The Ups and Downs of Nanobiotech

THERAPEUTICS

A number of nanobiotech companies have directed their energies towards the therapeutics market. Houston-based C Sixty Inc., for instance, focuses on nanomaterials called fullerenes. Hollow shells comprising 60 carbon atoms, fullerenes (or buckyballs) have several medically relevant properties. Most notably, "Fullerenes turn out to be very, very potent intracellular and extracellular antioxidants," says company president Russ Lebovitz.

Several neurodegenerative diseases, as well as normal aging processes, stem in part from oxidative injury. But as fullerenes are not normally biocompatible, C Sixty is tweaking their structure to develop the "next generation of small-molecule antioxidants," says Lebovitz. The company recently entered into an agreement with Merck & Co. to develop such drugs. Lebovitz says human trials are at least two years away.

Two companies, Nanospectra Biosciences of Houston and Triton Biosciences of Chelmsford, Mass., are developing anticancer therapies based on thermal ablation. Central to both companies' strategies are metallic nanoparticles activated by an exogenous energy source to heat and destroy the surrounding tumors.

Nanospectra's platform relies on nanometer-scale particles called nanoshells. With a gold shell surrounding an inert silica core, these nanoshells can be tuned to absorb or reflect light of various wavelengths depending on the thickness of the core and shell. Nanospectra's particles absorb near-infrared light that easily penetrates tissue. Triton employs targeted, polymer-coated iron oxide nanoparticles and an alternating magnetic field.

Both strategies have pros and cons. Gold nanoshell therapy implementation requires no expensive equipment. But it also requires direct line-of-sight from the laser to the tumor; it is ineffective against some tissues, such as bone; and it loses efficiency with tissue depth, though this can be overcome somewhat using fiber-optic lasers. Magnetic energy, in contrast, is unaffected by tissue, says Triton CEO Samuel Straface. "It sees tissue no differently that it does air; we can activate the particles anywhere in the body at any depth." But the patient also must lie between two magnetic poles, and whole-body treatment could be problematic if the particles accumulate in undesirable locations.

Nevertheless both approaches have shown early promise. Sally J. DeNardo and colleagues from the University of California, Davis, Medical Center presented evidence at the Society of Nuclear Medicine's annual meeting that Triton's nanoparticles, coupled to a monoclonal antibody specific for epithelial cancers, slowed the growth of a human breast cancer xenograft in nude mice.⁴

Nanospectra, meanwhile, has demonstrated complete tumor destruction using its gold nanoshells.⁵ "All of the tumors had completely regressed within 10 days, and even now, a year later, the mice are still alive with no regrowth of the tumors whatsoever," says Nanospectra cofounder Jennifer West, a professor of bio-engineering and chemical engineering at Rice University in Houston. The company plans to initiate human clinical trials for the treatment of mesothelioma in 18 months; Triton hopes to start its own trials in 2006.

DRUG DELIVERY

In the drug-delivery arena, companies are developing approaches to encapsulate drugs to minimize side effects, increase bioavailability, and enhance solubility. According to Paull, roughly 28 drugs are coming off patent in the next five years, representing some $46 billion in lost revenues to the patent owners. One way to extend a patent's effective lifetime is to reformulate an existing drug. "Nanotechnology is one of a few areas that [the drug industry is] really focusing on to do that," he says.

Elan Pharmaceuticals' NanoCrystal technology helps pharma companies improve drug solubility. At present two commercial products use NanoCrystal technology, Wyeth's Rapamune and Merck's Emend. But Dublin-based Elan recently announced it had licensed the technology to Roche for one of that company's drug candidates, and additional product launches are expected in the next few years.

Flamel Technologies of Lyon, France, uses its Medusa platform for drug encapsulation. According to Flamel's Web site, Medusa is a "self-assembled, polyamino acid nanoparticles system." The amphipathic material encapsulates protein drugs in a latticework of protein and carrier, which slowly breaks apart upon injection, delivering the drug slowly over time. The company's human insulin formulation, called Basulin, for instance, remains active in the blood for 24 hours after injection, according to Flamel's Web site. The company has entered into an agreement with Bristol-Myers Squibb Company to market and develop the drug.

Courtesy of Nanosphere

Nanosphere's diagnostics platform relies on gold nanoparticles and a silver precipitation reaction.

Another encapsulation approach involves nanomaterials called dendrimers. A dendrimer is basically like an onion, says Donald Tomalia, president and chief technical officer at Dendritic NanoTechnologies, Mt. Pleasant, Mich. "It has an information-bearing core that defines the nature of the shell or the onion layers that you put around it," says Tomalia. Like an onion, dendrimers grow from the inside out, layer-by-layer, growing 1 nm in diameter with each generation.

Dendritic Nanotechnologies is working to encapsulate anti-cancer drugs such as cisplatin inside the dendrimer's hollow interior, using a surface-bound targeting molecule to direct the complex to its intended target. Another dendrimer company, NanoCure, covalently attaches drug molecules to the dendrimer surface along with a targeting moiety.

Though NanoCure founder, James Baker says the company is at least two years away from human trials, dendrimers are working their way toward the clinic. Dendrimers covalently coupled to gadolinium make an effective contrast agent for magnetic resonance imaging, says Tomalia. "They have been used in vivo in animals for about 10 years with virtually zero side effects," he says. This past January Australian drug developer Starpharma initiated a Phase I human clinical trial of its VivaGel formulation, a dendrimer-based topical microbicide for the prevention of HIV, herpes, and other sexually transmitted viral diseases.

TISSUE RECONSTRUCTION

Another area being served by nanotech is tissue reconstruction. At the Institute for Bioengineering and Nanoscience in Advanced Medicine at Northwestern University, Chicago, director Sam Stupp's lab is developing self-assembling liquids that solidify upon injection. This tissue then forms structured scaffolds that present ordered biological signals (i.e., peptides) to cells.

Key to this material are long cylindrical nanofibers 6–8 nm in diameter and composed of peptide amphiphiles that aggregate noncovalently. In February Stupp's team demonstrated that one such scaffold could induce selective differentiation of neural progenitors into neurons, as opposed to astrocytes, a finding that ultimately could lead to a therapy for otherwise paralyzing central nervous system injuries.⁶ Other focus areas for Stupp's team include islet transplantation and bone regrowth.

"I think regenerative medicine is where nanotechnology will flourish," says Stupp. "Now nanotechnology is expensive, and so you have to solve extremely important problems with it. I think reversing paralysis or blindness is an example of something that deserves an expensive technology."

DIAGNOSTICS AND IMAGING

Diagnostics are perhaps as important as therapies. Northbrook, Ill.-based Nanosphere is using its Verigene nanotech platform to develop an accelerated test for methicillin-resistant Staphylococcus aureus. Standard protocols for MRSA can take 48 to 72 hours, says Vijaya Vasista, chief operating officer. "What we have in development is an assay that would take about an hour after the initial culture, and the next-generation assay will be directly from the sample."

Nanosphere builds molecular diagnostics modeled on sandwich-type assays. Surface-bound oligonucleotides capture specific target nucleic acids, which in turn capture oligonucleotide-bearing gold nanoparticles. Particle size is critical to their stability, says Vasista. The resulting complexes can be detected by a silver precipitation reaction that increases the signal intensity 1,000 to 10,000 times. Vasista says the company hopes to launch its first products in early 2005.

Immunicon of Huntingdon Valley, Pa., employs a "ferrofluid," a colloidal suspension of nanoscale ferrous oxide coupled to antibodies against the epithelial cell-adhesion molecule, EpCAM. These particles help concentrate rare human epithelial cells, such as circulating cancerous cells, in blood for subsequent automated staining and analysis.

Courtesy of Triton BioSystems

Triton BioSystems is developing an anticancer therapy using antibody-coated iron nanoparticles. Application of a magnetic field causes the particles to heat up and literally cook the tumors from the inside out

According to Carrie Mulherin, vice president of marketing, the system is sensitive enough to detect a single cancerous cell in 7.5 ml of human blood. "You can think of it as finding a grain of salt in a five-pound bag of sugar," she says. Immunicon plans to release its first in vitro diagnostic product this quarter.

Several researchers have recently used nanoparticles called quantum dots for live-animal imaging. Quantum dots are nanoscale semiconductor crystals, often of cadmium selenide or lead selenide, which exhibit tunable optical properties. By changing a crystal's diameter, it can be made to absorb and emit light of different wavelengths. But unlike organic fluorophores, each of which has a different absorption spectrum, quantum dots made from a specific material can all be excited by a single light source, making complicated fluorescence microscopy setups with multiple lasers obsolete. Quantum dots also are brighter than organic dyes, do not photobleach, and have narrow emission spectra (which makes multiplexing easier).

John Frangioni of Harvard Medical School and colleagues used quantum dots to locate "sentinel" lymph nodes through the skin of living mice (sentinel lymph nodes are often removed for cancer diagnostic screening, but they can be difficult to locate).⁷ "The size of the quantum dots turns out to be ideal for getting into the lymph system and then getting trapped in the sentinel lymph node," says Andy Watson, vice president of business development at Quantum Dot in Hayward, Calif.

More recently, Shuming Nie of the Winship Cancer Institute at Emory University in Atlanta and colleagues performed live-animal imaging of quantum dots targeted specifically to prostate cancer xenografts in mice.⁸ The team injected antibody-coupled nanoparticles into the animals' tail veins prior to imaging.

Despite these in vivo advances, quantum dots will likely find their greatest use in cultured cells and tissue specimens. They may also continue to be used in animal models, but Nie says he is not sure if they can ultimately be applied to human patients.

SAFETY CONCERNS

That's because quantum dots, like all nanoparticles, pose potential human health risks. The nanotech world collectively cringed in April when Eva Oberdörster, a researcher at Southern Methodist University in Dallas, reported at the American Chemical Society's national meeting that water-soluble fullerene molecules cause brain damage in largemouth bass. The story received considerable media coverage, despite its preliminary nature and not having been peer-reviewed (it has since been published⁹).

Other nanoparticles also are problematic. Dendrimers can cause osmotic damage, activate the clotting and complement systems, and even rip membranes off cells, says Baker. And quantum dots, composed of metals such as selenium, lead, and cadmium, would likely be toxic to most organisms if the metal leeched out of the particles. Developers add coatings to ensure safety and stability, but Alan Waggoner, director of the Molecular Biosensor and Imaging Center at Carnegie Mellon University in Pittsburgh, demonstrated recently that the movement, retention, and distribution of quantum dots varies greatly based on these surface coatings.¹⁰

Nanoscale materials, observes the NIEHS's Bucher, "don't act like particles and they don't act like chemicals. They take on properties that are either intermediate or they are unique, and we're just beginning to sort through this." The NIEHS recently initiated a series of studies designed to address these and other issues for quantum dots, nanoparticulate titanium dioxide, and buckyballs. Led by Bucher, the studies will concentrate on three questions: How do surface coatings and chemistries affect where nanomaterials go in the body; what are the immunologic properties of these nanomaterials; and what are their toxicological effects.

PUBLIC MISUNDERSTANDINGS

Courtesy of Dendritic NanoTechnologies

Researchers are working to convert dendrimers like these into useful drug-delivery tools. But dendrimers are already widely used in the lab. Qiagen's Superfect DNA transfection reagent is a dendrimer whose positively charged surface binds the nucleic acid's negatively charged phosphate backbone.

The results, say Bucher, can help guide product development to make them safer. But a pair of surveys conducted this year, both in the United States and in Britain, reveals a public largely ignorant of his efforts, and indeed of nanotech in general.

A nationwide survey from North Carolina State University (NCSU) in Raleigh found that more than 80% of Americans know "little" or "nothing" about nanotech.¹¹ In the UK, a March report released jointly by the Royal Society and Royal Academy of Engineering shows that only 29% of respondents have heard of nanotech, and only 19% could offer a definition, whether accurate or not.¹²

© 2004 Nature Publishing Group

Spectral imaging of quantum dots (QD) conjugated to a prostate-specific membrane antigen antibody in live animals harboring C4-2 tumor xenografts. Orange-red fluorescence signals indicate a prostate tumor growing in a live mouse (right). Control studies using a healthy mouse showed no localized fluorescence signals (left). (A) Original image; (B) unmixed autofluorescence image; (C) unmixed QD image; and (D) super-imposed image. (Reprinted with permission from Nat Biotechnol, 22:969–76, August 2004.)

On the other hand, both surveys recorded a positive attitude regarding nanotech. In the NCSU study, 40% of those surveyed believe nanotech will produce benefits exceeding risks, compared to 22% who believe the opposite was true. In Britain, 68% of those offering a definition of nanotech predicted it would improve the future, compared to 4% who said it would make things worse.

That's a sentiment echoed by those in the know, too. Says Theis: "If information technology is worth a trillion dollars a year in the economy, imagine what we're going to do when the benefits of this kind of miniaturization are extended to the life sciences and medicine, and just about every industry and ... manufactured object will incorporate this technology. How big will that be? It will be everything."

Jeffrey M. Perkel jperkel@the-scientist.com