Plasma Protein and Lipoprotein Binding of Cis- and Trans-Permethrin and Deltamethrin in Adult Humans and Rats.

The majority of residents of the United States, Canada, and Europe are exposed to pyrethroids, the most commonly used class of insecticides. Surprisingly little is known about key aspects of their pharmacokinetics, including their mode of transport in the systemic circulation. This study tested the hypothesis that pyrethroids are transported by both plasma lipoproteins and proteins, similarly to other highly lipophilic environmental contaminants. Other aims were to characterize the binding of representative type I and II pyrethroids, and to compare their binding to rat versus human plasma. Binding of 14C-labeled cis-permethrin (CIS), trans-permethrin (TRANS) and deltamethrin (DLM) to proteins and lipoproteins was measured by sequential extraction of spiked plasma with isooctane, 2-octanol, and acetonitrile. Binding of DLM, CIS, and TRANS to plasma proteins and lipoproteins was linear from 250 to 750 nM; concentrations present in the plasma of orally dosed rats. Binding of DLM to high-density lipoprotein was twice that to low-density lipoprotein. Binding of DLM, CIS, and TRANS was ∼2-fold greater to proteins than to lipoproteins of rat and human plasma; albumin was primarily responsible for protein binding. Higher total binding of each pyrethroid to human (∼90%) than to rat (∼80%) plasma resulted from higher protein binding in human plasma. This was attributable in part to the higher albumin/protein content of human plasma. Rat albumin exhibited lower pyrethroid binding capacity than did human albumin. The results of this investigation indicate that albumin and lipoproteins play a major role in binding and transport of pyrethroids in the systemic circulation of both rats and humans.

Effect of dose and exposure protocol on the toxicokinetics and first-pass elimination of trichloroethylene and 1,1,1-trichloroethane.

Trichloroethylene (TCE) and 1,1,1-trichloroethane (TRI) are frequent contaminants of drinking water and of groundwater at hazardous waste sites. There is relatively little information on the target organ deposition of TRI, despite its ingestion and common occurrence in humans. An important aim of the study was to delineate and contrast the toxicokinetics (TK) and bioavailability (F) of TRI and its well metabolized congener, TCE. Blood profiles were obtained from male Sprague-Dawley rats given aqueous emulsions of 6 or 48 mg TRI/kg and 10 or 50 mg TCE/kg as an oral bolus (po) or by gastric infusion (gi) over 2 h. TCE exhibited nonlinear TK, with a disproportionate increase in AUC and decrease in clearance and F with increase in dose. TRI exhibited linear TK. F did not vary significantly with TRI dose or dosage regimen. F values were substantially higher for TRI than for the respective TCE groups. TRI was distributed widely to tissues of rats gavaged with 6 mg TRI/kg, with accumulation in fat. This experiment yielded tissue uptake and elimination profiles and in vivo tissue:blood partition coefficients (PCs). Finally, additional rats were given 10 mg/kg of TCE and TRI po, ia and iv, so that first-pass hepatic (FPh) and pulmonary (FPp) elimination could be measured directly. Total and FPh elimination of TCE exceeded that of TRI. TRI, with its higher air:blood PC, exhibited the higher FPp. TCE and TRI, despite several common physical and chemical properties resulting in similar absorption and systemic distribution, displayed dissimilar dosage and dose rate effects on their TK.

Toxicokinetics of Deltamethrin: Dosage Dependency, Vehicle Effects, and Low-Dose Age-Equivalent Dosimetry in Rats.

There is increasing concern that infants and children may be at increased risk of neurological effects of pyrethroids, the most widely used class of insecticide. The objectives of this investigation were to (1) characterize the dose-dependent toxicokinetics (TK) of deltamethrin (DLM) for exposures ranging from environmentally relevant to acutely toxic; (2) determine the influence of an aqueous versus oil vehicle on oral absorption and bioavailability; and (3) determine whether DLM exhibits low-dose, age-equivalent internal dosimetry. Serial arterial plasma samples were obtained for 72 h from adult, male Sprague Dawley rats given 0.05-5.0 mg DLM/kg as an oral bolus in corn oil (CO). DLM exhibited linear, absorption rate-limited TK. Increases in maximum plasma concentration (Cmax) and AUC∘∞ were directly proportional to the dose. Oral bioavailability was quite limited. The vehicle and its volume had modest effect on the rate and extent of systemic absorption in adult rats. Postnatal day (PND) 15, 21, and 90 (adult) rats received 0.10, 0.25, or 0.50 mg DLM/kg orally in CO and were sacrificed periodically for plasma, brain, and liver collection. Age-dependent differences between PND 15 and 90 plasma Cmax and AUC∘24 values progressively diminished as the dose decreased, but there was a lack of low dose age equivalence in these brain and liver dosimeters. Other maturational factors may account for the lack of the low-dose age equivalence in brain and liver. This investigation provides support for the premise that the relatively low metabolic capacity of immature subjects may be adequate to effectively eliminate trace amounts of DLM and other pyrethroids from the plasma.

Brain uptake of deltamethrin in rats as a function of plasma protein binding and blood-brain barrier maturation.

Pyrethroids, including permethrin and deltamethrin (DLM), are very widely used of insecticides. It was hypothesized that lower plasma binding and increased blood-brain barrier (BBB) penetration of DLM in immature rats contribute to the higher brain concentrations of DLM and more pronounced neurotoxicity reported in this age group. The left brain of anesthetized adult rats was perfused for 2min via a carotid artery with 1µM 14C-DLM in: 2-5% human serum albumin (HSA); plasma from adult and 15- and 21-d-old rats; and plasma from human donors of: birth-1 week, 1-4 weeks, 4 weeks-1 year, 1-3 years and adults. The fraction of DLM bound and brain uptake of DLM did not vary significantly with the HSA concentration nor with the age of rat or human plasma donors. One, 10 and 50µM 14C-DLM were perfused into the left-brain of anesthetized adult, 15- and 21-d-old rats. DLM deposition in the brain was linear over this range of concentrations and inversely related to age. The results of this investigation indicate that increased BBB permeability in the youngest rats enhances brain deposition of the insecticide. Plasma protein binding of DLM in immature rats and humans is not sufficiently diminished to impact its brain uptake.

Influence of Maturation on In Vivo Tissue to Plasma Partition Coefficients for Cis- and Trans-Permethrin.

Permethrin, the most widely used household insecticide in the United States, is marketed as a mixture of its cis (CIS) and trans (TRANS) isomers. The major objective of this investigation is to develop and utilize a reliable approach to determine in vivo partition coefficients (PCs) for CIS and TRANS in immature and adult Sprague-Dawley rats. Adult, postnatal day (PND) 21, and PND 15 rats were infused with environmentally relevant concentrations of CIS or TRANS via a subcutaneous osmotic pump for 48 or 72 h. The adult and PND 21 rats also received an oral loading dose. Systemic steady-state or equilibrium was attained in each age group within 72 h of the protocol. CIS and TRANS were both distributed to tissues according to their neutral lipid content, with adipose tissue exhibiting much higher tissue:plasma PCs than skeletal muscle, liver, or brain. Liver:plasma and brain:plasma PCs were consistently at or lower than unity. Tissue:plasma PCs were generally higher for CIS than for TRANS, although the isomers are of comparable lipophilicity. Significantly higher blood levels of CIS apparently saturate plasma binding, resulting in greater tissue deposition of the isomer. CIS and TRANS tissue:plasma PCs were found to be inversely related to the rats' age, although TRANS brain:plasma PCs were comparable in immature and mature animals. These data support the conclusion that age-dependent partitioning is an important determinant of the pharmacokinetics of permethrin. Such partitioning could influence the risk assessment of these insecticides in infants and children when incorporated into physiologically based pharmacokinetic models.

The immature rat as a potential model for chemical risks to children: Ontogeny of selected hepatic P450s.

Concern about potential susceptibilities of infants and children to chemicals has led to the consideration of immature rodents as potential test surrogates. Maturation of some hepatic microsomal cytochrome P450s (CYPs), that participate in metabolic activation of organic solvents and polycyclic aromatic hydrocarbons (PAHs), may differ significantly between humans and rodents. The present investigation was undertaken to delineate the ontogeny of selected hepatic CYPs in male and female Sprague-Dawley (S-D) rats, and to contrast them with developmental profiles in humans. Microsomes were prepared from the liver of sexed and unsexed postnatal day (PND) 1-90 rats, and total CYP450 levels, as well as CYP1A1/2, CYP2E1 and CYP2B1/2 activities and protein, were quantified. CYP1A1/2 and CYP2E1 activity and expression rose rapidly after birth, peaked from PND 21-40/50, and declined substantially to adult values by PND 90. The same ontogenic profiles were manifested when the enzyme activities were expressed per entire liver or liver normalized to body weight. CYP1A1/2 and CYP2E1 activity and protein expression were well correlated. CYP2B1/2 activity peaked abruptly on PND 21 and declined irregularly to adult values. These patterns are in contrast to human CYP1A2 and CYP2E1, which are reported to progressively increase in liver during the first few months to years of life. The three CYP protein developmental profiles were largely gender independent in rats. The immature rat does not appear to be a suitable model for assessing risks posed to infants and children by chemicals metabolically activated by CYP2E1, based on the findings of greater carbon tetrachloride hepatotoxicity in preweanlings and weanlings than in adult animals. Additional studies are required to determine whether immature S-D rats may be used as an animal model for substrates of other CYPs, as total CYP450 levels in the liver progressively rose during maturation, similarly to humans.

Plasma protein binding limits the blood brain barrier permeation of the pyrethroid insecticide, deltamethrin.

Previous pharmacokinetic studies of deltamethrin (DLM) have revealed that brain levels of this highly lipophilic pyrethroid insecticide are only 15-20% of plasma levels. Experiments were performed to assess determinants limiting CNS access including plasma protein binding and the efflux transporter, P-gp. A human brain microvascular endothelial cell line, hCMEC/D3, was utilized as a model in vitro system to evaluate blood-brain barrier (BBB) permeation. Incubation of DLM with a series of human serum albumin (HSA) concentrations showed that unbound (fu) DLM ranged from 80% with 0.01% HSA to ∼20% at the physiologically-relevant 4% HSA. A positive correlation (R=0.987) was seen between fu and cellular uptake. Concentration-dependent uptake of DLM in 0.01% HSA was non-linear and was reduced at 4°C and by the P-gp inhibitor cyclosporine (CSA), indicative of a specific transport process. Cellular accumulation of [(3)H]-paclitaxel, a P-glycoprotein (P-gp) substrate, was increased by CSA but not by DLM, suggesting that DLM is neither a substrate nor an inhibitor of P-gp. The concentration-dependent uptake of DLM from 4% HSA was linear and not significantly impacted by temperature or CSA. In situ brain perfusion studies monitoring brain association of DLM at 0.01% and 4% HSA confirmed the aforementioned in vitro findings. This study demonstrates that brain uptake of DLM under normal physiological conditions appears to be a passive, non-saturable process, limited by the high protein binding of the pyrethroid.

Ontogeny of plasma proteins, albumin and binding of diazepam, cyclosporine, and deltamethrin.

BACKGROUND: To characterize the ontogeny of plasma albumin and total proteins, due to the lack of a comprehensive pediatric database. Secondly, to establish the magnitude and duration of maturational changes in binding of highly-bound drugs/chemicals. METHODS: Anonymized plasma samples from 296 donors were pooled in 6 age brackets from birth to adolescence. Total protein and albumin levels were measured in each age group, as was the age-dependency of plasma binding of diazepam (DZP), cyclosporine (CYC), and deltamethrin (DLM), a pyrethroid insecticide. RESULTS: Plasma levels of albumin and total proteins steadily increased for the first 1-3 y of life. Unbound DZP and CYC fractions were elevated three- to fourfold in neonates, but decreased to adult levels after 1 and 3 y, respectively. Unbound DLM levels exceeded those in adults for just 1 mo. CONCLUSION: Neonates and infants under 1-3 y may be at risk from increased amounts of free drug, when given standard doses of some highly-bound drugs. Pyrethroid insecticides might be anticipated to pose increased risk for 1 mo.

Association of brominated proteins and changes in protein expression in the rat kidney with subcarcinogenic to carcinogenic doses of bromate.

The water disinfection byproduct bromate (BrO3(-)) produces cytotoxic and carcinogenic effects in rat kidneys. Our previous studies demonstrated that BrO3(-) caused sex-dependent differences in renal gene and protein expression in rats and the elimination of brominated organic carbon in their urine. The present study examined changes in renal cell apoptosis and protein expression in male and female F344 rats treated with BrO3(-) and associated these changes with accumulation of 3-bromotyrosine (3-BT)-modified proteins. Rats were treated with 0, 11.5, 46 and 308 mg/L BrO3(-) in drinking water for 28 days and renal sections were prepared and examined for apoptosis (TUNEL-staining), 8-oxo-deoxyguanosine (8-oxoG), 3-BT, osteopontin, Kim-1, clusterin, and p-21 expression. TUNEL-staining in renal proximal tubules increased in a dose-related manner beginning at 11.5mg BrO3(-)/L in female rats and 46 mg/L in males. Increased 8-oxoG staining was observed at doses as low as 46 mg/L. Osteopontin expression also increased in a dose-related manner after treatment with 46 mg/L, in males only. In contrast, Kim-1 expression increased in a dose-related manner in both sexes, although to a greater extent in females at the highest dose. Clusterin and p21 expression also increased in a dose-related manner in both sexes. The expression of 3-BT-modified proteins only increased in male rats, following a pattern previously reported for accumulation of α-2u-globulin. Increases in apoptosis in renal proximal tubules of male and female rats at the lowest doses suggest a common mode of action for renal carcinogenesis for the two sexes that is independent of α-2u-globulin nephropathy.

Changes in mRNA and protein expression in the renal cortex of male and female F344 rats treated with bromate.

Bromate (BrO3(-)), a by-product of ozonation of drinking water, induces nephrotoxicity in male rats at much lower doses than in female rats. This difference appears to be related to the development of α-2u-globulin nephropathy in males. To determine sex-dependent changes in mRNA and protein expression in the renal cortex attributable to α-2u-globulin nephropathy, we performed microarray and immunohistochemical analyses in proximal renal tubules of male and female F344 rats treated with KBrO3 for 28 days. Particular attention was paid to molecular biomarkers of renal tubular injury. Microarray analysis of male and female rats treated with BrO3(-) at low doses (125 mg/L KBrO3) displayed marked sex-dependent changes in renal gene expression. The greatest differences were seen in genes encoding for cellular differentiation, apoptosis, ion transport, and cell proliferation. Differences by sex were especially prominent for the cell cycle checkpoint gene p21, the renal injury protein Kim-1, and the kidney injury and cancer biomarker protein osteopontin. Dose-related nephrotoxicity, assessed by hematoxylin and eosin staining, was greater in males compared to female rats, as was cellular proliferation, assessed by bromodeoxyuridine staining. The fraction of proximal renal cells with elevated 8-oxodeoxyguanosine (8-OH-dG) was only increased at the high dose and did not differ by sex. Dose-dependent increases in the expression of osteopontin were detected immunohistochemically only in male rats and were localized in proximal tubule cells. Similarly, BrO3(-) treatment increased clusterin and Kim-1 staining in the proximal tubules; however, staining for these proteins did not differ appreciably between males and females. These data demonstrate both qualitative and quantitative differences in the response of male versus female kidneys to BrO3(-)-treatment. These sex-dependent effects likely contribute to renal carcinogenesis of BrO3(-) in the male rat.

Absorption and disposition of bromate in F344 rats.

Bromate (BrO(3)(-)) is a ubiquitous by-product of using ozone to disinfect water containing bromide (Br(-)). The reactivity of BrO(3)(-) with biological reductants suggests that its systemic absorption and distribution to target tissues may display non-linear behavior as doses increase. The intent of this study is to determine the extent to which BrO(3)(-) is systemically bioavailable via oral exposure and broadly identify its pathways of degradation. In vitro experiments of BrO(3)(-) degradation in rat blood indicate a rapid initial degradation immediately upon addition that is >98% complete at concentrations up to 66µM in blood. As initial concentrations are increased, progressively lower fractions are lost prior to the first measurement. Secondary to this initial loss, a slower and predictable first order degradation rate was observed (10%/min). Losses during both phases were accompanied by increases in Br(-) concentrations indicating that the loss of BrO(3)(-) was due to its reduction. In vivo experiments were conducted using doses of BrO(3)(-) ranging from 0.077 to 15.3mg/kg, administered intravenously (IV) or orally (gavage) to female F344 rats. The variable nature and uncertain source of background concentrations of BrO(3)(-) limited derivation of terminal half-lives, but the initial half-life was approximately 10min for all dose groups. The area under the curve (AUC) and peak concentrations (C(t=5')) were linearly related to IV dose up to 0.77mg/kg; however, disproportionate increases in the AUC and C(t=5') and a large decrease in the volume of distribution was observed when IV doses of 1.9 and 3.8mg/kg were administered. The average terminal half-life of BrO(3)(-) from oral administration was 37min, but this was influenced by background levels of BrO(3)(-) at lower doses. With oral doses, the AUC and C(max) increased linearly with dose up to 15.3mgBrO(3)(-)/kg. BrO(3)(-) appeared to be 19-25% bioavailable without an obvious dose-dependency between 0.077 and 1.9mg/kg. The urinary elimination of BrO(3)(-) and Br(-) was measured from female F344 rats for four days following administration of single doses of 8.1mgKBrO(3)/kg and for 15 days after a single dose of 5.0mgKBr/kg. BrO(3)(-) elimination was detected over the first 12h, but Br(-) elimination from BrO(3)(-) over the first 48h was 18% lower than expected based on that eliminated from an equimolar dose of Br(-) (15.5±1.6 vs. 18.8±1.2µmol/kg, respectively). The cumulative excretion of Br(-) from KBr vs. KBrO(3) was equivalent 72h after administration. The recovery of unchanged administered BrO(3)(-) in the urine ranged between 6.0 and 11.3% (creatinine corrected) on the 27th day of treatment with concentrations of KBrO(3) of 15, 60, and 400mg/L of drinking water. The recovery of total urinary bromine as Br(-)+BrO(3)(-) ranged between 61 and 88%. An increase in the fraction of the daily BrO(3)(-) dose recovered in the urine was observed at the high dose to both sexes. The deficit in total bromine recovery raises the possibility that some brominated biochemicals may be produced in vivo and more slowly metabolized and eliminated. This was supported by measurements of dose-dependent increases of total organic bromine (TOBr) that was eliminated in the urine. The role these organic by-products play in BrO(3)(-)-induced cancer remains to be established.

Optimization, validation and application of a method for the determination of trichloroethylene in rat plasma by headspace solid-phase microextraction gas chromatography mass spectrometry.

Trichloroethylene (TCE) is a small halogenated compound that has been used extensively as a metal degreaser and a solvent for the past 100 years. As a result of its widespread use, TCE can be found in the groundwater of about one-third of the hazardous waste sites on the United States Environmental Protection Agency's National Priorities List. Human exposure to TCE in the environmental media is of concern because TCE has been found to be carcinogenic in laboratory animals. This paper describes the development and validation of a HS-SPME-GC/MS method for determination of TCE in rat plasma. The effects of different parameters such as sample volume, extraction and desorption conditions, fiber positions and salt addition were investigated and optimized. The method is rapid, simple, sensitive and requires a very small sample volume. The lower limit of quantitation was 0.25 ng/mL and correlation coefficient (r(2)) values for the linear range of 0.25-100 ng/mL were 0.996 or greater. The precision and accuracy for intra-day and inter-day were better than 8.0%. This validated method was successfully applied to study the toxicokinetic behavior of TCE following low levels of oral administration.

In situ derivatization/solid-phase microextraction coupled with gas chromatography/negative chemical ionization mass spectrometry for the determination of trichloroethylene metabolites in rat blood.

An in situ derivatization solid-phase microextraction (SPME) method has been developed for the determination of the trichloroethylene (TCE) metabolites, trichloroacetic acid (TCA), dichloroacetic acid (DCA) and trichloroethanol (TCOH), in rat blood. The analytical procedure involves derivatization of TCA and DCA to their ethyl esters with acidic ethanol, headspace sampling using SPME, and gas chromatography/negative chemical ionization mass spectrometry (GC/NCI-MS) determination. Parameters affecting both derivatization efficiency and the headspace SPME procedure, such as the concentration of sulfuric acid, amount of ethanol, derivatization-extraction temperature and time, sample preheating time, agitator speed and desorption conditions, were optimized. The method showed good linearity over the range of 1-1000 ng/mL in rat blood for each metabolite with correlation coefficients (R(2)) higher than 0.99. The intra-day and inter-day precision and accuracy were less than 10%. The relative recoveries for all analytes were greater than 84%. Validation results demonstrated that selected ion monitoring of the (35)Cl and (37)Cl isotopes using NCI resulted in reliable and sensitive quantitation of all three TCE metabolites. This validated method was successfully applied to study the toxicokinetic behavior of TCE metabolites following a 1 mg/kg oral dose of TCE.

Determination of trichloroethylene in biological samples by headspace solid-phase microextraction gas chromatography/mass spectrometry.

A simple, rapid and sensitive method for determination of trichloroethylene (TCE) in rat blood, liver, lung, kidney and brain, using headspace solid-phase microextraction (HS-SPME) and gas chromatography/mass spectrometry (GC/MS), is presented. A 100-microm polydimethylsiloxane (PDMS) fiber was selected for sampling. The major analytical parameters including extraction and desorption temperature, extraction and desorption time, salt addition, and sample preheating time were optimized for each of the biological matrices to enhance the extraction efficiency and sensitivity of the method. The lower limits of quantitation for TCE in blood and tissues were 0.25ng/ml and 0.75ng/g, respectively. The method showed good linearity over the range of 0.25-100ng TCE/ml in blood and 0.75-300ng TCE/g in tissues, with correlation coefficient (R(2)) values higher than 0.994. The precision and accuracy for intra-day and inter-day measurements were less than 10%. The relative recoveries of TCE respect to deionized water from all matrices were greater than 55%. Stability tests including autosampler temperature and freeze and thaw of specimens were also investigated. This validated method was successfully applied to study the toxicokinetics of TCE following administration of a low oral dose.

Trace level determination of trichloroethylene in biological samples by headspace solid-phase microextraction gas chromatography/negative chemical ionization mass spectrometry.

A gas chromatography/negative chemical ionization mass spectrometry (GC/NCIMS) method using headspace solid-phase microextraction (HS-SPME) was developed for the determination of trichloroethylene (TCE) in blood, liver, kidney, lung and brain. The method was optimized with respect to several parameters including extraction time, extraction temperature, desorption time and salt addition. The method showed good linearity over the range of 0.025-25 ng/mL in blood and 0.075-75 ng/g in tissues with correlation coefficient (R2) values higher than 0.99. The precision and accuracy for intra-day and inter-day measurements were less than 10%. The relative recoveries of all matrices were greater than 52%. Samples showed no significant loss during 8 h in the autosampler and following three freeze/thaw cycles. Validation results demonstrated that selected-ion monitoring of the 35Cl and 37Cl isotopes using NCI resulted in reliable and sensitive quantitation. This validated method was successfully applied to study the toxicokinetics of TCE following oral administration of extremely low doses of this potential human carcinogen to small test animals (rats).

Formulation-dependent toxicokinetics explains differences in the GI absorption, bioavailability and acute neurotoxicity of deltamethrin in rats.

The acute neurotoxicity of pyrethroid insecticides varies markedly with the dosage vehicle employed. The objective of the present study was to assess the influence of two common vehicles on the bioavailability and toxicokinetics (TK) of a representative pyrethroid insecticide, deltamethrin (DLM), to determine whether the vehicles influence toxic potency by modifying the chemical's TK. Adult, male Sprague-Dawley rats were administered DLM iv or po, either by dissolving it in glycerol formal (GF) or by suspending it in Alkamuls (AL). Groups of rats received 10mg DLM/kg by gavage in each vehicle, as well as 2 mg/kg in GF or 10mg/kg in AL by iv injection. Serial blood samples were collected over 96 h and analyzed for their DLM content by HPLC. In a second experiment, plasma, brain, fat, liver and lung DLM concentrations were measured 2h after giving 10mg DLM/kg orally in GF or AL. In a third experiment rats received 2 or 10mg DLM/kg iv in AL or 2mg DLM/kg iv in GF. Lung DLM content was determined 15 min post injection. DLM particle size in both formulations was measured under a phase contrast microscope. DLM appeared to be completely dissolved in GF, while particle size ranged from <5 to >50 microm in AL. The bioavailability of DLM in the aqueous AL suspension was approximately 9-fold lower than in GF (1.7% versus 15%). Blood C(max) (0.95+/-0.27 versus 0.09+/-0.01 microg/ml) and AUC(0)(48h) (5.49+/-0.22 versus 0.61+/-0.14 microg.h/ml) were markedly higher in the GF gavage group. Tissue DLM levels were also significantly higher in the GF animals at 2h. The 10mg/kg po and 2mg/kg iv doses of DLM in GF produced moderate salivation and slight tremors. Rats receiving the insecticide in AL were asymptomatic. IV injection of the AL suspension resulted in trapping of much of the dose in the pulmonary capillaries. As anticipated, the injected suspension had a longer half-life and slower clearance than did the GF formulation. In summary, limited dissolution of the highly lipophilic DLM particles in the AL suspension severely limited DLM's GI absorption, bioavailability, target organ deposition and acute neurotoxic potency.

Characterization of deltamethrin metabolism by rat plasma and liver microsomes.

Deltamethrin, a widely used type II pyrethroid insecticide, is a relatively potent neurotoxicant. While the toxicity has been extensively examined, toxicokinetic studies of deltamethrin and most other pyrethroids are very limited. The aims of this study were to identify, characterize, and assess the relative contributions of esterases and cytochrome P450s (CYP450s) responsible for deltamethrin metabolism by measuring deltamethrin disappearance following incubation of various concentrations (2 to 400 microM) in plasma (esterases) and liver microsomes (esterases and CYP450s) prepared from adult male rats. While the carboxylesterase metabolism in plasma and liver was characterized using an inhibitor, tetra isopropyl pyrophosphoramide (isoOMPA), CYP450 metabolism was characterized using the cofactor, NADPH. Michaelis-Menten rate constants were calculated using linear and nonlinear regression as applicable. The metabolic efficiency of these pathways was estimated by calculating intrinsic clearance (Vmax/Km). In plasma, isoOMPA completely inhibited deltamethrin biotransformation at concentrations (2 and 20 microM of deltamethrin) that are 2- to 10-fold higher than previously reported peak blood levels in deltamethrin-poisoned rats. For carboxylesterase-mediated deltamethrin metabolism in plasma, Vmax=325.3+/-53.4 nmol/h/ml and Km=165.4+/-41.9 microM. Calcium chelation by EGTA did not inhibit deltamethrin metabolism in plasma or liver microsomes, indicating that A-esterases do not metabolize deltamethrin. In liver microsomes, esterase-mediated deltamethrin metabolism was completely inhibited by isoOMPA, confirming the role of carboxylesterases. The rate constants for liver carboxylesterases were Vmax=1981.8+/-132.3 nmol/h/g liver and Km=172.5+/-22.5 microM. Liver microsomal CYP450-mediated biotransformation of deltamethrin was a higher capacity (Vmax=2611.3+/-134.1 nmol/h/g liver) and higher affinity (Km=74.9+/-5.9 microM) process than carboxylesterase (plasma or liver) detoxification. Genetically engineered individual rat CYP450s (Supersomes) were used to identify specific CYP450 isozyme(s) involved in the deltamethrin metabolism. CYP1A2, CYP1A1, and CYP2C11 in decreasing order of importance quantitatively, metabolized deltamethrin. Intrinsic clearance by liver CYP450s (35.5) was more efficient than that by liver (12.0) or plasma carboxylesterases (2.4).

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

Effect of PCB 126 on hepatic metabolism of thyroxine and perturbations in the hypothalamic-pituitary-thyroid axis in the rat.

The objective of this research was to examine the time- and dose- dependent disturbances in the hypothalamic-pituitary-thyroid (HPT) axis of adult male rats administered a potent coplanar (non-ortho) PCB, 3,3',4,4',5-pentachlorobiphenyl (PCB 126). Adult male Sprague-Dawley rats were administered a single oral bolus dose of 0, 7.5, 75, or 275 microg PCB 126/kg bw dissolved in corn oil. The rats were sacrificed periodically over 22 days. The 7.5-microg/kg dose induced hepatic ethoxyresorufin-O-deethylation EROD activity, but no changes were observed in hepatic uridine diphosphate glucuronyl transferases (UDPGTs) activity or serum TSH, T4, or fT4 concentrations. The two highest doses caused a modest decline in weight gain, induced hepatic EROD and UDPGT activities, increased serum TSH concentrations, and decreased serum T4 and fT4 concentrations. The amount of thyroxine glucuronide formed daily (pM/mg protein) increased linearly with the area-under-the-concentration-curve (AUCC) for PCB 126 in liver (microg/kg/day) and then slowed at the 275-microg/kg PCB 126 dose. Perturbations in the HPT axis were nonlinear with respect to PCB 126 dosing. As expected, an inverse relationship between the AUCC for serum T4 (microg/dl/day) and the AUCC for serum TSH (ng/dl/day) was observed; however, the relationship was highly nonlinear. These data support a mode of action for PCB 126 involving induction of hepatic UDPGTs by the aryl hydrocarbon receptor AhR. However, the dose-response characteristics of the HPT axis are nonlinear and complex, requiring sophisticated tools, such as PBPK models, to characterize dose response.