Cyclosporin A Enhances Susceptibility of Multi‐drug Resistant Human Cancer Cells to Anti‐P‐glycoprotein Antibody‐dependent Cytotoxicity of Monocytes, but Not of Lymphocytes

Cyclosporin A (CsA) was previously found to bind to P‐glycoprotein expressed on multidrug‐resistant (MDR) cancer cells. In the present study, the effect of CsA on anti‐P‐glycoprotein monoclonal antibody (mAb)‐dependent cell‐mediated cytotoxicity (ADCC) against human MDR cells was examined. The ADCC reaction was assessed by 4‐h (51)Cr‐release assay. Highly purified lymphocytes (> 99%) and monocytes (>99%) obtained from blood mononuclear cells (MNC) of healthy donors were used as effector cells. CsA decreased the cytotoxic activity of MNC against MDR cells, but enhanced their ADCC activity in the presence of anti‐P‐glycoprotein mAb MRK16. Lymphocyte‐mediated ADCC and natural killer activity against MDR cells were also suppressed by addition of CsA. CsA induced a significant dose‐dependent increase in monocyte‐mediated ADCC activity. Interestingly, pretreatment of MDR cancer cells, but not of monocytes, with CsA significantly enhanced ADCC activity mediated by monocytes, but not by lymphocytes. A CsA analog (PSC833) and FK‐506, but not verapamil also increased the sensitivity of MDR cells to ADCC by monocytes. CsA did not affect the binding of monocytes to MDR cells in the presence of MRK16 mAb. These results indicate that CsA may directly enhance the susceptibility of MDR cancer cells to the monocyte‐mediated ADCC reaction.