Using a technique known as “nucleic acid origami,” chemical engineers have built tiny particles made out of DNA and RNA that can deliver snippets of RNA directly to tumors, turning off genes expressed in cancer cells.
To achieve this type of gene shutdown, known as RNA interference, many researchers have tried — with some success — to deliver RNA with particles made from polymers or lipids. However, those materials can pose safety risks and are difficult to target, says Daniel Anderson, an associate professor of health sciences and technology and chemical engineering.
The new particles, developed by researchers at MIT, Alnylam Pharmaceuticals and Harvard Medical School, appear to overcome those challenges, Anderson says. Because the particles are made of DNA and RNA, they are biodegradable and pose no threat to the body. They can also be tagged with molecules of folate (vitamin B9) to target the abundance of folate receptors found on some tumors, including those associated with ovarian cancer — one of the deadliest, hardest-to-treat cancers.
Researchers successfully used this nanoparticle, made from several strands of DNA and RNA, to turn off a gene in tumor cells. Image: Hyukjin Lee and Ung Hee Lee
Nanoparticles are used for delivering therapeutics into cells. However, size, shape, surface chemistry and the presentation of targeting ligands on the surface of nanoparticles can affect circulation half-life and biodistribution, cell-specific internalization, excretion, toxicity and efficacy. A variety of materials have been explored for delivering small interfering RNAs (siRNAs)—a therapeutic agent that suppresses the expression of targeted genes. However, conventional delivery nanoparticles such as liposomes and polymeric systems are heterogeneous in size, composition and surface chemistry, and this can lead to suboptimal performance, a lack of tissue specificity and potential toxicity. Here, we show that self-assembled DNA tetrahedral nanoparticles with a well-defined size can deliver siRNAs into cells and silence target genes in tumours. Monodisperse nanoparticles are prepared through the self-assembly of complementary DNA strands. Because the DNA strands are easily programmable, the size of the nanoparticles and the spatial orientation and density of cancer-targeting ligands (such as peptides and folate) on the nanoparticle surface can be controlled precisely. We show that at least three folate molecules per nanoparticle are required for optimal delivery of the siRNAs into cells and, gene silencing occurs only when the ligands are in the appropriate spatial orientation. In vivo, these nanoparticles showed a longer blood circulation time (24.2 minute half life) than the parent siRNA (6 minute half life).
Programmable self-assembly of ONPs, Schematic of DNA strands for tetrahedron formation (arrow head represents 5′ end of the nucleic acid strand; each colour corresponds to one of the six edges of the tetrahedron) and representation showing site-specific hybridization of siRNA
This study has demonstrated that six single-stranded DNA fragments, and six double-stranded siRNAs, can self-assemble in a one-step reaction to generate DNA/siRNA tetrahedral nanoparticles for targeted in vivo delivery. The overhang design of the DNA strands allows specific hybridization of complementary siRNA sequences and provides full control over the spatial orientation of the siRNA and the locations and density of cancer-targeting ligands. The ONPs can be modified with different tumour-targeting ligands by simple conjugation chemistry, extending the use of these nanoparticles to the treatment of various cancers. We observed robust gene silencing with both intratumour and systemic injection of ONPs into KB xenograft tumours, without any detectable immune response. We believe these particles may also have utility in the treatment of other tissues by modification of their size and ligand type.
In vivo pharmacokinetic profile and gene silencing in tumour xenograft mouse model.