Bradley Collins

Embryonic zebrafish represent a useful test system to screen substances for their ability to perturb development. The exposure scenarios, endpoints captured, and data analysis vary among the laboratories who conduct screening. A lack of harmonization impedes the comparison of the substance potency and toxicity outcomes across laboratories and may hinder the broader adoption of this model for regulatory use. The Systematic Evaluation of the Application of Zebrafish in Toxicology (SEAZIT) initiative was developed to investigate the sources of variability in toxicity testing. This initiative involved an interlaboratory study to determine whether experimental parameters altered the developmental toxicity of a set of 42 substances (3 tested in duplicate) in three diverse laboratories. An initial dose-range-finding study using in-house protocols was followed by a definitive study using four experimental conditions: chorion-on and chorion-off using both static and static renewal exposures. We observed reasonable agreement across the three laboratories as 33 of 42 test substances (78.6%) had the same activity call. However, the differences in potency seen using variable in-house protocols emphasizes the importance of harmonization of the exposure variables under evaluation in the second phase of this study. The outcome of the Def will facilitate future practical discussions on harmonization within the zebrafish research community.

Phenolphthalein (PTH), which has been used as the active ingredient in a number of prescription and over-the-counter laxative products, is a rodent chemical carcinogen in multiple organs in the NTP 2-year bioassay at doses of 291–2927 mg/kg. This paper describes the toxicokinetics and estimates the internal dose of PTH administered as a single iv or gavage dose, or ad libitum for 14 days in feed to F344 rats, B6C3F1 mice, p53 (+/–) mice, and C57BL mice at doses that bracketed those used in the bioassay. Plasma concentrations for free phenolphthalein (PTH-F) and phenolphthalein glucuronide (PTH-G) were obtained for each dose regimen. Total phenolphthalein (PTH-T) was calculated as the sum of the molar concentrations of PTH-F and PTH-G. Noncompartmental pharmacokinetic models were used to calculate the area under the curve (AUC) from 0 h to infinity (AUC_∞), clearance (Cl), and oral bioavailability (F) for PTH-F; and were used to calculate AUC_∞, t_½, and relative absorption (Q) for PTH-T. After iv administration, PTH-F rapidly declined in rats and mice; PTH-T rose rapidly to Cmax and slowly declined 6–8 h after dosing, with no sex-related differences for rats or mice. For feed studies, mean plasma concentration (C̄_∞) and 24-h area under the curve (AUC_24h) values were calculated. Results from feed studies showed no dose response in rat plasma PTH-F above ∼50 mg/kg. Rat PTH-T AUC_24h and C̄_∞ were linear with doses up to ∼650 mg/kg. In B6C3F1 mice, PTH-F and PTH-T AUC_24h increased nonlinearly with doses above ∼165 mg/kg. PTH is well absorbed and readily converted to PTH-G when administered in feed to rats and mice, except at the highest bioassay doses, where PTH absorption may be saturated.