Carbon nanotubes in hyperthermia therapy

Abstract

Thermal tumor ablation therapies are being developed with a variety of nanomaterials, including single- and multiwalled carbon nanotubes. Carbon nanotubes (CNTs) have attracted interest due to their potential for simultaneous imaging and therapy. In this review, we highlight in vivo applications of carbon nanotube-mediated thermal therapy (CNMTT) and examine the rationale for use of this treatment in recurrent tumors or those resistant to conventional cancer therapies. Additionally, we discuss strategies to localize and enhance the cancer selectivity of this treatment and briefly examine issues relating the toxicity and long term fate of CNTs.

Keywords: Cancer therapy; Carbon nanotubes; Multiwalled nanotubes; Photothermal therapy; Single-walled nanotubes.

Figure 1

Treatment of tumor bearing mice by CNMTT reduces tumor growth and increases long term survival. Nu/nu mice were implanted s.c. with RENCA tumors and divided into groups of 10. Mice were either left untreated, treated with MWCNT alone, treated with laser alone, or treated with the combination of MWCNT and laser (3 W/cm²; 30 s). (A) Photographs at day 21 post-treatment of representative mice from groups treated with laser only, untreated controls, or mice treated with 100 μg of MWCNT plus laser. (B) Mice treated with the combination of MWCNTs and laser were injected with a range of MWCNT doses. Tumor sizes were measured every 2 days. Means and standard errors are shown. Control groups (untreated, treated with MWCNTs alone, or treated with laser alone) were statistically identical. There is a dose-dependent attenuation in tumor growth after 30 s of NIR laser treatment of MWCNT-loaded tumors (P < 0.0001). (C) MWCNT-based photothermal therapy increases long-term survival of tumor-bearing mice. Survival of mice treated as described in (C) was assessed for 10 months after treatment. Kaplan–Meier curves demonstrate a significant increase in survival in mice treated with all doses of MWCNTs plus laser (P < 0.0001 vs all controls). Survival curves for control groups were statistically identical (P > 0.775). Adapted and reproduced with permission from Burke et al. PNAS 106 (2009) 12897-12902.

Figure 2

Breast cancer stem cells are resistant to conventional hyperthermic cell death but sensitive to CNMTT. (A) Relative viability of cancer cells 24 h after water bath heat treatment as a model of conventional hyperthermia. Stem cell-like breast cancer cells (BCSCs) or bulk breast cancer (non-stem) cells were heated in a water bath at 47 °C for 0-60 min. MTT absorbance values were normalized to the untreated condition (“0” minutes heat shock). The results clearly show that BSCC subpopulations are more resistant to heat than breast cancer cells as a whole. (B) Sub-lethal hyperthermia enriches for the BCSC phenotype in bulk breast cancer cells. Changes in the CD44^high/CD24^low stem cell fraction of surviving bulk breast cancer cells 24 h after water bath heat shock at 43°C, 45°C or 47°C were determined by flow cytometry. Shown are mean percent changes in the CD44^high/CD24^low cell fraction normalized to the Untreated condition (which is set as 1.0, i.e. “0 percent change”). Dashed lines indicate the 95% C.I. for the Untreated condition. All heat treatments led to significant increases in the stem cell fraction (p < 0.0001) relative to Untreated. (C) In contrast, of BCSC and non-stem breast cancer cells are equally sensitive to CNMTT. Cancer cells were heat treated to specific temperatures by the combination of MWCNTs and NIR laser irradiation and the relative viability of BCSC and non-stem breast cancer cells was determined by MTT 24 h later and normalized to the “Untreated” conditions. “CNT Only” describes samples that were mixed with MWCNTs but were not laser treated. “CNT+ Laser” describes samples that were heat shocked to the indicated final temperatures by the combination of 50 mg/mL MWCNTs and 3W laser radiation. In contrast to the water bath heating results, no significant difference between the sensitivity of stem-like and non-stem breast cancer cells was observed, and (D) no enrichment of the stem cell phenotype (quantified as in (B)) in viable cells 24 h after CNMTT was detected. Adapted and reproduced with permission from Burke et al. Biomaterials 33 (2012) 2961-2970.

Figure 3

Effective delivery of CNTs to the tumor is a central challenge for using CNTs to treat cancer. . In spite of the dynamic growth of innovative nanoplatforms, the path from bench to bedside is still challenging. Barriers within the tumor microenvironment, including vascular heterogeneity, extracellar matrix pore size, high interstitial pressure, and immune cell infiltration, limit the uniform penetration of nanotherapeutics, lead to inefficient delivery and reduce the potential efficacy of many treatments. Currently, there is an insufficient understanding of how CNTs interact with the tumor microenvironment. As shown here, there is a patchwork of inter-related properties that are dependent not just on the characteristics of the nanoparticle, but also upon the environment in which the particle is to be used and the strategy by which it will be delivered. These properties influence the pharmacologic behavior of the CNTs in vitro and in vivo. Failure to appropriately characterize CNTs has led to a bottleneck in their clinical translation due to inadequately designed studies at the pre-clinical level that could bridge cell- culture-to-rodent-to-human studies. Identification of CNT physicochemical and surface characteristics under specific infusion parameters and consideration of how the pathophysiology and anatomy of the target site influence the pharmacologic properties of CNTs will be necessary for the development of pharmaceutical grade material for clinical use.

Figure 4

MR thermography combined with damage modeling algorithms can be used to optimize the delivery of therapeutic heat to tumor targets. (A) Schematic of experimental design. The entire set-up was placed within the magnet of a 7T MRI. A YAG laser tuned to 1064 nm served as the laser source and the phantom was exposed to a total fixed radiant energy of approximately 100 J/cm² using either a high power, short duration pulse (10 W/cm²; 9.6 s; refered to as the “short pulse”) or a low power, longer duration pulse (3 W/cm²; 32 s; referred to as the “long pulse”). (B) Time evolution of the average temperature measured by MR thermometry achieved in the MWCNT inclusion: Temperature increased approximately linearly with time in response to both long and short pulse irradiation with a rate of average temperature increase of 1.89 and 0.38 °C s^-1 and for the short and long pulse heating, respectively. Notably, the ratio of these heating rates is about 5 and is significantly greater than the ratio of heating powers (3.3), suggesting more heat diffuses away from the heated region during long pulse heating. (C) Temperature maps calculated by PRF MR thermometry of a coronal slice across the MWCNT inclusion and surrounding nanotube-free phantom generated at the time the laser was turned off. A temperature contour for which the peak temperature reached 43 °C (green curve) is shown. The images clearly show that heat is more localized following the short pulse treatment. Thermal damage contours calculated for 100 or 200 cumulative equivalent minutes at 43°C (100CEM43 (pink curve) and the 240CEM43 thresholds (blue curve)) and contours calculated using the Arrhenius damage integral are shown. Thermal necrosis is predicted to begin when the Arrhenius integral equals 1 (light blue curve), which is roughly equivalent to 100CEM43. Ninety-nine percent of cells are predicted to die when the Arrhenius integral equals 4.6 (dark blue curve), which is roughly equivalent to 240CEM43. Note that 99% cell death as calculated by the Arrhenius damage integral was not achieved using long laser pulse heating. Thus, for a fixed radiant exposure, short pulse heating of CNTs leads to a higher maximum temperature, a more rapid rate of temperature increase, more localized heating, and greater therapeutic efficacy. Adapted and reproduced with permission from Xie et al. Phys. Med. Biol. 57 (2012) 5765-75.

Figure 5

Summary of important parameters which must be assessed for successful clinical translation of CNMTT. To maximize treatment efficacy and minimize collateral damage due to heat spread, laser irradiation parameters (irradiance, duration, duty cycle) must be carefully matched to the NIR absorptive characteristics and distribution of the CNTs as well as the thermal properties of the targeted and surrounding tissue. These parameters are highly interrelated and must be balanced with the underlying biology and biophysics of the tumor target. An understanding of both the NIR absorptive and thermal dispersive qualities of the tumor target is needed. Specific issues related to the sensitivity of the tumor and surrounding tissue to thermal ablation must be identified. Treatment should be compatible with a non-invasive thermal mapping modality (such as proton resonance frequency shift magnetic resonance (MR) thermography) to monitor temperature localization.