In vitro and in vivo efficacy of the antimycobacterial molecule SQ109 against the human pathogenic fungus, Cryptococcus neoformans
Cryptococcosis is an opportunistic fungal infection affecting individuals with compromised immunity, particularly those with HIV. The limited accessibility to effective treatments, emerging resistance to current antifungals, and treatment-related toxicities underline the pressing need for more effective therapeutic options. In this study, we conducted a whole-cell screening of ~ 3,700 FDA-approved drugs and clinical molecules against the Cryptococcus neoformans H99 strain to identify potential antifungal candidates. The anti-mycobacterial agent SQ109 was identified as one of the most potent hits, with broad antifungal activity. SQ109 exhibited potent activity against Cryptococcus spp., with an MIC90 of 4 µg/mL. In the time-kill assay, SQ109 demonstrated fungicidal activity against proliferating cryptococcal cells in a concentration-dependent manner. Unlike fluconazole (FLC) and flucytosine (5-FC), C. neoformans showed minimal tendency to develop resistance to SQ109 during frequent passaging. Furthermore, SQ109 exhibited a potent efficiency in the murine model of cryptococcal infection, resulting in a 50% survival rate among the group treated with 25 mg/kg for 10 consecutive days. The transcriptomic analysis revealed that SQ109 disrupts ergosterol biosynthesis, affecting membrane integrity and oxidative stress homeostasis, which aligns with its interaction with other ergosterol pathway inhibitors, including FLC.  Additionally, molecular docking and structural analysis indicated that squalene synthase protein ERG9 is the most likely target of SQ109 within the ergosterol biosynthesis machinery of cryptococcal cells.  These findings highlight the therapeutic potential of SQ109 in combating cryptococcal infections, both as a standalone therapy and as an adjuvant to FLC monotherapy. Overall design: A total of 18 clinical isolates representing C. neoformans/gattii complex were included in this study. Additional 6 clinical isolates of clinically relevant fungi, including Candida parapsilosis, C. auris, and Aspergillus fumigatus were involved in susceptibility testing. Determination of the MIC for SQ109 and standard antifungals, AmB and FLC, was conducted as described in the Clinical and Laboratory Standards Institute (CLSI) guidelines M27-A3 for yeast and M38-A2 for filamentous fungi.