Our results demonstrate that gold nanoparticles coupled with a noninvasive radiofrequency generator are a potentially novel method for ablating tumors. Our in vitro cell culture tests demonstrate that colloidal gold nanoparticles are not directly cytotoxic but can, upon exposure to the RF field, be heated to a degree that results in cell death. We correlated our promising in vitro results with in vivo models in which gold nanoparticle injections resulted in ablation of either normal tissues or subcutaneous tumor nodules from a rat hepatoma model. The conceivable applications of this technology are broad and include the potential treatment of patients with nonoperative tumors as well as the possibility of treating multiple tumors in a single treatment session.
While the results of this study support further investigations into the use of gold nanoparticles in combination with the RF generator, there are a number of questions that remain to be answered as well as technical obstacles to be overcome in future studies. First and foremost, the aim of the project is to develop a noninvasive method of ablating tumors as an alternative to currently available RF probes. However, in our rat hepatoma model, gold nanoparticles were targeted to subcutaneous tumors via direct intra-tumoral injections. For this technology to be clinically applicable, nanoparticles will have to be administered noninvasively and subsequently targeted toward cancer cells. There are studies regarding the highly selective targeting of specific cells and tissues by nanoparticles through the modification of their physical properties (18
). Recently, nanoparticle delivery systems (20nm-100nm) capable of escaping phagocytic clearance by the reticuloendothelial system have been investigated (21
). Additionally, 33nm polyethylene glycol-coated gold nanoparticles have been incorporated with TNFα to enhance thermal induced tumor growth delay in a murine colon cancer model (24
). Furthermore, 1.9nm gold nanoparticles delivered by IV injection have been used to enhance radiotherapy induced tumor ablation in a mammary cancer model (25
). These reports suggest that research progress is rapidly being achieved for targeted nanoparticle delivery to cancer cells.
Glypican-3, a member of the glypican family of heparin sulfate proteoglycans, has been found to be overexpressed in hepatocellular cancer (26
). Furthermore, epidermal growth factor receptor (EGFR), a transmembrane tyrosine kinase, is overexpressed in the fibrolamellar variant of HCC as well as other cancers for which RFA is commonly used including breast, gastric and colorectal (27
). Anti-EGFR antibody conjugated gold nanoparticles have been described with applications including real time vital optical imaging as well as selective laser photo-thermal therapy. Therefore, future investigations will likely focus on developing antibody coated gold nanoparticles for targeted delivery. In addition to targeting gold nanoparticles to tumors, the radiofrequency generator has the potential to be modified in such a way that a more focused field is applied to specific sites of interest in a manner akin to cyberknife technology that is currently in clinical use for the delivery of targeted external beam radiation. Another limitation of the current study is that it does not address the diffusion of gold nanoparticles in vivo, or the size of the thermal ablation zone. However, the aim of the in vivo aspects of this study was simply to provide proof of concept that this novel technology can be used in conjunction with gold nanoparticles to cause thermally induced tissue injury. Certainly, future studies focusing on targeted delivery of gold nanoparticles will involve imaging to detect where systemically delivered nanoparticles accumulate as well as long-term follow-up to assess the ability of the non-invasive radiowave ablation to shrink or resolve established tumors.
Although at least 3-4 years away from clinical testing, it is important to mention that clinical monitoring that would be utilized with this non-invasive radiowave treatment. We envision that patients would be awake with IV sedation and would have cardiac and pulse oximetry monitoring during the procedure. Pain medications could be employed either prior to or during the procedure if the patient experiences pain. As for the strength of the current power generator, current radiofrequency ablation devices in clinical use are approved for up to 200 Watts. While the Kanzius power generator is capable of producing power up to 1000 W, our studies used 35-100 W for solution tests, and 35 W for cell culture and in vivo experiments, which is well below the current FDA limit of 200W.
We first described the use of this noninvasive radiowave technology for thermal ablation of cancer over a year ago (28
). In that study, we used metal ion (copper, iron, and magnesium) injections to focus the radiowave heating, and have subsequently used gold nanoparticles as described in the current study. Recently, another group has reported using a similar radiowave machine in a rabbit hepatoma model utilizing single walled carbon nanotubes (29
). However, like our study, the ablation model consisted of direct tumor injection of nanotubes, and thus selective, noninvasive cancer targeting remains elusive. Nonetheless, taken together these studies provide proof-of-principle that noninvasive radiowave ablation of cancer is technically feasible, particularly when facilitated by nanotechnologies. While this technology holds exciting potential for broad applications, significant further research is needed.