Cyclophosphamide-Induced Depression of the Antioxidant Defense Mechanisms of the Lung

Abstract

Cyclophosphamide causes lung toxicity in a wide variety of animals, including humans. Recent evidence suggests that oxygen (O₂) potentiates cyclophosphamide-induced pulmonary injury. We hypothesized that cyclophosphamide or one of its toxic metabolites, acrolein, may potentiate O₂ toxicity by depressing lung antioxidant defense mechanisms. To test this, we gave rats cyclophosphamide (100 mg/kg), acrolein (5 mg/kg), or a vehicle (control) in a single intraperitoneal injection and then killed them during a 5-day study period. Excised lungs were analyzed for reduced glutathione (GSH) content, glucose-6-phosphate dehydrogenase (G6PD), glutathione reductase (GSH-R), glutathione peroxidase (GSH-P), and superoxide dismutase (SOD) activities. In the lungs of cyclophosphamide-treated rats, GSH content was increased 48% (P < 0.001) on day 2 but progressively decreased to 50% of control values (P < 0.001) on day 5. Significant reductions (P < 0.005) in G6PD, GSH-R, and GSH-P activities occurred on days 1–5, and SOD activity was significantly decreased (P < 0.005) on days 4 and 5 by cyclophosphamide. In acrolein-treated rats, GSH content and GSH-R, GSH-P, and SOD activities were indistinguishable from those in controls. However, G6PD was increased (35–38%) on days 2 and 3 but returned to control values thereafter. To assess whether the cyclophosphamide-induced reduction in lung antioxidant defenses increased susceptibility to acute O₂ toxicity, we gave a separate group of rats cyclophosphamide, acrolein, or vehicle, and 4 days later exposed them to 100% O₂ or air at 1 atmosphere absolute. All cyclophosphamide-, acrolein-, and vehicle-treated rats survived 60 h air exposure, and all vehicle-treated rats exposed to 100% O₂ survived. In contrast, all of the cyclophosphamide-treated rats exposed to 100% O₂ died (P < 0.05) within 40 h. Acrolein had no effect on survival in 100% O₂. These results indicate that cyclophosphamide, but not acrolein, depresses lung antioxidant defense mechanisms, which may be responsible for increased mortality from O₂ toxicity in cyclophosphamide-treated animals.