Photodamaging effects of merocyanine 540 on neutrophils and HL-60 cells

Merocyanine 540 (MC540) is a photosensitizing dye that has been used in several preclinical models and in a phase I clinical trial for the extracorporeal purging of tumor cells from autologous bone marrow grafts. The mechanism of the cytotoxic activity of MC540 is not yet fully understood, and the subcellular targets of MC540-mediated photodynamic damage remain to be identified. The human neutrophil provides an attractive model with which to study the effects of photoactivated MC540 on several well-defined cellular functions. As we report in this paper, simultaneous exposure of neutrophils to MC540 and light inhibited phagocytosis, random migration, chemotaxis, hydrogen peroxide production, and oxygen consumption. By contrast, the ability of neutrophils to kill engulfed bacteria and to produce superoxide radical was not compromised. Intracellular ATP levels and the activities of the cytosolic enzymes superoxide dismutase, catalase, and myeloperoxidase were only slightly reduced. Even in HL-60 leukemia cells, which bind more dye and are more readily killed by MC540-mediated photodynamic therapy than neutrophils, superoxide dismutase, catalase, and myeloperoxidase activities remained at normal or near-normal levels. These results are compatible with the view that plasma membrane components are primary targets of MC540-mediated photodynamic damage.