Retrospective analysis of dermal absorption triple pack data

Abstract

Dermal toxicity is driven by the ability of a substance to penetrate the skin. The “triple pack” approach, which combines in vivo rat, in vitro rat, and in vitro human data, is used to calculate an estimated human dermal absorption factor (DAF). To assess the feasibility of deriving a DAF using only in vitro data, we retrospectively evaluated agrochemical formulations to compare the DAF derived from each individual method to the DAF generated from the triple pack approach. For most of the formulations evaluated, the in vitro rat method generated a similar or higher DAF value than the in vivo method. Absorption through in vitro human skin was similar to or less than that observed in rat skin for all formulations. For most of the formulations, the human in vitro method provided a similar or higher estimate of dermal absorption than the triple pack approach. For human health risk assessment, in vitro assays using human skin would be preferable, as they would be directly relevant to the species of interest and avoid overestimation of dermal absorption using rat models. However, rat in vitro studies would still have utility in the absence of human in vitro data. In vitro rat data provide estimates of dermal absorption that are at least as protective as in vivo rat data and thus could also be considered adequate for use in estab­lishing DAFs. The comparisons presented support potentially using in vitro data alone for DAF derivation for human health risk assessment of pesticides.

Keywords: agrochemicals testing; in vitro; skin absorption.

Fig. 1:. Absorbance ratios calculated for comparisons

Fig. 2:. Calculating the impact of variability on absorbance ratios

Maximum and minimum ratios were calculated to establish the range of possible outcomes. The maximum ratio is the sum of the mean and SD of replicate measurements in the numerator and the difference of the mean and SD in the denominator. The minimum ratio is based on the difference of the mean and SD in the numerator and the sum of the mean and SD of replicate measurements in the denominator. TS, tape strips; Pot Abs, potential absorption (the sum of direct absorption and the amount of chemical measured at the skin application site and in the stratum corneum).

Fig. 3:. Dermal absorption time-matched comparisons at 24 h: in vitro rat vs in vivo rat

(A) Low dose group; (B) high dose group. TS, tape strips. ^ and * indicate formulations that include the same active ingredient in a different formulation.

Fig. 4:. Impact of variability on absorbance ratios

Range of possible absorbance ratios based on the maximum and minimum ratios calculated with or without tape strips (TS) 1 and 2. Data presented are from low dose ratios of in vitro and in vivo rat data, time-matched at 24 h. Dots are mean absorbance ratios and each end of the line is the maximum (top) and minimum (bottom) absorbance ratio. See Figure 2 for an example of how these data were calculated. ^ and * indicate formulations that include the same active ingredient in a different formulation.

Fig. 5:. Dermal absorption comparisons: in vitro rat vs in vivo rat, maximum in vivo time point

(A) Low dose group; (B) high dose group; ^ and * indicate formulations that include the same active ingredient in a different formulation. TS, tape strips.

Fig. 6:. Dermal absorption time-matched comparisons at 24 h: in vitro human vs in vitro rat

(A) Low dose group; (B) high dose group; (C) impact of variability on absorbance ratios, low dose group; (D) impact of variability on absorbance ratios, high dose group. ^ and * indicate formulations that include the same active ingredient in a different formulation.

Fig. 7:. Dermal absorption comparisons: in vitro human add period to vs in vivo rat

(A) Low dose groups time-matched at 24 h; (B) high dose groups time-matched at 24 h; (C) low dose groups longest time points (not time-matched); (D) high dose groups longest time points (not time-matched). ^ and * indicate formulations that include the same active ingredient in a different formulation.

Fig. 8:. Impact of variability on absorbance ratios for in vitro human vs in vivo rat

(A) Low dose group time-matched at 24 h; (B) high dose group time-matched at 24 h. ^ and * indicate formulations that include the same active ingredient in a different formulation.