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

Abstract

Drug exposure or pharmacodynamic breakpoints refer to a magnitude of drug exposure which separates a population into groups with high and low probabilities of attaining a desired outcome. We used a pharmacodynamic model of disseminated candidiasis to define an in vivo drug exposure breakpoint for flucytosine (5FC) against Candida albicans. The results were bridged to humans by using population pharmacokinetics and Monte Carlo simulation. An in vivo drug exposure breakpoint for 5FC was apparent when serum levels were above the MIC for 45% of the dosing interval. The Monte Carlo simulations suggested that using a human dose of 100 mg/kg of body weight/day in four divided doses, 5FC resistance was defined at an MIC of 32 mg/liter. Target attainment rates following administration of 25, 50, and 100 mg/kg/day were similar, suggesting that the use of a lower dose of 5FC is possible. Using six isolates of C. albicans with MICs ranging from 0.06 to >64 mg/liter, we also explored the influence that the MIC, the fraction of the dosing interval that the serum levels of 5FC remained above the MIC (T>MIC), the 5FC resistance genotype, and the in vivo growth rate had on the response to 5FC. The MIC and T>MIC were both critical measures affecting the generation of a drug effect but had no bearing on the magnitude of the maximal kill induced by 5FC. The in vivo growth rate was a critical additional determinant of the exposure-response relationship. There was a relationship between the 5FC resistance genotype and the exposure-response relationship.

FIG. 1.

(A) Sigmoid-E_max relationship for isolate F/6862 (MIC, 0.125 mg/liter). Data are means ± SD. The doses producing 0.5-, 1-, and 2-log falls in log₁₀CFU/g were 0.18, 0.37, and 1 mg/kg administered every 8 h, respectively. (B) Kaplan-Meier survival curves for groups of mice infected with strain F/6862 (MIC, 0.125 mg/liter) following 24 h of 5FC therapy, to further examine the consequences of various degrees of antifungal killing. ‡, 2-log drop versus 1-log drop, P = 0.039; †, 1-log drop versus control, P = 0.002; §, 0.5-log drop versus control, P = 0.854.

FIG. 2.

Relationship between the continuous variable T>MIC and the probability of achieving a 2-log fall in log₁₀CFU/g in the kidneys of mice infected with strain F/6862 (MIC, 0.125 mg/liter), as defined using logistic regression. The regression coefficients for the constant and T>MIC were −7.506 (P = 0.008) and 22.08 (P = 0.004), respectively. The breakpoint value of T>MIC for 45% of the dosing interval was defined using classification and regression tree analysis. A serum level above the MIC for at least 45% of the dosing interval optimally separated the population into groups with high and low probabilities of a 2-log fall within the kidney. Circles represent raw data obtained from eight mice (except for controls and the group in which the T>MIC was 0.83, which consisted of four mice).

FIG. 3.

Fraction of the 9,999 simulated patients which achieved the drug exposure target (i.e., serum 5FC concentrations greater than the MIC for at least 45% of the dosing interval) following the administration of 5FC at 25, 50 and 100 mg/kg/day, depicted by open circles, triangles, and squares, respectively. The distribution of the MICs for 1,941 C. albicans isolates, obtained using the EUCAST method, is shown by the solid circles.

FIG. 4.

Dose-response relationships for each of the study strains. Data are means ± SD. (A) F/10141 (MIC, 0.06 mg/liter; homozygous susceptible); (B) F/6862 (MIC, 0.125 mg/liter; homozygous susceptible); (C) F/7421 (MIC, 0.5 mg/liter; heterozygous); (D) F/9651 (MIC, 4.0 mg/liter; resistance mechanism unknown); (E) F/9464 (MIC, 8.0 mg/liter; homozygous resistant); (F) F/8341 (MIC, 64 mg/liter; homozygous resistant) (for this strain only the maximum possible dose was studied and growth was observed). The model fits from the population analysis and mean parameter estimates for each strain are shown. The EC₅₀s vary 25-fold.

FIG. 5.

Relationship between the fraction of the dosing interval that serum 5FC concentrations are above the MIC (T>MIC) and the observed response for each of the study strains. The model was fitted to the data using a population methodology; r² = 99.2%. The importance of the T>MIC in accounting for the exposure-response relationships is highlighted by the fact that the estimates for the EC₅₀s now vary approximately twofold. The decline in log₁₀CFU/g varies between strains and is independent of the 5FC MIC.

FIG. 6.

Correlation between the growth constant (K_g) and the maximum effect induced by 5FC. The maximum effect was taken from the mean parameter estimates shown in Fig. 1.

FIG. 7.

Expanded model describing the effect of 5FC. The exposure-response relationships are influenced by both the fraction of the dosing interval that the serum concentrations of 5FC are above the MIC (T>MIC) and the growth rate (K_g).