Combatting Fungal Infections: Advances in Antifungal Polymeric Nanomaterials

Fungal pathogens cause over 6.5 million life-threatening systemic infections annually, with mortality rates ranging from 20 to 95%, even with medical intervention. The World Health Organization has recently emphasized the urgent need for new antifungal drugs. However, the range of effective antifungal agents remains limited and resistance is increasing. This Review explores the current landscape of fungal infections and antifungal drugs, focusing on synthetic polymeric nanomaterials like nanoparticles that enhance the physicochemical properties of existing drugs. Additionally, we examine intrinsically antifungal polymers that mimic naturally occurring peptides. Advances in polymer characterization and synthesis now allow precise design and screening for antifungal activity, biocompatibility, and drug interactions. These antifungal polymers represent a promising new class of drugs for combating fungal infections.

Biofilm: The invisible culprit in catheter-induced candidemia

Candidemia is the most common form of invasive fungal infection associated with several risk factors, and one of them is the use of medical devices, to which microbial biofilms can attach. Candidemia related to the use of peripheral intravascular and central venous catheters (CVC) is referred to as Candida catheter-related bloodstream infection, with more than 90% being related to CVC usage. The infection is associated with a higher morbidity and mortality rate than nosocomial bacterial infections. Candida spp. can protect themselves from the host immune system and antifungal drugs because of the biofilm structure, which is potentiated by the extracellular matrix (ECM). Candida albicans and Candida parapsilosis are the most pathogenic species often found to form biofilms associated with catheter usage. Biofilm formation of C. albicans includes four mechanisms: attachment, morphogenesis, maturation and dispersion. The biofilms formed between C. albicans and non-albicans spp. differ in ECM structure and composition and are associated with the persistence of colonization to infection for various catheter materials and antifungal resistance. Efforts to combat Candida spp. biofilm formation on catheters are still challenging because not all patients, especially those who are critically ill, can be recommended for catheter removal; also to be considered are the characteristics of the biofilm itself, which readily colonizes the permanent medical devices used. The limited choice and increasing systemic antifungal resistance also make treating it more difficult. Hence, alternative strategies have been developed to manage Candida biofilm. Current options for prevention or therapy in combination with systemic antifungal medications include lock therapy, catheter coating, natural peptide products and photodynamic inactivation.

Candida auris susceptibility on surfaces coated with the antifungal drug caspofungin

Candida auris is known to survive for weeks on solid material surfaces. Its longevity contributes to medical device contamination and spread through healthcare facilities. We fabricated antifungal surface coatings by coating plastic and glass surfaces with a thin polymer layer to which the antifungal drug caspofungin was covalently conjugated. Caspofungin-susceptible and -resistant C. auris strains were inhibited on these surfaces by 98.7 and 81.1%, respectively. Cell viability studies showed that this inhibition was fungicidal. Our findings indicate that C. auris strains can be killed on contact when exposed to caspofungin that is reformulated as a covalently-bound surface layer.

LAY SUMMARY

Candida auris is pathogenic, multidrug resistant yeast with the ability to survive on surfaces and remain transmissible for long periods of time in healthcare settings. In this study, we have prepared an antifungal surface coating and demonstrated its ability to kill adhering C. auris cells on contact.

Assessment of nonreleasing antifungal surface coatings bearing covalently attached pharmaceuticals

There are many reports of antimicrobial coatings bearing immobilized active agents on surfaces; however, strong analytical evidence is required to verify that the agents are indeed covalently attached to the surface. In the absence of such evidence, antimicrobial activity could result from a release of active agents. We report a detailed assessment of antifungal surface coatings prepared using covalent attachment chemistries, with the aim of establishing a set of instrumental and biological evidence required to convincingly demonstrate antimicrobial activity due to nonreleasing, surface active compounds and to exclude the alternate possibility of activity due to release. The strongest biological evidence initially supporting permanent antifungal activity was the demonstration of the ability to reuse samples in multiple, sequential pathogen challenges. However, additional supporting evidence from washing studies and instrumental analysis is also required to probe the possibility of gradual desorption of strongly physisorbed compounds versus covalently attached compounds. Potent antifungal surface coatings were prepared from approved pharmaceutical compounds from the echinocandin drug class (caspofungin, anidulafungin, and micafungin) and assessed by microbiological tests and instrumental methods. Carbonyl diimidazole linking chemistry enabled covalent attachment of caspofungin, anidulafungin, and micafungin to plasma polymer surfaces, with antifungal surface activity likely caused by molecular orientations that present the lipophilic tail toward interfacing fungal cells. This study demonstrates the instrumental and biological evidence required to convincingly ascertain activity due to nonreleasing, surface active compounds and summarize these as three criteria for assessing other reports on surface-immobilized antimicrobial compounds.

Candida albicans Can Survive Antifungal Surface Coatings on Surfaces with Microcone Topography

This study demonstrates the ability of Candida albicans, a medically significant human fungal pathogen, to minimize contact with an antifungal surface coating that on a flat surface is lethal on contact by growing on and between micron-sized surface topographical features, thus minimizing the contact area. Scanning electron microscopy showed that cells contacting the "floor" between microcones were killed, whereas cells attached to microcones survived and formed hyphal filaments. These spanned space between cones and avoided contact with the flat surface in-between cones. Thus, fungal cells managed to attach and grow despite the antifungal coating. This ability of Candida albicans to exploit topography features to minimize surface contact yet utilize the solid surface for anchoring reduces the effectiveness of the grafted antifungal surface coating. This suggests that biomedical devices with rough surfaces might be more challenging to protect against fungal biofilm formation via application of an antifungal coating.

Caspofungin Functionalized Polymethacrylates with Antifungal Properties

We describe the synthesis of hydrophilic poly(poly(ethylene glycol) methyl ether methacrylate) (PmPEGMA) and hydrophobic poly(methyl methacrylate) (PMMA) caspofungin conjugates by a post-polymerization modification of copolymers containing 10 mol % pentafluorophenyl methacrylate (PFPMA), which were obtained via reversible addition-fragmentation chain transfer copolymerization. The coupling of the clinically used antifungal caspofungin was confirmed and quantified in detail by a combination of ¹H-, ¹⁹F- and diffusion-ordered NMR spectroscopy, UV-vis spectroscopy, and size exclusion chromatography. The trifunctional amine-containing antifungal was attached via several amide bonds to the hydrophobic PMMA, but sterical hindrance induced by the mPEGMA side chains prohibited intramolecular double functionalization. Both polymer-drug conjugates revealed activity against important human-pathogenic fungi, that is, two strains of Aspergillus fumigatus and one strain of Candida albicans (2.5 mg L^-1 < MEC < 8 mg L^-1, MIC₅₀ = 4 mg L^-1), whereas RAW 264.7 macrophages as well as HeLa cells remained unaffected at these concentrations.

Characterization of Key Bio-Nano Interactions between Organosilica Nanoparticles and Candida albicans

Nanoparticle-cell interactions between silica nanomaterials and mammalian cells have been investigated extensively in the context of drug delivery, diagnostics, and imaging. While there are also opportunities for applications in infectious disease, the interactions of silica nanoparticles with pathogenic microbes are relatively underexplored. To bridge this knowledge gap, here, we investigate the effects of organosilica nanoparticles of different sizes, concentrations, and surface coatings on surface association and viability of the major human fungal pathogen Candida albicans. We show that uncoated and PEGylated organosilica nanoparticles associate with C. albicans in a size and concentration-dependent manner, but on their own, do not elicit antifungal activity. The particles are also shown to associate with human white blood cells, in a similar trend as observed with C. albicans, and remain noncytotoxic toward neutrophils. Smaller particles are shown to have low association with C. albicans in comparison to other sized particles and their association with blood cells was also observed to be minimal. We further demonstrate that by chemically immobilizing the clinically important echinocandin class antifungal drug, caspofungin, to PEGylated nanoparticles, the cell-material interaction changes from benign to antifungal, inhibiting C. albicans growth when provided in high local concentration on a surface. Our study provides the foundation for defining how organosilica particles could be tailored for clinical applications against C. albicans. Possible future developments include designing biomaterials that could detect, prevent, or treat bloodstream C. albicans infections, which at present have very high patient mortality.

The importance of fungal pathogens and antifungal coatings in medical device infections

In recent years, increasing evidence has been collated on the contributions of fungal species, particularly Candida, to medical device infections. Fungal species can form biofilms by themselves or by participating in polymicrobial biofilms with bacteria. Thus, there is a clear need for effective preventative measures, such as thin coatings that can be applied onto medical devices to stop the attachment, proliferation, and formation of device-associated biofilms. However, fungi being eukaryotes, the challenge is greater than for bacterial infections because antifungal agents are often toxic towards eukaryotic host cells. Whilst there is extensive literature on antibacterial coatings, a far lesser body of literature exists on surfaces or coatings that prevent attachment and biofilm formation on medical devices by fungal pathogens. Here we review strategies for the design and fabrication of medical devices with antifungal surfaces. We also survey the microbiology literature on fundamental mechanisms by which fungi attach and spread on natural and synthetic surfaces. Research in this field requires close collaboration between biomaterials scientists, microbiologists and clinicians; we consider progress in the molecular understanding of fungal recognition of, and attachment to, suitable surfaces, and of ensuing metabolic changes, to be essential for designing rational approaches towards effective antifungal coatings, rather than empirical trial of coatings.