Influence of circadian rhythms on radiosensitivity: single and fractionated dose studies in mouse skin

Chronobiology of the mammalian response to ionizing radiation. Potential applications in oncology

Ionizing radiation from all sources under appropriate conditions leads to cell death and tissue damage. It is used in cancer treatment under the assumption of a higher radiosensitivity of the fast dividing tumor cells as compared with adjacent host tissues. The radiosensitivities of proliferating host tissues like bone marrow and gastrointestinal lining epithelium are dose limiting. Since these host tissues and many tumors show circadian and other periodicities in their cell proliferation, the timing of radiation treatment according to host and/or tumor rhythms is expected to improve the toxic/therapeutic ratio of the treatment. The experimental data on the chronobiology of radiation exposure show circadian rhythmicity in radiation response after whole body irradiation in mice and rats with highest toxicity in light-dark 12h:12h synchronized animals during their daily activity span. Bone marrow toxicity as well as gastrointestinal epithelial damage show circadian rhythms in part due to radiation damage to the stem cells involved and especially in the intestine also due to damage to the microvasculature. Chronoradiotherapy of malignant tumors seems promising, alone or in combination with response modifiers, provided the host and potential tumor rhythms can be monitored.

Chronomodulated chemotherapy and irradiation: an idea whose time has come?

The chronomodulated delivery of systemic chemotherapy given with irradiation (chemoradiation) is driven by an understanding of: the chronobiology of normal tissue response to cytotoxic insult, chronopharmacology, and by technologic advances in vascular access and in the availability of portable programmable pumps. Since circadian variation exists in the proliferative activity of acute-reacting normal tissues like the gut and bone marrow, a potential therapeutic gain can be realized by the chronomodulated administration of S-phase chemotherapeutic agents at biological times when these normal tissues are in a different cell phase and thus relatively spared (chronotolerance). The reasons for this are complex and possibly include newly described time-keeping genes that may influence the cell cycle. Another important aspect of chronotolerance is based on chronopharmacologic behavior of S-phase chemotherapeutic radiation sensitizing agents, especially 5-fluorouracil (5-FU). In this review laboratory and clinical evidence is presented for using chronomodulated 5-FU or the topoisomerase-I inhibitor, camptothecin, when best tolerated biologically. Although the main body of this work has been accomplished with pure chemotherapy schedules, there is emerging clinical evidence this approach to treatment also applies to the application of chemoradiation. This knowledge has been exploited only recently in the clinic. These data should be viewed as a call for additional studies to investigate the precise timing of systemic chemotherapeutic radio sensitizers to ameliorate toxicity and maximize treatment effect, especially with newer and potentially more toxic chemoradiation programs.

Effect of diet and vitamin C on DNA strand breakage in freshly-isolated human white blood cells

We have measured DNA strand breaks induced by ionising radiation in nucleated cells from freshly isolated whole blood from normal human subjects. Samples were taken after subjects had fasted overnight and again 1 h after they had eaten breakfast in combination with approximately 35 mg/kg vitamin C. Damage was measured by single cell gel electrophoresis (the 'comet' assay), in which DNA single strand breaks generate a comet tail streaming from the nucleus. In repeat experiments on 6 subjects a reduction in DNA damage, as indicated by a highly significant decrease in overall comet length, was observed following vitamin C ingestion, both in the unirradiated control blood samples and in the dose response to ionising radiation damage. In addition, consistent differences in dose response between individual subjects were found. The peak effect was 4 h after intake of food and vitamin C. An effect was also seen with vitamin C alone and after breakfast without additional vitamin C. Protection against strand breakage was also seen in Ficoll-separated mononuclear cells but evidence was not obtained for protection of separated, mitogen stimulated T-lymphocytes either against ionising radiation cell killing in a clonal assay, or against clastogenicity assessed by micronucleus formation following one cell division. Exposure of separated lymphocytes in vitro to vitamin C, at doses greater than 200 microM, did not offer protection but induced strand breakage. Our results raise the possibility that variation in normal diet may not only affect susceptibility to endogenous oxidative damage, but may affect some responses of the individual to radiation.