The use of in vitro-in vivo extrapolation (IVIVE) of metabolic data in the prediction of population clearance has become an important tool in both discovery and preclinical phases of the drug development process (Rostami-Hodjegan and Tucker, 2007). Scaling of metabolic rate constants derived using microsomal protein (MSP) isolated from human livers or those from recombinantly expressed cytochrome P450 enzymes requires knowledge of the amount of microsomal protein per gram of liver (MPPGL) among other scaling factors (Barter et al., 2007). The most commonly used value of MPPGL in human IVIVE is 45 mg g Ϫ1 (Obach, 1997;Soars et al., 2002;Andersson et al., 2004;Uchaipichat et al., 2006) reported in a review by Houston (1994). However, this value is not obtained from human livers; instead it is a combination of data generated using rat tissue from a number of sources (Joly et al., 1975;Lin et al., 1978; Baarnhielm et al., 1986;Chiba et al., 1990). Several values of MPPGL determined using human tissue have been reported in the literature, and a detailed review of these studies has been the focus of a recent consensus article on the most appropriate values of MPPGL for use in IVIVE alongside other scaling factors such as human hepatocellularity (Barter et al., 2007). Collation of values of MPPGL from five studies (114 observations; age range 11-80 years, median 48 years; 47 females) has indicated a weak but statistically significant inverse relationship between MPPGL and donor age (Barter et al., 2007). Barter et al. (2007) assumed that different experimental procedures and technical staff carrying out the exercise ("operators") would not introduce bias into their analysis. However, these effects have not been assessed systematically. Assessing the operator effect also has implications for within-laboratory pooling of data. The preparation and analysis of replicate samples, which are required to differentiate between methodological and true biological variability in values of MPPGL, is time-consuming (2 days/liver). To increase output, processing of samples may be performed by more than one individual (operator); however, this will require an indication of consistency between estimates of MPPGL by these operators.In addition, determination of values of MPPGL from fresh tissue from large numbers of donors is hampered by the infrequent supply of human tissue and logistical problems associated with immediate analysis of the samples when they arrive at the laboratory. In theory, the use of tissue stored at Ϫ80°C solves this problem. However, the use of frozen samples assumes maintenance of protein structure and function through the freeze-thaw process. Thus, to ensure adequate compatibility between the results obtained within and between differ-Z.E.B. was supported by Simcyp Ltd., (Sheffield, UK) and European Union Framework 6 (BIOSIM).Article, publication date, and citation information can be found at http://dmd.aspetjournals.org. doi:10.1124/dmd.108.021311. ABBREVIATIONS:IVIVE, in vitro-in vivo extrapolation; MSP, micr...

In studies designed to simulate a clinical observation in which an individual became tolerant to normally lethal doses of acetaminophen (APAP), mice were pretreated with increasing doses of APAP for 8 days and challenged on day 9 with normally supralethal doses of APAP. These animals developed minimal hepatotoxicity after a challenge dose with a fourfold increase in LD50 to 1,350 mg/kg. The pretreatment regimen resulted in hepatic changes including: centrilobular localization of 3-(cysteine-S-yl)APAP protein adducts, selective down-regulation of cytochrome P4502E1 (CYP2E1) and CYP1A2 that produced the toxic metabolite, N-acetyl-p-benzoquinone imine, higher levels of reduced glutathione (GSH), centrilobular inflammation, and a fourfold increase in hepatocellular proliferation. The protection against the lethal APAP doses afforded by pretreatment is secondary to these changes and to the associated regional shift in the bioactivation of the APAP challenge dose from centrilobular to periportal regions where CYP2E1 is not found, protective GSH is more abundant, and where cell-proliferative responses are better able to sustain repair. This shift in APAP bioactivation results in less-intense covalent binding that is more diffuse and spread uniformly throughout the hepatic lobe, most likely contributing to protection by delaying the early onset of liver injury that has been generally associated with centrilobular localization of the adducts. Intervention of APAP pretreatment-induced cell division in mice with colchicine left them resistant to a 500-mg/kg (normally lethal) dose of APAP, but unable to survive a 1,000-mg/kg APAP challenge dose. The data demonstrate multiple mechanistic components to the protection afforded by APAP pretreatment. Whereas metabolic and physiological changes not dependent on cell proliferation are adequate to protect against 500 mg/kg APAP, these changes plus a potentiated cell-proliferative response are necessary for protection against the supralethal 1,000-mg/kg APAP dose. Furthermore, the data document an uncoupling of the traditional association between covalent binding and toxicity, and suggest that the assessment of toxicity following repeated or chronic APAP exposure must consider altered drug interactions and parameters besides those historically used to assess acute APAP overdose.

This report describes a previously uncharacterized occupational health hazard: work crew exposures to respirable crystalline silica during hydraulic fracturing. Hydraulic fracturing involves high pressure injection of large volumes of water and sand, and smaller quantities of well treatment chemicals, into a gas or oil well to fracture shale or other rock formations, allowing more efficient recovery of hydrocarbons from a petroleum-bearing reservoir. Crystalline silica ("frac sand") is commonly used as a proppant to hold open cracks and fissures created by hydraulic pressure. Each stage of the process requires hundreds of thousands of pounds of quartz-containing sand; millions of pounds may be needed for all zones of a well. Mechanical handling of frac sand creates respirable crystalline silica dust, a potential exposure hazard for workers. Researchers at the National Institute for Occupational Safety and Health collected 111 personal breathing zone samples at 11 sites in five states to evaluate worker exposures to respirable crystalline silica during hydraulic fracturing. At each of the 11 sites, full-shift samples exceeded occupational health criteria (e.g., the Occupational Safety and Health Administration calculated permissible exposure limit, the NIOSH recommended exposure limit, or the ACGIH threshold limit value), in some cases, by 10 or more times the occupational health criteria. Based on these evaluations, an occupational health hazard was determined to exist for workplace exposures to crystalline silica. Seven points of dust generation were identified, including sand handling machinery and dust generated from the work site itself. Recommendations to control exposures include product substitution (when feasible), engineering controls or modifications to sand handling machinery, administrative controls, and use of personal protective equipment. To our knowledge, this represents the first systematic study of work crew exposures to crystalline silica during hydraulic fracturing. Companies that conduct hydraulic fracturing using silica sand should evaluate their operations to determine the potential for worker exposure to respirable crystalline silica and implement controls as necessary to protect workers.

Turnout gear provides protection against dermal exposure to contaminants during firefighting; however, the level of protection is unknown. We explored the dermal contribution to the systemic dose of polycyclic aromatic hydrocarbons (PAHs) and other aromatic hydrocarbons in firefighters during suppression and overhaul of controlled structure burns. The study was organized into two rounds, three controlled burns per round, and five firefighters per burn. The firefighters wore new or laundered turnout gear tested before each burn to ensure lack of PAH contamination. To ensure that any increase in systemic PAH levels after the burn was the result of dermal rather than inhalation exposure, the firefighters did not remove their self-contained breathing apparatus until overhaul was completed and they were >30 m upwind from the burn structure. Specimens were collected before and at intervals after the burn for biomarker analysis. Urine was analyzed for phenanthrene equivalents using enzyme-linked immunosorbent assay and a benzene metabolite (s-phenylmercapturic acid) using liquid chromatography/tandem mass spectrometry; both were adjusted by creatinine. Exhaled breath collected on thermal desorption tubes was analyzed for PAHs and other aromatic hydrocarbons using gas chromatography/mass spectrometry. We collected personal air samples during the burn and skin wipe samples (corn oil medium) on several body sites before and after the burn. The air and wipe samples were analyzed for PAHs using a liquid chromatography with photodiode array detection. We explored possible changes in external exposures or biomarkers over time and the relationships between these variables using non-parametric sign tests and Spearman tests, respectively. We found significantly elevated (P < 0.05) post-exposure breath concentrations of benzene compared with pre-exposure concentrations for both rounds. We also found significantly elevated post-exposure levels of PAHs on the neck compared with pre-exposure levels for round 1. We found statistically significant positive correlations between external exposures (i.e. personal air concentrations of PAHs) and biomarkers (i.e. change in urinary PAH metabolite levels in round 1 and change in breath concentrations of benzene in round 2). The results suggest that firefighters wearing full protective ensembles absorbed combustion products into their bodies. The PAHs most likely entered firefighters’ bodies through their skin, with the neck being the primary site of exposure and absorption due to the lower level of dermal protection afforded by hoods. Aromatic hydrocarbons could have been absorbed dermally during firefighting or inhaled during the doffing of gear that was off-gassing contaminants.

Differences in the pharmacokinetics of xenobiotics among humans makes them differentially susceptible to risk. Differences in enzyme content can mediate pharmacokinetic differences. Microsomal protein is often isolated from liver to characterize enzyme content and activity, but no measures exist to extrapolate these data to the intact liver. Measures were developed from up to 60 samples of adult human liver to characterize the content of microsomal protein and cytochrome P450 (CYP) enzymes. Statistical evaluations are necessary to estimate values far from the mean value. Adult human liver contains 52.9 +/- 1.476 mg microsomal protein per g; 2587 +/- 1.84 pmoles CYP2E1 per g; and 5237 +/- 2.214 pmols CYP3A per g (geometric mean +/- geometric standard deviation). These values are useful for identifying and testing susceptibility as a function of enzyme content when used to extrapolate in vitro rates of chemical metabolism for input to physiologically based pharmacokinetic models which can then be exercised to quantify the effect of variance in enzyme expression on risk-relevant pharmacokinetic outcomes.