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

AbstractBackgroundContrary 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.ObjectivesThis study compares Ni2+, Co2+ and Pd2+ skin penetration from commonly used diagnostic patch test preparations (PTPs) and aqueous metal salt solutions.MethodsUsing 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.ResultsMetal 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).ConclusionsNi2+ 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.