Skin Sensitization: Modeling Based on Skin Metabolism Simulation and Formation of Protein Conjugates

A quantitative structure-activity relationship (QSAR) system for estimating skin sensitization potency has been developed that incorporates skin metabolism and considers the potential of parent chemicals and/or their activated metabolites to react with skin proteins. A training set of diverse chemicals was compiled and their skin sensitization potency assigned to one of three classes. These three classes were, significant, weak, or nonsensitizing. Because skin sensitization potential depends upon the ability of chemicals to react with skin proteins either directly or after appropriate metabolism, a metabolic simulator was constructed to mimic the enzyme activation of chemicals in the skin. This simulator contains 203 hierarchically ordered spontaneous and enzyme controlled reactions. Phase I and phase II metabolism were simulated by using 102 and 9 principal transformations, respectively. The covalent interactions of chemicals and their metabolites with skin proteins were described by 83 reactions that fall within 39 alerting groups. The SAR/QSAR system developed was able to correctly classify about 80% of the chemicals with significant sensitizing effect and 72% of nonsensitizing chemicals. For some alerting groups, three-dimensional (3D)-QSARs were developed to describe the multiplicity of physicochemical, steric, and electronic parameters. These 3D-QSARs, so-called pattern recognition-type models, were applied each time a latent alerting group was identified in a parent chemical or its generated metabolite(s). The concept of the mutual influence amongst atoms in a molecule was used to define the structural domain of the skin sensitization model. The utility of the structural model domain and the predictability of the model were evaluated using sensitization potency data for 96 chemicals not used in the model building. The TIssue MEtabolism Simulator (TIMES) software was used to integrate a skin metabolism simulator and 3D-QSARs to evaluate the reactivity of chemicals thus predicting their likely skin sensitization potency.

TIMES-SS—A Mechanistic Evaluation of an External Validation Study Using Reaction Chemistry Principles

The TImes MEtabolism Simulator platform used for predicting skin sensitization (TIMES-SS) is a hybrid expert system that was developed at Bourgas University using funding and data from a consortium comprised of industry and regulators. TIMES-SS encodes structure-toxicity and structure-skin metabolism relationships through a number of transformations, some of which are underpinned by mechanistic three-dimensional quantitative structure-activity relationships. Here, we describe an external validation exercise that was recently carried out. As part of this exercise, data were generated for 40 new chemicals in the murine local lymph node assay (LLNA) and then compared with predictions made by TIMES-SS. The results were promising with an overall good concordance (83%) between experimental and predicted values. The LLNA results were evaluated with respect to reaction chemistry principles for sensitization. Additional testing on a further four chemicals was carried out to explore some of the specific reaction chemistry findings in more detail. Improvements for TIMES-SS, where appropriate, were put forward together with proposals for further research work. TIMES-SS is a promising tool to aid in the evaluation of skin sensitization potential under legislative programs such as REACH.

TIMES-SS—A promising tool for the assessment of skin sensitization hazard. A characterization with respect to the OECD validation principles for (Q)SARs and an external evaluation for predictivity

The TImes MEtabolism Simulator platform used for predicting Skin Sensitization (TIMES-SS) is a hybrid expert system that was developed at Bourgas University using funding and data from a Consortium comprising industry and regulators. The model was developed with the aim of minimizing animal testing and to be scientifically valid in accordance with the OECD principles for (Q)SAR validation. TIMES-SS encodes structure-toxicity and structure-skin metabolism relationships through a number of transformations, some of which are underpinned by mechanistic 3D QSARs. Here, we describe the extent to which the five OECD principles are met and in particular the results from an external evaluation exercise that was recently carried out. As part of this exercise, data were generated for 40 new chemicals in the murine local lymph node assay (LLNA) and then compared with predictions made by TIMES-SS. The results were promising with an overall good concordance (83%) between experimental and predicted values. Further evaluation of these results highlighted certain inconsistencies which were rationalized by a consideration of reaction chemistry principles for sensitization. Improvements for TIMES-SS were proposed where appropriate. TIMES-SS is a promising tool to aid in the evaluation of skin sensitization hazard under legislative programs such as REACH.

Molecular modelling of the mouse cytochrome P450 CYP2F2 based on the CYP102 crystal structure template and selective CYP2F2 substrate interactions

The results of homology modelling of the mouse cytochrome P450, CYP2F2, are reported, based on the CYP102 crystallographic template. It is found that selective CYP2F2 substrates are able to fit the putative active site of the enzyme via aromatic pi-pi stacking and, in some cases, hydrogen-bonded interactions. Two alkylnaphthalenes were investigated via the model to evaluate whether they are likely to act as CYP2F2 substrates and, of these, 2-isopropyl-naphthalene was found to fit the putative active site, whereas 2-(2-hexadecyl)naphthalene was unable to do this, due to its bulky side-chain. Consequently, it is possible to utilize homology modelling to evaluate the likelihood of enzyme-substrate selectivity for CYP2F2 and predict routes of metabolism mediated by this enzyme. This procedure can therefore be used to investigate the potential for activation of xenobiotics via this enzyme, especially those related to known CYP2F substrates, such as naphthalene.

Dose-response assessments of Kathon biocide. (II). Threshold prophetic patch testing

Biocidal preservative agents are important in preventing the growth of pathogenic and nonpathogenic organisms in a variety of personal care, household and industrial products. Nearly all effective, commonly used preservative materials possess skin sensitization potential. In a series of 13 prophetic human repeat insult patch tests involving a total of 1450 subjects, no skin sensitization was induced at concentrations of 10 ppm, 6 ppm or 5 ppm (1121 subjects) or at 15 ppm (200 subjects) of a new isothiazolinone preservative mixture (Kathon CG). Delayed contact hypersensitivity was induced in 1 of 84 subjects at 12.5 ppm and in 2 of 45 subjects at 20 ppm. These results add further support to the previously reported conclusion that use of this new biocide at the very low concentrations required for effective preservation of rinse-off products involves an extremely low risk of clinical dermatoses.

Methiocarb as a bird repellent on wine grapes

An experimental bird repellent, rnethiocarb, was evaluated as a spray treatment for reducing damage to ripening grapes at Loxton, South Australia, by blackbirds (Turdus merula) and grey breasted silvereyes (Zosterops lateralis). Methiocarb was applied to three border rows in an effort to protect the entire vineyard from bird damage. Yields and the incidence of bird damage on treated and untreated sides of a vineyard were compared. One application of rnethiocarb at the rate of 1.9 kg a.i. ha-1 four weeks before harvest protected the entire side of the vineyard from damage. Grapes on the treated side of the vineyard were either undamaged, or showed minor pecking; most bunches of grapes in untreated border rows were damaged. Yields from treated border rows were about double those from untreated border rows. Methiocarb residues of 1.3 ppm were recovered from Gordo grapes 28 days after harvest. There was no evidence that methiocarb killed any birds. Before border row treatment with methiocarb can be commercially recommended, more extensive observations need to be made to determine whether birds learn to fly over the treated rows. Also, the effects of methiocarb on fermentation need to be studied.

In vitro and in vivo actions of zinc ion affecting cellular substances which influence host metabolic responses to inflammation

Glycogen-stimulated rabbit peritoneal exudate cells (polymorphonuclear leukocytes, PMN) produce prostaglandins (PG) and substances which induce alterations (mediators) in experimental animals characteristic of host metabolic responses to infection and other acute inflammatory stresses. The effect of Zn2+ on mediator production and PG synthesis was examined because: Zn homeostasis is perturbed during infection, Zn is known to regulate some cellular functions, and there appears to be an interrelationship between PG synthesis and mediator production. Using exudate cells, 2 mM Zn2+ caused complete inhibition of in vitro PG synthesis as assessed by conversion of [1-14C] arachidonic acid into PG. This concentration of Zn2+ also inhibited production of substances mediating plasma Zn depression, hepatic amino acid "uptake", fever, and neutrophil release from bone marrow. Conversely, Zn2+ did not inhibit in vivo metabolic responses to these mediators. Zn-pretreatment of rabbits or simultaneous injection of Zn2+ and crude PMN-derived pyrogenic activity resulted in prolongation of fever. It is suggested that this action of Zn2+ may be attributed to either stabilization of cyclic AMP through inhibition of phosphodiesterase or a Zn-mediator interaction which stabilizes crude endogenous pyrogen. The potential physiological significance of these results includes: possible potentiation of the host's defense mechanisms by Zn2+ and its utilization for prolongation of fever to determine its effect on potentially temperature-dependent host defense mechanisms.

Temporal and regional differences in brain concentrations of lithium in rats

Lithium (Li) concentrations have been estimated in several peripheral tissues (serum, heart, kidney, liver and skeletal muscle) and different brain areas (cortex, caudate nucleus, hypothalamus, diencephalon, midbrain, pons-medulla and cerebellum) at different intervals (0.5 to 48 h) after administrations of 3 mEq/kg of lithium chloride (LiCl). The Li concentrations in the brain areas increased gradually, reaching their peak at 8 h; the hypothalamus showed the highest concentration measured between 0.5 and 8 h; after 8 h the highest concentration was observed in the caudate nucleus, followed approximately by those of the cortex, rest of the diencephalon, and other areas. Concentration of Li in the skeletal muscle, although initially less, was maintained at a higher percentage of the initial level for more than 24 h compared to the other peripheral tissues.

Interactions of adrenergic stimulants and blockers on self-stimulation behavior in rats

Self-stimulation behavior in rats was facilitated by two adrenergic stimulants, amphetamine (0.5 or 1 mg/kg, intraperitoneal, i.p., or 100 mug, intracerebroventricular, i.c.v.) and cocaine (5 mg/kg, i.p.). Three alpha-adrenergic blockers (phenoxybenzamine, dibenamine, phentolamine) and a beta-adrenergic blocker (propranolol) decreased self-stimulation responding at 100 mug i.c.v. doses, but showed very little effect at small i.p. doses. Pretreatment with alpha- and beta-adrenergic blockers (i.c.v.) also decreased amphetamine-facilitated responding. The effects of amphetamine or cocaine (i.p.) were not significantly altered by these blockers at the doses used. The depressant effects of the alpha- and beta-adrenergic blockers on self-stimulation behavior appear to be nonspecific with respect to the type of adrenergic receptors.