Clotrimazole as a Potent Agent for Treating the Oomycete Fish Pathogen Saprolegnia parasitica through Inhibition of Sterol 14α-Demethylase (CYP51)

A candidate CYP51 gene encoding sterol 14α-demethylase from the fish oomycete pathogenSaprolegnia parasitica (SpCYP51) was identified based on conserved CYP51 residues among CYPs in the genome. It was heterologously expressed inEscherichia coli , purified, and characterized. Lanosterol, eburicol, and obtusifoliol bound to purified SpCYP51 with similar binding affinities (Ks , 3 to 5 μM). Eight pharmaceutical and six agricultural azole antifungal agents bound tightly to SpCYP51, with posaconazole displaying the highest apparent affinity (Kd , ≤3 nM) and prothioconazole-desthio the lowest (K d , ∼51 nM). The efficaciousness of azole antifungals as SpCYP51 inhibitors was confirmed by 50% inhibitory concentrations (IC50 s) of 0.17 to 2.27 μM using CYP51 reconstitution assays. However, most azole antifungal agents were less effective at inhibitingS. parasitica ,Saprolegnia diclina , andSaprolegnia ferax growth. Epoxiconazole, fluconazole, itraconazole, and posaconazole failed to inhibitSaprolegnia growth (MIC100 , >256 μg ml−1 ). The remaining azoles inhibitedSaprolegnia growth only at elevated concentrations (MIC100 [the lowest antifungal concentration at which growth remained completely inhibited after 72 h at 20°C], 16 to 64 μg ml−1 ) with the exception of clotrimazole, which was as potent as malachite green (MIC100 , ∼1 μg ml−1 ). Sterol profiles of azole-treatedSaprolegnia species confirmed that endogenous CYP51 enzymes were being inhibited with the accumulation of lanosterol in the sterol fraction. The effectiveness of clotrimazole against SpCYP51 activity (IC50 , ∼1 μM) and the concentration inhibiting the growth ofSaprolegnia speciesin vitro (MIC100 , ∼1 to 2 μg ml−1 ) suggest that clotrimazole could be used againstSaprolegnia infections, including as a preventative measure by pretreatment of fish eggs, and for freshwater-farmed fish as well as in leisure activities.