It has been reported that clarithromycin (CAM) augments the anti-tumor activity of thalidomide against multiple myeloma (MM) cells, while the mechanism remains unclear. A myeloma cell line or primary myeloma cells were treated with CAM. Autophagy was analyzed by morphological changes, LC3 expression and lysotracker staining. CAM induced vacuoles in the cytoplasm of MM cells which resembled autolysosomes. The manifestation of the CAM-induced vacuoles was blocked by treatment with PI3-kinase inhibitor. CAM induced an accumulation of LC3-II without affecting the mTOR or AKT pathways, eventually leading to cell death. CAM may halt the autophagy process after fusion of autophagosomes with lysosomes. This phenomenon may explain how CAM, combined with thalidomide, augments the cytotoxic effects of the latter on MM cells and suggests that modification of autophagy might represent a new approach for therapy of MM.

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