Larry P. Sheets

A Tiered Approach to Life Stages Testing for Agricultural Chemical Safety Assessment

Aproposal has been developed by the Agricultural Chemical Safety Assessment (ACSA) Technical Committee of the ILSI Health and Environmental Sciences Institute (HESI) for an improved approach to assessing the safety of crop protection chemicals. The goal is to ensure that studies are scientifically appropriate and necessary without being redundant, and that tests emphasize toxicological endpoints and exposure durations that are relevant for risk assessment. The ACSA Life Stages Task Force proposes a tiered approach to toxicity testing that assesses a compounds potential to cause adverse effects on reproduction, and that assesses the nature and severity of effects during development and adolescence, with consideration of the sensitivity of the elderly. While incorporating many features from current guideline studies, the proposed approach includes a novel rat reproduction and developmental study with enhanced endpoints and a rabbit development study. All available data, including toxicokinetics, ADMEdata, and systemic toxicity information, are considered in the design and interpretation of studies. Compared to existing testing strategies, the proposed approach uses fewer animals, provides information on the young animal, and includes an estimation of human exposure potential for making decisions about the extent of testing required.

A Retrospective Performance Assessment of the Developmental Neurotoxicity Study in Support of OECD Test Guideline 426

Objective We conducted a review of the history and performance of developmental neurotoxicity (DNT) testing in support of the finalization and implementation of Organisation of Economic Co-operation and Development (OECD) DNT test guideline 426 (TG 426). Information sources and analysis In this review we summarize extensive scientific efforts that form the foundation for this testing paradigm, including basic neurotoxicology research, interlaboratory collaborative studies, expert workshops, and validation studies, and we address the relevance, applicability, and use of the DNT study in risk assessment. Conclusions The OECD DNT guideline represents the best available science for assessing the potential for DNT in human health risk assessment, and data generated with this protocol are relevant and reliable for the assessment of these end points. The test methods used have been subjected to an extensive history of international validation, peer review, and evaluation, which is contained in the public record. The reproducibility, reliability, and sensitivity of these methods have been demonstrated, using a wide variety of test substances, in accordance with OECD guidance on the validation and international acceptance of new or updated test methods for hazard characterization. Multiple independent, expert scientific peer reviews affirm these conclusions.

Comparative functional observational battery study of twelve commercial pyrethroid insecticides in male rats following acute oral exposure.

Twelve commercial pyrethroid insecticides (technical-grade active ingredients) were evaluated individually for acute neurobehavioral manifestations of toxicity under conditions suited to assist with determining whether they act by a common mechanism of toxicity. The pyrethroids that were tested reflect a diversity of structures, including six with an alpha-cyano phenoxybenzyl moiety (beta-cyfluthrin, lambda-cyhalothrin, cypermethrin, deltamethrin, esfenvalerate and fenpropathrin) and six without this moiety (bifenthrin, S-bioallethrin, permethrin, pyrethrins, resmethrin and tefluthrin). These chemicals also present a variety of behavioral effects, including ones that are historically classified as causing a T (tremor), CS (choreoathetosis with salivation) or intermediate syndrome of intoxication, and others that have not previously been classified. Each pyrethroid that was tested consisted of the complement of isomers that occur in commercial products--a key factor for relevance for environmental and human exposure and for comparisons, since the biological activity of the individual isomers can vary tremendously. Young-adult male Sprague-Dawley rats (10 per dose group) were administered a single dose of pyrethroid by oral gavage, in corn oil, at a volume of 5 ml/kg. Each was tested at a range of two or three dose levels, including a minimally toxic dose, to establish the more sensitive manifestations of toxicity, and a more toxic dose, to establish a more complete spectrum of neurobehavioral manifestations. Animals were evaluated using a functional observational battery (FOB) that was designed to characterize and distinguish effects classically associated with T or CS syndromes of intoxication. The FOB was performed when manifestations of toxicity were most apparent at the time of peak effect (2, 4, or 8 h post-dosing) by observers who were blinded to dose group assignment, thus avoiding possible bias. The results from this study indicate that some pyrethroids clearly exhibit the historic classification symptoms of the T and CS syndromes while others do so less obviously. Use of the statistical technique of Principal Component Analysis (PCA) further helped interpret the study findings, as described in the accompanying paper (Breckenridge et al., 2009). These results establish manifestations of neurotoxicity in vivo that can be used as weight of evidence to determine whether pyrethroid insecticides act through a common mechanism of toxicity in mammals. Based on a review of the FOB data, analyzed by PCA, and other published data, two common mechanism groups are proposed. Group 1 would include pyrethrins, bifenthrin, resmethrin, permethrin, S-bioallethrin and tefluthrin. Group 2 would include cypermethrin, deltamethrin, esfenvalerate, beta-cyfluthrin and lambda-cyhalothrin. Fenpropathrin exhibited features of both groups.

Chronic toxicity and carcinogenicity testing in the Sprague–Dawley rat of a prospective insect repellant (KBR 3023) using the dermal route of exposure

The chronic toxicology and carcinogenic potential of 1-(1-methyl-propoxycarbonyl)-2-(2-hydroxyethyl)-piperidine (KBR 3023), a prospective new insect repellent intended for human use, was studied in rats using the dermal route of application. Relying upon the toxicology profile that emerged in the subchronic rat bioassay that was conducted using dermally applied dosages of 0, 80, 200, 500 and 1000 mg KBR 3023/kg body wt/day, it was determined, in concert with the Environmental Protection Agency (EPA), that dermally applied dosages of 0, 50, 100 or 200 mg KBR 3023/kg body wt/day would be used in the conduction of all definitive forms of subchronic, chronic, and lifetime descriptive testing performed with the chemical. Using this testing approach, the specific results of this 2-year study are as follows. All in-life parameters, which included body weight, food consumption, clinical observations, survival, ophthalmology, clinical chemistry, hematology, and urinalysis, were unaffected by exposure to KBR 3023. Similarly, postmortem analyses, which included organ weights and gross pathology, were also unchanged following exposure to KBR 3023. Histopathology at the dose site/skin was characterized by a pattern of acanthosis and/or hyperkeratosis across all doses in 1- and 2-year rats. Beyond the dosing site, cystic degeneration of the liver was described in 2-year 200-mg KBR 3023/kg body wt/day males. No other compound-related non-dosing site lesion was identified at any dose tested. No evidence of a compound-induced neoplasia was suggested in this bioassay.

Carcinogenicity testing in the CD-1 mouse of a prospective insect repellant (KBR 3023) using the dermal route of exposure

The carcinogenic potential of 1-(1-methyl-propoxycarbonyl)-2-(2-hydroxyethyl)-piperidine (KBR 3023), a prospective new insect repellent intended for human use, was studied in mice using the dermal route of application. Relying upon the toxicology profile that emerged in the subchronic rat bioassay that was conducted using dermally applied dosages of 0, 80, 200, 500, and 1000 mg KBR 3023/kg body weight per day, it was determined, in concert with the EPA, that dermally applied dosages of 0, 50, 100, or 200 mg KBR 3023/kg body weight per day would be used in the conduct of all definitive forms of subchronic, chronic, and lifetime descriptive testing performed with the chemical. Using this testing approach, the specific results of this 18-month study are as follows. All in-life parameters, which included body weight, food consumption, clinical observations, survival, and hematology were unaffected by exposure to KBR 3023. Similarly, postmortem analyses, which included organ weights and gross pathology, and histopathology were also unchanged following exposure to KBR 3023. No evidence of a compound-induced neoplasia was suggested in this bioassay.

CHAPTER 54 – Imidacloprid: A Neonicotinoid Insecticide

Imidacloprid is the principal representative of a new pesticide class, the neonicotinoid insecticides. These insecticides are designed to act on nicotinic receptors to control insect pests and, at the same time, to be relatively nontoxic to vertebrate species. Imidacloprid has been evaluated for mutagenicity using a full complement of in vitro and in vivo tests that is required for registration. The results from this database indicate that imidacloprid is not mutagenic. Acute exposure to imidacloprid was determined to produce minimal evidence of toxicity by dermal and inhalation routes of exposure, and moderate acute toxicity by oral administration. Imidacloprid is not an irritant and does not produce evidence of dermal sensitization. Imidacloprid is absorbed and widely distributed to organs within one hour following oral administration to rats. Whole body autoradiography indicates that imidacloprid is not distributed to fatty tissues, to tissues in the central nervous system (CNS), or to the mineral components of bone. By oral administration, imidacloprid is rapidly absorbed, metabolized in the liver, and excreted primarily via the urine. Results from long-term dietary-exposure studies support rapid metabolism, with little evidence of cumulative toxicity and minimal effects, even at maximum-tolerated doses. Imidacloprid is not mutagenic or carcinogenic.

Recommendations for harmonization of data collection and analysis of developmental neurotoxicity endpoints in regulatory guideline studies: Proceedings of workshops presented at Society of Toxicology and joint Teratology Society and Neurobehavioral Teratology Society meetings

The potential for developmental neurotoxicity (DNT) of environmental chemicals may be evaluated using specific test guidelines from the US Environmental Protection Agency or the Organisation for Economic Cooperation and Development (OECD). These guidelines generate neurobehavioral, neuropathological, and morphometric data that are evaluated by regulatory agencies globally. Data from these DNT guideline studies, or the more recent OECD extended one-generation reproductive toxicity guideline, play a pivotal role in childrens health risk assessment in different world areas. Data from the same study may be interpreted differently by regulatory authorities in different countries resulting in inconsistent evaluations that may lead to inconsistencies in risk assessment decisions internationally, resulting in regional differences in public health protection or in commercial trade barriers. These issues of data interpretation and reporting are also relevant to juvenile and pre-postnatal studies conducted more routinely for pharmaceuticals and veterinary medicines. There is a need for development of recommendations geared toward the operational needs of the regulatory scientific reviewers who apply these studies in risk assessments, as well as the scientists who generate DNT data sets. The workshops summarized here draw upon the experience of the authors representing government, industry, contract research organizations, and academia to discuss the scientific issues that have emerged from diverse regulatory evaluations. Although various regulatory bodies have different risk management decisions and labeling requirements that are difficult to harmonize, the workshops provided an opportunity to work toward more harmonized scientific approaches for evaluating DNT data within the context of different regulatory frameworks. Five speakers and their coauthors with neurotoxicology, neuropathology, and regulatory toxicology expertise discussed issues of variability, data reporting and analysis, and expectations in DNT data that are encountered by regulatory authorities. In addition, principles for harmonized evaluation of data were suggested using guideline DNT data as case studies.