Less efficient skin penetration of the metal allergen Pd compared to Ni and Co from patch test preparations.

Background: Contrary to Ni- and Co-induced allergic contact dermatitis (ACD), reactions against Pd are rare. However, Pd activates a larger T cell fraction in vitro, suggesting an inefficient skin penetration.

Objectives: This study compares Ni, Co and Pd skin penetration from commonly used diagnostic patch test preparations (PTPs) and aqueous metal salt solutions.

Methods: Using Franz diffusion cell assays, we applied the metals in PTPs (5% NiSO, 1% CoCl, 2% PdCl and 3% NaPdCl) and in solution to pigskin for 48 h, thereby mirroring the time frame of a patch test. The different compartments were analysed individually by inductively coupled plasma mass spectrometry.

Results: Metal ions were mainly retained in the upper stratum corneum layers. After application of PTPs, concentrations in the viable skin were lower for Pd (1 and 7 μM) compared to Ni and Co (54 and 17 μM).

Conclusions: Ni and Co penetrated the skin more efficiently than Pd and thus may sensitize and elicit ACD more easily. This was observed for ions applied in petrolatum and aqueous solutions. We hypothesize that the differently charged metal complexes are responsible for the varying skin penetration behaviours.