A new in-vitro kinetic model to study the pharmacodynamics of antifungal agents: inhibition of the fungicidal activity of amphotericin B against Candida albicans by voriconazole

Similarities and distinctions in the activation of the Candida glabrata Pdr1 regulatory pathway by azole and non-azole drugs

Incidences of fluconazole (FLC) resistance among Candida glabrata clinical isolates are a growing issue in clinics. The pleiotropic drug response network in C. glabrata confers azole resistance and is defined primarily by the Zn2Cys6 zinc cluster-containing transcription factor Pdr1 and target genes such as CDR1, which encodes an ATP-binding cassette transporter protein thought to act as an FLC efflux pump. Mutations in the PDR1 gene that render the transcription factor hyperactive are the most common cause of fluconazole resistance among clinical isolates. The phenothiazine class drug fluphenazine and a molecular derivative, CWHM-974, which both exhibit antifungal properties, have been shown to induce the expression of Cdr1 in Candida spp. We have used a firefly luciferase reporter gene driven by the CDR1 promoter to demonstrate two distinct patterns of CDR1 promoter activation kinetics: gradual promoter activation kinetics that occur in response to ergosterol limitations imposed by exposure to azole and polyene class antifungals and a robust and rapid CDR1 induction occurring in response to the stress imposed by fluphenazines. We can attribute these different patterns of CDR1 induction as proceeding through the promoter region of this gene since this is the only segment of the gene included in the luciferase reporter construct. Genetic analysis indicates that the signaling pathways responsible for phenothiazine and azole induction of CDR1 overlap but are not identical. The short time course of phenothiazine induction suggests that these compounds may act more directly on the Pdr1 protein to stimulate its activity.

IMPORTANCE

Candida glabrata has emerged as the second-leading cause of candidiasis due, in part, to its ability to acquire high-level resistance to azole drugs, a major class of antifungal that acts to block the biosynthesis of the fungal sterol ergosterol. The presence of azole drugs causes the induction of a variety of genes involved in controlling susceptibility to this drug class, including drug transporters and ergosterol biosynthetic genes such as ERG11. We found that the presence of azole drugs leads to an induction of genes encoding drug transporters and ERG11, while exposure of C. glabrata cells to antifungals of the phenothiazine class of drugs caused a much faster and larger induction of drug transporters but not ERG11. Coupled with further genetic analyses of the effects of azole and phenothiazine drugs, our data indicate that these compounds are sensed and responded to differentially in the yeast cell.

Similarities and distinctions in the activation of the Candida glabrata Pdr1 regulatory pathway by azole and non-azole drugs

Incidences of fluconazole (FLC) resistance among Candida glabrata clinical isolates is a growing issue in clinics. The pleiotropic drug response (PDR) network in C. glabrata confers azole resistance and is defined primarily by the Zn2Cys6 zinc cluster-containing transcription factor Pdr1 and target genes such as CDR1, that encodes an ATP-binding cassette transporter protein thought to act as a FLC efflux pump. Mutations in the PDR1 gene that render the transcription factor hyperactive are the most common cause of fluconazole resistance among clinical isolates. The phenothiazine class drug fluphenazine and a molecular derivative, CWHM-974, which both exhibit antifungal properties, have been shown to induce the expression of Cdr1 in Candida spp. We have used a firefly luciferase reporter gene driven by the CDR1 promoter to demonstrate two distinct patterns of CDR1 promoter activation kinetics: gradual promoter activation kinetics that occur in response to ergosterol limitations imposed by exposure to azole and polyene class antifungals and a robust and rapid CDR1 induction occurring in response to the stress imposed by fluphenazines. We can attribute these different patterns of CDR1 induction as proceeding through the promoter region of this gene since this is the only segment of the gene included in the luciferase reporter construct. Genetic analysis indicates that the signaling pathways responsible for phenothiazine and azole induction of CDR1 overlap but are not identical. The short time course of phenothiazine induction suggests that these compounds may act more directly on the Pdr1 protein to stimulate its activity.

Construction of an in vitro simulated one compartment extravascular administration model and its comparison with classic in vitro administration model in copper chloride induced HepG2 cell death

In this study, we designed an in vitro administration device based on compartment model theory and utilized it to construct an in vitro simulated one compartment extravascular administration model of copper chloride. Within the Cmax range of 3.91-1000.00 μM, the measured concentration-time curves of the simulated one compartment extravascular administration model almost coincide with the corresponding theoretical curves. The measured values of toxicokinetic parameters, including ke, T1/2, ka, T1/2a, Tmax, Cmax, CL, and AUC0-∞ are close to the corresponding theoretical values. The fitting coefficients are >0.9990. In simulated one compartment extravascular administration and classic in vitro administration, copper chloride dose-dependently induced HepG2 cell death. When Cmax/administration concentration is equal, classic in vitro administration induces a higher cell death rate than simulated one compartment extravascular administration. However, there is no significant difference in inducing cell death between the two administration models when area under the curve is equal. In conclusion, the device designed in this study can be used to in vitro simulate one compartment extravascular administration, making in vitro toxicity testing more similar to in vivo scenarios. There are differences in copper chloride induced HepG2 cell death between simulated one compartment extravascular administration and classic in vitro administration.

Sphingosine as a New Antifungal Agent against Candida and Aspergillus spp

This study investigated whether sphingosine is effective as prophylaxis against Aspergillus spp. and Candida spp. In vitro experiments showed that sphingosine is very efficacious against A. fumigatus and Nakeomyces glabrataa (formerly named C. glabrata). A mouse model of invasive aspergillosis showed that sphingosine exerts a prophylactic effect and that sphingosine-treated animals exhibit a strong survival advantage after infection. Furthermore, mechanistic studies showed that treatment with sphingosine leads to the early depolarization of the mitochondrial membrane potential (Δψm) and the generation of mitochondrial reactive oxygen species and to a release of cytochrome C within minutes, thereby presumably initiating apoptosis. Because of its very good tolerability and ease of application, inhaled sphingosine should be further developed as a possible prophylactic agent against pulmonary aspergillosis among severely immunocompromised patients.

Pharmacodynamic effects of simulated standard doses of antifungal drugs against Aspergillus species in a new in vitro pharmacokinetic/pharmacodynamic model

In conventional ΜΙC tests, fungi are exposed to constant drug concentrations, whereas in vivo, fungi are exposed to changing drug concentrations. Therefore, we developed a new in vitro pharmacokinetic/pharmacodynamic model where human plasma pharmacokinetics of standard doses of 1 mg/kg amphotericin B, 4 mg/kg voriconazole, and 1 mg/kg caspofungin were simulated and their pharmacodynamic characteristics were determined against three clinical isolates of Aspergillus fumigatus, Aspergillus flavus, and Aspergillus terreus with identical MICs (1 mg/liter for amphotericin B, 0.5 mg/liter for voriconazole) and minimum effective concentrations (0.5 mg/liter for caspofungin). This new model consists of an internal compartment (a 10-ml dialysis tube made out of a semipermeable cellulose membrane allowing the free diffusion of antifungals but not galactomannan) inoculated with Aspergillus conidia and placed inside an external compartment (a 700-ml glass beaker) whose content is diluted after the addition of antifungal drugs by a peristaltic pump at the same rate as the clearance of the antifungal drugs in human plasma. Fungal growth was assessed by galactomannan production. Despite demonstrating the same MICs, amphotericin B completely inhibited (100%) A. fumigatus but not A. flavus and A. terreus, whose growth was delayed for 7.53 and 22.8 h, respectively. Voriconazole partially inhibited A. fumigatus (49.5%) and Α. flavus (27.9%) but not Α. terreus; it delayed their growth by 3.99 h (A. fumigatus) and 5.37 h (Α. terreus). Caspofungin did not alter galactomannan production in all of the species but A. terreus. The new model simulated human pharmacokinetics of antifungal drugs and revealed important pharmacodynamic differences in their activity.

Posaconazole in human serum: a greater pharmacodynamic effect than predicted by the non-protein-bound serum concentration

It is generally accepted that only the unbound fraction of a drug is pharmacologically active. Posaconazole is an antifungal agent with a protein binding of 98 to 99%. Taking into account the degree of protein binding, plasma levels in patients, and MIC levels of susceptible strains, it can be assumed that the free concentration of posaconazole sometimes will be too low to exert the expected antifungal effect. The aim was therefore to test the activity of posaconazole in serum in comparison with that of the calculated unbound concentrations in protein-free media. Significant differences (P < 0.05) from the serum control were found at serum concentrations of posaconazole of 1.0 and 0.10 mg/liter, with calculated free concentrations corresponding to 1× MIC and 0.1× MIC, respectively, against one Candida lusitaniae strain selected for proof of principle. In RPMI 1640, the corresponding calculated unbound concentration of 0.015 mg/liter resulted in a significant effect, whereas that of 0.0015 mg/liter did not. Also, against seven additional Candida strains tested, there was an effect of the low posaconazole concentration in serum, in contrast to the results in RPMI 1640. Fluconazole, a low-grade-protein-bound antifungal, was used for comparison at corresponding concentrations in serum and RPMI 1640. No effect was observed at the serum concentration, resulting in a calculated unbound concentration of 0.1× MIC. In summary, there was a substantially greater pharmacodynamic effect of posaconazole in human serum than could be predicted by the non-protein-bound serum concentration. A flux from serum protein-bound to fungal lanosterol 14α-demethylase-bound posaconazole is suggested.

Voriconazole-induced inhibition of the fungicidal activity of amphotericin B in Candida strains with reduced susceptibility to voriconazole: an effect not predicted by the MIC value alone

An antagonistic effect of voriconazole on the fungicidal activity of sequential doses of amphotericin B has previously been demonstrated in Candida albicans strains susceptible to voriconazole. Because treatment failure and the need to switch to other antifungals are expected to occur more often in infections that are caused by resistant strains, it was of interest to study whether the antagonistic effect was still seen in Candida strains with reduced susceptibility to voriconazole. With the hypothesis that antagonism will not occur in voriconazole-resistant strains, C. albicans strains with characterized mechanisms of resistance against voriconazole, as well as Candida glabrata and Candida krusei strains with differences in their degrees of susceptibility to voriconazole were exposed to voriconazole or amphotericin B alone, to both drugs simultaneously, or to voriconazole followed by amphotericin B in an in vitro kinetic model. Amphotericin B administered alone or simultaneously with voriconazole resulted in fungicidal activity. When amphotericin B was administered after voriconazole, its activity was reduced (median reduction, 61%; range, 9 to 94%). Levels of voriconazole-dependent inhibition of amphotericin B activity differed significantly among the strains but were not correlated with the MIC values (correlation coefficient, -0.19; P = 0.65). Inhibition was found in C. albicans strains with increases in CDR1 and CDR2 expression but not in the strain with an increase in MDR1 expression. In summary, decreased susceptibility to voriconazole does not abolish voriconazole-dependent inhibition of the fungicidal activity of amphotericin B in voriconazole-resistant Candida strains. The degree of interaction could not be predicted by the MIC value alone.

In vitro pharmacodynamic models to determine the effect of antibacterial drugs

In vitro pharmacodynamic (PD) models are used to obtain useful quantitative information on the effect of either single drugs or drug combinations against bacteria. This review provides an overview of in vitro PD models and their experimental implementation. Models are categorized on the basis of whether the drug concentration remains constant or changes and whether there is a loss of bacteria from the system. Further subdifferentiation is based on whether bacterial loss involves dilution of the medium or is associated with dialysis or diffusion. For comprehension of the underlying principles, experimental settings are simplified and schematically illustrated, including the simulations of various in vivo routes of administration. The different model types are categorized and their (dis)advantages discussed. The application of in vitro models to special organs, infections and pathogens is comprehensively presented. Finally, the relevance and perspectives of in vitro investigations in drug discovery and clinical research are elucidated and discussed.

Characterization of the inhibitory effect of voriconazole on the fungicidal activity of amphotericin B against Candida albicans in an in vitro kinetic model

OBJECTIVES

The aim of the present investigation was to study and characterize the effect of voriconazole on the fungicidal activity of amphotericin B.

METHODS

Four strains of Candida albicans susceptible to voriconazole were exposed to voriconazole and amphotericin B, either alone, simultaneously or sequentially in an in vitro kinetic model. Bolus doses resulting in voriconazole and amphotericin B concentrations of 0.005-5 and 2.5 mg/L, respectively, were administered. Antifungal-containing RPMI 1640 was eliminated and replaced by a fresh medium using a peristaltic pump, with a flow rate adjusted to obtain the desired half-lives. With two drugs tested, a computer-controlled dosing pump compensated for differences in the elimination rates. Using static time-kill methodology, one C. albicans strain was exposed to 5 mg/L voriconazole for varying durations followed by 2.5 mg/L amphotericin B after three repeated washes of voriconazole.

RESULTS

Voriconazole and amphotericin B treatment alone resulted in fungistatic and fungicidal activities, respectively. Simultaneous administration of voriconazole and amphotericin B resulted in fungicidal activity, whereas only fungistatic activity was observed when repeated doses of amphotericin B were administered sequentially after voriconazole at 24-96 h. The inhibition of the fungicidal activity of amphotericin B was voriconazole dose-dependent, but seemed to be recovered once the voriconazole concentration fell below the MIC. The fungicidal activity was quickly regained after the removal of voriconazole, irrespective of the duration of voriconazole pre-exposure.

CONCLUSIONS

Voriconazole inhibited the fungicidal effect of sequentially administered amphotericin B in a concentration- and time-dependent manner; the clinical significance of this needs further investigation.