Does EU legislation allow the use of the Benchmark Dose (BMD) approach for risk assessment?

Hazard characterisation is largely based on an approach of (statistically) comparing dose groups with the controls in order to derive points of departure such as no-observed-adverse-effect levels (NOAELs) or lowest-observed-adverse-effect levels (LOAELs). This approach suggests the absence of any relevant effect at the NOAEL. The NOAEL approach has been debated for decades. A recent Scientific Opinion by the European Food Safety Authority (EFSA) concluded that the Benchmark Dose (BMD) approach should be preferred over the NOAEL approach for deriving human (health-based) limit or guidance values. Nonetheless, the BMD approach is used infrequently within European regulatory frameworks. The reason for this may lie in legislation or guidelines requiring the use of the NOAEL approach. In this context, various EU regulatory frameworks were examined on such demands. Interestingly, no single legislation was identified containing statutory requirements in conflict with the use of the BMD approach.

First tier modeling of consumer dermal exposure to substances in consumer articles under REACH: a quantitative evaluation of the ECETOC TRA for consumers tool.

The demonstration of safe use of chemicals in consumer products, as required under REACH, is proposed to follow a tiered process. In the first tier, simple conservative methods and assumptions should be made to quickly verify whether risks for a particular use are expected. The ECETOC TRA Consumer Exposure Tool was developed to assist in first tier risk assessments for substances in consumer products. The ECETOC TRA is not a prioritization tool, but is meant as a first screening. Therefore, the exposure assessment needs to cover all products/articles in a specific category. For the assessment of the dermal exposure for substances in articles, ECETOC TRA uses the concept of a 'contact layer', a hypothetical layer that limits the exposure to a substance contained in the product. For each product/article category, ECETOC TRA proposes default values for the thickness of this contact layer. As relevant experimental exposure data is currently lacking, default values are based on expert judgment alone. In this paper it is verified whether this concept meets the requirement of being a conservative exposure evaluation method. This is done by confronting the ECETOC TRA expert judgment based predictions with a mechanistic emission model, based on the well established theory of diffusion of substances in materials. Diffusion models have been applied and tested in many applications of emission modeling. Experimentally determined input data for a number of material and substance combinations are available. The estimated emissions provide information on the range of emissions that could occur in reality. First tier tools such as ECETOC TRA tool are required to cover all products/articles in a category and to provide estimates that are at least as high as is expected on the basis of current scientific knowledge. Since this was not the case, it is concluded that the ECETOC TRA does not provide a proper conservative estimation method for the dermal exposure to articles. An alternative method was proposed.

Consumer exposure scenarios: development, challenges and possible solutions.

Exposure scenarios (ES) under REACH (Registration, Evaluation, and Authorisation of Chemicals; new EU legislation) aim to describe safe conditions of product and substance use. Both operational conditions and risk management measures (RMMs) are part of the ES. For consumer use of chemicals, one of the challenges will be to identify all of the consumer uses of a given chemical and then quantify the exposure derived from each of them. Product use categories can be established to identify in a systematic fashion how products are used. These product categories comprise products that are used similarly (e.g. paints, adhesives). They deliver information about product use characteristics, and provide an easy-to-handle tool for exchanging standardised information. For practical reasons, broad ES will have to be developed, which cover a wide range of products and use. The challenge will be to define them broadly, but not in a way that they provide such an overestimation of exposure that a next iteration or a more complex model is always needed. Tiered and targeted approaches for estimation of exposure at the right level of detail may offer the best solution. RMMs relevant for consumers include those inherent to product design (controllable) and those that are communicated to consumers as directions for use (non-controllable). Quantification of the effect of non-controllable RMMs on consumer exposure can prove to be difficult. REACH requires aggregation of exposure from all relevant identified sources. Development of appropriate methodology for realistic aggregation of exposure will be no small challenge and will likely require probabilistic approaches and comprehensive databases on populations' habits, practices and behaviours. REACH regulation aims at controlling the use of chemicals so that exposure to every chemical can be demonstrated to be safe for consumers, workers, and the environment when considered separately, but also when considered in an integrated way. This integration will be another substantial challenge for the future.

Risk assessment of chemicals and pharmaceuticals in the pediatric population: a workshop report.

ATSDR and RIVM organized an Expert Panel Workshop on the Differences Between Children and Adults and Their Relevance to Risk Assessment. The workshop was held in June 2003, in Brussels, Belgium. The purpose of the workshop was to identify data gaps in current scientific knowledge related to children's health and to recognize areas of mutual interest that would serve as the basis for upcoming ATSDR/RIVM cooperative projects. The aim for both agencies is a better understanding of the issues related to children's health, and the improvement of scientifically based (chemical) risk assessment in children. Topics discussed included clinical trials/toxicity studies, testing in juvenile animals, PBPK modeling in children, and children's risk assessment.

Role of biokinetics in risk assessment of drugs and chemicals in children.

Whether children incur different risks from xenobiotics than adults will depend on the exposure, biokinetics, and dynamics of compound. In this paper, current knowledge on developmental physiology and possible effects on biokinetics are evaluated and the role of biokinetics in risk assessment both for drugs and chemicals is discussed. It is concluded that most dramatic age-related physiological changes that may affect biokinetics occur in the first 6-12 months of age. The difference in internal exposure between children and adults can generally be predicted from already known developmental physiological differences. However, for risk assessment it will also be necessary to determine whether internal exposure is within the drug's therapeutic window or if it will exceed the NOAEL of a chemical. Furthermore, the effects of internal exposure of potentially harmful compounds on developing organ systems is of utmost importance. However, knowledge on this aspect is very limited. Risk assessment in children could be improved by: (1) application of pediatric PBPK-models in order to gain insight into internal exposure in children, (2) studies in juvenile animals for studying effects on developing systems, and (3) extrapolation of knowledge on the relationship between internal exposure and dynamics for drugs to other chemicals.

Crossing the river stone by stone: approaches for residential risk assessment for consumers.

Consumer products may contain constituents that warrant a risk analysis if they raise toxicological concern. Risk assessments are performed a priori, e.g. for pesticides and biocides, and a posteriori, to diagnose risks of contaminants. An overview is presented of residential exposure assessment and risk characterization. For exposure assessment, predictive models are used to estimate exposure concentrations. The available data on product use are used to quantify the intensity of exposure. Often, both exposure concentration and product use show high variability. Worst case assessments cope with variability and uncertainty in data poor situations by selecting 'worst case' values for exposures and exposure factors. Probabilistic models may be used to quantify and model variability and uncertainty when appropriate data is available. The Margin Of Safety approach to characterize risk is discussed. Many biocides handled by consumers are used now and then and (sub)acute exposure and toxicology will be most relevant. Users and children are generally seen as critical groups during the application and post-application phases of exposure, respectively. Still, the diversity of consumer products requires consideration of the merits of each case. We conclude that residential risk assessment is still searching for methods, data and models. Probabilistic methods appear to be useful tools, but a major challenge is to integrate them in regulatory frameworks.

Effect of coexposure to methyl ethyl ketone (MEK) on n-hexane toxicokinetics in human volunteers.

In order to study the effects of methyl ethyl ketone (MEK) on the toxicokinetics of n-hexane and, in particular, the formation of 2,5-hexanedione from n-hexane in humans, volunteers were exposed to n-hexane (approx. 60 ppm, 2.4 microM in the inhaled air) with or without simultaneous inhalatory coexposure to MEK for 15.5 min. The concentration-time course of n-hexane (in exhaled alveolar air) and its neurotoxic metabolite, 2,5-hexanedione (in serum), were studied. The concentration-time courses obtained after exposure to n-hexane alone were compared with those obtained after coexposure to 200 or 300 ppm MEK in the same volunteer on the same day. No effect of MEK was observed on the concentration-time course of exhaled n-hexane. The concentration-time course of the metabolite, 2,5-hexanedione, revealed a decrease in the rate of formation of 2,5-hexanedione (about three-fold) after coexposure to MEK. Furthermore, the time to reach the peak concentration was increased from 18 to 30 min after the start of exposure. These changes in the concentration-time course of 2,5-hexanedione caused by MEK are most likely the result of inhibition of the biotransformation of one of the intermediate steps in the conversion of n-hexane to 2,5-hexanedione. These results indicate that the interaction of n-hexane and MEK leads to a decreased concentration of the neurotoxic metabolite 2,5-hexanedione (after short-term, acute exposure).

Determination of 2,5-hexanedione, a metabolite of n-hexane, in urine: evaluation and application of three analytical methods.

Three methods for the determination of 2,5-hexanedione (2,5-HD) in urine were compared in order to assess their applicability for toxicokinetic studies and biological monitoring of occupational exposure to n-hexane. Two of them were based on derivatization, followed by gas chromatography and electron-capture detection. Of these two, one is a modification of the other, already published, method. The third one involves direct extraction of 2,5-HD followed by gas chromatography and flame-ionization detection. To determine 2,5-HD in urine of workers occupationally exposed to n-hexane, the most straightforward method, direct extraction of 2,5-HD from urine, has been proven to be the most suitable. However, in case of very low concentrations of 2,5-HD in urine, or analysis of small samples of blood, e.g. in kinetic studies, it is necessary to use a more sensitive procedure. The sensitivity of the methods based on the derivatization of 2,5-HD followed by electron-capture detection, was, as expected, much higher in terms of analytical reliability. By using these methods, however, precautions are necessary to avoid a matrix effect.

Glucuronidation of thyroid hormone in rat liver: effects of in vivo treatment with microsomal enzyme inducers and in vitro assay conditions.

We investigated the effects of in vivo treatment with different microsomal enzyme inducers, including clofibrate (CLOF), hexachlorobenzene (HCB), 3-methylcholanthrene (MC), 3,3',4,4'-tetrachlorobiphenyl (TCB), and 2,3,7,8-tetrachloro-p-dioxin, as well as of in vitro addition of the detergent Brij 56 on the glucuronidation of T4, T3, and rT3 by UDP-glucuronyltransferase (UGT) activities of rat liver microsomes. The results were compared with measurements of UGT activities for bilirubin, p-nitrophenol (PNP), and androsterone. In general, glucuronidation rates were 5-fold or more higher with rT3 than with T4 or T3 as substrate. In liver microsomes from untreated rats, T4 UGT activity was stimulated by Brij 56 to a maximum of about 2-fold at 0.025% detergent. Treatment of Wistar rats for 4 days with CLOF (200 mg/kg BW.day) resulted in significant increases in UGT activities for T4 (to 154%), rT3 (to 155%), and bilirubin (to 194%), in particular if assayed in the presence of 0.025% Brij 56, but had little effect on the UGT activities for T3, PNP, and androsterone. The CLOF-induced increases in T4 and rT3 UGT activities were not observed in Gunn rats, which have a complete lack of bilirubin UGT activity and greatly impaired PNP UGT activity. Treatment of Wistar rats with a single injection of MC (50 mg/kg BW), TCB (50 mg/kg BW), or 2,3,7,8-tetrachloro-p-dioxin (6.25 micrograms/kg BW) resulted, after 4 days, in 6.3- to 7.3-fold increases in T4 UGT activity and 15.1- to 16.7-fold increases in rT3 UGT activity if determined in the absence of Brij 56, whereas T4 UGT activity was only increased by 33-68% when assayed in the presence of Brij 56. T3 glucuronidation was not affected (with Brij 56) or was increased by only 33-68% (without Brij 56) after treatment with these MC-type inducers. PNP UGT activity was induced 3.6- to 4.3-fold, whereas bilirubin and androsterone UGT activities were changed little by these treatments. Similar findings regarding T4, rT3, PNP, and bilirubin UGT activities were obtained after chronic treatment of WAG rats with HCB, another MC-type inducer. However, WAG rats lack androsterone UGT and show low T3 UGT activity, which was increased about 2.3-fold by HCB treatment. On the basis of these and previous findings it is concluded that at least three UGT isoenzymes are involved in the glucuronidation of thyroid hormone.(ABSTRACT TRUNCATED AT 400 WORDS)

Thyroxine and 3,3',5-triiodothyronine are glucuronidated in rat liver by different uridine diphosphate-glucuronyltransferases.

Male Wistar rats were treated with 50 mg 3,3',4,4'-tetrachlorobiphenyl (TCB)/kg BW or vehicle. After 4 days, the livers were isolated and perfused for 90 min with 2 nM [125I]T3 or 10 nM [125I]T4 in Krebs-Ringer medium containing 1% albumin. Deiodination and conjugation products and remaining substrates were determined in bile and medium samples by Sephadex LH-20 chromatography and HPLC. TCB treatment did not affect hepatic uptake and metabolism of T3. However, biliary excretion of T4 glucuronide was strongly increased by TCB, resulting in an augmented T4 disappearance from the medium, although initial hepatic uptake of T4 was not altered. Measurement of the microsomal UDP-glucuronyltransferase (UDPGT) activities confirmed that T4 UDPGT was induced by TCB, whereas T3 glucuronidation was unaffected. T3 UDPGT activity showed a discontinuous variation, which completely matched the genetic heterogeneity in androsterone glucuronidation in Wistar rats. These results indicate that different isozymes catalyze the glucuronidation of T3 and T4.