Natural Polyketides Act as Promising Antifungal Agents

Invasive fungal infections present a significant risk to human health. The current arsenal of antifungal drugs is hindered by drug resistance, limited antifungal range, inadequate safety profiles, and low oral bioavailability. Consequently, there is an urgent imperative to develop novel antifungal medications for clinical application. This comprehensive review provides a summary of the antifungal properties and mechanisms exhibited by natural polyketides, encompassing macrolide polyethers, polyether polyketides, xanthone polyketides, linear polyketides, hybrid polyketide non-ribosomal peptides, and pyridine derivatives. Investigating natural polyketide compounds and their derivatives has demonstrated their remarkable efficacy and promising clinical application as antifungal agents.


Introduction
Invasive fungal infections significantly threaten human health, resulting in approximately 1.5 million deaths annually [1,2].The primary culprits responsible for these fatalities are Candida, Cryptococcus, and Aspergillus species [3,4].The rise in severe underlying diseases and immunocompromised populations, such as those undergoing hematopoietic stem cell transplantation, organ transplantation, immunosuppressive therapy, acquired immune deficiency syndrome, cancer, advanced age, and preterm birth, has further exacerbated the morbidity and mortality associated with invasive fungal infections [5,6].The current antifungal agents utilized in clinical settings are associated with drawbacks such as drug resistance, limited bioavailability, nephrotoxicity, and a restricted antifungal spectrum [7].As a result, there is a pressing demand for developing novel antifungal agents to treat invasive fungal infections.
Among the three primary categories of antifungal medications presently accessible, amphotericin B and caspofungin are classified as polyketide compounds.Amphotericin B, a polyene macrolide polyketide, exhibits a broad spectrum of fungicidal activity against Candida, Aspergillus, and Cryptococcus species [8] and remains a preferred treatment option for severe invasive fungal infections [7].Caspofungin, a non-ribosomal polyketide derivative, selectively targets β-1,3-glucan synthase and impedes fungal cell wall biogenesis with notable selectivity and biological safety compared to amphotericin B [9].Amphotericin B and caspofungin, both polyketide compounds, have demonstrated clinical efficacy in treating invasive fungal infections.This suggests that developing polyketide compounds as antifungal drugs shows considerable potential.
Polyketides are synthesized through a series of Claisen decarboxylation condensation reactions, utilizing short-chain acyl starting substrates and extension units, including acetyl-CoA, propionyl-CoA, malonyl-CoA, and methylmalonyl-CoA [10].Polyketides, categorized as secondary metabolites, demonstrate a broad spectrum of structural diversity and are generated by various organisms, including bacteria, fungi, plants, and animals.The biosynthesis of polyketides involves a sequence of condensation reactions catalyzed by three types of polyketide synthases (PKSs): type I PKSs, type II PKSs, and type III PKSs.Type I PKSs are responsible for the biosynthesis of macrolides and related polyenes [11].
Within the category of type I PKSs, there are two distinct subtypes: modular and iterative.Modular type I PKSs are composed of enzyme complexes containing multiple modules, each consisting of linear domains.Each set of domains is utilized only once during the assembly of polyketides [12].In contrast, iterative type I PKSs possess a single reusable module, with the domains within this module being reused to catalyze multiple rounds of decarboxylation condensation reactions [13].Type II PKSs, also called aromatic PKSs, are comprised of multiple distinct proteins that function as enzyme complexes.These complexes facilitate repeating a specific chemical reaction by using reusable domains.Typically employing malonyl-CoA as a substrate, Type II PKSs incrementally add two carbon atoms to the polyketide intermediate following each round of decarboxylation condensation reaction.Subsequently, the polyketide is transformed into an aromatic compound under ketoreductase, an aromatase, and a cyclase.The resulting preliminary aromatic polyketide is further modified by an oxygenase, a glycosyltransferase, and a methyltransferase to yield the ultimate aromatic products [14].Type II PKSs produce aromatic polyketide compounds, including anthracyclines, anticyclones, aureolic acids, tetracyclines, anthracyclines [14], and polyenes [15].In contrast to the other two types of PKSs, type III PKSs are comprised of a single protein that directly utilizes simple carboxylic acids as substrates, which are activated by acyl-CoA and do not require acyl carrier protein-activated acyl-CoA.Type III PKSs primarily facilitate the biosynthesis of flavonoids, stilbenes, phenylpropanoids, pyrone-type aromatic polyketides, and resorcinoltype aromatic polyketides [16][17][18].
This review provides a comprehensive overview of the antifungal properties and mechanisms exhibited by a range of natural polyketide compounds, encompassing macrolide polyethers, polyether polyketides, xanthone polyketides, linear polyketides, hybrid polyketide nonribosomal peptides, and pyridine derivatives.The potential of these natural polyketide compounds in managing invasive fungal infections appears highly promising.

Macrolide Polyketides
Macrolide polyketides are mainly synthesized by the Type I PKSs.The structural diversity of these compounds is achieved through variations in starting substrates, extension units, modules, and domains, as well as a series of post-modifications that occur after their release.Macrolide polyketides can form glycosidic bonds with one or more sugar moieties.These compounds are classified based on the number of atoms present in the macrolide ring, which includes 12-membered, 14-membered, 24-membered, 26-membered, 32-membered, 36-membered, and 38-membered variants.
Biomolecules 2023, 13, x FOR PEER REVIEW 2 of 38 catalyzed by three types of polyketide synthases (PKSs): type I PKSs, type II PKSs, and type III PKSs.Type I PKSs are responsible for the biosynthesis of macrolides and related polyenes [11].Within the category of type I PKSs, there are two distinct subtypes: modular and iterative.Modular type I PKSs are composed of enzyme complexes containing multiple modules, each consisting of linear domains.Each set of domains is utilized only once during the assembly of polyketides [12].In contrast, iterative type I PKSs possess a single reusable module, with the domains within this module being reused to catalyze multiple rounds of decarboxylation condensation reactions [13].Type II PKSs, also called aromatic PKSs, are comprised of multiple distinct proteins that function as enzyme complexes.These complexes facilitate repeating a specific chemical reaction by using reusable domains.Typically employing malonyl-CoA as a substrate, Type II PKSs incrementally add two carbon atoms to the polyketide intermediate following each round of decarboxylation condensation reaction.Subsequently, the polyketide is transformed into an aromatic compound under ketoreductase, an aromatase, and a cyclase.The resulting preliminary aromatic polyketide is further modified by an oxygenase, a glycosyltransferase, and a methyltransferase to yield the ultimate aromatic products [14].Type II PKSs produce aromatic polyketide compounds, including anthracyclines, anticyclones, aureolic acids, tetracyclines, anthracyclines [14], and polyenes [15].In contrast to the other two types of PKSs, type III PKSs are comprised of a single protein that directly utilizes simple carboxylic acids as substrates, which are activated by acyl-CoA and do not require acyl carrier protein-activated acyl-CoA.Type III PKSs primarily facilitate the biosynthesis of flavonoids, stilbenes, phenylpropanoids, pyrone-type aromatic polyketides, and resorcinoltype aromatic polyketides [16][17][18].This review provides a comprehensive overview of the antifungal properties and mechanisms exhibited by a range of natural polyketide compounds, encompassing macrolide polyethers, polyether polyketides, xanthone polyketides, linear polyketides, hybrid polyketide nonribosomal peptides, and pyridine derivatives.The potential of these natural polyketide compounds in managing invasive fungal infections appears highly promising.

Macrolide Polyketides
Macrolide polyketides are mainly synthesized by the Type I PKSs.The structural diversity of these compounds is achieved through variations in starting substrates, extension units, modules, and domains, as well as a series of post-modifications that occur after their release.Macrolide polyketides can form glycosidic bonds with one or more sugar moieties.These compounds are classified based on the number of atoms present in the macrolide ring, which includes 12-membered, 14-membered, 24-membered, 26-membered, 32-membered, 36-membered, and 38-membered variants.
25 µg/mL).Compound 59 (Figure 7), removing the malonyl side chain at the C-23 position of methylguanidylfungin A (55), increases antifungal activity against C. albicans IAM 4888 (MIC = 3.12 µg/mL), C. albicans Yu 1200 (MIC = 6.25 µg/mL), and A. fumigatus IAM 2153 (MIC = 3.12 µg/mL) due to increased solubility in water [36].Compound 60 (Figure 7), the ring-opening structure of the tetrahydropyran ring of guanidylfungin A (54), loses anti fungal activity against C. albicans IAM 4888 (MIC > 100 µg/mL), C. albicans Yu 1200 (MIC > 100 µg/mL), and A. fumigatus IAM 2153 (MIC = 50 µg/mL).Compound 61 (Figure 7) removing the malonyl side chain at the C-23 position of compound 60, cannot restore an tifungal activity against C. albicans IAM 4888 (MIC = 100 µg/mL), C. albicans Yu 1200 (MIC = 100 µg/mL), and A. fumigatus IAM 2153 (MIC = 12.5 µg/mL) despite the increased wate solubility [36] (Table 1).These lines of evidence suggest that the tetrahydropyran ring is necessary for guanidylfungin A (54) activity, but the malonyl group is not.In the 1950s, amphotericin B (62) (Figure 8), a 38-membered macrolide isolated from Streptomyces nodosus, was introduced to the clinic as a natural antifungal agent, demonstrat-ing broad-spectrum antifungal activity against various invasive fungi [7,8] (Table 1).Its mechanism of action involves acting as a "sterol sponge", forming aggregates outside the cell membrane to extract ergosterol from the bilayer and kill yeasts [43].However, amphotericin B (62) exhibits dose-dependent renal and hematopoietic toxicity by targeting and extracting cholesterol from host cell membranes, damaging host cells [44].The introduction of lipid-based formulations of amphotericin B (62) during the mid-1990s significantly reduced its nephrotoxicity [44].However, the clinical application of amphotericin B ( 62) is restricted due to its requirement for intravenous administration.A new nanoparticle crystal encapsulated formulation of amphotericin B (62) called cochleated amphotericin B (CAmB) is a novel oral formulation of amphotericin B [45].CAmB demonstrates in vitro activity against C. neoformans, Candida species, and A. fumigatus [46].Intraperitoneal injection of CAmB significantly increases the survival rate of mice infected with C. albicans [47].Using a systemic aspergillosis model, survival is 70% after 14 days at oral doses of 20 mg/kg and 40 mg/kg of CAmB and the fungal burden of lung, liver and kidney is reduced by more than 100 times [48].Using a 3-day delayed model of murine cryptococcal meningoencephalitis and a large inoculum of a highly virulent strain of serotype A C. neoformans, CAmB, in combination with flucytosine, is found to have efficacy equivalent to parental amphotericin B deoxycholate with flucytosine and superior to oral fluconazole without untoward toxicity [49].In a Phase I trial, the safety and tolerability of CAmB treatment for cryptococcal meningitis in HIV-infected patients were assessed, revealing that oral CAmB was well tolerated and not nephrotoxic when compared to intravenous CAmB (NCT04031833) [46].Furthermore, a Phase II trial examined the efficacy of CAmB in patients with azole-resistant chronic mucocutaneous candidiasis, and subsequent clinical trials demonstrated favorable tolerance and safety outcomes (NCT02629419) [50].
marks the ring-opening structure of compound 60, which differs from the tetrahydropyran ring guanidylfungin A (54).
In the 1950s, amphotericin B (62) (Figure 8), a 38-membered macrolide isolated from Streptomyces nodosus, was introduced to the clinic as a natural antifungal agent, demon strating broad-spectrum antifungal activity against various invasive fungi [7,8] (Table 1) Its mechanism of action involves acting as a "sterol sponge", forming aggregates outside the cell membrane to extract ergosterol from the bilayer and kill yeasts [43].However amphotericin B (62) exhibits dose-dependent renal and hematopoietic toxicity by target ing and extracting cholesterol from host cell membranes, damaging host cells [44].The introduction of lipid-based formulations of amphotericin B (62) during the mid-1990s sig nificantly reduced its nephrotoxicity [44].However, the clinical application of amphoter icin B ( 62) is restricted due to its requirement for intravenous administration.A new na noparticle crystal encapsulated formulation of amphotericin B (62) called cochleated am photericin B (CAmB) is a novel oral formulation of amphotericin B [45].CAmB demon strates in vitro activity against C. neoformans, Candida species, and A. fumigatus [46].Intra peritoneal injection of CAmB significantly increases the survival rate of mice infected with C. albicans [47].Using a systemic aspergillosis model, survival is 70% after 14 days at ora doses of 20 mg/kg and 40 mg/kg of CAmB and the fungal burden of lung, liver and kidney is reduced by more than 100 times [48].Using a 3-day delayed model of murine cryptococ cal meningoencephalitis and a large inoculum of a highly virulent strain of serotype A C neoformans, CAmB, in combination with flucytosine, is found to have efficacy equivalen to parental amphotericin B deoxycholate with flucytosine and superior to oral fluconazole without untoward toxicity [49].In a Phase I trial, the safety and tolerability of CAmB treat ment for cryptococcal meningitis in HIV-infected patients were assessed, revealing that ora CAmB was well tolerated and not nephrotoxic when compared to intravenous CAmB (NCT04031833) [46].Furthermore, a Phase II trial examined the efficacy of CAmB in pa tients with azole-resistant chronic mucocutaneous candidiasis, and subsequent clinical tri als demonstrated favorable tolerance and safety outcomes (NCT02629419) [50].

Polyether Polyketides
Type I PKSs catalyze the decarboxylation reaction of the substrate to generate a polyketide skeleton.This skeleton then is undergone a series of post-modifications, in cluding epoxidation, epoxide ring opening cascade to ether, hydroxylation, methylation and glycosylation, ultimately forming polyether polyketides.Polyether polyketides are natural polyketide products with multiple asymmetric centers and two or more tetrahy drofuran and tetrahydropyran rings.Polyether polyketides can be categorized into three groups based on their distinct chemical structures: polyethers with long-chain and multi hydroxyl groups, macrolide polyethers, and ladder-like polyethers.

Polyether Polyketides
Type I PKSs catalyze the decarboxylation reaction of the substrate to generate a polyketide skeleton.This skeleton then is undergone a series of post-modifications, including epoxidation, epoxide ring opening cascade to ether, hydroxylation, methylation, and glycosylation, ultimately forming polyether polyketides.Polyether polyketides are natural polyketide products with multiple asymmetric centers and two or more tetrahydrofuran and tetrahydropyran rings.Polyether polyketides can be categorized into three groups based on their distinct chemical structures: polyethers with long-chain and multi-hydroxyl groups, macrolide polyethers, and ladder-like polyethers.

Ladder-like Polyethers
Ladder-like polyethers are composed of ether rings, which are composed mainly of six-membered rings.The ether rings are arranged into ladder-like structures by trans

Ladder-like Polyethers
Ladder-like polyethers are composed of ether rings, which are composed mainly of six-membered rings.The ether rings are arranged into ladder-like structures by trans configuration.The oxygen atoms of the adjacent ether rings are alternately located at the upper and lower ends of the ring.Ladder-like polyethers have low polarity and are lipid-soluble compounds.
Yessotoxin (76) (Figure 10), a compound derived from the dinoflagellate Protoceratium reticulatum found in Mutsu Bay, Japan, has been investigated regarding its structureactivity relationship (SAR) [58].Desulfated yessotoxin (77) and hydrogen-desulfated yessotoxin (78) (Figure 10), two derivatives of yessotoxin, have been synthesized for this purpose [58].Desulfated yessotoxin (77) has been found to exhibit reduced hydrophilicity and increased antifungal activity against A. niger [58] (Table 1).Hydrogen-desulfated yessotoxin ( 78) is a product of the hydrogenation of the polyene side chain of desulfated yessotoxin (77), and its antifungal activity is comparable to that of desulfated yessotoxin (77), indicating that the ladder-shaped polyether structure, as opposed to the polyene side chain, is critical for the antifungal activity of yessotoxin.Desulfated yessotoxin (77) has been found to bind to the transmembrane α-helix motif of the membrane integral protein glycophorin A, thereby inducing the dissociation of glycophorin A oligomers into dimers and monomers [58].Despite its antifungal activity, yessotoxin (76) has been observed to induce subacute cardiotoxicity [59].In vitro studies have shown that human ether-a-gogo related gene (hERG) Chinese hamster ovary cells treated with 100 nM yessotoxin for 12 or 24 h exhibit increased hERG potassium channels on the cell surface [59].In vivo experimentation involves the intraperitoneal injection of rats with either 50 µg/kg or 70 µg/kg yessotoxin (76) every 4 days, resulting in significant physiological changes such as bradycardia, hypotension, cardiac structural alterations, and elevated levels of plasma tissue metalloproteinase-1 inhibitor after 15 days [59].Additional studies on its structureactivity relationship are warranted to improve the antifungal efficacy of yessotoxin (76) while mitigating its cardiotoxicity.
Biomolecules 2023, 13, x FOR PEER REVIEW 13 of 38 configuration.The oxygen atoms of the adjacent ether rings are alternately located at the upper and lower ends of the ring.Ladder-like polyethers have low polarity and are lipidsoluble compounds.Yessotoxin (76) (Figure 10), a compound derived from the dinoflagellate Protoceratium reticulatum found in Mutsu Bay, Japan, has been investigated regarding its structure-activity relationship (SAR) [58].Desulfated yessotoxin (77) and hydrogen-desulfated yessotoxin (78) (Figure 10), two derivatives of yessotoxin, have been synthesized for this purpose [58].Desulfated yessotoxin (77) has been found to exhibit reduced hydrophilicity and increased antifungal activity against A. niger [58] (Table 1).Hydrogen-desulfated yessotoxin ( 78) is a product of the hydrogenation of the polyene side chain of desulfated yessotoxin (77), and its antifungal activity is comparable to that of desulfated yessotoxin (77), indicating that the ladder-shaped polyether structure, as opposed to the polyene side chain, is critical for the antifungal activity of yessotoxin.Desulfated yessotoxin (77) has been found to bind to the transmembrane α-helix motif of the membrane integral protein glycophorin A, thereby inducing the dissociation of glycophorin A oligomers into dimers and monomers [58].Despite its antifungal activity, yessotoxin (76) has been observed to induce subacute cardiotoxicity [59].In vitro studies have shown that human ether-a-gogo related gene (hERG) Chinese hamster ovary cells treated with 100 nM yessotoxin for 12 or 24 h exhibit increased hERG potassium channels on the cell surface [59].In vivo experimentation involves the intraperitoneal injection of rats with either 50 µg/kg or 70 µg/kg yessotoxin (76) every 4 days, resulting in significant physiological changes such as bradycardia, hypotension, cardiac structural alterations, and elevated levels of plasma tissue metalloproteinase-1 inhibitor after 15 days [59].Additional studies on its structureactivity relationship are warranted to improve the antifungal efficacy of yessotoxin (76) while mitigating its cardiotoxicity.

Macrolide Polyethers
Macrolide polyethers are end-to-end polyether products in the form of ester bonds.Forazoline A (79) (Figure 11) is obtained from Actinomadura species strain WMMB-499 isolated from the ascidian Ecteinascidia turbinate [60].Forazoline A (79) exhibits favorable water solubility, with a concentration of approximately 5 mg/mL [60].The chemogenomic approach suggests that forazoline A (79) may interfere with the integrity of the cell membrane by disrupting phospholipid homeostasis [60].Forazoline A (79) exhibits growth inhibition of C. albicans K1 with a MIC value of 16 µg/mL [60] (Table 1).In a mouse model of C. albicans infection, forazoline A (79) demonstrates comparable in vivo efficacy to amphotericin B (62) without toxicity [60].The administration of forazoline A (79) at a dose of 0.125 mg/kg reduced the colony-forming unit more than 10 times in the fungal burden of mice kidneys after 8 h, compared to the control group [60].

Macrolide Polyethers
Macrolide polyethers are end-to-end polyether products in the form o Forazoline A (79) (Figure 11) is obtained from Actinomadura species strai isolated from the ascidian Ecteinascidia turbinate [60].Forazoline A (79) exh water solubility, with a concentration of approximately 5 mg/mL [60].The c approach suggests that forazoline A (79) may interfere with the integrity of brane by disrupting phospholipid homeostasis [60].Forazoline A (79) exhib hibition of C. albicans K1 with a MIC value of 16 µg/mL [60] (Table 1).In a of C. albicans infection, forazoline A (79) demonstrates comparable in vivo photericin B (62) without toxicity [60].The administration of forazoline A (7 0.125 mg/kg reduced the colony-forming unit more than 10 times in the fun mice kidneys after 8 h, compared to the control group [60].

Xanthone Polyketides
Xanthone is synthesized through different pathways in plants, fungi, a plants, it is synthesized via the shikimate and acetate pathways, while in chens, it is synthesized through the polyketide pathway.This review focu synthesis of xanthone polyketides through the polyketide pathway.The su as acetyl-CoA and propionyl-CoA, undergo decarboxylation and conden polyketides.Ketoreductase, aromatases, and cyclases then catalyze these form aromatic polyketides.Finally, post-modification processes lead to th xanthone polyketides.
Xanthone is an aromatic oxygenated heterocyclic molecule with a dib scaffold [61].Xanthones can be categorized into three distinct structural g xanthones, O-heterocyclic xanthones, and polycyclic xanthones.Simple characterized by hydroxy, methyl, carboxyl, or methoxy substitutions.O-he thones incorporate O-heterocyclic groups, such as furan and pyran rings, i γ-pirone scaffold [62].Polycyclic xanthones are aromatic polyketide deriv bled from type II PKSs using malonyl-CoA as the substrate, and they ha hexacyclic framework that is highly oxygenated and contains a xanthone su an isoquinolone or isochromane moiety [63].The structural diversity of p thones depends on the variation in the oxidation state of the xanthone m diversity of substituents, including hydroxyl, halogen atoms, and sugar mo variation of the oxidation state of the quinone/hydroquinone in the isoqui

Xanthone Polyketides
Xanthone is synthesized through different pathways in plants, fungi, and lichens.In plants, it is synthesized via the shikimate and acetate pathways, while in fungi and lichens, it is synthesized through the polyketide pathway.This review focuses on the biosynthesis of xanthone polyketides through the polyketide pathway.The substrates, such as acetyl-CoA and propionyl-CoA, undergo decarboxylation and condensation to form polyketides.Ketoreductase, aromatases, and cyclases then catalyze these polyketides to form aromatic polyketides.Finally, post-modification processes lead to the formation of xanthone polyketides.
Xanthone is an aromatic oxygenated heterocyclic molecule with a dibenzo-γ-pirone scaffold [61].Xanthones can be categorized into three distinct structural groups: simple xanthones, O-heterocyclic xanthones, and polycyclic xanthones.Simple xanthones are characterized by hydroxy, methyl, carboxyl, or methoxy substitutions.O-heterocyclic xanthones incorporate O-heterocyclic groups, such as furan and pyran rings, into a dibenzo-γ-pirone scaffold [62].Polycyclic xanthones are aromatic polyketide derivatives assembled from type II PKSs using malonyl-CoA as the substrate, and they have an angular hexacyclic framework that is highly oxygenated and contains a xanthone substructure and an isoquinolone or isochromane moiety [63].The structural diversity of polycyclic xanthones depends on the variation in the oxidation state of the xanthone moiety and the diversity of substituents, including hydroxyl, halogen atoms, and sugar moieties [64].The variation of the oxidation state of the quinone/hydroquinone in the isoquinolone moiety and the diversity of substituents, including alkyl, hydroxyl, and halogen atoms, also contribute to the structural diversity of polycyclic xanthones [64].The methylene dioxybridge or the oxazolidine ring fused with the angular hexacyclic framework is also essential for the structural diversity of xanthones [64].Complex xanthones include dimeric, pseudo-dimeric (one xanthonic and a hydroxanthone nucleus connected by a C-C bond), and glycosylated xanthones [64].
The essentiality of the polyene side chain for the antifungal activity of turbinmicin ( 85) is demonstrated by the significant reduction in antifungal activity upon cleavage of the polyene side chain by hydrolysis [70].Turbinmicin (85) exhibits noteworthy broad-spectrum antifungal activity against C. albicans, C. glabrata, C. tropicalis, C. auris, A. fumigatus, Fusarium, and Scedosporium species, with MIC values ranging from 0.03 to 0.5 µg/mL [70] (Table 1).Turbinmicin (85) significantly diminished the fungal load of the neutropenic mouse model of C. auris injection and the neutropenic and corticosteroid immuno-suppressed mouse model of A. fumigatus injection [70].Turbinmicin (85), a potent antifungal lead compound, has shown significant in vivo and in vitro effectiveness, devoid of any apparent toxic effects, presenting a promising avenue for developing novel antifungal drugs.Parnafungins A and B are isolated from the acetone extract of lichenicolous strains fermentation of Fusarium larvarum (Ascomycota, Hypocreales) [73].These compounds exhibit the ability to interconvert, with the major syn relative configurations (parnafungins A1 (86) and B1 ( 87)) (Figure 13) and minor anti-relative configurations (parnafungins A2 (88) and B2 ( 89)) (Figure 13) being determined based on the relative configuration of C15hydroxyl and C15A-methyl carboxylate [73].The biological activity of the parnafungins A and B mixture is dependent on the presence of the intact isoxazolidinone ring.Despite the broad-spectrum antifungal activity exhibited by the mixture of parnafungins A and B against various Candida species, their efficacy is limited due to the inherent instability of the isoxazolidinone ring, resulting in the loss of antifungal activity upon ring-opening (90 and 91) (Figure 13) [73].Through the utilization of affinity selection/mass spectrometry, it has been discovered that the "straight" structural isomer of parnafungin A1 (86) exhibits a higher affinity for polyadenosine polymerase compared to the "bent" structural isomer of parnafungin B1 (87) [74].The C. albicans fitness test demonstrated that a parnafungin A and B mixture inhibits polyadenosine polymerase, a key component of the fungal mRNA cleavage and polyadenylation complex.In a mouse model of disseminated candidiasis, treatment with parnafungins at a 50 mg/kg dosage reduced renal fungal burden and showed in vivo efficacy without any observable toxicity [75].Parnafungins C (92) and D (93) (Figure 13), analogs of parnafungin A, are isolated from the acetone extract of Fusarium larvarum strain F-155, 597.Parnafungin C ( 92) is produced through the methylation of the C7-phenolic hydroxyl group of parnafungin A, and parnafungin D ( 93 1).Parnafungins A and B are isolated from the acetone extract of lichenicolous strains fermentation of Fusarium larvarum (Ascomycota, Hypocreales) [73].These compounds exhibit the ability to interconvert, with the major syn relative configurations (parnafungins A1 (86) and B1 ( 87)) (Figure 13) and minor anti-relative configurations (parnafungins A2 (88) and B2 ( 89)) (Figure 13) being determined based on the relative configuration of C15hydroxyl and C15A-methyl carboxylate [73].The biological activity of the parnafungins A and B mixture is dependent on the presence of the intact isoxazolidinone ring.Despite the broad-spectrum antifungal activity exhibited by the mixture of parnafungins A and B against various Candida species, their efficacy is limited due to the inherent instability of the isoxazolidinone ring, resulting in the loss of antifungal activity upon ring-opening (90 and 91) (Figure 13) [73].Through the utilization of affinity selection/mass spectrometry, it has been discovered that the "straight" structural isomer of parnafungin A1 (86) exhibits a higher affinity for polyadenosine polymerase compared to the "bent" structural isomer of parnafungin B1 (87) [74].The C. albicans fitness test demonstrated that a parnafungin A and B mixture inhibits polyadenosine polymerase, a key component of the fungal mRNA cleavage and polyadenylation complex.In a mouse model of disseminated candidiasis, treatment with parnafungins at a 50 mg/kg dosage reduced renal fungal burden and showed in vivo efficacy without any observable toxicity [75].Parnafungins C (92) and D (93) (Figure 13), analogs of parnafungin A, are isolated from the acetone extract of Fusarium larvarum strain F-155, 597.Parnafungin C ( 92) is produced through the methylation of the C7-phenolic hydroxyl group of parnafungin A, and parnafungin D ( 93 1).

Linear Polyketides
The polyketide chain skeleton was catalyzed by type I PKSs and then subjected complex post-modifications, including glycosylation, to form linear polyketide [77].The are differences in the sphingolipid synthesis pathways between fungi and mammal Fungi utilize a process whereby phosphoinositide is transferred to the 1-OH group ceramides to generate inositol phosphoceramide rather than directly producing sphi gesters [78].Khafrefungin (94) (Figure 14), isolated from endophytic fungi found in Costa Rican plant, has been found to inhibit the inositol phosphoceramide synthase of cerevisiae and pathogenic fungi.This inhibition leads to the blockage of the phosphoinos tide-to-ceramide pathway, thereby inhibiting fungal sphingolipid synthesis while leavin t mammalian sphingolipid synthesis unaffected [78].Khafrefungin (94) shows antifung activity against C. albicans, C. neoformans, and S. cerevisiae, with MIC values of 2, 2, an 15.6 µg/mL, respectively [78].Additionally, khafrefungin (94) has been shown to posse fungicidal activity against C. albicans, C. neoformans, and S. cerevisiae with minimum fu gicidal concentrations of 4, 4, and 15.6 µg/mL, respectively [78].Notably, the removal the aldonic acid group (95) (Figure 14) greatly attenuates its antifungal activity against cerevisiae (MIC > 200 µM), indicating the importance of the aldonic acid group for the a tifungal activity of khafrefungin ( 94) [79].The presence of the enantiomeric form of th aldonic acid group (96) (Figure 14) has been found to diminish the antifungal activity khafrefungin (94), indicating that the aldonic acid group not only enhances the water so ubility of khafrefungin (94) but also plays a role in its antifungal activity [79].Furthermor the enantiomer of the 4-methyl group (97) (Figure 14) has been shown to completely abo ish the activity of khafrefungin (94), highlighting the essentiality of the configuration the 4-methyl group for the antifungal activity of khafrefungin ( 94) [79].Additionall treating khafrefungin (94) under acidic conditions forms a six-membered lactone deriv tive (98) (Figure 14) that exhibits comparable antifungal activity against S. cerevisiae (MI = ~10 µM) to that of native khafrefungin (94) [80] (Table 1).

Linear Polyketides
The polyketide chain skeleton was catalyzed by type I PKSs and then subjected to complex post-modifications, including glycosylation, to form linear polyketide [77].There are differences in the sphingolipid synthesis pathways between fungi and mammals.Fungi utilize a process whereby phosphoinositide is transferred to the 1-OH group of ceramides to generate inositol phosphoceramide rather than directly producing sphingesters [78].Khafrefungin (94) (Figure 14), isolated from endophytic fungi found in a Costa Rican plant, has been found to inhibit the inositol phosphoceramide synthase of S. cerevisiae and pathogenic fungi.This inhibition leads to the blockage of the phosphoinositide-to-ceramide pathway, thereby inhibiting fungal sphingolipid synthesis while leaving t mammalian sphingolipid synthesis unaffected [78].Khafrefungin (94) shows antifungal activity against C. albicans, C. neoformans, and S. cerevisiae, with MIC values of 2, 2, and 15.6 µg/mL, respectively [78].Additionally, khafrefungin (94) has been shown to possess fungicidal activity against C. albicans, C. neoformans, and S. cerevisiae with minimum fungicidal concentrations of 4, 4, and 15.6 µg/mL, respectively [78].Notably, the removal of the aldonic acid group (95) (Figure 14) greatly attenuates its antifungal activity against S. cerevisiae (MIC > 200 µM), indicating the importance of the aldonic acid group for the antifungal activity of khafrefungin (94) [79].The presence of the enantiomeric form of the aldonic acid group (96) (Figure 14) has been found to diminish the antifungal activity of khafrefungin (94), indicating that the aldonic acid group not only enhances the water solubility of khafrefungin (94) but also plays a role in its antifungal activity [79].Furthermore, the enantiomer of the 4-methyl group (97) (Figure 14) has been shown to completely abolish the activity of khafrefungin (94), highlighting the essentiality of the configuration of the 4-methyl group for the antifungal activity of khafrefungin (94) [79].Additionally, treating khafrefungin (94) under acidic conditions forms a six-membered lactone derivative (98) (Figure 14) that exhibits comparable antifungal activity against S. cerevisiae (MIC = ~10 µM) to that of native khafrefungin (94) [80] (Table 1).

Hybrid Polyketide Nonribosomal Peptides
Echinocandins are novel lipopeptide antifungal products synthesized by a heterozy gous pathway of non-ribosomal peptides synthases (NRPSs)-PKSs.Compared with azole and polyene antibiotics, echinocandins have a completely different mechanism of action and exert their antifungal effect by destroying the cell wall.Non-competitive binding o   94) and its analogues.The red boxes mark the difference between khafrefungin (94) and 97 of the 4-methyl group.

Hybrid Polyketide Nonribosomal Peptides
Echinocandins are novel lipopeptide antifungal products synthesized by a heterozygous pathway of non-ribosomal peptides synthases (NRPSs)-PKSs.Compared with azole and polyene antibiotics, echinocandins have a completely different mechanism of action and exert their antifungal effect by destroying the cell wall.Non-competitive binding of echinocandins to the catalytic subunits of β-(1,3)-D-glucan synthetase encoded by the FKS1 and FKS2 genes results in the inhibition of biosynthesis of β-(1,3)-D-glucan, an important component of the fungal cell wall, which destroys the integrity of fungal cell wall and disrupts the osmotic balance, ultimately leading to fungal death [82,83].Echinocandins show great in vitro antifungal activity against various invasive fungal pathogens, including Candida and Aspergillus species, but they are ineffective against C. neoformans [84] (Table 1).Currently, four echinocandin antifungal drugs are on the market, including caspofungin, micafungin, anidulafungin, and rezafungin.Echinocandin B, the lead compound of Anidulafungin and rezafungin, and FR901379, the lead compound of micafungin, are assembled through the NRPs and fatty acid synthases heterozygous pathway [85,86].Only pneumocandin B 0 (102) (Figure 16), the lead compound of caspofungin (103) (Figure 16), is assembled by the NRPSs and PKSs heterozygous pathway [85].Only caspofungin and its lead compound pneumocandin B 0 (102) are discussed in this section.Pneumocandin B 0 (102) is isolated from the filamentous fungus Glarea lozoyensis [87,88].Pneumocandin B 0 (102) is a lipopeptide composed of myristic acid and a hexapeptide ring.PKSs catalyze the assembly of 10, 12-dimethylmyristic acid, and then, catalyzed by a series of enzymes, the polyketide intermediate localizes to NRPSs to acylate the 4, 5-dihydroxyornithine of pneumocandin B 0 (102), initiating cyclic hexapeptide elongation.

Conclusions
In conclusion, this review provides a comprehensive overview of natural antifungal polyketides encompassing various subclasses such as polyethers, macrolides, xanthones, linear polyketides, anthraquinone, polyphenols, pyridine derivatives, furan derivatives, pyranan derivatives, monophenyl derivatives, macrolactam polyketides, hybrid polyketide non-ribosomal peptides, and other polyketides.Additionally, this review discusses the origin, in vitro and in vivo antifungal activities, structure-activity relationship (SAR), safety profile, mechanism of action, and the impact of structural modifications on the SAR of these polyketides.Previous studies on polyketides have demonstrated the substantial antifungal properties exhibited by certain natural polyketides, such as amphotericin B and caspofungin.This observation suggests that polyketide lead compounds hold considerable potential for the future treatment of fungal infections.Given the remarkable antifungal activities displayed by natural polyketides, this class of compounds has garnered significant interest as a potential therapeutic avenue for fungal infections in the future.Currently, the predominant research emphasis lies in synthesizing novel polyketides, while the clinical utilization of pre-existing polyketide compounds as antifungal medications remains limited.Consequently, further comprehensive and meticulous clinical investigations are imperative to substantiate their efficacy in the future.
Moreover, the antifungal properties of unnatural polyketide compounds can potentially be harnessed through combinatorial biosynthesis.Numerous PKSs facilitate the production of primary polyketide compounds, which lack biological activity until they undergo modification by PKS post-modifying enzymes, thereby presenting a promising avenue for exploring new antifungal drugs.Polyketides' chemical composition and fungicidal properties can be altered by utilizing various post-modification enzymes, including cyclase, aromatase, glycosylase, and halogenase.Unconventional antifungal polyketides have been synthesized by modifying the modules, domains, and subunits of PKSs and employing sitedirected mutagenesis techniques.Furthermore, the enhancement of antifungal polyketide production or the acquisition of novel antifungal polyketides can be achieved by combining initiation substrates and elongation units from different hosts and implementing targeted modifications.Only a small number of antifungal natural polyketide compounds have been identified.Many habitats of microorganisms, plants, animals and marine organisms have not been explored, and many antifungal polyketide products urgently need to be discovered.Natural polyketides from microorganisms such as fungi are scarce and difficult to obtain.The solubility, safety, and in vivo bioavailability of natural polyketides should also be considered.Semisynthetic components of natural products will play an important role in developing antifungal candidates in the future.

Figure 4 .
Figure 4.Chemical structures of analogues of Oligomycin A.

Figure 7 .
Figure 7.Chemical structures of Guanidylfungin A (54) and its analogues.The red dotted box marks the ring-opening structure of compound 60, which differs from the tetrahydropyran ring guanidylfungin A (54).

Figure 14 .
Figure 14.Chemical structures of khafrefungin (94) and its analogues.The red boxes mark the difference between khafrefungin (94) and 97 of the 4-methyl group.

Figure 14 .
Figure 14.Chemical structures of khafrefungin (94) and its analogues.The red boxes mark the difference between khafrefungin (94) and 97 of the 4-methyl group.

Table 1 .
The general characteristic of poliketides having antifungal activity.