Bacillus anthracis is one of the most dangerous potential biological weapons. Currently, there is no effective treatment for inhalational anthrax, beyond the administration of antibiotics shortly after exposure. Time delay reduces the effectiveness of antibiotic treatment. Therefore, there is a need for new safe and efficient treatments to supplement the traditional antibiotic intervention. Major factors playing a role in anthrax infection are the cytotoxic effect of anthrax toxin, and bacteremia leading to oxygen and nutritional substance deprivation, accumulation of various bacterial and host toxic products with eventual organ failure and death.

Our research is based on the premise that the simultaneous blocking of bacterial growth by antibiotics and inhibition of anthrax toxin action would be beneficial for the treatment of anthrax. The two anthrax toxins are formed by three different proteins: protective antigen (PA) which either combines with lethal factor (LF) to form lethal toxin (LeTx), or with edema factor (EF) to form edema toxin (EdTx). LF and EF are enzymes targeting substrates within the cytosol, and PA facilitates their transport across the cell membrane forming a heptameric pore.

Recently, we demonstrated that treatment of Bacillus anthracis infected mice with a combination of the antibiotic ciprofloxacin and partially purified antibodies against anthrax protective antigen dramatically increased survival rates in comparison with antibiotic treatment alone (click here to see the paper). Although promising, antibodies are less attractive as potential drugs in comparison with low molecular weight compounds, which offer potentially better penetration through membranes and are not sensitive to proteases.

Our hypothesis is that low molecular weight compounds designed to block the pore formed by PA can inhibit anthrax toxin action. PA assembles into a ring-shaped heptamer and exposes a hydrophobic surface for the binding of LF and EF. The information available on the three-dimensional structure of the PA pore permits computer-assisted rational drug design.

As a starting point for the development of high-affinity blockers of the PA we decided to use beta- cyclodextrin (β-CD), which is a cyclic molecule, comprised of seven D-glucose units and having sevenfold symmetry, like the PA pore. We have designed and synthesized a number of β-cyclodextrin derivatives and evaluated their ability to inhibit lethal toxin action. Several compounds displayed anti-toxin activity at low micromolar concentrations in cell-based assays. Preliminary toxicity and efficacy studies in rodents produced very promising results.

The obtained results will serve as the basis for a structure-based drug discovery program with the eventual goal of finding new drug candidates for anthrax treatment. Persubstituted β-cyclodextrin derivatives can potentially also be utilized for blocking of other toxins that form heptameric transmembrane channels, such as staphylococcal α-hemolysin. More generally, this can be considered as a general approach for the discovery of new drugs by blockage of pores with molecules having the same dimensions and and symmetry as the pores.