Pharmacokinetic profile and placental transfer of a single intravenous injection of [14C]chlorpyrifos in pregnant rats

The pharmacokinetics and placental transfer of a single intravenous dose of 5.0 mg/kg (10 muCi/kg) ring-labeled [C-14]chlorpyrifos were investigated in pregnant Sprague-Dawley rats at 11-13 days of gestation. Three rats were killed at 5, 15 or 30 min, or 1, 2, 4, 8, 12, 18, 24, 36, 48, 72 or 96 h after dosing. Radioactivity and 3,5,6-trichloropyridinol (TCP) were detected in all tissues 5 min after dosing. Chlorpyrifos was only found in maternal plasma and liver. Peak maternal plasma concentration of radioactivity (mug chlorpyrifos equivalents/ml) was 157 at 5 min, compared with 1.9 for fetal plasma at 15 min. The maximum concentrations of radioactivity (mug chlorpyrifos equivalents/g), detected in most tissues within 12 h of dosing, were, in descending order: liver (30), brain (29), placenta (21), and fetus (2). All peaks occurred at 5 min except for fetus and fetal plasma, which were at 15 min. TCP was detected by HPLC as the major compound identified in plasma and tissues. The maximum concentration detected was in plasma, at 12.4 mug/ml, and for the following tissues was: liver 4.3 ng/g fresh tissue, fetus 4 ng/g, placenta 2.97 ng/g, brain 1.68 ng/g, and fetal plasma 0.52 ng/g. All TCP peaks occurred at 5 min except for fetus at 30 min and fetal plasma at 15 min. Parent chlorpyrifos was detected in maternal plasma and liver at maximum concentrations of 5.1 mug/ml and 0.40 mug/g, respectively, at 5 min. Chlorpyrifos was detectable in maternal plasma up to 36 h after dosing, and in liver up to 24 h after dosing. Pharmacokinetic analysis best described radioactivity, chlorpyrifos, and TCP as disappearing blexponentially from plasma and tissues. The terminal elimination half-lives of radioactivity, chlorpyrifos and TCP from maternal plasma were 16, 18, and 16 h, respectively. The results indicate that (1) chlorpyrifos undergoes a rapid metabolism to its major metabolites (TCP); (2) chlorpyrifos and its metabolites are distributed to all maternal and fetal tissues and plasma; and (3) the elimination of chlorpyrifos and TCP is slow, with redistribution from lipid stores a likely determinant of elimination rates.