Challenges for developing selective fungal/protozoal carbonic anhydrase inhibitors as anti-infectives

Carbonic anhydrases (CAs, EC 4.2.1.1) were characterized in several fungi (Cryptococcus neoformans, Candida albicans and C. glabrata, Saccharomyces cerevisiae, Malassezia globosa, M. restricta and M. pachydermatis, Sordaria macrospora, Aspergillus fumigatus and A. oryzae) and protozoans (Trypanosoma cruzi, Leishmania donovani chagasi, Plasmodium falciparum, Entamoeba histolytica, Trichomonas vaginalis, Toxoplasma gondii) being also shown that they are present in Acanthamoeba castellanii. These enzymes belong to various genetic families (α- and β-CAs for fungi, α-, β-, γ- and η-classes for protozoans), showed significant CO₂ hydrase activity and a vast number of inhibitors were detected belonging to the inorganic anions, sulfonamides, phenols, mono-/dithiocarbamates, boronic acids, benzoxaboroles, or coumarins. However, few of them showed anti-infective properties in vivo or ex vivo, due to the limited number of such studies. Promising results were however obtained with sulfonamides showing antimalarial, anti-Malassezia spp., anti-T. cruzi and anti-leishmanial action against various strains of these pathogens, sometimes resistant to clinically used drugs. The main challenges for obtaining effective antifungals/antiprotozoan agents based on CA inhibitors are: (i) the complex life cycles of most of these pathogens, which frequently have different stages, hosts and diverse gene expression and metabolic patterns; (ii) lack of detailed structural data for many such enzymes; (iii) lack of focused drug design campaigns for the specific enzymes found in these pathogens, and (iv) lack of simple, inexpensive in vivo models for their testing. Future work in the field that should address these limitations might lead to relevant developments for obtaining novel anti-infectives.