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Absorption and mass balance of piperonyl butoxide following an 8-h dermal exposure in human volunteers
Abstract
Dermal absorption, metabolism and excretion of piperonyl butoxide (PBO) was studied using 14C-PBO either by itself as a 3% (w/w) solution in isopropyl alcohol or as a 4% (w/w) solution in an aqueous end-use formulation. Each of these two formulations were tested on four young, healthy male volunteers, using a single topical application on the ventral forearm under non-occlusive conditions for an 8-h period. The application sites were thoroughly cleaned with cotton swabs moistened with isopropyl alcohol, then rinsed with isopropyl alcohol. Blood from the ipsilateral and contralateral arms, urine and feces were collected at selected intervals during the 8-h application and through a 120-h post-application period. The application area was also tape-stripped to determine if any of the test material accumulated in the stratum corneum. These samples provided data which permitted insight into the kinetics of penetration and elimination processes of PBO. The absorption of PBO either by itself or formulated was very poor, as demonstrated by the radioactivity excreted in the urine, and radioactivity in the ipsilateral plasma. When dosed by itself, approximately 1.78% of the dose was excreted in the urine. In contrast, only 0.47% of the formulated PBO was excreted in the urine. Trace radioactivity was detected in the feces from both formulations. The absorbed radioactivity was rapidly eliminated in the urine. There was no evidence of accumulation of PBO in the skin as evidenced by low amounts of radioactivity in the tape-strippings. The majority of the applied radioactivity was recovered from the skin surface. Total recovery of the applied radioactivity was 100.86 and 104.22% for PBO and the formulated product respectively. Absorbed PBO was completely metabolized to at least three major metabolites prior to its excretion in the urine. The three metabolites represented over 70% of the excreted radioactivity for PBO. The HPLC retention times for these metabolites are different than that seen in rats. The structures of these metabolites have not been elucidated.
... Piperonyl butoxide (PBO) was used as a reference compound, as it has been frequently formulated with pyrethroids and has reliable in vivo and in vitro human dermal absorption data available (Wester et al., 1994;Selim et al., 1999). [Phenyl-14 C-(UL)]piperonyl butoxide (99.5% radiolabel purity) at 36.5 mCi/mmol was obtained from Moravek Biochemicals, Inc., and stored at -20 • C until use. ...
... Previously, the percutaneous absorption of PBO dissolved in isopropyl alcohol was determined in 4 human subjects at a dose of 100 µg/cm 2 to the ventral forearm, with an 8-h exposure period (Selim et al., 1999). Only 1.8% of the applied dose appeared in the urine and feces, with the majority of the label recovered from the skin surface. ...
... Piperonyl butoxide in isopropanol solution was used as a reference substance in this study because in vivo human dermal absorption data are available with this solvent (Selim et al., 1999;Wester et al., 1994). Although a different solvent for permethrin was used, the purpose of the study was not a direct comparison of Values are mean ± SD, n = 6-9. ...
The objectives of this study were to (1) determine the percutaneous absorption of radiolabeled permethrin and piperonyl butoxide (PBO) in vivo in rats and in vitro to permit a calculation of the ratio of in vitro to in vivo values, and (2) test a method of estimating in vivo human absorption. Carbon-14 labeled permethrin in ethanol solution was applied to the clipped skin of rats in vivo at doses of 2.25, 20, or 200 μg/cm2. As a reference compound, 14C-labeled PBO in isopropanol solution was applied to rat skin in vivo at a dose of 100 μg/cm2. All applications were washed at 24 h postapplication, and rats were sacrificed either at 24 h for permethrin or 5 d for both compounds. The radiolabel recovered from carcass, urine including cage wash, and feces was summed to determine percent absorption. For the 24-h time point, at doses of 2.25, 20, and 200 μg/cm2 of permethrin, values of 22, 22, and 28%, respectively, were obtained for in vivo rat percutaneous absorption (n=6 per dose). For the 5-d time point, at doses of 2.25, 20, and 200 μg/cm2 of permethrin, values of 38, 38, and 30%, respectively, were obtained for in vivo rat percutaneous absorption (n=6 per dose). The 5-d percutaneous absorption of 14C-PBO at 100 μg/cm2 was determined to be 42% (n=6). Dose and test duration did not exert a statistically significant effect on percutaneous absorption of permethrin in the rat in vivo. For in vitro absorption determination, 14C-permethrin in ethanol solution was applied to freshly excised human skin in an in vitro test system predictive of skin absorption in humans. Twenty-four hours after application, the radiolabel recovered from dermis and receptor fluid was summed to determine percent absorption. At doses of approximately 2.25, 20, and 200 μg/cm2 permethrin, values of 1, 3, and 2%, respectively, were obtained for percutaneous absorption (n=9 per dose). Excised human skin absorption of 14C-PBO at 100 μg/cm2 was determined to be 7% (n=9). Excised rat skin absorptions of permethrin at 2.25, 20, and 200 μg/cm2 were found to be 20, 18, and 24%, respectively (n=6 per dose), approximately 10-fold higher than human skin absorption. Excised rat skin absorption of PBO was also higher (35%) than the value obtained for human skin by a factor of about 5.
... PBO was detected in muscular tissue of 9 fish species, with sardines having the highest mean concentration of 0.013 μg/kg (Polat et al., 2018). Although dermal PBO absorption is thought to be relatively low (Selim et al., 1999), these findings suggest ingestion of water and aquatic organisms as additional sources of human exposure. ...
... Interestingly, these studies have also demonstrated the presence of sex-dependent PBO metabolites (Byard and Needham, 2006). Current knowledge of PBO metabolites in humans is limited to a single study where 8 male volunteers were dermally exposed to PBO to determine its routes of elimination (Selim et al., 1999). PBO was found to be mainly eliminated in the urine, but the structure of the predominant metabolites and their activity could not be characterized. ...
Piperonyl butoxide (PBO) is a semisynthetic chemical present in hundreds of pesticide formulations used in agricultural, commercial, and residential settings. PBO acts as a pesticide synergist by inhibiting insect cytochrome P450 enzymes and is often present at much higher concentrations than active insecticidal ingredients. PBO was recently discovered to also inhibit Sonic hedgehog (Shh) signaling, a key molecular pathway in embryonic development and in brain and face morphogenesis. Recent animal model studies have shown that in utero PBO exposure can cause overt craniofacial malformations or more subtle neurodevelopmental abnormalities. Related adverse developmental outcomes in humans are etiologically heterogeneous, and, while studies are limited, PBO exposure during pregnancy has been linked to neurodevelopmental deficits. Contextualized in PBO’s newly recognized mechanism as a Shh signaling inhibitor, these findings support more rigorous examination of the developmental toxicity of PBO and its potential contribution to etiologically complex human birth defects. In this review, we highlight environmental sources of human PBO exposure and summarize existing animal studies examining the developmental impact of prenatal PBO exposure. Also presented are critical knowledge gaps in our understanding of PBO’s pharmacokinetics and potential role in gene-environment and environment-environment interactions that should be addressed to better understand the human health impact of environmental PBO exposure.
... Our understanding of PBO pharmacokinetics (PK) is limited, and apart from being detected in human umbilical cord fluid [19], to our knowledge PBO serum concentrations in humans have not been reported. Results from a study of four male volunteers suggest that dermal absorption is minimal over an eight-hour period [20]. Early rodent studies suggested that PBO is readily absorbed when administered orally [21,22]. ...
Piperonyl butoxide (PBO) is a popular insecticide synergist present in thousands of commercial, agricultural, and household products. PBO inhibits cytochrome P450 activity, impairing the ability of insects to detoxify insecticides. PBO was recently discovered to also inhibit Sonic hedgehog signaling, a pathway required for embryonic development, and rodent studies have demonstrated the potential for in utero PBO exposure to cause structural malformations of the brain, face, and limbs, or more subtle neurodevelopmental abnormalities. The current understanding of the pharmacokinetics of PBO in mice is limited, particularly with respect to dosing paradigms associated with developmental toxicity. To establish a pharmacokinetic (PK) model for oral exposure, PBO was administered to female C57BL/6J mice acutely by oral gavage (22–1800 mg/kg) or via diet (0.09 % PBO in chow). Serum and adipose samples were collected, and PBO concentrations were determined by HPLC-MS/MS. The serum concentrations of PBO were best fit by a linear one-compartment model. PBO concentrations in visceral adipose tissue greatly exceeded those in serum. PBO concentrations in both serum and adipose tissue decreased quickly after cessation of dietary exposure. The elimination half-life of PBO in the mouse after gavage dosing was 6.5 h (90 % CI 4.7–9.5 h), and systemic oral clearance was 83.3 ± 20.5 mL/h. The bioavailability of PBO in chow was 41 % that of PBO delivered in olive oil by gavage. Establishment of this PK model provides a foundation for relating PBO concentrations that cause developmental toxicity in the rodent models to Sonic hedgehog signaling pathway inhibition.
... At the same time, synthetic pyrethroids maintain a low toxicity to terrestrial vertebrates [7]. They are approximately 2,250 times more toxic to insects than to mammals [8]. ...
Background: Lambda-cyhalothrin (LCH) is a one of the type II synthetic pyrethroids which is widely used in veterinary medicine and in agriculture to protect crops from pest insects. In previous studies, there are few reports about the influence of pyrethroids on the liver and its damage. Analyzing numerous publications, nuclear factor-ĸB (NF-ĸB) and vascular endothelial growth factor 2 (VEGFR2) seem to be sensitive indicators of microdamages occurring at the cellular level in the liver. The aim of the study was to investigate the effect of subacute poisoning with LCH on the concentration of NFĸB and VEGFR2 in the livers.
Methods: The experiment was carried on 32 Albino Swiss mice (16 females and 16 males). The animals were divided into 4 groups. Controls received canola oil, the rest received LCH orally in oil at a dose of 2 mg/kg bw for 7 days. The NF-ĸB and VEGFR2 were mesuredin mice livers with ELISA kits. Results: The mean NF-ĸB concektration in control femals’ livers was 3.27ng/mL and after LCH it was 6.12ng/mL (p<0.05). In control males it was 5.49ng/mL and it did not significantly differ after LCH when it was 5.27ng/mL. The mean VEGFR2 in control females was 84.28ng/mL and after LCH it was 173.81ng/mL (p<0.05). In control males it was 170.61ng/mL and after LCH 170.06ng/mL.
Conclusion:The NF-ĸB and VEGFR2 can be used as markers of liver damage after subacute poisoning with LCH on female mice. Females are more sensitive to LCH than males.
... The exact mechanism of degradation in humans still lacks systematic analysis as minimal studies have been carried out on the same. Selim et al. (1999) reported that eight volunteers participated in a study for pyrethroids and were exposed to PBO through the dermal route. It is known to cause metabolic alterations and seriously affects the activity of the Cytochrome P450 enzymes. ...
Pyrethroids are a class of insecticides structurally similar to that of natural pyrethrins. The application of pyrethrins in agriculture and pest control lead to many kinds of environmental pollution affecting human health and loss of soil microbial population that affect soil fertility and health. Natural pyrethrins have been used since ancient times as insect repellers, and their synthetic versions especially type 2 pyrethroids could be highly toxic to humans. PBO (Piperonyl butoxide) is known to enhance the toxicity of prallethrin in humans due to the resistance in its metabolic degradation. Pyrethroids are also known to cause plasma biochemical profile changes in humans and they also lead to the production of high levels of reactive oxygen species. Further they are also known to increase SGPT activity in humans. Due to the toxicity of pyrethrins in water bodies, soils, and food products, there is an urgent need to develop sustainable approaches to reduce their levels in the respective fields, which are eco-friendly, economically viable, and socially acceptable for on-site remediation. Keeping this in view, an attempt has been made to analyse the advances and prospects in using pyrethrins and possible technologies to control their harmful effects. The pyrethroid types, composition and biochemistry of necessary pyrethroid insecticides have been discussed in detail, in the research paper, along with their effect on insects and humans. It also covers the impact of pyrethroids on different plants and soil microbial flora. The second part deals with the microbial degradation of the pyrethroids through different modes, i.e., bioaugmentation and biostimulation. Many microbes such as Acremonium, Aspergillus, Microsphaeropsis, Westerdykella, Pseudomonas, Staphylococcus have been used in the individual form for the degradation of pyrethroids, while some of them such as Bacillus are even used in the form of consortia.
... They determined the 1-day application rate of PBO to be 0.525 mg/kg which was below the no observed adverse effect level (NOAEL) of 630 mg/kg per day above which toxic effects are expected, but well above the acceptable daily intake (ADI) of 0.2 mg/kg (Moretto, 1995). A study of dermal absorption, metabolism, and excretion on exposed human volunteers (Selim et al., 1999) determined that absorption of PBO through the skin was very low as demonstrated by the radioactivity excreted in urine, and that there was no evidence of bioaccumulation in the skin. Fecal excretion was minimal. ...
Piperonyl butoxide (PBO) contains the methylenedioxyphenyl moiety, a molecule found in sesame oil and later named sesamin, and thus is classified as a member of the methylenedioxyphenyl group of chemicals. In 1947, it was semisynthesized from safrole, the principal component of camphor oil (Knowles, 1991; Tozzi, 1998). P0010 Its molecular formula is C 19 H 30 O 5 ; it has a molecular weight of 338.4 g/mol. The chemical structure is shown in Figure 116.1. P0015 PBO is a colorless to pale yellow liquid and is moderately stable in that it is resistant to hydrolysis, oxidation, and exposure to sunlight (Kidd and James, 1994). Manufacturers include MGK and Prentiss and Johnson (Tozzi, 1998). Since its discovery, and since pesticide resistance has emerged as a problem, PBO has been frequently used as an insecticide synergist, in over 1700 insecticides (USEPA, 2002). It is commonly blended with pyrethrins and synthetic pyrethroids, including in human and veterinary medicine; in agriculture, it is used with a wider range of pesticides including fipronil, parathion, dichlorvos, linalool, D-limonene, methoprene, hydroprene and fenox-ycarb, and alpha-naphthylthiourea (Levine and Murphy, 1977; Carpenter and Roth, 1988; Farnham, 1998; Hainzal et al., 1998; Keane, 1998; Tripathi and Agarwal, 1998).
... Although chlorpyrifos was not reported used, it was measured in all media, suggesting that it has a long residence time in the indoor environment (Fenske et al., 2000a, b). Piperonyl butoxide is added as a synergist to many pyrethroid mixtures for products used in the residential environment so it was not surprising to find it in all media (Selim et al., 1999). ...
A pilot observational exposure study was performed to evaluate methods for collecting multimedia measurements (air, dust, food, urine) and activity patterns to assess potential exposures of young children to pesticides in their homes. Nine children (mean age=5 years) and their caregivers participated in this study, performed in the Duval County, Florida, in collaboration with the Centers for Disease Control and Prevention and the Duval County Health Department. For all nine children, the total time reported for sleeping and napping ranged from 9.5 to 14 h per day, indoor quiet time from 0 to 5.5 h per day, indoor active time from 0.75 to 5.5 h per day, outdoor quiet time from 0 to 1.5 h per day, and outdoor active time from 0.5 to 6.5 h per day. Each home had one to three pesticide products present, with aerosols being most common. Pesticide inventories, however, were not useful for predicting pesticide levels in the home. Synthetic pyrethroids were the most frequently identified active ingredients in the products present in each home. Fifteen pesticide active ingredients were measured in the application area wipes (not detected (ND) to 580 ng/cm(2)), 13 in the play area wipes (ND-117 ng/cm(2)), and 14 in the indoor air samples (ND-378 ng/m(3)) and the socks (ND-1000 ng/cm(2)). Cis-permethrin, trans-permethrin, and cypermethrin were measured in all nine homes. Chlorpyrifos was measured in all nine homes even though it was not reported used by the participants. All urine samples contained measurable concentrations of 3-phenoxybenzoic acid (3-PBA). The median 3-PBA urinary concentration for the nine children was 2.2 mug/l. A wide variety of pesticide active ingredients were measured in these nine homes at median concentrations that were often higher than reported previously in similar studies. These data highlight the need for additional observational studies in regions where pesticides are used in order to understand the factors that affect young children's exposures and the education/mitigation strategies that can be used to reduce children's exposures.
Insecticides include a variety of compounds that act against different target species targeting different receptors, enzymes or functions. Hence, there are many chemical groups that also present different species specificity and toxicity for humans. They are used in agriculture, in public health to fight against vectors of disease or nuisance insects, and in veterinary medicine. Some, for example, avermectins, are also used in human medicine. In general, they act on different molecular targets within the nervous system with variable species specificity. For instance, carbamates inhibit acetylcholinesterase (AChE) without significant differences in potency between insects and mammals, while there is a significantly higher sensitivity to neonicotinoids of insects' nicotinic receptors as compared to mammalian ones. Also, significant differences in metabolism are responsible of the low toxicity of pyrethroids to mammalians as compared to insects. Application of insecticides is usually upwards; hence, human exposure during their application may be higher than that following the application of herbicides. Factors that impact on the level of exposure include the amount of active ingredient per hectare, water volume (dilution), the protective equipment used. Dermal exposure is considered to be the most relevant, inhalation accounting for about 10% of total exposure. The Insecticide Resistance Action Committee (IRAC) lists mode of actions and related compounds ( www.irac‐online.org ). It will not be possible to deal with all compounds in this chapter. Therefore, groups with significant current uses will be dealt with. In addition, organophosphates are considered in a separate chapter.
Biofouling is called “lessons from nature”. Currently, governments and industry spend more than 5.7 billion USD annually to control unwanted marine biofouling, aquatic flora and fauna on submerged construction leading to various technical, economical, and ecological problems. In turn, the Baltic Sea is defined as a “time machine” for the future coastal ocean, as processes occurring in the Baltic Sea are related to future changes. Our study describes the biofouling community at 12 sites located at different depths on the legs of the “Baltic Beta” oil platform that resulted in finding a maximum of 1,300 individuals on 400 cm2. We analyzed: spatial distribution of dominant marine organisms living on a steel platform surface, their abundance and mass. Our work showed no significant difference in the benthic samples mass among different depths or cardinal directions of the rig columns. Our research can help to predict offshore biofouling on other devices in the Baltic Sea, to control invasive species and to estimate environmental load.
Absorption of toxic substances via the skin is an important phenomenon in the assessment of the risk of exposure to these substances. People are exposed to a variety of substances and products via the skin, either directly or indirectly, while at work, at home or in public space. Pesticides, organic solvents and metalworking fluids are seen to be important contributors to adverse health effects due to occupational exposure via the skin. In daily life, cosmetics, clothing and household products are the most relevant commodities with respect to exposure via the skin. Given the importance of skin exposure in the assessment of the risk of toxic substances, the objective of this thesis was to further develop, evaluate and improve methods for including skin absorption data this assessment. In this thesis, four factors influencing dermal absorption, namely dermal loading (chapters 3 and 6), irritative/corrosive potential (chapters 3 and 4), frequency of exposure (chapters 3, 4 and 5) and the vehicle used (chapter 5), were investigated in more detail. Furthermore, a model to extrapolate infinite dose absorption data to finite dose conditions, baptized Dermal Absorption Model for Extrapolation (DAME), was developed and tested. I n chapter 2 of this thesis, the relationship between relative dermal absorption and dermal loading was investigated. Hundred-and-thirty-eight dermal publicly available absorption experiments with 98 substances were evaluated. The results obtained revealed that dermal loading ranged mostly between 0.001 and 10 mg/cm2. In 87 experiments (63%), an inverse relationship was observed between relative dermal absorption and dermal loading. On average, relative absorption at high dermal loading was 33 times lower than at low dermal loading. Known skin irritating and volatile substances less frequently showed an inverse relationship between dermal loading and relative absorption. It was concluded that when using relative dermal absorption in regulatory risk assessment, its value should be determined at or extrapolated to dermal loadings relevant for the exposure conditions being evaluated. I n chapter 3 of this thesis, a literature search was presented with the aim to investigate whether neglecting the effects of repeated exposure may lead to an incorrect estimate of dermal absorption. The results demonstrated that the effect of repeated versus single exposure does not demonstrate a unique trend. Nevertheless, an increase in daily absorption was frequently observed upon repeated daily exposure. The little information available mostly concerned pharmaceuticals. However, consumers and workers may be repeatedly exposed to other types of chemicals, like disinfectants and cleaning products, which often contain biocidal active substances that may decrease the barrier function of the skin, especially after repeated exposure. These biocidal products, therefore, may present a safety risk that is not covered by the current risk assessment practice since absorption data are usually obtained by single exposure experiments. Consequently, it was decided to investigate the importance of this issue for biocide safety evaluation. As the literature search revealed that hardly any data on absorption upon repeated dermal exposure to biocides are available, it was concluded that data need to be generated by testing. To cover the entire range of biocidal products in such testing, a representative series of biocidal substances should be tested, making in vitro testing of dermal absorption the preferred choice over in vivo testing. Based on an inventory made, it appeared that the 16 product types represented among the biocidal products authorised in the Netherlands could be clustered into 6 more or less homogeneous categories based on similarity in active substances. This result could facilitate experimental testing by providing a basis for selection of a limited number of representative compounds to be evaluated. I n chapter 4 of this thesis, the importance of the effect of repeated dermal exposure on skin permeability for biocide safety evaluation was investigated, using a selection of nine representative biocides from the inventory made in chapter 3. The in vitro dermal penetration of tritiated water and [14C]propoxur was chosen as a measure of the permeability and integrity of human abdominal skin after single and repeated exposure. The results indicated that single and repeated exposure to specific biocidal products (e.g. the quaternary ammonium chlorides DDAC and ADBAC) may significantly increase skin permeability, especially when the compounds are applied at high concentrations, while a substance like formaldehyde may reduce skin permeability under specific conditions. I n chapter 5 of this thesis, the in vitro dermal absorption kinetics of the quaternary ammonium compound didecyldimethylammonium chloride (DDAC) during single and repeated exposure was studied in more detail. In addition, the influence of biocidal formulations on the absorption of DDAC was investigated, because it was expected that formulation characteristics may be another factor influencing its dermal absorption. The analysis of biocidal products on the Dutch market, reported in chapter 3, indicated that DDAC is often used in combination with other active ingredients. DDAC was most frequently combined with formaldehyde, glutaraldehyde and/or alkyldimethylbenzylammonium chloride (ADBAC). Consequently, commercial formulations containing one or more of these additional active ingredients were selected, in addition to one formulation containing only DDAC as an active ingredient. The selected commercial formulations tended to reduce skin penetration of DDAC. This was most pronounced with the formulation containing the highest concentration of formaldehyde (196 mg/mL) and glutaraldehyde (106 mg/mL), which reduced the flux of DDAC across the skin by 95%. The reduction caused by the only tested formulation containing no other active ingredients than DDAC, and thus incorporating no aldehydes, was smallest, and did not reach statistical significance. I n chapter 6 of this thesis, a simple in silico model to predict finite dose dermal absorption from infinite dose data (kp and lag time) and the stratum corneum/water partition coefficient (KSC,W) was developed. This model was tentatively called Dermal Absorption Model for Extrapolation (DAME). As dermal exposure may occur under a large variety of conditions leading to quite different rates of absorption, such a predictive model using simple experimental or physicochemical inputs provides a cost-effective means to estimate dermal absorption under different conditions. To evaluate the DAME, a series of in vitro dermal absorption experiments was performed under both infinite and finite dose conditions using a variety of different substances. The kp’s and lag times determined in the infinite dose experiments were entered into DAME to predict relative dermal absorption value under finite dose conditions. For six substances, the predicted relative dermal absorption under finite dose conditions was not statistically different from the measured value. For all other substances, measured absorption was overpredicted by DAME, but most of the overpredicted values were still lower than 100%, the European default absorption value for the tested compounds. In conclusion, our finite dose prediction model (DAME) provides a useful and cost-effective estimate of in vitro dermal absorption, to be used in risk assessment for non-volatile substances dissolved in water at non-irritating concentrations. I n chapter 7 of this thesis, the results of the research reported in chapters 2 to 6 were put into perspective, the pitfalls and promises emanating from them discussed and general conclusions drawn. The possible influence of vehicles on absorption and the possible impact of irritative or corrosive vehicles or chemicals on the skin barrier have been demonstrated in this thesis. An in silico predictive model tentatively called DAME was developed, which enables the user to evaluate a variety of dermal exposure scenarios with limited experimental data (kp and lag time) and easy to obtain physicochemical properties (MW and log KOW). The predictions of our experiments reported in chapter 6 were compared to those of the Finite Dose Skin Permeation (FDSP) model published on the internet by the US Centers for Disease Control and Prevention (CDC). DAME outperformed FDSP (R2 of the correlation predicted/measured potential absorption 0.64 and 0.12, respectively). At present, the applicability domain of DAME is limited to non-volatile substances dissolved in aqueous solvents. However, in future the model will be adapted to include volatile substances as well. Altogether, it is concluded that dermal exposure can be an important factor in risks posed by chemicals and should be taken into account in risk assessment. The methods to actually do this are still open for further improvement to better account for the various factors influencing skin penetration and to develop adequate combinations of in vitro and in silico models that can accurately predict human dermal absorption.
The objective of this study was to develop an estimate of the percent dermal absorption of permethrin in humans to provide more accurate estimates of potential systemically absorbed dose associated with dermal exposure scenarios. Piperonyl butoxide (PBO) was used as a reference compound. The human percutaneous absorption estimate was based on the assumption that the ratio of in vivo dermal absorption (expressed as a percentage during a given time period) of permethrin through rat skin to in vitro dermal absorption through rat skin was the same as the ratio of in vivo dermal absorption in humans to in vitro dermal absorption with human skin, known as the parallelogram method. The ratio of dermal absorption by in vitro rat skin to absorption by in vitro human skin ranged from 6.7 to 15.4 (for a 24-h exposure period) with an average of 11. Data suggest in vivo human dermal absorption values for permethrin ranging from 1.4 to 3.3% when estimated based on 24-h in vivo rat values, and 2.5 to 5.7% based on 5-d in vivo rat values. The parallelogram method used to estimate dermal absorption of permethrin and PBO is supported by results from several other compounds for which in vivo and in vitro rat and human dermal absorption data exist. Collectively, these data indicate that estimating human dermal absorption from in vitro human and rat plus in vivo rat data are typically accurate within ±3-fold of the values measured in human subjects.
The controversy about the use of data from human volunteer studies involving experimental exposure to pesticides as part of regulatory risk assessment has been widely discussed, but the complex and interrelated scientific and ethical issues remain largely unresolved. This discussion paper, generated by authors who comprised a workgroup of the ICOH Scientific Committee on Rural Health, reviews the use of human experimental studies in regulatory risk assessment for pesticides with a view to advancing the debate as to when, if ever, such studies might be ethically justifiable. The discussion is based on three elements: (a) a review of discussion papers on the topic of human testing of pesticides and the positions adopted by regulatory agencies in developed countries; (b) an analysis of published and unpublished studies involving human testing with pesticides, both in the peer-reviewed literature and in the JMPR database; and (c) application of an ethical analysis to the problem. The paper identifies areas of agreement which include general principles that may provide a starting point on which to base criteria for judgements as to the ethical acceptability of such studies. However, the paper also highlights ongoing unresolved differences of opinion inherent in ethical analysis of contentious issues, which we propose should form a starting point for further debate and the development of guidelines to achieve better resolution of this matter.
p-Menthane-3,8-diol(38DIOL) was recently introduced as a natural topical insect repellent in the commercial product "OFF! Botanicals" lotion. The objective of this study was to provide an estimate of the potential for 38DIOL systemic absorption in humans. Carbon-14-labeled 38DIOL formulated in the lotion and in an ethanol solution was applied to excised pig skin in an in vitro flow-through test system predictive of skin absorption in humans. Twenty-four hours after application, radiolabel recovered from the dermis and receptor fluid was summed to determine percent absorption. At a dose of approximately 80 microg/cm(2) of 38DIOL in the lotion, a value of 3.5 +/- 0.8% of applied dose was obtained with pig skin. The corresponding value for 38DIOL in ethanol (90 microg/cm(2)) was not significantly different (3.0 +/- 1.2%). Most of the applied dose of 38DIOL was found to evaporate from pig skin (77 +/- 8% for the lotion and 87 +/- 1% for ethanol solution), thus limiting percutaneous absorption values. For reference purposes, the pig skin absorptions of piperonyl butoxide (PBO) at 100 microg/cm(2) in isopropanol, N,N-diethyl-m-toluamide (DEET) at 500 microg/cm(2) in ethanol, and neat isododecane at 650 microg/cm(2) (in order of increasing volatility) were 15 +/- 6%, 23 +/- 3%, and 0.09 +/- 0.05% of applied dose respectively. Isododecane was lost almost exclusively from the skin surface by evaporation. For additional reference, absorptions of PBO, DEET, and 38DIOL were found to be higher with excised rat skin.
The human percutaneous absorption of 2,4-dichlorophenoxyacetic acid (2,4-D) is well characterized. Five studies using human subjects have been published and the results of those studies showed remarkable reproducibility across a span of three decades and multiple laboratories, formulations, and methods. These human data provide valuable perspective for characterizing the variability (CV = 60%) and central tendency (mean = 5.7%) associated with dermal absorption of 2,4-D from 34 individuals. Mouse, rat, and rabbit absorption measurements all tend to be higher, while Rhesus monkeys provide data in the same range as humans. Inter-laboratory reproducibility for a range of other pesticides shows 60% difference in central tendency estimates of human dermal absorption, providing reassurance that commonly used methods of measurement are reliable. For purposes of estimating potential human health risks associated with systemic absorbed doses, there is far less uncertainty in using carefully collected human data than in using dermal absorption estimates from small numbers of inbred laboratory animals.
The dermal route of exposure is important in worker exposure to biocidal products. Many biocidal active substances which are used on a daily basis may decrease the barrier function of the skin to a larger extent than current risk assessment practice addresses, due to possible skin effects of repeated exposure. The influence of repeated and single exposure to representative biocidal active substances on the skin barrier was investigated in vitro. The biocidal active substances selected were alkyldimethylbenzylammonium chloride (ADBAC), boric acid, deltamethrin, dimethyldidecylammonium chloride (DDAC), formaldehyde, permethrin, piperonyl butoxide, sodium bromide, and tebuconazole. Of these nine compounds, only the quaternary ammonium chlorides ADBAC and DDAC had a clear and consistent influence on skin permeability of the marker compounds tritiated water and [(14)C]propoxur. For these compounds, repeated exposure increased skin permeability more than single exposure. At high concentrations the difference between single and repeated exposure was quantitatively significant: repeated exposure to 300 mg/L ADBAC increased skin permeability two to threefold in comparison to single exposure. Therefore, single and repeated exposure to specific biocidal products may significantly increase skin permeability, especially when used undiluted.
The pyrethrins have a long and fascinating history. They were derived from dried chrysanthemum flower heads that were found to have pesticidal activity centuries ago. They comprise a complex mixture of six main chemicals. Commercial formulations usually contain piperonyl butoxide, which inhibits metabolic degradation of the active ingredients. Pyrethrins are readily absorbed from the gut and respiratory tract but poorly absorbed through skin. The active components are rapidly and extensively metabolised in the liver. Pyrethrins probably act on sodium channels resulting in nervous system overactivity. The possibility that they also induce hypersensitivity, which may be fatal when the respiratory tract is involved, has been debated for many years. A few clinical reports support this suggestion but the limited epidemiological evidence available is against it. The number of reports of toxicity caused by pyrethrins has greatly decreased over recent years. The pyrethrins are generally of low acute toxicity but convulsions may occur if substantial amounts are ingested. Two deaths from acute asthma have been attributed to pyrethrins and clinical reports suggest that they may also cause a variety of forms of dermatitis. Ocular exposure has resulted in corneal erosions. Management of pyrethrin toxicity is supportive and symptomatic.
The first pyrethroid pesticide, allethrin, was identified in 1949. Allethrin and other pyrethroids with a basic cyclopropane carboxylic ester structure are type I pyrethroids. The insecticidal activity of these synthetic pyrethroids was enhanced further by the addition of a cyano group to give α-cyano (type II) pyrethroids, such as cypermethrin. The finding of insecticidal activity in a group of phenylacetic 3-phenoxybenzyl esters, which lacked the cyclopropane ring but contained the α-cyano group (and hence were type II pyrethroids) led to the development of fenvalerate and related compounds. All pyrethroids can exist as at least four stereoisomers, each with different biological activities. They are marketed as racemic mixtures or as single isomers. In commercial formulations, the activity of pyrethroids is usually enhanced by the addition of a synergist such as piperonyl butoxide, which inhibits metabolic degradation of the active ingredient. Pyrethroids are used widely as insecticides both in the home and commercially, and in medicine for the topical treatment of scabies and headlice. In tropical countries mosquito nets are commonly soaked in solutions of deltamethrin as part of antimalarial strategies.
Pyrethroids are some 2250 times more toxic to insects than mammals because insects have increased sodium channel sensitivity, smaller body size and lower body temperature. In addition, mammals are protected by poor dermal absorption and rapid metabolism to non-toxic metabolites. The mechanisms by which pyrethroids alone are toxic are complex and become more complicated when they are co-formulated with either piperonyl butoxide or an organophosphorus insecticide, or both, as these compounds inhibit pyrethroid metabolism. The main effects of pyrethroids are on sodium and chloride channels. Pyrethroids modify the gating characteristics of voltage-sensitive sodium channels to delay their closure. A protracted sodium influx (referred to as a sodium ‘tail current’) ensues which, if it is sufficiently large and/or long, lowers the action potential threshold and causes repetitive firing; this may be the mechanism causing paraesthesiae. At high pyrethroid concentrations, the sodium tail current may be sufficiently great to prevent further action potential generation and ‘conduction block’ ensues. Only low pyrethroid concentrations are necessary to modify sensory neurone function. Type II pyrethroids also decrease chloride currents through voltage-dependent chloride channels and this action probably contributes the most to the features of poisoning with type II pyrethroids. At relatively high concentrations, pyrethroids can also act on GABA-gated chloride channels, which may be responsible for the seizures seen with severe type II poisoning.
Despite their extensive world-wide use, there are relatively few reports of human pyrethroid poisoning. Less than ten deaths have been reported from ingestion or following occupational exposure. Occupationally, the main route of pyrethroid absorption is through the skin. Inhalation is much less important but increases when pyrethroids are used in confined spaces. The main adverse effect of dermal exposure is paraesthesiae, presumably due to hyperactivity of cutaneous sensory nerve fibres. The face is affected most commonly and the paraesthesiae are exacerbated by sensory stimulation such as heat, sunlight, scratching, sweating or the application of water.
Pyrethroid ingestion gives rise within minutes to a sore throat, nausea, vomiting and abdominal pain. There may be mouth ulceration, increased secretions and/or dysphagia. Systemic effects occur 4–8 hours after exposure. Dizziness, headache and fatigue are common, and palpitations, chest tightness and blurred vision less frequent. Coma and convulsions are the principal life-threatening features. Most patients recover within 6 days, although there were seven fatalities among 573 cases in one series and one among 48 cases in another.
Management is supportive. As paraesthesiae usually resolve in 12–24 hours, specific treatment is not generally required, although topical application of dl-α tocopherol acetate (vitamin E) may reduce their severity.
This chapter discusses the plant metabolism of piperonyl butoxide (PBO), crop residues, and its metabolites. PBO in combination with pyrethrum and pyrethroids is used to control insect pests on a wide variety of food crops. Defining the nature and magnitude of the residues in crops resulting from such uses, is an important part of the evaluation of safety of the product to humans and to the environment. The metabolism of PBO has been studied in three different crops: lettuce, cotton, and potatoes. Once the metabolites had been identified, crop residues of PBO and its closely related metabolites were determined in approximately 70 commodities from field studies on eight crop groups. To provide adequate accountability in metabolism studies of the distribution of parent compound and metabolites in each crop, 14C-radiolabelled PBO was used. The use of radiolabelled PBO allowed the distinction of parent compound and metabolites from naturally occurring plant compounds. A range of analytical methods was developed to characterize the nature of metabolites formed in crops both from the C metabolism and field crop residue studies.
In order to determine the human in vivo percutaneous absorption of pyrethrin and piperonyl butoxide, a commercial formulation containing either [14C]pyrethrin (3.8 mCi/mmol) or [14C]piperonyl butoxide (3.4 mCi/mmol) was applied to the ventral forearm of six human volunteers. The formulation contained 0.3% pyrethrin and 3.0% piperonyl butoxide. Spreadability studies showed that concentrations of 5.5 micrograms pyrethrin/cm2 and 75.8 micrograms piperonyl butoxide/cm2 (used in this study) would be consistent with levels found in actual use. The forearms were thoroughly cleansed with soap and water 30 min after application (as recommended for actual use). Percutaneous absorption was determined by urinary cumulative excretion following dose application. With a 7-day urinary accumulation, 1.9 +/- 1.2% (SD) of the dose of pyrethrin and 2.1 +/- 0.6% of the dose of piperonyl butoxide applied was absorbed through the forearm skin. 1 hr after application blood samples contained no detectable radioactivity. The percutaneous absorption of pyrethrin and piperonyl butoxide from the scalp was calculated to be 7.5% of the applied dose for pyrethrin and 8.3% for piperonyl butoxide. The calculated half-life of 14C excretion was 50 hr for pyrethrin and 32 hr for piperonyl butoxide. The data should be of relevance to appropriate risk assessment in extrapolating animal data to humans.
Pyrethroid esterases of Trichoplusia ni, Spodoptera littoralis and Bemisia tabaci hydrolyze the trans-isomers of various pyrethroids more extensively than the cis-isomers. Profenofos fed to T. ni larvae at a level inhibiting the gut pyrethroid esterases by 65% with trans-permethrin and of 95% with cis-cypermethrin increased the toxicity of topically applied trans-permethrin by fourfold and cis-cypermethrin by 20-fold. Similar assays with S. littoralis resulted in an increase of about threefold in the toxicity of both compounds. Monocrotophos, profenofos, acephate, and methidathion inhibited pyrethroid esterase activity in B. tabaci and synergized considerably the toxicity of cypermethrin. The remarkable tolerance of the predator Chrysopa carnea to pyrethroids is attributed to the presence of a high level of pyrethroid esterase activity with a unique specificity for hydrolyzing the cis-isomer. Phenyl saligenin cyclic phosphonate, a potent inhibitor for larval pyrethroid esterases synergized the toxicity of trans-permethrin by 68-fold from an LD50 of 17,000 micrograms/g to 250 micrograms/g. In contrast, oxidase inhibitors such as piperonyl butoxide, SV-1, and MPP synergized considerably the toxicity of pyrethroids in Tribolium castaneum and Musca domestica. Hence the predominant pathway for pyrethroid detoxification in insects, whether hydrolytic or oxidative, depends largely on the insect species. The high toxicity of the recent developed acylureas results from their high retention in the insects. Assays using radiolabeled diflubenzuron and chlorfluazuron applied to fourth instar T. castaneum larvae revealed a rapid elimination of diflubenzuron (T1/2 approximately equal to 7 h) as compared with chlorfluazuron (T1/2 > 100 h). Addition of 100 ppm DEF to the diet increased both the retention time and the toxicity of diflubenzuron in both T. castaneum and S. littoralis, which was due probably to the inhibition of diflubenzuron hydrolase activity. Esterases, hydrolyzing pyrethroids, and acylureas may serve as tools for evaluating potential synergists and for monitoring resistance in various agricultural pests due to increased metabolism.
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