In vivo skin absorption and distribution of the nerve agent VX (O–ethyl–S–[2(diisopropylamino)ethyl] methylphosphonothioate) in the domestic white pig

Baseline physiological data from anesthetized pigs in a VX intoxication model

Over the past fifty years, swine models have been used for organophosphorus intoxication studies. Among these studies and others on the swine model in general, some physiological data, especially cholinesterase activity highly impacted by organophosphorus compounds like nerve agent VX, still need to be completed. To support and compare our model to others, we have published the experimental protocol, the physiological values of 31 juvenile anesthetized pigs, and the 6 h-follow-up of six supplementary anesthetized control animals and 7 VX-intoxicated pigs. We reported hemodynamics and respiratory parameters, blood levels in several biochemical parameters, blood gas, and complete blood count and compared them to the literature. We also focused on tissue and blood cholinesterase activities and detailed them for acetylcholinesterase and butyrylcholinesterase. After establishing a broad physiological data set consistent with the literature, we reported several cardio-respiratory parameters that seem more affected by an organophosphate intoxication, like heart rate, arterial blood pressure, cardiac output, and respiratory rate. Within the blood, oxygen saturation (SpO2), lactatemia, base excess, and glycemia can also be measured and associated with the other parameters to evaluate the life-threatening status. This swine model is currently used to develop and evaluate medical countermeasures against organophosphate nerve agent intoxications.

"Catch-up" therapy: combining antidotal treatment with dermal application of AHA following percutaneous VX poisoning in the domestic swine

Low-volatility organophosphorus chemical warfare agents (OP CWAs) are cholinesterase inhibitors which easily absorb into the skin, leading to the formation of a dermal depot from which they slowly enter the bloodstream. This leads to sustained cholinergic hyperstimulation, which if untreated may lead to death. However, current available countermeasures are not adequate to neutralize the agent residing in the dermal depot. Accordingly, we evaluated the efficacy of the potassium salt of acetohydroxamic acid (880 mg/ml in DMSO/H2O 1:4, AHAK), as a potential "catch-up" therapy lotion intended to neutralize the dermal depot, by penetrating the skin and decomposing it before it reaches the bloodstream. To that end, we compared the clinical outcome following skin surface decontamination combined with antidotal treatment, to that following the same antidotal treatment combined with dermal application of AHAK at the site of VX exposure, against percutaneous poisoning by a lethal neat dose (4 mg/kg) of the low-volatility nerve agent VX, in an unanesthetized swine model. Following skin surface decontamination and antidotal treatment, recurrence of intoxication signs and a prolonged recovery time were observed. In contrast, similar antidotal treatment combined with dermal application of AHAK significantly reduced intoxication signs recurrences and accordingly medical supervision duration needed, paralleled by a significantly faster recovery of whole blood cholinesterase activity. An initial evaluation demonstrated the safety of prolonged whole-body AHAK application. Hence, the AHAK lotion may act as an efficient "catch-up" therapy against percutaneous poisoning by low-volatility OP CWAs, improving the clinical outcome and reducing the burden on medical staff.

Comparison of skin decontamination strategies in the initial operational response following chemical exposures

In mass casualty incidents including hazardous chemical skin exposure, decontamination is the primary intervention to avoid systemic uptake of the toxic compound. The protocol needs to be both simple and efficient to enable a rapid response and avoid delay of patient management. In the present study, decontamination strategies included in the initial operational response were evaluated following human skin exposure in vitro to four different contaminants. Results demonstrated that the efficacy of selected decontamination procedures was highly dependent on the chemical contaminant used. Dry removal of the sulfur mustard simulant methyl salicylate prior to wet decontamination was found beneficial compared to wet decontamination alone. Rapidly initiated wet decontamination was more efficient compared to dry and wet removal of the industrial chemical 2-butoxyethanol and the nerve agent tabun. Following VX-exposure, all wet decontamination procedures resulted in increased agent penetration compared to the control. In conclusion, challenges in establishing simple and efficient decontamination procedures for a broad-spectrum of chemicals have been demonstrated. The impact of including a dry removal step during decontamination was evidently agent specific. Despite the variation in efficacy, immediately initiated dry removal may facilitate patient management until wet decontamination resources are available and to reduce the risk of secondary contamination.

Toxicokinetics of plasmatic VX in a swine model: comparison of a simple enzymatic titration method with a mass spectrometry method

Recent events have shown that organophosphorus nerve agents (OPNAs) are a serious threat. Cholinesterase inhibition by OPNAs results in acetylcholine accumulation, a cholinergic crisis leading to death if untreated. Efficacy assessment of new medical countermeasures against OPNAs relies on translational animal models. We developed a swine model of percutaneous VX intoxication and a simple plate reader-based enzymatic method to quantify plasmatic VX over time. Juvenile pigs anesthetized with sevoflurane were poisoned with a single supralethal (n = 5; 1200 μg/kg) or sublethal (n = 6; 320 μg/kg) percutaneous dose of VX. These intoxicated animals were compared to 7 control animals. Repeated blood sampling was performed up to 6 h post-intoxication. Blood cholinesterase activities were measured using the Ellman assay. Nanomolar plasma concentrations of VX were measured by exogenous butyrylcholinesterase added to an aliquot of plasma. As expected, we observed a steady increase in plasma concentration of VX over time concomitant to a decrease in blood cholinesterase activities for all intoxicated pigs. Despite the simplicity of the enzymatic method, the results obtained are in good agreement with those of the liquid chromatography-mass spectrometry method. This method is also applicable to other OPNAs such as novichoks with minor adaptations.

Do cold weather temperatures affect the efficacy of skin decontamination?

Skin decontamination in cold weather temperatures might be challenging due to the aggravating circumstances. However, no information is available on the efficacy of commonly used procedures in winter conditions. Therefore, the efficacy of the reactive skin decontamination lotion (RSDL) and soapy water decontamination following skin exposure to the nerve agent VX was evaluated at three ambient air temperatures (-5°C, -15°C and room temperature). Experiments were performed in vitro using human dermatomed skin. The ability of RSDL to degrade VX at the three different air temperatures was separately evaluated. The ambient air temperature in experiments without decontamination did not influence the penetration rate of VX through skin. RSDL decontamination was highly efficient in removing VX from skin when performed in all three ambient temperatures, despite the slower agent degradation rate of VX at the lower temperatures. Decontamination with soapy water at RT resulted in an increased skin penetration of VX compared with the control without decontamination; however, in colder temperatures the VX skin penetration was similar to the corresponding control without decontamination. At RT, dry removal prior to washing with soapy water did not improve decontamination of VX compared with washing solely with soapy water. This study demonstrated high efficacy of RSDL decontamination following skin exposure to VX also at cold temperatures. The previously reported 'wash-in' effect of soapy water on VX skin penetration was reduced at cold temperatures. Altogether, this study found a scientific basis to establish guidelines for skin decontamination of chemical casualties at cold weather temperatures.

Towards catch-up therapy: evaluation of nucleophilic active pharmaceutical ingredients for the treatment of percutaneous VX poisoning, in-vial and in-vitro studies

Dermal exposure to low volatility organophosphorus chemical warfare agents (OP CWA) poses a great risk to the exposed person. Due to their lipophilic nature, these compounds rapidly absorb into the skin, leading to the formation of a "dermal reservoir" from which they slowly enter the bloodstream causing prolonged intoxication. Traditionally, strategies to counter the toxicity of such substances consist of chemical decontamination/physical removal of the residual agent from the skin surface (preferably as soon as possible following the exposure) and administration of antidotes in the case of intoxication signs. Hence, these strategies are unable to counter a substantial amount of the agent, which accumulates inthe dermal reservoir. More than a decade ago, the concept of a "catch-up therapy" intended to neutralize the dermal reservoir was suggested. Herein, we describe examples of potential "catch-up therapy" lotions - vehicles designed to deliver small nucleophilic molecules into the skin and potentially decompose the remaining CWA before it reaches the blood stream. Eleven nucleophilic compounds, based on approved drugs, were initially screened. They were then tested in various binary solutions, for their detoxification efficacy and degradation ability towards lipophilic OP CWA models such as dibutylphosphofluoridate and o-nitro-phenyl diphenyl phosphate, as well as the nerve agent VX, by means of kinetic ³¹P NMR and UV-Vis spectroscopy. Of these, the potassium and diethyl ammonium salts of acetohydroxamic acid (AHAK and AHA DEA) in (DMSO/H₂O 1:4) were found to be the most active nucleophiles, hydrolyzing VX in practical time scales (t_1/2 = 5.28 and 6.78 min, respectively). The vehicle solution DMSO/H₂O 1:4 promoted the penetration of substantial amounts of AHA K and AHA DEA through excised pig skin in in-vitro studies, suggesting that such formulations may serve as useful CWA nucleophilic scavengers for both on and within -skin detoxification. These findings may pave the way to a more efficacious treatment against low volatility OP CWA percutaneous poisoning.

Organophosphorus poisoning in animals and enzymatic antidotes

Organophosphorus compounds (OPs) are neurotoxic molecules developed as pesticides and chemical warfare nerve agents (CWNAs). Most of them are covalent inhibitors of acetylcholinesterase (AChE), a key enzyme in nervous systems, and are therefore responsible for numerous poisonings around the world. Many animal models have been studied over the years in order to decipher the toxicity of OPs and to provide insights for therapeutic and decontamination purposes. Environmental impact on wild animal species has been analyzed to understand the consequences of OP uses in agriculture. In complement, various laboratory models, from invertebrates to aquatic organisms, rodents and primates, have been chosen to study chronic and acute toxicity as well as neurobehavioral impact, immune response, developmental disruption, and other pathological signs. Several decontamination approaches were developed to counteract the poisoning effects of OPs. Among these, enzyme-based strategies are particularly attractive as they allow efficient external decontamination without toxicity or environmental impact and may be of interest for treatment. Approaches using bioscavengers for prophylaxis, treatment, and external decontamination are emphasized and their potential is discussed in the light of toxicological observations from various animal models. The relevance of animal models, regarding their cholinergic system and the abundance of naturally protecting enzymes, is also discussed for better extrapolation of results to human.

Efficient agent degradation within skin is important for decontamination of percutaneously exposed VX

AIM OF THE STUDY

Following percutaneous exposure to the nerve agent VX, the remaining intact agent within the skin after decontamination is of great concern. Consequently, this leads to prolonged agent release to the blood circulation resulting in sustained intoxication, which may complicate the medical management. The decontamination procedure used should therefore possess the ability for agent removal both on and within the skin. The efficacy of three decontamination procedures was evaluated by measuring VX and the primary degradation product ethyl methyl phosphonic acid (EMPA) penetrated through human skin and the amount remaining within the skin.

MATERIALS AND METHODS

Decontamination was initiated 5 min post-exposure to VX on human dermatomed skin. Experiments were conducted using an in vitro skin penetration model and the amount remaining within the skin was determined by combining the tape-stripping technique and acetylcholinesterase activity measurements.

RESULTS

In control experiments without decontamination, higher amounts of VX were recovered in the deeper layers of skin compared to EMPA, which was primarily located in the stratum corneum. Both Reactive Skin Decontamination Lotion (RSDL) and the RSDL training kit (TRSDL) significantly reduced the amount of VX within the skin and decreased the penetration through the skin. However, the degradation ability of RSDL was demonstrated to be beneficial by the reduction of intact agents remaining in the skin compared to TRSDL without agent degradation capability. Soapy water decontamination caused a "wash-in" effect of VX with decreased agent amounts within stratum corneum but increased the amount VX penetrated through the skin.

CONCLUSION

Efficient skin decontamination of VX requires skin decontaminants reaching deeper layers of the skin, and that both absorption and degradation properties are important. In addition, the "wash-in" effect by using soapy water may enhance VX release to the blood circulation.

Skin decontamination efficacy of sulfur mustard and VX in the pig model: A comparison between Fuller's earth and RSDL

Skin decontamination following exposure to chemical agents is a most important component of the individual defense doctrine, removing the agent, ceasing its penetration and preventing secondary contamination of the first responders. The goal of the current study was to compare the efficacy of Reactive Skin Decontaminant Lotion (RSDL) and Fuller's Earth (FE) following exposure to sulfur mustard (SM) and VX, aiming to find the optimal procedure for mass casualty decontamination protocol. Decontamination efficacy was evaluated in pigs by measurement of lesion area and erythema (SM) and cholinesterase inhibition and clinical symptoms (VX). FE and RSDL were highly effective against both agents. Following SM exposure, the two decontaminants demonstrated a significant decrease in lesions' size together with the decrease in exposure duration. Likewise, skin decontamination following exposure to VX with either FE or RSDL resulted in reduction in clinical symptoms and prevention of death. Decontamination was worthwhile even if postponed, up to 30 min (SM) and 2 h (VX). In conclusion, both decontamination products were efficient in ameliorating the toxic effects even though in a different mechanism. Finally, for mass casualty scenario, FE is preferred as a universal decontaminant, considering its safety, ease of use and longer shelf life.

The percutaneous toxicokinetics of VX in a damaged skin porcine model and the evaluation of WoundStat™ as a topical decontaminant

This study used a damaged skin, porcine model to evaluate the in vivo efficacy of WoundStat™ for the decontamination of superficial, nerve agent-contaminated wounds. Anaesthetized animals were randomly assigned to either control (n = 7), no decontamination (n = 12) or WoundStat™ (n = 12) treatment groups. Pigs were exposed to a 5× LD₅₀ dose of neat, radiolabelled S-[2-(diisopropylamino)ethyl]-O-ethyl methyl-phosphonothioate (VX; or equivalent volume of sterile saline for the control group) via an area of superficially damaged skin on the ear. WoundStat™ was applied at 30 seconds post-exposure to assigned animals. The VX contaminant (or saline) and decontaminant remained in place for the duration of the study (up to 6 hours). Physiological parameters and signs of intoxication were recorded during the exposure period. Skin and organ samples were taken post mortem for ¹⁴ C-VX distribution analyses. Blood samples were taken periodically for toxicokinetic and whole-blood acetylcholinesterase (AChE) activity analyses. VX exposure was accompanied by a rapid decrease in AChE activity in all animals, regardless of decontamination. However, decontamination significantly improved survival rate and time and reduced the severity of signs of intoxication. In addition, the distribution of ¹⁴ C-VX in key internal organs and post mortem blood samples was significantly lower in the WoundStat™ treatment group. This study demonstrates that WoundStat™ may be a suitable medical countermeasure for increasing both survival rate and time following VX exposure. The results also suggest that AChE activity is not a useful prognostic indicator.

Ex vivo localization and permeation of cisplatin from novel topical formulations through excised pig, goat, and mice skin and in vitro characterization for effective management of skin-cited malignancies

CONTEXT

It would be advantageous to administer cisplatin topically for treatment of cutaneous malignancies.

OBJECTIVES

Present work focuses on ex vivo and in vitro characterization of proultraflexible topical formulations.

MATERIALS AND METHODS

Permeation of cisplatin through the excised pig, goat, and mice skin was quantitatively determined.

RESULTS

Data indicate that protransfersome carbopol gel (pcg) formulation clearly delayed drug permeation through skin. Permeation of cisplatin from protransfersome system (ps) formulation was enhanced by approximately 1.5 fold compared with pcg for pig and goat skin.

DISCUSSION

Localization of drug from pcg was higher and showed less permeation.

CONCLUSION

Cisplatin-loaded pcg formulation is better to treat cutaneous malignancies.

Skin decontamination

Skin decontamination is the primary intervention needed in chemical, biological and radiological exposures, involving immediate removal of the contaminant from the skin performed in the most efficient way. The most readily available decontamination system on a practical basis is washing with soap and water or water only. Timely use of flushing with copious amounts of water may physically remove the contaminant. However, this traditional method may not be completely effective, and contaminants left on the skin after traditional washing procedures can have toxic consequences. This article focuses on the principles and practices of skin decontamination.

Interactions of organophosphates with keratins in the cornified epithelium of human skin

Methods to unequivocally assess and quantify exposure to organophosphate anti-cholinesterase agents are highly valuable, either from a biomonitoring or a forensic perspective. Since for both OP pesticides and various nerve agents the skin is a predominant route of entry, we hypothesized that proteins in the skin might represent an ideal source of unequivocal and persistent biomarkers for exposure to these compounds. In this exploratory study we show that keratin proteins in human skin are relevant binding sites for organophosphates. The thick cornified epithelium of human plantar skin (callus) was exposed to a selection of relevant organophosphorus compounds and keratin proteins were subsequently extracted. After carboxymethylation of cysteine residues, enzymatic digestion of the keratins with pronase and trypsin was performed and the resulting amino acid and peptides were analyzed to assess whether covalent adducts had formed. LC-tandem MS analysis of the pronase digests demonstrated that tyrosine and to a lesser extent serine residues were selectively modified by organophosphate pesticides (both phosphorothioates and the corresponding oxon forms) under physiological conditions. In addition, modification of tyrosine with the nerve agent VX was unequivocally assessed. In order to elucidate specific binding sites, LC-tandem MS analysis of trypsin digests showed two separate tryptic keratin fragments, i.e. LASY*LDK and SLY*GLGGSK, with Y* the modified tyrosine residues, originating from keratin 1/6 and keratin 10, respectively. These preliminary findings, revealing novel binding targets for anti-cholinesterase organophosphates, will form a firm basis for the development of novel (non-invasive) methods for assessment of exposure to organophosphates. Whether this binding will also have biological implications remains an issue for further investigations.

Human scalp permeability to the chemical warfare agent VX

The use of chemical warfare agents such as VX in terrorism act might lead to contamination of the civilian population. Human scalp decontamination may require appropriate products and procedures. Due to ethical reasons, skin decontamination studies usually involve in vitro skin models, but human scalp skin samples are uncommon and expensive. The purpose of this study was to characterize the in vitro permeability to VX of human scalp, and to compare it with (a) human abdominal skin, and (b) pig skin from two different anatomic sites: ear and skull roof, in order to design a relevant model. Based on the VX skin permeation kinetics and distribution, we demonstrated that (a) human scalp was significantly more permeable to VX than abdominal skin and (b) pig-ear skin was the most relevant model to predict the in vitro human scalp permeability. Our results indicated that the follicular pathway significantly contributed to the skin absorption of VX through human scalp. In addition, the hair follicles and the stratum corneum significantly contributed to the formation of a skin reservoir for VX.

Cutaneous challenge with chemical warfare agents in the SKH-1 hairless mouse. (I) Development of a model for screening studies in skin decontamination and protection

Exposure to lethal chemical warfare agents (CWAs) is no longer only a military issue due to the terrorist threat. Among the CWAs of concern are the organophosphorus nerve agent O-ethyl-S-(2[di-isopropylamino]ethyl)methyl-phosphonothioate (VX) and the vesicant sulfur mustard (SM). Although efficient means of decontamination are available, most of them lose their efficacy when decontamination is delayed after exposure of the bare skin. Alternatively, CWA skin penetration can be prevented by topical skin protectants. Active research in skin protection and decontamination is thus paramount. In vivo screening of decontaminants or skin protectants is usually time consuming and may be expensive depending on the animal species used. We were thus looking for a suitable, scientifically sound and cost-effective model, which is easy to handle. The euthymic hairless mouse Crl: SKH-1 (hr/hr) BR is widely used in some skin studies and has previously been described to be suitable for some experiments involving SM or SM analogs. To evaluate the response of this species, we studied the consequences of exposing male anaesthetized SKH-1 mice to either liquid VX or to SM, the latter being used in liquid form or as saturated vapours. Long-term effects of SM burn were also evaluated. The model was then used in the companion paper (Taysse et al.(1)).

Pulmonary deposition of aerosolized Bacillus atrophaeus in a Swine model due to exposure from a simulated anthrax letter incident

Dry anthrax spore powder is readily disseminated as an aerosol and it is possible that passive dispersion when opening a letter containing anthrax spores may result in lethal doses to humans. The specific aim of this study was to quantify the respirable aerosol hazard associated with opening an envelope/letter contaminated with a dry spore powder of the biological pathogen anthrax in a typical office environment. An envelope containing a letter contaminated with 1.0 g of dry Bacillus atrophaeus (BG) spores (pathogen simulant) was opened in the presence of an unrestrained swine model. Aerosolized spores were detected in the room in seconds and peak concentrations occurred by three minutes. The swine, located approximately 1.5 m from the source, was exposed to the aerosol for 28 min following the letter opening event and then moved to a clean room for 30 min. A necropsy was completed to determine the extent of in vivo spore deposition in the lungs. The median number of viable colony forming units (CFU) measured in the combined right and left lung was 21,200: the average mass of both lungs was 283 g. In excess of 100 CFU per gram of lung tissue was found at sites within the anterior, intermediate and posterior lobes. The results of this study confirmed that opening an envelope containing spores generated an aerosol spanning the respirable particle size range of 1-10 microm, and that normal respiration of swine led to spore deposition throughout the lungs. The observed deposition of spores in the lungs of the swine is within the LD(50) range of 2,500-55,000 estimated for humans for inhaled anthrax. Thus, there would appear to be a significant health risk to those individuals exposed to anthrax spores when opening a contaminated envelope.

An unexpected plasma cholinesterase activity rebound after challenge with a high dose of the nerve agent VX

Organophosphorus chemical warfare agents (nerve agents) are to be feared in military operations as well as in terrorist attacks. Among them, VX (O-ethyl-S-[2-(diisopropylamino)ethyl] methylphosphonothioate) is a low volatility liquid that represents a percutaneous as well as an inhalation hazard if aerosolized. It is a potent irreversible cholinesterase (ChE) inhibitor that causes severe signs and symptoms, including respiratory dysfunction that stems from different mechanisms. VX-induced pulmonary oedema was previously reported in dogs but mechanisms involved are not well understood, and its clinical significance remains to be assessed. An experimental model was thus developed to study VX-induced cardiovascular changes and pulmonary oedema in isoflurane-anaesthetized swine. In the course of this study, we observed a fast and unexpected rebound of plasma ChE activity following inhibition provoked by the intravenous injection of 6 and 12 microg kg(-1) of VX. In whole blood ChE activity, the rebound could stay unnoticed. Further investigations showed that the rebound of plasma esterase activity was neither related to spontaneous reactivation of ChE nor to VX-induced increase in paraoxonase/carboxylesterase activities. A bias in Ellman assay, haemoconcentration or severe liver cytolysis were also ruled out. All in all, these results suggest that the rebound was likely due to the release of butyrylcholinesterase into the blood stream from ChE producing organs. Nature of the organ(s) and mechanisms involved in enzyme release will need further investigations as it may represent a mechanism of defence, i.e. VX scavenging, that could advantageously be exploited.

Comparison of selected skin decontaminant products and regimens against VX in domestic swine

An anesthetized domestic swine model was used to compare the efficacy and cross-contamination potential of selected skin decontaminant products and regimens against the chemical warfare agent, VX. Animals topically exposed to 2×, 3× or 5× LD50 VX showed typical signs of organophosphate nerve agent poisoning, including miosis, salivation, mastication, dysrhythmias, and respiratory distress prior to death. Animals were exposed to 5× LD50 VX and then decontaminated 45 min later with the reactive skin decontamination lotion (RSDL®), Fuller’s earth (FE), 0.5% hypochlorite, or soapy water. Survival was 100% when the reactive skin decontamination lotion or FE was utilized, although 50% of Fuller’s earth-decontaminated animals exhibited serious signs of VX poisoning. Decontamination of VX-treated animals with 0.5% hypochlorite was less effective but also increased survival. Soapy water was ineffective in preventing lethality. Blood cholinesterase levels were not predictive of clinical outcome in decontaminated animals. The potential of “decontaminated” VX in open wounds to cause poisoning was assessed by vigorously mixing 5× LD50 VX with the test decontaminants for 5 min and then placing the mixture onto a full-thickness skin wound. Soapy water was ineffective in preventing lethality. Although treatment with dry Fuller’s earth prevented death and all signs of organophosphate poisoning, a significant proportion of treated animals decontaminated with Fuller’s earth in aqueous suspension exhibited serious signs of organophosphate poisoning, suggesting that live agent may be desorbed from Fuller’s earth when it is exposed to a liquid environment. Animals treated with reactive skin decontamination lotion or 0.5% hypochlorite-VX mixtures showed no signs of organophosphate poisoning during the 6- h test period.

Swine models in the design of more effective medical countermeasures against organophosphorus poisoning

Although the three most commonly used large mammal species in the safety assessment of drugs remain the dog, the macaque and the marmoset, swine, especially minipigs, have also been widely used over the years in many toxicological studies. Swine present a number of interesting biological and physiological characteristics. Similarities in skin properties with humans have led to extensive in vitro and in vivo studies. There is a specific interest in cardiovascular research, as well as in anaesthesiology and critical care medicine due to common features of swine and human physiology. Although knowledge of swine brain structure and functions remains incomplete, data does exist. The multiple blood sampling that is necessary in pharmacokinetic and toxicokinetic studies are possible, as well as multiparametric monitoring and interventions with equipment used in human clinical settings. Practicality (handling), scientific (stress reduction) and ethical (invasive monitoring) reasons have led research teams to incorporate anaesthesia into their paradigms which makes the analysis of data increasingly difficult. Although not substantiated by scientific data, the swine appears to have an intermediate position in the scale of public perception between non-human primates and animals commonly referred to as pets (i.e. dogs and cats) and rodents. The benefits of the swine model justify the use of these animals in the design of more effective medical countermeasures against known chemical warfare agents (nerve agents, vesicants and lung damaging agents). Exposure to organophosphorus (OP) pesticides represents a severe health issue in developing countries, while OP intoxication with the more lethal military nerve agents is not only of military concern but also a terrorist threat. Tailoring therapeutic regimens to the reality of OP poisoning is of the utmost importance when little experimental data and sparse human clinical data are available in the decision making process. We will present some of the advantages and disadvantages of the swine model in OP countermeasures elaborating on two examples. First, we will present the issues related to the use of anaesthesia during experimental OP poisoning and second we will show how results from experiments with swine can be integrated into a kinetic-based dynamic model to evaluate oxime efficacy. A better knowledge of OP poisoning in swine (comparative toxicokinetics, pharmacokinetics and biochemistry) is definitely necessary before accepting it as a first choice non-rodent model. However, there exists a large amount of data in the model on anaesthesia and different types of shock favouring their use for evaluation of complex situations such as the anaesthesia of OP poisoned patients and combined injuries.

A Physiologically Based Pharmacokinetic Model of Organophosphate Dermal Absorption

The rate and extent of dermal absorption are important in the analysis of risk from dermal exposure to toxic chemicals and for the development of topically applied drugs, barriers, insect repellents, and cosmetics. In vitro flow-through cells offer a convenient method for the study of dermal absorption that is relevant to the initial processes of dermal absorption. This study describes a physiologically based pharmacokinetic (PBPK) model developed to simulate the absorption of organophosphate pesticides, such as parathion, fenthion, and methyl parathion through porcine skin with flow-through cells. Parameters related to the structure of the stratum corneum and solvent evaporation rates were independently estimated. Three parameters were optimized based on experimental dermal absorption data, including solvent evaporation rate, diffusivity, and a mass transfer factor. Diffusion cell studies were conducted to validate the model under a variety of conditions, including different dose ranges (6.3-106.9 microg/cm2 for parathion; 0.8-23.6 microg/cm2 for fenthion; 1.6-39.3 microg/cm2 for methyl parathion), different solvents (ethanol, 2-propanol and acetone), different solvent volumes (5-120 microl for ethanol; 20-80 microl for 2-propanol and acetone), occlusion versus open to atmosphere dosing, and corneocyte removal by tape-stripping. The study demonstrated the utility of PBPK models for studying dermal absorption, which can be useful as explanatory and predictive tools that may be used for in silico hypotheses generation and limited hypotheses testing. The similarity between the overall shapes of the experimental and model-predicted flux/time curves and the successful simulation of altered system conditions for this series of small, lipophilic compounds indicated that the absorption processes that were described in the model successfully simulated important aspects of dermal absorption in flow-through cells. These data have direct relevance to topical organophosphate pesticide risk assessments.