The topical delivery of gene regulation technology to cells deep in the skin is extremely difficult because of the formidable defenses skin provides for the body. The Northwestern approach takes advantage of drugs consisting of novel spherical arrangements of nucleic acids. These structures, each about 1,000 times smaller than the diameter of a human hair, have the unique ability to recruit and bind to natural proteins that allow them to traverse the skin and enter cells.
PNAS - Topical delivery of siRNA-based spherical nucleic acid nanoparticle conjugates for gene regulation
Applied directly to the skin, the drug penetrates all of the skin’s layers and can selectively target disease-causing genes while sparing normal genes. Once in cells, the drug simply flips the switch of the troublesome genes to “off.”
A detailed study of a method that could dramatically redefine the field of gene regulation will be published online during the week of July 2 by the Proceedings of the National Academy of Sciences (PNAS).
Early targets of the novel treatment are melanoma and squamous cell carcinoma (two of the most common types of skin cancer), the common inflammatory skin disorder psoriasis, diabetic wound healing and a rare genetic skin disorder that has no effective treatment (epidermolytic ichthyosis). Other targets could even include wrinkles that come with aging skin.
“The technology developed by my collaborator Chad Mirkin and his lab is incredibly exciting because it can break through the skin barrier,” said co-senior author Amy S. Paller, M.D., the Walter J. Hamlin Professor, chair of dermatology and professor of pediatrics at Northwestern University Feinberg School of Medicine. She also is director of Northwestern’s Skin Disease Research Center.
“This allows us to treat a skin problem precisely where it is manifesting -- on the skin,” she said. “We can target our therapy to the drivers of disease, at a level so minute that it can distinguish mutant genes from normal genes. Risks are minimized, and side effects have not been seen to date in our human skin and mouse models.”
Mirkin first developed the nanostructure platform used in this study in 1996 at Northwestern, and the FDA-cleared technology now is the basis of powerful commercialized medical diagnostic tools. This, however, is the first realization that the nanostructures naturally enter skin and that they can deliver a large payload of therapeutics.
The key is the nanostructure’s spherical shape and nucleic acid density. Normal (linear) nucleic acids cannot get into cells, but these spherical nucleic acids can. Small interfering RNA (siRNA) surrounds a gold nanoparticle like a shell; the nucleic acids are highly oriented, densely packed and form a tiny sphere. The RNA’s sequence is programmed to target the disease-causing gene.
“We now can go after a whole new set of diseases,” Mirkin said. “Thanks to the Human Genome Project and all of the genomics research over the last two decades, we have an enormous number of known targets. And we can use the same tool for each, the spherical nucleic acid. We simply change the sequence to match the target gene. That’s the power of gene regulation technology.”
The nanostructures were developed in Mirkin’s lab on the Evanston campus and then combined with a commercial moisturizer. Next, down in Paller’s Chicago lab, the researchers applied the therapeutic ointment to the skin of mice and to human epidermis. The nanostructures were designed to target epidermal growth factor receptor (EGFR), a biomarker associated with a number of cancers.
In both cases, the drug broke through the epidermal layer and penetrated the skin very deeply, with cells taking up 100 percent of the nanostructures. They selectively knocked down the EGFR gene, decreasing the production of the problem proteins.
After a month of continued application of the ointment, there was no evidence of side effects, inappropriate triggering of the immune system or accumulation of the particles in organs. The treatment is skin specific and doesn’t interfere with other cells.
“This study is a landmark achievement in the area of gene regulation -- I believe our work has a chance to positively and irreversibly change the field,” Mirkin said. “The skin is a very tough barrier to go through, which is why this effective gene knockdown has not been accomplished before. The power and elegance of this system are in its simplicity.”
Abstract Topical application of nucleic acids offers many potential therapeutic advantages for suppressing genes in the skin, and potentially for systemic gene delivery. However, the epidermal barrier typically precludes entry of gene-suppressing therapy unless the barrier is disrupted. We now show that spherical nucleic acid nanoparticle conjugates (SNA-NCs), gold cores surrounded by a dense shell of highly oriented, covalently immobilized siRNA, freely penetrate almost 100% of keratinocytes in vitro, mouse skin, and human epidermis within hours after application. Significantly, these structures can be delivered in a commercial moisturizer or phosphate-buffered saline, and do not require barrier disruption or transfection agents, such as liposomes, peptides, or viruses. SNA-NCs targeting epidermal growth factor receptor (EGFR), an important gene for epidermal homeostasis, are over 100-fold more potent and suppress longer than siRNA delivered with commercial lipid agents in cultured keratinocytes. Topical delivery of 1.5 uM EGFR siRNA (50 nM SNA-NCs) for 3 wk to hairless mouse skin almost completely abolishes EGFR expression, suppresses downstream ERK phosphorylation, and reduces epidermal thickness by almost 40%. Similarly, EGFR mRNA in human skin equivalents is reduced by 52% after 60 h of treatment with 25 nM EGFR SNA-NCs. Treated skin shows no clinical or histological evidence of toxicity. No cytokine activation in mouse blood or tissue samples is observed, and after 3 wk of topical skin treatment, the SNA structures are virtually undetectable in internal organs. SNA conjugates may be promising agents for personalized, topically delivered gene therapy of cutaneous tumors, skin inflammation, and dominant negative genetic skin disorders.
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