Bob Safford

Predictive identification of human skin sensitization thresholds.

For years, methods have been available for the predictive identification of chemicals that possess the intrinsic potential to cause skin sensitization. However, many have proven less suitable for the determination of relative sensitizing potency. In this respect, the local lymph node assay (LLNA) has been shown to have a number of important advantages. Through interpolation of LLNA dose–response data, the concentration of a chemical required to produce a threshold positive response (a 3‐fold increase in activity compared with concurrent vehicle controls, the EC3 value) can be measured. The robustness of this parameter has been demonstrated rigorously in terms of inter‐ and intralaboratory reproducibility. Additionally, the relationship between potency estimates from the LLNA and an appreciation of human potency based on clinical experience has been reported previously. In the present investigations, we have sought to consolidate further our understanding of the association between EC3 values and human skin‐sensitization potency by undertaking a thorough and extensive analysis of existing human predictive assays, particularly where dose–response information is available, from historical human repeated insult patch tests (HRIPTs). From these human data, information on the approximate threshold for the induction of skin sensitization in the HRIPT was determined for 26 skin‐sensitizing chemicals. These data were then compared with LLNA‐derived EC3 values. The results from each assay, expressed as dose per unit area (μg/cm2), revealed a clear linear relationship between the 2 values, thereby substantiating further the utility of LLNA EC3 values for prediction of the relative human sensitizing potency of newly identified skin sensitizers.

The impact of exposure variables on the induction of skin sensitization.

Whereas many investigations of the variables associated with the elicitation of allergic contact dermatitis have been undertaken, to the point where we can begin to predict the likelihood of elicitation occurring in a given situation, the same is not true for the induction of skin sensitization. Studies have demonstrated that increasing dose has an impact; in an experimental setting, a number of variables received attention some decades ago. However, in the work reported here, the relative importance of the frequency and the duration of exposure is highlighted. In an investigation using a human repeated insult patch test, it was demonstrated that reduction of the exposure duration from 48 hr to 5 min decreased the rate of sensitization to 1% p‐phenylenediamine (PPD) from 54% to 3%. However, in an extended clinical study, it was observed that infrequent but longer duration and higher concentration exposure to PPD was significantly less likely to induce sensitization compared to more frequent, short duration, and lower concentration exposure. Detailed statistical analysis of the results indicated that the most important factor driving the induction of skin sensitization was the number of exposures.

Skin sensitization quantitative risk assessment: A review of underlying assumptions

Toxicological risk assessment informs exposure limits, so the potential for adverse effects to human health are minimised or avoided. For skin sensitisers, the situation is complicated by asymptomatic induction of contact allergy, a necessary prerequisite for expression of the disease allergic contact dermatitis (ACD). For fragrance skin sensitisers, the development of quantitative risk assessment (QRA) arose from the need to improve the extent to which contact allergy occurred. However, the perceived impact has been less than anticipated. Accordingly, the science and assumptions upon which QRA was founded have been scrutinised and proposals for refinement have been made. In addition, areas of uncertainty have been made explicit, e.g. inter-individual variability and the impact of concomitant disease, clarifying where numerical safety assessment factors are based on expert judgement. Also, the relatively small contribution of factors eg. age, gender, ethnic origin, vehicle matrix and skin permeability are highlighted by reference to the (now controversial) human experiments carried out in the second half of the last century. Adoption and widespread implementation of the current recommendations for QRA, taken in concert with improved assessment of aggregate exposure from multiple sources, should ensure that the frequency of contact allergy will decrease over the coming years.

The impact of vehicle on the relative potency of skin-sensitizing chemicals in the local lymph node assay

The identification and characterization of chemicals that possess skin-sensitizing potential are typically performed using predictive tests. However, human exposure to skin-sensitizing chemicals often occurs via a matrix (vehicle) that differs from that used in these tests. It is thus important to account for the potential impact of vehicle differences when undertaking quantitative risk assessment for skin sensitization. This is achieved through the application of a specific sensitization assessment factor (SAF), scaled between 1 and 10, when identifying an acceptable exposure level. The objective of the analysis described herein is to determine the impact of vehicle differences on local lymph node assay (LLNA) EC3 values (concentrations of test chemical required to provoke a 3-fold increase in lymph node cell proliferation). Initially, the inherent variability of the LLNA was investigated by examining the reproducibility of EC3 values for 14 chemicals that have been tested more than once in the same vehicle (4:1 acetone:olive oil, AOO). This analysis reveals that the variability in EC3 value for these chemicals following multiple assessments is <5-fold. Next, data from the literature and previously unpublished studies were compiled for 18 chemicals that had been assessed in the LLNA using at least 2 of 15 different vehicles. These data demonstrate that often the variability in EC3 values observed for a given chemical in different vehicles is no greater than the 5-fold inherent variability observed when assessing a chemical in the same vehicle on multiple occasions. However, there are examples where EC3 values for a chemical differ by a factor of more than 10 between different vehicles. These observations were often associated with an apparent underestimation of potency (higher EC3 values) with predominantly aqueous vehicles or propylene glycol. These data underscore the need to consider vehicle effects in the context of skin-sensitization risk assessments.

A model to estimate the oestrogen receptor mediated effects from exposure to soy isoflavones in food

The advantages that regular consumption of a diet containing soy may have on human health have been enshrined in a major health claim that has been approved by the Food and Drug Administration in the USA, regarding potential protection from heart disease by soy. This could have a major influence on the dietary consumption patterns of soy for consumers and lead to the development of soy enriched foods to enable consumers to achieve the benefits thought to be associated with increased soy consumption in a Western diet. If an increase in soy consumption is beneficial to particular disease conditions, there is always the possibility that there will be effects other than those that are desirable. For soy-containing foods there has been concern that the phytoestrogen content of soy, which is composed of several isoflavones, could be a separate health issue, due to the oestrogen-like activity of isoflavones. To address this, a method has been developed to estimate, relative to 17-beta oestradiol, the activity of the common isoflavones present in soy phytoestrogens, based on their binding to and transcriptional activation of the major oestrogen receptor sub-types alpha and beta. Using this approach, the additional oestrogen-like activity that would be expected from inclusion of soy supplemented foodstuffs in a Western diet, can be determined for different sub-populations, who may have different susceptibilities to the potential for the unwanted biological effects occurring with consumption of soy enriched foods. Because of the theoretical nature of this model, and the controversy over the nature of whether some of the oestrogen-like effects of phytoestrogens are adverse, the biological effects of soy isoflavones and their potential for adverse effects in man, is also reviewed. The question that is critical to the long term safe use of foods enriched in soy is, which observed biological effects in animal studies are likely to also occur in man and whether these would have an adverse effect on human health.

Thresholds of toxicological concern for endocrine active substances in the aquatic environment

The threshold of toxicological concern (TTC) concept proposes that an exposure threshold value can be derived for chemicals, below which no significant risk to human health or the environment is expected. This concept goes further than setting acceptable exposure levels for individual chemicals, because it attempts to set a de minimis value for chemicals, including those of unknown toxicity, by taking the chemicals structure or mode of action (MOA) into consideration. This study examines the use of the TTC concern concept for endocrine active substances (EAS) with an estrogenic MOA. A case study formed the basis for a workshop of regulatory, industry and academic scientists held to discuss the use of the TTC in aquatic environmental risk assessment. The feasibility and acceptability, general advantages and disadvantages, and the specific issues that need to be considered when applying the TTC concept for EAS in risk assessment were addressed. Issues surrounding the statistical approaches used to derive TTCs were also discussed. This study presents discussion points and consensus findings of the workshop.