Quantum physics helps destroy cancer cells

Cancer cell death is triggered within three days when x-rays are focused on tumor tissue containing iodine-carrying nanoparticles. Iodine releases electrons which break down tumor DNA, resulting in cell death. The findings, by scientists at the Institute for Integrated Cell-Material Sciences (iCeMS) at Kyoto University and colleagues in Japan and the United States, have been published in the journal Scientific reports.

“Exposure of a metal to light causes the release of electrons, a phenomenon called the photoelectric effect. An explanation of this phenomenon by Albert Einstein in 1905 heralded the birth of quantum physics,” explains molecular biologist iCeMS Fuyuhiko Tamanoi, who led the study. “Our research provides evidence that suggests it is possible to reproduce this effect inside cancer cells.”

A long-standing problem with cancer radiation therapy is that it is not effective in the center of tumors, where oxygen levels are low due to the lack of deeply penetrating blood vessels. X-ray irradiation needs oxygen to generate DNA-damaging reactive oxygen when the rays strike molecules inside the cell.

Tamanoi, together with Kotaro Matsumoto and his colleagues, tried to overcome this problem by finding more direct ways to damage cancer DNA. In previous work, they have shown that gadolinium-charged nanoparticles can kill cancer cells when irradiated with 50.25 kilovolts of x-ray electrons generated by the synchrotron.

In the present study, they designed porous organosilica nanoparticles that carry iodine. Iodine is cheaper than gadolinium and releases electrons at lower energy levels.

The researchers dispersed their nanoparticles through tumor spheroids, 3D tissues containing several cancer cells. Irradiation of the spheroids for 30 minutes with 33.2 keV of X-rays led to their complete destruction in three days. By systematically changing energy levels, they were able to demonstrate that the optimal tumor killing effect occurs with 33.2 keV X-rays.

Further analysis showed that the nanoparticles were taken up by tumor cells, localizing just outside their nuclei. Shining just the right amount of x-ray energy on the tissue prompted iodine to release electrons, which in turn caused double-stranded breaks in nuclear DNA, triggering cell death.

“Our study represents an important example of the use of a quantum physics phenomenon inside a cancer cell,” explains Matsumoto. “It appears that a low-energy electron cloud is generated near the DNA, causing difficult-to-repair double-stranded breaks, ultimately leading to programmed cell death.”

The team then wants to understand how electrons are released from iodine atoms when exposed to x-rays. They are also working to place iodine on DNA rather than nearby to increase efficiency, and testing the nanoparticles on mouse models of cancer.