Dermal absorption for pesticide health risk assessment: Harmonization of study design and data reporting for North American Regulatory submissions.

Although an internationally-adopted in vitro dermal absorption test guideline is available (OECD Test Guideline 428), the replacement of the in vivo approach in North America for pesticide formulations has not occurred due to concern over the reliability and consistency of the in vitro results. A 2012 workshop convened a panel of experts in the conduct of in vitro studies used for pesticide risk assessment, together with North American regulators, to examine techniques for in vitro dermal absorption testing. Discussions led to the recommended "best practices" for the conduct of in vitro dermal absorption studies provided herein. The workshop participants also developed recommendations for reporting study results in order to improve the quality and consistency of the data submitted to regulatory agencies in North America. Finally, a case study is presented that illustrates the use of the "triple-pack" approach; the studies, conducted for the registration of sulfoxaflor, follow the standardized recommendations provided at the workshop. In addressing the concerns of these regulators and of the regulated community, and providing harmonized recommendations to facilitate comparative data analyses, it is anticipated that wider acceptance of in vitro dermal absorption studies alone can be achieved for pesticide risk assessment.

Distribution and visualisation of chlorhexidine within the skin using ToF-SIMS: a potential platform for the design of more efficacious skin antiseptic formulations.

PURPOSE: In order to increase the efficacy of a topically applied antimicrobial compound the permeation profile, localisation and mechanism of action within the skin must first be investigated. METHODS: Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to visualise the distribution of a conventional antimicrobial compound, chlorhexidine digluconate, within porcine skin without the need for laborious preparation, radio-labels or fluorescent tags. RESULTS: High mass resolution and high spatial resolution mass spectra and chemical images were achieved when analysing chlorhexidine digluconate treated cryo-sectioned porcine skin sections by ToF-SIMS. The distribution of chlorhexidine digluconate was mapped throughout the skin sections and our studies indicate that the compound appears to be localised within the stratum corneum. In parallel, tape strips taken from chlorhexidine digluconate treated porcine skin were analysed by ToF-SIMS to support the distribution profile obtained from the skin sections. CONCLUSIONS: ToF-SIMS can act as a powerful complementary technique to map the distribution of topically applied compounds within the skin.

Antimicrobial efficacy and mechanism of action of poly(amidoamine) (PAMAM) dendrimers against opportunistic pathogens.

The aim of this study was to investigate a range of poly(amidoamine) (PAMAM) dendrimer generations against Gram-positive and Gram-negative skin pathogens and to determine any differences in antimicrobial potency for different generations, characterising how differences in physicochemical properties influence antimicrobial efficacy. A range of tests were carried out, including viable count assays to determine half maximal inhibitory concentration (IC50) values for each dendrimer, membrane integrity studies and an inner membrane permeabilisation assay. This is supported by scanning electron microscopy imaging of the interactions observed between dendrimers and bacteria. The results of this study indicate that the antimicrobial efficacy of native PAMAM dendrimers is dependent on generation, concentration and terminal functionalities, for example, the concentration at 50% growth inhibition (MIC50) (µg/mL), against Staphylococcus aureus was between 26.77 for the G2-PAMAM-NH2 dendrimer and 2.881 for the G5-PAMAM-NH2 dendrimer. There was a strong correlation between membrane disruption and the determined biocidal activity, making it a key contributing mechanism of action. This study demonstrates that selection of the type of PAMAM dendrimer is important as their inherent antimicrobial efficacy varies according to their individual physicochemical properties. This understanding may pave the way for the development of enhanced dendrimer-based antimicrobial formulations and drug-delivery systems.

Dermal absorption of testosterone in human and pig skin in vitro.

The OECD test guideline 428 for the assessment of dermal absorption in vitro has been in force for more than a decade. Various sectors of industry utilise the method for the registration of chemical products. These include the Agrochemical and Cosmetic sectors where the OECD test guideline and industry-specific guidance forms a key part of the human risk assessment process for new and existing products. This investigation has compared the dermal absorption characteristics of one of the OECD 428 reference chemicals, testosterone, in human and pig dermatomed skin. We used identical dosing and skin decontamination conditions for testosterone in Franz static diffusion cells. This included a full mass balance recovery of the dose applied and distribution of the compound in the different layers of the skin. Our investigation has shown that intact human skin provides a more effective barrier to the dermal absorption of testosterone compared with pig skin, when studied according to modern day in vitro dermal absorption guidance.

The ECVAM international validation study on in vitro tests for acute skin irritation: report on the validity of the EPISKIN and EpiDerm assays and on the Skin Integrity Function Test.

ECVAM sponsored a formal validation study on three in vitro tests for skin irritation, of which two employ reconstituted human epidermis models (EPISKIN, EpiDerm), and one, the skin integrity function test (SIFT), employs ex vivo mouse skin. The goal of the study was to assess whether the in vitro tests would correctly predict in vivo classifications according to the EU classification scheme, "R38" and "no label" (i.e. non-irritant). 58 chemicals (25 irritants and 33 non-irritants) were tested, having been selected to give broad coverage of physico-chemical properties, and an adequate distribution of irritancy scores derived from in vivo rabbit skin irritation tests. In Phase 1, 20 of these chemicals (9 irritants and 11 non-irritants) were tested with coded identities by a single lead laboratory for each of the methods, to confirm the suitability of the protocol improvements introduced after a prevalidation phase. When cell viability (evaluated by the MTT reduction test) was used as the endpoint, the predictive ability of both EpiDerm and EPISKIN was considered sufficient to justify their progression to Phase 2, while the predictive ability of the SIFT was judged to be inadequate. Since both the reconstituted skin models provided false predictions around the in vivo classification border (a rabbit Draize test score of 2), the release of a cytokine, interleukin-1alpha (IL-1alpha), was also determined. In Phase 2, each human skin model was tested in three laboratories, with 58 chemicals. The main endpoint measured for both EpiDerm and EPISKIN was cell viability. In samples from chemicals which gave MTT assay results above the threshold of 50% viability, IL-1alpha release was also measured, to determine whether the additional endpoint would improve the predictive ability of the tests. For EPISKIN, the sensitivity was 75% and the specificity was 81% (MTT assay only); with the combination of the MTT and IL-1alpha assays, the sensitivity increased to 91%, with a specificity of 79%. For EpiDerm, the sensitivity was 57% and the specificity was 85% (MTT assay only), while the predictive capacity of EpiDerm was not improved by the measurement of IL-1alpha release. Following independent peer review, in April 2007 the ECVAM Scientific Advisory Committee endorsed the scientific validity of the EPISKIN test as a replacement for the rabbit skin irritation method, and of the EpiDerm method for identifying skin irritants as part of a tiered testing strategy. This new alternative approach will probably be the first use of in vitro toxicity testing to replace the Draize rabbit skin irritation test in Europe and internationally, since, in the very near future, new EU and OECD Test Guidelines will be proposed for regulatory acceptance.

Further development of an in vitro model for studying the penetration of chemicals through compromised skin.

A new in vitro model based on the electrical resistance properties of the skin barrier has been established in this laboratory. The model utilises a tape stripping procedure in dermatomed pig skin that removes a specific proportion of the stratum corneum, mimicking impaired barrier function observed in humans with damaged skin. The skin penetration and distribution of chemicals with differing physicochemical properties, namely; Benzoic acid, 3-Aminophenol, Caffeine and Sucrose has been assessed in this model. Although, skin penetration over 24h differed for each chemical, compromising the skin did not alter the shape of the time course profile, although absorption into receptor fluid was higher for each chemical. Systemic exposure (receptor fluid, epidermis and dermis), was marginally higher in compromised skin following exposure to the fast penetrant, Benzoic acid, and the slow penetrant Sucrose. The systemically available dose of 3-Aminophenol increased to a greater extent and the absorption of Caffeine was more than double in compromised skin, suggesting that Molecular Weight and Log Pow, are not the only determinants for assessing systemic exposure under these conditions. Although further investigations are required, this in vitro model may be useful for prediction of dermal route exposure under conditions where skin barrier is impaired.

Dendrimer pre-treatment enhances the skin permeation of chlorhexidine digluconate: Characterisation by in vitro percutaneous absorption studies and Time-of-Flight Secondary Ion Mass Spectrometry.

Skin penetration and localisation of chlorhexidine digluconate (CHG) within the skin have been investigated in order to better understand and optimise the delivery using a nano polymeric delivery system of this topically-applied antimicrobial drug. Franz-type diffusion cell studies using in vitro porcine skin and tape stripping procedures were coupled with Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) to visualise the skin during various treatments with CHG and polyamidoamine dendrimers (PAMAM). Pre-treatment of the skin with PAMAM dendrimers significantly increased the amount and depth of permeation of CHG into the skin in vitro. The effect observed was not concentration dependant in the range 0.5-10mM PAMAM. This could be important in terms of the efficiency of treatment of bacterial infection in the skin. It appears that the mechanism of enhancement is due to the PAMAM dendrimer disrupting skin barrier lipid conformation or by occluding the skin surface. Franz-type diffusion cell experiments are complimented by the detailed visualisation offered by the semi-quantitative ToF-SIMS method which provides excellent benefits in terms of sensitivity and fragment ion specificity. This allows a more accurate depth profile of chlorhexidine permeation within the skin to be obtained and potentially affords the opportunity to map the co-localisation of permeants with skin structures, thus providing a greater ability to characterise skin absorption and to understand the mechanism of permeation, providing opportunities for new and more effective therapies.

Development of an in vitro model for studying the penetration of chemicals through compromised skin.

The conventional safety approach that includes dermal absorption of pharmaceutical or consumer products uses models that are based on intact skin. However, when products are intended for application to skin with a less effective barrier, such as in new-born infants, or in cases where the skin is mildly damaged or diseased, there are instances where absorption through compromised skin is also important. A tape stripping procedure was investigated using dermatomed pig skin to assess if an in vitro model could replicate the typical changes in barrier function observed in humans with compromised skin. The relationship between Trans-Epidermal Water Loss (TEWL), Electrical Resistance (ER) and Tritiated Water Flux(TWF), markers of skin barrier function in OECD 428 studies was investigated. There was a step-wise reduction in ER from normal (control) skin following 5, 10, 15 or 20 tape strips. This was mirrored by increases in both TWF and TEWL. An in vitro experimental protocol using 5 tape strips, ER and dermatomed pig skin provided a rapid, robust and reproducible approach equivalent to the 3­4-fold increases in TEWL observed clinically in compromised skin.

Follow-up to the ECVAM prevalidation study on in vitro tests for acute skin irritation. The European Centre for the Validation of Alternative Methods Skin Irritation Task Force report 2.

The European Centre for the Validation of Alternative Methods (ECVAM) Skin Irritation Task Force was established in 1996, to review the status of the development and validation of alternative tests for skin irritation and corrosion, and to identify appropriate non-animal tests for predicting human skin irritation that were sufficiently well-developed to be prevalidated and validated by ECVAM. The EpiDerm method, based on a reconstituted human skin model, was proposed as being sufficiently well advanced to enter a prevalidation (PV) study. Based on a review of test protocols, prediction models (PMs), and data submitted by test developers on ten specified chemicals, with 20% sodium lauryl sulphate as a reference standard, the task force recommended the inclusion of four other tests: EPISKIN and PREDISKIN, based on reconstituted human epidermis or on human skin; the non-perfused pig-ear test, based on pig skin; and the skin integrity function test (SIFT), with ex vivo mouse skin. The prevalidation study on these methods was funded by ECVAM, and took place during 1999-2000. The outcome of the PV study was that none of the methods was ready to enter a formal validation study, and that the protocols and PMs of the methods had to be improved in order to increase their predictive abilities. Improved protocols and PMs for the EpiDerm and EPISKIN methods, the pig ear test, and the SIFT were presented at an extended Task Force meeting held in May 2001. It was agreed that, in the short term, the performance of the revised and harmonised EpiDerm and EPISKIN methods, as well as the modified SIFT, should be evaluated in a further study with a new set of 20 test chemicals. In addition, it was decided that the SIFT and the pig ear test would be compared to see if common endpoints (transepidermal water loss, methyl green-pyronine stain) could be identified.

Skin sensitization potency of methyl methacrylate in the local lymph node assay: comparisons with guinea-pig data and human experience.

There is compelling evidence that contact allergens differ substantially (by 4 or 5 orders of magnitude) with respect to their inherent skin-sensitizing potency. Relative potency can now be measured effectively using the mouse local lymph node assay (LLNA) and such data form the basis of risk assessment and risk management strategies. Such determinations also facilitate distinctions being drawn between the prevalence of skin sensitization to a particular contact allergen and inherent potency. The distinction is important because chemicals that are implicated as common causes of contact allergy are not necessarily potent sensitizers. One example is provided by nickel that is undoubtedly a common cause of allergic contact dermatitis, but is a comparatively weak sensitizer in predictive tests. In an attempt to explore other examples of contact allergens where there may exist a discrepancy between prevalence and potency, we describe here analyses conducted with methyl methacrylate (MMA). Results of LLNA studies have been interpreted in the context of historical clinical data on occupational allergic contact dermatitis associated with exposure to MMA.