Pharmacodynamics of liposomal amphotericin B and flucytosine for cryptococcal meningoencephalitis: safe and effective regimens for immunocompromised patients

Abstract

Background: Cryptococcal meningoencephalitis is a lethal infection with relatively few therapeutic options. The optimal dosage of liposomal amphotericin B (LAmB) alone or in combination with flucytosine is not known.

Methods: A murine model of cryptococcal meningoencephalitis was used. The fungal density in the brain was determined using quantitative cultures. Pharmacokinetic-pharmacodynamic relationships were determined for LAmB and flucytosine administered alone. The effect of the combination was described using the Greco model and a mathematical model. The results were bridged to humans.

Results: Inoculation resulted in hematogenous dissemination and logarithmic growth within the central nervous system. There was histological evidence of multifocal infection throughout the brain. Both LAmB and flucytosine produced a dose-dependent reduction in fungal burden. The effect of the combination of agents in the brain was additive. Bridging studies suggested that a human dosage of LAmB 3 mg/kg/d resulted in a submaximal antifungal effect. Regimens of LAmB 6 mg/kg/d alone, LAmB 3 mg/kg/d plus flucytosine 50 mg/kg/d, and LAmB 3 mg/kg/d plus flucytosine 100 mg/kg/d all resulted in near-maximal antifungal activity.

Conclusions: Potential regimens for further study in clinical trials include LAmB 6 mg/kg/d alone, LAmB 3 mg/kg/d plus flucytosine 50 mg/kg/d, and LAmB 3 mg/kg/d plus flucytosine 100 mg/kg/d.

Keywords: Cryptococcus; flucytosine; liposomal amphotericin B; meningitis; meningoencephalitis.

Figure 1.

The rate of change of fungal density in the cerebrum of mice infected with Cryptococcus neoformans at time zero. Data are pooled from 5 experiments and are mean ± SD of 3–4 mice. The solid line is the fit of the model dN/dt = Kgmax*(N/POPMAX)*N to the data, where N is the number of organisms, Kgmax is the growth constant, and POPMAX is the maximal fungal density in the brain. The estimates for the parameter values are Kgmax = 0.08 log₁₀CFU/g brain/h and POPMAX = 6.41 log₁₀CFU/g brain and the number of organisms in the brain immediately after inoculation of 2.00 log₁₀CFU/g brain; r² = 0.88. The corresponding histological sections stained with hematoxylin and eosin and anti–C. neoformans antibody and taken on days 1, 3, 5, and 7 post infection are shown. A high power section shows the formation of cyst-like structures with leaching of cryptococcal capsular material into the contiguous parenchyma.

Figure 2.

The pharmacokinetics of liposomal amphotericin B (LAmB) 3, 10, and 20 mg/kg/d intravenously in plasma and cerebrum. Data are mean ± SD, and the solid line is the fit of the model to the data.

Figure 3.

The pharmacokinetics of flucytosine 25, 50, and 100 mg/kg every 8 hours orally in plasma and cerebrum. Data are mean ± SD, and the solid line is the fit of the model to the data.

Figure 4.

The pharmacodynamic data for liposomal amphotericin B and flucytosine administered alone. Data are mean ± SD. The total daily dosages of fluconazole are 75, 150, and 300 mg/kg/d.

Figure 5.

The fitted surface from the Greco model to the data from the combination of liposomal amphotericin B and flucytosine 168 hours post inoculation. As in this case, an additive surface is usually relatively flat (ie, not concave as seen in synergy nor convex as seen in antagonism). Because the interaction term is essentially zero, the surface shown is very close to the null interaction surface. The additive interaction extends over the entire surface. The total daily dose of flucytosine was administered in 3 divided dosages (every 8 hours). The raw data are not shown. Estimates for the parameter values are Econ 6.41 log₁₀CFU/g, E_50,5FC 487.9 mg/kg/d, m_5FC 1.11, E_50,LAmB 15.18 mg/kg/d, m_LAmB 0.65, and α = 0.74 × 10⁻⁶ (95% confidence interval,−2.38–2.38); r² = 0.84).

Figure 6.

The humanized pharmacokinetics and predicted pharmacodynamics for patients receiving various antifungal regimens. The dosages are those that would be used in a human. (A) The mean human concentration–time profile of patients receiving liposomal amphotericin B 3 and 6 mg/kg/d (solid line) with the humanized mouse profiles (dotted lines). (B) The mean human concentration–time profile of patients receiving flucytosine 50 and 100 mg/kg/d in 4 divided dosages (solid line) with the humanized mouse profiles. (C–F) The model-predicted pharmacodynamics of various antifungal regimens in comparison with untreated controls (dotted line).