Methodological issues related to antifungal drug interaction modelling for filamentous fungi:

In Vitro–In Vivo Correlation of Posaconazole–Amphotericin B Combination against Candida albicans: In Vitro Interacting Concentrations Are Associated with In Vivo Free Drug Levels

The in vitro/in vivo correlation of antifungal combination testing is necessary in order to assess the efficacy of combination regimens. We, therefore, attempted to correlate in vitro chequerboard testing of posaconazole (POS) and amphotericin B (AMB) with the in vivo outcome of combination therapy against experimental candidiasis in a neutropenic murine model. The AMB + POS combination was tested against a Candida albicans isolate. In vitro, a broth microdilution 8 × 12 chequerboard method with serial two-fold drug dilutions was used. In vivo, CD1 female neutropenic mice with experimental disseminated candidiasis were treated with i.p. AMB and p.o. POS alone and in combination at three effective doses (ED20, ED50 and ED80 corresponding to 20%, 50% and 80% of maximal effect, respectively). CFU/kidneys after 2 days were determined. The pharmacodynamic interactions were assessed based on Bliss independence interaction analysis. In vitro, a Bliss antagonism of −23% (−23% to −22%) was observed at 0.03–0.125 mg/L of AMB with 0.004–0.015 mg/L of POS, while a Bliss synergy of 27% (14%–58%) was observed at 0.008–0.03 mg/L of AMB with 0.000015–0.001 mg/L of POS. In vivo, Bliss synergy (13 ± 4%) was found when an AMB ED20 of 1 mg/kg was combined with all POS ED 0.2–0.9 mg/kg, while Bliss antagonism (35–83%) was found for the combinations of AMB ED50 2 mg/kg and ED80 3.2 mg/kg with POS ED80 of 0.9 mg/kg. Free drug serum levels of POS and AMB in in vivo synergistic and antagonistic combinations were correlated with the in vitro synergistic and antagonistic concentrations, respectively. Both synergistic and antagonistic interactions were found for the AMB + POS combination. POS compromised the efficacy of high effective AMB doses and enhanced low ineffective AMB doses. In vitro concentration-dependent interactions were correlated with in vivo dose-dependent interactions of the AMB + POS combination. In vivo interactions occurred at free drug serum levels close to in vitro interacting concentrations.

Multicenter Collaborative Study of the Interaction of Antifungal Combinations against Candida Spp. by Loewe Additivity and Bliss Independence-Based Response Surface Analysis

Combination antifungal therapy is widely used but not well understood. We analyzed the spectrophotometric readings from a multicenter study conducted by the New York State Department of Health to further characterize the in vitro interactions of the major classes of antifungal agents against Candida spp. Loewe additivity-based fractional inhibitory concentration index (FICi) analysis and Bliss independence-based response surface (BIRS) analysis were used to analyze two-drug inter- and intraclass combinations of triazoles (AZO) (voriconazole, posaconazole), echinocandins (ECH) (caspofungin, micafungin, anidulafungin), and a polyene (amphotericin B) against Candida albicans, C. parapsilosis, and C. glabrata. Although mean FIC indices did not differ statistically significantly from the additivity range of 0.5−4, indicating no significant pharmacodynamic interactions for all of the strain−combinations tested, BIRS analysis showed that significant pharmacodynamic interactions with the sum of percentages of interactions determined with this analysis were strongly associated with the FIC indices (Χ2 646, p < 0.0001). Using a narrower additivity range of 1−2 FIC index analysis, statistically significant pharmacodynamic interactions were also found with FICi and were in agreement with those found with BIRS analysis. All ECH+AB combinations were found to be synergistic against all Candida strains except C. glabrata. For the AZO+AB combinations, synergy was found mostly with the POS+AB combination. All AZO+ECH combinations except POS+CAS were synergistic against all Candida strains although with variable magnitude; significant antagonism was found for the POS+MIF combination against C. albicans. The AZO+AZO combination was additive for all strains except for a C. parapsilosis strain for which antagonism was also observed. The ECH+ECH combinations were synergistic for all Candida strains except C. glabrata for which they were additive; no antagonism was found.

Madurella mycetomatis, the main causative agent of eumycetoma, is highly susceptible to olorofim

Objectives

Eumycetoma is currently treated with a combination of itraconazole therapy and surgery, with limited success. Recently, olorofim, the lead candidate of the orotomides, a novel class of antifungal agents, entered a Phase II trial for the treatment of invasive fungal infections. Here we determined the activity of olorofim against Madurella mycetomatis, the main causative agent of eumycetoma.

Methods

Activity of olorofim against M. mycetomatis was determined by in silico comparison of the target gene, dihydroorotate dehydrogenase (DHODH), and in vitro susceptibility testing. We also investigated the in vitro interaction between olorofim and itraconazole against M. mycetomatis.

Results

M. mycetomatis and Aspergillus fumigatus share six out of seven predicted binding residues in their DHODH DNA sequence, predicting susceptibility to olorofim. Olorofim demonstrated excellent potency against M. mycetomatis in vivo with MICs ranging from 0.004 to 0.125 mg/L and an MIC₉₀ of 0.063 mg/L. Olorofim MICs were mostly one dilution step lower than the itraconazole MICs. In vitro interaction studies demonstrated that olorofim and itraconazole work indifferently when combined.

Conclusions

We demonstrated olorofim has potent in vitro activity against M. mycetomatis and should be further evaluated in vivo as a treatment option for this disease.

Antifungal activity of silver nanoparticles in combination with ketoconazole against Malassezia furfur

Malassezia furfur is lipophilic and lipid-dependent yeast, inhabitant of human skin microbiota associated with several dermal disorders. In recent years, along with the advances in nanotechnology and the incentive to find new antimicrobial drugs, there has been a growing interest in the utilization of nanoparticles for the treatment of skin microbial infections. This work aimed to study the in vitro inhibitory activity of silver nanoparticles (AgNP) against 41 M. furfur clinical isolates, visualize the interaction between AgNP-Malassezia, evaluate the synergism with ketoconazole (KTZ) and to produce an antimicrobial gel of AgNP–KTZ. The synthesized AgNP were randomly distributed around the yeast surface and showed a fungicidal action with low minimal inhibitory concentration values. AgNP showed no antagonistic effect with KTZ. The broad-spectrum antimicrobial property with fungicidal action of AgNP and its accumulation in affected areas with a sustained release profile, added to the great antifungal activity of KTZ against Malassezia infections and other superficial mycoses, allowed us to obtain a gel based on carbopol formulated with AgNP–KTZ with the potential to improve the topical therapy of superficial malasseziosis, reduce the number of applications and, also, prevent the recurrence.

Combination of Amphotericin B and Flucytosine against Neurotropic Species of Melanized Fungi Causing Primary Cerebral Phaeohyphomycosis

Primary central nervous system phaeohyphomycosis is a fatal fungal infection due mainly to the neurotropic melanized fungiCladophialophora bantiana,Rhinocladiella mackenziei, andExophiala dermatitidis.Despite the combination of surgery with antifungal treatment, the prognosis continues to be poor, with mortality rates ranging from 50 to 70%. Therefore, a search for a more-appropriate therapeutic approach is urgently needed. Ourin vitrostudies showed that with the combination of amphotericin B and flucytosine against these species, the median fractional inhibitory concentration (FIC) indices for strains ranged from 0.25 to 0.38, indicating synergy. By use of Bliss independence analysis, a significant degree of synergy was confirmed for all strains, with the sum ΔE ranging from 90.2 to 698.61%. No antagonism was observed. These results indicate that amphotericin B, in combination with flucytosine, may have a role in the treatment of primary cerebral infections caused by melanized fungi belonging to the orderChaetothyriales Furtherin vivostudies and clinical investigations to elucidate and confirm these observations are warranted.

Optimization of polyene-azole combination therapy against aspergillosis using an in vitro pharmacokinetic-pharmacodynamic model

Although amphotericin B-azole combination therapy has traditionally been questioned due to potential antagonistic interactions, it is often used successfully to treat refractory invasive aspergillosis. So far, pharmacodynamic (PD) interactions have been assessed with conventional in vitro tests, which do not mimic human serum concentrations and animal models using limited doses. We therefore simulated the human serum concentration profiles of amphotericin B and voriconazole in an in vitro dialysis/diffusion closed pharmacokinetic-pharmacodynamic (PK-PD) model and studied the pharmacodynamic interactions against an azole-resistant and an azole-susceptible Aspergillus fumigatus isolate, using Bliss independence and canonical mixture response surface analyses. Amphotericin B dosing regimens with the drug administered every 24 h (q24h) were combined with voriconazole q12h dosing regimens. In vitro PK-PD combination data were then combined with human PK data by using Monte Carlo analysis. The target attainment rate and the serum concentration/MIC ratio were calculated for isolates with different MICs. Synergy (20 to 31%) was observed at low amphotericin B-high voriconazole exposures, whereas antagonism (-6 to -16%) was found at high amphotericin B-low voriconazole exposures for both isolates. Combination therapy resulted in 17 to 48% higher target attainment rates than those of monotherapy regimens for isolates with voriconazole/amphotericin B MICs of 1 to 4 mg/liter. Optimal activity was found for combination regimens with a 1.1 total minimum concentration of drug in serum (tCmin)/MIC ratio for voriconazole and a 0.5 total maximum concentration of drug in serum (tCmax)/MIC ratio for amphotericin B, whereas the equally effective monotherapy regimens required a voriconazole tCmin/MIC ratio of 1.8 and an amphotericin B tCmax/MIC ratio of 2.8. Amphotericin B-voriconazole combination regimens were more effective than monotherapy regimens. Therapeutic drug monitoring can be employed to optimize antifungal combination therapy with low-dose (≤0.6 mg/kg) amphotericin B-based combination regimens against resistant isolates for minimal toxicity.

In vitro interaction of voriconazole and anidulafungin against triazole-resistant Aspergillus fumigatus

Voriconazole is the recommended drug of first choice to treat infections caused by Aspergillus fumigatus. The efficacy of voriconazole might be hampered by the emergence of azole resistance. However, the combination of voriconazole with anidulafungin could improve therapeutic outcomes in azole-resistant invasive aspergillosis (IA). The in vitro interaction between voriconazole and anidulafungin was determined against voriconazole-susceptible and voriconazole-resistant (substitutions in the cyp51A gene, including single point [M220I and G54W] and tandem repeat [34-bp tandem repeat in the promoter region of the cyp51A gene in combination with substitutions at codon L98 and 46-bp tandem repeat in the promoter region of the cyp51A gene in combination with mutation at codons Y121 and T289] mutations) clinical A. fumigatus isolates using a checkerboard microdilution method with spectrophotometric analysis and a viability-based XTT {2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)carbonyl]-2H-tetrazolium hydroxide} assay within 2 h of exposure after 24 and 48 h of incubation at 35 °C to 37 °C. Fractional inhibitory concentration (FIC) indexes (FICis) were determined using different MIC endpoints and Bliss independence analysis performed based on the response surface calculation of the no-drug interaction. Significant synergistic interactions obtained based on measuring the FIC index were dependent on the MIC endpoint, in which FICs were inversely related to voriconazole and anidulafungin MICs and were influenced by the CYP51A genotype. A statistically significant difference was observed between FIC indexes of isolates harboring tandem repeat mutations and wild-type controls (P = 0.006 by one-way analysis of variance [ANOVA]), indicating that synergy is decreased in azole-resistant strains. Our results indicated that a combination of voriconazole and anidulafungin might be effective against infections caused by both azole-susceptible and azole-resistant A. fumigatus isolates, but the combination could possibly be less effective in voriconazole-resistant strains with high MICs. Studies in vivo and in vitro-in vivo correlation investigations are required to validate the potential synergy of voriconazole and anidulafungin.

Micafungin in combination with voriconazole in Aspergillus species: a pharmacodynamic approach for detection of combined antifungal activity in vitro

OBJECTIVES

Preclinical and clinical evidence indicate that echinocandin-triazole combinations provide enhanced killing versus triazoles alone against some Aspergillus isolates, however, in vitro test results designed to detect this combined effect are difficult to interpret.

METHODS

We used a straightforward pharmacodynamic approach based on a microdilution format and a colorimetric analysis to harmonize growth end points.

RESULTS

We detected a fourfold decrease in the EC90 of voriconazole when tested in combination with micafungin (4 mg/L) against isolates of Aspergillus fumigatus and Aspergillus terreus, but not against an isolate of Aspergillus flavus. Echinocandin-enhanced voriconazole activity was confirmed in A. fumigatus and A. terreus but not A. flavus by fluorescent morbidity staining and fluorescence microscopic analysis of damaged hyphae.

CONCLUSIONS

A microdilution-based pharmacodynamic method for testing antifungal combinations provides a less ambiguous description of the combined effects of antifungals against moulds and could be useful in reference laboratories that routinely evaluate the activity of antifungal combinations in vitro and in vivo.

Assessing in vitro combinations of antifungal drugs against yeasts and filamentous fungi: comparison of different drug interaction models

Non-parametric and parametric approaches of two competing zero-interaction theories--the Loewe additivity and the Bliss independence - were evaluated for analyzing the in vitro interactions of various antifungal drugs. Fifty-one data sets, derived from three drug combinations, tested in triplicate against 17 clinical yeast and mold isolates with a two-dimensional checkerboard microdilution technique, were selected to span from strong synergy to strong antagonism. These were analyzed with the standard FIC index model and modern concentration-effect response surface models: the fully parametric model developed by Greco et al. and the 3-D analysis developed by Prichard et al. The FIC index model is subjective, sensitive to experimental errors and resulted in approximated results and variable conclusions depending on the MIC endpoints determined and interpretation endpoints used. By using the MIC-2 endpoint (lowest drug concentration showing 50% of growth) for calculating the FIC indices, problems due to trailing phenomena were reduced and weak interactions could be detected; higher levels of reproducibility and agreement with the other models were achieved using the MIC-0 and MIC-1 (lowest drug concentration showing 10 and 25% of growth, respectively). High reproducibility was achieved in interpreting the FIC indices when the cutoffs of 0.25 and 4 (for single experiments) and the cutoff of 1 (for replicates) were used for defining the limits of additivity/indifference. Although the fully parametric Greco model did not describe precisely the entire response surface of all antifungal drug interactions, it was able to differentiate synergistic from non-synergistic interactions with a non-unit, reproducible, concentration-independent interaction parameter, including its uncertainty, without requiring replication. The Bliss independence based models resulted in mosaics of synergistic and antagonistic combinations, raising questions about the concentration-dependent nature of antifungal drug interaction. The sum of all statistically significant interactions were used as a summary interaction parameter for the entire response surface, concluding synergy or antagonism when it was positive or negative, respectively. The cutoffs of 100% and 200% were used to distinguish weak and moderate interactions, respectively in 12-16 x 8-12 checkerboard formats. Semi-parametric approaches need particular care as experimental errors are not eliminated from the entire response surface.

In vitro interactions between amphotericin B, itraconazole, and flucytosine against 21 clinical Aspergillus isolates determined by two drug interaction models

Combination therapy of flucytosine (5FC) with other antifungal agents could be of use for the treatment of invasive aspergillosis. However, interpretation of the results of in vitro interactions is problematic. The fractional inhibitory concentration (FIC) index is the most commonly used method, but it has several major drawbacks in characterizing antifungal drug interaction. Alternatively, a response surface approach using the concentration-effect relationship over the whole concentration range instead of just the MIC can be used. We determined the in vitro interactions between amphotericin B (AMB), itraconazole, and 5FC against 21 Aspergillus isolates with a broth microdilution checkerboard method that employs the dye MTT [3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide]. FIC indices based on three different MIC endpoints (MIC-0, MIC-1, and MIC-2) and the interaction coefficient alpha were determined, the latter by estimation from the response surface approach described by Greco et al. (W. R. Greco, G. Bravo, and J. C. Parsons, Pharmacol. Rev. 47:331-385, 1995). The value obtained for the FIC index was found to be dependent on the MIC endpoint used and could be either synergistic, indifferent, or antagonistic. The response surface approach gave more consistent results. Of the three combinations tested, the AMB-5FC combination was the most potent in vitro against Aspergillus spp. We conclude that the use of the response surface approach for the interpretation of in vitro interaction studies of antifungals may be helpful in order to predict the nature and intensity of the drug interaction. However, the correlation of these results with clinical outcome remains difficult and needs to be further investigated.