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

Abstract

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.

Predicting in vivo absorption of chloramphenicol in frogs using in vitro percutaneous absorption data

“…This study represents the first attempt to use our model of in vitro percutaneous absorption in the cane toad, coupled with previous in vivo percutaneous absorption studies of model chemicals [ 35 ], to predict the in vivo absorption of a drug for treatment of infectious disease through frog skin. Serum concentrations of chloramphenicol achieved reached the MIC (12.5 μg.mL − 1 ) required for treatment of Batrachochytrium dendrobatidis infection, as predicted by the model.…”

“…Further, such refinement would also improve compliance and utility of this treatment in practice. The models described previously [ 35 ] may also be used to select and formulate other drugs for treatment of infectious disease in amphibians, thereby maintaining the health of captive insurance populations and hastening the successful reintroduction of these animals to their original environments.…”

“…Fifty-five adult male cane toads ( Rhinella marina ), weighing 75.8–140.1 g (mean 104.1 g) were wild-caught in the Townsville region (Australia). Cane toads were used as this species is the same as was used in previous in vitro and in vivo studies to produce the model used to predict absorption in this study [ 35 ]. Animals were transported to the laboratory and housed in groups of 3 or 4 in plastic tubs (60 × 36 × 40 cm) in a dedicated room maintained at 21 ± 2 °C.…”

“…Chloramphenicol was formulated in 20%v/v propylene glycol at a dose of 250 μg.mL − 1 , applied to the ventral pelvis of the toads. The dose and exposure time required were determined by firstly using our previously-described in vitro linear mixed-effect models of absorption in cane toads [ 35 ] to predict absorption parameters (flux and permeability coefficient (K p )) for chloramphenicol (Table 4 ). Briefly: the models were developed from in vitro percutaneous absorption data collected for three model chemicals of differing lipophilicity and molecular size, formulated as saturated solutions in ARS.…”

“…Recently, we developed in vitro models of absorption in the cane toad ( Rhinella marina ) based on three model chemicals, and determined how well the models’ predictions of absorption matched the in vivo absorption of the same chemicals [ 35 ]. We have also reported the impact of penetration enhancers on absorption through frog skin [ 36 ].…”