Fate and specific tissue retention of toxaphene in mice

Development of a reference dose for the persistent congeners of weathered toxaphene based on in vivo and in vitro effects related to tumor promotion

Toxaphene is a mixture of chlorinated camphenes and bornanes that was produced and used in the United States until 1982. 1.3 million tons of toxaphene have been released worldwide. "Technical" toxaphene (TT) consists of a mixture of up to 800 different chemicals, known as congeners. TT weathers in the environment by both biotic and abiotic processes. The human body burden of toxaphene consists of only five persistent congeners that are not metabolized; three of these occur in considerably greater amounts than the other two. Because of the rapid metabolism and excretion of the non-persistent congeners, the persistent congeners that make up the human body burden most likely play a role in eliciting any potential adverse effects. EPA's toxicity assessment for TT is based on the occurrence of liver cancer in rodents, and considerable doubt exists whether this assessment is applicable to weathered toxaphene (WT). Using experimental results from European Union scientists, a reference dose (RfD) was developed for WT based on the three most persistent congeners that comprise the human body burden. The critical effect chosen was tumor promotion and this endpoint is considered protective for other endpoints as well. Although RfDs are typically derived for non-carcinogenic effects, the endpoint of tumor promotion is appropriate for RfD development because the experimental data suggest a dose threshold. The RfD for weathered toxaphene represented by the sum of the three major persistent congeners ( summation 3PC) is 2E-05 mg/kg-day. To apply this reference dose to a particular WT mixture, information is needed regarding the percentage of summation 3PC in the mixture.

A pharmacokinetic model for predicting absorption, elimination, and tissue burden of toxaphene in rats

A two-compartment pharmacokinetic model was formulated to predict absorption, elimination, and tissue burden of toxaphene in rats. The model was constructed based on the database of Crowder and Dindal (Bull. Environ. Contam. Toxicol. 12, 320-327, 1974) and included six tissue compartments: blood, brain, liver, muscle, fat, and carcass. The pharmacokinetically based dosimetry indicated that absorption of toxaphene was fast in fat, whole body, carcass, and blood, relatively slow in liver and muscle, and slow in brain. In contrast, the elimination rate was rapid in whole body, muscle, and blood, moderate in carcass and brain, and slow in liver and fat. Tissue burden was highest in fat, whole body, and blood, intermediate in liver, and lowest in brain. The model performance was evaluated by the data set of Pollock and Hillstrand (J. Environ. Sci. Health B 17, 635-648, 1982) on toxaphene absorption and elimination in pregnant rats. Validity of the model was confirmed by the close agreement between the predicted and observed tissue burdens of toxaphene in target tissues. Disposition of toxaphene via feces was a dominant excretory pathway while urinary excretion was a minor elimination route in male rats. However, for pregnant rats, excretion of toxaphene both in urine and feces were of similar magnitude. These characteristics of elimination are valuable for understanding the metabolism of toxaphene in pregnant rats. The model serves as a starting point for a quantitative, mechanism-based understanding of the processes that influence the pharmacokinetics of toxaphene in mammalian systems.

Distribution of toxaphene, DDT, and PCB among lipoprotein fractions in rat and human plasma

The distribution of 14C-toxaphene, 14C-DDT, and 14C-PCB among lipoprotein fractions was studied in vitro and in vivo using rat and human plasma. The association of these substances with rat plasma fractions was similar in both in vitro and in vivo experiments. Thirty-seven to fifty-two per cent of the total radioactivity was associated with the cholesterol-rich high density lipoproteins (HDL2, d = 1.075-1.21 g/ml) and 18-52% was recovered in the albumin-rich bottom fraction (BF, d greater than 1.21 g/ml). A time-dependent redistribution of the radioactivity from the lipoprotein fractions to the BF was also observed in the in vivo studies. In human plasma, the distribution of the three compounds was different and uncorrelated to the cholesterol level of the individual lipoprotein fractions. Toxaphene was almost equally distributed between BF (d greater than 1.21 ml), HDL (d = 1.063-1.21 g/ml) and low density lipoproteins (LDL, d = 1.006-1.063 g/ml) (26%, 27% and 29%, respectively), while only 18% appeared in the very low density lipoprotein (VLDL, d less than 1.006) fraction. In contrast, a large proportion of DDT and PCB radioactivity was recovered in the BF (52% and 62%, respectively) while only 38-48% was present in lipoprotein fractions. The complex nature of the interaction between xenobiotics and plasma lipoproteins is discussed.

Tissue accumulation of lipoprotein associated toxaphene in normo- and hypolipidemic mice

Normo- and hypolipidemic mice were given a single i.v. injection of 14C-toxaphene associated with low density lipoprotein (LDL), high density lipoprotein (HDL) or dimethyl sulfoxide (DMSO). The tissue distribution of radioactivity was studied 20 min and 4 h after the application. In the normolipidemic mice at 20 min postinjection there was high uptake of the 14C-toxaphene preparations in the liver and adrenals followed after 4 h by a redistribution to the adipose tissues. In the hypolipidemic mice, proportionally less label accumulated initially in the liver and adrenals while more radioactivity was seen in the kidneys and heart. The radioactivity then redistributed to the liver with a very small uptake in the adipose tissue compared to the normolipidemic mice after 4 h. The results indicate that changes in the lipid pattern, e.g. hypolipidemic conditions, may influence the tissue distribution of lipophilic xenobiotics.

Toxaphene: accumulation in the adrenal cortex and effect on ACTH-stimulated corticosteroid synthesis in the rat

Uptake and distribution of [14C]toxaphene was studied in the adrenals of rats using whole-body autoradiography. An accumulation of radioactivity was seen in the adrenal cortex (zona fasciculata) 1-24 h after a single gavage of [14C]toxaphene (16 mg/kg b.w.). In in vitro studies toxaphene was found to inhibit ACTH-stimulated corticosterone synthesis in the cultured rat adrenocortical cells (IC50 2.8 X 10(-5) M). Moderate but significant inhibition (P less than 0.001) of ACTH-stimulated corticosterone synthesis was also observed in the adrenocortical cells isolated from rats after a prolonged exposure (5 weeks) to low levels (1.2 ppm) of toxaphene in feed. The results indicate a direct adrenotoxic effect of toxaphene.