Introduction: Chironex fleckeri is a large box jellyfish that has been labelled the ‘most venomous animal’ in the world. We have recently shown that the primary effect of C. fleckeri venom in vivo is cardiovascular collapse. This study utilised a cell-based assay to examine the effects of C. fleckeri venom on the proliferation of a rat aortic smooth muscle cell line. In addition, the ability of CSL box jellyfish antivenom and/or various potential treatment strategies to neutralise the effects of the venom was examined. Methods: A7r5 cells were cultured in media containing venom. The effect of CSL box jellyfish antivenom (5 U/mL), CSL polyvalent snake antivenom (5 U/mL), lanthanum (5 μM), MgSO4 (50 mM), verapamil (5 μM) or felodipine (5 μM) was examined. Cell viability was determined using a Cell titer 96 AQueous One Solution cell proliferation assay. Results: Incubation of A7r5 cells with serially diluted venom (2–0.004 µg/mL) caused a concentration-dependent inhibition of cell proliferation with an IC50 value of 0.056 μg/mL. This response was not affected by the absence of calcium or the presence of lanthanum in the media. Box jellyfish antivenom (5 U/mL) prevented the inhibition of cell proliferation caused by the venom. Verapamil (5 μM) had no significant effect on the inhibition. In contrast, felodipine (5 µM) or MgSO4 (50 mM) potentiated the effects of the venom and partially negated the protective effect of the antivenom. Discussion: This study displayed the ability to utilise a cell-based assay to determine the effects of C. fleckeri venom on vascular cell viability. It showed that CSL box jellyfish can neutralise the effects of the venom but only if added prior to the venom. In addition, potential adjunct therapies verapamil, felodipine and MgSO4 were found to be ineffective, with felodipine and MgSO4 potentiating the detrimental effects of the venom
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