Quinacrine Inhibits Candida albicans Growth and Filamentation at Neutral pH

Abstract

Candida albicans is a common cause of catheter-related bloodstream infections (CR-BSI), in part due to its strong propensity to form biofilms. Drug repurposing is an approach that might identify agents that are able to overcome antifungal drug resistance within biofilms. Quinacrine (QNC) is clinically active against the eukaryotic protozoan parasites Plasmodium and Giardia. We sought to investigate the antifungal activity of QNC against C. albicans biofilms. C. albicans biofilms were incubated with QNC at serially increasing concentrations (4 to 2,048 g/ml) and assessed using a 2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide (XTT) assay in a static microplate model. Combinations of QNC and standard antifungals were assayed using biofilm checkerboard analyses. To define a mechanism of action, QNC was assessed for the inhibition of filamentation, effects on endocytosis, and pH-dependent activity. High-dose QNC was effective for the prevention and treatment of C. albicans biofilms in vitro. QNC with fluconazole had no interaction, while the combination of QNC and either caspofungin or amphotericin B demonstrated synergy. QNC was most active against planktonic growth at alkaline pH. QNC dramatically inhibited filamentation. QNC accumulated within vacuoles as expected and caused defects in endocytosis. A tetracycline-regulated VMA3 mutant lacking vacuolar ATPase (V-ATPase) function demonstrated increased susceptibility to QNC. These experiments indicate that QNC is active against C. albicans growth in a pH-dependent manner. Although QNC activity is not biofilm specific, QNC is effective in the prevention and treatment of biofilms. QNC antibiofilm activity likely occurs via several independent mechanisms: vacuolar alkalinization, inhibition of endocytosis, and impaired filamentation. Further investigation of QNC for the treatment and prevention of biofilm-related Candida CR-BSI is warranted.

Proton Transport and pH Control in Fungi

“…Both proton pumps are critical for growth and virulence [175,172,176,173,174], and have been explored as anti-fungal targets [175,170,177]. Pma1 activity is upregulated in preparation for filamentous growth [171], and loss of cytosolic pH control is associated with loss of filamentation [178].…”

Combinatorial drug approaches to tackleCandida albicansbiofilms

“…Quinacrine is FDA-approved for the treatment of several diseases including malaria and giardiasis [108] and recently its antifungal activity against C. albicans biofilms was reported, although only when applying high doses and in a pHdependent manner [109]. Moreover, this compound acted synergistically with caspofungin but not with fluconazole.…”

“…antifungal effect [109], could explain these interactions, although a general mechanism underlying both effects is also plausible.…”

“…Moreover, this compound acted synergistically with caspofungin but not with fluconazole. Quinacrine's antibiofilm activity likely occurs via several independent mechanisms including vacuolar alkalinization, inhibition of endocytosis and impaired filamentation [109].…”

Candida albicans morphology: still in focus

“…This included filastatin [66], ascorbic acid [66], mucins [67], gymnemic acids [68], and quinacrine [69] to name only a few. Of note, even known antifungals seem to have specific effects on the different morphological forms of C. albicans, and one azole was shown to force the fungus into the yeast growth mode, potentially due to reduced ergosterol content of the cell membrane [70].…”