Percutaneous absorption of chemicals: developing an understanding for the treatment of disease in frogs

Drug Delivery and Safety Considerations

Anesthetic drugs must be delivered at the appropriate dose and route of administration to produce the expected anesthetic effects. This is important for patient safety because anesthetic drugs function by depressing the central nervous and cardiovascular systems, which if improperly dosed or administered could cause potentially life-threatening effects. Several routes of administration and different drug delivery methods are available to safely and reliably anesthetize zoologic companion animal patients. Because of the nature of zoologic companion animal practice, anesthetic procedures pose risks that the anesthetist should understand to carefully plan procedures that are as safe and efficient as possible.

Pharmacokinetics of ceftazidime in Northern leopard frogs (Lithobates pipiens) at two different doses and administration routes

OBJECTIVE

To determine an optimal ceftazidime dosing strategy in Northern leopard frogs (Lithobates pipiens) by evaluation of 2 different doses administered SC and 1 dose administered transcutaneously.

ANIMALS

44 Northern leopard frogs (including 10 that were replaced).

PROCEDURES

Ceftazidime was administered to frogs SC in a forelimb at 20 mg/kg (n = 10; SC20 group) and 40 mg/kg (10; SC40 group) or transcutaneously on the cranial dorsum at 20 mg/kg (10; TC20 group). Two frogs in each ceftazidime group were euthanized 12, 24, 48, 72, and 96 hours after drug administration. Plasma, renal, and skin concentrations of ceftazidime were measured by means of reversed-phase high-performance liquid chromatography. Four control frogs were used for assay validation.

RESULTS

Mean plasma half-life of ceftazidime in the SC20, SC40, and TC20 groups was 9.01 hours, 14.49 hours, and too low to determine, respectively. Mean maximum plasma ceftazidime concentration was 92.9, 96.0, and 1.3 μg/mL, respectively. For 24 hours after drug administration in the SC20 and SC40 groups, plasma ceftazidime concentration exceeded 8 μg/mL. Renal and skin concentrations were detectable at both doses and routes of administration; however, skin concentrations were significantly lower than renal and plasma concentrations.

CONCLUSIONS AND CLINICAL RELEVANCE

Findings indicated that ceftazidime administration to Northern leopard frogs at 20 mg/kg, SC, every 24 hours would achieve a plasma concentration exceeding the value considered effective against common amphibian pathogens. Transcutaneous administration of the injectable ceftazidime formulation at 20 mg/kg warrants further investigation but is not currently recommended because of a potential lack of efficacy.

Avoidance behavior of juvenile common toads (Bufo bufo) in response to surface contamination by different pesticides

Most agricultural soils are expected to be contaminated with agricultural chemicals. As the exposure to pesticides can have adverse effects on non-target organisms, avoiding contaminated areas would be advantageous on an individual level, but could lead to a chemical landscape fragmentation with disadvantages on the metapopulation level. We investigated the avoidance behavior of juvenile common toads (Bufo bufo) in response to seven pesticide formulations commonly used in German vineyards. We used test arenas filled with silica sand and oversprayed half of each with different pesticide formulations. We placed a toad in the middle of an arena, filmed its behavior over 24 hours, calculated the proportion of time a toad spent on the contaminated side and compared it to a random side choice. We found evidence for the avoidance of the folpet formulation Folpan® 500 SC, the metrafenone formulation Vivando® and the glyphosate formulation Taifun® forte at maximum recommended field rates for vine and a trend for avoidance of Wettable Sulphur Stulln (sulphur). No avoidance was observed when testing Folpan® 80 WDG (folpet), Funguran® progress (copper hydroxide), SpinTorTM (spinosad), or 10% of the maximum field rate of any formulation tested. In the choice-tests in which we observed an avoidance, toads also showed higher activity on the contaminated side of the arena. As video analysis with tracking software is not always feasible, we further tested the effect of reducing the sampling interval for manual data analyses. We showed that one data point every 15 or 60 minutes results in a risk of overlooking a weak avoidance behavior, but still allows to verify the absence/presence of an avoidance for six out of seven formulations. Our findings are important for an upcoming pesticide risk assessment for amphibians and could be a template for future standardized tests.

Can models of percutaneous absorption based on in vitro data in frogs predict in vivo absorption?

The primary aim of in vitro testing of chemicals delivered via the percutaneous route is to predict the absorption that would ensue if exposure occurred in live animals. While there is mounting evidence that in vitro diffusion studies in mammalian skin can provide valid information regarding likely in vivo absorption, little is known whether such a correlation exists between in vitro diffusion testing and in vivo blood levels in amphibians. The current study used previously-reported in vitro absorption data for caffeine, benzoic acid, and ibuprofen across isolated skin from the cane toad (Rhinella marina) to produce a series of linear mixed-effect models of the absorption parameters flux and permeability coefficient (K_p). Models investigated the relative impacts of animal weight, physicochemical characteristics of the applied chemical (logP or molecular weight), and site of application. The top models were then used to predict the flux, K_p and serum concentrations of the same three model chemicals. Finally, the absorption of these chemicals was determined in live cane toads, and results compared to the model predictions. LogP and site of application were included in all top models. In vivo absorption rates were lower than predicted for all chemicals, however, the models provided reasonable predictions of serum concentration, with factors of difference (FOD) ranging from 2.5–10.5. Ibuprofen, the chemical with the highest relative lipophilicity, had the poorest predictive performance, consistently having the highest FOD for all predictions. This report presents the first models of percutaneous absorption in an amphibian. These models provide a basic method to establish the approximate in vivo absorption of hydrophilic and moderately-lipophilic chemicals through frog skin, and could therefore be used to predict absorption when formulating such chemicals for treatment of disease in frogs, or for risk-assessments regarding chemical pollutants in frog habitats.

The effect of injection and topical application of hCG and GnRH agonist to induce sperm-release in the roseate frog, Geocrinia rosea

Reproductive technologies may assist amphibian conservation breeding programs (CBPs) to achieve propagation targets and genetic management goals. However, a trial-and-error approach to protocol refinement has led to few amphibian CBPs routinely employing reproductive technologies with predictable outcomes. Additionally, while injections can be safely administered to amphibians, perceived animal welfare risks, such as injury and disease transmission, warrant the development of alternative hormone administration protocols. The present study investigated the spermiation response of roseate frogs, Geocrinia rosea, administered various doses of human chorionic gonadotropin (hCG) and gonadotropin-releasing hormone agonist (GnRH-a) via subcutaneous injection. This study also quantified the spermiation response of frogs administered both hormones via topical application. Total sperm, sperm concentration and sperm viability were assessed over a 12-h period post hormone administration. Males released sperm in response to the injection of hCG (88-100% response; 5, 10 or 20 IU), but all samples collected from males administered hCG topically (100, 100 + DMSO or 200 IU hCG) were aspermic. In contrast, males consistently released sperm in response to both the injection (100% response; 1, 5 or 10 μg), or topical application (80-100% response; 50, 50 + DMSO or 100 μg) of GnRH-a. Overall, the administration of GnRH-a was more effective at inducing spermiation than hCG. Mean total sperm and sperm concentration were highest in response to the optimal topically applied dose of 100 μg GnRH-a (mean total sperm = 2.44 × 10³, sperm concentration = 1.48 × 10⁵ sperm/ml). We provide novel evidence that topical application provides a viable alternative to injection for the administration of GnRH-a to induce spermiation in amphibians.

Permeability of frog skin to chemicals: effect of penetration enhancers

Rarely do commercial chemical products contain solely the active chemical/ingredient. It is therefore important to consider whether ingredients other than the active may: 1) alter absorption of the active chemical, or 2) be absorbed themselves, resulting in systemic effects. Frogs have highly permeable skin and are routinely exposed to commercial chemical products in the environment or therapeutically. Ethanol and propylene glycol (PG), which have known penetration-enhancing effects, are commonly included in such products. The current study has therefore investigated the in vitro absorption kinetics through Rh. marina skin of three model chemicals – caffeine, benzoic acid, and ibuprofen – formulated individually as solutions containing: 1%, 10% or 30% v/v ethanol, or 20% v/v PG. Differential scanning calorimetry and histology were used to characterise fresh frog skin, investigate the mechanism of these enhancers in frog skin, and to determine whether these enhancers significantly affected skin structure. Results showed that the extent of absorption enhancement was influenced by chemical, enhancer and skin region, and that enhancement was generally not consistent for individual enhancers or skin regions. The exception was 1% v/v ethanol, which did not significantly alter flux across the skin for any of the chemicals evaluated. Caffeine absorption was not enhanced by any of the investigated penetration enhancers, and was in fact significantly reduced by 30% v/v ethanol and PG. Ethanol caused concentration-dependant changes in skin morphology and should be avoided in concentrations ≥10% v/v. PG, however, caused minimal changes to the skin and consistently improved absorption of benzoic acid and ibuprofen through all skin regions. Owing to the significant changes in skin structure following ≥10% v/v ethanol exposure, it is recommended to avoid its use in frogs. For enhancement of penetration of moderately-to-highly lipophilic chemicals, this study has identified 20% v/v PG should to be the enhancer of choice.

Effects of skin region and relative lipophilicity on percutaneous absorption in the toad Rhinella marina

Owing to the dynamic interaction between frog skin and the environment, xenobiotics in frog habitats are of particular concern, and knowledge of percutaneous absorption in frog skin is necessary for risk-mitigation purposes. Baseline transdermal kinetics in adult aquatic and arboreal frog species have recently been reported; however, there is little information regarding absorption kinetics in adult terrestrial species. The present study investigated the in vitro absorption kinetics of 3 model chemicals-caffeine, benzoic acid, and ibuprofen-through different skin regions in the terrestrial toad Rhinella marina. Caffeine flux was consistently higher than that of the other 2 chemicals (p < 0.001), whereas the fluxes of the moderately and highly lipophilic chemicals (benzoic acid and ibuprofen) were similar, regardless of skin region. When considering individual chemicals, caffeine demonstrated increased flux through the ventral pelvic skin compared with the ventral thoracic or dorsal skin regions. Flux did not differ between skin regions for either benzoic acid or ibuprofen. These findings have implications for management of environmental contamination in frog habitats, as many environmental xenobiotics are of moderate to high lipophilicity and would be expected to be equally absorbed from all skin surfaces in terrestrial toads. Environ Toxicol Chem 2019;38:361-367. © 2018 SETAC.

Regional variation in percutaneous absorption in the tree frog Litoria caerulea

Frog skin structure and physiology differs between skin regions, however little is known about how these differences affect transdermal absorption of chemicals. Further, no information is available regarding how the relative lipophilicity of a chemical influences its transdermal pharmacokinetics in frog skin. This study investigated the in vitro percutaneous absorption of three model chemicals - benzoic acid, caffeine, and ibuprofen - through dorsal and ventral skin of the tree frog Litoria caerulea. Flux was significantly higher through the ventral skin for all chemicals. Relative lipophilicity affected flux differently in different skin regions. These differences are likely due to significantly thicker dorsal skin increasing absorption path length, and also possibly owing to lipoid secretions on the dorsum providing an additional diffusional barrier. This knowledge can advise risk mitigation of xenobiotics in agricultural and industrial settings, and also guide selection of chemicals and doses when considering transdermal drug therapy in captive frogs.

An interspecies correlation model to predict acute dermal toxicity of plant protection products to terrestrial life stages of amphibians using fish acute toxicity and bioconcentration data

This paper presents a model to predict acute dermal toxicity of plant protection products (PPPs) to terrestrial amphibian life stages from (regulatory) fish data. By combining existing concepts, including interspecies correlation estimation (ICE), allometric relations, lethal body burden (LBB) and bioconcentration modelling, an equation was derived that predicts the amphibian median lethal dermal dose (LD₅₀) from standard acute toxicity values (96-h LC₅₀) for fish and bioconcentration factors (BCF) in fish. Where possible, fish BCF values were corrected to 5% lipid, and to parent compound. Then, BCF values were adjusted to an exposure duration of 96 h, in case steady state took longer to be achieved. The derived correlation equation is based on 32 LD₅₀ values from acute dermal toxicity experiments with 15 different species of anuran amphibians, comprising 15 different PPPs. The developed ICE model can be used in a screening approach to estimate the acute risk to amphibian terrestrial life stages from dermal exposures to PPPs with organic active substances. This has the potential to reduce unnecessary testing of vertebrates.