Are In Vitro Susceptibilities to Azole Antifungals Predictive of Clinical Outcome in the Treatment of Candidemia?

Alarming Increase of Azole-Resistant Candida Causing Blood Stream Infections in Oncology Patients in Egypt

Candidemia is a life-threatening invasive fungal infection in immunocompromised patients. The widespread use of azoles and the shift toward non-albicans Candida (NAC) species remarkably increase azole resistance in developing countries. We aimed to study candidemia trends and associated risk factors in oncology patients since they vary geographically, and rapid and appropriate treatment improves outcomes. Vitek 2 was used to identify the Candida species, and the E-test determined their susceptibility to azoles. Candida was the cause of 3.1% (n = 53/1701) of bloodstream infections (BSIs) during a 1-year study. Candida tropicalis was the most predominant species among the 30 candidemia episodes studied (36.7%), followed by C. albicans (33.3%). However, C. krusei, C. guilliermondii, C. pelliculosa, C. parapsilosis, C. famata, and C. inconspicua accounted for 30.0% of the isolates. An increased risk of NAC BSI was significantly associated with chemotherapy and leucopenia (P = 0.036 and 0.016, respectively). However, the multivariable analysis revealed that leucopenia was the only independent risk factor (P = 0.048). Fluconazole and voriconazole resistance were 58.3% and 16.7%, with NAC species showing higher resistance rates than C. albicans. Both fluconazole and voriconazole minimum inhibitory concentration (MIC) median values were higher in NAC than in C. albicans, but only voriconazole was significantly higher (0.220 versus 0.048 μg/ml, P = 0.047). In conclusion, the increased prevalence of NAC BSIs and incredibly high fluconazole resistance rates in cancer patients emphasize the necessity of antifungal stewardship to preserve voriconazole effectiveness, continued surveillance of candidemia, and future studies into azole resistance molecular mechanisms.

Pharmacokinetic/Pharmacodynamic Target Attainment of Different Antifungal Agents in De-escalation Treatment in Critically Ill Patients: a Step toward Dose Optimization Using Monte Carlo Simulation

Differences in pharmacokinetics/pharmacodynamics (PK/PD) target attainment are rarely considered when antifungals are switched in critically ill patients. This study intends to explore whether the antifungal de-escalation treatment strategy and the new intermittent dosing strategy of echinocandins in critically ill patients are able to achieve the corresponding PK/PD targets. The published population PK models of antifungals in critically ill patients and a public data set from the MIMIC-III database (n = 662) were employed to evaluate PK/PD target attainment of different dosing regimens of antifungals. Cumulative fraction of response (CFR) was calculated for each dosing regimen. Most guideline-recommended dosing regimens of fluconazole and voriconazole could achieve target exposure as de-escalation treatment in critically ill patients. For initial echinocandin treatment, achievement of the target exposure decreased as body weight increased, and the intermittent dosing strategy had a slightly higher CFR value in most simulations compared to conventional dosing strategy. For Candida albicans and Candida glabrata infection, caspofungin at the lowest dose achieved a CFR of >90%, while micafungin or anidulafungin required almost the highest doses simulated in this study to achieve the same effect. None of the echinocandins other than 150 mg every 24 h (q24h) or 200 mg q48h of caspofungin achieved the target CFR for Candida parapsilosis infection. These findings support the guideline-recommended dose of triazoles for antifungal de-escalation treatment and confirm the insufficient dosage of echinocandins in critically ill patients, indicating that a dosing regimen based on body weight or intermittent dosing of echinocandins may be required.

Antifungal Susceptibility Testing: A Primer for Clinicians

Clinicians treating patients with fungal infections may turn to susceptibility testing to obtain information regarding the activity of different antifungals against a specific fungus that has been cultured. These results may then be used to make decisions regarding a patient's therapy. However, for many fungal species that are capable of causing invasive infections, clinical breakpoints have not been established. Thus, interpretations of susceptible or resistant cannot be provided by clinical laboratories, and this is especially true for many molds capable of causing severe mycoses. The purpose of this review is to provide an overview of susceptibility testing for clinicians, including the methods used to perform these assays, their limitations, how clinical breakpoints are established, and how the results may be put into context in the absence of interpretive criteria. Examples of when susceptibility testing is not warranted are also provided.

Antifungal Susceptibility Testing and Identification

The requirement for antifungal susceptibility testing is increasing given the availability of new drugs, increasing populations of individuals at risk for fungal infection, and emerging multiresistant fungi. Rapid and accurate fungal identification remains at the forefront of laboratory efforts to guide empiric therapy. Antifungal susceptibility testing methods have greatly improved, but are subject to variation in results between methods. Careful standardization, validation, and extensive training of users is essential to ensure susceptibility results are clinically useful and interpreted appropriately. Interpretive criteria for many drugs and species are still lacking, but this will continue to evolve.

Functional information from clinically-derived drug resistant forms of the Candida glabrata Pdr1 transcription factor

Azole drugs are the most frequently used antifungal agents. The pathogenic yeast Candida glabrata acquires resistance to azole drugs via single amino acid substitution mutations eliciting a gain-of-function (GOF) hyperactive phenotype in the Pdr1 transcription factor. These GOF mutants constitutively drive high transcription of target genes such as the ATP-binding cassette transporter-encoding CDR1 locus. Previous characterization of Pdr1 has demonstrated that this factor is negatively controlled by the action of a central regulatory domain (CRD) of ~700 amino acids, in which GOF mutations are often found. Our earlier experiments demonstrated that a Pdr1 derivative in which the CRD was deleted gave rise to a transcriptional regulator that could not be maintained as the sole copy of PDR1 in the cell owing to its toxically high activity. Using a set of GOF PDR1 alleles from azole-resistant clinical isolates, we have analyzed the mechanisms acting to repress Pdr1 transcriptional activity. Our data support the view that Pdr1-dependent transactivation is mediated by a complex network of transcriptional coactivators interacting with the extreme C-terminal part of Pdr1. These coactivators include but are not limited to the Mediator component Med15A. Activity of this C-terminal domain is controlled by the CRD and requires multiple regions across the C-terminus for normal function. We also provide genetic evidence for an element within the transactivation domain that mediates the interaction of Pdr1 with coactivators on one hand while restricting Pdr1 activity on the other hand. These data indicate that GOF mutations in PDR1 block nonidentical negative inputs that would otherwise restrain Pdr1 transcriptional activation. The strong C-terminal transactivation domain of Pdr1 uses multiple different protein regions to recruit coactivators.

Resistance to antibiotics is a major threat to the continued use of these lifesaving chemotherapeutic drugs. This problem is especially acute in the case of antifungal drugs as only 3 classes of these compounds exist. The pathogenic yeast Candida glabrata acquires resistance to the azole class of antifungal drugs by developing hyperactive alleles of the PDR1 gene, encoding a major inducer of azole resistance. We provide evidence that these hyperactive mutant proteins identify different negative inputs that would otherwise repress the transcriptional activity of Pdr1. Mutational analysis of the extreme C-terminus of Pdr1 indicated that this region exhibited multiple different interactions with coactivator proteins required for normal transcriptional activation of target gene expression. The data reported here shed light on the complicated nature of regulation of Pdr1 activity and identify domains in this protein that are bifunctional in their role to ensure normal factor activity. A detailed understanding of the molecular control of Pdr1 will allow strategies to be devised to reverse the azole resistance triggered by mutant forms of this protein.

Antifungal susceptibility testing in Candida species: current methods and promising new tools for shortening the turnaround time

INTRODUCTION

Invasive fungal diseases (IFDs) have received attention as an emerging public health threat, are difficult to diagnose and to treat, and are associated with substantial morbidity and mortality. The standard of care in IFD management requires an early and targeted antifungal treatment, hence covers - amongst others - species identification and antifungal susceptibility testing (AFST).

AREAS COVERED

This review gives an overview of methods currently applied in AFST and highlights promising new tools for shortening the turnaround time focusing on Candida species.

EXPERT OPINION

The performance of the broth microdilution reference methods for AFST is not suitable for daily laboratory practice as they are too labor-intensive and time-consuming. Other conventional approaches such as disk diffusion assays, epsilometer tests, colorimetric or automated approaches are easier in handling, and in part, show good correlations with the reference methods. Promising results for shortening the turnaround time in providing MIC data or resistance detection include matrix-assisted laser desorption/ionization-time of flight mass spectrometer (MALDI-TOF MS) assisted AFST, molecular-based techniques and modified conventional approaches applying direct inoculation methods. These underlying AFST concepts are promising but in part completely different, have their own advantages and disadvantages, and need further clinical validation.

The antifungal pipeline: the need is established. Are there new compounds?

Our review summarizes and compares the temporal development (eras) of antifungal drug discovery as well as antibacterial ventures. The innovation gap that occurred in antibacterial discovery from 1960 to 2000 was likely due to tailoring of existing compounds to have better activity than predecessors. Antifungal discovery also faced innovation gaps. The semi-synthetic antibiotic era was followed closely by the resistance era and the heightened need for new compounds and targets. With the immense contribution of comparative genomics, antifungal targets became part of the discovery focus. These targets by definition are absolutely required to be fungal- or even lineage (clade) specific. Importantly, targets need to be essential for growth and/or have important roles in disease and pathogenesis. Two types of antifungals are discussed that are mostly in the FDA phase I–III clinical trials. New antifungals are either modified to increase bioavailability and stability for instance, or are new compounds that inhibit new targets. One of the important developments in incentivizing new antifungal discovery has been the prolific number of publications of global and country-specific incidence. International efforts that champion global antimicrobial drug discovery are discussed. Still, interventions are needed. The current pipeline of antifungals and alternatives to antifungals are discussed including vaccines.

MIC distributions for amphotericin B, fluconazole, itraconazole, voriconazole, flucytosine and anidulafungin and 35 uncommon pathogenic yeast species from the UK determined using the CLSI broth microdilution method

Background

Epidemiological cut-off values and clinical interpretive breakpoints have been developed for a number of antifungal agents with the most common Candida species that account for the majority of infections due to pathogenic yeasts species. However, less-common species, for which susceptibility data are limited, are increasingly reported in high-risk patients and breakthrough infections.

Methods

The UK National Mycology Reference Laboratory performs routine antifungal susceptibility testing of clinical yeast isolates submitted from across the UK. Between 2002 and 2016, >32 000 isolates representing 94 different yeast species were referred to the laboratory. Here we present antifungal susceptibility profiles generated over this period for amphotericin B, fluconazole, voriconazole, itraconazole, anidulafungin and flucytosine against 35 species of uncommon yeast using CLSI methodologies. MIC data were interpreted against epidemiological cut-off values and clinical breakpoints developed with Candida albicans, in order to identify species with unusually skewed MIC distributions that potentially indicate resistance.

Results

Potential resistance to at least one antifungal agent (>10% of isolates with MICs greater than the epidemiological cut-off or clinical breakpoint) was evidenced for 29/35 species examined here. Four species exhibited elevated MICs with all of the triazole antifungal drugs against which they were tested, and 21 species exhibited antifungal resistance to agents from at least two different classes of antifungal agent.

Conclusions

This study highlights a number of yeast species with unusual MIC distributions and provides data to aid clinicians in deciding which antifungal regimens may be appropriate when confronted with infections with rarer yeasts.

Fluconazole Resistance in Isolates of Uncommon Pathogenic Yeast Species from the United Kingdom

The triazole drug fluconazole remains one of the most commonly prescribed antifungal drugs, both for prophylaxis in high-risk patients and also as a second-line treatment option for invasive Candida infections. Established susceptibility profiles and clinical interpretive breakpoints are available for fluconazole with Candida albicans, Candida glabrata, Candida tropicalis, and Candida parapsilosis, which account for the majority of infections due to pathogenic yeast species. However, less common species for which only limited susceptibility data are available are increasingly reported in high-risk patients and from breakthrough infections. The UK National Mycology Reference Laboratory performs routine antifungal susceptibility testing of clinical isolates of pathogenic yeast submitted from across the United Kingdom. Between 2002 and 2016, ∼32,000 isolates were referred, encompassing 94 different yeast species. Here, we present fluconazole antifungal susceptibility data generated using a CLSI methodology over this 15-year period for 82 species (2,004 isolates) of less common yeast and yeast-like fungi, and amphotericin B, fluconazole, itraconazole, voriconazole, posaconazole, and anidulafungin, with members of the Nakaseomyces clade (C. glabrata, Candida nivariensis, and Candida bracarensis). At least 22 different teleomorph genera, comprising 45 species, exhibited high MICs when tested with fluconazole (>20% of isolates with MICs higher than the clinical breakpoint [≥8 mg/liter] proposed for C. albicans). Since several of these species have been reported anecdotally from breakthrough infections and therapeutic failures in patients receiving fluconazole, the current study underscores the importance of rapid and accurate yeast identification and may aid clinicians dealing with infections with rarer yeasts to decide whether fluconazole would be appropriate.