Calcimycin Inhibits Cryptococcus neoformans in Vitro and in Vivo by Targeting the Prp8 Intein Splicing

Drug resistance is a significant concern in the treatment of diseases, including cryptococcosis caused by Cryptococcus neoformans (Cne) and Cryptococcus gattii (Cga). Alternative drug targets are necessary to overcome drug resistance before it attains a critical stage. Splicing of inteins from pro-protein precursors is crucial for activities of essential proteins hosting intein elements in many organisms, including human pathogens such as Cne and Cga. Through a high-throughput screening, we identified calcimycin (CMN) as a potent Prp8 intein splicing inhibitor with a minimum inhibitory concentration (MIC) of 1.5 μg/mL against the wild-type Cne-H99 (Cne-WT or Cne). In contrast, CMN inhibited the intein-less mutant strain (Cne-Mut) with a 16-fold higher MIC. Interestingly, Aspergillus fumigatus and a few Candida species were resistant to CMN. Further studies indicated that CMN reduced virulence factors such as urease activity, melanin production, and biofilm formation in Cne. CMN also inhibited Cne intracellular infection in macrophages. In a target-specific split nanoluciferase assay, the IC₅₀of CMN was 4.6 μg/mL. Binding of CMN to recombinant Prp8 intein was demonstrated by thermal shift assay and microscale thermophoresis. Treating Cne cells with CMN reduced intein splicing. CMN was fungistatic and showed a synergistic effect with the known antifungal drug amphotericin B. Finally, CMN treatment at 20 mg/kg body weight led to 60% reduction in lung fungal load in a cryptococcal pulmonary infection mouse model. Overall, CMN represents a potent antifungal with a novel mechanism of action to treat Cne and possibly Cga infections.