[Merocyanine 540-mediated photodynamic therapy inhibits P-glycoprotein (P-gp) activity in adriamycin-resistant K562...

The photosensitizing dye merocyanine 540 (MC540) has been used in preclinical models and in a phase I clinical trial in the U.S.A. for the extracorporeal purging of autologous bone marrow grafts contaminated with leukemia or lymphoma. In this communication, we report MC540-mediated photodynamic therapy (PDT) was effective in purging leukemic cells expressing P-gp. When K562 and K562/ADM were exposed to MC540 (15 micrograms/ml) and white light (145.8 kJ/m2), the concentration of K562 and K 562/ADR was reduced by 1.8 and 3.0 log, respectively. Using flow cytometry and confocal laser scan microscopy, MC540 and calcein-AM were bound intracellularly and effluxed by P-gp in K562/ADM. In K562/ADM, calcein-AM efflux was inhibited by P-gp modulator, cyclosporin A (5 microM) and verapamil (15 micrograms/ml). In contrast, MC540 efflux was inhibited by cyclosporin A but not verapamil. Furthermore, MC540-mediated PDT inhibited efflux of calcein-AM and MC540, and induced the accumulation of dyes in K562/ADM. We conclude that MC540 is a substrate of P-gp and that MC540-mediated PDT is useful for purging MDR cells through inhibition of P-gp activity.