Neurotoxicity Evaluation of N,N-Diethyl-m-toluamide (DEET) in Rats

The neurotoxic potential of N,N-diethyl-m-toluamide (DEET) wasevaluated following acute oral administration or following multigenerationplus chronic dietary administration to the rat. For the acutestudy, rats were administered undiluted DEET at dose levelsof 50, 200, or 500 mg/kg by gavage. A dose level of 500 mg/kgwas considered to be the highest practical dose that could beevaluated in this study based upon observations of overt toxicityat 500 mg/kg and mortality at 1000 mg/ kg in a dose range-findingstudy. The two measures of neurotoxicity evaluated in the acutestudy were functional observational battery (FOB) and motoractivity measurements. An apparent treatment-related effectin thermal response time (increased) was noted for both sexes1 hr after dosing at the 500 mg/kg dose level. A questionableeffect on rearing activity (decreased) also was noted at thesame dose level. For the multigeneration plus chronic dietaryadministration study, rats were administered DEET at dietaryconcentrations of 0, 500, 2000, or 5000 ppm continuously overtwo generations and then chronically for 9 months. A dietaryconcentration of 5000 ppm meets the criteria for a maximum tolerateddose (MTD) based on traditional chronic toxicology assessments.Evaluations included FOB, motor activity, discriminative acquisitionand reversal in an Mmaze, acoustic startle habituation, passiveavoidance acquisition and retention, and microscopic examinationof central and peripheral nervous tissue. The only effect thatwas considered to be possibly treatment-related was a slightincrease in exploratory locomotor activity at the 5000 ppm doselevel. Based on the results of these studies, the nervous systemdoes not appear to be a selective target when DEET is administeredto rats either as a single oral dose at high dose levels orchronically at the MTD.     

Review of the biodistribution and toxicity of the insect repellent N,N-diethyl-m-toluamide (DEET)

A review of the biodistribution and toxicity of the insect repellent N,N-diethyl-m-toluamide (DEET) is presented. Workers using repellent containing this compound may be exposed to greater than 442 g in 6 mo. In human studies, variable penetration into the skin of from 9 to 56% of a topically applied dose and absorption into the circulatory system of approximately 17% have been reported. Excretion of DEET by humans was initially rapid but not as complete as in animal models. Only about one-half of the absorbed DEET was excreted by humans over 5 d. Depot storage of DEET in the skin was also documented. Skin irritant effects, including scarring bullous dermatitis in humans, were reported. One animal study that reported embryotoxicity could not be confirmed by other investigators. The limited testing for mutagenicity and carcinogenicity provided negative results. Neurotoxic effects were observed in workers exposed to 4 g or more per week. Six young girls developed encephalopathies after exposure to unspecified amounts of DEET ranging from small to massive doses. Three of these girls later died. The cause of their death has not been resolved. Because of the lack of information, further research into the absorption, carcinogenicity, and neurotoxic effects is needed.     

Acute toxicity assessment of N,N-diethyl-m-toluamide (DEET) on the oxygen flux of the dinoflagellate Gymnodinium instriatum

Ecotoxicology - Despite the ubiquitous occurrence of N,N-diethyl-m-toluamide (DEET) in aquatic systems, assessments evaluating the toxicity of DEET on phytoplankton species are summed to a single...     

Some cardiovascular effects of the insect repellent N,N-diethyl-m-toluamide (DEET)

Initial toxicological safety evaluations of the insect repellent N,N-diethyl-m-toluamide (DEET) indicated a potential hypotensive effect. The current study was initiated in order to pursue this aspect of DEET toxicity and to elucidate potential mechanisms for this response. Sublethal intraperitoneal injections of DEET in anesthetized rats were found to decrease mean blood pressure and heart rate in a dose-related fashion. Doses ranged from 56 to 225 mg/kg. Dogs treated with 225 mg/kg of DEET exhibited a similar hypotension and bradycardia. Cardiac output was also significantly reduced but stroke volume and total peripheral resistance were not altered. Lead II ECG changes included small increases in P-R and Q-T intervals. In a series of pharmacological studies in rats, DEET was found to decrease the hypotension and bradycardia associated with acetylcholine injection; epinephrine, norepinephrine, and histamine responses were not altered. Atropine pretreatment reduced but did not eliminate the hypotensive effects of DEET.     

Teratologic Evaluations of N,N-Diethyl-m-toluamide (DEET) in Rats and Rabbits

The potential for DEET to produce developmental toxicity wasevaluated in Charles River CD rats and New Zealand White rabbits.Rats were administered undiluted DEET by gavage on GestationalDays (gd) 6–15 at dosage levels of 0, 125, 250, and 750mg/kg/day. Rabbits were administered undiluted DEET by gavageon gd 6–18 at dosage levels of 0, 30, 100, and 325 mg/kg/day.Group sizes were 25 females per group for rats and 16 femalesper group for rabbits. Control rats and rabbits were ad ministeredcorn oil at the same dosage volumes administered in the high-doseDEET groups. In rats, maternal toxicity in the form of clinicalsigns including two deaths and depressed body weight and foodconsumption was observed at the high-dose level of 750 mg/kg/day.Rat fetal body weights per litter also were reduced at 750 mg/kg/day.In rabbits, maternal toxicity in the form of depressed bodyweight and food consumption was observed at the high-dose levelof 325 mg/kg/day. No maternal toxicity was observed at the low-or mid-dose groups for rats or rabbits. With the exception ofthe reduced fetal weights in rats at 750 mg/kg, there was noevidence of fetal toxicity, no effects on any of the gestationalparameters, nor were there any treat ment-related increasesin external, visceral, or skeletal variations or malformationsin the offspring from the rats and rabbits from these studies.1994 Society of Toxicology.     

In-vitro permeation of the insect repellent N,N-diethyl-m-toluamide (DEET) and the sunscreen oxybenzone

The permeation behaviours of the insect repellent N,N-diethyl-m-toluamide (DEET) and the sunscreen oxybenzone were assessed in a series of in-vitro diffusion studies, using piglet skin and poly (dimethylsiloxane) (PDMS) membrane. The transmembrane permeability of DEET and oxybenzone across piglet skin and PDMS membrane was dependent on dissolving vehicles and test concentrations. An enhanced permeation increase across piglet skin was found for DEET and oxybenzone when both compounds were present in the same medium (DEET: 289% in propylene glycol, 243% in ethanol and 112% in poly(ethylene glycol) (PEG-400); oxybenzone: 139% in PEG-400, 120% in propylene glycol and 112% in ethanol). Permeation enhancement was also observed in PDMS membrane (DEET: 207% in ethanol, 124% in PEG-400 and 107% in propylene glycol; oxybenzone: 254% in PEG-400, 154% in ethanol and 105% in propylene glycol). PDMS membrane was found to be a suitable candidate for in-vitro diffusion evaluations. This study shows that the permeations of the insect repellent DEET and the sunscreen oxybenzone were synergistically enhanced when they were applied simultaneously.     

Anticonvulsant-resistant seizures following pyridostigmine bromide (PB) and N,N-diethyl-m-toluamide (DEET).

An acute toxic interaction has been described, in which sublethal doses of pyridostigmine bromide (PB) and the insect repellent N,N-diethyl-m-toluamide (DEET), when administered concomitantly, resulted in seizures and lethality. To investigate the possible relationships between seizures and lethality and the role of the cholinergic system in this interaction, PB (5 mg/kg), DEET (200 mg/kg) or PB (3 mg/kg) + DEET (200 mg/kg) were administered i.p. to male ICR mice, alone or following i.p. pretreatment, with one of several anticonvulsant agents: diazepam, 10 mg/kg; fosphenytoin, 40 mg/kg; phenobarbital, 45 mg/kg; or dextrophan, 25 mg/kg), or the anticholinergic agents, atropine (5 mg/kg), atropine methyl nitrate (2.7 mg/kg), or mecamylamine (2.5 mg/kg). The anticonvulsants selected for this study act through different mechanisms to reduce seizures. None of the anticonvulsants was able to reduce the incidence of seizures following treatment with PB, DEET or PB + DEET. Only diazepam delayed the onset of seizures. Fosphenytoin or diazepam significantly prolonged the time to lethality following PB, but only fosphenytoin reduced the incidence of PB-induced lethality. Diazepam or phenobarbital significantly prolonged the time to lethality following PB + DEET. Both atropine and atropine methyl nitrate protected against PB and PB + DEET-induced lethality and PB-induced seizures. Neither agent blocked seizures resulting from DEET or PB + DEET. Mecamylamine reduced seizures and lethality in PB-treated mice, but not in mice treated with DEET or PB + DEET. The results indicate that seizure activity is not a causative factor in the toxic interaction between PB and DEET. Furthermore, PB, DEET and PB + DEET induce seizures that are resistant to standard anticonvulsants, and each appears to operate through different mechanisms to produce seizures. Peripheral muscarinic receptors may play a specific role in lethality caused by PB + DEET.     

Microsomal metabolism of N,N-diethyl-m-toluamide (DEET,DET): the extended network of metabolites

1. The aim was to set out to establish the complete network of metabolites arising from the phenobarbital-treated rat liver microsomal oxidation of N,N-diethyl-m-toluamide (DEET). The products formed from DEET and all its subsequent metabolites were identified by HPLC retention times, UV spectroscopy, mass spectrometry and by comparison with authentic standards. 2. DEET (1a) produces three major metabolites, N-ethyl-m-toluamide (1b), N,N-diethyl-m-(hydroxymethyl)benzamide (2a) and N-ethyl-m-(hydroxymethyl)benzamide (2b), and, at low substrate concentrations or extended reaction times, two minor metabolites, toluamide (1c) and N,N-diethyl-m-formylbenzamide (3a). 1b and 2a are primary metabolites and their formation follows Michaelis-Menten-type kinetics. At low DEET concentrations, ring methyl group oxidation is favoured; at saturation concentrations, methyl group oxidation and N-deethylation proceed at similar rates. The rate of formation of 2b decreases with increasing DEET concentration; 2b is therefore a secondary metabolite of DEET and DEET acts as a competitive inhibitor of the metabolism of 1b and 2a. 3. Except for the primary amides, where N-dealkylation is impossible, metabolism of all subsequent compounds, 1b,c, 2a-c, 3a-c and 4a,b, involves an N-deethylation (NEt₂ → NHEt or NHEt → NH₂) competitive with a ring substituent oxidation (CH₃ → CH₂OH, CH₂OH → CHO or CHO → CO₂H). Surprisingly, the aldehydes 3a-c are also reduced to the corresponding alcohols 2a-c (CHO → CH₂OH); CO inhibits the oxidative metabolism of 3a-c, but reduction to 2a-c continues uninhibited. 4. The outcomes of this work are that (1) previously unreported aldehydes 3b and 3c form part of the DEET network of metabolites, (2) the reduction of the aldehydes 3a-c has the potential to inhibit the formation of the more highly oxidized DEET metabolites, (3) amide hydrolysis was not observed for any substrate and (4) no evidence was obtained for N-(1-hydroxyethyl)amide intermediates.     

Microencapsulation decreases the skin absorption of N,N-diethyl-m-toluamide (DEET).

The insect repellent N,N-diethyl-3-methylbenzamide (DEET) is widely used and is generally regarded as safe when used according to label instructions. Yet many studies have shown it to be absorbed through the skin. The objective of this study was to determine whether the skin absorption rate of DEET could be decreased while maintaining an evaporation rate consistent with effective repellency. To this end, an aqueous suspension containing 14C-DEET (15%w/w) entrapped in walled polysaccharide microcapsules was prepared and tested for skin absorption in vitro using modified Franz cells maintained in a fume hood. The control formulation was 15%w/w DEET in ethanol. Two doses (3 microL and 5 microL per 0.79 cm2 cell) of each formulation were applied to split-thickness human cadaver skin (n=8/dose), and permeation was monitored for 24h. The microencapsulated DEET formulation lead to a 25-35% reduction of radiolabel permeation compared to the ethanolic DEET formulation. Skin levels of radioactivity at 24h were comparable, indicating that DEET evaporation from the microencapsulated formulation was comparable to or greater than that from ethanol. Hence microencapsulation increased the ratio of DEET evaporation rate to skin penetration rate relative to unencapsulated control in this in vitro study.     

Topical formulation studies with DEET (N,N-diethyl-3-methylbenzamide) and cyclodextrins.

The purpose of this study was to investigate the utility of cyclodextrins as a safer alternative to the commonly used cosolvent ethanol in topical N,N-diethyl-3-methylbenzamide (DEET) formulations. Ethanol is a known penetration enhancer and would ideally not be used in a formulation containing DEET, a compound known for its potential toxicity. DEET formulations were prepared containing up to 20% w/w gamma-cyclodextrin (GCD) or 30% w/w hydroxypropyl-beta-cyclodextrin (HPBCD). These formulations were evaluated by determining the release rate of DEET from its vehicle through a synthetic membrane into a receiver medium with a high affinity for DEET using a fully automated Franz-diffusion cell system. DEET release was measured by ultraviolet spectrophotometry at 260 nm. Both the traditional Higuchi release model and an alternate repeated measures analysis of variance with nested factors were used to analyze the data. The influence of the cyclodextrins on the volatility of DEET was determined by solid-phase microextraction. Morphological characterization of the formulations was performed by light microscopy. Physical stability of the formulations was evaluated by rheology in an accelerated study performed at 50 degrees C for 28 days. The cyclodextrins caused a decrease in the release of DEET from its vehicle compared with an ethanol-containing formulation. This effect became more pronounced as the amount of cyclodextrin in the formulation was increased. The addition of the cyclodextrins resulted in a cream-like product compared with the ethanol formulation, which was a lotion. The DEET formulations had the following rank order of volatility: ethanol < DEET = GCD < HPBCD. Examination of the formulations by light microscopy indicated that the CD-based formulations produced a smaller and more homogeneous droplet size distribution than the ethanol-based formulations. The accelerated physical stability study indicated that cyclodextrin-based formulations are both technically and commercially feasible. Cyclodextrins are a promising alternative to ethanol in DEET-containing formulations. The reduced release rate of DEET from these formulations suggests that the toxic potential of DEET may be decreased in these products, while its repellency and stability are maintained.     

Microsomal metabolism of N,N-diethyl-m-toluamide (DEET, DET): the extended network of metabolites.

1. The aim was to set out to establish the complete network of metabolites arising from the phenobarbital-treated rat liver microsomal oxidation of N,N-diethyl-m-toluamide (DEET). The products formed from DEET and all its subsequent metabolites were identified by HPLC retention times, UV spectroscopy, mass spectrometry and by comparison with authentic standards. 2. DEET (1a) produces three major metabolites, N-ethyl-m-toluamide (1b), N,N-diethyl-m-(hydroxymethyl)benzamide (2a) and N-ethyl-m-(hydroxymethyl)benzamide (2b), and, at low substrate concentrations or extended reaction times, two minor metabolites, toluamide (1c) and N,N-diethyl-m-formylbenzamide (3a). 1b and 2a are primary metabolites and their formation follows Michaelis-Menten-type kinetics. At low DEET concentrations, ring methyl group oxidation is favoured; at saturation concentrations, methyl group oxidation and N-deethylation proceed at similar rates. The rate of formation of 2b decreases with increasing DEET concentration; 2b is therefore a secondary metabolite of DEET and DEET acts as a competitive inhibitor of the metabolism of 1b and 2a. 3. Except for the primary amides, where N-dealkylation is impossible, metabolism of all subsequent compounds, 1b,c, 2a-c, 3a-c and 4a,b, involves an N-deethylation (NEt2 --> NHEt or NHEt --> NH2) competitive with a ring substituent oxidation (CH3 --> CH2OH, CH2OH --> CHO or CHO --> CO2H). Surprisingly, the aldehydes 3a-c are also reduced to the corresponding alcohols 2a-c (CHO --> CH2OH); CO inhibits the oxidative metabolism of 3a-c, but reduction to 2a-c continues uninhibited. 4. The outcomes of this work are that (1) previously unreported aldehydes 3b and 3c form part of the DEET network of metabolites, (2) the reduction of the aldehydes 3a-c has the potential to inhibit the formation of the more highly oxidized DEET metabolites, (3) amide hydrolysis was not observed for any substrate and (4) no evidence was obtained for N-(1-hydroxyethyl)amide intermediates.     

Dinitrotoluene: acute toxicity, oncogenicity, genotoxicity, and metabolism

Dinitrotoluene (DNT) is a major commodity chemical; over six hundred million pounds of DNT were used in the chemical industry in 1980. Interest in the toxicology of this important chemical was greatly increased when separate oncogenicity assays yielded the conflicting results that DNT was either not hepatocarcinogenic or produced a 100% incidence of hepatocellular carcinomas in male rats in one year. Research revealed pronounced differences in the activity of the DNT isomers and provided the reason for the dissimilar results of the various carcinogenicity studies. Cell culture genetic toxicology assays failed to predict the potent carcinogenic activity of any isomer of DNT. Only when the complex pattern of metabolic activation of DNT began to unfold and genotoxic activity was assessed in the appropriate target organ in the intact treated animal was the potent genotoxic activity of DNT revealed, and the reasons for the negative in vitro results understood. The DNTs have been extensively tested for reproductive effects in animals and humans, and the metabolism and disposition of each of the six possible isomers have been studied. This work has provided valuable information in establishing the risk of these compounds to humans.     

Acute manic psychosis following the dermal application of N,N-diethyl-m-toluamide (DEET) in an adult

Extensive animal testing and 30 years of human experience have established the general safety of DEET when applied episodically to skin or bedclothes. Local and systemic toxic and allergic reactions to DEET have been observed in man. Three weeks prior to admission, for the purpose of self-medication, a 30 year old man began daily applications of the insect repellant, DEET followed by a 1-2 hour period in a light-bulb heated box. Sedation and incoherence were noted for short periods following each application session. Aggressiveness and psychotic ideation led to hospital admission where he displayed psychomotor hyperactivity, rapid and pressured speech, tangentiality, flight of ideas, and grandiose delusions. Treatment was begun with haloperidol. Clinical improvement was complete within 6 days, atypical for classic endogenous mania. Drug and metabolites were identified in the urine more than 2 weeks after the last drug application.     

Chronic toxicity and oncogenicity of N-methylpyrrolidone (NMP) in rats and mice by dietary administration

A two-year feeding study in rats and an 18-month feeding study in mice were conducted to evaluate the potential chronic toxicity and oncogenicity of NMP in Crl:CD (SD)BR rats and B6C3F1/CrlBR mice. Groups of 62 male and female rats were administered diets containing 0, 1600, 5000, or 15,000 ppm of NMP for approximately 2 years. Groups of 50 male and female mice were administered diets containing 0, 600, 1200, or 7200 ppm NMP for approximately 18 months. In vivo parameters were evaluated weekly during the first 3 months of the study, and every other week or monthly during the remainder of the study. For rats, an ophthalmoscopic examination was conducted prior to study start and near the end of the study. Periodically, blood samples were collected from rats and mice for determination of leukocyte differential counts, and from mice for red blood cell morphology. After approximately 2 years of dietary administration in rats and 18 months in mice, all surviving animals were sacrificed. Selected tissues were processed for morphological evaluation. Over the course of the two-year study in rats, test substance-related decrements in body weight and weight gain occurred in 15,000 ppm males and females, which correlated with decreased food consumption and food efficiency. A toxicologically significant, test substance-related increase in the incidence of severe chronic progressive nephropathy occurred in 15,000 ppm males. Several morphological changes noted grossly and/or microscopically were secondary to the increased severity of chronic progressive nephropathy. NMP was not oncogenic in male or female rats at dietary concentrations of 15,000 ppm and below. A test substance-related decrease in the percentage of 15,000 ppm males surviving to the end of the two-year study compared to the control group resulted from the higher incidence of severe chronic progressive nephropathy. However, a sufficient population of 15,000 ppm rats were at risk for potential oncogenicity, so the lower survival did not impair the ability to detect an oncogenic response in this study. There were no adverse, test substance-related effects on the incidences of clinical observations, ophthalmic observations, or differential leukocyte counts in males or females, or on survival of females at any dietary concentration. Male and female mice administered dietary concentrations of 7200 ppm had significantly increased liver weight, significantly increased incidence of hepatocellular adenoma, and significantly increased foci of cellular alteration in the liver. At 7200 ppm, male mice also had an increased incidence of hepatocellular carcinoma while the increased incidence of hepatocellular carcinoma in female mice fell within the historical control range. In addition, the incidence of hepatocellular hypertrophy was increased in 7200 ppm males. Liver weight and hepatocellular hypertrophy were also increased in 1200 ppm males. There were no adverse, test substance-related effects on the incidences of clinical observations, food consumption, body weight, differential leukocyte counts, red blood cell morphology, or survival in either males or females at any dietary concentration. Under the conditions of the study, the no-observed-effect level for NMP was 5000 ppm for male and female rats, 600 ppm for male mice, and 1200 ppm for female mice.     

Percutaneous absorption of topical N,N-diethyl-m-toluamide (DEET): effects of exposure variables and coadministered toxicants

Exposure to N,N-diethyl-m-toluamide (DEET) commonly occurs in the general population and has been implicated as a contributory factor to the Gulf War Illness. The focus of the present studies was to determine the effect of coexposure factors, potentially encountered in a military environment, that could modulate transdermal flux of topically applied DEET. Factors investigated were vehicle, dose, coexposure to permethrin, low-level sulfur mustard, occlusion, and simultaneous systemic exposure to pyridostigmine bromide and the nerve agent stimulant diisopropylfluorophosphate (DFP). Studies were conducted using the isolated perfused porcine skin flap (IPPSF), with a few mechanistically oriented studies conducted using in vitro porcine skin and silastic membrane diffusion cells. DEET was quantitated using high-performance liquid chromatography. The vehicle-control transdermal DEET flux in the IPPSF was approximately 2 micrograms/cm2/h for both 7.5 and 75% DEET concentrations, a value similar to that reported in humans. DEET absorption was enhanced by coinfusion of pyridostigmine bromide and DFP, by the presence of sulfur mustard, or by dosing under complete occlusion. The greatest increase in baseline flux was fivefold. In vitro diffusion cell studies indicated that silastic membranes had two orders of magnitude greater permeability than porcine skin, and showed vehicle effects on flux that were not detected in the IPPSF. These results suggest that coexposure to a number of chemicals that potentially could be encountered in a military environment may modulate the percutaneous absorption of topically applied DEET beyond that seen for normal vehicles at typically applied concentrations.     

Metabolism of N,N-diethyl-m-toluamide (DEET) by liver microsomes from male and female rats. Simultaneous quantitation of DEET and its metabolites by high performance liquid chromatography

A simple and sensitive HPLC procedure was developed to separate and quantitate N,N-diethyl-m-toluamide (DEET) and its in vitro metabolites using a gradient elution, a reversed phase C18 column, and an internal standard. This procedure was applied to examine the metabolism of this insect repellent in liver microsomes from normal (untreated) male and female Wistar rats. At a pH of 8.6 and at a substrate concentration of 200 microM, the microsomal preparations from males degraded DEET faster than did those from females. The half-life of DEET was 10 min and 15 min, respectively. The benzylic methyl hydroxylated metabolite and the N-deethylated metabolite were determined over 2 hr in fortified microsomal suspensions. Rate constants for appearance of the metabolites showed significantly higher values for males than for females. These observations suggests that a sex difference may be present in the metabolism of DEET.     

Combined exposure to DEET (N,N-diethyl-m-toluamide) and permethrin-induced release of rat brain mitochondrial cytochrome c.

The release of cytochrome c from the mitochondrial intermembrane space can induce apoptosis. The levels of mitochondrial cytochrome c in rat brain following a single dermal dose of 400 mg/kg of DEET, and of 1.3 mg/kg of permethrin, alone or in combination were determined. Rats were sacrificed at a time interval of 0.5, 1, 2, 4, 8, 16, 24, 48, or 72 h after dosing. Brain mitochondria were isolated and the levels of cytochrome c were measured using reversed-phase high-performance liquid chromatography (HPLC) with ultraviolet (UV) detection. Average percentage recovery of cytochrome c spiked with control rat brain mitochondria was 83.2 +/- 8.9%. Limits of detection and quantitation were 1 and 5 ng, respectively. The results showed that a single dermal dose of a combination of DEET and permethrin significantly increased the release of brain mitochondrial cytochrome c starting 24 h after treatment. DEET and permethrin alone did not affect the release of cytochrome c. The results indicate that combined exposure to DEET and permethrin might induce the apoptotic processes in rat brain as seen by the release of cytochrome c.     

Cardiorespiratory effects following acute exposure to pyridostigmine bromide and/or N,N-diethyl-m-toluamide (DEET) in rats

The acute lethal interaction that occurs in rodents when high doses of a peripherally restricted cholinesterase inhibitor, pyridostigmine bromide (PB), and the insect repellent N, N-diethyl-m-toluamide (DEET) are combined was first described during studies of chemical mixtures that were targeted as potential causative agents of Gulf War illnesses. This study was intended to provide insight into possible mechanisms of that lethal interaction. Following a single intraperitoneal injection of PB (2 mg/kg) and/or DEET (300 or 500 mg/kg), respiratory activity was measured in conscious freely moving rats using whole-body plethysmography. Cardiovascular function was also monitored simultaneously through an arterial catheter. PB (2 mg/kg) given alone stimulated respiration and increased blood pressure. Arterial pH levels were decreased, whereas pO(2) and pCO(2) remained at control levels. Administration of DEET (300 mg/kg) alone increased tidal volume and decreased blood pressure. Blood gases and pH levels were unaltered. A higher dose of DEET (500 mg/kg) also decreased respiratory and heart rate. Coadministration of PB (2 mg/kg) and DEET (300 mg/kg) increased tidal volume, decreased arterial pH, and elevated pCO(2). Heart rate and blood pressure declined progressively after drug coadministration. Pretreatment with atropine methyl nitrate (AMN), a peripherally selective competitive antagonist at nicotinic and muscarinic receptor sites, reduced the individual effects of PB or DEET, and significantly increased survival after coexposure to these agents. Although changes in respiratory function may have contributed to the lethal interaction, it was concluded that the primary cause of death was circulatory failure.     

Chronic dietary toxicity and oncogenicity evaluation of MCPA (4-chloro-2-methylphenoxyacetic acid) in rodents

The chronic toxicity and oncogenicity of 4-chloro-2-methylphenoxyacetic acid (MCPA) were evaluated in Wistar rats at target doses of 20, 80, and 320 ppm for 2 years. Chronic effects were noted in male and/or female rats in the 80- and 320-ppm dose groups, namely elevations in triglycerides and serum glutamic transaminase levels. Nephrotoxicity was confined to male rats in the 320-ppm dose group. The systemic NOEL was determined to be 20 ppm for male and female rats. No oncogenic potential was observed. Doses in the 2-year oncogenicity study in mice were 20, 100, and 500 ppm. Kidney weight changes with corresponding minor histopathological findings in the kidney were evident in females in the 500-ppm dose group. MCPA was determined to have no oncogenic potential in B6C3F1 mice. In summary, there is no evidence of any oncogenic potential after dietary exposure of MCPA in rats or mice even at doses where limited chronic toxicity is seen.