(Nanowerk Spotlight) Cancer treatments have traditionally involved a difficult balance between effectiveness and side effects. For skin cancers, this challenge is particularly pronounced. Researchers have worked to find ways to selectively target cancer cells while sparing healthy tissue, but progress has been limited by several persistent obstacles. Drug delivery systems typically lacked the precision needed to concentrate only in tumors.
Early targeting strategies often failed because cancer cells and normal cells share many biochemical similarities, making true selectivity difficult to achieve. For photodynamic therapy specifically, the low-oxygen environment inside tumors severely limited efficacy, while the inability to confine photosensitizers solely to cancer cells led to painful skin reactions when patients were exposed to light.
These challenges are especially problematic when treating cutaneous squamous cell carcinoma (cSCC), which often develops on functionally and cosmetically sensitive areas. Tumors on the fingers, lips, eyelids, or face are usually removed surgically, but this can result in disfigurement or functional loss. The high recurrence rate of cSCC further compounds the problem, requiring repeated interventions and causing ongoing physical and emotional stress.
Recent scientific advances have created new options for tackling these challenges. Researchers have made progress in DNA nanotechnology, developing methods to fold DNA strands into precise nanoscale structures that can carry therapeutic payloads. At the same time, cancer biologists have identified microRNAs—short regulatory molecules that show distinct expression patterns in cancer cells—as promising triggers for targeted therapies. Advances in photosensitizer chemistry and catalytic oxygen generation have also helped to overcome previous barriers in photodynamic therapy.
Schematic diagram of the working principle of the tetrahedra DNA nanomachine precisely targeting nucleolin protein overexpressed on the surface of tumor cell membrane and specifically responding to miRNA to achieve combined photodynamic and chemotherapy in cSCC tumor. (Image: Reprinted from DOI:10.1002/advs.202415296, CC BY)
The research team built tetrahedral nanostructures—each about 10 nanometers across—by folding four long DNA strands into a pyramid-like shape. At one vertex, they placed a short DNA sequence known as AS1411, which binds specifically to nucleolin, a protein found in high abundance on the surface of tumor cells but largely absent from normal cells. This enables the nanostructure to recognize and accumulate in cancerous tissue.
To achieve intracellular activation, the researchers attached two hairpin-shaped DNA strands at other vertices. These hairpins remain folded and inactive until they encounter microRNA-7, a molecule that their sequencing data revealed to be highly expressed in cSCC but scarce in healthy skin. Once inside a cancer cell, microRNA-7 binds to one of the hairpins, unfolding it and triggering the release of a photosensitizer called Chlorin e6 (Ce6). This molecule is initially kept inactive by close proximity to a quencher, but when the hairpin unfolds, the quencher is displaced, allowing Ce6 to respond to light and generate reactive oxygen species.
To enhance the effect, the nanomachine is designed to recycle the microRNA-7 trigger through a catalytic reaction. After activating one hairpin, the microRNA is released and free to activate others, setting off a chain reaction that amplifies the therapeutic output even at low microRNA levels.
The researchers also addressed the low-oxygen environment of tumors, which usually hinders photodynamic therapy. They incorporated an iron-based compound called hemin into the AS1411 aptamer region. Hemin mimics the behavior of catalase, an enzyme that converts hydrogen peroxide into oxygen. This self-generated oxygen boosts the activity of Ce6 and ensures more consistent treatment effects.
To add a second therapeutic mode, the researchers loaded the chemotherapy drug doxorubicin into the double-stranded regions of the hairpins. When triggered by microRNA-7, the nanomachine not only activates phototherapy but also releases doxorubicin directly inside the cancer cell, achieving a synergistic combination treatment.
Laboratory tests showed that this nanomachine remains largely inactive in normal cells but responds robustly in cSCC cells. Confocal imaging and qPCR confirmed that microRNA-7 expression was about six times higher in cancer cells, correlating with significantly stronger activation. When exposed to red light, treated cancer cells showed extensive death, while normal skin cells were largely unaffected.
Further experiments confirmed that the nanomachine produced more reactive oxygen than equivalent concentrations of free Ce6. The system also showed high specificity, responding to microRNA-7 but not to unrelated RNA sequences. Tests in low-oxygen environments demonstrated that the hemin component reduced hypoxia and enhanced intracellular oxygen levels, validating its catalytic function.
In mice with cSCC tumors, the full nanomachine—containing Ce6, doxorubicin, and hemin—produced a marked reduction in tumor size over 12 days of treatment. Fluorescence imaging showed that the nanomachine accumulated at tumor sites and became activated only in the presence of microRNA-7. Control versions without this targeting component showed weak or diffuse signals. Examination of major organs found no evidence of toxicity, and body weight remained stable throughout treatment.
The team also tested the risk of unintended phototoxicity. Mice treated with standard photosensitizer delivery methods developed significant skin inflammation after light exposure, but those receiving the DNA nanomachine showed minimal side effects. This safety margin results from the combined targeting requirements—both nucleolin recognition and microRNA activation must occur for therapy to proceed.
This dual-trigger strategy could be adapted to other cancers by identifying alternative microRNA signatures, making the approach broadly applicable. Unlike surgery, it preserves tissue function and appearance. Unlike traditional photodynamic therapy, it minimizes side effects by localizing treatment to cancer cells. And unlike prior nanoparticle-based strategies, it integrates multiple therapeutic elements into a single, programmable platform.
While the results are promising, further studies will be needed to optimize dosing schedules, evaluate long-term outcomes, and address regulatory considerations. Still, this work demonstrates how molecular engineering can overcome long-standing barriers in cancer treatment. By combining tumor recognition, microRNA responsiveness, oxygen generation, and drug release into a single nanoscale system, the researchers have taken a significant step toward safer, more precise therapies for difficult-to-treat skin cancers.
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