The response of murine stem spermatogonia to radiation combined with 3-aminobenzamide

Nicotinamide and other benzamide analogs as agents for overcoming hypoxic cell radiation resistance in tumours. A review

Oxygen deficient hypoxic cells, which are resistant to sparsely ionising radiation, have now been identified in most animal and some human solid tumours and will influence the response of those tumours to radiation treatment. This hypoxia can be either chronic, arising from an oxygen diffusion limitation, or acute, resulting from transient stoppages in microregional blood flow. Although clinical attempts to overcome hypoxia have met with some success, the results have been far from satisfactory, and efforts are still being made to find better methods. Extensive experimental studies, especially in the last decade, have shown that nicotinamide and structurally related analogs can effectively sensitise murine tumours to both single and fractionated radiation treatments and that they do so in preference to the effects seen in mouse normal tissues. The earliest studies suggested that this enhancement of radiation damage was the result of an inhibition of the repair mechanisms, as was well documented in vitro. However, recent studies in mouse tumours have shown that the primary mode of action actually involves a reduction in tumour hypoxia. More specifically, these drugs prevent transient cessations in blood flow, thus inhibiting the development of acute hypoxia. This novel discovery led to the suggestion that the potential role of these agents as radiosensitizers would be when combined with treatments that overcame chronic hypoxia. The first attempt to demonstrate this combined nicotinamide with hyperthermia and found that the enhancement of radiation damage by both agents together was greater than that seen with each agent alone. Similar results were later seen for nicotinamide combined with a perfluorochemical emulsion, carbogen breathing, and pentoxifylline, and in all these studies the effects in tumours were always greater than those seen in appropriate normal tissues. Of all the analogs, it is nicotinamide itself which has been the most extensively studied as a radiosensitizer in vivo and the one that shows the greatest effect in animal tumours. It is also an agent that has been well established clinically for the treatment of a variety of disorders, with daily doses of up to 6 g being considered reasonably safe and associated with a low incidence of side effects. This human dose is equivalent to 100-200 mg/kg in mice and such doses will maximally sensitize murine tumours to radiation. These findings have now resulted in phase I/II clinical trials of nicotinamide, in combination with carbogen breathing, as a potential radiosensitizing treatment.

The induction of chromosomal damage and cell killing in mouse spermatogonial stem cells following combined treatments with hydroxyurea, 3-aminobenzamide and X-rays

To analyze in more detail the relation between the sensitivity of spermatogonial stem cells to killing and the induction of genetic damage, mature male mice received combined treatments with hydroxyurea (HU), 3-amino-benzamide (3-AB) and X-rays. Stem cell killing was determined using the repopulation index method and translocations were studied via spermatocyte analysis. HU was administered at 16 or at 48 h before further treatment in order to create stem cell populations with different sensitivities in which the translocation induction and stem cell killing could be studied and compared. The sensitivities for cell death and genetic damage appeared to be strongly correlated: at 16 h after HU significantly higher values were found than at 48 h or in controls without HU pretreatment. By using 3-AB in the treatment schedules we were able to investigate whether the sensitization of stem cells towards cell death and genetic damage is the outcome of a radiation- or drug-induced G1 delay. The effect of 3-AB was most pronounced at 16 h after HU. This confirms that at this interval a large fraction of stem cells is in G1. Our data therefore indicate that all treatments that induce an enrichment of G1 cells also result in a sensitization of stem cells to cell killing or the induction of mutagenic damage.

Metoclopramide enhances the effect of ionizing radiation on xenografted squamous cell carcinoma of the head and neck

The commonly used drug metoclopramide, a benzamide derivative, has been shown previously in our laboratory to enhance the effect of cisplatin on xenografted squamous cell carcinomas of the head and neck. In the present study, we show that metoclopramide also enhances the effect of ionizing radiation. Two human squamous cell carcinoma lines of the head and neck xenografted to nude mice have been used. Doses of radiation were chosen (5 and 8 Gy single doses) which caused only a slight retardation of tumor growth when administered alone. Tumor response to ionizing radiation was assessed with and without metoclopramide (2.0 mg kg-1), and administered at the time of radiation and 24 and 48 hr after treatment. The effects of these schedules on the tumors were compared using the reduction of the area under the growth curves and specific growth delay. The dose schedule with metoclopramide alone did not induce any significant reduction in the area under the growth curves. The addition of metoclopramide to the radiated groups caused a significant enhancement of the radiation-induced reduction of the area under the growth curves in both of the tumor lines studied.

Dose-effect relationship for X-ray-induced reciprocal translocations in mouse spermatogonia following pretreatment with 3-aminobenzamide

The influence of 3-aminobenzamide (3-AB) pretreatment on the dose-response relationship for radiation-induced reciprocal translocations in mouse spermatogonial stem cells was studied. The results show that at doses of 3-10 Gy of X-rays the frequencies of translocations were higher in 3-AB-pretreated animals as compared to animals that received X-rays only. The 3-AB pretreatment was not effective at dose levels of 1 and 2 Gy. The shape of the dose-effect curve was similar to that obtained without 3-AB pretreatment, i.e., a humped curve, but the initial slope was clearly steeper and the position of the peak was shifted from 7 to 9 Gy. The effects observed can be explained by a 3-AB-mediated sensitization of normally radioresistant stem cells that are at the stage of stimulation to enter the mitotic cycle, thus increasing the population of radiosensitive spermatogonial stem cells.

The effect of 3-aminobenzamide in the radiation response of three human cervix carcinoma xenografts

By means of growth delay measurements, the in vivo radiation response of three recently established human cervix carcinoma xenograft lines has been determined at both high (70 cGy/min) and low dose rate (5 cGy/min). In addition, we have investigated the role of the polyADP-ribosylation inhibitor, 3-aminobenzamide, (3-AB, administered at 450 mg/kg) in modifying the xenograft response to continuous low dose rate irradiation (5 cGy/min). For all three lines, less growth delay was observed for 9 Gy administered at 5 cGy/min compared to 70 cGy/min; in terms of the times for tumours to reach twice their original weight (T2 values), low dose rate effect sparing ratios of 2.1 for HX 155, 1.5 for HX 156 and 2.4 for HX 160 were observed. 3-AB exerted no significant effect on the growth of unirradiated tumours. When 3-AB was given during irradiation at 5 cGy/min to 9 Gy, an enhancement in growth delay was observed for each line. However, the degree of radioenhancement by 3-AB varied among the lines; significant enhancement (p less than 0.01) at all time points tested was observed for HX 156, while for the other two lines, the degree of enhancement was not significant. Enhancement effect ratios (in terms of T2 values) were 1.37 for HX 155, 1.55 for HX 156 and 1.02 for HX 160. Similar differential radiosensitizing effects with 3-AB have been observed previously in vitro. More potent poly(ADP-ribosylation) inhibitors are probably required along with additional normal tissue studies before such an approach may be proposed at the clinical level.

Radiosensitization by nicotinamide in tumors and normal tissues: the importance of tissue oxygenation status

Nicotinamide induced radiosensitization of tumors has been suggested to be a consequence of a reduction in tumor hypoxia. We have investigated the possibility that nicotinamide may produce significant radiosensitization in a normal tissue in which the radiation response is also influenced by hypoxia. The normal tissue studied was testis and radiation damage was assessed by measuring survival of spermatogonial stem cells. The radiosensitizing action of nicotinamide in testis was compared to that observed in a C3H mammary carcinoma when assayed by both regrowth delay and local tumor control. Our results show that nicotinamide (1000 mg/kg; i.p.) enhanced radiation damage in both tissue types when the radiation was given up to at least 3 hr after drug injection. Enhancement ratios obtained when the drug and radiation were separated by a 1 hr time interval were between 1.1 to 1.2 for the testis and 1.0 to 1.5 for the tumor. The results suggest that nicotinamide will produce radiosensitization in testis, but the effect is small and less than that observed in tumors.

Differential radiosensitization by the poly(ADP-ribose) transferase inhibitor 3-aminobenzamide in human tumor cells of varying radiosensitivity

Four newly-established human tumor cell lines, have been irradiated at dose rates of 150 and 3.2 cGy/min to compare their capacity to repair radiation damage. They included a neuroblastoma, a germ-cell carcinoma of the testis, a large cell carcinoma of the lung, and a carcinoma of the cervix. The four lines varied in their sensitivity to high dose-rate irradiation, with the neuroblastoma being most radiosensitive and the lung and cervix tumors the most radioresistant. The extent of dose sparing associated with lowering the dose rate to 3.2 cGy/min was similar in three of the lines but somewhat greater in the case of the cervix carcinoma cell line. The presence of non-toxic concentrations of the poly(ADP-ribose) transferase inhibitor, 3-aminobenzamide (3-AB), enhanced the response of 3 of the 4 tumors to irradiation; it failed to modify the sensitivity of a lung carcinoma cell line. The extent of sensitization was generally similar at high and low dose rate. Measurement of poly(ADP-ribose) transferase activity in control and irradiated cells showed the neuroblastoma cells to contain much higher initial levels than the other three lines but there were no significant differences in the extent of stimulation in enzyme levels after irradiation. Survival curves obtained at low dose-rate help define the initial slope of the acute curve and it appears that 3-AB may exert a differential effect among human tumors in modifying this component.