S3.5d Identification of fungicidal activity of otilonium bromide by the repurposing approach
Authors: Zhen Cheng , Lu Hui , Yang Feng , Jiang Yuanying 
Published: September 2022

Abstract

S3.5 Environmental exposure - risk for human fungal disease, September 21, 2022, 4:45 PM - 6:15 PM

Objectives
The development of new antifungal drugs will help to expand the limited antifungal arsenal and curb the emergence of drug resistance. To meet the demand for novel antifungal drugs, we performed a high-throughput screen of an FDA-approved compound library (HY-L022, MCE®) and identified a promising lead antifungal compound, otilonium bromide (OTB).

Methods
We determined antifungal activity of each compound from the library and investigated the antifungal spectrum of OTB using broth microdilution assays (Fig. 1a). We tested the fungicidal effect of OTB by the spot assay and disk diffusion assay. The inhibitory effect of OTB on mycelial growth was identified by observing hyphal growth in hyphal-inducing media containing OTB. The antibiofilm activity of OTB was evaluated by XTT method. We further checked the antifungal activity of OTB in vivo by a murine C. albicans infection model, which was constructed by immunosuppression of mice with cyclophosphamide, inhibiting the intestinal microorganisms by levofloxacin (14 mg/ml), and then gavage with C. albicans cells (1 × 108 cells per mouse). A total of 24 C57 mice were randomly divided into three groups: one group was gavaged with saline, one group with 1 mg/kg fluconazole, and one group with 50 mg/kg OTB. In all 2 h after infection, drugs were given once a day for 4 consecutive days, with 15 days of observation. The death of a mouse was assessed daily and statistically analyzed by Log-rank (Mantel-Cox) test. The toxicity of OTB to human cloned colon adenocarcinoma cells was assessed with MTT method.

Results
We found that OTB is the most promising lead antifungal molecule among FDA-approved drugs (Fig. 1a). OTB has a broad-spectrum antifungal activity with minimum inhibitory concentration for inhibiting the growth of 50% of strains (MIC50) values ranging from 0.39 to 6.25 μm (Fig. 1b). We found that OTB (≥3.5 μm) has fungicidal effect on C. albicans using spot assay (Fig. 1c). We further found that OTB (25 μg/disk) can form a clear inhibition zone, but not fluconazole (25 μg/disk) (Fig. 1d). Besides, we spotted C. albicans cells from the broth microdilution assays onto YPD solid medium and found that cells treated with OTB (≥1.56 μm) could not recover on YPD solid medium, while cells treated with FLC could (Fig. 1e). It is worth noting that OTB has inhibitory effects on filamentation and biofilm formation at 12.5 μm (Fig. 2a, b), which are key virulence factors of pathogenic fungi. Besides, OTB (50 mg/kg) significantly prolonged the median survival time from 5 days (control group) to 15 days and improved the survival rate from 0% (control group) to 62.5% (P = .0078) (Fig. 2c). Furthermore, we found that OTB exhibited relative low toxicity to mammal cells with IC50 28-57 μm (Fig. 2d), which was much higher than MIC50 against pathogenic fungi.

Conclusion
Our study highlights that OTB has a broad-spectrum antifungal activity with low MIC50 values and high safety in vitro and potent antifungal efficacy in vivo and will be beneficial to the treatment of devastating invasive fungal infections.