Radiosensitization of primary human tumor cell cultures by N-methylformamide

Induction of myogenic differentiation in human rhabdomyosarcoma cells by ionising radiation, N,N-dimethylformamide and their combination

Differentiation-inducing ability of gamma-radiation, N,N-dimethylformamide and their combination has been tested on human rhabdomyosarcoma RMZ-RC2 clone cells. Ionising radiation at 2-5 Gy doses induced a more differentiated morphology, with the appearance of an increased proportion of multinuclear myotube-like cells, and a significant increase in myosin-positive and multinuclear cells. Radiation appeared to act by inducing de novo differentiated elements. N,N-dimethylformamide was able to induce an increased myosin expression, but did not affect multinuclear cell proportion. The combined treatment (ionising radiation and N,N-dimethylformamide) resulted in an additive increase in the proportion of myosin-positive cells, approaching 25-35%, but de novo differentiated elements were not increased above the levels obtained with irradiation alone.

Enhancement of the radiosensitivity of two human tumour cell lines by hexamethylene bisacetamide

The effect of the maturation-inducing polar solvent, hexamethylene bisacetamide (HMBA), on the radiosensitivity of two human tumour cell lines (clone A, a colon carcinoma; and EJ, a bladder carcinoma) was investigated. Exposure of clone A or EJ cells to HMBA resulted in a concentration-dependent increase in doubling time, a decreased plating efficiency and changes in cell morphology, which are consistent with the formation of a better-differentiated phenotype. Growth of clone A cells in 2 or 3 mM HMBA, followed by irradiation and plating into HMBA-free medium, resulted in a significant enhancement in radiosensitivity, as determined by colony-forming ability. A similar increase in radiosensitivity was detected for EJ cells; however, for these cells a concentration of 7 mM HMBA was required. The increased radiosensitivity caused by HMBA was observed primarily in the low-dose, shoulder region of the gamma-ray cell survival curves for both cell lines, which is reflected by an increase in the alpha component of the survival curve with essentially no effect on beta. These data indicate that HMBA can radiosensitise human tumour cells at concentrations and for exposure periods that can be achieved in the clinic.

Inhibition of recovery from damage induced by ionizing radiation in mammalian cells

Recent studies indicate the importance for radiotherapy of the inherent radiosensitivity of tumour cells in the low-dose region; a region where recovery is probably almost complete. Improvements in radiotherapy may therefore depend on the search for specific inhibitors of cellular recovery. This review summarizes data from studies in which use was made of a variety of mammalian, including human cell systems, where attempts have been made to inhibit recovery using chemical agents. Inhibition of sublethal damage repair, potentially lethal damage repair and low dose-rate sparing has been observed to varying extents by several agents including those thought to act by interfering with DNA repair processes (e.g. ara-C, 3-aminobenzamide) differentiation-inducing agents (e.g. N-methylformamide), halogenated pyrimidines (e.g. iododeoxyuridine), caffeine and chemotherapeutic agents (e.g. adriamycin). No individual agent stands out as exerting an exceptionally dramatic effect on recovery. However, of the agents used clinically, cis-platinum appears to hold some promise, while iododeoxyuridine, N-methylformamide, beta ara-A and caffeine all appear to inhibit to some extent the recovery of ionizing radiation-induced damage. There is an urgent need for further study to determine, in particular, relative effects in tumour versus normal cell types and whether any agents found to be effective in vitro show similar effects in vivo.