Enhanced DNA Double-strand Break Induction by Carbon Ions Under Intratumoral Hypoxia

Background/Aim: Intratumoral hypoxia contributes to tumor resistance to X-rays. Carbon ions exhibit a strong antitumor effect in radioresistant tumors; however, their ability to induce DNA double-strand breaks (DSBs) under hypoxic conditions remains unclear. This study aimed to compare the ability of carbon ions and X-rays to induce DSBs under hypoxic conditions.

Materials and Methods: HeLa cells cultured in high (4.5 g/l)- or low (0.45 g/l)-glucose media under normoxic (21% O₂) or hypoxic (0.1% O₂) conditions, or HeLa mouse xenografts, were irradiated with X-rays (4 Gy) or carbon ions (4 Gy, approximately 50 keV/μm) and 10 min later subjected to immunofluorescence staining to detect 53BP1 (a DSB marker) and pimonidazole (a hypoxia marker).

Results: A modest reduction in the number of 53BP1 foci was observed post-irradiation [by approximately 10-30% (median value)] under normoxic low-glucose or hypoxic high-glucose conditions, with no significant difference between X-rays and carbon ions. By contrast, hypoxic low-glucose conditions led to a marked reduction in 53BP1 foci after X-ray irradiation (88% reduction in the median value), whereas carbon ions induced a 4.7-fold greater number of foci than X-rays (p<0.001). In xenografts, X-rays induced significantly fewer foci in pimonidazole-positive regions than in pimonidazole-negative regions (36% difference in the median value, p<0.001), whereas carbon ions induced a comparable number of foci in both regions.

Conclusion: Carbon ions are more efficient than X-rays at inducing DSBs under hypoxia both in vitro and in vivo.