Clinical oxygen enhancement ratio of tumors in carbon ion radiotherapy: the influence of local oxygenation changes

The effect of carbon ion radiotherapy on hypoxic tumors has recently been questioned because of low linear energy transfer (LET) values in the spread-out Bragg peak (SOBP). The aim of this study was to investigate the role of hypoxia and local oxygenation changes (LOCs) in fractionated carbon ion radiotherapy. Three-dimensional tumors with hypoxic subvolumes were simulated assuming interfraction LOCs. Different fractionations were applied using a clinically relevant treatment plan with a known LET distribution. The surviving fraction was calculated, taking oxygen tension, dose and LET into account, using the repairable–conditionally repairable (RCR) damage model with parameters for human salivary gland tumor cells. The clinical oxygen enhancement ratio (OER) was defined as the ratio of doses required for a tumor control probability of 50% for hypoxic and well-oxygenated tumors. The resulting OER was well above unity for all fractionations. For the hypoxic tumor, the tumor control probability was considerably higher if LOCs were assumed, rather than static oxygenation. The beneficial effect of LOCs increased with the number of fractions. However, for very low fraction doses, the improvement related to LOCs did not compensate for the increase in total dose required for tumor control. In conclusion, our results suggest that hypoxia can influence the outcome of carbon ion radiotherapy because of the non-negligible oxygen effect at the low LETs in the SOBP. However, if LOCs occur, a relatively high level of tumor control probability is achievable with a large range of fractionation schedules for tumors with hypoxic subvolumes, but both hyperfractionation and hypofractionation should be pursued with caution.     

The variation of OER with dose rate

The oxygen enhancement ratio (OER) has been measured as a function of dose rate from 276 Gy/hr to 0.89 Gy/hr for V-79 cells irradiated at 23 degrees C or 37 degrees C. As dose rate is decreased, the OER initially increases, from a value of 3.0, to a maximum value of 3.7 to 4.0, at a dose rate between 20 and 60 Gy/hr. The OER subsequently decreases with further dose rate reduction to a minimum value of 2.4 at the lowest dose rate. Similar experiments conducted with cells in nutrient deprived conditions exhibited a monotonic decrease in OER from 3.0 to 1.7 with dose rate reduction. These experimental findings can be understood in terms of the sublethal damage repair capability of cells under different pO2 and nutrient conditions.     

High-performance AEM unitized regenerative fuel cell using Pt-pyrochlore as bifunctional oxygen electrocatalyst

The performance of fixed-gas unitized regenerative fuel cells (FG-URFCs) are limited by the bifunctional activity of the oxygen electrocatalyst. It is essential to have a good bifunctional oxygen electrocatalyst which can exhibit high activity for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). In this regard, Pt-Pb₂Ru₂O_7-x is synthesized by depositing Pt on Pb₂Ru₂O_7-x wherein Pt individually exhibits high ORR while Pb₂Ru₂O_7-x shows high OER and moderate ORR activity. Pt-Pb₂Ru₂O_7-x exhibits higher OER (η_@10mAcm-2 = 0.25 ± 0.01 V) and ORR (η_@-3mAcm-2 = -0.31 ± 0.02 V) activity in comparison to benchmark OER (IrO₂, η_@10mAcm-2 = 0.35 ± 0.02 V) and ORR (Pt/C, η_@-3mAcm-2 = -0.33 ± 0.02 V) electrocatalysts, respectively. Pt-Pb₂Ru₂O_7-x shows a lower bifunctionality index (η_{@10mAcm-2, OER} ⁻ η_{@-3mAcm-2, ORR}) of 0.56 V with more symmetric OER–ORR activity profile than both Pt (>1.0 V) and Pb₂Ru₂O_7-x (0.69 V) making it more useful for the AEM (anion exchange membrane) URFC or metal-air battery applications. FG-URFC tested with Pt-Pb₂Ru₂O_7-x and Pt/C as bifunctional oxygen electrocatalyst and bifunctional hydrogen electrocatalyst, respectively, yields a mass-specific current density of 715 ± 11 A/g_cat^-1 at 1.8 V and 56 ± 2 A/g_cat^-1 at 0.9 V under electrolyzer mode and fuel-cell mode, respectively. The FG-URFC shows a round-trip efficiency of 75% at 0.1 A/cm⁻², underlying improvement in AEM FG-URFC electrocatalyst design.

Energy storage has gained increased attention for flexible electrical grid operation as conventional constant and variable-energy sources converge on the electrical grid. In this context, hydrogen has emerged as a clean energy carrier/source (energy density: 120 to 142 MJ/kg), which is produced via water splitting in an electrolyzer using external power source. The generated hydrogen is used as a fuel in a fuel cell (FC) to convert chemical energy into electrical energy. Combining FC and electrolyzer in a single device, known as a regenerative FC (RFC), offers certain advantages over conventional rechargeable batteries such as fast start-up/shut down, low self-discharge, low environmental effect, high energy density, and long duration/lifetime. To date, RFCs have been extensively used for unmanned underwater vehicles, high-altitude, long-duration aircraft, off-grid power storage, and emergency power generators (1). Unitized RFCs (URFCs) are the modification of RFCs, which offer the same benefits as RFCs with less weight, less volume, and less capital cost, as a single cell (FC + electrolyzer) in URFC does not require auxiliary equipment for an additional cell stack used in the RFCs (2–4). Despite the wide application of proton exchange membrane–URFCs (PEM-URFCs) for their high power density, anion exchange membrane (AEM)–URFCs have gained attention due to their cost-effectiveness as use of costly noble metals with high loading can be avoided (3, 4). Typically the URFCs have two different configurations: 1) fixed-gas (FG-URFC) in which oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) happen at one electrode (oxygen electrode) whereas hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) occur on the other electrode (hydrogen electrode) under water-electrolyzer (WE) and FC mode, respectively, and 2) fixed-polarity (FP-URFC) in which OER (WE mode) and HOR (FC mode) occur at the same electrode, whereas HER (WE mode) and ORR (FC mode) occur at the other (5). The FG-URFCs offer the advantage of efficient and easy gas management over FP-URFCs. However, FG-URFCs suffer from sluggish oxygen electrode reactions (ORR during FC mode and OER during WE mode) on the same side of the cell resulting in a convergence of inefficiencies which act as a bottleneck for the wide application of AEM FG-URFCs. Therefore, the development of highly active ORR/OER bifunctional electrocatalyst is necessary for AEM FG-URFCs. An ideal bifunctional oxygen electrocatalyst should show converging OER and ORR onset potential toward an equilibrium potential of 1.23 V versus RHE with a low bifunctionality index (BI = difference between OER potential at 10 mA/cm²_geo and ORR potential at −3.0 mA/cm²_geo). However, benchmark OER (RuO₂ and IrO₂) and ORR (Pt) electrocatalyst exhibit a BI greater than 1.0 V making them unsuitable individually for use in URFC (6, 7). Hence, they are used along with other electrocatalyst which can reduce their asymmetric electrocatalytic behavior [e.g., Pt-IrO2-(RuO₂-TiO₂ {RTO}) (8), Pt-IrO₂ (9, 10), and Pt-Ru-Ir (11)]. The use of AEM-URFC enables expansion of the material space of electrocatalysts as a wide spectrum of materials is stable in alkaline medium.In this context, an OER electrocatalyst which can also act as a support, in lieu of benchmark ORR electrocatalyst Pt, could be a good candidate as a bifunctional oxygen electrocatalyst. The support-material for Pt plays a critical role as it should provide high surface area, high OER activity, high OER–ORR stability, high electronic charge transport, efficient catalyst dispersion, and help in facet engineering (12–15). The most commonly used catalyst supports are Vulcan XC-72 (16), TiC (17, 18), RTO (8, 12, 19), Sb-doped SnO₂ (ATO) (20), doped-TiO₂ (21, 22), metal (13, 14), and Ti_nO_2n-1 (23), which offer moderate-to-high conductivity and moderate surface area but offer no OER activity of note, thereby effectively ruling them out. In this regard, use of alkaline OER-active and OER/ORR-stable electrocatalyst as a support for Pt could be a solution. Among them, lead ruthenate pyrochlore (Pb₂Ru₂O_7-x) has shown excellent OER activity–stability (24–26) and moderate ORR activity–stability in alkaline medium (24).In this perspective, we have deposited Pt on Pb₂Ru₂O_7-x, which shows a very low BI with highly symmetric OER–ORR activity profile, making it useful for the alkaline-URFC as well as metal-air battery applications. Pt-Pb₂Ru₂O_7-x exhibits higher OER and ORR activity in comparison to IrO₂ and Pt/C, respectively. The high OER activity is ascribed to a high Ru(V): Ru(IV) ratio in Pt-Pb₂Ru₂O_7-x, which is confirmed through X-ray photoelectron spectroscopy (XPS) study. The high ORR activity of Pt-Pb₂Ru₂O_7-x is attributed to the high dispersion of Pt on Pb₂Ru₂O_7-x support. An FG-URFC tested with Pt-Pb₂Ru₂O_7-x and Pt/C as bifunctional oxygen electrocatalyst and bifunctional hydrogen electrocatalyst, respectively, yields a mass-specific current density of 715 ± 11 A/g⁻¹ at 1.8 V and 56 ± 2 A/g_cat^-1 at 0.9 V under WE mode and FC mode, respectively. The FG-URFC shows a round-trip efficiency (RTE) of 75% at 0.1 A/cm⁻², which is the highest-reported RTE in an AEM FG-URFC to our knowledge, thereby signifying the usefulness of Pt-Pb₂Ru₂O_7-x as OER/ORR bifunctional electrocatalyst for future energy and fuel production applications.     

Radiation-induced Lipid Peroxidation: Influence of Oxygen Concentration and Membrane Lipid Composition

Summary

Radiation-induced lipid peroxidation in phospholipid liposomes was investigated in terms of its dependence on lipid composition and oxygen concentration. Non-peroxidizable lipid incorporated in the liposomes reduced the rate of peroxidation of the peroxidizable phospholipid acyl chains, possibly by restricting the length of chain reactions. The latter effect is believed to be caused by interference of the non-peroxidizable lipids in the bilayer. At low oxygen concentration lipid peroxidation was reduced. The cause of this limited peroxidation may be a reduced number of radical initiation reactions possibly involving oxygen-derived superoxide radicals. Killing of proliferating mammalian cells, irradiated at oxygen concentrations ranging from 0 to 100 per cent, appeared to be independent of the concentration of peroxidizable phospholipids in the cell membranes. This indicates that lipid peroxidation is not the determining process in radiation-induced reproductive cell death.     

Relative Biological Effectiveness and Oxygen Enhancement Ratio of 50 Mv X-Rays

Experiments in vitro, using V-79 Chinese hamster fibroblasts, were carried out in order to establish whether or not there was a difference between the relative biological efficiency (RBE) and/or the oxygen enhancement ratio (OER) of 50 and 4 MV bremsstrahlung photons. Dosimetry was performed with both Fricke dosimetry and ionization chamber. Cells were irradiated in an all glassmetal system under both oxic and anoxic conditions and subsequently plated for cloning assay. Dose corrections were made for the backscatter from the glass to which the cells were attached during irradiation. A statistically significant difference between RBE of the two energies was found. the RBE was estimated to be 1.1. for 50 MV photons. No difference in OER was found. This RBE difference is of clinical interest as it is generally accepted that it should be possible to determine the dose level with a precision of about $5% in radiation treatment.     

Radiosensitization of hypoxic cells in vivo by SR 2508 at low radiation doses: A preliminary report

We have used the RIF-1 tumor implanted intradermally in the lower dorsum of C3H mice to explore to what extent the radiosensitizer SR 2508 is capable of sensitizing hypoxic cells at clinically relevant doses of 1 and 2 Gy per fraction. We injected SR 2508 (1000 mg/kg) 45 min prior to each radiation dose in fractionated regimens of 2 or 4 doses/day for up to 5 days (1 or 2 Gy/fraction) given locally to the tumors, which were clamped to occlude the blood supply prior to each radiation exposure. This necessitated the design of clamps which (a) totally occluded blood flow (b) could be applied to nonanesthetized mice without obvious discomfort, and (c) could be applied up to 20 times without compromising the tumor blood supply on removal of the clamps. We have performed various experiments which confirm the validity of these 3 requirements. The response of the tumor cells with and without clamping and with and without SR 2508 was determined by constructing multifraction cell survival curves using the in vivo-in vitro assay. The initial results demonstrate significant radiosensitization of artificially hypoxic tumor cells at 1 and 2 Gy/fraction by SR 2508 (1000 mg/kg). Using the ratio of the D₀'s of the exponential, multifraction survival curves, we obtained an SER for SR 2508 of 1.6 (3 experiments pooled) compared to an OER(D₀ clampled/D₀ air-breathing mice) of 2.3 (4 experiments pooled). These data suggest that SR 2508 (and presumably other electron-affinic sensitizers) can radiosensitize hypoxic cells at low radiation doses, and indicate that this and similar drugs may be useful in the radiotherapy of those tumors for which hypoxis limits curability.     

Study of acute 60Co, low dose rate CF-252 and CS-137 radiation on LSA ascites lymphoma in vivo

Dose response curves were determined for the LSA lymphoma for acute 60Co, low dose rate Cs-137 and Cf-252 radiations using in vivo survival time bioassay. Mean survival times increased with dose with a prominent oxygen effect noted for acute 60Co and Cs-137. OER was lowest for Cf-252 where it was approximately 1.4. The RBEn for oxic LSA cells to Cf-252 neutrons was 3.1 for acute 60Co and 4.2 for Cs-137. It was larger for hypoxic tumor and RBE was 5.3 for 60Co and 5.8 for Cs-137. Survival curves based on survival data used a multitarget dose-response model for photon radiation and exponential dose-response for Cf-252 radiation. When LSA was irradiated in advanced tumor stages in vivo, Cf-252 was much more effective than acute 60Co or LDR Cs-137 for increasing survival time. Tumor response in vivo matched the in vitro irradiated tumor data. No schedule dependence was observed for mixing of 60Co and Cf-252 radiations.     

Effects of D,L-buthionine-S,R-sulfoximine on cellular thiol levels and the oxygen effect in Chinese hamster V79 cells

The role of glutathione (GSH) and total non-protein thiols (NPSH) in repairing radiation-induced free radical damage incurred under aerated and hypoxic conditions was investigated using Chinese hamster V79 cells cultured in vitro. GSH and NPSH levels were depleted in V79 cells of varying cell densities using the gamma-glutamyl-cysteine-synthetase inhibitor, D,L-Buthionine-S,R-sulfoximine (BSO). A small change in hypoxic cell radiosensitivity could be attributed to the loss of GSH while depletion of thiols to lower levels affected both aerated and hypoxic cell radiosensitivity, resulting in no change in the OER. Only a long term incubation with BSO produced a large change in the OER, by which time many other biochemical pathways using GSH and amino acids are likely to be affected.     

Robust wrinkled MoS2/N-C bifunctional electrocatalysts interfaced with single Fe atoms for wearable zinc-air batteries

The ability to create highly efficient and stable bifunctional electrocatalysts, capable of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the same electrolyte, represents an important endeavor toward high-performance zinc-air batteries (ZABs). Herein, we report a facile strategy for crafting wrinkled MoS₂/N-doped carbon core/shell nanospheres interfaced with single Fe atoms (denoted MoS₂@Fe-N-C) as superior ORR/OER bifunctional electrocatalysts for robust wearable ZABs with a high capacity and outstanding cycling stability. Specifically, the highly crumpled MoS₂ nanosphere core is wrapped with a layer of single-Fe-atom-impregnated, N-doped carbon shell (i.e., Fe-N-C shell with well-dispersed FeN₄ sites). Intriguingly, MoS₂@Fe-N-C nanospheres manifest an ORR half-wave potential of 0.84 V and an OER overpotential of 360 mV at 10 mA⋅cm⁻². More importantly, density functional theory calculations reveal the lowered energy barriers for both ORR and OER, accounting for marked enhanced catalytic performance of MoS₂@Fe-N-C nanospheres. Remarkably, wearable ZABs assembled by capitalizing on MoS₂@Fe-N-C nanospheres as an air electrode with an ultralow area loading (i.e., 0.25 mg⋅cm⁻²) display excellent stability against deformation, high special capacity (i.e., 442 mAh⋅g⁻¹_Zn), excellent power density (i.e., 78 mW⋅cm⁻²) and attractive cycling stability (e.g., 50 cycles at current density of 5 mA⋅cm⁻²). This study provides a platform to rationally design single-atom-interfaced core/shell bifunctional electrocatalysts for efficient metal-air batteries.

Metal-air batteries represent a class of promising energy storage devices composed of a metal negative electrode electrochemically coupled to an air-breathing positive electrode through a suitable electrolyte (1). Among them, aqueous zinc-air batteries (ZABs) are widely recognized as one of the most promising devices due to their high theoretical energy density (1,086 Wh⋅kg⁻¹), low cost (<$10 kW⁻¹⋅h⁻¹) and inherent safety (2). Despite these advantageous attributes, the commercialization of rechargeable ZABs is plagued by their limited energy density and poor cycle life due to the inefficiency of air catalysts. In rechargeable ZABs, the oxygen reduction reaction (ORR) and oxygen evolution reactions (OER) take place at the air electrode in discharging and charging processes, respectively. Thus, the overall energy efficiency of ZABs is dictated by ORR/OER at the air electrode, which involves multiple proton-coupled electron transfers that are sluggish in nature, thereby resulting in small current density and large electrode polarization of ZABs (3). Despite the prominent electrocatalytic activity of Pt-based metals and Ir- and Ru-based metals toward ORR and OER, respectively, the scarce abundance, high cost, poor physical stability, and insufficient bifunctionaility hinder their large-scale use in sustainable energy devices (4). Clearly, the ability to develop inexpensive bifunctional electrocatalysts with high kinetics and long durability is the key to their utility for constructing efficient and stable ZABs.Two-dimensional transition-metal dichalcogenides (e.g., MoS₂, WS₂, and MoSe₂) have garnered much attention in the context of catalysis due to their reduced dimensionality and a set of intriguing chemical properties (e.g., high catalytic activity, outstanding chemical stability, etc.) (5, 6). Particularly, MoS₂ has been extensively studied as a unique electrocatalyst for hydrogen evolution reaction (HER) (7). It is notable that S-vacancies and sulfur-terminated edges of MoS₂ flakes not only activate HER but also accelerate OER (8, 9). Intriguingly, by creating MoS₂-containing heterojunctions (e.g., MoS₂/WS₂, MoS₂/Ni₃S₂, etc.) (10–12), the resulting composites manifest the enhanced OER activity over the pristine MoS₂. Nonetheless, due to direct exposure to alkaline electrolytes, MoS₂-based composites may be severely oxidized during the OER process, leading to the rapid decay in the activity (13). In addition, regardless of high OER activity, these composites still suffer from sluggish ORR kinetics, thereby limiting their use in ZABs. In sharp contrast, nitrogen-coordinated transition-metal atoms–anchored carbon nanomaterials (denoted M–N–C), in particular Fe–N–C, have emerged as a class of appealing ORR electrocatalysts owing to their earth abundance, tunable surface chemistry, modified electronic structure, and optimal oxygen absorption (14). Generally, downsizing active species of Fe–N–C catalysts to the single-atom scale could promote maximum atom-utilization efficiency via fully exposing the active sites, thereby greatly enhancing the intrinsic nature of catalysts (15, 16). Furthermore, the nitrogen-doped carbon matrix could not only firmly stabilize the highly energetic single atoms through the metal-nitrogen interaction to mitigate the aggregation of metal atoms but also effectively facilitate the transport of ORR-relevant species (i.e., O*, OH*, OOH*, and O₂*) during the electrocatalytic process (17, 18). In this context, the capability of creating MoS₂/Fe-N-C heterostructures composed of atomic Fe catalysts positioned at the interface may enable the construction of electrocatalysts with enhanced bifunctional ORR/OER activities. This, however, has yet to be largely explored.Herein, we report a general and robust route to crafting highly crumpled nanospheres composed of a MoS₂ core blanketed by a single-metal-atom-coordinated, N-doped carbon shell (i.e., MoS₂@M-N-C; M = Fe, Co, Ni) that function as stable ORR/OER bifunctional electrocatalysts for wearable, high-capacity, and outstanding-cycling-stability ZABs. Taking MoS₂@Fe-N-C as an example, one Fe single atom coordinates with four N atoms into a Fe-N₄ site at the MoS₂/Fe-N-C interface, as corroborated by X-ray absorption fine structure study. The Fe-N₄ sites are supported by carbon matrix as Fe-N-C shell, which drapes on the surface of MoS₂ nanospheres, forming MoS₂@Fe-N-C nanospheres that possess ample MoS₂/Fe-N-C interface. Notably, the spherical structure of MoS₂@Fe-N-C nanospheres could greatly prevent the aggregation of active sites, exhibiting sufficient electrochemical stability. Moreover, density functional theory (DFT) calculations signify that MoS₂/Fe-N-C interface could lower the reaction barriers of ORR and OER. Consequently, MoS₂@Fe-N-C nanospheres deliver superior bifunctional catalytic activity with a reversible oxygen overpotential of 0.86 V and long-term durability in the alkaline solution. More importantly, the Fe-N-C shell not only accelerates the electrocatalytic activity of MoS₂ nanospheres but also minimizes the corrosion in alkaline electrolyte. Finally, MoS₂@Fe-N-C nanospheres are employed as an air cathode in a flexible zinc-air battery, displaying a high power density of 78 mW⋅cm⁻², an excellent special capacity of 442 mAh⋅g⁻¹_Zn, and an outstanding stability with an ultralow area loading of 0.25 mg⋅cm⁻². As such, creating MoS₂@N-doped carbon nanospheres containing metal single atoms constrained at the MoS₂/N-doped carbon interface represents a facile strategy for constructing active and durable ORR/OER bifunctional electrocatalysts and in turn high-performance air electrodes for ZABs.     

Chemical Competition in Target Radical Reactions: Numerical Simulation of the Theory and Comparison with Measured Oxygen Effect on DNA Damage in Cells

Summary

An intracellular radiation-chemical reaction scheme is tested in which solute and solvent radicals R· react with non-target molecules Sa (scavengers) or with target molecules (presumed to be DNA) to produce target radicals T·, which may also be produced by direct ionization of DNA. The rate of target radical decomposition to become ‘uncommitted damage’ that the cell may repair is affected by the concentration of oxygen (O₂), thiols (S) and electronaffinic sensitizers (F), which compete with one another to form, respectively, target products TO₂, TS and TF. This uncommitted damage is then subject to biochemical modification, including molecular repair, by the cell. The rate equations for this competing reaction scheme were written and programmed for computer simulations of changes in oxygen, thiol and electronaffinic sensitizer concentrations. A reaction scheme that also includes some non-radical target damage was also simulated. Simulations were made using available experimental data concerning intranuclear concentrations and reaction rate constants, respectively, k_o, k_s and k₁ for the reactions T· + O₂→TO₂, T· + S→TS and T· + F→TF, which produce uncommitted chemical damage. Experimental data on strand-break induction in glutathione-proficient and glutathione-deficient cells, in cells treated with thiol active agents, and in cells treated with hypoxic sensitizers, along with the computer simulations, generally agree that thiol molecules can react with target radicals to reverse T· in competition with O₂ and/or electronaffinic sensitizers.

Forward reaction rate constants k_o, k_s (dithiothreitol), k_s (glutathione) and k₁ (misonidazole) in the approximate ratio 10 : 0·3 : 0·02 : 0·4 satisfied the above reaction scheme, and approximately 5 per cent non-radical target molecule damage could be included with satisfactory agreement with experimental data.