The evolutionary relationship between plants and the malarial parasite Plasmodium falciparum is well established and underscored by the P. falciparum apicoplast, an essential chloroplast-like organelle. As a result of this relationship, studies have demonstrated that herbicides active against plants are also active against P. falciparum and thus could act as antimalarial drug leads. Here we show the converse is also true; many antimalarial compounds developed for human use are highly herbicidal. We found that human antimalarial drugs (e.g. sulfadiazine, sulfadoxine, pyrimethamine, cycloguanil) were lethal to the model plant Arabidopsis thaliana at similar concentrations to market herbicides glufosinate and glyphosate. Furthermore, the physicochemical properties of these herbicidal antimalarial compounds were similar to commercially used herbicides. The implications of this finding that many antimalarial compounds are herbicidal proffers two novel applications: (i) using the genetically tractable A. thaliana to reveal mode-of-action for understudied antimalarial drugs, and (ii) co-opting antimalarial compounds as a new source for much needed herbicide lead molecules.

Herbicides are an essential tool not only in weed management, but also in conservation tillage approaches to cropping. The first commercial herbicides were released in the 1940s and hundreds more since then, although genetic resistance to them is an issue. Here, we review the experimental and estimated physicochemical properties of 334 successful herbicidal compounds and make available a dynamic electronic database containing detailed analyses of the main chemical properties for herbicides and which adopts the Simplified Molecular-Input Line-Entry System (SMILES) for describing the structure of chemical molecules. This fully available resource allows for the rapid comparison of potential new herbicidal compounds to the chemical properties of known herbicides.