A thin layer of gold wrapped around nanotubes may be the secret formula for a better contrast imaging agent – one that enhances absorption of laser radiation and simultaneously reduces toxicity. This new imaging agent being developed at the University of Arkansas and University of Arkansas for Medical Sciences (UAMS; Little Rock) is capable of molecular mapping of lymphatic endothelial cells and detecting cancer metastasis in sentinel lymph nodes.

"The absorption of near-infrared (NIR) radiation is an important issue for non-invasive photoacoustic [laser-induced sound wave] detection and photothermal [laser-induced heat] treatment," Jin-Woo Kim, associate professor in the department of biological and agricultural engineering at the University of Arkansas, told Medical Device Daily. "Indeed, because most biotissues are relatively transparent to NIR radiation, targeting of tumor cells with strongly NIR absorbing nanoparticles could allow both highly sensitive diagnosis and targeted killing of tumors noninvasively in a whole body at the laser energy, which is safe for surrounding healthy tissue."

Many researchers have avoided using nanotubes as part of imaging agents because of the potential toxicity that comes with these little hollow particles. But Kim and his collaborator, Vladimir Zharov, professor in the Winthrop P. Rockefeller Cancer Institute at UAMS, found that gold solved the problem and even enhanced the effectiveness of radiation. The gold nanotubes required low laser-energy levels for detection and low concentrations were required for effective diagnostic and therapeutic applications.

Their work – which targeted imaging lymphatic vessels in mice – is reported in the current issue of Nature Nanotechnology.

In a previous study, Kim and Zharov demonstrated that carbon nanotubes held a great promise as NIR contrast agents for photoacoustic detection and photothermal killing of individual bacteria in the blood system. However, they suffer from relatively poor NIR absorption, and questions abound about their toxicity.

"We addressed this problem by depositing a thin layer of gold around the carbon nanotubes. The gold layer enhanced absorption of laser radiation and reduced toxicity," Kim said. "In vitro tests showed only minimal toxicity associated with the golden carbon nanotubes (GNTs). Furthermore the synthesis process is very robust and simple, inexpensive and environmentally friendly green one. The reaction of the carbon nanotubes and gold chloride occurs in water and happens at ambient temperature. No other chemicals or special conditions, such as heating, are required."

The team's GNTs synthesized in this study are shorter than carbon nanotubes used in the previous study, but they absorb NIR radiation at least twice as effectively.

"Two-order higher concentrations of carbon nanotubes will be required to have the same photothermal responsiveness as GNTs," Kim said. "Taking into account the issue of toxicity – i.e., as long as the controversy exists and until full-scale studies prove one way or the other, we should not assume the particles to be safe. The amount of nanoparticles applied for biomedical applications, such as in vivo clinical diagnosis in human, becomes more important. The less, the better. Furthermore, gold is chemically inert, so it is highly possible to rule out potential toxicity."

Kim said that recently gold-based nanoparticles, in particular gold nanoshells AuroLase, made by Nanospectra Biosciences (Houston) and colloidal gold nanospheres conjugated with tumor necrosis factor alpha, made by Cytimmune Sciences (Rockville, Maryland), have been approved for pilot clinical trials for cancer treatments. That led his team to assume a gold coating could potentially improve the biocompatibility of carbon nanotubes.

Some of the unique features of GNTs compared with existing nanoparticles include:

One of the highest near-infrared absorption at a minuscule diameter (up to 3 nm-5 nm).

Absorption can be adjusted in NIR window of transparency of biotissue to provide deeper laser penetration and advanced diagnosis.

They provide multimodal function as triple contrast agents for photoacoustic and photothermal detection and photothermal therapy.

"With the GNT with such unique property, we successfully demonstrated in vivo molecular mapping of lymphatic vessels, and targeted detection and purging of metastasis in lymph nodes, which is an important site of tumor spreading," Kim said. "This new nanomaterial could be an effective alternative to existing nanoparticles and fluorescent labels for non-invasive targeted imaging of molecular structures in vivo."

In addition to diagnosing cancer, Kim said the GNTs could also be used therapeutically for cancer as well as bacterial and viral infections, such as antibiotic-resistant staphylococcus aureus.

"For example, in this study, we demonstrated molecular detection of lymphatic endothelial cells and highly precise targeted destruction of lymphatic wall in vivo," he said. "This holds promise for mapping and destruction of intra- or peri-tumor lymph vessels that provide initial dissemination of detached tumor cells to metastatic sites. Another example is to apply our developed technique for the detection and purging of cancer metastasis in so-called sentinel lymph nodes that is important for early cancer staging with potential to improve cancer treatment and reducing patient's morbidity through replacement of conventional surgical approach with noninvasive laser ablation."

Kim and Zharov are continuing to explore the optical properties of this nanoparticle with different dimensions and to use double- and multi-walled carbon nanotubes as core and different bioconjugation for targeting normal and abnormal cells.

"Specifically, we are currently investigating the potential of golden carbon nanotubes for molecular detection and eradication of metastasis in the sentinel lymph nodes and real-time tracking of golden carbon nanotubes in ear and skin animal vasculatures, as well as an toxicity study of golden carbon nanotubes in vivo," he said.

Although the team's preliminary toxicity results reveal minimal toxicity, comprehensive toxicity evaluations are required before any human trials could be initiated.

"We expect the completion of toxicity evaluation in a few years," he said.

Their research was supported by the National Institutes of Health, the National Science Foundation and the University of Arkansas System's Division of Agriculture and the Arkansas Biosciences Institute. n

Lynn Yoffee, 770-361-4789; lynn.yoffee@ahcmedia.com