the target tissues of an exposed organism. The exposure concentration of a toxic substance is usually not the same as the concentration of the active form of the toxicant that reaches the target tissues following absorption, distribution, and biotransformation of the parent toxicant. Biotransformation modulates the biological activity of chemicals through bioactivation and detoxication pathways. Many toxicants require biotransformation to exert their adverse biological effects. Considerable species differences in biotransformation and other pharmacokinetic processes can make extrapolation of toxicity data from laboratory animals to humans problematic. Additionally, interindividual differences in biotransformation among human populations with diverse genetics and lifestyles can lead to considerable variability in the bioactivation of toxic chemicals. Compartmental pharmacokinetic models of animals and humans are needed to understand the quantitative relationships between chemical exposure and target tissue dose as well as animal to human differences and interindividual differences in human populations. The data-based compartmental pharmacokinetic models widely used in clinical pharmacology have little utility for human health risk assessment because they cannot extrapolate across dose route or species. Physiologically based pharmacokinetic (PBPK) models allow such extrapolations because they are based on anatomy, physiology, and biochemistry. In PBPK models, the compartments represent organs or groups of organs and the flows between compartments are actual blood flows. The concentration of a toxicant in a target tissue is a function of the solubility of the toxicant in blood and tissues (partition coefficients), blood flow into the tissue, metabolism of the toxicant in the tissue, and blood flow out of the tissue. The appropriate degree of biochemical detail can be added to the PBPK models as needed. Comparison of model simulations with experimental data provides a means of hypothesis testing and model refinement. In vitro biotransformation data from studies with isolated liver cells or subcellular fractions from animals or humans can be extrapolated to the intact organism based upon protein content or cell number. In vitro biotransformation studies with human liver preparations can provide quantitative data on human interindividual differences in chemical bioactivation. These in vitro data must be integrated into physiological models to understand the true impact of interindividual differences in chemical biotransformation on the target organ bioactivation of chemical contaminants in air and drinking water.