Pentasaccharide anticoagulant fondaparinux inhibits diphtheria toxin

Diphtheria toxin (DT), an AB-type protein exotoxin, is the main virulence factor of Corynebacterium diphtheriae and the causative agent of diphtheria, a life-threatening disease especially in children. DT binds to its receptor heparin-binding EGF-like growth factor on human target cells via its binding subunit DTB, enters cells by receptor-mediated endocytosis and delivers its enzyme subunit DTA into the cytosol. There, DTA catalyzes the ADP-ribosylation of elongation factor 2, which inhibits protein synthesis and leads to cell death. The number of diphtheria cases is increasing worldwide despite routine vaccination against DT in many countries. Although diphtheria mortality is significantly reduced by standard diphtheria antitoxin therapy, there are drawbacks to this therapy and new therapeutic strategies are highly desirable. In this study, we identified the approved anticoagulant drug fondaparinux, the pharmacologically active pentasaccharide sequence within heparin, as potent inhibitor against DT in vitro. Fondaparinux protected eukaryotic cells from intoxication with DT, whereas unfractionated and low molecular weight heparins did not. When DT was applied to cells in the presence of fondaparinux, the ADP-ribosylation of elongation factor 2 was significantly reduced in these cells and their protein synthesis was maintained. By investigating the inhibitory mechanisms of fondaparinux against DT, we found that fondaparinux prevented DT binding to cells and thus DT uptake. As fondaparinux is a licensed drug with well-known toxicity and pharmacokinetic profiles, these findings should provide a starting point for the development of novel pharmacological strategies to treat diphtheria, which is considered a re-emerging disease, also in western countries.