Derivation of an in vivo drug exposure breakpoint for flucytosine against Candida albicans and Impact of the MIC, growth rate, and resistance genotype on the antifungal effect

A ketogenic diet enhances fluconazole efficacy in murine models of systemic fungal infection

Invasive fungal infections are a significant public health concern, with mortality rates ranging from 20% to 85% despite current treatments. Therefore, we examined whether a ketogenic diet could serve as a successful treatment intervention in murine models of Cryptococcus neoformans and Candida albicans infection in combination with fluconazole-a low-cost, readily available antifungal therapy. The ketogenic diet is a high-fat, low-carbohydrate diet that promotes fatty acid oxidation as an alternative to glycolysis through the production of ketone bodies. In this series of experiments, mice fed a ketogenic diet prior to infection with C. neoformans and treated with fluconazole had a significant decrease in fungal burden in both the brain (mean 2.66 ± 0.289 log10 reduction) and lung (mean 1.72 ± 0.399 log10 reduction) compared to fluconazole treatment on a conventional diet. During C. albicans infection, kidney fungal burden of mice in the keto-fluconazole combination group was significantly decreased compared to fluconazole alone (2.37 ± 0.770 log10-reduction). Along with higher concentrations of fluconazole in the plasma and brain tissue, fluconazole efficacy was maximized at a significantly lower concentration on a keto diet compared to a conventional diet, indicating a dramatic effect on fluconazole pharmacodynamics. Our findings indicate that a ketogenic diet potentiates the effect of fluconazole at multiple body sites during both C. neoformans and C. albicans infection and could have practical and promising treatment implications.IMPORTANCEInvasive fungal infections cause over 2.5 million deaths per year around the world. Treatments for fungal infections are limited, and there is a significant need to develop strategies to enhance antifungal efficacy, combat antifungal resistance, and mitigate treatment side effects. We determined that a high-fat, low-carbohydrate ketogenic diet significantly potentiated the therapeutic effect of fluconazole, which resulted in a substantial decrease in tissue fungal burden of both C. neoformans and C. albicans in experimental animal models. We believe this work is the first of its kind to demonstrate that diet can dramatically influence the treatment of fungal infections. These results highlight a novel strategy of antifungal drug enhancement and emphasize the need for future investigation into dietary effects on antifungal drug activity.

Therapeutic drug monitoring of antifungal therapies: do we really need it and what are the best practices?

INTRODUCTION

Despite advancements, invasive fungal infections (IFI) still carry high mortality rates, often exceeding 30%. The challenges in diagnosis, coupled with limited effective antifungal options, make managing IFIs complex. Antifungal drugs are essential for IFI management, but their efficacy can be diminished by drug-drug interactions and pharmacokinetic variability. Therapeutic Drug Monitoring (TDM), especially in the context of triazole use, has emerged as a valuable strategy to optimize antifungal therapy.

AREAS COVERED

This review provides current evidence regarding the potential benefits of TDM in IFI management. It discusses how TDM can enhance treatment response, safety, and address altered pharmacokinetics in specific patient populations.

EXPERT OPINION

TDM plays a crucial role in achieving optimal therapeutic outcomes in IFI management, particularly for certain antifungal agents. Preclinical studies consistently show a link between therapeutic drug levels and antifungal efficacy. However, clinical research in mycology faces challenges due to patient heterogeneity and the diversity of fungal infections. TDM's potential advantages in guiding Echinocandin therapy for critically ill patients warrant further investigation. Additionally, for drugs like Posaconazole, assessing whether serum levels or alternative markers like saliva offer the best measure of efficacy is an intriguing question.

5-Flucytosine Longitudinal Antifungal Susceptibility Testing of Cryptococcus neoformans: A Substudy of the EnACT Trial Testing Oral Amphotericin

Background

The EnACT trial was a phase 2 randomized clinical trial conducted in Uganda, which evaluated a novel orally delivered lipid nanocrystal (LNC) amphotericin B in combination with flucytosine for the treatment of cryptococcal meningitis. When flucytosine (5FC) is used as monotherapy in cryptococcosis, 5FC can induce resistant Cryptococcus mutants. Oral amphotericin B uses a novel drug delivery mechanism, and we assessed whether resistance to 5FC develops during oral LNC-amphotericin B therapy.

Methods

We enrolled Ugandans with HIV diagnosed with cryptococcal meningitis and who were randomized to receive 5FC and either standard intravenous (IV) amphotericin B or oral LNC-amphotericin B. We used broth microdilution to measure the minimum inhibitory concentration (MIC) of the first and last cryptococcal isolates in each participant. Breakpoints are inferred from 5FC in Candida albicans. We measured cerebral spinal fluid (CSF) 5FC concentrations by liquid chromatography and tandem mass spectrometry.

Results

Cryptococcus 5FC MIC50 was 4 µg/mL, and MIC90 was 8 µg/mL. After 2 weeks of therapy, there was no evidence of 5FC resistance developing, defined as a >4-fold change in susceptibility in any Cryptococcus isolate tested. The median CSF 5FC concentration to MIC ratio (interquartile range) was 3.0 (1.7-5.5) µg/mL. There was no association between 5FC/MIC ratio and early fungicidal activity of the quantitative rate of CSF yeast clearance (R2 = 0.004; P = .63).

Conclusions

There is no evidence of baseline resistance to 5FC or incident resistance during combination therapy with oral or IV amphotericin B in Uganda. Oral amphotericin B can safely be used in combination with 5FC.

Population pharmacokinetics and CSF penetration of flucytosine in adults with HIV-associated cryptococcal meningoencephalitis

BACKGROUND

There are limited data describing clinical flucytosine pharmacokinetics (PK). The variability of flucytosine partitioning into the CNS is not known. We described the interindividual variability in flucytosine PK in patients with HIV-associated cryptococcal meningoencephalitis. In addition, we quantified the extent and variability of CSF partitioning of flucytosine.

METHODS

A PK study was conducted in 64 patients with confirmed HIV-associated cryptococcal meningoencephalitis in Blantyre, Malawi. A four-compartment PK model was developed, and Monte Carlo simulations were performed with flucytosine administered at different doses and in different schedules.

RESULTS

The estimated mean apparent volume of the central compartment was 17.50 (SD 9.99) L; mean apparent clearance was 5.88 (SD 3.35) L/h; mean apparent volume of the CNS compartment was 41.73 (SD 13.66) L. From the Bayesian posterior estimates, AUC24 values at steady state (144-168 h) with doses of 25 mg/kg q6h were median (IQR) 890.38 (603.81-1213.70) mg.h/L in plasma and 595.66 (425.69-776.64) mg.h/L in CSF. The ratio of CSF:plasma AUC24 was 0.69 (IQR 0.58-0.82).

CONCLUSIONS

This study revealed significant interindividual variability in flucytosine PK in plasma and CSF in patients with HIV-associated cryptococcal meningoencephalitis. The population PK model is a first critical step for revised flucytosine regimens that maximize fungal killing and minimize toxicity and the emergence of resistance.

Osteoarticular Mycoses

Osteoarticular mycoses are chronic debilitating infections that require extended courses of antifungal therapy and may warrant expert surgical intervention. As there has been no comprehensive review of these diseases, the International Consortium for Osteoarticular Mycoses prepared a definitive treatise for this important class of infections. Among the etiologies of osteoarticular mycoses are Candida spp., Aspergillus spp., Mucorales, dematiaceous fungi, non-Aspergillus hyaline molds, and endemic mycoses, including those caused by Histoplasma capsulatum, Blastomyces dermatitidis, and Coccidioides species. This review analyzes the history, epidemiology, pathogenesis, clinical manifestations, diagnostic approaches, inflammatory biomarkers, diagnostic imaging modalities, treatments, and outcomes of osteomyelitis and septic arthritis caused by these organisms. Candida osteomyelitis and Candida arthritis are associated with greater events of hematogenous dissemination than those of most other osteoarticular mycoses. Traumatic inoculation is more commonly associated with osteoarticular mycoses caused by Aspergillus and non-Aspergillus molds. Synovial fluid cultures are highly sensitive in the detection of Candida and Aspergillus arthritis. Relapsed infection, particularly in Candida arthritis, may develop in relation to an inadequate duration of therapy. Overall mortality reflects survival from disseminated infection and underlying host factors.

Therapeutic Drug Monitoring of Antifungal Agents in Critically Ill Patients: Is There a Need for Dose Optimisation?

Invasive fungal infections are an important cause of morbidity and mortality, especially in critically ill patients. Increasing resistance rates and inadequate antifungal exposure have been documented in these patients, due to clinically relevant pharmacokinetic (PK) and pharmacodynamic (PD) alterations, leading to treatment failure. Physiological changes such as third spacing (movement of fluid from the intravascular compartment to the interstitial space), hypoalbuminemia, renal failure and hepatic failure, as well as common interventions in the intensive care unit, such as renal replacement therapy and extracorporeal membrane oxygenation, can lead to these PK and PD alterations. Consequently, a therapeutic target concentration that may be useful for one patient may not be appropriate for another. Regular doses do not take into account the important PK variations in the critically ill, and the need to select an effective dose while minimising toxicity advocates for the use of therapeutic drug monitoring (TDM). This review aims to describe the current evidence regarding optimal PK/PD indices associated with the clinical efficacy of the most commonly used antifungal agents in critically ill patients (azoles, echinocandins, lipid complexes of amphotericin B, and flucytosine), provide a comprehensive understanding of the factors affecting the PK of each agent, document the PK parameters of critically ill patients compared to healthy volunteers, and, finally, make recommendations for therapeutic drug monitoring (TDM) of antifungals in critically ill patients.

Cocrystallization-driven self-assembly with vanillic acid offers a new opportunity for surmounting fast and excessive absorption issues of antifungal drug 5-fluorocytosine: a combined theoretical and experimental research

The cocrystal of 5-fluorocytosine (FCY) with vanillic acid (VAA) was assembled via a cocrystallization technique, giving a novel understanding for conquering the dose-limited hepatotoxicity caused by the rapid and almost complete absorption of FCY.

Drug Resistance and Novel Therapeutic Approaches in Invasive Candidiasis

Candida species are the leading cause of invasive fungal infections worldwide and are associated with acute mortality rates of ~50%. Mortality rates are further augmented in the context of host immunosuppression and infection with drug-resistant Candida species. In this review, we outline antifungal drugs already in clinical use for invasive candidiasis and candidaemia, their targets and mechanisms of resistance in clinically relevant Candida species, encompassing not only classical resistance, but also heteroresistance and tolerance. We describe novel antifungal agents and targets in pre-clinical and clinical development, including their spectrum of activity, antifungal target, clinical trial data and potential in treatment of drug-resistant Candida. Lastly, we discuss the use of combination therapy between conventional and repurposed agents as a potential strategy to combat the threat of emerging resistance in Candida.

Potent antifungal agents and use of nanocarriers to improve delivery to the infected site: A systematic review

There are four major classes of antifungals with the predominant mechanism of action being targeting of cell wall or cell membrane. As in other drugs, low solubility of these compounds has led to low bioavailability in target tissues. Enhanced drug dosages have effects such as toxicity, drug-drug interactions, and increased drug resistance by fungi. This article reviews the current state-of-the-art of antifungals, structure, mechanism of action, other usages, and toxic side effects. The emergence of nanoformulations to transport and uniformly release cargo at the target site is a boon in antifungal treatment. The article details research that lead to the development of nanoformulations of antifungals and potential advantages and avoidance of the lacunae characterizing conventional drugs. A range of nanoformulations based on liposomes, polymers are in various stages of research and their potential advantages have been brought out. It could be observed that under similar dosages, test models, and duration, nanoformulations provided enhanced activity, reduced toxicity, higher uptake and higher immunostimulatory effects. In most instances, the mechanism of antifungal activity of nanoformulations was similar to that of regular antifungal. There are possibilities of coupling multiple antifungals on the same nano-platform. Increased activity coupled with multiple mechanisms of action presents for nanoformulations a tremendous opportunity to overcome antifungal resistance. In the years to come, robust methods for the preparation of nanoformulations taking into account the repeatability and reproducibility in action, furthering the studies on nanoformulation toxicity and studies of human models are required before extensive use of nanoformulations as a prescribed drug.

Managing uncertainty in antifungal dosing: antibiograms, therapeutic drug monitoring and drug-drug interactions

PURPOSE OF REVIEW

A number of pharmacokinetic and pharmacodynamic factors in critically ill or severely immunosuppressed patients influence the effectiveness of antifungal therapy making dosing less certain. Recent position papers from infectious diseases societies and working groups have proposed methods for dosage individualization of antibiotics in critically ill patients using a combination of population pharmacokinetic models, Monte-Carlo simulation and therapeutic drug monitoring (TDM) to guide dosing. In this review, we examine the current limitations and practical issues of adapting a pharmacometrics-guided dosing approaches to dosing of antifungals in critically ill or severely immunosuppressed populations.

RECENT FINDINGS

We review the current status of antifungal susceptibility testing and challenges in incorporating TDM into Bayesian dose prediction models. We also discuss issues facing pharmacometrics dosage adjustment of newer targeted chemotherapies that exhibit severe pharmacokinetic drug-drug interactions with triazole antifungals.

SUMMARY

Although knowledge of antifungal pharmacokinetic/pharmacodynamic is maturing, the practical application of these concepts towards point-of-care dosage individualization is still limited. User-friendly pharmacometric models are needed to improve the utility of TDM and management of a growing number of severe pharmacokinetic antifungal drug-drug interactions with targeted chemotherapies.

Therapeutic drug monitoring of systemic antifungal agents: a pragmatic approach for adult and pediatric patients

Introduction: Therapeutic drug monitoring (TDM) has been shown to optimize the management of invasive fungal infections (IFIs), particularly for select antifungal agents with a well-defined exposure-response relationship and an unpredictable pharmacokinetic profile or a narrow therapeutic index. Select triazoles (itraconazole, voriconazole, and posaconazole) and flucytosine fulfill these criteria, while the echinocandins, fluconazole, isavuconazole, and amphotericin B generally do not do so. Given the morbidity and mortality associated with IFIs and the challenges surrounding the use of currently available antifungal agents, TDM plays an important role in therapy.Areas covered: This review seeks to describe the rationale for TDM of antifungal agents, summarize their pharmacokinetic and pharmacodynamic properties, identify treatment goals for efficacy and safety, and provide recommendations for optimal dosing and therapeutic monitoring strategies.Expert opinion: Several new antifungal agents are currently in development, including compounds from existing antifungal classes with enhanced pharmacokinetic or safety profiles as well as agents with novel targets for the treatment of IFIs. Given the predictable pharmacokinetics of these newly developed agents, use of routine TDM is not anticipated. However, expanded knowledge of exposure-response relationships of these compounds may yield a role for TDM to improve outcomes for adult and pediatric patients.

Pharmacodynamics for antifungal drug development: an approach for acceleration, risk minimization and demonstration of causality

The treatment of invasive fungal diseases constitutes a significant unmet medical need. There are relatively few antifungal agents in clinical development and a paucity of novel targets. Morbidity and mortality remain high and clinical outcomes are compromised by submaximal efficacy, emergence of drug resistance and drug-related toxicity. Thus, new antifungal agents are urgently required. A deep understanding of exposure-response relationships underpins the development of safe and effective clinical regimens of any therapeutic agent. Pharmacokinetics (PK) and pharmacodynamics (PD) is increasingly recognized as a vital tool in the development of new antimicrobial agents and maximizes the probability that the right dose will be studied the first time. There is currently no information or agreement as to what constitutes an adequate PK/PD package for the development of a new antifungal agent. This review provides a summary of the achievements of antifungal PK/PD for the treatment of invasive candidiasis, invasive aspergillosis and cryptococcal meningoencephalitis, and outlines the necessary components of a PK/PD package for a new antifungal agent. Such information is critical for the accelerated and efficient development of new agents and enables improved clinical outcomes to be secured.

Clinical pharmacology of antifungal agents to overcome drug resistance in pediatric patients

INTRODUCTION

Antifungal resistance is an emerging problem that increases morbidity and mortality in immunosuppressed pediatric patients, who suffer from invasive fungal diseases. Optimal pharmacological management is critical for the successful treatment of invasive fungal infections by resistant strains.

AREAS COVERED

This paper reviews the mechanisms of resistance of different classes of antifungal agents and the current understanding of pediatric antifungal pharmacology for overcoming antifungal resistance in children based on laboratory and clinical studies in the English literature. The therapeutic choices against fungal pathogens with intrinsic or acquired resistance are further reviewed.

EXPERT OPINION

There is a paucity of data in the pediatric population regarding the epidemiology of the resistant organisms to different antifungal agents. It is also unknown if there are more prevalent molecular mechanisms that promote antifungal resistance. Selection and dosages of the most effective antifungal agent for overcoming the antifungal resistance is crucial. However, there are limited studies guiding the optimal dosage and duration of treatment for management of emergent antifungal resistance. Further studies are warranted to elucidate the optimal pharmacology of the current antifungal agents against resistant organisms and to advance the development of new antifungal agents.

Antifungal pharmacokinetics and pharmacodynamics

Successful treatment of infectious diseases requires choice of the most suitable antimicrobial agent, comprising consideration of drug pharmacokinetics (PK), including penetration into infection site, pathogen susceptibility, optimal route of drug administration, drug dose, frequency of administration, duration of therapy, and drug toxicity. Antimicrobial pharmacokinetic/pharmacodynamic (PK/PD) studies consider these variables and have been useful in drug development, optimizing dosing regimens, determining susceptibility breakpoints, and limiting toxicity of antifungal therapy. Here the concepts of antifungal PK/PD studies are reviewed, with emphasis on methodology and application. The initial sections of this review focus on principles and methodology. Then the pharmacodynamics of each major antifungal drug class (polyenes, flucytosine, azoles, and echinocandins) is discussed. Finally, the review discusses novel areas of pharmacodynamic investigation in the study and application of combination therapy.

Therapeutic drug monitoring (TDM) of antifungal agents: guidelines from the British Society for Medical Mycology

The burden of human disease related to medically important fungal pathogens is substantial. An improved understanding of antifungal pharmacology and antifungal pharmacokinetics-pharmacodynamics has resulted in therapeutic drug monitoring (TDM) becoming a valuable adjunct to the routine administration of some antifungal agents. TDM may increase the probability of a successful outcome, prevent drug-related toxicity and potentially prevent the emergence of antifungal drug resistance. Much of the evidence that supports TDM is circumstantial. This document reviews the available literature and provides a series of recommendations for TDM of antifungal agents.

The initial 96 hours of invasive pulmonary aspergillosis: histopathology, comparative kinetics of galactomannan and (1->3) β-d-glucan and consequences of delayed antifungal therapy

Acute invasive pulmonary aspergillosis is a rapidly progressive and frequently lethal infection. Relatively little is known about early events in the pathogenesis and relationship between the cell wall biomarkers galactomannan and (1→3)-β-d-glucan. The consequences of delayed antifungal therapy are also poorly defined. A persistently neutropenic rabbit model of invasive pulmonary aspergillosis was used to describe the histopathology of early invasive pulmonary aspergillosis and the kinetics of galactomannan and (1→3)-β-d-glucan. The time course of both molecules was mathematically modeled by using a population methodology, and Monte Carlo simulations were performed. The effect of progressive delay in the administration of amphotericin B deoxycholate 1 mg/kg at 24, 48, 72, and 96 h postinoculation on fungal burden, lung weight, pulmonary infarct score, and survival was determined. Histopathology showed phagocytosis of conidia by pulmonary alveolar macrophages at 4 h postinoculation. At 12 to 24 h, there was a progressive focal inflammatory response with conidial germination and hyphal extension. Subsequently, hyphae invaded into the contiguous lung. Galactomannan and (1→3)-β-d-glucan had similar trajectories, and both exhibited considerable interindividual variability, which was reflected in Monte Carlo simulations. Concentrations of both molecules began to rise <24 h postinoculation before pulmonary hemorrhagic infarction was present. Delays of 72 and 96 h in the administration of amphotericin B resulted in fungal burdens and lung weights that were indistinguishable from those of controls, respectively. Galactomannan and (1→3)-β-d-glucan have similar kinetics and are comparable biomarkers of early invasive pulmonary aspergillosis. Antifungal treatment at ≥48 h postinoculation is associated with suboptimal therapeutic outcomes.

What is the role of therapeutic drug monitoring in antifungal therapy?

Despite an expanding number of therapeutic options for treating invasive fungal infection (IFI), the mortality rate from these infections remains high. Many risk factors for poor outcome from IFI (eg, the compromised immune status of the host) are not modifiable by the treating clinician. Thus, elevated interest exists in any modifiable factor that might improve outcome. Many of the new antifungal agents have marked variability in drug concentration based on either inconsistent absorption or elimination, leading to very wide interpatient variability. Many agents also have a narrow therapeutic index, meaning a small range between drug levels too low to achieve the desired clinical benefit and high enough to produce unwanted or toxic effects. Therefore, therapeutic drug monitoring is useful to maximize efficacy while minimizing drug toxicity of some antifungal agents.

Clinical pharmacokinetics of oral controlled-release 5-fluorocytosine

5-Fluorocytosine (5FC) is an oral antifungal that is currently used in combination with amphotericin B to treat Cryptococcus neoformans meningoencephalitis. The oral dosing of 5FC could be optimized by the use of a controlled-release (CR) formulation. The objective of the current study was to develop two prototype 5FC-CR formulations and evaluate the single-dose (1,500-mg) serum pharmacokinetic profiles of those formulations relative to the profile of the commercially available, immediate-release 5FC product (Ancobon) by the use of a phase 1, open-label, randomized, three-phase, crossover pharmacokinetic study design. Hydroxypropyl methylcellulose was utilized as the rate-controlling matrix to compound the 5FC-CR tablets. The two prototype 5FC-CR formulations demonstrated 80% release at 13.0 and 18.4 h, respectively, whereas the immediate-release product demonstrated 80% release at 0.28 h, as determined in vitro by the United States Pharmacopeia apparatus 2 dissolution method. Five subjects completed all three phases of the study without any adverse events. The mean maximum concentration, the area under the curve from time zero to 24 h, and the area under the curve from time zero to infinity were approximately 50% lower (P < 0.01) with the 5FC-CR formulations than with the immediate-release 5FC product. However, no statistically significant differences in the minimum concentrations at 24 h were noted between the formulations. The gastric absorption profile of 5FC-CR was well predicted by in vitro dissolution. Future exploration of a gastroretentive 5FC-CR formulation could overcome the marked lack of bioequivalence observed in the present study.

Antifungal pharmacokinetics and pharmacodynamics: bridging from the bench to bedside

This review considers a way in which experimental data can be used to identify safe and effective antifungal regimens for humans. The process begins with experimental models of invasive fungal infections that enable definition of optimal dosages and schedules of antifungal drug administration to be defined. These preclinical models also enable the identification of drug exposure targets that are associated with therapeutic outcomes of interest. Human pharmacokinetic variability results in a considerable range of drug exposures following the use of fixed antifungal drug regimens. This variability can be quantified using population pharmacokinetic modeling techniques. Monte Carlo simulation can then be used to simulate pharmacokinetic variability and thereby estimate the proportion of patients with a therapeutic outcome of interest. Effective and safe regimens can thus be studied appropriately in clinical settings. This approach can, and should, be used to optimize antifungal therapy for a large number of clinical scenarios.

Pharmacokinetics and pharmacodynamics of a novel triazole, isavuconazole: mathematical modeling, importance of tissue concentrations, and impact of immune status on antifungal effect

Isavuconazole is a triazole with broad-spectrum activity against medically important fungal pathogens. We investigated the pharmacokinetics and pharmacodynamics of isavuconazole in a murine model of disseminated candidiasis. We determined the pharmacokinetics in both plasma and kidney. The relationship between tissue concentrations and the resultant antifungal effect was described using a mathematical model. The pharmacodynamic parameter that optimally links drug exposure with the antifungal effect was determined using dose fractionation studies. The impact of the immune status of mice receiving isavuconazole was determined in persistently and temporarily neutropenic animals. The pharmacokinetics of 1.6 to 28 mg isavuconazole/kg of body weight were linear. Exposure-response relationships demonstrated near-maximal effect following the administration of >15 mg/kg. The mathematical model showed that exposures in the kidney were 5.77 times higher than those in plasma, and there was persistence of the drug at this site despite concentrations in plasma falling to undetectable levels. The in vitro and in vivo postantifungal effects were 2 to 5 and 8.41 h, respectively. The area under the concentration-time curve (AUC)/MIC ratio was the parameter that optimally linked drug exposure with the observed antifungal effect. The total drug AUC/MIC ratios associated with a 90% probability of survival in temporarily and persistently neutropenic mice were 270 and 670, respectively. Once corrected for protein binding, these values are similar to the magnitude of drug exposure associated with a high probability of a successful therapeutic outcome for other triazoles. This study provides the experimental foundation for the use of isavuconazole in patients with disseminated candidiasis.

Therapeutic drug monitoring of antifungals: pharmacokinetic and pharmacodynamic considerations

Therapeutic drug monitoring of any pharmacologic agent should be considered when there is both significant pharmacokinetic variability and strong, clinically relevant, exposure-effect relationships. Many antifungal drugs exhibit marked variability in drug concentration as a result of inconsistent absorption, metabolism, elimination, or interaction with concomitant medications. For each of the available antifungal drugs, both preclinical and clinical trials have exhibited a relationship between serum concentrations and treatment efficacy. For a smaller subset of compounds, a similar relationship has been identified for the toxicity. The kinetic variability among patients falls outside the therapeutic window for a group of four antifungal compounds. This review summarizes the current literature on therapeutic drug monitoring for these antifungal agents.

Optimization of the dosage of flucytosine in combination with amphotericin B for disseminated candidiasis: a pharmacodynamic rationale for reduced dosing

Amphotericin B and flucytosine (5FC) have an additive effect when used for disseminated candidiasis. Here, we bridge the results of an experimental pharmacodynamic study to humans and demonstrate that a 5FC dosage of 25 mg/kg of body weight/day in four divided doses in combination with amphotericin B produces near-maximal effect.

Flucytosine therapeutic monitoring: 15 years experience from the UK

BACKGROUND

There is uniform consensus that flucytosine blood concentrations should be measured to avoid toxicity and ensure adequate efficacy.

OBJECTIVES AND METHODS

The purpose of this study was to evaluate all flucytosine levels performed in a regional centre in the UK from October 1991 to May 2006. Concentrations were measured by bioassay.

RESULTS

We reviewed 1071 flucytosine levels in 233 patients, including 33 neonates. Overall, only 20.5% of levels were in the expected therapeutic range. Low levels were observed in 40.5%, of which 5.1% were undetectable levels (<12.5 mg/L). High levels occurred in 38.9%, of which 9.9% were considered potentially toxic (>100 mg/L). High flucytosine levels occurred more frequently amongst neonates, which could be related to an immature renal system resulting in drug accumulation.

CONCLUSIONS

Our findings reveal that the vast majority of patients were out of range for flucytosine levels. These data emphasize the importance of monitoring flucytosine levels.

Effect of neutropenia and treatment delay on the response to antifungal agents in experimental disseminated candidiasis

Disseminated candidiasis is associated with a high rate of morbidity and mortality. The presence of neutrophils and the timely administration of antifungal agents are likely to be critical factors for a favorable therapeutic outcome of this syndrome. The effect of neutropenia on the temporal profile of the burden of Candida albicans in untreated mice and those treated with amphotericin B was determined using a pharmacodynamic model of disseminated candidiasis. A mathematical model was developed to describe the rate and extent of the C. albicans killing attributable to neutrophils and to amphotericin B. The consequences of a delay in the administration of amphotericin B, flucytosine, or micafungin were studied by defining dose-response relationships. Neutrophils caused a logarithmic decline in fungal burden in treated and untreated mice. The combination of amphotericin B and neutrophils resulted in a high rate of Candida killing and a sustained anti-C. albicans effect. In neutropenic mice, 5 mg/kg of body weight of amphotericin B was required to prevent progressive logarithmic growth. An increased delay in drug administration resulted in a reduction in the maximum effect to a point at which no drug effect could be observed. Neutrophils and the timely initiation of antifungal agents are critical determinants in the treatment of experimental disseminated candidiasis.