The short-term toxicity and metabolome of dicyclopentadiene.

Dicyclopentadiene (DCPD) was investigated in a 14-day oral rat toxicity study based on the OECD 407 guideline in combination with plasma metabolomics. Wistar rats received the compound daily via gavage at dose levels of 0, 50 and 150 mg/kg bw. The high dose induced transient clinical signs of toxicity and in males only reduced body weight gain. High dose liver changes were characterized by altered clinical chemistry parameters in both sexes and pathological changes in females. In high dose males an accumulation of alpha-2 u-globulin in the kidney was noted. Comparing the DCPD metabolome with previously established specific metabolome patterns in the MetaMap® Tox data base suggested that the high dose would result in liver enzyme induction leading to increased breakdown of thyroid hormones for males and females. An indication for liver toxicity in males was also noted. Metabolomics also suggested an effect on the functionality of the adrenals in high dose males, which together with published data, is suggestive of a stress related effect in this organ. The results of the present 14-day combined toxicity and metabolome investigations were qualitatively in line with literature data from subchronic oral studies in rats with DCPD. Importantly no other types of organ toxicity, or hormone dysregulation beyond the ones associated with liver enzyme induction and stress were indicated, again in line with results of published 90-day studies. It is therefore suggested that short term "smart" studies, combining classical toxicity with 'omics technologies, could be a 2 R (refine and reduce) new approach method allowing for the reduction of in vivo toxicity testing.

Micronuclei in human peripheral blood and bone marrow as genotoxicity markers: A systematic review and meta-analysis.

Can human peripheral blood cells be used as a surrogate for bone marrow cells, in evaluating the genotoxic effects of stressors? We searched the Pubmed/Medline and PubChem databases to identify publications relevant to this question. Micronucleus formation was the genotoxicity endpoint. Three publications comparing exposed vs. non-exposed individuals are included in this analysis; the exposures were to ethylene oxide or ionising radiation (atomic bomb, thorotrast, or radioiodine therapy). Information was extracted on the types of exposure, the numbers of participants, and the micronucleus frequencies. Relative differences (odds ratios) and absolute differences (risk differences) in the numbers of micronuclei between exposed and non-exposed persons were calculated separately for individual cell types (peripheral blood and bone marrow). Random effects meta-analyses for the relative differences in cell abnormalities were performed. The results showed very small differences in the frequencies of micronuclei between exposed and non-exposed individuals, as measured in either peripheral blood or bone marrow cell populations, on both absolute and relative scales. No definite conclusion concerning the relative sensitivities of bone marrow and peripheral blood cells can be made, based on these publications.

Recommendation for an occupational exposure limit for toluene.

The Lower Olefins and Aromatics (LOA) REACH Consortium, which includes toluene registrants in the EU, established a Working Group (WG) to conduct a review of the occupational exposure limit (OEL) for toluene. The review focussed on CNS and neuro-behavioural toxicity, ototoxicity, effects on colour vision, reproductive and developmental effects, as safety signals for these effects were identified. The WG also examined the need for a skin notation and/or a short-term exposure limit (STEL). The WG critically reviewed and discussed the strengths and weaknesses of the available published information describing the effects of toluene in animals and humans, to assess its adequacy as a potential point of departure for the establishment of an OEL for toluene and to derive an OEL. As a result, the WG recommendation for a toluene OEL is 20 ppm 8-h TWA, with a 15-min STEL of 100 ppm and a skin notation.

Application of physiologically-based pharmacokinetic modeled toluene blood concentration in the assessment of short term exposure limits.

Toluene is a volatile hydrocarbon with solvent applications in several industries. Acute neurological effects in workers exposed to toluene have been reported in various publications. To inform the basis for a toluene Short Term Exposure Limit (STEL), studies of toluene-exposed workers were modeled using customized exposure scenarios within an existing physiologically-based pharmacokinetic (PBPK) model to simulate blood concentrations during individual studies. Maximum simulated blood concentration ranged from 0.3 to 1.7 (mean = 0.74 mg/L, median = 0.73, upper 95th percentile = 1.07) at the studies identified No Observed Adverse Effect Concentration (NOAEC). Maximum simulated blood concentration ranged from 0.7 to 4.1 mg/L (mean = 1.81, median = 1.63, lower 95th percentile = 0.92) at the studies identified Lowest Observed Adverse Effect Concentration (LOAEC). The maximum blood concentration for a 100 ppm STEL-like simulation was 0.4 mg/L, at the lower end of the NOAEC range and below the 95th percentile of the LOAEC. Therefore, it appears that a STEL <100 ppm would be unnecessary to protect workers due to peak occupational exposures to toluene.

DARTpaths, an in silico platform to investigate molecular mechanisms of compounds.

SUMMARY: Xpaths is a collection of algorithms that allow for the prediction of compound-induced molecular mechanisms of action by integrating phenotypic endpoints of different species; and proposes follow-up tests for model organisms to validate these pathway predictions. The Xpaths algorithms are applied to predict developmental and reproductive toxicity (DART) and implemented into an in silico platform, called DARTpaths. AVAILABILITY AND IMPLEMENTATION: All code is available on GitHub https://github.com/Xpaths/dartpaths-app under Apache license 2.0, detailed overview with demo is available at https://www.vivaltes.com/dartpaths/. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.

"Commandeuring" Xenobiotic Metabolism: Advances in Understanding Xenobiotic Metabolism.

The understanding of how exogenous chemicals (xenobiotics) are metabolized, distributed, and eliminated is critical to determine the impact of the chemical and its metabolites to the (human) organism. This is part of the research and educational discipline ADMET (absorption, distribution, metabolism, elimination, and toxicity). Here, we review the work of Jan Commandeur and colleagues who have not only made a significant impact in understanding of phase I and phase II metabolism of several important compounds but also contributed greatly to the development of experimental techniques for the study of xenobiotic metabolism. Jan Commandeur's work has covered a broad area of research, such as the development of online screening methodologies, the use of a combination of enzyme mutagenesis and molecular modeling for structure-activity relationship (SAR) studies, and the development of novel probe substrates. This work is the bedrock of current activities and brings the field closer to personalized (cohort-based) pharmacology, toxicology, and hazard/risk assessment.

RespiraTox - Development of a QSAR model to predict human respiratory irritants.

Respiratory irritation is an important human health endpoint in chemical risk assessment. There are two established modes of action of respiratory irritation, 1) sensory irritation mediated by the interaction with sensory neurons, potentially stimulating trigeminal nerve, and 2) direct tissue irritation. The aim of our research was to, develop a QSAR method to predict human respiratory irritants, and to potentially reduce the reliance on animal testing for the identification of respiratory irritants. Compounds are classified as irritating based on combined evidence from different types of toxicological data, including inhalation studies with acute and repeated exposure. The curated project database comprised 1997 organic substances, 1553 being classified as irritating and 444 as non-irritating. A comparison of machine learning approaches, including Logistic Regression (LR), Random Forests (RFs), and Gradient Boosted Decision Trees (GBTs), showed, the best classification was obtained by GBTs. The LR model resulted in an area under the curve (AUC) of 0.65, while the optimal performance for both RFs and GBTs gives an AUC of 0.71. In addition to the classification and the information on the applicability domain, the web-based tool provides a list of structurally similar analogues together with their experimental data to facilitate expert review for read-across purposes.

Towards a reporting guideline for developmental and reproductive toxicology testing in C. elegans and other nematodes.

Implementation of reliable methodologies allowing Reduction, Refinement, and Replacement (3Rs) of animal testing is a process that takes several decades and is still not complete. Reliable methods are essential for regulatory hazard assessment of chemicals where differences in test protocol can influence the test outcomes and thus affect the confidence in the predictive value of the organisms used as an alternative for mammals. Although test guidelines are common for mammalian studies, they are scarce for non-vertebrate organisms that would allow for the 3Rs of animal testing. Here, we present a set of 30 reporting criteria as the basis for such a guideline for Developmental and Reproductive Toxicology (DART) testing in the nematode Caenorhabditis elegans. Small organisms like C. elegans are upcoming in new approach methodologies for hazard assessment; thus, reliable and robust test protocols are urgently needed. A literature assessment of the fulfilment of the reporting criteria demonstrates that although studies describe methodological details, essential information such as compound purity and lot/batch number or type of container is often not reported. The formulated set of reporting criteria for C. elegans testing can be used by (i) researchers to describe essential experimental details (ii) data scientists that aggregate information to assess data quality and include data in aggregated databases (iii) regulators to assess study data for inclusion in regulatory hazard assessment of chemicals.

Appraisal of the human health related toxicological information available on dicyclopentadiene (DCPD) in view of assessing the substance's potential to cause endocrine disruption.

Dicyclopentadiene (DCPD) is an olefinic hydrocarbon which is manufactured and imported into the European Union (EU) at greater than 1000 tons per year. Concerns related to fetotoxic effects observed in reproductive toxicity studies at high doses led the REACH registrants to self-classify DCPD as a Category 2 reproductive toxicant under the EU CLP Regulation. DCPD was also reviewed in the European Union in the frame of an ongoing European Chemical Agency (ECHA) Community Rolling Action Plan (CoRAP) procedure and under the French National Strategy on Endocrine Disruptors (SNPE). To elucidate whether the developmental effects may be triggered by an endocrine mode of action, the Lower Olefins Sector Group (LOSG) of the European Chemical Industry Council (CEFIC) formed an ad hoc expert team to review the available scientific information pertaining to the potential endocrine activity and adversity of DCPD. Existing experimental data was complemented with structure activity modelling using ECHA-recommended (Q)SAR tools. Overall, considering the available information from (Q)SAR, mechanistic in vitro and in vivo studies, no indication of endocrine-mediated adversity was found. Hence, the available evidence supports the conclusion that DCPD does not cause developmental toxicity via an endocrine mode of action. Further work is ongoing to support this conclusion.

Key event-informed risk models for benzene-induced acute myeloid leukaemia.

Occupational exposure to benzene at levels of 10 ppm or more has been associated with increased risk of acute myeloid leukaemia (AML). The mode of action (MOA) for AML development leading to mortality is anticipated to include multiple earlier key events, which can be observed in hematotoxicity and genetic toxicity in peripheral blood of exposed workers. Prevention of these early events would lead to prevention of the apical, adverse outcomes, the morbidity and mortality caused by the myelodysplastic syndromes (MDS) and AML. Incorporation of key event information should modify the risk model, but few modification approaches have been suggested. To that end, two approaches to risk model modification are described that use sub-linear and segmented linear increases in risk below key events, while maintaining a linear increase in AML mortality risk beginning at 2 ppm, the lowest observed adverse effect concentration (LOAEC) identified for hemato- and geno- toxicity in high quality studies of human occupational exposure. Below 2 ppm two different modification approaches to quantitative risk models were applied: a continuously decreasing slope model and a segmented modification in slope. These two approaches provide greater flexibility to incorporate MOA information in risk model development and selection.

Modes of action considerations in threshold expectations for health effects of benzene.

Understanding the Mode of Action (MOA) for a chemical can help guide decisions in development of Occupational Exposure Limits (OELs). Where sufficient information exists, it can provide the OEL developer the basis for selecting either a health-based or risk-based approach. To support the development of an OEL for benzene, scientific information relevant to MOA assessment for risk-based and health-based OEL approaches was reviewed. Direct-acting mutagenicity was considered as a basis for a risk-based OEL, versus MOAs consistent with a health-based approach: indirect mutagenicity via topoisomerase II inhibition, indirect mutagenicity via reactive oxygen species generation, or an immune-based bone marrow dysfunction. Based on the evidence against direct DNA reactivity, threshold expectations for remaining MOAs, and evidence for dose rate affecting acute myeloid leukemia and myelodysplastic syndrome risk, the weight of evidence favors a health-based OEL approach. In the case of benzene, development of an OEL based on observations of earlier key events (i.e., hematologic changes and genetic toxicity) is anticipated to provide protection from later adverse outcomes such as leukemia.

New approach methodologies (NAMs) for human-relevant biokinetics predictions. Meeting the paradigm shift in toxicology towards an animal-free chemical risk assessment

For almost fifteen years, the availability and regulatory acceptance of new approach methodologies (NAMs) to assess the absorption, distribution, metabolism and excretion (ADME/biokinetics) in chemical risk evaluations are a bottleneck. To enhance the field, a team of 24 experts from science, industry, and regulatory bodies, including new generation toxicologists, met at the Lorentz Centre in Leiden, The Netherlands. A range of possibilities for the use of NAMs for biokinetics in risk evaluations were formulated (for example to define species differences and human variation or to perform quantitative in vitro-in vivo extrapolations). To increase the regulatory use and acceptance of NAMs for biokinetics for these ADME considerations within risk evaluations, the development of test guidelines (protocols) and of overarching guidance documents is considered a critical step. To this end, a need for an expert group on biokinetics within the Organisation of Economic Cooperation and Development (OECD) to supervise this process was formulated. The workshop discussions revealed that method development is still required, particularly to adequately capture transporter mediated processes as well as to obtain cell models that reflect the physiology and kinetic characteristics of relevant organs. Developments in the fields of stem cells, organoids and organ-on-a-chip models provide promising tools to meet these research needs in the future.

Derivation of an occupational exposure limit for benzene using epidemiological study quality assessment tools.

This paper derives an occupational exposure limit for benzene using quality assessed data. Seventy-seven genotoxicity and 36 haematotoxicity studies in workers were scored for study quality with an adapted tool based on that of Vlaanderen et al., 2008 (Environ Health. Perspect. 116 1700-5). These endpoints were selected as they are the most sensitive and relevant to the proposed mode of action (MOA) and protecting against these will protect against benzene carcinogenicity. Lowest and No- Adverse Effect Concentrations (LOAECs and NOAECs) were derived from the highest quality studies (i.e. those ranked in the top tertile or top half) and further assessed as being "more certain" or "less certain". Several sensitivity analyses were conducted to assess whether alternative "high quality" constructs affected conclusions. The lowest haematotoxicity LOAECs showed effects near 2 ppm (8 h TWA), and no effects at 0.59 ppm. For genotoxicity, studies also showed effects near 2 ppm and showed no effects at about 0.69 ppm. Several sensitivity analyses supported these observations. These data define a benzene LOAEC of 2 ppm (8 h TWA) and a NOAEC of 0.5 ppm (8 h TWA). Allowing for possible subclinical effects in bone marrow not apparent in studies of peripheral blood endpoints, an OEL of 0.25 ppm (8 h TWA) is proposed.

The current status of exposure-driven approaches for chemical safety assessment: A cross-sector perspective.

For the purposes of chemical safety assessment, the value of using non-animal (in silico and in vitro) approaches and generating mechanistic information on toxic effects is being increasingly recognised. For sectors where in vivo toxicity tests continue to be a regulatory requirement, there has been a parallel focus on how to refine studies (i.e. reduce suffering and improve animal welfare) and increase the value that in vivo data adds to the safety assessment process, as well as where to reduce animal numbers where possible. A key element necessary to ensure the transition towards successfully utilising both non-animal and refined safety testing is the better understanding of chemical exposure. This includes approaches such as measuring chemical concentrations within cell-based assays and during in vivo studies, understanding how predicted human exposures relate to levels tested, and using existing information on human exposures to aid in toxicity study design. Such approaches promise to increase the human relevance of safety assessment, and shift the focus from hazard-driven to risk-driven strategies similar to those used in the pharmaceutical sectors. Human exposure-based safety assessment offers scientific and 3Rs benefits across all sectors marketing chemical or medicinal products. The UK's National Centre for the Replacement, Refinement and Reduction of Animals in Research (NC3Rs) convened an expert working group of scientists across the agrochemical, industrial chemical and pharmaceutical industries plus a contract research organisation (CRO) to discuss the current status of the utilisation of exposure-driven approaches, and the challenges and potential next steps for wider uptake and acceptance. This paper summarises these discussions, highlights the challenges - particularly those identified by industry - and proposes initial steps for moving the field forward.

Application of Caenorhabditis elegans (nematode) and Danio rerio embryo (zebrafish) as model systems to screen for developmental and reproductive toxicity of Piperazine compounds.

To enable selection of novel chemicals for new processes, there is a recognized need for alternative toxicity screening assays to assess potential risks to man and the environment. For human health hazard assessment these screening assays need to be translational to humans, have high throughput capability, and from an animal welfare perspective be harmonized with the principles of the 3Rs (Reduction, Refinement, Replacement). In the area of toxicology a number of cell culture systems are available but while these have some predictive value, they are not ideally suited for the prediction of developmental and reproductive toxicology (DART). This is because they often lack biotransformation capacity, multicellular or multi- organ complexity, for example, the hypothalamus pituitary gonad (HPG) axis and the complete life cycle of whole organisms. To try to overcome some of these limitations in this study, we have used Caenorhabditis elegans (nematode) and Danio rerio embryos (zebrafish) as alternative assays for DART hazard assessment of some candidate chemicals being considered for a new commercial application. Nematodes exposed to Piperazine and one of the analogs tested showed a slight delay in development compared to untreated animals but only at high concentrations and with Piperazine as the most sensitive compound. Total brood size of the nematodes was also reduced primarily by Piperazine and one of the analogs. In zebrafish Piperazine and analogs showed developmental delays. Malformations and mortality in individual fish were also scored. Significant malformations were most sensitively identified with Piperazine, significant mortality was only observed in Piperazine and only at the higest dose. Thus, Piperazine seemed the most toxic compound for both nematodes and zebrafish. The results of the nematode and zebrafish studies were in alignment with data obtained from conventional mammalian toxicity studies indicating that these have potential as developmental toxicity screening systems. The results of these studies also provided reassurance that none of the Piperazines tested are likely to have any significant developmental and/or reproductive toxicity issues to humans when used in their commercial applications.

Comparison of drug transporter gene expression and functionality in Caco-2 cells from 10 different laboratories.

Caco-2 cells, widely used to study carrier mediated uptake and efflux mechanisms, are known to have different properties when cultured under different conditions. In this study, Caco-2 cells from 10 different laboratories were compared in terms of mRNA expression levels of 72 drug and nutrient transporters, and 17 other target genes, including drug metabolising enzymes, using real-time PCR. The rank order of the top five expressed genes was: HPT1>GLUT3>GLUT5>GST1A>OATP-B. Rank correlation showed that for most of the samples, the gene ranking was not significantly different. Functionality of transporters and the permeability of passive transport markers metoprolol (transcellular) and atenolol (paracellular) were also compared. MDR1 and PepT1 function was investigated using talinolol and Gly-Sar transport, respectively. Sulfobromophthalein (BSP) was used as a marker for MRP2 and OATP-B functionality. Atenolol permeability was more variable across laboratories than metoprolol permeability. Talinolol efflux was observed by all the laboratories, whereas only five laboratories observed significant apical uptake of Gly-Sar. Three laboratories observed significant efflux of BSP. MDR1 expression significantly correlated to the efflux ratio and net active efflux of talinolol. PepT1 mRNA levels showed significant correlation to the uptake ratio and net active uptake of Gly-Sar. MRP2 and OATP-B showed no correlation to BSP transport parameters. Heterogeneity in transporter activity may thus be due to differences in transporter expression as shown for PepT1 and MDR1 which in turn is determined by the culture conditions. Absolute expression of genes was variable indicating that small differences in culture conditions have a significant impact on gene expression, although the overall expression patterns were similar.

Functional replacement of murine CXCR2 by its human homologue in the development of atherosclerosis in LDLR knockout mice.

The CXC chemokine receptor CXCR2 has been implicated in the pathogenesis of several chronic diseases including atherosclerosis. To enable animal studies towards understanding the role of human CXCR2 (hCXCR2) in disease development, we previously generated hCXCR2 knockin (hCXCR2(+/+)) mice. We have demonstrated that the phenotype and the acute immune response of the hCXCR2(+/+) mice was identical to that of wild-type mice, indicating that hCXCR2 indeed takes over the function of endogenous mouse CXCR2 (mCXCR2). In the present paper, we extend these findings by studying whether hCXCR2 functionally replaces the role of mCXCR2 in a chronic disease model for atherosclerosis. We first defined which of two well-described atherosclerosis models (ApoE(-/-) or LDLR(-/-) mice) is most suited for this purpose. When expression of mCXCR2 and that of its ligands in atherosclerotic lesions were compared in these mice, increased expression levels were observed only in LDLR(-/-) mice. Further, cultured atherosclerotic aortas from LDLR(-/-) mice did secrete significantly higher levels of CXCR2 ligands compared to aortas from healthy controls. Since these results support the role of CXCR2 in the atherogenesis in the LDLR(-/-) mice, double mutant hCXCR2(+/+)/LDLR(-/-) mice were generated and diet-induced atherosclerosis in these mice was compared to that in LDLR(-/-) mice. Upon an atherogenic diet, the hCXCR2(+/+)/LDLR(-/-) mice developed plaque lesions in a similar manner to those in LDLR(-/-) mice, indicating successful functional replacement of mCXCR2 by hCXCR2 in this disease model. We conclude that hCXCR2(+/+)/LDLR(-/-) mice present an attractive model to study the role of hCXCR2 in atherosclerosis development and for future testing of novel pharmaceuticals designed to antagonize hCXCR2.

Imidazolylpyrimidine based CXCR2 chemokine receptor antagonists.

An imidazolylpyrimidine was identified in a CXCR2 chemokine receptor antagonist screen and was optimized for potency, in vitro metabolic stability, and oral bioavailability. It was found that subtle structural modification within the series affected the oral bioavailability. Potent and orally available CXCR2 antagonists are herein reported.

Human CXCR2 (hCXCR2) takes over functionalities of its murine homolog in hCXCR2 knockin mice.

Human CXCR2 (hCXCR2) has been implicated in diverse inflammatory diseases. When roles of this receptor studied in animal models are extrapolated into men, large species differences in expression of the receptor and its ligands must be considered. These differences seriously weaken conclusions toward the role of hCXCR2 in the development of human diseases. It furthermore hampers straightforward testing of CXCR2 antagonists, especially when compounds discriminate between human and other species' receptors. Using gene targeting in embryonic stem cells, a hCXCR2 knockin mouse strain was generated in which endogenous murine CXCR2 (mCXCR2) sequences are replaced by the hCXCR2 gene. Correct targeting and expression on neutrophils were confirmed by Southern blot and immunohistochemical analyses. A phenotypic analysis of the hCXCR2 knockin mice, in comparison to wild-type and CXCR2 knockout mice, confirmed proper function of the hCXCR2 gene. In vivo migratory responses of neutrophils were intact in hCXCR2 knockin mice. Finally, an experiment with a CXCR2 antagonist demonstrated that the knockin model is indeed useful for in vivo evaluation of low-molecular weight compounds. In conclusion, our data unequivocally show that hCXCR2 can functionally replace mCXCR2, making this an attractive model to test novel pharmaceuticals designed to antagonize human CXCR2 in vivo.

Enzyme-catalyzed activation of anticancer prodrugs.

The rationale fo the development of prodrugs relies upon delivery of higher concentrations of a drug to target cells compared to administration of the drug itself. In the last decades, numerous prodrugs that are enzymatically activated into anti-cancer agents have been developed. This review describes the most important enzymes involved in prodrug activation notably with respect to tissue distribution, up-regulation in tumor cells and turnover rates. The following endogenous enzymes are discussed: aldehyde oxidase, amino acid oxidase, cytochrome P450 reductase, DT-diaphorase, cytochrome P450, tyrosinase, thymidylate synthase, thymidine phosphorylase, glutathione S-transferase, deoxycytidine kinase, carboxylesterase, alkaline phosphatase, beta-glucuronidase and cysteine conjugate beta-lyase. In relation to each of these enzymes, several prodrugs are discussed regarding organ- or tumor-selective activation of clinically relevant prodrugs of 5-fluorouracil, axazaphosphorines (cyclophosphamide, ifosfamide, and trofosfamide), paclitaxel, etoposide, anthracyclines (doxorubicin, daunorubicin, epirubicin), mercaptopurine, thioguanine, cisplatin, melphalan, and other important prodrugs such as menadione, mitomycin C, tirapazamine, 5-(aziridin-1-yl)-2,4-dinitrobenzamide, ganciclovir, irinotecan, dacarbazine, and amifostine. In addition to endogenous enzymes, a number of nonendogenous enzymes, used in antibody-, gene-, and virus-directed enzyme prodrug therapies, are described. It is concluded that the development of prodrugs has been relatively successful; however, all prodrugs lack a complete selectivity. Therefore, more work is needed to explore the differences between tumor and nontumor cells and to develop optimal substrates in terms of substrate affinity and enzyme turnover rates fo prodrug-activating enzymes resulting in more rapid and selective cleavage of the prodrug inside the tumor cells.