Nystatin is a polyene antifungal agent and was the first active antifungal drug discovered for clinical use.
Polyenes are broad-spectrum antifungal compounds characterized biochemically by:
A macrolide structure, consisting of a macrocyclic lactone ring containing conjugated double bonds.
One or more sugar moieties attached to the macrolide structure.
Multiple hydroxyl groups, contributing to their amphiphilic nature.
The amphiphilic structure of polyene antifungals enables them to interact with fungal cell membranes, particularly by binding to ergosterol, the major sterol component of fungal membranes.
After binding to ergosterol, nystatin can disrupt the integrity of the fungal cell membrane by forming pores. This increases membrane permeability, causes leakage of cellular components, and ultimately leads to membrane depolarization and fungal cell death.
Nystatin has activity against several medically important fungal pathogens, including organisms listed by the World Health Organization (WHO) among critical- and high-priority fungal pathogens, such as:
Critical-priority pathogens: Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus.
High-priority pathogens: Histoplasma spp., Candida tropicalis, and Candida glabrata.
Nystatin was discovered in 1950 from the fermentation product of Streptomyces noursei.
The discovery was made by Rachel Fuller Brown, a chemist, and Elizabeth Lee Hazen, a microbiologist. They subsequently identified, characterized, and purified the antifungal compound.
Nystatin is marketed under several product names and formulations, including:
Mycostatin – oral suspension.
Nystop – topical powder.
Nilstat – oral drops.
Nystatin is primarily used as a topical antifungal agent for the treatment of superficial Candida infections, including:
Oral candidiasis (oral thrush)
Vaginal candidiasis
Diaper candidiasis
Nystatin has low aqueous solubility, which contributes to its limited systemic absorption and restricts its usefulness for treating invasive fungal infections.
Based on its low solubility and permeability characteristics, nystatin is classified as a Biopharmaceutics Classification System (BCS) Class IV drug, a category characterized by low solubility and low permeability.
Consequently, nystatin is mainly suitable for localized or superficial fungal infections, rather than systemic treatment of invasive mycoses.
Chemical Properties of Nystatin
Nystatin is a tetraene polyene antifungal, meaning that its macrolide structure contains four conjugated double bonds. These conjugated double bonds are an important structural feature of the molecule and contribute to its interaction with ergosterol in fungal cell membranes.
The molecule has an amphiphilic structure, consisting of distinct hydrophobic and hydrophilic regions:
A hydrophobic polyene region containing the conjugated double bonds.
A hydrophilic polyol region containing multiple hydroxyl (–OH) groups.
A polar amino sugar, D-deoxycosamine (mycosamine), which is linked to the macrolactone ring through a β-glycosidic bond.
The mycosamine sugar moiety plays an important role in the antifungal activity of nystatin by contributing to its interaction with ergosterol in the fungal cell membrane.
Nystatin contains seven hydroxyl groups within its hydrophilic polyol region. These hydroxyl groups contribute to the molecule's polarity and amphiphilic character.
Structurally, nystatin is closely related to amphotericin B, another important polyene antifungal. However, the two compounds differ in their number of conjugated double bonds and the arrangement of hydroxyl groups within their polyol regions.
Important structural distinction: Nystatin contains four conjugated double bonds, which is why it is classified as a tetraene, whereas amphotericin B contains seven conjugated double bonds and is classified as a heptaene.
Nystatin also contains a carboxylic acid group and an amino group. Because it possesses both acidic and basic functional groups, nystatin exhibits amphoteric behavior.
At approximately physiological pH (7.2–7.4), these ionizable groups can exist in charged forms, influencing the molecule's solubility, ionization, and interaction with biological membranes.
Physical Properties of Nystatin
Appearance: Nystatin is typically a light-yellow to yellow powder.
Hygroscopic nature: Nystatin is hygroscopic, meaning it can absorb moisture from the surrounding environment. Therefore, it should be protected from humidity and excessive moisture during storage.
Stability: Nystatin is sensitive to environmental conditions, particularly heat, oxygen, moisture, and light, which can affect its stability and potency. Appropriate storage conditions are therefore important for maintaining its activity.
Taste: Nystatin has a characteristically bitter taste, which can influence the palatability of oral formulations.
Isomer of Nystatin
Nystatin obtained from natural sources is present as a mixture of closely related structural variants, rather than as a single completely uniform compound.
These naturally occurring nystatin variants share important structural features, particularly the tetraene polyene macrolide structure and the amino-sugar moiety.
The major naturally occurring forms of nystatin are:
Nystatin A1
Nystatin A2
Nystatin A3
Nystatin A1: This is the major and most extensively studied form of nystatin. Its overall structure is closely related to other polyene antifungals, including amphotericin B.
Nystatin A2: This is a structural variant of nystatin A1 that contains a deoxy group at the C10 position, resulting in a difference in the molecular structure.
Nystatin A3: This variant differs structurally through the presence of an L-digitoxose sugar moiety, which is involved in a glycosidic linkage at C35.
Although these isomers have closely related structures, differences in their sugar substituents and functional groups can influence their physicochemical properties and biological activity.
Biosynthesis of Nystatin
Nystatin is a naturally occurring polyene antifungal produced by the actinomycete Streptomyces noursei. In addition to nystatin, S. noursei has been reported to produce other bioactive compounds, including cycloheximide, phalamycin, and nourseothricin.
Nystatin biosynthesis involves the stepwise assembly of a 38-carbon macrolactone backbone through successive condensation reactions of 16 malonyl-CoA units and 3 methylmalonyl-CoA units.
Malonyl-CoA is produced from acetyl-CoA through a carboxylation reaction catalyzed by acetyl-CoA carboxylase (ACC), whereas methylmalonyl-CoA is generated from propionyl-CoA through a carboxylation reaction catalyzed by methylmalonyl-CoA carboxyltransferase.
Polyene antifungals such as nystatin are synthesized by type I modular polyketide synthases (PKS-I). These multifunctional enzyme systems contain several catalytic domains that coordinate the selection, transfer, condensation, and modification of the building blocks required for macrolactone formation.
The principal domains involved in polyketide chain assembly are acyltransferase (AT), acyl carrier protein (ACP), and ketosynthase (KS). The AT domain selects the appropriate chain-building units, including malonyl-CoA and methylmalonyl-CoA, and transfers them to the ACP domain.
The ACP domain carries the activated chain-building units and the growing polyketide intermediate. The substrates are attached to ACP through a 4′-phosphopantetheine prosthetic group, which provides a flexible molecular arm for transferring intermediates between catalytic domains.
The KS domain catalyzes the condensation of the activated extender unit with the previously assembled polyketide chain, resulting in progressive extension of the carbon skeleton.
Additional catalytic domains, including ketoreductase (KR), dehydratase (DH), and enoyl reductase (ER), determine the degree of reduction of individual ketide units and consequently influence the final chemical structure of the polyketide.
The KR domain reduces a keto group to a hydroxyl group, while the DH domain can promote dehydration to generate a carbon–carbon double bond. The ER domain can subsequently reduce this double bond to form a saturated carbon–carbon bond.
Thus, the presence or absence of these reductive domains determines the chemical structure of each region of the polyketide. KR activity results in hydroxyl groups, KR and DH activities can generate hydroxyl groups and double bonds, while the combined action of KR, DH, and ER can produce hydroxyl groups and saturated bonds. This programmed enzymatic activity contributes to the characteristic tetraene structure of nystatin.
Nystatin production occurs as part of secondary metabolism in S. noursei and is strongly influenced by the nutritional environment. High concentrations of inorganic phosphate, glucose, and ammonia can suppress nystatin biosynthesis by favoring vegetative growth (trophophase) and reducing secondary metabolic activity, including expression of the nystatin biosynthetic pathway.
Following formation of the macrolactone backbone by the PKS system, several post-polyketide synthase (post-PKS) modifications are required to generate biologically active nystatin.
These post-PKS modifications include the attachment of the mycosamine (D-deoxyamino sugar) moiety, oxidation of the C16 methyl group to a carboxylic acid group, and hydroxylation at the C10 position.
The mycosamine sugar is attached to the macrolactone core through a glycosidic linkage and is essential for the biological activity of nystatin. The combined action of these post-PKS tailoring reactions produces the mature nystatin molecule capable of interacting with ergosterol in fungal cell membranes.
Mechanism of Action of Nystatin
Nystatin exerts its antifungal activity through a mechanism characteristic of polyene antifungals, primarily involving interaction with ergosterol, the principal sterol present in fungal cell membranes.
Because nystatin is amphiphilic, its hydrophobic polyene region interacts preferentially with ergosterol, while its hydrophilic polyol region remains oriented toward the aqueous environment.
Following interaction with ergosterol, nystatin molecules associate within the fungal membrane and form transmembrane channels or pores. The proposed pore has an approximate diameter of 0.36 nm, allowing ions and small intracellular components to pass through the membrane.
Pore formation disrupts the structural integrity, permeability, and homeostasis of the fungal cell membrane. This results in the uncontrolled leakage of ions and other cellular contents, particularly potassium and other small intracellular molecules.
Loss of membrane integrity causes ionic imbalance, disruption of cellular metabolism, depletion of essential intracellular components, and ultimately fungal cell death.
Although the pore-forming model has traditionally been used to explain nystatin's antifungal activity, several other mechanisms have also been proposed. These include the sterol sponge model, half-pore model, and surface adsorption model.
The sterol sponge model proposes that polyene molecules can extract or sequester ergosterol from the membrane, thereby disturbing membrane organization without requiring the formation of conventional transmembrane pores.
The half-pore model suggests that membrane disruption may involve partial pore-like structures or assemblies rather than a complete, symmetrical transmembrane channel.
The surface adsorption model proposes that nystatin molecules initially associate with the membrane surface and interact with membrane sterols, producing alterations in membrane organization and function.
In addition to direct membrane damage, oxidative stress has also been proposed as a contributing mechanism of polyene-induced fungal cell death. Nystatin exposure may promote the generation of reactive oxygen species (ROS), which can cause cellular damage.
Increased oxidative stress may result in DNA damage, lipid peroxidation, and protein carbonylation, further impairing essential cellular functions and contributing to fungal cell death.
Therefore, nystatin's antifungal activity is best understood as a multifaceted process involving ergosterol-mediated membrane disruption, altered membrane permeability, cellular leakage, metabolic dysfunction, and potentially oxidative damage.
Toxicity of Nystatin
Nystatin is generally considered less toxic than amphotericin B, particularly because its poor systemic absorption limits exposure of internal organs when administered orally or topically.
Both oral and topical formulations may cause local or gastrointestinal adverse effects. Reported reactions include itching, rash, burning sensation, and, rarely, acute generalized exanthematous pustulosis (AGEP).
Oral nystatin may additionally cause gastrointestinal adverse effects, including nausea, vomiting, diarrhea, and abdominal discomfort or stomachache.
Parenteral administration of nystatin has been associated with significant infusion-related reactions and dose-limiting toxicity. Because of its toxicity and poor tolerability when administered systemically, conventional nystatin is primarily used for local treatment rather than parenteral therapy.
Infusion-related reactions have been linked to the potential immunomodulatory activity of nystatin. Proposed mechanisms involve Toll-like receptor (TLR)-dependent signaling, which can stimulate macrophages and other immune cells to release proinflammatory cytokines, including IL-1β, IL-8, and TNF-α.
Self-aggregation of nystatin molecules may also contribute to systemic toxicity. When nystatin forms aggregates, its normal preference for binding fungal ergosterol may be reduced, potentially increasing nonspecific interactions with mammalian sterols and cellular membranes and thereby contributing to cytotoxicity.
Nystatin can also interact with low-density lipoproteins (LDL). Its association with LDL has been proposed as a potential mechanism contributing to renal toxicity, because LDL receptors are expressed on glomerular endothelial cells, potentially facilitating renal exposure to nystatin.
Overall, the toxicity of nystatin is strongly influenced by its route of administration, aggregation behavior, interactions with mammalian membranes, immune activation, and systemic exposure. Its limited absorption following oral and topical administration is therefore an important factor underlying its comparatively favorable safety profile.
Conclusion
Nystatin is a potent polyene antifungal agent that has remained clinically relevant since its discovery in 1950, particularly for the treatment of superficial candidiasis.
Despite being an older antifungal, nystatin continues to be valuable because of its effective antifungal activity, established safety profile for topical and oral use, low cost, and limited systemic absorption.
Nystatin generally has a low propensity for clinically significant resistance, although resistance can develop, particularly with prolonged or repeated exposure.
Parenteral nystatin is no longer used clinically because of significant toxicity and poor tolerability. Historical systemic administration was associated with adverse effects, although its toxicity profile differed from that of amphotericin B.
Nystatin has also demonstrated potential as a prophylactic agent against oral candidiasis in immunocompromised populations, with some studies reporting favorable outcomes compared with fluconazole in specific patient groups, including people with HIV.
Its cost-effectiveness and broad accessibility make nystatin an important option, particularly where access to newer or more expensive antifungal therapies is limited.
However, its poor solubility, limited systemic absorption, and inability to adequately treat invasive fungal infections restrict its therapeutic applications.
Further research into structural modification, improved formulations, and drug-delivery strategies could help reduce adverse effects, improve pharmacological properties, and expand the therapeutic potential of nystatin.
Overall, nystatin remains an important antifungal despite its age, and continued research may help transform this established drug into a safer, more effective, and more versatile antifungal therapy.
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