Abstract

Background: tert-Butylphenyl diphenyl phosphate (BPDP) 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 BPDP.

Methods: Scientists at the Division of Translational Toxicology, National Institute of Environmental Health Sciences conducted this short-term in vivo biological potency study on BPDP in young adult male Sprague Dawley (Hsd:Sprague Dawley® SD®) rats. BPDP was formulated in corn oil and administered once daily for 4 consecutive days by gavage. BPDP was tested at six doses (0, 65, 129, 258, 516, and 1,033 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 increased relative and absolute liver weights and decreased serum bile salt/acid concentration. The BMDs and benchmark dose lower confidence limits (BMDLs) were 288.7 (223.5), 313.0 (121.5), and 763.1 (434.1) mg/kg, respectively. Although serum cholinesterase activity was significantly decreased in all dosed groups (19%–56% decrease), beginning with 65 mg/kg (the lowest-observed-effect level), a BMD (BMDL) was not determined because no viable model was available.

Two Gene Ontology biological processes had BMD median values below the lower limit of extrapolation (<21.7 mg/kg), which relate to entrainment of circadian clock by photoperiod and cellular response to thyroid hormone stimulus. The most sensitive gene sets for which a reliable estimate of the BMD could be made were neutral amino acid transport and negative regulation of collagen biosynthetic process with median BMDs of 23.8 and 29.4 mg/kg and median BMDLs of 15.2 and 18.8 mg/kg, respectively. The top 10 most sensitive individual genes exhibited changes in expression at dose levels below which a reliable estimate of potency could be achieved (<21.7 mg/kg). Of these genes, eight were upregulated: Hsd17b2, Nr1d2, Jade1, Sdr42e1, Tef, Per3, Bcar3 and Akr7a3. Two genes, Lgalsl and Nfil3, were downregulated.

Summary: Taken together, the most sensitive gene set BMD (BMDL) median and apical endpoint BMD (BMDL) values that could be reliably determined occurred at 23.8 (15.2) and 288.7 (223.5) mg/kg, respectively. The BMDs (and BMDLs) could not be determined for the top 10 most sensitive individual genes and were estimated to be <21.7 mg/kg. Serum cholinesterase activity was significantly and markedly decreased for all dosed groups and appeared to be one of the most sensitive apical measures, although a BMD (BMDL) was not determined because no viable model was available. Future studies investigating lower doses would be helpful to obtain more accurate estimates of BMD values for the most sensitive gene sets and genes.

Citation: Auerbach SS, Behl MV, Collins BJ, Cora MC, Fostel JM, Ingle BL, 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 tert-butylphenyl diphenyl phosphate (CASRN 56803-37-3) in male Sprague Dawley (Hsd:Sprague Dawley^® SD^®) rats (gavage studies). Research Triangle Park, NC: National Institute of Environmental Health Sciences. NIEHS Report 03. [https://doi.org/10.22427/NIEHS-03 Auerbach SS, Behl MV, Collins BJ, Cora MC, Fostel JM, Ingle BL, 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 tert-butylphenyl diphenyl phosphate (CASRN 56803-37-3) in male Sprague Dawley (Hsd:Sprague Dawley^® SD^®) rats (gavage studies). Research Triangle Park, NC: National Institute of Environmental Health Sciences. NIEHS Report 03.]