The postantibiotic effect of antifungal agents against common pathogenic yeasts

Postantifungal Effect of Antifungal Drugs against Candida: What Do We Know and How Can We Apply This Knowledge in the Clinical Setting?

The study of the pharmacological properties of an antifungal agent integrates the drug pharmacokinetics, the fungal growth inhibition, the fungicidal effect and the postantifungal activity, laying the basis to guide optimal dosing regimen selection. The current manuscript reviews concepts regarding the postantifungal effect (PAFE) of the main classes of drugs used to treat Candida infections or candidiasis. The existence of PAFE and its magnitude are highly dependent on both the fungal species and the class of the antifungal agent. Therefore, the aim of this article was to compile the information described in the literature concerning the PAFE of polyenes, azoles and echinocandins against the Candida species of medical interest. In addition, the mechanisms involved in these phenomena, methods of study, and finally, the clinical applicability of these studies relating to the design of dosing regimens were reviewed and discussed. Additionally, different factors that could determine the variability in the PAFE were described. Most PAFE studies were conducted in vitro, and a scarcity of PAFE studies in animal models was observed. It can be stated that the echinocandins cause the most prolonged PAFE, followed by polyenes and azoles. In the case of the triazoles, it is worth noting the inconsistency found between in vitro and in vivo studies.

Extended Dosing Regimens for Fungal Prophylaxis

Invasive fungal diseases carry high morbidity and mortality in patients undergoing chemotherapy for hematological malignancies or allogeneic hematopoietic stem cell transplantation. In order to prevent these life-threatening infections, antifungal chemoprophylaxis plays an important role in daily clinical practice. Broad-spectrum antifungal triazoles are widely used but exhibit disadvantages such as relevant drug-drug interactions. Therefore, amphotericin B products or echinocandins can be an alternative in selected patient populations. As these compounds are available as intravenous formulations only, there is growing interest in extended dosing regimens. Although not approved for these agents, this strategy is a rational option, as these compounds have properties suitable for this strategy, including dose-proportional pharmacokinetics, prolonged elimination half-life, and a large therapeutic window. As the use of extended dosing regimens in antifungal prophylaxis is expanding in clinical practice, we reviewed the pharmacokinetic and pharmacodynamic rationale for this strategy, animal model data, dose escalation studies, and clinical trials supporting this concept.

Miltefosine Has a Postantifungal Effect and Induces Apoptosis in Cryptococcus Yeasts

Cryptococcus spp. are common opportunistic fungal pathogens, particularly in HIV patients. The approved drug miltefosine (MFS) has potential as an alternative antifungal against cryptococcosis; however, the mechanism of action of MFS in Cryptococcus is poorly understood. Here, we examined the effects of MFS on C. neoformans and C. gattii yeasts (planktonic and biofilm lifestyles) to clarify its mechanism of action. MFS presented inhibitory and fungicidal effects against planktonic Cryptococcus cells, with similar activities against dispersion biofilm cells, while sessile biofilm cells were less sensitive to MFS. Interestingly, MFS had postantifungal effect on Cryptococcus, with a proliferation delay of up to 8.15 h after a short exposure to fungicidal doses. MFS at fungicidal concentrations increased the plasma membrane permeability, likely due to a direct interaction with ergosterol, as suggested by competition assays with exogenous ergosterol. Moreover, MFS reduced the mitochondrial membrane potential, increased reactive oxygen species (ROS) production, and induced DNA fragmentation and condensation, all of which are hallmarks of apoptosis. Transmission electron microscopy analysis showed that MFS-treated yeasts had a reduced mucopolysaccharide capsule (confirmed by morphometry with light microscopy), plasma membrane irregularities, mitochondrial swelling, and a less conspicuous cell wall. Our results suggest that MFS increases the plasma membrane permeability in Cryptococcus via an interaction with ergosterol and also affects the mitochondrial membrane, eventually leading to apoptosis, in line with its fungicidal activity. These findings confirm the potential of MFS as an antifungal against C. neoformans and C. gattii and warrant further studies to establish clinical protocols for MFS use against cryptococcosis.

Antifungal Combinations for Treatment of Cryptococcal Meningitis in Africa

BACKGROUND

Cryptococcal meningitis accounts for more than 100,000 human immunodeficiency virus (HIV)-related deaths per year. We tested two treatment strategies that could be more sustainable in Africa than the standard of 2 weeks of amphotericin B plus flucytosine and more effective than the widely used fluconazole monotherapy.

METHODS

We randomly assigned HIV-infected adults with cryptococcal meningitis to receive an oral regimen (fluconazole [1200 mg per day] plus flucytosine [100 mg per kilogram of body weight per day] for 2 weeks), 1 week of amphotericin B (1 mg per kilogram per day), or 2 weeks of amphotericin B (1 mg per kilogram per day). Each patient assigned to receive amphotericin B was also randomly assigned to receive fluconazole or flucytosine as a partner drug. After induction treatment, all the patients received fluconazole consolidation therapy and were followed to 10 weeks.

RESULTS

A total of 721 patients underwent randomization. Mortality in the oral-regimen, 1-week amphotericin B, and 2-week amphotericin B groups was 18.2% (41 of 225), 21.9% (49 of 224), and 21.4% (49 of 229), respectively, at 2 weeks and was 35.1% (79 of 225), 36.2% (81 of 224), and 39.7% (91 of 229), respectively, at 10 weeks. The upper limit of the one-sided 97.5% confidence interval for the difference in 2-week mortality was 4.2 percentage points for the oral-regimen group versus the 2-week amphotericin B groups and 8.1 percentage points for the 1-week amphotericin B groups versus the 2-week amphotericin B groups, both of which were below the predefined 10-percentage-point noninferiority margin. As a partner drug with amphotericin B, flucytosine was superior to fluconazole (71 deaths [31.1%] vs. 101 deaths [45.0%]; hazard ratio for death at 10 weeks, 0.62; 95% confidence interval [CI], 0.45 to 0.84; P=0.002). One week of amphotericin B plus flucytosine was associated with the lowest 10-week mortality (24.2%; 95% CI, 16.2 to 32.1). Side effects, such as severe anemia, were more frequent with 2 weeks than with 1 week of amphotericin B or with the oral regimen.

CONCLUSIONS

One week of amphotericin B plus flucytosine and 2 weeks of fluconazole plus flucytosine were effective as induction therapy for cryptococcal meningitis in resource-limited settings. (ACTA Current Controlled Trials number, ISRCTN45035509 .).

40 years of medical mycology at JAC

In the past 40 years, medical mycology has gone from a curiosity in the basements of medical schools to a mainstream branch of clinical microbiology and infectious diseases. Long gone are the days of carefully curated collections of organisms identified purely based on morphology and skill, the lack of therapeutic interventions beyond amphotericin B and the occasional strange case in the ward of a diabetic patient with mucormycosis. We highlight advances in medical mycology as reflected in the past 40 years of JAC.

CD101, a long-acting echinocandin, and comparator antifungal agents tested against a global collection of invasive fungal isolates in the SENTRY 2015 Antifungal Surveillance Program

CD101 is a novel echinocandin with exceptional chemical stability and long-acting pharmacokinetics. The activity of CD101 and comparators was evaluated using CLSI broth microdilution methods against 713 invasive fungal isolates, including 589 Candida spp. (6 species), 14 C. neoformans, 97 A. fumigatus and 13 A. flavus species complex collected worldwide during 2015. All C. tropicalis, C. krusei and C. dubliniensis, 99.7% of C. albicans and 98.3% of C. glabrata were inhibited by ≤0.12 µg/mL of CD101, and these isolates were susceptible/wild type to other echinocandins using CLSI clinical breakpoint and epidemiological cutoff value (ECV) interpretive criteria. C. parapsilosis displayed higher MIC values (range 0.25-2 µg/mL), but similar results were observed for other echinocandins. One C. glabrata and one C. albicans with CD101 MIC value at 1 and 0.25 µg/mL possessed F625S and S645P alterations on FKS1, respectively. These isolates also displayed elevated MIC values for at least one clinically available echinocandin. Fluconazole resistance was noted for 6.6% of C. glabrata and 3.6% C. parapsilosis. Echinocandins had limited activity against C. neoformans. CD101 activity against A. fumigatus and A. flavus (MEC ≤0.03 µg/mL) was comparable to other echinocandins (MEC ≤0.03 µg/mL). These moulds had MIC values below ECVs for the mould-active azoles. CD101 was as active as other echinocandins against common fungal organisms recovered from invasive fungal infections. The extended half-life profile is very desirable as less frequent dosing of this agent should facilitate shorter and more cost-effective hospital stays, improve compliance for outpatients, and provide more convenient outpatient prophylaxis.

Antifungal pharmacodynamics: Latin America's perspective

The current increment of invasive fungal infections and the availability of new broad-spectrum antifungal agents has increased the use of these agents by non-expert practitioners, without an impact on mortality. To improve efficacy while minimizing prescription errors and to reduce the high monetary cost to the health systems, the principles of pharmacokinetics (PK) and pharmacodynamics (PD) are necessary. A systematic review of the PD of antifungals agents was performed aiming at the practicing physician without expertise in this field. The initial section of this review focuses on the general concepts of antimicrobial PD. In vitro studies, fungal susceptibility and antifungal serum concentrations are related with different doses and dosing schedules, determining the PD indices and the magnitude required to obtain a specific outcome. Herein the PD of the most used antifungal drug classes in Latin America (polyenes, azoles, and echinocandins) is discussed.

CD101: a novel long-acting echinocandin

CD101 is a novel echinocandin drug being developed to treat severe fungal infections including invasive candidiasis. We have performed a series of studies to evaluate the antifungal properties of CD101 against both echinocandin‐susceptible and ‐resistant Candida strains. Antifungal susceptibility testing performed on a collection of 95 Candida strains including 30 caspofungin‐resistant isolates containing fks mutations demonstrated comparable antifungal potency of CD101 relative to micafungin (MCF) across different Candida species. Comparable kinetic inhibition of glucan synthase activity was also observed for CD101 and MCF on both wild‐type (WT) and resistant fks mutant Candida strains. Similarly, both drugs yielded nearly identical values for a mutant prevention concentration. In a murine model of invasive candidiasis, CD101 displayed better or at least comparable efficacy relative to MCF in treating WT or fks mutant Candida albicans. An exceptional long‐lived pharmacokinetic profile was observed in mice following a single dose of CD101. Collectively, CD101 has great potential not only in treating invasive Candida infections but also in preventing emergence of resistance to currently approved echinocandin drugs.

Efficacy of Extended-Interval Dosing of Micafungin Evaluated Using a Pharmacokinetic/Pharmacodynamic Study with Humanized Doses in Mice

The pharmacokinetic/pharmacodynamic (PK/PD) characteristics of the echinocandins favor infrequent administration of large doses. The in vivo investigation reported here tested the utility of a range of humanized dose levels of micafungin using a variety of prolonged dosing intervals for the prevention and therapy of established disseminated candidiasis. Humanized doses of 600 mg administered every 6 days prevented fungal growth in prophylaxis. Humanized doses of 300 to 1,000 mg administered every 6 days demonstrated efficacy for established infections.

A new look at the antibiotic amphotericin B effect on Candida albicans plasma membrane permeability and cell viability functions

Amphotericin B (AmB) is an antifungal polyene for which the most accepted mode of action is formation of protein-like ion channels in the cell membrane. Patch-clamp research on Candida albicans protoplasts carried out in the outside-out configuration showed that application of 0.05 and 0.1 μM AmB caused a decrease in seal resistance. Such a phenomenon can be correlated with a decrease in membrane tightness. AmB applied at a 0.05 μM concentration also caused a decrease in the number of active TOK1 (two-pore outward rectifiers) potassium channels, but did not significantly change their open probability. The results indicate that in C. albicans protoplast AmB causes a decrease in cell membrane integrity by interaction with its lipid phase but not with ion channels. Fluorescence microscopy techniques showed that AmB treatment, in clinical concentrations, had no effect on the percentage of PI-positive protoplasts. AmB treatment in the concentrations tested did not cause a rapid reduction of the number of C. albicans protoplasts. However, there was a significant loss of replication competency and numerous morphological and physiological disorders, including cytoplasm shrinking, abnormal morphology of the nucleus and mitochondria, a sudden decrease in the MTT reduction level and oxidative stress. Our results show that the induction of yeast cell death by AmB, at therapeutic doses, is a multistage and long-term process involving multiple intracellular pathways.

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.

The Postantifungal Effect (PAFE) of Itraconazole, in Comparison with Those of Miconazole and Fluconazole, onCandidaSpecies

To study the postantifungal effect (PAFE) of itraconazole (ITCZ) against Candida species, we quantitated it using a newly introduced parameter T/C(T=time required for the drug-treated culture to reach 5-fold increase in turbidity; C=time required for the drug-free control culture to reach 5-fold increase in turbidity) referred as PAFE index and compared the results with those obtained for two other azole antifungal drugs fluconazole (FLCZ) and miconazole (MCZ). The mean values of PAFE index for ITCZ against three C. albicans strains were 1.28, 1.45, and 1.60 when exposed to the drug for 1 hr at concentrations of 1 x MIC, 2 x MIC and 4 x MIC, respectively. These values are similar to those for MCZ and appreciably higher than those for FLCZ. PAFE index values for ITCZ against each single strain of three non-albicans Candida species exposed to 2 x MIC of the drug for 1 hr were: 1.22 with C. glabrata, 1.63 with C. parapsilosis, and 3.90 with C. tropicalis. Again, comparable values were obtained for MCZ and lower values for FLCZ. The drug concentration for exposure and the duration of exposure only slightly or scarcely affected the PAFE of ITCZ, although the magnitude of the PAFE considerably varied among different strains or species tested. In general, ITCZ, as well as MCZ, produced greater extents of PAFE than those for FLCZ against C. albicans and several non-albicans Candida species.

Pharmacodynamics and dose-response relationships of liposomal amphotericin B against different azole-resistant Aspergillus fumigatus isolates in a murine model of disseminated aspergillosis

The management of invasive aspergillosis (IA) has become more complicated due to the emergence of acquired azole resistance in Aspergillus fumigatus, which is associated with treatment failure and a mortality rate of 88%. Treatment with liposomal amphotericin B (L-AmB) may be a useful alternative to improve therapeutic outcome in azole-resistant IA. Four clinical A. fumigatus isolates obtained from patients with proven IA were studied in a nonneutropenic murine model of infection: a wild-type isolate without mutations in the cyp51A gene and three azole-resistant isolates harboring a single mutation at codon 220 (M220I) and tandem repeat mutations (a 34-bp tandem repeat mutation in the promoter region of the cyp51A gene in combination with substitutions at codon L98 [TR(34)/L98H] and a 46-bp tandem repeat mutation in the promoter region of the cyp51A gene in combination with mutation at codons Y121 and T289 [TR(46)/Y121F/T289A]), respectively. Female CD-1 mice were infected intravenously 24 h prior to the start of therapy. Groups of 11 mice were treated at days 1, 2, and 5 postchallenge with increasing 4-fold doses of L-AmB ranging from 0.004 to 16 mg/kg/day and observed for 14 days. Survival for all 4 isolates at day 14 was significantly better than that of controls. A dose-response relationship was observed independent of the azole resistance mechanism. The Hill-type model with a variable slope fitted the relationship between the dose and 14-day survival well for all isolates, with R(2) values of 0.95 (wild-type), 0.97 (M220I), 0.85 (TR(34)/L98H), and 0.94 (TR(46)/Y121F/T289A), respectively. Multiple logistic regression analysis confirmed that there was no significant difference between groups. The results of these experiments indicate that L-AmB was able to prolong survival in vivo in disseminated IA independent of the presence of an azole resistance mechanism in a dose-dependent manner, and therefore, they support a role for L-AmB in the treatment of azole-resistant IA.

Amphotericin B-induced in vitro postantifungal effect on Candida species of oral origin

OBJECTIVE

The aim of this investigation was to measure the postantifungal effect (PAFE) of 6 different oral Candida species following exposure to amphotericin B.

MATERIALS AND METHODS

Five oral isolates each of Candida albicans, Candida tropicalis, Candida krusei, Candida parapsilosis, Candida glabrata and Candida guilliermondii (total of 30 isolates) were examined for the presence of PAFE after 1 h of exposure to the minimum inhibitory concentration of amphotericin B. The PAFE was determined as the difference in time (hours) required for the growth of the drug-free control and the drug-exposed test cultures to increase to 0.05 absorbance level following removal of amphotericin B.

RESULTS

The mean duration of amphotericin B-induced PAFE was lowest for C. albicans (5.91 ± 0.31 h) and greatest for C. parapsilosis (12.72 ± 0.11 h), while C. guilliermondii (8.32 ± 0.33 h), C. glabrata (8.43 ± 0.21 h), C. krusei (9.68 ± 0.23 h) and C. tropicalis (10.98 ± 0.18 h) elicited intermediate values.

CONCLUSION

Even a limited exposure to sublethal concentrations of amphotericin B suppressed growth of Candida species of oral origin. The significant variation in amphotericin B-induced PAFE amongst different Candida species may have clinical implications in terms of amphotericin B regimens used in the management of oral candidiasis.

Miconazole induces fungistasis and increases killing of Candida albicans subjected to photodynamic therapy

Cutaneous and mucocutaneous Candida infections are considered to be important targets for antimicrobial photodynamic therapy (PDT). Clinical application of antimicrobial PDT will require strategies that enhance microbial killing while minimizing damage to host tissue. Increasing the sensitivity of infectious agents to PDT will help achieve this goal. Our previous studies demonstrated that raising the level of oxidative stress in Candida by interfering with fungal respiration increased the efficiency of PDT. Therefore, we sought to identify compounds in clinical use that would augment the oxidative stress caused by PDT by contributing to reactive oxygen species (ROS) formation themselves. Based on the ability of the antifungal miconazole to induce ROS in Candida, we tested several azole antifungals for their ability to augment PDT in vitro. Although miconazole and ketoconazole both stimulated ROS production in Candida albicans, only miconazole enhanced the killing of C. albicans and induced prolonged fungistasis in organisms that survived PDT using the porphyrin TMP-1363 and the phenothiazine methylene blue as photosensitizers. The data suggest that miconazole could be used to increase the efficacy of PDT against C. albicans, and its mechanism of action is likely to be multifactorial.

Overview of treatment options for invasive fungal infections

The introduction of several new antifungals has significantly expanded both prophylaxis and treatment options for invasive fungal infections (IFIs). Relative to amphotericin B deoxycholate, lipid-based formulations of amphotericin B have significantly reduced the incidence of nephrotoxicity, but at a significant increase in drug acquisition cost. Newer, broad-spectrum triazoles (notably voriconazole and posaconazole) have added significantly to both the prevention and treatment of IFIs, most notably Aspergillus spp. (with voriconazole) and the treatment of some emerging fungal pathogens. Finally, a new class of parenteral antifungals, the echinocandins, is employed most frequently against invasive candidal infections. While the role of these newer agents continues to evolve, this review summarizes the activity, safety and clinical applications of agents most commonly employed in the treatment of IFIs.

The post-antifungal effect (PAFE) of amphotericin B, nystatin, ketoconazole and 5-fluorocytosine and its impact on the colonization traits of Candida glabrata

The post-antifungal effect (PAFE) has been shown to affect Candida pathogenicity, but there is little information on either PAFE or its association with the colonization traits of Candida glabrata. The objective of this study was to determine, in vitro, the PAFE on 14 C. glabrata isolates following exposure to amphotericin B (AMB), nystatin (NYS), ketoconazole (KETO) and 5-fluorocytosine (5FC). In addition, we evaluated the impact of PAFE on yeast adherence to buccal epithelial cells (BEC), cell-surface-hydrophobicity (CSH) and biofilm growth (BG) on denture acrylic surfaces. PAFE was induced following a 1-h exposure of yeasts to (x1-x4MIC) of AMB, NYS, KETO and 5FC in RPMI medium and, measured using automated turbidometry. The BEC adhesion, CSH and BG assays were performed by the methods of Kimura & Pearsall, Sweet et al., and Jin et al., respectively. Significant differences in PAFE (P < 0.001) were observed after exposure to AMB and NYS, but not KETO and 5FC. Following exposure to AMB, NYS, KETO and 5FC, significant inter-strain differences (P < 0.001) were observed in percentage terms in adhesion (39.0%, 43.48%, 38.28%, 35.07%) and biofilm growth (42.86%, 39.86%, 42.81%, 36.38%), respectively. Short exposure of C. glabrata to sub-cidal concentrations of antifungals modulates yeast growth and also affects some of their colonization traits.

Invasive fungal infections in the ICU

Invasive fungal infections are major causes of morbidity and mortality in critically ill patients. Foremost among these is invasive candidiasis. In recent years, invasive aspergillosis (IA) and zygomycosis have emerged as major problems in susceptible, critically ill patients. Risk factors for invasive fungal infections, including disrupted anatomic barriers, suppressed antifungal host responses, and exposure to potentially opportunistic fungi are common in critically ill patients. The expanded antifungal armamentarium and advent of rapid diagnostic techniques are altering the approach to invasive fungal infections in the intensive care unit (ICU). Herein, we review recent developments in the field of antifungal host defenses, the changing epidemiology of fungal infections in the ICU, the pharmacology of antifungal agents of importance to critically ill patients, and the evolving approaches to therapy in this setting.

In vivo pharmacodynamic characterization of anidulafungin in a neutropenic murine candidiasis model

Multiple in vivo studies have characterized the pharmacodynamics of drugs from the triazole and polyene antifungal drug classes. Fewer studies have investigated these pharmacodynamic relationships for the echinocandin drug class. We used a neutropenic murine model of disseminated Candida albicans, Candida tropicalis, and Candida glabrata infection to characterize the time course of activity of the new echinocandin anidulafungin. The pharmacokinetic-pharmacodynamic (PK-PD) indices (the percentage of time that the drug concentration was above the MIC, the ratio of the area under the concentration-time curve from 0 to 24 h [AUC(0-24)] to the MIC, and the ratio of the maximum serum drug concentration [C(max)] to the MIC) were correlated with in vivo efficacy, as measured by organism numbers in kidney cultures after 96 h of therapy. The kinetics following intraperitoneal anidulafungin dosing in neutropenic infected mice were monitored. Peak levels and AUCs were linear over the 16-fold dose range studied. The drug elimination half-life in serum ranged from 14 to 24 h. Single-dose postantifungal-effect studies demonstrated prolonged suppression of organism regrowth after serum anidulafungin levels had fallen below the MIC. Of the four dosing intervals studied, treatment with the more widely spaced dosing regimens was most efficacious, suggesting the C(max)/MIC ratio as the PK-PD index most predictive of efficacy. Nonlinear regression analysis suggested that both the C(max)/MIC and AUC/MIC ratios were strongly predictive of treatment success. Studies were then conducted with 13 additional C. albicans, C. tropicalis, and C. glabrata isolates with various anidulafungin susceptibilities (MICs of anidulafungin for these strains, 0.015 to 2.0 microg/ml) to determine if similar C(max)/MIC and AUC(0-24)/MIC ratios for these isolates were associated with efficacy. The anidulafungin exposures associated with efficacy were similar among Candida species.

Dormancy of Candida albicans cells in the presence of the polyene antibiotic amphotericin B: simple demonstration by flow cytometry

Flow cytometry light scattering was used to monitor size increase of Candida albicans (isolate ATCC 10231) cells in the presence or absence of the antifungal drug amphotericin B (AmB). This non-invasive and descriptive method allowed for the differentiation of dead and dormant sub-populations of cells. When inoculated into a growth medium without AmB, a progressive increase in light scattering was observed over a period of approximately 4 h, but without proliferation of the yeast. After this period, the light scattering distribution regressed to baseline level, whereas cell proliferation started. In the presence of AmB, all the cells shrank in size within approximately 4 h and proliferation was temporarily halted. However, in the presence of 0.4 microM AmB, a progressive increase of light scattering occurred after 21 h which was similar to that observed within the first 4 h in the absence of the antifungal. After approximately 24 h of incubation at this concentration of AmB, proliferation resumed. These observations indicate that this renewed cell proliferation was due to the reawakening of dormant cells in the presence of AmB (45% in the presence of 0.4 microM AmB) rather than the result of the development of viable cells that had escaped detection. This simple descriptive approach could be extended to other fungal strains or species, to other antifungal drugs and possibly to bacteria.

Post-antifungal effects of the antifungal compound 2-(3,4-dimethyl-2,5-dihydro-1H-pyrrol-2-yl)-1-methylethyl pentanoate on Aspergillus fumigatus

The post-antifungal effect (PAFE) of the antifungal compound 2-(3,4-dimethyl-2,5-dihydro-1H-pyrrol-2-yl)-1-methylethyl pentanoate (DHP) upon Aspergillus fumigatus was investigated. The conidia of A. fumigatus were exposed to DHP at concentrations of 1× and 4× MIC90 for variable times at 37 °C. Amphotericin B (AmB)-treated or drug-free controls were included in the study. DHP as well as AmB exposure resulted in prolonged lag phases of the turbidimetric growth curves. Both the treatments gave rise to delayed growth, with lag phases of 11 h upon treatment with a concentration of 4× MIC90 for 4 h. Furthermore, it was observed that DHP inhibited the expression of three A. fumigatus secretory proteins of 18, 42 and 55 kDa. One protein of 42 kDa was found to be a metalloprotease, which is an important virulence factor. Analysis of time-dependent antigenic profiles showed the early expression of high-molecular-mass antigens. Expression of low-molecular-mass antigens started after 24 h culture. The antigens of A. fumigatus that are expressed during the early phase of growth were observed to be adversely affected after treatment with DHP. Although the mechanism of action of DHP to inhibit these proteins/antigens is unknown, the observations may be valuable to understand their role in the virulence of the pathogen, as well as the antigen-mediated responses caused by A. fumigatus.

Pharmacokinetics and pharmacodynamics of antifungals

Application of pharmacodynamic principles to antifungal drug therapy of Candida and Aspergillus infections has provided and understanding of the relationship between drug dosing and treatment efficacy. Observations of the pharmacodynamics of triazoles and AmB have correlated with the results of clinical trials and have proven useful for validation of in vitro susceptibility breakpoints. Although there remain many unanswered questions regarding antifungal pharmacodynamics, available data suggest usefulness in the application of pharmacodynamics to antifungal clinical development. Future application of these principles should aid in the design of optimal combination antifungal therapies.

Characterizing the effects of caspofungin on Candida albicans, Candida parapsilosis, and Candida glabrata isolates by simultaneous time-kill and postantifungal-effect experiments

We measured time-kills and postantifungal effects (PAFEs) of caspofungin against Candida albicans, C. parapsilosis, and C. glabrata isolates. One-hour exposure to caspofungin during PAFE experiments accounted for the majority of killing during time-kill experiments. Regrowth of all isolates was inhibited for at least 24 h following drug washout.

Time course of microbiologic outcome and gene expression in Candida albicans during and following in vitro and in vivo exposure to fluconazole

Pharmacodynamics (PD) considers the relationship between drug exposure and effect. The two factors that have been used to distinguish the PD behaviors of antimicrobials are the impact of concentration on the extent of organism killing and the duration of persistent microbiologic suppression (postantibiotic effect). The goals of these studies were (i) to examine the relationship between antimicrobial PD and gene expression and (ii) to gain insight into the mechanism of fluconazole effects persisting following exposure. Microarrays were used to estimate the transcriptional response of Candida albicans to a supra-MIC F exposure over time in vitro. Fluconazole at four times the MIC was added to a log-phase C. albicans culture, and cells were collected to determine viable growth and for microarray analyses. We identified differential expression of 18% of all genes for at least one of the time points. More genes were upregulated (n=1,053 [16%]) than downregulated (174 [3%]). Of genes with known function that were upregulated during exposure, most were related to plasma membrane/cell wall synthesis (18%), stress responses (7%), and metabolism (6%). The categories of downregulated genes during exposure included protein synthesis (15%), DNA synthesis/repair (7%), and transport (7%) genes. The majority of genes identified at the postexposure time points were from the protein (17%) and DNA (7%) synthesis categories. In subsequent studies, three genes (CDR1, CDR2, and ERG11) were examined in greater detail (more concentration and time points) following fluconazole exposure in vitro and in vivo. Expression levels from the in vitro and in vivo studies were congruent. CDR1 and CDR2 transcripts were reduced during in vitro fluconazole exposure and during supra-MIC exposure in vivo. However, in the postexposure period, the mRNA abundance of both pumps increased. ERG11 expression increased during exposure and fell in the postexposure period. The expression of the three genes responded in a dose-dependent manner. In sum, the microarray data obtained during and following fluconazole exposure identified genes both known and unknown to be affected by this drug class. The expanded in vitro and in vivo expression data set underscores the importance of considering the time course of exposure in pharmacogenomic investigations.

In vitro and in vivo antifungal activities of the marine sponge constituent spongistatin

Spongistatin 1 is a macrocyclic lactone polyether from the marine sponge Hyrtios erecta. The aim of this study was to evaluate the in vitro and in vivo antifungal efficacies and mechanism of spongistatin 1. Spongistatin 1 was fungicidal for the majority of 74 reference strains and clinical isolates, including those resistant to flucytosine, ketoconazole or fluconazole, and retained activity in the presence of human serum or at lowered pH. The duration of the postantifungal effect following 1 h exposure to one, four and eight times the minimal inhibitory concentration was strain-dependent. Spongistatin 1 was significantly more efficacious than amphotericin B in reducing kidney infectious burden in a murine model of disseminated candidiasis, and reduced the lung burden in a murine model of pulmonary cryptococcosis. When cryptococcal microtubules were visualized by fluorescence microscopy and iterative deconvolution, spongistatin 1 was shown to disrupt first cytoplasmic and then spindle microtubules in a time- and concentration-dependent manner. Microtubule disruption was accompanied by an abnormal distribution of nuclei in budding cells and an inhibition of cell division, resulting in cells arrested in a large-budded stage. Spongistatin 1 should be pursued as a potential antifungal agent and as a probe to study, major cellular processes.

Clinical utility of antifungal pharmacokinetics and pharmacodynamics

PURPOSE OF REVIEW

Historically the anti-infective dose and dosing interval chosen in clinical trials have been based on an arbitrary goal of maintaining drug levels in serum above the minimum inhibitory concentration of infecting pathogens for most if not all of the dosing interval. Subsequent United States Food and Drug Administration approval of a dosing regimen is then based on clinical success in treatment trials. Over the past decade, the emergence of drug resistance has limited the clinical utility of an increasing number of antimicrobial agents. However, early in drug development clinical trials do not often define the impact of infection with these less susceptible pathogens. The field of pharmacodynamics provides analysis tools that can help predict the likelihood of treatment success with various antimicrobial treatment regimens against susceptible and resistant pathogens.

RECENT FINDINGS

In-vitro and animal model studies have begun to define the pharmacodynamic characteristics of a variety of antifungal compounds. In-vivo studies have demonstrated that the pharmacodynamic target associated with efficacy is similar among antifungal drugs within the same class and have shown the importance of considering protein. Analysis of clinical trial data suggests that the pharmacodynamic target identified in animal model studies is predictive of outcomes in humans.

SUMMARY

Antifungal pharmacodynamics can be used to understand the relationship between drug dosing, in-vitro susceptibility and treatment efficacy. Consideration of these relationships can be used to optimize dosing regimens with current antifungal agents, to develop susceptibility breakpoint guidelines, and in the design of dosing regimens for drugs in early development.

Antifungal pharmacokinetics and pharmacodynamics: understanding the implications for antifungal drug resistance

Pharmacodynamics (PDs) describe the relationship between drug exposure and outcome. The drug exposures in these analyses are most commonly expressed in a variety of pharmacokinetic terms. The outcome of interest with anti-infective therapy is either microbiologic resolution or a clinical surrogate of treatment efficacy. An in vitro measure of drug potency, such as the minimum inhibitory concentration (MIC) is also frequently considered in this relationship. Examination of the relationships among drug pharmacokinetics, MIC, and efficacy has provided a framework for choice of antifungal drug and dose. These analyses provide a PD target for drug class/organism combinations. The PD target can be useful for defining the upper MIC limit for a drug-dosing regimen that would be expected to result in treatment efficacy. The PD target can be used to optimize dosing regimens and to aid in defining susceptibility breakpoints.

In vitro pharmacodynamic characteristics of amphotericin B, caspofungin, fluconazole, and voriconazole against bloodstream isolates of infrequent Candida species from patients with hematologic malignancies

Time-kill and postantifungal effect (PAFE) of amphotericin B, caspofungin, fluconazole, and voriconazole were determined against clinical isolates of Candida guilliermondii, Candida kefyr, and Candida lusitaniae. Azoles displayed fungistatic activity and no measurable PAFE, regardless of the concentration tested. Amphotericin B and caspofungin demonstrated concentration-dependent fungicidal activity, although amphotericin B only produced a significant dose-dependent PAFE against all isolates tested.

Antibiotic pharmacokinetic and pharmacodynamic considerations in critical illness

BACKGROUND

Many factors over which there may be little control may influence the response of a patient to therapy. However, therapy with antibiotics can be readily optimised.

DISCUSSION

Concentration-dependent agents such as aminoglycosides appear effective and to entail fewer side effects when given in large, infrequent doses. There is also evidence that time-dependent antibiotics often fail to reach adequate concentrations throughout the treatment period. To date no randomised controlled prospective trial has demonstrated improvement in clinical outcome following infusion rather than intermittent boluses of time-dependent antibiotics. Critical illness alters antibiotic pharmacokinetics principally through increases in volume of distribution. Other than glycopeptides and aminoglycosides, antibiotic blood concentrations are rarely monitored and therefore adequate concentrations can only be inferred from clinical response.

CONCLUSIONS

Failure to respond within the first few days of empirical treatment may be due to antibiotic resistance or inadequate doses. Therefore the same rigor should be applied to achieving adequate antibiotic concentrations as is applied to inotropes, which are titrated to achieve predetermined physiological targets.

Post-antifungal effect of amphotericin B and voriconazole against Aspergillus fumigatus analysed by an automated method based on fungal CO2 production: dependence on exposure time and drug concentration

The post-antifungal effect (PAFE) of amphotericin B and voriconazole, either alone or in combination, on Aspergillus fumigatus was studied using an automated system based on fungal CO2 production.

METHODS

Conidia of A. fumigatus were exposed to concentrations of 1-10 x MIC of amphotericin B and 1-40 x MIC of voriconazole for 1, 2 and 4 h. After a washing step, exposed and control conidia were inoculated into Pedi-BacT culture bottles. CO2 production was automatically monitored until the bottles signalled positive. The difference in time span for positive signals in drug-exposed and control bottles was used to calculate PAFE.

RESULTS

There was a linear relationship between inoculum size and time to positive signal (r2=0.99). The precision of duplicate analyses was 1.5%. Longer exposure times increased the amphotericin B-induced PAFE (P<0.001), whereas concentrations above the MIC did not. Voriconazole after 4 h of exposure induced a short dose-independent PAFE. The combination with amphotericin B did not prolong the PAFE over that caused by amphotericin B alone.

CONCLUSIONS

This automated method can be used for determination of PAFE. In contrast to Candida spp., in which amphotericin B-induced PAFE is mainly related to the area under the curve, the effect on A. fumigatus was more dependent on the exposure time. This implies that pharmacodynamic data obtained from Candida experiments cannot be directly extrapolated to Aspergillus.

Clinical pharmacodynamics of antifungals

Application of pharmacodynamic principles to antifungal drugs has provided an understanding of the relationship between drug dosing and treatment outcomes similar to that observed in antibacterial pharmacodynamics. Initial observations with triazole pharmacodynamics have correlated with clinical trial results and proved useful for validation of in vitro susceptibility breakpoints. Pharmacodynamic studies have been invaluable for clinical trial dosing design for numerous antibacterial drugs in the development stage. More recently, pharmacodynamics has been used for the development of treatment guidelines. Although there remain many unanswered questions regarding antifungal pharmacodynamics, available data suggest usefulness in the application of pharmacodynamics to antifungal clinical development.

In vitro activity of amphotericin B, itraconazole, terbinafine and 5-fluocytosine against Exophiala spinifera and evaluation of post-antifungal effects

Antifungal susceptibility profiles were determined for 16 strains of the black yeast Exophiala spinifera applying different temperature regimens. Fluconazole was the least effective in vitro. Lowest minimal inhibitory concentration (MIC) values were found with itraconazole. The activities of antifungal agents against environmental and clinical strains were similar. Post-antifungal effect (PAFE) of four drugs was determined for 11 clinical strains. PAFE was observed only for amphotericin B, with extended inhibition times seen at high drug concentrations.

Sublethal injury and resuscitation of Candida albicans after amphotericin B treatment

Amphotericin B treatment was previously shown to inhibit Candida albicans reproduction and reduce the fluorescence of vitality-specific dyes without causing a corresponding increase in the fluorescence of the mortality-specific dyes bis-(1,3-dibutylbarbituric acid)trimethine oxonol and SYBR Green I. In the present study, we have confirmed these results and have shown that the numbers of CFU are reduced by 99.9% by treatment with 0.5 micro g of amphotericin B per ml for 10 h at 35 degrees C. This reduction was not due to fungal cell death. First, the level of reduction of the tetrazolium salt 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)carbonyl]-2H-tetrazolium hydroxide increased in the presence of concentrations of amphotericin B that caused greater than 90% reductions in the numbers of CFU. Second, fungal cells treated with amphotericin B at a concentration of 0.5 micro g/ml were resuscitated by further incubation at 22 degrees C for 15 h in the continued presence of amphotericin B. Third, recovery of the ability to replicate was prevented by sequential treatment with 20 micro g of miconazole per ml, which also increased the fluorescence of mortality-specific dyes to near the maximal levels achieved with 0.9 micro g of amphotericin B per ml. Sequential treatment with fluconazole and flucytosine did not increase the levels of staining with the mortality-specific dyes. Itraconazole was less effective than ketoconazole, which was less effective than miconazole. The practice of equating the loss of the capacity of C. albicans to form colonies with fungal cell death may give incorrect results in assays with amphotericin B, and the results of assays with caution with other antifungal agents that are lipophilic or that possess significant postantifungal effects may need to be interpreted.

In vivo pharmacodynamics of HMR 3270, a glucan synthase inhibitor, in a murine candidiasis model

In vivo pharmacokinetic/pharmacodynamic characterization for numerous antibacterial compounds has had a significant impact upon optimal dosing regimen design and the development of in vivo susceptibility breakpoints. More recently, similar characterization has been undertaken for antifungal drug classes. Very little is known of these pharmacodynamic relationships for the new echinocandin class of compounds. We utilized a neutropenic murine model of disseminated candidiasis to describe the time course antifungal activity of HMR 3270, a new glucan synthase inhibitor. Single-dose in vivo time kill studies with four 16-fold escalating doses demonstrated concentration-dependent killing when drug levels in serum were more than four times the MIC. Postantifungal effects were dose dependent, ranging from 8 to 80 h duration. Multiple dosing regimen studies utilized six total doses, four dosing intervals, and a treatment duration of 6 days. Shortening the dosing interval from every 144 h (q144h) to q36h resulted in a fourfold rise in the dose necessary to achieve a net fungistatic effect. The peak/MIC ratio most strongly correlated with treatment outcomes (peak/MIC ratio, R(2) = 98%; ratio of the area under the concentration-time curve from 0 to 24 h to the MIC, R(2) = 79%, percentage of time above the MIC, R(2) = 61%). Studies were also conducted with five additional Candida albicans isolates to determine if a similar peak/MIC ratio was associated with efficacy. In vivo concentration-dependent killing was similarly observed in studies with each of the additional isolates. The peak/MIC ratio necessary to produce efficacy was relatively similar among the strains studied (P = 0.42). The peak/MIC ratio (mean +/- standard deviation) necessary to achieve a fungistatic effect was 3.72 +/- 1.84, and the ratio necessary to achieve maximal killing was near 10.

Post-antifungal effect and effects of sub-MIC concentrations on previously treated Candida spp.: influence of exposure time and concentration

This study evaluates the influence of exposure time and concentration on the post-antifungal effect (PAFE) and the effect of sub-MIC concentrations (1/4 x MIC) on Candida albicans and C. glabrata in the PAFE stage (PAFSE). This stage was induced by pretreatment for 1.5, 3 or 12 h with 1 x, 4 x or 8 x MIC of 4 antifungal agents fundamental to modern candidiasis therapy. The length of the 2 effects studied was dependent on the concentration of the antifungal agent applied during pretreatment, as well as on the exposure time. An increase in the dose and/or longer pretreatment prolonged the duration of the PAFE and PAFSE in both species and with all the antifungal agents. Significant PAFEs were always observed for amphotericin B and 5-fluorocytosine (0.8-13 h and 0.6-10.8 h, respectively). These values were increased (by 2.3-8.7 h and 1.5-7.8 h, respectively) by posterior exposure to 1/4 x MIC of the respective antifungal agent. Neither ketoconazole nor fluconazole were able to induce significant PAFEs, even with exposures of up to 12 h duration and a dose of 8 x MIC. However, treatment with 1/4 x MIC of each of the 2 azoles led to significant PAFSEs in both yeast species, of up to 6.5 h duration with ketoconazole and 1.7 h with fluconazole, if the concentrations and/or exposure times were sufficiently high.

New automated method for determining postantifungal effect of amphotericin B against Candida species: effects of concentration, exposure time, and area under the curve

The postantifungal effect (PAFE) of amphotericin B was determined with a BacT/Alert automated system, which is based on the colorimetric detection of CO(2). The levels of accuracy and precision of the automated method were high. Longer PAFEs were obtained for Candida albicans (P < 0.001), C. glabrata (P < 0.01), and C. krusei (P < 0.01) at increasing amphotericin B concentrations and exposure times.

Postantifungal effect and effects of sub-MIC concentrations on previously treated Candida sp. influence of growth phase

OBJECTIVES

This study evaluates the influence of growth phase on the postantifungal effect (PAFE) and on the effect of sub-MIC concentrations (1/4x MIC) on Candida sp. in PAFE stage (PAFSE).

METHODS

This stage was induced by pre-treatments of 1.5 h of C. albicans or C. glabrata in their exponential or stationary phase, with 1x, 4x or 8x MIC of four antifungal agents that are fundamental for modern candidiasis therapy.

RESULTS

Ketoconazole and fluconazole induced longer PAFSEs on microorganisms in logarithmic growth phase. However, this influence did not exist in the case of PAFSEs induced by AmB and 5-Fc or with the postantifungal effect induced by the four antifungal agents. In any way, significant PAFEs were always observed for Amphotericin B and 5-fluorocytosine (0.8-4.8 and 0.5-3 h, respectively, depending on the treatment dose). These values were increased (2.3-3.6 and 1.4-3.2 h respectively, depending on the pre-treatment dose) by posterior exposition to 1/4x MIC of the respective antifungal agent. In the case of ketoconazole and fluconazole, both antimycotics were not able to induce significant PAFEs, but posterior treatments to 1/4x MIC of each of the two azoles led in both yeast species to significant PAFSE of up to 2.6 h duration with ketoconazole, and 0.8 h with fluconazole, depending on the pre-treatment concentration.

CONCLUSION

The growth phase of microorganisms should be considered in the planning of dosage protocols with azoles, because if the concentration applied is not high enough, the sub-MIC effects could be no significant for fungi in stationary phase of large wounds. Amphotericin B and 5-fluorocytosine induced significant postantifungal effect onCandida sp. This effect was increased by posterior exposition to sub-MIC concentration of the antifungal agents. Ketoconazole and fuconazole were not able to induce significant PAFEs at the concentrations tested, but posterior treatments to sub-MIC concentrations led to significant PAFSE. The growth phase of the culture at the time of its pre-treatment did not influence the length of the PAFE induced in it. However, the effect of the sub-MIC concentrations of Kz or Flu in yeast in PAFE phase was greater on yeast in exponential phase than for cultures in stationary phase.

Method for measuring postantifungal effect in Aspergillus species

An in vitro method for determination of postantifungal effect (PAFE) in molds was developed by using three isolates each of Aspergillus fumigatus, A. flavus, A. terreus, A. nidulans, and A. ustus. MICs of amphotericin B and itraconazole were determined by using National Committee for Clinical Laboratory Standards guidelines (M38-P). The inoculum was prepared in RPMI 1640 broth buffered with MOPS (morpholinepropanesulfonic acid) at pH 7.0, and conidia were exposed to amphotericin B and itraconazole at concentrations of 4, 1, and 0.25 times the MIC, each for 4, 2, and 1 h at 37 degrees C. The same procedure was followed for controls with drug-free medium. Following exposure, the conidia were washed three times in saline and the numbers of CFU per milliliter were determined. Exposed and control conidia were then inoculated into microtitration plates and incubated at 37 degrees C for 48 h in a spectrophotometer reader. The optical density (OD) was measured automatically at 10-min intervals, resulting in growth curves. PAFE was quantified by comparing three arbitrary points in the control growth curve, the first increase of OD and the points when 20 and 50% of the maximal growth were reached, with the growth curve of drug-exposed conidia. Amphotericin B induced PAFE in A. fumigatus at four times the MIC after 2 and 4 h of exposure ranging from 1.83 to 6.00 h and 9.33 to 10.80 h, respectively. Significantly shorter PAFEs or lack of PAFE was observed for A. terreus, A. ustus, and A. nidulans. Itraconazole did not induce measurable PAFE in the Aspergillus isolates at any concentration or exposure time tested. Further studies are warranted to investigate the implications of PAFE in relation to clinical efficacy and dosing frequency.

Oral candidal infections and antimycotics

The advent of the human immunodeficiency virus infection and the increasing prevalence of compromised individuals in the community due to modern therapeutic advances have resulted in a resurgence of opportunistic infections, including oral candidoses. One form of the latter presents classically as a white lesion of "thrush" and is usually easily diagnosed and cured. Nonetheless, a minority of these lesions appears in new guises such as erythematous candidosis, thereby confounding the unwary clinician and complicating its management. Despite the availability of several effective antimycotics for the treatment of oral candidoses, failure of therapy is not uncommon due to the unique environment of the oral cavity, where the flushing effect of saliva and the cleansing action of the oral musculature tend to reduce the drug concentration to sub-therapeutic levels. This problem has been partly circumvented by the introduction of the triazole agents, which initially appeared to be highly effective. However, an alarming increase of organisms resistant to the triazoles has been reported recently. In this review, an overview of clinical manifestations of oral candidoses and recent advances in antimycotic therapy is given, together with newer concepts, such as the post-antifungal effect (PAFE) and its possible therapeutic implications.

Influence of pH and concentration on the postantifungal effect and on the effects of sub-MIC concentrations of 4 antifungal agents on previously treated Candida spp

This study investigates the impact of different pH values (5.5 and 7.4) on the postantifungal effect (PAFE) and the effect of sub-MIC concentrations (1/4 x MIC) on C. albicans and C. glabrata in the PAFE stage (PAFSE). The PAFE stage was induced by a 1.5 h pretreatment with different doses (1, 4 and 8 x MIC) of 4 antifungal agents. An increase in the pH and/or an increase in the dose of the antimycotic prolonged the duration of the PAFE induced by amphotericin B or 5-fluorocytosine and the PAFSE induced by all 4 antifungal agents in both species. 5-Fluorocytosine and amphotericin B (except for treatment with 1 x MIC at pH 5.5) induced significant PAFEs (0.5-3.0 h and 1.4-4.8 h, respectively), which were increased (to 0.9-3.2 h and 0.8-3.4 h, respectively) by posterior (PLEASE EXPLAIN WHAT YOU MEAN BY THE WORD "POSTERIOR" HERE) exposure to 1/4 X MIC of the respective antifungal agent. Although ketoconazole and fluconazole were not able to induce significant PAFEs, posterior exposure to 1/4 x MIC of each of these 2 azoles led to significant PAFSEs of up to 2.6 h in both yeast species when the concentrations and pH were high enough.