NIEHS Report on the In Vivo Repeat Dose Biological Potency Studies of 1,4-Dichlorobenzene (CASRN 106-46-7) in Female Sprague Dawley (Hsd:Sprague Dawley® SD®) Rats and B6D2F1/Crl Mice (Whole-body Inhalation Studies)

Abstract

Background: 1,4‑Dichlorobenzene (1,4‑DCB) is a chlorinated aromatic hydrocarbon that is used in various industrial and household applications, including as a fumigant for moth control, a deodorizer in urinal cakes, and an air freshener in domestic and public settings. Short-term in vivo transcriptomic studies were used to assess the biological potency of 1,4‑DCB. The data from these studies are intended to support risk assessment and establishment of acceptable exposure levels of 1,4‑DCB in environmental and occupational settings.

Methods: Short-term in vivo biological potency studies on 1,4‑DCB in adult female Sprague Dawley (Hsd:Sprague Dawley® SD®) rats and B6D2F1/Crl mice were conducted. Animals were exposed to 1,4‑DCB via whole-body inhalation for 6 hours plus the time to achieve 90% of the target concentration after the beginning of vapor generation (T90) per day for 5 consecutive days (study days 0–4) at exposure concentrations of 0, 1, 10, 50, 150, 400, or 800 ppm for rats and 0, 1, 10, 50, 150, or 400 ppm for mice. Blood was collected from animals dedicated to internal concentration assessment in all groups. On study day 5, the day after the final day of exposure, animals were euthanized, standard toxicological measures were assessed, and the heart, kidney, liver, lung, and ovary were assayed in gene expression studies using the TempO-Seq assay. Modeling was conducted to identify the benchmark doses (BMDs) associated with apical toxicological endpoints and transcriptional changes in the heart, kidney, liver, lung, and ovary. A benchmark response of 1 standard deviation from the mean was used to model all apical endpoints, whereas a benchmark response set to a 25% change in the median response was used to model the gene expression data.

Results: Several clinical pathology and organ weight measurements showed exposure-related changes from which BMD values were calculated. In rats, the effects, and their BMDs and benchmark dose lower confidence limits [BMDLs] in ppm, included significantly increased relative liver weight (191.4 [127.6]), increased relative left kidney weight (219.2 [89.4]), increased absolute liver weight (245.5 [163.4]), increased cholesterol concentration (259.4 [126.2]), increased absolute left kidney weight (259.8 [93.3]), increased reticulocyte count (437.0 [307.0]), and increased triglyceride concentration (489.4 [332.3]). In mice, the effects, and their BMDs (BMDLs) in ppm, included significantly decreased albumin/globulin ratio (15.1 [2.2]), increased relative liver weight (70.2 [47.4]), increased cholesterol concentration (71.0 [43.1]), and increased absolute liver weight (83.7 [59.2]).

Following the last exposure (study day 4), 1,4-DCB concentrations in rats increased proportionally to the exposure concentration up to 150 ppm in blood, 50 ppm in liver, and 400 ppm in lung. At higher exposure concentrations, blood, liver, and lung concentrations increased more than proportionally to the exposure concentration, demonstrating saturation of metabolism and/or clearance processes. In mice, 1,4-DCB concentrations increased proportionally to the exposure concentration in blood and tissues up to 10 ppm. At exposure concentrations >50 ppm, blood and tissue concentration increased more than proportionally to the exposure concentration. In general, 1,4-DCB tissue concentrations were similar to blood concentrations in rats and mice, demonstrating low tissue distribution and/or retention. Blood and tissue concentrations on the day after the last exposure (study day 5) were much lower than those on the last day of exposure (study day 4) in both rats and mice, demonstrating rapid elimination of 1,4-DCB. In general, when normalized to the theoretical inhaled dose, rats had higher blood and tissue concentrations of 1,4-DCB than mice at the lower exposure concentrations, although the difference was smaller at 400 ppm.

In both rats and mice, no Gene Ontology (GO) biological process in the heart, kidney, liver, lung, or ovary had BMD values below the lower limit of extrapolation (<0.333 ppm). The most sensitive gene sets for which a reliable estimate of the BMD could be made are given below for each tissue and species, with their BMDs (BMDLs) in ppm.

In the heart, the most sensitive gene sets in rats were cell cycle phase transition (141.4 [55.9]) and mitotic cell cycle phase transition (141.4 [55.9]), whereas in mice, the most sensitive gene set was organic hydroxy compound biosynthetic process (91.0 [25.1]). In the kidney, the most sensitive gene sets in rats were cell cycle phase transition (121.0 [42.2]) and mitotic cell cycle phase transition (121.0 [42.2]), and in mice, the most sensitive gene set was circadian rhythm (16.9 [2.0]). In the liver, the most sensitive gene set in rats was xenobiotic metabolic process (87.6 [59.6]), whereas the most sensitive gene sets in mice were regulation of chromosome segregation (6.3 [0.9]), positive regulation of cell cycle phase transition (6.3 [0.9]), and positive regulation of mitotic cell cycle phase transition (6.3 [0.9]). In the lung, the most sensitive gene sets in rats and mice were chromosome segregation (208.6 [55.4]) and DNA replication (1.4 [0.5]), respectively. In the ovary, the most sensitive gene set in rats was ameboidal-type cell migration (223.1 [90.9]). There were no active GO terms (i.e., no gene expression response at the gene set level) in the ovaries of mice exposed to 1,4‑DCB.

Summary: Taken together, in rats, the most sensitive gene set and apical endpoint BMD (BMDL) values that could be reliably determined occurred at 87.6 (59.6) and 191.4 (127.6) ppm, respectively. In mice, the most sensitive gene set and apical endpoint BMD (BMDL) values that could be reliably determined occurred at 1.4 (0.5) and 15.1 (2.2) ppm, respectively.

Official citation: Auerbach SS, Cora MC, Liu YF, Luh J, Prince LM, Roberts GK, Shipkowski KA, Waidyanatha S. 2026. NIEHS report on the in vivo repeat dose biological potency studies of 1,4-dichlorobenzene (CASRN 106-46-7) in female Sprague Dawley (Hsd:Sprague Dawley® SD®) rats and B6D2F1/Crl mice (whole-body inhalation studies). Research Triangle Park, NC: National Institute of Environmental Health Sciences. NIEHS Report 13.