The Activity of FDA-Approved Prodrug Isavuconazonium Sulfate and Its Major Metabolite Isavuconazole Against Naegleria fowleri

<b>Objectives</b>: Free-living amoeba <i>Naegleria fowleri</i> causes primary amoebic meningoencephalitis (PAM). While infection is rare, PAM's fatality rate exceeds 97%. The recommended treatment includes combination therapy, which does not result in uniform survival. Thus, there is a critical unmet need for finding better therapy for PAM. Drug repurposing can expedite the discovery of effective treatment for PAM. Isavuconazonium is approved for the treatment of fungal infections. Given that isavuconazole is the major metabolite of isavuconazonium and isavuconazole penetrates into the brain with high efficiency, our objective was to determine the activity of both isavuconazonium and isavuconazole on <i>N. fowleri</i> trophozoites. <b>Methods</b>: To test the effect of both compounds, we determined their dose-responses against <i>N. fowleri</i> and two mammalian cells. To establish how fast the prodrug and the metabolite kill the trophozoites, we measured potency at different time points. Finally, we investigated the effect of combining isavuconazonium or isavuconazole with amphotericin B on both <i>N. fowleri</i> and mammalian cells. <b>Results</b>: Both isavuconazonium and the metabolite isavuconazole were active against multiple strains, with clinically relevant isavuconazole exhibiting potency ranging between 0.1 and 0.6 µM. They were less toxic on mammalian cells. Isavuconazonium and isavuconazole required 24 h to achieve nanomolar potency. Combination with amphotericin B was synergistic without eliciting toxicity on mammalian cells. <b>Conclusions</b>: Our findings, together with the use of intravenous and oral formulations of isavuconazonium to treat pediatric and adult patients, support further in vivo efficacy study of isavuconazonium for its potential use for the treatment of PAM.