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Biological Chemistry

Taking Aim At Brain Tumors

Nanomedicine: Gold spheres decorated with RNA block cancer gene expression in mice

by Bethany Halford
November 4, 2013 | A version of this story appeared in Volume 91, Issue 44

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Credit: Northwestern University
This spherical nucleic acid has cancer-fighting siRNA (green) anchored to a gold nanoparticle core via a thiol linker (blue). Polyethylene glycol (red) stabilizes the particle.
This is a rendition of a gold nanoparticle decorated with RNA (green), known as a spherical nucleic acid.
Credit: Northwestern University
This spherical nucleic acid has cancer-fighting siRNA (green) anchored to a gold nanoparticle core via a thiol linker (blue). Polyethylene glycol (red) stabilizes the particle.

Scientists have created a new weapon to test in the fight against the deadly brain cancer glioblastoma multiforme. By packing strands of small interfering ribonucleic acid, or siRNA, onto the surfaces of gold nanoparticles, they made a nanomedicine that entered the brains of mice and then slipped into tumor cells. There, the siRNA neutralized genes that make those cells flourish.

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Credit: Sci. Transl. Med.
SNAs, functionalized with a gadolinium contrast agent, accumulate in a glioblastoma multiforme tumor in a mouse brain, letting researchers image the mass.
This is a glioblastoma multiforme tumor imaged in a mouse brain using Gd-modified SNAs.
Credit: Sci. Transl. Med.
SNAs, functionalized with a gadolinium contrast agent, accumulate in a glioblastoma multiforme tumor in a mouse brain, letting researchers image the mass.

“Glioblastoma multiforme is one of the worst cancers one can get,” says Chad A. Mirkin, a chemistry professor at Northwestern University who helped invent the cancer-fighting nanoparticles. The disease kills approximately 13,000 people in the U.S. each year, and life expectancy once diagnosed is 14 to 16 months. Doctors currently treat the disease by surgically removing the tumor, followed by radiation and chemotherapy. But this aggressive approach extends life expectancy by only a few months.

The team led by Mirkin and his colleague Alexander H. Stegh thought it might have a better method for fighting this tenacious cancer. In 1996, Mirkin developed spherical nucleic acids, or SNAs, which are nanoparticles covered with either DNA or RNA. The nucleic acids in turn can interact with genes and proteins. SNAs have been used for medical diagnostic tests and as topical medicines. In 2007, Stegh and coworkers identified a gene that is overexpressed in glioblastoma tumors. Mirkin and Stegh decided to build an SNA equipped with siRNA that is capable of interfering with that gene, a process known as RNA interference (RNAi).

When injected into the bloodstream of mice with glioblastoma multiforme, the SNA nanoparticles travel through the body. About 1% of them cross the blood-brain barrier, the barricade that protects the brain from toxic agents. The nanoparticles accumulate in tumors, rather than healthy brain tissue, because they are trapped in the tumors’ unusually distorted blood vessels. The researchers found that treatment shrank tumors and increased the survival rates of the mice. Unlike other therapies based on RNAi, no other agent was required to deliver the RNA (Sci. Transl. Med. 2013, DOI: 10.1126/scitranslmed.3006839).

“The RNAi approach shown here has huge potential,” comments Omid C. Farokhzad, a nanomedicine expert at Harvard Medical School. “The effectiveness of SNAs for uptake into cells makes them uniquely suitable for RNAi applications.”

“Crossing the blood-brain barrier with an siRNA-based approach that does not involve a separate pharmacological agent to open the blood-brain barrier is significant,” adds John J. Rossi, an RNAi expert at the Beckman Research Institute of City of Hope, a cancer center in Duarte, Calif. “The SNA approach could be applied to neurological diseases such as Alzheimer’s or Parkinson’s, using different siRNAs specific for those diseases.”

AuraSense Therapeutics, a company cofounded by Mirkin, has licensed the technology. The next step, the researchers say, is to do safety and efficacy studies in primates. They hope to begin human clinical trials by the end of 2014.

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