Emerging and future strategies in the management of recalcitrant Candida auris

Abstract Candida auris is an emerging, multi drug resistant fungal pathogen that has caused infectious outbreaks in over 45 countries since its first isolation over a decade ago, leading to in-hospital crude mortality rates as high as 72%. The fungus is also acclimated to disinfection procedures and persists for weeks in nosocomial ecosystems. Alarmingly, the outbreaks of C. auris infections in Coronavirus Disease-2019 (COVID-19) patients have also been reported. The pathogenicity, drug resistance and global spread of C. auris have led to an urgent exploration of novel, candidate antifungal agents for C. auris therapeutics. This narrative review codifies the emerging data on the following new/emerging antifungal compounds and strategies: antimicrobial peptides, combinational therapy, immunotherapy, metals and nano particles, natural compounds, and repurposed drugs. Encouragingly, a vast majority of these exhibit excellent anti- C. auris properties, with promising drugs now in the pipeline in various stages of development. Nevertheless, further research on the modes of action, toxicity, and the dosage of the new formulations are warranted. Studies are needed with representation from all five C. auris clades, so as to produce data of grater relevance, and broader significance and validity. Lay Summary Elimination of Candida auris that causes deadly infections to susceptible individuals is extremely challenging due to the lack of effective treatment options. Promising, new antifungal agents and strategies are being developed and further refinement will facilitate their clinical use in the near future.


Introduction
With the arrival of Coronavirus Disease -2019 (COVID-19) pandemic, and the associated superinfections, such as mucormycosis, there is a renewed interest in another rapidly escalating, silent pandemic of antimicrobial resistance (AMR). The latter phenomenon has been declared by the World Health Organisation, 1 as one of the top ten global public health threats facing humanity today. It has been estimated that there will be over 10 million deaths associated with AMR infections, and its cu-mulative global economic burden will increase up to $100.2 trillion by 2050, if immediate action is not taken to counter the crisis ( https://amr-review.org/). 2 The AMR research and surveillance to date has been largely focused on antibacterial, antiviral, and antimalarial resistance. Nevertheless, it is well known that fungal infections too are a significant contributor to AMR. 1 The recent surges in systemic fungal infections, particularly in immunocompromised populations, and those due to pan-resistant Candida auris as well as the current 'black fungus' disease associated with COVID-19 due to Mucorales species, are examples of fungal diseases that are resurging due to the sparsity of effective antifungals. 3 Since its first isolation in 2009, from an ear discharge of a female patient in Japan, 4 C. auris infections have been reported in 45 countries in far-Eastern Asia, the Middle East, Africa, Europe, North America and South America 5 highlighting its alarming global spread. Due to its rapid emergence within the past decade or so, the Centers for Disease Control and Prevention (CDC) European Centre for Disease Prevention and Control (ECDC), and Public Health England have released a clinical alert to healthcare facilities in 2016 to warn of the global threat of pan-resistant C. auris . 6 -8 Outbreaks of C. auris associated with COVID-19 continue to be in the limelight, as we write. 9 -12 The origins of C. auris have been intriguing. In-depth analyses of 18S internal transcribed spacer (ITS) and rDNA sequences of the 28S D1/D2 regions and sequences of 50 different proteins have revealed that C. auris is a member of the Metschnikowiaceae family within the Candida/Clavispora clade. 4 , 13 , 14 Based on their genetic and genomic information and first isolated locations, C. auris has been clustered into four major discrete genetic clades (Clades I-IV): the South Asian Clade, the East Asian Clade, the South African Clade, and the South American Clade. 15 A fifth Clade originated from Iran, separated from the other clades by over 200 000 SNPs, has been recently reported. 16 Pitfalls in disparate identification techniques, reporting, and surveillance of C. auris , may have underestimated its prevalence and genomic diversity, hence, caution must be exercised when interpreting the current epidemiological data of C. auris infections.
The commonest human habitat of C . auris is the skin, however, several studies have reported their isolation from the gut, and the oral, and esophageal mucosae of infected individuals. 15 Similar to other major Candida pathogens, C. auris primarily infects the usual spectrum of compromised individuals including those with uncontrolled diabetes mellitus, chronic renal diseases, neutropenia, and those on immunosuppressive therapy, broadspectrum antimicrobials, and those with indwelling medical devices, or at extremes of age. 17 Although, first isolated from an ear infection, C. auris is now known to cause an array of human diseases ranging from fungemias, surgical/nonsurgical wound infections, urinary tract infections, meningitis, myocarditis, skin abscesses, to bone infections. 18 -21 Biologically, the behavior of C. auris is similar to most other Candida pathogens such as C . albicans, C . tropicalis, C. parapsilosis of the, so called, CTG clade [Species that translate the CTG codon into serine instead of leucine 22 ] possessing shared virulence attributes such as biofilm formation, yeast to hyphae transition, and phenotypic switching. 23 , 24 Hence, these characteristics are likely to modulate C. auris virulence including antifungal tolerance, and survival in diverse habitats in the host, as well as on in inert, abiotic, environmental surfaces.

C. auris antifungal resistance
The major reason why C. auris has been in the spotlight, is their notorious and dogged antifungal resistance, relative to other Candida species, and consequent persistence causing chronic infections with poor prognosis, particularly in compromised hosts. Indeed, the consensus is that C. auris associated candidemias and septicemias could lead to in-hospital crude mortality rates as high as 72%. 25 A study by Lockhart et al. (2017) clearly illustrates the alarming breadth and depth of this problem. 15 They noted that almost one half of all C. auris isolates from various studies were resistant to the most widely used, azole antifungal, fluconazole (44.29% resistance), followed by the polyene, amphotericin B (15.46%), and other antifungal drugs such as voriconazole (12.67%), caspofungin (3.48%) and flucytosine (1.95%). 26 Thankfully, it appears that the yeast is still reasonably sensitive to echinocandin, and the DNA-analog flucytosine with only a small minority of 5% isolates being, disconcertingly pan-resistant, thus far.
C. auris employs several different molecular mechanisms to evade the action of antifungals. In brief, the azole resistance is associated with the overexpression of drug efflux pumps belonging to ATP Binding Cassette (ABC) and Major Facilitator Superfamily (MFS) transporters, and alterations of the ergosterol synthesis pathway (overexpression of ERG11, and point mutations in ERG11 , i.e., Y132F or K143R). The echinocandin resistance in C. auris is shown to be attributable to mutations of FKS1 , a gene that codes the enzyme responsible for the key fungal cell wall component, β(1,3)D-glucan. Single nucleotide polymorphisms in genes related to the ergosterol synthesis pathway leading to altered sterol composition and potential amino acids substitution in the FUR1 gene (i.e., F211I) have been linked to C. auris resistance to polyenes (e.g., amphotericin B) and nucleoside analogs (e.g., flucytosine) respectively. 27 -36 Emerging evidence suggests novel mechanisms such as mutations in the zinc-cluster transcription factor-encoding gene TAC1B may also have a role in C. auris antifungal resistance. 37 Further indepth discussion of the resistance mechanisms of C. auris is beyond the remit of this review and could be found elsewhere. 38 , 39 The principal mechanisms of resistance are summarized in Table 1 .
The alarming severity of the infection, and the associated global morbidity and mortality rates, coupled with the emergence of the resistance to multiple antifungal classes, have all led to an urgent exploration of novel, and promising candidate antifungal agents for C. auris therapeutics. The main objective of this narrative review, therefore, is to codify the emerging data on the new and emerging antifungal classes for C. auris. For ease of review, we discuss these in alphabetical order in the following sections. The main findings are also summarized in Table 2 . Increased copy number of ERG11 Overexpression of CDR genes members of the ATP-binding cassette (ABC) family and MDR1 member of the major facilitator superfamily (MFS) transporters Increased copy numbers of transcription factor that regulate expression of ABC and MFS family transporters 14 , 29 , 244 Echinocandins Inhibits 1,3-beta-glucan synthase leading to defective cell wall formation Mutations in the FKS1 gene encoding 1,3-beta-glucan synthase FKS1 gene substitutions S639F, S639P, S639Y, S652Y 29 , 35 , 36 Polyene Binds to the membrane ergosterol and forms multimeric pores in the plasma membrane Single nucleotide polymorphisms (SNPs) in different genomic loci 32 Nucleoside analogs Competitive inhibition of purine and pyrimidine uptake Incorporation into fungal RNA to inhibit DNA and RNA synthesis Amino acid substitution F211I in the FUR1 gene 36

Antimicrobial combination therapy
As opposed to prokaryotic bacteria, human beings and yeasts possess structurally similar nuclear material and cell membranes and are classified as eukaryotes. Hence antifungal drugs, in general, adversely affect the host cells, due to the molecular mimicry of the drug targets of the fungi and the host cells. 39 There is, therefore, a dearth of antifungals in comparison to a plethora of antibacterial drugs currently available to manage infectious diseases due to prokaryotes. It is now well known that C. auris displays a remarkable resistance to most of the current antifungal drug classes. Although there are some variations among different clades, C. auris is generally resistant to azole agents, but remain susceptible to echinocandins. Hence, recommendations for safe antifungal therapy dictates, reverting back to azoles from echinocandins whenever possible, so as to prevent the development of pan resistant organisms. 40 This is due to the higher margin of safety, and the lower adverse effects of the azole class drugs such as fluconazole.
One such well-established strategy to synergise the activity of antimicrobial drugs is combination therapy, a technique effective against difficult to treat infections such as tuberculosis. 41 A similar approach has been evaluated in order to improve the efficacy of commonly used, first line, antifungal agents against C. auris . In the following section, we review these approaches, their advantages, and disadvantages as well as their potential utility against C. auris infections .
In contrast, synergy of anidulafungin with isavuconazole or voriconazole, flucytosine with voriconazole or posaconazole has been witnessed in recent research, implying the isolate specific diversity of antifungal synergism in C. auris . 45 , 46 Antifungal and antibiotic combinations Eldesouky et al. (2018) reported increased sensitivity of azoles when paired with sulfonamides. Particularly, 37% of voriconazole resistant C. auris (3/8 isolates) and 75% of itraconazole resistant isolates (3/4 isolates) become sensitive to the corresponding azole when delivered with the sulfamethoxazole, in vitro , although co-delivery of sulfamethoxazole with fluconazole reversed the resistance of only a single fluconazole resistant isolate (1/8 isolates). The 70% survival of Caenorhabditis elegans infected with voriconazole resistant C. auris further confirmed the favorable in vivo activity of sulfamethoxazole-voriconazole combination. 47 Although the exact mechanism of the synergism of the antibacterial-antifungal combination is yet to be determined, the data suggest that such inhibitory activity is likely to be due to the interruption of C. auris folate pathway. The sulfamethoxazole-voriconazole combination, for instance, was only effective against azole resistance associated with, either overproduction of or decreased affinity for azole target (Erg11p), but not with the overexpression of multidrug efflux pumps, 47 , 48 Further, the addition of trimethoprim, (usually co-administered with sulfamethoxazole) to the sulfamethoxazole and fluconazole combinations showed synergies (27%, 3/11 isolates), 49 whilst other antifungal and antibiotic combinations such as isavuconazole and colistin have also shown promising synergy against C. auris . 50

Antifungal and non-antimicrobial drug combinations
Arguably, the most notable approach that has been experimented to improve the efficacy of azole agents against C. auris is to combine them with non-antimicrobial drugs or compounds. For example, Eldesouky et al . (2020) reported potential synergism of pitavastatin, an antihyperlipidemic statin, with azole antifungals, against C. auris . The former combined with fluconazole, voriconazole or itraconazole was shown to lower the MIC by 4-16 folds in five different C. auris isolates (fractional inhibitory concentration indices: FICI < 0.5). However, the MIC reduction was insufficient to restore the azole sensitivity of the most C. auris isolates. Additionally, when tested in vivo on a C. elegans model, the pitavastatin-fluconazole drug combination lowered fluconazole resistant C. auris burden up to 92%. Yet, in vitro , the combination was only effective in suppressing biofilm development of a single C. auris isolate by only 41% and failed to eliminate established biofilms. 51 In a series of further follow-up studies, Eldesouky et al. (2020) reported that the combined therapy of azole agents, itraconazole in particular, with various other drugs potentiate the activity of the antifungal against C. auris . Itraconazole combined with ospemifene, a selective oestrogen receptor modulator, usually indicated for dyspareunia, or with aprepitant, an antiemetic agent, or with lopinavir, an HIV protease inhibitor, was demonstrated to lower MIC of itraconazole sensitive and resistant C. auris isolates by 4 to 8-folds (n = 5, FICI 0.14-0.27), 8-folds (n = 10, FICI 0.14-0.31) and 32 to 256-fold, (n = 10, FICI 0.04-0.09) respectively. All three combinations were highly effective in lowering itraconazole resistant C. auris fungal load in C. elegans model by 96%, ∼92%, ∼88.5% respectively and extended the nematode survival significantly. 52 -54 Investigators claim that the increased activity of itraconazole is likely to be associated with the increased affinity of ospemifene to multidrug efflux pumps 53 whereas, aprepitant/itraconazole and lopinavir/itraconazole exposure appeared to impact on C. auris membrane transport processes, ions homeostasis and subsequent reactive oxygen species (ROS) detoxifying mechanisms and ergosterol biosynthesis, as well as fungal glucose transport. 52 , 54 Latest research on combinatorial therapies have revealed the ability of azoffluxin, a novel oxindole efflux pump inhibitor, to produce a synergistic antifungal effect by reducing C. auris infection in mice by lowering the fungal burden by ∼1000 -folds. 55 This is thought to be due to enhanced intracellular fluconazole accumulation through inhibition of CDR1 , a major multidrug efflux transporter in Candida .
There are several other reports showing the potential of combinatorial antifungal/non-antimicrobial drugs/compound therapy. For instance, a recent report from Cheng et al. (2021) has revealed a synergistic anti-C. auris activity of flucytosine and myriocin, a serine palmitoyltransferase inhibitor that impede sphingolipid biosynthesis in eukaryotic cells. 45 However, myriocin combination with flucytosine, another antifungal with known toxic effects at higher doses, will need to be closely watched prior to their clinical use. This is because, a derivative of myriocin, fingolimod, approved for management for multiple sclerosis, has a significant toxicity against mammalian cells.

Antifungal and antiseptic combinations
The data on antifungal and antiseptic combinatorial therapy in managing C. auris is scarce. A topical antifungal miconazole in combination with a well-known antiseptic quaternary ammonium compound domiphen bromide elicited significant inhibi-tion of C. auris biofilms. 57 Co-delivery of 150 μM of miconazole with 37.5 μM domiphen bromide decreased C. auris biofilm viability by ∼3 Log 10 colony forming units (CFUs). The authors suggested that the surfactant properties of domiphen bromide is likely to increases the efficacy of azoles by increasing the permeability of the yeast vacuolar membrane, thereby releasing sequestered azoles. The latter combination holds much promise as a topical antifungal after relevant optimisation.
Antifungal and miscellaneous drug combinations Wu et al. (2020) demonstrated that the antiparasitic drug miltefosine, an alkyl-phospholipid analog for leishmaniasis, is fungicidal against 12 isolates of fluconazole or voriconazole resistant C. auris at clinically safe concentrations of 2 μg/ml. 58 This is in contrast to its high dosage of 17.2 to 42.4 μg/ml required to manage leishmaniasis. 59 , 60 Other researchers have also noted comparable MICs of miltefosine against planktonic (1-4 μg/ml) and biofilm (0.25-4 μg/ml), modes of C. auris, although higher concentrations were required to inhibit preformed biofilms (16-32 μg/ml) and reducing the fungal burden in Galleria mellonella . 61 Nevertheless, miltefosine showed only marginal synergy with amphotericin B (FICI = 0.5) against 3/12 C. auris isolates in vitro . 58 In a similar study, Shaban et al . (2020) evaluated a monoterpenoid phenolic compound carvacrol, against C. auris and demonstrated its antifungal attributes, as well as suppression of yeast adherence, and proteinase synthesis. Also, a combination of carvacrol and fluconazole (16% synergy), and amphotericin B and nystatin (28% synergy each) elicited a reasonable degree of synergy. 62 There are several other reports where combinatorial antifungal drug therapies against C. auris have been attempted but with rather modest outcomes. These include attempts to explore synergy between antifungal drugs and antitumor agents (e.g., geldanamycin), and nonsteroidal anti-inflammatory drugs (e.g., ibuprofen, diclofenac sodium, aspirin). 63 -65 To conclude, the interesting outcomes of the elegant series of studies of Eldesouky et al., and various others, show the promise of combinatorial azole/antibiotic/drug therapy in overcoming C. auris infections, particularly in the absence of novel, efficacious antifungal agents in the horizon. The current body of data is largely preliminary, focused on a relatively few isolates and based mainly on in vitro assessments. Further in vivo studies, incorporating a broader geographical coverage of C. auris isolates/clades, and the molecular basis underpinning their activity, are essential to take this field forwards.

Antimicrobial peptides
Antimicrobial peptides (AMPs) are naturally existing, positively charged, amphipathic molecules secreted by all eukaryotes and are an integral component of the innate immune system of vertebrates. 66 , 67 They have been shown to possess broad, rather nonspecific antimicrobial properties against various pathogens, and they primarily kill microbes by disrupting the microbial cellular membranes. AMPs can also target some important cellular processes such as cell wall synthesis, DNA and protein synthesis, protein folding, and multiple enzymatic activities. 68 , 69 AMPs vary among different species by virtue of their amino acid sequence, size, and structure, making them an attractive option in for managing microbial infections. 67 , 70 , 71 Up to now, AMP therapy has been largely experimented against bacterial, and not fungal, pathogens although data on fungal pathogens are slowly emerging, as discussed below.

PepBiotics
Recently, van Eijk et al. (2020) demonstrated strong anti-C. auris activity of 'PepBiotics', a group of custom synthesized cathelicidin-inspired AMPs. In particular, two of these peptides, CR172 (amino acid sequence: RRWVQRWIRRWRPKVAAARRWVQRWIRRWRPKV-NH2) and CR184 (amino acid sequence: APKAMRRWVQRWI RRWRPKVFQVTGSSA-NH2) significantly suppressed C. auris metabolic activity, at a concentration < 1 μM, suggesting their potential as an anti-C. auris agent. 72 Lipopeptide -AF 4 Some bacteria are known to produce native peptide molecules with a broad antimicrobial spectrum. 73 , 74 Of these, lipopeptides such as bacillomycin, mojavensin, mycosubtilin, iturin A, bacillopeptin, and fengycin homologues secreted by Bacillus species, are notable as they generate ion-conducting pores in the lipid component of the fungal cell membranes leading to cell death. Despite this attractive antimicrobial property, native lipopeptides are deemed undesirable for clinical use due to their strong hemolytic activities, and the high MICs required for microbial kill. 74 However, Ramachandran et al. (2018) purified three lipopeptides (AF 3 , AF 4 , and AF 5 ) from Bacillus subtilis RLID 12.1 with promising antifungal properties against Candida species, including C. auris, with relatively low MICs and as well as hemolytic activity. 73 Structurally, they are homologs of bacillomycin D with a shared amino acid sequence of Asn-Pro-Tyr-Asn-Gln-Thr-Ser-Xaa, where Xaa is a β-amino fatty acid variant. 73 The lipopeptide molecule-AF 4 , in particular, is noteworthy as it demonstrated a geometric MIC of 3.48 μg/ml against clinical C. auris isolates with negligible hemolysis. These three molecules also showed promising inhibitory effects against C. auris biofilms.

Cm-p5
AMPs derived from primitive life forms such as sea mollusks are now known to have antifungal activity very low toxicity. One such, Cm-p5, derived from a peptide isolated from the mollusk Cenchritis muricatus efficiently kills C . auris, C . albicans, C. parapsilosis, Cryptococcus neoformans, and Trichophyton rubrum and demonstrated only negligible toxicity against mammalian cells. 75 , 76 In order to demonstrate the antifungal effect of Cm-p5, Kubiczek et al. (2020) developed an elegant two-layered hydrogel containing a base layer loaded with mannose-specific lectin B (LecB) that immobilizes C. auris cells, and a second, sandwich layer loaded with Cm-p5. The released of free Cm-p5 in this model, was anti-candidal at an MIC of 11 μg/ml, while the loaded hydrogel was effective in eradicating C. auris at sub-MIC concentrations of Cm-p5. 77 These findings suggest the hydrogel functionalized with AMPs such as Cm-p5 would likely to be an attractive solution for decontamination of chronic wound infections due to C. auris.

Crotamine
New research has also shown the surprising potential of antimicrobial potency of various components of snake venom. For instance, Crotamine, a native polypeptide isolated from the venom of the South American rattlesnake Crotalus durissus terrificus has been studied for its antifungal properties. 78 80 However, the weak antifungal effect of crotamine was evident by its high MIC values compared with micafungin (MIC of 0.06-0.12 μg/ml). The exact antimicrobial mechanism of Crotamine is not known, as of yet. 81 , 82 Interestingly, other research has shown structural and functional similarities of crotamine to human AMP, β defensins. 83 Particularly, crotamine and β defensins share similar avidity to lipids such as phosphatidylserine of the fungal plasma membrane, involved in cellular transport and if interfered with may lead to apoptotic cell death. 84 -86 However, further data on compatibility of crotamine with mammalian cells, their stability, and PK/PD values and adverse effects in immunocompromised individuals are essential prior to its clinical use as an anti-C. auris agent.

θ defensins
The discovery of mammalian AMPs such as cathelicidins, salivary histatins and α-, βand θ -defensins, have prompted investigators to explore their use as therapeutic agents against infections including those caused by fungi. However, the lack of data on their in vivo potency, safety, and stability, to date has hindered such approval. 87 -91 Interestingly, θ -defensins, an 18-amino-acid macrocyclic peptide only expressed in Old World monkeys, appear to possess remarkable antifungal activities against C. albicans and Cryptococcus neoformans . 92 -95 Basso et al. (2018) reported that θdefensins isoforms from rhesus macaque (RTD-1 and RTD-2) and olive baboon (BTD-2 and BTD-8) displayed significant fungicidal properties against fluconazole and caspofungin resistant C. auris (MICs 3.125-6.25 μg/ml). The investigators claimed that the θ -defensins were more stable than the salivary histatin-5 (Hst 5), and up to 1 000 times more effective in antifungal assays.
θ -defensins isoforms appear to induce oxidative stress and death in C. auris , and yet, they are nontoxic to mice, and nonhemolytic in whole blood suggesting the possibility of their use as an anti-C. auris agent. 96 Hence, antifungal properties of naturally occurring θ -defensins, with further optimizations, may prove valuable in managing multidrug resistant fungal pathogens such as C. auris , in future.

Histatin 5
Although less stable than θ -defensins, histatin-5 (Hst 5) is a wellrecognized salivary cationic peptide with anti-candidal properties. Its effect against C. auris infections has been investigated by Pathirana et al. (2018) who observed up to 90% killing of 7 out of 10 C. auris isolates tested, on exposure to 7.5 μM of Hst 5 for a period of 1 h. Hst 5 appeared to be rapidly translocated into the yeast cells which led to their rapid kill, as seen with C. albicans . 97 Nevertheless, due to the higher threshold of oxidative stress tolerance of C. auris, it is unlikely to respond as well as C. albicans in vivo. Similarly, the systemic stability of Hst 5 may be diminished by blood serum components and salts, and therefore it may be less likely to be effective in managing systemic C. auris infections such as in sepsis. 97 However, considering its natural presence in saliva, the antifungal properties of Hst 5 may be valuable for topical management of mucosal C. auris infections, perhaps particularly as an antiseptic against oral candidiasis.

Ceragenins
Despite their potent antifungal properties, a major impediment for the clinical use of AMPs has been the relatively high cost of their large-scale synthesis/preparation. To overcome this limitation, some researchers have investigated the possibility of developing low cost, non-peptide molecules to mimic the amphiphilic morphology of common AMPs. 98 , 99 Molecules of this class, ceragenins, have been shown to mimic 'secondary' activities of AMPs during wound healing. 100 Ceragenins possess broad-spectrum antimicrobial properties and have been tested against C. auris . 101 , 102 The MIC of ceragenins-CSA-131 ranged from 0.5 to 1 μg/ml against 107 different isolates of C. auris regardless of their clade and or the antifungal susceptibility. Two tested ceragenins (CSA-44 and CSA-131) demonstrated potent antibiofilm activity against all C. auris isolates studied (n = 5; MBIC50 = 2-4 μg/ml, MBIC80 = 4-64 μg/ml) and a significant reduction of C. auris CFUs (by ∼3fold log 10 ) in porcine vaginal mucosal explants infected with C. auris . 103 As the favorable tolerability of ceragenins have been established, 104 further investigations into its molecular mechanisms of actions and in vivo pharmacokinetics and pharmacodynamics will likely assist their application as a future, cost-effective anti-C. auris agent, particularly against variants that are nonresponsive to empirical antifungal therapy.
Although the repertoire of the anti-C. auris properties of AMPs is yet to be fully explored, the foregoing clearly indicates their potential therapeutic utility. Although antimicrobial potential of AMPs has been explored for decades, their further development for clinical use appears to be notably slow as shown by the absence of any such, clinical trials at present. Therefore, further in-depth studies are warranted to explore, refine, and establish these AMPs as effective therapeutic agents against C. auris .

Immunotherapy
Antibodies and various other immune molecules have been used over the last few decades to manage infectious diseases. Hence, some workers have attempted to use this approach for treating C. auris infections using antibodies and interleukins, with some degree of success.

Anti-CR3-RP polyclonal antibody
It is well known that Candida species express a multitude of surface proteins to survive in hostile habitats and to enrich their biofilm lifestyle. Complement receptor 3-related protein (CR3-RP) is one such surface protein expressed by many Candida species including C. auris . Manipulation of the expression of these surface antigens have been shown to be effective in controlling C. albicans and C. dubliniensis biofilms. 105 A study by Dekkerová et al. (2019) suggested that blockage CR3-RP expression through an anti-CR3-RP polyclonal antibody could disrupt C. auris biofilms development (36-73% inhibition) as well as established biofilms (28-46% inhibition) to a level comparable to those of fluconazole, amphotericin B, and caspofungin. 106 These preliminary data implies that antibody therapies against C. auris infections may be feasible in the future.

Antibodies against agglutinin-like sequence-3 (Als3p)
Using an approach similar to the above, Singh et al. (2019) demonstrated, in a mouse model, that antibodies against agglutinin-like sequence-3 (Als3 protein) surface adhesins of Candida species, i.e., anti-Als3p antibodies, could impede C. auris biofilm formation, and enhance the macrophage-mediated killing of the yeast. 107 The anti-Als3p antibody formation in mice was triggered using a vaccine based on the N-terminus of Als3 protein. Importantly, the investigators observed that the vaccine was capable of inducing anti-Als3p antibodies (humoral immune response) as well as CD4 + T helper cells mediated activation of tissue macrophages (cellular immune response) that were highly effective against C. auris disseminated infections. In addition, coadministering the same vaccine in tandem with antifungal micafungin synergized the elimination of C. auris induced murine candidemia. 107 Clearly, vaccines conferring lifelong immunity are the ultimate goal for protection against multidrug resistant yeasts including C. auris , and the foregoing brings this vision a step closer to realization.

Interleukins
The armory of complex immune molecules that protect the body from invading pathogens, includes an array of various cytokines. Therefore, some have evaluated the antifungal properties of such agents against C. auris . In a preliminary study, Schneider et al. (2018) reported that human uterine cervical stem cells conditioned medium rich in several cytokines (IL-6, IL-8, IL-17, IP-10, CXCL-16, CCL-5, and CCL-6) with known antifungal activity could suppress fluconazole resistant C. auris isolated from a urine sample. 108 Although it is not clear the specific components of the medium that elicited the inhibitory response, identifying such cytokines, may pave the way for developing immunological interventions against C. auris mucosal infections.

Metals and nanoparticles
Elimination strategies of a new pathogen are primarily based on evaluating the effects of currently approved drugs, and repurposing those that are already approved against the other pathogens, against the newcomer. Essentially, this approach circumvents the extremely lengthy test protocols required for introducing a brand-new antimicrobial drug as a new therapeutic agent. Nevertheless, when all else fails, unconventional strategies of drug discovery have been increasingly experimented with a few recorded successes. This section addresses such approaches that have been taken for managing C. auris infections.

Gallium
Certain metallic elements such as gallium have been proven to be valuable in managing cancers, and disorders of calcium and bone metabolism. 109 In addition, compounds such as gallium ni-trate (Ga(NO 3 ) 3 ) are known to be effective against various bacterial pathogens suggesting the possibility of their broader utility as antifungals. 110 -113 Their antimicrobial activity is due to the ability of gallium to replace iron, in iron containing proteins, thereby altering the functionality of the latter, arresting cellular metabolism and growth. In this context Gallium, acts through a 'Trojan horse' mechanism to alter the protein structure and the functionality of both bacterial and cancer cells. 113 Bastos et al. (2019 ) were the first to investigate the effect of Ga(NO 3 ) 3 on C. auris , and they reported MICs ranging from 128 to 256 μg/ml for several strains of the multidrug resistant C. auris isolates. Although the MICs were considerably higher than C. albicans (16 μg/ml), the authors suggested its potential value as a fungistatic agent. 114 However, the toxicity of such high gallium doses has not been evaluated thus questioning their value as a safe drug.

Distorted gold(I) −phosphine complexes
Auranofin, a traditional anti-arthritis drug containing a gold complex, is known to possess anticancer, antibacterial, and antifungal properties. 115 118 The chiral square-planar gold(I) complexes were also fungicidal and exhibited excellent anti-biofilm activity (MBIC 90 3.9 μg/ml for developing biofilms, and 7.8-15.6 μg/ml against preformed biofilms). Unfortunately, the compounds were cytotoxic and elicited hemolysis indicating their limited potential, if any, in managing C. auris infections. Although the exact mechanism of their action is not yet known, it is likely that gold complexes may inhibit mitochondrial functions of the fungus. 119

Silver nanoparticles
Recent advances in nanotechnology and the realization that nanoparticles could be used as an effective drug delivery vehicle have led a number of workers to evaluate the potential of antimicrobial-laced nanoparticles in managing infectious diseases. 120 Elemental silver, a key element that has been in use for many years as an antimicrobial agent due to its broad-spectrum antimicrobial activity has been used in this context, in the form of silver nanoparticles (AgNPs).
The latter is now known to possess potent activity against C. albicans biofilms., 121  The biofilms treated with AgNP demonstrated cells with altered and, distorted morphology, and damaged cell walls. 123 The investigators further demonstrated the potency of AgNPs in eliminating C. auris biofilm on colonized catheters, as well as on hospital fabrics. More importantly, the AgNP-interlaced fabric fibers (e.g., elastic bandage wraps) retained the fungicidal effect even after repeated washes indicating its lasting antifungal potency, and economy of use. The same group of workers, in a follow-up study, confirmed their previous findings (MIC < 0.5-1 μg/ml, MFC 1 ≤ 32 μg/ml), as well as the potency of AgNPs in preventing C. auris biofilm formation (IC 50 0.5-4.9 μg/ml) and eliminating preformed biofilms (IC 50 1.2-6.2 μg/ml) regardless of their clade. Interestingly, the biofilm phenotypes of all isolates were highly susceptible for AgNPs although a single C. auris isolate of one clade, showed a higher MFC value. 124 It is believed that AgNPs exert their anti-candidal activity by attaching to the yeast cell surface, increasing the cell wall/membrane permeability, and disrupting the cell membrane integrity, leading to cellular apoptosis. In addition, others have noted a reduction of cell wall ergosterol content and hydrolytic enzyme production in C. albicans in response to AgNPs. 125 -127 Bismuth nanoparticles Similar to silver, elemental bismuth is also renowned for its antimicrobial properties including its anti-candidal properties, particularly against C. albicans . 128 Vazquez-Munoz et al. (2020) demonstrated nanoparticles synthesized from bismuth (BiNPs), also exhibit promising anti-C. auris activity against planktonic C. auris regardless of their clades (MIC 1-4 μg/ml). However, its inhibitory effect on the biofilm phenotype appeared to be rather moderate as the IC 50 values for biofilms ranging from 5.1 to 113.1 μg/ml. 129

Trimetallic nanoparticles
Nanoparticle comprising a combination of three components, so called trimetallic composites, such as silver copper and cobalt (Ag-Cu-Co), are now known to exhibit superior antimicrobial properties compared to their mono-and bi-metallic counterparts. 130 The trimetallic nanoparticles also exhibit properties such as higher catalytic activity, better stability and selective and sensitive detection, increased drug encapsulation efficacy, in comparison to their monometallic equivalents. 131 Kamli et al. (2021) recently investigated the effect of Ag-Cu-Co trimetallic nanoparticles against C. auris and noted a MIC range of 0.39-0.78 μg/ml against 25 different C. auris isolates. The authors also suggested that the nanoparticles are likely to exert their effect on C. auris by inducing cellular apoptosis and arresting its cell cycle. 131 Thus, nanoparticle technology appears to show promise in combating C. auris both in infectious foci as well as in environmental ecosystems. Nevertheless, such applications will be premature until their PK/PD profiles, physicochemical interactions, toxicities, and their specific modes of action are determined.

Silver nanoparticles and curcumin
Curcumin, well-known for its antimicrobial, anti-inflammatory, and antioxidant properties, has also been used in combination with AgNP as an antimicrobial agent. 132 , 133 In particular, AgNPs laced with aqueous curcumin: hydroxypropyl-β-cyclodextrin complex (cAgNPs; average size 42.71 ± 17.97 nm), has shown to be effective against C. auris . Curiously though, free cAgNPs were toxic to mammalian cells, and poorly microbicidal, but when combined with a bacterial cellulose hydrogel, their cytotoxicity was reduced, and a significant antimicrobial effect was noted. 134 Additionally, cAgNPs were found to be hemolytic, yet the Investigators suggested that loading such nanoparticles into bacterial cellulose hydrogel would be an attractive approach for eliminating pathogens such as C. auris , Staphylococcus aureus and Pseudomonas aeruginosa polymicrobial, chronic wound infections.

Nanoparticle and nitric oxide (NO) combinations
Nitric oxide (NO), an important component of the mammalian innate immune system, possesses both cytostatic and cytotoxic properties against a broad-spectrum of microorganisms. 135 , 136 The antifungal properties of NO have been previously demonstrated against C. albicans . 137 In order to assess the anti-C. auris properties of NO, Cleare et al. (2020) developed nanoparticles that induce a sustained release of NO. They noted that 10 mg/ml of the nanoparticles were sufficient to completely arrest the growth of both fluconazolesusceptible and -resistant planktonic C. auris (a reduction of planktonic CFU by 1.49-10.2 log 10 ). Interestingly, there was a significant reduction in biofilm viability when exposed to similar nanoparticle concentrations (a reduction of biofilm CFU by 0.98-9.68 log 10 ). 138 Of note, the nanoparticle scaffold itself displayed intrinsic inhibition of C. auris suggesting that the antifungal activity was a combined outcome due to the nanoparticle structure as well as the released NO.
Although NO nanoparticles are capable of disrupting fungal growth and morphogenesis by inducing cellular apoptosis and necrosis, 139 there is no data on the cellular toxicity of the novel, combined formulations described above. Hence, further investigations are necessary to evaluate the potential clinical value of such combinations.

Phenylthiazole family
Also, other molecules such as those from phenylthiazole family, with no previously reported antifungal activities, have been shown to eliminate C. auris planktonic phenotypes at 2 μg/ml (MIC) and display fungicidal activity, even after a short, 30-min exposure (2-4 folds of MIC). 140 Another derivative of phenylthiazole family suppressed C. auris biofilm formation by 91.2% reduction at 2 μg/ml and reduced preformed biofilms by 50.7% at 8 μg/ml. Further, the phenylthiazole compound prolonged the survival of C. auris infected C. elegans nematode model and displayed no toxicity for mammalian cells indicating its potential as a future antifungal agent. 140 Others have shown a related group of thiazoles, oxadiazolylthiazoles, and demonstrated anti-C. auris properties similar to phenylthiazole to (MIC 2-4 mg/ml). 141

VT-1598
VT-1598 is an investigational tetrazole with promising inhibitory properties against various fungi, particularly in experimental models of invasive candidiasis. 143 -145 Using C. auris isolates from South Asian, South American, East Asian, and African clades, Wiederhold et al. (2019) demonstrated that VT-1598 successfully eliminates the fungus both in vitro and in vivo . The overall MICs for all isolates ranged from 0.03-8 μg/ml (MIC50: 0.25 μg/ml and MIC90: 1 μg/ml). However, the isolates from South Asian clade displayed lesser sensitivity to VT-1598. When treated with up to 50 mg/kg, a longer survival rates ( > 21 days) and lower fungal burdens in the kidneys (mean log 10 CFU/g, treated vs. control: 3.67 vs 7.26) were observed in a neutropenic murine model infected with C. auris . 146 Despite the lack of C. auris specific data, VT-1598 is considered a selective inhibitor of fungal Cyp51A (Erg11p; 14 αdemethylase) but not its mammalian counterpart of cytochrome P -450 enzymes. 147 , 148 Thus, providing increased margin of safety for human use, if and when approved for such.

Arylamidine T-2307
In a subsequent study, using similar experimental approach, the latter investigators demonstrated the anti-C. auris properties of another novel compound, arylamidine (T-2307). T-2307 destroyed 100% of planktonic C. auris at 0.125 to > 4 μg/ml and significantly improved survival and reduced the kidney fungal burden in an animal model. 149 T-2307 elicits mitochondrial membrane collapse in other fungi 150 although, it is not yet known whether it deploys a similar mechanism for killing C. auris.

Drimenol
In other published work, drimenol, a synthetic drimane sesquiterpenoids, was demonstrated to completely inhibit C. auris growth at 50 μg/ml (MIC 30 μg/ml). 151 Although exact mechanism of C. auris inhibition is yet to be known, drimenol is likely to affect fungal cellular activities that regulate protein secretion, vacuolar functions, chromatin remodeling and cyclin dependent protein kinase (CDK)-associated functions. 151

2-Aryloxazoline deri vati ves
Similarly, two of the 2-aryloxazoline derivatives generated from a reaction between L-threonine, and derivatives of naphthoic or salicylic acid also exhibited MIC of 0.06-2 μg/ml against fluconazole and amphotericin B sensitive and resistant C. auris isolates, respectively. 152 However, their mode of action, tolerability/toxicity and in-depth PK/PD is yet to be determined.

Cuminaldehyde deri vati ve
Soliman et al. (2018) has shown a promising ant-C. auris activity of derivatives of cuminaldehyde, an essential oil isolated from Calligonum comosum. Cuminaldehyde is known for its antifungal properties, however, its toxicity has impeded its use as an antifungal agent. 153 In order to improve its biocompatibility, Hamdy et al. (2020) synthesized cuminaldehyde derivatives by replacing the toxic aldehyde group of the essential oil. The derivatives elicited antifungal activity against C. auris at a concentration range of MIC 50 2-15 μg/ml, and the compounds were well tolerated by mammalian cells. 154

Amidinourease compounds
In another study, Orofino et al. (2020) noted the putative anti-C. auris properties of a synthetic compound from macrocyclic amidinourease, a novel class of antifungal agents. They exhibited activity against fluconazole resistant strains of C. auris , with a MIC range of 8-64 μg/ml, and were well tolerated in a murine model. The exact mechanism of their action is yet to be determined. 155

Aryl-and heteroaryl-substituted hydrazones
Thamban- Chandrika et al. (2021) recently proposed a potential new class of antifungal agents, fluorinated, aryl-and heteroarylsubstituted hydrazones. 156 When an array of synthetic monohydrazones (family IV) were screened for their antifungal properties against C. albicans and a panel of C. auris isolates (n = 10), they noted seven compounds to be highly active against all C. auris isolates with a MIC 0.015-7.8 μg/ml. Two of the novel compounds significantly suppressed biofilm formation (15.6-31.3 μg/ml; 4-16x MIC) as well and exhibited better MBIC 50 than voriconazole. These compounds appeared to be well tolerated by the host cells as murine red blood cells were not adversely affected when exposed to these monohydrazones. 156 Although the mechanisms of action of the monohydrazones are yet to be unraveled, the latter authors suggested that they are likely to interfere with fungal DNA-protein interactions.

Acetohydroxyacid synthase inhibitors
Acetohydroxyacid synthase (AHAS) has been used as the target for over 50 commercial herbicides and is considered a promising target for antimicrobial drug discovery.  suggested the suppression of AHAS, the first enzyme in the branched-chain amino acid biosynthesis pathway, as an effective strategy for managing invasive antifungal infections. 157 Using purified AHAS derived from C. auris , Agnew-Francis et al. (2020) identified 13 different AHAS inhibitors. Among these, bensulfuron methyl appeared to be the most potent (MIC 50 of 0.09 μM) and fungicidal against two C. auris strains tested. In addition, both bensulfuron methyl and chlorimuron ethyl, exhibited potent antibiofilm effects (MBIC 50 0.596-1.98 μM). 158 Several AHAS inhibitors identified in this study (chlorimuron ethyl, bensulfuron methyl, metosulam, diclosulam, cloransulam methyl) maintained growth inhibition for a period of 14 days indicating their potential long-term efficacy. 158 The AHAS inhibitors have exceptionally low toxicity against mammalian cells, 159 and could prove to be an another viable therapy for managing C. auris infections.
The foregoing clearly indicates the explosion of interest in the drug discovery against C. auris, and the multitude of agents evaluated as anti-C. auris agents, although, unfortunately, none appear to have been approved for therapeutic use, thus far. Further, in depth , in vitro and in vivo studies, particularly with broader coverage of all its clades form different geographic regions, are essential to achieve this ultimate goal.

Natural compounds
Apart from the synthetic compounds described above a number of natural metabolites derived from microbes have been another focus of study in the quest for anti-C. auris agents, and these are described below.

Quorum sensing molecules (QSMs) -Farnesol
Microbial quorum sensing (QS) is the phenomenon of gene expression within a microbial community through chemical signals, termed QS molecules, in response to fluctuations in communal cell-population density. 160 QSMs are also known to possess potent antimicrobial properties, particularly against competing organisms within microbial communities, and this attribute has been exploited in the search for potential antimicrobial agents. 161 -163 Farnesol, for instance, a major QSM secreted by some Candida spp. inhibits the morphological transition of C . albicans and nonalbicans Candida species from the yeast to the hyphal phase thereby impeding their virulence. 164 -166 Interesting in the current context is the work of Nagy et al. (2020) who demonstrated concentration dependent (from 10 to 300 μM) inhibition of C. auris biofilm growth and development, up to 24 h. 167 The latter researchers also reported the synergism between farnesol and triazole antifungals (fluconazole, voriconazole, isavuconazole, itraconazole and Posaconazole), on preformed 24 h C. auris biofilms, (with a FICI ranging from 0.038 to 0.375). In an earlier study, the same group reported synergism between farnesol and three echinocandins; anidulafungin, caspofungin, or micafungin in inhibiting biofilms of four C. auris isolates of South Asian/Indian lineage. 168 They witnessed 64-128 folds reduction in biofilm inhibitory concentrations ( ≥64 μg/ml vs. 1 μg/ml) of all three echinocandins (by means of 50% reduction in biofilm metabolism) when treated with farnesol. 168 Using 25 different C. auris isolates from South Africa, including 22 fluconazole and five amphotericin B resistant strains, Srivastava et al. (2020) demonstrated similar inhibitory properties of farnesol on planktonic and biofilm phenotypes of the yeast, although the overall inhibitory concentration of the QSM was markedly higher than noted in previous studies. 167 -169 The researchers observed planktonic MIC of farnesol ranged between 62.5 and 125 mM. Farnesol concentrations of 125 mM inhibited C. auris adhesion, 7.81 mM inhibited > 50% of forming biofilms, and 500 mM inhibited 12 and 24 h biofilms. 169 As regards the morphologic effects of farnesol on C. auris biofilms , Srivastava et al. (2020) noted thin, scanty, and sparse biofilm development on exposure to farnesol, in comparison to thicker and robust biofilm growth in farnesol-free controls. 167 , 169 The exact mechanisms by which farnesol impacts C. auris planktonic and biofilm phenotypes are not yet known, although, it has been speculated that the reduced activity of drug efflux pumps and the downregulation of the genes coding them ( CDR1, CDR2 and SNQ2 ) may be the reasons for such observations. 169 It is well established that farnesol is actively involved in ergosterol biosynthesis, induce intracellular ROS, thus disrupting mitochondrial metabolism in several Candida species. 170 Hence it is highly likely that the inhibitory properties of QSM against C. auris, including the synergism with echinocandins, are likely to be associated with one or more of these mechanisms. Further work is warranted however, to confirm the foregoing hypotheses, evaluate the bio-safety, and realize the promising potential of farnesol as an anti C. auris agent.

Chitosan
Chitosan (poly-( β-1 → 4)-2-amino-2-deoxy-D-glucopyranose), a naturally occurring, nontoxic polymer derived from deacetylated chitin, commonly found in fungal cell walls, as well as in crustacean exoskeletons is known to exhibit broad spectrum antimicrobial activity. 171 Chitosan has been tested against both aggregating and nonaggregating phenotypes of C. auris (from Southern Asian/Indian and South African clades) for their anti-candidal properties. 172 The latter study reported that fungicidal concentrations of chitosan for planktonic C. auris ranged between 5-20 μg/ml, while for the biofilm phenotype MIC50 and MIC80 ranged between 10 and 80 μg/ml and 40 and 160 μg/ml, respectively. Two aggregating phenotypes with known resistance to caspofungin exhibited the highest planktonic and sessile MIC. 23 In ultrastructural studies, the chitosan treated nonaggregating phenotype, unlike the aggregating counterpart, exhibit ruptured cell walls, implying phenotype dependent response of C. auris to the polymer. 172 Further analyses of the differential expression of genes associated with stress-like response ( ALS5, HYR3 , ERG2, KRE6, EXG, ENG1 , SAP5, PLB1 ), the latter researchers observed upregulation of all genes in non-aggregating phenotype (only SAP5 in aggregating phenotype). In another study, chitosan treatment (200 mg of chitosan/kg of body weight) significantly increased the survival rate (up to 84%) of G. mellonella wax worm infected with C. auris compared to untreated controls. Although the link between the chitosan exposure and the foregoing gene expression is unclear, particularly for aggregating and non-aggregating phenotypes, the investigators suggested that the direct interactions of chitosan with the cell surface of the fungus may have contributed to this phenomenon. 172 Nevertheless, C. auris inhibitory properties of chitosan appear promising and further investigations are warranted to confirm their clinical utility as a therapeutic agent.

Plant products
Medicinal plants, plant products and essential oil derivatives have been used for millennia as components of various traditional medicines, due to their potent antimicrobial activities. 173 -176 The active anti-microbial ingredients of these plant products are complex and varies intensely but are minimally toxic to human cells. In general, the active components of the herbal products are monomers such as phenylpropanoids, flavonoids, alkaloids, terpenoids, and quinones. 177 , 178 The following section summarizes some of the plant products evaluated as anti-C. auris agents.

Herbal monomers
Liu et al. (2020) explored the anti-C. auris properties of five common traditional herbal monomers used in Chinese medicine, berberine, sodium houttuyfonate, jatrorrhizine, palmatine, and cinnamaldehyde, that are already known to be antifungal in nature. 179 They reported planktonic MICs of 64 μg/ml for sodium houttuyfonate, and 50 μg/ml for cinnamaldehyde, in contrast to the other three monomers which displayed higher MIC values of 256 μg/ml. The investigators also noted a significant reduction in C. auris colonization and aggregation, and a greater degree of cell wall β-glucan exposure in response to a combination of cinnamaldehyde, jatrorrhizine, and palmatine, as opposed to exposure to the individual compounds. Although the exact mechanism is not known, their effects are thought to be associated with either the cell wall development mechanics and/or the fungal stress response. This hypothesis is further supported by the cell wall remodeling properties of some of the traditional herbal monomers. 180

Cinnamaldehyde
The essential oil extracted from Cinnamomum zeylanicum Blume (Sri Lankan cinnamon) leaves and bark have been widely studied for their antiseptic, immunostimulant, detoxifying, analgesic, and antidepressant effects. Previous studies have shown the potential of this essential oil in inhibiting germ tube formation, adhesion to epithelial cells and proteinase production in C. albicans . 181 Essential oil extracted from cinnamon bark that contains ( ∼66%) trans -cinnamaldehyde is now known to be fungicidal at 0.03% (v/v) concentrations, and to lower C. auris associated hemolysis. 182 The latter workers further suggested that the active compound, cinnamaldehyde is likely to exert its activity by compromising the yeast cell membrane and cell wall integrity.

α-Cyperone
An essential oil containing α-Cyperone extracted from rhizome of Cyperus rotundus seem to possess beneficial properties such as protecting host cells from lipopolysaccharide mediated cellular damage and from H 2 O 2 -induced oxidative stress and apoptosis in neuronal cells. 183 Recent study by Horn et al. (2021) demonstrated the inhibitory properties of α-Cyperone on C. auris growth at a concentration of 150 μg/ml. 184 Their anti-fungal mechanism is yet to be elucidated.

6-Shogaol
6-shogaol is a biologically active phytochemicals in ginger and displays potent C. auris anti-biofilm activity. 6-shogaol has a low MIC (MIC 50 16-32 μg/ml, and MIC 80 32-64 μg/ml) for the planktonic form, with over 97% suppression of forming and preformed biofilms of C. auris on exposure to 64 μg/ml. Although the molecular mechanism of its anti-C. auris activity is yet to be defined, the authors suggested that 6-shogaol is likely to act on drug efflux machinery of the fungus. 185 Clearly, not all the tested essential oils or their active compounds seem to possess effective and strong anti-C. auris properties. For example, an essential oil extracted from native eastern north American plant Thuja plicata, commonly known as American arborvitae , exhibited only a marginal reduction in intrinsic growth rate of C. auris, possibly due to increased cellular death or decreased cell division. 186 From the foregoing it is evident that plant oils, extracts and monomers have significant, yet highly variable anti -C auris properties. Further in-depth investigations into such phytochemicals with promising potential should pay dividends in the quest for natural and green, pharmacological products that are antifungal in nature.

Bee honey
Medical-grade honey contains various phytochemicals such as alkaloids and flavonoids, in addition to bee-derived peptides, such as bee-defensin-1 and apidaecin. 187 , 188 Honey is also known to have over 200 different components that may vary from one shipment to another depending on the source of origin, the geographical location, as well as the bee species. 187 Most of the honey constituents are known to possess antimicrobial properties although these may vary among different formulations. Despite the latter drawback, de Groot et al. (2021) tested a medical grade honey formulation against 32 isolates of C. auris from five different clades and observed that exposure to 40% honey for 24 h results in a notable reduction in CFU counts (by 2 Log 10 ). However, as pure honey, at similar concentrations, had inferior activity, the authors concluded that the observed yeast inhibition was likely to be due to a combined effect of honey components, as well as other commercial additives in their samples. 189 Although bees honey may be a promising anti-C. auris candidate compound, quality control of such naturally derived products would be a major impediment that needs to be overcome in future studies.

Probiotics
Probiotics therapy is now a widely accepted alternative strategy for improving the overall human health due to their beneficial, synergistic interactions with the commensal flora while inhibiting colonisation by extraneous pathogens. In this context, probiotics have been studied for their potential in preventing and managing fungal infections including those caused by Candida spp . 190 , 191 Rossoni et al. (2020) investigated the impact of the probiotic Lactobacillus paracasei 28.4 and its supernatant on C. auris. The co-culture of the probiotic with C. auris led to a significant reduction of yeast counts for up to 3 days. The crude extracts of the supernatant ( > 15 mg/ml) and its first fraction (3.75 -> 7.5 mg/ml) also demonstrated a significant suppression of all 10 C. auris isolates tested (up to 6 log 10 CFU reduction). 192 In a subsequent study, co-inoculation of C. auris with two different probiotic yeasts; Saccharomyces cerevisiae and Issatchenkia occidentalis resulted in reduction of C. auris adhesion by 44-62%. 193 Additionally, both the probiotic and the supernatant were effective in impeding C. auris biofilm growth, significantly prolonged the survival of C. auris infected G. mellonella . 192 As the anti-fungal components of the probiotic supernatants are yet to be deciphered, futures workers should seek to isolate, identify, and characterize these bioactive molecules to evaluate their translational potential as therapeutics against C. auris. Probiotics are particularly attractive potential therapeutics as they are generally regarded as safe by FDA and have been tested for their clinical efficacy against C. albicans . 194

Novel compounds with antifungal properties
' Fungerp' group of antifungal agents Since the first reports of the stubborn resistance profile C. auris to the current antifungal armamentarium, a number of workers have attempted to evaluate novel anti-C. auris compounds, and as a result, significant new discoveries have been made, as discussed below.
Several studies have been conducted for evaluating compounds that are structurally and/or functionally similar to the most effective family of anti-C. auris drugs, namely glucan synthase inhibitors such as echinocandins. One such compound, so called ' fungerp' antifungal agents, has been tested against C. auris , is ibrexafungerp ( syn . SCY-078) is an orally bioavailable triterpene glucan synthase inhibitor. It is known to possess antifungal properties against common pathogenic Candida species, including those resistant to echinocandins. 195 Berkow et al. (2017) demonstrated the efficacy of SCY-078 against a panel of 100 different C. auris isolates from four known clades from India, Pakistan, Colombia, South Africa, and the United States. 15 , 196 The MIC values of SCY-078 in these studies ranged from 0.0625 to 2 μg/ml (mode MIC 50 0.5 μg/ml and MIC 90 1 μg/ml). 196 Importantly, seven isolates that were resistant to anidulafungin, caspofungin or micafungin exhibited susceptibility to SCY-078 (MIC 0.5-1 μg/ml). Zhu et al. (2020) have also confirmed these MIC profiles in a large-scale screening study of 200 different C. auris isolates. 197 Using 16 isolates, Larkin et al. (2017) further demonstrated the potent antibiofilm activity of the new compound is likely to be due to the reduction of the metabolic activity as well as the thickness of C. auris biofilms treated with SCY-078. These investigators also suggested that the compound is likely to act on different cellular targets as they observed, in SEM studies, severely altered yeast morphology and cell division arrest in SCY-078 treated C. auris . 198 In addition to being bioavailable orally, SCY-078 differs from other echinocandins as its activity is not compromised by the most common yeast mutations within the protein target, Fks. 199 This has been confirmed in a study conducted on a panel of 122 C. auris isolates from various geographical clades. There were eight echinocandin resistant isolates within the tested panel of the isolates with a S639F Fks1 alteration. Interestingly, all 122 isolates were susceptible to SCY-078 with a modal MIC and MIC 50 of 0.5 μg/ml (a range of 0.06-2 μg/ml). 200 In practical terms, the range of MICs reported in these studies is well within the serum concentrations recorded in murine models of disseminated candidiasis, and preclinical pharmacokinetic and pharmacodynamic (PK/PD) studies. 201 Ghannoum et al . (2020) demonstrated using a guinea pig model, that oral administration of (10 mg/kg) of SCY-078 lowers the severity of lesions as well as the fungal burden in infected animals compared to the controls. 202 Therefore, SCY-078 is highly likely to be one of the more promising new antifungal agents against echinocandin-resistant C. auris infections, and currently it is undergoing phase III clinical trials (NCT04029116). 203 The potent antifungal activity of SCY-078 has led to further experiments using its analogs SCY-247. One such secondgeneration ' fungerp' antifungal compound, elicited an MIC range of 0.06-1 μg/ml; MIC 50 and MIC 90 0.5 μg/ml which was comparable to SCY-078, against a panel of different C. auris isolates. 204 Interestingly, SCY-247 exhibited fungicidal effects (MFC range 0.5-8 μg/ml; MFC 50 and MFC 90 4 μg/ml) on a larger percentage of C. auris isolates than SCY-078 (14 vs. 7 isolates). Further investigations on SCY-247 in vivo responses, the potential for developing resistance, the molecular mechanisms of action, and their side effects will be important to validate the fitness of SCY-247 as an anti-C. auris antifungal agent. In summary, the family of fungerp antifungal compounds SCY-078 and its analogs are likely to be a potent antifungal drug class in future.
Fosmanogepix or APX0 01/APX0 01A APX001/APX011A (Fosmanogepix, formerly E1211), a watersoluble small molecule with a novel and unique mechanism of action, has drawn the attention of a number of workers searching for effective therapies against C. auris . Unlike other classes of antifungal agents, the active moiety of the APX001, which is released by its rapid metabolism by systemic phosphatases, targets a highly conserved fungal enzyme Gwt1 (glycosylphosphatidylinositol-anchored wall transfer protein 1). Gwt1 catalyses the inositol acylation step of glycosylphosphatidylinositol (GPI) anchored cell wall mannoproteins synthesis. These mannoproteins play a significant role in anchoring the fungus to eukaryotic cell surface proteins. 205 , 206 The inhibition of Gwt1 is known to affect maturation and localization of fungal cell wall mannoproteins, leading to compromised cell wall integrity, defective filamentation and biofilm formation, and severe retardation of fungal growth. 205 , 207 Interestingly, the PIG-W protein, the mammalian ortholog of Gwt1, appeared to be insensitive APX001 mediated inhibition, thus, significantly improving the fungal specificity of the drug target. 205 One of the early studies conducted by  on the effects of APX001 on 16 isolates of C. auris have shown that all isolates were susceptible to the novel compound, with MIC 50 and MIC 90 values of 0.004 and 0.031 μg/ml, respectively. 208 In another study, using a large array of C. auris isolates (n = 100) from four different geographical clades, Berkow and Lockhart (2018) further confirmed and validated these results across the clades (MIC range < 0.005-0.015 μg/ml, overall modal MIC 0.005 μg/ml, MIC 50 0.002 μg/ml and MIC 90 0.008 μg/ml). 209 The efficacy of APX001 against six echinocandin resistant isolates included in this study suggests its value in treating echinocandin resistant C. auris infections. Furthermore, Arendrup et al . (2018) also noted in an in vitro study that APX001 is highly effective against a collection of 122 C. auris isolates (MIC 50 = 0.016 μg/ml). 210 Their data also indicated that APX001 was equally or more active than anidulafungin, micafungin, voriconazole, fluconazole, and amphotericin B. 197 , 200 , 210 APX001 has been proven to be very effective in in vivo models. For instance, the exposure of immunocompromised mice infected with C. auris to APX001 resulted in a significantly higher 16-day survival rate (as high as 100%) compared to the treatment with anidulafungin. APX001 treated mice had significantly lower CFU counts in kidney, lung, and brain tissue (with a log 10 reduction range of 1.03-1.83) versus the vehicle control. 208 In a parallel study, using a similar disseminated candidiasis murine model, Zhao et al. (2018) confirmed the efficacy of the new compound in eliminating C. auris infection in vivo and described that the outcome was dependent on the concentration of APX001. 211 The foregoing clearly testifies to the fact that APX001 is likely to be a highly effective compound against C. auris infections. Indeed, as a novel drug, currently in the clinical development phase, ( https://clinicaltrials.gov/ct2/ show/NCT04240886 ), APX001/APX001A would likely to be one of the key last resort drugs in managing multi-resistant C. auris infections in the not-too-distant future.

New echinocandin agents -Rezafungin (CD101)
Rezafungin (CD101) is yet another echinocandin currently in the clinical development pipeline. Compared to current echinocandins, rezafungin offers enhanced pharmacokinetic properties and has an improved safety profile, 212 -214 and the efficacy of this compound is proven against isolates of several other Candida species. 215 -217 In one such study, Berkhow and Lockhart (2018) noted that MIC values of rezafungin ranged between 0.03 and 8 μg/ml (mode MIC50 = 0.125 μg/ml, MIC90 = 0.5 μg/ml) among 100 different C. auris isolates. Similar to previous investigations with SCY-078 and APX001A, there were no notable variations among four clades tested. However, 4 of 8 echinocandin resistant isolates exhibited higher MICs to rezafungin (MIC range 0.06-8 μg/ml with an MIC 50 of 0.5 μg/ml). 218 Further analyses revealed this to be due to S639P amino acid substitution in Fks1 hot spot 1, a mutation corresponding to the echinocandin resistance in other Candida species (e.g., S645P in C. albicans and S629P in C. glabrata ). 219 Generally, the relative increase in the MIC of rezafungin conferred by Fks mutations was comparable to or slightly less than those for anidulafungin and micafungin. 218 , 220 In accordance with cross resistance between rezafungin and the comparator echinocandins observed previously, Helleberg et al. (2020) also reported increased MIC of rezafungin in C. auris isolates with Fks1 hot spot S639F mutations (MIC 8-16 μg/ml). 220 However, this increase in MIC was 3-4-folds greater than for the anidulafungin and micafungin. It is clear that FKS mutations would likely to impact the efficacy of rezafungin and the increase in the rezafungin MIC is likely to be dependent on the codon and the substitution involved in the mutation. Therefore, further investigations are warranted in understanding rezafungin resistance in C. auris. On a positive note, the azole resistance in C. auris did not appear to impact the efficacy of rezafungin. 221 Rezafungin has also displayed some encouraging results in in vivo disseminated C. auris candidiasis models. Using a murine model,  demonstrated potent reduction of C. auris in kidney tissues compared with controls, and those treated with amphotericin B, up to 10 days posttreatment and compared to those treated with micafungin on 10th day posttreatment. 222 In a similar disseminated candidiasis murine model, Lepak et al. (2018) further validated the efficacy of rezafungin in eliminating C. auris infection in vivo . By integrating their pharmacokinetics/pharmacodynamics (PK/PD) targets with human PK studies, the investigators further suggested that intravenously administered rezafungin dose of 400 mg once a week would likely to meet or exceed the PD target for > 90% of C. auris isolates. 223 To conclude, rezafungin shows promising activity against C. auris and its once-weekly intravenous therapy would likely to be an attractive therapeutic strategy for critically ill with the fungal infection. The prolong half-life, greater safety margin with chemical stability provide rezafungin a notable advantage of preventing the development of resistance to echinocandins class of antifungal agents. Large scale clinical investigations are, however, needed to translate these findings into the therapeutic domain.
PC945, similar to other azoles, acts on ergosterol synthesis pathway by inhibiting lanosterol 14a-demethylase enzyme coded by ERG11 . Yet the data indicated that PC945 acts independent of any mutations in the ERG11 that may be associated with azole resistance in C. auris , indicating its superior effectiveness over other azoles against azole resistant C. auris . 224

Novel candidate agents -Ebselen
Rather than following the conventional route of repurposing the currently available antimicrobials, some have taken the more challenging route of evaluating totally new compounds for their antifungal activity. In one such study, by screening a library of 1 280 small molecules, Wall et al. (2018) identified nine candidate molecules with potential antifungal activities against C. auris . 225 One of these molecules with no recorded antifungal activity, to date, Ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)-one), exhibited 100% growth inhibition of C. auris at physiologically achievable concentrations (as low as 2.5 μM). 225 -227 These investigators further noted that ebselen actively impedes the biofilm phenotype of the fungus at physiological concentrations of 5.9 to 9.8 μg/ml. The novel molecule appeared to be highly effective against C. auris irrespective of its resistance to fluconazole, amphotericin B or caspofungin. Although its exact antifungal mechanism is not yet known, ebselen is known to induce ROS mediated cytotoxicity and the membrane H + -ATPase pump (Pma1p) in Saccharomyces cerevisiae, and also deplete the fungal intracellular glutathione levels. 228 , 229 Same investigators have also noted synergistic interactions between ebselen and anidulafungin. 225 , 230 Novel candidate agents -Miltefosine and Iodoquinol Wall et al. (2019) in a subsequent study, identified two further compounds, miltefosine and iodoquinol, with anti-C. auris properties by screening another chemical library (Pathogen Box®). They noted the potent in vitro inhibitory activity of iodoquinol against planktonic C. auris, and miltefosine against both planktonic and biofilm phase C. auris at 4 μg/ml, irrespective of the antifungal resistance profiles of the chosen isolates. 231 The same group of researchers in another study, identified 26 different compounds with anti-C. auris activity by screening of over 12 000 small molecules (ReFRAME library). These included antiseptics, disinfectants, antibacterials and most interestingly five repositionable compounds including miltefosine (Tazomeline, Lonafarnib, AM-24, Miltefosine and Provecta). However, the mechanisms of antifungal actions of the latter compounds are yet to be defined. 232 Novel candidate agents -Suloctidil Concurrently, de Oliveira et al. (2019), by screening another chemical library (Prestwick Chemical Library), identified 12 different compounds with ≥90% growth inhibition of C. auris, and seven of these demonstrated reproducible antifungal activities. These were suloctidil, trifluoperazine dihydrochloride, ciclopirox ethanolamine, tamoxifen citrate, ebselen, pyrvinium pamoate, and thiethylperazine dimalate. 230 Among these, suloctidil inhibited C. auris growth by > 78% at a concentration of 16 μg/ml. They also demonstrated a synergy between suloctidil and voriconazole, that led to 2-32-fold lowering of MIC of voriconazole against C. auris . 230 Novel candidate agents -Niclosamide and Halogenated Salicylanilide Furthermore, a screen of a chemical library of 678 small molecules revealed that niclosamide (5-chloro-salicyl-(2chloro-4-nitro) anilide), an FDA-approved anthelmintic drug for humans, and a halogenated salicylanilide (N1-(3,5dichlorophenyl)-5-chloro-2-hydroxybenzamide), another anthelmintic drug for veterinary use, as compounds with potential antibiofilm properties against C. auris. Both anthelmintic drugs demonstrated anti-C. auris biofilm properties at 1 μM. Although their C. auris specific molecular mechanisms are yet to be known, the two anthelmintic drugs are known to subdue C. albicans virulence suppressing morphological transition and mitochondrial protein import machinery. 233

Repurposed drugs
As discussed above, screening compound libraries have revealed hitherto unknown non-antimicrobial drugs such as miltefosine, niclosamide, and halogenated salicylanilide with promising antimicrobial potential. 232 , 234 As a consequence, there has been a renewed interest in repurposing older drugs as antifungal agents. For instance, Hao et al. (2020) witnessed the potent inhibition of C. auris planktonic phase by 4-8 μg/ml of disulfiram, a drug used for treating chronic alcoholism, however, its ability to inhibit the biofilm phenotype appeared to be modest (MBIC 80 64-128 μg/ml). 234 The following section discusses some key non-antimicrobial drugs that could be repurposed and exhibited promising anti-C. auris properties.

Sertraline
In another work, Gowri et al. (2020) reported that an antidepressant sertraline is capable of suppressing C. auris . 235 The MIC of sertraline against three different C. auris isolates ranged between 20 and 40 μg/ml. The antidepressant exhibited its fungicidal activity as early as 6 h, and suppressed biofilm formation by 71%, at doses of 20 μg/ml. Through in silico studies, authors noted that sertraline elicits its deleterious effect on C. auris by binding to the Erg11p in the ergosterol biosynthesis pathway, as they noted a 32-fold reduction in the ergosterol content of the test samples. 235 The anti-Candida properties of sertraline have been previously recorded, and findings of this study further validates its broad spectrum of activity although the collateral repercussions of administering such doses to those that are otherwise healthy needs further investigations. 236

Alexidine dihydrochloride
Alexidine dihydrochloride (a bis-biguanide dihydrochloride), an anticancer drug that targets a mitochondrial tyrosine phosphatase in mammalian cells which drive mitochondrial apoptosis, 237 has been noted to inhibit C. auris by Mamouei et al. (2018). They reported > 80% inhibition of fluconazole resistant planktonic phase (at 1.5 μg/ml), and developing and mature biofilm phase (at 3-6 μg/ml) of C. auris when exposed to alexidine dihydrochloride. 238 The anticancer drug appeared to be well tolerated by mammalian epithelial cells (5-10 × planktonic MIC is needed for 50% killing of HUVEC), although its toxicity for some immune cell components such as macrophages, appeared to be high (50% cytotoxic concentration, CC 50 , of over 5 μg/ml). 238 Interestingly, the drug is already used in dentistry as an antiplaque agent and root canal irrigant due to its antibacterial properties. 239 -241 Therefore, its further optimization into a compound with a higher efficacy and bioavailability, and low toxicity would likely to generate a successful repurposed antifungal agent.

Mefloquine deri vati ves
In another study that investigated the potential of repurposed approved drugs, derivatives of mefloquine, an orally prescribed, 4quinoline-methanol antimalarial drug, was noted to possess anti-C. auris properties. Montoya et al. (2020) tested a small group of mefloquine derivatives against fluconazole susceptible and resistant C. auris and noted their planktonic MICs to range between 2 and 8 μg/ml. 242 The derivatives were also effective against fluconazole resistant C. auris isolates (MIC 4-8 μg/ml). Interestingly, despite this observation mefloquine itself was largely ineffective against a tested C. auris isolate (MIC 128 μg/ml). They suggested that the antifungal effect of the derivatives may be associated with their ability to disrupt the mitochondrial membrane, vacuolar disruption and interfere with DNA stabilitya mode of action distinctive from existing antifungal drugs. 242 As the antifungal activity of these derivatives have been previously shown against with Cryptococcus neoformans and C. albicans 243 further optimization of their pharmacokinetics is likely to be fruitful.

Perspectives and concluding remarks
The multitude of reports on the new therapeutic strategies against C. auris infections reported here is rather startling, considering the fact that the organism was first described over a decade ago in 2009. Its alarming global spread, multi-resistance to almost all of the currently available antifungals, and the morbidity and mortality it causes are the clear reasons for such great interest in this inveterate 'new kid on the block'. Fortunately, there is hope, as a vast majority of compounds reported thus far appear to exhibit encouraging anti-C. auris properties, with promising drugs now in the pipeline in various stages of development. Nevertheless, there are several areas that need immediate attention in anti-C. auris therapeutics. The lack of data on the modes of action, toxicity, dosage, and the potential of C. auris to develop resistance to the new therapeutic modes is clear, and further studies are urgently warranted. As per EU Clinical Trials Register and NIH ClinicalTrials.gov, several novel antifungal compounds are currently undergoing clinical trials, however, only two of the aforementioned compounds (SCY-078, APX001) have been specifically tested on C. auris . Considering the severity of the infection and the incidence of antifungal resistance of C. auris, urgent attention is needed to minimize the lag between laboratory testing and clinical validation of anti-C. auris compounds. This yeast is also unique in its bi-pronged action of being a harmful human pathogen, and unlike most of its counterparts, possessing the ability to survive in the environment, and fomite surfaces, for weeks, maintaining its ability to cause infection. Hence strategies to tackle the organism should include not only the development of appropriate antifungal therapeutics, but also good environmental disinfectants effective in healthcare ecosystems. Finally, due to the isolation of the pathogen in over 45 different countries, future studies are needed with an inclusion of wide range of clinical isolates and a greater clade representation so as to produce data of grater relevance, significance, and validity.

Funding
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.