## 5 Mechanism of Action  
Ciclopirox and its salt form, ciclopirox olamine, are classified within the hydropyri-done class, and are unlike other antifungal agents such as polyenes, azoles, imidazoles, flucytosine, and echinocandins, which generally act through ergosterol inhibition [5]. Although hydroxypyridones have been in clinical use for nearly three decades, their mechanisms of action are still not completely understood. Because azole-resistant strains remain sensitive to ciclopirox, this suggests that the inhibition of ergosterol biosynthesis is not the primary mechanism of action for ciclopirox. Early research indicated that ciclopirox disrupts fungal cell membrane function, compromising its integrity and leading to the leakage of cellular contents. It inhibits energy production in fungal cells, thereby blocking the uptake of essential components needed for growth. Early studies also suggested that ciclopirox interferes with DNA repair, cell division, and mitotic spindle formation, which inhibits fungal growth and ultimately results in cell death [1; 4; 6; 21]. In mammalian systems, ciclopirox olamine inhibits cell proliferation by chelating intracellular iron and targeting iron-dependent or iron-containing enzymes, such as ribonucleotide reductase or deoxyhypusine hydroxylase [3; 9]. This intracellular action distinguishes it from extracellular iron chelators like deferoxamine, which are less effective in assays due to their inability to penetrate cell membranes. This highlights the potential of CPX as a promising therapeutic strategy targeting intracellular iron [9]. So, a key aspect of CPO's mechanism of action is its ability to chelate polyvalent metal cations, particularly iron (Fe), in both its oxidation states, and it demonstrates a strong affinity for trivalent cations like aluminum (AI3+) [1,3]. Additionally, ciclopirox inhibits the activity of metal-dependent enzymes, including cytochromes, catalases, and peroxididases, which may disrupt mitochondrial function, energy production, and membrane transport. Given that both catalases and respiratory chain enzymes are iron-dependent proteins, this supports the hypothesis that iron binding plays a crucial role in the mechanism of action of hydroxypyridone antifungal agents [7]. Research on various ascomycete species suggests that the inhibition of iron permeases and transporters contributes to their toxic effects on fungi [3,21]. Studies that analyzed the effect of divalent metal chelators on the sensitivity of _C. neoformans_ to CPO and its derivatives were conducted. A synergistic interaction was observed with iron chelators, but not with copper-specific chelators. The addition of exogenous iron negated the inhibitory effects of ciclopirox olamine and its derivatives, while exogenous copper reduced inhibition only by CPO. Exogenous zinc had no effect. These findings suggest that the described compounds target iron-dependent enzymes, with excess iron or copper potentially competing with iron at the active site, thereby reducing the binding and inhibition of CPO analogs. The study also indicates that CPO and its analogs likely target multiple enzymes, as combinations of CPO with other compounds exhibited additive or synergistic effects, rather than antagonistic effects, which would be expected if they targeted the same enzyme [3]. A sample study investigated the expression patterns of several genes involved in iron metabolism. Two iron permease genes, FTR1 and FTR2, exhibited differential regulation in comparison to untreated cells. FTR1, responsible for a high-affinity Fe2+ permease, was strongly up-regulated, while FTR2, coding for a low-affinity Fe2+ permease, was significantly down-regulated. This inverse regulation suggests there are iron-limiting conditions in cells treated with ciclopirox olamine. Additionally, the upregulation of the FTH1 gene, encoding a putative iron transporter, further supports this notion.