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IARC MONOGRAPHS ON THE EVALUATION OF CARCINOGENIC ...

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CYP2E1 and CYP2F2) inhibited nasal metabolism. Subsequent metabolism of styrene<br />

7,8-oxide by epoxide hydrolase and GST was much higher in rat nasal tissue than in<br />

mouse nasal tissue.<br />

(d ) Excretion<br />

Sumner et al. (1997) compared the metabolism of [7- 14 C]styrene administered by<br />

inhalation (250 ppm [1060 mg/m 3 ], 6 h per day for 1–5 days) to B6C3F 1 mice, CD-1<br />

mice and Fischer 344 rats. Rats and CD-1 mice excreted the radioactivity faster than did<br />

B6C3F 1 mice following a single exposure, but the rates were similar for the three groups<br />

after repeated exposures on 3 or 5 days. Boogaard et al. (2000a) studied the disposition<br />

of [ring-U- 14 C]styrene in Sprague-Dawley rats and CD-1 mice exposed by recirculating<br />

nose-only exposure (160 ppm [682 mg/m 3 ] for 6 h). Urinary excretion was the primary<br />

route of elimination (amounting to 75% of the retained styrene for rats and 63% for<br />

mice), and there was little quantitative difference between rats and mice except that mice<br />

exhaled 14 CO 2 which amounted to 6.4–8.0% of the retained styrene, while for rats this<br />

was only 2%. Whole-body autoradiography showed significantly higher non-specific<br />

binding of radioactivity in mouse lung and nasal passages compared with rat lung.<br />

4.1.3 Pharmacokinetic modelling<br />

STYRENE 491<br />

Table 11. Metabolism of styrene to styrene 7,8-oxide enantiomers<br />

by mouse and rat lung microsomes and isolated<br />

Clara cell-enriched fraction<br />

Lung microsomes<br />

Mouse (n = 4)<br />

Rat (n = 5)<br />

Clara cell fraction<br />

Mouse (n = 4)<br />

Rat (n = 3)<br />

R-enantiomer S-enantiomer R/S<br />

1.50 ± 0.23 a<br />

0.49 ± 0.09<br />

83.3 ± 27.7<br />

11.2 ± 3.6<br />

0.63 ± 0.06<br />

0.95 ± 0.18<br />

23.0 ± 8.2<br />

11.0 ± 3.2<br />

2.40 ± 0.36<br />

0.52 ± 0.01<br />

3.98 ± 0.75<br />

1.02 ± 0.09<br />

Data from Hynes et al. (1999)<br />

Values for R- and S-enantiomers are in nmol/mg protein/min for lung microsomes<br />

and in pmol/10 6 cells/min (not corrected for cell type) for Clara cell<br />

fractions. The percentages of Clara cells in these fractions were 56% for<br />

mouse and 37% for rat.<br />

a Mean ± standard error<br />

Ramsey and Young (1978) and Ramsey et al. (1980) analysed the pharmacokinetic<br />

profile of inhaled styrene in male rats [strain unspecified] and humans. In human<br />

volunteers exposed to 80 ppm [341 mg/m 3 ] for 6 h, styrene was cleared from the blood

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