In recent years, there has been a growing interest in nanomedicine, an interdisciplinary field which aims to use various nanomaterials to tackle a range of biomedical applications and medical ailments.

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One such application is the production of radiosensitizers for cancer treatments, with gold nanoparticles (GNPs) leading the way. However, with the human body being as complex as it is, GNP radiosensitizers haven’t quite hit the heights that were initially expected, and have yet to make it to the clinic. This is despite promising preclinical in-vitro and in-vivo evidence.

A team of Irish researchers have published a review paper into the underlying biological mechanisms of GNP radiosensitizers and how the barriers to clinical trials can be broken down.

Radiation is a common form of cancer treatment, but the levels of toxicity associated with the treatments limit the dosage. There has been much research to sensitise the cancerous tissue to the radiation, whilst leaving the surrounding healthy cells alone.

One such way is through therapeutic ratios that introduces a material with a high atomic number to the target cells. With its high mass number, strong photoelectron coefficient and high mass energy coefficient, gold is a very promising candidate for such mechanistic targeting approaches.

Response of Stress and Oxidative Stress Mechanisms

Whilst inert, gold is thought to possess an active surface which can be utilised to promote and increase the catalytic efficiency of a reaction, which can lead to an increase in response of stress (ROS) mechanisms. The effect is greater in nanoparticles with a diameter less than 5 nm as particles on this scale present a greater surface area-to-volume ratio.

However, some of these mechanisms are believed to be responsible for the cytotoxicity effects that GNPs radiosensitization methods can exhibit. The interaction between the surface of the nanoparticles and oxygen molecules facilitates the transfer of donor electrons to the oxygen species and generates superoxide radicals. This can lead to ROS production through dismutation.

Other oxidation stresses can also contribute to the cytotoxicity by causing damage to the DNA and cell membrane proteins in a cell. There are many reasons for the increase of oxidative stress, but the most common are the presence of redox groups in the coating, contaminants from the production method and oxidant-inducing properties from the nanoparticles.

Cell Cycle Effects

The sensitivity and biological effects of radiation exposure are dependent upon the cell cycle phase. GNPs can enhance radiosensitization through cell cycle disruption and induce apoptosis (cell death). In response to radiation, cells respond to certain checkpoints and repair their genomic defects, preventing cell death. GNPs, unlike other metals, have been shown many alter cell cycle distribution mechanisms, rather than just through induced cell cycle arrest.

GNPs have been found to promote a certain phase, known as the G2/M phase, to accelerate the cell cycle arrest in cancer cells (DU-145) and decrease the expression of the tumour proteins found in these cells.

Thiolated-GNPs have been used as efficient detectors of tumour cells. The coated nanoparticles invoke a response in the G2/M phases of tumour cells and induce apoptosis. Ultimately, this has been found to give an increase in the detection sensitivity under X-ray exposure. Nuclear-targeted GNPs alone can also disrupt tumour cell transition and populations, to induce apoptosis in cancerous cells.

The main driving factors to obtain distinct responses in cells through these mechanisms are defined by the choice of coating and size of the nanoparticles. However, the various concentrations, coatings, materials and cell lines make it hard to determine the actual mechanism of action in play during these processes. It is known that the presence of GNPs induces alterations in the cell kinetics due to the accumulation of G2/M phases. G2/M is known to be the most radiosensitive, so such accumulations lead to an overall increase in the radiosensitization.

DNA Damage and Repair

GNP-induced radiosensitization can provide an alternate mechanism through DNA damage and repair. Radiation itself induces double-strand breaks in DNA and their subsequent repair is essential to maintain cell life. Because DNA is so essential for cell division, it also makes it a key therapeutic target in helping to stop the multiplication of cancer cells.

DNA damage through GNP-induced radiosensitization occurs in two stages- early and late damage. Early DNA damage, i.e. 1 hour after radiation exposure, is due to the GNPs presence in the perinuclear region at the time of irradiation. Whereas, late DNA damage, i.e. after 24 hours’ post irradiation, occurs through other indirect processes such as radical production.

Through various research efforts, it is has been shown that GNPs can influence the repair mechanism of the cell and cause residual damage. However, it is thought that not all GNP processes follow the same mechanism and may induce distinct repair kinetics in different cell lines.

GNPs can promote the dose enhancement and increase double strand breaks in the DNA through radiosensitization approaches, but the lack of consistency in cell lines, radiation sources and energies, treatment conditions and nanoparticle properties can lead to varying results, which has made it hard for researchers to draw an overall conclusion regarding these mechanisms. In the future, the understanding of how the various parameters can affect the DNA damage and repair could potentially shed light on how the GNPs invoke a DNA damage and repair response in cancerous cells.

Bystander Effects of GNP Radiosensitization

Aside from the direct radiation effects, communication between cells is very important after radiation exposure. Even if cells haven’t been directly affected by the radiation, if they communicate with nearby exposed cells then they can receive signals which cause them to act as if they have been subjected to direct radiation exposure. This is known as the bystander effect, and can occur in many different cell types.

The signals involved in bystander processes can cause an alteration in the gene expression, damage to the DNA and chromosomes, cell proliferation alterations, apoptosis or changes in the translation process in non-irradiated cells.

There are many types of signalling molecules involved in these processes that are released into the surrounding environment and reach the bystander cells through either passive diffusion, binding to receptors or direct cell-to-cell contact.

Exosomes (vesicles) carrying microRNA (miRNA) are believed to be the catalyst for mediating intracellular signals between tumour cells and bystander cells. MicroRNAs can be up- or down-regulating after radiation exposure, with some strains multiplying after a radiation dose which enhances the proliferation and resistance of the cancer cells by targeting the death receptors.

GNPs, alongside other metal-NPs, have been found to interrupt the intracellular pathways associated with cell signalling, even when no radiation is present. The presence of GNPs can lead to a series of responses depending on their size, shape and coating. The understanding of which signalling pathways are affected is a future consideration, but could lead to a greater understanding of bystander and radiosensitization effects.

Toxicity of GNPs

As with any form of therapeutic treatment, toxicity, and more importantly cytotoxicity, is a key factor that can affect the success of the treatment. There is currently a level of uncertainty surrounding GNPs level of toxicity. Bulk gold is very safe but certain functionalised GNPs have exhibited unusable levels of cytotoxicity.

Size, concentration, cell type and treatment time are the basic parameters that researchers consider when examining the cytotoxicity of GNPs. Size is an important factor, as very small particles can be highly toxic, whereas larger particles are relatively nontoxic. A high concentration of GNPs have found to cause a decrease in the cell viability, but low concentrations do not appear to have any influence.

Some researchers have measured the uptake and localisation of the nanoparticles in the cell by transmission electron microscopy (TEM). These methods led the researchers to the conclusion that nanoparticles are not inherently toxic to human cells. However, it was also noted that the potential modification of nanoparticles by their environment is an important factor, as this may result in significant variations that could change their applicability for clinical applications.

A potential way to check the toxicity and clinical viability of GNPs in the future is by modification of existing technology. Researchers have developed a fast and efficient vivo assay known as the “ToxTracker”. Currently it used to identify DNA damage caused by direct DNA interaction, oxidative stress and general cellular stress from other metal oxide and silver-based nanoparticles. It could be adapted in the future to include GNPs and help to elucidate not only some of the underlying mechanisms, but also their cytotoxic properties.

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This information has been sourced, reviewed and adapted from materials provided by SpingerOpen.