Abstract

Background: Tricresyl phosphate (TCP) is an organophosphate flame retardant currently on the market that is used as a replacement for phased-out polybrominated diphenyl ethers. TCP is a mixture of three isomeric compounds, including TCP, dicresyl phenyl phosphate, and cresyl diphenyl phosphate. Toxicological information on this class of chemicals is sparse. A short-term, in vivo transcriptomic study was used to assess the biological potency of TCP.

Methods: Scientists at the Division of Translational Toxicology, National Institute of Environmental Health Sciences conducted this short-term in vivo biological potency study on TCP in young adult male Sprague Dawley (Hsd:Sprague Dawley® SD®) rats. TCP was formulated in corn oil and administered once daily for 4 consecutive days by gavage. TCP was tested at six doses (0, 62, 125, 249, 497, and 995 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 serum alanine aminotransferase activity, decreased serum albumin concentration, increased relative liver weight, decreased serum albumin/globulin ratio, increased absolute liver weight, increased serum high-density lipoprotein cholesterol concentration, increased serum total cholesterol concentration, increased serum lowdensity lipoprotein cholesterol concentration, and increased serum globulin concentration. The BMDs and benchmark dose lower confidence limits (BMDLs) were 31.1 (10.1), 62.5 (28.4), 129.5 (78.4), 172.8 (134.5), 223.8 (97.1), 241.8 (175.1), 243.8 (193.1), 335.6 (245.7), and 344.8 (255.8) mg/kg, respectively. Although serum cholinesterase activity was significantly decreased in all dosed groups (43%–78% decrease), beginning with 62 mg/kg (the lowest-observed-effect level), its BMD value was below the lower limit of extrapolation (<20.7 mg/kg).

No Gene Ontology biological processes had BMD median values <20.7 mg/kg. The most sensitive gene sets for which a reliable estimate of the BMD could be made were cellular response to dexamethasone stimulus and cellular response to glucocorticoid stimulus, both with median BMDs of 58.5 mg/kg and median BMDLs of 34.0 mg/kg. Two individual genes, Fer1l5 and Gsta3, had median BMD values <20.7 mg/kg. Gsta3 was upregulated, while Fer1l5 was downregulated. The most sensitive upregulated genes with reliable BMD estimates included Ces2c, Abcc3, Orm1, Mnd1, App, Cyp7a1, Ddit4, and Gstt3, with BMDs (BMDLs) of 27.7 (18.7), 32.1 (22.0), 32.6 (10.4), 33.8 (14.8), 37.7 (20.8), 39.4 (17.3), 41.6 (9.5), and 66.0 (28.7) mg/kg, respectively. Other than Fer1l5, no other responding genes had decreased expression among the top 10 most sensitive genes.

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 58.5 (34.0), 27.7 (18.7), and 31.1 (10.1) mg/kg, respectively. The BMDs (BMDLs) could not be determined for 2 of the top 10 most sensitive individual genes and were estimated to be <20.7 mg/kg. Serum cholinesterase inhibition was also estimated to be <20.7 mg/kg. Future studies investigating lower doses would be helpful to obtain more accurate estimates of BMD values for the most sensitive individual genes and for serum 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 tricresyl phosphate (CASRN 1330-78-5) in male Sprague Dawley (Hsd:Sprague Dawley^® SD^®) rats (gavage studies). Research Triangle Park, NC: National Institute of Environmental Health Sciences. NIEHS Report 06. [https://doi.org/10.22427/NIEHS-06 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 tricresyl phosphate (CASRN 1330-78-5) in male Sprague Dawley (Hsd:Sprague Dawley^® SD^®) rats (gavage studies). Research Triangle Park, NC: National Institute of Environmental Health Sciences. NIEHS Report 06.]