Abstract

Background: 2-Ethylhexyl diphenyl phosphate (EHDP) is an organophosphate flame retardant currently on the market that is used as a replacement for phased-out polybrominated diphenyl ethers. Toxicological information on this class of chemicals is sparse. A short-term, in vivo transcriptomic study was used to assess the biological potency of EHDP.

Methods: Scientists at the Division of Translational Toxicology, National Institute of Environmental Health Sciences conducted this short-term in vivo biological potency study on EHDP in young adult male Sprague Dawley (Hsd:Sprague Dawley® SD®) rats. EHDP was formulated in corn oil and administered once daily for 4 consecutive days by gavage. EHDP was tested at six doses (0, 61, 123, 245, 489, and 979 mg/kg body weight [mg/kg] corresponding to 0, 0.169, 0.338, 0.675, 1.35, and 2.7 mmol/kg). On study day 4, animals were euthanized, standard toxicological measures were assessed, and the liver was assayed in gene expression studies using Affymetrix microarrays. Modeling was conducted to identify the benchmark doses (BMDs) associated with apical toxicological endpoints and transcriptional changes in the liver. A benchmark response of one standard deviation was used to model all endpoints.

Results: Several clinical pathology and organ weight measurements showed dose-related changes from which BMD values could be obtained. The effects include decreased serum bile salt/acid concentration, increased serum high-density lipoprotein cholesterol concentration, and increased relative liver weight. The BMDs and benchmark dose lower confidence limits (BMDLs) were 20.3 (9.3), 36.8 (18.3), and 39.4 (17.2), respectively. Although serum cholinesterase activity was significantly decreased in all dosed groups (31%–54% decrease), beginning with 61 mg/kg (the lowest-observed-effect level), its BMD value was below the lower limit of extrapolation (<20.3 mg/kg).

Two Gene Ontology biological processes had BMD median values <20.3 mg/kg, which relate to prostaglandin metabolic process and xenobiotic catabolic process. The most sensitive gene sets for which a reliable estimate of the BMD could be made were long-chain fatty acid biosynthetic process and glutathione metabolic process with median BMDs of 23.8 and 29.4 mg/kg, respectively, and both with a median BMDL of 15.2 mg/kg. Four individual genes, Ddit4, Ugt2b17, Ces2c, and Gstt3, had BMD values <20.3 mg/kg. All four genes were upregulated. The most sensitive upregulated genes with reliable BMD estimates included Aldh1a1, Cryl1, Me1, and App with BMDs (BMDLs) of 21.0 (12.8), 23.0 (14.0), 23.6 (14.5), and 28.8 (18.4) mg/kg, respectively. The most sensitive downregulated genes with reliable BMD estimates were Slc34a2 and Slc6a6 with BMDs (BMDLs) of 25.8 (14.9) and 30.7 (20.2) mg/kg, respectively.

Summary: Taken together, the most sensitive gene set BMD (BMDL) median, individual gene BMD (BMDL), and apical endpoint BMD (BMDL) values that could be reliably determined occurred at 23.8 (15.2), 21.0 (12.8), and 20.3 (9.3) mg/kg, respectively. The BMDs (BMDLs)could not be determined for two gene sets and four individual genes and were estimated to be <20.3 mg/kg. Serum cholinesterase inhibition was also estimated to be <20.3 mg/kg. Future studies investigating lower doses would be helpful to obtain more accurate estimates of BMD values for the most sensitive gene sets and genes and for cholinesterase inhibition.

Citation: Auerbach SS, Behl MV, Collins BJ, Cora MC, Fostel JM, Liu YF, Luh J, Roberts GK, Shipkowski KA, Waidyanatha S, Watson ATD. 2022. NIEHS report on the in vivo repeat dose biological potency study of 2-ethylhexyl diphenyl phosphate (CASRN 1241-94-7) in male Sprague Dawley (Hsd:Sprague Dawley^® SD^®) rats (gavage studies). Research Triangle Park, NC: National Institute of Environmental Health Sciences. NIEHS Report 02. [https://doi.org/10.22427/NIEHS-02 Citation: Auerbach SS, Behl MV, Collins BJ, Cora MC, Fostel JM, Liu YF, Luh J, Roberts GK, Shipkowski KA, Waidyanatha S, Watson ATD. 2022. NIEHS report on the in vivo repeat dose biological potency study of 2-ethylhexyl diphenyl phosphate (CASRN 1241-94-7) in male Sprague Dawley (Hsd:Sprague Dawley^® SD^®) rats (gavage studies). Research Triangle Park, NC: National Institute of Environmental Health Sciences. NIEHS Report 02.]