Influence of dosing volume on the neurotoxicity of bifenthrin

Use of subcutaneous transponders to monitor body temperature in laboratory rats

Implantable radiofrequency transponders may be adequate for the characterization of hazardous chemicals targeting body temperature control in experimental animals when colonic probes and automated monitoring systems based on intraperitoneal transmitters are not available, installable or applicable for any reason. In this work, we summarize a series of experiments showing the implantation protocol and utility of rice-grain size transponders to monitor subcutaneous temperature (T_sc) after exposure to pharmacological or toxicological treatments targeting body temperature control in laboratory rats. In addition, to explore the responsiveness of this thermometric system, the influence of physiological activity on T_sc readings was examined by monitoring T_sc after a motor exercise in a RotaRod system. Moreover, we characterized the effects of acute oral administration of the pyrethroid insecticide permethrin (PRM) in corn oil (1 mL/kg) on T_sc. PRM has been previously reported to cause dose-related increases in core temperature after administering oral doses ≥75 mg/kg, with peak effects at 2-4 h in adult rats. We monitored T_sc at 30 min intervals over a 4 h period after exposure to PRM (40-160 mg/kg). PRM caused a moderate increase in T_sc starting at ~3.5 h. Overall, T_sc assays showed minimal animal stress (if any) and rapid animal recovery from transponder implantation, simplicity to collect data, convenient testing room space requirements, and a competitive global cost per animal examined. However, various experimental factors may greatly influence the variability within and between individuals, some of which can be controlled by carefully setting up experimental conditions.

Bifenthrin reduces pregnancy potential via induction of oxidative stress in porcine trophectoderm and uterine luminal epithelial cells

Exposure to pesticides has become a serious concern for the environment and human health. Bifenthrin, a synthetic pyrethroid pesticide, is one of the most frequently used pesticides worldwide. Despite the toxic potential of bifenthrin, no studies have elucidated the cytotoxic response of bifenthrin in maternal and fetal cells that are involved in the implantation process. In this study, the cytotoxic effect of bifenthrin was investigated using porcine trophectoderm (pTr) and uterine luminal epithelial (pLE) cells. The results showed that bifenthrin suppressed cell proliferation and viability in pTr and pLE cells. In particular, bifenthrin induced cell cycle arrest, resulting in apoptosis in both cell lines. We found that bifenthrin damaged the mitochondria and induced the production of reactive oxygen species, causing endoplasmic reticulum stress and calcium dysregulation in pTr and pLE cells. Finally, bifenthrin altered the MAPK/PI3K signaling pathway and pregnancy-related gene expression. Collectively, our results suggest that bifenthrin reduces the implantation potential of embryos and may help elucidate the mechanisms underlying toxin-derived cytotoxicity in maternal and fetal cells.

Thermal effects on tissue distribution, liver biotransformation, metabolism and toxic responses in Mongolia racerunner (Eremias argus) after oral administration of beta-cyfluthrin

Effects of temperature on metabolism/biotransformation and toxicokinetics to lizards are significant, but frequently ignored in toxicology studies. Beta-cyfluthrin (BC) is a pyrethroid insecticide and has been widely used globally. The study aimed to understand the diverse adverse effects of BC to the lizard (Eremias argus) at different temperature regimes. We carried out a single oral BC treatment (20 mg/kg bw) for toxicokinetic study and a 7-day BC (10 mg/kg bw) gavage to look at toxicology by monitoring changes in the biomarkers HSP70, SOD, MDA, CarE, UDPGT, GST, cyp genes, and other metabolic responses. Results showed that BC was lethal to lizards, showing oxidative damages in the liver at ambient temperature (25 °C). Heat stress (35 °C) could exacerbate the oxidative damage (MDA increased) caused by BC, due to the disorder of the antioxidant defense system. The result of tissue distribution and toxicokinetic study also showed that temperature affected the BC biotransformation in lizards. The biotransformation of BC maybe relates to the activation of CarE and UDGPT by heat stress. However, the cyp system and GST didn't increase under BC or/and heat treatments. ¹H-NMR metabolomics analysis showed that BC or/and heat stress interfered with energy and amino acid metabolism of the liver. Unlike acute lethal toxicity, the occurrence of the BC and heat stresses has detrimental effects on lizard individuals and populations on sub-lethal levels. Our results indicate that pollution and global warming (or some other extremely weather) may generate significant and harmful effects on lizards.

Effects of Acute Deltamethrin Exposure in Adult and Developing Sprague Dawley Rats on Acoustic Startle Response in Relation to Deltamethrin Brain and Plasma Concentrations

Deltamethrin (DLM) is a commonly used pesticide that helps to control crop destruction, disease, and nuisance insects. In rodents DLM can produce choreoathetosis, salivation, and decreased acoustic startle responses (ASR). Herein, adult Sprague Dawley rats were assessed for ASR 2 h after DLM delivered in 5 ml/kg corn oil, however no decrease was observed. Therefore, a test-retest protocol was used to reduce variability, and the effects on ASR on postnatal day 15 (P15) and adult rats were assessed 2, 4, 6, and 8 h after DLM administration (0, 1, 2, or 4 mg/kg for P15 rats and 0, 2, 8, or 25 mg/kg for adults). In a separate set of rats identically treated, DLM levels were determined in blood and brain. DLM (8 or 25 mg/kg) in adult rats decreased ASR up to 4 h, whereas in P15 rats decreases were observed between 2 and 8 h. The adult 25 mg/kg group showed consistent signs of salivation and tremor, whereas in P15 rats salivation was observed in the 2 and 4 mg/kg groups and tremor was observed at all doses over the 8-h period. Mortality was observed in all P15 dose groups but not in adults. Dose-dependent increases of DLM in blood and brain regardless of age were observed. At approximately equivalent whole brain concentrations, effects were more pronounced in P15 rats than in adult rats. Comparable brain levels of DLM do not explain differences in ASR and tremor between the P15 and adult rats. These data indicate age-dependent differences in sensitivity to DLM.

Relationship Between the Dose Administered, Target Tissue Dose, and Toxicity Level After Acute Oral Exposure to Bifenthrin and Tefluthrin in Young Adult Rats

Most pyrethroid insecticides (PYRs) share a similar primary target site in mammals. However, the potency estimates of the lethal and sublethal effects of these compounds differ up to 103-fold. The aim of this study was to evaluate the relationship between the dose administered, the target tissue dose, and the effect of 2 highly toxic PYRs, tefluthrin (TEF; 0.1-9 mg/kg) and bifenthrin (BIF; 0.5-12 mg/kg), by using the oral route, a corn oil vehicle (1 ml/kg) and subcutaneous temperature (Tsc) monitoring assays in adult rats. The Tsc was determined at 30-min intervals for 5 h (TEF) or 4.5 h (BIF) after dosing. Rats were sacrificed at 6 h after dosing, and BIF and TEF concentrations were determined in blood (Bd), liver (Lv), and cerebellum (Cb) by using a GC-ECD system. The minimal effective dose of BIF (3 mg/kg) affecting Tsc was similar to that found in prior studies using other testing paradigms. Regarding TEF, a very steep relationship between the dose administered and toxicity was observed, with a near-threshold to low-effective range for Tsc at 0.1-6 mg/kg, and a near lethal syndrome at ≥ 7.5 mg/kg. At 6-7.5 mg/kg TEF, the Cb/Bd and Cb/Lv concentration ratios were both > 1. Conversely, for BIF, the Cb concentration was barely over the Bd concentration and the Cb/Lv concentration ratio remained < 1. Our results and previous findings call for more comprehensive consideration to establish the relevance of the distribution into target tissues and the tissue dosimetry for health risks through the exposure to PYRs in humans.

Effect of β-cyfluthrin (synthetic pyrethroid) on learning, muscular coordination and oxidative stress in Swiss albino mice

The present study was planned to evaluate neurotoxic effects of β-cyfluthrin in female Swiss albino mice. Two doses of β-cyfluthrin, specifically, one-tenth of median lethal dose (LD₅₀) and one-twentieth of LD₅₀, were selected for the study. Open-field behaviour, exploratory behaviour and emotional status were affected, and animals showed anxiety-like behaviour after β-cyfluthrin administration. Spatial learning was decreased using the Hebb-Wiliams maze. Acetylcholinesterase enzyme activity significantly decreased in the treated animals. The administration of β-cyfluthrin caused increased lipid peroxidation (malondialdehyde) and decreased superoxide dismutase, catalase and glutathione peroxidase activity in brain tissue. In conclusion, β-cyfluthrin caused neurotoxicity as well as oxidative damage in the brain of Swiss albino mice at the tested dose levels.

Lethality of synthetic and natural acaricides to worker honey bees (Apis mellifera) and their impact on the expression of health and detoxification-related genes

In this study, honey bees (Apis mellifera L.) were exposed to LD₀₅ and LD₅₀ doses of five commonly used acaricides for controlling the parasitic mite, Varroa destructor. LD₅₀ values at 48 h post-treatment showed that tau-fluvalinate was the most toxic, followed by amitraz, coumaphos, thymol, and formic acid. However, the hazard ratios, which estimate the hive risk level based on a ratio of a standard dose of acaricide per hive to the LD₅₀ of the acaricide, revealed that tau-fluvalinate was the most hazardous followed by formic acid, coumaphos, amitraz, and thymol. The expression of the honey bee acetylcholinesterase gene increased after treatment with the LD₀₅ and LD₅₀ acaricide doses and could distinguish three patterns in the timing and level of increased expression between acaricides: one for amitraz, one for tau-fluvalinate and formic acid, and one for coumaphos and thymol. Conversely, changes in cytochrome P450 gene expression could also be detected in response to all five acaricides, but there were no significant differences between them. Changes in vitellogenin gene expression could only detect the effects of tau-fluvalinate, amitraz, or coumaphos treatment, which were not significantly different from each other. Among the acaricides tested, coumaphos, amitraz, and thymol appear to be the safest acaricides based on their hazard ratios, and a good marker to detect differences between the effects of sub-lethal doses of acaricides is monitoring changes in acetylcholinesterase gene expression.

From the Cover: Vulnerability of C6 Astrocytoma Cells After Single-Compound and Joint Exposure to Type I and Type II Pyrethroid Insecticides

A primary mode-of-action of all pyrethroid insecticides (PYRs) is the disruption of the voltage-gated sodium channel electrophysiology in neurons of target pests and nontarget species. The neurological actions of PYRs on non-neuronal cells of the nervous system remain poorly investigated. In the present work, we used C6 astrocytoma cells to study PYR actions (0.1-50 μM) under the hypothesis that glial cells may be targeted by and vulnerable to PYRs. To this end, we characterized the effects of bifenthrin (BF), tefluthrin (TF), α-cypermethrin (α-CYP), and deltamethrin (DM) on the integrity of nuclear, mitochondrial, and lysosomal compartments. In general, 24- to 48-h exposures produced concentration-related impairment of cell viability. In single-compound, 24-h exposure experiments, effective concentration (EC)₁₅s 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT assay) were computed as follows (in μM): BF, 16.1; TF, 37.3; α-CYP, 7.8; DM, 5.0. We found concentration-related damage in several C6-cell subcellular compartments (mitochondria, nuclei, and lysosomes) at ≥ 10^-1 μM levels. Last, we examined a mixture of all PYRs (ie, Σ individual EC₁₅) using MTT assays and subcellular analyses. Our findings indicate that C6 cells are responsive to nM levels of PYRs, suggesting that astroglial susceptibility may contribute to the low-dose neurological effects caused by these insecticides. This research further suggests that C6 cells may provide relevant information as a screening platform for pesticide mixtures targeting nervous system cells by expected and unexpected toxicogenic pathways potentially contributing to clinical neurotoxicity.

The pharmacokinetic properties of bifenthrin in the rat following multiple routes of exposure

BACKGROUND

Pyrethroids generally have relatively low oral toxicity but variable inhalation toxicity. The pharmacokinetics of bifenthrin in the rat after oral, inhalation and intravenous administration is described. Pyrethroid acute toxicity via oral and inhalation routes is also presented.

RESULTS

Groups of male rats were dosed by oral gavage at 3.1 mg kg(-1) in 1 mL kg(-1) of corn oil (the critical, acute, oral benchmark dose lower limit, BMDL) and at an equivalent dose by inhalation (0.018 mg L(-1)) for 4 h.  At 2, 4, 6, 8 and 12 h after dosing initiation, blood plasma and brain bifenthrin concentrations were measured. The maximum concentrations of bifenthrin in plasma were 361 ng mL(-1) or 0.853 μM (oral) and 232 ng mL(-1) or 0.548 μM (inhalation), and in brain they were 83 and 73 ng g(-1). The area under the concentration versus time curve (AUC) values were 1969 h ng mL(-1) (plasma) and 763 h ng mL(-1) (brain) following oral gavage dosing, and 1584 h ng mL(-1) (plasma) and 619 h ng mL(-1) (brain) after inhalation. Intravenous dosing resulted in apparent terminal half-life (t1/2 ) values of 13.4 h (plasma) and 11.1 h (brain) and in AUC0-∞ values of 454 and 1566 h ng mL(-1) for plasma and brain. Clearance from plasma was 37 mL min(-1) kg(-1).

CONCLUSION

Peak plasma nd brain concentrations were generally a little higher after oral dosing (by ca 14%). Inhalation administration of bifenthrin did not cause increases in exposure in plasma or brain by avoiding first-pass effects in the liver. The elimination t1/2 was comparable with other pyrethroids and indicated little bioaccumulation potential. These pharmokinetics data allow risks following inhalation exposure to be modeled using oral toxicity data.

Exposure to bifenthrin causes immunotoxicity and oxidative stress in male mice

Bifenthrin (BF) is one of the most commonly used pesticides among the synthetic pyrethroids. The effects of BF exposure on the induction of immunotoxicity and oxidative stress were studied both in adolescent and adult male ICR mice. Both the weights of the spleen and thymus decreased significantly in the adolescent mice when they were treated with 20 mg/kg BF for 3 weeks. We found that the 3-week oral administration of BF during puberty increased the transcriptional levels of the genes TNF and IL2 in the spleen and IL2 as well as IL4 in the thymus. The effect of BF exposure on the induction of oxidative stress was also studied in serum and liver samples. The total antioxidant capacity and activity of superoxide dismutase were altered significantly in the serum of the 20 mg/kg BF-treated adolescent mice, and the activity of glutathione peroxidase (GPX) decreased significantly in the serum of adolescent and adult mice after 3 weeks of oral administration of 20 mg/kg BF. Compared to serum, hepatic GSH content increased significantly in both the adolescent and adult mice exposed to 20 mg/kg BF; hepatic CAT and GPX activities were altered significantly, even in adolescent mice, after treatment with 10 mg/kg BF. Taken together, the results of this study suggest that exposure to BF, especially during puberty, has the potential to induce immunotoxicity accompanied by oxidative stress in male mice. These findings will help in elucidating the mechanism of toxicity induced by BF in mice.

Nanomolar bifenthrin alters synchronous Ca2+ oscillations and cortical neuron development independent of sodium channel activity

Bifenthrin, a relatively stable type I pyrethroid that causes tremors and impairs motor activity in rodents, is broadly used. We investigated whether nanomolar bifenthrin alters synchronous Ca(2+) oscillations (SCOs) necessary for activity-dependent dendritic development. Primary mouse cortical neurons were cultured 8 or 9 days in vitro (DIV), loaded with the Ca(2+) indicator Fluo-4, and imaged using a Fluorescence Imaging Plate Reader Tetra. Acute exposure to bifenthrin rapidly increased the frequency of SCOs by 2.7-fold (EC50 = 58 nM) and decreased SCO amplitude by 36%. Changes in SCO properties were independent of modifications in voltage-gated sodium channels since 100 nM bifenthrin had no effect on the whole-cell Na(+) current, nor did it influence neuronal resting membrane potential. The L-type Ca(2+) channel blocker nifedipine failed to ameliorate bifenthrin-triggered SCO activity. By contrast, the metabotropic glutamate receptor (mGluR)5 antagonist MPEP [2-methyl-6-(phenylethynyl)pyridine] normalized bifenthrin-triggered increase in SCO frequency without altering baseline SCO activity, indicating that bifenthrin amplifies mGluR5 signaling independent of Na(+) channel modification. Competitive [AP-5; (-)-2-amino-5-phosphonopentanoic acid] and noncompetitive (dizocilpine, or MK-801 [(5S,10R)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate]) N-methyl-d-aspartate antagonists partially decreased both basal and bifenthrin-triggered SCO frequency increase. Bifenthrin-modified SCO rapidly enhanced the phosphorylation of cAMP response element-binding protein (CREB). Subacute (48 hours) exposure to bifenthrin commencing 2 DIV-enhanced neurite outgrowth and persistently increased SCO frequency and reduced SCO amplitude. Bifenthrin-stimulated neurite outgrowth and CREB phosphorylation were dependent on mGluR5 activity since MPEP normalized both responses. Collectively these data identify a new mechanism by which bifenthrin potently alters Ca(2+) dynamics and Ca(2+)-dependent signaling in cortical neurons that have long term impacts on activity driven neuronal plasticity.

cis-Bifenthrin enantioselectively induces hepatic oxidative stress in mice

Bifenthrin (BF), as a chiral synthetic pyrethroid, is widely used to control field and household pests. In China, the commercial cis-BF contained two enantiomers including 1R-cis-BF and 1S-cis-BF. However, the difference in oxidative stress induced by the two enantiomers in mice still remains unclear. In the present study, 4 week-old adolescent male ICR mice were orally administered cis-BF, 1R-cis-BF or 1S-cis-BF daily for 2, 4 and 6 weeks at doses of 5 mg/kg/day, respectively. We found that the hepatic reactive oxygen species (ROS) levels, as well as the malondialdehyde (MDA) and glutathione (GSH) content both in the serum and liver increased significantly in the 4 or 6 weeks 1S-cis-BF treated groups. The activities of superoxide dismutase (SOD) and catalase (CAT) also changed significantly in the serum and liver of 1S-cis-BF treated mice. More importantly, the significant differences in MDA content and CAT activity both in the serum and liver, and the activities of total antioxidant capacity (T-AOC) and SOD in serum were also observed between the 1S-cis-BF and 1R-cis-BF treated groups. Moreover, the transcription of oxidative stress response related genes including Sod1, Cat and heme oxygenase-1(Ho-1) in the liver of 1S-cis-BF treated groups were also significant higher than those in 1R-cis-BF treated group. Thus, it was concluded that cis-BF induced hepatic oxidative stress in an enantiomer specific manner in mice when exposed during the puberty, and that 1S-cis-BF showed much more toxic in hepatic oxidative stress than 1R-cis-BF.

Critical consideration of the multiplicity of experimental and organismic determinants of pyrethroid neurotoxicity: a proof of concept

Pyrethroids (PYR) are pesticides with high insecticidal activity that may disrupt neuronal excitability in target and nontarget species. The accumulated evidence consistently showed that this neurophysiologic action is followed by alterations in motor, sensorimotor, neuromuscular, and thermoregulatory responses. Nevertheless, there are some equivocal results regarding the potency of PYR in lab animals. The estimation of potency is an important step in pesticide chemical risk assessment. In order to identify the variables influencing neurobehavioral findings across PYR studies, evidence on experimental and organismic determinants of acute PYR-induced neurotoxicity was reviewed in rodents. A comprehensive analysis of these studies was conducted focusing on test material and dosing conditions, testing conditions, animal models, and other determinants such as testing room temperature. Variations in the severity of the neurotoxicity, under lab-controlled conditions, was explained based upon factors including influence of animal species and age, test material features such as chemical structure and stereochemistry, and dosing conditions such as vehicle, route of exposure, and dose volume. If not controlled, the interplay of these factors may lead to large variance in potency estimation. This review examined the scope of acute toxicological data required to determine the safety of pesticide products, and factors and covariates that need to be controlled in order to ensure that predictivity and precaution are balanced in a risk assessment process within a reasonable time-frame, using acute PYR-induced neurotoxicity in rodents as an exemplar.

Pesticide exposure and neurodevelopmental outcomes: review of the epidemiologic and animal studies

Assessment of whether pesticide exposure is associated with neurodevelopmental outcomes in children can best be addressed with a systematic review of both the human and animal peer-reviewed literature. This review analyzed epidemiologic studies testing the hypothesis that exposure to pesticides during pregnancy and/or early childhood is associated with neurodevelopmental outcomes in children. Studies that directly queried pesticide exposure (e.g., via questionnaire or interview) or measured pesticide or metabolite levels in biological specimens from study participants (e.g., blood, urine, etc.) or their immediate environment (e.g., personal air monitoring, home dust samples, etc.) were eligible for inclusion. Consistency, strength of association, and dose response were key elements of the framework utilized for evaluating epidemiologic studies. As a whole, the epidemiologic studies did not strongly implicate any particular pesticide as being causally related to adverse neurodevelopmental outcomes in infants and children. A few associations were unique for a health outcome and specific pesticide, and alternative hypotheses could not be ruled out. Our survey of the in vivo peer-reviewed published mammalian literature focused on effects of the specific active ingredient of pesticides on functional neurodevelopmental endpoints (i.e., behavior, neuropharmacology and neuropathology). In most cases, effects were noted at dose levels within the same order of magnitude or higher compared to the point of departure used for chronic risk assessments in the United States. Thus, although the published animal studies may have characterized potential neurodevelopmental outcomes using endpoints not required by guideline studies, the effects were generally observed at or above effect levels measured in repeated-dose toxicology studies submitted to the U.S. Environmental Protection Agency (EPA). Suggestions for improved exposure assessment in epidemiology studies and more effective and tiered approaches in animal testing are discussed.

Correlation of tissue concentrations of the pyrethroid bifenthrin with neurotoxicity in the rat

The potential for human exposure to pyrethroid pesticides has prompted pharmacodynamic and pharmacokinetic research to better characterize risk. This work tested the hypothesis that blood and brain concentrations of the pyrethroid bifenthrin are predictive of neurotoxic effects. Adult male Long Evans rats received a single oral dose of bifenthrin dissolved in corn oil. Using figure-eight mazes, motor activity was measured for 1h at 4- and 7-h following exposure to bifenthrin (0-16mg/kg or 0-9mg/kg, respectively; n=4-8/group). Whole blood and brains were collected immediately following motor activity assays. Bifenthrin concentrations in blood and brain were quantified using HPLC/MS/MS. Bifenthrin exposure decreased motor activity from 20% to 70% in a dose-dependent manner at both time points. The relationship between motor activity data and administered dose, and blood and brain bifenthrin concentrations were described using a sigmoidal E(max) model. The relationships between motor activity and administered dose or blood concentrations were different between the 4- and 7-h time points. The relationship between motor activity and brain concentration was not significantly different between the two time points. These data suggest that momentary brain concentration of bifenthrin may be a more precise dose metric for predicting behavioral effects because the relationship between brain concentration and locomotor activity is independent of the time of exposure.

Formulated Beta-Cyfluthrin Shows Wide Divergence in Toxicity among Bird Species

It is generally assumed that the toxicity of pyrethroid insecticides to birds is negligible, though few species have been tested. The oral acute toxicity of formulated beta-cyfluthrin was determined for canaries (Serinus sp.), shiny cowbirds (Molothrus bonariensis), and eared doves (Zenaida auriculata). Single doses were administered to adults by gavage. Approximate lethal doses 50 (LD₅₀) and their confidence intervals were determined by approximate D-optimal design. Canaries were found to be substantially more sensitive to formulated beta-cyfluthrin (LD₅₀ = (170 ± 41) mg/kg) than the other two species tested (LD₅₀ = (2234 ± 544) mg/kg and LD₅₀ = (2271 ± 433) mg/kg, resp.). The LD₅₀ obtained for canaries was also considerably lower than typical toxicity values available in the literature for pyrethroids. This study emphasizes the need for testing a broader range of species with potentially toxic insecticides, using modern up and down test designs with minimal numbers of birds.

Physiologically based pharmacokinetic modeling of deltamethrin: development of a rat and human diffusion-limited model

Mirfazaelian et al. developed a physiologically based pharmacokinetic (PBPK) model for the pyrethroid pesticide deltamethrin in the rat. This model describes gastrointestinal (GI) tract absorption as a saturable process mediated by phase III efflux transporters which pump deltamethrin out of the intestinal enterocytes into the GI tract lumen, resulting in minimal net absorption at low concentrations and increasing absorption at higher concentrations. In the present study, the dose dependency in absorption of deltamethrin was examined in male Long Evans rats using po exposures predicted by the Mirfazaelian model to yield different po bioavailability values. No difference in the bioavailability from single po doses of 0.3 and 3.0 mg/kg deltamethrin was observed. Based on this finding, the Mirfazaelian PBPK model was modified to exclude a saturable absorption process. Other changes to the Mirfazaelian model included describing all tissue compartments with diffusion-limited kinetics and a single blood compartment. These changes improved model predictions of deltamethrin tissue concentration data from the present study and the literature. The rat model was then scaled to humans. The model predicted a twofold greater peak deltamethrin brain concentration and threefold greater area under the curve (AUC(0-48 h)) for humans following an po exposure of 1 mg/kg. Based on this model, humans would have greater distribution of deltamethrin to the brain for the same administered po dose compared to rats. The relative sensitivity to deltamethrin between rats and humans depends on both pharmacokinetic and pharmacodynamic differences. Species differences in the pharmacodynamic responses to deltamethrin between rats and humans remain uncharacterized.

Evidence for dose-additive effects of pyrethroids on motor activity in rats

BACKGROUND

Pyrethroids are neurotoxic insecticides used in a variety of indoor and outdoor applications. Previous research characterized the acute dose-effect functions for 11 pyrethroids administered orally in corn oil (1 mL/kg) based on assessment of motor activity.

OBJECTIVES

We used a mixture of these 11 pyrethroids and the same testing paradigm used in single-compound assays to test the hypothesis that cumulative neurotoxic effects of pyrethroid mixtures can be predicted using the default dose-addition theory.

METHODS

Mixing ratios of the 11 pyrethroids in the tested mixture were based on the ED30 (effective dose that produces a 30% decrease in response) of the individual chemical (i.e., the mixture comprised equipotent amounts of each pyrethroid). The highest concentration of each individual chemical in the mixture was less than the threshold for inducing behavioral effects. Adult male rats received acute oral exposure to corn oil (control) or dilutions of the stock mixture solution. The mixture of 11 pyrethroids was administered either simultaneously (2 hr before testing) or after a sequence based on times of peak effect for the individual chemicals (4, 2, and 1 hr before testing). A threshold additivity model was fit to the single-chemical data to predict the theoretical dose-effect relationship for the mixture under the assumption of dose additivity.

RESULTS

When subthreshold doses of individual chemicals were combined in the mixtures, we found significant dose-related decreases in motor activity. Further, we found no departure from the predicted dose-additive curve regardless of the mixture dosing protocol used.

CONCLUSION

In this article we present the first in vivo evidence on pyrethroid cumulative effects supporting the default assumption of dose addition.

Neurobehavioral toxicology of pyrethroid insecticides in adult animals: a critical review

Pyrethroids are pesticides with high selectivity for insects. In order to identify strengths and gaps in the database for pyrethroid neurobehavioral toxicology, we have critically analyzed the data from peer-reviewed literature. This review includes dose-response data that have been recently generated demonstrating consistent findings for low-dose, acute, oral exposure to pyrethroids in small rodents. All pyrethroids tested (i.e., about twenty compounds), regardless of structure, produce a decrease in motor activity in a variety of test protocols. The range of relative potencies varies more than two orders of magnitude, and thresholds for motor activity were found well below doses that produce overt signs of poisoning. Six compounds (allethrin, permethrin, cis-permethrin, deltamethrin, cypermethrin, and fenvalerate) impair schedule-controlled operant responding, seven compounds (pyrethrum, bifenthrin, S-bioallethrin, permethrin, beta-cyfluthrin, cypermethrin, and deltamethrin) decrease grip strength, and two compounds (deltamethrin and alpha-cypermethrin) produce incoordination using the rotarod. In addition, while compounds lacking an alpha-cyano group (e.g., cismethrin, permethrin, bifenthrin) induce an increase in acoustic-evoked startle response amplitude, cyano compounds (e.g., deltamethrin, cypermethrin, cyfluthrin) produce the opposite outcome. Other endpoints (e.g., tremor intensity, sensory response) have been only occasionally explored. A synthesis of the neurobehavioral evidence relating to the action of pyrethroids indicates that some differences in the experimental findings across compounds are also present in the low-effective dose range. For risk assessment purposes, a strategy that takes into account data from an array of neurobehavioral endpoints is needed to capture the heterogeneity of pyrethroid-induced adverse effects and accurately inform policy decisions.

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.