siRNA-gold nanoparticle conjugates efficiently penetrate skin barrier to effect gene knockdown.

Scientists have developed an siRNA-carrying gold nanoparticle that they claim can be mixed with a commercial moisturizing cream to enable the topical delivery of gene-targeting therapeutics directly into deep skin layers. The Northwestern University team says the platform could lead to the development of topical forms of gene therapy for diseases such as skin cancer, inflammation, and genetic skin disorders.

The spherical nucleic acid nanoparticle conjugate (SNA-NC) constructs essentially comprise a gold core surrounded by a dense, highly orientated shell of covalently immobilized siRNAs, which can easily penetrate the epidermis without the need for transfection agents such as liposomes, peptides, or viruses. Initial in vitro studies by Amy S. Paller, M.D., Dan Zheng, Ph.D., and colleagues first showed that dye-labeled SNA-NCs were rapidly taken up by typically difficult-to-transfect primary normal human keratinocytes (hKCs), without impacting normal cell behavior or gene expression, or resulting in any evidence of cytotoxicity.

When applied to the skin of experimental mice using a commercial moisurizer, the SNA-NCs rapidly penetrated through the stratum corneum and into the epidermis and dermis within just three hours of a single application. Again, treatment appeared safe, as the nanoparticles didn’t cause any unwanted immune reactions, or accumulate in any other organs. Further murine experiments using an EGFR-targeting SNA-NC showed that repeated application of the topical therapy resulted in a 65% reduction in EGFR mRNA expression and almost complete suppression of EGFR protein levels, accompanied by a 40% reduction in skin thickness.

Encouragingly, tests on human skin equivalents confirmed that low concentrations of EGFR-targeting nanoparticles penetrated into the basal layer of the epidermis within just a couple of hours of administration, and led to a 52% reduction in EGFR mRNA expression and up to 75% reduction in EGFR protein levels. The authors report their work in a paper titled “Topical delivery of siRNA-based spherical nucleic acid nanoparticle conjugates for gene regulation.”

The SNA-NCs platform was developed some years ago in the laboratory of Chad Mirkin, Ph.D., director of Northwestern’s International Institute of Nanotechnology, and now forms the basis of FDA-approved medical diagnostics. During the course of development the researchers found that the nanoparticles could directly penetrate over 50 cell lines and primary cells tested, as well as cultured tissues and whole organs. Importantly, SNA-NCs retain stability, are resistant to nuclease degradation, and exhibit low immunogenicity as a result of the high-density oligonucleotide shells.

The latest work published in PNAS is the first to demonstrate that the nanoparticles can get through the skin barrier to penetrate into skin cells for the delivery of oligonucleotide therapeutic payloads. This could open up a whole new opportunity for treating skin-related diseases, the team states. “We can target our therapy to the drivers of disease, at a level so minute that it can distinguish mutant genes from normal genes,” Dr. Paller explains. “Risks are minimized, and side effects have not been seen to date in our human skin and mouse models.”

An added bonus is that producing siRNAs is much faster and more cost effective than the discovery of new small molecule or antibody therapies, so the platform may eventually allow for personalized, genotype-based therapy, the authors write. Moreover, they state, “the SNA-NC structure is highly tailorable, and the gold core can serve as a scaffold for multicomponent functionalization, not just with siRNAs, but with targeting antibodies or peptides, and small molecule drugs as well … these SNA conjugates are part of the growing arsenal of nanomaterials that are showing promise as alternatives to conventional molecular formats for the development of novel and effective therapies for a wide variety of diseases.”

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