Dermal absorption and short-term biological impact in hairless mice from sunscreens containing zinc oxide nano- or larger particles

Previous studies have shown no, or very limited, skin penetration of metal oxide nanoparticles following topical application of sunscreens, yet concerns remain about their safety compared to larger particles. Here, we assessed the comparative dermal absorption of a traceable form of Zn (Zn-68) from (ZnO)-Zn-68 nano-sized and larger particles in sunscreens. Sunscreens were applied to the backs of virgin or pregnant hairless mice over four days. Control groups received topical applications of the sunscreen formulation containing no ZnO particles, or no treatment. Major organs were assessed for changes in Zn-68/Zn-64 ratios, Zn-68 tracer and total Zn concentrations. Short-term biological impact was assessed by measuring levels of serum amyloid A in blood, and by performing whole-genome transcriptional profiling on livers from each group. Increased concentrations of Zn-68 tracer were detected in internal organs of mice receiving topical applications of (ZnO)-Zn-68 (nano-sized and larger particles), as well as in fetal livers from treated dams, compared with controls. Furthermore, concentrations of Zn-68 in organs of virgin mice treated with sunscreen containing (ZnO)-Zn-68 nanoparticles were found to be significantly higher than in mice treated with sunscreen containing larger (ZnO)-Zn-68 particles. However, no ZnO-mediated change in total Zn concentration in any of the major organs was observed. Thus, despite Zn-68 absorption, which may have been in the form of soluble Zn-68 species or (ZnO)-Zn-68 particles (not known), Zn homeostasis was largely maintained, and the presence of ZnO particles in sunscreen did not elicit an adverse biological response in the mice following short-term topical applications.