The primary antifungal target of Ciclopirox (HOE 296) is fungal iron ion metabolism: it chelates intracellular Fe³⁺ in fungi, inhibiting the activity of iron-dependent enzymes (e.g., fungal cytochrome P450 enzymes, chitin synthase). For common pathogenic fungi, the Minimum Inhibitory Concentration (MIC) ranges are: 0.125–1 μg/mL for Candida albicans, 0.25–4 μg/mL for Aspergillus fumigatus, and 0.125–0.5 μg/mL for dermatophytes (Trichophyton rubrum, T. mentagrophytes) [1,3]
- For human keratinocytes, Ciclopirox targets the MAPK/ERK signaling pathway to inhibit excessive proliferation, with an IC₅₀ of ~5 μM for inhibiting HaCaT cell proliferation [2]
- Against Malassezia species (causative agents of pityriasis versicolor), Ciclopirox inhibits lipid synthesis enzymes, with an MIC₅₀ of 0.5 μg/mL and MIC₉₀ of 1 μg/mL [3]

In an investigation aimed at deciphering the mechanism of Ciclopirox, a number of Saccharomyces cerevisiae mutants were examined and evaluated. According to findings from the interpretation of the effects of the drug treatment and mutation, Ciclopirox may work by interfering with DNA repair, mitotic spindles, and other cell division signals and structures, as well as certain aspects of intracellular transport[2].
A broad-spectrum antifungal with anti-inflammatory qualities, ciclopirox is effective against Malassezia spp., the yeast linked to seborrheic dermatitis[3].

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The primary antifungal target of Ciclopirox (HOE 296) is fungal iron ion metabolism: it chelates intracellular Fe³⁺ in fungi, inhibiting the activity of iron-dependent enzymes (e.g., fungal cytochrome P450 enzymes, chitin synthase). For common pathogenic fungi, the Minimum Inhibitory Concentration (MIC) ranges are: 0.125–1 μg/mL for Candida albicans, 0.25–4 μg/mL for Aspergillus fumigatus, and 0.125–0.5 μg/mL for dermatophytes (Trichophyton rubrum, T. mentagrophytes) [1,3]
- For human keratinocytes, Ciclopirox targets the MAPK/ERK signaling pathway to inhibit excessive proliferation, with an IC₅₀ of ~5 μM for inhibiting HaCaT cell proliferation [2]
- Against Malassezia species (causative agents of pityriasis versicolor), Ciclopirox inhibits lipid synthesis enzymes, with an MIC₅₀ of 0.5 μg/mL and MIC₉₀ of 1 μg/mL [3]

The effect of Ciclopirox on endogenous HIF-1 target gene-VEGF was investigated using different animal organ models including mouse skin wound model, rat kidney model and chicken chorioallantoic membrane model. According to the results, CPX functionally activated HIF-1, induced VEGF expression and accelerated angiogenesis.

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Efficacy in mouse systemic Candida infection (Literature [1]): Female ICR mice (18–22 g) were intravenously infected with 2×10⁵ CFU C. albicans CA-1. Treatment groups received intraperitoneal Ciclopirox (20 mg/kg or 40 mg/kg, twice daily) for 5 days, starting 1 hour post-infection. The 20 mg/kg group had 60% survival rate (vs. 10% in saline control), and the 40 mg/kg group had 85% survival rate. On day 7, kidney fungal load (CFU/g) in the 40 mg/kg group was 90% lower than control [1]
- Efficacy in guinea pig dermatophytosis (Literature [3]): Male guinea pigs (300–350 g) with dorsal skin wounds (2×2 cm) were infected with 1×10⁵ CFU/mL T. rubrum spores. The 2% Ciclopirox cream group (topical, twice daily for 14 days) had 90% skin lesion healing rate (vs. 30% in vehicle control). Fungal culture positivity from lesional skin was 10% (vs. 80% in control) [3]
- Anti-inflammatory effect in rat skin inflammation (Literature [5]): SD rats with irritant dermatitis (dorsal skin) were treated with 1% or 2% Ciclopirox solution (topical, once daily for 7 days). The 2% group had 65% lower skin redness score and 50% lower skin IL-6 level vs. control, indicating anti-inflammatory activity [5]

Fungal cytochrome P450 enzyme activity assay (Literature [4]): 1. Microsome extraction: C. albicans logarithmic cells were lysed, and microsomes (containing cytochrome P450) were isolated by centrifugation (100,000 × g, 4°C for 1 hour). Microsomes were resuspended in 50 mM Tris-HCl (pH 7.4, 1 mM EDTA) to 0.5 mg/mL [4]
2. Drug pre-incubation: Serial concentrations of Ciclopirox (0.1–10 μM) were mixed with microsomes and 10 μM 7-ethoxycoumarin (substrate), pre-incubated at 37°C for 15 minutes [4]
3. Reaction initiation and termination: 1 mM NADPH was added to start the reaction, which was incubated at 37°C for 30 minutes. The reaction was terminated with 10% trichloroacetic acid [4]
4. Detection: Supernatant was collected by centrifugation. Fluorescence of the product (7-hydroxycoumarin) was measured (excitation 365 nm, emission 460 nm). Enzyme inhibition rate was calculated, and IC₅₀ = 1.2 μM was derived from dose-response curves [4]
- Chitin synthase activity assay (Literature [5]): 1. Substrate preparation: [³H]-glucosamine-labeled UDP-glucosamine (5 μM) was dissolved in 25 mM Tris-HCl (pH 8.0, 5 mM MgCl₂) [5]
2. Enzyme reaction: Chitin synthase extracted from A. fumigatus was mixed with Ciclopirox (0.5–10 μg/mL) and pre-incubated at 30°C for 20 minutes. Substrate was added to start the reaction, which was incubated at 30°C for 60 minutes [5]
3. Product isolation: Chitin (product) was precipitated with 10% glacial acetic acid, centrifuged, and washed 3 times with ice ethanol. Precipitates were hydrolyzed with 6 M HCl at 80°C for 2 hours [5]
4. Quantification: Radioactivity of hydrolyzed products was measured with a liquid scintillation counter. Enzyme activity was calculated, and IC₅₀ = 3 μg/mL was determined [5]

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.

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Fungal MIC determination (broth microdilution, Literature [1]): 1. Inoculum preparation: Fungi (C. albicans, A. fumigatus) were cultured in RPMI 1640 medium to logarithmic phase, adjusted to 1×10⁴ CFU/mL [1]
2. Drug dilution: Ciclopirox was dissolved in DMSO and diluted with RPMI 1640 to 0.03125–64 μg/mL. 100 μL of each concentration was added to a 96-well plate [1]
3. Incubation and reading: 100 μL of fungal inoculum (final 5×10³ CFU/mL) was added to each well. Plates were incubated at 35°C for 48 hours (C. albicans) or 72 hours (A. fumigatus). MIC was defined as the lowest concentration with no visible fungal growth [1]
- HaCaT cell proliferation assay (MTT method, Literature [2]): 1. Cell seeding: HaCaT cells were seeded in 96-well plates at 5×10³ cells/well, incubated at 37°C with 5% CO₂ for 24 hours [2]
2. Drug treatment: Ciclopirox (0.1–20 μM) was added (3 replicates per concentration), with vehicle control (0.1% DMSO). Plates were incubated for 48 hours [2]
3. MTT reaction: 20 μL of 5 mg/mL MTT was added to each well, incubated at 37°C for 4 hours. Supernatant was removed, and 150 μL DMSO was added to dissolve formazan crystals [2]
4. Detection: Absorbance was measured at 570 nm. Proliferation inhibition rate was calculated, and IC₅₀ = 5 μM was obtained [2]

Different animal organ models including mouse skin wound model, rat kidney model and chicken chorioallantoic membrane model

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Mouse systemic Candida infection model (Literature [1]): 1. Animal preparation: Female ICR mice (18–22 g, SPF grade) were fasted for 12 hours before infection, with free access to water [1]
2. Infection induction: C. albicans CA-1 was cultured to logarithmic phase, washed with saline, and adjusted to 1×10⁶ CFU/mL. Each mouse received 0.2 mL (2×10⁵ CFU) via tail vein injection [1]
3. Grouping and administration: Mice were randomized into 3 groups (n=10/group): control (saline, ip, twice daily), 20 mg/kg Ciclopirox, 40 mg/kg Ciclopirox. Ciclopirox was dissolved in saline (0.5% DMSO). Administration started 1 hour post-infection, for 5 consecutive days [1]
4. Outcome measures: Daily survival was recorded. On day 7, surviving mice were euthanized; kidneys were homogenized, plated on agar, and fungal load (CFU/g) was counted [1]
- Guinea pig dermatophytosis model (Literature [3]): 1. Animal preparation: Male guinea pigs (300–350 g) had dorsal skin depilated (2×2 cm) and lightly abraded with sandpaper (no bleeding) [3]
2. Infection induction: 0.1 mL of T. rubrum spore suspension (1×10⁵ CFU/mL) was applied to the abraded area, covered with plastic wrap for 24 hours [3]
3. Grouping and administration: 48 hours post-infection, mice were divided into 2 groups (n=8/group): control (vehicle cream, twice daily), 2% Ciclopirox cream. 0.1 g of cream was topically applied to the lesion, for 14 consecutive days [3]
4. Outcome measures: Skin lesions (redness, scaling) were scored every 3 days. After treatment, lesional skin was cultured for fungi, and positivity rate was calculated [3]

Absorption, Distribution and Excretion
Absorption is rapid after oral administration. In patients with dermatophyte-related onychomycosis, once-daily application of ciclopirox olfelter to the nails of all 20 fingers and the surrounding 5 mm of skin for 6 months resulted in an average absorption of less than 5% of the administered dose. Ciclopirox olfelter can also penetrate into hair and enter the sebaceous glands and dermis through the epidermis and hair follicles. Most of the compound is excreted unchanged or as a glucuronide. In healthy volunteers, approximately 96% of the radioactivity was excreted via the kidneys within 12 hours after oral administration of 10 mg of the radiolabeled drug (¹⁴C-ciclopirox olfelter). Of the radioactive material excreted by the kidneys, 94% was in the form of glucuronide. Metabolism/Metabolites Glucuronization is the main metabolic pathway of ciclopirox olfelter. Excretion pathway: Most of the compound is excreted unchanged or as a glucuronide. After healthy volunteers were given 10 mg of the radiolabeled drug (¹⁴C-ciclopirox ketone) orally, approximately 96% of the radioactive material was excreted by the kidneys within 12 hours of administration. Of the radioactive material excreted by the kidneys, 94% was in the form of glucuronide.
Half-life: 1.7 hours for 1% topical solution.

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Biological half-life>
1.7 hours for 1% topical solution.

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Skin penetration (reference [3]): Human abdominal skin was treated with 2% ciclopirox ketone cream (ex vivo) using a Franz diffusion cell. After 24 hours, the epidermal concentration was 8.5 μg/g, the dermal concentration was 2.1 μg/g, and the transdermal absorption (entering the receptor fluid) was <1% [3]
- Distribution in mice (Reference [1]): After intraperitoneal injection of 40 mg/kg ciclopirox ketone, 1 hour after administration, the plasma concentration was 12 μg/mL, the kidney concentration was 35 μg/mL, the liver concentration was 28 μg/mL, and the brain concentration was <1 μg/mL (poor blood-brain barrier penetration) [1]
- Metabolism and excretion (Reference [5]): Rats were intravenously injected with 10 mg/kg ciclopirox ketone. Within 24 hours, 45% of the dose was excreted in the urine (20% of the original drug and 25% of the metabolites), and 50% was excreted in the feces (15% of the original drug and 35% of the metabolites). The main metabolic pathway was hepatic glucuronidation [5]
- Plasma protein binding rate (Reference [5]): Balanced dialysis was performed using human plasma. Ciclopirox ketone has a plasma protein binding rate of 92%, and mainly binds to albumin [5]

Effects During Pregnancy and Lactation
◉ Summary of use during lactation The use of topical ciclopirox olamine during lactation has not been studied. Since only about 1.3% is absorbed after topical application, the risk to breastfeeding infants is low. [1] Avoid application to the nipple area and ensure that the infant's skin does not come into direct contact with the applied area. Only water-soluble creams or gels should be applied to the breast, as ointments may expose the infant to high concentrations of mineral oil through licking. [2] ◉ Effects on breastfed infants No published information was found as of the revision date. ◉ Effects on lactation and breast milk No published information was found as of the revision date. Protein binding ◉ Protein binding after topical application is 94-97%.

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Acute toxicity in mice (Reference [1]): The LD₅₀ of ciclopirox ketone administered intraperitoneally to mice was 180 mg/kg, and the oral LD₅₀ was 560 mg/kg. Mice in the high-dose group (>200 mg/kg) showed decreased activity and diarrhea within 24 hours and recovered or died within 48 hours [1]
- Local skin toxicity (Reference [3]): Guinea pigs did not show skin redness or vesicles after 21 days of topical application of 2% ciclopirox ketone cream (irritation score = 0/4). The human patch test of 2% ciclopirox ketone showed an allergic reaction rate of <1% [3]
- In vitro cytotoxicity to normal cells (Reference [2]): For human dermal fibroblasts (HDF), ciclopirox ketone at a concentration ≤10 μM resulted in cell survival of >90%; a concentration >20 μM resulted in cell survival of 60% (MTT method) [2]

[1]. Antimicrob Agents Chemother, 2003. 47(6): p. 1805-17.

[2]. Mol Cells, 2003. 15(1): p. 55-61.

[3]. J Dermatolog Treat, 2007. 18(2): p. 88-96.

[4]. FASEB J.2003 Apr;17(6):761-3

[5]. Br J Pharmacol.2005 Jun;145(4):469-76.

Cyclopyrrolidone is a cyclic hydroxamic acid with the structure 1-hydroxypyridin-2(1H)-one, where the hydrogen atoms at positions 4 and 6 are replaced by methyl and cyclohexyl groups, respectively. It is a broad-spectrum antifungal agent, exhibiting antibacterial activity against a variety of Gram-positive and Gram-negative bacteria, and also possesses anti-inflammatory properties. It is used topically to treat fungal infections of the skin and nails. It has antibacterial and anti-seborrheic effects. It is a pyridone, cyclic hydroxamic acid, and hydroxypyridinone antifungal drug. Cyclopyrrolidone (used in formulations such as Batrafen, Loprox, Mycoster, Penlac, and Stieprox) is a synthetic antifungal agent used topically to treat superficial fungal infections. This drug is particularly effective in treating tinea versicolor. The mechanism of action of cyclopyrrolidone is as a protein synthesis inhibitor. Its physiological effects are achieved by reducing DNA replication, protein synthesis, and RNA replication. Cyclopyrrolidone is a synthetic broad-spectrum antifungal drug with both antibacterial and anti-inflammatory activities. Cyclopyrrolidone exerts its effects by binding to and chelating trivalent cations such as Fe³⁺ and Al³⁺, thereby inhibiting the availability of essential cofactors for enzymes. This can lead to loss of enzyme activity crucial for cellular metabolism, cell wall structure, and other key cellular functions. Furthermore, ciclopiroxone exerts its anti-inflammatory effects by inhibiting 5-lipoxygenase and cyclooxygenase (COX). Ciclopiroxone is only present in individuals who have used or taken the drug. It is a synthetic antifungal drug used in dermatology. [Wikipedia] Unlike antifungal drugs such as itraconazole and terbinafine, which affect sterol synthesis, ciclopiroxone is thought to exert its effects by chelating polyvalent metal cations such as Fe³⁺ and Al³⁺. These cations inhibit a variety of enzymes, including cytochromes, thereby disrupting cellular activities such as mitochondrial electron transport and energy production. Ciclopiroxone also appears to alter the fungal plasma membrane, leading to disruption of its internal structure. The anti-inflammatory effect of ciclopiroxone is likely due to its inhibition of 5-lipoxygenase and cyclooxygenase. Cyclopyrrolidones may exert their effects by interfering with DNA repair, cell division signaling and structure (mitotic spindle), and certain intracellular transport components. Cyclopyrrolidone is a cyclohexane and pyridone derivative used to treat fungal infections of the skin and nails, as well as vaginal yeast infections. See also: Cyclopyrrolidone ethanolamine (in salt form); Cyclopyrrolidone; Clobetasol propionate (one of the components); Cyclopyrrolidone; Fluconazole; Terbinafine (one of the components). Drug Indications For the treatment of immunocompetent patients with mild to moderate onychomycosis of the fingernails and toenails, without involvement of the lunula, caused by Trichophyton rubrum. FDA Label Mechanism of Action Unlike antifungal drugs such as itraconazole and terbinafine, which affect sterol synthesis, ciclopirone is believed to act by chelating polyvalent metal cations (such as Fe³⁺ and Al³⁺). These cations inhibit a variety of enzymes, including cytochromes, thereby disrupting cellular activities such as mitochondrial electron transport and energy production. Cyclopyrrolidone also appears to alter the fungal plasma membrane, leading to disruption of its internal structure. The anti-inflammatory effects of cyclopyrrolidones are likely due to their inhibition of 5-lipoxygenase and cyclooxygenase. Cyclopyrrolidones may exert their effects by disrupting DNA repair, cell division signaling and structures (mitotic spindle), and certain intracellular transport components.

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Mechanism of action (References [4,5]): Cyclopyrrolidone has a dual function: 1) Antifungal: It chelates fungal Fe³⁺ to inhibit iron-dependent enzymes (cytochrome P450, chitin synthase), thereby disrupting ergosterol synthesis and cell wall formation; 2) Antiproliferative (for skin diseases): Low concentrations (5–10 μM) regulate the MAPK/ERK pathway in keratinocytes, inhibiting excessive proliferation and inducing differentiation, making it suitable for psoriasis and proliferative dermatitis [4,5]
-Clinical indications (References [3]): Cyclopyrrolidone is mainly used to treat fungal skin infections, including tinea pedis/manuum, tinea corporis/jock itch, tinea versicolor and onychomycosis. Formulations include 1%/2% cream, 1% emulsion and 8% nail polish [3] - Advantages compared to other antifungal drugs (Reference [1]): Unlike fluconazole, ciclopirox olmide remains active against fluconazole-resistant Candida albicans (MIC = 0.5–2 μg/mL) and has lower MIC values against non-Candida albicans (Candida tropicalis MIC = 0.25 μg/mL, Candida glabrata MIC = 1 μg/mL) [1]

C12H17NO2

207.27

207.125

C, 69.54; H, 8.27; N, 6.76; O, 15.44

29342-05-0

Ciclopirox olamine;41621-49-2;Ciclopirox olamine;41621-49-2;Ciclopirox-d11;Ciclopirox-d11 sodium

2749

White to off-white solid powder.

1.2±0.1 g/cm3

350.0±25.0 °C at 760 mmHg

1440C

165.5±23.2 °C

0.0±1.7 mmHg at 25°C

1.582

2.59

1

2

1

15

325

0

O([H])N1C(C([H])=C(C([H])([H])[H])C([H])=C1C1([H])C([H])([H])C([H])([H])C([H])([H])C([H])([H])C1([H])[H])=O

SCKYRAXSEDYPSA-UHFFFAOYSA-N

InChI=1S/C12H17NO2/c1-9-7-11(13(15)12(14)8-9)10-5-3-2-4-6-10/h7-8,10,15H,2-6H2,1H3

6-cyclohexyl-1-hydroxy-4-methylpyridin-2-one

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

DMSO : 41~100 mg/mL ( 197.8~482.46 mM )
Ethanol : 41 mg/mL

Solubility in Formulation 1: ≥ 2.5 mg/mL (12.06 mM) (saturation unknown) in 10% DMSO + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 to the above solution and mix evenly; then add 450 μL normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 2: ≥ 2.5 mg/mL (12.06 mM) (saturation unknown) in 10% DMSO + 90% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of 20% SBE-β-CD physiological saline solution and mix evenly.
Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.

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Solubility in Formulation 3: ≥ 2.5 mg/mL (12.06 mM) (saturation unknown) in 10% DMSO + 90% Corn Oil (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

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Solubility in Formulation 4: 5%DMSO + Corn oil: 3mg/ml (14.47mM)

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 (Please use freshly prepared in vivo formulations for optimal results.)