Combination antifungal therapy: what can and should we expect?

Powering up antifungal treatment: using small molecules to unlock the potential of existing therapies

Fungal pathogens are increasingly appreciated as a significant infectious disease challenge. Compared to bacteria, fungal cells are more closely related to human cells, and few classes of antifungal drugs are available. Combination therapy offers a potential solution to reduce the likelihood of resistance acquisition and extend the lifespan of existing antifungals. There has been recent interest in combining first-line drugs with small-molecule adjuvants. In a recent article, Alabi et al. identified 1,4-benzodiazepines as promising molecules to enhance azole activity in pathogenic Candida spp. (P. E. Alabi, C. Gautier, T. P. Murphy, X. Gu, M. Lepas, V. Aimanianda, J. K. Sello, I. V. Ene, 2023, mBio https://doi.org/10.1128/mbio.00479-23). These molecules have no antifungal activity on their own but exhibited significant potentiation of fluconazole in azole-susceptible and -resistant isolates. Additionally, the 1,4-benzodiazepines increased the fungicidal activity of azoles that are typically fungistatic to Candida spp., inhibited filamentation (a virulence-associated trait), and accordingly increased host survival in Galleria mellonella. This research thus provides another encouraging step on the critical pathway toward reducing mortality due to antimicrobial resistance.

Calcineurin Inhibitors Synergize with Manogepix to Kill Diverse Human Fungal Pathogens

Invasive fungal infections have mortality rates of 30–90%, depending on patient co-morbidities and the causative pathogen. The frequent emergence of drug resistance reduces the efficacy of currently approved treatment options, highlighting an urgent need for antifungals with new modes of action. Addressing this need, fosmanogepix (N-phosphonooxymethylene prodrug of manogepix; MGX) is the first in a new class of gepix drugs, and acts as a broad-spectrum, orally bioavailable inhibitor of the essential fungal glycosylphosphatidylinositol (GPI) acyltransferase Gwt1. MGX inhibits the growth of diverse fungal pathogens and causes accumulation of immature GPI-anchored proteins in the fungal endoplasmic reticulum. Relevant to the ongoing clinical development of fosmanogepix, we report a synergistic, fungicidal interaction between MGX and inhibitors of the protein phosphatase calcineurin against important human fungal pathogens. To investigate this synergy further, we evaluated a library of 124 conditional expression mutants covering 95% of the genes encoding proteins involved in GPI-anchor biosynthesis or proteins predicted to be GPI-anchored. Strong negative chemical-genetic interactions between the calcineurin inhibitor FK506 and eleven GPI-anchor biosynthesis genes were identified, indicating that calcineurin signalling is required for fungal tolerance to not only MGX, but to inhibition of the GPI-anchor biosynthesis pathway more broadly. Depletion of these GPI-anchor biosynthesis genes, like MGX treatment, also exposed fungal cell wall (1→3)-β-D-glucans. Taken together, these findings suggest the increased risk of invasive fungal infections associated with use of calcineurin inhibitors as immunosuppressants may be mitigated by their synergistic fungicidal interaction with (fos)manogepix and its ability to enhance exposure of immunostimulatory glucans.

Potential of marine natural products against drug-resistant fungal, viral, and parasitic infections

Antibiotics have revolutionised medicine in many aspects, and their discovery is considered a turning point in human history. However, the most serious consequence of the use of antibiotics is the concomitant development of resistance against them. The marine environment has proven to be a very rich source of diverse natural products with significant antibacterial, antifungal, antiviral, antiparasitic, antitumour, anti-inflammatory, antioxidant, and immunomodulatory activities. Many marine natural products (MNPs)-for example, neoechinulin B-have been found to be promising drug candidates to alleviate the mortality and morbidity rates caused by drug-resistant infections, and several MNP-based anti-infectives have already entered phase 1, 2, and 3 clinical trials, with six approved for usage by the US Food and Drug Administration and one by the EU. In this Review, we discuss the diversity of marine natural products that have shown in-vivo efficacy or in-vitro potential against drug-resistant infections of fungal, viral, and parasitic origin, and describe their mechanism of action. We highlight the drug-like physicochemical properties of the reported natural products that have bioactivity against drug-resistant pathogens in order to assess their drug potential. Difficulty in isolation and purification procedures, toxicity associated with the active compound, ecological impacts on natural environment, and insufficient investments by pharmaceutical companies are some of the clear reasons behind market failures and a poor pipeline of MNPs available to date. However, the diverse abundance of natural products in the marine environment could serve as a ray of light for the therapy of drug-resistant infections. Development of resistance-resistant antibiotics could be achieved via the coordinated networking of clinicians, microbiologists, natural product chemists, and pharmacologists together with pharmaceutical venture capitalist companies.

Multicentre surveillance study on feasibility, safety and efficacy of antifungal combination therapy for proven or probable invasive fungal diseases in haematological patients: the SEIFEM real-life combo study

This multicentre observational study evaluated the feasibility, efficacy and toxicity of antifungal combination therapy (combo) as treatment of proven or probable invasive fungal diseases (IFDs) in patients with haematological malignancies. Between January 2005 and January 2010, 84 cases of IFDs (39 proven and 45 probable) treated with combo were collected in 20 Hematological Italian Centres, in patients who underwent chemotherapy or allogeneic haematopoietic stem cell transplantation for haematological diseases. Median age of patients was 34 years (range 1-73) and 37% had less than 18 years. Acute leukaemia was the most common underlying haematological disease (68/84; 81%). The phase of treatment was as follows: first induction in 21/84 (25%), consolidation phase in 18/84 (21%) and reinduction/salvage in 45/84 (54%). The main site of infection was lung with or without other sites. The principal fungal pathogens were as follows: Aspergillus sp. 68 cases (81%), Candida sp. six cases (8%), Zygomycetes four cases (5%) and Fusarium sp. four cases (5%). The most used combo was caspofungin+voriconazole 35/84 (42%), caspofungin + liposomal amphotericin B (L-AmB) 20/84 (24%) and L-AmB+voriconazole 15/84 (18%). The median duration of combo was 19 days (range 3-180). The overall response rate (ORR) was 73% (61/84 responders) without significant differences between the combo regimens. The most important factor that significantly influenced the response was granulocyte (PMN) recovery (P 0.009). Only one patient discontinued therapy (voriconazole-related neurotoxicity) and 22% experienced mild and reversible adverse events (hypokalaemia, ALT/AST increase and creatinine increase). The IFDs-attributable mortality was 17%. This study indicates that combo was both well tolerated and effective in haematological patients. The most used combo regimens were caspofungin + voriconazole (ORR 80%) and caspofungin + L-AmB (ORR 70%). The ORR was 73% and the mortality IFD related was 17%. PMN recovery during combo predicts a favourable outcome.

CLINICAL TRIALS REGISTRATION

NCT00906633.

Comparison of real-time florescence quantitative PCR measurements of VAD1 mRNA with three conventional methods in diagnosis and follow-up treatment of Cryptococcus neoformans infection

This study was to develop a real-time florescence quantitative PCR (RT-FQ-PCR) assay to measure virulence-associated DEAD-box RNA helicase (VAD1) mRNA from Cryptococcus neoformans and evaluate its potential use in diagnosis and follow-up treatment of C. neoformans meningitis (CNM). Cryptococcus neoformans was detected using RT-FQ-PCR, ink staining, fungal culturing and C. neoformans antigen detection in CNM compared with a normal control. VAD1 mRNA was measured in both acute and stable CNM patients. The sensitivity of RT-FQ-PCR (96%) is higher than ink staining (72%) and culture culturing (64%) (P<0.05, P<0.05 respectively), but its sensitivity is the same as antigen detection (96%, P>0.05). The levels of VAD1 mRNA in the acute and stable phase of a C. neoformans infection are 3.042±0.906 and 2.187±0.665 respectively (P<0.01). The levels of VAD1 mRNA are correlated to the numbers of C. neoformans, intracranial pressure and glucose concentration in cerebrospinal fluid (CSF; P<0.01, P<0.01 and P<0.05 respectively). The levels of expression of VAD1 mRNA in the group of patients who received an AmB/5-FC/FZC drug regimen decreased more than in patients taking a 5-FC/AmB or 5-FC/FCZ drug combination. Quantitative measurements of VAD1 mRNA are valuable and reliable in diagnosing C. neoformans infection and evaluating a therapy response.

Rhizomucor and scedosporium infection post hematopoietic stem-cell transplant

Hematopoietic stem-cell transplant recipients are at increased risk of developing invasive fungal infections. This is a major cause of morbidity and mortality. We report a case of a 17-year-old male patient diagnosed with severe idiopathic acquired aplastic anemia who developed fungal pneumonitis due to Rhizomucor sp. and rhinoencephalitis due to Scedosporium apiospermum 6 and 8 months after undergoing allogeneic hematopoietic stem-cell transplant from an HLA-matched unrelated donor. Discussion highlights risk factors for invasive fungal infections (i.e., mucormycosis and scedosporiosis), its clinical features, and the factors that must be taken into account to successfully treat them (early diagnosis, correction of predisposing factors, aggressive surgical debridement, and antifungal and adjunctive therapies).

Enhanced activity of antifungal drugs using natural phenolics against yeast strains of Candida and Cryptococcus

AIMS

Determine whether certain, natural phenolic compounds enhance activity of commercial antifungal drugs against yeast strains of Candida and Cryptococcus neoformans.

METHODS AND RESULTS

Twelve natural phenolics were examined for fungicidal activity against nine reference strains of Candida and one of C. neoformans. Six compounds were selected for synergistic enhancement of antifungal drugs, amphotericin B (AMB), fluconazole (FLU) and itraconazole (ITR). Matrix assays of phenolic and drug combinations conducted against one reference strain, each, of Candida albicans and C. neoformans showed cinnamic and benzoic acids, thymol, and 2,3- and 2,5-dihydroxybenzaldehydes (-DBA) had synergistic interactions depending upon drug and yeast strain. 2,5-DBA was synergistic with almost all drug and strain combinations. Thymol was synergistic with all drugs against Ca. albicans and with AMB in C. neoformans. Combinations of benzoic acid or thymol with ITR showed highest synergistic activity. Of 36 combinations of natural product and drug tested, none were antagonistic.

CONCLUSIONS

Relatively nontoxic natural products can synergistically enhance antifungal drug activity, in vitro.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is a proof-of-concept, having clinical implications. Natural chemosensitizing agents could lower dosages needed for effective chemotherapy of invasive mycoses. Further studies against clinical yeast strains and use of animal models are warranted.

Aspergillosis in hematopoietic stem cell transplant recipients: risk factors, prophylaxis, and treatment

This article discusses newer antifungal agents, recent randomized controlled trials, and the 2008 guidelines for treatment of aspergillosis in reference to hematopoietic stem cell transplantation (HSCT). Strategies such as reduced-intensity conditioning and agents such as infliximab shed new light on aspergillosis risk. The association between Toll-like receptor polymorphisms and aspergillosis is an exciting development. Posaconazole was evaluated in two randomized prophylaxis trials, and a large, randomized trial established voriconazole's therapeutic superiority to amphotericin. However, many questions remain regarding which patients benefit most from prophylaxis; resistance to newer antifungals; and combination, salvage, and immunomodulatory therapies. Current therapies and strategies have improved the outlook of HSCT recipients with invasive aspergillosis. Future directions include increasingly sophisticated risk stratification, clinical testing of combination therapies, and adjunctive immunomodulatory therapies.

Lipid formulations of amphotericin B as first-line treatment of zygomycosis

Zygomycosis is a difficult to treat and frequently fatal infection affecting immunocompromised and, rarely, immunocompetent patients. The early diagnosis and immediate initiation of treatment with an antifungal agent in combination with surgical intervention has proved critical for the favourable outcome of the disease. Few antifungal agents are available for treatment. Amphotericin B (AmB) deoxycholate has been the drug of choice for many years and is usually given at high daily doses which can result in renal toxicity. Currently, lipid formulations of AmB (liposomal AmB (L-AmB), AmB lipid complex (ABLC), AmB colloidal dispersion (ABCD)), mainly L-AmB, rather than conventional AmB have become the standard therapy. The rationale behind the use of lipid formulations is that they decrease the nephrotoxicity associated with longterm AmB use. Although there is a developing consensus that high doses of lipid formulations of AmB should be the antifungal therapy of choice for all patients with zygomycosis, until now there have been no data available with which to define the appropriate dose. The duration of therapy remains an unresolved issue, regarding both lipid formulations of AmB as well as sequential or combination treatments consisting of lipid formulations of AmB with posaconazole, a drug which has now emerged as a new therapeutic option.

Efflux-mediated antifungal drug resistance

Fungi cause serious infections in the immunocompromised and debilitated, and the incidence of invasive mycoses has increased significantly over the last 3 decades. Slow diagnosis and the relatively few classes of antifungal drugs result in high attributable mortality for systemic fungal infections. Azole antifungals are commonly used for fungal infections, but azole resistance can be a problem for some patient groups. High-level, clinically significant azole resistance usually involves overexpression of plasma membrane efflux pumps belonging to the ATP-binding cassette (ABC) or the major facilitator superfamily class of transporters. The heterologous expression of efflux pumps in model systems, such Saccharomyces cerevisiae, has enabled the functional analysis of efflux pumps from a variety of fungi. Phylogenetic analysis of the ABC pleiotropic drug resistance family has provided a new view of the evolution of this important class of efflux pumps. There are several ways in which the clinical significance of efflux-mediated antifungal drug resistance can be mitigated. Alternative antifungal drugs, such as the echinocandins, that are not efflux pump substrates provide one option. Potential therapeutic approaches that could overcome azole resistance include targeting efflux pump transcriptional regulators and fungal stress response pathways, blockade of energy supply, and direct inhibition of efflux pumps.

Comparative pharmacodynamic interaction analysis of triple combinations of caspofungin and voriconazole or ravuconazole with subinhibitory concentrations of amphotericin B against Aspergillus spp

Triple combination therapy with an antifungal triazole, echinocandin and amphotericin B (AmB) is used in some centres to treat refractory aspergillosis. The objective of this study was to investigate the effect of subinhibitory concentrations of AmB on the double combinations of caspofungin (CAS) + voriconazole (VOR) or ravuconazole (RAV) against Aspergillus fumigatus, Aspergillus flavus and Aspergillus terreus. Isolates were studied in triplicate against CAS/VOR and CAS/RAV combinations by chequerboard broth microdilution. AmB was added to each double combination at concentrations of 0, 0.1 and 0.2 microg ml(-1). The fractional inhibitory concentration (FIC) index was calculated for the double and triple combinations. Comparative analysis was performed by repeated measures analysis followed by Dunnett's post-test. The double combinations of CAS/RAV and CAS/VOR were synergistic or additive in most conditions. Addition of AmB to the double combinations resulted in increased FIC indices for A. fumigatus and A. flavus. By contrast, AmB increased the synergism of the double combinations decreasing FIC indices for A. terreus (P < 0.05). RAV and VOR displayed similar synergistic activity with CAS. The addition of sub-inhibitory amphotericin B concentrations reduced but did not eliminate the synergistic interaction between the echinocandin and triazole against A. fumigatus and A. flavus, while it increased the synergy against A. terreus.

The evolution of fungal drug resistance: modulating the trajectory from genotype to phenotype

The emergence of drug resistance in pathogenic microorganisms provides an excellent example of microbial evolution that has had profound consequences for human health. The widespread use of antimicrobial agents in medicine and agriculture exerts strong selection for the evolution of drug resistance. Selection acts on the phenotypic consequences of resistance mutations, which are influenced by the genetic variation in particular genomes. Recent studies have revealed a mechanism by which the molecular chaperone heat shock protein 90 (Hsp90) can alter the relationship between genotype and phenotype in an environmentally contingent manner, thereby 'sculpting' the course of evolution. Harnessing Hsp90 holds great promise for treating life-threatening infectious diseases.

Synergistic Antifungal Activities of Thymol Analogues with Propolis

In this study, an attempt was made to elucidate the combined effect of thymol analogues with propolis on fungi that are involved in human diseases. Antifungal activity of thymol analogues was examined by the agar dilution method, using antibiotic medium-3-enriched with 0.2% dextrose. The activity of thymol analogues against Penicillium rastrickii appeared to be more marked than against Candida albicans and Saccharomyces cerevisiae. Various thymol analogues and propolis, when tested alone, did not exhibit significant inhibitory activity. However, in the presence of sub-inhibitory concentrations of propolis, the relative antifungal potency of the thymol analogues was greatly enhanced. Natural thymol was found to be superior to the synthetic thymol analogues. In particular, the MIC of thymol was considerably reduced (191-fold) against Candida albicans by combination with propolis. This study highlights the potential of the combination of either thymol or synthetic thymol analogues with propolis to inhibit fungi.