Assessment of FDA-approved drugs against Strongyloides ratti in vitro and in vivo to identify potentially active drugs against strongyloidiasis

Background: Infections with Strongyloides stercoralis belong to the most neglected helminth diseases, and research and development (R&D) efforts on novel drugs are inadequate. Methods: A commercially available library containing 1600 FDA-approved drugs was tested in vitro against Strongyloides ratti larvae (L3) at 100 mu M. Hits (activity > 70%) were then evaluated against S. ratti adult worms at 10 mu M. Morantel, prasterone, and levamisole were tested in the S. ratti rat model using dosages of 1-100 mg/kg. Results: Seventy-one of the 1600 compounds tested against S. ratti L3 revealed activity above 70%. Of 64 compounds which progressed into the adult screen, seven compounds achieved death of all worms (benzethonium chloride, cetylpyridinium chloride, Gentian violet, methylbenzethonium chloride, morantel citrate, ivermectin, coumaphos), and another eight compounds had activity > 70%. Excluding topical and toxic compounds, three drugs progressed into in vivo studies. Prasterone lacked activity in vivo, while treatment with 100 mg/kg morantel and levamisole cured all rats. The highest in vivo activity was observed with levamisole, yielding a median effective dose (ED50) of 1.1 mg/kg. Conclusions: Using a drug repurposing approach, our study identified levamisole as a potential backup drug for strongyloidiasis. Levamisole should be evaluated in exploratory clinical trials.