An F1-Extended One-Generation Reproductive Toxicity Study in Crl:CD(SD) Rats With 2,4-Dichlorophenoxyacetic Acid

Abstract

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 (16.6mg/kg/day) and females (20.6mg/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 evaluation of 2,4-D potential for interactions with the estrogen, androgen and thyroid pathways and steroidogenesis

“…There were no effects in the Marty et al (2010) EOGRT study (published in Marty et al 2013) considered to reflect an androgenic or anti-androgenic mode of action. Although there were several study findings that could support potential anti-androgenicity, these were found either not exposurerelated, not replicated across generations, or were attributable to other factors such as randomization artifacts.…”

“…The single exception to the guidelines for these assays was that it was considered appropriate to limit the high concentration in the first four EDSP in vitro assays to 100 lM, rather than the guideline-recommended 1 mM, because the 100 lM concentration was equivalent to serum concentrations at or slightly above the TSRC (slightly above the inflection point for non-linear TK) in rats dosed with 2,4-D in the diet in the EOGRT study (Marty et al 2010;Marty et al 2013;Saghir et al 2013). As noted previously, responses seen only in the non-linear TK range are not regarded as relevant to human risk assessment.…”

“…No Hershberger or male pubertal assays were required for 2,4-D because of the recently completed EOGRT study Marty et al (2010) (published in Marty et al 2013). The EOGRT study provides information on all endpoints included in the male pubertal assay, with the exception of serum testosterone levels, and provides additional endpoints sensitive to androgen deficiency not assessed in the pubertal study including AGD and nipple retention in males.…”

“…As noted in Marty et al 2013, "These endpoints are considered highly sensitive to altered androgen status (Clark 1999;McIntyre et al 2001;Wolf et al 2002;Hotchkiss et al 2004). "…”

“…Based on its production volume, 2,4-D was recently screened using the US Environmental Protection Agency (US EPA) Tier I Endocrine Disruptor Screening Program (EDSP) assays for potential interactions with the estrogen, androgen, thyroid (EAT) pathways or with steroidogenesis (Coady et al 2013. The endocrine activity of 2,4-D also has been comprehensively evaluated in a state-of-art apical in vivo extended one-generation reproduction study (EOGRT; Marty et al 2013). Overall, the toxicological, epidemiological, exposure and biomonitoring information available for 2,4-D represents a useful and comprehensive dataset to further assess the potential for endocrine interactions.…”