Validation of the plasmid study to relate DNA damaging effects of radionuclides to those from external beam radiotherapy

Abstract

Introduction: The biological consequences of absorbed radiation doses are ill-defined for radiopharmaceuticals, unlike for external beam radiotherapy (EBRT). A reliable assay that assesses the biological consequences of any radionuclide is much needed. Here, we evaluated the cell-free plasmid DNA assay to determine the relative biological effects of radionuclides such as Auger electron-emitting [⁶⁷Ga]GaCl₃ or [¹¹¹In]InCl₃ compared to EBRT.

Methods: Supercoiled pBR322 plasmid DNA (1.25 or 5 ng/μL) was incubated with 0.5 or 1 MBq [⁶⁷Ga]GaCl₃ or [¹¹¹In]InCl₃ for up to 73 h or was exposed to EBRT (¹³⁷Cs; 5 Gy/min; 0-40 Gy). The induction of relaxed and linear plasmid DNA, representing single and double strand breaks, respectively, was assessed by gel electrophoresis. Chelated forms of ⁶⁷Ga were also investigated using DOTA and THP. Topological conversion rates for supercoiled-to-relaxed (k_sr^x) or relaxed-to-linear (k_rl^x) DNA were obtained by fitting a kinetic model.

Results: DNA damage increased both with EBRT dose and incubation time for [⁶⁷Ga]GaCl₃ and [¹¹¹In]InCl₃. Damage caused by [⁶⁷Ga]GaCl₃ decreased when chelated. [⁶⁷Ga]GaCl₃ proved more damaging than [¹¹¹In]InCl₃; 1.25 ng/μL DNA incubated with 0.5 MBq [⁶⁷Ga]GaCl₃ for 2 h led to a 70% decrease of intact plasmid DNA as opposed to only a 19% decrease for [¹¹¹In]InCl₃. For both EBRT and radionuclides, conversion rates were slower for 5 ng/μL than 1.25 ng/μL plasmid DNA. DNA damage caused by 1 Gy EBRT was the equivalent to damage caused by 0.5 MBq unchelated [⁶⁷Ga]GaCl₃ and [¹¹¹In]InCl₃ after 2.05 ± 0.36 and 9.3 ± 0.77 h of incubation, respectively.

Conclusions: This work has highlighted the power of the plasmid DNA assay for a rapid determination of the relative biological effects of radionuclides compared to external beam radiotherapy. It is envisaged this approach will enable the systematic assessment of imaging and therapeutic radionuclides, including Auger electron-emitters, to further inform radiopharmaceutical design and application.

Keywords: Auger electrons; Biological effects; External beam radiation; Radiobiology; Radionuclides.