Howard B. Michaels

A Model for Radiation Damage in Cells by Direct Effect and by Indirect Effect: A Radiation Chemistry Approach

A model is presented to describe the contributions of direct and indirect effects to the radiation damage of cells. The model is derived using principles of radiation chemistry and of pulse radiolysis in particular. From data available in the literature, parameters for cellular composition and values of rate constants for indirect action have been used in preliminary applications of the model. The results obtained in calculations of the protective effect of ·OH and ·H scavengers are consistent with experimental data. Possible modifications and improvements to the model are suggested, along with proposed future applications of the model in radiobiological studies.

Oxygen Sensitization of Mammalian Cells under Different Irradiation Conditions

The oxygen dependence of the radiosensitivity of cultured CHO cells was examined in detail with particular attention paid to avoiding possible artifacts due to radiolytic oxygen depletion. Two methods of gas equilibration and irradiation were used. In the first approach, cells were irradiated with 50-kVp X rays in a thin-layer geometry which offered maximum interchange between the cells and the surrounding gas. The second technique employed 280-kVp X irradiation of cells under full-medium conditions with mechanical agitation to minimize the effect of radiochemical oxygen consumption by promoting rapid oxygen replenishment. With these techniques oxygen radiosensitization was clearly resolved at an oxygen concentration of 0.03% in the gas phase. The oxygen K curves measured by these two methods were similar in shape over a wide range of oxygen concentration.

Dosimetry, field shaping and other considerations for intra-operative electron therapy

Abstract In the early part of 1978, a pilot program of intra-operative radiotherapy was initiated at the Massachusetts General Hospital (MGH), using beams of high energy electrons. A technique has been employed to irradiate the tumor bearing area using a sterilized acrylic resin cone that slides into a metal holder attached to the head of the accelerator. The acrylic resin cone is inserted into the patient directly over the tumor; the patient couch is adjusted until the cone is correctly aligned inside the holder. The dosimetry for this procedure has been investigated as a function of the primary collimator setting of the linear accelerator. A fixed setting was chosen as a compromise between increased bremsstrahlung, low effective electron dose rate observed with narrower settings, and more rounded beam profiles together with somewhat poorer depth dose characteristics found with larger settings. Field shaping and blocking of critical organs was achieved using sterile lead sheets that are cut to the appropriate size. Consideration has been given to improved beam design by increasing the incident electron dose rate and by improving the depth dose at each energy. The design of a dedicated intra-operative facility, using a high energy linear accelerator, is presented with respect to shielding requirements for the machine and the room.

Interaction of misonidazole and oxygen in the radiosensitization of mammalian cells.

The radiosensitizing effect of misonidazole in the presence of various oxygen concentrations was studied using cultured Chinese hamster ovary (CHO) cells. Survival curves were measured for cells equilibrated with various combinations of oxygen and misonidazole concentrations and irradiated with 50 kVp x-rays. In cases where the two agents independently yield two moderately different enhancement ratios (ER), the combined ER is simply the higher of the two. Only when the two individual ERs are approximately equal is there some degree of increase in the overall ER. An empirical technique was derived, based on the concept of ER-equivalence and the respective K-curves of the two sensitizers, to predict the combinative effect of the agents in radiosensitization. These experimental and theoretical considerations suggest that oxygen and misonidazole share in the same sensitization mechanism and act on the same target site or sites. Since it is likely that there is a distribution of oxygen tension in a tumor sensitizing tumor cells to varying degrees, knowledge of the additive effects of misonidazole and oxygen in radiosensitization may be valuable in understanding the efficacy of the application of electron affinic drugs to cancer radiotherapy.

Reaction of oxygen with radiation-induced free radicals on single-stranded polynucleotides.

Pulse radiolysis experiments have been carried out to investigate the reaction of O/sub 2/ with the radicals produced by .OH attack on nucleic acid derivatives and single-stranded polynucleotides. The .OH adducts of the pyrimidine polynucleotides poly C and poly U react with O/sub 2/ giving rise to species whose absorption spectra are similar to the peroxy adducts of the monomers CMP and UMP. There may be two forms of polynucleotide radicals, reacting with O/sub 2/ at different rates. The situation for purine polynucleotides is complex. Radicals on single-stranded DNA react with O/sub 2/ on the same time scale as the reaction of O/sub 2/ with .OH adducts on pyrimidine polynucleotides. Typical values of the rate constant for this reaction are approximately 4 x 10/sup 8/ M/sup -1/ sec/sup -1/, a value which agrees approximately with an estimated collision frequency of O/sub 2/ with the polynucleotide radicals.

Comparison of hyperbaric oxygen and misonidazole in fractionated irradiation of murine tumors.

The enhancement ratios for hyperbaric oxygen (O/sub 2/3ATA) and for misonidazole (0.3 mg/g body wt) for fractionated irradiation (5 or 10 equal doses) of two spontaneous tumors and of normal skin of the C3H mouse have been determined. Acute skin reactions were scored for mice irradiated 18 days after plucking hair from the leg. Enhancement ratios for the TCD/sub 50/ values were virtually the same for O/sub 2/3ATA and misonidazole, 1.45 to 1.55. For acute skin reaction the enhancement ratio was higher for O/sub 2/3ATA, i.e., 1.94 vs 1.54 for misonidazole.

Interaction of the Radiosensitizer Paranitroacetophenone with Radiation-Induced Radicals on Nucleic Acid Components

The techniques of optical and conductiometric pulse radiolysis were used to investigate the reactions of PNAP with the free radicals produced by .OH attack on uracil, dihydrouracil, ribose, and deoxyribose. Electron transfer from sugar radicals to PNAP occurs with moderate efficiency, the yields being 37 and 23 percent for deoxyribose and ribose, respectively. In the case of uracil and dihydrouracil, however, the radicals produced by reaction with .OH do not transfer an electron to PNAP; instead, an addition product with the sensitizer is probably formed. This transient product absorbs strongly in the near uv. A comparison with other electron-affinic sensitizers, namely, nifuroxime and menaquinone, indicates that radical addition reactions, and not electron-transfer reactions, are of major significance in the interaction of these sensitizers with the radicals produced by .OH attack on nucleic acid bases. Reports in the literature used as evidence in support of the electron-transfer mechanism are shown to be faulty, due to improper analysis of the experimental data. An attempt is made to rationalize the various results reported for electron-affinic sensitizers. (auth)

Influence of low pH on the development and decay of 42°C thermotolerance in cho cells☆

Abstract The development, magnitude and decay of thermotolerance in Chinese hamster ovary cells heated to 42°C at pH 6.7 was examined. The cells were exposed to single or fractionated heat treatments with 0 to 168 hours elapsing between the treatments. Administration of a specified heat treatment in two fractions at low pH substantially reduced the lethal effect of heat. The rate of hyperthermic cell killing was reduced by a factor of approximately 2.5 in preheated cells compared to cells not receiving prior heat treatment. Tolerance to second heat treatments was apparent when 3 hours at 37°C separated the treatments, and was near maximum when 6 hours separated the treatments. Beginning about 96 hours after the initial heat treatment, resistance to second heat treatments began to decline, and was not evident when 168 hours at 37°C separated the treatments. In contrast to these results, a sparing effect as a result of dose fraction was not observed at pH 7.4. At pH 7.4, cells developed thermotolerance during the initial heat treatment and additional culturing at 37°C was without additional effect. Nevertheless, the sensitivity of cells to hyperthermia was greater at pH 6.7 than at pH 7.4. This pH sensitizing effect was more pronounced in cells exposed to single heat treatments than in cells exposed to fractionated treatments at 42°C.

Radiolysis of the Double-Stranded Polynucleotides Poly(A ? U) and DNA in the Presence of Oxygen

MICHAELS, H. B., AND HUNT, J. W. Radiolysis of the Double-Stranded Polynucleotides Poly(A + U) and DNA in the Presence of Oxygen. Radiat. Res. 72, 32-47 (1977). The reaction of 02 with -OH adducts produced on poly(A + U) was studied by pulse radiolysis. In the presence of 0O, the absorption spectrum of the poly(A + U) - OH adduct decays to a new spectrum, with a rate constant of 5 X 107 M-1 sec-1. Solutions of poly(A + U) and DNA, in common with other polynucleotides, are sensitive to repetitive pulsing, and in the case of DNA, denaturation takes place and single-stranded regions and pieces are produced. The .OH adduct of native doublestranded DNA does not appear to react with O on a time scale from microseconds to seconds, yet steady-state radiolysis experiments and analysis for peroxides show that DNA hydroperoxides are produced at low doses. The *OH adduct of heat denatured, single-stranded DNA reacts with O as expected. The apparent lack of reaction of double-stranded DNA -OH adducts with 02 is hard to understand, since a reaction with the radiosensitizer NPPN is clearly observable under similar conditions. Possible explanations for the experimental results are discussed.

Effects of modifiers of the yield of hydroxyl radicals on the radiosensitivity of mammalian cells at ultrahigh dose rates

Experiments were conducted to investigate the effects of increasing or decreasing the yield of hydroxyl radicals (.OH) reacting with target molecules on the survival of CHO cells irradiated in a thin layer with single 3-nsec pulses of electrons. Dimethyl sulfoxide (DMSO), a known radioprotector, was used as an .OH scavenger. The gas nitrous oxide (N2O), which scavenges hydrated electrons and in the process generates an additional yield of .OH, was used in an attempt to produce sensitization by increasing the amount of .OH-induced cellular damage. It was found that DMSO at high concentration was an effective radioprotective agent in cells equilibrated with nitrogen, air, and N2O and irradiated at ultrahigh dose rates. Sensitization by N2O was observed, but only under certain conditions, specifically, when a high concentration of the .OH scavenger DMSO was also present. The enhancement ratio (ER) for oxygen sensitization was reduced in the presence of DMSO, as was the ER for sensitization by misonidazole. Interpretation of these results according to radiation chemistry considerations will be discussed.