SMALL RNA AS ANTIAGING COSMECEUTICALS 463 clinical applications has been achieved in several investigations. Ritprajak et al. proposed a new siRNA-based therapy using cream-emulsifi ed CD86 siRNA, targeting DCs for murine contact hypersensitivity and atopic dermatitis–like disease (85). Topical applica- tion of CD86 siRNA reduced effectively antigen-specifi c local infl ammation in mice skin. Wang et al. demonstrated that chitosan nanoparticles containing imiquimod and siRNA exerted an anti-infl ammatory effect and may provide a new and simple therapy for asthma (86). In particular, researchers at Northwestern University have identifi ed a way of delivering gene-regulating molecules through topical moisturizers (87). This impor- tant study on nanoparticle transdermal delivery demonstrated that spherical nucleic acid nanoparticle conjugates (SNA-NCs), which consisted of gold cores surrounded by a dense shell of highly oriented and covalently immobilized siRNA, penetrated freely almost 100% of keratinocytes in vitro, mouse skin, and human epidermis within hours after ap- plication. Applying SNA conjugates may offer a promising method for transdermal de- livery of small RNA as therapeutics and cosmetics. CELL-PENETRATING PEPTIDES On the other hand, cell-penetrating peptides (CPP) have been increasingly used to medi- ate delivery of small interfering RNA nucleotides in vitro and in vivo. Application of CPP in topical and transdermal delivery systems has recently garnered a tremendous attention in both cosmeceutical and pharmaceutical research and development (88). A variety of studies reported that conjugation of peptide to siRNA led to enhanced absorption into skin and knockdown of corresponding protein targets (89–92). In particular, Yi et al. demonstrated that conjugating TD1 to R8 covalently can be a powerful way to deliver siRNAs into epidermis and dermis of mammals (25). High effi cacy of the MITF-siR cream formulation could be because of ionic interactions between R7 and siRNA aided by TD1, thus creating a transient opening in the skin to facilitate access of the siRNA to melanocytes. Indeed, such a noninvasive delivery of small RNA for skin whitening has been clinically tested with good effi cacy (25,26). SAFETY OF SMALL RNA COSMECEUTICALS Natural RNAs have been used as cosmetic ingredients with a long history. Several cos- metic end products launched recently in the market contain natural RNA ingredients, such as sodium RNA or Saccharomyces cerevisiae extract. It is believed that natural RNA not only has a moisture retaining property derived from the nature of its high molecular anionic polymer but also has a UV-preventing property derived from its chemical struc- ture (93,94). In addition, natural RNA is genetic material and has some unknown antiag- ing benefi ts (95). With these properties, RNA has generally been considered safe for use as a functional ingredient for a long time. Besides having a specifi c gene target, small RNA is different from natural RNA in several aspects, such as length of oligonucleotide and effective dosages. Small RNA is chemically synthesized and may induce toxicities such as activation of innate immunity as well as off-target gene silencing. In general, double-stranded RNA can induce cellular changes through induction of interferons and other cytokines by the innate immune

JOURNAL OF COSMETIC SCIENCE 464 system (96–98). The interferon response is especially obvious for duplex RNAs greater than 30 bases long. At a dose less than 1 nM, the dose-dependent interferon response to small RNA could be negligible. In a design of small silencing RNA, avoiding certain sequence motifs can reduce interferon responses, and immunostimulatory aspects from small RNAs can be further reduced through chemical modifi cations of nucleotides (97,98). Although other safety concern such as “off-target” effects that small RNAs may pose (i.e., unintended alteration of nontarget gene expression), designing modifi ed small RNA to maximize selectivity while retaining potency could minimize the off-target ef- fects (99). In summary, toxicities from the use of small RNA could be minimized by better designs to avoid immunostimulatory motifs and to reduce amounts required for effi cacy through more selective delivery to desired tissue or target genes. To date, several studies have demonstrated that small RNA is safe for cosmetic applica- tions. Zheng et al. reported that there was no clinical or histological evidence of toxicity on the small RNA–treated skin. No cytokine activation in mouse blood or tissue samples was observed, and after three weeks’ of topical skin treatment, the SNA structures were virtually undetectable in internal organs (87). Similarly in the Valentine’s study (100), it was believed that small RNA formulation had no signifi cant skin irritation. No histo- pathological alterations in the skin of the animals treated with either nanodispersion in- corporating small RNA or naked small RNA were observed at 48 h post application. No signifi cant difference was observed in the epidermal thickness after treatment with the nanodispersions compared to saline-treated animals. In a clinical study of topical applica- tion, neither toxicity nor serious side effect had been observed on use of MITF-siR that was chemically synthesized and modifi ed (25,26). PERSPECTIVES ON ANTIAGING SMALL RNA COSMECEUTICALS Cosmeceuticals are the fastest growing sector in the cosmetic industry and the future of antiaging cosmeceuticals in particular is very promising. Nowadays, the major remaining challenges of small RNA cosmeceuticals are to elucidate molecular mechanisms of aging and to develop safe and effective noninvasive transdermal delivery. As scientists continue to search for breakthroughs, in-depth understanding of biological pathways that distin- guish young and aging skin continues to provide new insights into the process of skin aging, which will in turn lead to better preventive measures and antiaging treatments. We believe that cosmeceuticals in the antiaging field will be fl ourishing in the future through applications of sound science and advanced technologies, including RNA inter- ference, nanotechnology, biomimicry in cosmetics, stem cell technology, novel extraction techniques, and advances in biopolymers. REFERENCES (1) C.M. Choi, and D.S. Berson, Cosmeceuticals, Semin. Cutan. Med. Surg., 25(3), 163–168 (2006). (2) S. Bruce, Cosmeceuticals for the attenuation of extrinsic and intrinsic dermal aging, J. Drugs Derma- tol., 7(2 Suppl), s17–22 (2008). (3) F.S. Brandt, A. Cazzaniga, and M. Hann, Cosmeceuticals: Current trends and market analysis, Semin. Cutan. Med. Surg., 30(3), 141–143 (2011). (4) A.F. Stallings, and M.P. Lupo, Practical Uses of Botanicals in Skin Care, J. Clin. Aesthet. Dermatol., 2(1), 36–40 (2009).