An in vitro method to quantify dermal absorption of pesticide resi- dues

All pesticides must go through a rigorous risk assessment process in order to show that they are safe for use. With respect to dermal risk assessment for re-entry workers, the absorption value applied to predict systemic dose from this external exposure is obtained by testing liquid forms of the pesticide in vivo and/or in vitro. However, in a real exposure scenario, the worker would be exposed to a dried residue, for which little or no absorption data are available. This study has developed a novel methodology for assessing the dermal absorption of pesticides from dried residues, and aims ultimately to use this methodology to obtain more realistic absorption values for the risk assessment. A principal function of the skin is to act as a barrier, both to the loss of endogenous water and to absorption of exogenous compounds. The skin comprises two major components: the innermost dermis and the superficial epidermis. Barrier function resides in the outer layer of the epidermis, the stratum corneum (SC), the thickness of which is typically on the order of 20 μm. When a crop is treated with pesticide, a residue is left behind on surfaces such as leaves. An individual who subsequently enters the area may then be exposed to these residues via contact with their skin; this is most common with ‘re-entry workers’ who may enter the treated area after application of the pesticide formulation to carry out tasks such as crop inspection or manual harvest. A risk assessment must be carried out for these re-entry workers and their Potential Dermal Exposure (PDE, μg/day) calculated: PDE = DFR x TC x T where DFR is the Dislodgeable Foliar Residue (μg/cm), the quantity of substance remaining on the surface of the leaf that can be dislodged and transferred to skin; TC is the Transfer Coefficient (cm/h), which is specific to the particular re-entry task and refers only to the amount of contact between skin and the contaminated surface; T (h/day) is the Exposure Time, typically 2 hours for crop inspection and 8 hours for harvest. Once the potential exposure has been calculated, the percentage of the applied ‘dose’ that becomes available systemically is estimated. In vitro skin absorption studies are carried out for most pesticides, determining the compound’s uptake from a finite dose of both the concentrate and from a relevant in-use spray dilution (described below). The higher of the calculated percentage absorption values (generally the most dilute solution) is then used to represent a worst-case scenario for the re-entry worker. To pass risk assessment, this value must be below the maximum acceptable value identified for the compound during toxicology testing. In a real exposure scenario, the re-entry worker would most likely come into contact with a dried residue rather than a liquid form of the product. Unfortunately, no acceptable methodology exists for the acquisition of absorption data from such residues. It is likely that the use of data from liquid applications represents an overestimate and that the dried residues left on plant surfaces, to which workers are exposed, would not be absorbed to the same extent. This may lead to the pesticide failing the risk assessment process meaning that safe and effective products may not be approved for use. Previous work has shown that pesticide absorption from a residue, when applied in the form of a coated disk pressed against the skin, was different to that of an aqueous solution. However, this occlusive and long-term exposure was not fully representative of a re-entry worker scenario, where only a brief contact between skin and foliage would occur. Furthermore, the ‘doses’ used (100-1000 μg/cm of pesticide) were an order of magnitude higher than would occur in a re-entry exposure scenario and were delivered as neat active ingredients either in solution or as a suspension (as opposed to a commercially relevant formulation) The aim of this study, therefore, is to develop a robust methodology for assessing the dermal absorption of pesti2 cides from dried foliar residues that addresses these limitations and is more relevant to the re-entry scenario. The ultimate aim is to use this approach to obtain more realistic absorption values for risk assessment. It is important that this method is as close to a real exposure scenario as possible and is easily reproducible. A standard in vitro protocol was used to measure pesticide dermal absorption . Experiments (n = 4-5) were performed in static Franz diffusion cells (Permegear, Hellertown, USA), with dermatomed porcine skin (diffusion area = 2 cm) maintained at 32°C. The receptor chamber contained 7.4 ml of a 6% (w/v) solution of polyoxyethylene glycol (10) oleyl ether (Sigma, UK) in phosphate-buffered saline at pH 7.4. The pesticide Trinexapac-ethyl (TXP, Syngenta plc, Jealott’s Hill, U.K.) was applied to the skin as an emulsifiable concentrate (10% w/w) diluted 100-fold in water, or as a dried residue (see below). In the case of the liquid formulation, 20 μl (1 μg/μl) were applied directly and evenly to the skin surface. For the residue, 40 μl of the diluted concentrate were first applied to a 12 mm diameter steel disc (SPM specimen discs, TAAB Laboratories Equipment Ltd., Aldermaston, U.K.) and allowed to dry for 24 hours to a dried residue. The disc was then attached to a weighted vial (~10 g) that was rotated on the skin surface. The procedure involved three complete rotations in both the clockwise and anticlockwise directions, followed by moving the disc laterally in a “+” configuration (see Supporting Information 1). After application, non-transferred residue remaining on the disk was extracted and quantified to confirm the amount actually transferred to skin, specifically 21.7 ± 3.3 μg, (mean ± S.D.; i.e., ~54% of that applied to the disk) with the aim being to match the 20 μg application from the liquid. Post-application of the formulations, the receptor solution was sampled at 2, 4, 6 and 8 hours. The skin surface was washed at 8 hours (to represent a typical working day) with 100 μl of a mild (0.1% w/v) soap solution and dried with two cotton buds. An additional receptor solution sample was taken at 24 hours, after which the stratum corneum was sequentially removed by adhesive tape stripping . The first two tape-strips were not discarded and the chemical thereon was quantified; however, the quantities found were not included in the total absorption calculations as this material is generally not assumed to be bioavailable. The skin uptake and absorption of TXP (Table 1; Figure 1) was determined following HPLC analysis (see Supporting Information 2) of the receptor solution samples, the SC tape-strips, the washing solution, viable tissue and cotton buds. The pesticide was efficiently extracted from the SC using 60:40, acetonitrile: water. Total TXP absorption (i.e., quantity of pesticide in tape strips 3-15 + skin extraction + quantity permeated to receptor) was significantly lower for the residue than for the liquid; permeation of pesticide into the receptor solution continued after skin was washed. It is noteworthy that, while significantly more TXP permeated into the receptor phase in 8 hours following liquid application, there was no significant difference after 24 hours between the liquid and dried residue exposures. Table 1: Skin uptake of TXP (mean±SD); surface cleaned at 8 hr. Liquid (μg) Residue (μg) p-value Receptor 2 hr 0.08±0.17 0.00±0.00 0.35 Receptor 4 hr 0.52±0.18 0.00±0.00 < 0.01 Receptor 6 hr 0.72±0.12 0.00±0.00 < 0.01 Receptor 8 hr 0.90±0.16 0.07±0.15 < 0.01 Receptor 24 hr 1.70±0.46 1.15±0.40 0.08 Tapes 1 & 2 0.86±0.45 0.56±0.16 0.19 Tapes 3-15 0.33±0.11 0.23±0.12 0.29 Surface wash 11.0±1.78 12.2±2.67 0.42 Skin 1.08±0.66 0.30±0.10 0.03 Total absorbed 3.11±0.86 1.68±0.56 0.015 % ‘dose’ absorbed 15.54±4.36 7.63±1.65* < 0.01 * Expressed as a percentage of the estimated ‘dose’ applied for each replicate. Figure 1: Permeation of TXP into receptor (μg) as a function of time. Skin surface cleaned at 8 hr.