Gold nanoparticles kill cancer, but not as thought

Gold particles the size of billionths of a meter are fatal to cancer cells. This fact has long been known, as well as a simple correlation: the smaller the nanoparticles used to fight cancer cells, the faster they die. However, a more interesting, complex picture of these interactions emerges from the latest research, conducted at the Institute of Nuclear Physics of the Polish Academy of Sciences, using a new microscopic technique.

Smaller kills faster: This is what was previously thought about gold nanoparticles used to fight cancer cells. Scientists thought that small nanoparticles would simply find it easier to penetrate the interior of a cancer cell, where their presence would lead to metabolic disruptions and ultimately cell death.

However, the reality turns out to be more complex, as shown by research carried out by scientists at the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Krakow, supported by theoretical analyzes carried out at the University of Rzeszow (UR) and the Technical University from Rzeszow.

“Our institute operates a state-of-the-art medical and accelerator center for proton radiotherapy. When reports emerged a few years ago that gold nanoparticles could be good radiosensitizers and improve the effectiveness of this type of therapy, we started synthesizing them ourselves and studying their interaction with cancer cells tests. We quickly discovered that the toxicity of nanoparticles was not always as expected,” says Dr. Joanna Depciuch-Czarny (IFJ PAN), initiator of the study and first author of an article discussing the results, published in the journal Small.

Nanoparticles can be produced using a variety of methods, resulting in particles of different sizes and shapes. Shortly after they began their own experiments with gold nanoparticles, the IFJ PAN physicists noted that biology does not follow the popular rule that their toxicity is greater the smaller they are.

Spherical nanoparticles of 10 nanometers in size, produced in Krakow, turned out to be practically harmless to the glioma cell line studied. However, high mortality was observed in cells exposed to nanoparticles as large as 200 nanometers, but with a star-shaped structure.

The clarification of the mentioned contradiction became possible thanks to the use of the first holotomographic microscope in Poland, at IFJ PAN.

A typical CT scanner scans the human body using X-rays and reconstructs its spatial internal structure section by section. In biology, a similar function has recently been performed by the holotomographic microscope. Cells are also irradiated with a beam of radiation, but not high-energy radiation, but electromagnetic radiation. Its energy is chosen so that the photons do not disrupt cell metabolism.

The result of the scan is a series of holographic cross-sections that contain information about the distribution of refractive index changes. Because light refracts differently on the cytoplasm and differently on the cell membrane or nucleus, it is possible to reconstruct a three-dimensional image of both the cell itself and its interior.

“Unlike other high-resolution microscopy techniques, holotomography does not require sample preparation or the introduction of foreign substances into the cells. The interactions of gold nanoparticles with cancer cells could therefore be observed directly in the incubator where they were grown, in an undisturbed environment – ​​and with nanometric resolution – from all sides simultaneously and practically in real time,” Dr. Depciuch-Czarny summarizes.

The unique features of holotomography allowed physicists to determine the causes of the unexpected behavior of cancer cells in the presence of gold nanoparticles. A series of experiments were performed on three cell lines: two gliomas and one colon. Among other things, it was observed that although the small, spherical nanoparticles easily penetrated the cancer cells, the cells regenerated and even began to divide again, despite the initial stress.

In the case of colon cancer cells, the gold nanoparticles were quickly pushed out. For the large star-shaped nanoparticles, this was different. Their sharp points perforated the cell membranes, which most likely resulted in increasing oxidative stress in the cells. When these cells could no longer repair the increasing damage, the mechanism of apoptosis, or programmed death, was activated.

“We used the data from the experiments in Krakow to build a theoretical model of the process of nanoparticle deposition in the studied cells. The final result is a differential equation into which appropriately processed parameters can be substituted – for now only the shape and size of nanoparticles descriptively – to quickly determine how the uptake of the analyzed particles by cancer cells will proceed over a given period of time,” says Dr. Pawel Jakubczyk, professor at UR and co-author of the model.

He emphasizes: “Any scientist can already use our model in the design phase of their own research to immediately limit the number of nanoparticle variants that require experimental verification.”

The ability to easily reduce the number of potential experiments that need to be performed means a reduction in the costs associated with purchasing cell lines and reagents, as well as a marked reduction in research time (it typically takes about two weeks to grow a commercially available cell line alone). Furthermore, the model can be used to design better targeted therapies than before – therapies in which the nanoparticles are particularly well absorbed by selected cancer cells, while the toxicity to healthy cells in the patient’s other organs remains relatively low or even zero.

The Krakow-Rzeszow group of scientists is already preparing to continue their research. New experiments should soon make it possible to extend the model of the interaction of nanoparticles with cancer cells with further parameters, such as the chemical composition of the particles or further tumor types. Later plans also include supplementing the model with mathematical elements to optimize the efficacy of photo- or proton therapy for indicated combinations of nanoparticles and tumors.