Mechanistic Investigation of Toxaphene Induced Mouse Liver Tumors.

Chronic exposure to toxaphene resulted in an increase in liver tumors in B6C3F1 mice. This study was performed to investigate the mode of action of toxaphene induced mouse liver tumors. Following an initial 14 day dietary dose range-finding study in male mice, a mechanistic study (0, 3, 32, and 320 ppm toxaphene in diet for 7, 14, and 28 days of treatment) was performed to examine the potential mechanisms of toxaphene induced mouse liver tumors. Toxaphene induced a significant increase in expression of constitutive androstane receptor (CAR) target genes (Cyp2b10, Cyp3a11) at 32 and 320 ppm toxaphene. aryl hydrocarbon receptor (AhR) target genes (Cyp1a1 and Cyp1a2) were slightly increased in expression at the highest toxaphene dose (320 ppm). No increase in peroxisome proliferator-activated receptor alpha activity or related genes was seen following toxaphene treatment. Lipid peroxidation was seen following treatment with 320 ppm toxaphene. These changes correlated with increases in hepatic DNA synthesis. To confirm the role of CAR in this mode of action, CAR knockout mice (CAR(-/-)) treated with toxaphene confirmed that the induction of CAR responsive genes seen in wild-type mice was abolished following treatment with toxaphene for 14 days. These findings, taken together with previously reported studies, support the mode of action of toxaphene induced mouse liver tumors is through a nongenotoxic mechanism involving primarily a CAR-mediated processes that results in an increase in cell proliferation in the liver, promotes the clonal expansion of preneoplastic lesions leading to adenoma formation.

Relevance weighting of tier 1 endocrine screening endpoints by rank order.

Weight of evidence (WoE) approaches are recommended for interpreting various toxicological data, but few systematic and transparent procedures exist. A hypothesis-based WoE framework was recently published focusing on the U.S. EPA's Tier 1 Endocrine Screening Battery (ESB) as an example. The framework recommends weighting each experimental endpoint according to its relevance for deciding eight hypotheses addressed by the ESB. Here we present detailed rationale for weighting the ESB endpoints according to three rank ordered categories and an interpretive process for using the rankings to reach WoE determinations. Rank 1 was assigned to in vivo endpoints that characterize the fundamental physiological actions for androgen, estrogen, and thyroid activities. Rank 1 endpoints are specific and sensitive for the hypothesis, interpretable without ancillary data, and rarely confounded by artifacts or nonspecific activity. Rank 2 endpoints are specific and interpretable for the hypothesis but less informative than Rank 1, often due to oversensitivity, inclusion of narrowly context-dependent components of the hormonal system (e.g., in vitro endpoints), or confounding by nonspecific activity. Rank 3 endpoints are relevant for the hypothesis but only corroborative of Ranks 1 and 2 endpoints. Rank 3 includes many apical in vivo endpoints that can be affected by systemic toxicity and nonhormonal activity. Although these relevance weight rankings (WREL ) necessarily involve professional judgment, their a priori derivation enhances transparency and renders WoE determinations amenable to methodological scrutiny according to basic scientific premises, characteristics that cannot be assured by processes in which the rationale for decisions is provided post hoc.

Life-stage-, sex-, and dose-dependent dietary toxicokinetics and relationship to toxicity of 2,4-dichlorophenoxyacetic acid (2,4-D) in rats: implications for toxicity test dose selection, design, and interpretation.

Life-stage-dependent toxicity and dose-dependent toxicokinetics (TK) were evaluated in Sprague Dawley rats following dietary exposure to 2,4-dichlorophenoxyacetic acid (2,4-D). 2,4-D renal clearance is impacted by dose-dependent saturation of the renal organic anion transporter; thus, this study focused on identifying inflection points of onset of dietary nonlinear TK to inform dose selection decisions for toxicity studies. Male and female rats were fed 2,4-D-fortified diets at doses to 1600 ppm for 4-weeks premating, <2 weeks during mating, and to test day (TD) 71 to parental (P1) males and to P1 females through gestation/lactation to TD 96. F1 offspring were exposed via milk with continuing diet exposure until postnatal day (PND) 35. As assessed by plasma area under the curve for the time-course plasma concentration, nonlinear TK was observed ≥ 1200 ppm (63 mg/kg/day) for P1 males and between 200 and 400 ppm (14-27 mg/kg/day) for P1 females. Dam milk and pup plasma levels were higher on lactation day (LD) 14 than LD 4. Relative to P1 adults, 2,4-D levels were higher in dams during late gestation/lactation and postweaning pups (PND 21-35) and coincided with elevated intake of diet/kg body weight. Using conventional maximum tolerated dose (MTD) criteria based on body weight changes for dose selection would have resulted in excessive top doses approximately 2-fold higher than those identified incorporating critical TK data. These data indicate that demonstration of nonlinear TK, if present at dose levels substantially above real-world human exposures, is a key dose selection consideration for improving the human relevance of toxicity studies compared with studies employing conventional MTD dose selection strategies.

An F1-extended one-generation reproductive toxicity study in Crl:CD(SD) rats with 2,4-dichlorophenoxyacetic acid.

2,4-Dichlorophenoxyacetic acid (2,4-D) was assessed for systemic toxicity, reproductive toxicity, developmental neurotoxicity (DNT), developmental immunotoxicity (DIT), and endocrine toxicity. CD rats (27/sex/dose) were exposed to 0, 100, 300, 600 (female), or 800 (male) ppm 2,4-D in diet. Nonlinear toxicokinetic behavior was shown at high doses; the renal clearance saturation threshold for 2,4-D was exceeded markedly in females and slightly exceeded in males. Exposure was 4 weeks premating, 7 weeks postmating for P1 males and through lactation for P1 females. F1 offspring were examined for survival and development, and at weaning, pups were divided in cohorts, by sex and dose, and by systemic toxicity (10), DNT (10), DIT (20), and reproductive toxicity (≥ 23). Remaining weanlings were evaluated for systemic toxicity and neuropathology (10-12). Body weight decreased during lactation in high-dose P1 females and in F1 pups. Kidney was the primary target organ, with slight degeneration of proximal convoluted tubules observed in high-dose P1 males and in high-dose F1 males and females. A slight intergenerational difference in kidney toxicity was attributed to increased intake of 2,4-D in F1 offspring. Decreased weanling testes weights and delayed preputial separation in F1 males were attributed to decreased body weights. Endocrine-related effects were limited to slight thyroid hormone changes and adaptive histopathology in high-dose GD 17 dams seen only at a nonlinear toxicokinetic dose. 2,4-D did not cause reproductive toxicity, DNT, or DIT. The "No Observed Adverse Effect Level" for systemic toxicity was 300 ppm in both males (16.6 mg/kg/day) and females (20.6 mg/kg/day), which is approximately 6700- to 93 000-fold higher than that reported for 2,4-D exposures in human biomonitoring studies.

Weight-of-the-evidence review of acrylonitrile reproductive and developmental toxicity studies.

Risk assessment of acrylonitrile (AN) toxicity to humans has focused on potential carcinogenicity and acute toxicity. Epidemiological studies from China reported reproductive and developmental effects in AN workers, including infertility, birth defects, and spontaneous abortions. A weight-of-the-evidence (WoE) evaluation of the AN database assessed study strength, characterized toxicity, and identified no-observed-adverse-effect levels (NOAELs). The epidemiological studies do not demonstrate causality and are not sufficiently robust to be used for risk assessment. Rodent developmental studies showed fetotoxicity and malformations at maternally toxic levels; there was no unique developmental susceptibility. NOAELs for oral and inhalation exposures were 10 mg/kg/day and 12 ppm (6 h/day), respectively. Drinking-water and inhalation reproductive toxicity studies showed no clear effects on reproductive performance or fertility. Maternally toxic concentrations caused decreased pup growth. The drinking-water reproductive NOAEL was 100 ppm (moderate confidence due to study limitations). The inhalation exposure reproductive and neonatal toxicity high confidence NOAEL was 45 ppm (first generation 90 ppm) (6 h/day). The inhalation reproductive toxicity study provides the most robust data for risk assessment. Based on the WoE evaluation, AN is not expected to be a developmental or reproductive toxicant in the absence of significant maternal toxicity.

Risk assessment of toxaphene and its breakdown products: time for a change?

Technical toxaphene (TT) was last used in commerce in about 1982. Any environmental exposure to toxaphene in this century is to environmentally degraded forms of toxaphene, termed weathered toxaphene. Several hundred chlorinated bornane congeners have been identified in technical toxaphene. The degradation of technical toxaphene to weathered toxaphene can result in various congener mixtures, but the primary mode of degradation is dechlorination. The U.S. Environmental Protection Agency (EPA) presently estimates the risk of exposure to toxaphene by relying upon rat and mouse toxicology studies performed on technical toxaphene. No adjustment is made for the dechlorination of toxaphene in the environment. The European Union (EU), however, has modeled toxaphene risks from eating fish with chlorinated bornane residues through a series of studies on toxaphene degraded by either ultraviolet light, or biodegradation in fish. The EU risk assessment relies upon rat liver studies in vivo and mouse in vitro studies on the inhibition of gap junction intercellular communication (GJIC). This article reviews the current state of knowledge of technical and weathered toxaphene toxicology. We discuss the various current methods and opportunities to advance the risk assessment of weathered toxaphene beyond the existing U.S. EPA assessment of technical toxaphene.