Nystatin

Nystatin

John F. Mohr, Pharm.D., Luis Ostrosky-Zeichner, M.D., FACP

CLASS

Structure-Activity Relationships

Nystatin (Figure 1) is a polyene antifungal produced by Streptomyces noursei that was discovered in the 1950’s and was the first antifungal drug available for human use. Approximately 100 polyene antifungals have been isolated, but only nystatin, amphotericin B, natamycin, harnycin, trichomycin and candicidin have been used clinically (55). The polyenes are characterized by common structural features, including a macrolide ring of carbon atoms closed by an ester or lactone, a large number of hydroxyls on the macrolide ring on alternate carbon atoms, an extensive double-bond system, and an amino sugar. No extensive structure-activity studies of nystatin derivatives have been performed. Methyl, ethyl, propyl, and butyl esters of nystatin have been synthesized but showed no distinctive advantages over nystatin (11) and were never used clinically.

The use of nystatin has been limited to the topical treatment of cutaneous, vaginal, and oral candidiasis (5). Nyotran™ (Antigenics, New York, New York) is a liposomal formulation of nystatin that contains nystatin, dimyristoyl phosphatidyl choline and dimyristoyl phosphatidyl glycerol in a ratio (by weight) of 1:7:3 (2b). By utilizing liposomes as carriers, nystatin was able to be administered intravenously to mice with good evidence of activity and reduced toxicity (56, 57). However, at the time of the writing of this chapter, the development plan of this formulation is uncertain.

Mechanisms of Action

Similar to other polyene antifungal agents, nystatin binds to ergosterol in the fungal cell membrane, resulting in altered membrane permeability that allows release of intracellular potassium, sugars, and metabolites (82). Disruption of the membrane is believed to be responsible for fungal cell death (see "Mechanisms of Action," below).

ANTIFUNGAL ACTIVITY

Spectrum

In Vitro: Nystatin is active against a broad spectrum of filamentous fungi in vitro, including yeast and moulds (31, 77, 79). When nystatin is placed on solid test media and in other biological fluids, a second active isomer develops (63b). This second isomer is not as active against Candida as the commercially available nystatin; however, it likely contributes to the overall in vitro activity of the compound. The in vitro spectrum of nystatin is much broader than that of azole antifungals and flucytosine. Nystatin has also been reported to be active in vitro against HIV (73). Table 1 shows the in vitro activity of nystatin and liposomal nystatin against multiple fungal isolates obtained from Europe and the United States (13a, 16a, 23a, 39a, 57, 57a, 62a, 68a, 68b). In vitro activities of nystatin and liposomal nystatin are similar, except that in some cases, the minimum inhibitory concentration (MIC) for liposomal nystatin is one dilution lower. In vitro, MICs of nystatin and liposomal nystatin are also similar to those of amphotericin B desoxycholate (Fungizone®), amphotericin B lipid complex (Abelcet®), and liposomal amphotericin B (AmBisome®) against clinical fungal isolates (2a). However, nystatin and liposomal nystatin are active against all fluconazole, all flucytosine and certain amphotericin B resistant clinical isolates (2a).

In Vivo: Nystatin has demonstrated antifungal activity against Candida albicans (10,22,32,76), Cryptococcus neoformans (76), Histoplasma capsulatum (12, 18), and Coccidioides immitis (18,24) in animals. Liposomal nystatin has been reported to be active against C. albicans (56) and Aspergillus fumigatus (86) in mice as well as C. albicans (27a) and Aspergillus fumigatus (27b) in rabbits. In addition to its antifungal activity in vitro and in animals, nystatin has been shown effective against a variety of fungi in humans, including Candida (54), Aspergillus (81), Histoplasma (67), and Coccidioides (61). Nystatin has demonstrated antifungal activity in humans via different routes of administration, including oral (13, 71), pleural (48,63), inhalation (22,74,78,85), and topical routes (43,64,81). Liposomal nystatin has antifungal activity similar to that of amphotericin B desoxycholate in candidemia in humans with reduced toxicity (24). Liposomal nystatin may be beneficial in some patients who have failed therapy with amphotericin B (7, 43a).

Pharmacodynamic Effects

Fungicidal Effects: Nystatin exhibits concentration dependent, fungicidal effects against Candida sp (27c, 64a). Susceptibility of fungi to nystatin has been reported to increase with growth rate, lower culture temperature, and lower pH (39).

Postantibiotic Effects: A significant post anti fungal effect has been reported with nystatin against Candida species (27c, 18d). The higher the nystatin concentration exposure, the longer the post antifungal effect (27c). These in vitro findings imply that even a short period of exposure to a high concentration of nystatin prevent the regrowth of Candida. Further studies with liposomal nystatin would be warranted to verify this phenomenon in vivo against organisms causing systemic infections.

Effects of Subinhibitory Concentrations: Subinhibitory concentrations of nystatin have been reported to block adherence of Candida species to human vaginal (8) and buccal (1, 18c) cells in vitro and to reduce the concentration of peptidomannans in C. albicans (6). In addition, short exposure of Candida albicans to nystatin in vitro completely inhibits germ tube formation (18a) and sub-lethal exposure of Candida albicans has been shown to block the adherence of organisms to denture acrylic (18b). Subinhibitory concentrations have been used to develop fungi resistant to nystatin (2), although with great difficulty.

Important Effects on Host Immunity (Positive or Negative): Nystatin has been reported to stimulate thymus-independent B cells and polyclonal antibody synthesis in vitro (29, 38), but there have been no reports of an effect of nystatin or liposomal nystatin on immune function in humans.

Pharmacodynamic Correlates with Outcome: There are no published studies on the relationship between pharmacodynamic effects of nystatin and clinical outcomes. However, due to the concentration dependent fungicidal activity (27d), long post anti-fungal effect in vitro (27d, 18d) and an improved safety profile of the liposomal nystatin formulation (56,57), an infrequent administration may allow for a prolonged drug free interval, potentially reducing toxicities without compromising efficacy. More clinical data with liposomal nystatin is needed to confirm these hypotheses.

MECHANISMS OF ACTION

The activity of nystatin has been attributed to binding to ergosterol in the fungal membrane, resulting in altered membrane permeability that allows release of K+, sugars, and metabolites (82). Ergosterol is the main sterol in fungi but is not found in human cells; cholesterol is the main sterol in mammalian cells. Nystatin has been shown to have greater affinity for ergosterol than for cholesterol (45) and to bind directly to ergosterol in the fungal cell membrane in an irreversible manner (44). Exogenously added sterol can block the antifungal effect of nystatin (26). In nystatin-resistant, ergosterol-deficient C. albicans and Candida krusei isolates obtained from human patients, addition of ergosterol to the culture medium restored nystatin-induced K+ leakage (51). Similarly, in ergosterol-deficient Saccharomyces cerevisiae there was a direct correlation between the amount of ergosterol replaced in the fungus and sensitivity to nystatin (69). When ergosterol was replaced in ergosterol-deficient S. cerevisiae by cholesterol or cholestanol, sensitivity to nystatin was markedly decreased (69). Disruption of the membrane by nystatin results in fungal death, although it is not clear if ionic changes are responsible for membrane disruption (14).

Despite the close structural resemblance of nystatin to amphotericin B, there are data indicating that nystatin and amphotericin B have different biologic properties. (a) The rate at which nystatin causes K+ release (i.e., membrane disruption) from C. albicans in vitro is much faster than that of amphotericin, but fungal death occurs at a faster rate with amphotericin B than with nystatin (14). However, in 24-hour susceptibility assays, the two drugs have nearly identical MICs. (b) Nystatin has been reported to be markedly less toxic to mammalian cells than amphotericin at equimolar concentrations (82). Kinsky (42) reported that amphotericin B (5 ug/mL) produced 78% lysis in human red blood cells in 40 minutes, while nystatin (5 ug/mL) produced no lysis in 40 minutes. Hemolytic activity is known to correlate approximately with mammalian toxicity (30). (c) K+ or NH4+ reverses amphotericin B inhibition of fungal glycolysis but does not reverse nystatin inhibition of fungal glycolysis (30). Inhibition of glycolysis is a sequelae to polyene-induced K+ leakage. (d) Nystatin and amphotericin B differ in their ability to inhibit ATPase (82). (e) Amphotericin B-resistant fungi are sometimes not cross-resistant to nystatin. IIebeka and Solotorovsky (33) developed two C. albicans strains with 50-fold increased resistance to amphotericin B but normal sensitivity to nystatin. Broughton et al. (9) reported isolation of a C. albicans strain that was resistant to amphotericin B but not to nystatin. On the basis of these data, the mechanisms of action of nystatin and amphotericin B may differ in certain fungal isolates or strains. This suggests that nystatin may be useful in clinical situations in which amphotericin B has failed.

It is assumed that the in vitro and in vivo antifungal mechanism of action of liposomal nystatin and nystatin are the same, since nystatin is the active ingredient in liposomal nystatin. In vivo, it is believed that the liposome encapsulation of liposomal nystatin is responsible for directing the drug toward the cells of the reticuloendothelial system (e.g., macrophages) that localize in inflammatory areas and away from the kidney, which is the primary organ of toxicity. The mechanism by which nystatin is released from the liposomes in vitro or in vivo is not understood but may involve fusion of the liposomes containing nystatin with the fungal cell or transfer of the nystatin from the liposome to the fungi because of the greater affinity of nystatin for ergosterol in the fungal cell. A stability study conducted in vitro in human serum at 37°C showed that 75% of nystatin is retained in liposomes after 4 h, 65% after 24 hours, and 30% after 72 hours of incubation (57).

MECHANISMS OF RESISTANCE

Commonly Resistant Organisms

Although nystatin-resistant fungi have been created in the laboratory, resistance to nystatin in humans is rare. However, resistance has been reported in a small number of clinical cases. See Table 2. Fungi resistant to amphotericin B are not always resistant to nystatin (see "Mechanisms of Action," above), but there are many cases of cross-resistance between amphotericin B and nystatin (88). Although a strain of C. albicans obtained in the laboratory was reported to be cross-resistant to azoles and polyenes (34), no cross-resistance between these classes of antifungals has been reported in humans. Resistance to moulds has not been reported for nystatin. There are no published data on possible cross-resistance of nystatin to amphotericin B-resistant moulds.

Mechanisms of Resistance

There are probably multiple mechanisms of resistance to nystatin (35) (Table 3). A major contributor to resistance appears to be reduction in ergosterol content of fungi, although some nystatin-resistant fungi do not have reduced ergosterol content (22a). A reduction in ergosterol molecules would decrease binding sites for nystatin. The mechanism by which nystatin causes a decrease in ergosterol content is probably by mutation of one or more enzymes involved in ergosterol biosynthesis. Mutations in demethylase, desaturase, and isomerase biosynthetic enzymes in various isolates have been associated with resistance (41, 60, 65). Most of the resistance studies have been conducted in fungal mutants created in vitro in the laboratory. It is not clear if the mechanism of resistance to nystatin in vitro is the same as that seen clinically.

PHARMACOKINETICS

Absorption

Nystatin is not absorbed to any significant extent following oral, cutaneous, or vaginal administration. Oral administration of nystatin in a murine infection model had little effect on the size of cutaneous candida lesions as compared to subcutaneous administration (63a). The drug is excreted in the feces following oral therapy. Nystatin has been measured in feces and saliva following oral administration. In one study, the concentration of nystatin in the feces following oral administration of 3, 4, 5 and 6 X 106 USP international units (IU) was approximately 70, 200, 200, and 250 IU, respectively (37). The concentration of nystatin in the saliva of patients who received 2x105 IU of nystatin using a mucosal oral delivery system was 279, 654, and 532 ug/mL at 0.5, 1, and 2 hours, respectively (20). Absorption is not applicable to liposomal nystatin, which is administered as an intravenous infusion.

Distribution

No distribution studies have been published on nystatin. The pharmacokinetics of intravenous liposomal nystatin have been studied in rabbits (27c) and humans with HIV (15, 70). In the rabbit, liposomal nystatin is widely distributed into tissues and tissue concentrations increase with increased doses. Concentrations of nystatin were highest in the lung, liver, spleen and urine. Increases in dosage coincided with a proportional increase in Cmax and a greater than proportional increase in AUC0-24 and greater than proportional decrease in total clearance.

Liposomal nystatin pharmacokinetics were evaluated in HIV-infected patients at doses of 2, 3, 4, 5 mg/kg infused at a rate of 2 mg/min (15). The concentration at the end of infusion (Co) and blood AUC values increased with increasing dose. Maximal blood concentrations were 4.8, 9.5, 24.3 and 24.1 ug/mL for doses of 2, 3, 4, or 5 mg/kg respectively. Following repeated doses of liposomal nystatin every other day, there was minimal accumulation in the blood.

Routes of Elimination

Metabolism: No data are available on the metabolism of nystatin or liposomal nystatin in vitro, in animals, or in humans. No metabolites have been found in the blood of humans following administration of liposomal nystatin.

Elimination: No elimination studies have been published on nystatin as the primary use is limited to the topical route. After administration of intravenous liposomal nystatin, there is a rapid initial clearance of nystatin from the central compartment (15, 27c, 70). After a single dose of liposomal nystatin the clearance rate from blood ranged from 0.6 to 1.0 ml/kg/min and the terminal–phase half-life ranged from 2.6 hours to 5.5 hours (27c). There are also substantial concentrations of nystatin in the urine of rabbits following intravenous administration suggesting renal elimination may play a significant role in elimination (27c).

DOSAGE REGIMENS

Adults and Children

Topical Nystatin: Nystatin (Mycostatin®) is available as a cream, ointment, and powder containing 100,000 USP units/g for topical use in adults and children in treatment of local forms of candidiasis. It is also available as a cream and ointment in combination with the corticosteroid triamcinolone (Mytrex®). It is applied topically to the affected area two to four times per day.

Oral Nystatin: The oral suspension of nystatin contains 100,000 USP units/ml and should be swished and swallowed three to four times per day for the treatment of oral candidiasis. Premature and low birth-weight neonates are given 1 ml, infants are given 2 ml, and children or adults are given 4 to 6-ml doses. A pastille formulation (Mycostatin®) for oral candidiasis contains 200,000 USP units of nystatin. One to two pastilles are given 4 to 5 times daily for up to 14 days.

Intravenous Nystatin: The lyophilized powder should be reconstituted with sterile saline yielding a final concentration of 1 mg/rnL of nystatin. Liposomal nystatin has been dosed from 2 to 6 mg/kg intravenously once daily in clinical trials for the treatment of systemic fungal infections. At the time of writing this chapter, intravenous liposomal nystatin is not commercially available.

Other: No data are available on use of nystatin in patients with renal failure, hepatic failure, obesity, or ascites/edema. Topical nystatin has been used safely in pregnant women (40a). However, liposomal nystatin has been shown to be toxic to rat and rabbit fetuses and should be avoided in pregnant women (45a).

ADVERSE EFFECTS

Oral nystatin is virtually nontoxic and is well tolerated. Nausea, vomiting, diarrhea, and gastrointestinal distress are occasional side effects of oral nystatin; allergic reactions are very uncommon. Serious toxic reactions due to systemic administration of liposomal nystatin appear to be less frequent than non-encapsulated nystatin. The most common side effect was hypokalemia. Nephrotoxicity also occurs, although the true frequency of this adverse effect with larger numbers of treated patients is not yet available (86a).

DRUG INTERACTIONS

None are known.

CLINICAL INDICATIONS

Nystatin oral suspension and lozenges are indicated for the treatment and prevention of candidiasis in the oral cavity. Topical nystatin is indicated for the treatment of cutaneous or mucocutaneous mycotic infections caused by Candida albicans and other Candida species. Nystatin oral suspension has also been used for selective gut decontamination prior to gastrointestinal surgery although it’s role here is controversial. The role of intravenous nystatin (liposomal nystatin) is not yet clearly defined.

TABLES AND FIGURES

Table 1. In-vitro Activity of Nystatin and Liposomal Nystatin

Table 2. Clinical Fungal Isolates Reported to be Resistant to Nystatin

Table 3. Proposed Mechanisms of Fungal Resistance to Nystatin

Figure 1. Nystatin A1 (MW, 926.13)

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