Oxygen Fixation Research Articles

CTNI-30. A PHASE 2 DOUBLE-BLIND, RANDOMIZED, PROSPECTIVE, PLACEBO CONTROLLED STUDY OF NANO₂TM COMBINED WITH RADIATION AND TEMOZOLOMIDE IN PATIENTS WITH NEWLY-DIAGNOSED GLIOBLASTOMA: RESTORE

Abstract INTRODUCTION Glioblastoma (GBM) commonly has extensive hypoxic regions, possibly due to thrombosis from tumor-produced pro-coagulant factors. Radiation-induced cell death relies on oxygen free radicals and oxygen fixation of DNA-strand breaks. In the absence of oxygen tumor cells can repair the DNA strand breaks leading to treatment resistance. Hypoxic cells also exhibit chemoresistance due to decreased drug efficacy, limited drug diffusion, and poor delivery to distant cells. This study hypothesizes that increasing oxygen delivery to hypoxic GBM tissue using NanO₂ (2% w/v dodecafluoropentane emulsion) infusion will enhance the effectiveness of radiation therapy (RT) and chemotherapy (CT). METHODS Patients diagnosed with GBM per CNS5-WHO criteria receive standard chemoradiation: 30 fractions of focal RT with 60 Gy, delivered as 2 Gy fractions five days per week for six weeks with concurrent oral TMZ at 75 mg/m²/day for six weeks. Patients are randomized in a 1:2 ratio to receive either a placebo (0.9% Sodium Chloride) or NanO₂ infusion administered immediately prior to each RT dose. Patients continuously breathe oxygen during infusion and RT. Following six weeks of RT with temozolomide, patients have a four-week break before starting adjuvant temozolomide. RESULTS Ten patients have been enrolled in the RESTORE trial. Seven continue to be followed through maintenance treatment and long term follow up. There have been 23 non-hematologic adverse events described as CTCAE severity grade 3 and above. Grade 4, hematologic toxicity occurred in two patients, with one patient experiencing prolonged pancytopenia necessitating hematologic support and discontinuation of temozolomide. Two patients discontinued therapy due to early tumor progression. CONCLUSION This study is ongoing, assessing the safety and potential efficacy of NanO₂ in improving treatment outcomes for GBM by enhancing oxygen delivery to hypoxic tumor regions.

In-situ FTIR spectroscopy of epoxy resin degradation: kinetics and mechanisms.

We report on an in situ FTIR study of the thermo-oxidative degradation of a flexible epoxy resin. Different and complementary approaches to the analysis of the spectral data were employed, providing a detailed description of the process in terms of kinetics and mechanisms. A preliminary normal coordinate analysis, based on the DFT method, allowed for a reliable interpretation of the observed spectrum, increasing the amount of available structural information. Two-dimensional correlation spectroscopy provided details on the evolution of the reacting network structure. The relative stability of the various functional groups was ranked, and the most likely sites of initiation were identified. Oxygen fixation on the network chains produced amide and ketone groups, with the latter developing at a higher rate. The kinetic profiles of various functional groups were accurately simulated by a first-order, biexponential model, which allowed a quantitative comparison among their relative stabilities. The spectroscopic analysis allowed us to propose likely mechanisms and to identify those that occur preferentially.

Modeling the oxygen effect in DNA strand break induced by gamma-rays with TOPAS-nBio

Objective.To present and validate a method to simulate from first principles the effect of oxygen on radiation-induced double-strand breaks (DSBs) using the Monte Carlo Track-structure code TOPAS-nBio.Approach.Two chemical models based on the oxygen fixation hypothesis (OFH) were developed in TOPAS-nBio by considering an oxygen adduct state of DNA and creating a competition kinetic mechanism between oxygen and the radioprotective molecule WR-1065. We named these models 'simple' and 'detailed' due to the way they handle the hydrogen abstraction pathways. We used the simple model to obtain additional information for the •OH-DNA hydrogen abstraction pathway probability for the detailed model. These models were calibrated and compared with published experimental data of linear and supercoiling fractions obtained with R6K plasmids, suspended in dioxane as a hydroxyl scavenger, and irradiated with137Cs gamma-rays. The reaction rates for WR-1065 and O2with DNA were taken from experimental works. Single-Strand Breaks (SSBs) and DSBs as a function of the dose for a range of oxygen concentrations [O2] (0.021%-21%) were obtained. Finally, the hypoxia reduction factor (HRF) was obtained from DSBs.Main Results.Validation results followed the trend of the experimental within 12% for the supercoiled and linear plasmid fractions for both models. The HRF agreed with measurements obtained with137Cs and 200-280 kVp x-ray within experimental uncertainties. However, the HRF at an oxygen concentration of 2.1% overestimated experimental results by a factor of 1.7 ± 0.1. Increasing the concentration of WR-1065 from 1 mM to 10-100 mM resulted in a HRF difference of 0.01, within the 8% statistical uncertainty between TOPAS-nBio and experimental data. This highlights the possibility of using these chemical models to recreate experimental HRF results.Significance.Results support the OFH as a leading cause of oxygen radio-sensitization effects given a competition between oxygen and chemical DNA repair molecules like WR-1065.

Enhancing thermal conductivity of AlN ceramics via vat photopolymerization through refractive index coupling and oxygen fixation

Despite the growing development of ceramic fabrication by vat photopolymerization (VP), major gaps remain in application. Particularly in the case of VP-printed aluminum nitride (AlN) ceramic, the thermal conductivity is still below 170 W·m−1·K−1, a critical benchmark for efficient heat dissipation. To address this challenge, here we prepared an AlN slurry with high curing thickness through RI coupling between liquid-solid phase, and took into account of the rheological property under high solid loading. Full dense AlN green bodies with solid loading up to 50 vol% were successfully printed. Aiming to to tackle the degradation of thermal conductivity issue caused by oxygen increment of AlN via VP, we systemically studied the form of oxygen in AlN preparation processes. Through the sintering optimization, the oxygen element from the hydrolysis of AlN surface was fixated in the sintering aid Y2O3. Eventually, without any special process control or additional treatment, the final sintered AlN ceramics prepared by the developed slurry present a full densification and the highest thermal conductivity (up to 187.9 W∙m−1∙K−1) of any known additive manufactured ceramics.

Electron Aggregation and Oxygen Fixation Reinforced Microwave Dynamic and Thermal Therapy for Effective Treatment of MRSA-Induced Osteomyelitis.

Antibiotics are frequently used to clinically treat osteomyelitis caused by bacterial infections. However, extended antibiotic use may result in drug resistance, which can be life threatening. Here, a heterojunction comprising Fe2O3/Fe3S4 magnetic composite is constructed to achieve short-term and efficient treat osteomyelitis caused by methicillin-resistant Staphylococcus aureus (MRSA). The Fe2O3/Fe3S4 composite exhibits powerful microwave (MW) absorption properties, thereby effectively converting incident electromagnetic energy into thermal energy. Density functional theory calculations demonstrate that Fe2O3/Fe3S4 possesses significant charge accumulation and oxygen-fixing capacity at the heterogeneous interface, which provides more active sites and oxygen sources for trapping electromagnetic hotspots. The finite element analysis indicates that Fe2O3/Fe3S4 displays a larger electromagnetism field enhancement parameter than Fe2O3 owing to a significant increase in electromagnetic hotspots. These hotspots contribute to charge differential accumulation and depletion motions at the interface, thereby augmenting the release of free electrons that subsequently combine with the oxygen adsorbed by Fe2O3/Fe3S4 to generate reactive oxygen species (ROS) and heat. This research, which achieves extraordinary bacterial eradication through the synergistic effect of microwave thermal therapy (MWTT) and microwave dynamic therapy (MDT), presents a novel strategy for treating deep-tissue bacterial infections.

Influence of some abiotic parameters on the biodiversity of macroarthropods in the "lep-liye" forest stream

A study was carried in the locality of Ngog-mapubi. The objectif was to evaluate the spatio- temporal dynamics of macro-arthropods in relation to seasonal and physico-chemical quality of forest stream. Benthic macro-arthropods were sampled during the short dry season (SDS) and the long rainy season (LRS). Physico-chemical parameters such as temperature, oxygen and carbon dioxide fixation, oxydability of oxygen, carbon dioxide, suspended solids, nitrate, pH were measured and analyzed follow standard methods. A sampling net was used to collect macro-arthropods at (Lep1) up-stream, (Lep 2 and Lep3) med-stream and (Lep 4) down-stream. 313 macro-arthropods predominated by Hexapoda (89%) and Malacostraca (11%) were collected and mainly identified as: Odonata (43.13%), Hemiptera (15.34%), Coleoptera (13.74%), followed by Décapoda (11%), Trichoptera (8.3%), then Ephemeroptera (7.33%) and Diptera (0.98%). Concerning biocenotic analysis, the diversity and abundance index were higher during the LRS, Shannon index showed significant difference between the two seasons, while from a spatial point of view, the Mann-Whitney U-test revealed a significant difference between the coarse substrate stations compared to those with fine substrate stations.

Mechanistic modelling of oxygen enhancement ratio of radiation via Monte Carlo simulation-based DNA damage calculation

Objective. Oxygen plays an important role in affecting the cellular radio-sensitivity to ionizing radiation. The objective of this study is to build a mechanistic model to compute oxygen enhancement ratio (OER) using a GPU-based Monte Carlo (MC) simulation package gMicroMC for microscopic radiation transport simulation and DNA damage calculation. Approach. We first simulated the water radiolysis process in the presence of DNA and oxygen for 1 ns and recorded the produced DNA damages. In this process, chemical reactions among oxygen, water radiolysis free radicals and DNA molecules were considered. We then applied a probabilistic approach to model the reactions between oxygen and indirect DNA damages for a maximal reaction time of t 0. Finally, we defined two parameters P 0 and P 1, representing probabilities for DNA damages without and with oxygen fixation effect not being restored in the repair process, to compute the final DNA double strand breaks (DSBs). As cell survival fraction is mainly determined by the number of DSBs, we assumed that the same numbers of DSBs resulted in the same cell survival rates, which enabled us to compute the OER as the ratio of doses producing the same number of DSBs without and with oxygen. We determined the three parameters (t 0, P 0 and P 1) by fitting the OERs obtained in our computation to a set of published experimental data under x-ray irradiation. We then validated the model by performing OER studies under proton irradiation and studied model sensitivity to parameter values. Main results. We obtained the model parameters as t 0 = 3.8 ms, P 0 = 0.08, and P 1 = 0.28 with a mean difference of 3.8% between the OERs computed by our model and that obtained from experimental measurements under x-ray irradiation. Applying the established model to proton irradiation, we obtained OERs as functions of oxygen concentration, LET, and dose values, which generally agreed with published experimental data. The parameter sensitivity analysis revealed that the absolute magnitude of the OER curve relied on the values of P 0 and P 1, while the curve was subject to a horizontal shift when adjusting t 0. Significance. This study developed a mechanistic model that fully relies on microscopic MC simulations to compute OER.

Using oxygen dose histograms to quantify voxelised ultra-high dose rate (FLASH) effects in multiple radiation modalities

Purpose. To introduce a methodology to predict tissue sparing effects in pulsed ultra-high dose rate radiation exposures which could be included in a dose-effect prediction system or treatment planning system and to illustrate it by using three published experiments. Methods and materials. The proposed system formalises the variability of oxygen levels as an oxygen dose histogram (ODH), which provides an instantaneous oxygen level at a delivered dose. The histogram concept alleviates the need for a mechanistic approach. At each given oxygen level the oxygen fixation concept is used to calculate the change in DNA-damage induction compared to the fully hypoxic case. Using the ODH concept it is possible to estimate the effect even in the case of multiple pulses, partial oxygen depletion, and spatial oxygen depletion. The system is illustrated by applying it to the seminal results by Town (Nat. 1967) on cell cultures and the pre-clinical experiment on cognitive effects by Montay-Gruel et al (2017 Radiother. Oncol. 124 365–9). Results. The proposed system predicts that a possible FLASH-effect depends on the initial oxygenation level in tissue, the total dose delivered, pulse length and pulse repetition rate. The magnitude of the FLASH-effect is the result of a redundant system, in that it will have the same specific value for a different combination of these dependencies. The cell culture data are well represented, while a correlation between the pre-clinical experiments and the calculated values is highly significant (p < 0.01). Conclusions. A system based only on oxygen related effects is able to quantify most of the effects currently observed in FLASH-radiation.

A Modified Lethal and Potentially Lethal Model With Explicit Oxygen Tension Dependence for FLASH RT

To modify Curtis Lethal and Potentially Lethal (LPL) model to extend it from conventional (CONV) to ultra-high dose rate (FLASH) Radiotherapy (RT) domain with explicit intracellular oxygen tension dependence.After combining radiation-induced oxygen depletion, oxygen fixation hypothesis, and the Oxygen Enhancement Ratio (OER) concept, we incorporate intracellular oxygen tension dependence into the coefficients of the ordinary differential equations describing the original Curtis LPL model. Two biochemical pre-lesion states for the LPL lesions are amended into the model to explain the oxygen post-effect on the cell radiation survival fraction (SF). We perform analytical and numerical analyses for the solutions of the average numbers of LPL lesions. Using Poisson statistics, we can use the average number of LPL lesions to predict the cell SF for FLASH RT.Our modified LPL model maintains all original LPL model characteristics at fixed intracellular oxygen tension. For variable intracellular oxygen tensions, our new model extends prediction capabilities to FLASH RT. The comparisons of the two models are summarized below: CONCLUSION: Our modified LPL model has the potential to explain the observed FLASH phenomena, i.e., FLASH RT provides better normal tissue sparing while maintaining similar tumor control compared with CONV RT. Future studies are necessary to quantitatively validate our model predictions in systematically conducted FLASH irradiation experiments.

In silico simulation of the effect of hypoxia on MCF-7 cell cycle kinetics under fractionated radiotherapy.

The treatment outcome of a given fractionated radiotherapy scheme is affected by oxygen tension and cell cycle kinetics of the tumor population. Numerous experimental studies have supported the variability of radiosensitivity with cell cycle phase. Oxygen modulates the radiosensitivity through hypoxia-inducible factor (HIF) stabilization and oxygen fixation hypothesis (OFH) mechanism. In this study, an existing mathematical model describing cell cycle kinetics was modified to include the oxygen-dependent G1/S transition rate and radiation inactivation rate. The radiation inactivation rate used was derived from the linear-quadratic (LQ) model with dependence on oxygen enhancement ratio (OER), while the oxygen-dependent correction for the G1/S phase transition was obtained from numerically solving the ODE system of cyclin D-HIF dynamics at different oxygen tensions. The corresponding cell cycle phase fractions of aerated MCF-7 tumor population, and the resulting growth curve obtained from numerically solving the developed mathematical model were found to be comparable to experimental data. Two breast radiotherapy fractionation schemes were investigated using the mathematical model. Results show that hypoxia causes the tumor to be more predominated by the tumor subpopulation in the G1 phase and decrease the fractional contribution of the more radioresistant tumor cells in the S phase. However, the advantage provided by hypoxia in terms of cell cycle phase distribution is largely offset by the radioresistance developed through OFH. The delayed proliferation caused by severe hypoxia slightly improves the radiotherapy efficacy compared to that with mild hypoxia for a high overall treatment duration as demonstrated in the 40-Gy fractionation scheme.

Development of a DNA damage model that accommodates different cellular oxygen concentrations and radiation qualities

Research regarding cellular responses at different oxygen concentrations (OCs) is of immense interest within the field of radiobiology. Therefore, this study aimed to develop a mechanistic model to analyze cellular responses at different OCs. A DNA damage model (the different cell oxygen level DNA damage [DICOLDD] model) that examines the oxygen effect was developed based on the oxygen fixation hypothesis, which states that dissolved oxygen can modify the reaction kinetics of DNA-derived radicals generated by ionizing radiation. The generation of DNA-derived radicals was simulated using the Monte Carlo method. The decay of DNA-derived radicals due to the competing processes of chemical repair, oxygen fixation, and intrinsic damaging was described using differential equations. The DICOLDD model was fitted to the previous experimental data obtained under different irradiation configurations and validated by calculating the yields of DNA double-strand breaks (DSBs) after exposure to 137 Cs as well as cell survival fractions (SFs) using a mechanistic model of cellular survival. Moreover, we used the DICOLDD model to calculate DNA DSB damage yields after irradiation with 0.5-50MeV protons. Generally, DSB yields calculated after exposure to 137 Cs at different OCs correspond to statistical uncertainties of previous experimental results. Calculated SFs of CHO and V79 cells exposed to photons, protons, and alpha particles at different OCs generally concur with those obtained in previous studies. Our results demonstrated that the variation in DSB yields was less than 10% when the cellular OC decreased from 21% to 5%. Additionally, DSB yields changed drastically when OC dropped below 1%. We developed a DNA damage model to evaluate the oxygen effect and provide evidence that a reaction-kinetic model of DNA-derived radicals induced by ionizing radiation suffices to explain the observed oxygen effects. Therefore, the DICOLDD model is a powerful tool for the analysis of cellular responses at different OCs after exposure to different types of radiation.

PO-1793 Quantifying the oxygen fixation mechanism in charged particle beams

PO-1793 Quantifying the oxygen fixation mechanism in charged particle beams

Incorporating oxygenation levels in analytical DNA-damage models—quantifying the oxygen fixation mechanism

Purpose. To develop a framework to include oxygenation effects in radiation therapy treatment planning which is valid for all modalities, energy spectra and oxygen levels. The framework is based on predicting the difference in DNA-damage resulting from ionising radiation at variable oxygenation levels. Methods. Oxygen fixation is treated as a statistical process in a simplified model of complex and simple damage. We show that a linear transformation of the microscopic oxygen fixation process allows to extend this to all energies and modalities, resulting in a relatively simple rational polynomial expression. The model is expanded such that it can be applied for polyenergetic beams. The methodology is validated using Microdosimetric Monte Carlo Damage Simulation code (MCDS). This serves as a bootstrap to determine relevant parameters in the analytical expression, as MCDS is shown to be extensively verified with published empirical data. Double-strand break induction as calculated by this methodology is compared to published proton experiments. Finally, an example is worked out where the oxygen enhancement ratio (OER) is calculated at different positions in a clinically relevant spread out Bragg peak (SOBP) dose deposition in water. This dose deposition is obtained using a general Monte Carlo code (FLUKA) to determine dose deposition and locate fluence spectra. Results. For all modalities (electrons, protons), the damage categorised as complex could be parameterised to within 0.3% of the value calculated using microdosimetric Monte Carlo. The proton beam implementation showed some variation in OERs which differed slightly depending on where the assessment was made; before the SOBP, mid-SOBP or at the distal edge. Environment oxygenation was seen to be the more important variable. Conclusions. An analytic expression calculating complex damage depending on modality, energy spectrum, and oxygenation levels was shown to be effective and can be readily incorporated in treatment planning software, to take into account the impact of variable oxygenation, forming a first step to an optimised treatment based on biological factors.

Transport Proteins Enabling Plant Photorespiratory Metabolism.

Photorespiration (PR) is a metabolic repair pathway that acts in oxygenic photosynthetic organisms to degrade a toxic product of oxygen fixation generated by the enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase. Within the metabolic pathway, energy is consumed and carbon dioxide released. Consequently, PR is seen as a wasteful process making it a promising target for engineering to enhance plant productivity. Transport and channel proteins connect the organelles accomplishing the PR pathway—chloroplast, peroxisome, and mitochondrion—and thus enable efficient flux of PR metabolites. Although the pathway and the enzymes catalyzing the biochemical reactions have been the focus of research for the last several decades, the knowledge about transport proteins involved in PR is still limited. This review presents a timely state of knowledge with regard to metabolite channeling in PR and the participating proteins. The significance of transporters for implementation of synthetic bypasses to PR is highlighted. As an excursion, the physiological contribution of transport proteins that are involved in C4 metabolism is discussed.

Evaluation of Ecological Benefits of Corn Straw Back in Liaozhong North Based on Fuzzy Evaluation

Taking Cainiu Town, Tieling City, Liaoning Province as the main research area, the four evaluation index factors of water, fertilizer, gas and soil were selected through investigation and data collection. The method of combining fuzzy evaluation method and analytic hierarchy process is used to analyze the ecological aspects of comprehensive ecological links such as straw returning to grass, clear ridge returning to field, strip returning to field, deep rotation returning to field and strip returning to field. The final EBI index is obtained by multiplying the matrix by fuzzy analysis method, and then the EBI index and the grade corresponding to the score of the review are compared to get the final judgment result, and the comprehensive ecological benefit index is the final result. Obviously from the results, the comprehensive ecological benefits of soil and water conservation, carbon and oxygen fixation, and water bodies have improved significantly

Oxidation of graphite by different modified Hummers methods

Graphite oxides with different oxygen contents and specifications were prepared from a commercial graphite by different versions of the Hummers method and by oxidation with sodium dichromate. The chemical and structural characteristics of the graphite oxides were studied by elemental analysis, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Dichromate oxidation provided a low oxygen fixation, mainly in the form of hydroxyl and epoxy groups. A much more efficient oxidation was achieved by using the Hummers method. The use of NaNO3 and a reaction time of 2 h led to the highest oxygen content in the graphite oxide, over 40 wt%, and oxygen was found to be single- and double-bonded to carbon. SEM and XRD observations showed a high spacing of the graphitic layers under these conditions. These results prove that, even using the same oxidizing method, the chemical structure of graphite oxides can be tailored by changing reaction conditions.

Pyramiding of three C4 specific genes towards yield enhancement in rice

C4 plants can efficiently accumulate CO2 in leaves and thus reduce wasteful oxygen fixation by the RuBisCO enzyme. Three C4 enzymes, namely carbonic anhydrase (CA), phosphoenol pyruvate (PEPC) and pyruvate orthophosphate dikinase (PPDK), were over expressed in Oryza sativa L. ssp. indica var. Khitish under the control of green tissue specific promoters PD54o, PEPC and PPDK, respectively. Integration of these genes was confirmed by Southern hybridization. The relative expression of PEPC, CA and PPDK were, respectively, 6.75, 6.57 and 3.6-fold higher in transgenic plants compared to wild type plants (control). Photosynthetic efficiency of the transgenic plants increased significantly along with a 12 % increase in grain yield compared to wild type plants. Compared to control plants, transgenic plants also showed phenotypic changes such as increased leaf blade size, root biomass, and plant height and anatomical changes such as greater leaf vein number, bundle sheath cells, and bulliform cells. Our findings indicate that the combined over expression of these three enzymes is an efficient strategy for incorporating beneficial physiological and anatomical features that will enable subsequent yield enhancement in C3 rice plants.

Time as a Variable in Radiation Biology: The Oxygen Effect

It has long been known that time is an important parameter in radiobiological effects, especially the intervals between radiation exposures in fractionated radiotherapy. But time is also an important experimental variable, which can be used to distinguish between possible contributions of differing pathways in radiation effects. A recent review of mechanisms underlying oxygen-dependent radiosensitivity by Liu, Lin and Yun (1) prompted this author to remind readers of seminal contributions utilizing time as a variable. Although these contributions were groundbreaking in the 1960s (2), it is important that they remain absolutely central to our thoughts in this area of research even today. If we define the ‘‘oxygen effect’’ as the almost universal, major increase in radiosensitivity (around a factor of two to three reduction in dose required for the same effect) that results from the presence of oxygen (compared to anoxia) in cells of widely-varying absolute radiosensitivity exposed to low-linear energy transfer radiation, then a testable hypothesis is that the effect largely reflects the concentration of oxygen present in cells at the instant of irradiation. As discussed below, this hypothesis is supported by experimental data obtained several decades ago. This is important because, as discussed in the recent review (1), there are numerous oxygen-dependent biological changes that can influence radiosensitivity. However, time is a key variable by which the relative importance of these changes to oxygen-dependent radiosensitivity can be identified. Well over 50 years ago, Howard-Flanders and Moore showed cells physically transported from a nitrogen atmosphere to oxygen around 20 ms after irradiation showed no radiosensitizing effect from the added oxygen (3). Using a liquid-phase rapid-mix flow technique, Adams and Michael et al. shortened the time interval between irradiation of anoxic cells and subsequently adding oxygenated buffer to ;5 ms but still found very little radiosensitizing effect; conversely, adding oxygen to previously anoxic cells for just a few milliseconds before irradiation gave good radiosensitization (2, 4). Much better time resolution was achieved by Michael and colleagues, who managed to change the cellular environment in bacteria from anoxic to oxic in times of about 0.1 ms using a ‘‘gas explosion’’ technique (5). [Ultimately, the time required for oxygen to diffuse to the target site is resolution limiting; and by using an alternative approach in which oxygen was first depleted by a radiation pulse, followed by a second pulse with an interval allowing for oxygen replenishment by diffusion, the same timescale of around 0.1 ms for some oxygen-dependent effect was detected and deduced (6, 7).] By varying the interval between (sub-microsecond) irradiation pulses and admission of oxygen, the lifetimes of oxygen-dependent damage and the effects of chemical modifiers could be properly resolved for the first time in mammalian cells (8). Crucially, these lifetimes could be compared using end points in addition to clonogenic survival, such as DNA strand break measurements. For example, it was shown that the timescales of action of chemicals influencing clonogenic survival and the yield of DNA double-strand breaks were similar (9). Other alternative fast mixing methods were developed and applied (10). Briefly, the key conclusion from these time-resolved studies is that the oxygen effect in radiobiology must arise to a very large extent from events that are completed in a few milliseconds, and by far the dominant factor is the concentration of oxygen in cells at the instant of irradiation. Because of this timescale, reactions involving short-lived free radicals are likely to be the major contributors to the oxygen effect, and chemists have presented plausible mechanistic schemes by which reactions involving oxygen lead to DNA strand breaks, differing in nature and extent from those found in anoxia. In fact, our understanding of these pathways is far advanced beyond that which might be surmised from the description of ‘‘oxygen fixation’’ and formation of ‘‘peroxides that cannot be structurally restored’’ in the review which prompted this letter (1). Oxygen is thought not to ‘‘fix’’ damage directly, but to modify the chemical pathways to DNA strand breaks, and peroxides are incidental by-products of these pathways 1 Address for correspondence: 20 Highover Park, Amersham, Buck inghamsh i r e HP7 0BN, Un i t ed Kingdom; ema i l : peterwardman@btinternet.com.

A mechanistic investigation of the oxygen fixation hypothesis and oxygen enhancement ratio

The presence of oxygen in tumours has substantial impact on treatment outcome; relative to anoxic regions, well-oxygenated cells respond better to radiotherapy by a factor 2.5–3. This increased radio-response is known as the oxygen enhancement ratio. The oxygen effect is most commonly explained by the oxygen fixation hypothesis, which postulates that radical-induced DNA damage can be permanently ‘fixed’ by molecular oxygen, rendering DNA damage irreparable. While this oxygen effect is important in both existing therapy and for future modalities such a radiation dose-painting, the majority of existing mathematical models for oxygen enhancement are empirical rather than based on the underlying physics and radiochemistry. Here we propose a model of oxygen-enhanced damage from physical first principles, investigating factors that might influence the cell kill. This is fitted to a range of experimental oxygen curves from literature and shown to describe them well, yielding a single robust term for oxygen interaction obtained. The model also reveals a small thermal dependency exists but that this is unlikely to be exploitable.

Low-energy electron-induced ‘‘oxygen fixation’’ to DNA SAMs studied by stimulated anion desorption

Reactions of 18O2 with self-assembled monolayer films of thiolated DNA oligomers are studied by the electron stimulated desorption of anions. Electrons with energies < 20 eV initiate dehydrogenation of the DNA by resonant and non-resonant processes, to form radical sites for oxygenation. This damage mechanism may underlie the ‘‘oxygen fixation’’ process of radiobiology.