PBPK models in risk assessment--A focus on chloroprene

Mathematical models are increasingly being used to simulate events in the exposure-response continuum, and to support quantitative predictions of risks to human health. Physiologically based pharmacokinetic (PBPK) models address that portion of the continuum from an external chemical exposure to an internal dose at a target site. Essential data needed to develop a PBPK model include values of key physiological parameters (e.g., tissue volumes, blood flow rates) and chemical specific parameters (rate of chemical absorption, distribution, metabolism, and elimination) for the species of interest. PBPK models are commonly used to: (1) predict concentrations of an internal dose over time at a target site following external exposure via different routes and/or durations; (2) predict human internal concentration at a target site based on animal data by accounting for toxicokinetic and physiological differences; and (3) estimate variability in the internal dose within a human population resulting from differences in individual pharmacokinetics. Himmelstein et al. [M.W. Himmelstein, S.C. Carpenter, P.M. Hinderliter, Kinetic modeling of beta-chloroprene metabolism. I. In vitro rates in liver and lung tissue fractions from mice, rats, hamsters, and humans, Toxicol. Sci. 79 (1) (2004) 18-27; M.W. Himmelstein, S.C. Carpenter, M.V. Evans, P.M. Hinderliter, E.M. Kenyon, Kinetic modeling of beta-chloroprene metabolism. II. The application of physiologically based modeling for cancer dose response analysis, Toxicol. Sci. 79 (1) (2004) 28-37] developed a PBPK model for chloroprene (2-chloro-1,3-butadiene; CD) that simulates chloroprene disposition in rats, mice, hamsters, or humans following an inhalation exposure. Values for the CD-PBPK model metabolic parameters were obtained from in vitro studies, and model simulations compared to data from in vivo gas uptake studies in rats, hamsters, and mice. The model estimate for total amount of metabolite in lung correlated better with rodent tumor incidence than did the external dose. Based on this PBPK model analytical approach, Himmelstein et al. [M.W. Himmelstein, S.C. Carpenter, M.V. Evans, P.M. Hinderliter, E.M. Kenyon, Kinetic modeling of beta-chloroprene metabolism. II. The application of physiologically based modeling for cancer dose response analysis, Toxicol. Sci. 79 (1) (2004) 28-37; M.W. Himmelstein, R. Leonard, R. Valentine, Kinetic modeling of beta-chloroprene metabolism: default and physiologically-based modeling approaches for cancer dose response, in: IISRP Symposium on Evaluation of Butadiene & Chloroprene Health Effects, September 21, 2005, TBD--reference in this proceedings issue of Chemical-Biological Interactions] propose that observed species differences in the lung tumor dose-response result from differences in CD metabolic rates. The CD-PBPK model has not yet been submitted to EPA for use in developing the IRIS assessment for chloroprene, but is sufficiently developed to be considered. The process that EPA uses to evaluate PBPK models is discussed, as well as potential applications for the CD-PBPK model in an IRIS assessment.     

The Biology of Tardigrades: An Introduction to the 9th International Symposium on Tardigrada*

The 9th International Symposium on Tardigrada took place in Tampa, Florida, USA from 28 July to 1 August 2003. Fifty-four participants representing thirteen countries attended and there were fifty-two presentations of which fourteen were chosen for the publication in these proceedings. Topics include cryptobiosis, ecology, taxonomy and systematics of tardigrades. * This symposiumvolume is dedicated to Nigel Marley (Fig. 4) for his courage and persistence in pursuing research on tardigrades, despite ongoing medical challenges. His optimism and positive attitude are an inspiration to all of us, and his willingness to help other tardigradologists is gratefully acknowledged and appreciated.     

Butadiene production process overview

Over 95% of butadiene is produced as a by-product of ethylene production from steam crackers. The crude C4 stream isolated from the steam cracking process is fed to butadiene extraction units, where butadiene is separated from the other C4s by extractive distillation. The amount of crude C4s produced in steam cracking is dependent on the composition of the feed to the cracking unit. Heavier feeds, such as naphtha, yield higher amounts of C4s and butadiene than do lighter feeds. Crackers using light feeds typically produce low quantities of C4s and do not have butadiene extraction units. Overall butadiene capacity is determined by ethylene cracker operating rates, the type of feed being cracked, and availability of butadiene extraction capacity. Global butadiene capacity is approximately 10.5 million metric tons, and global production is approximately 9 million metric tons [Chemical Marketing Associates, Inc. (CMAI), 2005 World Butadiene Analysis, Chemical Marketing Associates, Inc. (CMAI), 2005]. Crude C4s are traded globally, with the United States being the only significant net importer. Finished butadiene is also traded globally, with the largest exporters being Canada, Western Europe, Saudi Arabia and Korea. The largest net importers are Mexico, the United States and China. The global demand for butadiene is approximately 9 million metric tons [Chemical Marketing Associates, Inc. (CMAI), 2005 World Butadiene Analysis, Chemical Marketing Associates, Inc. (CMAI), 2005]. Production of styrene-butadiene rubber and polybutadiene rubber accounts for about 54% of global butadiene demand, with tire production being the single most important end use of butadiene synthetic rubbers. Other major butadiene derivatives are acrylonitrile-butadiene-styrene (ABS) and styrene butadiene latex (about 24% of demand combined).     

The cytogenetic evaluation of in vivo genotoxic and cytotoxic activity using rodent somatic cells

With the growing realization that in vitro short-term tests for genotoxicity can never fully mimic in vivo conditions, the evaluation of genotoxic damage in somatic cells of rodents has played an increasingly important role in assessing the carcinogenic potential of suspect compounds. Among the various genotoxic endpoints assessed in in vivo somatic cell assays, cytogenetic endpoints (e.g., chromosomal aberrations, micronuclei, sister chromatid exchanges) continue to be used most frequently. The purpose of this paper is to demonstrate the utility of evaluating different cytogenetic endpoints in the same animal, using as examples studies to evaluate the in vivo genotoxic potential of benzene, of methylisocyanate, and of butadiene, chloroprene and isoprene.Abbreviations CA chromosomal aberrations - MI mitotic index - MIC methylisocyanate - MN-NCE micronucleated monochromatic erythrocytes - MN-PCE micronucleated polychromatic erythrocytes - SCE sister chromatid exchange     

The metabolism and molecular toxicology of chloroprene

Chloroprene (2-chloro-1,3-butadiene, 1) is oxidised by cytochrome P450 enzymes in mammalian liver microsomes to several metabolites, some of which are reactive towards DNA and are mutagenic. Much less of the metabolite (1-chloroethenyl)oxirane (2a/2b) was formed by human liver microsomes compared with microsomes from Sprague-Dawley rats and B6C3F1 mice. Epoxide (2a/2b) was a substrate for mammalian microsomal epoxide hydrolases, which showed preferential hydrolysis of the (S)-enantiomer (2b). The metabolite 2-chloro-2-ethenyloxirane (3a/3b) was rapidly hydrolysed to 1-hydroxybut-3-en-2-one (4) and in competing processes rearranged to 1-chlorobut-3-en-2-one (5) and 2-chlorobut-3-en-1-al (6). The latter compound isomerised to (Z)-2-chlorobut-2-en-1-al (7). In microsomal preparations from human, rat and mouse liver, compounds 4, 5 and 7 were conjugated by glutathione both in the absence and presence of glutathione transferases. There was no evidence for the formation of a chloroprene diepoxide metabolite in any of the microsomal systems. The major adducts from the reaction of (1-chloroethenyl)oxirane (2a/2b) with calf thymus DNA were identified as N7-(3-chloro-2-hydroxy-3-buten-1-yl)-guanine (20) and N3-(3-chloro-2-hydroxy-3-buten-1-yl)-2'-deoxyuridine (23), with the latter being derived by alkylation at N-3 of 2'-deoxycytidine, followed by deamination. Adducts in DNA were identified by comparison with those derived from individual deoxyribonucleosides. The metabolite (Z)-2-chlorobut-2-en-1-al (7) formed principally two adducts with 2'-deoxyadenosine which were identified as a pair of diastereoisomers of 3-(2'-deoxy-beta-d-ribofuranosyl)-7-(1-hydroxyethyl)-3H-imidazo[2,1-i]purine (25). The chlorine atom of chloroprene thus leads to different intoxication and detoxication profiles compared with those for butadiene and isoprene. The results infer that in vivo oxidations of chloroprene catalysed by cytochrome P450 are more important in rodents, whereas hydrolytic processes catalysed by epoxide hydrolases are more pronounced in humans. The reactivity of chloroprene metabolites towards DNA is important for the toxicology of chloroprene, especially when detoxication is incomplete.     

Call for papers oral and poster presentations are invited for the International Symposium: Exploring the Greet Lakes of the World: Food Web Dynamics,Health & Integrity

Aquatic Ecosystem Health and Management Society

Call for papers oral and poster presentations are invited for the International Symposium: Exploring the Greet Lakes of the World: Food Web Dynamics, Health & Integrity     

Transgenic animals as models for human diseasereport of an EC Study Group

ContributorsThis report results from the discussion of an Expert Group convened in Edinburgh on 29–30 October 1992 for a workshop on that subject sponsored and organized by the Commission of the European Communities, Directorate General XII (CEC-DG XII). The experts taking part in the workshop were: R. Lathe and J.J. Mullins, Coordinators (AFRC Centre for Genome Research, University of Edinburgh); G.N. Fracchia, Secretary (Medical Research-Pharmaceuticals, CEC-DG XII, Brussels); and the participants; C. Babinet (Dept d'Immunologie, Institut Pasteur, Paris); P. Eliard (EFPIA, Brussels); C. Benoist (LGME du CNRS/INSERM, Strasbourg); G. Bianchi (Ospedale San Raffaele, Universita di Milano, Milan); E. Boncinelli (DIBIT, Ospedale San Raffaele, Milan); G. Brem (Universitat München); G. Cossu (Institute of Histology, School of Medicine, University of Rome); N. Dillon (MRC National Institute for Medical Research, London); V. Episkopou (Dept of Biochemistry & Molecular Genetics, St Mary's Hospital Medical School, London); M. Evans (Wellcome/CRC Institute, Cambridge); R. Forster (Italfarmaco Research Centre, Cinisello Balsamo, Milan); D. Ganten (Max-Delbrück-Zentrum für Molekulare Medizin, Berlin); A. Gossler (Max-Delbrück-Laboratorium in der Max-Planck-Gesellschaft, Köln); J. Gray (Dept Psychology, Institute of Psychiatry, London); R. Hammer (Howard Hughes Medical Institute, University of Texas, Dallas, USA); A. Hobden (Genetics Unit, Glaxo Group Research Ltd, Middlesex); G. Kollias (Laboratory of Molecular Genetics, Hellenic Pasteur Institute, Athens); D. Lamy (Transgène SA, Strasbourg); D. Lincoln (MRC Reproductive Biology Unit, Edinburgh); J. Mallet (CNRS/LNCM, Gif-sur-Yvette); D. Melton (ICMB, University of Edinburgh, Edinburgh); J.M. Moalic (U127 INSERM, Hôpital Laraboisire, Paris); S. Mockrin (Dept Health & Human Services, National Institutes of Health, Bethesda, MA, USA); J. Ottesen (Biopharmaceuticals Division, Dept of Gene Technology and Virology, Novo Industrie, Denmark); D. Porteous (MRC Human Genetics Unit, Western General Hospital, Edinburgh); P. Rae (Pharmaceutical Division, Miles, West Haven, USA); F. Theuring (Schering AG, Pharmaceutical Research, Berlin); G. Tremp (Rhone-Poulenc Rorer SA, Centre de Recherche de Vitry-Alfortville, Vitry-sur-Seine); H. Van der Putten (Dept Biotechnology, Ciba-Geigy AG, Basel); R. Wolf (ICRF Molecular Pharmacology Group, Biochemistry Dept, Edinburgh). Further supporting documentation and commentary were received from P. Dement (Amsterdam), U. Habenicht (Berlin), P. Grüss (Gottingen), M. Lyon (Oxford), C.C.J. Miller (London), W.-D. Schleuning (Berlin) and R. Williamson (London) and their contributions are gratefully acknowledged.     

Book review

PRANCE, G.T. ed. (1986): Tropical Rain Forests and the World Atmosphere. American Association for the Advancement of Science Selected Symposium 101. Westview Press, Boulder, Colorado and London. 105 pp., some figs., paper. 20.50.

LOWE‐McCONNELL, R.H. (1987): Ecological Studies in Tropical Fish Communities. Cambridge University Press, Cambridge, London, New York, Melbourne. 382 pp. with tables and figs., paper. 12.95 (hardbound 35.00).     

Future directions in butadiene risk assessment and the role of cross-species internal dosimetry

The 2005 International Symposium on the evaluation of butadiene and chloroprene health risks provided the opportunity to consider the past, present and future state of research issues for 1,3-butadiene. Considerable advancements have been made in our knowledge of exposure, metabolism, biomarkers of exposure and effect, and epidemiology. Despite this, uncertainties remain which will impact the human health risk assessment for current worker and environmental exposures. This paper reviews key aspects of recent studies and the role that biomarkers of internal dosimetry can play in addressing low to high exposure, gender, and cross-species differences in butadiene toxicity and metabolism. Considerable information is now available on the detection and quantification of protein adducts formed from the mono-, di- and dihydroxy-epoxide metabolites of butadiene. The diepoxide metabolite appears to play a key role in mutagenesis. Species differences in production of this critical metabolite are reflected by the diepoxybutane-specific hemoglobin adduct, pry-Val. To date, the pry-Val adduct has not been quantifiable in human blood samples from workers with cumulative occupational exposures up to 6.3 ppm-weeks; whereas, the pry-Val was quantifiable in the blood of mice and rats with similar cumulative exposures. Levels in mice were much higher than in rats. Further improvements in analytical sensitivity for the pyr-Val adduct are on the horizon. Epidemiology studies are also described and ongoing efforts promise to help bridge our understanding of past and future risks.     

Effect of n-hexane on the disposition and toxicity of the 1,3-butadiene metabolite 3-butene-1,2-diol

3-Butene-1,2-diol (butenediol), a major metabolite of 1,3-butadiene (butadiene), can undergo either detoxification or biotransformation to potentially toxic metabolites, including 3,4-epoxy-1,2-butanediol and hydroxymethylvinyl ketone (HMVK). Butadiene exposure can occur concomitantly with hexanes, which share common biotransformation pathways with butadiene. To determine the potential influence of hexane co-exposure on butadiene toxicity, the present study examined the effect of n-hexane on butenediol disposition [as measured by urinary excretion of (N-acetyl-S-(3,4-dihydroxybutyl)-L-cysteine) (MI level)] and genotoxicity (as measured by the frequency of bone marrow micronucleated erythrocytes) and acute toxicity (as measured by body weight changes) in the rat. The results show that butenediol was not genotoxic to adult or immature rats but was acutely toxic to adult but not immature rats. The results also suggest that n-hexane co-exposure may attenuate the acute toxicity by butenediol in adult rats and that immature rats may be less sensitive than adults to the acute toxicity.     

Mutagenicity of butadiene and butadiene monoxide.

Incubation of $\text{S. typhimurium}$ strains TA1530 and TA1535 in the presence of gaseous butadiene increased the number of his⁺ revertants/plate. This mutagenic effect occured in absence of fortified S-9 rat liver fraction. In its presence, the mutagenic effect seemed to be dependent on its composition. With butadiene monoxide, a reversion to histidine prototrophy was obtained without metabolic activation with strains TA1530, TA1535 and TA100. Butadiene monoxide might be a possible primary metabolite of butadiene.     

Stem cell powwow in Squaw Valley

The Keystone Symposium entitled 'The Life of a Stem Cell: from Birth to Death' was held at Squaw Valley, CA, USA in March 2012. The meeting brought together researchers from across the world and showcased the most recent developments in stem cell research. Here, we review the proceedings at this meeting and discuss the major advances in fundamental and applied stem cell biology that emerged.     

Why only some plants emit isoprene

Isoprene (2‐methyl‐1,3‐butadiene) is emitted from many plants and it appears to have an adaptive role in protecting leaves from abiotic stress. However, only some species emit isoprene. Isoprene emission has appeared and been lost many times independently during the evolution of plants. As an example, our phylogenetic analysis shows that isoprene emission is likely ancestral within the family Fabaceae (= Leguminosae), but that it has been lost at least 16 times and secondarily gained at least 10 times through independent evolutionary events. Within the division Pteridophyta (ferns), we conservatively estimate that isoprene emissions have been gained five times and lost two times through independent evolutionary events. Within the genus Quercus (oaks), isoprene emissions have been lost from one clade, but replaced by a novel type of light‐dependent monoterpene emissions that uses the same metabolic pathways and substrates as isoprene emissions. This novel type of monoterpene emissions has appeared at least twice independently within Quercus, and has been lost from 9% of the individuals within a single population of Quercus suber. Gain and loss of gene function for isoprene synthase is possible through relatively few mutations. Thus, this trait appears frequently in lineages; but, once it appears, the time available for evolutionary radiation into environments that select for the trait is short relative to the time required for mutations capable of producing a non‐functional isoprene synthase gene. The high frequency of gains and losses of the trait and its heterogeneous taxonomic distribution in plants may be explained by the relatively few mutations necessary to produce or lose the isoprene synthase gene combined with the assumption that isoprene emission is advantageous in a narrow range of environments and phenotypes.     

Book Reviews

Natural Antioxidants in Human Health and Disease Edited by Balz Frei Academic Press, San Diego

In Vitro Toxicology Edited by: Shayne Cox Gad Raven Press Ltd.: New York, 290pp ISBN: 0-88167-974-7

Active Oxygen, Lipid Peroxidation and Antioxidants Edited by Kunio Yagi Japan Scientific Society Press: Tokyo ISBN 4-7622-6738-4, 1993 CRC Press: Boca Raton ISBN 0-8493-7769-2, 1993 x +372 pages Y 13, 000

Free Radicals, Cardiovascular Dysfunction and Protection Strategies R C Kukreja and M L Hess R G Landes Company: Austin, Texas, USA, 1994

Oxygen and Environmental Stress in Plants (Special issue of the Proceedings of the Royal Society of Edinburgh, volume 102 1994) eds R M M Crawford, G A F Hendry and B A Goodman

Mitochondria: DNA, Proteins and Disease Eds. V. Darley Usmar and A.H.V. Schapira Portland Press Research Monograph V, London, 1994

Human Medicinal Agents From Plants Edited by A. Douglas Kinghorn and Manuel F. Balandrin ACS Symposium Series 534 American Chemical Society: Washington DC, 1993, pp. xii + 356. ISBN 08412 2705 5. $89.95.     

Selected topics in probiotics and prebiotics: meeting report for the 2004 international scientific association for probiotics and prebiotics

On August 29-31, 2004, 84 academic and industry scientists from 16 countries gathered in Copper Mountain, Colorado USA to discuss certain issues at the forefront of the science of probiotics and prebiotics. The format for this invitation only meeting included six featured lectures: engineering human vaginal lactobacilli to express HIV-inhibitory molecules (Peter Lee, Stanford University), programming the gut for health (Thaddeus Stappenbeck, Washington University School of Medicine), immune modulation by intestinal helminthes (Joel Weinstock, University of Iowa Hospitals and Clinics), hygiene as a cause of autoimmune disorders (G. A. Rook, University College London), prebiotics and bone health (Connie Weaver, Purdue University) and prebiotics and colorectal cancer risk (Ian Rowland, Northern Ireland Centre for Food and Health). In addition, all participants were included in one of eight discussion groups on the topics of engineered probiotics, host-commensal bacteria communication, 'omics' technologies, hygiene and immune regulation, biomarkers for healthy people, prebiotic and probiotic applications to companion animals, development of a probiotic dossier, and physiological relevance of prebiotic activity. Brief conclusions from these discussion groups are summarized in this paper.     

Report from the First Snake Genomics and Integrative Biology Meeting

This report summarizes the proceedings of the 1st Snake Genomics and Integrative Biology Meeting held in Vail, CO USA, 5-8 October 2011. The meeting had over twenty registered participants, and was conducted as a single session of presentations. Goals of the meeting included coordination of genomic data collection and fostering collaborative interactions among researchers using snakes as model systems.     

Basic defect in the expression of adenosine deaminase in ADA- SCID disease investigated through the cells of an obligate heterozygote

Summary The nature of the defect of a female baby who died of severe combined immunodeficiency (SCID) disease associated with adenosine deaminase deficiency (ADA^-) was investigated. Since tissue or tissue culture material was not available for subsequent studies, the expression of ADA in her cells was investigated in the somatic cell hybrid clones derived from a fusion between the lymphocytes from one of her two obligate heterozygote parents and thymidine kinase deficient Chinese hamster (a3) fibroblasts. The results of analyses of the human chromosomes and biochemical markers in 12 independent clones and 27 subclones indicated that the ADA deficiency in the patient is determined probably by a mutation in the structural gene for ADA in chromosome 20 leading either to the production of catalytically defective molecules or to the cessation of the production of ADA. Incidentally, the involvement of chromosome 2, which carries a gene for adenosine deaminase complexing protein (ADCP), in the causation of ADA deficiency was excluded. The in vitro approach through the cells from an obligate heterozygote described in this paper may have a general application in pursuing studies on other cases of inborn errors of metabolism whenever the material from the affected individuals (i.e., the homozygotes) is not available or not suitable for direct investigations.A part of this work was presented at the New York State Department of Health, Birth Defects Institute Symposium IX (Inborn Errors of Specific Immunity), Albany, October 16–18, 1978 and reported as an abstract in the proceedings of the Fifth International Conference on Human Gene Mapping, Edinburgh, July 1979. Cytogenet Cell Genet 22:164     

Epigenetics in Rome: Breaking news from the Chromatin Remodeling and Human Disease Workshop

In 2009 the Istituto Regina Elena (IRE) and Istituto Dermopatico dell' Immacolata (IDI), joined their efforts to organize the “First IRE Annual Workshop on Chromatin Remodeling and Human Disease, which had place in Rome on the 3-4 of December 2009. The Workshop program listed a number of presentations on various epigenetic phenomena believed to have an impact on human diseases. Internationally recognized leaders in this field from Europe and USA have brilliantly accomplished this highly compelling task. Special emphasis has been placed on emerging understanding of epigenetic mechanisms as they relate to the physiopathology of numerous human diseases. How this field scientifically and technologically recently progressed in this direction, was clearly evident from the presentations and discussions having place during the workshop.     

Peering down the sink: A review of isoprene metabolism by bacteria

Isoprene (2-methyl-1,3-butadiene) is emitted to the atmosphere each year in sufficient quantities to rival methane (>500 Tg C yr⁻¹), primarily due to emission by trees and other plants. Chemical reactions of isoprene with other atmospheric compounds, such as hydroxyl radicals and inorganic nitrogen species (NO_x), have implications for global warming and local air quality, respectively. For many years, it has been estimated that soil-dwelling bacteria consume a significant amount of isoprene (~20 Tg C yr⁻¹), but the mechanisms underlying the biological sink for isoprene have been poorly understood. Studies have indicated or confirmed the ability of diverse bacterial genera to degrade isoprene, whether by the canonical iso-type isoprene degradation pathway or through other less well-characterized mechanisms. Here, we review current knowledge of isoprene metabolism and highlight key areas for further research. In particular, examples of isoprene-degraders that do not utilize the isoprene monooxygenase have been identified in recent years. This has fascinating implications both for the mechanism of isoprene uptake by bacteria, and also for the ecology of isoprene-degraders in the environments.     

Microbial consumption of atmospheric isoprene in a temperate forest soil

Isoprene (2-methyl-1,3 butadiene) is a low-molecular-weight hydrocarbon emitted in large quantities to the atmosphere by vegetation and plays a large role in regulating atmospheric chemistry. Until now, the atmosphere has been considered the only significant sink for isoprene. However, in this study we performed both in situ and in vitro experiments with soil from a temperate forest near Ithaca, N.Y., that indicate that the soil provides a sink for atmospheric isoprene and that the consumption of isoprene is carried out by microorganisms. Consumption occurred rapidly in field chambers (672.60 +/- 30.12 to 2,718.36 +/- 86.40 pmol gdw day) (gdw is grams [dry weight] of soil; values are means +/- standard deviations). Subsequent laboratory experiments confirmed that isoprene loss was due to biological processes: consumption was stopped by autoclaving the soil; consumption rates increased with repeated exposure to isoprene; and consumption showed a temperature response consistent with biological activity (with an optimum temperature of 30 degrees C). Isoprene consumption was diminished under low oxygen conditions (120 +/- 7.44 versus 528.36 +/- 7.68 pmol gdw day under ambient O(2) concentrations) and showed a strong relationship with soil moisture. Isoprene-degrading microorganisms were isolated from the site, and abundance was calculated as 5.8 x 10 +/- 3.2 x 10 cells gdw. Our results indicate that soil may provide a significant biological sink for atmospheric isoprene.