IMRT has been performed for cancer patients to alleviate the adverse effects associated with increased oxidative stress and inflammation ; however, bone marrow damage still occurs in patients with multiple tumor lesions and large irradiation volumes. The present study investigated whether H2, a selective ·OH scavenger, mitigates IMRT-induced bone marrow damage in end-stage cancer patients. The results obtained demonstrated that the H2 gas treatment alleviated IMRT-induced bone marrow damage without compromising the anti-tumor effects of IMRT. To the best of our knowledge, this is the first study to report the benefits of H2 gas on IMRT in cancer patients.
Since the number of radiation courses and total exposure doses of radiation need to be selected according to the size and number of tumors in individual patients, the number of radiation courses performed in the present study ranged between a minimum of 5 (1 week) and maximum of 20 (4 weeks). Therefore, blood sampling to examine the adverse effects of radiation was performed from 1 to 4 weeks. However, the average number of radiation courses and the average total exposure doses of radiation (× 103 cm3·Gy) in the control and H2 groups were equivalent, indicating that the design of the present study allowed for the protective effects of H2 gas inhalation to be evaluated.
The present study demonstrated that the QOL, such as fatigue, depression, sleep, and gastrointestinal symptoms, was similar between the control and H2 groups. These results indicate that the inhalation of H2 gas did not improve the QOL. However, since the main purpose of the present study was to mitigate radiation-induced damage, long-term inhalation may be needed to improve the QOL. Moreover, although the effects of H2 gas inhalation were not examined in a non-radiated control group (patients not undergoing radiotherapy), based on the findings of an animal study showing that H2-rich saline did not affect hematological data in non-radiated control mice , H2 gas inhalation may not have an impact on hematological data in patients.
The radioprotective effects of H2 have been reported in different systems, including a cell-free system and various organs. In the cell-free system, Chuai et al. showed that the levels of ·OH produced by water radiolysis and the Fenton reaction were reduced by H2 solution . Moreover, Yang et al. demonstrated that a H2-rich medium pretreatment decreased ·OH levels in AHH-1 cells, a human lymphocyte cell line . On the other hand, Yang et al. also noted radiation-induced hematological changes in WBC and PLT, but not in RBC, HGB, or the mean corpuscular volume (MCV) in mice subjected to total body radiation, and found that an intraperitoneal injection of H2-rich saline significantly attenuated the depletion of WBC and PLT . They also showed that H2 reduced radiation-induced apoptosis in thymocytes and splenocytes in mice. These findings suggest that H2 reduced radiation-induced ·OH levels by directly affecting ·OH levels, and simultaneously reduced radiation-induced oxidative stress, apoptosis, and inflammation by indirect effects on ·OH. In the present study, the H2 gas treatment in combination with HCC therapy did not suppress the anti-tumor effects of IMRT in cancer patients because the response rates between the two groups were similar. This result is supported by the findings of Kang et al., who demonstrated that the consumption of H2-rich water reduced radiation-induced oxidative stress and the QOL in patients treated with radiotherapy for liver tumors without compromising anti-tumor effects . Therefore, the mechanisms underlying the radioprotective effects of H2 gas may involve not only direct effects on ·OH, but also indirect effects on ·OH via the activation of the host-mediated antioxidant and anti-inflammation systems.
In the present study, patients received the H2 gas treatment after each course of IMRT. Previous studies reported the preventive (H2 gas inhalation before radiation), but not therapeutic effects of this treatment. Furthermore, H2 selectively scavenges ·OH and ONOO− ; however, since these ROS are rapidly generated, many chemical agents generally exert stronger effects via prophylactic rather than therapeutic administration. Thus, the therapeutic administration protocol employed in the present study appears to accurately reflect the effects of H2 gas inhalation because the mechanisms underlying the radioprotective effects of H2 gas may involve not only direct, but also indirect effects on ·OH via the activation of host defense systems.
Hyperbaric oxygen therapy that involves housing patients for 1 hour in a chamber containing 100% O2 at 2 atm has been performed, and is effective for patients with decompression, peripheral circulation failure, wound dysfunction, and radiation-induced damage. In contrast, therapy using HCC is based on the principle of hyperbaric oxygen therapy, and patients are housed in a chamber containing 1.35 atm and air . In the present study, based on Henry’s law, patients in the H2 group received 1.35-fold more H2 and O2 under 1.35 atm environmental conditions, which is equivalent to patients receiving 6.8% H2 gas inhalation therapy under normal pressure. In additional experiments, the radioprotective effects of 6.5% H2 gas inhalation therapy at normal pressure (data not shown) were found to be similar, suggesting the importance of inhaling higher H2 concentrations to attenuate bone marrow damage. Although H2 gas concentrations for the detonation limit in a mixture of H2 and air are less than 4%, we recently demonstrated that the detonation limit was less than 10% in our experiment  and a literature search . Therefore, 6.5% H2 gas therapy without HCC appears to be a clinically convenient, effective, and safe method for mitigating IMRT-induced bone marrow damage.
The development of safe and more effective radioprotective agents is very important in view of their potential application during radiotherapy for cancer patients. The radioprotective effects of many synthetic and natural agents have been investigated in the past 50 years. Nutraceuticals, including vitamin C, vitamin E succinate, α-lipoic acid, and N-acetyl cystine, in addition to hematopoietic growth factors and cytokines, such as stem cell factor, granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, and interleukin 3, have been reported to exert radioprotective effects in animal models [16–20]. Amifostine, known as Ethyol or WR2721, is the only clinically accepted radioprotective agent, but is not considered to be a viable option as a radioprotective agent because of its inherent dose-limiting toxicities [21–23]. Although several drugs are in different stages of evaluation, none possess all of the requisite qualities of an optimum radioprotective agent. Thus, there are no safe and effective non-toxic radioprotective agents available for human use. Previous studies reported that H2 exerts radioprotective effects in various animal models [10, 11]. Moreover, H2 improved the QOL of patients treated with radiotherapy for liver tumors . However, there is currently no definitive therapy to improve radiation-induced bone marrow damage in cancer patients. Therefore, the present study investigated whether H2 gas mitigates IMRT-induced bone marrow damage in cancer patients, and the results obtained demonstrated that H2 gas inhalation therapy with or without HCC alleviated IMRT-induced bone marrow damage without compromising the anti-tumor effects of IMRT. Clinical studies have demonstrated that H2 has no adverse effects [7–9]. Therefore, these findings may provide the foundation for a clinically applicable, effective, and safe strategy for a H2 gas to mitigate IMRT-induced bone marrow damage.
Bone marrow damage, such as reductions in WBC (leukopenia) and PLT (thrombocytopenia), frequently occurs during cancer radiotherapy, including IMRT, and this is a limiting factor for radiotherapy [24, 25]. Since leukopenia and thrombocytopenia may cause infection and gastrointestinal bleeding, caution is required. If bone marrow damage may be attenuated by H2 gas inhalation, it will lead to the prevention of infection and gastrointestinal bleeding. Therefore, H2 gas inhalation may improve the prognosis of cancer patients.