Hyo J. Kim

Ontogeny of hepatic and plasma metabolism of deltamethrin in vitro: role in age-dependent acute neurotoxicity.

Deltamethrin (DLM) is a relatively potent and widely used pyrethroid insecticide. Inefficient detoxification has been proposed to be the primary reason for the greater sensitivity of immature rats to the acute neurotoxicity of DLM. The objective of this study was to test this hypothesis by characterizing the age dependence of DLM metabolism in vitro, as well as toxic signs and blood levels of the neurotoxic parent compound following administration of 10 mg DLM/kg p.o. in glycerol formal. Metabolism was quantified in vitro by monitoring the disappearance of the parent compound from plasma [via carboxylesterases (CaEs)] and liver microsomes [via CaEs and cytochromes P450 (P450s)] obtained from 10-, 21-, and 40-day-old male Sprague-Dawley rats. Mean (±S.E.) intrinsic clearances (Vmax/Km) in these respective age groups by liver P450s (4.99 ± 0.32, 16.99 ± 1.85, and 38.45 ± 7.03) and by liver CaEs (0.34 ± 0.05, 1.77 ± 0.38, and 2.53 ± 0.19) and plasma CaEs (0.39 ± 0.06, 0.80 ± 0.09, and 2.28 ± 0.56) increased significantly (p ≤ 0.05) with age, because of progressive increases in Vmax. Intrinsic clearance of DLM by plasma CaEs and liver P450s reached adult levels by 40 days, but clearance by liver CaEs did not. Hepatic P450s played the predominant role in DLM biotransformation in young and adult rats. The incidence and severity of neurotoxic effects varied inversely with age. Correspondingly, blood DLM areas under the concentration versus time curve (AUCs) and Cmax values progressively decreased with increasing age. Internal exposure to DLM (blood AUCs) was closely correlated with toxic signs (salivation and tremors). The present study provides evidence that the limited metabolic capacity of immature rats contributes to elevated systemic exposure and ensuing neurotoxic effects of DLM.

Age, Dose, and Time-Dependency of Plasma and Tissue Distribution of Deltamethrin in Immature Rats

The major objective of this project was to characterize the systemic disposition of the pyrethroid, deltamethrin (DLT), in immature rats, with emphasis on the age dependence of target organ (brain) dosimetry. Postnatal day (PND) 10, 21, and 40 male Sprague-Dawley rats received 0.4, 2, or 10 mg DLT/kg by gavage in glycerol formal. Serial plasma, brain, fat, liver, and skeletal muscle samples were collected for up to 510 h and analyzed for DLT and/or 3-phenoxybenzoic acid (PBA) content by high-performance liquid chromatography. Toxicokinetic data from previous experiments of the same design with young adult (PND 90) rats (Kim, K.-B., Anand, S. S., Kim, H. J., White, C. A., and Bruckner, J. V. [2008]. Toxicokinetics and tissue distribution of deltamethrin in adult Sprague-Dawley rats. Toxicol. Sci. 101, 197-205) were used to compare to immature rat data. Plasma and tissue DLT levels were inversely related to age. Preweanlings and weanlings showed markedly elevated brain concentrations and pronounced salivation, tremors, choreoathetosis, and eventual fatalities. Plasma DLT levels did not reliably reflect brain levels over time. Plasma:brain ratios were time and dose dependent, but apparently not age dependent. Brain levels were better correlated with the magnitude of salivation and tremors than plasma levels. Hepatic intrinsic clearance of DLT progressively increased during maturation, as did the hepatic extraction ratio. Thus, limited capacity to metabolically inactivate DLT appeared primarily responsible for the inordinately high target organ doses and acute neurotoxicity in pups and weanling rats. Hepatic blood flow was not rate limiting in any age group. Limited DLT hydrolysis was manifest in vivo in the pups by relatively low plasma PBA levels. Elevated exposure of the immature brain to a pyrethroid may prove to be of consequence for long-term, as well as short-term neurotoxicity.

Effect of oral dosing vehicles on the acute hepatotoxicity of carbon tetrachloride in rats

Although carbon tetrachloride (CCl4) is of concern as a drinking water contaminant, it has been necessary in most oral toxicity studies to give CCl4 in an oil vehicle due to its limited water solubility. The primary objective of our study was to assess the influence of dosing vehicles on the acute hepatotoxicity of CCl4. Fasted 200- to 230-g rats were generally found to be more susceptible to CCl4 hepatotoxicity than fasted 300- to 330-g rats. A time-course study revealed that corn oil did not delay the onset or time of maximal liver injury by an oral 100 mg/kg dose of CCl4, but did reduce the extent of injury relative to that when the chemical was given undiluted or as an aqueous emulsion. Fasted 200- to 230-g male Sprague-Dawley rats were given 0, 10, 25, 50, 100, 250, 500, or 1000 mg CCl4/kg body wt by gavage: in corn oil; as an aqueous emulsion; as the undiluted chemical; and in the 10 and 25 mg/kg doses only, in water. Blood and liver samples were taken 24 hr after dosing for measurement of serum and microsomal enzymes. Pathological examination of liver samples was also conducted. Dose-dependent increases in serum enzyme levels and pathological changes and dose-dependent decreases in microsomal P450 and glucose-6-phosphatase activity were observed in each vehicle group. Both the 10 and 25 mg/kg oral doses of CCl4 in water caused significant elevations in serum enzymes and hepatic centrolobular vacuolation. The study revealed that acute hepatotoxicity was less pronounced at each dosage level in rats given CCl4 in corn oil than in other vehicle groups. These findings demonstrate that dosing vehicles can significantly influence the acute hepatotoxicity of CCl4 in rats and are a cause for additional consideration and review of the practice of routinely using vegetable oils as a diluent in studies of volatile organic compound (VOC) toxicity. The use of aqueous Emulphor emulsions appears more appropriate in acute toxicity studies of VOC drinking water contaminants such as CCl4, in that the emulsion did not substantially alter the toxicity of CCl4 from that of undiluted CCl4 or CCl4 ingested in water.

Oral Toxicity of 1,2-Dichloropropane: Acute, Short-Term, and Long-Term Studies in Rats

The objective of this investigation was to characterize the acute and short- and long-term toxic potency of orally administered 1,2-dichloropropane (DCP). In the acute and short-term studies, male rats of 250-300 g were gavaged with 0, 100, 250, 500, or 1000 mg DCP/kg in corn oil once daily for up to 10 consecutive days. Although ingestion of DCP caused body weight loss and CNS depression, few other toxic effects were manifest 24 hr after a single dose of the chemical. Morphological changes were limited to liver centrilobular cells in 500 and 1000 mg/kg rats. Similarly, elevated activity of some serum enzymes occurred only at these two highest dose levels. Hepatic nonprotein sulfhydryl (NPS) levels were decreased and renal NPS levels increased at 24 hr. In the short-term study resistance developed to DCP hepatotoxicity over the 10 consecutive days of exposure, as reflected by progressively lower serum enzyme levels and by decreases in the severity and incidence of toxic hepatitis and periportal vacuolization. Nucleolar enlargement in hepatocytes, however, was observed at all dosage levels at 5 and 10 days. There were a number of manifestations of hemolytic anemia, including erythrophagocytosis in the liver, splenic hemosiderosis and hyperplasia of erythropoietic elements of the red pulp, renal tubular cell hemosiderosis, and hyperbilirubinemia. Urinalyses and histopathology revealed no evidence of nephrotoxicity. In the long-term study, male rats initially weighing 180-200 g were gavaged five times weekly for up to 13 weeks with 0, 100, 250, 500, or 750 mg DCP/kg. As over one-half the 750 mg/kg group died within 10 days, the survivors were sacrificed. Histopathological changes in the 750 mg/kg animals included mild hepatitis and splenic hemosiderosis, as well as adrenal medullary vacuolization and cortical lipidosis, testicular degeneration and a reduction in sperm, and increased number of degenerate spermatogonia in the epididymis in some members of the group. Similar testicular and epididymal degenerative change also were observed in some 500 mg/kg animals after 13 weeks of dosing. There was a progressive increase in the number of deaths in the 500 mg/kg group, such that more than 50% were dead by 13 weeks. No deaths occurred in the 100 or 250 mg/kg groups. The DCP dosage regimen also produced a dose-dependent decrease in body weight gain. DCP exhibited very limited hepatotoxic potential and no apparent nephrotoxic potential in the long-term study. Slight elevations in serum ornithine-carbamyltransferase activity, periportal vacuolization, and active fibroplasia in the liver were seen in the 500 mg/kg animals.

A physiological and system analysis hybrid pharmacokinetic model to characterize carbon tetrachloride blood concentrations following administration in different oral vehicles

Oral absorption of chemicals can be influenced significantly by the administration vehicle or diluent. It has been observed that the oral absorption of carbon tetrachloride (CCl4)and other volatile organic chemicals is markedly affected by the dosing vehicle, with administration in oils producing erratic blood concentration-time profiles with multiple peaks. Analysis of this type of data by a compartmental modeling approach can be difficult, and requires numerous assumptions about the absorption processes. Alternatively, a system analysis method with few assumptions may provide a more accurate description of the observed data. In the current investigations, a nonlinear system analysis approach was applied to blood CCl4concentration-time data obtained following iv and oral administration. The oral regimens consisted of 25 mg CCl4/kgbody wt given as an aqueous emulsion, in water, as pure chemicals, and in corn oil. The system analysis procedure, based upon a disposition decomposition method, provided an absorption input rate function, F,for each regimen. A physiological pharmacokinetic model, based primarily on parameters available in the literature, and the Finput functions, formed a hybrid model that adequately described the observed blood CCl4concentration-time data. The same physiological pharmacokinetic model, employing conventional first-order absorption input schemes, did not predict the data as well. Overall, the system analysis approach allowed the oral absorption of CCl4to be characterized accurately, regardless of the vehicle. Though system analysis is based on general mathematical properties of a systems behavior rather than on its causal mechanisms, this work demonstrates that it can be a useful adjunct to physiological pharmacokinetic models.

Effect of oral dosing vehicles on the subchronic hepatotoxicity of carbon tetrachloride in the rat

Previous studies in this laboratory have shown that corn oil delayed and prolonged the gastrointestinal absorption of carbon tetrachloride (CCl4) and reduced its acute hepatotoxicity in rats. The objective of the present study was to extend the duration of ingestion of CCl4 to assess vehicle effects on the subchronic oral toxicity of CCl4. Male Harlan Sprague-Dawley rats were given doses of 0, 25, or 100 mg CCl4/kg body weight by gavage in either corn oil or a 1% Emulphor aqueous emulsion 5 times a week for 13 wk. Blood was collected at 4, 8, and 13 wk for measurement of serum enzymes. Liver samples were also taken at 13 wk for measurement of triglyceride and microsomal enzyme levels, as well as for histopathological examination. Serum enzyme levels peaked at 8 wk in the high-dose groups, but not until 13 wk in the low-dose animals. Effects of CCl4 on serum and microsomal enzymes were of similar magnitude in the two vehicle groups. A comprehensive histopathological examination revealed no qualitative or quantitative differences between the corn oil and aqueous vehicle groups in hepatic lesions. Although CCl4 and chloroform have been reported by other investigators to be more hepatotoxic to mice when given for 90 d in corn oil, current findings indicate that corn oil does not significantly alter the subchronic hepatotoxicity of CCl4 in rats from that when the halocarbon is given in an aqueous medium.