Invasive Aspergillosis and the Impact of Azole-resistance

Purpose of Review

IA (invasive aspergillosis) caused by azole-resistant strains has been associated with higher clinical burden and mortality rates. We review the current epidemiology, diagnostic, and therapeutic strategies of this clinical entity, with a special focus on patients with hematologic malignancies.

Recent Findings

There is an increase of azole resistance in Aspergillus spp. worldwide, probably due to environmental pressure and the increase of long-term azole prophylaxis and treatment in immunocompromised patients (e.g., in hematopoietic stem cell transplant recipients). The therapeutic approaches are challenging, due to multidrug-resistant strains, drug interactions, side effects, and patient-related conditions.

Summary

Rapid recognition of resistant Aspergillus spp. strains is fundamental to initiate an appropriate antifungal regimen, above all for allogeneic hematopoietic cell transplantation recipients. Clearly, more studies are needed in order to better understand the resistance mechanisms and optimize the diagnostic methods to identify Aspergillus spp. resistance to the existing antifungal agents/classes. More data on the susceptibility profile of Aspergillus spp. against the new classes of antifungal agents may allow for better treatment options and improved clinical outcomes in the coming years. In the meantime, continuous surveillance studies to monitor the prevalence of environmental and patient prevalence of azole resistance among Aspergillus spp. is absolutely crucial.

Proteomic Perspective of Azole Resistance in Aspergillus fumigatus Biofilm Extracellular Matrix in Response to Itraconazole

Azole-resistant Aspergillus fumigatus makes a major challenge to the chemotherapy for invasive aspergillosis, whereas cyp51A gene mutation is the most dominant mechanism for azole resistance. Moreover, biofilm contributes to drug resistance for A. fumigatus, and extracellular matrix (ECM) is essential to protect live cells from antifungal drugs. Therefore, we performed a comparative proteomic study on the biofilm ECM of both the wild-type and azole-resistant strains of A. fumigatus under azole pressure. In total, 2377 proteins were identified, of which 480 and 604 proteins with differential expression were obtained from the wild-type and azole-resistant A. fumigatus in exposure to itraconazole respectively (fold change > 2 or < 0.5, P-value < 0.05). We found that a high proportion of regulated proteins were located in cytoplasm, nucleus, and mitochondria. Meanwhile, GO and KEGG analyses revealed that metabolic process and ribosome pathway were significantly enriched. Particularly, differentially expressed proteins in response to azole pressure of both the wild-type and resistant strains were further analyzed. Our results indicated that these changes in biofilm ECM proteins were related to ergosterol synthesis, oxidative stress, efflux pumps, DNA repair, DNA replication, and transcription.

Aspergillus fumigatus and aspergillosis: From basics to clinics

The airborne fungus Aspergillus fumigatus poses a serious health threat to humans by causing numerous invasive infections and a notable mortality in humans, especially in immunocompromised patients. Mould-active azoles are the frontline therapeutics employed to treat aspergillosis. The global emergence of azole-resistant A. fumigatus isolates in clinic and environment, however, notoriously limits the therapeutic options of mould-active antifungals and potentially can be attributed to a mortality rate reaching up to 100 %. Although specific mutations in CYP 51A are the main cause of azole resistance, there is a new wave of azole-resistant isolates with wild-type CYP 51A genotype challenging the efficacy of the current diagnostic tools. Therefore, applications of whole-genome sequencing are increasingly gaining popularity to overcome such challenges. Prominent echinocandin tolerance, as well as liver and kidney toxicity posed by amphotericin B, necessitate a continuous quest for novel antifungal drugs to combat emerging azole-resistant A. fumigatus isolates. Animal models and the tools used for genetic engineering require further refinement to facilitate a better understanding about the resistance mechanisms, virulence, and immune reactions orchestrated against A. fumigatus. This review paper comprehensively discusses the current clinical challenges caused by A. fumigatus and provides insights on how to address them.

Azole-triphenylphosphonium conjugates combat antifungal resistance and alleviate the development of drug-resistance

Azole antifungals are commonly used to treat fungal infections but have resulted in the occurrence of drug resistance. Therefore, developing azole derivatives (AZDs) that can both combat established drug-resistant fungal strains and evade drug resistance is of great importance. In this study, we synthesized a series of AZDs with a fluconazole (FLC) skeleton conjugated with a mitochondria-targeting triphenylphosphonium cation (TPP⁺). These AZDs displayed potent activity against both azole-sensitive and azole-resistant Candida strains without eliciting obvious resistance. Moreover, two representative AZDs, 20 and 25, exerted synergistic antifungal activity with Hsp90 inhibitors against C. albicans strains resistant to the combination treatment of FLC and Hsp90 inhibitors. AZD 25, which had minimal cytotoxicity, was effective in preventing C. albicans biofilm formation. Mechanistic investigation revealed that AZD 25 inhibited the biosynthesis of the fungal membrane component ergosterol and interfered with mitochondrial function. Our findings provide an alternative approach to address fungal resistance problems.

How to: EUCAST recommendations on the screening procedure E.Def 10.1 for the detection of azole resistance in Aspergillus fumigatus isolates using four-well azole-containing agar plates

BACKGROUND

The emergence of azole-resistant Aspergillus fumigatus isolates is a matter of significant concern in Europe, with countries reporting resistance rates (which can be as high as 30%) in hospitalized patients. Consequently, the treatment guidelines in The Netherlands, the country with the highest documented prevalence of azole-resistant A. fumigatus, has just been revised to now recommend initial therapy with combination therapy until the susceptibility pattern is known. Therefore, susceptibility testing of clinically relevant isolates has been strongly recommended in the ESCMID-EFISG aspergillosis guidelines. Furthermore, mixed azole-susceptible and azole-resistant (isogenic as well as non-isogenic) infections have been reported to occur, which implies that colonies of clinical cultures may harbour various phenotypes of azole susceptibility.

OBJECTIVES

The EUCAST-AFST (European Committee on Antimicrobial Susceptibility Testing Subcommittee on Antifungal Susceptibility Testing) has released a new screening method document (E.Def 10.1) for the detection of azole-resistant A. fumigatus isolates and updated the QC tables for antifungal susceptibility testing with associated QC endpoints. This review described in detail how to perform the screening test.

SOURCES

This "How to document" is based on the EUCAST azole agar screening method document E.Def 10.1 and the QC tables for antifungal susceptibility testing document, v 2.0 (available at http://www.eucast.org/ast_of_fungi/qcafsttables/) CONTENTS: The method is based on the inoculation of azole-containing and azole-free agars and visual determination of fungal growth after one and two days of incubation. It can easily be implemented in routine laboratories of clinical microbiology and has been validated for simultaneous testing of up to five A. fumigatus colonies using itraconazole and voriconazole (mandatory), and posaconazole (optional).

IMPLICATIONS

This easy-to-use screening procedure for the detection of azole resistance in clinical A. fumigatus isolates will allow rapid testing in the daily routine of the microbiology laboratory and thus facilitate earlier appropriate therapy.

In-host microevolution of Aspergillus fumigatus: A phenotypic and genotypic analysis

In order to survive, Aspergillus fumigatus must adapt to specific niche environments. Adaptation to the human host includes modifications facilitating persistent colonisation and the development of azole resistance. The aim of this study is to advance understanding of the genetic and physiological adaptation of A. fumigatus in patients during infection and treatment. Thirteen A. fumigatus strains were isolated from a single chronic granulomatous disease patient suffering from persistent and recurrent invasive aspergillosis over a period of 2 years. All strains had identical microsatellite genotypes and were considered isogenic. Whole genome comparisons identified 248 non-synonymous single nucleotide polymorphisms. These non-synonymous mutations have potential to play a role in in-host adaptation. The first 2 strains isolated were azole susceptible, whereas later isolates were itraconazole, voriconazole and/or posaconazole resistant. Growth assays in the presence and absence of various antifungal stressors highlighted minor changes in growth rate and stress resistance, with exception of one isolate showing a significant growth defect. Poor conidiation was observed in later isolates. In certain drug resistant isolates conidiation was restored in the presence of itraconazole. Differences in virulence were observed as demonstrated in a Galleria mellonella infection model. We conclude that the microevolution of A. fumigatus in this patient has driven the emergence of both Cyp51A-independent and Cyp51A-dependent, azole resistance mechanisms, and additional phenotypes that are likely to have promoted fungal persistence.

The role of azoles in the management of azole-resistant aspergillosis: from the bench to the bedside

Azole resistance is an emerging problem in Aspergillus fumigatus and is associated with a high probability of treatment failure. An azole resistance mechanism typically decreases the activity of multiple azole compounds, depending on the mutation. As alternative treatment options are limited and in some isolates the minimum inhibitory concentration (MIC) increases by only a few two-fold dilutions steps, we investigated if voriconazole and posaconazole have a role in treating azole-resistant Aspergillus disease. The relation between resistance genotype and phenotype, pharmacokinetic and pharmacodynamic properties, and (pre)clinical treatment efficacy were reviewed. The results were used to estimate the exposure needed to achieve the pharmacodynamic target for each MIC. For posaconazole adequate exposure can be achieved only for wild type isolates as dose escalation does not allow PD target attainment. However, the new intravenous formulation might result in sufficient exposure to treat isolates with a MIC of 0.5 mg/L. For voriconazole our analysis indicated that the exposure needed to treat infection due to isolates with a MIC of 2 mg/L is feasible and maybe isolates with a MIC of 4 mg/L. However, extreme caution and strict monitoring of drug levels would be required, as the probability of toxicity will also increase.