Hemodynamic and anti-Toxin Treatments in Anthrax Lethal Toxin Challenged Canines

尽管在2001年美国炭疽热爆发中，对传统血液动力学支持和血管加压剂的休克抵抗导致了所有5名非幸存者的死亡，但其机制和最佳管理仍不清楚。致死毒素（LeTx）和水肿毒素（ETx）是炭疽的主要致病因子。LeTx和ETx都含有保护性抗原（PA），这是细胞摄取有毒物质所必需的；LeTx为致死因子（LF）， ETx为水肿因子（EF）。致死因子是一种抑制丝裂原活化蛋白激酶（mapkk1 - 4,6和7）的锌金属蛋白酶，而EF是一种钙调素依赖的腺苷环化酶。最近的研究表明，LeTx和ETx产生的细胞变化不同于更常见的细菌毒素，如脂多糖（LPS）或革兰氏阳性生物（如金黄色葡萄球菌）的外毒素。尽管进行了这项研究，但确定这些毒素对心血管影响的体内工作有限。此外，尽管在开发和测试新的毒素导向药物方面已经做出了重大努力，但在炭疽毒素相关休克期间，传统的液体、血管加压剂和肌力支持的效果却很少受到关注。我们在实验室所做的大部分工作表明，这些反应可能很难预测。

Although shock resistant to conventional hemodynamic support with fluids and vasopressors contributed to death in all five non-survivors in the 2001 United States anthrax outbreak, its mechanisms and optimal management remain unclear. Lethal (LeTx) and edema (ETx) toxin are major contributors to the pathogenesis of anthrax. Both LeTx and ETx consist of protective antigen (PA), which is necessary for cellular uptake of the toxic moities; lethal factor (LF) for LeTx and edema factor (EF) for ETx. Lethal factor is a zinc metalloproteinase that inhibits mitogen activated protein kinase kinases (MAPKK1-4, 6 and 7) while EF is a calmodulin dependent adenyl cyclase. Recent research has shown that LeTx and ETx produce cellular changes different from those associated with more commonly encountered bacterial toxins such as lipopolysaccharide (LPS) or the exotoxins of gram-positive organisms like S. aureus. Despite this research, there has been limited in vivo work defining these toxins cardiovascular effects. Furthermore, although there have been major efforts to develop and test new toxin directed agents, little attention has been paid to the efficacy of conventional fluid, vasopressor and inotrope support during anthrax toxin associated shock. Work done largely in our laboratory suggests these responses may be difficult to predict. In a rat model, 24 h LeTx infusion produced lethality and shock, but it did not alter circulating inflammatory cytokines, nitric oxide or catecholamines. In this model, while PA directed monoclonal antibody (PA-mAb, 5H3 or Raxibacumab) (HGS, Rockville, MD) was protective, regimens of fluid or norepinephrine support beneficial with LPS or E. coli, were not with LeTx (4 - 6). In fact fluids not only worsened survival with LeTx but also negated the effects of PA-mAb. Although informative, the rat model did not permit the duration (i.e. > 24h) or type of hemodynamic monitoring (i.e. systemic and pulmonary arterial catheter measures and serial cardiac imaging) necessary to fully characterize the cardiovascular effects of LeTx and ETx. Therefore a canine model was developed with support from the Trans-NIH/FDA Biodefense Program. In initial studies, LeTx challenge in the canine produced many of the same changes noted in the rat. LeTx was also associated with myocardial depression in the canine. The purpose of present study is to use the now developed canine anthrax LeTx model to comprehensively investigate the effects of fluid and vasopressor treatment alone and in combination with a toxin directed treatment. An advantage of this model not possible in smaller animal models is the ability to titrate hemodynamic support based on measures of preload and perfusion pressure just as it is done in critically ill patients. Specifically, this study will address whether a standard regimen (fluid and norepinephrine infusion) or a toxin directed therapy (i.e. 5H3-mAb or Raxibacumab) are beneficial when used alone or together in canines challenged with 24 h infusions of LeTx. Based on earlier studies with this canine model, treatment algorithms have been developed for the sequential administration of fluid and norepinephrine titrated using preload measures. Studies examining the effects of hemodynamic support and Raxibacumab with LeTx challenge are complete. Addition of Raxibacumab to hemodynamic support improved hemodynamics, reduced vasopressor needs and fluid retention and increased survival following LeTx challenge. This data was presented at the International Conference of the American Thoracic Society 2011 and has now been published. (Barochia et al. Protective antigen antibody augments hemodynamic support in anthrax lethal toxin shock in canines. J Infect Dis. 2012; 205: 818-829.) Studies are now also complete examining the effects of Raxibacumab and hemodynamic support alone or together with ETx challenge alone and with ETx and LeTx in combination inthis same canine model. Raxibacumab also augmented hemodynamic support and improved survival with ETx alone and with ETx and LeTx together. These findings were presented at the International Conference of the American Thoracic Society in 2013 and 2014. Additional agents are now under consideration for study in this model.