Fundamental Studies of the Effects of Auger Electrons Emitted in Different Subcellular Compartments of Human Cells

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Auger electrons (AEs) are very low energy electrons (<25 keV) emitted by radionuclides (e.g. 99mTc, 111In, 67Ga) that decay by electron capture (EC). These radionuclides are used in nuclear medicine for SPECT since they also emit ?-photons. In addition, AEs are being studied for precise irradiation of cancerous cells because they have a subcellular range (nm to µm) and high linear energy transfer (LET=1-23 keV/µm) that inflicts lethal damage on sensitive cell compartments. It was assumed that AE emission in close proximity to the nucleus was required to kill cells, by inflicting DNA double-strand breaks (DSBs), but it was recently reported that AEs also kill cells by causing oxidative damage to the cell membrane. This raises the intriguing question about whether or not there are other targets for AE damage. This is an unexplored area. We propose a program of research to study the effects of AEs emitted at the cell membrane, in the cytoplasm, mitochondria, nuclear envelope or nucleus of cancerous and normal cells. These fundamental studies have important health physics implications for AE-emitting radionuclides used in nuclear medicine, and will also inform on the design of AE-emitting radiotherapeutic agents in the future. We will engineer novel "nano-shuttle" delivery vehicles to route 111In, 67Ga or 99mTc to the selected subcellular compartments. The nano-shuttles will consist of gold nanoparticles (AuNPs) modified with peptides that target cell surface avß3 integrins, and with homing peptides that route the internalized AuNPs to subcellular compartments. The AuNPs will be fluorescently-labeled to visualize their subcellular distribution and will be labeled with polymers that complex 111In, 67Ga or 99mTc. The amount of radioactivity deposited in subcellular compartments will be measured by cell fractionation and used to estimate the absorbed doses. Doses will be calculated by Monte Carlo N-Particle (MCNP6) modeling, and then correlated with the radiobiological effects of AE emission in the subcellular compartments. The radiobiological effects of AE emission will be studied by determining the clonogenic survival of the cells. The mechanism of cell death will be examined by probing the cells for cell membrane damage, disruption of mitochondrial membrane potential, micronucleus formation due to nuclear envelope damage and DNA DSBs by immunofluorescence for ?-H2AX. Our long-term aim is to develop the tools for studying the effects of short-range radiations in cells, and to understand the radiobiological and dosimetric properties of radiations in human cells. The proposed research program will provide an outstanding trans-disciplinary HQP training environment for 3 new graduate students (two PhD and one MSc) with different expertise who will work collaboratively as a team to develop and characterize the nano-shuttles, study the radiobiological effects of AEs emitted in the different subcellular compartments, and model the cellular dosimetry.