Studies of the LSA Ascites Lymphoma of C57B1 Mice

Bromodeoxyuridine-mediated radiosensitization in human glioma: the effect of concentration, duration, and fluoropyrimidine modulation

PURPOSE

To define the relative influence of duration of exposure, concentration, and modulation by fluorodeoxyuridines (FdUrd) on the incorporation of 5-bromo-2-deoxyuridine (BrdUrd) into DNA of a human malignant glioma line (D-54) in vitro and in vivo.

MATERIALS AND METHODS

IN VITRO STUDIES

an established human malignant glioma line (D-54) was exposed to a clinically achievable concentration of BrdUrd to model intravenous (1 microM BrdUrd) and intraarterial (4 microM BrdUrd) conditions. The influence of modulation was assessed using 1 nM FdUrd. Incorporation of BrdUrd, radiosensitization, and cytotoxicity were determined after 24, 72, and 120 h drug exposures. In Vivo studies: nude mice bearing D-54 xenografts were infused with BrdUrd at 100 mg/kg/day for 7 and 14 days or BrdUrd at 400 mg/kg/day for 5 days. The influence of modulation was assessed by combining 100 mg/kg/day of BrdUrd with 0.1, 0.3, and 1 mg/kg/day FdUrd for 7 days. Incorporation of BrdUrd into the DNA of tumor, gut, and marrow were determined.

RESULTS

In Vitro: thymidine replacement and radiosensitization were a function of concentration, and incorporation began to plateau after 2 to 3 population doublings. Modulation with 1 nM FdUrd significantly increased incorporation. Radiosensitization was a linear function of thymidine replacement under all conditions tested. In Vivo: infusion with 400 mg/kg/day for 5 days resulted in greater tumor incorporation (10.3 +/- 0.4% thymidine replaced) than treatment with 100 mg/kg/day for 14 days (6.0 +/- 0.6% of thymidine replaced). Infusion of FdUrd with BrdUrd increased normal tissue incorporation of BrdUrd, but failed to increase BrdUrd incorporation in tumor cells.

CONCLUSION

These results suggest that relatively short, high dose rate infusions may be preferable to long, low dose rate infusions. The potential benefit of FdUrd modulation demonstrated in vitro may be difficult to realize using continuous systemic infusions.

Effects of intralesional injection of cisplatin dissolved in urografin and lipiodol on Ehrlich ascites tumor and normal tissues of CD-1 mice

The response of Ehrlich ascites tumor and the effect on normal tissues (kidney and small intestine) of CD-1 mice were evaluated after intralesional (i.l.) injection of cisplatin dissolved in urografin and lipiodol, which is henceforth termed CUL suspension. The results obtained were compared with the effects of i.p. and i.l. injections of cisplatin dissolved in sterile distilled water. Each of these treatment modalities involves the injection of 10 mg/kg cisplatin. The tumor response was evaluated by tumor growth-delay studies as well as by determining the percentage of cells in the S phase. Toxicity studies were accomplished by evaluation of the change in the body weight of mice and also by S-phase studies. S-phase fraction analyses were done with the use of the Cell Proliferation Kit. This commercial kit was used to measure bromodeoxyuridine (BrdU), a thymidine analogue that is incorporated into cells synthesizing DNA. Tumor, kidney, and small-intestine platinum concentrations were determined by measurement with a flameless atomic absorption spectrophotometer. The results of the tumor growth-delay studies showed that i.p. injection, with water being the drug carrier, produced the weakest antitumor effect, whereas i.l. injection of cisplatin, with lipiodol being the drug carrier, evoked the most enhanced effect. This finding was substantiated by BrdU-uptake analysis of tumor cells, wherein i.p. injections yielded the highest S-phase fraction and CUL treatment gave the lowest. Toxicity studies showed that a very significant decrease in body weight occurred in mice receiving i.p. treatment. No significant decrease in body weight was noted after i.l. treatment. BrdU analysis revealed that DNA synthesis in kidney cells and crypt cells of the small intestine was depressed after i.p. treatment. On the other hand, no significant effect was observed in the kidney or small intestine of CUL-treated mice. A correlation between the effects of the various treatment modalities (on tumors, kidney, and small intestine) and the retention of cisplatin was found.

Correlation of sensitizer enhancement ratio with bromodeoxyuridine concentration and exposure time in human cervical carcinoma cells treated with low dose rate irradiation

The effect of a range of bromodeoxyuridine concentrations and exposure times on the sensitizer enhancement ratio of two human cervical carcinoma cell lines treated with low rate irradiation was evaluated. Both cell lines show a linear, though different, dose response to low dose rate irradiation at dose rates up to 0.62 Gy/hr. Cells were pre-incubated with 0, 1, 5, or 10 microM bromodeoxyuridine for 0, 24, or 48 hr prior to low dose rate irradiation. Based on the survival of cells exposed to 4 Gy, delivered at 0.62 Gy/hr, a slope was determined for an exponential survival curve for each bromodeoxyuridine concentration and exposure time. Sensitizer enhancement ratios were calculated as the ratio of the slopes of the treated and untreated cells from a particular exposure time. Ratios increased linearly initially with a plateauing at higher levels with respect to the product of bromodeoxyuridine concentration and time of exposure for both cell lines with similar degrees of enhancement. A linear relationship has been noted previously for acute irradiation of hamster cells over a similar range of sensitizer concentrations and exposure times. At the highest dose-time points (540 microM-hr) ratios of greater than 2 were attained, which were comparable to results obtained following acute radiation exposures of V-79 cells. These results demonstrate that significant sensitizer enhancement ratios with low dose irradiation can be obtained in human tumor cells after exposures of 2 to 3 doubling times using relatively low concentrations of sensitizer.

Modulation of iododeoxyuridine-mediated radiosensitization by 5-fluorouracil in human colon cancer cells

The incorporation of 5-iodo-2'-deoxyuridine (IdUrd), a thymidine analog radiosensitizer, can be increased by the use of modulators such as 5-fluorouracil (FUra). FUra is a particularly attractive potential modulator to use against colorectal cancer, as it is the most active single agent in the treatment of this disease. To begin to define the conditions for the optimal combination of IdUrd and FUra in the treatment of patients with colorectal cancer, a study was conducted of the effect of FUra on IdUrd-mediated radiosensitization in cultured HT29 human colon cancer cells. It was found that when cells were exposed to concentrations of IdUrd typical of those obtained through intravenous exposure (1-3 microM), FUra (1 microM) increased radiosensitization beyond that which would be predicted for the same extent of incorporation produced by incubation with IdUrd alone. This increase appeared to result from a combination of at least two effects: FUra-mediated cell cycle redistribution and increased IdUrd incorporation. When a higher concentration of IdUrd (10 microM) was used with FUra (1 microM), cell cycle distribution returned to nearly normal, and radiosensitization was equal to that predicted by the extent of incorporation of IdUrd. These data demonstrate that the combination of FUra and IdUrd can produce radiosensitization both through increased IdUrd incorporation and cell cycle redistribution. Furthermore, they suggest that, in the presence of a modulator, it may not be necessary to achieve high levels of IdUrd incorporation to produce significant tumor radiosensitization.

Low dose rate irradiation and halogenated pyrimidine effects on human cervical carcinoma cells

The response of two human cervical carcinoma cell lines to the combination of bromodeoxyuridine (BrdUrd) incorporation and low dose rate irradiation (LDRI) was assessed. Survival curves were generated following both acute (67.2 Gy/hr) and low dose rate (0.11-0.62 Gy/hr) 137Cs gamma-ray irradiation. The two cell lines exhibited markedly different radiation responses, but for the chronic irradiation, there was no significant difference in response between the low dose rates for either cell line. In all cases, the low dose rate radiation response could be described using a single exponential y = e-alpha D. The sensitizer enhancement ratio (SER) at 10% survival was determined and for the first cell line the low dose rate slope (alpha) was approximately 0.7 Gy-1 with an SER of 1.3-1.5 and an SER of approximately 1.6 for the acute response. In contrast, the second cell line had a low dose rate slope (alpha) of approximately 0.4 Gy-1, an SER of 1.3-1.5, and an SER of approximately 1.7 for the acute response. The similarity of the SER's is notable in light of the marked differences between the intrinsic radiation response of the cells. As the concentration and/or time of exposure to BrdUrd increased, both cytotoxicity and the degree of sensitization also increased. This study demonstrates that SER's with BrdUrd and LDRI are comparable to those obtained for acute exposures in two cervical carcinoma cell lines. The results suggest that this combination of modalities may have clinical applications.

Enhanced IUdR radiosensitization by 241Am photons relative to 226Ra and 125I photons at 0.72 Gy/hr

The dependence of IUdR radiosensitization on photon energy was investigated by irradiating Chinese hamster cells in vitro under aerobic conditions at a dose rate of 0.72 Gy/hr which is typical of temporary brachytherapy implants. It had been observed previously that the IUdR radiosensitization with the 60 keV photons from 241Am is about 1.5 times greater than that with 830 keV (average) photons from 226Ra. It was hypothesized that the enhanced IUdR radiosensitization for 60 keV photons was a result of a larger production of Auger electron cascades from the filling of K-shell vacancies in the iodine atoms, which have a K-shell binding energy of 33.2 keV. Since most of the photons from a 125I source have energies below 33.2 keV, it would be expected that IUdR radiosensitization with 28 keV (average) photons from 125I and 830 keV (average) photons from 226Ra would both be smaller than the radiosensitization with the 60 keV photons from 241Am. To test this hypothesis we compared IUdR radiosensitization for 226Ra, 241Am, and 125I at 0.72 Gy/hr, using Chinese hamster lung cells in vitro. The measured survival curves led to RBEs of 1.20 +/- 0.10 and 1.30 +/- 0.11 for 241Am and 125I photons relative to 226Ra; to IUdR radiosensitization factors at a 10(-5) M concentration of 1.35 +/- 0.11, 1.67 +/- 0.09, and 1.47 +/- 0.08 for 226Ra, 241Am, and 125I, respectively; and to radiosensitization factors at a 10(-4) M concentration of 1.89 +/- 0.16, 3.04 +/- 0.13, and 2.48 +/- 0.17 for 226Ra, 241Am, and 125I, respectively. These results indicate that IUdR produces significant radiosensitization with all three isotopes (226Ra, 241Am, and 125I) for continuous low dose rate irradiations at 0.72 Gy/hr. Also, we observed greater radiosensitization with 241Am photons compared to 226Ra on the higher energy side and to 125I on the lower energy side. These findings support the concept that photon-induced Auger electrons produce a significant increase in IUdR radiosensitization when photons with energies just above the K-edge of the iodine atom are employed for continuous low dose rate irradiations. These findings suggest that regimens combining IUdR infusion with temporary brachytherapy implants using low energy photons in relatively quiescent sites such as brain tumors may have clinical potential, and indicate the need for rigorous preclinical evaluation of this approach.

Enhancement of IUdR radiosensitization by low energy photons

The effect of the photon energy on the radiosensitization produced by iododeoxyuridine (IUdR) was examined using Chinese hamster cells in vitro. Radiosensitization by IUdR was considerably higher for 60 keV photons from 241Am sources than for the 860 keV photons (average energy) from 226Ra sources, under continuous low dose rate conditions applicable to intracavitary brachytherapy (a dose rate of 0.57 Gy/hr). Also, IUdR radiosensitization was higher for 250 kV X rays than for 4 MV X rays under the acute exposure conditions used in external beam radiation therapy (dose rates of 1 to 2 Gy/min). These data support the hypothesis that photons with energies just greater than 32.2 keV, the K-absorption edge of iodine, are more effective in causing cell damage than are photons of other energies, because their absorption results in the production of Auger electron cascades and therefore in the production of high linear energy transfer (LET) radiations.