Table 1.

hCE1b IC₅₀ Values of ITP and TBPP Isomers and Related FR Mixtures

Test Chemical	Structure or Description	Presence in FR Mixturea	IC50 Value (nM)b	R 2 of Model
BNPP		NA	69.3 ± 9.4c	0.998
TPHP		19.8% in FM 550 44.6% in ITP Mix 1.7% in FM 600 24.0% in TBPP Mix	65.4 ± 8.3	0.993
EHDPHP		NA	99.4 ± 65.6	0.975
FM 550	EH-TBB, BEH-TEBP and ITP mixture	NA	31.2 ± 2.9	0.999
BZ-54	EH-TBB and BEH-TEBP only	NA	> 35 000	NA
ITP Mix	ITP mixture	NA	28.0 ± 15.4	0.979
K 50	ITP mixture	NA	27.6 ± 16.9	0.978
2IPPDPP		11.8% in FM 550 26.9% in ITP Mix	574 ± 127	0.996
4IPPDPP		2.3% in FM 550 4.9% in ITP Mix	49.9 ± 21.9	0.984
TBPP Mix	TBPP mixture	NA	17.9 ± 6.9	0.988
4tBPDPP		2.6% in FM 600 35.5% in TBPP Mix	22.3 ± 6.9	0.977
T4tBPP		22.0% in FM 600 2.5% in TBPP Mix	1740 ± 1010	0.979
TBP		NA	> 19 000	NA
DPHP		NA	> 90 400	NA
ip-PPP		NA	3420 ± 1620	0.985
tb-PPP		NA	4820 ± 2690	0.979

Abbreviations: hCE1b, human hepatic carboxylesterase; ITP and TBPP, isopropylated and tert-butylated triarylphosphate esters; FR, flame retardant; BNPP, bis(4-nitrophenyl) phosphate; TPHP, triphenyl phosphate; EHDPHP, 2-ethylhexyl diphenyl phosphate; FM 550, Firemaster 550; BZ-54, Firemaster BZ-54; ITP, isopropylated triaryl phosphate commercial mixture; 2IPPDPP, 2-isopropylphenyl diphenyl phosphate; 4IPPDPP, 4-isopropylphenyl diphenyl phosphate; TBPP, tert-butylated triarylphosphate commercial mixture; 4tBPDPP, 4-tert-butylphenyl diphenyl phosphate; T4tBPP, tris(4-tert-butylphenyl) phosphate; TBP, tributyl phosphate; DPHP, diphenyl phosphate; ip-PPP, isopropylated phenyl phenylphosphate; tb-PPP, tert-butylated phenyl phenylphosphate; NA, not applicable.

^aPercent composition is expressed on a wt/wt basis (Phillips et al., 2017).

^bIC₅₀ values were calculated using a 3 parameter nonlinear model in JMP Pro 11 (Roberts et al., 2015). More information regarding the model equation can be found in the Supplementary Material.

^cThis value is comparable to the IC₅₀ value determined by the manufacturer (24 nM) (Wang et al., 2011).