Nanotechnology, which applies gathered knowledge on the characteristics of matter to design new products on the nanoscale (<1,000 nm), emerged in the 1980s and has made great strides since then. Dermatology is a prime area of interest for nanotech applications, as numerous products using nanotechnology have been marketed. In fact, the sixth-largest U.S. patent holder in nanotechnology is a cosmetics company (Skin Therapy Lett. 2010;15:1-4). The newest generation of skin products is characterized by improved skin penetration (Arch. Dermatol. Res. 2011;303:533-50), and these products may have a role in enhancing the treatment of several skin disorders; however, toxicological studies must establish the safety of formulations increasingly likely to penetrate multiple skin layers.
Zinc oxide (ZnO) and titanium dioxide (TiO2) are two of the most prominent ingredients in the dermatologic armamentarium that are used in micro- and nanoparticle forms. Efficacy has been well established for these ingredients as inorganic sunscreens, but their relative safety has been debated and remains somewhat controversial. This column discusses findings regarding the safety of ZnO nanoparticles.
Elevated risk
Absorption and effects of zinc ions. In a small study (n = 20) in humans conducted in 2010, Gulson et al. found that small amounts of zinc from ZnO in sunscreens applied for five consecutive days outdoors were absorbed in the skin, with levels of the stable isotope tracer (68)Zn in blood and urine from females receiving the nano sunscreen higher than in males receiving the same sunscreen and higher than in all participants who received the bulk sunscreen (Toxicol. Sci. 2010;118:140-9).
In 2010, Martorano et al. examined the separation of zinc ions from ZnO in commercial sunscreens under UVB exposure and studied the effects of zinc ion accumulation in human epidermal keratinocytes. They noted that UVB light exposure led to a significant concentration-dependent and radiation intensity–dependent rise in zinc ion levels. In turn, a late- or delayed-type cytotoxicity in human epidermal keratinocytes was observed, as was the induction of reactive oxygen species (ROS) in the keratinocytes. The investigators concluded that UVB exposure leads to an elevation in zinc ion dissociation in ZnO sunscreen, yielding cytotoxic effects and oxidative stress (J. Cosmet. Dermatol. 2010;9:276-86).
Genotoxic potential. As Wang and Tooley aptly noted, the concerns regarding the safety of nanoparticles in sunscreens pertain to potential toxicity and capacity to penetrate the skin (Sem. Cutan. Med. Surg. 2011;30:210-13).
In a 2010 in vitro study of the toxicity of ZnO and TiO2 on keratinocytes over short- and long-term application periods, Kocbek et al. found that ZnO nanoparticles conferred more adverse effects than TiO2, with ZnO inhibiting cell viability above 15 mcg/mL after brief exposure while TiO2 exerted no effect up to 100 mcg/mL. Prolonged exposure to ZnO nanoparticles at 10 mcg/mL yielded diminished mitochondrial activity as well as changes in cell morphology and cell-cycle distribution; no such changes were associated with TiO2 nanoparticles. The researchers also reported more nanotubular intercellular structures in keratinocytes exposed to either nanoparticle type as compared to unexposed cells and nanoparticles present in vesicles within the cell cytoplasm. Finally, they observed that partially soluble ZnO spurred the synthesis of ROS, as opposed to insoluble TiO2. They concluded that their findings of an adverse effect on human keratinocytes suggest that long-term exposure to ZnO and TiO2 nanoparticles poses a possible health risk (Small 2010;6:1908-17).
In early 2011, Sharma et al. studied the cytotoxic and genotoxic potential of ZnO nanoparticles in the human liver carcinoma cell line HepG2, given what they argued was the pervasiveness of ZnO in consumer products and industrial applications and the concomitant likelihood of transmission to the liver. Their various assays revealed a significant concentration- and time-dependent toxicity after 12 and 24 hours at 14 and 20 mcg/mL, as well as a significant surge in DNA damage in cells exposed to ZnO nanoparticles for 6 hours (J. Biomed. Nanotechnol. 2011;7:98-9).
Previously, in 2009, Sharma et al. had investigated the potential genotoxicity of ZnO nanoparticles in the human epidermal cell line A431. They found concentration- and time-dependent decreases in cell viability as well as DNA damage potential, as revealed by Comet assay results. In addition, oxidative stress was provoked by ZnO nanoparticles, as evidenced by significant reductions in glutathione, catalase, and superoxide dismutase. The investigators urged caution related to dermatologic formulations containing ZnO nanoparticles, suggesting that their findings indicate a genotoxic potential in human epidermal cells, possibly mediated via lipid peroxidation and oxidative stress (Toxicol. Lett. 2009;185:211-8).
In May 2011, Sharma et al. investigated the biological interactions of ZnO nanoparticles in human epidermal keratinocytes, where electron microscopy showed the internalization of the nanoparticles after 6 hours of exposure at a concentration of 14 mcg/mL. Various assays revealed a time- and concentration-dependent suppression of mitochondrial activity as well as DNA damage in cells, compared with controls. The investigators concluded that ZnO nanoparticles are internalized by human epidermal keratinocytes and provoke a cytotoxic and genotoxic response, providing reason for caution when using consumer products containing nanoparticles. Specifically, they warned that any disruptions in the stratum corneum (SC) could allow the exposure of internal cells to nanoparticles (J. Nanosci. Nanotechnol. 2011;11:3782-8).