Evaluation and Development of Metal-Binding Agents that Alter Copper Bioavailability

During microbial infections, cells of the innate immune system ingest pathogens into phagosomal compartments that present a hostile environment that is low in pH, high in reactive oxygen species (ROS), deplete of iron (Fe), but high in copper (Cu). The battleground for control of metal distribution between host and pathogen suggests that small molecules capable of selectively manipulating metals at the host/pathogen interface might boost the immune system¿s response to infection. Here, we present a screen of small molecule Fe and Cu chelators to determine if altering bioavailability of these essential metal ions inhibits growth of the fungal pathogen, Cryptococcus neoformans. Results show that select chelating agents that facilitate the increase of intracellular Cu levels inhibited growth of C. neoformans, while traditional metal sequestering agents had no effect on growth at concentrations tested. After identifying chelating agents that inhibit C. neoformans in a Cu-dependent manner, we take a closer look at 8-hydroxyquinoline (8HQ) as a model Cu ionophore. In spite of its promising biological activity of 8HQ, the metal-dependent toxicity of 8HQ extends to mammalian cells as well. To overcome this challenge, we used a prochelator form of 8HQ, which allows for targeted Cu-dependent microbial killing at site of infection. The prochelator, QBP, is deprotected via ROS produced by activated macrophages, creating 8HQ and eliciting Cu-dependent killing of C. neoformans in vitro.