Structure-dependent reactive oxygen species generation by scintillator-free X-ray-activated porphyrins: insights into charge effects and internal conversion quantum yield

TL;DR

This study explores how the structure of porphyrin molecules affects their ability to generate reactive oxygen species under X-rays for cancer therapy without using toxic nanoparticles.

Contribution

The study identifies molecular charge and conformational flexibility as key design factors for scintillator-free radiosensitizers.

Findings

Anionic porphyrins like tetrakis (4-carboxyphenyl) porphyrin significantly enhance ROS generation under X-rays.

X-ray-mediated ROS production primarily generates superoxide and hydrogen peroxide, not singlet oxygen.

Molecular flexibility, in addition to charge, is crucial for effective scintillator-free radiosensitization.

Abstract

Radiodynamic therapy enables treatment of deep-seated tumors but typically requires scintillator nanoparticles with associated toxicity concerns. While porphyrin photosensitizers (PS) have been shown to enhance reactive oxygen species (ROS) generation under X-ray irradiation without scintillators, the molecular features governing this effect remain unclear. Here, we systematically examined structure–activity relationships of nine porphyrin derivatives to elucidate the design principles for scintillator-free radiosensitizers. Molecular charge critically influenced ROS modulation: anionic tetrakis (4-carboxyphenyl) porphyrin showed the highest efficacy with approximately 7-fold enhancement over X-ray alone, whereas cationic tetra (N-methyl-4-pyridyl) porphyrin suppressed ROS below control levels. Notably, heavy-atom coordination yielded structure-dependent effects rather than uniform enhancement, distinguishing this process from conventional photodynamic therapy. Using complementary ROS probes, we found that X-ray-mediated ROS generation produces predominantly superoxide and hydrogen peroxide rather than singlet oxygen, suggesting electron transfer rather than energy transfer as the operative pathway. Dose–response analysis demonstrated that ROS generation scales linearly with radiation dose, indicating no saturation within the therapeutic window. Validation with clinically relevant PS showed that Visudyne and protoporphyrin IX—both bearing anionic carboxylate groups—were active. However, the inactivity of anionic but rigid rose bengal suggests that electrostatic properties alone are insufficient. These findings suggest that along with molecular charge, conformational flexibility—potentially facilitating internal conversion from high-energy excited states—represents a key design parameter for scintillator-free radiosensitizers. This approach offers a simplified, biocompatible formulation strategy for deep-tissue cancer therapy without relying on heavy-metal nanoscintillators.

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