Fungal infections have become a major public health problem and are growing in number and severity over the past three decades. The development of new medical treatments, including therapy with immunosuppressive agents and chemotherapy of cancer, led to a dramatic increase in the number of immunocompromised individuals who are vulnerable to infections which otherwise would have been easily resolved (Karkowska-Kuleta et al., 2009). More than 1.7 billion people worldwide are estimated to suffer from fungal diseases, ranging from superficial to invasive infections (Brown et al., 2012), and some of these infections cause more deaths per year than tuberculosis or malaria (Gaffi – Global Action Fund for Fungal Infections, 2016). However, deaths resulting from invasive or chronic fungal infections are often overlooked, and most public health agencies present little or no mycological surveillance programs (Brown et al., 2012). Preventive measures, premature diagnosis, and the availability of appropriate antifungal treatment could reduce death rates by fungal infections.

The current antifungal therapy is based on three major chemical classes: echinocandins, polyenes, and azoles. Echinocandins are β(1,3)-glucan synthase inhibitors, disrupting the synthesis of the cell wall β(1,3)-D-glucan polymer. Polyenes and azoles alter cytoplasm membrane permeability; polyenes bind irreversibly to ergosterol whilst azoles inhibit ergosterol biosynthesis specifically inhibiting cytochrome P450 dependent enzyme lanosterol 14-α-demethylase. Other antifungals as allylamines and pyrimidine analogs find use against a few fungal infections, mainly to treat cutaneous mycosis (Ostrosky-Zeichner et al., 2010). It is important to emphasize that most antifungal compounds are approved for treatment of superficial mycoses, especially in topical formulations (Denning and Bromley, 2015). In contrast, the clinical repertoire for treatment of invasive fungal infections (IFIs) is much smaller, with only nine approved compounds: amphotericin B, fluconazole, voriconazole, itraconazole, posaconazole, caspofungin, anidulafungin, micafungin, and flucytosine (Denning and Bromley, 2015).

However, the use of these compounds for IFIs is complicated by their poor solubility and bioavailability due to problems in drug absorption and distribution. The situation is aggravated by difficulties to deliver the drug to its target, which leads to an increased incidence of adverse effects resultant from the lack of drug selectivity, which compromise efficacy and safety. An example is the polyene amphotericin B; the drug displays a broad spectrum of action and fungicidal effect, but the significant number of serious adverse effects (which include nephrotoxicity) limits its wider therapeutic use (Hamill, 2013). Triazole agents also exhibit adverse effects associated mainly to hepatic disorders, and their interaction with other drugs have been described due to their ability to modulate the activity of cytochrome P450 enzymes and the hepatic oxidative metabolism of many drugs (Peyton et al., 2015). Even the newest antifungal agents, the echinocandins, present low oral bioavailability and cause gastrointestinal effects after intravenous administration (Chen et al., 2011).

This scenario becomes more complicated if we take into consideration the challenges related to the development of new antifungal agents. The fact that fungi are evolutionarily close to their human host impairs the discovery of new compounds with selective action (Seneviratne and Rosa, 2016). These factors associated with increases in antifungal resistance (Pfaller, 2012), emergence of uncommon fungi (Davoudi et al., 2015; Roilides, 2016), and high mortality rates (Brown et al., 2012; Gaffi – Global Action Fund for Fungal Infections, 2016) have led the scientific community to search for other ways to improve drug targeting and efficacy while reducing adverse effects.

In this context, drug delivery systems emerge as promising alternatives to improve therapy and overcome the above-mentioned limitations (Figure 1A). They have been demonstrated to reduce toxicity caused by conventional drug treatments, improve drug efficacy and bioavailability, reduce dose and administration frequency, and might allow oral administration of compounds with unfavorable pharmacokinetic characteristics, formerly used in topical and/or intravenous forms (Kumar et al., 2013). Various nanocarriers, such as dendrimers, polymers, liposomes, nanoemulsions, and micelles have been investigated for drug delivery (Jeevanandam et al., 2016).

Amphotericin B lipid formulations are examples of the successful application of drug nanocarriers to improve therapeutic outcomes. Three lipid formulations of amphotericin B are available for medical use: liposomal (L-AMB); colloidal dispersion (ABCD); and a lipid complex (ABLC). All were developed to reduce nephrotoxicity without compromising the antifungal efficacy (Falci et al., 2015). The L-AMB formulation (Ambisome^®) incorporates the drug into liposomes, lipid vesicles of 55–75 nm in diameter composed of soy lecithin, cholesterol, and distearoylphosphatidylglycerol. In the ABCD formulation (Amphocil^®), amphotericin B is co-incorporated with cholesterol sulfate in microdiscs of an average diameter of 122 nm. The ABLC formulation (Abelcet^®) is a macromolecular complex of amphotericin B with dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol forming ribbon-like particles ranging in length from 1,600 to 6,000 nm (Tiphine et al., 1999; Martinez, 2006; Voltan et al., 2016).

Amphotericin B association with lipids improves drug dissolution, facilitates parenteral infusion, protects the drug from destruction by enzymatic degradation and/or host immune factor inactivation and changes the pharmacokinetic profile of amphotericin B by slow drug release resulting in the protection of potentially vulnerable tissues (most importantly, the kidneys) (Hamill, 2013). In a review, the lipid-based compounds were associated with less risk of changes in renal function, while the risk of a doubling in serum creatinine from baseline was 58% less likely with the lipid based formulations (Barrett et al., 2003). Moreover, the slow and continuous release of amphotericin B from lipid formulations allow the administration of higher doses and longer treatment (Hamill, 2013). However, although lipid-based amphotericin B formulations have shown equivalent efficacy and reduced toxicity compared with the conventional amphotericin B formulation, they are often reserved as a secondary treatment option, presumably because of the differences in costs (Wade et al., 2013).

Various other antifungal agents could benefit from the ability of delivery systems to modify pharmacokinetics and reduce adverse effects. In fact, several studies have been conducted on incorporation of other antifungal agents in various types of delivery systems in the past years, suggesting a demand for novel cost-effective and safe delivery platforms. In this review, we will discuss non-lipid drug carriers for antifungal agents, emphasizing the relevance of alginate-based carriers as an interesting approach for delivery of antifungal agents.

Among the many types of modified release systems developed, polymer-based systems have attracted attention as they offer significant advantages over other carrier platforms primarily due to the tremendous versatility of polymer matrices, which allows for tailoring of the carrier properties to meet the specific intended need. Other advantages of polymeric systems include ease of production, high encapsulation efficiency of the molecule of interest, protection of the drug against physicochemical degradation, flexibility of their physicochemical properties (such as size, surface charge, and hydrophobicity), slow or fast polymer degradation and stimuli-responsive polymer erosion for temporal control over the release of drugs (Van Vlerken et al., 2007; Voltan et al., 2016).

Bulk gels, films, micro and nanocarriers have been prepared using natural polymers, such as alginate (Liakos et al., 2013; Elsayed et al., 2015), chitosan (Al-Kassas et al., 2016; Islam and Ferro, 2016), gelatin (Mahor et al., 2016), dextran (Gallovic et al., 2016), or synthetic polymers, such as polylactic acid (PLA) (Tyler et al., 2016), poly(lactide-co-glycolide) (PLGA) (Han et al., 2016; Thomas et al., 2016) and others. Natural polysaccharides are advantageous compared to the synthetic polymers due to their abundance in nature, low processing cost, biocompatibility, biodegradability, water solubility, bioactivity, and environmental safety (Sundar et al., 2010; Debele et al., 2016). Among the biopolymers used, alginate represents an excellent alternative for obtainment of drug carriers and is the main focus of this review.

Alginate is a generic name assigned to a series of natural unbranched polyanionic polysaccharides of β-D-mannuronic acid (M) and α-L-guluronic acid (G) (Figure 2) repeating units linked by a 1→4 linkage and displaying chain homosequences of MMM and GGG, interdispersed with MGM heterosequences (Lee and Mooney, 2012; Sosnik, 2014). Molecular weights in the range of 32–400 kg/mol together with different relative G/M compositions and variations in the proportion and chain arrangements of M and G blocks might be found depending on type of seaweed from which alginate was extracted (Sosnik, 2014; Cardoso et al., 2016).

This polysaccharide is frequently obtained from brown seaweeds, including Laminaria hyperborea, Laminaria digitata, Laminaria japonica, Ascophyllum nodosum, and Macrocystis pyrifera (Lee and Mooney, 2012). Alginate extraction is conducted by treatment with aqueous alkali solutions, typically sodium hydroxide, in which the natural alginate in various salt forms is converted into water-soluble sodium alginate; the extract is filtered, and either sodium or calcium chloride is added to the filtrate in order to precipitate alginate. After further purification and conversion, water-soluble sodium alginate powder is produced (Qin, 2008; Lee and Mooney, 2012). On a dry weight basis, the alginate contents are 22–30% for A. nodosum, 25–44% for L. digitata, and 17–33 and 25–30%, respectively, for the fronds and stems of L. hyperborea (Qin, 2008).

Another alginate source is bacteria from the Azotobacter and Pseudomonas species. The alginate biosynthesis pathway in bacteria can be divided into four different stages: (i) synthesis of precursor substrate, (ii) polymerization and cytoplasmic membrane transfer, (iii) periplasmic transfer and modification, and (iv) export through the outer membrane. However, even under optimized fermentation conditions, production yields are quite low (around 4 g/l) (Remminghorst and Rehm, 2006), and the process is not considered economically viable for commercial applications (Goh et al., 2012).

Alginate has important characteristics that make it one of the most used polymers for drug delivery application: it is non-toxic, biocompatible, non-immunogenic, biodegradable, mucoadhesive, readily available and has low cost (Figure 1B) (Hosseini et al., 2013; Paques et al., 2014). Various modified drug delivery systems can be obtained using alginate, such as hydrogels (polymeric networks with three-dimensional configuration capable of imbibing high amounts of water or biological fluid), microparticles (diameter from 1 to 200 μm) and nanoparticles (diameter from 10 to 1000 nm) (Hamidi et al., 2008; Sosnik, 2014). Depending on the desired application and formulation characteristics, different techniques can be used to produce the carriers. The ionotropic gelation is the most common method used to obtain alginate-based nano and microparticles, beads and hydrogels, among other systems, due to its simplicity and non-toxic procedure.

In this process, alginate is crosslinked in aqueous solutions ionotropically by a mechanism whereby pendant carboxylic acid moieties of G units chelate Ca²⁺ or other divalent cations (e.g., Sr²⁺, Ba²⁺) to generate 3D networks that allow the incorporation of compounds (Tønnesen and Karlsen, 2002; Sosnik, 2014). This gelation is known as the “egg box model” and has been demonstrated to result from specific and strong interactions between calcium ions and blocks of guluronic acid residues (Braccini and Pérez, 2001). Biomolecules and other drugs can be loaded under mild conditions and retained in the 3D structure (Paques et al., 2014; Lopes et al., 2016). Ionotropic gelation can be divided into external and internal gelation. The main difference between these methods is the type of calcium salt used and consequently the gelling kinetics (Draget et al., 2005).

In the external gelation method, which is also referred to as the “diffusion method” particles are produced by dropping a drug-loaded alginate solution into the aqueous solution of a soluble salt of calcium, such as calcium chloride. The internal gelation method, also referred to as “internal setting,” is characterized by the release of calcium ions in a controlled manner from an insoluble calcium source, such as calcium carbonate, within the alginate solution. Controlled release of calcium is usually obtained by a change in pH, by a limited solubility of the calcium salt source, and/or by the presence of chelating agents (Draget et al., 2005; Paques et al., 2014; Lopes et al., 2016).

Micro and nanoparticles of alginate are usually obtained using emulsification (W/O or O/W) followed by external or internal gelation (Hosseini et al., 2013; Paques et al., 2013; Song et al., 2013). Emulsions are basically obtained by dispersing a fluid into another immiscible fluid and are produced by fragmenting, through various processes, one phase into another, and leading to a dispersion of droplets in a continuous phase. The recombination of the freshly formed droplets may be partially suppressed by the addition of surfactants that locate at the droplet interface reducing the interfacial tension (Bibette and Leal-Calderon, 1996; Pillai et al., 1999).

Several types of alginate-based delivery systems for antifungal agents have been studied in the last few years, such as nanoparticles and microparticles, beads, hydrogels, tablets, and films. In the next sections, alginate–based carriers for two classes of antifungal agents will be addressed: azoles and polyenes (Table 1).

Alginate formulations have been used as delivery systems for other molecules with antifungal activity potential, such as essential oils, antiseptics, and others (Table 2).

Turmeric oil from the rhizome of Curcuma longa is an example of an essential oil encapsulated in alginate-based carriers. Turmeric oil is widely used as a food additive, condiment and household medicine in Southern Asia, and presents several pharmacological properties as an antibacterial, an antifungal, an antioxidant, an antimutagenic, and a repellent (Lertsutthiwong et al., 2008). Lertsutthiwong et al. (2008) encapsulated this oil in alginate nanocapsules, while Lertsutthiwong and Rojsitthisak (2011) produced chitosan-alginate nanocapsules containing turmeric oil. Chitosan-alginate nanocapsules increased the skin permeation of turmeric oil compared with an ethanolic solution (Lertsutthiwong and Rojsitthisak, 2011), representing a new approach for topical delivery and treatment of cutaneous infections (Lertsutthiwong and Rojsitthisak, 2011). These studies also demonstrated that alginate is an effective biopolymer for encapsulation of a volatile essential oil, producing nanocarriers with long-term physical stability (120 days) when stored at 4°C.

Hosseini et al. (2013) encapsulated Satureja hortensis essential oil (known for its antimicrobial and antioxidant activity) in alginate microparticles, and concluded that these microparticles could be used for antioxidant and antimicrobial purposes with controlled release properties in the food and nutraceuticals industries. Other essential oils, such as elicriso italic, chamomile blue, cinnamon, lavender, tea tree, peppermint, eucalyptus, lemongrass, and lemon were dispersed in sodium alginate films and presented remarkable antibacterial and antifungal activities (Liakos et al., 2014). These alginate films enriched with essential oils should find multiple applications, including as disposable wound dressings, in food packaging, medical device protection and disinfection, and indoor air quality improvement materials (Liakos et al., 2014).

Soliman et al. (2013) encapsulated clove, thyme and cinnamon oils in alginate microspheres in order to achieve their optimal antifungal activity against two of mycotoxigenic fungi species, Aspergillus niger and Fusarium verticillioides, and prevent fungal contamination in stored grains. These microspheres showed antifungal activity in addition to increasing the storage time of oil with antifungal activity for up to 8 days, while the free oil lost activity within 2 days. Thus, the main advantage of encapsulation was to reduce the rate of evaporation of these essential oils prolonging antifungal activity (Soliman et al., 2013).

Plant extracts have also been incorporated in alginate-based nanocarriers. The anise crude extracts and active compounds manifest antimicrobial, antioxidant, insecticidal, analgesic, sedative, and convulsive activities (Wang et al., 2011). Anise-based bioadhesive alginate gels for vaginal application showed antibacterial activity, and no effect against Candida species was observed (Gafiţanu et al., 2016). Alginate microparticles containing pomegranate extract (Punica granatum) with encapsulation efficiency of 81.9% (Endo et al., 2012) displayed antifungal activity against C. albicans (minimum inhibitory concentration of 3.9 μg/ml), with the advantage of providing controlled extract release to achieve the desired therapeutic effect against oral fungal infection caused by C. albicans (Endo et al., 2012).

Sodium alginate tablets containing lactoferrin, a potential antifungal candidate for candidiasis treatment, displayed release-controlling and mucoadhesive properties (Kuipers et al., 2002). In vitro microbiological studies demonstrated that the formulation had the same antifungal properties as the free lactoferrin against several clinical isolates of C. albicans and Candida glabrata (Kuipers et al., 2002). The tablets (250 mg lactoferrin) on oral mucosa of human volunteers were able to keep the lactoferrin concentration in the saliva for at least 2 h, suggesting that they might improve the therapeutic efficacy of lactoferrin in oral candidiasis treatment (Kuipers et al., 2002).

Research showed that the use of sodium alginate as a stabilizer provided a highly stable solution of silver nanoparticles (nanosilver) that has significant antibacterial and antifungal activity. The nanosilver aqueous solution at concentrations of 0.0005–0.005% with sodium alginate completely inhibited C. albicans viability within 24 h (Kubyshkin et al., 2016). Shao et al. (2015) developed silver sulfadiazine-loaded cellulose/sodium alginate films and observed that these films had excellent antibacterial performances on E. coli and S. aureus and antifungal activity on C. albicans. They also showed good biocompatibility and accepted cytotoxicity (less than 20% using 0.1% of silver sulfadiazine). The authors suggested that these films containing silver sulfadiazine could be a potential antimicrobial for use in wound dressings.

Antiseptics have also been incorporated in alginate carriers for controlled release (Juliano et al., 2008; Liakos et al., 2013). Films of sodium alginate or sodium alginate/chitosan were well tolerated, and able to sustain the release of chlorhexidine diacetate, providing active concentrations against C. albicans for prolonged periods of time (approximately 3 h) on the buccal mucosa of human volunteers, which is in agreement with results showing in vitro mucoadhesion (Juliano et al., 2008). Another antiseptic incorporated in alginate beads and films was povidone iodine (PVPI) (Liakos et al., 2013). Controlled release of PVPI was possible when the composite films and beads were brought into direct contact with aqueous media, maintaining the bactericidal and fungicidal properties. This characteristic is highly desired in clinical applications to avoid toxic doses of PVPI (Liakos et al., 2013). A wide variety of applications of these systems are envisioned, such as wound dressings, hygienic and protective packaging films for medical devices, and polymer beads as water disinfectants (Liakos et al., 2013).

Alginate is a natural polymer widely used for obtainment of drug delivery systems due to its non-toxicity, biodegradability, biocompatibility, low cost, mucoadhesive, and non-immunogenic properties, in addition to the availability of simple protocols for production of these systems. In spite of the popularity of alginate-based carriers in the drug delivery field, few studies have been conducted with antifungals, mostly dealing with azoles and polyenes. Thus, there is a gap when it comes to the advantages and limitations of alginate-based carriers for other types of antifungal agents. To the best of our knowledge, this review is the first to directly focus on alginate carriers for antifungal drugs. Based on the studies discussed here, alginate-based antifungal delivery systems have shown great potential in the treatment of fungal infections, providing modified release, and decreasing administration frequency, toxicity and the dose necessary for antifungal efficacy. Therefore, the use of alginate-based carriers represents a promising approach for delivery of antifungal agents to improve the treatment protocols for cutaneous and systemic mycosis.

CS did the literature review and wrote the manuscript. KI and LL designed and co-wrote the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.