Inhibitors of the glyoxylate cycle enzyme ICL1 in Candida albicans for potential use as antifungal agents

Abstract

Candida albicans is an opportunistic pathogen that causes candidiasis in humans. In recent years, metabolic pathways in C. albicans have been explored as potential antifungal targets to treat candidiasis. The glyoxylate cycle, which enables C. albicans to survive in nutrient-limited host niches and its. Key enzymes (e.g., isocitrate lyase (ICL1), are particularly attractive antifungal targets for C. albicans. In this study, we used a new screening approach that better reflects the physiological environment that C. albicans cells experience during infection to identify potential inhibitors of ICL. Three compounds (caffeic acid (CAFF), rosmarinic acid (ROS), and apigenin (API)) were found to have antifungal activity against C. albicans when tested under glucose-depleted conditions. We further confirmed the inhibitory potential of these compounds against ICL using the ICL enzyme assay. Lastly, we assessed the bioavailability and toxicity of these compounds using Lipinski's rule-of-five and ADMET analysis.

Figure 1. TCA cycle (black arrows) and glyoxylate cycle (dashed arrows).

In both cycles, oxaloacetate serves as the precursor for gluconeogenesis, but the glyoxylate cycle bypasses the carbon dioxide generating steps of the TCA cycle via isocitrate lyase and malate synthase, thus conserving the carbons for gluconeogenesis. Adapted from Lorenz and Fink (2002) .

Figure 2. Drugs and plant reference compounds used in this study with their PubChem IDs.

Figure 3. Alternative antifungal screening approach for C. albicans in YNB broth supplemented with glucose or lactate as the sole carbon source.

The chart represents the averaged growth percentage with error bars of representation standard deviation. ITC is the known ICL inhibitor that served as the positive control in this experiment: it reduced the growth of C. albicans only in the lactate-supplemented medium. CAFF, ROS, and API caused growth reduction similar to that ITC, and hence were selected as potential ICL inhibitors for further analysis. QCT and CINN showed growth reduction in both media, which indicates non-specific inhibition or different targets. RT was the only compound that showed a glucose-specific pattern of growth reduction.

Figure 4. Inhibitory percentage of tested compounds in the ICL enzyme inhibition assay.

The chart represents the averaged inhibitory percentage with error bars representing standard deviation. ITC is the known ICL inhibitor that served as the positive control in this experiment. CAFF, API, and ROS are the potential ICL inhibitors identified in the alternative screening experiment; they showed inhibition of ICL activity that was similar to that of ITC, with inhibitory percentage higher than 40% (in contrast to QCT, CINN, GALL, and RT).