Rappleye's team searched a library of commercially-available small molecules used by other investigators to find new antivirals or anticancer drugs. They performed a high-throughput phenotypic screen of 3,600 compounds looking for agents that inhibited fungal, but not human, cells.
To speed the selection process, Rappleye and Edwards engineered Histoplasma cells with a fluorescent protein that made the cells glow red while inside of a living macrophage -- the type of mammalian immune cell that Histoplasma attacks and in which it reproduces.
As the number of fungal cells increased inside the macrophage, so did the fluorescence and consequently, the cells would glow brighter. However, when a macrophage was exposed to an active compound that prevents Histoplasma reproduction, it maintained the same level of brightness. This allowed the scientists to quickly determine efficacy and toxicity of the drug candidate in a natural environment.
"Not only were we able to visually screen thousands of compounds in just a few weeks, but we were also able to measure the compound's impact in a real, live host cell," said Edwards.
The team narrowed down to a primary candidate called 41F5, which is 60 times more toxic to fungal cells than human cells. Their work was published in the September Antimicrobial Agents and Chemotherapy.
The team is currently working with Werner Tjarks, PhD, a medicinal chemist at Ohio State, to see if the selectivity and toxicity profile can be enhanced further for additional testing. Rappleye is also working with the Ohio State's Technology Commercialization Office (TCO) to potentially commercialize the derivatives from 41F5.
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