In vitro activity: Ciclopirox significantly inhibits the growth of C. albicans strain SC5314 cells, with MIC80s of 1.0-2.0 μg/mL, growth decreased dramatically at the concentrations of>0.6 μg/mL, and almost complete growth inhibition at concentration of 0.7 μg/mL, unlike fluconazole which shows a much wider range of concentrations with intermediate inhibition. Like iron chelator bipyridine, Ciclopirox reduces cell growth by binding to iron ions, which can be reversed by addition of FeCl3. Moreover, Ciclopirox treatment at subinhibitory concentration (0.6 μg/mL) only moderately reduces the virulence genes such as genes encoding secreted proteinases or lipases, but leads to a distinct up- or down-regulation of genes encoding iron permeases or transporters (FTR1, FTR2, and FTH1), a copper permease (CCC2), an iron reductase (CFL1), and a siderophore transporter (SIT1). Although the Candida drug resistance genes CDR1 and CDR2 are up-regulated after Ciclopirox treatment, no change in resistance or increased tolerance could be observed even after an incubation period of 6 months, in contrast to fluconazole in which the MICs for cells noticeably increase after 2 months. Ciclopirox inhibits the growth of Aspergillus fumigatus strain B5233 with IC50 of 4.22 μM, more potently compared with deferiprone with IC50 of 1.29 mM.

Cell Assay: Sabouraud glucose medium (2%) is used for cell culture growth, and RPMI 2% glucose medium and 2% Sabouraud glucose medium are used for MIC determinations. For cell culture growth curves, 220 mL of 2% Sabouraud glucose medium containing different concentrations of Ciclopirox are inoculated with 105 cells/mL, and the mixture is shaken at 160 rpm and 37 °C for 1-10 hours. Growth is measured photometrically at 630 nm. FeCl3 or 2,2'-bipyridine is added to the medium at different concentrations for inhibition studie.