This is not your grandmother’s castor oil. In a new twist on an old remedy, a protein toxin derived from the hard, glossy seeds of castor plants may have a powerful high-tech application in fighting cancer, thanks to a novel drug-carrying nanoparticle, the “protocell,” created by researchers at the University of New Mexico and Sandia National Laboratories. A new paper in the scientific journal Advanced Healthcare Materials describes how the researchers used protocells to selectively deliver the castor seed toxin, called ricin, to liver cancer cells in the lab. Their work has important implications for the eventual clinical testing and use of these toxins, whose therapeutic potential has been limited, until now, by a lack of safe and effective delivery mechanisms.
“Protein toxins have long been proposed as therapeutic agents for cancer and other diseases. What’s been missing is a way to safely deliver these toxic molecules to diseased cells,” says paper co-author Eric Carnes, PhD, Assistant Research Professor of Chemical and Nuclear Engineering at UNM and a member of the interdisciplinary research team investigating medical applications for the protocell.
“In this latest study, we’re able to show that encapsulating ricin in targeted protocells addresses many of the issues that currently limit the clinical use of protein toxins,” adds Katharine Epler, the paper’s lead author. Remarkably, Ms. Epler is a junior in the UNM Department of Chemical Engineering, one of several undergraduate investigators who have made significant contributions to the protocell research.
Ricin works by inhibiting protein synthesis inside cells. Highly toxic to human beings in its natural form, ricin can be fused to antibodies that promote specific binding to designated cells, a strategy that therapeutically harnesses and focuses its destructive effects. However, hurdles remain: both toxins and antibodies can invoke an immune response within the body, which limits the number of possible treatment cycles and prevents patients from tolerating concentrations high enough to successfully eradicate cancer cells. Because of these limitations, no cancer therapy approved to-date contains ricin.
But this could change. The UNM-Sandia team’s paper suggests that ricin can be safely and efficiently delivered to liver cancer cells via targeted protocells, with very little damage to normal cells. Billions of protocells, each containing vanishingly tiny amounts of ricin, were used in the experiments. Measuring just 165 nanometers across (a nanometer is one-billionth of a meter), the protocell is an elegant fusion of two nanostructures: a sponge-like silica core, rich in tiny pockets for holding therapeutic molecules, and a fluid outer membrane called a lipid bilayer that surrounds and protects the core’s contents. The lipid bilayer also provides a suitable medium for attaching a variety of indispensable molecules that help the protocell target and enter cancer cells and neutralize it in the body to avoid triggering an immune response.
In the recent study, the researchers used special nanoscale engineering techniques to load ricin into the protocells’ cores and attach targeting molecules to the lipid bilayer. Like keys fitting into locks, these molecules bind to receptors overexpressed on the liver cancer cell surface, but have no particular affinity for normal cells. The protocells are thus primed to selectively attach themselves to liver cancer cells; other molecules in the lipid bilayer help transport the protocell into the liver cancer cell interior, where the ricin is released, halting protein production and paving the way to cell death.
To benchmark their novel nanoparticle’s performance, the researchers tested the ricin-loaded protocells against another type of nanocarrier, the liposome, prepared using state-of-the-art techniques. Amazingly, the protocells proved to be 100,000 times more effective in delivering ricin to liver cancer cells. Each protocell was able to carry 500 times the amount of ricin as comparably sized liposomes, thanks to its porous interior, a feature that could help researchers overcome current dose limits that prevent testing ricin in optimally effective amounts. Protocells were also far more stable, retaining virtually all of their toxic cargo in simulated body fluid, yet releasing it efficiently once experimental conditions were changed to mimic the inside of a cell. Finally, protocells showed a much greater ability to efficiently and precisely target cancer cells; few “off-target” effects were observed, and the majority of normal liver cells remained viable in the presence of ricin-loaded protocells. (Translated into a clinical setting, this suggests ricin delivered via protocells could have minimal side effects.)
While further research and testing is required, the UNM-Sandia team’s results are an exciting step forward in the development of protein toxin-based therapeutics. More broadly, their latest study offers further confirmation that the protocell constitutes a uniquely versatile and highly customizable platform for delivering a wide range of therapies to cancer cells. The UNM-Sandia team’s previous research demonstrated the protocell’s superior performance in encapsulating and delivering both conventional chemotherapy drugs and experimental therapeutic RNA. Two prior publications (in Nature Materials last May and ACS Nano last month) detailed these results; this third paper, published online earlier this month and forthcoming in print on the cover of Advanced Healthcare Materials, rounds out the picture by describing the protocell’s success in delivering yet another type of therapeutic molecule. Each class of molecule presents different and daunting challenges (related to molecular weight, charge and other factors) that researchers have successfully overcome.
“With each hurdle cleared, we’re coming closer to realizing our goal of creating a ‘universal nanocarrier’ capable of making conventional cancer therapies safer and more effective while unlocking the potential of promising new therapies,” explains Dr. Carnes. Several protocell projects are underway, including a collaboration with UNM Cancer Center pediatric leukemia researchers led by Cheryl Willman, MD, the Center’s Director and CEO, to bring a version of the protocell targeted to acute lymphoblastic leukemia (ALL) into clinical trials within the next few years.
“Delivery of Ricin Toxin A-Chain by Peptide-Targeted Mesoporous Silica Nanoparticle-Supported Lipid Bilayers” was first published online April 2, 2012, in the journal Advanced Healthcare Materials. Print publication is forthcoming. Authors are (in order of citation) Katharine Epler, David Padilla, Genevieve Phillips, Peter Crowder, Robert Castillo, Dan Wilkinson, Brian Wilkinson, Cameron Burgard, Robin Kalinich, Jason Townson, Bryce Chackerian, Cheryl Willman, David Peabody, Walker Wharton, C. Jeffrey Brinker, Carlee Ashley and Eric Carnes. Read the abstract.