NIEHS Report on the In Vivo Repeat Dose Biological Potency Studies of 1,2-Dichlorobenzene (CASRN 95-50-1) in Female Sprague Dawley (Hsd:Sprague Dawley® SD®) Rats and B6D2F1/Crl Mice (Whole-body Inhalation Studies)

Abstract

Background: 1,2‑Dichlorobenzene (1,2‑DCB) is a chlorinated aromatic hydrocarbon that is widely used in industrial applications, including as a solvent for waxes, resins, and paints, and it serves as an ingredient in the production of disinfectants and deodorants. Short-term in vivo transcriptomic studies were used to assess the biological potency of 1,2‑DCB. The data from these studies are intended to support risk assessment and establishment of acceptable exposure levels of 1,2‑DCB in environmental and occupational settings.

Methods: Short-term in vivo biological potency studies on 1,2‑DCB in adult female Sprague Dawley (Hsd:Sprague Dawley® SD®) rats and B6D2F1/Crl mice were conducted. Animals were exposed via whole-body inhalation to 1,2‑DCB vapor 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, 30, 100, 250, or 500 ppm for rats and 0, 1, 10, 30, 100, or 250 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, body weight, 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 (57.9 [29.9]), increased relative right kidney weight (93.1 [43.2]), increased relative left kidney weight (101.4 [40.9]), increased absolute liver weight (114.0 [41.8]), increased bile salt/acid concentration (143.8 [45.9]), increased albumin/globulin ratio (182.3 [99.4]), decreased terminal body weight (227.1 [116.4]), increased cholesterol concentration (238.4 [147.3]), decreased body weight gain over study duration (273.4 [215.6]), decreased body weight on study day 4 (298.0 [137.9]), and decreased glucose concentration (360.3 [183.9]). In mice, the effects, and their BMDs (BMDLs) in ppm, included significantly decreased alkaline phosphatase activity (33.4 [10.9]), increased absolute liver weight (57.4 [36.4]), increased relative liver weight (60.5 [37.7]), increased sorbitol dehydrogenase activity (93.0 [56.8]), decreased body weight on study day 2 (107.8 [62.4]), decreased body weight on study day 3 (118.5 [69.5]), decreased body weight on study day 4 (133.3 [82.8]), increased alanine aminotransferase activity (157.8 [108.9]), decreased body weight gain over study duration (159.8 [115.4]), and increased relative right kidney weight (161.5 [82.3]).

In rats and mice, 1,2-DCB blood, liver, and lung concentrations following the last exposure (study day 4) increased proportionally to the exposure concentration up to 100 ppm. At higher exposure concentrations, blood and tissue concentrations increased more than proportionally to the exposure concentration, demonstrating saturation of metabolism and/or clearance processes above 100 ppm. In general, tissue concentrations were similar to blood concentrations, 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), demonstrating rapid elimination of 1,2-DCB. In general, when normalized to the theoretical inhaled dose, there were no apparent species differences in blood and tissue concentrations of 1,2-DCB.

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 set in rats was cellular response to hydrogen peroxide (148.7 [58.2]), whereas in mice, the most sensitive gene set was cellular modified amino acid metabolic process (4.4 [1.7]). In the kidney, the most sensitive gene sets in rats and mice were embryonic organ development (114.0 [73.9]) and organic hydroxy compound transport (2.3 [0.6]), respectively. In the liver, the most sensitive gene set in rats was xenobiotic metabolic process (29.8 [25.3]) and the most sensitive gene set in mice was positive regulation of lipid biosynthetic process (1.5 [0.5]). In the lung, the most sensitive gene sets in rats were detoxification (175.9 [109.3]), cellular oxidant detoxification (175.9 [109.3]), and cellular detoxification (175.9 [109.3]), whereas in mice, the most sensitive gene set was unsaturated fatty acid metabolic process (2.1 [0.7]). In the ovary, the most sensitive gene sets in rats were kidney development (156.7 [50.8]), gonad development (156.7 [50.8]), and male gonad development (156.7 [50.8]), and the most sensitive gene set in mice was hemopoiesis (108.0 [35.9]).

Summary: Taken together, in rats, the most sensitive gene set and apical endpoint BMD (BMDL) values that could be reliably determined occurred at 29.8 (25.3) and 57.9 (29.9) ppm, respectively. In mice, the most sensitive gene set and apical endpoint BMD (BMDL) values that could be reliably determined occurred at 1.5 (0.5) and 33.4 (10.9) 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 1,2-dichlorobenzene (CASRN 95-50-1) 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 12.