The effect of P‐glycoprotein antagonists on short‐chain ceramide cytotoxicity in cancer cells

P‐glycoprotein (P‐gp) antagonists block ceramide (cer) glycosylation. We tested whether this property would enhance cer‐driven cell death by partnering short‐chain cer (C6‐cer) with commonly employed P‐gp antagonists. In leukemia cells (KG‐1), C6‐cer (10 μM) and tamoxifen (tam) (5 μM) reduced viability to 62 and 95% of control, respectively; when co‐administered, viability was reduced to zero. Although both tam and ethylenedioxy‐P4 (P4), a potent glucosylceramide synthase (GCS) inhibitor, blocked C6‐cer glycosylation by 80–90%, P4 did not enhance C6‐cer cytotoxicity. Combination C6‐cer/cyclosporin A (cycA) was also cytotoxic in KG‐1 and in vincristine‐resistant HL‐60 cells. In LoVo colon cancer cells, tam, verapamil, and cycA enhanced C6‐cer cytotoxicity over single agents, and C6‐cer/tam was synergistic for caspase 3 cleavage. Nanoliposomal formulations of C6‐cer and tam were synergistic (Combination Index = 0.37) for cell killing. Multidrug resistant ovarian cancer cells (2780AD) were refractory to C6‐cer and tam, but co‐administration halted cell growth and produced a 4‐fold increase in the levels of long‐chain cer. Addition of a cer synthase inhibitor failed to rescue 2780AD cells. Although the mechanism of action remains unclear, we conclude that the use of P‐gp antagonists is an effective alternative to targeting GCS for enhancing cytotoxicity of exogenous cer in cancer cells. NIH GM77391