In vitro percutaneous absorption of benzidine in complex mechanistically defined chemical mixtures

Little work has been done on the topical absorption of the bladder carcinogen benzidine. Since humans are more likely to be exposed to chemical mixtures than to a single chemical, a program was developed in these laboratories to examine the cumulative effect of complex mixtures on percutaneous absorption of important toxicants such as benzidine. In this investigation, a mixture is defined as a physical combination consisting of a marker chemical and several other chemicals, each of which can have independent and/or synergistic effects on dermal penetration and absorption of the marker chemical. Ten mixtures, consisting of a marker chemical (benzidine, B), a solvent (acetone, A or DMSO, D), a surfactant (0 or 10% sodium lauryl sulfate, SL), a vasodilator (0 or 180 mu g methyl nicotinate, M), and a reducing agent (0 or 2% SnCl2, s) were employed in this study. Isolated perfused porcine skin flaps (IPPSFs), which have proven to be a suitable in vitro model for assessing dermal absorption and toxicity, and flow-through diffusion cell systems were utilized. The extent of benzidine absorption in skin sections dosed with either B + A (0.94% dose) or B + D (1.01% dose) was similar to that when IPPSFs were dosed with either B + A (0.54% dose) or B + D (1.31% dose). However, flux vs time profiles were different when the two in vitro methods were compared. For mixtures containing (1) DMSO only or acetone only or (2) solvents containing SL + M, benzidine absorption was enhanced when compared with other mixtures. Compared to acetone, DMSO appears to enhance dermal penetration of benzidine in most of the mixtures. Compared to other mixtures evaluated, SnCl2 inhibited benzidine absorption irrespective of solvent present. SnCl2 also appears to inhibit benzidine penetration in DMSO mixtures containing SL only, but not in acetone mixtures. It is proposed that chemical-chemical interactions between benzidine and SnCl2 may be inhibiting benzidine absorption and chemical-biological interactions between M + SL and skin may be enhancing benzidine absorption. Across all mixtures, maximum observed benzidine absorption was almost 3% of the topical dose over 8 hr, but maximum penetration was 22% over the same time period which would suggest a potential for greater systemic exposure over longer time frames. This work underscores the need to study potentially toxic chemicals in mixture exposure scenarios since the interactions observed would confound risk assessment based on single chemical data. (C) 1996 Academic Press, Inc.