In vitro pharmacokinetic/pharmacodynamic modeling of voriconazole activity against Aspergillus species in a new in vitro dynamic model

Abstract

The pharmacodynamics (PD) of voriconazole activity against Aspergillus spp. were studied using a new in vitro dynamic model simulating voriconazole human pharmacokinetics (PK), and the PK-PD data were bridged with human drug exposure to assess the percent target (near-maximum activity) attainment of different voriconazole dosages. Three Aspergillus clinical isolates (1 A. fumigatus, 1 A. flavus, and 1 A. terreus isolate) with CLSI MICs of 0.5 mg/liter were tested in an in vitro model simulating voriconazole PK in human plasma with C(max) values of 7, 3.5, and 1.75 mg/liter and a t(1/2) of 6 h. The area under the galactomannan index-time curve (AUC(GI)) was used as the PD parameter. In vitro PK-PD data were bridged with population human PK of voriconazole exposure, and the percent target attainment was calculated. The in vitro PK-PD relationship of fAUC(0-24)-AUC(GI) followed a sigmoid pattern (global R(2) = 0.97), with near-maximum activities (10% fungal growth) observed at an fAUC(0-24) (95% confidence interval [CI]) of 18.9 (14.4 to 23.1) mg · h/liter against A. fumigatus, 26.6 (21.1 to 32.9) mg · h/liter against A. flavus, and 36.2 (27.8 to 45.7) mg · h/liter against A. terreus (F test; P < 0.0001). Target attainment for 3, 4, and 5 mg/kg-of-body-weight voriconazole dosages was 24% (11 to 45%), 80% (32 to 97%), and 93% (86 to 97%) for A. fumigatus, 12% (5 to 26%), 63% (17 to 93%), and 86% (73 to 94%) for A. flavus, and 4% (2 to 11%), 36% (6 to 83%), and 68% (47 to 83%) for A. terreus. Based on the in vitro exposure-effect relationships, a standard dosage of voriconazole may be adequate for most patients with A. fumigatus but not A. flavus and A. terreus infections, for which a higher drug exposure may be required. This could be achieved using a higher voriconazole dosage, thus highlighting the usefulness of therapeutic drug monitoring in patients receiving a standard dosage.

Fig 1

In vitro pharmacokinetic/pharmacodynamic model that simulates the pharmacokinetics of voriconazole in humans and determines a drug's effect on Aspergillus growth. The model consists of an external compartment (EC), a glass beaker containing 700 ml broth medium, and an internal compartment (IC), a dialysis tube containing 10 ml broth medium; the tube is made of a semipermeable cellulose membrane that allows free passage of small molecules (<20 kDa), such as like antifungals, but not galactomannan. The EC is placed on a heated magnetic stirrer (37°C and 2 rpm). A peristaltic pump introduces drug-free medium in the EC and removes its content concurrently in order to maintain a constant volume. The flow rate is adjusted to achieve drug concentrations corresponding to their clearance from human plasma. At time zero, 10⁵ CFU/ml of Aspergillus conidia are inoculated into the IC, while the drug is introduced into both the EC and the IC for rapid concentration equilibration. Subsequently, the drug concentration declines over time. Galactomannan levels of the IC medium are measured at regular time points (adapted from ref. 11).

Fig 2

Microbiological method for determining voriconazole levels. Linear regression analysis between the diameter of the inhibition zone (y axis) and the drug concentration (mg/liter) (x axis). The intra- and interexperimental variation was <20% among all drug concentrations. Error bars represent standard deviations.

Fig 3

Use of the area under the galactomannan index-time curve (AUC_GI) as a surrogate marker of fungal growth. Kinetics of galactomannan production by increasing inocula of A. fumigatus (left graph) and correlation of the AUC_GI with the initial inoculum (right graph) are shown.

Fig 4

Pharmacokinetic analysis of simulated doses with C_max values of 7, 3.5, and 1.75 mg/liter of voriconazole in the in vitro pharmacokinetic/pharmacodynamic system. The horizontal dotted line represents the lower limit of detection of the bioassay. Error bars represent standard deviations.

Fig 5

Galactomannan index-time curves in the internal compartment of the in vitro PKPD model for each voriconazole simulated dose against the three Aspergillus species.

Fig 6

In vitro PKPD relationship of voriconazole. The relationship between the area under the galactomannan index curve (AUC_GI) (left graph) or normalized AUC_GI (right graph) and the area under the concentration-time curve (fAUC_0–24) for each Aspergillus species. The AUC_GI for the first 24 h was used as the surrogate marker of fungal growth.