Triazolopyrimidine herbicides are potent inhibitors of Aspergillus fumigatus acetohydroxyacid synthase and potential antifungal drug leads

Aspergillus fumigatus is a fungal pathogen whose effects can be debilitating and potentially fatal in immunocompromised patients. Current drug treatment options for this infectious disease are limited to just a few choices (e.g. voriconazole and amphotericin B) and these themselves have limitations due to potentially adverse side effects. Furthermore, the likelihood of the development of resistance to these current drugs is ever present. Thus, new treatment options are needed for this infection. A new potential antifungal drug target is acetohydroxyacid synthase (AHAS; EC 2.2.1.6), the first enzyme in the branched chain amino acid biosynthesis pathway, and a target for many commercial herbicides. In this study, we have expressed, purified and characterised the catalytic subunit of AHAS from A. fumigatus and determined the inhibition constants for several known herbicides. The most potent of these, penoxsulam and metosulam, have K-i values of 1.8 +/- 0.9 nM and 1.4 +/- 0.2 nM, respectively. Molecular modelling shows that these compounds are likely to bind into the herbicide binding pocket in a mode similar to Candida albicans AHAS. We have also shown that these two compounds inhibit A. fumigatus growth at a concentration of 25 mu g/mL. Thus, AHAS inhibitors are promising leads for the development of new anti-aspergillosis therapeutics.