Residual damage in mouse lungs at long intervals after cyclophosphamide treatment

The purpose of these studies was to quantify the effects of radiation given to mouse lungs at intervals up to 6 months after injection of the maximally tolerated dose of cyclophosphamide. In one set of experiments a single i.p. injection of 300 mg/kg of cyclophosphamide was followed at either 1, 3, or 6 months by a range of single doses of gamma-rays delivered to the whole thorax only. In a second set of experiments mice were given five daily i.p. injections of cyclophosphamide, 100 mg/kg, followed at 1, 3, and 6 months by a range of fractionated doses of X-rays. Breathing rate, histology, and mortality were used to assess lung damage. These data were compared with age-matched animals given either the drug alone or single doses of radiation alone. Dose-response curves of lethality were constructed and fitted by a logit program, and 50% lethal doses with 95% confidence limits were determined at monthly intervals after irradiation. Dose enhancement factors were then calculated at this isoeffect for the mice given the drug and radiation. Deaths from radiation pneumonitis occurred as early as 6 weeks in mice given cyclophosphamide before irradiation; few deaths occurred after 26 weeks. However, in the mice given radiation alone, deaths from pneumonitis did not occur before 12 weeks. Cyclophosphamide given as either single doses or fractionated doses at all three times before irradiation enhanced radiation pneumonitis in mouse lung. Dose enhancement factors of 1.2, 1.4, and 1.3 were obtained when single doses of radiation followed single doses of cyclophosphamide at 1, 3, and 6 months, respectively. The dose enhancement factor for radiation pneumonitis after the fractionated exposures was less, 1.1, and was independent of time between the two treatments. An enhancement factor of 1.2 was observed for the later wave of lung damage in those few studies available for analysis at this time. These data clearly show that prior treatment of the animal with cyclophosphamide significantly reduces the radiation dose that can be given to the lung for as long as 6 months after drug treatment. In addition, lung damage occurred sooner when the drug was given prior to irradiation. These data indicate that the lung will be sensitive to retreatment with radiation when a full tolerance dose of cyclophosphamide precedes radiation.