Evidence for an important role of neutrophils in the efficacy of photodynamic therapy in vivo

Abstract

To investigate the role of neutrophils in the efficacy of photodynamic therapy (PDT) in rhabdomyosarcoma-bearing rats, the number of these circulating phagocytes was decreased or increased before interstitial PDT by use of rabbit anti-rat neutrophil serum or granulocyte colony-stimulating factor, respectively. After administration of the antiserum, the number of circulating neutrophils decreased by 99.9%. However, the number of monocytes, lymphocytes, and platelets decreased as well (by 100%, 80%, and 25%, respectively). Under these conditions, PDT did not retard tumor growth at all. However, after cessation of the antiserum treatment 5 days after PDT, a striking decrease in the growth rate occurred subsequent to an increase above the normal range of the number of circulating neutrophils. Administration of the granulocyte colony-stimulating factor led to a specific 4-fold increase in the number of circulating neutrophils. In these rats, the tumor growth at day 2 after PDT was retarded as compared with PDT-treated rats that received saline only. Statistical evaluation of both experimental conditions showed that the efficacy of PDT, expressed as the percentage of change in tumor volume at day 2 after treatment, was dependent on the number of circulating neutrophils present at the day of PDT (P = 0.001; r2 = 0.482). Apparently, neutrophils are indispensable for successful PDT in vivo.