Sarah F. Bliven

Response of Rat Spinal Cord to Single and Fractionated Doses of Accelerated Heavy Ions

region, RBE values of 1.45 ? 0.25 (propagated 95% confidence limits) and 1.46 ? 0.33, respectively, were obtained. In the spread peak regions for carbon and neon ions, the RBE values were 1.48 ? 0.18 and 1.86 ? 0.42, respectively. These values were obtained using the dose needed to produce 50% paralysis in a group of irradiated rats as the isoeffect comparison dose (ED50 dose). Similarly, in groups of rats receiving four daily exposures, the RBE values for carbon and neon ions in the plateau ionization region were 1.31 ? 0.27 and 1.80 + 0.24, respectively. In the spread peak regions of ionization for carbon and neon ions, the RBE values were 1.95 ? 0.19 and 2.18 ? 0.23, respectively. Similar values for RBE were obtained using changes in the activity of enzymes in spinal cord tissue (cyclic nucleotide phosphohydrolase and y--glutamyl transpeptidase). Also, it was estimated that, for X irradiation, the fractional amount of dose repaired (at the EDso dose) was 0.64 ? 0.10 (95% confidence limits). For carbon and neon ions in the plateau ionization region, the values for the fractional amount of dose repaired were 0.70 ? 0.27 and 0.48 ? 0.20, and for carbon and neon ions in the spread peak region of ionization, the fractional repair values were 0.40 ? 0.10 and 0.52 ? 0.17. Spinal cord tissue therefore shows a high capacity for subeffective damage repair, and even at the highest LET investigated (neon ions in the spread peak region) there is still indication of a significant amount of repair of radiation damage.

Effects of the differentiating agents sodium butyrate and N-methylformamide on the oxygen enhancement ratio of human colon tumor cells.

We have previously shown that chronic adaptation of human tumor cells to the differentiation-inducing agents N-methylformamide (NMF) and sodium butyrate (NAB) increases the sensitivity of oxic cells to graded single doses of X rays. These studies were carried out to define the sensitivity of hypoxic cells after adaptation. Clone A colon tumor cells were grown for three passages in medium containing 170 mM NMF or 2 mM NAB and irradiated in suspension culture, after gassing with either oxygen (60 min) or ultrapure nitrogen (90 min), and complete survival curves were generated. Using the linear-quadratic equation to describe the data, it was found that NMF and NAB produced increased X-ray killing of hypoxic cells. At the 10% level of survival, the dose-modifying factors were about 1.20 and 1.25 for NMF- and NAB-adapted hypoxic cells, respectively, as compared to hypoxic control cells. However, since both oxic and hypoxic cells exhibited increased sensitivity after NMF and NAB adaptation, there was no major change in the oxygen enhancement ratio.

Changes in X-Ray Sensitivity and Glutathione Content of Human Colon Tumor Cells after Exposure to the Differentiation-Inducing Agent Sodium Butyrate

Clone A human colon cancer cells were exposed to concentrations of sodium butyrate (NAB, 0-2 mM) for three passages in vitro, and responses to either graded single doses or split doses of 250 kVp X rays were determined. The survival data were fit to the single-hit, multitarget model of inactivation. For the graded single dose experiments, we found that NAB produced a decrease in the magnitude of the quasi-threshold (Dq) parameter after a concentration of about 0.9 mM was exceeded. Similarly, in split dose experiments, the amount of sublethal damage recovery (SLDR) was reduced in a concentration-dependent manner as shown by a decrease in the Dq parameter. However, the inhibition of SLDR occurred with no apparent threshold NAB concentration. NAB did not affect potentially lethal damage recovery. Paradoxically, increasing concentrations of NAB produced an exponential increase in the intracellular glutathione content, which could be blocked by exposure of the cells to buthionine sulfoximine (BSO). BSO treatment of NAB-adapted cells led to additional cell killing, again most noted by changes in the Dq parameter. We postulate that these responses are associated with NAB-induced changes in chromatin structure, particularly the association between DNA and nucleosomal histones H3 and H4.

X RAY RESPONSES OF A HUMAN COLON TUMOR CELL LINE AFTER EXPOSURE TO THE DIFFERENTIATION-INDUCING AGENT N-METHYLFORMAMIDE: CONCENTRATION DEPENDENCE AND REVERSIBILITY CHARACTERISTICS

The combination of differentiation-inducing agents with conventional cytotoxic agents has been suggested as a potential cancer therapeutic strategy. In this regard, we have chronically exposed (3 passages) a human colon tumor line (clone A) to varying concentrations (0-170 mM) of N-methylformamide and examined the change in sensitivity to ionizing radiation in vitro. The linear-quadratic formalism of survival was used to characterize the single graded dose survival curves. This equation yields two constants (alpha and beta) relating to cellular inactivation produced by either single events (alpha) or by the combination of two events (beta). As the N-methylformamide concentration increased, the alpha parameter increased while the beta parameter concomitantly decreased, yielding a concentration dependent radiosensitization which was most marked in the low dose region of the survival curve. Upon removal of NMF, the original radiation resistance was regained within 2-3 cell culture doubling times.

Similarity of Thermotolerance Characteristics in Heterogeneous Human Colon Tumor Subpopulations after Exposure to Fractionated Heat Doses (44°C)

We have studied the thermotolerance (TTR) responses of three tumor cell populations obtained from a heterogeneous human colon cancer after exposure to 44 degrees C water bath hyperthermia given in a split dose regimen. The cell populations studied consist of the original tumor line (DLD-1) obtained from surgical biopsy, and two subcloned populations termed clones A and D. All tumor lines were treated with initial heat doses at 44 degrees C which reduced the initial survival level to about 30, 10, or 1%, after which cultures were returned to 37 degrees C. Complete survival curves were then determined for all three lines as a function of time after the initial treatment (0, 2, 4, 8, 12, 14, 20, and 24 h), and the magnitude and kinetics of thermotolerance development and its decay were determined. It was found that the DLD-1 line was more heat sensitive and exhibited a lesser degree of induced thermotolerance than did the A and D subpopulations, indicating intraclonal heterogeneity. Also, the rate of thermotolerance induction, the maximal thermotolerance reached (TTRmax), and the rate of thermotolerance decay were strongly dependent upon the survival level produced by the initial heat exposure. The importance of these findings to the treatment of heterogeneous cancers by heat is discussed.

Modification of the response of mouse skin to X-irradiation by a polar solvent, N,N-dimethylformamide

The modification of the response of mouse skin to either single or split (24 hrs) graded doses of X rays by topically applied N,N-dimethylformamide (DMF) was investigated. DMF was applied daily for 5 days prior to irradiation. At a radiation dose level producing dry desquamation, DMF enhanced the X ray response by a factor of 1.3. Also, at the same level of response, the fraction of X ray dose repaired in 24 hours was 0.57, whereas for the DMF-treated and irradiated skin, this factor was 0.41, indicating a reduction of about 28% in subeffective damage repair. The times of maximal involvement of the skin reactions were not different in the X ray plus DMF treated mice versus mice receiving x-irradiation only. The data indicate that DMF is able to modify intrinsic radiation sensitivity of mouse skin epithelial cells, possibly through a reduction in the magnitude of the shoulder region of the survival curve.