(1/186) Complex hprt deletion events are recovered after exposure of human lymphoblastoid cells to high-LET carbon and neon ion beams.

Hypoxanthine phosphoribosyltransferase gene (hprt) mutations were induced in human TK-6 lymphoblastoid cells by irradiation at a linear energy transfer (LET) of 250 or 310 keV/micron for carbon and neon ions, respectively. At such a high level of LET, ions will lose most of their total energy and stop shortly after passing through the cell. The hprt mutations were analyzed by multiplex PCR, long-PCR and DNA sequencing of both genomic and cDNA. Over half of the C ion-induced hprt mutations (10 of 19) were point mutations, in contrast to 15% of the mutations induced by Ne ions (three of 20). The remaining 47 and 85% of the C and Ne ion-induced mutants, respectively, are deletion events. The latter events include three complex losses of multiple non-contiguous exon regions in both ion irradiation collections. We note that mutations involving the exon 6 region are frequent in the Ne ion collection: all three of the complex events retained the exon 6 region with flanking deletion of sequence and three other mutants involved deletion of this region. It may be concluded that these high-LET C and Ne ion irradiations produce different mutational spectra.  (+info)

(2/186) High-linear energy transfer (LET) alpha versus low-LET beta emitters in radioimmunotherapy of solid tumors: therapeutic efficacy and dose-limiting toxicity of 213Bi- versus 90Y-labeled CO17-1A Fab' fragments in a human colonic cancer model.

Recent studies suggest that radioimmunotherapy (RIT) with high-linear energy transfer (LET) radiation may have therapeutic advantages over conventional low-LET (e.g., beta-) emissions. Furthermore, fragments may be more effective in controlling tumor growth than complete IgG. However, to the best of our knowledge, no investigators have attempted a direct comparison of the therapeutic efficacy and toxicity of a systemic targeted therapeutic strategy, using high-LET alpha versus low-LET beta emitters in vivo. The aim of this study was, therefore, to assess the toxicity and antitumor efficacy of RIT with the alpha emitter 213Bi/213Po, as compared to the beta emitter 90Y, linked to a monovalent Fab' fragment in a human colonic cancer xenograft model in nude mice. Biodistribution studies of 213Bi- or 88Y-labeled benzyl-diethylene-triamine-pentaacetate-conjugated Fab' fragments of the murine monoclonal antibody CO17-1A were performed in nude mice bearing s.c. human colon cancer xenografts. 213Bi was readily obtained from an "in-house" 225Ac/213Bi generator. It decays by beta- and 440-keV gamma emission, with a t(1/2) of 45.6 min, as compared to the ultra-short-lived alpha emitter, 213Po (t(1/2) = 4.2 micros). For therapy, the mice were injected either with 213Bi- or 90Y-labeled CO17-1A Fab', whereas control groups were left untreated or were given a radiolabeled irrelevant control antibody. The maximum tolerated dose (MTD) of each agent was determined. The mice were treated with or without inhibition of the renal accretion of antibody fragments by D-lysine (T. M. Behr et al., Cancer Res., 55: 3825-3834, 1995), bone marrow transplantation, or combinations thereof. Myelotoxicity and potential second-organ toxicities, as well as tumor growth, were monitored at weekly intervals. Additionally, the therapeutic efficacy of both 213Bi- and 90Y-labeled CO17-1A Fab' was compared in a GW-39 model metastatic to the liver of nude mice. In accordance with kidney uptake values of as high as > or = 80% of the injected dose per gram, the kidney was the first dose-limiting organ using both 90Y- and 213Bi-labeled Fab' fragments. Application of D-lysine decreased the renal dose by >3-fold. Accordingly, myelotoxicity became dose limiting with both conjugates. By using lysine protection, the MTD of 90Y-Fab' was 250 microCi and the MTD of 213Bi-Fab' was 700 microCi, corresponding to blood doses of 5-8 Gy. Additional bone marrow transplantation allowed for an increase of the MTD of 90Y-Fab' to 400 microCi and for 213Bi-Fab' to 1100 microCi, respectively. At these very dose levels, no biochemical or histological evidence of renal damage was observed (kidney doses of <35 Gy). At equitoxic dosing, 213Bi-labeled Fab' fragments were significantly more effective than the respective 90Y-labeled conjugates. In the metastatic model, all untreated controls died from rapidly progressing hepatic metastases at 6-8 weeks after tumor inoculation, whereas a histologically confirmed cure was observed in 95% of those animals treated with 700 microCi of 213Bi-Fab' 10 days after model induction, which is in contrast to an only 20% cure rate in mice treated with 250 microCi of 90Y-Fab'. These data show that RIT with alpha emitters may be therapeutically more effective than conventional beta emitters. Surprisingly, maximum tolerated blood doses were, at 5-8 Gy, very similar between high-LET alpha and low-LET beta emitters. Due to its short physical half-life, 213Bi appears to be especially suitable for use in conjunction with fast-clearing fragments.  (+info)

(3/186) Generalized concept of the LET-RBE relationship of radiation-induced chromosome aberration and cell death.

The frequency of chromosome aberrations per traversal of a nucleus by a charged particle at the low dose limit increases proportionally to the square of the linear energy transfer (LET), peaks at about 100 keV/micron and then decreases with further increase of LET. This has long been interpreted as an excessive energy deposition over the necessary energy required to produce a biologically effective event. Here, we present an alternative interpretation. Cell traversed by a charged particle has certain probability to receive lethal damage leading to direct death. Such events may increase with an increase of LET and the number of charged particles traversing the cell. Assuming that the lethal damage is distributed according to a Poisson distribution, the probability that a cell has no such damage is expressed by e-cLx, where c is a constant, L is LET, and x is the number of charged particles traversing the cell. From these assumptions, the frequency of chromosome aberration in surviving cells can be described by Y = alpha SD + beta S2D2 with the empirical relation Y = alpha D + beta D2 in the low LET region, where S = e-cL, alpha is a value proportional to LET, beta is a constant, and D is the absorbed dose. This model readily explains the empirically established relationship between LET and relative biological effectiveness (RBE). The model can also be applied to clonogenic survival. If cells can survive and they have neither unstable chromosome aberrations nor other lethal damage, the LET-RBE relationship for clonogenic survival forms a humped curve. The relationship between LET and inactivation cross-section becomes proportional to the square of LET in the low LET region when the frequency of a directly lethal events is sufficiently smaller than unity, and the inactivation cross-section saturates to the cell nucleus cross-sectional area with an increase in LET in the high LET region.  (+info)

(4/186) Establishment of a radiation- and estrogen-induced breast cancer model.

It is well accepted that cancer arises in a multistep fashion in which exposure to environmental carcinogens is a major etiological factor. The aim of this work was to establish an experimental breast cancer model in order to understand the mechanism of neoplastic transformation induced by high LET radiation in the presence of 17beta-estradiol (E). Immortalized human breast cells (MCF-10F) were exposed to low doses of high LET alpha particles (150 keV/microm) and subsequently cultured in the presence or absence of E for periods of up to 10 months post-irradiation. MCF-10F cells irradiated with either a single 60 cGy dose or 60/60 cGy doses of alpha particles showed gradual phenotypic changes including altered morphology, increase in cell proliferation relative to the control, anchorage-independent growth and invasive capability before becoming tumorigenic in nude mice. In alpha particle-irradiated cells and in those cells subsequently cultured in the presence of E, increased BRCA1, BRCA2 and RAD51 expression were detected by immunofluorescence staining and quantified by confocal microscopy. These studies showed that high LET radiation such as that emitted by radon progeny, in the presence of estrogen, induced a cascade of events indicative of cell transformation and tumorigenicity in human breast epithelial cells.  (+info)

(5/186) Inverse radiation dose-rate effects on somatic and germ-line mutations and DNA damage rates.

The mutagenic effect of low linear energy transfer ionizing radiation is reduced for a given dose as the dose rate (DR) is reduced to a low level, a phenomenon known as the direct DR effect. Our reanalysis of published data shows that for both somatic and germ-line mutations there is an opposite, inverse DR effect, with reduction from low to very low DR, the overall dependence of induced mutations being parabolically related to DR, with a minimum in the range of 0.1 to 1.0 cGy/min (rule 1). This general pattern can be attributed to an optimal induction of error-free DNA repair in a DR region of minimal mutability (MMDR region). The diminished activation of repair at very low DRs may reflect a low ratio of induced ("signal") to spontaneous background DNA damage ("noise"). Because two common DNA lesions, 8-oxoguanine and thymine glycol, were already known to activate repair in irradiated mammalian cells, we estimated how their rates of production are altered upon radiation exposure in the MMDR region. For these and other abundant lesions (abasic sites and single-strand breaks), the DNA damage rate increment in the MMDR region is in the range of 10% to 100% (rule 2). These estimates suggest a genetically programmed optimatization of response to radiation in the MMDR region.  (+info)

(6/186) Mechanisms for the biological effectiveness of high-LET radiations.

Radiations of high linear energy transfer (LET) have long been known to have greater biological effectiveness per unit dose than those of low LET, for a wide variety of biological effects. However, values of relative biological effectiveness depend considerably on the biological system and in some instances the values are clearly below unity. The differences between high- and low-LET radiations may be due to many factors, almost all of which are related to radiation track structure in one way or another, and some can in principle lead to qualitative as well as quantitative differences between the radiations. Explanations for LET-dependent differences in effectiveness are discussed over a variety of levels from the multicellular and cellular scale down to the DNA scale, with illustrations from radiobiological data. Information from well-defined slow light ions provide particularly useful analytic data, but practical issues extend also to neutrons and fast heavy ions, which may compound high- and low-LET features. It is suggested that effectiveness of the radiation is determined predominantly by the complex clustered damage that it produces in DNA, but that for high-LET radiations long-term effects are in some instances limited by single-track-survival probabilities of the traversed cells.  (+info)

(7/186) Neutron generator (HIRRAC) and dosimetry study.

Dosimetry studies have been made for neutrons from a neutron generator at Hiroshima University (HIRRAC) which is designed for radiobiological research. Neutrons in an energy range from 0.07 to 2.7 MeV are available for biological irradiations. The produced neutron energies were measured and evaluated by a 3He-gas proportional counter. Energy spread was made certain to be small enough for radiobiological studies. Dose evaluations were performed by two different methods, namely use of tissue equivalent paired ionization chambers and activation of method with indium foils. Moreover, energy deposition spectra in small targets of tissue equivalent materials, so-called lineal energy spectrum, were also measured and are discussed. Specifications for biological irradiation are presented in terms of monoenergetic beam conditions, dose rates and deposited energy spectra.  (+info)

(8/186) Cell cycle and LET dependence for radiation-induced mutation: a possible mechanism for reversed dose-rate effect.

A previous study of the mutagenic action of 252Cf radiation in mouse L5178Y cells showed that the mutation frequency was higher when the dose was chronic rather than acute, which was in sharp contrast to the effects reported for gamma-rays (Nakamura and Sawada, 1988). A subsequent study using synchronized cells revealed that the cells at the G2/M stage were uniquely sensitive to mutation induction by 252Cf radiation but not to gamma-rays (Tauchi et al., 1993). A long phase cell population was first subjected to conditioning gamma or 252Cf radiation doses at different dose-rates. The cell cycle distribution of these cells was then observed, and they were then exposed to 252Cf radiation, and the mutation rate was determined. The G2/M fraction increased by 3- to 4-fold when the conditioning doses (2 Gy of gamma or 1 Gy of 252Cf radiation) were delivered chronically over 10 h, but only slightly when the same doses were delivered over a 1 h period or less. Subsequent 252Cf irradiation gave higher mutation frequencies in the cells pre-irradiated with gamma-rays over a protracted period of time than in those exposed with the higher dose-rate gamma-rays. These results suggest that the radiation-induced G2 block could be at least partly (but not totally) responsible for this reverse dose-rate effect (Tauchi et al. 1996). Possible factors which cause the hyper-sensitivity of G2/M cells to mutation induction by neutrons will be discussed.  (+info)