Potential toxicity of polymyxins in human lung epithelial cells [PublishAheadOfPrint]

Inhaled polymyxins are of considerable utility to achieve optimal exposure in the respiratory tract for the treatment of lung infections caused by multidrug-resistant Gram-negative pathogens. Current inhaled polymyxin therapy is empirical and often high doses are used which may lead to potential pulmonary adverse effects. This study aimed to investigate the effect of polymyxins on human lung epithelial cells (A549). Viability of A549 cells was examined after treatment with polymyxins using flow cytometry. Activation of caspase-3, -8, -9, expression of Fas ligand (FasL), loss of mitochondrial membrane potential and mitochondrial oxidative stress induced by polymyxin B was evaluated. The EC₅₀ (95% CI) value of polymyxin B induced cell death was 1.74 mM (1.60 to 1.90 mM). Colistin was at least two-fold less toxic than polymyxin B while colistimethate (CMS) was non-toxic. With 2.0 mM polymyxin B, apoptotic cells were 30.6 ± 11.5% (mean ± SD) at 8 h and increased to 71.3 ± 3.72% at 24 h. Concentration- and time-dependent activation of caspase-3, -8 and -9 were evident while the activation of caspase-9 was more dramatic. Furthermore, polymyxin B caused concentration- and time-dependent FasL expression, production of mitochondrial reactive oxygen species and changes in mitochondrial membrane potential. This is the first study to demonstrate that both extrinsic death receptor pathway and intrinsic mitochondrial pathway are involved in polymyxin-induced toxicity in A549 cells. This knowledge base is critical for the development of novel strategies for the safe and effective inhalation therapy of polymyxins against Gram-negative 'superbugs'.

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