Biomedical subjects

R S Yang

Publications and source records attributed to R S Yang.

At least 73 records · Page 4Linked to original sources

PubMed1997

A proposed approach to study the toxicology of complex mixtures of petroleum products: the integrated use of QSAR, lumping analysis and PBPK/PD modeling.

Mixture toxicity is a topic that has become a matter of concern during the last two decades. One of the major problems with assessing the toxicity of mixtures and the associated human and environmental risk is the large number of possible mixtures, as well as the fact that the actual mixture effect for a given set of constituents might strongly depend on the actual composition of the mixture, i.e., the ratios of the constituent, as well as their nature. This paper presents a possible approach to describe and thereby better understand the pharmacokinetics and dynamics of complex mixtures by combining quantitative structure-activity relationships to predict needed parameters, lumping to reduce the complexity of the problem, and physiologically based pharmacokinetic/pharmacodynamic modeling to integrate all this information into a complete toxicological description of the mixture. It is our hope that by presenting this conceptual approach we might be able to stimulate some criticisms and discussions in the toxicology community regarding this complex and yet very important area of research.

Animals↗

PubMed1997

Integrated approaches for the analysis of toxicologic interactions of chemical mixtures.

Although an overwhelmingly large portion of the resources in toxicologic research is devoted to single chemical studies, the toxicology of chemical mixtures, not single chemicals, is the real issue regarding health effects of environmental and/or occupational exposure to chemicals. The relative lack of activities in the area of toxicology of chemical mixtures does not suggest ignorance of the importance of the issue by the toxicology community. Instead, it is a reflection of the difficulty, complexity, and controversy surrounding this area of research. Until recently, much of the literature on the toxicology of chemical mixtures has been either very focused on certain specific interaction studies or slanted toward broad-based, relatively vague theoretical deliberation. The typical interaction study involved binary mixtures at relatively high dose levels with acute toxicities as endpoints. Although the theoretical papers have been valuable contributions, little is available on actual, practical experimental approaches toward a systematic solution of this immensely complex area of research. We present here a broad discussion on the important issues of the toxicology of chemical mixtures. First, we provide some background information with respect to the problem and significance of toxicology of chemical mixtures in relation to some of the real life issues. Second, we review and compare the existing experimental approaches relevant to toxicologic interactions of chemical mixtures. Third, we propose three integrated approaches that involve the combination of physiologically based pharmacokinetic/pharmacodynamic (PBPK/PD) modeling with: (1) Monte Carlo simulation, (2) median effect principle (MEP), and (3) response surface methodology (RSM). These modeling approaches, coupled with very focused mechanistically based toxicology studies, could be the basis for solving the problems of toxicology and risk assessment of chemical mixtures.

Analysis of Variance↗

PubMed1996

A novel arylsulfatase A protein variant and genotype in two patients with major depression.

A new, 'diffuse, multiple banding', electrophoretic variant of arylsulfatase A protein was found in two patients with major depression. Protein analyses showed that this variant and the normal enzyme differed in amino acid sequence and/or post-translational modifications unrelated to phosphate groups and oligomannose glycans. Analysis of the arylsulfatase A genes from a subject with the new variant identified three mutations; one gene had the two mutations associated with arylsulfatase A pseudodeficiency, and the other had a G to T transversion which changes a tryptophan to cysteine in the protein. These mutations result in an arylsulfatase A protein heteromer with diffuse electrophoretic banding. The possible association of these mutations with major depression is discussed.

Adult↗

PubMed1996

Integrin alpha v beta 3 and phospholipase C regulate prostacyclin formation of endothelial cells caused by ancrod-generated fibrin.

Ancrod-generated fibrin has been shown to stimulate prostacyclin synthesis of human umbilical vein endothelial cells (Chang et al., 1994, Biochem. Biophys. Res. Commun. 203, 1920). We further investigated its mechanism of action. The increment of 6-keto prostaglandin F1 alpha stimulated by ancrod-generated fibrin was almost completely inhibited when endothelial cells were either pretreated with 50 microM 8-(N,N'-diethylamino)octyl-3,4,5- trimethoxybenzoate (TMB-8) or preloaded with 15 microM 1,2-bis(2- aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). 6-Keto prostaglandin F1 alpha production during 2 and 10 h incubation period was also inhibited by 1.2 mM ethyleneglycol-bis-(beta-aminoethylether)-N,N,N',N'-tetraace tic acid (EGTA) (41 +/- 12 and 53 +/- 17% inhibition, respectively). Further, ancrod-generated fibrin caused a rapid-onset increase in [3h]inositol monophosphate (IP1) formation in endothelial cells. This increase in IP1 was significantly inhibited by 1 mM Gly-Pro-Arg-Pro, 1 mM neomycin or 100 ng/ml pertussis toxin. At the same time, neomycin and pertussis toxin also significantly inhibited 6-keto prostaglandin F1 alpha synthesis of endothelial cells stimulated by ancrod-generated fibrin. Additionally, the increment of IP1 production as well as prostacyclin production were significantly inhibited by monoclonal antibodies directed against alpha v beta 3. These results suggest that intra- and extra-cellular Ca2+ participate in prostacyclin synthesis stimulated by ancrod-generated fibrin. Ancrod-generated fibrin stimulates pertussis toxin-sensitive G-protein regulated phosphoinositide breakdown, which is responsible for prostacyclin synthesis. This augmentation in prostacyclin synthesis and phosphoinositide breakdown caused by ancrod-generated fibrin area, at least in part, mediated by fibrin binding to integrin alpha v beta 3 on endothelial cells.

6-Ketoprostaglandin F1 alpha↗

PubMed1996

Primary immunolocalization of estrogen and progesterone target cells in the human anterior cruciate ligament.

To identify estrogen and progesterone target cells in the human anterior cruciate ligament immunohistochemical localization of both estrogen and progesterone receptors was performed in 17 specimens of human anterior cruciate ligament. All ligament specimens were obtained at surgery. Thirteen specimens were from women, and four were from men: the average age was 57 years (range, 18-78 years). Eleven specimens (from nine women and two men) came from total knee replacements for osteoarthritis of the knee: three (from two women and one man), from reconstructions of the anterior cruciate ligament: two (both from women), from medial meniscectomies; and one (from a man), from an amputation secondary to chondrosarcoma of the pelvis. An immunoperoxidase method using monoclonal antibodies to the estrogen and progesterone receptors was employed to identify estrogen and progesterone target cells in the anterior cruciate ligament. Staining of both receptors was demonstrable in 14 specimens and in the remaining three specimens less than 15% of the cells were stained. Both estrogen and progesterone receptors were localized to synoviocytes in the synovial lining, fibroblasts in the anterior cruciate ligament stroma and cells in the blood vessel walls of the ligament. This demonstration of receptors for estrogen and progesterone in the cells of anterior cruciate ligament suggests that female sex hormones may have an effect on its structure and composition.

Adolescent↗

PubMed1996

Incorporating Monte Carlo simulation into physiologically based pharmacokinetic models using advanced continuous simulation language (ACSL): a computational method.

Biologically based models with physiological parameters are becoming more popular as a tool to estimate target tissue doses from chemical exposures. However, the majority of current physiologically based pharmacokinetic (PBPK) models do not take into account the uncertainty and/or variability within the various model parameters. Consideration of uncertainty is important to evaluate the predictive ability and complexity of a model as well as identification of parameters which contribute disproportionately to variability in model output. In order to estimate the uncertainty in PBPK model output, a versatile and simple computational method is presented which can be readily incorporated into the majority of PBPK models without extensive additions to model computer code. In this paper, a separate computer program for Monte Carlo simulation is furnished that randomly samples values for model parameters and writes them into a run-time language (command file) format which can then be utilized to execute individual PBPK models. Modifications to the PBPK model allow the desired output to be written to a data file for statistical analysis. The method presented in this paper is applied to a simple PBPK model for benzene disposition.

Algorithms↗

PubMed1996

Physiologically based pharmacodynamic modeling of an interaction threshold between trichloroethylene and 1,1-dichloroethylene in Fischer 344 rats.

Physiologically based pharmacokinetic modeling (PBPK) and gas uptake experiments have been used by researchers to demonstrate the competitive inhibition mechanism between trichloroethylene (TCE) and 1,1-dichloroethylene (DCE). Expanding on their work, we showed that this pharmacokinetic interaction was absent at levels of 100 ppm or less of either chemical in gas uptake systems. In this study, we further illustrate the presence of such an interaction threshold at the pharmacodynamic level by examining the interaction effect of either chemical on the other's ability to bind and deplete hepatic glutathione (GSH) in Fischer 344 rats. However, at this end point, the pharmacodynamic interaction is complicated by the ability of the liver to resynthesize GSH in response to its depletion. To quantitatively resolve the interaction effects on GSH content from the resynthesis effects, physiologically based pharmacodynamic (PBPD) modeling is applied. Initially, the PBPD model description of hepatic GSH kinetics was calibrated against previously published data and by gas uptake experiments conducted in our laboratory. Then, the model was used to determine the duration of the gas uptake exposure experiments by identifying the critical time point at which hepatic GSH is at a minimum in response to both chemicals. Subsequently, gas uptake experiments were designed following the PBPK/PD model predictions. In these model-directed experiments, DCE was the only chemical capable of significantly depleting hepatic GSH. The application of TCE to the rats at concentrations higher than 100 ppm obstructed the ability of DCE to deplete hepatic GSH. Since the metabolites of DCE bind to hepatic GSH, this obstruction signaled the presence of metabolic inhibition by TCE. However, TCE, at concentrations less than 100 ppm, was not effective in inhibiting DCE from significantly depleting hepatic GSH. The same observations were made when the ability of DCE to cause hepatic injury, as measured by aspartate aminotransferase serum activity, was assessed. Both conclusions validated the previous findings of the presence of the interaction threshold at the pharmacokinetic level.

Animals↗

PubMed1996

Sexual differences in bone markers and bone mineral density of normal Chinese.

We measured bone mineral density (BMD) at lumbar (L2-L4) vertebrae and proximal femurs of 385 healthy Chinese women aged 40-70 years and 156 healthy Chinese men aged 20-85, and four markers-bone alkaline phosphatase isozyme (BAP), procollagen-I C terminal propeptide (PICP), osteocalcin (BGP) in serum, and a bone resorption marker, urinary cross-linked N-telopeptide of type I collagen (NTX), of these subjects. The results indicate that in postmenopausal women, levels of all the markers increased with age. In men, serum BAP, PICP, and urinary NTX decreased significantly, and serum BGP decreased with borderline significance (P = 0.08). With increasing age, bone density decreased at both sites in postmenopausal women and at the proximal femur in men. The lumbar bone density showed no significant age-related changes in men. In premenopausal women, BMD at either site showed no significant change with increasing age. Despite the different trends between men and women of age-related changes in BMD and bone markers, bone density of both proximal femur and spine in both sexes correlated inversely with levels of the bone markers in a manner independent of age or body weight. The meaning of opposite age effects on bone markers in men and women needs further investigation. In addition, higher bone marker levels, implying faster bone turnover rate, are associated with lower BMD in both sexes.

Adult↗

PubMed1996

Bone mineral density in Chinese elderly women with hip fracture.

In order to examine the status of osteoporosis of the patients with hip fracture, we assessed the bone mineral density (BMD) of the contralateral hip of 81 elderly females with hip fracture and compared those with 77 normal Chinese women. The age of fracture subjects was 73.5 +/- 6.6 years (mean +/- SD), and 69.2 +/- 6.9 years for the controls. All of these fractures were caused by minor trauma, such as falls from a standing position or slipping to the ground. The Norland 2600 dual-photon absorptiometer (DPA) was used to evaluate the BMD in the femoral neck, trochanter, and Ward's triangle areas. The BMD for the fracture subjects was significantly lower than those of the controls. By linear regression, the probability of fracture increased exponentially with age and low BMD. The mean BMD for femoral neck of the fracture subjects versus controls was 0.556 versus 0.624 g/cm2; for trochanter: 0.505 versus 0.566 g/cm2; for Ward's triangle: 0.432 versus 0.485 g/cm2. Both negative predictive value (NPV) and positive predictive value (PPV) were acceptable at the prevalence of hip fracture of 5% or 20% and at a cutoff point of 0.65 g/cm2. These data revealed that the degree of relative osteoporosis in the patients with hip fractures was more severe than that of controls.

Absorptiometry, Photon↗

PubMed1996

Exploration of an interaction threshold for the joint toxicity of trichloroethylene and 1,1-dichloroethylene: utilization of a PBPK model.

Physiologically based pharmacokinetic (PBPK) modeling and gas uptake experiments were utilized to verify the competitive inhibition mechanism of interaction between trichloroethylene (TCE) and 1,1-dichloroethylene (DCE) and to investigate the presence of an interaction threshold between the two chemicals. Initially, gas uptake experiments were conducted on Fischer 344 rats where the initial concentrations of both DCE and TCE were 2000:0, 0:2000, 2000:2000, 1000:0, 1000:1000, and 500:500 ppm, respectively. When the different modes of inhibition interactions (competitive, uncompetitive and noncompetitive) were employed in the PBPK model, the model description of the competitive inhibition provided the best description of the declining concentrations in the gas uptake chamber. Furthermore, to predict the range at which the interaction threshold would be found, the PBPK model included a mathematical description of the percentage of enzyme sites occupied by either chemical in the presence or the absence of the other. By comparing the percentage of occupied sites by one chemical, in the presence of the other, to those sites occupied in the absence of the latter, the PBPK model predicted a range of concentrations (100 ppm or less) of either chemical where the competitive inhibition interaction would not be observed. Consequently, gas uptake experiments were designed where the initial concentration was selected at 2000 ppm for one chemical while the other chemical was set at 100 in one experiment and 50 ppm in another. Under these conditions, the best stimulation to the concentration depletion curves in the gas uptake system of the chemical in the higher concentration was obtained when the PBPK model was run under the assumption of no-interaction. This substantiated the model predictions of the presence of observable interaction only at concentrations higher than 100 ppm.

Animals↗

PubMed1996

Physiologically based pharmacokinetic/pharmacodynamic modeling of the toxicologic interaction between carbon tetrachloride and Kepone.

Carbon tetrachloride (CCl4) lethality in Sprague-Dawley rats is greatly amplified by pretreatment of Kepone (decachlorooctahydro-1,3,2-metheno-2H-cyclobuta[cd] pentalen-2-one). The increase in lethality was attributed to the obstruction of liver regenerative processes. These processes are essential for restoring the liver to its full functional capacity following injury by CCl4. Based on the available mechanistic information on Kepone/CCl4 interaction, a physiologically based pharmacokinetic/pharmacodynamic (PBPK/PD) model was constructed where the following effects of Kepone on CCl4 toxicity are incorporated: (1) inhibition of mitosis; (2) reduction of repair mechanism of hepatocellular injury; (3) suppression of phagocytosis. The PBPK/PD model provided computer simulation consistent with previously published time-course results of hepatotoxicity (i.e., pyknotic, injured and mitotic cells) of CCl4 with or without Kepone. As a further verification of this model, the computer simulations were also consistent with exhalation kinetic data for rats injected with different intraperitoneal (i.p.) doses of CCl4 in our laboratory. Subsequently, the PBPK/PD model, coupled with Monte Carlo simulation, was used to predict lethalities of rats treated with CCl4 alone and CCl4 in combination with Kepone. The experimental lethality studies performed in our laboratories were as follows: Sprague-Dawley rats were given either control diet or diet containing 10 ppm Kepone for 15 days. On day 16, rats in the Kepone treated group were given i.p. doses of 0, 10, 50, and 100 microliters/kg CCl4 (n = 9) while control rats were exposed to 0, 100, 1000, 3000, and 6000 microliters/kg CCl4 (n = 9). Lethality was observed at the 1000 (1/9), 3000 (4/9), and 6000 (8/9) microliters/kg doses for the control group and at the 50 (4/9) and 100 (8/9) microliters/kg for the treated group. Based on Monte Carlo simulation, which was used to run electronically 1000 lethality experiments for each dosing situation, the LD50 estimates for CCl4 toxicity with and without Kepone pretreatment were 47 and 2890 microliters/kg, respectively. Monte Carlo simulation coupled with the PBPK/PD model produced lethality rates which were not significantly different from the observed mortality, with the exception of CCl4 at very high doses (e.g., 6000 microliters/kg, p = 0.014). Deviation at very high doses of the predicted mortality from the observed may be attributed to extrahepatic systemic toxicities of CCl4, or solvent effects on tissues at high concentrations, which were not presently included in the model. Our modeling and experimental results verified the earlier findings of Mehendale (1990) for the 67-fold amplification of CCl4 lethality in the presence of Kepone. However, much of this amplification of CCl4 lethality with Kepone pretreatment was probably due to pharmacokinetic factors, because when target tissue dose (i.e., model estimated amount of CCl4 metabolites) was used to evaluate lethality, this amplification was reduced to 4-fold.

Animals↗

PubMed1996

Potentiation of spontaneous acetylcholine release from motor nerve terminals by glutamate in Xenopus tadpoles.

Extracellular application of glutamate (100 microM) increased the spontaneous secretion of acetylcholine, as well as the amplitude and decay time of miniature endplate potentials at developing neuromuscular synapses in Xenopus tadpoles. Kainate, quisqualate and N-methyl-D-aspartate (100 microM each) increased miniature endplate potential frequency by 26-, 13- and four-fold, respectively. The rank order of efficacy at 100 microM was kainate > quisqualate > N-methyl-D-aspartate > glutamate. The effect of kainate on miniature endplate potential frequency was inhibited by 6-cyano-2,3-dihydroxy-7-nitroquinoxaline (20 microM), but not by (+/-)-2-amino-5-phosphonovalerate (20 microM). Treatment with the voltage-dependent Ca2+ channel blockers verapamil (10 microM), Cd2+ (100 microM) or omega-conotoxin (1 microM) inhibited the potentiating action of kainate on miniature endplate potential frequency. On the other hand, 1S,3R-1-aminocyclopentane-1,3-dicarboxylate (300 microM), a glutamate metabotropic receptor agonist, inhibited the spontaneous acetylcholine release, which was antagonized by the application of 2-amino-3-phosphonopropionate (500 microM). The potentiating effect of glutamate receptor agonists on the miniature endplate potential frequency declined or disappeared in older Xenopus tadpoles. Quisqualate (100 microM) and N-methyl-D-aspartate (100 microM) but not kainate (30 microM) increased the amplitude and decay time of miniature endplate potential, whereas 1S, 3R-1-aminocyclopentane-1, 3-dicarboxylate (300 microM) only increased the decay time of miniature endplate potentials. These results suggest that there are kainate/quisqualate and N-methyl-D-aspartate receptors existing in the motor nerve terminals of younger Xenopus tadpoles and the activation of these receptors potentiates spontaneous acetylcholine release through increasing Ca2+ influx. Our data suggest that the presynaptic glutamate receptors on cholinergic terminals may be involved in feedback regulation of acetylcholine secretion at earlier embryonic stages.

Acetylcholine↗

PubMed1996

Thrombin enhances the adhesion and migration of human colon adenocarcinoma cells via increased beta 3-integrin expression on the tumour cell surface and their inhibition by the snake venom peptide, rhodostomin.

The interactions between tumour cells and the microvasculature, including the adhesion of tumour cells to endothelium and extracellular matrix (ECM) as well as their migratory ability, are prerequisites for metastasis to occur. In this study we showed that thrombin is capable of enhancing in vitro tumour cell metastatic potential in terms of adhesive properties and migratory response. Following exposure to subclotting concentrations of thrombin, SW-480 human colon adenocarcinoma cells exhibited increased adhesion to both the endothelium and ECM component (i.e. fibronectin). Likewise, the pretreatment of thrombin enhanced the migratory ability of SW-480 cells. The enhanced adhesion was significantly inhibited by complexing of thrombin with its inhibitor hirudin, or by serine proteinase inhibition with 3,4-DCI, but was unaffected by pretreatment of tumour cells with actinomycin D or cycloheximide. The effect of thrombin resulted in an upregulated cell-surface expression of beta 3 integrins, a group of receptors mediating interactions between tumour cells and endothelial cells, and between tumour cells and ECM. Antibodies against beta 3 integrins effectively blocked both the enhanced adhesion and migration. This thrombin-mediated up-regulation of beta 3 integrins involved the activation of protein kinase C (PKC) as thrombin-enhanced adhesion was diminished by PKC inhibition. Rhodostomin, an Arg-Gly-Asp-containing antiplatelet snake venom peptide that antagonises the binding of ECM toward beta 3 integrins on SW-480 cells, was about 600 and 500 times, more potent that RGDS in inhibiting thrombin-enhanced adhesion and migration respectively. Our data suggest that PKC inhibitors as well as rhodostomin may serve as inhibitory agents in the prevention of thrombin-enhanced metastasis.

Adenocarcinoma↗

PubMed1996

Variability in biological exposure indices using physiologically based pharmacokinetic modeling and Monte Carlo simulation.

By using physiologically based pharmacokinetic (PBPK) modeling coupled with Monte Carlo simulation, the interindividual variability in the concentrations of chemicals in a worker's exhaled breath and urine were estimated and compared with existing biological exposure indices (BEIs). The PBPK model simulated an exposure regimen similar to a typical workday, while exposure concentrations were set to equal the ambient threshold limit values (TLVs) of six industrial solvents (benzene, chloroform, carbon tetrachloride, methylene chloride, methyl chloroform, and trichloroethylene). Based on model predictions incorporating interindividual variability, the percentage of population protected was derived using TLVs as the basis for worker protection. Results showed that current BEIs may not protect the majority or all of the workers in an occupational setting. For instance, current end-expired air indices for benzene and methyl chloroform protect 95% and less than 10% of the worker population, respectively. Urinary metabolite concentrations for benzene, methyl chloroform, and trichloroethylene were also estimated. The current BEI recommendation for phenol metabolite concentration at the end-of-shift sampling interval was estimated to protect 68% of the worker population, while trichloroacetic acid (TCAA) and trichloroethanol (TCOH) concentrations for methyl chloroform exposure were estimated to protect 54% and 97%, respectively. The recommended concentration of TCAA in urine as a determinant of trichloroethylene exposure protects an estimated 84% of the workers. Although many of the existing BEIs considered appear to protect a majority of the worker population, an inconsistent proportion of the population is protected. The information presented in this study may provide a new approach for administrative decisions establishing BEIs and allow uniform application of biological monitoring among different chemicals.

Humans↗

PubMed1996

Symmetry of bone mineral density in the hips is not affected by age.

To assess the symmetry of bone mineral density (BMD) of the hips, we used dual photon absorptiometry (DPA) to asses BMD in 266 normal Chinese women in the bilateral femoral neck, trochanter and Ward's triangle regions. Linear regression analysis of the results showed excellent correlation between the hips. The correlation coefficients for the femoral neck, trochanter and Ward's triangle regions were 0.899, 0.892 and 0.935 respectively, and the standard errors of the estimate (S.E.E.) were 0.029, 0.027 and 0.022 g cm-2 respectively. Pooling the BMD data of corresponding regions (n = 532), the correlation coefficient was 0.813 for femoral neck versus trochanter, 0.889 for femoral neck versus Ward's triangle, and 0.833 for trochanter versus Ward's triangle. Further analysis showed mean correlation coefficients of 0.854 for elderly subjects (age > or = 65 years, n = 51) and 0.887 for younger subjects (age < 65 years, n = 215). In conclusion, assessment of left or right hip BMD may be used to represent the BMD of the contralateral hip and can reduce both the cost and radiation exposure during mass screening for osteoporosis.

Absorptiometry, Photon↗

PubMed1996

PBPK modeling/Monte Carlo simulation of methylene chloride kinetic changes in mice in relation to age and acute, subchronic, and chronic inhalation exposure.

During a 2-year chronic inhalation study on methylene chloride (2000 or 0 ppm; 6 hr/day, 5 days/week), gas-uptake pharmacokinetic studies and tissue partition coefficient determinations were conducted on female B6C3F1, mice after 1 day, 1 month, 1 year, and 2 years of exposure. Using physiologically based pharmacokinetic (PBPK) modeling coupled with Monte Carlo simulation and bootstrap resampling for data analyses, a significant induction in the mixed function oxidase (MFO) rate constant (Vmaxc) was observed at the 1-day and 1-month exposure points when compared to concurrent control mice while decreases in glutathione S-transferase (GST) rate constant (Kfc) were observed in the 1-day and 1-month exposed mice. Within exposure groups, the apparent Vmaxc maintained significant increases in the 1-month and 2-year control groups. Although the same initial increase exists in the exposed group, the 2-year Vmaxc is significantly smaller than the 1-month group (p < 0.001). Within group differences in median Kfc values show a significant decrease in both 1-month and 2-year groups among control and exposed mice (p < 0.001). Although no changes in methylene chloride solubility as a result of prior exposure were observed in blood, muscle, liver, or lung, a marginal decrease in the fat:air partition coefficient was found in the exposed mice at p = 0.053. Age related solubility differences were found in muscle:air, liver:air, lung:air, and fat:air partition coefficients at p < 0.001, while the solubility of methylene chloride in blood was not affected by age (p = 0.461). As a result of this study, we conclude that age and prior exposure to methylene chloride can produce notable changes in disposition and metabolism and may represent important factors in the interpretation for toxicologic data and its application to risk assessment.

Administration, Inhalation↗

PubMed1996

A gas-liquid system for enzyme kinetic studies of volatile organic chemicals. Determination of enzyme kinetic constants and partition coefficients of trichloroethylene.

A gas-liquid system was developed for enzyme kinetic study with volatile organic chemicals (VOCs) by modification of the gas uptake method for the in vivo physiologically based pharmacokinetic experiment. This gas-liquid system, designed in our laboratory, is composed of: 1) a diffusion chamber for adjusting initial vapor concentration by mixing ambient air and the VOCs; 2) a condenser for maintaining the liquid level in the incubation chamber; 3) a stainless-steel metal bellows pump for recirculating vapor in this system; 4) a gas chromatograph equipped with an autosampler and a flame ionization detector; and 5) a computer for controlling automation and data processing. Trichloroethylene (TCE) was used as a model chemical, and enzyme kinetics were studied by measuring the depletion of TCE in the gas phase of the system. TCE-at initial concentrations of 56, 620, and 1240 ppm-was incubated with rat liver microsomes and a NADPH regenerating system in a 100-ml round-bottom flask. Based on parallel enzyme assays using p-nitrophenol as a substrate, cytochrome P450IIE1, activity remained stable up to 3 hr under the incubation conditions (37 degrees C and pH 7.4) whereas addition of glutathione into the incubation mixture did not affect TCE metabolism. Kinetic constants were analyzed using a two-compartment pharmacokinetic model and the computer software SimuSolv. Statistical optimization using the maximum-likelihood method produced apparent in vitro Vmax and KM values of 0.55 nmol/mg protein/min and 0.9 microM, respectively. In addition, this newly developed methodology has a number of advantages over those reported in the literature, including the potential utility of determining tissue partition coefficients of VOCs for physiologically based pharmacokinetic modeling. We conclude that this gas-liquid system is suitable for determination of kinetic constants near realistic environmental concentrations of VOCs including TCE.

Animals↗

PubMed1995

Physiologically based pharmacokinetic/pharmacodynamic modeling of chemical mixtures and possible applications in risk assessment.

Human exposure to chemicals, be it environmental or occupational, is rarely, if ever, limited to a single chemical. Therefore, it is essential that we consider multiple chemical effects and interactions in our risk assessment process. However, with the almost infinitely large number of chemical mixtures in the environment, systematic studies of the toxicology of these chemical mixtures with conventional methodologies and approaches are impossible because of the immense resources and unrealistically long durations required. Thus, the development of predictive and alternative toxicology method is imperative. In order to have a reasonable chance to deal with the complex issue of toxicology of chemical mixtures, we believe that the following concepts must be considered: (1) the exploitation of recent advances in computational technology; (2) the utilization of mathematical/statistical modeling; (3) coupling computer modeling with very focused, mechanistically based, and short-term toxicology studies. Our approach is, therefore, the utilization of physiologically based pharmacokinetic/pharmacodynamic (PB-PK/PD) modeling, coupled with very focused, model-directed toxicology experiments as well as other statistical/mathematical modeling such as isobolographic analysis and response surface methodology. Tissue dosimetry at the pharmacokinetic and pharmacodynamic levels is achievable with simple and complex, but chemically defined, mixtures. Our long-term goal is to formulate innovative risk assessment methodologies for chemical mixtures. In this presentation, one of our specific research projects is described: PB-PK/PD modeling of toxicologic interactions between Kepone and carbon tetrachloride (CCl4) and the coupling of Monte Carlo simulation for the prediction of acute toxicity.

Animals↗