Less efficient skin penetration of the metal allergen Pd²⁺ compared to Ni²⁺ and Co²⁺ from patch test preparations

Contrary to Ni²⁺- and Co²⁺-induced allergic contact dermatitis (ACD), reactions against Pd²⁺ are rare. However, Pd²⁺ activates a larger T cell fraction in vitro, suggesting an inefficient skin penetration.

This study compares Ni²⁺, Co²⁺ and Pd²⁺ skin penetration from commonly used diagnostic patch test preparations (PTPs) and aqueous metal salt solutions.

Using Franz diffusion cell assays, we applied the metals in PTPs (5% NiSO₄, 1% CoCl₂, 2% PdCl₂ and 3% Na₂PdCl₄) 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.

Metal ions were mainly retained in the upper stratum corneum layers. After application of PTPs, concentrations in the viable skin were lower for Pd²⁺ (1 and 7 μM) compared to Ni²⁺ and Co²⁺ (54 and 17 μM).

Ni²⁺ and Co²⁺ penetrated the skin more efficiently than Pd²⁺ 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.