Dermal absorption of the phenoxy herbicides 2,4-D, 2,4-D amine, 2,4-D isooctyl, and 2,4,5-T in rabbits, rats, rhesus monkeys, and humans: a cross-species comparison

A Comprehensive, Integrated Review and Evaluation of the Scientific Evidence Relating to the Safety of the Herbicide 2,4-D

The safety of 2,4-D to farm and forestry workers, commercial applicators and the general public has been of continuing concern because certain epidemiological studies of groups potentially exposed to 2,4-D have suggested a relationship between 2,4-D use and increased risk of soft tissue sarcoma, Hodgkin's disease or non-Hodgkin's lymphoma. This review on 2,4-D is unique in that the approach taken was to integrate data from worker exposure studies, whole animals, metabolic and other relevant laboratory studies with the epidemiological findings to assess the extent to which there is scientific support for the hypothesis that 2,4-D exposure is associated with any increased risk of human cancer.

The case-control epidemiological studies that have been the source of the cancer risk hypothesis are inconclusive. Problems in assessing exposure based on patients' memories make these studies difficult to interpret. Cohort studies of exposed workers do not generally support the specific hypothesis that 2,4-D causes cancer. Taken together, the epidemiological studies provide, at best, only weak evidence of an association between 2,4-D and the risk of cancer.

Importantly, the cancer hypothesis is not supported by other data. A critical evaluation of the exposure data indicates that exposure to 2,4-D in user groups is intermittent and much lower than the doses tested chronically in long-term animal studies that have not shown evidence of tumor induction. Moreover, the structure of 2,4-D does not suggest it would be a carcinogen. 2,4-D is a simple organic acid, that is largely excreted unchanged, and there is no evidence that it is metabolized to critically reactive metabolites or accumulates in tissues. This evidence is supported by a large body of negative studies on genotoxicity, which taken together with the metabolic studies, clearly indicates that 2,4-D is highly unlikely to be a genotoxic carcinogen. Furthermore, 2,4-D has no known hormonal activity and does not induce proliferative changes in any tissue or organ, indicating that it does not possess any of the characteristics of non-genotoxic animal carcinogens. Thus the available mechanistic studies provide no plausible basis for a hypothesis of carcinogenicity.

In this review, data relating to potential neurotoxicity, immunotoxicity and reproductive toxicity also were evaluated. There is no evidence that 2,4-D adversely affects the immune system and neurotoxic and reproductive effects only have been associated with high toxic doses that would not be encountered by 2,4-D users.

Historical exposures to 2,4-D by user groups, particularly farmers, forestry workers and commercial applicators, would be higher than those sustained under present rigorous standards for application which involve the use of protective clothing and other measures to reduce exposure. Proposed label changes indicate that in the future exposures will be even further reduced. Viewed in this context, the available data indicate that the potential public health impact of 2,4-D, including the risk of human cancer, was negligible in the past and would be expected to be even smaller in the present and future.

Absorption of hydrocortisone acetate in human connective tissue using phonophoresis

Background:

Therapeutic ultrasound to drive medication (phonophoresis) has been a mainstay in physical therapy. The most common drug used in phonophoresis is hydrocortisone acetate (HA). A number of studies have been done examining phonophoresis in the delivery of HA through the skin to underlying tissues; however, a study has never been done examining the absorption of HA using phonophoresis on human connective tissue.

Hypothesis:

Phonophoresis will facilitate the transmission of HA in human connective tissue.

Study Design:

Randomized controlled study.

Methods:

Twenty-one patients undergoing anterior cruciate ligament reconstruction surgery were randomly assigned to either a sham or true phonophoresis treatment group. The latter group received 6 minutes of 10% HA ultrasound at a point consistent with the gastrocnemius slip of the semitendinosis tendon (treatment site). The sham group received 6 minutes of 10% HA ultrasound to the same area, but the ultrasound was not turned on. The slip and a sample of the distal attachment of the tendon (control) were removed. Samples were analyzed for HA levels.

Results:

Although the mean and median levels of HA found at the treatment site were greater than those of the control site (means, 34.1 vs 22.9 parts per billion; medians, 7 vs 0 parts per billion), the levels of HA found at the treatment site were not significantly greater than those at the control site (P = 0.15). There were no statistically significant differences between the true and sham phonophoresis groups in HA levels (P = 0.80) nor in age, sex, or skin thickness.

Conclusion:

Phonophoresis does not appear to facilitate the absorption of HA in connective tissue when compared with simple absorption (sham).

Clinical Relevance:

Phonophoresis does not appear to enhance the transmission of HA in human connective tissue; therefore, use of phonophoresis should be reconsidered in inflammatory conditions.

The ethics of human volunteer studies involving experimental exposure to pesticides: unanswered dilemmas

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.

Cutaneous challenge with chemical warfare agents in the SKH-1 hairless mouse (II): Effects of some currently used skin decontaminants (RSDL and Fuller’s earth) against liquid sulphur mustard and VX exposure

Using the hairless mouse screening model presented in the companion paper1 the aim of this study was to assess two skin decontaminating systems: Fuller’s earth (FE) and Reactive Skin Decontamination Lotion (RSDL) against two extremely toxic chemical warfare agents that represent a special percutaneous hazard, sulphur mustard (SM) and O-ethyl-S-(2[di-isopropylamino]ethyl)methyl-phosphonothioate (VX). Five minutes after being exposed on the back to either 2 µL of neat sulphur mustard or 50 µg.kg-1 of diluted VX, mice were decontaminated. Both systems were able to reduce blisters 3 days after SM exposure. However, RSDL was found to be more efficient than FE in reducing the necrosis of the epidermis and erosion. In the case of VX exposure, RSDL, whatever the ratio of decontaminant to toxicant used (RSDL 10, 20, 50), was not able to sufficiently prevent the inhibition of plasma cholinesterases taken as a surrogate marker of exposure and toxicity. Only FE reduced significantly the ChE inhibition. Some of these observations are different from our previous results obtained in domestic swine and these changes are thus discussed in the perspective of using SKH-1 hairless mice for the initial in vivo screening of decontaminants.

Percutaneous absorption and exposure assessment of pesticides

Dermal exposure to a diverse range of chemicals may result from various uses. In order to assess exposure and estimate potential risks, accurate quantitative data on absorption are required. Various factors will influence the final results and interpretations of studies designed to assess the ability of compounds to penetrate the skin. This overview will discuss skin penetration by pesticides, emphasizing key parameters to be considered from the perspective of exposure assessment.

The effect of skin thickness and time in the absorption of dexamethasone in human tendons using iontophoresis

STUDY DESIGN

Experimental laboratory study.

OBJECTIVES

To measure the transmission of dexamethasone sodium phosphate (DEX-P) using iontophoresis as a function of skinfold tissue thickness and time elapsed between treatment and tissue extraction.

BACKGROUND

Iontophoresis is a modality used in physical therapy with the intent to drive medications through the skin to underlying tissues using a direct electrical current. DEX-P is the most commonly used medication with iontophoresis and is used to treat a variety of connective tissue conditions.

METHODS AND MEASURES

Sixteen adults undergoing anterior cruciate ligament reconstructive surgery using the semitendinosis/gracilis autograft received a 40-mA-min dose of iontophoresis with 0.4% DEX-P superficial to a slip of the distal semitendinosis tendon prior to surgery. The tendon slip was extracted within 4 hours. Time between treatment and tissue extraction and skinfold thickness were measured. Analysis was performed on the slip of the semitendinosis using high-performance liquid chromatography mass spectrum.

RESULTS

Of the 16 subjects (10 female, 6 male; mean age, 33 years), 7 had measurable amounts of DEX-P in the tendon slip (4 female, 3 male; mean age, 34 years). The average concentration in the 16 subjects was 2.9 ng/g of tendon tissue. There was no correlation between DEX-P absorbed and skinfold thickness (r = -0.08, P = .79) or time elapsed (r = 0.25, P = .38). In a subset of the 7 individuals that showed measurable levels of DEX-P absorbed, the average concentration of DEX-P was 6.6 ng/g of tendon tissue, and there was a relationship between DEX-P concentrations and time elapsed that did not reach statistical significance (r = 0.71, P = .11).

CONCLUSIONS

Iontophoresis appears to facilitate the transmission of dexamethasone to connective tissues in humans with skinfold thickness up to at least 30 mm. The absorption of the dexamethasone seemed to continue to occur for up to 4 hours after delivery. It is not clear why DEX-P was measured in only 7 of the 16 subjects.

LEVEL OF EVIDENCE

Therapy, level 5.

Absorption of dexamethasone sodium phosphate in human connective tissue using iontophoresis

BACKGROUND

Iontophoresis ostensibly facilitates the delivery of medications through the skin to underlying tissues using a direct electrical current. Dexamethasone is the most commonly used medication with iontophoresis to treat a variety of connective tissue disorders.

HYPOTHESIS

Iontophoresis will facilitate the absorption of dexamethasone into connective tissue compared with diffusion.

STUDY DESIGN

Controlled laboratory study.

METHODS

Twenty-nine adults undergoing anterior cruciate ligament reconstructive surgery using the semitendinosus/gracilis autograft were randomly assigned to either a true iontophoresis (TI) or sham iontophoresis (SI). In the TI group, a 40-mA/min dose of iontophoresis using a 0.4% (4 mg/mL) solution of dexamethasone was used targeting the semitendinosus tendon just before surgery. The SI group underwent the same treatment, but the machine was not turned on. Tissue was extracted within 4 hours of treatment and analyzed for dexamethasone. In addition, 2 control samples were sent to the laboratory for analysis.

RESULTS

There was a statistically significant difference in dexamethasone concentrations between the groups (P = .0216). Of the 16 samples in the TI group, 8 had measurable amounts of dexamethasone, with an average concentration of 2.906 ng/g of tendon tissue. In the SI group, 1 of the 13 samples had measurable amounts of dexamethasone with an average concentration of 0.205 ng/g of tendon tissue. The control samples contained no dexamethasone.

CONCLUSION

Iontophoresis facilitates the transmission of dexamethasone to connective tissues in humans.

CLINICAL RELEVANCE

Iontophoresis can deliver dexamethasone to connective tissues in humans.

Estimating absorbed dose of pesticides in a field setting using biomonitoring data and pharmacokinetic models

Linking biomarker data to pharmacokinetic (PK) models permits comparison of absorbed dose with a toxicological benchmark, which is an important step to understanding the health implications of pesticide exposure. The purpose of this analysis was to evaluate the feasibility of reconstructing the absorbed dose of two pesticides using PK models developed from biomarker data in a study of occupational application of these compounds. Twenty-four-hour urine samples were collected from farmers 24 h before through 96 h after a typical application of chlorpyrifos or 2,4-D. PK models were used to link the amounts found in urine samples to absorbed dose. Modeled total body dose estimates (in micrograms) were compared to measured dose from time 0-96 h. Despite the complexities surrounding the interpretation of biomonitoring data from a field setting, the models developed as part of this analysis accurately estimated the absorbed dose of 2,4-D and chlorpyrifos when collection of urine samples was largely complete. Over half of the farmers were excluded from modeling due to suspected noncompliance with urine collection or confounding exposure events, which highlights the importance of these issues for designing and interpreting biomonitoring data in future studies. Further evaluation of PK models in scenarios using single void samples is warranted for improving field-based dose assessments.

Skin decontamination of mustards and organophosphates: comparative efficiency of RSDL and Fuller's earth in domestic swine

Research in skin decontamination and therapy of chemical warfare agents has been a difficult problem due to the simultaneous requirement of rapid action and non-aggressive behaviour. The aim of this study was to compare the performance of two decontaminating systems: the Canadian Reactive Skin Decontaminant Lotion (RSDL) and the Fuller's Earth (FE). The experiment was conducted with domestic swine, as a good model for extrapolation to human skin. RSDL and FE were tested against sulphur mustard (SM), a powerful vesicant, and VX, a potent and persistent cholinesterase inhibitor. When used 5 min after contamination, the results clearly showed that both systems were active against SM (10.1 mg/cm(2)) and VX (0.06 mg/cm(2)). The potency of the RSDL/sponge was statistically better than FE against skin injury induced by SM, observed 3 days post-exposure. RSDL was rather more efficient than FE in reducing the formation of perinuclear vacuoles and inflammation processes in the epidermis and dermis. Against a severe inhibition (67%) of plasmatic cholinesterases induced by VX poisoning, the potencies of the RSDL/sponge and FE were similar. Both systems completely prevented cholinesterase inhibition, which indirectly indicates a prevention of toxic absorption through the skin.

Dermal absorption of 2,4-D: a review of species differences

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.

Review of 2,4-dichlorophenoxyacetic acid (2,4-D) epidemiology and toxicology

The scientific evidence in humans and animals relevant to cancer risks, neurologic disease, reproductive risks, and immunotoxicity of 2,4-D was reviewed. Despite several thorough in vitro and in vivo animal studies, no experimental evidence exists supporting the theory that 2,4-D or any of its salts and esters damages DNA under physiologic conditions. Studies in rodents demonstrate a lack of oncogenic or carcinogenic effects following a lifetime dietary administration of 2,4-D. Epidemiologic studies provide scant evidence that exposure to 2,4-D is associated with soft tissue sarcoma, non-Hodgkin's lymphoma, Hodgkin's disease, or any other cancer. Overall, the available evidence from epidemiologic studies is not adequate to conclude that any form of cancer is causally associated with 2,4-D exposure. There is no human evidence of adverse reproductive outcomes related to 2,4-D. The available data from animal studies of acute, subchronic, and chronic exposure to 2,4-D, its salts, and esters show an unequivocal lack of systemic toxicity at doses that do not exceed renal clearance mechanisms. There is no evidence that 2,4-D in any of its forms activates or transforms the immune system in animals at any dose. At high doses, 2,4-D damages the liver and kidney and irritates mucous membranes. Although myotonia and alterations in gait and behavioral indices are observed after overwhelming doses of 2,4-D, alterations in the neurologic system of experimental animals are not observed with the administration of doses in the microgram/kg/day range. It is unlikely that 2,4-D has any neurotoxic potential at doses below those required to induce systemic toxicity.

Site-specific percutaneous absorption of methyl salicylate and VX in domestic swine

The site specificity of the percutaneous absorption of methyl salicylate (MeS) and the organophosphate nerve agent VX (O-ethyl S-(2-diisopropylaminoethyl) methylphosphonothioate) was examined in anaesthetized domestic swine that were fully instrumented for physiological endpoints. Four different anatomical sites (ear, perineum, inguinal crease and epigastrium) were exposed to the MeS and the serum levels were measured over a 6-h time period. The dose absorbed at the ear region was 11 microg cm(-2) with an initial flux of 0.063 microg cm(-2)min(-1), whereas at the epigastrium region the dose absorbed was 3 microg cm(-2) with an initial flux of 0.025 microg cm(-2)min(-1). For this reason further studies were carried out with VX on the ear and the epigastrium only. In animals treated with agent on the epigastrium, blood cholinesterase (ChE) activity began to drop 90 min after application and continued to decline at a constant rate for the remainder of the experiment to ca. 25% of awake control activity. At this time there were negligible signs of poisoning and the medical prognosis was judged to be good. In contrast, the ChE activity in animals receiving VX on the ear decreased to 25% of awake control values within 45 min and levelled out at 5-6% by 120 min. Clinical signs of VX poisoning paralleled the ChE inhibition, progressing in severity over the duration of the exposure. It was judged that these animals would not survive. The dramatic site dependence of agent absorption leading to vastly different toxicological endpoints demonstrated in this model system has important ramifications for chemical protective suit development, threat assessment, medical countermeasures and contamination control protocols.

Chemical uptake into human stratum corneum in vivo from volatile and non-volatile solvents

PURPOSE

Simple, safe and quick in vivo methods for estimating chemical uptake into the stratum corneum (SC) from volatile and non-volatile solvents are invaluable to health risk assessors. This study compares the human in vivo SC uptake of a model compound (4-cyanophenol) from water and acetone using quantitative attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy.

METHODS

Small areas on the ventral forearms of human volunteers were treated with 4-cyanophenol (CP) dissolved either in water or acetone. After the skin was cleansed of remaining surface CP, SC samples were taken by a standard tape-stripping method. CP concentration profiles across the SC were quantitated by direct measurement of the permeant on the individual tape-strips using ATR-FTIR.

RESULTS

Increasing the duration of exposure to CP aqueous solutions resulted in increasing CP uptake into the SC; the kinetics of uptake correlated well with predictive diffusion equations. Increasing the 'dose' of CP in acetone also resulted in increasing uptake into the SC, but uptake eventually plateaued at a maximum level. The amount of CP taken up into the SC from acetone was 2 to 8-fold greater than that from water following similar short-time exposures.

CONCLUSIONS

These safe, simple experimental methods provide practical and predictive assessments of chemical uptake into human SC in vivo.

Percutaneous absorption of salicylic acid, theophylline, 2, 4-dimethylamine, diethyl hexyl phthalic acid, and p-aminobenzoic acid in the isolated perfused porcine skin flap compared to man in vivo

Human risk assessment for topical exposure requires percutaneous absorption data to link environmental contamination to potential systemic dose. Human absorption data are not readily available, so absorption models are used. In vitro diffusion systems are easy to use but have proved to be somewhat unreliable and are not validated to man. This study compares percutaneous absorption in the isolated perfused porcine skin flap (IPPSF) system with that in man in vivo. The study design utilized the same compounds and the same dose concentration and vehicle in both systems. Methodology for each system was that which is routinely used ineach system. The skin surface was not protected during the absorption dosing period. Percutaneous absorption values were, for man and the IPPSF system, respectively: salicylic acid (6.5 +/- 5.0%; 7.5 +/- 2.6%), theophylline (16.9 +/- 11.3%; 11.8 +/- 3.8%), 2,4-dimethylamine (1.1 +/- 0.3%; 3.8 +/- 0.6%), diethyl hexyl phthalic acid (1.8 +/- 0.5%; 3.9 +/- 2.4%), and p-aminobenzoic acid (11.5 +/- 6.3%; 5.9 +/- 3.7%) (correlation coefficient was 0.78; p < 0.04). The skin surface wash recovery postapplication was similar for salicylic acid in man (53.4 +/- 6.3%) and the IPPSF system (48.2 +/- 4.9%). With the other compounds the majority of surface chemical was recovered in the surface wash and skin tape strip in the IPPSF system. With man, other than salicylic acid, only a few percent applied dose was recovered with surface washing and tape stripping. Since the wash procedure was effective with pig skin, we can assume that these chemicals in man were lost to adsorption to any clothing or bedding with the volunteers. The absorption in man was not less than that in the IPPSF. Assuming the dose was lost in man, it seems plausible that whatever compound was to penetrate human skin in solvent vehicle did so in the period of time before the chemical was removed. The IPPSF system appears to be a good model for predicting percutaneous absorption relative to man. This study design should be used to validate other systems to humans in vivo.

Dermal penetration of 4"-(epi-methylamino)-4"-deoxyavermectin B1a benzoate in the rhesus monkey

The dermal absorption of the experimental avermectin insecticide emamectin benzoate was studied in the Rhesus monkey. Dermal absorption was calculated by comparing radioactivity levels in excreta following dermal application of the compound with those following administration of an equivalent intravenous dose. After i.v. administration of 300 micrograms [3H]MAB1a (prepared as a 1:1 solution of propylene glycol:saline) to three monkeys, plasma levels decreased biphasically with a rapid decline in radioactivity during the first 15 min followed by a slower decline to background. By 7 days post-dose, approximately 90% and 5% of the administered radioactivity was recovered in the faeces and urine, respectively. After a washout period, 300 micrograms [3H]MAB1a (dissolved in emulsifiable concentrate) was applied topically to the shaved forearm of the same monkeys. Following a 10-hr exposure period, approximately 90% of the radioactivity was recovered in a soap and water wash of the exposed forearms. Although plasma radioactivity levels generally remained below background levels, approximately 1.5% of the applied dose was recovered in the excreta. Dermal absorption of [3H]emamectin benzoate was calculated as 1.6%. The low dermal penetration of emamectin benzoate indicates that minimal actual exposure of agricultural workers to this compound will occur.

Assessment of exposure to 2,4-dichlorophenoxyacetic acid in the chemical industry: results of a five year biological monitoring study

Data on individual exposure to 2,4-dichlorophenoxyacetic acid (2,4-D) in herbicide production plants are limited. Hence, the urinary excretion of this herbicide was measured during a five year (1985-1989) biological monitoring study of 27 men and 18 women employees exposed during the production and formulation of 2,4-D and related sodium and dimethylamine salts. In separate studies, specimens of urine were collected in the morning, or during the last three hours of a working shift, or over a 24 hour period (1200 to 1200 or 0800 to 0800) and were analysed by an immunochemical method (2,4-D radioimmunoassay (RIA)). Urinary 2,4-D concentrations varied within a large scale from only a few micrograms/l to several 10s of mg/l. During a week, herbicide excretion increased, culminating on Friday. At the weekend, when no work was done, 2,4-D elimination decreased but did not return to zero in any case. After an interruption of exposure for about three weeks, urinary 2,4-D was no longer detectable. About five days after restarting work, body concentrations had built up again. Measurements of 2,4-D concentrations in air at different work-places showed that herbicide concentrations did not exceed 0.5 mg/m3. As well as inhalation, dermal 2,4-D absorption seemed to play an important part in total uptake of herbicide. Furthermore, a strong correlation was found (r = 0.9628) between 2,4-D urinary concentration, adjusted for endogenous creatinine, and the estimated amount of absorbed herbicide. Estimated absorbed doses were, in most cases, well below 0.1 mg 2,4-D/kg body wt/day. Sometimes this concentration was greatly exceeded. Thyroid hormone concentrations in blood were measured as well. No notable abnormalities were found. Exposed subjects were also typed for histocompatibility locus antigens (ABC antigens). The immunochemical determination of 2,4-D in specimens of urine proved to be a simple, cost effective, and non-invasive method to measure human exposure.

Pig ear skin as an in-vitro model for human skin permeability

Pig skin has been shown to have similar histological and physiological properties to human skin and has been suggested as a good model for human skin permeability. In this series of experiments, the in-vitro permeability of pig ear skin was compared with human (abdominal) skin and rat (dorsal) skin using both hydrophilic (water, mannitol, paraquat) and lipophilic (aldrin, carbaryl, fluazifop-butyl) penetrants. Pig skin was found to have a closer permeability character than rat skin to human skin, particularly for lipophilic penetrants. Electrical conductivity measurements across pig skin membranes showed that skin conductivity could be a useful method for assessing the integrity of membranes, particularly when used in conjunction with water permeability assessments.

The influence of different strains and age on in vitro rat skin permeability to water and mannitol

Water and mannitol were used as test penetrants to study the effect of age on the skin permeability of the Wistar-derived Alderley Park (AP) rat and Sprague-Dawley (SD) rat. Whole-skin membranes were prepared from rats aged 10 to 120 days, while epidermal membranes were prepared from rats aged 24 to 32 days. The results indicated that the skin permeabilities of the two strains were very similar for either whole-skin or epidermal membranes. The influence of age on skin permeability was found to be negligible for the AP rat, and a small decrease in whole-skin permeability was observed for SD rats above 80 days of age. A statistically derived expression ("the separation efficiency factor") was used to determine the optimum age for preparing intact epidermal membranes; these were 26 days for AP rats and 28 days for SD rats. Histological examination of whole-skin membranes for both strains revealed that the stratum corneum and epidermal thickness did not alter significantly with age (10 to 120 days old). Dermal thickness, hair follicle depth, and, to a lesser extent, the surface area occupied by hair follicles all appeared to be influenced by age, although these changes had no detectable effect on skin permeability.

Dermal absorption of the phenoxy herbicide 2,4-D dimethylamine in humans: effect of DEET and anatomic site

Percutaneous absorption of the 14C-ring-labeled phenoxy herbicide 2,4-D-amine (2,4-dichlorophenoxyacetic acid dimethylamine) was examined following topical applications of the herbicide to the palm and forearm of human volunteers. The effect of two vehicles (water and acetone) and the mosquito repellent DEET (N,N-diethyl-m-toluamide) on dermal absorption of 2,4-D-amine also was investigated. The total percent dermal absorption was calculated from the mean percent urinary recoveries and was not corrected for nonurinary excretion. The data revealed 14 +/- 4.5% (standard deviation) and 10 +/- 11.5% palmar absorption of 2,4-D-amine applied in water, with and without DEET, respectively, and 7 +/- 6.2% and 13 +/- 5.0% forearm absorption of the herbicide applied in water or acetone, respectively. Soap-and-water skin washes conducted at 24 h posttreatment removed up to 34% of the applied dose. Successive tape strips of skin taken at 24 h posttreatment demonstrated generally decreasing herbicide levels in the outer layers. The data bring into question the complete validity of the rhesus monkey model to predict human dermal absorption.

Oral and dermal application of 2,4-dichlorophenoxyacetic acid sodium and dimethylamine salts to male rats: investigations on absorption and excretion as well as induction of hepatic mixed-function oxidase activities

Absorption and urinary excretion of 2,4-dichlorophenoxyacetic acid sodium (sodium 2,4-D) and 2,4-dichlorophenoxyacetic acid dimethylammonium (2,4-DMA) salts were examined after single oral and mid-dorsal skin applications of the herbicides to male rats. Doses of 2.6 mg 2,4-D/kg body wt (sodium 2,4-D) and 1.9 mg 2,4-D/kg body wt (2,4-DMA) were applied. Quantitatively, with both salts, most of the orally administered herbicide (over 90%) was excreted in urine within 28 h. However, 2,4-D urinary peak concentrations were measured 4.5 and 20.5 h after dosing with 2,4-DMA and sodium 2,4-D, respectively. Additionally, the volume of urine in the oral study was significantly increased with both salts when compared with the controls or the dermal exposure. After topical application, 2,4-D absorption was much lower than in the oral study. Urinary excretion only reached about 10 and 15% of the applied dose for sodium 2,4-D and 2,4-DMA, respectively, by 5 days post-treatment. Further, we found some elevations in hepatic cytochrome P-450 activities. Ethylmorphine N-demethylase was only slightly induced by the herbicides while ethoxyresorufin O-deethylase activity was increased nearly 2-fold by sodium 2,4-D.